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THE EVALUATION OF WASTE MINIMIZATION/WASTE TREATMENT STRATEGIES FOR A COMMERCIAL PRODUCTION PROCESS OF 4-METHYL-3- THIOSEMICARBAZIDE by
WILROY BENNEN
Bachelor Degree Hogeschool Drenthe (The Netherlands) A dissertation submitted in partial fulfilment of the requirements for the degree of MASTER TECHNOLOGIAE in the faculty of Applied Science at the PORT ELIZABETH TECHNIKON January 2002 Promotor : Prof B. Zeelie
CORE Metadata, citation and similar papers at core.ac.uk Co-promotor : Mr G. Rubidge Provided by South East Academic Libraries System (SEALS)
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H i n g s t a k k e r s 4 4 9 4 1 1 N P B e i l e n T h e N e t h e r l a n d s
T h e F a c u l t y C o m m i t t e e F a c u l t y o f A p p l i e d S c i e n c e P E T e c h n i k o n P r i v a t e B a g X 6 0 1 1 P o r t E l i z a b e t h 6 0 0 0
D e a r S i r / M a d a m
¢¡¤£¦¥¨§ © ¡¤¡¥¨¡¤¥¨¡¤©£ © ¤ £¦© ¨
I h e r e b y c o n f i r m t h a t t h e p r o p o s e d a m e n d m e n t s h a v e b e e n m a d e t o m y d i s s e r t a t i o n i n c o m p l i a n c e w i t h t h e r u l e s s e t o u t b y t h e P E T e c h n i k o n E x a m i n a t i o n D e p a r t m e n t a n d F a c u l t y C o m m i t t e e . T h e f i n a l d o c u m e n t w i l l b e d e l i v e r e d t o t h e E x a m i n a t i o n D e p a r t m e n t a s s p e c i f i e d i n t h e l e t t e r o f a p p r o v a l .
Y o u r s F a i t h f u l l y
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38
SUMMARY
Chemical synthesis is closely related to waste minimization. There is no chemical process that does not produce waste. The methods used by industry to deal with this waste is a major environmental concern. This thesis describes the laboratory scale waste minimization and waste treatment strategies for the commercial production process of 4-methyl-3-thiosemicarbazide (MTSC). The production process of 4-methyl-3- thiosemicarbazide was investigated with the aim of increasing the isolated yield of MTSC and at the same time decrease the amount and toxicity of effluent obtained. During this study, parameters were investigated such as the use of excess DIPEA and the temperature of the reaction. Preliminary studies clearly showed that both factors have a significant influence on the final yield of the product. The next part of the investigation was to optimize the two parameters influencing the isolated yield of the MTSC. For this investigation, a multi factorial design was used to determine the optimum conditions in the MTSC yield response. From the results obtained, it was clear that the excess of DIPEA and the temperature of the reaction both need to be high to obtain high yields. These theoretical results were confirmed by results obtained practically, where yields of up 82 % were obtained, but it became clear that even higher yields could be obtained since chromatographic results showed yeilds as high as 90 %. The mass balance of the MTSC synthesis showed a loss of approximately 30 grams per reaction. This loss may have an influence on the final yield.
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The effluent obtained during the synthesis of MTSC was investigated and a waste treatment protocol was established to reduce the high COD value of the MTSC effluent. The protocol consists of two steps used for the clean up of the effluent. The first being a cooling step; the effluent was cooled at 0oC to induce precipitation of a solid, consisting mostly of MTSC. The second step is a high pressure wet oxidation of the effluent with oxygen in a high pressure reactor. The remaining compounds in the effluent were oxidized, resulting in another precipitate, consisting mostly of sulphur. After the oxidation the COD value of the effluent was decreased by 98 % to a value of 0.4 %. The MTSC present in the precipitate obtained after cooling could be isolated and purified, to add to the yield of the synthesis. The sulphur obtained during the oxidation could also be isolated and reused, or sold to prevent it from contaminating the environment.
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The Chemistry of Arsenic
A t o m i c N u m b e r : 3 3 A t o m i c m a s s : 7 4 . 9 2 1 5 9 A M U E l e m e n t a l C l a s s i f i c a t i o n : M e t a l l o i d J . D . L e e , C o n c i s e I n o r g a n i c C h e m i s t r y , f i f t h e d i t i o n , C h a p m a n a n d H a l l , M a d r a s , p 4 7 7 , ( 1 9 9 6 ) .
O x i d a t i o n S t a t e s : - 3 , 0 , + 3 , + 5 V a l e n c e e l e c t r o n s : t w o S a n d t h r e e P E l e c t r o n i c s t r u c t u r e : [ A r ] 3 d 1 0 4 s 2 4 p 3 J . D . L e e , C o n c i s e I n o r g a n i c C h e m i s t r y , f i f t h e d i t i o n , C h a p m a n a n d H a l l , M a d r a s , p 9 8 1 , ( 1 9 9 6 ) . B o i l i n g p o i n t : 8 1 6 o C J . D . L e e , C o n c i s e I n o r g a n i c C h e m i s t r y , f i f t h e d i t i o n , C h a p m a n a n d H a l l , M a d r a s , p 9 7 7 , ( 1 9 9 6 ) . M e l t i n g p o i n t : 6 1 5 o C J . D . L e e , C o n c i s e I n o r g a n i c C h e m i s t r y , f i f t h e d i t i o n , C h a p m a n a n d H a l l , M a d r a s , p 9 7 5 , ( 1 9 9 6 ) . V a p o u r p r e s s u r e a t 3 7 2 o C : 1 m m H g A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 . S p e c i f i c g r a v i t y : 5 . 7 3 A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 . .
A b u n d a n c e i n e a r t h s c r u s t : 5 2 c n d o f 7 7 e l e m e n t s 1 . 8 g / m e t r i c t o n J . D . L e e , C o n c i s e I n o r g a n i c C h e m i s t r y , f i f t h e d i t i o n , C h a p m a n a n d H a l l , M a d r a s , p 9 7 3 , ( 1 9 9 6 ) . A r s e n i c l e v e l s o f s o i l s , r o c k s a n d o t h e r s u b s t a n c e s a r e g i v e n i n t a b l e ? ?
T a b l e ? ? A r s e n i c l e v e l s o f s o i l s , r o c k s a n d o t h e r s u b s t a n c e s J . G u l l e d g e , a n d J O ’ C o n n e r , “ r e m o v a l o f a r s e n i c ( V ) f r o m w a t e r b y a d s o r p t i o n o n a l u m i n i u m a n d f e r r i c h y d r o x i d e s ” , J o u r n a l o f t h e
A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . 021 , I s s u e 8 , p p 5 4 8 - 5 5 2 , ( 1 9 7 3 ) . M a t e r i a l C o n c e n t r a t i o n ( m g / k g i f n o t s t a t e d o t h e r w i s e ) O c e a n 1 4 t o n s / c u b i c m i l e E a r t h s ’ c r u s t 5 0 ( 0 . 0 0 0 0 5 % ) I g n e o u s r o c k s 1 - 9 S o i l d ( U S ) 7 . 1 P h o s p h a t e r o c k s 2 0 . 9 C o a l s ( E u r o p e , U S ) 4 5 P y r i t e ( U S ) 5 6 5 0 S p h a l e r i t e ( G e r m a n y ) 6 1 2 9
41
D e e p s e a s e d i m e n t s “ f e w p p m ” - 4 5 5 S o i l s i n v o l c a n i c r e g i o n s 2 0 P h o s p h a t e b a s e d f e r t i l i z e r s 2 0 D e t e r g e n t s 7 0 - 8 0
S t a n d a r d r e d u c t i o n p o t e n t i a l s J . D . L e e , C o n c i s e I n o r g a n i c C h e m i s t r y , f i f t h e d i t i o n , C h a p m a n a n d H a l l , M a d r a s , p 1 6 7 , ( 1 9 9 6 ) . : - A s + 3 e I A s H 3 - 0 . 6 0 V
- H A s O 2 + 3 e I A s + 0 . 2 5 V
- H A s O 3 + 2 e I H A s O 2 + 0 . 5 6 V R e d o x c o n d i t i o n s a n d p H c o n t r o l t h e s p e c i a t i o n o f a r s e n i c b e t w e e n t h e v a r o i u s
o x i d a t i o n s t a t e s . P o u r b a i x d i a g r a m s F . C o l e m a n , P o u r b a i x D i a g r a m s a n d R e a c t i o n s i n A q u e o u s
4 45656587 5:9<; ; 9<=>; 9JLK>?<4 MDB9
W o r k s A s s o c i a t i o n , V o l . U , p p 6 4 - 7 7 , ( 1 9 9 4 ) . T h e c o n v e r s i o n o f a r s e n i t e t o a r s e n a t e i n t h e p r e s e n c e o f d i s s o l v e d o x y g e n , i s o n e s u c h e x a m p l e ; t h e a r s e n a t e m a y o n l y b e f o r m e d o v e r p e r i o d s r a n g i n g f r o m d a y s t o m o n t h s , d e p e n d i n g o n t h e c o n d i t i o n s . U n d e r o x i c c o n d i t i o n s a r s e n a t e p r e d o m i n a t e s a n d m a y b e a d s o r b e d o r c o p r e c i p i t a t e d b y i r o n o r m a n g a n e s e c o m p o u n d s . A r s e n a t e m a y b e r e l e a s e d i n t h e p r e s e n c e o f o r t h o p h o s p h a t e o r n a t u r a l o r g a n i c m a t t e r ( N O M ) s i n c e t h e s e s p e c i e s c o m p e t e f o r s o r p t i o n s i t e s o n t h e s u r f a c e s o f t h e s o r p t i v e m a t r i x . U n d e r a n o x i c c o n d i t i o n s , i n t h e a b s e n c e o f s u l p h i d e s , a r s e n i t e i s s t a b l e ; i n t h e p r e s e n c e o f s u l p h i d e s , a r s e n i t e b e c o m e s i m m o b i l i z e d b y t h e f o r m a t i o n o f o r p i m e n t , r e a l g a r , a r s e n o p y r i t e , o r i s c o p r e c i p i t a t e d w i t h i r o n p y r i t e . M . E d w a r d s , “ C h e m i s t r y o f a r s e n i c r e m o v a l d u r i n g c o a g u l a t i o n a n d F e - M n o x i d a t i o n ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s
A s s o c i a t i o n , V o l . U , p p 6 4 - 7 7 , ( 1 9 9 4 ) .
o o S o l u b i l i t y o f a r s e n i c t r i o x i d e ( A s 2 O 3 ) i s 1 2 g / L a t 2 C a n d 1 1 4 . 6 g / L a t 1 0 0 C U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 4 4 , 1 9 9 9 .
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A r s e n i c t r i o x i d e i s t h e s t a r t i n g m a t e r i a l f o r a l l u s e s U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 5 5 , 1 9 9 9 . o f a r s e n i c ; i n c l u d i n g : • A g r i c u l t u r e a n d f o r e s t r y : h e r b i c i d e s i n c o t t o n , r i c e g r o w i n g , c o f f e e , w o o d p r e s e r v a t i o n , f e e d a d d i t i v e s t o a i d f a t t e n i n g a n d p r e v e n t i n f e c t i o n , • I n d u s t r i a l c h e m i c a l s : e l e c t r o l y t i c p u r i f i c a t i o n i n e l e c t r o l y s i s o f z i n c ; m e t a l p i c k l e s c o n t a i n i n g p h o s p h o r i c a c i d , • G l a s s i n d u s t r y : f i n i n g a g e n t s a n d d e c o l o u r i z e r s
The Uses of Arsenic W o o d p r e s e r v a t i v e – c h r o m a t e d c o p p e r a r s e n a t e : A s l e v e l s i n w o o d 8 1 1 1 m g A s / K g w o o d A . B . R i b e i r o , E . P . M a t e u s , L . M . O t t o s e n , a n d G . B e c h - N i e l s e n , “ E l e c t r o d y a l i t i c R e m o v a l o f C u , C r , a n d A s f r o m C h r o m a t e d C o p p e r A r s e n a t e - T r e a t e d T i m b e r W a s t e ” , E n v i r o n m e n t a l S c i e n c e a n d T e c h n o l o g y , V o l . 3 4 , N o . 5 , p p 7 8 4 - 7 8 9 , 2 0 0 0 . / / / / / / J . G u l l e d g e , a n d J O ’ C o n n e r , “ r e m o v a l o f a r s e n i c ( V ) f r o m w a t e r b y
a d s o r p t i o n o n a l u m i n i u m a n d f e r r i c h y d r o x i d e s ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . 021 , I s s u e 8 , p p 5 4 8 - 5 5 2 , ( 1 9 7 3 ) . A r s e n i t e s a r e u s e d a s a w o o d p r e s e r v a t i v e s a n d h e r b i c i d e s w h i l e a r s e n a t e s f i n d u s e a s p e s t i c i d e s o r p i g m e n t s A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 . / / / / / / S . A r r y k u l , N . M a h a r a t c h a p o n g , K . K o o p t a r n o n , P . B u n n a u l , “ A r s e n i c r e m o v a l f r o m p o t a l b l e w a t e r ” , i n t e r n e t s i t e : h t t p : / / w w w . p s u . a c . t h / e p i d e m i o l o g y / c o n f e r / 5 s u r a p o e . h t m l .
A l l o y i n g a g e n t f o r h e a v y m e t a l s , l e a d s h o t p r o d u c t i o n J . G u l l e d g e , a n d J O ’ C o n n e r , “ r e m o v a l o f a r s e n i c ( V ) f r o m w a t e r b y a d s o r p t i o n o n a l u m i n i u m a n d f e r r i c h y d r o x i d e s ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s
A s s o c i a t i o n , V o l . 021 , I s s u e 8 , p p 5 4 8 - 5 5 2 , ( 1 9 7 3 ) . , s p e c i a l s o l d e r i n g a g e n t s , a n d a s a d o p i n g a g e n t i n s i l i c o n a n d g e r m a n i u m s o l i d s t a t e c o n d u c t o r s A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 . .
G r e e n p i g m e n t s s u c h a s S c h e e l e ’ s g r e e n ( C u 2 A s 2 O 5 ) a n d P a r i s g r e e n
[ ( C H 3 C O O C u 2 ( A s O 3 ) ] w e r e o n c e c o m m o n b u t a r e l e s s u s e d t o d a y d u e t o t h e i r t o x i c i t y , a n d e v e n w o r s e , i n d a m p p l a c e s b a c t e r i a a n d m o u l d s c a n p r o d u c e J . D . L e e , C o n c i s e I n o r g a n i c p o i s o n o u s v o l a t i l e s u b s t a n c e s s u c h a s A s H 3 a n d A s ( C H 3 ) 3 C h e m i s t r y , f i f t h e d i t i o n , C h a p m a n a n d H a l l , M a d r a s , p 9 9 1 , ( 1 9 9 6 ) / / / / / / J . G u l l e d g e , a n d J O ’ C o n n e r , “ r e m o v a l o f a r s e n i c ( V ) f r o m w a t e r b y a d s o r p t i o n o n a l u m i n i u m a n d f e r r i c h y d r o x i d e s ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s
A s s o c i a t i o n , V o l . 021 , I s s u e 8 , p p 5 4 8 - 5 5 2 , ( 1 9 7 3 ) . .
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L e a d a r s e n a t e ( P b H A s O 4 ) w a s f o r m e r l y u s e d i n E u r o p e a s a p l a n t p r o t e c t i o n a g e n t , a n d i s s t i l l u s e d i n A s i a . U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 4 8 , 1 9 9 9 .
A r s e n i c t r i o x i d e h a s b e e n u s e d t o p r e s e r v e a n i m a l h i d e s J . G u l l e d g e , a n d J O ’ C o n n e r , “ r e m o v a l o f a r s e n i c ( V ) f r o m w a t e r b y a d s o r p t i o n o n a l u m i n i u m a n d f e r r i c h y d r o x i d e s ” , J o u r n a l o f t h e A m e r i c a n W a t e r
W o r k s A s s o c i a t i o n , V o l . 021 , I s s u e 8 , p p 5 4 8 - 5 5 2 , ( 1 9 7 3 ) . . T h e f e l l m o n g e r y ( t a n n i n g o f s h e e p s k i n s ) i n d u s t r y h a s u s e d a r s e n i c i n t h e f o r m o f A s 2 S 2 t o l o o s e n w o o l o n s k i n s K . A . M u r r a y , W a s t e w a t e r T r e a t m e n t a n d P o l l u t i o n C o n t r o l , W a t e r R e s e a r c h C o m m i s s i o n , P r e t o i a , p 2 4 3 . 1 9 8 7 . .
T h e e l e c t r o n i c s i n d u s t r y r e q u i r e s h i g h p u r i t y a r s e n i c ( 9 9 . 9 9 9 % ) f o r t h e p r o d u c t i o n o f d i o d e s , i n f r a r e d d e t e c t o r s , a n d l a s e r s . G a l l i u m a r s e n i d e i s u s e d i n s o l a r c e l l s , w h i c h h a v e a n e f f i c i e n c y o f 2 0 % U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 5 5 , 1 9 9 9 . .
C a l c i u m a n d c o p p e r a r s e n a t e s ( t r i v a l e n t ) w e r e f o r m e r l y u s e d a s “ c o p p e r - a r s e n i c l i q u o r ” t o c o n t r o l p e s t s i n v i t i c u l t u r e U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 4 5 , 1 9 9 9 .
W o r d w i d e p r o d u c t i o n : 4 7 0 0 0 t o n s / a n n u m J . D . L e e , C o n c i s e I n o r g a n i c C h e m i s t r y , f i f t h e d i t i o n , C h a p m a n a n d H a l l , M a d r a s , p 9 9 1 , ( 1 9 9 6 ) .
A r s e n i c t r i o x i d e i s a n e x c e l l e n t p o i s o n d u e t o i s l a c k o f e i t h e r a t a s t e o r o d o u r J . G u l l e d g e , a n d J O ’ C o n n e r , “ r e m o v a l o f a r s e n i c ( V ) f r o m w a t e r b y a d s o r p t i o n o n a l u m i n i u m a n d f e r r i c h y d r o x i d e s ” ,
J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . 021 , I s s u e 8 , p p 5 4 8 - 5 5 2 , ( 1 9 7 3 ) . . .
T h e m a j o r u s e o f a r s e n i c t r i o x i d e ( 9 0 % o f A s 2 O 3 ) i n t h e U . S . A . i n 1 9 9 8 w a s f o r t h e p r o d u c t i o n o f c h r o m a t e d c o p p e r a r s e n a t e , a w o o d p r e s e r v a t i v e . A n o t h e r w o o d p r e s e r v a t i v e i s a m m o n i a c a l c o p p e r - z i n c a r s e n a t e M . L e i s t , R . J . C a s e y , a n d D . C a r i d i , “
T h e m a n a g e m e n t o f a r s e n i c w a s t e s : p r o b l e m s a n d p r o s p e c t s ” , J o u r n a l o f H a z a r d o u s M a t e r i a l s , V o l . W20 , N o . 1 , p p 1 2 5 -
1 3 8 , 2 0 0 0 . A r s e n i c u s a g e i n a g r i c u l t u r a l c h e m i c a l s h a s s e e n a s t e a d y d e c r e a s e M . L e i s t , R . J . C a s e y , a n d D . C a r i d i , “ T h e m a n a g e m e n t o f a r s e n i c w a s t e s : p r o b l e m s a n d p r o s p e c t s ” , J o u r n a l o f H a z a r d o u s M a t e r i a l s ,
V o l . W20 , N o . 1 , p p 1 2 5 - 1 3 8 , 2 0 0 0 . T h e m o s t l i k e l y c a u s e o f s u c h a r e d u c e d u s a g e i s t h e p r o d u c t i o n o f e n v i r o n m e n t a l l y f r i e n d l y o r g a n i c m o l e c u l e s t h a t a r e
44 b i o d e g r a d a b l e . A r s e n i c , b e i n g a n e l e m e n t , i s i n d e s t r u c t i b l e i n t h e n a t u r a l e n v i r o n m e n t .
D i m e t h y l a r s i n i c a c i d [ ( C H 3 ) 2 A s O 2 H ] i s a n o n - s e l e c t i v e a r s e n i c a l h e r b i c i d e u s e d f o r w e e d c o n t r o l o n n o n - c r o p l a n d , l a w n r e n o v a t i o n , d e f o l i a n t a n d d e s i c c a n t f o r c o t t o n , a n d k i l l i n g u n w a n t e d t r e e s b y i n j e c t i o n . I t s L D 5 0 f o r r a t s i s 7 0 0 m g / k g . T h e B r i t i s h C r o p P r o t e c t i o n C o u n c i l a n d T h e R o y a l S o c i e t y o f C h e m i s t r y , T h e P e s t i c i d e M a n u a l , 1 0 t h E d i t i o n , C .
T o m l i n ( E d . ) , T h e B a t h P r e s s , B a t h , p 3 5 2 , 1 9 9 5 .
C a c o d y l i c a c i d , [ ( C H 3 ) 2 A s O ( O H ) ] , i s e f f e c t i v e a g a i n s t w e e d s a n d f u n c t i o n s a s a c o n t a c t h e r b i c i d e J . G u l l e d g e , a n d J O ’ C o n n e r , “ r e m o v a l o f a r s e n i c ( V ) f r o m w a t e r b y a d s o r p t i o n o n
a l u m i n i u m a n d f e r r i c h y d r o x i d e s ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . 021 , I s s u e 8 , p p 5 4 8 - 5 5 2 , ( 1 9 7 3 ) . . M e t h y l a r s o n i c a c i d , o r i t s s a l t s , h a v e b e e n u s e d a s s e l e c t i v e c o n t a c t h e r b i c i d e s w i t h s o m e s y s t e m i c p r o p e r t i e s . M a j o r f i e l d s o f a p p l i c a t i o n b e i n g : ( M S M A ) g r a s s w e e d s c o n t r o l i n c o t t o n , s u g a r c a n e , s p r a y i n g u n d e r t r e e c r o p s , a n d n o n - c r o p a r e a s , ( D M S A ) g r a s s c o n t r o l f o r c i t r u s , c o t t o n , t u r f , a n d u n c r o p p e d l a n d . T h e B r i t i s h C r o p P r o t e c t i o n C o u n c i l a n d T h e R o y a l S o c i e t y o f C h e m i s t r y , T h e P e s t i c i d e M a n u a l , 1 0 t h E d i t i o n , C . T o m l i n ( E d . ) , T h e B a t h P r e s s , B a t h , p 6 8 4 , 1 9 9 5 . A r s e n i c s e e m s t o b e o f n u t r i t i o n a l v a l u e a s e x e m p l i f i e d b y i s u s e i n f e e d s t o a u g m e n t t h e g r o w t h o f j u v e n i l e p i g s . A d d i t i o n o f 0 . 2 % 3 - n i t r o , 4 - h y d r o x y p h e n y l a r s o n i c a c i d l e d t o a 5 7 . 4 % i n c r e a s e i n g r o w t h r a t e a n d a 3 3 . 4 % f e e d - c o n v e r s i o n e f f i c i e n c y J . G u l l e d g e , a n d J O ’ C o n n e r , “ r e m o v a l o f a r s e n i c ( V ) f r o m w a t e r b y
a d s o r p t i o n o n a l u m i n i u m a n d f e r r i c h y d r o x i d e s ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . 021 , I s s u e 8 , p p 5 4 8 - 5 5 2 , ( 1 9 7 3 ) . .
Environmental Fate of Arsenical Pesticides
Occurrence of As
A r s e n i c o c c u r s n a t u r a l l y i n r o c k s , w a t e r , s o i l , a i r , p l a n t s a n d m a m m a l s . A r s e n i c m a y b e r e l e a s e d i n t o t h e e n v i r o n m e n t t h r o u g h a w i d e v a r i e t y o f a c t i v i t i e s , o f n a t u r a l a n d a n t h r o p o g e n i c o r i g i n – v o l c a n i c a c t i o n , e r o s i o n o f r o c k s , f o r e s t f i r e s , b u r n i n g f o s s i l f u e l s , p a p e r p r o d u c t i o n , c e m e n t m a n u f a c t u r i n g , m i n i n g ,
45 p e s t i c i d e a p p l i c a t i o n , s p i l l s , a n d l a n d f i l l s A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 . .
Arsenic Bearing Minerals T a b l e ? / A r s e n i c B e a r i n g M i n e r a l s U l l m a n s E n c y c l o p e d i a : U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 4 4 , 1 9 9 9 . M i n e r a l F o r m u l a % A r s e n i c
A r s e n o p y r i t e F e A s S ( F e S 2 . F e A s 2 ) 4 6
L o l l i n g i t e F e A s 2 7 3
O r p i m e n t A s 2 S 3 6 1 R e a l g a r A s S 7 0 N a t i v e a r s e n i c A s 9 0 - 1 0 0
M a j o r s u p p l i e r s o f a r s e n i c a r e S w e d e n , B e l g i u m , M e x i c o , U . S . A . , C h i n a , S o u t h - w e s t A f r i c a , a n d P e r u , w h i l e t h e m a j o r c o n s u m e r s a r e M a l a s i a , U . S . A . , a n d U . K . . U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 5 5 , 1 9 9 9 .
A r s e n i c a n d a n t i m o n y m a y b o t h o c c u r a s c o n t a m i n a n t s i n c o p p e r o r e s E . A . C r e c e l i u s , a n d C . J . J o h n s o n , a n d G . C . H o f e r , “ C o n t a m i n a t i o n o f s o i l s n e a r c o p p e r s m e l t e r b y a r s e n i c , a n t i m o n y a n d l e a d ” , W a t e r ,
A i r a n d S o i l P o l l u t i o n , v o l . X , p p 3 3 7 - 3 4 2 , 1 9 7 4 . C h i n a i s t h e l a r g e s t p r o d u c e r o f a r s e n i c t r i o x i d e ( 1 5 0 0 0 t o n e s ) w i t h B e g i u m s e c o n d , p r o d u c i n g 9 0 0 0 t o n e p e r a n n u m M . L e i s t , R . J . C a s e y , a n d D . C a r i d i , “ T h e m a n a g e m e n t o f a r s e n i c w a s t e s : p r o b l e m s a n d p r o s p e c t s ” , J o u r n a l o f H a z a r d o u s M a t e r i a l s , V o l . W20 , N o . 1 , p p 1 2 5 - 1 3 8 , 2 0 0 0 .
T a b l e ? ? D e p i c t s t h e U . S . A . u s a g e U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 5 6 , 1 9 9 9 . o f a r s e n i c :
T a b l e ? ? A b r e a k d o w n o f t h e U . S . A . u s e s f o r a r s e n i c ( Y e a r ? ? ? ) U s e P e r c e n t a g e o f 1 9 4 2 0 t o n s / a n n u m A g r i c u l t u r e a n d f o r e s t r y 7 0 I n d u s t r i a l c h e m i c a l s 2 0 G l a s s 5 A l l o y s a n d e l e c t r o n i c s 3 P h a r m a c e u t i c a l s , f e e d a d d i t i v e s a n d 2 c a t a l y s t s
46
A p p r o x i m a t e l y 9 0 % o f i n d u s t r i a l a r s e n i c i s u s e d a s a w o o d p r e s e r v a t i v e , w h i l e o t h e r m i n o r u s e s i n c l u d e d y e s , d r u g s , w a r g a s e s a n d s o a p s A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 . .
Water treatment
Y © © ¨ §Z¤¥[© \¡¤§ w a t e r t r e a t m e n t s y s t e m s m a y b e d i v i d e d i n t o t w o c a t e g o r i e s , n a m e l y p o i n t - o f - u s e ( P O U ) a n d p o i n t o f e n t r y ( P O E ) s y s t e m s G . C r a u n , J . G o o d r i c h , “ S e l e c t i n g r e s i d e n t i a l o r p e r s o n a l w a t e r t r e a t m e n t s y s t e m s ” i n : P r o v i d i n g S a f e D r i n k i n g W a t e r i n S m a l l S y s t e m s , p p 2 9 7 - 3 0 7 T E C H L I B R A R Y < < < < < < < < < < < g e t p u b s , y e a r a n d p l a c e o f p u b s P O U s y s t e m s , w h i c h a r e t y p i c a l l y u s e d i n h o u s e h o l d s o r b y t r a v e l e r s f e a r i n g t h a t t h e i r w a t e r m a y b e c o n t a m i n a t e d , a r e f u r t h e r d i v i d e d i n t o p o u r o r p u m p t h r o u g h u n i t s , f a u c e t u n i t s , l i n e b y p a s s u n i t s o r s t a t i o n a r y t r e a t m e n t u n i t s . P O E s y s t e m s a r e u s e d i n l i n e b e f o r e o n e o r m o r e r e s i d e n t i a l u n i t s .
T a b l e ? ? W a t e r t r e a t m e n t t e c h n o l o g i e s a n d t h e i r r e l a t i v e c o s t s G . C r a u n , J . G o o d r i c h , “ S e l e c t i n g r e s i d e n t i a l o r p e r s o n a l w a t e r t r e a t m e n t s y s t e m s ” i n : P r o v i d i n g S a f e D r i n k i n g W a t e r i n S m a l l S y s t e m s , p p 2 9 7 - 3 0 7 T E C H L I B R A R Y < < < < < < < < < < < g e t p u b s , y e a r a n d p l a c e o f p u b s T e c h n o l o g y C o n t a m i n a n t s R e m o v e d I n i t i a l O p e r a t i n g L e v e l o f C o s t C o s t s s k i l l r e q u i r e d f o r O p e r a t i o n a n d M a i n t e n a n c e c h l o r i n e , i o d i n e m i c r o b i a l L L L U V , o z o n e m i c r o b i a l M L M s u b m i c r o n p r o t o z o a , b a c t e r i a L L - M L c a r t r i d g e f i l t e r r e v e r s e m i c r o b i a l , M H H o s m o s i s i n o r g a n i c c h e m i c a l s a n d m e t a l s , m i n e r a l s , s o m e o r g a n i c s c h e m i c a l s , r a d i u m , d i s t i l l a t i o n m i c r o b i a l , M M L i n o r g a n i c c h e m i c a l s a n d m e t a l s , m i n e r a l s , s o m e
47
o r g a n i c s c h e m i c a l s , r a d i u m , u r a n i u m a c t i v a t e d o r g a n i c c h e m i c a l s , r a d o n , M M - H L c a r b o n o d o u r s , p r o t o z o a a n d s o m e b a c t e r i a p a c k e d t o w e r r a d o n , v o l a t i l e o r g a n i c M L H a e r a t i o n c o m p o u n d s , t a s t e s , o d o u r s i o n e x c h a n g e I n o r g a n i c c h e m i c a l s , M M - H M r a d i u m , n i t r a t e , a c t i v a t e d a r s e n i c , s e l e n i u m , H H H a l u m i n a f l u o r i d e e l e c t r o d y a l y s i s I n o r g a n i c c h e m i c a l s , H H H r a d i u m , u r a n i u m ,
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T h e t w o o x i d a t i o n s t a t e s c o m m o n l y f o u n d i n d r i n k i n g w a t e r a r e a r s e n a t e ( A s V ) a n d a r s e n i t e ( A s I I I ) . B o t h a r s e n a t e a n d a r s e n i t e a r e p a r t o f t h e a r s e n i c
( H 3 A s O 4 ) a n d a r s e n i o u s ( H 3 A s O 3 ) s y s t e m s . H e n c e t h e p H o f t h e s y s t e m w i l l c o n t r o l t h e d e g r e e o f p r o t o n a t i o n o f t h e a c i d s . T h e p K a v a l u e s o f t h e s e a c i d s a r e h e l p f u l i n d e t e r m i n i n g t h e f o r m t h a t t h e a c i d s m a n i f e s t a t v a r i o u s p H l e v e l s . T a b l e ? ? l i s t s t h e p K a v a l u e s o f a r s e n i o u s a n d a r s e n i c a c i d s K . R a v e n , A . J a i n , R . L o e p p e r t , “ A r s e n i t e a n d a r s e n a t e a d s o r p t i o n o n f e r r i h y d r i t e : k i n e t i c s , e q u i l i b r i u m , a n d a d s o r p t i o n e n v e l o p e s ” ,
E n v i r o n m e n t a l S c i e n c e a n d T e c h n o l o g y , V o l . X2a , p p 3 4 4 - 3 4 9 , 1 9 9 8 . . T h e s e v a l u e s a r e i n v e r y c l o s e a g r e e m e n t w i t h t h o s e r e p o r t e d b y M e n g X . M e n g , S . B a n g , a n d G . K o r f i a t i s , “ E f f e c t s o f s i l i c a t e ,
s u l p h a t e , a n d c a r b o n a t e o n a r s e n i r e m o v a l b y f e r r i c c h l o r i d e ” , W a t e r R e s e a r c h , V o l . X2b , I s s u e 4 , p p 1 2 5 5 - 1 2 6 2 , ( 2 0 0 0 ) . .
T a b l e ? ? ? p K a v a l u e s f o r a r s e n i o u s a n d a r s e n i c a c i d s A c i d p K 1 p K 2 p K 3
H 3 A s O 3 9 . 2 2 1 2 . 1 3 1 3 . 4 0
H 3 A s O 4 2 . 2 0 6 . 9 7 1 1 . 5 3
48
A p p e n d i x ? ? S h o w s t h e d i s t r i b u t i o n a r s e n i c a l s p e c i e s t h a t e x i s t i n s o l u t i o n a s a f u n c t i o n o f X . M e n g , S . B a n g , a n d G . K o r f i a t i s , “ E f f e c t s o f s i l i c a t e , s u l p h a t e , a n d c a r b o n a t e o n a r s e n i r e m o v a l
b y f e r r i c c h l o r i d e ” , W a t e r R e s e a r c h , V o l . X2b , I s s u e 4 , p p 1 2 5 5 - 1 2 6 2 , ( 2 0 0 0 ) . ( i n t e r n e t l o x c a t i o n : h t t p : / / w w w . s c i e n c e d i r e t c t . c o m / s c i e n c e ? _ o b = M i a m i C a p t i o n U R L & _ m e t h o d = r e t r i e v e & _ u d i = B 6 V 7 3 - 3 Y F 3 V X J - N & _ i m a g e = f i g 2 & _ b a = 4 & _ u s e … )
A t t y p i c a l p H ’ s ( p H 5 - 8 ) i n n a t u r a l w a t e r s a r s e n a t e e x i s t s a s a n a n i o n w h i l e a r s e n i t e r e m a i n s a s a f u l l y p r o t o n a t e d m o l e c u l e P . B r a n d h u b e r , a n d G . A m y , “ A l t e r n a t i v e m e t h o d s f o r m e m b r a n e f i l t r a t i o n o f a r s e n i c f r o m d r i n k i n g w a t e r ” , D e s a l i n a t i o n , V o l . V2VTW , p p 1 - 1 0 , 1 9 9 8 . . T h i s b e h a v i o u r i s d u e t o t h e h i g h p K a v a l u e s o f H 3 A s O 3 c o m p a r e d t o t h o s e o f
H 3 A s O 4 . p H , h o w e v e r , i s n o t t h e o n l y v a r i a b l e a f f e c t i n g t h e f o r m t h a t a r s e n i c m a n i f e s t s ; t h e f o u r o x i d a t i o n s t a t e s ( - 3 , 0 , + 3 , a n d + 5 ) m a k e s i g n i f i c a n t c o n t r i b u t i o n s t o t h e c h e m i s t r y o f a r s e n i c a s d i c t a t e d b y t h e r e d o x p o t e n t i a l o f t h e s o l u t i o n . A P o u r b a i x d i a g r a m m a y b e u s e d t o p r e d i c t t h e f o r m o f a n e l e m e n t b a s e d o n t h e p H a n d r e d o x p o t e n t i a l o f i t s e n v i r o n m e n t . A p p e n d i x ? ? “ Pourbaix Diagrams and Reactions in Aqueous Solution”, Internet site: h t t p : / / w w w . w e l l e s l e y . e d u / C h e m i s t r y / c h e m 1 2 0 / p o u r . h t m l d e p i c t s P o u r b a i x d i a g r a m s f o r a r s e n i c , i r o n , a l u m i n i u m , a n d s u l p h u r . A p p e n d i x ? / c o n t a i n s c o m b i n e d P o u r b a i x d i a g r a m s f o r i r o n , s u l p h u r a n d a r s e n i c c o m p o u n d s G o k e n , 1 9 8 8 , I S B N 0 - 8 7 3 3 9 - 0 3 7 - 7 , i n t e r n e t s i t e : h t t p : / / m e m e b e r s . i i n e t . n e t . a u / ~ m e n e / f i g a s 3 b . h t m / / / / / / / / / / / O s s e o - A s a r e a o , 1 9 8 4 , I S B N 0 8 9 5 2 0 - 4 6 9 x , i n t e r n e t s i t e : h t t p : / / m e m e b e r s . i i n e t . n e t . a u / ~ m e n e / f i g a s 2 b . h t m T h i s u s e f u l d i a g r a m d e s c r i b e s t h e e x i s t e n c e o f a r s e n i c , i r o n - a r s e n i c , i r o n - s u l p h u r a n d r e l a t e d c o m p o u n d s .
C o n v e n t i o n a l o x i d a n t s u s e d i n w a t e r t r e a t m e n t ( e . g . c h l o r i n e , p e r m a n g a n a t e , o z o n e a n d p e r o x i d e ) r e a d i l y c o n v e r t a r s e n i t e t o a r s e n a t e J . H e r i n g . P . C h e n , J . W i l k i e , M .
E l i m e l c h , a n d S . L i a n g , “ A r s e n i c r e m o v a l b y f e r r i c c h l o r i d e ” , J o u r n a l o f A m e r i c a n W a t e r W o r k s A s s o c o a i t i o n , V o l . c2c , 4 , p p 1 5 5 - 1 6 7 , ( 1 9 9 6 ) . T h e r a t e o f o x i d a t i o n o f a r s e n i t e t o a r s e n a t e b y d i s s o l v e d o x y g e n i s s l o w M . E m e t t , a n d G . K h o e , “ P h o t o c h e m i c a l o x i d a t i o n o f a r s e n i c b y o x y g e n a n d i r o n i n a c i d i c s o l u t i o n s , W a t e r R e s e a r c h ,
V o l . X21 , I s s u e 3 , p p 6 4 9 - 6 5 6 , ( 2 0 0 1 ) . T h e a r s e n i c t h a t i s f o u n d i n n a t u r a l w a t e r s i s n o t s i m p l y a q u e o u s a r s e n i c i o n s , b u t r a t h e r a c o m b i n a t i o n o f d i s s o l v e d f o r m s a n d p a r t i c u l a t e o r c o l l o i d a l a r s e n i c . B r a n d h u b e r P . B r a n d h u b e r , a n d G . A m y , “ A l t e r n a t i v e m e t h o d s f o r m e m b r a n e f i l t r a t i o n o f a r s e n i c f r o m d r i n k i n g w a t e r ” , D e s a l i n a t i o n , V o l . V2VTW , p p 1 - 1 0 , 1 9 9 8 s h o w e d t h a t g r o u n d w a t e r s t e n d t o h a v e a m o r e h o m o g e n e o u s a r s e n i c a l p a r t i c l e s i z e a s c o m p a r e d t o s u r f a c e w a t e r s . T a b l e ? ? ( c o m p i l e d f r o m t h e w o r k o f B r a n d h u b e r P . B r a n d h u b e r , a n d G . A m y ,
“ A l t e r n a t i v e m e t h o d s f o r m e m b r a n e f i l t r a t i o n o f a r s e n i c f r o m d r i n k i n g w a t e r ” , D e s a l i n a t i o n , V o l . V2VTW , p p 1 - 1 0 , 1 9 9 8 ) s h o w s t h e t y p i c a l s i z e r a n g e s o f p a r t i c u l a t e a r s e n i c i n s u r f a c e a n d
49 g r o u n d w a t e r s . T h i s w o r k s h o w e d t h a t t h e d i s s o l v e d a r s e n i c c o n c e n t r a t i o n i n s u r f a c e w a t e r s v a r i e d f r o m l e s s t h a n 1 % - 8 2 % , i n d i c a t i n g t h a t e a c h w a t e r m a y b e d i f f e r e n t d e p e n d i n g o n i t s e n v i r o n m e n t s u r f a c e w a t e r s . W a t e r s a l r e a d y c o n t a i n i n g h i g h l e v e l s o f p a r t i c l a t e o f c o l l o i d a l a r s e n i c m a y b e m o r e r e a d i l y t r e a t e d t h a n t h o s e c o n t a i n i n g e s s e n t i a l l y d i s s o l v e d a r s e n i c . A n a l y s i s o f a r s e n i c b e a r i n g w a t e r s f o r t h e i r a r s e n i c a l p a r t i c l e s i z e d i s t r i b u t i o n r a n g e m a y t h u s b e b e n e f i c i a l f o r e c o n o m i c a l c o n s i d e r a t i o n s .
A r s e n i c o c c u r s i n g r o u n d w a t e r a t t y p i c a l l e v e l s o f 0 . 0 2 - 0 . 0 1 p p m i n t h e M i d w e s t a n d N e w E n g l a n d a n d g r e a t e r t h a n 0 . 0 1 p p m i n o t h e r a r e a s i n t h e U S i n d u s t r y A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 . .
N a t u r a l l y e l e v a t e d c o n c e n t r a t i o n s o f a r s e n i c o c c u r i n g r o u n d w a t e r ’ s o f t h e w e s t e r n U n i t e d S t a t e s , l a k e s m a y r a n g e f r o m < 1 – 4 0 V g / L . J . G . H e r i n g , P . C h e n , J . A . W i l k i e , M . E l i m e l c h , a n d S . L i a n g , “ A r s e n i c R e o m v a l b y F e r r i c C h l o r i d e ” , J o u r n a l o f A m e r i c a n W a t e r W o r k s
A s s o c i a t i o n , V o l c2c , N o . 4 , p p 1 5 5 – 1 6 7 , 1 9 9 6 E x t r e m e l y e l e v a t e d l e v e l s o f a r s e n i c , i . e . 1 7 m g / L h a v e b e e n r e p o r t e d i n M o n o l a k e , C a l i f o r n i a . L . C . D . A n d e r s o n , a n d K . W . B r u l a n d , B i o c h e m i s t r y o f A r s e n i c i n N a t u r a l W a t e r s : T h e I m p o r t a n c e o f M e t h y l a t e d S p e c i e s ” , E n v i r o n m e n t a l S c i e n c e a n d
T e c h n o l o g y , V o l . a21 , p p 4 2 0 - 4 2 9 E d w a r d s e t a l a s c i t e d b y L . M N e i l , a n d M . E d w a r d s , “ A r s e n i c r e m o v a l d u r i n g p r e c i p i t a t i v e s o f t e n i n g ” , J o u r n a l o f E n v i r o n m e n t a l E n g i n e e r i n g , V o l . 1 2 3 , N o . 5 , p p 4 5 3 - 4 6 0 , 1 9 9 7 . s u r v e y e d U . S . w a t e r s y s t e m s a n d r e l a t e d a r s e n i c l e v e l s t o t h e h a r d n e s s o f w a t e r ; h e f o u n d t h a h a r d w a t e r s ( > 1 5 0 m g C a C O 3 / L ) c o n t a i n e d 4 3 % m o r e a r s e n i c t h a n s o f t w a t e r s ( < 1 5 0 m g C a C O 3 / L ) .
D i s s o l u t i o n o f a r s e n i c f r o m n a t u r a l l y o c c u r r i n g a r s e n i c - b e a r i n g m i n e r a l s a n d s e d i m e n t s i n I n d i a a n d B a n g l a d e s h h a s p u t m o r e t h a n 2 0 m i l l i o n p e o p l e a t r i s k f r o m d r i n k i n g c o n t a m i n a t e d w a t e r . M . T . E m e t t , a n d G . H . K h o e , “ P h o t o c h e m i c a l O x i d a t i o n o f A r s e n i c b y O x y g e n a n d I r o n i n A c i d i c S o l u t i o n s ” , W a t e r R e s e a r c h , V o l . X21 , N o . 3 , p p 6 4 9 - 6 5 6 , 2 0 0 1 . T y p i c a l a r s e n i c l e v e l s i n t h e g r o u n d w a t e r o f t h e s e a r e a s r a n g e d f r o m 0 . 3 - 0 . 7 p p m w i t h s o m e w a t e r s c o n t a i n i n g a s m u c h a s 5 p p m A . D e b , A G u p t a , P . B r a n d y o p a d h y a y , R B i s w a s , a n d S . R o y , “ A p p r o p r i a t e t e c h n o l o g y f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r o f r u r a l W e s t B e n g a l ” , i n P r o v i d i n g S a f e D r i n k i n g
W a t e r i n S m a l l S y s t e m : T e c h n o l o g y , O p e r a t i o n s , a n d E c o n o m i c s . C o t r u v o , G . C r a u n , a n d N . H e a r n e ( E d s ) , W o r l d H e a l t h O r g a n i z a t i o n , C R C P r e s s , p p 2 7 3 - 2 7 8 , 1 9 9 9 . .
50
C h i l e a n d T a i w a n a l s o e x p e r i e n c e e l e v a t e d g r o u n d w a t e r l e v e l s o f a r s e n i c i n c e r t a i n r e g i o n s A . D e b , A G u p t a , P . B r a n d y o p a d h y a y , R B i s w a s , a n d S . R o y , “ A p p r o p r i a t e t e c h n o l o g y f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r o f r u r a l W e s t B e n g a l ” , i n P r o v i d i n g S a f e D r i n k i n g W a t e r i n S m a l l S y s t e m : T e c h n o l o g y ,
O p e r a t i o n s , a n d E c o n o m i c s . C o t r u v o , G . C r a u n , a n d N . H e a r n e ( E d s ) , W o r l d H e a l t h O r g a n i z a t i o n , C R C P r e s s , p p 2 7 3 - 2 7 8 , 1 9 9 9 . A r s e n i c i s a n u n w a n t e d h a z a r d o u s w a s t e g e n e r a t e d f r o m t h e p r o c e s s i n g o f a v a r i e t y o f o r e s i n c l u d i n g t h o s e o f c o p p e r , g o l d , n i c k e l , l e a d a n d z i n c . M . L e i s t , R . J . C a s e y , a n d D . C a r i d i , “ T h e m a n a g e m e n t o f a r s e n i c w a s t e s : p r o b l e m s a n d p r o s p e c t s ” , J o u r n a l o f H a z a r d o u s M a t e r i a l s ,
V o l . W20 , N o . 1 , p p 1 2 5 - 1 3 8 , 2 0 0 0 .
L e i s t , e t a l M . L e i s t , R . J . C a s e y , a n d D . C a r i d i , “ T h e m a n a g e m e n t o f a r s e n i c w a s t e s : p r o b l e m s a n d p r o s p e c t s ” , J o u r n a l o f H a z a r d o u s M a t e r i a l s , V o l . W20 , N o . 1 , p p 1 2 5 - 1 3 8 , 2 0 0 0 . , p r e d i c t s a n i n c r e a s e i n A s w a s t e a s i n d u s t r y p r o c e s s e s m o r e c o m p l i c a t e d s u l p h i d e o r e s s u c h a s l o w g r a d e g o l d a s s o c i a t e d w i t h a r s e n o p y r i t e a n d n i c k e l o r e s w i t h h i g h a r s e n i c c o n t e n t s .
Oxidation States Arsenic
T h e c o m m o n o x i d a t i o n s t a t e s a r e 0 , + 3 , + 5 , a n d – 3 . E m e t t e t a l M . E m e t t , a n d G . K h o e ,
“ P h o t o c h e m i c a l o x i d a t i o n o f a r s e n i c b y o x y g e n a n d i r o n i n a c i d i c s o l u t i o n s , W a t e r R e s e a r c h , V o l . X21 , I s s u e 3 , p p 6 4 9 - 6 5 6 , ( 2 0 0 1 ) . r e p o r t e d t h e e x i s t e n c e o f a t e t r a v a l e n t a r s e n i c i n t e r m e d i a t e d e t e c t e d i n r a d i c a l r e a c t i o n s . D i s s o l v e d t e t r a v a l e n t a r s e n i c w a s o b s e r v e d a n d c h a r a c t e r i z e d i n a r s e n i o u s a c i d , a r s e n i t e a n d a r s e n a t e s o l u t i o n s .
Analysis for As
T o t a l a r s e n i c : T o t a l a r s e n i c m a y b e d e t e r m i n e d b y a t o m i c s p e c t r o s c o p y ( U S E P A a p p r o v e d ) , e i t h e r d i r e c t a n a l y s i s b y p l a t f o r m g r a p h i t e f u r n a c e a t o m i c a b s o r p t i o n ( P G F - A A ) , o r b y a s h i n g ( w e t o r d r y ) a n d a s p i r a t i o n i n t o a f l a m e a t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t e r o r a n i n d u c t i v e l y c o u p l e d p l a s m a e m i s s i o n s p e c t r o m e t e r ( I C P ) , I C P - m a s s s p e c t r o m e t e r .
O t h e r m e t h o d s i n c l u d e : h y d r i d e g e n e r a t i o n o n t o H g B r 2 a b s o r p t i o n f i l t e r s t h a t t u r n y e l l o w o n e x p o s u r e t o a r s e n i c – i n t e r p r e t e d b y c o m p a r i s o n t o a c o l o u r c h a r t ( d e t e c t i o n l i m i t = 0 . 1 p p m ) H . H u a n g , P . D a s g u p t a , “ A f i e l d d e p l o y a b l e i n s t r u m e n t f o r t h e m e a s u r e m e n t a n d s p e c i a t i o n o f a r s e n i c i n p o t a b l e w a t e r ” , A n a l y t i c a C h i m i c a A c t , V o l . X2c2d , p p 2 7 - 3 7 , 1 9 9 9 . , m o l y b d a t e c o m p l e x a t i o n t o f o r m c o l o u r e d m o l e c u l a r s p e c i e s t h a t a b s o r b v i s i b l e r a d i a t i o n H . H u a n g , P . D a s g u p t a , “ A f i e l d d e p l o y a b l e i n s t r u m e n t f o r t h e m e a s u r e m e n t a n d s p e c i a t i o n o f
51
a r s e n i c i n p o t a b l e w a t e r ” , A n a l y t i c a C h i m i c a A c t , V o l . X2c2d , p p 2 7 - 3 7 , 1 9 9 9 / / / / / D . J o h n s o n , M . P i l s o n ,
“ S p e c t r o p h o t o m e t r i c d e t e r m i n a t i o n o f a r s e n i t e , a r s e n a t e a n d p h o s p h a t e i n n a t u r a l w a t e r s ” , A n a l y t i c a C h i m i c a A c t a ,
V o l . 12c , p p 2 8 9 - 2 9 9 , ( 1 9 7 2 ) .
S p e c i a t i o n : T o d e t e r m i n e t h e a r s e n i c s p e c i e s o f t o x i c o l o g i c a l r e l e v a n c e t o m a n ( A s I I I , A s V , m o n o m e t h y l a r s i n i c a c i d a n d d i m e t h y l a r s i n i c a c i d ) t h e h y d r i d e g e n e r a t i o n m e t h o d i s r e c o m m e n d e d T. Gebel, “Confounding Variables in the Environmental Toxicology of Arsenic”, Toxicology, Vol 144, pp 155-162, 2000.. Organo-arsenicals and trimethylated forms are not detected using this method. S h o u l d t h e n o n t o x i c p o r t i o n o r a r s e n i c n e e d t o b e d e t e r m i n e d t h e n t h e t o x i c o l o g i c a l l y r e l e v a n t h y d r i d e c o n c e n t r a t i o n s h o u l d b e s u b t r a c t e d f r o m t o t a l a r s e n i c a n a l y s i s . A s I I I m a y a l s o b e d e t e r m i n e d b y p o l a r o g r a p h i c m e a n s , b u t t h e p e n t a v a l e n t f o r m i s n o t r e d u c e d . M o l e c u l a r s p e c t r o s c o p i c m e t h o d s a r e a l s o a v a i l a b l e . A r s e n a t e ( n o t a r s e n i t e ) r e a c t s w i t h m o l y b d a t e t o f o r m a b l u e c o m p l e x ( a b s o r b i n g a t 8 2 5 n m ) – t h e l i m i t o f d e t e c t i o n i s 0 . 0 0 1 5 m g A s / L . s p e c i a t i o n i s p o s s i b l e i f t h e a r s e n i t e i s f i r s t c o n v e r t e d t o a r s e n a t e . M . E d w a r d s , “ C h e m i s t r y o f a r s e n i c r e m o v a l d u r i n g c o a g u l a t i o n a n d F e - M n o x i d a t i o n ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s
A s s o c i a t i o n , V o l . U , p p 6 4 - 7 7 , ( 1 9 9 4 ) .
H u a n g e t a l H . H u a n g , P . D a s g u p t a , “ A f i e l d d e p l o y a b l e i n s t r u m e n t f o r t h e m e a s u r e m e n t a n d s p e c i a t i o n o f a r s e n i c i n p o t a b l e w a t e r ” , A n a l y t i c a C h i m i c a A c t , V o l . X2ced , p p 2 7 - 3 7 , 1 9 9 9 . d e v e l o p e d a l i g h t w e i g h t ( 6 . 5 k i l o g r a m s ) p o r t a b l e s t r i p p i n g v o l t a m m e t r i c i n s t r u m e n t t h a t w a s c a p a b l e o f d e t e r m i n a t i o n o f t r i a n d p e r n t a v a l e n t a r s e n i c w i t h a l i m i t o f d e t e c t i o n ( L O D ) o f 0 . 5 V g / L .
L e e t a l X . L e , a n d W C u l l e n , “ S p e c i a t i o n o f A r s e n i c i n W a t e r a n d B i o l o g i c a l M a t r i c e s ” , A W W A R F R e s e a r c h N e w s , p r o j e c t # 2 8 7 , i n t e r n e t s i t e h t t p : / / w w w . a w w a r f . c p m / e x u m s / 2 8 7 . h t m , 2 0 0 1 . d e v e l o p e d t h r e e m e t h o d s o f a r s e n i c s p e c i a t i o n i n a q u e o u s m a t r i c e s , t h e y a n a l y z e d f o r a r s e n i t e , a r s e n a t e , m o n o m e t h y l a r s o n i c a c i d ( M M A ) , a n d d i m e t h y l a r s o n i c a c i d ( D M A ) . T h e i r m o s t e f f e c t i v e m e t h o d w a s h i g h - p r e s s u r e l i q u i d c h r o m a t o g r a p h i c s e p a r a t i o n o f t h e s p e c i e s f o l l o w e d b y h y d r i d e g e n e r a t i o n a t o m i c f l u o r e s c e n c e s p e c t r o m e t r i c d e t e c t i o n – t h e c o m p l e t e a n a l y s i s t o o k 3 - 4 m i n u t e a n d t h e L O D w a s 0 . 4 - 0 . 8 V g / L . T h e y f o u n d t h a t w a t e r a n d u r i n e s a m p l e s w e r e s t a b l e w i t h r e s p e c t t o a r s e n i c f o r t w o m o n t h s a t 4 o C . Arsenic Waste Treatment T h r e e g e n e r a l m e t h o d s o f w a s t e t r e a t m e n t i n c l u d e :
52 c o n c e n t r a t i o n a n d c o n t a i n m e n t – s a f e t y a n d c o s t f a c t o r s t e n d t o l i m i t t h i s m e t h o d , d i l u t i o n a n d d i s p e r s i o n – t h i s u n d e s i r a b l e p r o c e d u r e i s m e r e l y l e g i s l a t i v e , a n d e n c a p s u l a t i o n - . M . L e i s t , R . J . C a s e y , a n d D . C a r i d i , “ T h e m a n a g e m e n t o f a r s e n i c w a s t e s : p r o b l e m s a n d p r o s p e c t s ” , J o u r n a l o f H a z a r d o u s M a t e r i a l s , V o l . W20 , N o . 1 , p p 1 2 5 - 1 3 8 , 2 0 0 0 .
Methods of Aqueous Arsenic Removal
Precipitation and coagulation
f ©©\ §g h ©¡i!$ ¤j¥¨ `¤ ¥¨§ ©k¡¤l¥¨\©` ¥¨ m ¨n ¨ ¡¤ ¤j`¡¤ op'§ ¡¤
D e f i n i t i o n s : M . E d w a r d s , “ C h e m i s t r y o f a r s e n i c r e m o v a l d u r i n g c o a g u l a t i o n a n d F e - M n o x i d a t i o n ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . U , p p 6 4 - 7 7 , ( 1 9 9 4 ) . P r e c i p i t a t i o n i s t h e i n s o l u b i l i z a t i o n o f c o n t a m i n a n t s b y e x c e e d i n g t h e i r s o l u b i l i t y p r o d u c t . C o p r e c i p i t a t i o n i s a n i n c o r p o r a t i o n o f s o l u b l e s p e c i e s i n t o a g r o w i n g [ h y d r o x i d e ] p h a s e b y i n c l u s i o n , o c c l u s i o n , o r a d s o r p t i o n . A d s o r p t i o n r e f e r s t o f o r m a t i o n o f s u r f a c e c o m p l e x e s b e t w e e n s o l u b l e s p e c i e s [ a r s e n i c ] a n d t h e s o l i d s u r f a c e [ o x y h y d r o x i d e ]
Coagulation has been practiced since ancient times. Extracts of seeds, leaves, and roots were used in the far east for clarification of water R.Droste, Theory and Practice of Water and Wastewater Treatment, John Wiley & Sons, New York, P387, 1997.. These processes involve the removal of arsenic that exists in the aqueous system as suspended particles, or 3- negatively charged anions such as AsO4 . Agglomeration is aided by metals salts such as ferric or aluminium sulphates, the settling of suspended solid may be augmented by addition of long chain molecular flocculants. The long chain molecular flocculants are more efficient than the inorganic salts, but they are more costly. Such compounds include nonionic polymers such as polyacrilamide(molecular masses of1-30 million), anionic molecules such as polyacrylamide carboxylate salts(molecular masses of several million), and cationic molecules including polyethylamines or polyvinylamines (molecular masses of less than 1 million) R.Droste, Theory and Practice of Water and Wastewater Treatment, John Wiley & Sons, New York, P387, 1997 . Activated silica (acidified Na2SiO3) is also a commonly used coagulant; it is believed to form polysilicates that may cross link and precipitate. Such solutions are only stable for a few weeks R.Droste, Theory and Practice of Water and Wastewater Treatment, John Wiley & Sons, New York, P387, 1997.. T h e c o a g u l a t i o n p o w e r o f a n a g e n t d e p e n d s o n i t s c h a r g e a n d t h e s i z e o f a s y n t h e t i c p o l y m e r a l s o p l a y s a r o l e . T a b l e ? ? l i s t s t h e r e l a t i v e p o w e r o f s o m e i n o r g a n i c e l e c t r o l y t e s
53
T a b l e ? ? l i s t s t h e p r o p e r t i e s a n d r e l a t i v e p o w e r o f s o m e i n o r g a n i c e l e c t r o l y t e s R.Droste, Theory and Practice of Water and Wastewater Treatment, John Wiley & Sons, New York, P385, 1997. C h e m i c a l E l e c t r o l y t e F o r m u l a M o l e c u l a r P o s i t i v e N e g a t i v e a g e n t m a s s C o l l o i d s C o l l o i d s s o d i u m N a C l N a C l 5 8 . 5 1 1 c h l o r i d e a l u m i n i u m A l C l 3 A l C l 3 1 3 3 . 4 1 1 0 0 0 c h l o r i d e s o d i u m N a 3 P O 4 N a 3 P O 4 1000 1 p h o s p h a t e a l u m A l 2 ( S O 4 ) 3 A l 2 ( S O 4 ) 3 . 1 8 H 2 O 6 6 6 . 7 3 0 > 1 0 0 0 f e r r i c F e 2 ( S O ) 4 ) 3 Fe2(SO)4)3 4 0 0 3 0 > 1 0 0 0 s u l p h a t e
A l u m i s t h e m o s t c o m m o n l y u s e d s a l t w i t h i r o n b e i n g t h e s e c o n d m o s t p o p u l a r R.Droste, Theory and Practice of Water and Wastewater Treatment, John Wiley & Sons, New York, P385, 1997.. T h e a c i d i t y o f t h e s e i o n s l e a d t o l o w e r i n g o f p H a n d i t m a y n e c e s s a r y t o a d d a l k a l i n i t y t o m a i n t a i n t h e p H . D r o e s t e r e p o r t e d t h a t h t e o p t i m u m p H r a n g e s f o r c o a g u l a t i o n w i t h i r o n a n d a l u m a r e 4 . 5 - 5 . 5 a n d 5 . 5 - 6 . 3 r e s p e c t i v e l y R.Droste, Theory and Practice of Water and Wastewater Treatment, John Wiley & Sons, New York, P384, 1997.. T h e u s e o f i r o n i s a d v a n t a g e o u s o v e r a l u m i n i u m i n t h a t t h e r e s i d u a l i r o n l e f t i n t h e w a t e r a f t e r c o a g u l a t i o n i s l e s s t o x i c t h a n t h e a l u m i n i u m . A l u m i n i u m i s a s s o c i a t e d w i t h A l z h e i m e r ’ s d i s e a s e a n d t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n ( A W W A ) h a s s e t a g o a l o f t h e m a x i m u m c o n t a m i n a n t l e v e l ( M C L ) f o r a l u m i n i u m o f 0 . 0 5 0 p p m , ( t h e s a m e a s t h e c u r r e n t M C L f o r a r s e n i c ) R.Droste, Theory and Practice of Water and Wastewater Treatment, John Wiley & Sons, New York, P182, 1997..
T h e r e a c t i o n s o c c u r r i n g d u r i n g o r d i n a r y w a t e r c o a g u l a t i o n i n v o l v e t h e f o r m a t i o n o f h y d r o x i d e s , T a b l e ? ? l i s t s t h e r e a c t i o n s o f a l u m i n i u m a n d i r o n s a l t s o c c u r r i n g d u r i n g a q u e o u s c o a g u l a t i o n G . T c h o b a n o g l o u s , W a s t e w a t e r E n g i n e e r i n g : T r e a t m e n t D i s p o s a l a n d R e u s e , S e c o n d E d i t i o n , M c G r a w - H i l l B o o k C o m p a n y , N e w Y o r k , p p 2 6 1 - 2 6 7 , 1 9 7 9 . ///////R.Droste, Theory and Practice of Water and Wastewater Treatment, John Wiley & Sons, New York, P262, 1997.. Table ?? The reactions of aluminium and iron salts occurring during aqueous coagulation ¢¡¤ op'§ Reaction + 2 - A l u m A l 2 ( S O 4 ) 3 . 1 8 H 2 O + 6 H 2 O I 2 A l ( O H ) 3 ( S ) + 6 H + 3 S O 4 + 1 8 H 2 O
A l 2 ( S O 4 ) 3 . 1 8 H 2 O + 3 C a ( H C O 3 ) 2 * I 2 A l ( O H ) 3 ( S ) + 3 C a S O 4 + 6 C O 2
54
+ 1 8 H 2 O
Ferric F e 2 ( S O 4 ) 3 + 3 C a ( H C O 3 ) 2 * I 2 F e ( O H ) 3 ( S ) + 3 C a S O 4 + 6 C O 2 sulphate
* N a t u r a l h a r d n e s s i s t h e s o u r c e o f 3 C a ( H C O 3 ) 2 The hydroxides that form have a large surface area prior to coagulation and neagitvely charged ions may be adsorbed onto the hydroxide surfaces. Precipitation
P r e c i p i t a t i o n o f a r s e n i c m a y i n v o l v e t h e r e a c t i o n o f a r s e n i c a l s p e c i e s w i t h a a n i o n t h a t f o r m s a n i n s o l u b l e ( o r s p a r i n g l y s o l u b l e ) p r o d u c t ; i t m a a l s o i n c l u d e a g g l o m e r a t i o n o f s u s p e n d e d a r s e n i c - c o n t a i n g p a r t i c l e s o r t h e e n m e s h m e n t i n t o a f l o c c u l a t e d p r e c i p i t a t e .
T h e U S E n v i r o n m e n t a l P r o t e c t i o n A g e n c y ( E P A ) c o m p i l e d a r e v i e w U S E P A , T e c h n o l o g i e s a n d c o s t s f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r , i n t e r n e t s i t e : h t t p : / / w w w . e p a . g o v / s a f e w a t e r / a r s / t r e a t m e n t s _ a n d _ c o s t s . p d f , P 2 - 1 5 , 2 0 0 0 o f p r e c i p i t a t i v e p r o c e s s e s u s e d t o r e m o v e a r s e n i c f r o m a q u e o u s m e d i a . T h e s a l i e n t p o i n t s a p p l i c a b l e t o t h i s r e s e a r c h a r e d i s c u s s e d .
E q u i p m e n t r e q u i r e d f o r p r e c i p i t a t i v e p r o c e s s e s i n c l u d e s c h e m i c a l f e e d s y s t e m s , m i x i n g d e v i c e s , b a s i n s f o r m i x i n g , f l o c c u l a t i o n , s e t t l i n g , f i l t e r m e d i a , s l u d g e h a n d l i n g e q u i p m e n t , a n d f i l t e r b a c k w a s h s y s t e m s . A d d i t i o n a l a s p e c t s t o b e c o n s i d e r e d i n c l u d e w a s t e d i s p o s a l o f a r s e n i c b e a r i n g s l u d g e s . N o r m a l l y a l e a c h i n g t e s t s u c h a s t h e T L C P t e s t w o u l d b e r u n o n s l u d g e s a m p l e s a t r e g u l a r i n t e r v a l s t o e n s u r e t h a t t h e a r s e n i c i s n o t r e a d i l y l e a c h e d . A n a l y t i c a l e q u i p m e n t , a p p a r a t u s a n d a n o p e r a t o r a r e a l s o r e q u i r e d .
I t i s w e l l k n o w n t h a t t h e p e n t a v a l e n t s t a t e o f a r s e n i c i s m o r e r e a d i l y r e m o v e d t h a t t h e t r i v a l e n t f o r m . C o m m o n c o a g u l a n t s i n c l u d e f e r r i c s u l p h a t e o r t h e c h l o r i d e a n d a l u m .
A l u m i n i u m b a s e d c o a g u l a t i o n w i t h d i s i n f e c t i o n b y c h l o r i n a t i o n i s m o s t c o m m o n l y u s e d i n N e w Z e a l a n d J . G r e g o r , “ A r s e n i c d u r i n g a l u m i n i u m - b a s e d d r i n k i n g w a t e r t r e a t m e n t ” , W a t e r R e s e a r c h , V o l . 3 5 , N o . 7 , p p 1 6 5 9 - 1 6 6 4 , 2 0 0 1 . . I n t h i s p r o c e s s A s I I I i s n o t r e m o v e d b y V t h e t r e a t m e n t p r o c e s s a n d e v e n t u a l l y b e c o m e s o x i d i z e d t o A s i n t h e f i n a l C l 2 d i s i n f e c t i o n s t e p . A r s e n i c V f o r m s s t a b l e p r e c i p i t a t e s w i t h m a n y i o n s . S o m e o f
55 t h e s e c o m p o u n d s a r e l i s t e d i n t a b l e ? ? a l o n g w i t h t h e i r s o l u b i l i t y p r o d u c t s
( K S P ) .
z {B|y}E~(s¨r62~En|y~6{B gEvy}6x [vyE6|yz<~y:mx }E
Table ?? Ksp values of pentavalent arsenates qer6sutwvyx
6¬ ¬y¯6°
v66x x |: x 6v¨T vy}6x }E6{TE2Bm:6B:i" ¡66¢ £T¤ 6EIF¥e £:£Tm F¥I{§¦"z r¨<©6ª2{6tw2rB«i{ «6ª6® {¨<±6±y²r§³ ³ ³ ³ ³ ³ ³y´rBµ2¶T:}E6|2{B´r
·
y¶F6¸E|y|2{F|y}E~¨´r<¹º|6E|6¸{B »8vyE6|yz§Ey|6vEvy}6x 6¼¾½ ½ ½ ¿2|y}E~¨|yTv}6x ¼ ¦ ¿Ix }Enmy|6ÀyEv:yE¤Eyz Ex y}Ex Á[z |y} ÁE|}66¨¼¾½ ½ ½ ¿2E|yz
6
|}E~¨T |ymx Ey}x Á|yz 6¨x }:x :[¼ ½ ½ ½ ¿>{BE|yz 6x 6¨¼¾½ ½ ¿>{F|y}E~x y}6¼ ½ ½ ½ ¿2T|z EÂ{Ää¤ £T i" ¡66¢ £T¤6Å£TÆ:£T6 ÇBÈ{<¦ yz rTÉOÊ6{6tw2rBªi{ ©6±6±6®
°6¯ {T¼L¨<±6±6±6¿Lr ª F o r m u l a - l o g K s p
C a 3 ( A s O 4 ) 2 1 8 . 1 7
M g 3 ( A s O 4 ) 2 1 9 . 6 8
M n 3 ( A s O 4 ) 2 2 8 . 7 2
F e 3 ( A s O 4 ) 2 2 0 . 2 4
L a 3 A s O 4 21.03
M c N e i l e t a l L . M N e i l , a n d M . E d w a r d s , “ A r s e n i c r e m o v a l d u r i n g p r e c i p i t a t i v e s o f t e n i n g ” , J o u r n a l o f E n v i r o n m e n t a l E n g i n e e r i n g , V o l . 1 2 3 , N o . 5 , p p 4 5 3 - 4 6 0 , 1 9 9 7 . s h o w e d t h a t s i m p l e f o r m a t i o n o f t h e a r s e n a t e s w a s n o t t h e m e c h a n i s m f o r a r s e n a t e r e m o v a l f o r c a l c i u m a n d m a n g a n e s e a r s e n a t e s ; i n s t e a d t h e a r s e n a t e w a s a d s o r b e d o n c a l c i u m c a r b o n a t e o r m a n g a n e s e h y d r o x i d e . I t w a s a l s o s h o w n t h a t t h e c o r r e s p o n d i n g a r s e n i t e r e m o v a l s w e r e a b o u t a n o r d e r o f m a g n i t u d e M N e i l , a n d M . E d w a r d s , “ A r s e n i c r e m o v a l d u r i n g p r e c i p i t a t i v e s o f t e n i n g ” , J o u r n a l o f E n v i r o n m e n t a l E n g i n e e r i n g , V o l . 1 2 3 , N o . 5 , p p 4 5 3 - 4 6 0 , 1 9 9 7 . l o w e r t h a n t h e a r s e n a t e r e m o v a l s ; t h e p r e o x i d a t i o n o f a r s e n a t e i s t h u s f a v o u r a b l e .
B r a n d h u b e r P . B r a n d h u b e r , a n d G . A m y , “ A l t e r n a t i v e m e t h o d s f o r m e m b r a n e f i l t r a t i o n o f a r s e n i c f r o m d r i n k i n g w a t e r ” , D e s a l i n a t i o n , V o l . V2VTW , p p 1 - 1 0 , 1 9 9 8 t e s t e d a r s e n i c r e m o v a l s ( b y v a r y i n g f e r r i c c h l o r i d e c o a g u l a t i o n s o n a 0 . 0 2 0 m g / L a r s e n a t e s o l u t i o n o f 7 . 6 5 a t 9 . 8 o C i n c l u d i n g f i l t r a t i o n t h r o u g h a 0 . 2 V f i l t e r ) f o r t h e i r f i t o f t h e L a n g m u i r a d s o r p t i o n m o d e l a n d f o u n d a p l o t o f c o a g u l a n t d o s e v s . % a r s e n i c r e m o v e d w a s a h y p e r b o l i c f u n c t i o n . T h e e q u a t i o n t h a t d e s c r i b e d t h e p l o t i s
% a r s e n i c r e m o v a l = 1 0 0 * ( k x d o s e ) / ( 1 + k x d o s e ) ,
w h e r e k = 0 . 3 3 2 L / m g a n d d o s e = F e C l 3 d o s e i n m g / L
56
T h e s e d a t a s u g g e s t t h a t i n c r e a s i n g t h e c o a g u l a n t d o s e b e y o n d a c e r t a i n p o i n t w o u l d r e s u l t i n a r e d u c e d a r s e n i c r e m o v a l d e s p i t e u n i f o r m l y i n c r e a s e d f e r r i c c h l o r i d e d o s e s .
W a t e r s o f t e n i n g b y r a i s i n g t h e p H t o p r e c i p i t a t e t h e m a g n e s i u m a n d c a l c i u m r e m o v e s a r s e n i c m o r e e f f e c t i v e l y a s t h e p H i s r a i s e d ( p H 9 – p H 1 1 ) p r o d u c e g o o d r e m o v a l s e v e n o n a r s e n i c c o n c e n t r a t i o n s a s l o w a s 0 . 0 0 8 m g / L . R a i s i n g t h e p H e v e n f u r t h e r ( 1 1 – 1 2 ) p r e c i p i t a t e s m a g n e s i u m a s i t h y d r o x i d e , h o w e v e r , t h e r e i s e v i d e n c e t h a t m a g n e s i u m h y d r o x i d e s s e t t l e p o o r l y M N e i l , a n d M . E d w a r d s , “ A r s e n i c r e m o v a l d u r i n g p r e c i p i t a t i v e s o f t e n i n g ” , J o u r n a l o f E n v i r o n m e n t a l E n g i n e e r i n g , V o l . 1 2 3 , N o . 5 , p p 4 5 3 - 4 6 0 , 1 9 9 7 . t h u s i n c r e a s i n g t h e c o s t o f t h e o p e r a t i o n . T h e m a x i m u m a c c e p t a b l e v a l u e ( M A V ) f o r N e w Z e a l a n d i s 0 . 0 1 m g / L ; t h i s M A V i s n o t a l w a y s a c h i e v e d a n d c o u l d b e a t t a i n e d b y d i s i n f e c t i o n p r i o r t o A l u m c o a g u l a t i o n . A p o s s i b l e d i s a d v a n t a g e o f p r i o r c h l o r i n a t i o n i s t h a t o r g a n i c m a t t e r m a y b e o x i d i z e d t o f o r m f o u l t a s t i n g b y - p r o d u c t t h u s i n h i b i t i n g t h e a e s t h e t i c q u a l i t y o f t h e w a t e r .
H e r i n g J . H e r i n g , “ A r s e n i c R e m o v a l b y E n h a n c e d C o a g u l a t i o n a n d M e m b r a n e P r o c e s s e s ” , M e n a c h e m E l i m e l e c h , D e p a r t m e n t o f C i v i l a n d E n v i r o n m e n t a l E n g i n e e r i n g , U n i v e r s i t y o f C a l i f o r n i a , L o s A n g e l e s , , i n t e r n e t s i t e : h t t p : / / w w w . a w w a r f . c o m / e x s u m s / 9 0 7 0 6 . h t m , 1 9 9 6 . i n v e s t i g a t e d a l u m a n d f e r r i c c o a g u l a t i o n i n a r s e n i c r e m o v a l f r o m d r i n k i n g w a t e r h a v i n g 0 . 0 0 2 – 0 . 1 0 0 m g / L a r s e n i c a n d r e a c h e d t h e f o l l o w i n g c o n c l u s i o n s :
• O p t i m i u m p H r a n g e s : a l u m 6 - 7
F e C l 3 1 - 8 • O n a m a s s t o m a s s b a s i s f e r r i c c h l o r i d e i s m o r e e f f e c t i v e t h a n a l u m , b u t o n a m o l e t o m o l e b a s i s s i m i l a r r e s u l t s a r e a c h i e v e d • A l u m c a n n o t r e m o v e t r i v a l e n t a r s e n i c , b u t f e r r i c c h l o r i d e c a n b u t l e s s e f f e c t i v e l y t h a n p e n t a v a l e n t a r s e n i c • V a r i a t i o n o f f i l t e r p o r e s i z e f r o m 0 . 1 - 1 . 0 m i c r o m e t e r s h a d v e r y l i t t l e e f f e c t o n t h e a r s e n i c r e m o v a l s ! ! ! ! ! ! ! N o t e t h a t t h i s f a c t i s i n a g r e e m e n t w i t h t h i s r e s e a r c h w o r k i n w h i c h s a m p l e s o f a l u m c o a g u l a t e d a r s e n i c w e r e d e c a n t e d a n d f i l t e r e d ( t h r o u g h a 0 . 2 V m f i l t e r ) a n d b o t h t h e d e c a n t a t e a n d t h e f i l t r a t e y i e l d e d v e r y c l o s e r e m o v a l s • U n d e r o p t i m a l c o n d i t i o n s o r g a n i c m a t t e r a n d i n o r g a n i c i o n s s u c h a s C a , s u p h a t e a n d p h o s p h a t e h a d m i n i m a l i n f l u e n c e s o n a r s e n i c r e m o v a l s . ! ! ! ! O t h e r w o r k e r s f o u n d i n t e r f e r e n c e s b y p h o s p h a t e , a n e x p e c t a b l e
57
p h e n o m e n a s i n c e p h o s p h o r u s i s a l s o a g r o u p V A e l e m e n t a n d t h e i r c h e m i s t r i e s a r e s i m i l a r .
The aluminium hydroxide is more soluble than the ferric hydroxide hence the reduced effective pH range J. Hering, “Arsenic Removal by Enhanced Coagulation and Membrane Processes”, Menachem Elimelech, Department of Civil and Environmental Engineering, University of California, Los Angeles, , internet site: http://www.awwarf.com/exsums/90706.htm , 1996. Recovery of coagulants
After the coagulation process is complete the sludge may be acidified with sulphuric acid to solubilize the metal ions used for coagulation. Edwards and Benjamin, as cited by Droste R.Droste, Theory and Practice of Water and Wastewater Treatment, John Wiley & Sons, New York, P262, 1997, regenerated ferric sludges 50 times by treating the sludge at a ph of 3.5-4.5 to redissolve the ferric ions. If toxic ions such as lead or cadmium are present in the sludge, careful monitoring is required to prevent contamination of the treated water.
T a b l e ? ? A C o m p a r i s o n o f A r s e n i c R e m o v a l T e c h n i q u e s
M e t h o d [ A s ] i n i t i a A s S u b s t r a t e % A s L e a c h I n t e r f e r D a t a
l m g / L V a l e n c y D o s e R e m o t e s t e n t v a l p a s s i o n s = P , F a i l = F
F e C l 3 & C a ( O H ) 2 9 . 8 ? 0 . 5 - 1 . 0 g / L 9 8 - R e f - L e i s t 9 9 L . M N e i l , a n d M . E d w a r d s , “ A r s e n i c r e m o v a l d u r i n g p r e c i p i t a t i v e s o f t e n i n g ” , C a C O 3 a n d i r o n 0 . 0 1 0 5 9 m g / L 8 2 ( v a l e n c e n o t o f E n v i r o n m e n t a l E n g i g i v e n )
F e C l 3 3 1 ? 0 . 2 - 1 . 0 g / L 8 6 - - L e i s t 9 3 S . A r r y k u l , N . M a h a r a t c h a p o n g , K . K o o p t a r n o n , P . B u n n a u l , “ A r s e n i c r e m o v a l f r o m F e C l 3 0 . 1 5 F e / A s 9 0 % ? - > 4 / 1 p o t a l b l e w a t e r ” , i n t e r n e t s i t e : h t t p : / / w w w . p s u . a c . t h / e p i d e m i o l o g y / c o n f e r / 5 s u r a p o e . h t m l
J . H e r i n g . P . C h e n , J . W i l k i e , M . E l i m e l c h , a n d S . L i a n g , “ A r s e n i c r e m o v a l b y f e r r i c F e C l 3 0 . 0 2 5 F e / A s 9 7 % ? = 1 1 6 / 1 c h l o r i d e ” , J o u r n a l o f A m e r i c a n W a t e r ( 1 9 9 6 ) .
J . H e r i n g . P . C h e n , J . W i l k i e , M . E l i m e l c h , a n d S . L i a n g , “ A r s e n i c r e m o v a l b y f e r r i c F e C l 3 0 . 0 2 3 F e / A s 6 5 % ? = 5 8 / 1 c h l o r i d e ” , J o u r n a l o f A m e r i c a n W a t e r W o r k s A s s o c i a t i o n ( 1 9 9 6 ) .
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( 1 9 9 6 ) .
S . T o k u n g a , S . Y o k o y a m a , a n d S . W a s a y , “ R e m o v a l o f a e r s e n i c ( I I I ) a n d a r s e n i c ( V ) L a C l 3 18.75 3 1 4 0 i o n s f r o m a q u e o u s s o l u t i o n s w i t h l a n t h a n u m ( I I I ) s a l t a n d c o m p a r i s o n w i t h
a l u m i n i u m ( I I I ) , c a l c i u m ( I I ) , a n d i r o n ( I I I ) s a l t s ” ,
WV , N o . 3 , p p 2 9 9
S . T o k u n g a , S . Y o k o y a m a , a n d S . W a s a y , “ R e m o v a l o f a e r s e n i c ( I I I ) a n d a r s e n i c ( V ) L a C l 3 18.75 5 1 9 9 . 5 i o n s f r o m a q u e o u s s o l u t i o n s w i t h l a n t h a n u m ( I I I ) s a l t a n d c o m p a r i s o n w i t h
a l u m i n i u m ( I I I ) , c a l c i u m ( I I ) , a n d i r o n ( I I I )
WV , N o . 3 , p p 2 9 9 T. Harper, and. Kingham, “removal of arswenic from wastewater using precipitative methods”, F e C l 3 & C a ( O H ) 2 9 . 8 - Fe/As = 46 9 8 - ? ? Water Environment researc 9 9 23
9 8 T. Harper, and. Kingham, “removal of arswenic from wastewater using precipitative methods”, N a 2 S & C a ( O H ) 2 9 . 8 - Na2S/As = 8 1 ? ? Water Environment research, Vol. 64, pp 200 140 T . H a r p e r , a n d . K i n g h a m , “ r e m o v a l o f a r s w e n i c f r o m w a s t e w a t e r u s i n g p r e c i p i t a t i v e A l u m & C a ( O H ) 2 9 . 8 ? Al/As = 6.0 7 9 ? ? m e t h o d s ” , W a t e r E n v i r o n m e n t r e s e a r c h , V o l . 6 4 , p p 2 0 0 P r e f o r m e d 0.010 5 3 7 5 9 8 ? J . H e r i n g . P . C h e n , J . W i l k i e , M . E l i m e l c h , a n d S . L i a n g c h l o r i d e ” , J o u r n a l o f A m e r i c a n W a t e r W o r k s A s s o c o a i t i o n F e ( O H ) 3 ( 1 9 9 6 ) . P r e f o r m e d 0.010 3 3 7 5 9 2 ? J . H e r i n g . P . C h e n , J . W i l k i e , M . E l i m e l c h , a n d S . L i a n g , “ A r s e n i c r e m o v a l b y f e r r i c c h l o r i d e ” , J o u r n F e ( O H ) 3 ( 1 9 9 6 ) .
3 - L . M N e i l , a n d M . E d w a r d s , “ A r s e n i c r e m o v a l d u r i n g p r e c i p i t a t i v e s o f t e n i n g ” , M g ( O H ) 2 f o r m e d 0 . 0 1 5 5 5 0 m g / L > 9 5 P O 4 , i n s i t u M g 2 + O H - a t o f E n v i r o n m e n t a l E n g i n e e r p H > 1 1 . 5 6
ËÍÌ(ÎBÏÑÐ_ÒFÓ 2 - L . M N e i l , a n d M . E d w a r d s , “ A r s e n i c r e m o v a l d u r i n g p r e c i p i t a t i v e s o f t e n i n g ” , 0 . 0 2 5 5 2 0 m g / L > 9 5 C O 3 2 + o f E n v i r o n m e n t a l E n g i n e e r i n g Ô Õ§Ö[קØ(ÕEÙÚÖÛ M g
ËÑÜÎBÏÑÐ_ÒFÓ 0 . 0 7 5 5 1 m g / L 8 0 L . M N e i l , a n d M . E d w a r d s , “ A r s e n i c r e m o v a l d u r i n g p r e c i p i t a t i v e s o f t e n i n g ” , 2 + o f E n v i r o n m e n t a l E n g i n e e r i n g Ô Õ§Ö[קØ(ÕEÙÚÖÛ M n
N a 2 S , p H 8 , F e ? ? ? 9 8 - l e i s t 9 9 . 6 T r i v a l e n t F e & C r 1 0 5 8 g / L 9 7 . 8 L e i s t h y d r o x i d e s M n + t r e a t e d G A C 1 5 5 ? 9 9 ( B a , C u , F e I I , I I I
59
F e I I I ) M n g r e e n s a n d & 0 . 2 3 F e : A s = 2 0 8 7 . 5 V i r a r a g h a v a n F e M n g r e e n s a n d 0 . 0 5 2 5 8 9 M . E d w a r d s , “ C h e m i s t r y o f a r s e n i c r e m o v a l d u r i n g c o a g u l a t i o n a n d F e o x i d a t i o n ” , J o u r n a l o f t h e A m e r i c a n W a ( 1 9 9 4 ) . GSA Resources Inc., Application data sheet: Removal of arsenic and other heavy metals from I r o n ( I I ) l a d e n ? 3 & 5 * * ? ? P water, website: www.gsaresources.com, (200 C h a b a z i t e ( a f i n a l z e o l i t e ) c o n c . < 5 p p b I r o n o x i d e c o a t e d - 3 , 5 P U S E P A , T e c h n o l o g i e s a n d c o s t s f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r , s a n d i n t e r n e t s i t e : 4 3 , 2 0 0 0 . I r o n f i l i n g s 2 . 0 - 5 9 5 - ? U S E P A , T e c h n o l o g i e s a n d c o s t s f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r , 2 0 . 0 8 1 * i n t e r n e t s i t e : 4 7 , 2 0 0 0 . E l e m e n t a l i r o n & - 5 - # P U S E P A , T e c h n o l o g i e s a n d c o s t s f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r , i n t e r n e t s i t e : S u l p h u r a n d H 2 O 2 4 4 , 2 0 0 0 . C e m e n t l e a c h a t ? 0 . 4 4 g 9 5 P V . D u t r e , C . C h e s t e n s , J . S c h a e p , C . V a n d e c a t e e l e , “ S t u d y o f t h e r e m e d i a t i o n o f a s i t e c o n t a m i n a t e d w i t h a r s e n i c ” , s t a b i l i z a t i o n ( o f e [ A s ] = c e m e n t ( o r p p 1 8 5 - 1 9 4 . 1 9 9 8 . s o l i d s ) 1 6 . 1 p p l i m e ) / g m s o i l P C e m e n t & l i m e & 1 . 0 g 9 9 s t a b i l i z a t i o n ( o f w a t e r \ s o l i d s ) P l i m e [ o n l y ] 9 6 s t a b i l i z a t i o n ( o f s o l i d s ) I r o n I I l a d e n - 3 , 5 2 . 2 g A s V - P G S A R e s o u r c e s I n c . , “ R e m o v a l o f a r s e n i c a n d o t h e r h e a v y m e t a l s f r o m w a t e r ” , Z e o l i t e o r 7 . 5 g C o r t a r o , A r i z o n a 8 5 6 5 2 , i n t e r n e t s i t e : A s I I I / k g m e d i a M e m b r a n e 0 . 0 2 5 5 5 - > 9 5 P P . B r a n d h u b e r , a n d G . A m y , “ A l t e r n a t i v e m e t h o d s f o r m e m b r a n e f i l t r a t i o n o f a r s e n i c f i l t r a t i o n 0 . 0 1 8 5 3 - 5 7 f r o m d r i n k i n g w a t e r ” , T e c h n o l o g i e s a n d c o s t s f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r ,
h t t p : / / w w w . e p a . g o v / s a f e w a t e r / a r s / t r e a t m e n t s _ a n d _ c o s t s
60
* 7 d a y r e a c t i o n s t o a t t a i n l e s s t h a n 0 . 0 5 p p m A s * * C a p a c i t y f o r a r s e n i t e a n d a r s e n a t e a r e 6 - 9 a n d 2 - 2 . 5 g A s / k g m e d i a # e q u i l i b r i u m c o n c e n t r a t i o n s b e l o w 0 . 0 5 p p m w e r e a t t a i n e d w i t h u p t o 5 0 m g A s a d s o r b e d / g F e ? i n d i c a t e t h a t t h e s a m p l e s w e r e n o t s p e c i a t e d \ 2 w e e k s s e t t i n g t i m e
P r e c i p i t a t i o n o f s m a l l a m o u n t s h i g h l y c o n c e n t r a t e d a r s e n i c w a s t e s i s c o s t e f f e c t i v e M . L e i s t , R . J . C a s e y , a n d D . C a r i d i , “ T h e m a n a g e m e n t o f a r s e n i c w a s t e s : p r o b l e m s a n d p r o s p e c t s ” ,
J o u r n a l o f H a z a r d o u s M a t e r i a l s , V o l . W20 , N o . 1 , p p 1 2 5 - 1 3 8 , 2 0 0 0 .
A r s e n i c i s m o s t e f f e c t i v e l y r e m o v e d i n t h e p e n t a v a l e n t s t a t e , i r o n c o a g u l a t i o n b e i n g t h e p r e f e r r e d m e t h o d o f r e m o v a l . M . L e i s t , R . J . C a s e y , a n d D . C a r i d i , “ T h e m a n a g e m e n t o f
a r s e n i c w a s t e s : p r o b l e m s a n d p r o s p e c t s ” , J o u r n a l o f H a z a r d o u s M a t e r i a l s , V o l . W20 , N o . 1 , p p 1 2 5 - 1 3 8 , 2 0 0 0 . / / / / / J .
H e r i n g . P . C h e n , J . W i l k i e , M . E l i m e l c h , a n d S . L i a n g , “ A r s e n i c r e m o v a l b y f e r r i c c h l o r i d e ” , J o u r n a l o f A m e r i c a n W a t e r
W o r k s A s s o c o a i t i o n , V o l . c2c , 4 , p p 1 5 5 - 1 6 7 , ( 1 9 9 6 ) .
Interference by anions during ferric precipitation of arsenic
A r s e n i t e r e m o v a l ( p r e c i p i t a t i o n , o r a d s o r p t i o n o n t o p r e f o r m e d f e r r i c h y d r o x i d e ) i s m o r e s e n s i t i v e t o i n t e r f e r e n c e b y c o m p e t i n g a n i o n s ( s u c h a s s u l p h a t e o r p h o s p h a t e ) t h a n a r s e n a t e r e m o v a l J . H e r i n g . P . C h e n , J . W i l k i e , M . E l i m e l c h , a n d S .
L i a n g , “ A r s e n i c r e m o v a l b y f e r r i c c h l o r i d e ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c o a i t i o n , V o l . c2c , 4 , p p 1 5 5 - 1 6 7 , ( 1 9 9 6 ) . . T h e p r e s e n c e o f 3 m M c a l c i u m ( a t p H 9 . 0 ) a u g m e n t e d t h e a r s e n a t e r e m o v a l f o r b o t h p r e c i p i t a t i o n b y f e r r i c c h l o r i d e a n d a d s o r p t i o n o n t o p r e f o r m e d f e r r i c h y d r o x i d e a n d i t r e d u c e d t h e p h o s p h a t e i n t e r f e r e n c e J . H e r i n g . P . C h e n , J . W i l k i e , M . E l i m e l c h , a n d S . L i a n g , “ A r s e n i c r e m o v a l b y f e r r i c c h l o r i d e ” , J o u r n a l o f t h e A m e r i c a n W a t e r
W o r k s A s s o c o a i t i o n , V o l . c2c , 4 , p p 1 5 5 - 1 6 7 , ( 1 9 9 6 ) . . T h i s o b s e r v a t i o n i s i n a g r e e m e n t w i t h t h e w o r k o f M c N e i l a n d E d w a r d s L . M N e i l , a n d M . E d w a r d s , “ A r s e n i c r e m o v a l d u r i n g p r e c i p i t a t i v e s o f t e n i n g ” , J o u r n a l o f E n v i r o n m e n t a l E n g i n e e r i n g , V o l . 1 2 3 , N o . 5 , p p 4 5 3 - 4 6 0 , 1 9 9 7 . . M e n g e t a l X . M e n g , S . B a n g , a n d G . K o r f i a t i s , “ E f f e c t s o f s i l i c a t e , s u l p h a t e , a n d c a r b o n a t e o n a r s e n i r e m o v a l b y f e r r i c c h l o r i d e ” , W a t e r R e s e a r c h , V o l . X2b , I s s u e 4 , p p 1 2 5 5 - 1 2 6 2 , ( 2 0 0 0 ) . f o u n d t h a t s i l i c a t e s i g n i f i c a n t l y d e c r e a s e d a r s e n i t e r e m o v a l w h e n t h e S i c o n c e n t r a t i o n e x c e e d e d 1 m g / L a n t h e p H w a s g r e a t e r t h a n 5 , w h i l e a r s e n a t e w a s o n l y s l i g h t l y r e d u c e d ; c a l c i u m a n d m a g n e s i u m i o n s c o u n t e r a c t e d t h i s e f f e c t i n a r s e n a t e c o a g u l a t i o n . S u l p h a t e a n d c a r b o n a t e h a d a n e g l i g i b l e e f f e c t o n b o t h a r s e n i t e a n d a r s e n a t e r e m o v a l s . C a C O 3 , M g ( O H ) 2 , M n ( O H ) 2 , a n d F e ( O H ) 3 w e r e f a c i l i t a t e d t h e
61 r e m o v a l o f a r s e n i c d u r i n g p r e c i p i t a t i v e s o f t e n i n g o f w a t e r A m e r i c a W a t e r W o r k s A s s o c i a t i o n R e s e a r c h F o u n d a t i o n , i n t e r n e t s i t e : h t t p : / / w w w . a w w a r f . c o m / e x u m s / 1 5 3 . h t m , ( 2 0 0 0 ) . .
D a v i s a n d S t u m , a s c i t e d b y M e n g e t a l X . M e n g , S . B a n g , a n d G . K o r f i a t i s , “ E f f e c t s o f s i l i c a t e ,
s u l p h a t e , a n d c a r b o n a t e o n a r s e n i r e m o v a l b y f e r r i c c h l o r i d e ” , W a t e r R e s e a r c h , V o l . X2b , I s s u e 4 , p p 1 2 5 5 - 1 2 6 2 , ( 2 0 0 0 ) . , s u c c e s s f u l l y d e s c r i b e d t h e a d s o r p t i o n o f c a t i o n s a n d a n i o n s o n t h e s u r f a c e s o f m e t a l o x i d e s a n d h y d r o x i d e s u s i n g s u r f a c e c o m p l e x a t i o n m o d e l s . M e n g u s e d t h e t r i p l e l a y e r m o d e l ( T L M ) t o d e s c r i b e t h e i n t e r f e r e n c e o f s i l i c a t e i n t h e a d s o r p t i o n o f a r s e n i t e a n d a r s e n a t e o n t h e s u r f a c e o f f e r r i c h y d r o x i d e s f o r m e d d u r i n g f e r r i c c o a g u l a t i o n . T h e t r i p l e l a y e r m o d e l s i m u l a t e s t h e f o r m a t i o n o f i n n e r a n d o u t e r s u r f a c e c o m p l e x e s b y p l a c i n g t h e a d s o r b e d i o n s i n d i f f e r e n t p l a n e s i n t h e e l e c t r i c a l l a y e r K . H a y e s , a n d J . L e c k i e , “ M o d e l l i n g i o n i c s t r e n g t h
e f f e c t s o n a n i o n a d s o r p t i o n a t h y d r o u s o x i d e / s o l u t i o n i n t e r f a c e s ” , J o u r n a l o f C o l l o i d I n t e r f a c e S c i e n c e , V o l . VTa21 , p p
7 1 7 - 7 2 6 , 1 9 8 8 . T h e r e m o v a l o f a r s e n i t e a n d a r s e n a t e ( 0 . 1 m g A s / L a t p H 6 . 8 ) d e c r e a s e d f r o m a p p r o x i m a t e l y 9 0 t o 4 5 % w h e n t h e s i l i c a t e c o n c e n t r a t i o n w a s i n c r e a s e d f r o m 1 - 1 0 m g / L d u r i n g c o a g u l a t i o n s w i t h 5 m g / L f e r r i c c h l o r i d e X . M e n g , S . B a n g , a n d G .
K o r f i a t i s , “ E f f e c t s o f s i l i c a t e , s u l p h a t e , a n d c a r b o n a t e o n a r s e n i r e m o v a l b y f e r r i c c h l o r i d e ” , W a t e r R e s e a r c h , V o l . X2b , I s s u e 4 , p p 1 2 5 5 - 1 2 6 2 , ( 2 0 0 0 ) . A t p H l e v e l s l e s s t h a n 6 . 8 a r s e n a t e r e m o v a l s w e r e n o t a f f e c t e d b y s i l i c a t e . T h e p r e s e n c e o f p o s i t i v e l y c h a r g e d c a l c i u m a n d m a g n e s i u m i o n s c o u n t e r a c t e d t h e s i l i c a t e i n t e r f e r e n c e – t h e s e i o n s w o u l d h a v e r e d u c e d t h e n e g a t i v e s u r f a c e c h a r g e t h u s p e r m i t t i n g t h e a r s e n a t e t o c o n t a c t t h e h y d r o x i d e s u r f a c e X . M e n g , S . B a n g , a n d G . K o r f i a t i s , “ E f f e c t s o f s i l i c a t e , s u l p h a t e , a n d c a r b o n a t e o n a r s e n i r e m o v a l b y f e r r i c c h l o r i d e ” , W a t e r R e s e a r c h , V o l . X2b , I s s u e 4 , p p 1 2 5 5 - 1 2 6 2 , ( 2 0 0 0 ) . . R e p l a c i n g s o d i u m h y d r o x i d e w i t h c a l c i u m h y d r o x i d e l e a d t o e n h a n c e d f e r r i c p r e c i p i t a t i o n o f a r s e n a t e i n t h e p r e s e n c e o f s u l p h a t e ; t h i s i m p r o v e m e n t i s m o s t l i k e l y d u e t o t h e f o r m a t i o n o f t h e g y p s u m N . P a p a s s i o p i , E . V i r c i k o v a , V . N e n o v , A . K o n t o p o u l o s , L . M o l n a r , “ R e m o v a l a n d f i x a t i o n o f a r s e n i c i n t h e f o r m o f f e r r i c a r s e n a t e s ” , H y d r o m e t a l l u r g y , V o l . bV , p p 2 4 3 - 2 5 3 , ( 1 9 9 6 ) . .
T h e T L M X . M e n g , S . B a n g , a n d G . K o r f i a t i s , “ E f f e c t s o f s i l i c a t e , s u l p h a t e , a n d c a r b o n a t e o n a r s e n i r e m o v a l b y f e r r i c c h l o r i d e ” , W a t e r R e s e a r c h , V o l . X2b , I s s u e 4 , p p 1 2 5 5 - 1 2 6 2 , ( 2 0 0 0 ) . i n c l u d e d s u r f a c e r e a c t i o n s s u c h a s p r o t o l y s i s , a n d e l e c t r o l y t e b i n d i n g : L o g K + + • S u r f a c e i o n i z a t i o n s : S O H + H I S O H 2 5 . 1 S O H I S O - + H + - 1 0 . 7
62
• E l e c t r o l y t e a d s o r p t i o n : S O H + K + I S O - K + H + - 9 . 0 - S O H + N O 3 I S O H 2 - N O 3 6 . 9 ( S O H r e f e r s t o s u r f a c e h y d r o x i d e )
• I n n e r s p h e r e s u r f a c e c o m p l e x a t i o n r e a c t i o n s i n c l u d e : L o g K 2 - + S O H + H 3 A s O 4 I S A s O 4 + H 2 O + 2 H 0 . 6 - + S O H + H 3 A s O 3 I S H A s O 3 + H 2 O + H - 3 . 1 + + S O H + H 3 A s O 3 + K I S A s O 3 K + H 2 O + H - 2 . 1 + + S O H + H 4 S i O 4 + K I S H 2 S i O 4 K + H 2 O + H - 2 . 1
T h e s i z e o f F e ( O H ) 3 p r e c i p i t a t e s t h a t f o r m u p o n d i s s o l u t i o n o f f e r r i c c h l o r i d e w a s s e e n t o v a r y a c r o s s t h e p H r a n g e 4 - 9 w i t h t h e l a r g e r p a r t i c l e s e x i s t i n g a t t h e e x t r e m e s a n d t h u s p o s s e s s i n g g r e a t e r a d s o r p t i v e c a p a c i t i e s f o r a r s e n i t e a t t h e s e e x t r e m e s d u e t o t h e g r e a t e r a v a i l a b l e s u r f a c e a r e a J . H e r i n g . P . C h e n , J . W i l k i e , M . E l i m e l c h , a n d S . L i a n g , “ A r s e n i c r e m o v a l b y f e r r i c c h l o r i d e ” , J o u r n a l o f A m e r i c a n W a t e r W o r k s
A s s o c o a i t i o n , V o l . c2c , 4 , p p 1 5 5 - 1 6 7 , ( 1 9 9 6 ) . .
P a p a s s i o p i e t a l N . P a p a s s i o p i , E . V i r c i k o v a , V . N e n o v , A . K o n t o p o u l o s , L . M o l n a r , “ R e m o v a l a n d f i x a t i o n o f a r s e n i c i n t h e f o r m o f f e r r i c a r s e n a t e s ” , H y d r o m e t a l l u r g y , V o l . bV , p p 2 4 3 - 2 5 3 , ( 1 9 9 6 ) . f o u n d t h a t p H , s u l p h a t e c o n t e n t , a n d i n i t i a l a r s e n i c c o n c e n t r a t i o n i n f l u e n c e d t h e a q u e o u s r e m o v a l o f a r s e n i c . T h e e f f e c t i v e n e s s o f a r s e n i c r e m o v a l i s m a i n l y d e p e n d a n t o n t h e F e / A s r a t i o . T h e o p t i m u m p H f o r p r e c i p i t a t i o n d e p e n d s o n t h e F e / A s m o l a r r a t i o n p r e s e n t i n t h e i n i t i a l s o l u t i o n . T h e p H f o r m a x i m u m r e m o v a l s h i f t e d f r o m 3 t o 5 t o 6 a s t h e F e / A s m o l a r r a t i o i n c r e a s e d f r o m 2 t o 4 t o 6 , r e s p e c t i v e l y . T h e e f f e c t i v e n e s s o f a r s e n i c p r e c i p i t a t i o n a n d s e t t l i n g r a t e i n c r e a s e d w i t h F e / A s m o l a r r a t i o a n d t h e s e p r e c i p i t a t e s w e r e m o s t s t a b l e t o , l e a c h i n g a t p H 5 . 0 a t 2 5 o C . T h e y a l s o f o u n d t h a t t h e f e r r i c a r s e n a t e s w e r e m o s t s t a b l e i n t h e p H r a n g e o f 3 - 8 w i t h a r s e n i c s o l u b i l i t y ’ s r e m a i n i n g l e s s t h a n 0 . 1 0 0 m g / L . F e r r i c a r s e n a t e p r e c i p i t a t e s w i t h F e / A s r a t i o s a b o v e 3 a r e c o n s i d e r e d s a f e f o r d i s p o s a l u p t o n e u t r a l p H v a l u e s w h i l e t h o s e w i t h F e / A s r a t i o s > 5 . 9 m a y b e d i s p o s e d e v e n a t a l k a l i n e p H v a l u e s . T h e p r e c i p i t a t e r e l e a s e d f r o m 0 . 0 1 - a p p r o x i m a t e l y 0 . 1 m g A s / L . T h e s e f a c t o r i n d i c a t e t h a t s u i t a b l y f o r m e d f e r r i c a r s e n a t e s a r e s t a b l e i n t h e p H r a n g e f r o m 3 - 8 u n d e r l a b o r a t o r y c o n d i t i o n s , h o w e v e r , t h e e n v i r o n m e n t s u c h a p r e c i p i t a t e w i l l
63 e x p e r i e n c e i n a l a n d f i l l i s u n p r e d i c t a b l e a n d d i f f e r e n t b e h a v i o r m a y b e e n c o u n t e r e d .
L a n t h a n u m c h l o r i d e e x h i b i t s a w i d e r p H w i n d o w c o m p a r e d t o f e r r i c c h l o r i d e I t h t c o a g u l a t i o n o f a r s e n a t e ; t h e r e s p e c t i v e p H w i n d o w s a r e 5 - 1 0 a n d 5 - 7 S . T o k u n g a , S . Y o k o y a m a , a n d S . W a s a y , “ R e m o v a l o f a e r s e n i c ( I I I ) a n d a r s e n i c ( V ) i o n s f r o m a q u e o u s s o l u t i o n s w i t h l a n t h a n u m ( I I I ) s a l t a n d c o m p a r i s o n w i t h a l u m i n i u m ( I I I ) , c a l c i u m ( I I ) , a n d i r o n ( I I I ) s a l t s ” , W a t e r E n v i r o n m e n t
R e s e a r c h , V o l . WV , N o . 3 , p p 2 9 9 - 3 0 6 , ( 1 9 9 9 ) . . T h i s w i d e r w i n d o w i s a d v a n t a g e o u s t o w a t e r t r e a t m e n t s i n c e w a t e r s m a y r e q u i r e l e s s p r e t r e a t m e n t s i n c e m o s t n a t u r a l w a t e r s p H r a n g e s a r e i n t h e p H r a n g e o f 5 - 1 0 .
M a n g a n e s e g r e e n s a n d ( M n O 2 - c o a t e d s a n d ) c o m b i n e d w i t h F e a d d i t i o n ( F e : A s = 2 0 ) r e d u c e s A s I I I l e v e l s f r o m 0 . 2 0 0 m g / L t o 0 . 0 2 5 m g / L . T . V i r a r a g h a v a n , K . S . S u b r a m a n i a n , a n d J . A . A r u l d o s s , “ A r s e n i c i n d r i n k i n g w a t e r : p r o b l e m s a m d s o l u t i o n s ” , W a t e r S c i e n c e a n d T e c h n o l o g y ,
V o l . b2d , N o . 2 , 1 9 9 9 .
T a b l e ? ? A c o m p a r i s o n o f t h e r e m o v a l o f T r i v a l e n t a n d P e n t a v a l e n t A r s e n i c R e m o v a l A g e n t % A s I I I R e m o v e d % A s V R e m o v e d r e f
F e C l 3 3 0 9 0 l e i s t F e I I I 4 0 7 6 l e i s t L a I I I 6 0 9 9 S . T o k u n g a , S . Y o k o y a m a , a n d S .
W a s a y , “ R e m o v a l o f
a e r s e n i c ( I I I ) a n d
a r s e n i c ( V ) i o n s f r o m
a q u e o u s s o l u t i o n s w i t h
l a n t h a n u m ( I I I ) s a l t
a n d c o m p a r i s o n w i t h
a l u m i n i u m ( I I I ) ,
c a l c i u m ( I I ) , a n d
i r o n ( I I I ) s a l t s ” ,
W a t e r E n v i r o n m e n t
R e s e a r c h , V o l . WV , N o .
3 , p p 2 9 9 - 3 0 6 , ( 1 9 9 9 ) . A l I I I & p o l y A l 0 4 0 ? c h r o i d e
64
R e v e r s e o s m o s i s & 5 7 > 9 5 P . B r a n d h u b e r , a n d G . n a n o f i l t r a t i o n A m y , “ A l t e r n a t i v e m e t h o d s f o r m e m b r a n e
f i l t r a t i o n o f a r s e n i c
f r o m d r i n k i n g w a t e r ” ,
D e s a l i n a t i o n , V o l . V2VTW , p p 1 - 1 0 , 1 9 9 8 .
T a b l e ? ? A C o m p a r a t i v e R e v i e w o f A l u m v e r s u s F e r r i c C o a g u l a t i o n [ F e I I I ] [ F e I [ A l u m ] p H [ A s ] A s C o a g u l a n t % A s r e f m g / L I ] m g / L p p m o x i d a t i : A s m o l e r e m o v e m g / o n r a t i o d L s t a t e 3 3 5 1 0 - - 5 7 5 0 0 5 6 ~ 1 0 0 N . P a p a s s i o p i , E . V i r c i k o v a , V . 1 6 8 0 0 - - 5 3 7 7 5 5 6 ~ 1 0 0 N e n o v , A . K o n t o p o u l o s , L . 8 5 5 0 - - 5 1 9 1 3 5 6 ~ 1 0 0 M o l n a r , “ R e m o v a l a n d f i x a t i o n 2 2 3 - - 5 5 0 5 6 9 9 . 9 5 o f a r s e n i c i n t h e f o r m o f f e r r i c a r s e n a t e s ” ,
H y d r o m e t a l l u r g y , V o l . b 2 4 3 - 2 5 3 , ( 1 9 9 6 ) . - - 2 . 4 5.5 0 . 3 3 2 2 . 5 4 1 # M . E d w a r d s , “ C h e m i s t r y o f -9 a r s e n i c r e m o v a l d u r i n g
c o a g u l a t i o n a n d F e - M n
o x i d a t i o n ” , J o u r n a l o f t h e
A m e r i c a n W a t e r W o r k s
A s s o c i a t i o n , V o l . U , p p 6 4
( 1 9 9 4 ) . 5 . 0 - - 5.5 0 . 3 3 2 2 . 5 8 3 M . E d w a r d s , “ C h e m i s t r y o f -9 a r s e n i c r e m o v a l d u r i n g
c o a g u l a t i o n a n d F e - M n
o x i d a t i o n ” , J o u r n a l o f t h e
A m e r i c a n W a t e r W o r k s
A s s o c i a t i o n , V o l . U , p p 6 4
( 1 9 9 4 ) . 1 0 - 5 0 - 5 - ? 5 8 9 - 9 9 J . G u l l e d g e , a n d J O ’ C o n n e r , “ r e m o v a l o f a r s e n i c ( V ) f r o m F e 2 ( S O 4 8 w a t e r b y a d s o r p t i o n o n ) . 9 H 2 O a l u m i n i u m a n d f e r r i c
65
a l u m i n i u m a n d f e r r i c
h y d r o x i d e s ” , J o u r n a l o f t h e
A m e r i c a n W a t e r W o r k s
A s s o c i a t i o n , V o l . 021 , I s s u e 8 ,
p p 5 4 8 - 5 5 2 , ( 1 9 7 3 ) . - - 1 0 - 5 - ? 5 1 9 - 9 4 J . G u l l e d g e , a n d J O ’ C o n n e r , 5 0 * * 8 “ r e m o v a l o f a r s e n i c ( V ) f r o m w a t e r b y a d s o r p t i o n o n
a l u m i n i u m a n d f e r r i c
h y d r o x i d e s ” , J o u r n a l o f t h e
A m e r i c a n W a t e r W o r k s
A s s o c i a t i o n , V o l . 021 , I s s u e 8 ,
p p 5 4 8 - 5 5 3 , ( 1 9 7 3 ) . 3 – 1 0 * * - - 5 - 0 . 0 0 1 6 5 8 3 4 - 2 7 5 2 8 3 - 9 6 K . S c o t t , J . G r e e n , H . D o , a n d S M c L e a n , “ A r s e n i c r e m o v a l b y ( F e C l 3 ) 8 c o a g u l a t i o n ” , J o u r n a l o f t h e
A m e r i c a n W a t e r W o r k s
A s s o c i a t i o n , V o l . b , p p 1 1 4
( 1 9 9 5 ) . - - 6 - 2 0 5 - - 5 9 4 3 - 3 1 4 5 2 3 - 6 7 K . S c o t t , J . G r e e n , H . D o , a n d 8 S M c L e a n , “ A r s e n i c r e m o v a l b y c o a g u l a t i o n ” , J o u r n a l o f t h e
A m e r i c a n W a t e r W o r k s
A s s o c i a t i o n , V o l . b , p p 1 1 4 ( 1 9 9 5 ) . - - 3 0 + 5 - 0 . 0 5 5 > 9 0 E d w a r d s c i t e d b y 7 L e i s t I n d e p e n C h e n g d e n t o f p H a t 5 . 5 – 7 . 0 * a s f e r r i c s u l p h a t e * * P o l y m e r ( c a t i o n i c p o l y D A D M A C ) d o s e s o f 2 - 3 m g / L w e r e u s e d , a n d 0 . 0 1 m g / L n o n i o n i c p o l y a c r y l a m i d e w a s a d d e d a s a f i l t e r a i d . # s o r p t i o n d e n s i t i e s w e r e s i g n i f i c a n t l y h i g h e r f o r i r o n ( m i n i m u m v a l u e > 0 . 0 1 M A s / M F e ) t h a n a l u m i n i u m ( m a x i m u m v a l u e < M A s / M A l ) # # w a s t e w a t e r s a m p l e
66
B e l o w a p H o f 7 . 5 a l u m a n d f e r r i c i o n s ( o n a m o l a r b a s i s ) a r e c o m p a r a b l e c o a g u l a n t s , b u t f e r r i c i o n s a r e s u p e r i o r i n a r s e n i t e r e m o v a l a t p H l e v e l s g r e a t e r t h a n 7 . 5 . A b o v e p H 8 a l u m i n i u m h y d r o x i d e i s s o l u b l e . M . E d w a r d s , “ C h e m i s t r y o f
a r s e n i c r e m o v a l d u r i n g c o a g u l a t i o n a n d F e - M n o x i d a t i o n ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . U , p p 6 4 - 7 7 , ( 1 9 9 4 ) .
W h e n f e r r i c c o a g u l a n t s a r e a d d e d a l l t h e i r o n f o r m s F e ( O H ) 3 . H o w e v e r , n o t a l l L . M N e i l , a n d M . t h e a l u m i n i u m a d d e d a s a l u m c o a g u l a n t p r e c i p i t a t e s a s A l ( O H ) 3 . E d w a r d s , “ P r e d i c t i n g r s e n i c r e m o v a l d u r i n g m e t a l h y d r o x i d e p r e c i p i t a t i o n ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s
A s s o c i a t i o n , V o l . c2U , N o . 1 , p p 7 5 - 8 6 , ( 1 9 9 7 ) . S i n c e o n l y p a r t i c u l a t e m e t a l ( A l a n d F e ) h y d r o x i d e s c a n m e d i a t e a r s e n i c r e m o v a l , p H s h o u l d b e c a r e f u l l y a d j u s t e d i n a l u m c o a g u l a t i o n t r e a t m e n t u n i t s .
A l l f e r r i c i o n s a r e c o n v e r t e d t o t h e h y d r o x i d e , b u t n o t a l l t h e a l u m i n i u m i s c o n v e r t e d t o t h e c o r r e s p o n d i n g h y d r o x i d e , h e n c e t h e g r e a t e r c o a g u l a t i o n p o w e r o f f e r r i c i o n s . L . M N e i l , a n d M . E d w a r d s , “ P r e d i c t i n g r s e n i c r e m o v a l d u r i n g m e t a l h y d r o x i d e p r e c i p i t a t i o n ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . c2U , N o . 1 , p p 7 5 - 8 6 , ( 1 9 9 7 ) . F o r e x a m p l e , o f t h e 0 . 7 7 m g / L A l d o s e d a t a w a t e r t r e a t m e n t p l a n t , o n l y 0 . 1 8 m g / L w e r e p r e c i p i t a t e d . L . M N e i l , a n d M . E d w a r d s , “ P r e d i c t i n g r s e n i c r e m o v a l d u r i n g m e t a l h y d r o x i d e
p r e c i p i t a t i o n ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . c2U , N o . 1 , p p 7 5 - 8 6 , ( 1 9 9 7 ) .
R a m b e r g , a s c i t e d b y M c N e i l e t a l L . M N e i l , a n d M . E d w a r d s , “ P r e d i c t i n g r s e n i c r e m o v a l d u r i n g m e t a l h y d r o x i d e p r e c i p i t a t i o n ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . c2U , N o . 1 , p p 7 5 - 8 6 , ( 1 9 9 7 ) . , u s e d b o t h f e r r i c c h l o r i d e a n d s u l p h a t e o n a n e q u i m o l a r b a s i s a s c o a g u l a n t s i n p i l o t s c a l e t e s t s , a n d t h e s u l p h a t e w a s o n a v e r a g e f e w p e r c e n t m o r e e f f i c i e n t t h a n t h e c h l o r i d e i n c o a g u l a t i o n o f a 0 . 0 8 0 m g / L a r s e n a t e s o l u t i o n .
T a b l e ? ? T h e E f f e c t o f A d d i t i o n a l I o n s o n C o a g u l a t i o n C o a g u l a n t [ A s ] & A d d i t i o n a l E f f e c t C a u s e o f R e f v a l e n c e I o n E f f e c t 2 + F e C l 3 5 , 3 . 0 m M C a E n h a n c e d C a 3 ( P O 4 ) 2 L e i s t
0 . 0 2 m g / L f o r m a t i o n Ý%Ü Þ[ß(Ü[à C a 2 + a s ÖÛ C a C O 3
A c t i v a t e d 5 A s h i n 5 x I n o r g a n i c i o n s L e i s t c i t i n g C a r b o n C a r b o n e n h a n c e d c o n t r i b u t e d t o H u a n g A s a d s o r p t i o n r e m o v a l
67
A l u m i n a s t r o n g l y a d s o r b s a r s e n i c . W h e n r e g e n e r a t i o n o f t h e a l u m i n a i s p e r f o r m e d b y e l u t i o n w i t h 2 M N a O H o n l y 5 0 - 7 0 % o f t h e a r s e n i c i s r e l e a s e d . U S E P A , T e c h n o l o g i e s a n d c o s t s f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r , i n t e r n e t s i t e : h t t p : / / w w w . e p a . g o v / s a f e w a t e r / a r s / t r e a t m e n t s _ a n d _ c o s t s . , P 4 - 1 9 , 2 0 0 0 .
E d w a r d s M . E d w a r d s , “ C h e m i s t r y o f a r s e n i c r e m o v a l d u r i n g c o a g u l a t i o n a n d F e - M n o x i d a t i o n ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . U , p p 6 4 - 7 7 , ( 1 9 9 4 ) . d i f f e r e n t i a t e d b e t w e e n c o p r e c i p i t a t i o n a n d a d s o r p t i o n b y p e r f o r m i n g t h e h y d r o x i d e s a n d t h e n a d d i n g t h e a r s e n i c ( t h i s p r o c e d u r e e x c l u d e c o p r e c i p i t a t i o n ) . I f t h e c o a g u l a n t i s a d d e d t o t h e s o l u t i o n c o n t a i n i n g t h e a r s e n i c t h e n c o p r e c i p i t a t i o n a n d a d s o r p t i o n m a y o c c u r .
R e g e n e r a t i o n o f a c t i v a t e d a l u m i n a i s a c c o m p l i s h e d w i t h 4 % N a O H , h o w e v e r r e c o v e r i e s r a n g e f r o m 5 0 - 7 0 % a n d t h e e f f i c i e n c y o f a c t i v a t e d a l u m i n a i s r e d u c e d a f t e r e a c h u s e , h e n c e t h e c o s t l i n e s s o f a c t i v a t e d a l u m i n a s y s t e m s U S E P A , T e c h n o l o g i e s a n d c o s t s f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r , i n t e r n e t s i t e : h t t p : / / w w w . e p a . g o v / s a f e w a t e r / a r s / t r e a t m e n t s _ a n d _ c o s t s . p d f , P 2 - 1 8 , 2 0 0 0 . . S u s p e n d e d s o l i d s c l o g a l u m i n a c o l u m n s a n d m a y b e r e m o v e d b y b a c k w a s h i n g . C l i f f o r d a n d L i n , a s c i t e d b y t h e E P A U S E P A , T e c h n o l o g i e s a n d c o s t s f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r , i n t e r n e t s i t e : h t t p : / / w w w . e p a . g o v / s a f e w a t e r / a r s / t r e a t m e n t s _ a n d _ c o s t s . p d f , P 2 - 1 8 , 2 0 0 0 . f o u n d t h a t s i l i c a a n d m i c a w e r e p a r t i c u l a r l y p r o b l e m a t i c i n a c t i v a t e d a l u m i n a s y s t e m s . A r s e n i c l e v e l s o f 0 . 0 0 3 m g / L m a y b e a c h i e v e d u s i n g a c t i v a t e d a l u m i n a . R e g e n e r a t i o n s b y s t r o n g a c i d a n d s t r o n g b a s e t e n d t o “ c e m e n t ” t h e a c t i v a t e d a l u m i n a c o l u m n s - t h e s o l u b i l i z a t i o n o f t h e a l u m i n a l e a d s t o r e - p r e c i p i t a t i o n o f a l u m i n i u m i n t h e c o l u m n U S E P A , T e c h n o l o g i e s a n d c o s t s f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r , i n t e r n e t s i t e : h t t p : / / w w w . e p a . g o v / s a f e w a t e r / a r s / t r e a t m e n t s _ a n d _ c o s t s . p d f , P 2 - 1 9 , 2 0 0 0 . .
T a b l e ? ? T h e t y p i c a l s i z e r a n g e s o f p a r t i c u l a t e a r s e n i c i n s u r f a c e a n d g r o u n d w a t e r s L . M N e i l , a n d M . E d w a r d s , “ P r e d i c t i n g r s e n i c r e m o v a l d u r i n g m e t a l h y d r o x i d e p r e c i p i t a t i o n ” , J o u r n a l o f t h e
A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . c2U , N o . 1 , p p 7 5 - 8 6 , ( 1 9 9 7 ) . / / / / / / T . H a r p e r , a n d . K i n g h a m , “ R e m o v a l o f a r s w e n i c f r o m w a s t e w a t e r u s i n g p r e c i p i t a t i v e m e t h o d s ” , W a t e r E n v i r o n m e n t r e s e a r c h , V o l . 6 4 , p p 2 0 0 - 2 0 3 , ( 1 9 9 2 ) . P a r t i c l e s i z e w i n d o w F o r m o f G r o u n d w a t e r S u r f a c e W a t e r a r s e n i c > 0 . 4 5 V > A s s i z e > 3 K D a l t o n c o l l o i d a l 1 4 % 1 1 A s s i z e > 3 K D a l t o n d i s s o l v e d 8 3 5 4
68
A s s i z e > 0 . 4 5 V p a r t i c u l a t e 3 3 4 H a r p e r e t a l T . H a r p e r , a n d . K i n g h a m , “ R e m o v a l o f a r s w e n i c f r o m w a s t e w a t e r u s i n g p r e c i p i t a t i v e m e t h o d s ” , W a t e r E n v i r o n m e n t r e s e a r c h , V o l . 6 4 , p p 2 0 0 - 2 0 3 , ( 1 9 9 2 ) . c o m p a r e d d e c a n t a t i o n t o f i l t r a t i o n f o r p h a s e s e p a r a t i o n o f f e r r i c c o a g u l a t e d w a s t e w a t e r c o n t a i n i n g 3 1 m g / L a r s e n i c a n d f o u n d t h a t a n V m f i l t e r r e m o v e d 5 5 m o r e a r s e n i c t h a t w a s l i k e l y t o i n t h e c o l l o i d a l f o r m . R e c e n t w o r k L . M N e i l , a n d M . E d w a r d s , “ P r e d i c t i n g r s e n i c r e m o v a l d u r i n g m e t a l h y d r o x i d e p r e c i p i t a t i o n ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . c2U , N o . 1 , p p 7 5 - 8 6 , ( 1 9 9 7 ) . h a s d e m o n s t r a t e d t h a t p a r t i c u l a t e a r s e n i c i s n e a r l y a l w a y s a s s o c i a t e d w i t h p a r t i c u l a t e i r o n a n d t h a t p a r t i c u l a t e a r s e n i c i s a s i g n i f i c a n t f r a c t i o n o f t h e t o t a l a r s e n i c i n m a n y r a w w a t e r s . S t u d i e s p e r f o r m e d b y H e r i n g e t a l J . H e r i n g . P . C h e n , J . W i l k i e , M . E l i m e l c h , a n d S . L i a n g , “ A r s e n i c r e m o v a l b y f e r r i c c h l o r i d e ” , J o u r n a l o f A m e r i c a n W a t e r W o r k s A s s o c o a i t i o n , V o l . c2c , 4 , p p 1 5 5 - 1 6 7 , ( 1 9 9 6 ) . o n f e r r i c c o a g u l a t i o n o f l o w l e v e l s o f a r s e n i c ( 0 . 0 2 m g / L a r s e n a t e a n d a r s e n i t e s e p a r a t e l y ) r e v e a l e d t h a t f i l t r a t i o n t h r o u g h 0 . 5 a n d 1 . 0 V m f i l t e r s h a d a n e g l i g i b l e e f f e c t o n t h e % a r s e n i c r e m o v e d , b u t u n f i l t e r e d s o l u t i o n s s h o w e d % a r s e n i c r e m o v a l s 2 - 7 2 % l o w e r t h a n t h e f i l t e r e d s o l u t i o n s . N o o r g a n i c f l o c c u l a n t s w e r e a d d e d i n t h i s p r o t o c o l .
M c N e i l a n d E d w a r d s L . M N e i l , a n d M . E d w a r d s , “ P r e d i c t i n g a r s e n i c r e m o v a l d u r i n g m e t a l h y d r o x i d e p r e c i p i t a t i o n ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . c2U , N o . 1 , p p 7 5 - 8 6 , ( 1 9 9 7 ) . c o m p i l e d a u s e f u l d e c i s i o n t r e e t h a t m a y b e u s e d t o o p t i m i z e a r s e n i c r e m o v a l – t h e t r e e d i s c u s s e s f a c t o r l i m i t i n g a r s e n i c r e m o v a l , t h e i r c a u s e s , r e m e d i e s a n d p o t e n t i a l d r a w b a c k s a s a f u n c t i o n o f c o m p l e x i t y a n d c o s t . T h e d e c i s i o n t r e e * i n v o l v e s t h e f o l l o w i n g : • C o v e r t a r s e n i t e t o a r s e n a t e p r i o r t o a d d i t i o n o f c o a g u l a n t • I m p r o v e f l o c s e p a r a t i o n b y s e l e c t i o n o f p H , p o l y m e r i c f l o c c u l e n t , a n d / o r c h a n g i n g t h e c o a g u l a n t d o s e • I f a l u m i n i u m f l o c s a r e s t i l l s o l u b l e , i n v e s t i g a t e p H r a n g e o r c o n s i d e r i r o n • I f a r s e n i c s t i l l p a s s e s t h r o u g h t h e s y s t e m i n c r e a s e c o a g u l a n t d o s e a t t h e e x p e n s e o f m o r e s l u d g e a n d a d d i t i o n p H a d j u s t m e n t b y l i m e .
* T h i s d e c i s i o n t r e e i s a p p l i c a b l e t o d r i n k i n g w a t e r t r e a t m e n t s i n c e t h e s e d a t e w e r e g l e a n e d f r o m s u c h w a t e r t r e a t m e n t u n i t s . T h e t r e e i s m o s t l i k e l y t o b e a p p l i c a b l e t o e f f l u e n t t r e a t m e n t w h e r e h i g h a r s e n i c l e v e l s a r e e n c o u n t e r e d s i n c e t h e b a s i c m e c h a n i s m s h o u l d r e m a i n s i m i l a r .
69
Membrane Filtration
M e m b r a n e f i l t r a t i o n i s d i v i d e d i n t o f o u r c a t e g o r i e s ; r e v e r s e o s m o s i s ( R O ) , n a n o f i l t r a t i o n ( N F ) , u l t r a f i l t r a t i o n ( U F ) , a n d m i c r o f i l t r a t i o n ( M F ) . T a b l e ? ? d i s p l a y s t h e c h a r a c t e r i s t i c s o f t h e s e f i l t r a t i o n t e c h n i q u e s D e g r e m o n t , a n d L y o n n a i s e d e E a u x - D u m e z , “ B a s i c p h y s i c a l - c h e m i c a l p r o c e s s e s i n w a t e r t r e a t m e n t ” , W a t e r T r e a t m e n t H a n d b o o k , S i x t h E d i t i o n , V o l u m e 1 , L a v o i s i e r P u b l i s h i n g , P a r i s , p p 5 9 5 - 6 3 5 , ( 1 9 9 1 ) . T a b l e ? ? C h a r a c t e r i s t i c s o f s e l e c t e d f i l t r a t i o n t e c h n i q u e s f i l t r a t i o n r e v e r s e n a n o f i l t r a t i o n u l t r a f i l t r a t i o n m i c r o f i l t r a t i o n m e t h o d o s m o s i s p o r e s i z e ( V m ) 0 . 0 0 2 - 0 . 0 0 2 - 0 . 0 0 0 1 0 . 0 2 - 0 . 0 0 2 V m 2 - 0 . 0 2 V m 0 . 0 0 0 1 s p e c i e s i o n s , c o a r s e f e w e r i o n s , c o a r s e o r g a n i c S o m e v i r u s e s , f i l t e r e d o r g a n i c p a s s a g e o f 3 0 m o l e c u l e s , b a c t e r i a , m o l e c u l e s – 6 0 % o f m a c r o m o l e c u l e s , y e a s t s , a l g a e , m o n o v a l e n t p o l y m e r s , s u s p e n d e d s a l t s , 5 - 1 5 % p r o t e i n s , s o l i d s , c o l l o i d s . p a s s a g e o f v i r u s e s b i v a l e n t s a l t s , c o a r s e o r g a n i c m o l e c u l e s f l u x r a t e o n 1 - 1 0 1 - 1 0 2 0 - 4 0 0 2 0 0 - 5 0 0 p u r e w a t e r ( L . h r - 1 . m - 2 . b a r - 1 )
T a b l e ? ? s u m m a r i z e s m e m b r a n e a p p l i c a t i o n s o f t h e w o r k o f J . H e r i n g J . H e r i n g , “ A r s e n i c R e m o v a l b y E n h a n c e d C o a g u l a t i o n a n d M e m b r a n e P r o c e s s e s ” , M e n a c h e m E l i m e l e c h , D e p a r t m e n t o f C i v i l a n d
E n v i r o n m e n t a l E n g i n e e r i n g , U n i v e r s i t y o f C a l i f o r n i a , L o s A n g e l e s , , i n t e r n e t s i t e : h t t p : / / w w w . a w w a r f . c o m / e x s u m s / 9 0 7 0 6 . h t m , 1 9 9 6 . i n v e s t i g a t i n g a r s e n i c r e m o v a l b y e n h a n c e d c o a g u l a t i o n a n d m e m b r a n e p r o c e s s e s .
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T a b l e ? ? w a s c o m p i l e d f r o m U S E P A , T e c h n o l o g i e s a n d c o s t s f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r , P 4 - 1 9 , 2 0 0 0 . I t c o m p a r e s t h e c a p i t a l a n d o p e r a t i o n a n d m a n a g e m e n t ( O & M ) c o s t s p r o j e c t e d b y t h e U S E P A .
T a b l e ? ? A c o m p a r i s o n o f t h e c a p i t a l a n d o p e r a t i o n a n d m a i n t e n a n c e c o s t s f o r v a r i o u s a r s e n i c r e m o v a l t e c h n i q u e s U S E P A , T e c h n o l o g i e s a n d c o s t s f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r , i n t e r n e t s i t e : h t t p : / / w w w . e p a . g o v / s a f e w a t e r / a r s / t r e a t m e n t s _ a n d _ c o s t s . p d f , p p 3 - 1 – 3 - 5 7 , 2 0 0 0 . A r s e n i c R e m o v a l F l o w r a t e o f C a p i t a l c o s t s O p e r a t i o n a n d T e c h n i q u e t r e a t e d M a i n t e n a n c e ( e f f l u e n t m e g a c o s t s g a l l o n s / d a y ) A c t i v a t e d a l u m i n a 0 . 0 1 $ 1 0 6 0 0 p H 6 $ 5 0 0 0 0 p H 7 - 8 $ 8 0 0 0 L i m e s o f t e n i n g 0 . 0 1 $ 1 0 3 0 0 $ 6 0 0 C o a g u l a t i o n a s s i s t e d 0 . 0 1 $ 1 5 0 0 0 0 $ 1 1 2 0 0 m i c r o f i l t r a t i o n E n h a n c e d 0 . 0 1 $ 9 0 0 0 $ 3 0 0 c o a g u l a t i o n / f i l t r a t i o n P r e o x i d a t i o n ( 1 . 5 0 . 0 1 $ 7 3 0 0 $ 1 1 0 0 p p m ) C l 2
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A r s e n i c m a y b e a d s o r b e d b y n u m e r o u s m a t e r i a l s i n c l u d i n g p h y l l o s i l i c a t e s , s i l i c a , h y d r o u s o x i d e s o f F e a n d A l , t r e a t e d a c t i v a t e d c a r b o n , t r e a t e d Z e o l i t e s , s e a s a n d , m a g n e s i u m h y d r o x i d e a n d m a n g a n e s e d i o x i d e R . L . D r o s t e , T h e o r y a n d P r a c t i c e o f W a t e r a n d W a s t e w a t e r T r e a t m e n t , J o h n W i l e y a n d S o n s , N e w Y o r k , p 4 7 8 , 1 9 9 7 . / / / K . R a v e n , A . J a i n ,
R . L o e p p e r t , “ A r s e n i t e a n d a r s e n a t e a d s o r p t i o n o n f e r r i h y d r i t e : k i n e t i c s , e q u i l i b r i u m , a n d a d s o r p t i o n e n v e l o p e s ” ,
E n v i r o n m e n t a l S c i e n c e a n d T e c h n o l o g y , V o l . X2a , p p 3 4 4 - 3 4 9 , 1 9 9 8 . / / / / U S E P A , T e c h n o l o g i e s a n d c o s t s f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r , i n t e r n e t s i t e : h t t p : / / w w w . e p a . g o v / s a f e w a t e r / a r s / t r e a t m e n t s _ a n d _ c o s t s . p d f , P 2 - 1 4 - 2 - 1 9 . 2 0 0 0 . . / / / L . M c N e i l , M . E d w a r d s , “ A r s e n i c r e m o v a l d u r i n g p r e c i p i t a t i v e s o f t e n i n g ” , J o u r n a l o f E n v i r o n m e n t a l E n g i n e e r i n g , V o l VTa2X , n o . 5 , 1 9 9 7 .
` ¨ mõ¨n ¨©§'¤£¦ ] i s a g r a n u l a r n a t u r a l z e o l i t e t h a t i s p r e f e r e n t i a l l y s e l e c t i v e f o r f l u o r i d e i o n s , A s , S e , P h o s p h a t e , a n d s i l i c a . T h e o r d e r o f p r e f e r e n t i a l - - - - - r e m o v a l o f a n i o n s a t p H 6 i s O H > H 2 P O 4 > F > H 2 A s O 4 > H S e O 3 . T h e o p t i m u m p H f o r r e m o v a l o f t h e a f o r e m e n t i o n e d i o n s i s 5 . 5 – 6 . 0 R . L . D r o s t e , T h e o r y a n d P r a c t i c e o f W a t e r a n d W a s t e w a t e r T r e a t m e n t , J o h n W i l e y a n d S o n s , N e w Y o r k , p 4 7 8 , 1 9 9 7 . / / / / A . D e b , A G u p t a , P . B r a n d y o p a d h y a y , R B i s w a s , a n d S . R o y , “ A p p r o p r i a t e t e c h n o l o g y f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r o f r u r a l W e s t B e n g a l ” , i n
P r o v i d i n g S a f e D r i n k i n g W a t e r i n S m a l l S y s t e m : T e c h n o l o g y , O p e r a t i o n s , a n d E c o n o m i c s . C o t r u v o , G . C r a u n , a n d N . H e a r n e ( E d s ) , W o r l d H e a l t h O r g a n i z a t i o n , C R C P r e s s , p p 2 7 3 - 2 7 8 , 1 9 9 9 . S h o u l d t h e p H b e r a i s e d b e y o n d 9 . 5 , t h e a c t i v a t e d a l u m i n a b e c o m e s a c a t i o n e x c h a n g e r R . L . D r o s t e , T h e o r y a n d P r a c t i c e o f W a t e r a n d W a s t e w a t e r T r e a t m e n t , J o h n W i l e y a n d S o n s , N e w Y o r k , p 4 7 8 , 1 9 9 7 . . T h e m e c h a n i s m o f a r s e n i c r e m o v a l , t h o u g h i t i s t e r m e d a d s o r p t i o n , i s m o r e c o r r e c t l y r e f e r r e d t o a s c h e m i s o r p t i o n a n d l i g a n d e x c h a n g e . A . D e b , A G u p t a , P . B r a n d y o p a d h y a y , R B i s w a s , a n d S . R o y , “ A p p r o p r i a t e t e c h n o l o g y f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r o f r u r a l W e s t B e n g a l ” , i n P r o v i d i n g S a f e
D r i n k i n g W a t e r i n S m a l l S y s t e m : T e c h n o l o g y , O p e r a t i o n s , a n d E c o n o m i c s . C o t r u v o , G . C r a u n , a n d N . H e a r n e ( E d s ) , W o r l d H e a l t h O r g a n i z a t i o n , C R C P r e s s , p p 2 7 3 - 2 7 8 , 1 9 9 9 . . T y p i c a l c a p a c i t i e s o f a c t i v a t e d a l u m i n a f o r a r s e n i c f r o m 5 - 1 5 g / k g , a n d t h e a q u e o u s e q u i l i b r i u m a r s e n i c c o n c e n t r a t i o n s r a n g e f r o m 0 . 0 0 5 – 0 . 2 p p m . A . D e b , A G u p t a , P . B r a n d y o p a d h y a y , R B i s w a s , a n d S . R o y , “ A p p r o p r i a t e
72
t e c h n o l o g y f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r o f r u r a l W e s t B e n g a l ” , i n P r o v i d i n g S a f e D r i n k i n g W a t e r i n
S m a l l S y s t e m : T e c h n o l o g y , O p e r a t i o n s , a n d E c o n o m i c s . C o t r u v o , G . C r a u n , a n d N . H e a r n e ( E d s ) , W o r l d H e a l t h O r g a n i z a t i o n , C R C P r e s s , p p 2 7 3 - 2 7 8 , 1 9 9 9 . . T h e a c t i v a t e d a l u m i n a i s p r e p a r e d b y l o w t e m p e r a t u r e ( 3 0 0 - 6 0 0 o C ) d e h y d r a t i o n o f a m o r p h o u s a n d g a m m a a l u m i n i u m o x i d e s A . D e b , A G u p t a , P . B r a n d y o p a d h y a y , R B i s w a s , a n d S . R o y , “ A p p r o p r i a t e t e c h n o l o g y f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r o f r u r a l W e s t B e n g a l ” , i n P r o v i d i n g S a f e D r i n k i n g W a t e r i n S m a l l S y s t e m : T e c h n o l o g y , O p e r a t i o n s , a n d E c o n o m i c s . C o t r u v o , G . C r a u n , a n d N . H e a r n e ( E d s ) , W o r l d H e a l t h O r g a n i z a t i o n , C R C P r e s s , p p 2 7 3 - 2 7 8 , 1 9 9 9 . . R e g e n e r a t i o n i s a c c o m p l i s h e d b y t h e u s e o f 4 % c a u s t i c s o l u t i o n s , f o l l o w e d b y a n e u t r a l i z a t i o n r i n s e w i t h d i l u t e d H C l . A d i s a d v a n t a g e o f t h e a c t i v a t e d a l u m i n a r e g e n e r a t i o n i s t h a t a t o x i c , a r s e n i c r i c h s l u d g e t e n d s t o s e t t l e f r o m u s e d t h e r e g e n e r a n t s u p e r n a t a n t ; t h e s l u d g e m a y b e s t a b i l i z e d u s i n g 5 – 1 0 % c e m e n t - s a n d ( b a s e d o n d r y s l u d g e m a s s ) . T h e c e m e n t - s t a b i l i z e d s l u d g e p a s s e d l e a c h i n g t e s t s u n d e r a c i d c o n d i t i o n s A . D e b , A G u p t a , P . B r a n d y o p a d h y a y , R B i s w a s , a n d S . R o y , “ A p p r o p r i a t e t e c h n o l o g y f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r o f r u r a l W e s t B e n g a l ” , i n P r o v i d i n g S a f e D r i n k i n g W a t e r i n
S m a l l S y s t e m : T e c h n o l o g y , O p e r a t i o n s , a n d E c o n o m i c s . C o t r u v o , G . C r a u n , a n d N . H e a r n e ( E d s ) , W o r l d H e a l t h O r g a n i z a t i o n , C R C P r e s s , p p 2 7 3 - 2 7 8 , 1 9 9 9 . . N a t u r a l l y a l t e r n a t e a r s e n i c r e m o v a l t e c h n i q u e s m a y b e u s e d t o t r e a t t h e s l u d g e . W h e r e e x p e r t i s e a n d f a c i l i t i e s a r e a v a i l a b l e t h e a r s e n i c m a y b e r e c l a i m e d s o a s t o p r e v e n t f u t u r e l e a k a g e f r o m t h e “ s t a b i l i z e d ” f o r m .
F e r g u s o n e t a l a s c i t e d b y R a v e n K . R a v e n , A . J a i n , R . L o e p p e r t , “ A r s e n i t e a n d a r s e n a t e a d s o r p t i o n o n
f e r r i h y d r i t e : k i n e t i c s , e q u i l i b r i u m , a n d a d s o r p t i o n e n v e l o p e s ” , E n v i r o n m e n t a l S c i e n c e a n d T e c h n o l o g y , V o l . X2a , p p 3 4 4 - 3 4 9 , 1 9 9 8 . o b s e r v e d t h a t a r s e n i c a d s o r p t i o n i s h i g h l y d e p e n d a n t o n i t s o x i d a t i o n s t a t e i n t h e p H r a n g e o f 5 . 5 – 7 . 5 . T h i s f a c t h a s a l s o b e e n r e p o r t e d b y o t h e r w o r k e r s R . L . D r o s t e , T h e o r y a n d P r a c t i c e o f W a t e r a n d W a s t e w a t e r T r e a t m e n t , J o h n W i l e y a n d S o n s , N e w Y o r k , p 4 7 8 , 1 9 9 7 . . / / / / U S E P A , T e c h n o l o g i e s a n d c o s t s f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r , i n t e r n e t s i t e : h t t p : / / w w w . e p a . g o v / s a f e w a t e r / a r s / t r e a t m e n t s _ a n d _ c o s t s . p d f , P 2 - 1 5 , 2 0 0 0 . / / / M . L e i s t , R . J . C a s e y , a n d D . C a r i d i , “
T h e m a n a g e m e n t o f a r s e n i c w a s t e s : p r o b l e m s a n d p r o s p e c t s ” , J o u r n a l o f H a z a r d o u s M a t e r i a l s , V o l . W20 , N o . 1 , p p 1 2 5 - 1 3 8 , 2 0 0 0 . .
Adsorption of arsenic on iron hydroxides
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§¥ üûyüFú6û ¥Eû ý ý ÷ ý¥¦¥ ý ÷ ù investigated the adsorption of arsenate and arsenite on preformed ferrihydrite Fe(OH)3 with a surface area of 202 m2/g, and a zero point of salt effect (ZPSE) at pH 8.5. Close fits of the Freundlich and Langmuir adsorption isotherms were observed. Broad adsorption maxima were exhibited for both the trivalent and pentavalent arsenic; extending from pH 6.8-9.4 and 5.2- 7.4 respectively. The arsenite adsorption only reduced significantly above pH values of 9.4
73
where the free hydroxide levels may permit dissolution of the H3AsO3 (pKa = 9.22) and the formation of a negatively charged ion. The reduction of arsenate adsorption as pH is increased is attributed to repulsion of the negatively charged pentavalent species by the negative surface sites on the ferrihydrite K.Raven, A. Jain, R. Loeppert, “ Arsenite and arsenate adsorption on ferrihydrite: kinetics, equilibrium, and adsorption envelopes”, Environmental Science and Technology, Vol. 32, pp 344-349, 1998..Ferguson and Gavis, as cited by Raven K.Raven, A. Jain, R. Loeppert, “ Arsenite and arsenate adsorption on ferrihydrite: kinetics, equilibrium, and adsorption envelopes”, Environmental Science and Technology, Vol. 32, pp 344-349, 1998., also obtained similar results. Arsenic adsorption reactions involve the following surface complexation reactions: - + • Arsenate adsorption Fe-OH + H2AsO4 + H Fe-H2AsO4 + H2O • Arsenate adsorption Fe-OH + H3AsO3 Fe-H2AsO3 + H2O
The precise mechanism of the arsenite adsorption was not determined. Adsorption
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of arsenite and arsenite onto metal hydroxide were reviewed by Edwards ÷
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û¨ L § ù¡§M Eûyü N§û¡ ú6ù ¦§ ü¤PL ù P¡ ù üùyü û ü ù ü ý
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÷ ý¥¦¥ ý ÷ ý Who found arsenite adsorptions of >0.4 M As/M oxide at pH 8; it was proposed that arsenite adsorption could be based on one or more of the following phenomena: • Arsenite reduction of ferric solids and re-oxidation of the ferrous solid by dissolved oxygen (this phenomena has not been observed) • Formation of an unknown ferric arsenite • Formation of arsenic polymers analogous to polphosphates • Formation of ferric arsenite or arsenate complexes that could resorb to the surface, as reported for polyphosphates on iron oxide.
Adsorption of arsenate onto preformed media is about five times weaker than the
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ù¨§ ý û¡ N § ù¨§M Eûyü¦ L§û¡ ú6ù ¦§ üPN ù¦P¡ ù üùyü û ü ù ü ý
in situ adsorption. ÷
)hV¦- 9h+ W *,+(& 13.¦"7Y[.¨- +(ZY4)¦(\]iW ]])¨13& .¨- & )¦" <=£¡ ^ _A¡Bj`¦` bfD8C¦CA¡c
÷ ý¥¦¥ ý ÷ ý This observation suggests that the surface area of the in situ medium is greater or that coprecipitation occurs.
74
A research project G. Amy, M. Edwards, M. Benjamin, K Carlson, J. Chwirka, P. Brandhuber, L. McNeil, and F. Vagliasindi, “Arsenic Treatability Options and Evaluation of Residuals Management Issues”, American Water Works Association Research Foundation Project # 153, internet site: http://www.awwarf.cpm/exums/153.htm, 2000. funded by the American Water Works Association Research Foundation (AWWARF) tested a number of arsenic removal techniques. Their major findings are listed below • Iron and alum coagulants had approximately equal capacities for sorption of arsenate • A r s e n a t e w a s m o r e e f f e c t i v e l y r e m o v e d t h a n a r s e n i t e • A r s e n i c r e m o v a l v i a p r e c i p i t a t i v e s o f t e n i n g c a n b e f a c i l i t a t e d v i a a
v a r i e t y o f s o l i d s t h a t f o r m i n c l u d i n g C a C O 3 , M g ( O H ) 2 , M n ( O H ) 2 , a n d
F e ( O H ) 3 • I o n e x c h a n g e r e s i n p e r f o r m a n c e w a s d e p e n d e d o n t h e s o u r c e w a t e r , n a t u r a l o r g a n i c m a t t e r ( N O M ) , s u l p h a t e a n d p H •
Regulatory Limits for Arsenic
M o s t o f t h e i n t e r n a t i o n a l d r i n k i n g w a t e r l i m i t s f o r a r s e n i c r a n g e f r o m 0 . 0 1 t o 0 . 0 5 p p m .
I n 1 9 4 2 t h e U . S . P u b l i c H e a l t h S e r v i c e s e t a m a x i m u m c o n t a m i n a n t l e v e l o f 0 . 0 5 0 p p m f o r a r s e n i c A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 . . T h e W o r l d H e a l t h O r g a n i z a t i o n ( W H O ) r a i s e d t h e g u i d e l i n e v a l u e f o r A s f r o m 0 . 0 5 t o 0 . 0 1 m g / L . M . L e i s t , R . J . C a s e y , a n d D . C a r i d i , “ T h e m a n a g e m e n t o f a r s e n i c w a s t e s : p r o b l e m s a n d p r o s p e c t s ” , J o u r n a l o f H a z a r d o u s M a t e r i a l s , V o l . W20 , N o . 1 , p p 1 2 5 - 1 3 8 , 2 0 0 0 .
T h e C a n a d i a n m a x i m u m c o n t a m i n a n t l e v e l ( M C L ) f o r A s i s 2 5 V g / L . T . V i r a r a g h a v a n , K . S . S u b r a m a n i a n , a n d J . A . A r u l d o s s , “ A r s e n i c i n d r i n k i n g w a t e r : p r o b l e m s a m d s o l u t i o n s ” , W a t e r S c i e n c e a n d
T e c h n o l o g y , V o l . b2d , N o . 2 , 1 9 9 9 . A s s o c i a t i o n f o r T o x i c s u b s t a n c e s a n d D i s e a s e R e g i s t r y ( A S T D R ) M i n i m u m R i s k L e v e l ( M R L ) f o r c h r o n i c i n o r g a n i c a r s e n i c i n t a k e i s 0 . 0 0 0 3 m g / k g / d a y . I n t e r n e t S i t e : h t t p : / / w w w . a s t d r . c d c . g o v / 9 9 l i s t . h t m l , ( 1 9 9 9 ) . T h e m a x i m u m a l l o w a b l e c o n c e n t r a t i o n ( M A K ) f o r t o t a l a r s e n i c c o m p o u n d s i n 3 3 U l l m a n s E n c y l o p e d i a , G e r m a n y i s 0 . 2 m g / m . F o r a r s i n e ( A s H 3 ) t h e M A K i s 1 . 2 m g / m . I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 5 8 , 1 9 9 9 .
75
W a t e r q u a l i t y l i m i t s ( V g / L ) f o r t o x i c p o l l u t a n t s f o r t h r e e u s e s - A r s e n i c l e v e l s :
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H u m a n H e a l t h - 0 . 0 0 0 2 – 0 . 0 2 2
T h e U . S . E n v i r o n m e n t a l P r o t e c t i o n A g e n c y ( E P A ) c o n s i d e r e d l o w e r i n g t h e c u r r e n t l i m i t f o r a r s e n i c i n d r i n k i n g w a t e r f r o m 0 . 0 5 p p m t o i n t h e r e g i o n o f 0 . 0 2 0 – 0 . 0 0 2 p p m M N e i l , a n d M . E d w a r d s , “ A r s e n i c r e m o v a l d u r i n g p r e c i p i t a t i v e s o f t e n i n g ” , J o u r n a l o f E n v i r o n m e n t a l E n g i n e e r i n g , V o l . 1 2 3 , N o . 5 , p p 4 5 3 - 4 6 0 , 1 9 9 7 . T h e c o s t i m p l i c a t i o n s o f r e d u c i n g t h e a r s e n i c l i m i t t o 0 . 0 0 2 p p m i n t h e U . S . a r e $ 6 4 0 0 0 0 0 0 0 0 .
T a b l e ? ? A r s e n i c r e g u l a t o r y l i m i t s U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s ,
W i l e y - V C H , N e w Y o r k , V o l . V , p 3 6 2 , 1 9 9 9 . C o m p o u n d C o u n t r y [ A s ] [ A s ] w a t e r b o u r n e [ A s ] i n s o l i d s ( s o i l , a i r b o u r n e m g / L R . L . D r o s t e , T h e o r y r o c k ) m g / k g m g / m 3 a n d P r a c t i c e o f W a t e r a n d W a s t e w a t e r T r e a t m e n t ,
J o h n W i l e y a n d S o n s , N e w Y o r k , p 1 9 9 , 2 0 2 , 1 9 9 7 . .
A s H 3 G e r m a n y 0 . 2 - U . S . A . 0 . 2 T o t a l A s I n d i a 0 . 0 5 A . D e b , A G u p t a , P . B r a n d y o p a d h y a y , R B i s w a s ,
a n d S . R o y , “ A p p r o p r i a t e
t e c h n o l o g y f o r r e m o v a l o f
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E c o n o m i c s . C o t r u v o , G .
C r a u n , a n d N . H e a r n e ( E d s ) ,
W o r l d H e a l t h O r g a n i z a t i o n ,
C R C P r e s s , p p 2 7 3 - 2 7 8 , 1 9 9 9 .
C a 3 ( A s O 4 ) 2 U . S . A . 1 . 0 - A l l o t h e r U . S . A . 0 . 0 1 0 . 0 5 i n o r g a n i c a r s e n i c W . H . O . 0 . 0 1 C a n a d a M A K 0 . 0 5 # 0 . 0 2 5 U . K . 0 . 2 ø B a n g l a d e s h * 0 . 0 5 0 O r g a n i c U . S . A . 0 . 5 a r s e n i c
A s 2 O 3 J a p a n 0 . 5
A s H 3 J a p a n 0 . 2 A s – a l l S w e d e n 0 . 0 5 c o m p o u n d s T o t a l A s B e l g i u m N a t u r a l a r e a 4 5 R u r a l a r e a 4 5 B u i l d i n g - e s t a t e 1 1 0 R e c r e a t i o n a l a r e a 2 0 0 I n d u s t r i a l s i t e 3 0 0 V . D u t r e , C . C h e s t e n s , J . S c h a e p , C . V a n d e c a t e e l e ,
“ S t u d y o f t h e r e m e d i a t i o n o f a
s i t e c o n t a m i n a t e d w i t h
a r s e n i c ” , T h e S c i e n c e o f t h e
T o t a l E n v i r o n m e n t , V o l . a2a2d , 1 9 9 8 .
* A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 .
77
# a 1 5 m i n u t e e x p o s u r e A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 . Ø d u e t o b e r e d u c e d t o 0 . 0 5 m g / m 3 p e n d i n g t h e p o t e n t i a l i m p a c t o n i n d u s t r y A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 . .
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T r i v a l e n t a r s e n i c i s 2 5 – 6 0 t i m e s m o r e t o x i c K . R a v e n , A . J a i n , R . L o e p p e r t , “ A r s e n i t e a n d a r s e n a t e a d s o r p t i o n o n f e r r i h y d r i t e : k i n e t i c s , e q u i l i b r i u m , a n d a d s o r p t i o n e n v e l o p e s ” , E n v i r o n m e n t a l S c i e n c e a n d
T e c h n o l o g y , V o l . X2a , p p 3 4 4 - 3 4 9 , 1 9 9 8 . t h a n p e n t a v a l e n t a r s e n i c w h i c h i s m u c h m o r e t o x i c t h a n z e r o v a l e n t a r s e n i c , h o w e v e r z e r o v a l e n t a r s e n i c r e a d i l y f o r m s t h e t o x i c h i g h e r o x i d a t i o n s t a t e s i n a n o r g a n i s m s b o d y . B e n z e n e a r s e n i c c o m p o u n d s h a v e s t r o n g A s - C b o n d s t h u s r e d u c i n g t h e i r t o x i c i t y . U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c
C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 5 8 , 1 9 9 9 .
S o m e o r g a n i c a r s e n i c c o m p o u n d s s u c h a s w a r g a s e s d o , h o w e v e r , e x e r t a h i g h t o x i c i t y a g a i n s t t h e e y e s , s k i n , r e s p i r a t o r y t r a c t , g a s t r o i n t e s t i n a l t r a c t a n d t h e c e n t r a l n e r v o u s s y s t e m o f h u m a n s . T y p i c a l c o m p o u n d s U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . 1 , p 3 6 1 , 1 9 9 9 . a r e R A s C l 2 , R 2 A s C l a n d
R 2 A s C N ; e . g . C l C H = C H A s C l 2 ( d i c h l o r o - 2 - c h l o r o v i n y l a r s i n e ) , ( C 6 H 5 ) 2 A s C l
( c h l o r o d i p h e n y l a r s i n e ) , a n d ( C 6 H 5 ) 2 A s C N ( c y a n o d i p h e n y l a r s i n e ) r e s p e c t i v e l y .
78
A r s e n i c h a s b e e n k n o w n t o c a u s e s k i n c a n c e r , t u m o r s , b l a c k f o o t d i s e a s e , a n d n e o p l a s m ( f o r m a t i o n o f a b n o r m a l m a s s i n t h e b o d y ) E n c y c l o p e d i a o f E n v i r o n m e n t a l P o l l u t i o n a n d C l e a n u p , J o h n W i l e y a n d S o n s , I n c . , N e w Y o r k , p 1 8 3 8 , 1 9 9 9 . . T r i v a l e n t a r s e n i c c o m p o u n d s t h a t a r e m o r e t o x i c t h a n p e n t a v a l e n t a r s e n i c a r e
A s 2 O 3 , A s F 3 , A s C l 3 , A s H 3 . Z e r o v a l e n t a r s e n i c i s n o t t o x i c b u t b e c o m e s t o x i c w i t h i n t h e o r g a n i s m a s i t c h a n g e s o x i d a t i o n s t a t e s . B e n z e n e a r s e n i c c o m p o u n d s p o s s e s s s t r o n g A s - C b o n d s a n d a r e t h u s l e s s t o x i c . U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 5 8 , 1 9 9 9 . . N a t u r a l w a t e r s a m l e s t y p i c a l l y c o n t a i n i n g o r g a n i c a r s e n i t e , a r s e n a t e m o n o m e h t y l a r s o n i c a c i d , a n d d i m e t h y l a r s o n i c a c i d ; l i s t e d i n o r d e r o f d e c r e a s i n g t o x i c i t y H . H u a n g , P . D a s g u p t a , “ A f i e l d d e p l o y a b l e i n s t r u m e n t f o r t h e m e a s u r e m e n t a n d s p e c i a t i o n o f a r s e n i c i n p o t a b l e w a t e r ” , A n a l y t i c a C h i m i c a A c t , V o l . X2c2d , p p 2 7 - 3 7 , 1 9 9 9 . .
B e n z e n e a r s e n i c c o m p o u n d s p o s s e s s s t r o n g A s - C b o n d s a n d a r e t h u s l e s s t o x i c . U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 5 9 , 1 9 9 9 . .
Toxicology of Arsenic L o n g - t e r m e x p o s u r e t o a r s e n i c i n d r i n k i n g w a t e r i s a s s o c i a t e d w i t h l i v e r , l u n g , k i d n e y , a n d b l a d d e r c a n c e r s , a s w e l l a s s k i n c a n c e r . M . M . W u , T . L . K u o , Y . H . H w a n g , a n d C . J . C h e n , “ D o s e - R e s p o n s e R e l a t i o n B e t w e e n A r s e n i c W e l l W a t e r a n d M o r t a l i t y f r o m C a n c e r . ” A m e r i c a n J o u r n a l o f
E p i d e r m i o l o g y , V o l . VTX2d , p p 1 1 2 3 - 1 1 3 2 , 1 9 9 1 . / / / / / / H . H u a n g , P . D a s g u p t a , “ A f i e l d d e p l o y a b l e i n s t r u m e n t f o r t h e m e a s u r e m e n t a n d s p e c i a t i o n o f a r s e n i c i n p o t a b l e w a t e r ” , A n a l y t i c a C h i m i c a A c t , V o l . X2c2d , p p 2 7 - 3 7 , 1 9 9 9 .
U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c I n g e s t i o n o f 0 . 1 g o f A s 2 O 3 c a n b e f a t a l t o a h u m a n
C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 4 4 , 1 9 9 9 .
Sources of arsenic pollution T h e f o l l o w i n g a c t i v i t i e s U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H ,
N e w Y o r k , V o l . V , p 3 5 8 , 1 9 9 9 . / / / / / / / / A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 . t h a t i n v o l v e a r s e n i c a r e c o n s i d e r e d t o b e h a z a r d o u s i n t h e a b s e n c e o f s u i t a b l e s a f e t y e q u i p m e n t : • M e t a l r e c o v e r y / c o n t a m i n a t i o n b y a r s e n i c • R o a s t i n g o f i r o n p y r i t e s • S m e l t i n g o f n o n - f e r r o u s m e t a l s , e s p e c i a l l y c o p p e r • C l e a n i n g o f l e a d c h a m b e r s i n s u l p h u r i c a c i d m a n u f a c t u r e
79
• P r o c e s s i n g o f a r s e n i c a l m i n e r a l s • P r o d u c t i o n o f a r s e n i c c o n t a i n i n g p h a r m a c e u t i c a l s • U s e i n p y r o t e c h n i c s • B u r n i s h i n g o f m e t a l s u r f a c e s • U s e o f r a w m a t e r i a l s c o n t a i n i n g a r s e n i c i n t h e g l a s s i n d u s t r y • R e p a i r o r c l e a n i n g o f f l u e - d u s t p l a n t s , o r f i l t e r s • U s e i n w o o d p r e s e r v a t i v e s • A p p l i c a t i o n o f p e s t i c i d e s • G o l d m i n i n g • B u r n i n g f o s s i l f u e l s *
* I n 1 9 0 2 , a n e p i d e m i c o f a r s e n i c a l p o i s o n i n g w a s c a u s e d b y d r y i n g b e e r m a l t i n a i r h e a t e b y b u r n i n g c o a l t h a t c o n t a i n e d a r s e n i c J . G u l l e d g e , a n d J O ’ C o n n e r , “ r e m o v a l o f a r s e n i c ( V ) f r o m w a t e r b y a d s o r p t i o n o n a l u m i n i u m a n d f e r r i c h y d r o x i d e s ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s
A s s o c i a t i o n , V o l . 021 , I s s u e 8 , p p 5 4 8 - 5 5 2 , ( 1 9 7 3 ) . .
A n t h r o p o g e n i c a c t i v i t i e s t h a t r e l e a s e a r s e n i c i n t o t h e a t m o s p h e r e i n c l u d e b u r n i n g o f f o s s i l f u e l s , p a p e r p r o d u c t i o n , c e m e n t m a n u f a c t u r e r , a n d m i n i n g i n d u s t r y A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 . .
O t h e r l e s s a p p a r e n t e x p o s u r e s m a y i n c l u d e A g e n c y f o T o x i c S u b s t a n c e s a n d D i s e a s e R e g i s t r y ( A S T D R ) . 2 0 0 0 . T o x i c o l o g i c a l p r o f i l e f o r a r s e n i c ( U P D A T E ) . A t l a n t a , G A : U . S . D e p a r t m e n t o f H e a l t h a n d H u m a n S e r v i c e s , P u b l i c H e a l t h S e r v i c e s , I n t e r n e t S i t e : h t t p : / / w w w . a s t d r . c d c . g o v / t f a c t s 2 . h t m l • E a t i n g f o o d , d r i n k i n g w a t e r , o r b r e a t h i n g a i r c o n t a i n i n g a r s e n i c • B r e a t h i n g s m o k e o r s a w d u s t f r o m w o o d t r e a t e d w i t h a r s e n i c • L i v i n g n e a r u n c o n t r o l l e d h a z a r d o u s w a s t e s i t e s c o n t a i n i n g g a s e s • L i v i n g i n a r e a s w i t h u n u s u a l l y h i g h n a t u r a l l e v e l s o f a r s e n i c i n r o c k a n d s o i l s
S e a f o o d s c o n t a i n c o m p a r a t i v e l y h i g h a m o u n t s o f a r s e n i c T. Gebel, “Confounding Variables in the Environmental Toxicology of Arsenic”, Toxicology, Vol 144, pp 155-162, 2000. ( f r e s h w a t e r f i s h a r e e x c l u d e d ) . P h i l l i p s a n d B u c h e t e t a l , a s c i t e d b y G e b e l T. Gebel, “Confounding Variables in the Environmental Toxicology of Arsenic”, Toxicology, Vol 144, pp 155-162, 2000. , found that a portion of seafood-bourne arsenic is bound to sugars and proteins or can exist as arsenocholine or arsenobetaine; they established that these seafood- bourne compounds release minute amounts of inorganic arsenic(the more toxic forms U l l m a n s
E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 5 8 , 1 9 9 9 . ) thus these compounds may be considered insignificantly toxic. Despite the low toxicity of the aforementioned compounds a few percent of the seafood-bourne arsenic exists as dimethylarsinic acid, a
80 compound of low but not irrelevant toxicity G e b e l T. Gebel, “Confounding Variables in the Environmental Toxicology of Arsenic”, Toxicology, Vol 144, pp 155-162, 2000..The highly toxic inorganic forms of arsenic prevail in fish and shellfish T. Gebel, “Confounding Variables in the Environmental Toxicology of Arsenic”, Toxicology, Vol 144, pp 155-162, 2000..
p © r ¡¤§ \£ ¤r \¡\¥\ ¡¡¤l\© ¤ ` o c c u r s m a i n l y t h r o u g h i n g e s t i o n a n d i n h a l a t i o n , b u t m a y i t a l s o b e a b s o r b e d t h r o u g h t h e s k i n . A r s e n i c i s f o u n d i n d i f f e r e n t t i s s u e s 2 4 h o u r s a f t e r i n t a k e ; l i v e r , k i d n e y , l u n g , s p l e e n , b o n e m a r r o w , s k i n , a n d t o a l e s s e r e x t e n t t h e b r a i n , h e a r t a n d u t e r u s . S m a l l q u a n t i t i e s c a n b e f o u n d i n h a i r a n d n a i l s s e v e r a l m o n t h s a f t e r t h e m a j o r p o t i o n h a s b e e n e l i m i n a t e d . U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 5 9 , 1 9 9 9 . . M e t a b o l i s m o f a r s e n i c m a y r e s u l t i n i n h i b i t i o n o f e s s e n t i a l e n z y m e s i s t h e f i r s t s t e p o f c e l l d a m a g e U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 5 9 , 1 9 9 9 . . A number of workers (cited by Gebel T. Gebel, “Confounding Variables in the Environmental Toxicology of Arsenic”, Toxicology, Vol 144, pp 155-162, 2000..) have shown that the half-life of arsenic in man is brief - on the order of 3-4 days. This is a fortunate phenomenon hinting that the bioaccumulation of arsenic in man is slow as indicated by its rapid loss in urine T. Gebel, “Confounding Variables in the Environmental Toxicology of Arsenic”, Toxicology, Vol 144, pp 155-162, 2000... Thompson D.J. Thompson, “A Chemical Hypothesis for Arsenic Methylation in Mammals”, Chemical and Biological Interaction, Vol 88, pp 89-114, 1993. as cited by Gebel, T. Gebel, “Confounding Variables in the Environmental Toxicology of Arsenic”, Toxicology, Vol 144, pp 155-162, 2000. claims that mammals metabolize arsenate via reduction and glutathione (GSH) conjugation to arsenite, which is subsequently mono- or dimethylated; a process referred to as detoxication by Moore M.M. Moore, K. Harrington, and C.L. Doerr, “Genotoxicity of Arsenic and its methylated metabolites” in Arsenic Exposure and Health. Science and Technology Letters, W.R. Chapell, C.O. Abernathy, and C.R. Cothern(Eds.), Norhtwood, UK, pp 191-198, 1994. et al M.Vahter, “Methylation of Inorganic Arsenic in Different Mamalian Species an Population Groups”, Scientific Progress, Vol 82, pp 62-88, 1999. as cited by Gebel T. Gebel, “Confounding Variables in the Environmental Toxicology of Arsenic”, Toxicology, Vol 144, pp 155-162, 2000..
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k `¤© ¡¡¤ \© ` ] o c c u r s v i a u r i n e , f e c e s , h a i r a n d s k i n U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 5 9 , 1 9 9 9 . . Airbourne arsenic deposited in the lung is rapidly taken up and soon apperars in the urine, however some of the absorbed arsenic is distributed in the rissues including the liver, abdominal viscera, bone, skin, and particularly the hair and nails A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 . . The accumulation in the latter tissues assist the analyst in detecting the recent history(6-12months Agency for Toxic Substances and Disease Registry {ASTDR} . 2000 Toxicological profile for arsenic Atlanta,GA: U.S. Department of Health and Human Services, Public Health Services Internet site:http://www.astdr.cdc.gov/facts2.html) of arsenic poisoning.
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A r s e n i c h a s l o n g s e r v e d a s a c h o i c e p o i s o n a n d i s o f l e g e n d a r y t o x i c i t y . M . E d w a r d s , “ C h e m i s t r y o f a r s e n i c r e m o v a l d u r i n g c o a g u l a t i o n a n d F e - M n o x i d a t i o n ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s
A s s o c i a t i o n , V o l . U , p p 6 4 - 7 7 , ( 1 9 9 4 ) .
81
H e a l t h e f f e c t s a r e n o r m a l l y c h a r a c t e r i z e d a c c o r d i n g t o t h e d o s e o f t h e c o m p o u n d i n q u e s t i o n . T h e d o s e s m a y b e a o n c e o f f o r i n t e r m i t t e n t e x p o s u r e ( a c u t e ) o r a r e g u l a r d o s e ( c h r o n i c ) . T h e l a t t e r i s u s u a l l y o f r e l a t i v e l y l o w c o n c e n t r a t i o n a s m a y b e e x p e r i e n c e d t h r o u g h c o n s u m p t i o n o f c o n t a m i n a t e d w a t e r o r p e r h a p s i n h a l a t i o n o f c o n t a m i n a t e d a i r d u r i n g r e g u l a r w o r k s h i f t s i n a s m e l t i n g m i n e .
U l l m a n s A c u t e p o i s o n i n g - A f a t a l d o s e o f A s 2 O 3 t o a h u m a n i s 7 0 – 1 8 0 m g E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 5 9 , 1 9 9 9 . .
`¤' ¨© S y m p t o m s o f a r s e n i c p o i s o n i n g U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d
P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 5 9 , 1 9 9 9 . , A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 . / / / / / H . H u a n g , P . D a s g u p t a , “ A f i e l d d e p l o y a b l e
i n s t r u m e n t f o r t h e m e a s u r e m e n t a n d s p e c i a t i o n o f a r s e n i c i n p o t a b l e w a t e r ” , A n a l y t i c a C h i m i c a A c t , V o l . X2c2d , p p 2 7 -
3 7 , 1 9 9 9 . The symptoms have been classified according to the rout of exposure U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 5 9 , 1 9 9 9 . . • I n g e s t i o n - v o m i t i n g , d i a r r h e a , c r a m p s , f a c i a l e d e m a , c a r d i a c a b n o r m a l i t i e s , d e h y d r a t i o n , a n d s h o c k . • I n h a l a t i o n - c o u g h , d y s p n o e a , c h e s t p a i n , r e s p i r a t o r y t r a c t d a m a g e , h e a d a c h e , g e n e r a l w e a k n e s s , n a u s e a , v o m i t i n g , c o l i c d i a r r h e a , g i d d i n e s s , e x t r e m e g e n e r a l w e a k n e s s , a n d e p i g a s t r i c p a i n . • S k i n c o n t a c t – i r r i t a t i o n o f s k i n a n d m u c o u s m e m b r a n e s , b r o n c h i t i s , c o n j u n c t i v i t i s , l a r y n g i t i s , a n d d e r m a t i t i s .
`¤h(\¡¤ ` S y m p t o m s o f a r s e n i c p o i s o n i n g U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d
P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p 3 5 9 , 1 9 9 9 . / / / / / / A . D e b , A G u p t a , P . B r a n d y o p a d h y a y , R B i s w a s , a n d S . R o y ,
“ A p p r o p r i a t e t e c h n o l o g y f o r r e m o v a l o f a r s e n i c f r o m d r i n k i n g w a t e r o f r u r a l W e s t B e n g a l ” , i n P r o v i d i n g S a f e D r i n k i n g
W a t e r i n S m a l l S y s t e m : T e c h n o l o g y , O p e r a t i o n s , a n d E c o n o m i c s . C o t r u v o , G . C r a u n , a n d N . H e a r n e ( E d s ) , W o r l d H e a l t h
O r g a n i z a t i o n , C R C P r e s s , p p 2 7 3 - 2 7 8 , 1 9 9 9 . C h r o n i c e f f e c t s m a y b e d i v i d e d i n t o t h r e e p h a s e s A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 . . A r s e n i c i n t a k e t h r o u g h c o n t a m i n a t e d f o o d , w a t e r a n d a i r i n s m e l t i n g p l a n t s , i n s e c t i c i d e f a c t o r i e s , a n d v i n e y a r d w o r k e r s – s y m m e t r i c a l p a l m e r a n d p l a n t a r h y p e r k e r a t o s i s , w h i t e s t r i a e o f f i n g e r n a i l s , c a r d i o v a s c u l a r m a n i f e s t a t i o n s ,
82 d i a b e t e s , m y o r c d i a c i s c h e m i a , h y p e r t e n s i o n , l i v e r d y s f u n t i o n s , h e m a t o l o g i c a l c h a n g e s , a n d v a s c u l a r d i s o r d e r s r e s u l t i n g i n g a n g r e n e s o f t h e l o w e r e x t r e m i t i e s ( b l a c k f o o t d i s e a s e ) .
C h r o n i c e f f e c t s m a y b e d i v i d e d i n t o t h r e e p h a s e s A r s e n i c P o i s o n i n g i n B a n g l a d e s h / I n d i a , I n t e r n e t s i t e : h t t p : / / w w w . s o s - a r s e n i c . n e t / e n g l i s h / c o n t a m i n / 1 . h t m l , M a y 2 0 0 0 . : F i r s t p h a s e : w o r k e r s o r e x p o s e d p e r s o n s e x p e r i e n c e w e a k n e s s , l o s s o f a p p e t i t e , n a u s e a , o c c a s i o n a l v o m i t i n g , s o m e d i a r r h e a , a n d a s e n s e o f h e a v i n e s s i n t h e s t o m a c h ; S e c o n d p h a s e : s y m p t o m s i n c l u d e c o n j u n c t i v i t i s , a c a t a r r h a l s t a t e o f t h e m u c o u s m e m b r a n e s o f t h e n o s e , l a r y n x , a n d r e s p i r a t o r y p a s s a g e s . C o r y z a , h o a r s e n e s s , a n d m i l d t r a c h e o b r o n c h i t i s m a y o c c u r . P e r f o r a t i o n o f t h e n a s a l s e p t u m i s c o m m o n , a n d i s p r o b a b l y t h e m o s t t y p i c a l l e s i o n o f t h e u p p e r r e s p i r a t o r y t r a c t i n o c c u p a t i o n a l e x p o s u r e t o a r s e n i c a l d u s t . S k i n l e s i o n s o f t h e e c z e m a t o i d a n d a l l e r g i c t y p e c o m m o n l y o c c u r . T h i r d p h a s e : S y m p t o m s s u c h a s p e r i p h e r a l n e u t r i t i s , i n i t i a l l y o f t h e h a n d s a n d f e e t , w h i c h i s e s s e n t i a l l y s e n s o r y . M o t o r p a r a l y s i s o c c u r s i n t h e m o r e s e v e r e c a s e s – t h e f i r s t m u s c l e s t o b e a f f e c t e d a r e u s u a l l y t h e t o e e x t e n s o r s a n d t h e p e r o n e i . I n o n l y t h e m o s t s e v e r e c a s e s w i l l p a r a l y s i s o f t h e f l e x o r m u s c l e s o f t h e f e e t a n d t h e e x t e n s o r m u s c l e s o f t h e h a n d s o r t h e f e e t o c c u r .
N o w t h a t t h e a c u t e a n d c h r o n i c p o i s o n i n g e f f e c t s h a v e b e e n d i s c u s s e d f r o m a c h r o n o l o g i c a l p o i n t o f v i e w i s a l s o a p t t o c o n s i d e r t h e s p a t i a l ( b o d i l y ) s y m p t o m s o f a r s e n i c p o i s o n i n g a s r e p o r t e d b y B r o w n i n g ( c i t e d b y G u l l e d g e J . G u l l e d g e , a n d J O ’ C o n n e r , “ r e m o v a l o f a r s e n i c ( V ) f r o m w a t e r b y a d s o r p t i o n o n a l u m i n i u m a n d f e r r i c h y d r o x i d e s ” , J o u r n a l o f t h e
A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . 021 , I s s u e 8 , p p 5 4 8 - 5 5 2 , ( 1 9 7 3 ) . ) : A r s e n i c p o i s o n i n g i n v o l v e s f o u r m a j o r a r e a s : • T h e d i g e s t i v e s y s t e m : s y m p t o m s i n c l u d e v o m i t i n g o f b l o o d s t a i n e d o r b i l e - s t a i n e d m u c o u s a n d c o n s t i p a t i o n , a c c o m p a n i e d b y i n c r e a s e d p u l s e • T h e s k i n : e r u p t i o n s c h a r a c t e r i z e d b y r e d n e s s a n s s w e l l i n g o f t h e e y e l i d s a n d s c r o t u m , e r y t h e m a o v e r t h e e n t i r e b o d y , a n d l o s s o f n a i l s a n d h a i r • D i s t u r b a n c e s o f s e n s i b i l i t y : h e a d a c h e , n u m b n e s s , s t i f f n e s s : c r a w l i n g a n d p r i c k l i n g s e n s a t i o n s i n t h e t o e s , f e e t , a n d l e g s ; a n d p a i n f u l c r a m p i n g o f t h e m u s c l e s • M o t o r p a r a l y s i s , d e a t h u p o n p a r a l y s i s o f t h e h e a r t .
83
T a b l e ? ? A r s e n i c p o i s o n i n g d i s o r d e r s a n d t h e i r r e l a t e d o c c u p a t i o n s o r `¤'" © a c t i v i t i e s . S y m p t o m s o f a r s e n i c p o i s o n i n g U l l m a n s E n c y l o p e d i a , I n d u s t r i a l I n o r g a n i c C h e m i c a l s a n d P r o d u c t s , W i l e y - V C H , N e w Y o r k , V o l . V , p p 3 6 0 - 3 6 1 , 1 9 9 9 . D i s o r d e r O c c u p a t i o n / A c t i v i t y S k i n d i s o r d e r s : l e s i o n s , w a r t s , I n s e c t i c i d e p r o d u c t i o n , c o p p e r o r e m e l a n o s i s , c o n t a c t d e r m a t i t i s , s m e l t i n g e c z e m a t o u s f e a t u r e s , d e p i g m e n t a t i o n .
M u c o u s m e m b r a n e d e f e c t s : I n s e c t i c i d e e x p o s u r e - C a 3 ( A s O 4 ) 2 c o n j u n c t i v i t i s w i t h s w e l l i n g a n d p a i n ,
P e r f o r a t i o n o f t h e n a s a l s e p t u m C o p p e r s m e l t e r w o r k e r s P e r i p h e r a l n e u r i t i s : s y m p t o m s s u c h a s L o n g - t e r m e x p o s u r e t o a r s e n a t e p a i n , d i f f i c u l t i e s i n w a l k i n g , b u r n i n g d u s t s s e n s a t i o n s , t e n d e r n e s s i n a f f e c t e d l i m b s , a n d l a t e r o n s e v e r e w e a k n e s s i n l e g s a n d f e e t .
H e a d a c h e , a p h a s i a , d r o w s i n e s s , S e v e r e c h r o n i c i n t o x i c a t i o n s d i s o r i e n t a t i o n , a n d c h a n g e s i n p e r s o n a l i t y . C i r c u l a r s y s t e m : C h r o n i c a r s e n i c p o i s o n i n g A b n o r m a l i t i e s i n e l e c t r o c a r d i o g r a m i n d i c a t i n g m y o c a r d i a l e f f e c t , p e r i p h e r a l v a s c u l a r d i s t u r b a n c e s , g a n g r e n e o f e x t r e m i t i e s , a t r o p h i c a c r o d e r m i t i s , e n d a o n g i t i s , a n d b l a c k f o o t d i s e a s e L i v e r d a m a g e s : C i r r h o s i s , h e p a t i t i s , V i n t a g e s u s i n g a r s e n i c a l h e r b i c i d e s a s c i t e s , e n l a r g e d l i v e r a n d a f t e r l o n g t e r m c o n s u m p t i o n o f a r s e n i c c o n t a i n i n g w i n e . G a s t r o i n t e s t i n a l d i s t u r b a n c e s : C h r o n i c i n t o x i c a t i o n b y i n g e s t i o n n a u s e a , v o m i t i n g , l o s s o f a p p e t i t e H e m a t o l o g i c a l c h a n g e s : a n e m i a , C h r o n i c a i r b o r n e e x p o s u r e l e u c o p o e n i a , t h r o m o c y t o p o e n i a ( t h e s e s y m p t o m s d i s a p p e a r e d 2 - 3 w e e k s a f t e r i n g e s t i o n w a s t e r m i n a t e d )
84
C a n c e r : O r a l a n d p h a r y n g e a l c a n c e r s W o o l f i b e r w o r k e r s – s h e e p h a d b e e n d i s i n f e c t e d u s i n g a r s e n i c a l a g e n t s
S k i n c a n c e r w i t h m u l t i f o c a l l e s i o n s A r s e n i c c o n t a m i n a t e d w a t e r i n A r g e n t i n a , C h i l e a n d T a i w a n M u a t g e n i c i t y : s i g n i f i c a n t i n c r e a s e i n C o n t a c t w i t h a r s e n i c a l c o m p o u n d s c h r o m o s o m a l a b e r r a t i o n s T e r a t o g e n i c i t y : S p o n t a n e o u s a b o r t i o n s E m p l o y e e s o f a s m e l t e r a n d n e a r i n h a b i t a n t s
H i g h A r s e n i c l e v e l s i n s t i l l b o r n A r s e n i c i n g e s t i o n i n f a n t 1 1 h o u r s a f t e r a r s e n i c i n g e s t i o n b y m o t h e r I n g e s t i o n o f d i s o d i u m a r s e n a t e E f f e c t s o b s e r v e d i n h a m s t e r f e t u s e s : e x e n c e p h a l y , c l e f t p a l a e a n d l i p , m i c r o a n o p h t h a l m i a , g e n i t o u r i n a r y a b n o r m a l i t i e s , e a r d e f o r m i t i e s a n d r e n a l a g e n e s i s
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ØMÚÛ¡Ü¦Ý Þ¨ï ܤۦë èMÛ¡Üá¨å Ý Ú¦çgèMÞà¡Üë Ý Üè òï çë å ï¥) I n o r g a n i c a r s e n i c i n h i b i t s r e p l i c a t i o n o f D N A a n d i n t e r r u p t s t h e r e p a i r m e c h a n i s m s . A p o s s i b l e c a u s e i s t h e b l o c k a g e o f D N A p o l y m e r a s e b y t h e l i n k a g e o f a r s e n a t e t o s u l p h i d e g r o u p s J . G u l l e d g e , a n d J O ’ C o n n e r , “ r e m o v a l o f a r s e n i c ( V ) f r o m
w a t e r b y a d s o r p t i o n o n a l u m i n i u m a n d f e r r i c h y d r o x i d e s ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . 021 , I s s u e 8 , p p 5 4 8 - 5 5 2 , ( 1 9 7 3 ) . . A t t h e c u r r e n t m a x i m u m c o n t a m i n a n t l e v e l ( M C L ) f o r a r s e n i c o f 0 . 0 5 0 m g / L , t h e e x p e c t e d h u m a n i n c i d e n c e o f c a n c e r , d u e t o t h e p r e s e n c e o f a r s e n i c , i s e x p e c t e d t o b e 1 3 p e r s o n s p e r t h o u s a n d . M . E d w a r d s , “ C h e m i s t r y o f a r s e n i c r e m o v a l d u r i n g c o a g u l a t i o n a n d F e - M n o x i d a t i o n ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . U , p p 6 4 - 7 7 , ( 1 9 9 4 ) .
Aluminium
86
T o x i c o l o g y o f a l u m i n i u m : I n t a k e o f a l u m i n i u m i s l i n k e d t o A l z h e i m e r s d i s e a s e a n d d i s c o l o u r a t i o n R . A . M e y e r s , a n d D . K . D i t t r i c k ( E d i t o r s ) , E n c y c l o p e d i a o f E n v i r o n m e n t a l P o l l u t i o n a n d C l e a n u p , , J o h n W i l e y a n d S o n s , I n c . , N e w Y o r k , p 1 8 3 8 , 1 9 9 9 .
Iron
A l t h o u g h i r o n h a s n o h e a l t h e f f e c t s , i t m a y i m p a r t a n u n a t t r a c t i v e t a s t e a n d c o l o u r t o w a t e r , i n a d d i t i o n i t p r o d u c e s s t a i n s a n d d e p o s i t s a n d p r o m o t e s g r o w t h o f i r o n b a c t e r i a R . A . M e y e r s , a n d D . K . D i t t r i c k ( E d i t o r s ) , E n c y c l o p e d i a o f E n v i r o n m e n t a l P o l l u t i o n a n d C l e a n u p , , J o h n W i l e y a n d S o n s , I n c . , N e w Y o r k , p 1 8 3 8 , 1 9 9 9 .
Experimental design and methodology
T h e u s e o f m o d e l i n g p r o g r a m s h a s b e e n u s e d M . E d w a r d s , “ C h e m i s t r y o f a r s e n i c r e m o v a l d u r i n g c o a g u l a t i o n a n d F e - M n o x i d a t i o n ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . U , p p 6 4 - 7 7 , ( 1 9 9 4 ) . / / / / / b u t t h e s e a r e n o t f a c t o r i a l l y d e s i g n e d , b u t r a t h e r b a s e d o n s i n g l e v a r i a b l e s t u d i e s w h i l e h o l d i n g t h e r e m a i n d e r c o n s t a n t . E d w a r d s M . E d w a r d s , “ C h e m i s t r y o f a r s e n i c
r e m o v a l d u r i n g c o a g u l a t i o n a n d F e - M n o x i d a t i o n ” , J o u r n a l o f t h e A m e r i c a n W a t e r W o r k s A s s o c i a t i o n , V o l . U , p p 6 4 - 7 7 , ( 1 9 9 4 ) . C o m p i l e d c o n t o u r d i a g r a m s d e p i c t i n g p e r c e n t a g e a r s e n i c r e m o v a l a s a f u n t i o n o f p H a n d c o a g u l a n t d o s e . T h e s e d i a g r a m s w e r e b a s e d o n t h e M i n t e q a 2 ( U S E P A s o f t w a r e ) a n d d a t a w a s t a k e n f r o m a d a t a b a s e .
Advantages of factorially designed experiments
87
CHAPTER 1
INTRODUCTION 1.1 General The future of the chemical industry is closely tied to waste reduction. It is rare that a process does not generate one or more by-products. Some of these effluents contain catalysts or other compounds that can be recovered and reused. However, not all by-products or other compounds in these effluents can be recovered or reused, and the “part” that remains is termed as waste. Waste therefore includes streams or materials that are vented to air, discharged to water, landfilled, incinerated or sent to a biological treatment facility.1 The problem with waste is that once it has been generated, it cannot be destroyed. Waste can be concentrated or diluted; its physical properties can be changed as well as its chemical form, but it cannot be destroyed. Waste is frequently relocated rather than treated. For example, solutions containing heavy metals can be treated by chemical precipitation to remove the metals. After the
88 metals are removed, the stream can continue to further treatment or be discharged into rivers or the ocean. The precipitated metals (or sludge) may then be landfilled. The waste problem is best dealt with by not producing waste at all, i.e. waste elimination. However, it is impossible to produce no waste at all, and so the best alternative is the reduction of the amount of waste produced. Pollution prevention is defined as all those activities that reduce the generation of hazardous waste.2 Many other terms are used to describe these activities: waste minimization, waste reduction, source reduction, waste diversion, pollution prevention, recycling and reuse. If waste can be eliminated at the source, no waste treatment is necessary and no waste disposal in any form is necessary. 1.1.1 Historical waste disasters While the chemical industry was exploring and growing, their innovations into new processes and research were not without danger. A long list of major historical waste disasters exists and, with the exception of the many oil spills into the sea, most happened at chemical industries. Below are some important dates and facts about historical waste disasters in the 1900’s. In May 1928, a poison gas (phosgene) incident at the Stolzenberg chemical factory in Hamburg (Germany) caused 10 deaths and 150 severe poisonings.3 From 1938, wastewater containing mercury was discharged into the bay of Minimata in Japan. As a consequence, several hundred deaths from the so-called “Itai-Itai” disease (literally “ouch-ouch” disease) resulted in the following decades due to consumption of fish
89 contaminated with mercury. The end of 1972 saw 292 proven cases of illness with 92 fatal cases and a total of several thousands victims.3 In 1951, a factory discharged, with government permission, some 200 t of radioactive uranium dust into the environment. In July 1976, the largest dioxin accident ever took place in the northern Italian town of Seveso: approximately 2.5 kg of 2,3,7,8-TCDD (dioxin) were released from the reactor of a chemical factory producing trichlorophenol. It resulted in 183 cases of chlorine poisoning, 3000 dead pets and farm animals, and 70 000 animals were slaughtered afterwards. The long term consequences of this disaster remain uncertain. Two very serious and long lasting disasters are respectively the Bhopal disaster4 and the Love Canal disaster.5,6 At midnight of 2-3 December 1984, over 40 t of deadly methyl isocyanate, hydrogen cyanide and other gases leaked from a poorly designed and recklessly managed pesticide factory (CarbideAE) in Bhopal (India). Over 500 000 men, women and children were exposed to the poison clouds and at least six thousand people died within the first week of the disaster.4 The health situation in Bhopal continues to be affected by this tragedy. According to the latest reports of the Indian Counsel of Medical Research (ICMR), nearly one quarter of the exposed population is chronically ill with diseases of all kinds, such as those affecting the neurological and respiratory systems. Exposure to CarbideAE gases has made people vulnerable to secondary infections. According to the ICMR, the figure for tuberculosis patients is more than three times higher than the national figure for the urban population. The International Medical Commission visited Bhopal in January 1994 and their report showed that more than 50 000 people in Bhopal
90 were suffering from total or partial disability because of their exposure to the toxic chemicals of Union Carbide.3 The future of the people in Bhopal is not certain, and there are hopes that at least healthy children may soon be born following this disaster. A similar situation occured in the Love Canal tragedy. In the beginning of 1942, the Love Canal landfill site was used by Hooker Chemicals and Plastics for the disposal of over 21 000 tons of various chemical wastes, including halogenated organics, pesticides, chlorobenzenes and dioxin.5 Dumping ceased in 1952, and in 1953 the landfill was covered. The area near the covered landfill was extensively developed, including the construction of an elementary school and numerous homes. The first reports of odors and problems with residues were heard in 1960, and the number of such reports increased during the 1970’s as the water table rose, bringing contaminated groundwater to the surface.5 By 1979, a scientist claimed to find a high rate of birth defects and miscarriages among Love Canal residents and he urged an evacuation. 6 As a result, approximately 950 families were evacuated from a 10 square block area surrounding the landfill. On 21 December 1995, a consent decree, as a cost recovery settlement between the United States and OCC (former Hooker Chemicals and Plastics), was lodged with the United States district court. As part of the settlement, OCC and the United States Army have agreed to reimburse the federal government’s post response cost, related directly to response activities taken at the site. The primary portion of OCC was $129 million. OCC has also agreed to reimburse certain other federal costs.5 This is just a small portion of the money spent on this disaster.
91
As this and previous examples show, no risks can be taken when working in the chemical industry, and good care must be exercised when dealing with waste handling, treatment and storage. It must be emphasized that the best way to deal with waste is not to produce it at all. Waste minimization and elimination are therefore very important considerations in every part of the chemical industry. 1.2 Waste minimization 1.2.1 Historical overview Waste minimization has been going on for quite some time already. There are numerous examples, some dating from as early as the nineteenth century, of industrial factories that reduced waste generation.2 Also, early twentieth century history contains many examples of waste reduction efforts. A book on waste utilization was published in London in 1915.2 Some industries have long since adopted the “prevention is better than cure” approach as exemplified by the following quote from Rocheleau and Taylor of E. I. Du Pont de Nemours & Co.: Emphasis has been placed on in-plant control measures, because operation of a wastewater treatment works is only part, often a small part of industrial control problems… It is now standard policy with nearly all chemical companies to apply effective wastewater controls as soon as possible in the development of a new or improved process. If a potential waste problem can be recognized early enough, it can often be prevented from becoming a problem, or at least it can be minimized.2 An ironic observation is that industries that were early pioneers in waste reduction are now being “penalized”
92 for being too forward in their thinking. As an example, “save-alls” were originally installed in paper mills to recover fibers. They became so effective that they are now considered as being part of the manufacturing process and no credit is given to the device as a waste minimization technique.2 1.2.2 Strategies for minimizing waste A waste minimization procedure begins with the need to minimize waste. The best place to start waste minimization is at the source. The U.S. Environmental Protection Agency (EPA) has published a waste minimization procedure,7 which can be summarized into the following phases: 1. planning and organization phase 2. assessment phase 3. feasibility analysis phase 4. implementation phase. 1.2.2.1 Planning and Organization To develop a successful waste minimization program, support from management is necessary. Management will only agree with such a procedure if the benefits obtained from the procedure, such as economic advantages, compliance with regulations, reduced environmental impact and improved public image, will outweigh the costs that have to be spent on implementing the system or procedure. If management is convinced, a policy about the waste minimization program should be given to the employees. They can apply the policy by identifying the tasks they have to carry out while striving to minimize waste. An employee from every area of the factory should then be involved in a committee to set tasks and objectives. This
93 employee from each area will know specifically what is necessary in his/her part of the factory and such a person will be very useful for planning purposes. 1.2.2.2 Assessment phase After setting the tasks and objectives, the assessment phase begins with identifying the different waste streams in the company. Every chemical- producing company obviously has a different set up of circumstances but in general, two classes of waste from chemical processes can be identified, namely process waste and utility waste.8 Process waste consists basically of three types of waste. First is the reactor waste, which is waste created in reactors through the formation of waste by-products etc. Second is waste produced from separation and recycling systems. This is waste produced through the incomplete recovery and recycle of valuable products in waste streams. The last source of process waste is the waste from process separations. This basically covers wastes such as those obtained during the start-up and shut- down of continuous processes, tank-filling and equipment cleaning for maintenance. Utility waste is associated with hot and cold utilities. Furnaces, diesel engines, gas turbines etc. all produce gaseous combustion products. These combustion products contain carbon dioxides, oxides of sulphur and nitrogen and particulates, which contribute to the greenhouse effect.8 In addition to the gaseous waste, steam generation, for instance, produces aqueous waste from boiler feed water treatment etc.
94
Once the wastes have been identified, the wastes have to be placed in order of greatest potential risk. Assessment teams have to be selected to classify individual waste streams or processes. The team should include personnel from the relevant plants as they should know the processes and the waste streams produced. Outside consultants may also be used to add their experience about a specific process. The various task forces should then compile a set of options for each specific waste stream. The options must be screened to decide which of these options are real possibilities in minimizing the waste. 1.2.2.3 Feasibility analysis phase During the feasibility analysis phase, the list of the options produced has to be evaluated from not only a technical point of view, but also to determine if the option for the specific application is workable. Economic considerations such as the payback period, return on investment etc. will also be addressed. Another obstacle here is of course the funding, and so financial teams have to be formed, together with technical personnel, to evaluate the proposed plan. The plan can be implemented if it is financially viable. 1.2.2.4 Implementation phase If the evaluation indicates that the plan is feasible, it should be moved to the fourth and final phase, the implementation phase. After implementation, re- evaluation should continuously be performed. The ultimate goal will be maximization of the reduction of waste. 1.2.3 Procedures for minimizing waste
95
The procedures which exist in a waste reduction program covers a very broad area.9 This procedure includes employee training, changing current methods, preventing spills and inventory control. The most important aspect of these is employee training. Not only is management commitment crucial to a waste minimization program, but also the commitment throughout the whole organization is crucial. Bonuses, awards, plaques and other forms of recognition are the best way to provide motivation and to boost employee cooperation in waste minimization.10 Without these, employers may maintain the status quo. Some control methods that can be implemented are: • Minimizing the number of raw materials, supplies used and other materials such as cleaning fluids, oils etc. This helps to clear up shelf-life problems of products exceeding their date of final use, and reduces the number of partially filled containers requiring disposal. Purchasing quantities of materials that are more appropriate to amounts actually used in the process. It can also be less expensive to not buy bulk quantities, especially if materials are often discarded because they are not used before the expiry date. • Reducing the inventory of hazardous materials to a minimum, and ensuring that oldest materials are used first to prevent these exceeding their shelf-life. 1.3 Waste treatment 1.3.1 Introduction The objectives of treating hazardous waste or waste streams in South Africa are:11 • To reduce the toxicity of the harmful components so as to minimize the impact of the waste on the environment, and
96
• To comply with relevant acts and to meet the minimum requirements for treatment and disposal. The environmental health risk, be it natural or anthropogenic (of human origin), is an ongoing danger to mankind. As modern industry has grown over the past years, so the human health risks arising from human activity has grown too, and to a considerable extent. The focus on these human risks has also changed.12 Initially the infectious diseases were the main focus. With more industries being born, more people were drawn to one place, resulting in sanitary problems and higher potentials of epidemics stemming from drinking water and food. As microbiological knowledge has advanced, the infectious diseases could be monitored, and so the health risk caused by chemical exposure and radiation became the more major issue. At first, these chemical and radiation problems were considered only in places with high exposure, but nowadays public health concerns are investigated in all places having human activities. Currently, industry has such a major influence on mankind’s health that a serious environmental problem is arising, resulting in concern for mankind’s health in every part of society. This problem is partly caused by the amount of waste, especially hazardous wastes, generated by industry and disposed of in still waters, rivers, seas and landfills. Therefore, waste treatment is essential to protect human health against the enormous amount of waste produced every day, as well as to protect the environment in general. 1.3.2 Preliminary steps for waste treatment Waste generated by an industry has to be classified according to what kind of waste it is, e.g. hazardous waste, recycled waste etc. Once the waste has been
97 classified, the minimum allowable requirement for the waste has to be determined.13 On the basis of these requirements, it is decided whether or not waste treatment is required. An important factor for waste treatment is to choose the exact treatment that is required. The choice of which technology to use is dependent on the nature of the waste, the availability of treatment and disposal facilities, and the cost effectiveness of the treatment. The technology chosen for the treatment of a hazardous waste is highly influenced by its chemical and physical properties. Properties such as state (solid, liquid, gas or vapor), nature (inorganic or organic), the concentration of hazardous and non-hazardous components and, of course, toxicity, mobility etc. are very important considerations when choosing the treatment. 1.4 Herbicides and Pesticides 1.4.1 History Any chemical used to destroy or inhibit plant growth, especially of weeds or other undesirable vegetation, is called a herbicide. The use of chemicals to kill pests is a very old process. In about 70 A.D., Pliny the Elder suggested that arsenic could be used to kill insects; the Chinese used arsenic sulphide as an insecticide as early as the late sixteenth century.14 During 1896 – 1908, metal salts and mineral acids were introduced as selective sprays for controlling broad-leafed weeds in cereals. During 1915 – 1925, acid arsenical sprays, sodium chlorate and other chemicals were recognized as herbicides, and in 1933 – 1934, sodium dinitrocresylate became the first organic-selective herbicide to be used in cereals, flax and peas. Since the discovery of 2,4-
98 dichlorophenoxyacetic acid in 1942,15,16 a growth regulator type of herbicide, a wide variety of organic herbicides have been developed and have enjoyed wide usage in agriculture, forestry and other industries. 1.4.2 Problems related to herbicides and pesticides During the 1950’s and early 1960’s, the first reports of large residues of herbicides in soil, and small quantities in water and at the bottom of streams began to appear in literature.14 Dead birds and fish were found near sprayed fields and in treated water. More and more reports were published about herbicides not only killing pests, but also concentrating in upper levels of the food chain. The discoveries started to cause concern about possible long term ecological effects. There is now little doubt that certain herbicides are major long term contaminants of the entire environment. One of the problems is that not all of the chemicals remain in the area of treatment. The physico-chemical properties of the substance, together with environmental transport processes, result in a portion of the chemical moving elsewhere in the environment. Traces of certain of the more persistent chemicals have been found well removed from areas of treatment.17 Several hundred miles of aerial transport has been reported to have occurred in certain cases. In 1980, about 3000 t of the herbicide 2,4,5-trichlorophenol (2,4,5- T) was used in the United States and 1000 t in the United Kingdom, chiefly to control weeds and bushes in plantations of young conifers. An impure form of this herbicide, known as “Agent Orange”, was used by the Americans in the Vietnam war to remove forestry cover which could conceal enemy troops. Agent Orange appeared to contain dioxin as impurity, which has teratogenic (foetus-deforming) effects. Experiments in
99 the United States showed that the use of 2,4,5-T on food crops resulted in teratogenic effects on rats and mice that had consumed the treated crops.15 It seems inevitable that the ultimate solution to our environmental pesticide problem must encompass a compromise, one in which the smallest possible quantities of pesticide is used, and this needs to be combined with other control measures so that environmental pollution by pesticides is kept at a minimum. There seems little likelihood of being able to dispense with the use of pesticides in the foreseeable future, but intelligent use of them will greatly reduce the hazards implicit in their continued use.14 1.4.3 Benefits of herbicides and pesticides Freeing agricultural crops from weeds has resulted in higher food production, reduced harvesting costs, improved food quality, et cetera. These factors have all contributed to reduced costs in labor, energy, irrigation and insect and disease control. Additional benefits when appropriate herbicides are used include that millions of people are relieved of allergies to pollens and exposure to poisonous plants. Recreational areas, roadsides, forests and parks have been largely freed from weeds and gardens improving their aesthetic appeal. Modern herbicides even benefit the construction industry, where chemicals applied under asphalt extend pavement life by preventing weed penetration of the surface.16 1.4.4 Herbicidal and pesticidal classification There are well over a hundred compounds in use as herbicides. Many of these have different names or formulations. The variety of materials is classified according to the properties of the active ingredient as either selective or non-selective. Selective herbicides are
100 those that kill certain members of a plant population with no injury to others. Non-selective herbicides are those that kill all vegetation to which they are applied. They are commonly used to keep roadsides weed-free. 1.4.4.1 Examples of herbicides and pesticides A wide variety of herbicides are currently available these days. Not all of them are harmful to human beings or animals. Urea, carbamate and triazine herbicides act by interference with photosynthesis in plants, causing them to starve to death. Humans do not photosynthesize and are therefore not directly affected. The described chemicals have low mammalian toxicity and do not build up in food chains. Dithiocarbamate fungicides such as Zineb, Maneb and Mancozeb are also widely used and have relative low mammalian toxicities.15 Two natural insecticides were introduced in the nineteenth century, these being rotenone from the derris plant and pyrethrum extracted from a species of chrysanthemum. Both were very safe insecticides, but were also very expensive. Newer synthetic organic insecticides now replace these insecticides, mainly because of the cost factor. These synthetic insecticides fall into three major classes: organophosphorous, organochlorine and the synthetic pyrethroids.15 1.4.4.1.1 Organophosphorus compounds Organophosphorus compounds were developed after World War II in the search for nerve gases. These compounds were therefore very toxic to man and other mammals. Parathion [1] was the first such insecticide used, followed by tetraethyl pyrophosphate (TEPP) and Schradan [2]. TEPP was the most toxic material ever to be used on farms.15 Organophosphorus insecticides act by disrupting effective nervous transmissions in the
S C2H5O P O NO2 C2H5O 101 insect, and in theory they would work in a similar way in mammals. [1]
O O
(CH3)2N P O P N(CH3)2 (CH3)2N N(CH3)2 [2] Table 1.1: Lethal doses of certain organophosphorous compounds * Name LD50 in rats (mg/kg) schradan 5 - 55 parathion 13 * LD50 = (lethal dose). This value represents the amount of poison per unit weight, which will kill 50% of the particular population of the animal species employed for the tests.18 1.4.4.1.2 Organochlorine compounds Organochlorine compounds such as DDT [3], HCH and Dieldrin [4] also act on the insect’s nervous system. The first and most widely used organochlorine compound H
Cl Cl
CCl3 102 was DDT, which was introduced in 1942.14,15 A very serious feature of organochlorine compounds is their ability to become concentrated along food chains, causing death to organisms at the end of the chain.14 [3]
Cl Cl Cl Cl
Cl Cl
O [4] Table 1.2: Lethal doses of certain organochlorine compounds Name LD50 in rats (mg/kg) DDT 250 dieldrin 60 TEPP 1 1.4.4.1.3 Synthetic pyrethroids The synthetic pyrethroids generally have low mammalian toxicity combined with very high levels of insecticidal potency. They can hence be used in very low doses. The pyrethroids do not concentrate along food chains
103 because they readily degrade. Allethrin [5] was the first synthetic pyrethrin analogue.18 The synthetic pyrethrins are more stable than the natural pyrethrins due to the lower reactivity of the side chains.
CH3 H H CH2CH CH2 (CH ) C CCOO 3 2 O (CH3)2C CHC H2 H [5] Table 1.3: Lethal doses of certain pyrethrins Name LD50 in rats (mg/kg) Natural pyrethrins 1500 Allethrin 680 – 920 Phtalthrin 1000 1.5 The production of 4-methyl-3-thiosemicarbazide (MTSC) 1.5.1 General 4-Methyl-3-thiosemicarbazide [6] (MTSC) is an intermediate in the synthesis of 5-tert-butyl-2- methylamino-1,3,4-thiadiazole (BTDA), the precursor of tebuthiuron, a broad spectrum herbicide. 19 H H
N N C CH3 NH2
S
MTSC 104
[6] 1.5.2 Synthesis of MTSC The synthesis of MTSC [6] at Dow Agrosciences in Sasolburg consists of two reactions. The first reaction is the synthesis of N-methyldithiocarbamate [9] by the reaction of methylamine [7], carbon disulfide and a base. This base may be a primary amine, tertiary amine or an alkali metal hydroxide.20,21 Dow Agrosciences first used ammonia as the base, but with every kilogram of product produced, 5 kg of waste resulted. Research was performed to find a base that could be recovered so that a minimum of waste was produced. N,N- diisopropylethylamine [8] (DIPEA) was found to be a suitable base for the reaction.19 Initially N- methyldithiocarbamic acid is formed, which then reacts with the base to form the corresponding N- methyldithiocarbamate salt (DTC) [9].22
H H CS2 + H3C N + N N S-+NH H H3C C
S [7] [8] [9] Scheme 1.1: Synthesis of the N-methyldithiocarbamate salt
105
The main impurity formed during the production of N- methyldithiocarbamate [9] is N,N-dimethylthiourea [10] (DMTU) (see scheme 1.2). This product is formed when methyl isothiocyanate [11], the decomposition product of N-methyldithiocarbamate, reacts with unreacted H H H N N H3C N C S + H3C N H3C CH H C 3
S methylamine [7]. 21 [11] [7] [10] Scheme 1.2 : The formation of DMTU in the side reaction MTSC [6] is formed by reacting N-methyldithiocarbamate [9] with hydrazine hydrate. Methyl isothiocyanate [11] is presumably formed in-situ by the base-catalysed decomposition of the N-methyldithiocarbamate [9]. Hydrazine hydrate then reacts with methyl isothiocyanate [11] to form MTSC [6]. 23
106
H H -+ H2NNH2 N S NH H3C N C S + H2S + R N H C C 3 12 11 H S
9
H H
N N H3C C NH2
S [6] Scheme 1.3: Synthesis of MTSC 1.5.3 Starting materials 1.5.3.1 Methylamines Monomethylamine [7], dimethylamine and trimethylamine are all produced by the reaction of methanol and ammonia over solid acid catalysts.24 Monomethylamines are the simplest members of the amine family. They are valuable in medicines, agriculture, rubber, plastics and synthetic fibers. Monomethylamine [7] is an intermediate in the synthesis of pharmaceuticals, pesticides, surfactants, photographic developers, explosives etc.24 1.5.3.2 Carbon disulfide
107
Lampadius discovered carbon disulfide in 1796 while heating a mixture of iron pyrite and carbon.25 Industrial interest grew with the use of carbon disulfide for the extraction of oil and fats and, in 1984, 1.1 million t of carbon disulfide was produced worldwide. Carbon disulfide reacts with ammonia and, depending on the ammonia concentration and temperature, forms ammonium dithiocarbamate, ammonium trithiocarbamate and ammonium thiocyanate 1.5.3.3 N,N-diisopropylethylamine [8] (DIPEA) Hünig’s base (DIPEA) can be used in most applications that require a hindered tertiary amine as a proton acceptor. The low nucleophilicity of DIPEA ensures that it does not react with reagents and intermediates to form by-products.19 Some properties of DIPEA include:19 • Proton specific, “non-nucleophilic” base • Auxiliary reagent in organic synthesis • Proton acceptor/scavenger • Replaces triethylamine and dimethylaniline as proton acceptors • Low self-alkylation • Low water solubility (0.4% w/w) • Recyclable • Increases process efficiency • Reduces process cost DIPEA is easily recovered from the reaction mixture because of its low solubility in water. DIPEA can be recovered by azeotropic distillation. After the MTSC is formed, the azeotropic distillation separates the DIPEA- water mixture from the reactor. After cooling, the
108 azeotrope separates as DIPEA with 99.8% purity and a water phase containing 0.4% DIPEA.19 1.5.3.4 Hydrazine Emil Fischer predicted the existence of hydrazine in 1875 but it was first isolated in 1887 by Curtius.26 Hydrazine is produced by variations of the Raschig process, the oxidation of ammonia by hypochlorite. It is available as anhydrous hydrazine, as an aqueous solution and as a solid dihydrazinium sulfate. Most hydrazine is sold as an aqueous solution. The only application for anhydrous hydrazine is as a rocket fuel or as a monopropellant for satellites and spacecraft. The most important uses for hydrazine are as blowing agents for foamed plastics and the production of herbicides. It is also used in the production of hydrazine- based pesticides. Maleic hydrazide was the first such pesticide used.26 1.5.4 Intermediates formed during MTSC synthesis 1.5.4.3 Dithiocarbamates The reaction of carbon disulfide with primary or secondary amines, both aliphatic and aromatic, affords dithiocarbamate salts. The free dithiocarbamate acids are unstable and very few of them have been isolated. The best way to isolate the acid is to convert them into the salt. Dithiocarbamates derived from primary amines are unstable and, in the presence of a base (B-), are converted to the corresponding isothiocyanates (Scheme 1.4).20,27 B- RHNCSS- RNCS + SH- Scheme 1.4: Conversion of dithiocarbamate to the isothiocyanate
109
1.5.4.4 Isothiocyanates Aliphatic and aromatic isothiocyanates are important starting materials for the production of thiourea derivatives such as substituted thiosemicarbazides20,21,23(see scheme 1.5)
S R N C S + NH3 R NH C NH2
S R N C S + H2NNH2 R NH C
NHNH2 (Scheme 1.5: Isothiocyanates used for the production of substituted thiosemicarbazides) Isothiocyanates can be formed using several methods and the most important of these are listed below: 27 1. Reaction of alkyl and acyl halides with salts of thiocyanic acid 2. Isomerisation of esters of thiocyanic acid 3. Reaction of dithiocarbamates with hydrogen peroxide, cyanogen chloride, cyanuric chloride, chloroformates, alkali hypochlorite or alkali chloride, phosgene, heavy metal salts, atmospheric oxygen and phosphorous oxychloride 4. Reaction of primary amines with thiophosgene 5. Reaction of thiourea derivatives with mineral acids, acetic anhydride or by heating 6. Addition of thiocyanic acid to unsaturated compounds. Methyl isothiocyanate is commercially produced by the following two processes:
110
1. Rearrangement of methyl thiocyanate 2. Oxidation of sodium N-methyldithiocarbamate with hydrogen peroxide.27 Methyl isothiocyanate may be reacted with hydrazine to form 4-methyl-3-thiosemicarbazide [6] in high yield (>90%) with almost analytical purity. 28 1.5.5 The MTSC product 1.5.5.1 General A wide range of thiosemicarbazides are known, and Jensen et. al. have published the preparation procedures for a large number of them.23 Because of the wide variety of thiosemicarbazides, a traditional numbering system of the compound has been introduced.
4 3 2 1 H2N CS NH NH2 Figure 1.1: traditional numbering of the thiosemicarbazides. Eight methods have been used to produce different types of thiosemicarbazides:23 1 Isomerisation of hydrazinium thiocyanates 2 Reduction of thiosemicarbazones 3 Reaction of isothiocyanates with hydrazine or hydrazines 4 Reaction of hydrazine or hydrazines with reactive thiocarbamic acid derivatives 5 Ammonolysis or aminolysis of monothiocarbazates or dithiocarbazates 6 Hydrazinolysis of monothiocarbazates or dithiocarbazates
111
7 Reaction of cyanohydrazines with hydrogen sulfide 8 Special methods for preparing 1-methyl derivatives such as: • hydrolysis of 1-acylthiosemicarbazides, obtained from acylhydrazines and isothiocyanates • reduction of formaldehyde-2- phenylthiosemicarbazone with diborane 1.5.5.2 Thiosemicarbazides For thiosemicarbazides monosubstituted in the 4– position, two important methods are mentioned. The general method for the production of these derivatives is the reaction between alkyl isothiocyanate and hydrazine or substituted hydrazines.23 The reaction proceeds very rapidly with monoalkylhydrazines, irrespective of the substituents on the hydrazine, and the yields are very satisfactory. The reaction between isothiocyanates and hydrazine is not significantly influenced by steric factors. The radical of the isothiocyanate seems to have little affect on the course of the reaction. Normally the 2- isomer is obtained as the only product. The second method discussed by Jensen et. al. for the synthesis of thiosemicarbazides monosubstituted in the 4-position is the hydrazinolysis of N- alkyldithiocarbamates.23 It was found that methyl N- methyl dithiocarbamate, CH3NH-CS-SCH3, which reacts rapidly with alkylhydrazines, always gave 2,4- dialkylthiosemicarbazides. The reason is that the hydrazines induce a base-catalyzed decomposition of the alkyl dithiocarbamate to form an isothiocyanate. The formation of the thiosemicarbazide actually proceeds through the reaction between the hydrazine and an isothiocyanate. The yields are comparable to those
112 obtained in the reactions between hydrazines and isothiocyanates so that an S-alkyl N-alkyl dithiocarbamate can be used instead of an isothiocyanate to prepare 4-alkylthiosemicarbazides. The monothiocarbamates on the other hand reacted only slowly with hydrazines, and the yield of 4- alkylthiosemicarbazide was low because of side reactions. Some 4-alkylthiosemicarbazides were obtained by aminolysis of S-methyl dithiocarbazate, CH3S-CS-NHNH2, but the yields were also low (10 – 40 %). From alkoxythiocarbonylhydrazines and primary amines, it has only been possible to obtain 4- alkylthiosemicarbazides in very low yields in some cases. Another method that has been investigated is the aminolysis of thiosemicarbazones to produce 4- alkylthiosemicarbazides, but that method cannot compete with the use of an isothiocyanate as starting material. 1.5.6 By-products 1.5.6.1 Thioureas The main by-product obtained when preparing the dithiocarbamate (DTC) salt is N,N-dimethylthiourea (DMTU). A typical thiourea system is shown in Figure 1.2.
R' R''' 1 2 3 N CS N R'' R'''' Figure 1.2: A typical thiourea system The reaction to prepare 1,3-disubstituted thioureas is very similar to the preparation of the DTC, the only real difference is the heat applied to the reaction. The most reasonable course of the thiourea reaction is:
113
S
RNH 2 + CS 2 RHNCSH S S
- RHNCSH + RNH 2 + RHNCS NRH3 S
- + heat RHNCS NRH3 RNCS + RNH 2 + H2 S
RNCS + RNH 2 RHNCSNHR
2 RNH 2 + CS 2 RHNCSNHR + H 2 S Scheme 1.6: Reaction scheme for thiourea synthesis As can be seen from Scheme 1.6, 3rd reaction, the application of heat will initiate the production of thioureas. This is also the main problem when producing the DTC salt. The temperature should be maintained below 30oC to prevent DMTU forming. 1.5.6.2 Hydrazides Carboxylic acid hydrazides are generally prepared by reactions of hydrazine hydrate with the corresponding methyl or ethyl ester. Hydrazine carboxamide and carbohydrazide are produced from hydrazine hydrate and urea, depending on the amount of urea used. 26 Hydrazine carbothioamide (thiosemicarbazide) is made from the reaction of carbon disulfide with hydrazine and thiocarbohydrazide (TCH), a by-product formed during
114 the MTSC synthesis, is produced from carbon disulfide, ammonia and hydrazine. 1.6 Objectives of this study 1.6.1 Introduction Dow Agrosciences was producing MTSC as an intermediate for the production of Tebuthiuron. During the synthesis of MTSC, an effluent was obtained with a very high Chemical Oxygen Demand (COD) level. The level is too high for the effluent to be disposed of into the Sasol waste stream. As a result Dow Agrosciences diluted the effluent about 20 000 times, to decrease the COD level to such an extent that the effluent could be disposed off into the Sasol waste stream. The dilution was a very expensive measure, which did not really solve the problem. Research had to be peformed, to determine if it was possible to treat the waste so that the effluent could be disposed off. Research had to be performed to investigate the synthesis reaction as well to determine whether the synthesis could be performed whilst producing less waste. 1.6.2 Objectives of this study Two objectives were investigated: • Develop a waste treatment protocol, that would improve the effluent to the extent that it meets the requirements for disposal into the Sasol stream, and
115
• Use waste minimization techniques to improve the MTSC synthesis in such a way, that less waste was produced and a cleaner effluent was obtained. CHAPTER 2 EXPERIMENTAL 2.1 Materials
116
The reagents and methods described here were used for the synthesis of 4- methyl-3-thiosemicarbazide and the treatment of the resulting effluent. 2.1.1 Reagents for synthesis All material used for the synthesis of 4-methyl-3-thiosemicarbazide, with their sources and respective grades, are listed in Table 2.1 and were used as received. Table 2.1: Reagents used for synthesis Formula Source Grade Chemical name
Monomethylamine CH3NH2 Aldrich Industrial grade (40 wt % solution in water) Diisopropylethylamine [(CH3)2CH]2NCH2CH3 Whyte >98% Chemicals Carbon disulfide CS2 Saarchem CP . Hydrazine Hydrate N2H4 H2O Saarchem CP 2.1.2 Analytical standards The reagents used as standard materials for High Performance Liquid Chromatography (HPLC) and the Chemical Oxygen Demand (COD) determination are listed in Table 2.2. All standard materials were used as received.
117
Table 2.2: Reagents used for analytical work Chemical name Formula Source Grade
4-Methyl-3- CH3NHCSNHNH2 Aldrich 99 % thiosemicarbazide N,N-Dimethylthiourea (CH3NH)2CS Aldrich >98 % Thiocarbohydrazide (NH2NH)2CS Aldrich >98 % Potassium dichromate K2Cr2O7 NT laboratory AR supplies Silver sulphate Ag2SO4 M and B Ferrous ammonium NH4Fe(SO4)2 NT lab supplies AR sulphate Ferroin indicator Merck Sulfuric acid H2SO4 Merck AnalaR Acetonitrile CH3CN BDH HiPerSolv 2.2 Apparatus used for the synthesis of 4-methyl-3- thiosemicarbazide The synthesis of 4-methyl-3-thiosemicarbazide was performed in a 250 mL glass reactor equipped with an overhead stirrer, condenser, and thermometer. A Labcon circulator was used to pump glycerine through the outside mantel of the reactor to heat the system to the required temperature (see Figure 2.1). When the reaction was complete, the diisopropylethylamine was removed using azeotropic distillation. For this distillation, the condenser was replaced by a distillation set-up, connected to a separating funnel which was used as the receiving flask (see Figure 2.2).
118
Figure 2.1: Reaction set-up for the synthesis of 4-methyl-3-thiosemicarbazide
119
Figure 2.2: Reaction set-up for the azeotropic distillation of diisopropylethylamine 2.3 Synthetic procedure for the synthesis of 4-methyl-3- thiosemicarbazide The synthesis of 4-methyl-3-thiosemicarbazide was performed according to a two step procedure. Firstly, the dithiocarbamate salt is made using diisopropylethylamine as a base. The dithiocarbamate was then used as a starting material for the synthesis of 4-methyl-3-thiosemicarbazide. 2.3.1 Synthesis of the dithiocarbamate salt (DIPEA-DTC) Methylamine (MMA) (40%) (38.75 g; 0.50 mol) and N,N-diisopropylethylamine (DIPEA) (71.5 g; 0.68 mol) was charged to a 250 mL glass reactor equipped with a reflux condenser and an overhead stirrer. Carbon disulfide (38.0 g;0.50 mol) was added dropwise, using a dropping funnel, while stirring the solution. The temperature was maintained below 30 oC. Water (32.4 g) was then added and the reaction mixture was left stirring for an additional 3 hours. 2.3.2 Synthesis of 4-methyl-3-thiosemicarbazide The distillate (distillate was obtained together with the DIPEA after azeotropic distillation) (30.0 g) from a previous reaction was added to the DTC mixture, and
120 hydrazine hydrate (30.05 g; 0.60 mol) was introduced over a 5 minute period at a flow rate of 5.8 mL/min, using a Beckman110B solvent delivery module (HPLC pump). The reaction mixture was heated to 90 oC using a Labcon circulator pumping glycerine, for heating up the reactor (Figure 2.1). The reaction was allowed to reflux for two hours and 20 minutes after which time the reflux apparatus was replaced by a distillation apparatus (Figure 2.2). Diisopropylethylamine was recovered by azeotropic distillation at 95 oC. The reaction mixture was cooled while stirring. The solid 4-methyl-3-thiosemicarbazide crystals were removed from the solution using vacuum filtration and dried in a vacuum oven at 60 oC for three hours to yield a light yellow product. 2.4 Analytical techniques The analytical methods used for the analysis of 4-methyl-3-thiosemicarbazide, as well as the method to determine the chemical oxygen demand, were obtained from Karbochem. Any modifications made to these methods are specifically mentioned in the relevant sections. The other methods described are according to the standard operating procedures of the specific apparatus used. 2.4.1 High Performance Liquid Chromatography (HPLC) Analysis of 4-methyl-3-thiosemicarbazide obtained after synthesis The analysis of the obtained 4-methyl-3-thiosemicarbazide by HPLC was performed using an Agilent 100 series High Pressure Liquid Chromatograph equipped with a 3.9 x 300 mm ubondapack C18 column and a UV detector. The external standard method was employed and an injection volume of 10 uL was used. The external standard was prepared by accurately weighing approximately 0.01 g of 4-methyl-3-thiosemicarbazide standard and 0.001 g of each of thiocarbohydrazide (TCH) and N,N-dimethylthiourea (DMTU), and making up to a volume of 250 mL with mobile phase. The sample was prepared by weighing
121 approximately 0.01 g of dry sample and making up to 250 mL with mobile phase. The samples were eluted with an isocratic mobile phase consisting of 6.4 % acetonitrile and 93.6 % deionised water at a flow rate of 1 mL/min. The detector was set at a fixed wavelength of 240 nm. Analysis of the composition of 4-methyl-3-thiosemicarbazide effluent HPLC analysis was performed on a Beckman System Gold HPLC system equipped with a programmable solvent module 126, a 250 x 4.00 mm 10-micron Lichrosorb 10 rp-18 column and programmable detector module 166 with an ultraviolet detector. Data were acquired from the detector by means of a personal computer equipped with the System Gold Data Acquisition software, version 810. The external standard method was employed and an injection volume of 20 uL (manual loop injector) was used. The external standard was prepared by accurately weighing approximately 0.1 g of 4-methyl-3-thiosemicarbazide standard and 0.1 g of each of thiocarbohydrazide (TCH) and N,N-dimethylthiourea (DMTU), and making up to a volume of 250 mL with mobile phase. The sample was prepared by diluting the 4-methyl-3-thiosemicarbazide effluent 1000 times. The diluted effluent was used for analysis. The samples were eluted with an isocratic mobile phase consisting of 6.4 % acetonitrile and 93.6 % de-ionised water containing 30 g of phosphoric acid
(H3PO4) at a flow rate of 1 mL/min. The detector was set at a fixed wavelength of 240 nm. The standards were stored in a refrigarator at 3 oC. (The standards were not stable at temperatures above 20 oC). Once stored in the fridge, no reaction occured and the standards remained useable for up to 1 month. 2.4.2 Chemical Oxygen Demand
122
The analysis of the COD value of the 4-methyl-3-thiosemicarbazide effluent was performed using a potassium dichromate reflux method. An aliquot of 4-methyl-3- thiosemicarbazide effluent (1.00 mL) and de-ionised water (49 mL) were pipetted into a 250 mL round-bottomed flask. De-ionised water (50 mL) was used as a blank determination. Potassium dichromate (0.25 N; 25.00 mL) was added to the round-bottomed flask. Sulfuric acid reagent (40 mL) was slowly added while cooling the mixture, after which the mixture was refluxed for two hours. The mixture was cooled to room temperature and five drops of ferroin indicator were added. The mixture was titrated with ferrous ammonium sulphate (0.25 N) until the color of the mixture turned from blue-green to reddish brown. The following equation was used to calculate the COD value in parts per million: (A − B)N COD = ×8000 V With A = volume of ferrous ammonium sulphate used for blank B = volume of ferrous ammonium sulphate used for sample N = normality of ferrous ammonium sulphate, and V = Volume of sample 2.4.3 Differential Scanning Calorimetry (DSC) Differential Scanning Calorimetry was used for the determination of the composition of the “stone-like” precipitate obtained after the oxidation of the 4- methyl-3-thiosemicarbazide effluent (see Chapter 4). Differential Scanning Calorimetry was performed using a Mettler DSC 820 calorimeter. Analysis was performed using standard aluminium pans, into which approximately 10 mg of sample was accurately weighed, and the pan hermetically sealed. An empty aluminium pan with a punctured lid was used as a reference. The processor was used in the screen mode and the heat flow was monitored between 80 oC and 130 oC at a heating rate of 2oC/min. This technique was successfully used to determine
123 the melting point of the precipitated samples obtained after oxidation of the effluent in the Parr high pressure reactor (see Chapter 4). 2.4.4 X-ray Fluorescence Spectroscopy (XRF) XRF Analysis was used to determine the composition of the stone-like precipitate obtained after the oxidation of the 4-methyl-3-thiosemicarbazide effluent (see Chapter 4). XRF analysis was performed using an Oxford 2000 instrument equipped with Link ISIS Software. Dispersion of the X-rays generated in the samples takes place by means of an Energy Dispersive X-ray analyzer (EDX). The samples were mounted in plastic holders with a Myler sheet at the bottom. General vacuum conditions were used for XRF analysis. The peak intensity is indicative of the quantity of the element. XRF was successfully used to determine qualitatively, the major elemental content of the precipitate obtained after oxidation of the MTSC effluent in the Parr high-pressure reactor (see chapter 4). CHAPTER 3 The synthesis of 4-methyl-3-thiosemicarbazide 3.1 Introduction Dow Agrosciences started the production of 4-methyl-3- thiosemicarbazide (MTSC) with ammonia as a base. The use of ammonia resulted in large amounts of ammonium salts. These salts could not be recovered after the reaction, resulting in a situation where for every kg of product 5 kg of waste was produced. When MTSC was produced at industrial scale, a very large amount of waste was produced, resulting in a huge waste problem. Naturally, research was performed into the synthesis of MTSC, to allow the reaction to produce the same quantity
124
of product, but less waste per kg of product. N,N- diisopropylethylamine (DIPEA) was introduced as the new base instead of ammonia. A big advantage of DIPEA is the ability to recover the DIPEA almost completely after the reaction. When performing reactions using DIPEA, the amount of waste decreased almost three times. However, because the volume of waste decreased, the concentration of the waste increased, causing a major waste disposal problem. A waste treatment protocol has been established (Chapter 4) to treat the effluent resulting from the synthesis. However, a treatment procedure after the synthesis means extra cost and expansion of the plant to add the waste treatment. Ideal conditions would be to reduce the amount of waste and waste concentration during the synthesis of the MTSC. This chapter described the synthesis of MTSC and the evaluation of the synthesis as performed at Dow Agrosciences. 3.2 Objectives From the preceding discussion, it is clear that the waste obtained during the synthesis of MTSC is a major concern. Another point of interest is the optimization of the synthesis. Effluent qualities could be improved by optimizing the reaction and thus reduce the amount of effluent by treating at source through improving the reaction yeilds and selectivity. The objectives of this chapter may thus be summarized as follows:
125
• To synthesize MTSC on laboratory scale as performed at Dow Agrosciences. • To evaluate and adjust the levels of reaction variables such as the reaction temperature and the amount DIPEA used by means of a statistical experimental design, with the view to optimize the yield of MTSC. • To evaluate the waste produced during the synthesis by using a statistical experimental design to optimize reaction conditions so less concentrated waste is produced. 3.3 The synthesis of 4-methyl-3-thiosemicarbazide The synthesis of MTSC consists of two reactions. The first one involves the reaction of monomethylamine and N,N-diisopropylethylamine with carbon disulfide. Carbondisulfide is added by means of a dropping funnel also containing water. While adding the carbon disulfide, the reaction temperature was maintained below 300C. Directly after the addition of the carbondisulfide the water is added and the reaction was left stirring overnight. The next morning hydrazine hydrate and distillate from a previous reaction was added and the reaction was allowed to react at 900C. The DIPEA was recovered by means of azeotropic distillation. Table 3.1 shows the results of the synthesis of MTSC performed in a similar manner as performed at Dow Agrosciences. Table 3.1: Results of MTSC synthesis performed similar to Dow Agrosciences Reaction Yield MTSC Recovery DIPEA 1 74.88% 67.3%
126
2 68.80% 91.7% 3 68.25% 89.0% 3.4 Selection of reaction variables and the determination of the experimental domain. 3.4.1 General The synthesis of MTSC was evaluated using a full rotatable central composite design. It should be noted that the optimum points obtained from this design are only the optimum points for this specific reaction under these specific conditions. These optimum points therefore provide no information about process details and possible up-scaling of the reaction, but it can only give an idea about the behavior of this specific reaction. 3.4.2 Identification of experimental variables The optimization of chemical reactions requires that all experimental variables that may affect the reaction in any way, be identified and investigated.29 Such variables would normally include many chemical engineering factors, such as reactor design, heat transfer, feed of materials etc. for the investigation of large scale production processes. In this case, however, the primary aim was to establish if the chosen variables had a significant effect on the design responses on laboratory scale, and an attempt was made to move as close as possible to the optimum conditions. For this reason, chemical engineering factors were not included in this design. For the present design, the following variables could have an impact on the synthesis of MTSC: excess of DIPEA used, reaction temperature, addition rate of hydrazine hydrate, concentration of dithiocarbamate (DTC) before the second reaction and excess of hydrazine hydrate used. In order to reduce the number of variables to be investigated, it was decided to use DIPEA loading together with reaction temperature as reaction variables, as the excess of hydrazine hydrate and
127 the concentration of DTC have already been investigated previously at Dow Agrosciences.30 The addition of hydrazine hydrate was not considered very important as the reaction only started at high temperature when all the hydrazine hydrate was already added. The stirring rate during the synthesis of MTSC proved to be a very important factor in the reaction kinetics by influencing the frequency of “collisions” of the reactive species. For best results of this synthesis, the stirring rate should therefore be as high as possible, but should not be varied between the different reactions. The stirring speed was therefore maintained at 700 rpm for all reactions in this design. 3.4.3 Selection of the experimental domain
Table 3.2: Factors and factor levels for the experimental design Code Variable Unit Variable level Low (-) Centre (0) High (+) T Temperature oC 80.0 85.0 90.0 D DIPEA g 75.0 82.5 90.0 Fixed variables Variable Unit Level Rate of stirring rpm 700 MMA loading gram 38.75 g
CS2 loading gram 38.0 g Hydrazine hydrate loading gram 30.1 g 3.5 Design and responses The response variable selected for this design was the yield of MTSC, as optimization of this yield is the very purpose of the whole experiment. This yield of MTSC is
128 dependent on the fixed variables and in this design highly dependent on the two variables investigated. Table 3.3 illustrates the experimental design and the responses obtained. A full, rotatable central composite design was used for the determination of the second- order response surface model. All reactions, including the replicated experiments at the design center were run in random order. The factor levels shown in Table 3.3 were coded using equation 3.1: u − u x = 1 o (3.1) i δ u where xi = coded value of variable Xi, ui = natural value of variable Xi at a factorial δ point, u0 = the natural value of Xi at the design center, and u = step size in the natural values of variable Xi.
Table 3.3: Design and experimental responses Variable settingsa
129
Variable settingsa
Yield MTSCb
V XW?Y Run No. £ ¡§
1 T-1 ((o8C0). 0) D-1 ((g7)5 .0) 65.30 2 +1 (90.0) -1(75.0) 73.33 3 -1 (80.0) +1(90.0) 70.30 4 +1 (90.0) +1(90.0) 64.58 5 -1.414 (77.9) 0 (82.5) 69.42 6 1.414 92.07) 0 (82.5) 63.62 7 0 (85.0) -1.414 (71.9) 60.00 8 0 (85.0) 1.414 (93.1) 66.90 9 0 (85.0) 0 (82.5) 65.60 10 0 (85.0) 0 (82.5) 60.20 11 0 (85.0) 0 (82.5) 64.19 12 0 (85.0) 0 (82.5) 69.70 13 0 (85.0) 0 (82.5) 64.50 a – Natural values are given in parentheses; b – Yield based on initial MMA 3.6 Statistical analysis 3.6.1 Calculation of response surface model The model parameters of the second-order response surface model was calculated by means of linear least squares regression. These calculations are briefly outline below. A model matrix, X, was first constructed by augmenting the design matrix β (Table 3.4) with a column, I, for the constant term ( 0 ), columns for the squared β β model parameters ( ii ), and columns for all two-factor interaction terms ( ij ) in the model. The model matrix, X, for the experiment will therefore be as shown in Table 3.4. (For this discussion, it will be assumed that the experimental settings of the reaction variables were exactly as specified by the design, i.e., either +1, or –1).
130
Table 3.4: Model matrix for the yield response 2 2 Run I X1 X2 X1X2 X1 X2 1 1 -1 -1 1 1 1 2 1 1 -1 -1 1 1 3 1 -1 1 -1 1 1 4 1 1 1 1 1 1 5 1 -1.414 0 0 1.9996 0 6 1 1.414 0 0 1.9996 0 7 1 0 -1.414 0 0 1.9996 8 1 0 1.414 0 0 1.9996 9 1 0 0 0 0 0 10 1 0 0 0 0 0 11 1 0 0 0 0 0 12 1 0 0 0 0 0 13 1 0 0 0 0 0 The overall calculation may be summarized by the matrix equation (eq. 3.2) (illustrated in full by eq. 3.3) Y = Xb + e (3.2) (y = the column vector of the experimental responses, X = the model matrix, b = the column vector of model parameters, and e = the column vector of error terms associated with each model parameter).
131
_ _
\ \
] ] Z
63.22 Z 1 −1 −1 1 1 1
] ] Z
Z − −
] ] Z
71.61 Z 1 1 1 1 1 1
] ] Z
68.63 Z 1 −1 1 −1 1 1
] Z ] Z
e
b
] ] Z
63.25 Z 1 1 1 1 1 1
` c k
β h
] ] Z
Z o
i f `
− c
64.08 1 1.414 0 0 1.9996 0 e1
] Z ] Z i
β f `
c 1
] Z ] Z i
66.21 1 1.414 0 0 1.9996 0 e2 f `
c β
] ] Z
Z 2
i f ` − c
69.25 = 1 0 1.414 0 0 1.9996 + e12 ] Z ] Z i
β f `
c 12 2
] Z ] Z i
67.17 1 0 1.414 0 0 1.9996 e f `
c β 2 1
] Z ] Z
j g `
c 1 2 a
64.83 1 0 0 0 0 0 d e
] ] Z Z β 2 2
2
] ] Z
64.83 Z 1 0 0 0 0 0
] Z ] Z
] ] Z
64.83 Z 1 0 0 0 0 0
] Z ] Z
Z Z ]
] 64.83 1 0 0 0 0 0
^ ^ [ 64.83 [ 1 0 0 0 0 0
( 3 . 3 )
The procedure used during the least square’s fit of the data is summarized by eq. 3.4 and illustrated in full in eqn’s 3.5 to 3.7. (X’X)-1X’Y = b (3.4) The second order response surface model calculated for the yield responses is summarized below (eq 3.8 – 3.9). In these models x1 = temperature and x2 = DIPEA loading.
132 q n o 1 −1 −1 1 1 1 l o − − l o 1 1 1 1 1 1 l o 1 −1 1 −1 1 1 l o l q n o 1 1 1 1 1 1 l o 1 1 1 1 1 1 1 1 1 1 1 1 1 l o l
o l 1 −1.414 0 0 1.9996 0 o − − − l
o 1 1 1 1 1.414 1.414 0 0 0 0 0 0 0 l o 1 1.414 0 0 1.9996 0 l o − − − l o 1 1 1 1 0 0 1.414 1.414 0 0 0 0 0 l X ’ X = o l 1 0 −1.414 0 0 1.9996 o − − l o 1 1 1 1 0 0 0 0 0 0 0 0 0 l o 1 0 1.414 0 0 1.9996 l o l o l
o 1 1 1 1 1.9996 1.9996 0 0 0 0 0 0 0 l
p m 1 0 0 0 0 0 o 1 1 1 1 0 0 1.9996 1.999 0 0 0 0 0 l o 1 0 0 0 0 0 l o l o 1 0 0 0 0 0 l o l o 1 0 0 0 0 0 l p 1 0 0 0 0 0 m
n q l o 13 0 0 0 7.999 7.999 l o l o 0 7.999 0 0 0 0 l o 0 0 7.999 0 0 0 l = o l o 0 0 0 4 0 0 l o l o 7.999 0 0 0 11.997 4 m p 7.999 0 0 0 4 11.997 (3.5)
133 q n q n
o 0.2 0 0 0 − 0.10001 − 0.10001 l o 1 1 1 1 1 1 1 1 1 1 1 1 1 l o l o − − − l
o 0 0.125 0 0 0 0 l o 1 1 1 1 1.414 1.414 0 0 0 0 0 0 0 l
o l o − − − l -1 0 0 0.125 0 0 0 1 1 1 1 0 0 1.414 1.414 0 0 0 0 0 (X’X) X’ = o l o l − − o 0 0 0 0.25 0 0 l o 1 1 1 1 0 0 0 0 0 0 0 0 0 l
o − l o l
o 0.10001 0 0 0 0.144 0.0187 l o 1 1 1 1 1.9996 1.9996 0 0 0 0 0 0 0 l p − 0.10001 0 0 0 0.0187 0.144 m p 1 1 1 1 0 0 1.9996 1.999 0 0 0 0 0 m q n
o −1.8E − 5 −1.8E − 5 −1.8E − 5 −1.8E − 5 1.8E − 5 1.8E − 5 1.8E − 5 1.8E − 5 0.2 0.2 0.2 0.2 0.2 l o − 0.125 0.125 − 0.125 0.125 − 0.177 0.177 0 0 0 0 0 0 0 l o l
o − 0.125 − 0.125 0.125 0.125 0 0 − 0.177 0.188 0 0 0 0 0 l = o l 0.25 − 0.25 − 0.25 0.25 0 0 0 0 0 0 0 0 0 o l − − − − − − − o 0.0625 0.0625 0.0625 0.0625 0.188 0.188 0.0625 0.0625 0.10001 0.10001 0.10001 0.10001 0.10001 l p m 0.0625 0.0625 0.0625 0.0625 − 0.0625 − 0.0625 0.188 0.188 − 0.10001 − 0.10001 − 0.10001 − 0.10001 − 0.10001
( 3 . 6 )
134
w t
u − 1.8E − 5 − 1.8E − 5 − 1.8E − 5 − 1.8E − 5 1.8E − 5 1.8E − 5 1.8E − 5 1.8E − 5 0.2 0.2 0.2 0.2 0.2 r u − 0.125 0.125 − 0.125 0.125 − 0.177 0.177 0 0 0 0 0 0 0 r u − − − r
u 0.125 0.125 0.125 0.125 0 0 0.177 0.188 0 0 0 0 0 r ( X ’ X ) - 1X’y = u 0.25 − 0.25 − 0.25 0.25 0 0 0 0 0 0 0 0 0 r u r
u 0.0625 0.0625 0.0625 0.0625 0.188 0.188 − 0.0625 − 0.0625 − 0.10001 − 0.10001 − 0.10001 − 0.10001 − 0.10001 r v s 0.0625 0.0625 0.0625 0.0625 − 0.0625 − 0.0625 0.188 0.188 − 0.10001 − 0.10001 − 0.10001 − 0.10001 − 0.10001
q n o 63.22 l o l o 71.61 l o 68.63 l o l } z o 63.25 l { b x o l o
64.08 { x o l
{ b1 x o 66.21 l { x o l b2 X 69.25 = { x (3.7) o l
{ b12 x o 67.17 l { 2 x o l { b1 x
64.83 | y o l 2 b2 o 64.83 l o l o 64.83 l o l o 64.83 l p 64.83 m
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The response surface model obtained for the yield of the reaction is: 2 2 Yield = 64.838+ 0.751x1 – 0.737x2 – 3.438x1 + 0.154x2 + 1.690x1x2 (3.8) 3.6.2 Statistical evaluation of results Several statistical tests have been developed to test if the obtained response surface model is indeed a good description of the actual response surface. Many of these tests have been applied to the response surface model given in eq. 3.8, and are described below. 3.6.2.1 Analysis of Variance (ANOVA) An important part of any experimental design is the analysis of variance. This is a powerful statistical technique that can be used to separate variances during the deliberate variation of experimental variables from pure experimental variance. The analysis of variance involves determining the sum of squares for each component in the model and the degrees of freedom associated with each sum of squares.31 To construct appropriate test statistics, the expected mean squares must also be calculated.
Calculation of the total sum of squares (SST) From the series of n experiments used to determine a response surface model, the sum of squared responses, called the total sum of squares (SST), can be calculated by obtaining the sum of the squared responses (eqn’s 3.9 and 3.10). The values
~ 2 obtained can be corrected for the mean by subtracting the quantity ( (xi yi ) / n ) to give the total sum of squares corrected for the mean. The total sum of squares is calculated with (n-1) degrees of freedom, where n is the total number of experiments in the design. = ~ 2 = SST yi y' y (3.9)
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SSTCorr = [65.302 + 70.302 + 73.332 + 64.582 + 60.002 + 66.902 + 69.422 + 63.622 + 65.602 + 60.202 + 64.192 + 69.702 + 64.502] – {(65.30 + 70.30 + 73.33 + 64.58 + 60.00 +66.90 + 69.42 + 63.62 + 65.60 + 60.20 + 64.19 + 69.70 + 64.50)2 /12} = 181.90 (3.10) Calculations of the sum of squares due to regression (SSR) The sum of squares due to regression is calculated by first calculating predicted responses for every experiment in the original design by substituting the individual model elements into the experimental response surface model (eq. 3.8), and then finding the sum of the squared predicted responses (eq. 3.11 and 3.12). The sum of squares due to regression is calculated with (p-1) degrees of freedom, where p is the number of parameters in the response surface model. As in the case of SST, the sum ( )2 of squares can be corrected for the mean by subtracting the quantity, xi yi / n . = ~ pred = SSR yi Xb (3.11) SSRCorr = [63.232 + 71.612 + 68.632 + 63.262 + 64.082 + 66.212 + 69.262 + 67.172 + 64.842 + 64.842 + 64.842 + 64.842 + 64.842] – {(63.23 + 71.61 + 68.63 + 63.26 + 64.08 + 66.21 + 69.26 + 67.17 + 64.84 + 64.84 + 64.84 + 64.84 + 64.84)2/12} = 76.02 (3.12) The sum of squares due to regression can be broken down into three components, namely the sum of squares resulting form the linear, interaction and squared components of the response surface model. The sum of squares due to the linear component of the regression may be found from eq. 3.13, the interaction component from eq. 3.14, and the squared component from eq. 3.15.
2 2
( ) Linear SS = xi y j / xi (3.13)
2 2 ( ) ( ) Interaction SS = xi y j yi / xi y j (3.14)
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Square SS = RSS – (linear SS + Interaction SS) (3.15) Calculation of the residual sum of squares (RSS) The residual sum of squares is calculated by finding the sum of the squared differences between the experimental responses and the predicted responses (eq. 3.16). Alternatively, the residual sum of squares can be obtained from the difference between SSR and SST (eq. 3.17). The residual sum of squares is calculated with (n-p) degrees of freedom, where n = the number of experiments and p = the number of parameters in the response surface model.
= ( Pr ed )2 RSS yi _ yi (3.16) RSS = SST – SSR (3.17) The residual sum of squares can be divided into two components, namely the sum of squares due to lack-of-fit and the sum of squares due to pure error. The residual sum of squares due to pure error is obtained as the sum of the squared differences between the experimental and predicted responses at the design center (eq. 3.18), while the residual sum of squares due to lack-of-fit is obtained as the sum of the squared differences between experimental and predicted responses for the factorial experiments. Alternatively, the residual sum of squares due to lack-of-fit may be obtained as the difference between the residual sum of squares (RSS) and the residual sum of squares due to pure error (eq. 3.19).
PureError = ( Pr ed )2 RSS yi0_ yi0 (3.18) RSSLack-of-fit = RSS – RSSPure Error (3.19)
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Statistical evaluation – significance of individual model parameters The significance of the individual model parameters in the response surface model can be estimated by either comparing the value of each parameter with its standard error, or by means of an F-test.32,33 The first method was used here. The standard errors of the individual model parameters are calculated by multiplying the mean square error (MSE) (eq. 3.20), with the dispersion matrix ((X’X)-1) (eq. 3.21) and finding the roots of the diagonal of the resulting product matrix (eq. 3.22). MSE = RSS / (n-p) = 181.9 – 76.02 (13-1-5) = 15.13 (3.20)
0.2 0 0 0 − 0.10001 − 0.10001
0 0.125 0 0 0 0
0 0 0.125 0 0 0
MSE = 15.13 x
0 0 0 0.25 0 0
−
0.10001 0 0 0 0.144 0.0187
− 0.10001 0 0 0 0.0187 0.144
− −
3.02 0 0 0 1.513 1.513
0 1.891 0 0 0 0
0 0 1.891 0 0 0
= (3.21)
0 0 0 3.783 0 0
−1.513 0 0 0 2.174 0.284
−1.513 0 0 0 0.284 2.174
Root MSE = S (β0) = 3.023 = 1.74
S (β1) = 1.891 = 1.375
S (β2) = 1.891 = 1.375 (3.22)
S (β12) = 3.783 = 1.945
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S (β11) = 2.174 = 1.474
S (β22) = 2.174 = 1.474
Statistical evaluation – Model The degree of fit of the experimentally determined response surface model is determined with an F-test by comparing the mean squares for lack-of-fit with the mean squares for pure error. For the model to give an adequate description of the actual response surface, the calculated F-value should be smaller than the critical F- value from the standard F-table for the corresponding degrees of freedom. The analysis described above is summarized in Table 3.5.
Table 3.5: Response surface Regression Yield Term Symbol Value Std Dev
Constant β0 64.84 1.74
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Mass DIPEA β1 0.75 1.38
Temperature β2 -0.74 1.38
2 (Mass DIPEA) β11 0.15 1.47 2 (Temperature) β22 1.69 1.47
DIPEA x Temp β12 -3.44 1.94 ANOVA Source DF SS MS F-value P Total 12 181.90 Regression 7 76.02 15.20 1.00 0.4792 Linear 2 8.85 4.43 0.26 0.7792 Square 3 67.14 22.38 1.48 0.3007 Residual Error 7 105.91 15.13 Lack of Fit 3 59.64 19.88 1.72 0.3005 Pure Error 4 46.26 11.57 3.6.2.2 Graphical analysis A fitted response surface model is normally used to give a good description of the actual response surface by means of an analysis of variance, but this is normally not sufficient. To be completely trustworthy, a response surface model should be able to withstand all possible diagnostic tests that could indicate inadequacies in the model. In this work, only the two most important diagnostic tests, namely a normal probability plot of the residuals, and a plot of residuals versus predicted response, will be considered.
Normal probability plot of residuals The statistical tests that have so far been carried out, were done with the assumption that the experimental data that was used was normally distributed. Normality of data can be verified by means of a normal probability plot of the residuals, since the residuals can be expected to be the result of random experimental error. Since random error is always normally distributed, a plot of the residuals on
141 normal probability paper should always give a straight line. Deviation from a straight line, like a curvature, can be interpreted as non-normally distributed experimental error. In such cases, care should be taken in drawing any conclusions from fitted response surface models. The residuals, calculated as the difference between the predicted and experimental responses, are shown in Table 3.6, while Figure 3.1 illustrates the normal probability plot of the residuals. The figure shows good normality.
Table 3.6: Calculated residuals Experimental Predicted Residuals responses responses e=Robs – R pred 65.30 63.23 2.07 70.30 71.61 -1.31 73.33 68.63 4.70 64.58 63.26 1.32 60.00 64.08 -4.08 66.90 66.21 0.69 69.42 69.26 0.16 63.62 67.17 -3.55 60.20 64.84 -4.64 69.70 64.84 4.86 65.60 64.84 0.76 64.50 64.84 -0.34 64.19 64.84 -0.65
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99
95 90 80 70 y t i l i 50 b a
b 30 o r
p 20 %
l 10 a 5
m r o N 1
-5.77231 -2.11792 1.53647 5.19087 8.84526
Residual Figure 3.1: Normal probability plot of residuals Plot of residuals versus predicted responses A second diagnostic test, which is always recommended, is to plot the residuals from one experiment against the predicted response from the same experiment.34 An adequate model should give a plot in which the points show a random scatter such that the upper and lower bands of the pattern in the plot form two parallel horizontal lines at approximately equal distance from the zero line. Certain features that can indicate problems with fitted response surface models include instances when: • The upper and lower bands diverge such that a pattern appears funnel-like. This is normally an indication that the experimental error differs in different parts of the experimental domain and, hence, the assumption of constant error variance is violated. • The upper and lower bands are parallel, but not horizontal. This is normally an indication of the presence of a systematic error. • The upper and lower bands are curved so as to give a pattern that results in an arch. This could indicate that a second-order term has been left out of the response surface model. Plot of the residuals against the predicted response is given in Figure 3.2.
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8.84526
5.19087 s l 1.53647 a u d i s e R
-2.11792
-5.77231
64.48 65.23 65.97 66.72 67.46
Predicted Figure 3.2: Plot of residuals versus predicted response The plot of predicted response versus residuals does not show any abnormality. 3.7 Analysis of the fitted response surfaces Having produced and evaluated the validity of a second-order response surface model, it becomes necessary to analyze this surface with the aim of finding the optimum, or stationary point, on the response surface. The stationary point is defined as the point where there will be no change in the response with a variation in any of ∂y the factor levels, i.e., the point where for any factor xi, the partial derivative = o . ∂xi For a second-order response surface model in two experimental variables of the type:
y= + x + x + x x + x 2 + x2 (3.23) 0 1 1 2 2 12 1 2 11 1 22
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Partial derivation with respect to x1 and x2, respectively, gives the following two simultaneous equations:
1 + 2 11x1 + 12x2 = 0 (3.24)
2 + 2 22x2 + 12x1 = 0 (3.25) Since the values of β1, β11, β2, β22 and β12 are known from Table 3.5, the position of the stationary point on the response surface can readily be determined. The experimentally determined response surface model (eq. 3.8) was: 2 2 y = 64.838+ 0.751x1 – 0.737x2 – 3.438x1 + 0.154x2 + 1.690x1x2 (3.8) Partial derivation with respect to x1 and x2, respectively gives:
0.751 – 6,876x1 + 1.690x2 = 0 (3.26)
-0.737 + 1.690x1 + 0.308x2 = 0 (3.27) These two simultaneous equations may be written in matrix format as in eq. 3.28:
6.876 1.690 X 1 0.751
= 1.690 0.308 X 2 0.737 (3.28)
Solving for x1 and x2 ( eq. 3.29) gives:
− −1
6.876 1.690 0.751 0.2969
= 1.690 0.308 0.737 0.7673 (3.29)
The values of x1 and x2 are in coded form and may be decoded using eq. 3.1 to give: x1: Temperature = 0.2969 x 5 + 85 = 86.48 x2: DIPEA loading = 07637 x 7.5 + 82.5 = 88.23
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The nature of the experimentally determined response surface model (eq. 3.8) is such that one of the squared terms is negative and the other positive. This is a clear indication of a saddle-shaped response surface. The three-dimensional plot of the response surface within the experimental domain investigated is shown in Fig. 3.3, clearly indicating the saddle-shaped nature of the response surface.
80 78
P 76
r
e d 74 i
c
t e 72
d
r
e 70
s p 68 o
n s 66 2 e
64 A 1 E 62 IP 0 D 60 f o 1 s -1 s 0 a -1 M Te -2 -2 mper ature Figure 3.3 Three-dimensional response surface for the MTSC yield
Optimum conditions for these types of response surfaces may be approached in two ways, namely by analyzing the canonical form of the response surface equation,33 or by means of graphical methods.33,34 The latter method was used as it allows one to obtain an idea of the actual shape of the response surface by plotting contour diagrams in two or three factors, the third being kept constant at its centre-point
146 value. Figure 3.4 shows the contour diagram obtained by this method for the MTSC yield.
70 72 68 66 62 64 1 70 68 66
64 66 e r u t a r 66 e 0 64 p
m 66 e
T 68 64
66 68 70 72 -1 62 70 74 64 68 66 72 76 78 -1 0 1 Mass of DIPEA
Figure 3.4: Contour diagram for the MTSC yield
The contour diagram shown in Figure 3.4 confirms the existence of a saddle-like response surface, and it also shows very clearly, that optimum conditions must be sought towards the two high corners of the saddle-like surface. Thus, higher yields can be obtained using a high DIPEA loading in combination with a low temperature or
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with a high temperature and a low DIPEA loading. This graph shows that both temperature and DIPEA loading are of essential importance to the MTSC yield. 3.8 Confirmatory Experiments Table 3.7 summarizes the results of a number of experiments carried out in order to prove the reaction condition closer to and at the optimum.
Table 3.7: Reaction conditions closer to the optimum DIPEA loading Reaction temp yield yield HPLC Isolated 90 g 60 0C 7.36% 90 g 80 0C 40.90% 100 g 90 0C 73.83% 74.10% 100 g 90 0C 66.27% 67.03% 100 g 90 0C 64.75% 88 g 87 0C 82.26 %* * At the predicted optimum 3.9 Discussion 3.9.1 Statistical evaluation of individual response surface models In order for a fitted response surface model to adequately describe the actual response surface, the residuals at the design points (obtained by subtracting the predicted responses, using the fitted response surface model, from the experimental responses) should not be significantly larger than the residuals at the replicated center points. In addition, the variation in the predicted responses at the design points should be adequately described by the fitted response surface (i.e. the regression). The variation can be evaluated by means of F-test, and in order for the fitted response surface models to be good, the F-values calculated for “regression” and “lack-of-fit” should not exceed the critical F-values for the corresponding number of degrees of freedom. Therefore we have:
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MS Re gress Re gress 15.20 F7,7 = = = 0.993 (3.30) MSRe sidual 15.30
lof MSlof 19.88 F3,4 = = = 1.718 (3.31) MSpe 11.57 The P-value for “regression” in the ANOVA table should be as small as possible (at P = 0.05, the confidence level that the model is good will be 95%), while the P-value for “lack-of-fit” in the ANOVA table should be as high as possible. From the analysis summarized in Table 3.5, it can be deduced that the fitted response shows a significant lack-of-fit, as the F-values are lower than the tabulated F-values for the above mentioned degrees of freedom. Therefore the fitted response surface model falls within the 95% confidence level. The observed lack of fit of the experimental response surface is most likely the result of the lack of leak* in the reactor discovered during confirmatory experiments. Nevertheless, the observed trends were such that the experimental yield, albeit an unisolated yield could be increased significantly compared to those in the design experiments. These experiments also clearly show the importance of mixing during this reaction where a viscous proton-acceptor (DIPEA) has to be mixed intimately with an aqueous solution of reagents. *The stirrer rod entered the reaction vessel through a seal which became worn with use and may have permitted leakage of a portion of the vaporized reaction mixture.
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Chapter 4
WASTE TREATMENT PROTOCOL
4.1 History of waste management As far back as 8000 to 9000 B.C., people learned to dispose of their waste outside of their own settlements.37 Waste was composed of food scraps, mussel shells, bones, broken household items, and clay shards. It may be assumed that those people established dump sites to escape the nuisances of vermin, odour and wild animals. In many cities in Europe and Asia, waste was collected in clay containers and hauled away, while in other areas, pits were used to collect waste and faeces, which were emptied and cleaned periodically. There are also records of regulations (Athens, 320 B.C.) for the daily sweeping of streets by residents, even though at that time the relationship between hygiene and the scourges of mankind such as the plague, smallpox, cholera and the like, were not known.37 Physicians such as the Greek scholar Hippocrates (around 400 B.C.) and the Arab Avicenna (around 1000 A.D.) were perhaps the first to suspect the link between hygiene, contaminated water, spoiled food and epidemics.37 Not until the 15th century did city councils require the paving of streets, so that no one would have to wade through faeces and waste. After that, garbage containers were introduced, the streets were cleaned regularly, animal carcasses were collected, etc. Between 1850 and 1890, a breakthrough in “Waste management” occurred when researchers revealed bacteria and viruses as the cause of diseases. Their research demonstrated that the spread of diseases could be controlled by the presence or absence of public health measures. The need for public health measures was dramatically highlighted in 1892 in the city of Hamburg, when about 9000 people fell victim to a cholera epidemic. The city had been pumping water contaminated with waste and faeces from the Elbe River into the city’s water supply system, encouraging the spread of the epidemic.37 To this day, the direct relationship between environmental hygiene and life- threatening diseases is observable in the developing world. Wherever waterways are used for waste disposal, where people are forced to eke out a living in garbage dumps, and where sanitary waste disposal, water and sewage treatment are nonexistent because of natural catastrophes, wars or underdevelopment, epidemics can spread virtually unchecked, even today.
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4.2 Waste treatment technologies 4.2.1 Wastewater treatment Development of an effective wastewater treatment system for an industrial plant involves characterizing all of the plant’s wastewater streams and evaluating the treatment needs of each stream with respect to the applicable regulatory standards.35 Waste streams requiring the same type of treatment are combined and treated as composite streams. This approach improves the cost-effectiveness of the overall treatment scheme. 4.2.1.1 Wastewater characterization Waste water can be characterized based on its bulk organic parameters, physical characteristics and specific contaminants (Table 4.1):35
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Table 4.1 Characterisation of wastewater contaminants and their corresponding hazards Parameter Concern Bulk organic TOC (Total Organic Carbon) Can be toxic; depletes oxygen COD (Chemical Oxygen Can be toxic; depletes oxygen Demand) BOD (Biological Oxygen Depletes oxygen in receiving waters Demand) Oil & Grease/TPH (Total Petroleum Damages vegetation and wildlife Hydrocarbons) Physical TSS (Total Suspended Solids) Turbidity; toxic to aquatic life pH Acidity or alkalinity is toxic to aquatic life Temperature Harmfull to aquatic life Color Aesthetic Odor Can be toxic to aquatic life and humans; aesthetic Redox potential Can be toxic to aquatic life Contaminant Specific
NH3/NO3 Toxic to aquatic life; eutrophication Phosphates Eutrophication Heavy metals Toxic to aquatic life and humans Surfactants Toxic to aquatic life and humans; aesthetic Sulfides Toxic to aquatic life and humans; aesthetic Phenol Toxic to aquatic life and humans; aesthetic Toxic organics Toxic to aquatic life and humans Cyanide Toxic to aquatic life and humans Table 4.2 Typical industrial wastewater effluent limitations35
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Parameters Concentration (mg/L) COD 300 – 2000 BOD 100 – 300 Oil&Grease 15 – 55 TSS 15 – 45 pH 6.0 – 9.0 Temperature Less than 40 0C Color 2 color units
NH3/NO3 1.0 – 10 Phosphates 0.2 Heavy Metals 0.1 – 5.0 Surfactants (total) 0.5 – 1.0 Sulfides 0.01 – 0.1 Phenol 0.1 – 1.0 Toxic Organics (total) 1.0 Cyanide 0.1 4.2.2 Waste treatment Waste treatment is generally classified into three major levels.36 Each treatment level is aimed at removing a more specific class of contaminants: 1) Primary treatment involves simple physical processes that remove suspended solids and entrained oils from a waste stream. It is generally regarded as preparation for further treatment although it can result in the removal of by-products and reduction of the quantity and hazard of the waste. 2) Secondary treatment is designed to remove soluble material from waste streams that cannot be removed by simple physical means. Secondary treatment detoxifies, destroys and/or removes hazardous constituents. 3) Tertiary and quaternary treatment is used to polish an effluent and to remove a specific contaminant that was not removed in the first two steps. Polishing usually refers to treatment of water that is removed from wastes so that it may be safely discharged. Major phases of waste treatment:36
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PrimParirmy atrreya tmreeanttment Dissolution Blending Disposal, resource, recovery Slurrying Phase separation
Secondary treatment
Neutralization Oxidation/Reduction Precipitation Adsorption Extraction Emulsion breaking Coagulation Disposal, resource, recovery Sedimentation Centrifugation Air flotation Sludge dewatering
Polishing
Filtration Discharge, recycling Activated carbon sorption ecycling Reverse osmosis
Fig 4.1: Major phases of waste treatment 4.2.3 Treatment technologies There are many ways to define waste treatment. One of the definitions mentioned in the literature describes waste treatment as follows: ‘waste treatment is any method, technique or process that changes the physical, chemical or biological character of any hazardous waste in a way that neutralizes the waste, or renders such waste non-hazardous, less hazardous, safer to manage, able to be recovered, able to be stored or reduced in volume.37 This definition makes mention of three classes of treatment technologies, but in fact there are four classes. These are physical treatment, chemical treatment, biological treatment and thermal treatment.35,36 Each of the treatment strategies are discussed in more detail below.
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4.2.3.1 Physical treatment Physical treatment processes bring about a physical change in the properties of the contaminants while the chemical nature of the compounds remain unaffected.1 Physical treatment of waste depends upon the physical properties of the material treated. These properties include state of matter, solubility in water and organic solvents, density, volatility, boiling point and melting point. Knowledge of the physical behaviour of wastes has been used to develop various unit operations for waste treatment that are based upon physical properties.36 The physical treatment processes typically employed in industrial wastewater treatment are gravity separation, air flotation, oil coalescence, evaporation, filtration, activated carbon adsorption, air or steam stripping and liquid/liquid extraction. 4.2.3.2 Chemical treatment The applicability of chemical treatment to wastes depends upon the chemical properties of the waste constituents. These properties include pH, reactivity, precipitation and complexation behaviour, redox potential, flammability/combustibility, corrosivity and compatibility with other wastes.36 The chemical treatment processes most often employed in industrial wastewater treatment are chemical precipitation and coagulation, electrolytic recovery, ion exchange, reverse osmosis and chemical oxidations or reductions.35 Chemical reactions of various types are used for the treatment and destruction of hazardous wastes. An attractive feature of chemical treatment is the opportunity to treat wastes with other wastes. 4.2.3.3 Biological treatment Biological treatment processes utilize biological and biochemical mechanisms to bring about a chemical change in the properties of the contaminants of interest. The chemical properties are changed under the action of a wide variety of microorganisms to cause the decomposition of the compound within the bulk waste stream. The major objective of biological treatment is to stabilize the organic matter in a waste stream so that biological degradation does not occur in the wastewater distribution system or in the receiving water body. The ideal end products of microbial degradation are more cell mass, carbon dioxide, water, halides, elemental nitrogen and sulphur, heat and excess energy. The biological treatment processes typically encountered in industrial wastewater treatment include activated sludge processes, aerated lagoons or stabilization ponds, trickling filters or fixed-film reactors and anaerobic processes. 4.2.3.4 Thermal treatment Thermal treatment processes utilize elevated temperatures to bring about the decomposition of contaminants. Thermal treatment of hazardous wastes can be used to accomplish most of the common objectives of waste treatment. For example, a reactive, combustible material can be burned to produce a much lower volume of ash with recovery of heat from the waste. Volatile, mobile organic
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materials can be eliminated and toxic compounds and pathogens can be destroyed. The thermal treatment processes employed in industrial wastewater treatment are wet-oxidation, supercritical oxidation and liquid injection incineration. 4.2.4 Treatment strategies used on MTSC effluent The MTSC effluent obtained after the MTSC synthesis is environmentally unfriendly and has to be treated before it can be disposed of. This section describes the treatment procedures used to treat the effluent. The treatment involves reducing the chemical oxygen demand (COD) value of the effluent, because the COD value is too high for safe disposal. COD (Chemical Oxygen Demand) The Department of Water Affairs and Forestry, South Africa,38 defines COD as a measure of the oxygen equivalent of the oxidizable matter in a sample that is susceptible to oxidation by a strong chemical oxidant. The COD measurements, therefore, represent everything in the sample that can be oxidised. It is, however, non-specific and does not indicate what proportion is biodegradable, nor does it distinguish between organic and inorganic matter. 4.2.4.1 Cooling Procedure Preliminary investigations showed that by cooling the effluent down to a temperature of zero degrees (or lower), a precipitate is formed, containing MTSC and some by- products formed during the MTSC production process.43 The COD value of the effluent is expected to decrease because of the formation of the precipitate. The explanation for the decrease in COD value of the effluent lies in the definition of the COD: the measure of the oxygen equivalent of the oxidisable matter in a sample. This means that all oxidizable compounds dissolved in the effluent are responsible for the COD value. When the effluent is cooled, a certain amount of the compounds dissolved in the effluent precipitate out of this solution. These precipitated compounds now do not contribute to the COD value any longer and so the COD value drops. Theoretically, the greater the mass of precipitate formed by cooling the effluent, the more the COD value of the effluent will drop. 4.2.4.2 Wet Air Oxidation Wet air oxidation refers to the aqueous phase oxidation of dissolved or suspended organic and inorganic substances at elevated temperatures and pressures.39 The oxidation process requires oxygen from compressed air or pure oxygen as the oxidising agent. The enhanced solubility of oxygen in aqueous solution at elevated
156 pressure (even though temperature is high) provides a strong driving force for oxidation.39 Typical operating pressures for the wet oxidation system vary from 100 to 3000 psig (690 to 20700 kPa),39 and are dependent on the operating temperature. Wet air oxidation systems that are used for industrial wastewater treatment generally operate at temperatures ranging from 150oC to 320oC. 39 4.3 Problem description Dow Agrosciences produced 4-methyl-3-thiosemicarbazide (MTSC) as an intermediate for the production of herbicides. The production process of the MTSC used ammonia, resulting in large amounts of ammonium salts that could not be recovered after the reaction, resulting in five tonnes of waste being produced per tonne of product. An alternate, recoverable base, diisopropylethylamine (DIPEA) was then used for the reaction. This resulted in a fourfold decrease in the amount of waste produced. Although the waste production was decreased four times, the waste was more concentrated, because a lot of liquid was removed with the recovery of the base, but not the actual waste products. Serious waste disposal problems resulted, because the COD value of the effluent has to be less than 1000 ppm to comply with the water regulations of Water Act 54 of 1956 (amended 1996). The COD value of the MTSC effluent was 240000 ppm. Other compounds such as sulphide also have maximum permissible concentrations prior to their disposal into the Sasol waste stream. The restriction for compounds such as sulphides, or substances from which hydrogen sulphide can be formed is 5 mg/L. After introducing this new base, the effluent generated contained 5 - 8% MTSC, which also meant a substantial loss of product. Analysis of the effluent showed total sulphur ranging from 7 - 9%, total sulphide 0.3 - 0.35 %, and a COD value of 24%. The effluent, besides being wasteful of product, is also environmentally unacceptable (high total [S], [S2-], and COD). Although the effluent generated using this process was free of ammonium salts, the high COD and sulphur content presented an
157 environmental disposal problem. In light of the above, objectives were set to clean up the process and make it environmentally friendly. 4.3.1 Waste Treatment Objective The main waste treatment objective was to produce environmentally acceptable effluent by cleaning up the effluent, i.e., decreasing the COD value in the effluent, removing sulphide and reclaiming product. 4.3.1.1 Strategy used to achieve the objective - Cooling the effluent and maintaining a low temperature in order to precipitate solids from the solution and to determine the optimum working temperature and the optimum cooling time would possibly lower the COD value. - Wet air oxidation would possibly further reduce the COD level of the effluent. A high-pressure reactor would be used to determine the optimum temperature, pressure, oxidising agent and perhaps catalyst for the oxidation reaction. 4.4 Experimental 4.4.1 Cooling Method The cooling of the MTSC effluent was performed using crushed ice to cool the effluent down and maintain a constant low temperature. With this low temperature, the solubility of the compounds in the effluent decreased, causing some of them to precipitate out. The compounds in the precipitate contribute to the COD value causing a decrease in COD value of the solution. The precipitate was recovered using vacuum filtration, and analysed by means of HPLC analysis. MTSC, DMTU and TCH standards were used to determine the exact composition of the precipitate. By means of GC-MS, IR and UV-VIS spectrometry, the composition of the precipitate was also verified. The remaining solution after removal of the precipitate was analysed for COD. 4.4.1.1 Set-up of cooling system A 30 litre cooler box was filled with crushed ice. Seven sample bottles, each containing 100 mL of effluent, were cooled in the cooler box together with one larger bottle containing 500 mL of effluent. The ice in the cooler box was refilled daily to maintain a constant temperature. The sample bottles remained in the cooler box until they were removed from the box to be analysed. The entire cooling procedure lasted ten days.
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Sampling: One small sample bottle was removed daily from the cooler box. A precipitate of yellow crystals had formed a cake of interlocked crystals in all the bottles. It was broken up and vacuum filtered through a 30 mL, porosity 4 vacuum filter crucible. The precipitate was dried in a vacuum oven for three hours at 60oC, and weighed to constant mass. Daily, a 1.00 mL aliquot of effluent was pipetted out of the 500 mL sample bottle into a 50.00 mL volumetric flask and made up to the mark with de- ionised water. A 1.00 mL aliquot of this diluted solution was used to determine COD values.
The rationale behind this methodology was that the samples had to be cooled independently of one another, to prevent interference due to volume change as a result of aliquot removal. To prevent complications, a 500 mL bottle was used for COD determinations and small aliquots, 1.00 mL, were removed from the bottle to study the change in COD each day. The small bottles containing 100 mL of effluent were only used to monitor precipitation. Every sample precipitated independently of one another, thus they could not influence each other. 4.4.1.2 Analysis of MTSC effluent COD of supernatant The mother liquor (remaining after the filtration) was diluted by pipetting 1.00 mL into a 50.00 mL volumetric flask and making up to volume with de-ionised water. A volume of 1.00 mL of this diluted solution was used for the COD determination. The sample taken out of the large bottle was analysed for its COD in the same manner. HPLC Characterisation of the precipitate formed on cooling Each of the seven precipitate samples were analysed for MTSC and reaction by-products that may have formed in the production process. A mass of 0.100 g of each precipitate was weighed into 250.00 mL volumetric flasks and made up to volume with de-ionised water. A standard solution was prepared by weighing 0.100 g of each of the by-products into a 250.00 mL volumetric flask and made up with de-ionised water. The standard and the samples were injected three times. The standard deviation8 was calculated using the peak areas of the chromatograms. The concentrations of unknowns were determined by direct comparison of peak areas of standards and unknowns. The samples and standards were stored at 3oC in a refrigerator in order to prevent decomposition (without cooled storing, the standards changed colour and some additional peaks appeared on the chromatogram).
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4.4.2 Oxidation method Initial experiments involving the oxidation of the MTSC effluent sample42,43were performed. The sample was heated in a round bottomed flask and oxygen/air was bubbled through. The results of these experiments looked promising since the COD value of the effluent decreased substantially. These initial results were obtained at atmospheric pressure, because the apparatus would only allow for atmospheric pressure. According to these experiments, higher temperatures and pressures would reduce the COD value and convert some organic sulphur (and sulphide) to either 2- elemental sulphur, SO2 or SO4 . Oxidation of the effluent was then performed using a Parr high pressure reactor. To this reactor was connected a gas cylinder containing air or oxygen with a regulator that was capable of delivering a maximum pressure of 700 Kpa. On completion of the oxidation in the reactor, a ‘stone-like’ precipitate was formed, and a melting point analysis was performed to confirm the assumption of its constitution. The remaining solution was analysed for COD.
4.4.2.1 Reactor features Wet oxidation was performed on a Parr 4522 Bench Top High Pressure reactor with a 4841 controller. This reactor contains a 2000 mL glass-lined reaction vessel. The temperature upper limit is 350 oC and the maximum pressure is 14000 KPa. The following reaction conditions were used for oxidation: • Temperature: 120 oC • Pressure: 7 bar • Time: 5 hrs 4.4.2.2 Oxidation reactions A 200.00 mL effluent sample was pipetted into the vessel. The reactor vessel was lowered down onto the heating mantle and the reactor closed. The heater was switched on and the temperature controller was set to 120 oC. A pressure of 700 KPa was applied through the regulator using the pure (medical grade) oxygen, this being the maximum pressure permitted through the regulator. A mechanical stirrer was used to stir the solution at a constant speed during the five-hour reaction. The stirrer was cooled by tap water running constantly inside the stirrer tube. After 5 hours, the reactor was switched off and left to cool overnight. The sample was
160 poured out of the reactor vessel into a 250.00 mL volumetric flask and diluted up to volume with de-ionised water. The sample was analysed for COD. The oxidation reactions were initially performed using air as the oxidant. Because air only contains 20% oxygen, the air was replaced with pure oxygen to prove the assumption that oxygen would have a higher impact on the oxidation. 4.5 Results 4.5.1 Cooling procedure The MTSC effluent was cooled for five days in a cooler box using ice water to maintain a constant temperature. The decrease in COD value was monitored to determine the time necessary for cooling. Figure 4.2 depicts the decrease in COD value during the cooling process. The first day of cooling had the largest effect, where the COD value fell to almost half of its initial value. This reduced a little more after the next three days of cooling, where the COD value was 10.22% and where it remained constant. Figure 4.3 shows a cooling curve of eight days, where the same trend was observed. The first day showed the largest decrease in COD, and after day 2, only small decreases were seen. In both cases, the ultimate COD value was less than half its initial value, falling from over 24% (240 000 ppm) to 12% or lower.
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Effect of cooling on COD value
30
25
) 20 % (
D 15 O
C 10
5
0 0 1 2 3 4 5 Days
Figure 4.2: Decrease in COD value after cooling the MTSC effluent for 4 days
Decrease of COD value in effluent
30 25
) 20 % (
D 15 O
C 10 5 0 0 2 4 6 8 10 Days
Figure 4.3: Decrease in COD value after cooling the MTSC effluent for eight days The precipitate obtained after filtration of the cooled samples was analysed by HPLC to determine its composition. It was observed that approximately 83% of the solid
162 was MTSC. Also small amounts of DMTU, TCH and TSC were observed which accounts for the other 17 percent. 4.5.2 Oxidation of MTSC effluent Both the cooled effluent (treated) and the untreated effluent were used for the wet air oxidation in the Parr reactor. Experiments were performed using air and oxygen as the oxidant. Table 4.3 shows the results obtained after five hours of oxidation at a pressure of 700 KPa and 120oC.
Table 4.3: Oxidation of effluent under different conditions Sample Oxidant Treated/untreated Initial COD value COD effluent COD after reduction value oxidation (%) (%) (%) 1 Air untreated 24.23 11.23 53.7 2 Air treated 10 8.5 15.0 3 Oxygen untreated 24.23 6.35 73.8 4 Oxygen untreated 24.23 6.20 74.4 5 Oxygen treated 12.20 <0.5 95.9 6 Oxygen treated 12.20 0.4 96.7 It is apparent from the results that the use of oxygen is highly favoured over the use of air. The air oxidation of treated effluent has a less significant effect (15% COD reduction) than the oxygen oxidation of the same effluent (96%). These results additionally imply that when the effluent is treated with both cooling and oxidation with oxygen, a total COD reduction of 98 % can be obtained, which assumes that almost every compound present in the effluent has either been removed or oxidized. Once the oxidation of the effluent in the Parr reactor was complete, a porous, yellow, stone-like precipitate was obtained. The precipitate was analysed using X-Ray Fluorescence (XRF) and Differential Scanning calorimetry (DSC). The XRF spectrum showed the main component to be sulphur. The melting point of the obtained precipitate was 109oC, while the DCS scan showed a melting point at 112 oC. The melting point of sulphur is 112.8oC.46 The substance deposited in the reactor vessel was therefore impure elemental sulphur.
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4.6 Discussion 4.6.1 Cooling The cooling of the effluent was a useful method to significantly reduce the COD value of the effluent. The precipitate obtained during the cooling process contained 84 % MTSC, implying that a significant amount of product was lost in the waste. The remaining material was confirmed to be all or mostly by-products (TCH, DMTU and TSC) of the MTSC synthesis.
To confirm the obtained HPLC results, a comparison was made between the precipitate obtained, and pure MTSC by means of IR and UV-VIS spectrometry. The IR spectra of the precipitate formed during cooling of the effluent, and the pure MTSC showed no significant difference. UV-VIS scans showed two identical spectra when a wavelength scan was performed on both the precipitate and the pure MTSC and the two absorption peak wavelengths were the same, indicating that the same major functional groups were present. 4.6.2 Oxidation of the sample The final reduction in the COD value was 98 %, but further reduction may be possible. The regulator had pressure limitations, so reactions could only be done with a maximum pressure of 7 bar. The pressure automatically increased as the temperature inside the reactor rose above 100 oC. The temperature could therefore not be raised higher than 120oC as the pressure at that temperature was already 700 KPa. At this pressure, no more oxygen could be added to the reactor either, as the pressure inside the reactor was the same or maybe even higher than the pressure from the regulator. Pressure was thus dependent on temperature. 4.6.3 Economic considerations Before the start of this project, the effluent obtained from the production process was diluted to obtain its COD value below the required level of 1000 ppm. Three tonnes of MTSC waste per day were diluted into 11000 tonnes of total waste per day. Dilution is relatively cheap, but it is of course more environmentally acceptable to
164 clean up the effluent, prior to dilution. It may be an advantage, from a public relations point of view, to clean up the effluent as much as possible and then to publish this achievement. Naturally the cost of the clean up would have to be taken into account.
The COD requirement for the effluent waste to be disposed of in the Sasol waste stream is 1000 ppm. The results obtained from this research approach this value, reaching 4000 ppm (0.4%). It is now possible for Dow Agrosciences to dilute and dispose of it. It may be financially advantageous to use only the cooling step and then dilute the effluent. Figure 4.4 summarizes the entire process in a mass balance, with the results achieved for the COD value and all other values. After diluting three tonnes of MTSC waste in 11000 tonnes of total waste stream, the COD value of the total waste stream was 1051 ppm. The results obtained from the treatment investigation were then projected onto the total Sasol waste stream and this is shown in Figure 4.5. The following possibilities are hence a summary of these findings: 1. Cooling affords a 49.6% reduction on the MTSC waste and a 3.5% improvement in COD of the total Sasol waste stream. 2. Wet oxidation using air at 7 bar and 120oC during a five hour reaction results in a 3.2% improvement in COD of the total Sasol waste stream. The same reaction using oxygen instead of air will result in a 74.4% reduction in COD on the MTSC waste and a 4.5% improvement in the COD of the total Sasol waste stream. 3. A combination of cooling and oxidation provides the best results. The improvement in COD of the MTSC waste is 98% and that of the total Sasol waste stream is 6%. The maximum improvement possible, i.e. a complete removal of COD of the MTSC waste, results in a 6.1% improvement of the total waste stream.
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4.6.3.1 Product recovery The amount of MTSC in the precipitate obtained by cooling was approximately 83%. If this MTSC could be purified after cooling, the mass of MTSC recoverable per day would be 70.5 kg, and the total recoverable mass of MTSC per year would then be 26 tonnes. The market value of MTSC is $9.50/kg, which means that 26 tonnes of MTSC is worth R1.48 million.
The amount of sulphur obtained during the oxidations was 14.6 kg per 1000 L effluent; the total recoverable mass of sulphur per year would then be 16 tonnes. The retail price for sulphur is R2/kg. This sulphur may be sold for R 32 000 a year. These economic benefits resulting from product recovery are also shown in Figure 4.5. It should be mentioned that no consideration has been given to the costs that have to be made to perform the treatment (cooling and oxidation) of the effluent.
MTSC effluent (100 g esstt). )e st) COD = 24.23%
Cooling
Solid Liquid (95 g est.) Mass = 4.5 g e st. COD = 12.20 %
HPLC Pressure oxidation with O2
MTSC = 83.3 % (3.75 g e st.)
TCH = 5.40 % (0.24 ge st.) Liquid Solid DMTU= 10.1( 0.45 g) Mass = 93.0 g e st. Mass = 1.43 g e st. % est.)
COD = 0.4 % S ulphur r
Fig 4.4: Flow diagram summarising results of treatment investigation est = estimated – based on the waste having a density of 1.00 g/L, though it is slightly higher
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Overall view
3 tonnes of MTSC effluent is diluted in 11000 tonnes of waste. The COD value of this total waste stream is 1051 ppm
Air: 3.2 % MTSC Cooling 3.5 % improvement in improvement in Wet COD COD effluent 26 T MTSC per Year O2: 4.5 % Ox. improvement in Cooling Wet oxidation COD Financial 13 t sulphur @ COD = ? R2/kg 6 % improvement in MTSC = R1.5m COD 16 t Sulphur @ R2/kg
Financial (O2) Energy = ? COD = ? Financial O = ? Sulphur = R26500 COD = ? 2 MTSC = R1.5 m Equipment = ? Sulphur: R32000
Figure 4.5: Environmental impacts and economical benefits from waste treatment protocol
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4.7 Conclusions A method has been developed to decrease the COD value of the MTSC effluent. This method is comprised of two steps. The first step is to cool the effluent to 0oC in ice-water. This decreases the COD value from 24% to 12.20%. After the cooling, a wet oxidation with pure oxygen in a high pressure reactor may be performed, and the final result is a COD value of 0.4%, a 98% reduction of the initial value. Dow Agrosciences may use these results to improve the COD value of their effluent and, in so doing, develop a reputation for being environmentally aware and concerned. In monetary terms, the process may only be cost-effective if the cooling step alone was carried out followed by dilution and disposal into the Sasol waste stream. Furthermore, product recovery from the precipitate formed during cooling, and containing approximately 84% of MTSC, may also be considered. The MTSC may then be recovered and purified and the by-products removed.
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CHAPTER 5 SUMMARY AND CONCLUSIVE REMARKS 5.1 Treatment versus waste minimization Several technologies for the treatment of waste are available: • Physical treatment; to remove, separate and concentrate hazardous and toxic materials. • Chemical treatment; to assist in the application of the physical treatment, and to lower the toxicity of a hazardous waste by changing its chemical nature, for example, neutralization of an acid. • Biological treatment; microbes are used to destroy or at least reduce the toxicity of a waste stream. • Immobilization, solidification and encapsulation; the prime objective is to convert the hazardous waste into an inert, physically stable mass. • Incineration; this is an option for both treatment and disposal. These different treatment technologies are used to solve the problem of high concentrated wastewater or toxic and hazardous by-product and effluent streams from industry. Waste treatment however only solves the problem of the specific waste water stream, but it doesn’t solve the problem at the cause. Waste minimization however deals with the problem at the source as can be seen by the waste minimization procedure mentioned before summarized into the following phases: 5. planning and organization phase 6. assessment phase 7. feasibility analysis phase 8. implementation phase
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The procedures which exist in a waste minimization program cover a very broad area.9 This procedure includes employee training, changing current methods, preventing spills and inventory control Some control methods that can be implemented are: • Minimizing the number of raw materials, supplies used and other materials such as cleaning fluids, oils etc. • Reducing the inventory of hazardous materials to a minimum, and ensuring that oldest materials are used first to prevent these exceeding their shelf-life. While waste treatment is necessary to treat the effluent if the concentration of certain compounds is too high, it can be considered unnecessary if a waste minimization is established. As waste minimization deals with all kind of factors throughout the process, it will clean up the process and it will save a lot of money as the use of certain chemicals will be reduced and the total amount and toxicity of the effluent will be reduced. Therefore no additional money has to be spend on a treatment procedure as additional costs to the production process. 5.2 Waste minimization The evaluation of the synthesis of 4-methyl-3- thiosemicarbazide on lab-scale has shown a number of interesting points: • Multi-factorial analysis showed that the two significant variables for the optimum conditions for the MTSC synthesis were the excess of DIPEA used and the temperature of the second reaction. Optimum conditions must be sought towards the two high corners of the saddle-like surface in the 3d graph (see figure 3.3). Thus, higher yields can thus be obtained using a high DIPEA loading in combination with a low temperature or with a
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high temperature and a low DIPEA loading. These graphs (figure 3.3 and 3.4) show that both temperature and DIPEA loading are of essential importance to the MTSC yield. A yield of 64.66 % was predicted by the statistical design under optimum conditions. • Practically, the predicted yield could easily be obtained. Due to a leak during the reactions, the yield was not as high as expected. A mass of on average 30 grams was lost during the reactions. As a result, the yield could not be at its optimum as the reagents needed for the production of MTSC, might have disappeared during the reaction. The highest yield obtained during the reactions was 82.26%. The chromatograms however, indicated that a yield of 90 % should be obtained, as the amount of by-products is minimal, as shown by the relative peak areas. It is assumed that no starting material is left after the reaction, which indicates that a much higher yield can be obtained if all the initial begin products take part in the reaction. 5.3 Waste treatment A method has been developed to decrease the COD value of the MTSC effluent. This method is comprised of two steps: • The first step is to cool the effluent to 0oC in ice-water. This decreases the COD value from 24% to 12.20% as a precipitate occurs, that consist mostly of MTSC still present in the effluent. Thus, product recovery from the precipitate formed during cooling, and containing approximately 84% of MTSC, may also be considered. The MTSC may then be recovered and purified and the by-products removed. • After the cooling, a wet oxidation with pure oxygen in a high-pressure reactor may be performed, and the final result is a COD value of 0.4%, a 98% reduction of the initial value. During this reaction a precipitate containing mostly sulphur is recovered. Product recovery could again be considered and the sulphur might be purified.
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Dow Agrosciences may use these results to improve the COD value of their effluent and, in so doing, develop a reputation for being environmentally aware and concerned. In monetary terms, the process may only be cost-effective if the cooling step alone was carried out followed by dilution and disposal into the Sasol waste stream. 5.4 Overall conclusion The optimum solution would be a combination of waste minimization and waste treatment; a high yield of MTSC and an environmentally friendly effluent. Optimally there should be essentially no MTSC in the effluent due to the optimized synthesis, resulting in a cleaner effluent as the MTSC is predominantly responsible for the high COD of the effluent. Therefore it is very important that the synthesis of MTSC be carried out in a sealed reactor, resulting in all the starting material taking part in the reaction and therefore a higher yield. An isolation step needs to be developed to isolate all the MTSC out of the reaction mixture, as the amount of MTSC still present in the effluent indicates that the isolation step is not very efficient. A combination of the isolation used during the synthesis and the isolation used during effluent treatment might be an option although it will be time consuming. If all the MTSC is isolated from the reaction mixture an essentially clean effluent will be left behind and minimal treatment will be required prior to disposal. Implementation of the above mentioned steps would result in an environmentally friendly process with high MTSC yeilds.
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