ö’3 The Mechanisms for Free Chlorine Oxidation of Reduced Manganese in Mixed—Media Filters by Suzanne Christine Occiano '

Thesis submitted to the Faculty of the Virginia Polytechnic Institute and State University of“ in partial Fulfillment oF the requirements For the degree _ Master of Science in EnvironmentalAPPROVED:Sciences and Engineering I I Dr. William R. nocke, Chairman

zé ; Üregory D. Boardman Dr. Robert C. Hoehn $ II

December, 1988 H Blacksburg, Virginia

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The Mechanisms for Free Chlorine Oxidation of Reduced l , Manganese in Mixed·Media Filters by Suzanne Christine Occiano Dr. William R. Knocke, Chairman Environmental Sciences and Engineering (ABSTRACT)

' The removal mechanisms of soluble manganese [Mn (1l)] through mixed-media filters were investigated. Experimentation was directed toward the continuous supply of an oxidant during column filter studies. Free chlorine (llOCl, OC1‘) was chosen to increase soluble manganese removal efficiency because chlorine is readily available and inexpen- sive.

Filter media from four different water treatment plants were used in this study. Continuous-flow filter columns were operated in the presence and absence of 2.0 milli- grams per Liter (mg/ L) free chlorine. Maintaining constant influent manganese con- l centrations of 1.0 mg/L and flow rates of 2.5 gallons per minute per foot squared ' (gpm/ftz), the operational pH values of 6-6.2, 7.8 and 8.8 were investigated. : 1 ‘ Results indicate that a continuous feed of free chlorine (2 mg/L) applied to the filter columns could increase manganese (Il) removal efficiency. However, the amount andoxidationstate of the MnOx(S) surface coating initially on the media and the influent pH 1 had major influences upon the uptake of soluble manganese. From numerous Mn (II)uptakestudies with different media and varying pH conditions, oxide-coated filter media i continuously regenerated with free chlorine could result in increased soluble manganeseP

l I removal through adsorption upon the MnOx(s) surface coating and subsequent oxidation directly on the media surface. The relationships of manganese removal and chlorine consumed were also explored.

To further investigate the mechanisms of free chlorine oxidation for the removal of re- duced manganese, pl—I 5.0 backtitrations were conducted following exhaustion of the filter media. The exposure of such low pll conditions to columns operated in the pres- ence and abscnce of HOCI would ascertain if oxidation of the adsorbed Mn2+ was al- ways occurring, regardless of an oxidant feed. Results indicated that in the abscnce of I HOCI, the mechanisms for manganese removal on oxide-coated filter media were adsorption only. With the additional of HOCI, the adsorbed Mn2+ is oxidized directly on the surface of the media, thereby, continuously regenerating the surface oxide coat- ing.

Additional work was begun to ascertain if free chlorine could be used as a viable alter- native to potassium permanganate (KMnO4) regeneration of oxide~coated filter media. Preliminary findings indicate from column cycling experiments that free chlorine could be used to regcnerate oxide-coated filter media prior to backwashing.

I I

” I I I II I I

Acknowledgements

iv INTRODUCTION ...... 1

LITERATURE REVIEW...... 4 Water Quality Concerns for Manganese ...... 4 Chemistry of Manganese ...... 5 Reactions of Mn (II) with Oxygen ...... {...... 5 Reactions of Mn (II) with Alternative Oxidants ...... 7 l. Free Chlorine ...... 7 2. Potassium Permanganate ...... 10 3. Chlorine Dioxide ...... 10 4. Ozone ...... 12 Removal and Control Techniques ...... I2 Ion Exchange ...... °...... 12 Mn (II) Removal by Mn Greensand ...... 14 Mn (II) Removal by Oxide—Coated Filter Media ...... 16 Stabilization ...... 1 8 METHODS AND MATERIALS...... 22 Experimental Design ...... 22 Laboratory Procedures ...... 24 'Preparation of Stock Solutions and Reagents ...... 24 Characterizing Water Treatment Filter Media ...... 25 Procedures for HAS Extraction ...... 25 Manganese Removal Studies by Oxide·Coated Filter Media ...... 26 Potassium Permanganate Demands of Oxide·Coated Filter Media ...... 27 Release of Mn2+ from Oxide·Coated Filter Media ...... 30 Repeated Mn2+ Uptake Studies with KMnO4 and HOCI Regeneration ...... 31

RESULTS ...... 33 Mn (II) Removal Studies ...... ,...... 33 Characterization of Water Treatment Plant Filter Media ...... 34 . Column Experiments in the Absence of Free Chlorine ...... ,...... 34 Effects of Surface Oxide Concentration on Adsorption Capacity at Constant pH 34 Effects of pH on Adsorption Capacity for Oxide·Coated Filter Media ...... 37 Column Studies in the Absence and Presence of Free Chlorine ...... 37 Column Experiments in the Presence of Free Chlorine ...... 51 Effects of Surface Oxide Concentration on Adsorption Capacity at Constant pH 51 Effects of pH on Adsorption Capacity for Oxide-Coated Filter Media ...... 51 Manganese and Chlorine Relationships51 Potassium Permanganate Demands of Oxide-Coated Filter Media ...... 57 Release of Mn2+ from Oxide·Coated Filter Media ...... 64 Repcated Mn2+ Uptake Studies with KMnO4 and HOCI Regeneration ...... 67 l vi1 I I DISCUSSION . . .‘...... 79 Mn (II) Removal Studies ...... 79 Characterization of Water Treatment Plant Filter Media ...... 79 Removal of Mn (II) in the Absence of Free Chlorine ...... 81 Effects of Surface Oxide Coating and Solution pH on Adsorption Capacity . . . 82 Mn (II) Removal in the Presence of Free Chlorine . . .‘...... 86 Repeated Mn2+ Uptake Studies with KMnO4 or I—IOCl Regeneration ...... 90 CONCLUSIONS...... 93 REFERENCES ...... 95 VITA...... 98

vii List of Figures

Figure 1. pE—pIl Stability Diagram for Various Manganese Species in Water [Temp = 25°C; from (7)] ...... 6 Figure 2. lnfluence of pH on the Sorption of Mn (Il) on MnO; at 25°C [from (19)] ...... 11 Figure 3. Ozonation Effects on Mn (II) Removal by Sand Filtration in the Presence of Organics ...... 13 Figure 4. Relationship of Chlorine Applied, Residual Chlorine and Effectiveness of Mn (II) Oxidation and Removal ...... 17 Figure 5. Effect of pH upon Mn (Il) Removal by Oxide·Coated Filter Media in the Absence of an Oxidant [from (3)] ...... 19 Figure 6. Effect of Solution pH and Free Chlorine Addition on Mn (II) Removal by Oxide·Coated Media [from (3)] ...... 20 Figure 7. Experimental continuous-flow column apparatus ...... 23 Figure 8. Mn (II) Uptake in the Absence of2 mg/L Free Chlorine for Media #3 at pH 6-6.2 (lnfluent Mn (II) = 1.0 mg,’L) ...... 28 Figure 9. KMnOa Demand Performed on Exhausted, Oxide-Coated Filter Media 29 Figure 10. Mn (II) Sorption by Fe(OH); as a function of pH [from Morgan (19)] . 35 Figure I1. Effect of Surface Oxide Concentration on Mn (ll) Uptake by Different Media at pl-I 6-6.2 ...... Y...... 38 Figure 12. Effect of Surface Oxide Concentration on Mn (I1) Uptake by Different Media at pl-I 7.8 ...... 39 Figure 13. Effect of Surface Oxide Concentration on Mn (Il) Uptake by Different Media at pH 8.8 ...... 40 Figure 14. Effect of Solution pH upon Mn (II) Removal by Media #1 [3 mg/g Mn (II) on surfaee] in the Absence of Free Chlorine ...... 41

viii Figure 15. Effect of Solution pH upon Mn (II) Removal by Media #2 [4 mg/g Mn(II)on surface] in the Absence of Free Chlorine ...... 42 · I Figure 16. Effect of Solution pl·I upon Mn (II) Removal by Media #3 [14 mg/g Mn (II) on surface] in the Absence of Free Chlorine ...... 43 Figure 17. Effect of Solution pH upon Mn (II) Removal by Media #4 [42 mg/g Mn (II) on surface] in the Absence of Free Chlorine ...... 44 Figure 18. Mn (II) Uptake for Media #2 at pH 6-6.2 in the Absence and Presence of 2 mg/L IIOCI in the Filter Applied Water ...... 46 Figure 19. Mn (II) Uptake for Media #2 at pl·I 7.8 in the Absence and Presence of 2 mg/ L IIOCL in the Filter Applied Water...... 47 Figure 20. Mu (II) Uptake for Media #2 at pH 8.8 in the Absence and Presence of 2 mg/L HOCI in the Filter Applied Water ...... 48 Figure 21. Mn (II) Uptake for Media #3 at pH 6-6.2 in the Absence and Presence of 2 mg/L HOCI in the Filter Applied Water ...... 49 • Figure 22. Effect of Surface Oxide Concentration on Mn (II) Uptake by Different Media at pl·1 6-6.2 in the Presence of2 mg/L l—I()Cl ...... 52 Figure 23. Effect of Surface Oxide Concentration on Mn (II) Uptake by Different Media at pH 7.8 in the Presence of 2 mg/L IIOCI ...... 53 Figure 24. Effect of Solution pH upon Mn (I1) Removal by Media #1 ...... 54 Figure 25. Effect of Solution pH upon Mn (II) Removal by Media #2 ...... 55 Figure 26. Effect of Solution pH upon Mn (II) Removal by Media #3 ...... 56 Figure 27. Effluent Manganese and Residual Chlorine Relationships for Media #1 at pH 6-6.2 ...... 58 Figure 28. Effluent Manganese and Residual Chlorine Relationships for Media #1 at pH 7.8 ...... 59 Figure 29. Effluent Manganese and Residual Chlorine Relationships for Media #3 at pH 6-6.2 ...... 60 Figure 30. Effluent Manganese and Residual Chlorine Relationships for Media #3 at pH 7.8 ...... 61 Figure 31. Effluent Manganese and Residual Chlorine Relationships for Media #4 at p1—I 6-6.2 ...... 62 Mn2‘*’ Figure 32. pl-IRelease5.0 Conditionsof From...... 68IOxide-Coated Filter Media During Exposure to ix Ii , l

Figure 33. Oxide-CoatedFilterEffluent pH during Exposure to pH 5.0 Conditions on Media ...... 69 Figure 34. Release of Mn2+ From Oxide-Coated Filter Media During Exposure to pH 5.0 Conditions ...... 70 Figure 35. Release of Mn2+ From Oxide-Coated Filter Media During Exposure to pH 5.0 Conditions ...... 71 Figure 36. Release of Mn2+ From Oxide-Coated Filter Media During Exposure to pll 5.0 Conditions ...... 72 Figure 37. Repeated Mn2+ Uptake Studies Conducted at pl·l 7.8 with KMnO4 Re- generation of Exhausted Oxide-Coated Filter Media ...... 74 Figure 38. Repeated KMnO4 Regeneration of Exhausted Oxide-Coated Filter Media 75 Figure 39. Repeated Mn2+ Uptake Studies Conducted at pH 7.8 with HOCl Re- generation of Exhausted Oxide-Coated Filter Media ...... 76 Figure 40. Repeated HOCI Regeneration of Exhausted Oxide-Coated Filter Media 77 Figure 41. Surface Oxide Concentration and Solution pll Effects upon the Removal . of Nin (ll) in the Absence ofIIOCl ...... 84 1 Figure 42. Surface Oxide Concentration and Solution pH Effects upon the Removal of Mn (ll) in the Presence of2 mg/L HOCI ...... 89

l

Ä X I List of Tables

Table l. Reactions of Mn (II) with Alternative Oxidants and the Theoretical Amounts of Oxidant to Oxidize 1.0 mg/L Mn (Il) ...... 8 ion Table 2. Amount of Extracted Manganese and lron Present Filter Media Sur- face after 2% KMnO4 Regeneration ...... 36 Table 3. Amount of Mn (II) Removal for Different Media at Varying pH in the Absence of Free Chlorine (expressed in meq) ...... 45 Table 4. Mn (II) Uptake in the Presence and Absence of 2 mg/L Free Chlorine (expressed in meq; mg/g denote surface Mn conc.) ...... 50 Table 5. Relationship of Mn (II) Uptake to Free Chlorine Consumed for Different Media at Varying pH ...... 63 T Table 6. KMnO4 Demands of Oxide-Coated Filter Media Exerted During Mn (ll) Removal in the Presence and Absence of 2 mg/L (expressed in meq) .. . 65 Table 7. Extended Free Chlorine Feed and Subsequent KMnO4 Demand for Oxide-Coated Filter Media #l at pH 6-6.2 (expressed in meq) ...... 66 Table 8. Repetitive Mn (II) Uptake Studies for Media #2 at pH 7.8 in the Absence of Free Chlorine (expressed in meq) ...... 78 Table 9. HOCI and KMnO4 Demands Exerted During Mn (II) Removal in the Presence and Absence of 2 mg/L HOCI; Mn (Il) Release Data (expressed in meq) ...... 80 Table 10. Amount of Extracted Manganese after Repeated Mn2+ Uptake Studies with HOCI Regeneration of Exhausted Media (expressed in mg Mn/g of media) ...... 92

” r I . I xl I I INTRODUCTION

Concern for problems associated with high manganese concentrations in public water I supplies has increased steadily over the years. This is due to the fact that exceeding the recommended secondary maximum contaminant level of 0.05 mg/L for manganese will often cause aesthetic problems such as discoloration of water, staining of laundry and plumbing fixtures, and increased turbidity. Elevated manganese concentrations can also accelerate biological growths in the distribution systems, clogging pipelines and further contributing to taste, color and odor problems.

lligher concentrations of manganese are more prevalent in groundwaters and in the hypolimnetic region of lakes and reserviors. Under anaerobic conditions, reduced Mn (II) is released into solution by biological activity. Water treatment plants typically re- move this reduced manganese using an oxidation process followed by coagulation, sedimentation and/or filtration of the resulting precipitates. The most common oxidizing ägßnts used by water utilities are oxygen, chlorine and potassium permanganate.

In aeration—filtration processes, oxygen from the atmosphere reacts with the manganese in raw water, producing insoluble manganic oxides. The rate of oxidation depends on iV 1 Q E j solution pH; at higher pH values ( > pI·I 9.5) the reaction occurs fairly rapidly (l). Since ‘ manganese has such a slow oxidation rate via O2(aq), this technique is not very effective treatmentarefor Mn (ll) removal. Additional retention time and supplemental chemical often required to decrease manganese concentrations to the desired level.

Manganese (II) removal by the addition of chlorine has been used for many years be- 1 cause chlorine is readily available and inexpensive. However, the need to modify treat- ment practices to minimize the formation of organics has rendcred chlorination an j inefficient process for Mn (Il) removal in certain facilities. Elimination of chlorine until just prior to filtration does not allow sufficient contact time for complete oxidation of soluble manganese; therefore, stronger oxidants are often employed. For example, potassium permanganate is effective in oxidizing soluble manganese; however, studies by Knocke er al. (2) have indicated that high TOC concentrations, low oxidation pH and/or low temperature conditions can adversely affect permanganate oxidation effi- cicncy. ‘

A few water utilities have replaced typical, dual-filter media of sand and anthracite coal with commerically produced manganese greensand. This particular filter media is a zeolite mineral impregnated with manganese oxide to promote the exchange of electrons and, thereby, oxidize the manganese to its insoluble and filterable state (1). Potassium permanganate is used in conjunction with this Mn greensand. This process has an ad- vantage in that if the KMnOa does not completely oxidize all the soluble manganese, the greensand will allow for completion of the oxidation process. A major drawback to Mn greendsand filtration is its limited oxidative capacity and the need for the bed to be re- gencrated with KMnO4. j 2 ‘ I I I I Over the years, certain publications have reported on an apparent naturally-occurring I "greensand effect". This phenomenon is observed when soluble manganese is removed I by mixed-media filters in the absence of commerical Mn greensand. Knocke er al. (3) reported the "natural greensand effect" is due to a surface manganese oxidize coating that develops on filter media over time. The removal efliciency of Mn (II) is a function of this surface manganese dioxide concentration, its oxidation state and the applied wa- ter pH. Furthermore, the addition of chlorine prior to filters resulted in Mn (II) being directly oxidlized on the media’s oxide surface coating (3).

From the results of the documentation cited above, it was hypothesized that perhaps the presence of free chlorine upon the oxide-coated filter media would be an inexpensive and viable altemative to soluble manganese removal. To evaluate the above hypothesis, the following research objectives were proposed: l. Characterize the Mn (II) removal potential of oxide·coated, water treatment plant filter media as a function of solution pH and media surface, oxide-coating concen- tration;

2. Gain a better understanding of the Mn (II) removal mechanisms by oxide-coated filter media in the presence and absence of free chlorine;

3. Determine if free chlorine can continually oxidize the oxide surface coating on the filter media during Mn (II) removal; and

4. Determine if free chlorine can be used as a viable alternative to KMnO4 for the re- generation of oxide-coated filter media.

I 3 I I LITERATURE REVIEW

Wülß? Qllülily CüüC8?HS fü? Alllllgfliléß?

In roughly 40% of the public water supply systems in the United States, soluble manganese concentrations exceed the secondary maximum contaminant level (SMCL) of 0.05 mg/L (4). This MCL was established primarily for aesthetic concems, since the problems associated with elevated manganese concentrations include staining of laundry and plumbing fixtures, discoloration of water and taste and odor problems. The growth of manganese-induced microorganisms in the distribution system can also reduce the carrying capacity of pipelines and clog meters and valves (5). Despite these annoying a problems experienced by consumers and industries, there have been no reported harmful effects from drinking waters containing elevated manganese concentrations.

Manganese is a common constituent ofimpounded surface waters and many well waters. The earth's crust consists of aproximately 0.1% manganese distributed evenly through- E out (6). Chemical and biological processes contribute to the occurrence of manganese in water. Chemical reactions account for the greater part of manganese in groundwater; in lakes and reserviors, elevated Mn (II) concentrations often occur due to a combina- tion of biologic and chemical processes working simultaneously (6). Within these K 4 impounded waters, the problems of increased Mn (Il) levels are often seasonal condi- tions occurring during the overturn of the lake or reservior.

The occurrence of manganese problems in water supplies depends on the chararacter of the source supply, the type of treatment and the usage of the finished water (domestic or industrial). The availability of information on the removal methods to be employed for a specific water supply is difficult to obtain. The demand for answers to questions conceming manganese removal is an indication that research is still needed to solve many of thesemanganese-related problems. Clzemistigy 0fMa11ga11cse To gain a better understanding of the control techniques available for Mn (ll) removal in water supplies, the aqueous chemistry of manganese must first be explored. Manganese has eight oxidation states: (O) Mn, (lI):Mn2+, (2.67): Mn3O4(S) , (III): ' Mn2O3(s) , (IV):MnO2(S), (V):MnOj‘, (VI): MnO42' and (VII):MnO4'; however, not all are relevant to the field of water treatment (5). Most utilities are concerned with manganese in the 2+ , 4+ and 7+ oxidation states. Aqueous manganese in the Mn (Il) valence state occupies a large portion of the pE·pl·{ regime associated with natural wa- ters (See Figure l). The oxidation and control of manganese is complicated by factors that range from misunderstanding the reaction chemistry to slow kinetics and numerous oxidation states that result from the oxidation reaction. Manganese precipitates can also be affected by formation of organic complexes (8). Reactions of Mn (II) with Oxygen Morgan (9) has suggested the following sequence of reactions to describe Mn (II) ‘ oxidation by O2(aq): Mn (Il) + 0.5 O2 [1] A s IZ I I

+1.6

+1.2 WATER oxmuzso Miio +.8+.4

$3'3 > ° Ewa ui __8 WATER REDUCED

__ ..1_2 [unlt -10"6u 42

0 2 4 6 6 • 12 14 PH

Figure 1. pE~pH Stability Diagram for Various Manganese Species in Water [Temp = 25°C; from (7)]

6 M11 (11) + Mnozmää M11 (11)- MHÜ2(s) [2] MH • MHO2(s)

+Experimentaldata collected by Stumm and organ (10) showed the extent of observed Mn (ll) removal from solutions containing Omq) did not follow the stoichiometry of equations 1-3, indicating an autocatalytic model. In slightly alkaline solutions, signif- icant amounts of Mn (ll) is adsorbed onto higher oxides of manganese. Since the solid phase from the oxygenation may range from MnO1_3 to MnO1_9 oxidation to MnOg is I not complete (11). I

The autocatalytic model proposed by Morgan (12) to describe overall removal in this system is:

= 11,[ M11 (11)] + 11,[ M11 (11)][M11o„,)] [4]

where kg and kg are oxidation and adsorption removal rate constants, respectively. Reactions of Mn (II) with Altemative Oxidants Since manganese reactions with oxygen are often slow and pH dependent, other oxidants are employed to effectively remove manganese. These include chlorine, potassium permanganate, chlorine dioxide and ozone. Table l shows the reactions of Mn (II) with alternative oxidants and the theoretical reaction stoichiometry. I. Free Chloriue The removal of Mn (II) by free chlorine has been reported in publication over forty years ago due to its availability and inexpensiveness. In 1946, Edwards and McCaH (16) reported that breakpoint chlorination could effectively eliminate manganese problems, and the manganese coated sand ir1 the filters was in part responsible for the complete I I 7

I _ _ _ %I Table 1. Reactions of Mn (II) with Alternative Oxidants and the Theoretical Amounts of Oxidant to Oxidize 1.0 mg/L Mn (II)

REACTION NUMBEREQUATION Mn2+ + HOC1 + H20 3 M¤02(s) + Cl' + 3H+ [5] 3Mn2+ + 2Mn0;, + 2H20 3 5Mn02(S) + 4H+ [6] M¤2+ + ZCIÜZ + 2H20 3 M¤02(s) + 20102+ 12H+ [7]

2H+HOCI1.30 mg HOCI/mg Mn KMnO4 ~ 1.92 mg KMnO4/mg Mn C10; 2.45 mg CIÜ3/mg MH O; 0.88 mg O;/mg Mn

s i e j manganese removal. In the following year, however, Mathews (17) noted the ability to remove Mn (II) by free residual chlorination without the necessity of a coating of manganese dioxide accumulated on the sand grains of the filter. In contrast, others have reported the importance of a "seasoning" or "aging" of the filter media but little infor- mation on the deposited MnO; on the sand grains was available at that time (6). Very recently, Knocke er al. (3) have directed their studies on the importance of the manganese dioxide coating on filter media as an efficient mechanism for Mn (II) re- moval. Weng (18) also reported that to remove manganese to an acceptable level, an i autocatalytic reaction on the sand or anthracite media coated with manganese oxide must take place during filtration.

For many years it was common practice to control Mn (II) by the addition of chlorine under alkaline conditions. In recent years, water treatment facilities have directed their attention to the removal of organics. The reaction of chlorine with organic compounds results in the formation of trihalomethanes (THMs). Since these are potentially carcinogenic compounds, the USEPA has established a maximum level of 0.1 mg/L for THMs. In response to this THM regulation, many water treatment plants have modi- fied prechlorination practices and have moved the point of chlorination to just prior to filtration. This drastically reduces the time available for Mn (II) oxidation by free chlorine.

Recently Knocke et al. (2) have studied new strategies for removing dissolved manganese while optimizing organics removal, and have concluded that manganese oxidation by free chlorine is severely inhibited at low temperatures and acidic pH conditions.

9 PermanganatcAlthough2. Potassium more expensive than chorine, potassium permanganate has been used for Mn (II) removal and reduction of taste and odors. To help reduce chemical costs, KMnO4 is often used in combination with chlorine. Unlike chlorine, the reaction of KMnO4 with Mn (II) is rapid and complete at pH 5.5-9.0 (1, 2). Knocke er al. (2) showed that increased organic carbon concentrations required greater doses of KMnOa to accom- plish Mn (II) oxidation. Results from this study also indicated a substantial decrease in Mn (II) oxidation efliciency under low temperature (5°C) conditions. V As with chlorine, the importance of the oxide coating on the media surface greatly in- Iluenced Mn (II) removal. Morgan and Stumm (19) showed that sorption of Mn (II) on MnO2 is pII dependcnt (See Figure 2). Weng et al. (17) showed that when the sorption capacity of the manganese oxide coating on the media had been exhausted, the media could be regenerated with HOC1 or KMnO4.

The use of KMnO4 discussed in the previous paragraphs is for more conventional gravity systems used by large municipalities. Ilowever, KMnOa is also utilized in pres- sure systems in conjunction with manganese-treated greensand. This removal technique is discussed further in a subsequent section. 3. Chlorine Dioxide Application of chlorine dioxide can be beneficial to water utilities due to its disinfectant capability, control of taste and odors, THM precursor reduction and soluble manganese removal (20-22). The aqueous chemistry of CIO; is somewhat more complicated than HOCI and KMnO4 (23). In most water treatment plants, the conditions are such that CIO; reduces to chlorite (CIO2 ') and the complete reduction to chloride (Cl‘) does not occur. The total sum of chlorine dioxide, chlorite and chloride concentrations must not

10 — 1 J 1

Q?. O Q 26°c 0.6 6Q, sE éB äE2 O4° 6 E gg<_• E O.2

I 4 5 6 7 8 SOLUTION pH

Figure 2. gir1f]ue?§;)]oI“ pl-I on the Sorption of Mu (II) on MnO; at 25°C

ll exceed 1 mg/L in the finished water due to reported health concerns (24). This concern i limits the CIO; dose that can be applied and may not be sufiicient to meet the required oxidant demand of the raw water. It may be necessary to use CIO; in conjunction with other oxidants to control persistent Mn (II) problems (2, 20). 4. Ozone A common oxidant used in Europe, ozone is gaining more attention within the U.S.. Ozonation can produce a finished water with improved taste and odor and less sludge production (18). An extremely strong oxidant, ozone is also used for disinfection, algal control and iron and manganese removal (20). Legube er al. (25) have shown the effects of ozonation on manganese removal by sand filtration in the presence of organics (See Figure 3). The required ozone dose to obtain effective manganese removal increases with larger residual TOC concentrations. The presence of humic substances lowers the efficiency of ozone in the removal of manganese. Major concerns that limit the use of ozone are the lack of residual after application and the costs associated with ozone generation (21).

REIIIOVGI (Uld COIlli'0l T€CllIliqll€S Ion Exchange The method of ion exchange using a sodium resin or a hydrogen cation exchange mate- rial can be employed to remove Mn (II) from solution. This technique is limited to waters with low dissolved solids content because ions such as Mg2"’and Ca2+ will compete with the Mn (II) for sites on the exchange medium. Under certain conditions, this method is applicable and econornical because the process can be almost completely automated except for periodic chemical cleaning of the ion exchange resin (26). One of the major difficulties involved in this control method is that if the Mn (ll) is oxidized by dissolved oxygen in the water, the media will become coated and fouled (8). As a

I2 I

residual manganese (1.tg/I) 6 Q 110 100 87 87 60 60 :.7 ¤ ¤ ¤ ¤ ¤ n M.A.L. ioo // Ä 5 // /Ece /

Figure 3. Ozonation Effects on Mn (ll) Removal by Sand Filtration in the Pres- ence of Organics: Following Coagulation, Sedimentation and Remineralization [Guide Levels (G.L.); Maximum Admissible Levels (M.A.L.) from (24)]

13 resinthatresult, it is important to control the levels of dissolved oxygen in an ion exchange is used to remove elevated Mn (ll) levels. 2 1 Mn (II) Removal by Mn Greensand 2 Manganese greensand is a zeolite mineral processed with manganese sulfide and potassium permanganate in alternating steps to produce a black precipitate of manganese dioxide on the grandules (27). The hydrous oxides of manganese deposited on the filter media have a large sorption capacity for Mn (ll) (5). Morgan and Stumm (l9) have shown the sorption of Mn (ll) on Mn()2 is pll dependent (Refer to Figure 2). 2 The greensand coated with MnO; is capable of exchanging electrons and thereby oxidizing the adsorbed Mn (II) to its filterable state (I). In essence, greensand acts as _ an adsorber of soluble manganese, an oxidizing contact medium and a filter medium (ll). Drawbacks to this technique for Mn (II) removal arc the limited oxidative capacity of the greensand and the exhausted media must be regenerated with l

The KMnO4 regeneration oxidizes the lower oxides present on the media surface to higher oxides to restore the catalytic properties of the bed. These catalytic properties do not pertain to the actual greensand granules but is a property of the oxidized manganese present on the surface (27). This autocatalytic nature has been previously shown by Morgan (l2) to be the O2(aq) removal mechanism of Mn (ll) from aqueous solution at pH of about 9.0. The autocatalytic properties are produced by the n-type, extrinsic, semi-conductive properties of the oxygen deficient manganese dioxide crystals formed and precipitated. The vacancies produced from insufiicient oxygen increase the 2 n I4 2 I I conductivity of the quasi-free Mn electrons to the surface. These electrons on the sur- face promote the ionization of O2 which is held to the manganese surface, thereby in- creasing the velocity of the Mn oxidation reaction. However, once the Mn surface has been oxidized to MnO2(S), no further autocatalytic oxidation will occur and regeneration is necessary. (28)

Manganese greensand is either continuously or interrnittently rcgenerated with KMnO4. Intermittent regeneration consists of backwashing, regeneration with 0.5-1% I dilute solution of KMnO4, and subsequcnt rinsing. This regeneration allows for longer filter runs at higher flow rates because head losses build up more slowly. Lower head losses are due to the fact that solid particulate manganese is not retained within the filter bed. Instead, manganese is being directly oxidized on the filter media which helps pre- vent clogging of the filter media. However, all the manganese removal takes place on the filter and the growth of the filter media must be accounted for in the design (29).

The oxidative capacity of the greensand is used more slowly in continuous regeneration I because most of the manganese is oxidized before entering the filter. This reduces the amount of soluble manganese going to the filter. The greensand then acts as a "bu1Ter" zone. If the KMnO4 feed does not oxidize all the Mn (II), the excess manganese can be oxidized by the greensand. In contrast, if the KMnOa feed is in ex- cess of the demand, it is used for regenerating the greensand. The Mn greensand is usually capped with anthracite coal to remove most of the precipitates so as not to bind the greensand (13) and thereby increase head loss. Backwashing is often utilized once sufiicient head loss has built up. In continuous regeneration of Mn greensand, chlorine may be used in conjunction with KMnO4 to reduce chemical‘ costs. I e I I 15

I _ _ I 4 4 44 E Mn (II) Removal by Oxide—Coated Filter Media { As mentioned in the previous section, catalytic properties are associated with the oxidized manganese and not the actual zeolite material. In this section, the effective retention of Mn (II) by hydrous oxide coatings and subsequent oxidation by chlorine applied prior to the filters in the absence of Mn·treated greensand is discussed. Many water facilities employ dual-media filters of anthracite coal upon sand instead of the greensand.Imore costly Mn

Over the years, the removal of Mn (ll) by the use of free residual chlorination has been documented. For example Edwards and McCall (16) reported in 1946 that a catalytic effect of the manganese·coated sand in the filters was responsible in part for Mn (II) removal. They-alsuognoted that chlorine dosages beyond the breakpointwcould eliminate manganese problems in the filter effluent. Data presented in Figure 4 show the re- lationship of chlorine applied, residual chlorine and effectivenes of Mn (II) oxidation from their study.

‘ In 1986, Weng er al. (18) noted the importance of an old or aged media coated with manganese oxide for efficient Mn (Il) removal. 1-lowever, the removal mechanisms by the "aging" or "seasoning" (6) of the filter media were not well documented until recent publications. Morgan and Stumm (19) had initially showed the significanceof Mn (II) removal by sorption on MHÜ2(s), but experiences within water treatment filters were — occurring without being recognized. Although black “coatings" were often reported on the sand grains in filters, it was not until recently that Knocke er al. (3) studied this "natural greensand" phenomenon. This "naturally·occurring greensand effect" involves the removal of soluble manganese across mixed-media filters in the absence of a { commerical zeolite material and is a viable, functional treatment mechanism aiding Mn16 1 III I I I

— APPUED CRLORINZ -—·-— FILTER EFFLUENT CIILORINE RESIDUAL zu :*0 -j ·- RAW WATER MANGANESE 2.0 I MANGANESEEPIANTEFFLUENT E Iéns 3.L5 eéa

Elza hl EL!)_Q3° 2.0 5 Q l·0 E E I0 11[_ § 0.s 1.0 ,..,\ /I I \_ M zä

’ ‘- \ / 6 ¢ § ‘ *’ — "• 6 ‘ ,0.0 0.0 ··" " " E M' MN FEB MAI AFI MAY WWE1945MY Aue sm ocr nov nec

Figure 4. Relationship of Chlorine Applied, Residual Chlorine and Effectiveness of Mn (II) Oxidation and Removal: [Note: The dash and two·dot lme (Cmüßnt InanganCSC content) lies on the Z¢I°O line and is OÖSCUIC on 1118 graph; adapted from Edwards & McCal1 from (16)] I 17 I I I removal operations. The removal efficiency is a direct function of the surface MnOX(S) concentration, its oxidation state and the applied water pll. Filter media with larger amounts of surface oxide-coating typically yielded efficient removal of manganese for longer periods of operation. Highly oxidized forms (MnO)_8 (S) - MnO2 (S)) of the oxide coating had an increased, soluble manganese removability. Acidic pH conditions effec- tively negate this removal mechanism (see Figure 5). The authors also reported on the addition of chlorine just prior to the filters resulted in Nin (Il) oxidation directly on the oxide surface coating of the media, a reaction which is highly efficient at pI·I 6.0 or above. In the presence of significant free chlorine (> I-2 mg/L as Clz), soluble manganese uptake within oxide—coated filters will be rapid (see Figure 6). Stabilization An alternative to removal of manganese is to sequester soluble manganese with polyphosphates. Polyphosphates [can be implemented quickly and economically to control the soluble manganese (8). O'(Ionnor (5) reported this treatment will stabilize manganese in suspension but is not suitable for metal concentrations in excess of l mg/L. lle also showed that higher water temperatures, causes the polyphosphates to revert to orthophosphates and lose this dispersing property. With mixed reactions from various water suppliers, this process is limited to polyphosphate dosages of IO mg/L since the availability of phosphorous may stimulate bacterial growths in the distribution systems. If this occurs, chlorine residuals must be sufficient to control these bacterial slime growths (5).

In summary, it is apparent that no single treatment chemical nor treatment process will solve all reduced manganese problems. Because every water type has its own charac- teristics, on-site studies are recommended to develop the most reliable and cost effective method for manganese removal. Each treatment facility should determine which 18 i i I

OC; • • • • PH 8.8 0 2 ’ Q „ E ——' gg; .g_g C . .--_._.,____·___*_.g_- 0 § 0 C 2 PH 7.2 .*3 „ _ gä 0.1 _ Q . E..: · . Fg 0.0 - · PH 6.1

OCZ ¤ Iurtusur 0•l ¤ EPP¤.usur ° TOP Poar 0.0 0 200 Q00 600 THROUGHPUT Vowna (L)

Figure 5. Effect of pH upon Mn (II) Removal by Oxide·Coated Filter Media in the Absence of an Oxidant [from (3)]

I9 · 1

O O 3 pH 6. 1 • Influent O f— O O 2 • "

' g . •e-( 0 • 1 Q . Top Port $5 · v I +) V fluent

0 Q pH 8. 5 Influentä O O 2’ 2 $° •-1 cu E-1*8 0. 1 Top Port „

I..·._...... -•.-1-l—-._-1 0 1+00 800 1200 Throughput Volume (L)

Figure 6. Effect of Solution pH and Free Chlorine Addition on Mn (II) Removal by Oxide·Coated Media [from (3)] , 20 oxidants will promote effective manganese oxidation under their specific operating con- ditions. .

2l METHODS AND MATERIALS

l Expßfliilßllltll. D€SlgIl.

Continuous-flow, laboratory scale filtration experiments were conducted using a four column apparatus (see Figure 7). Each 1 ft. long, 3/4 in. inner diameter column had an upper infiuent port and a lower effluent port; the distance between ports was approxi— mately l0 inches. Two large, 35·gallon holding tanks were used for the influent manganese feed solution. A Cole·Palmer Masterflex pump was used to pump the manganese-containing feed water to an elevated constant head tank. The elevated tank was used to insure a consistent flowrate to the columns. A separate 50 L carboy was used for the chlorine feed solution which was pumped into the manganese-feed line with a Cole-Palmer diaphragm pump.

The configuration of the filter media within each column consisted of an inch of base gravel topped with 50 g of filter media from various water treatment plants in Virginia and North Carolina. Dual-media samples of anthracite coal and sand from four differ- ent treatment plants were used in the experiments. Unlike the typical 2 ft. configuration of dual-media filters with 2 distinct layers of coal and sand, the columns contained a rnixture of the coal and sand ranging from 5 to 7.5 inches in depth. Such shallow media

22 N N

'? HEAD TANK ”

) BACKFLOW

PUM PUMP

SH CHLORINE FEED

MANGANESE FEED

Figure 7. Experimental continuous·flow column apparatus

23 depths were used because a 24 inch column would have taken weeks to exhaust with respect to Mn2+capacity. Failure of the filter media results ir1 elevated manganese con- centrations within the effluent. This study investigated 4 different media with varying amounts of manganese oxide coatings on the surface of the coal and sand. Duplicate tests were conducted with 2 columns receiving the 2 mg/L HOC1 feed and 2 colurnns without the oxidant present (See Figure 8). A downflow filtration scheme of the manganese·treated tap water was typically operated at flowrates of 2.5 Laboratory Procedures Preparation of Stock Solutions and Reagents The main source of feed water to the colutnns was dechlorinated Blacksburg tap water augumented by various chemical additions. In preparing the soluble manganese feed solution, the tap water was first dechlorinated using 0.028 N sodium thiosulfate solution to prevent oxidation of the added manganese. Chlorine mßasutßmßnts were made using a Wallace & Tieman amperometric titrator following the techniques outlined in Stand- ard Methods (31). The 1.0 mg/L Mn (II) solution was prepared from a stock solution of manganous sulfate. An 8 mg/mL stock solution of manganous sulfate was prepared by adding 6.15 g MnS0.t• H20 to 250 mL of distilled water. The pH of this solution was adjusted using appropriate amounts of concentrated HN02 and/or Na0H. A Fisher Accumet pH Model Meter was used to monitor influent and effluent pH values. Manganese concentrations were deterrnined using a Perkin-Elmer Atomic Absorption Spectrophotometer.

A stock solution of free chlorine was made by 1:1 dilution of a 5.25% by weight solution of Na0Cl with distilled water. A concentrated chlorine feed solution (14 mg/L) was generated from this stock solution and subsequently diluted to 2 mg/L when mixed with 2

24 u accordinglytothe Mn (ll) feed solution. The pump rate for the chlorine feed was adjusted assure that a proper dilution was being made. lnfluent and eflluent chlorine concen- trations were measured using the amperometric titrator [refer to Standard Methods (30)]. The pH of this solution was also monitored and adjusted if necessary. Characterizing Water Treatment Filter Media A 2% by weight solution of KMnO., was used to regenerate four different media. _ Fol- , lowing preparation of the KMnOa, this solution was fcd in an upflow manncr through a column packed with media directly from packages sent from the water treatment y plants. To insure the surface coating was completely oxidized, the permanganate re- generation was conducted until the effluent was similar in color to the influent. The KN/lnOa regeneration took approximately l hour after which the column was rinsed with distilled water until the efiluent was clear. The media was then air dried for 24 hours before being packed into the columns.

To determine the total amount of manganese initially on the filter media surface fol- lowing KMnO., regeneration, an extraction was performed. A strong reducing agent, hydroxyl amine sulfate (IIAS), was used to reduce all the manganese to its most soluble oxidation state, Mn2"'. Procedures for HAS Extraction ' A 4 gram media sample was placed into a 250 mL Erlenmeyer llask. To provide the necessary acidic conditions, 100 mL of 0.5% HNO; was added prior to the addition of an excess amount of HAS (approximately 1 g HAS). After being covered with parafilm, the flasks were shaken and allowed to react for about 4 hours. This allowed for the manganese on the surface of the filter media to be solublized and, thereby, able to be analyzed on the Atomic Absorption Spectrophotometer (AA). Prior to analysis on the25 E E 1 on the AA, 10 mL of the extracted liquid were filtered through a 0.2 am (25mm) Gelman membrane filter in order to remove any particulate matter.

The amount of total manganese on the media surface was determined using the follow- ing equation: Nin on surface (d~€)(V)(¢¤¤<=-) unit mass media = sample mass [9] where:

I df = dilution factor for AA analysis I V = volume of acid added to flask (i.e. 100 mL) conc. = Mn conc. in acid solution after HAS addition Manganese Removal Studies by Oxide-Coated Filter llledia To ascertain if a continuous oxidant supply would have an increased Nin removal ca- pacity, two of the four columns were exposed to 2 mg/L free chlorine. All four columns contained the same amount and type of media. By operating duplicate columns, data

V on the reproducibility of the column tests could be achieved. Different experimental column tests were conducted at three different pH values (6.0-6.2, 7.8 and 8.8) using four different media samples. The continuous-flow columns were operated until the columns without chlorine failed or until five days had elapsed. Initially, frequent samples for effiuent manganese and chlorine concentrations were collected. However, after the first day, samples were collected 3-4 times daily. All manganese samples were first filtered through 0.2 um Gelman membrane filter to remove any particulate matter before AA analysis. Influent Mn and HOCI concentrations were periodically measured to insure the correct amounts were being fed through the columns. Flowrates were also moni- tored for each of the columns. 26 Column studies were continued until eflluent manganese concentrations of columns without the HOCI present were 95% of the 1.0 mg/L Mn (II) influent. This amount is referred to as "exhaustion" of the media whereas the term "breakthrough" denotes the effluent concentration as being equal to 5% of the influent. Following exhaustion of the filter media without I-IOCI, all column flow was stopped and the amount oftotal Mn (II) uptake calculated. The calculation of Mn (II) uptake was determined by plotting eflluent manganese concentrations versus volume of water treated (refer to Figure 8). The area between the ellluent Mn curve and the inlluent 1.0 mg/L Mn (II) represents the total amount of Mn (II) uptake. A fortran program utilizing the trapezoidal method for determining areas above and below curves was executed for estimating these areas. The Mn (11) uptake was then expressed in milliequivalents for easier comparison with other data. Potassium Permanganate Demands of Oxide·Coated Filter Media Alter column operations ceased for Mn (II) removal, KMnO4 demands on oxide-coated filter media were conducted on columns with and without the HOC1 feed to evaluate the oxidation state of the MnOx(s) coating. A 100 mg/L KMnO4 solution was fed in an upflow manner through the exhausted media. The KMnO.„ demand was the amount of oxidant (KMnOa) required to oxidize the sorbed Mn2+ and is calculated similarly to the Mn (II) uptake. Eflluent KMnO4 concentration versus volume data were plotted and the area between the influent and ellluent KMnO4 concentration lines were estimated to represent the amount of KMnO.·. demand (see Figure 9). The amount of Mn (I1) taken up, expressed in meq, should be theoretically equal to the demand for KMnO4. The oxidation state of the surface coating can be quantified by relating the KMnO4 demand to the amount of Mn (II) taken up on the surface. KMnO4 concentrations were determined using the Wallace & Tieman amperometric titrator. The following equation

27 i 1 .25

‘§:1\ 1.00 V }‘ {|EEZ ¤ “ E „ 0 E _)" z . LIJz 0.75 , le ¤ » oL)z . U (J .A u _ ä ll 0 %U- 0 * Ü .25 ,::5 ustt gf.1i‘

O 50 100 150 200 VOLUME OF HQTER TREQTED (L)

Figure 8. Mu (II) Uptake in the Abseuce of 2 mg/L Free Chlorine for Media #3 at pl-I 6-6.2 (lnfluent Mh (II) = 1.0 mg/L) zs L ’“ 100 · 1 Z Z !' *— I4 gi •·‘ 1- 75 U-'Z U u OZ U ra E u DB ü / OZz u Z E-| „0. u 8.225C / =»1-1-C. / Lu2 [ 21; “‘ ..4 g *%.0 1 1.0 2.0 2.0 VOLUME OF NQTER TREQTEO (L)

Figure 9. KMnO.1 Demand Performed on Exhausted, Oxide-Coated Filter Media

29 3 I was used to calculate the concentration of KMnO4 present from the amount of titrant I required (20).

M V V2 [10] or 31/ig C = [1 1] Vwhere: M = molarity of permanganate solution

I C = conc. of permanganate solution (mg/L as KMnOa) N = normality of titrant (eq/ L) G = gram molecular weight of KMnOa (158 x 103 mg/mole) V1 = volume of titrant (L) v = electron equivalents transferred per mole of KMnOa (3.5) V; = volume of sample (L) I

Samples for eflluent KMnO4 determination were first filtered through Gelman 0.2 um filters to remove any particulatcs. A 200 mL volume of distilled water was added to the filtered, effluent KMnO.i sample (2 mL). The proper reagents (KI and pH 7 buffer) were added and the sample titrated using phenylarsine oxide (PAO). Release of Mn2+ from Oxide·Coated Filter Media In order to further investigate the mechanisms of Mn2+ uptake, filter columns were backtitrated with a pH 5.0 acid solution free of chlorine and manganese following the exhaustion of the columns without HOC1. This pl·1 adjustment would help to clarify if an oxidation process of the adsorbed Mn (ll) was occurring on the oxide surface both in the presence and absence of free chlorine. The pH 5.0 backtitration was used because at this pH value the Mn2+ species can be extracted without solublizing the MnOx(S)30 I l

coating (refer to Figure 1). Appropriate amounts of 10% HNO; were added to dechlorinated Blacksburg tap water to achieve a solution pH of 5.0, which was fed in a downflow manner at flowrates of 2.5 gpm/ftz. The amount of soluble Mn in this water was less than 0.03 mg/L. Eflluent Mn (II) concentrations and pll values were moni- tored. Efiluent Mn (II) concentrations versus volume data were plotted and the area under this curve was estimated to represent the amount of sorbed Mn2+ released from the NlnOX(S) coating. These cxperimcnts were conducted for 2-3 days to determine if the sorbed Mn in the the absenee of HOCI would be released back into solution due to the fact that an oxidation process was not occurring. This would theoretically be ac- companied by a rise in pH due to the protonation of the oxide surface coating. In contrast, columns operated with HOCI should not exhibit a release effect upon the ad- dition of pH 5.0 solution due to oxidation of sorbed Mn2+. Repeated Mn2+ Uptake Studies with K1\«InO4 and HOCI Regeneration This particular study area was conducted to investigate the regeneration capabilities of KMnO.; and HOCI of exhausted, oxide-coated filter media. These tests would determine if it was possible to recover all the removal capacity of the oxide coating by KMnO4 addition during backwashing. An Mn (ll) uptake study in the absenee of I·lOCl was conducted on media #2 at plI 7.8. The influent Mn (II) concentrations were maintained at 1.0 mg/L with flowrates of 2.5 gpm/ftz. Due to decreased availability of sorption sites upon the MnOx(s) coating, media #1 becomes exhausted. A 100 mg/L KMnO.; solution was fed in an upfiow manner to oxidize the adsorbed Mn2+, thereby, eontributing to the MnOx($) coating. The amount of Mn (II) taken up and the KMnO.; demand of the Mn2+’s oxide coating was calculated as previously shown. Once the demand for KMnO4 had been met, a second Mn (II) uptake study was performed using the same initial parameters (i.e. influent Mn (Il) concentration, pH and flowrate). The amount _ of Mn (II) was again calculated and a final KMnO.; demand of the oxide-coated filter

Q 31

l media was performed. The amount of KMnO4 taken up was again calculated. The objective of this study was to evaluate the the possiblity of recovering the removal ca- pacity of the oxide coating by KMnO4 addition following exhaustion of the filter media.

One final experiment was to ascertain if free chlorine could be used as a viable altema- tive to using KMnO4 to regenerate the surface coating during backwashing. Since KMnO4 is a more costly oxidant, the cycling studies using l1OCl were conducted to in- vestigate the possible potential of its use for regeneration of the oxide coating during backwashing of the filters. Media #1 was again used in this study with the operational pH maintained at 7.8. Following the media’s exhaustion, a 50 mg/ L free chlorine (ex- pressed as Cl;) solution was fed at 15 mL/rnin in an upflow manner until the sorbed Mn2+ was oxidized. The Mn (ll) taken up and the amount of 11OCl used were calcu- lated. Calculations made for determining llOCl demand were similar to those for KMnO4 demands. Effluent HOCI concentrations versus volume were plotted and the area between the influent and effluent HOC1 concentrations were estimated to represent . the amount of HOCI demand. Once the HOCl demand for the adsorbed Mn2‘*‘ was completed, a second Mn (ll) uptake study was performed. The amount of Mn (11) and subsequent HOCI taken up were calculated again. A third Mn (11) uptake test was conducted followed by a third and final HOCl regeneration of the oxide—coated filter media. Regeneration in this case is referred to the oxidation of only the adsorbed Mn2‘*‘ upon the MnOx(s) coating on media surface. By conducting these studies, the possiblity of employing an inexpensive oxidant, free chlorine, for regeneration of oxide- coated filter media during backwashing operations would be ascertained.

32 RESULTS

ln the following section, experimental results from continuous—flow laboratory filter columns are reported. These results are summarized into four subsections which include Mn (II) removal studies, potassium permanganate demands of oxide-coated filter media, release of Mn2+ from the oxide coating and repeated Mn2+ uptake studies with KMnO4 and l·lOCl regeneration of exhausted media. These subsections are further di- vided into various subheadings to present the data in a lucid and concise manner.

Mrz (II) Removal Studies Most of the research emphasis was placed on this area of experimentation. Columns were operated in the absence and presence of free chlorine. Free chlorine was the oxidant employed in all studies because it is readily available and inexpensive. The ef- fects of varying media and influent pH were extensively investigated. This particular subsection of Mn (Il) uptake is further divided into the following subheadings: charac- terization of the filter media; column testing in the absence of chlorine; combined data of column studies with and without chlorine; and column studies in the presence of chlorine. Within the last subheading listed above, chlorine and manganese interactions

i are described through the presentation of combined plots of effluent Mn (Il) and i chlorine residual data; 33 L Characterization of Water Treatment Plant Filter Media Following a 2% KMnOa regeneration to fully oxidize the media surface coating, a hydroxyl amine sulfate (1-IAS) extraction was performed. These results are reported in Table 2. The total amount of manganese extracted per gram of media is listed for each of the different media investigated. Extracted iron concentrations are also shown since Morgan showed that ferric oxide solids also have a limited affinity for Mn2"’ uptake (32). The data by Morgan (19) presented in Figure 10 show the pH effects on Mn (II) sorption by Fe(()l1)3. Also presented in Table 2 are water treatment plant (\V.T.P.) lo- j cations which supplied media and the media depths corresponding to 50 g of media within each column. Note that unlike typical water treatment plant filter depths of 12-24 inches, the height of media within the columns was much less. Column Experiments in the Absence of Free Chlorine Effects of Surface Oxide Concentratiorr ort Adsorption Capacity at Corzstant pH Unless otherwise noted, all data presented in figures describe eflluent manganese con- centration (in mg/L) pattems as a function of the volume of water filtered through the column, (expressed in liters). Influent Mn2+ concentrations were maintained at 1.0 mg/L in all studies.

Data presented in Figure ll indicate the importance of characterizing the media surface p for the amount of oxide coating present. This particular figure shows the manganese uptake capacity of four different media at pH 6.0-6.2. Media having 3-4 mg/g of surface oxide coating show similar breakthrough curves. However, sample media #4 (42 mg/g of oxide coating) indicates a greater Mn2+ uptake capacity in the absence of free chlorine at pH 6-6.2. Subsequent studies for pH 7.8 and 8.8 conditions are reported in Figures 12 and 13, respectively. Again the figures indicate a greater capacity for uptake with increasing amounts of surface oxide coating. It is also evident that solution pH

34 E E 6 g _55»rz. “änä O. 5 Ö O. 4 géosZ In : ä°· :2 0. 2

Figure 10. Mn (II) Sorption by Fe(OH)3 as a function of pH [from Morgan (19)] [ ss I

Table 2. Amount of Extracted Manganese and Iron Present on Filter Media Sur- face after 2% KMnO4 Regeneration

MEDIA LOCATION Mu CONC. Fe CONC. MEDIA SAMPLE # OF W.T.P. · (mg/g) (mg/g) DEPTH (in) Cheaspeake 3.0 6.5 Va.

2 Roanoake 2.5 5 Va.

3 Newport 4.2 6.5 News, Va.

Durham 42 7.5 N.C.

P 36 also has an important role in defining the manganese removal capacity of oxide-coated media.Ejfectsfilter MediaTheofpH oa Adsorptiorz Capacity for Oxicle-Coated Filter Mn (I1) uptake of the four different water treatment plant filter media was highly dependent upon the pH of the influent water. Data presented in Figures 14-17 show the effect pH had upon manganese removal efficiency. Uptake was severely impeded at pH values of 6-6.2; greater capacities for removal were observed as solution pl-I increased. The results of the particular media containing 3-4 mg/g of surface oxide coating are presented in Figures 14 and 15. ln contrast, Figures 15 and 16 indicate observed manganese removal capacity for the filter media samples which had larger amounts of surface oxide coating.

Data from Figures 14-17 are summarized in Table 3 to show the total amount of soluble manganese removal for each media at the three pH values investigated. Note that the removal amount is expressed in total manganese milliequivalents (meq). ‘ Column Studies in the Absence and Presence of Free Chlorine I In operating a four column apparatus, two columns could be exposed to 2.0 mg/L free chlorine. Two different media could therefore be studied, one column receiving theHOCIfeed and the other column operated in the absence of the oxidant. The data pre- sented in this section indicate a continuous feed of an oxidant could enhance Mn (ll) removal (see Figures 18-21). These figures show overlay plots of simultaneous tests with and without chlorine. The effect of pH upon a media (media #2) with a less amount of oxide coating (4 mg/g) is shown in Figures 18-20. Media #3 which has a larger amount I of oxide coating (14 mg/g) is presented in Figure 21. Table 4 follows these figures and summarizes the data collected during these experimentations. . 37 1 1 u __ lä ii A ' O __ 0.00 ·' _ _„_ A . A A _x_ .J A '- A ‘ •“ ·- ·· ·· 6Z 0.00 " A · A z ° E 0 .70 III Ü,_ I- A ä „ •• AA “ A u.:E 0.60 Q u A 3 0.60 ¤ 3 ¤¤/G 0 0 4 MG/G 14äM H “ A Ü 0.40 o 42 nc/0 CJ tw nb"·—Z .. ‘I Z 3 020 gu o ., 0 u- I, V V 9 ; _ v " 0.10 J! ” „ ° 'E, 6* " ' ° ° 0 .000 50 100 150 200 VOLUME OF HQTER TREQTE0 (L1

Figure ll. ElTect of Surface Oxide C011ce11tratio11 011 M11 (Il) Uptake by Differerit Media at pl-I 6-6.2: [l11f1ue11t M11 (ll)= 1.0 mg/L; 110 I-lOCl 111 filter _applied water; symbols dcuote surface extracted concentration]

ss I

1 .00 ...2u __ E] 0 0 .90 Q .1 U‘ Öz 0 .80 1.1 Q O ° _ — 9 0 .70 · txE u E 0 .60 “ A U*4-* ZI.1.1 z g_5g 8 u ·- E13 MG/G Lu _ 0 4 MG/0 ’11 · [(-3 0.40 gn oA 4214 no/0HG/G 0ä E fg; *‘ [I “ I_•.'1:. ä 0 .20 ,* 3 hp A n LL A hä 0.10 2g* - _; l ‘” v ;:‘é€v.v$:E'§‘Ko°Ii ·— ·: I 0 _000 S0 ° 100 ° * * •» '150° ·· 200VOLUMEOF HQTER TRERTE0 (L) I

Figure 12. ElTcct of Surface Oxide Conccntration on Mu (II) Uptake by Differerit Media at pI—I 7.8: [Influent Mn (II) = 1.0 mg/L; no HOC1 in filter I applied water; symbols denote surface extracted concentratiorxsl 39 1

l

0.50 =: Q1 ' u u Q 0_40 III 3 Me/6 "' 0 4 MG/G Z

1- CKE u E uJ2 0.30 U Z O U ‘ LU u _ H •: “ |• $2 0.20 __ g G u u ä u Z I- u " 5;•J:~ 0.10 I: U p: -· Li- :4 Lu IL.: • Üliv U “ 2 u fg?] ~= :; ' 0 .00 ¤ 0 25 50 75 100 VOLUME OF HQTER TREQTED (L)

E Figure 13. Effect of Surface Oxide Concerxtration on Mn (II) Uptake by Different Media at pH 8.8: [lnfluent Mn (II) = l.0 mg/L; no HOCI in filter applied water; symbols denote surface extracted concentrations] I 40 1 .00 5; 0 : O 3 'A uu Z IZ Ill E „ O ,5 O (2,E _ n 9 U 8 g_5g m PH 6-6.2 Lu 0 PH 7.8 A PH ZEB " sz 8.8 0 ¢ Z n ,; 0.25 he ä _ D II A u_..1 0 U- u l-IJ 1; _ A y n A _. . - A O .00 Ü ^ - - O 25 50 75 100 VOLUME OF HQTER TREQTED (L)

Figure 14. Effect of Solution _pH upon Mn (II) Removal by Media #1 [3 mg/g Mn (11) on surface] in the Absence of Free Chlorine: [Iuf1uent Mn (II) = 1.0 mg/L] 41 i I 1

l .00 “ ü H ü ü ".. n U .I g O U EJ uu 9 E “ Z 0.75 _ 2 V 0 PH 6-6.2 0:E 12* 04 PH 8.87.8 u.• I: ä0 0 0.50 ·· ui " Ü u A - z ‘° z sr . A · 0:Z Il 0.25 „_I „ E ‘ A A 3 *4 “ .. A lt ·· w l“.:1 ’ ^ ‘ _,-.·:-··’ A 0 .00 •ö' 25 50. 75 100 VOLUME OF HQTER TREHTED (L)

Figure 15. Effect of Solution pH upon Mn (II) Removal by Media #2 [4 mg/g Mn (II) on surface] in the Absence of Free Chlorine: [lnfluent Mn (II) = l.O mg/L] I 42 I s 1 .00 El ___ 0.90 Ü Ü 0 Ea., u u un ,,4 **0g.•:•u?: u 2 u ül 6 0.80 5* E EJ _) , 0 ä 5 •-• 0.70 u ‘·' äE 6..6 [D ° LI g 0 0.50 ‘* U E3 ¤ “‘ 'ä 0_40 0 pn 6-8.2 g 0 PH 7.8 EZ 0.80 J'II „ •• E 0.20 _ _ u.ä 0 ti': u ° · 'ilju··"‘ 0.00 7 60 100 150 200 VOLUME OF HQTER TREQTED 11.1

Figure 16. Effect of Solution pl-I upon Mn (II) Removal by Media #3 [I4 mg/g (11)Mn =(II)1.0onmg/1.1surface] in the Absence of Free Chlorinez [lnfluent Mn

43 I I

0030

3x Q 0 PH 6-6.2 “ 0 PH 7.8 Ü -·Z 2 E 0.20 E 0 E 0 3Z s: 0Z Ü U ““ —· EJ äZ II “ IB g• é 0.10 „ •—Z “ “ u U Z g IE uu"" O sz EJ El Ba . ¤• 0 '“·' 0 0 0 0 0 ·· :7 U Ü 0 0Ü 99 ¤ 1: 0 Ü , •C 0 00 50 100 150 200 VOLUME OF HQTER TREQTED (L)

Figure 17. Effect of Solution pH upon Mn (II) Removal by Media #4 [42 mg/g Mn (II) on surface] in the Absence of Free Chlorine: [lnfluent Mn (II) = 1.0 mg/L] 44 I

Table 3. Amount of Mn (II) Removal for Different Media at Varying pH in the Absence of Free Chlorine (expressed in meq)

MEDIA INFLUENT TOTAL Mn (II) SAMPLE # pH UPTAKE (meq) 1 6-6.2 0.30 (3 mg/g) 7.8 1.09 8.8 2.80 ° 2 6 · 6.2 0.60 (4 mg/g) 7.8 0.99 8.8 1.94 3 6 - 6.2 . 1.53 (14 mg/g) 7.8 3.00 4 6 - 6.2 14.1 (42 mg/g) 7.8 13.3* *Note: Column study halted before complete exhaustion.

as 1 .00 rz uuu an Ü

3\ CJ m Z Z 0 .75 '·' O

E Ilen K IZ? II LU U Z 8 0_50 E1 HITHOUT HOCL Lu n 0 HITH HOCL 3 Z u CI O II Z u 1* 0 || O 9'“ u 0 Q Ü 0O O 0 QI Lu;]ä au C) CD O O °

gaIP 0 .00 ¢ 0 50 100 150 200 VOLUME OF HQTER TREQTEO (L1

. Figure 18. Mn (II) Uptake for Media #2 at pH 6-6.2 in the Absence and Presence of 2 mg/L HOCI in mg/L1 the Filter Applied Water: [lnfluent Mn (I1) = 1.0

46 1 .00

II ,.„ E I-! O<' EI ¤ " Z

•20 Z 1- (I Ir- < Lu U Z 3 0 _g0 0EJ HITHNITHOUTHOCLHOCL 03LL, Lu Z S u Z

Figure 19. Mn (II) Uptake for Media #2 at pil 7.8 in the Absence and Presence of 2 mg/L I-IOCL in the Filter Applied Water: [lntluent Mn (ll) = 1.0 mg/ L]

47 ' l I

1 .00

\3 EJ HITHOUT HOCL rz" 0.,75 O HITH HOCL E . 1- G (I 1- Z L1J äL) 0 0.50 an LU nä:U) (JJ u

. ZE5 Ü G Z 0.25 E1" „ El 1 III2 u V (Il El L1. Ü Lu •: 'ingyils• G; u4~- " O 2 _ 0_00 U " 3 9 2 3 u V J 0 25 50 75 100 VOLUME OF NFJTER TREFITEO (L)

Figure 20. o1“2Mn (II) Uptake for Media #2 at pH 8.8 in the Absence and Presence mg/L l·lOCl in the Filter Applicd Water: llnfluent Mn (II) = 1.0 mg/Ll I ° 48 I I

1 .00 _ u A Ü an E] 0<' u Ü Z “¤ g 0.75 •— ä ECE 05 ·· " Z u 8 g_5g E1 HITHOUT HOCL E] 0 HITH HOCL ä E] E5 E] GZZ Z Q 0.25 Jg, ä_; sz'M LI-IJu. u g-• ug °°;;;“ ö U U U ö 9 O 0 .00 ¤ Ü ö 0 50 100 150 200 VOLUME OF HQTER TREFITEO (L) I I I Figure 21. Mn (II) Uptake for Media #3 at pH 6-6.2 in the Absence and Presence I of2 mg/L HOCI in the Filter Applied Water: [lnlluent Mn (Il) = 1.0 mg/L} I I 49 I I I Table 4. Mn (Il) Uptake in the Presence and Absence of 2 mg/L Free Chlorine (expressed in meq; mg/g denote surface Mn conc.)

MEDIA INFLUENT Mn (II) Mn (II) SAMPLE # pH UPTAKE UPTAKE w/HOCI (meq) w/0 HOC1 (meq) 1 6-6.2 3.20 0.30 (3 mg/g) 7.8 6.50 1.09 8.8 ··~ 2.80 2 6-6.2 4.15 0.60 (4 mg/g) 7.8 3.10 0.99 8.8 1.70* 1.94 3 6-6.2 7.36 1.53 (14 mg/g) 7.8 7.80 3.00 4 6-6.2 10.7* 14.1 (42 mg./g) 7.8 —-- 13.3 *Note: Column study halted before complete exhaustion.

I so I I Column Experiments in the Presence of Free Chlorine Eßects of Surface Oxide Concentration on Adsorption Capacity at Constant pH The influence of free chlorine on manganese uptake by oxide—coated media is apparent in the data presented in this section. Note that a scale change in the y-axis was made to better illustrate the effluent manganese concentrations. The influent manganese concentration remained at 1.0 mg/L for all studies; free chlorine concentrations were applied at 2.0 mg/L. Figures 22 and 23 represcnt manganese removal data for different media at pl-! 6-6.2 and 7.8, respectively. Research into the effects of higher pl-! condi- tions on Mn2+ removal was not conducted due to problems of Mn2+ oxidation by HOCI prior to the filter which plugged the columns. This issue will be expanded on further in the discussion section. Effects ofpH on Adsorption Capacity for Oxide-Coated Filter llledia Three of the four media were examined for their uptake ability in the presence of free chlorine at different pH values. Figure 24 presents results from media #1 (3 mg/g surface oxide concentration) with free chlorine at pl-! 6-6.2 and 7.8. Again the y-axis has been expanded to 0.3 mg! L effluent manganese concentration to better show the results from these particular studies. Data on media #2 (4 mg/g surface oxide concentration) with 2 mg/ L HOCI in the filter applied water are represented in Figure 25 at pl-! 6-6.2, 7.8 and 8.8. However, Figure 26 depicts media #3 which has a much larger amount of oxide coating (14 mg/g) under the effects of a continuous HOCI feed solution at pH 6-6.2 and 7.8. A summary of the data from studies with and without chlorine for different media at varying pH conditions has been presented in Table 4. Alanganese and Chlorine Relationships The relationship of Mn (ll) uptake with the amount of free chlorine consumed is re- p presented in Figures 27-31. In these plots the right y-axis presents the effluent chlorine51 0 .30 _ us „ 0 O< vZ “ 2 :• ,_ Z 9 U " I2 — ;‘ E 0 .20 •-OC gn Ü u L)ä 8 ” 0 a nom ' •; 0 4 no/0 w " . “ " 04 42I4 H3/0nc/0 G. O . IO Z uE•__ "u. n· · I _ “ '!“/ U uu ä IZ ‘ ga * "· vu gs1„ ‘a — -' ° ‘* ' .1„. _ ...... -,.¢§__. -¢. AVA = * " 0 •00 • SO IOO ISO 200 VOLUME 0F HRTER TREHTEO (L)

Figure 22. Effect of Surface Oxide Concentration on Mn (II) Uptake by Different Media at pH 6-6.2 in the Presence of 2 mg/L HOC1: [lnfluent Mn (I1) f—= 1.0 mg/L; symbols denote surface extracted concentration]

52 n

Ü O 30

x3 2"’ 0 a :1010 0A 414MG/GHG/G 0 .20 (Z•-*5 1.1 z z II 0 w u 1: 3 Z u an u S LI u • ‘ ä 0 " :Ü A ,_Z ·~ u u Ä _ n u /*1 — .. -; ‘U-1:z es — ·· - .. w-‘ -:1‘ ·· Lu Y "II pp ( u . *.1 u ° 0 .00• u Q ä *5 U ; u S0 100 150 200 VOLUME OF HQTER TREQTEO (L1

Figure 23. Effect of Surface Oxide Concentration on Mrz (II) Uptake by Different Media at pI·I 7.8 in the Prcscnce of 2 mg/L HOCI: [lnflucnt Mu (II) = 1.0 mg/L; symbols denote surface extractcd concerxtrationl V ss I I

I 0.30 Q Q ::1 am 6-8.2 " 0 PH 7.8 2Z 0.20 ¢5 •.• 1- •• - 3 Z O U ’] Ü: (D li. 0 0 0 ;• an § 0.10 c3; ·_ 0 c · II In Q 1% P12: u D ‘ _ -·' " 0 0 ‘ u-( 2 I U —. 2 ;‘- ss . ·4 __ ·- 0 * as 2 ., an Q ·- e 0 .00 •¢ 50 100 150 200 VOLUME OF HRTER TRERTEO (L)

Figure 24. Effect of Solution pH upon Mn (ll) Removal by Media #1: [3 mg Mn (Il)/g of media on the surface; 2 mg/L HOCI in the filter applied water; Influent Mn (Il) = 1.0 54

I_ _ _. _ __„ e [ E

0.30

_ us Q! a• 0E I: u- “ •: ä _ u u 2 °"" äu 5 0.20 an 0:•— L, *' U u (_)ä ·¤ Ü PH B—6.2 _ ggU, " ,, = =· 60 PHPI-I 8.87.8 Z " u O.lO n Z ·_ n '• LUZ _;D „ ca U. ,_ ‘ MLL g ‘— u W 'Ä, / L! 0 °=“ • ·ä" ' SO IOO ISO 200 VOLUME OF NRTER TREFITED (L)

Figure 25. Effect of Solution pH upon Mn (II) Removal by Media #2: [4 mg Mn (II)/g of media on the surface; 2 mg/L HOCI in the filter applied water; Influent Mn (II) = 1.0 mg/L]

[ SS

1 - _. I

0.30 Q "’E cz pn 6-8.2 0 PH 7.8 I EZ 'E 0.20 •-0: Z (11 U Z (JO [IJ (0 I1.! Z

O, IO 0: :1 51 ä •_• G ·,_Z :1 [M. Ö Q v c 2 H ü 9 5; é IF u Q E Ü ‘.~ N‘ .1 y_• ° L..J '·' g n--:1 & H P 0 E J· J· IIJ ··4 W : ä *.1 0 .00 •1: 50 100 150 200 VOLUME OF NGTER TRERTED (L)

Figure 26. Effect of Solution pl-1 upon Mn (II) Removal by Media #3: [14 mg Mn (II)/g of media on the surface; 2 mg/L HOC1 in the filter applied water; Influent Mn (II) = 1.0 S6 I I I concentrations or residual chlorine. lnfluent chlorine concentrations were maintained 2 mg/ L. Data included in Figures 27 and 28 show effluent Mn (ll) and residual chlorine relationships for media #1 at pH 6-6.2 and 7.8, respectively. The results of Mn (I1) up- take with the amount of chlorine consumed for media #3 at plel 6-6.2 and 7.8 are de- picted in Figures 29 and 30. Again the difference in media #1 from media #3 is the amount of oxide coating present on the surface of the media. Results for the largest, oxide-coated media (media #4; 42 mg/g) are reported in Figure 31. The results from these experiments are summarized in Table 5 and compare the Mn (Il) uptake in the presence of free chlorine with the amount of free chlorine consumed during the Mn (I1) removal process.

The research emphasis was aimed at Mn (II) removal studies in the presence and ab- sence of free chlorine. The effect of this oxidant was quite apparent in the reported data. The two basic parameters investigated were the effects of media oxide coating concen- tration and influent pH. Relationships between the amount of manganese removed with the amount of free chlorine consumed were also presented. To further investigate the mcchanisms for Mn (II) uptake, KMnO., demands on oxide-coated filter media and acid backtitrations of the exhausted filter columns were also performed. Data from these ‘ experiments are reported in the next two subsections. Potassium Permanganate Demands of Oxide-Coated Filter Media The KMnO.„ demands of the surface oxide coating of the filter media were performed for both columns with and without chlorine. With a continuous supply of chlorine fed to the column, the KMnOa demand of the oxide-coating for these columns should theore- tically be less than the columns operated without chlorine as the demand for an oxidant should have been met by the continuous supply of chlorine. The data reported in this { section are summarized in Table 6. The KMnO., demands of the oxide-coated filter 57 i ___ 1.00 2.00 Q . El EFFLLENT mmass ·a„ ·-· 0 cs-Lomus Rsstounn. d S an : 0.75 6 = =· 1.50 ct U „ Z 5 •• lg 0ä 0.50 na v 1.00. oE ä an 0 é U § 11 ". Q UJ 1: o . 25 •_·- o . so Qä E 0'Zf {B *6*...1 1 *6; zu cli;aü¤i‘.il'ß·•;;‘_ = :,=.:..„“ =餄 _00 ¤_00 • 50 100 150 200 v01..UHE 0F HRTER TRERTE0 (L)

Figure 27. Effluent Manganese and Residual Chlorine Relationships for Media 1 #1 at pl-I 6-6.2: [Influent Mn (ll) = 1.0 mg/L; lnfluent HOCI = 2

58 E _ I

__ l •ÜÜ 2 .00 < ··a• E° .1 Z LJ (D E 0_75 E EFFLLENT HRNGRNESE 1 _50 G: E 0 CI·I.CR!I~E RESIDUFJL ß J E E?x «—LJBZ [IJ 0.50 1.00 I1.! 9 OE ä'co '; 9 .1 •• .. » .. .. “ =· —· F5 |:ä J" " 6; "1h—•__ f- •• '| " ~ ‘ Pl Z U1}-! ll I u 1.1 gp “ “ “ äl lf " -1;,11 :; 0 00 • ° 50 100 150 2000 ° 00 V(].UHE 0F HPITER TREFITED (L)

Figure 28. Eflluent Manganese and Residual Chlorine Relationships for Media #1 at pl-! 7.8: °[In1luent Mn (II) = 1.0 mg/L; Influent HOCI = 2 mg/L] 59 I I I I

___ 1.00 2.00 .¤x 0 EFFLIEMT HRNOQNESE ^„ *·* 0 CI·I.CR1P£ RESIOLH. d _ „‘ cu ä 0.75 v ¢= 1.50 mCE _ Z U Ä. _] E I3 EU]Z ¢ Z Ii] Qä •=·‘_,„'¤ 0.50 1 .00 E H Q * Q6 “U . : 0.25 0.60 é ISI UI:U g E In-q _ *-1 L1 ‘u “ ¤‘V2: 2 S =‘ru u °‘1;.1 u_,_·;_' .; 0=·._•- ;= 0: “‘ N IUILZ 0 • 50 100 150 208 ' 00 VOLUME OF NRTER TREQTE0 (L1

Figure 29. Effluent Manganese and Residual Chlorine Relationships for Media #3 at pHI ‘ 616.2: [Influent Mn (II) = 1.0 mg/L; Influent HOCI = 260 I I I

I I ___ 1.00 2.00 x.1 I ä :1 I z 1.1 I 2 0.75 El EFFLLENT HNNIGRNESE _50 E 0 CPLGRINE Rssxoum. 1 2_N x x G IfI··· Z :1 lg .ÜOhäU Ü.5Ü II 1 „ '·‘__ “ = = N ·—·Q °‘äZ ° ce =~ “" ..1 0 25 9 S: 9 " Ü.5Ü g NI;.}vw E I Z U (D -JS E E 2 •·.. {I U, I! -•. —... 3 __ Saw‘ ·‘ 1- *··· :3; .. *13 :i‘·· II 0.00 7 60 100 150 2000.00 VOLUHE 0F HQTER TREQTE0 (LJ I I I I I Figure 30. Eflluent Manganese and Residual Chlorine Relationships for Media I #3 at pH 7.8: [lniluent Mn (II) = 1.0 mg/L; Influent HOCI = 2 1 mg/L] I · I 61 I I I I ¥ __ 1 .00 2,90 ä<' 2: Z u E ¤•75 m EF1=1.u6u1* nmcnusss 1 50 Q E 0 CILCRINE RESIDURL J E G ä Z ä B2 0 0.50 1.00 E _ ä 3 5 ’ „ __ ’. =· 5} E 0.25 _ — 0.60 J g ·* c Q Ü Ü I-! nn..16 .. v ; .. °= 'Ä; p- -• .. vg U" 2::•Qin: :2:*: 50 100 150 200 VCLLIHE 0F HHTER TREQTE0 (L1

Figure 31. Eflluent Manganese and Residual Chlorine Relationships for Media #4 at pH 6-6.2: [lntluent Mn (I1) = 1.0 mg/L; Influent HOCI = 2 mg/L1 62 Table 5. Relationship of Mn (II) Uptake to Free Chlorine Consumed for Different 1 Media at X/'arying pH

MEDIA INFLUENT Mn (II) HOCI SAMPLE # pH UPTAKE DEMAND w/HOCI (meq) (meq) 1 6-6.2 4.15 4.40 (3 mg/g) 7.8 3.10 2.80 ‘ „ 8.8 1.70 1.78 2 6-6.2 3.20 1.73 (4 mg/g) 7.8 6.50 7.50 3 6-6.2 7.36 5.75 (14 mg/g) 7.8 7.80 9.10 4 6-6.2 10.7* 10.3 (42 mg/2) *Note: Column study halted before complete exhaustion

63 media for columns with and without chlorine are given for different media at varying pI—I. ; , Note that the amount of chlorine taken up or the chlorine demand is also given for columns operated with chlorine.

A final experiment in this area of study was condueted to help explain the KMnO4 de- mand seen in columns operated in the presence of chlorine. An extended chlorine feed with no influent Mn (Il) and subsequent KMnOa demand was performed to ascertain if the manganese adsorbed just prior to stopping the column was accounting for the observed KMnO4 demand in columns with IIOCI present in the feed water. Table 7 represents the data from this experiment. The results indicate that an extended chlorine feed would help to oxidize the remaining adsorbed Mn2+ and the demand of the oxide— coated filter media for KMnO4 would decrease. Release of Mn2+ from Oxide-Coated Filter Media Data further investigating the mechanismrs for manganese uptake in the presence and absence of chlorine are presented in this section. Once KA/lnOa demand testing had been completed, studies investigating the potential release of surface adsorbed Mn (I I) were condueted. A pH 5.0 solution of dechlorinated Blacksburg tap water was fed through the column following completion of Mn (II) uptake experiments. Comparison of results from columns operated with and without free chlorine, would help to clarify if an oxidation process of the adsorbed Mn (I I) was always occurring, regardless of an oxidant feed. Data presented in Figures 32·36 show overlay plots of the release of Mn (II) back into solution for colurrms operated in the presence and absence of free chlorine, ‘ with Figure 33 representing the effluent pH pattern for media #2 during this pH 5.0 backtitration. Note that in these particular figures, a scale change has been made in the x-axis (volume filtered) to help clarify any trends in the data. Curves representing those columns operated withoutI chlorine being present during Mn (II) uptake show a strong64 i 6 9. Ä c: E Q äg""gg mc ccoo$2*3 $7:;mc esr: 2 ¤ 2G 2 Ü:}E ¤ °§ gg;•§ 66:'SS': 666$$2} $3.-:6 .6:3: 1-:3 Z2;} 2 Ö:¢:Q -1=‘i3 c. °· m g · .E g , E MQ • 233QM-• !~l~33 •—•{S: §Q °‘ §z Z E.~: -2ä .2 '8 *2:;:2E ::22 222:;..;,5 :2 22 13 ‘“Zxa 5..:..: ..:..: :2 2 v: · =62 ¤..„_2 E0 ggIS in “·=g 2 Z 333 332 $3 $2 2 ;•_· ·¤ Z °•-· ‘*23 E:: ä2 0 2 E E O E oa oa N oa Q ~b1~gsE dn.ä: von ~b:~¤o ~éx~ 5 gg s 2..2222 zu is *3 Sä A A 5 1::, ä' 2 52 2 2 g g 2 , ¢=¤2 Em< ...ECL **2E **2 **2 2>~ = S Ef 65 i I Table 7. Extended Free Chlorine Feed and Subsequent KMnO.. Demand for Oxide-Coated Filter Media #1 at pH 6-6.2 (expressed in meq)

MnUPTAKE(II) DEMANDHOCI EXTENDEDHOCI DEMAND.KMnO4 w/HOC1 DEMAND w/HOC1 7.07 4.93 1.24 0.30

66 initial peak of Mn (II) being released back into solution. Effluent pH values were also monitored during experimentation and a greater rise in pll occurred for columns without free chlorine (see Figure 33). In comparison, columns operated with chlorine tended to have much lower efiluent manganese concentrations throughout this release study. These results indicate that unless an oxidant such as free chlorine is present in the filter-applied water, the adsorbed Mn (Il) will not be oxidized directly on the media surface. These results agree with the work of Morgan and Stumm (19) who concluded that no reaction between adsorbed Mn2+ and MnO2(s) occurred unless the pll was greater than 9.0. Repeated Mn2+ Uptake Studies with KMnO4 and HOC1 Regeneration One particular media [media #2; (4 mg/g) surface Mn (11) concentration] was investi- gated for its regeneration capacity using KMnOa and llOC1 to oxidize the adsorbed Mn2+ on the oxide-coated media. A repetitive Mn2+ uptake study in the absence of free chlorine was performed on this media at pll 7.8. As the column began to fail during the first uptake study, a 100 mg/L KMnOa regeneneration on the exhausted media was , performed. Following the KMnO.i demand of the oxide-coated media, a second Mn2+ uptake study was conducted. Again following media exhaustion, a final KMnOa of the oxide-coating was performed. The data presented in Figure 37 show the results of re- peated Mn2+ uptake studies condueted at pH 7.8 with influent manganese concen- trations maintained at 1.0 mg/L. In the following figure (see Figure 38), the results from the KMnOa demand of the exhausted media are plotted. Oxidation of the adsorbed Mn2+ upon the oxide-coated surface by the 100 mg/L KMnOa feed resulted in addi- tional MnO2(s) being precipitated directly on the media surface. This lead to a greater amount of Mn2+ uptake during the second study (refer to Figure 37) due to additional sites for adsorption upon the oxide surface. The results from these experiments and Q subsequent studies to be addressed next are summarized into Table 8. 67 i 1 000

0Q „‘'M Z 0.75 2 u an E II Ez u (2,Q] N II ¤ °·5° 5} __ scm.6 cnnuos U]co ·| · 0Z

2 ||

‘ U. ng?v; :2 ° " ' ' ; ••_ 2 Q: ;g_g: gv e': u uu l ‘ 0.00 i' 6 10 30 60 VOLUME OF HQTER TRERTE0 (L)

Figure 32. Release of Mn2+ From Oxide-Coated Filter Media During Exposure to pH 5.0 Conditions: [Media #2; initially operated for Mn + removal ‘ at pH 6-6.2 in the presence (O) and absence ( E1) of HOCI] 68 I 7.0

u I:1 scnts cs-tones U “ ä ° ——- = ..-‘ P : * ‘~ _'.J6•0 : ä gas! — y · . U- •: J LL:] ¢ ;~ ‘ i · U I} ,: Ö! I · [ ¢ :4 u 0 |„

S .0 ••= S 10 30 S0 VOLUME OF HRTER TRERTE0 (I.)

Figure 33. Eflluent pH during Exposure to pH 5.0 Conditions on Oxide·Coated Filter Media: [Media #2; initially operated for Mn2+ removal at pH 6-6.2 in the presence (O) and absence (El) of HOCI] y 69 _ · I

I I

II E rs u ~·• Ü •75 I? 2Z Iu U CCE I?· P nl« ° H ä ·5° . [IJ° sc:-11.6 cnnwos W ° I ä .. (IZZ

ä U

•• ne,. : ’ 2 ; 2;; b- u o .00 •c 25 50 100 150 200 VOLUME OF HQTER TREQTEO (L)

Figure 34. Release of Mn2+ From Oxide·Coated Filter Media During Exposure to pH 5.0 Conditions: [Media #3; initially opcrated t°or°Mn + removal at pH 6-6.2 in the presence (O) and absence (CI) of HOC1] { { 70 1

2.00 Q OE Z 1.50 ‘,:*..=*„ ·-E III H |__g II H EJ 1 Il n ä ‘· U 00 " ·· SCRLE 01-11:1101; 2äé " * é· 2 GIZ 5 0.50 .1 ¤ = 1.;u. —. LU · ZI

we “ ...... •· 9 0 _00 ‘_«...... „„;A-:;;;..11•1;*::;::;.;-:.:-: :2;.;--* ··- 5 10 15 20 25 50 75 100 VOLUME OF HFITER TREQTE0 (L)

Figure 35. Release of Mn2+ From Oxide-Coated Filter Media During Exposure to pH 5.0 Conditions: [Media #3; initially operated for Mn + removal at pH 7.8 in the presence (O) and absence (El) of HOC1]

7l 1

1 .00 g

3\ EO •9. Z 0 •- CI ¤·—(Z E5 „ E 2 Ü E 8 0 .60 ·“Fg? äé Ü Ü El *2 rn ~ II ui “ Ü äz ¤ III 0.25 In 2 u..J tuLL

_ 0 00 c*éiiä:-...... :.„:$„-3.1-..::-§..:.:„-§d·£-é*=-S2: * 50 100 150 200 VOLUME OF HQTER TREGTED (L)

Figure 36. Release of Mn2+ From Oxide-Coated Filter Media During Exposure to pl-I 5.0 Conditions: [Media #4; initially operated for Mn + removal at pI—I 6-6.2 in the presence (O) and absence (Ü) of HOCI] 1 72

1- - Following regeneration of exhausted media using KMnO.,, the possiblity of utilizing free chlorine for regeneration during backwashing was investigated due to its inexpensiveness. The studies with free chlorine were conducted in a similar manner as in the previous experiments utiltizing KMnO.i. However, instead of only two Mn2+ uptake studies, a third and final exhaustion study was performed. Rather than employ- ingthe more costly KMnO.i, a 50 mg/L HOCI solution was fed to the columns to oxidize Mn2+ the adsorbed following depletion of adsorption sites upon the oxide coating. The HOCI demand of the oxide-coated filter media was conducted after each of the three Mn2+ uptake studies. The three M n2+ breakthrough curves during these studies utilizing HOCI to regenerate the media are presented in Figure 39. Data from the three HOC1 demands of the adsorbed Mn2+ are plotted in Figure 40. From data generated from these final experiments (refer to Figures 39 and 40), it is possible to recover the removal capacity of the oxide-coating for media #2 using free chlorine. Table 8 again summarizes data from the KMnO., and HOCI regeneration of repeated Mn2+ uptake studies. These preliminary results indicate that chlorine may be used as a viable alternative to KMnOa for the regeneration of oxide-coated filter media. However, further experiments on the effects of surface oxide concentration on the media and pH infiuences are still _ needed.

73I i 1 .00 U == = ‘ in , ° 0 a~ LI U Q, u O E v •7S Z 0 1-9 G II1- Z ‘2’ul 8 0_50 ·B FIRST EXHRUSTION STLDY w 0 SECOND EXHRUSTION STUDY t3Z § u G 2: ä '] fl U ‘ VI rg? ¢‘·’[VNV 0 .00 • 25 50 75 100 VOLUME OF HFITER TRERTEO (L)

Mn2‘*‘ Figure 37. Repcated Uptake Studies Conducted at pH 7.8 with KMr1O4 , Regeneration of Exhausted Oxide-Coated Filter Media: [Media #2; surface extracted concentration (4 mg/g); Influent Mn (II) = 1.0 mg/L] i I 74 l L I

100.0

u 0 3 E “ 0 0 Z 75.0 E „ 0 E 0 I- ‘ GI ZE 0 U U 0 ä 60.0 . U 0 FIRST PEHFIEFDTE EHNJ § _ 0 SECIID PERDIIEIHTE EHN3 i I- Z u *5,* 25.0 '·· J ln. lg. u ul NU 0 .. '

OO $0~ 0.5 1.0 1.5 2.0 VG.U|‘|E OF HRTER TRERTED (L)

Figure 38. Repeated KMnOa Regeneration of Exhausted Oxide·Coated Filter Media: [Media #2; surface extracted concentration (4 mg/g); Influent KMnOa = 100 mg/L]

' 75 i I 1 .00 A II 6.J „ ~ ·=» *.1 .- 9 E " ·· 75EZ 0 O _/ u E0: __. ZWI- U .. Z Q_5Q Lu8 ‘ 0E SECON]FIRST EXHÄIJSTIONEXPIFIISTIONSTLDYSTUOY ß '_ 0 A THIRD EX|·HL|STICI¤I STLDY Z u EZ . CK Z

O 25 EI ··/ 6* 0 „¢/ 0 .00l0 25 50 75 100 VOLUME OF NQTER TREFITE0 (L)

Figure 39. Repeated Mn2+ Uptake Studies Conducted at pH 7.8 with HOCI Re- ; generation of Exhausted Oxide·Coated Filter Media: [Media #2; sur- face extracted concentration (4 mg/g); Influent Mn (II) = 1.0 mg/L] } Ü 76 ‘ _u °

¤e :; (9

6 / 0 Q(D _ sz S ../ Z LI ·- U

E1 FIRST I-DCI. DEHPNJ g 2S_0 _ 0 SECGD |·|.'JCL 06+100 '·‘ |·¢L ä L A THIRD DEIND U U -· Qä '· Ä . I-I

¢

1, A { U 2: Q! - OO 2 lo VOLLHE OF HRTER TRERTED (L)

Figure 40. Repeated HOC1 Regeneration of Exhausted Oxide·Coated Filter Media: [Media #2; surface extracted concentration (4 mg/g); Influent KMnO4 = 100 mg/L] ß

77 6

Table 8. Repetitive Mn(I1) Uptake Studies for Media #2 at pH 7.8 in the Absence of Free Chloririe (expressed in meq)

Mn (II) KMnO4 HOC1 UPTAKE DEMAND DEMAND w/HOC1 IST EXHAUSTION 0.79 0.96 2ND EXHAUSTION 1.18 1.25 IST EXHAUSTION 1.05 0.60 2ND EXHAUSTION 1.44 0.74 3RD EXHAUSTION 1.42 0.55

78 DISCUSSIONExperimental

results are further explained ir1 the following section. This chapter pre- sents a detailed discussion into the results of the previous section and is organized into similar subsections: Mn (II) removal studies, KMnO4 demands of oxide·coated filter media, release of Mn2+ from the oxide·coated filter media and repeated Mn2+ uptake studies using KMnO4 or HOCI for regeneration of the exhausted media. A summary of most of the research results has been provided in Table 9.

Mrz (II) Removal Studies Many factors can affect the uptake of soluble manganese by oxide-coated filter media. Three parameters were extensively researched during this project: surface oxide con- centration, solution pH and oxidant effects. Characterization of Water Treatment Plant Filter Media The manganese oxide coating present on each of the media samples had a vital role in the uptake of soluble manganese. By solubilizing the manganese coating with a strong reducing agent, (HAS), the amount of oxide coating upon the filter media surface could be quantified (see Table 2). Four different media of varying amounts of oxide coating were investigated. Following a 2% KMnO4 regeneration of the media to completely _oxidize the surface coating to manganese dioxide (MnO2(s)), the HAS extractions were { 79 8 ,2S ·*~6 O, E 6£„ gg =«~ ·—·: Ög Q2 62 ..:3, O° "ivi E< ,;*2 gg E ·¢...FN ~ 6N "°„g *6“* Ö Mä0 @0 . unnc 2 I Ä •-r; "‘ Sg·c_ - ..:: E =.-. • 8*:**,2 :§ E fg 04 { °° *2 S • ‘<{!' { N;"Z*3:}Zä {c~-9 62 $2 *22 6 .„,-§ · 3;..I ~·$«S <=*· Q —:~$ Q ···2~¤: r~$ gc {g Q .„ : 3 {ggg }• ' gd; g 93 .1 : *2:2 =2 I ° ::2 6:.* 8 Z;c:Ü :: 6 *2.. E3 QÖ 26.-3 { QM5 O •c\ -6 E1: 1- oo ··*Ngg z $2 E E S. ..2 9:6=*g6 {Sg 8 "" E. I S go : E 2.- <~ä· <·'€ -: 2 C vicc *5E *2:6:o O 2 **vg,.¤ *8 I8 {Q "" ¤¤ ¤¤ 6:: N N E- °° ‘ö S~é oo §^S' 2,-=: °6 2:=2§Säge;3: S 3; 2:*SE $:2 EQ ___ 2 Z, 2... * E" c. u e; ... 0 :2::' ·—·ö··;—:2ö :8 0:*<' *26: &:""‘Uö E 3 80 ,_,§:>¥§gEgg- ~=r?°"" 2* S}? O 55 ’O 2: 3 3; S é Ü performed. Similar amounts of oxide coating were present on media #1 and #2; samples of media #3 and #4 _were included to investigate the effects of larger amounts of surface oxide coating upon Mn (II) removal.

The manganese oxide coating on the media surface acts as a surface catalyst and an adsorption site for manganese. The results from previous work by Palmer (30) indicated that the amount of oxide coating as well as the oxidation state ofthe MnOX(s) are crucial parameters for effective Mn (II) removal. That is, Mn (Il) removal efliciency is greatly enhanced by having the surface coating fully oxidized. In this study, this was accom- plished by a 2% KMnO.; regeneration prior to use of the media in the Mu (II) removal studies. In its fully oxidized state, the media surface would be expressed as a manganese dioxide (MnO2(s)) coating. As soluble Mn (II) sorbs to this MnO2($) coating, the sites for adsorption begin to decrease. Saturation of adsorption sites leads to Mn (II) breakthrough and eventual exhaustion of the oxide-coated filter media. The term often used to describe this type of Mn (Il) removal mechanism occurring in dua1—media filters is referred to as the "natural greensand effect" (2). ln the absence of an oxidant, the media's natural greensand process begins to fail and the media must be regenerated to yield new adsorption sites.

Following extraction experiments to quantify the amount of oxide coating present in its fully oxidized state, column studies were conducted in the absence and presence of a ‘ continuous feed offree chlorine. Two parameters under consideration during this testing . were media and pH effects. Removal of Mh (II) in the Absence of Free Chlorine Results obtained from column studies in the absence of free chlorine are first discussed. Dechlorinated Blacksburg, Virginia tap water containing 1.0 mg/L Mn2+ was filteredsi l I

through media-packed columns. Also serving as the "control", these column studies I could be compared with columns operated with a continuous feed of an oxidant. The influence of surface oxide concentration and pH effects were emphasized in this partic- ular phase of the study. Effects ofSurface Oxide Coating and Solution pH on Adsorption Capacity Data on the influences of surface oxide coating and solution pH on the removal of Mn (II) have been presented in Figures 1l-17. Table 3 summarized the amount of Mn (Il) uptake in relation to the amount of surface oxide coating and influent pH. An increase in both oxide coating concentration and solution pH resulted in greater capacity for Mn (Il) uptake due to the availability of more adsorption sites.

Hydrous Mn oxides presented in a microcrystalline form are characterized by high spe- cific surface areas up to 300 mz/g for MnO;. The sorption of Mn (II) to manganese oxides have been interpreted by Morgan and Stumm (10) as a surface complexation and/or ion exchange process where hydrogen ions or other cations are released as Mn2+ is adsorbed. Transition and heavy metal ions become specifically attached to the surface. This adsorption process is strongly dependent upon solution pH (See Figure 2). (10)

In this study, adsorption of Mn (II) on the oxide coating was also highly pH dependent with Mn (II) uptake increasing at higher pH conditions. One explanation for this mechanism is that as the influent pH increased, additional sites for Mn2+ adsorption were made available due to the release of H+ ions from such sites. The largest oxide- coated media [media #4; 42 mg Mn (Il)/g media] at pH 6-6.2 and 7.8 had tremendous capacity for Mn (II) removal even in the absence of an oxidant due to increased avail- II ability of adsorption sites [refer to Figure 17 (note scale change of y-axis)]. sz I Morgan and Stumm (19) showed that the net surface charge of MnO2(S) became more negative as solution pll increased, indicating that the number of adsorption sites avail- able per unit weight of MHO2(S) also would increase. During the present study, attcmpts were not made to directly determine the surface charge density of the oxide-coated me- dia. However, inferences regarding the impact that the number of available adsorption sites can be developed.

„ Consider the Mn (Il) breakthrough curves plotted in Figure 41 for two column studies, I each receiving a 1.0 mg/L Mn (Il) feed solution without HOCI present. One column had a higher manganese oxide surface concentration (14 mg/g as Mn) and was operated at pH 6-6.2. The second colurrm had a lower manganese oxide surface concentration (3 mg/g as Mn) but was operated at a higher pH condition. The pl·1 7.8 system would have produced a greater specific number of adsorption sites when considering the num- ber of negative sites per unit weight of manganese oxide. Thus, it was hypothesized that these combinations of surface oxide concentrations and operating pH conditions may have produced two columns for manganese removal with similar numbers of available adsorption sites. The breakthrough curves show that the capacity of these two columns for manganese removal is indeed similar, supporting the hypothesis that the number of available adsorption sites has a key role in defining the Mn (ll) removal capacity of manganese oxide coatings.

The surface, oxide coating concentration of the filter media and influent pH greatly in- fluenced the removal mechanisms of soluble manganese. In the absence of free chlorine, exhaustion of the filter media was observed due to the decreased capacity of the oxide coating to adsorb additional Mn2+. Calculation of the KMnOa demand of the oxide coating following filter media exhaustion showed that the KMnO4 requirement for me- ,} 83 1.00 _: III II O 0.90 ¤ ·="’··.. O = 0 2;: 3 ut! ..·0u ·· 0 " 0 E Ü.8Ü u Z Il E I'- 0.70 11 gf onen;/"’ '·‘·'z 0.60

Q {jl..41 U 0,50 02 116,6 nr 1>1~1 za „_,_, Ü 0 11 MG/G nr P1-1 66.2 ß Ü $5Z 0.40 v z 0.30 ,, E .;· „" E-1* 0.20 cf M2 Iäa fg tb 0. 10lävfzr::1 ~ 0 .00 li 0 S0 100 160 200 VOLUME OF HQTER TREQTED (L1

Figure 41. Surface Oxide Concentration and Solution pH Effects upon the Re- Absence of HOC1: [Mn (II) = 1.0 mg/L1moval of Mn (II) in the Influent l sa 1

dia regeneration was reasonably equivalent to the amount of Mn2+ uptake observed (refer to Table 9 for comparison). This result is not necessarily surprising since the Mn (Il) removed was the only oxidant demand applied to the filter column. Ilowever, the r question concerning auto-oxidation of the adsorbed Mn2"‘ by the MnOx(s) coating was yet to be answered. Was the KMnOa regeneration oxidizing only the adsorbed Mn2+ or was the demand for KMnO4 being attributed to a reduced oxide coating (MnOOll) that had been reduced during the auto-oxidation of the adsorbed Mn2+?

To help answer the question concerning the mechanisms responsible for removing Mn2+ on oxide-coated filter media, a pI·l 5.0 backtitration was performed following completion of column operations. The release data at pH 5.0 in the absence of HOCI showed sig- nificant amounts of Mn (ll) were desorbed ( > 70%; refer to Table 9). This implied that Mn2+ was adsorbed but never oxidized by the oxide coating. These results refute Weng er al. (18) who concluded that the oxide surface did auto-oxidize the Mn2+ with the concurrent reduction of the oxide surface. If this auto-oxidation process was occurring, ' the exposure to pll 5.0 conditions would not have yiclded the release of Mn2+ back into solution. This conclusion is supported by Morgan and Stumm (19) who found no evi- dence of auto-oxidation of Mn2+ by the oxide surface in the absence of an oxidant when the solution pl-I was less than 9.0.

In summary, surface adsorption appears to be the mechanism responsible for Mn2+ re- moval by oxide-coated filter media in the absence of an oxidant. The results of the pH 5.0 titration‘of the oxide-coated media also point to a potential problem in water treat- ment operations. A decreased in the pH of the filter-applied water would theoretically produce a corresponding reduction in the specific surface concentration of adsorption sites available on the oxide coating. Such a reduction in sites could lead to the possible I ss desorption of previously adsorbed Mn2+ as H+ ions replace the manganese ions. This release, occurring at the end of the water treatment process would result in elevated soluble manganese concentrations leaving the plant.

Column studies for Mn (II) desorption were only conducted at pH 5.0, so there is no direct desorption data to support this hypothesis. However, column studies did indicate that the Mn (ll) adsorption capacity of each media sample decreased as the solution pI—I decreased. Such results would give credence to the possiblity that a reduction in the pH of the filter-applied water may lead to Mn (II) desorption.

Mn (II) Removal in the Presence of Free Chlorine The presence of free chlorine promoted the efficient removal of soluble manganese across the oxide·coated filter media for extended periods of column operation. There was no evidence of significant column exhaustion occurring, even when operating under pH 6-6.2 conditions (refer to Figures 22-26; results are surnmarized in Table 9).

The question to be addressed was ascertaining the exact role of free chlorine in pro- moting the long·term manganese removal capacity of the filter media. One possibility was that free chlorine was oxidizing the Mn (ll) between the time that HOCI was added to the water and the water actually reached the filter media. However, water samples collected just prior to the top of the filter column showed no indication of manganese oxidation. Further, Knocke et al. (2) determined_that free chlorine was very inefiicient for manganese oxidation unless the solution pH was above 8.0-8.5. Instead, it was hy- pothesized that HOCI was oxidizing previously adsorbed Mn (II) on the oxide surface, with the surface serving as a catalyst for the oxidation reaction. This hypothesis is supported by considering the HOCI demand versus Mn (ll) uptake data presented in ß 86 s I Table 9 for several column studies. These results showed a reasonably close correlation between these two parameters when considered on an equivalence basis.

Previous work by Knocke er al. (3) also considered the role of free chlorine in such sys- tems under higher influent HOCI situations where the observed HOCI demand was above the stoichiometric requirements, free chlorine was thought to be reacting to re- generate the oxidation state of the MnOx(S) surface coating. In situations where chlorine consumption was less the stoichiometric amount, the complete removal of Mn2+ indi- cated that sorptive removal capacity of the filter was contributing to overall Mn (ll) removal. The authors also noted that the presence of significant free chlorine (> 1-2 mg/L as Cl;) promoted rapid, continuous uptake of soluble manganese within oxide· coated filters. (3) '

The role of HOCI in maintaining the viability of oxide surfaces for manganese removal may also be seen through data included in Figure 42. In this figure, the manganese breakthrough graphs are plotted for the same column configurations presented in Figure 41 (media #1 at pH 7.8; media #3 at pH 6-6.2). However, in these side-by-side column studies, HOCI is present in the filter-applied water. The interpretation of Figure 42 is that these sites are continually regenerated since the effluent Mn (II) concentration re- mains fairly constant for both colunms. Thus, HOCI must be functioning to oxide pre- viously adsorbed manganese ions.

Continuous regeneration of the oxide coating by HOCI has also been observed at the Williams Water Treatment Plant in Durham, North Carolina (33). This facility ceased prechlorination of its raw water in 1976 due to concems related to the formation of chlorinated organics. The point of chlorine addition was moved to just prior to the fil- ters; no other oxidants are routinely added to the water. Monitoring Mn2+ concen-

87 l trations following filtration at the plant had indicated that HOCI addition prior to the oxide-coated filters has consistently produced effluent manganese concentrations below 0.01 mg,’L, even when treating waters with solublc manganese concentrations above 0.05 mg,’ L. lt should be noted that media #4 used during this research study was in fact collected from the Williams Water Treatment Plant.

mg,’ The demand for KN/inO4 in the presence of 2 L 11OCl was attributed to the Mn2+ sorbed just prior to stopping the column. The contact time with the IIOCI was not long enough to oxidize the sorbed Mn2+. The data from the extended 11OCl study (see Table 7) resulted in a a grcatly reduced l{MnO4 demand of 0.30 meq compared to 1.73 meq ‘ for the column without the extended 1-lOCl feed. This demonstrated the need for HOC1 to fully regenerate the oxide coating during backwashing even though a continuous oxidant was being supplied.

The pl—I 5.0 backtitration studies in the presence of HOCI showed minimal Mn2+ release, again indicating that Mn2+ was being oxidized on the surface by the 1»lOCl (see Table 9). Figures 32-35 presented data from exposure to these acidic conditions in the pres- ence and absence of HOCI. Columns operated with HOCI had significantly less amount of sorbed manganese released back into solution during the pH adjustment. These re- sults again indicated that in the presence of HOCI in the feedwater, the adsorbed Mn2+ was directly oxidized on the media surface by the HOC1. Utilizing this manganese, "greensand-like" removal mechanism, the oxide—coated filter media acted to initially provide sorption sites for Mn2+ and subsequently, aided in catalyzing the oxidation of Mn2+ to MnOx(S) by HOCI.

ss 0.50

3 "’ g_4g E13 H0/0 RT PH 7.8 0 I4 MG/G RT PH 6-6.2 Z E•- CE Ez 0.30 Utu 0Z U . (*3tu 0.20 é N 0:z u •• N u u H •,• “ ° N '/ uu uu uv H .;,§· Bü ei?“ Ü “ ug u mn} 0.0 0 ¤ 0 50 IOO I50 200 VOLUME OF HHTER TREQTE0 (L)

Figure 42. Surface Oxide Concentration and Solution pH Effects upon the Re- moval of Mn (II) in the Presence of 2 mg/L HOCI: [Mn (I1) Influent = 1.0 mg/L]

89 i “ Repeated Mn2"' Uptake Studies with KMnO4 or HOCI Regeneration The final area of research included a repetitive Mn2+ uptake study of oxide-coated filter media in the absence of free chlorine followed by KMnOa or HOCI regeneration. lt was apparcnt from the data presented in Figure 37, the Mn2+ uptake capacity of the media increased during the second Mn2+ removal study. This is due to the fact that during KMnO4 regeneration of the exhausted media, not only is the adsorbed Mn2+ being oxidized to MnO2(S) but the reduction of KMnO.t is also contributing to additional MnO2(S) on the surface. This additional oxide coating allows for increased adsorption sites. The results presented in Table 8 and Figures 37 and 38 indicated that it was pos- sible to recover all of the removal capacity of the oxide coating by KMnO4 regeneration and additional Mn (ll) removal was apparent after the second exhaustion/regeneration study.

Data on the use of free chlorine to regenerate exhausted filter media were presented in Figures 39 and 40. This testing included an additional exhaustion via Mn2+ uptake study and subsequent regeneration using HOCL Results from these experiments indi- cated that free chlorine was also able to recover the removal capacity of the oxide- coating even after the third exhaustion study. Although there was a slight increase in the amount of Mn2+ removed during the second exhaustion study, the uptake capacity during the third exhaustion study remained the same. This is due to the fact that HOCI only oxidized the adsorbed Mn2+ to MnOx(s); unlike KMnO4, the reduction of HOCI did not contribute to additional MnO2(s). However, it was possible to recover the re- moval capacity of the oxide coating after three exhaustion studies with no loss of Mn (Il) uptake capacity observed. i 90 Ä Extractions of the media were conducted following each exhaustion and HOCI regener- « ation cycles; results are presented in Table 10. The amount of surface oxide concen· tration increased after each exhaustion study due to additional MnO(S) on the surface from the llOCl oxidation of the adsorbed Mn2+. From the experiments conducted, it would appear that l;IOCl can be used as a viable alternative to KMnOa for regeneration of oxide—coated filter media during backwashing. The concentration of IIOCI needed to achieve such regeneration during a typical backwash cycle would be site specific.

l 9l Table 10. Amount of Extracted Manganese after Repeated Mn2+ Uptake Studies with HOC1 Regeneration of Exhausted Media (expressed in mg Mn/g of media)

MEDIA INITIAL Mn FIRST SECOND THIRD ‘ SAMPLE # CONC (mg/g) EXHAUSTION EXHAUSTION EXHAUSTIO 2 3.37 3.53 4.15 5.05 Note: [lntluent Mn (II) = 1.0 mg/L; Operational pH 7.8]

’ 92 To investigate more efficient techniques for soluble manganese removal, continuous-flow colurnns were operated in the presence and absence of free chlorine. Characterization of four different water treatment plant filter media for their Mn2+ removal capacity, KMnO4 demands of the oxide-coated filter media and studies on the release of Mn2+ from the oxide surface were conducted. In addition, KMnO4 and HOCI regenerations of exhausted media were undertaken. Based on these results and discussion of the ac- cumulated data, the following conclusions were reached:

l. The amount of soluble manganese uptake on oxide-coated filter media is dependent upon the MnOx(S) surface coating and the extent of its oxidation state on the filter media.

2. Soluble manganese removal efficiency by oxide-coated filter media is also a function of the pH of the filter-applied water. The removal process can be significantly im- peded by acidic pH conditions. Solution pH appears to define the specific concen- tration of adsorption sites on the oxide coating.

3. Continuous application of free chlorine to oxide-coated filter media results in soluble manganese being directly oxidized upon the MnOx(S) surface coating. 93 I Ä I 4. "Aged" filter media continuously regenerated with free chlorine can result in in— creased soluble manganese removal through adsorption and subsequent oxidation directly on the media surface.

5. Repetitive Mn2+ uptake studies indicate that free chlorine can be used to regenerate exhausted oxide—coated filter media.

V In summary, the removal of soluble manganese using oxide·coated, dual-media filters I can be greatly enhanced by the presence of an oxidant, free chlorine. This continuous, free chlorine regeneration would aid in maintaining filter, eflluent manganese concen- tration below the SMCL of 0.05 mg/L and eliminate the constraints that operating in the auto—oxidative mode places upon the filter—applied pH water (30). The results of the repetitive column cycling studies indicate that strategies could be developed to maintain acceptable manganese removal capacity in oxide-coated filters while eliminating free · chlorine from the filter—applied water. By using free chlorine for regeneration during backwashing operations only, reductions in the formation of chlorinated organic com- pounds could also be realized. This would be an important consideration for many wa- ter treatment facilities facing the need to control both manganese and chlorinated organic concentrations in their finished water.

94 1

1. Wong, J. M., “Ch1orination-Filtration for Iron and Manganese Remova1." Journal of the American Water Works Asssociation., 76, 76-79 (1984). 2. Knocke, W. R., Iloehn, R. C., and Sinsabaugh, R. L., "Using Alternative Oxidants to Remove Dissolved Manganese From Waters Laden with Organics." Journal of the American Water Works Association., 79, 75-79 (1987). 3. Knocke, W. R., Hamon, J. R., and Palmer, C. C., "Solub1e Manganese Removal on Oxide-Coated Filter Media." Journal of the American Water Works Association., 80, 65-70 (1988). 4. Trace Inorganic Substances Committee, "C0mmittee Report: Research Needs For the Treatment of Iron and Manganese." Journal of the American Water Works As- sociation., 79, 119-122 (1987). 5. O’Connor, J. T., "Iron and Manganese." in Water Quality and Treatment., Prepared and Published by the American Water Works Association (1971). 6. Griflin, A. E., "Signilicance and Removal of Manganese in Water Supplies." Journal of the American Water Works Association., 52, 1326-1334 (1960). 7. Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Solutions. Pergamon Press, New York (1966). 8. Montgomery, J. M., "Iron and Manganese Removal." in Water Treatment Princi- ples and Design. John Wiley & Sons, Inc., New York, 339-342 (1985). 9. Morgan, J. J., "Chemistry of Aqueous Manganese ll and IV." Ph.D Dissertation, Harvard University, Cambridge, MA (1964). 10. Stumm, W. and Morgan J. J., Aquatic Chemistry, 2nd ed., John Wiley & Sons, Inc., New York (1981). 11. Faust, S. D. and Aly O. M., "Removal of Inorganic Contaminants." in Chemistry of Water Treatment. Butterworth Publishers, MA, 483-493 (1983). 12. Morgan, J. J., "Chemica1 Equilibria and Kinetic Properties of Manganese in Natural \Vaters.” in Principles and Applications of Water Chemistry, Edited by S.D. Faust and J .V. Hunter, John Wiley & Sons, Inc., New York, 561-624 (1967). 13. Ficek, K. J., "P0tassium Permanganate for Iron and Manganese Removal and Taste and Odor Contr0l.” in Water Treatment Plant Design for the Practicing Engineer,

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— — —— S Edited by Robert L. Sanks, Ann Arbor Science Publishers, Ann Arbor, Mich. (1980). 14. White, G. C., “Chemistry of Ch1orination." in The Handbook of Chlorination. 2nd edition, Van Nostrand Reinhold Co., New York, 182-210 (1972). 15. Sawyer, C. N., and McCarty P. L., "Residua1 Chlorine and Chlorine Demand." in Chemistryfor Environmental Engineering. 3rd edition, McGraw-Hill Book Co., New York, 385-404 (1978). 16. Edwards, S. E. and McCa11 G. B., "Manganese Removal by Breakpoint Ch1orination." Water and Sewage Works., 93, 303-305 (1946). 17. Mathews, E. R., "Iron and Manganese Removal by Free Residual Ch1orination." Journal of the American Water Works Association., 39, 680-686 (1947). 18. Weng, C., Hoven, D. L., and Schwartz, B. J., "Ozonation: An Economic Choice for Water Treatment." Journal of the American Water Works Association., 78, 83-89 (1986). 19. Morgan, J. J., and Stumm, W., "Co11oid-Chemical Properties of Manganese Dioxide." Journal ofthe American Water Works., 19, 347-359 (1964). 20. Carlson, M. A., “Oxidation of Trihalomehthane-Precursors and Manganese (II) by Chlorine Dioxide and Permanganate", Doctoral Dissertation, Virginia Polytechnic Institute and State University (1988). 21. Viessman, W. and Hammer, M. J., Water Supply and Polluzion Control, 4th Edition. Harper and Row Publishers, New York (1985). . “A 22. Aieta, E. M. and Berg, J. D., Review of Chlorine Dioxide in Drinking Water Treatment." Journal of the American Water Works Association., 78, 62-72 (1986). 23. Hair, D. H., "An Investigation of the Potassium Permanganate and Chlorine Dioxide During the Oxidation of Reduced Manganese", Masters Thesis, Virginia Polytechnic Institute and State University (1987). 24. Condie, L. W., "Toxico1ogica1 Problems Associated with Chlorine Dioxide." Journal of the American Water Works Association., 78, 73-77 (1986). 25. Legube, B., Lefebvre, E., Barbier J., Paillard, H. and Bourbigot, M. M., "Removal of Humic Substances and Manganese in Slightly Mineralized Water by Means of Iron Salts and Ozone.” Proceedings of the 1988 Annual Conference of the American Water Works Association, Orlando, FL, 1343-1359 (1988). 26. American Water Works Association, “Iron and Manganese Remova1." , Water Treatment Plant Design, Published by American Water Works Association, (1969). 27. Wilmarth, W. A., “Remova1 of Iron, Manganese and Su11ides." Water Wasres En- gineering., 5, 52-54 (1968). 28. Gaveland, A. and Heertjes, P. M., "Removal of Manganese from Groundwater by Hetergeneous Autocatalytic Oxidation", Transacrions of zhe Insrizure of Chemical Engineers., 53, 154-164 (1975).

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' 29. Sayell, K. M. and Davis, R. R., "Removal oflron and Manganese from Raw Water Supplies Using Manganese Greensand Zeolite." Industrial Water Engineering., I2, 20-24 (1975). 30. Palmer, C. C., “Manganese Removal by Oxidation and Mixed-Media Filtration", Masters Thesis, Virginia Polytechnic Institute and State University (1986). 31. Standard Methods for the Examination of Water and Wastewater, 16th edition, APIIA-AWWA-WPCF, (1985). 32. Weber, W. J., "Chemical Oxidation." in Physicochemical Processesfor Water Quality Control. John Wiley & Sons, Inc., New York, 363-411 (1972). 33. Rolan, T., Private Communication, Dept. of Public Works, Durham, N.C. (1988).

l 97 ~