Water Research 36 (2002) 4959–4962 Effectiveness of ferrate (VI) preoxidation in enhancing the coagulation of surface waters Jun Ma*, Wei Liu School of Municipal and Environmental Engineering, Harbin Institute of Technology, P.O. Box 2627, Harbin 150090, People’s Republic of China Received 6 August 2001; received in revised form 18 April 2002; accepted 17 May 2002 Abstract Standard jar tests were conducted to evaluate the effectiveness of ferrate preoxidation in enhancing the coagulation of surface waters. A substantial reduction of residual turbidity after sedimentation and filtration was achieved by ferrate preoxidation in all cases of the investigation of various water qualities at low ferrate dosage (0.5–1.0 mg/l). The enhancement of the coagulation was more obvious when the organic content in the waters were relatively high. r 2002 Published by Elsevier Science Ltd. Keywords: Potassium ferrate; Preoxidation; Coagulation; Turbidity; Enhanced coagulation 1. Introduction pH range (ranging from –2.2 V in acid to –0.7 V in base) [4], has received more and more attention recently. Preoxidation has been one of the principle means for The results of initial studies, in which ferrate improving the coagulation process, which is generally preoxidation was applied in alum coagulation of the aimed at destroying the organic coating on the surface of surface waters, showed that ferrate preoxidation could particles. Traditionally, chlorine has been used to aid the decrease residual algae concentration [5–7], destroy coagulation of waters with a high organic content. phenol and coprecipitate heavy metal ions [8], and aid However, the negative effect of using chlorine, resulting the removal of turbidity [9,10]. Due to the unique from the formation of hazardous by-products, is limit- characteristics of combining oxidant and coagulant ing the use of chlorine as a preoxidant [1]. qualities of ferrate (VI), it is a desirable preoxidant in It has been found that at very low ozone dosages drinking-water treatment. preoxidation has a positive effect on the coagulation of surface water by improving the removal of turbidity, whilst greater doses have a negative effect, resulting in 2. Materials and methods an increase of residual turbidity [2]. Ma et al. [3] reported that permanganate preoxidation 2.1. Raw water quality (at low permanganate dosage, i.e., 0.5–1.5 mg/l) greatly enhanced the coagulation of surface water as expressed Three kinds of Chinese surface waters with different in terms of residual turbidity. characteristics were selected in this study. A summary of Another potential oxidant, potassium ferrate (VI), the water quality of the three kinds of raw waters is which has a strong redox potential through the entire shown in Table 1. 2.2. Experimental procedure *Corresponding author. Tel.: +86-451-6282-292; fax: +86- 451-2368-074. Preoxidation with ferrate followed by aluminium E-mail address: majun [email protected] (J. Ma). sulphate coagulation–sedimentation was conducted in 0043-1354/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 0043-1354(02)00224-5 4960 J. Ma, W. Liu / Water Research 36 (2002) 4959–4962 Table 1 Water-quality characteristics of the three kinds of surface water Water source Turbidity Colour Temperature pH Hardness Alkalinity Permanganate (NTU) (CU) (1C) (mgCaCO3/L) (mgCaCO3/L) index (mg/l) Reservoir Shi 282 25 20 7.5 135.6 163 35.5 River Songaria 50–60 15 22 7.0 67.1 110 8 River Songarib 26–28 35 2 7.1 68 52 12.2 a In summer. b In winter. a series of 1 l glass beakers in which water samples were 20 agitated with a six-unit stirrer apparatus. Some of the Alum 50 mg/L Alum 60 mg/L water samples and a certain dosage of potassium ferrate 15 solution were mixed at a speed of 300 rpm for a period Alum 70 mg/L Alum 80 mg/L of time. Potassium ferrate [K FeO ] solid was prepared 2 4 10 by modifying the method of reaction between OClÀ and (NTU) Fe(OH)3 (gel) in strongly basic media and isolated from the saturated KOH solution [11], and stored in the 5 desiccator. Potassium ferrate solution (0.3 g/l, calculated in K2FeO4) was prepared by dissolving potassium 0 ferrate solid in distilled water just before use in order Residual turbidity after sedimentation 00.51 to minimise the loss of ferrate as a result of rapid (a) Ferrate dosage (K2FeO4, mg/L) decomposition rate in solution. Then, all of the water 1 samples were subjected to coagulation with the addition Alum 50 mg/L Alum 60 mg/L of specific dose of aluminium sulphate solution (10 g/l), 0.8 which was prepared by dissolving the analytical reagent Alum 70 mg/L Alum 80 mg/L (Al2(SO4)3 Á 18H2O, Tianjin chemical Inc., Tianjin, 0.6 China) in distilled water, at 300 rpm for 1 min. Subse- 0.4 quently, the samples were slowly stirred with the (NTU) coagulant at 60 rpm for 10 min, and settled for 30 min. Samples of supernatant after sedimentation were 0.2 siphoned from 1 cm below the water surface, and filtered with a filter paper (1–2 mm pore size). The residual Residual turbidity after filtration 0 00.51 turbidity of the settled and filtered water was analysed (b) Ferrate dosage (K FeO , mg/L) using a turbidity meter (2100A, HACH Chemical 2 4 Company, Loveland, USA). The residual iron and Fig. 1. Influence of ferrate preoxidation on the residual manganese concentrations in water were measured using turbidity after sedimentation and filtration of Reservoir Shi an Atomic Absorption Spectrophotometer equipped water. pH: 7.5; temperature: 201C. with a graphite furnace (AA-670, Shanghai Analytical Instrument Factory, Shanghai, China). lation performance of the surface water, with a substantial reduction of residual turbidity of the settled 3. Results and discussion water. At low alum dosages (50, 60 mg/l) adopted in the tests, a substantial reduction of residual turbidity was Residual turbidity was used as the principle indicator observed (more than 4 NTU) when the water samples for the evaluation of the effectiveness of ferrate were pretreated by ferrate even at the low dosage of preoxidation in enhancing the coagulation of surface ferrate. water. The remarkable reduction of residual turbidity after Fig. 1 shows the comparative results of the coagula- filtration (filtered by 1–2 mm pore size filter papers) was tion of the water from Reservoir Shi with and without also observed (Fig. 1b) when pretreated with ferrate. ferrate preoxidation. Alum coagulation has an optimum Thus, the filtration process accentuated the effects of alum dosage at 70 mg/l, since a higher alum dose ferrate preoxidation, indicating that the floc particle size resulted in an increase of the residual turbidity of the in the process of coagulation with ferrate preoxidation is settled water. Ferrate preoxidation enhanced the coagu- larger than those without preoxidation. J. Ma, W. Liu / Water Research 36 (2002) 4959–4962 4961 Table 2 alone, residual manganese after sedimentation and Variation of residual Fe and Mn with and without ferrate filtration was markedly reduced by the ferrate preoxida- preoxidation tion. Parameter index Ferrate (mg/l) Fig. 2 shows the results of the coagulation tests for Songhua River water in summer. The residual turbidity 0 0.5 1.0 after alum coagulation and sedimentation were rela- Residual Fe after sedimentation (mg/l) 220.5 117.5 115 tively low (below 5 NTU) (Fig. 2a). The residual Residual Fe after filtration (mg/l) 53.2 32.6 22.4 turbidity with ferrate preoxidation after sedimentation Residual Mn after sedimentation (mg/l) 37.5 32.5 28 was only slightly lower than in the case without ferrate Residual Mn after filtration (mg/l) 36.8 12.5 8 preoxidation (about 2 NTU) at low alum dosage (20 mg/ l), and did not have further reductions with the increase Note: Reservoir Shi water. Alum dose 70 mg/l; initial concen- tration of Mn and Fe in raw water are 80 and 350 mg/l, of ferrate dosage. The residual turbidity after filtration respectively. was rather low (below 0.5 NTU), and ferrate preoxida- tion shows no improvement on residual turbidity (Fig. 2b). The same coagulation test was conducted using 10 Songhua river water in winter (Fig. 3). Songhua River Alum 20 mg/L water in winter is the typical surface water with low Alum 30 mg/L temperature (21C) and low turbidity (26–28 NTU), Alum 40 mg/L Alum 50 mg/L containing more organic materials (Permanganate in- dex, as COD, 12.2 mg/l) than that in summer (8 mg/l). 5 The low temperature and low turbidity water is rather (NTU) difficult to coagulate due to the slower rate of hydrolysis of coagulant and the difficulty in flocculation because of the low concentration of particles in the water. 0 Residual turbidity after sedimentation 00.51 (a) Ferrate dosage (K2FeO4, mg/L) 12 Alum 30 mg/L 1 10 Alum 40 mg/L Alum 20 mg/L Alum 50 mg/L 0.8 Alum 30 mg/L 8 Alum 60 mg/L Alum 40 mg/L Alum 50 mg/L 6 0.6 (NTU) (NTU) 4 0.4 2 0.2 Residual turbidity after filtration 0 Residual turbidity after sedimentation 00.51 0 00.51 (a) Ferrate dosage (mg/L) (b) Ferrate dosage (K2FeO4, mg/L) 3 Fig. 2. Effect of ferrate dosage on various water-quality Alum 30 mg/L parameters for Songhua River water in summer. pH: 7.0; Alum 40 mg/L temperature: 221C. 2 Alum 50 mg/L Alum 60 mg/L When the water was pretreated with ferrate, a certain 1 amount of iron was introduced into the water as the initial form of ferrate; the residual iron concentration in the water is of special concern.