Effectiveness and Mechanism of Potassium

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Water Research 36 (2002) 871–878

Eﬀectiveness and mechanism of potassium ferrate(VI) preoxidation for algae removal by coagulation Jun Ma*, Wei Liu

School of Municipal and Environmental Engineering, Harbin Institute of Technology, PO Box 2627, 202 Haihe Road, Harbin 150090, People’s Republic of China

Received 17 October 2000; received in revised form 1 June 2001; accepted 18 June 2001

Abstract

Jar tests were conducted to evaluate the effectiveness of potassium ferrate preoxidation on algae removal by coagulation. Laboratory studies demonstrated that pretreatment with potassium ferrate obviously enhanced the algae removal by coagulation with alum [Al2(SO4)3 Á 18H2O]. Algae removal efficiency increased remarkably when the water was pretreated with ferrate. A very short time of preoxidation was enough to achieve substantial algae removal efficiency, and the effectiveness was further increased at a prolonged pretreatment time. Pretreatment with ferrate resulted in a reduction of alum dosage required to cause an efficient coagulation for algae removal. The obvious impact of cell architecture by potassium ferrate was found through scanning electron microscopy. Upon oxidation with ferrate, the cells were inactivated and some intracellular and extracelluar components were released into the water, which may be helpful to the coagulation by their bridging effect. Efficient removal of algae by potassium ferrate preoxidation is believed to be a consequence of several process mechanisms. Ferrate preoxidation inactivated algae, induced the formation of coagulant aid, which are the cellular components secreted by algal cells. The coagulation was also improved by increasing particle concentration in water, because of the formation of the intermediate forms of precipitant iron species during preoxidation. In addition, it was also observed that ferrate preoxidation caused algae agglomerate formation before the addition of coagulant, the subsequent application of alum resulted in further coagulation. r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Potassium ferrate; Algae; Preoxidation; Coagulation; Enhanced coagulation; Oxidation

1. Introduction impact of some chemicals such as copper sulphate, potassium permanganate, on algae were studied. Copper The eutrophication of surface water is a worldwide sulphate [CuSO4 Á 5H2O] has been used to control problem, which is increasing in significance. Eutrophica- nuisance algae in lakes and reservoirs for more than 80 tion is caused by excessive inputs of nutrients, especially years, and is considered to be an effective algicide phosphorus, that stimulate nuisance growth of algae. available [1]. Potassium permanganate was also studied The omnipresence of algae caused by the eutrophica- on the specific use as an algicide for reservoirs [2,3]. tion of surface water is the current and growing problem In drinking water treatment, conventional coagula- in the production of drinking water. To control the tion is still the main treatment process for algae removal. massive growth of algae in lakes and reservoirs, the Whereas other treatment processes, for example, dissolved air flotation [4], sand filtration [5], direct filtration *Corresponding author. Tel.: +86-451-628-2292, +86-451- [6], which are aimed at algae removal, have also been 628-2820; fax: +86-451-236-8074. researched. Massive growths of algae have caused many E-mail address: majun [email protected], [email protected] problems. Some algae cause uncomfortable tastes and cea.edu.cn (J. Ma). odors, some algae cause filter clogging, some algae can

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penetrate the filter, leading to the deterioration of addition, recent study found that ferrate treatment did drinking water quality. Algae is also a precursor of not produce any mutagenic by-products during the disinfection by-products. Algae removal from water treatment process [15]. The effects of pre-treatment with treatment process is difficult because of their small size ferrate(VI) ion on algae coagulation by aluminium and the low specific gravity. sulphate was investigated in this paper. Also, the effects It was reported that pre-treatment with oxidants may of the pre-oxidation by ferrate on algae cell surface enhance the coagulation process and specifically en- architecture and dissolved organic materials (DOM) of hance the removal of algae and other particulate matters samples were studied by scanning electron microscopy in subsequent treatment steps [7–9]. The effects of (SEM) and UV spectrophotometer, respectively. chlorine, ozone and chlorine dioxide on Scenedesmus sp. cultures were studied [10]. Algal cells activity and chlorophyll concentration decreased, and the concentra- 2. Materials and methods tion of dissolved organic substances increased with increasing applied oxidant concentration. It was found 2.1. Raw lake water that pretreatment with chlorine dioxide (1, 3 or 5 mg/l) or ozone (2.6, 4.6 or 8.1 mg/l) on algal cultures enhanced Raw water of shallow lake located in northeast part of algal coagulation with aluminium sulphate, while pre- China, which is in deep green color indicating high algal chlorination with 10 or 20 mg/l chlorine increased the concentration, was selected in this study. Observation required dosage of alum by 15%. However, the negative results by microscopy shows that the lake water effect of using chlorine and chlorine dioxide resulting principally contains green algae, such as Chlorella, from the formation of by-products are limiting the use Spirogyra, Chlorococoum, Scenedesmus etc. of these chemicals as pre-oxidants. In addition, it was The raw water quality was listed as follows: Turbidity recently recognized that the ozonation of waters 10–30 NTU; pH 7.5–7.7; temperature 15–181C; algal 6 7 containing bromide may lead to the formation of concentration 8 Â 10 –2 Â 10 cells/l, CODMn(perman- bromate at a level suspected of being hazardous to ganate index) 10.5 mg/l (measured after filtration with health, which is a negative aspect for using ozone as a 0.45 mm membrane). preoxidant. Potassium permanganate has been investigated as an 2.2. Culture conditions alternative preoxidant for the direct filtration of impounded surface water. The experiments of modified A solution containing cultured green algae species was jar test apparatus and pilot plants showed that also used in this study in order to overcome the influence permanganate pre-treatment followed by coagulation of other materials except algae in natural surface water with dual coagulants (ferric sulphate and cationic on experimental results. Algae species chosen in cultured polymer) distinctly improved the particle and algae process are Chlorococoum and Scenedesmus, because removal commonly achieved in direct filtration [6]. It is they are commonly found in natural waters and are suggested that the common mechanism of algae removal typical green algaes. It is also because that they are easily by oxidant is the destruction of the algae architecture to available and are easily cultured in the laboratory. The various extent through different ways of oxidation. algae seeds were cultured in the plastic culture tank Potassium ferrate [K2FeO4] is another strong oxidiz- containing total 280 l modified inorganic nutrient solu- ing agent, which has a strong redox potential through tion (the concentration of inorganic salts that contained the entire pH range, ranging from À2.2 V in acid to in the solution is listed in Table 1), in which 200 ml À0.7 V in base [11]. Several investigations have been soil exudation liquid was mixed. The starting cell conducted in applying potassium ferrate as a favorable concentration in the water was 4 Â 106 cells/l at a alternative disinfection to chlorine for the disinfection of constant temperature of 15711C, pH 7.3. Continuous water and wastewater [12,13]. It is found that ferrate(VI) light was provided by incandescent lamp and day- ion appeared to be an effective antifoulant [14], as only light lamp. A gas mixture (1% CO2 in air) was bubbled short contact times were required for ferrate concentra- into the medium for a period of 15 min every other day. tion of 10À5 M to control the biofilm growth. In After grown for 25 days, algal concentration in nutrient

Table 1 Inorganic salts contained in the nutrient solution

Inorganic salts KNO3 Ca(NO3)2 MgSO4 Á 7H2OKH2PO4 FeCl3 Concentration (mg/l) 20 60 20 20 0.2 中国科技论文在线 http://www.paper.edu.cn

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solution achieved 3.5–4.2 Â 108 cells/l, pH 9.1, turbidity of the concentration of natural organic materials 20–40 NTU, CODMn(permanganate index) 5.2 mg/l (NOM) in water. In water treatment practice, the use (measured after filtration with 0.45 mm). of absorbance at 254 nm has been found to be useful for monitoring the concentration of DOC [17]. UV absor- 2.3. Coagulation tests bance was also used to characterise NOM by the degree of its aromaticity. UVA at 254 nm and UVA scanning Standard jar tests were conducted in a mixer equipped were used in this study to indicate the variation of with six-paddle jar test apparatus. The effects of various dissolved organic materials (DOM) in cultured solutions dosages of aluminium sulphate [Al2(SO4)3 Á 18H2O] on during different treatment processes. cell coagulation with and without preoxidation was tested in six 0.5 l beakers. The pH of each 0.5 l cultured sample was adjusted to 7.1 with 1 N HCl. Potassium 3. Results and discussion ferrate [K2FeO4] was prepared by the modification of – the method of reaction between OCl and Fe(OH)3 (gel) 3.1. Effects of ferrate preoxidation on coagulation in strongly basic media and isolated from saturated KOH solution [16]. The chemical solution was trans- Fig. 1 shows the effects of increasing ferrate concen- ferred into six test tubes fixed on one stick and injected tration on algae removal by coagulation with alum. The into the individual sample simultaneously by rotating removal efficiency is expressed as the ratio of the algal the stick in order to minimize the systematic errors concentration before addition of chemicals to the algal resulting from differences in the time of addition. A concentration measured at the end of the coagulation- carefully calculated amount of potassium ferrate solu- sedimentation test. It can be seen that alum coagulation tion was injected into beakers a certain time before the partially removed the algal cells in lake water, 20–30% addition of alum solution. The freshly prepared alum of algae removal was achieved at the low alum dosage of solution using analytical reagent was predetermined 20–50 mg/l; 50% of algae removal was observed at (10 mg/l aluminium sulphate). During ferrate and alum higher alum dosage (e.g. 80 mg/l, see Fig. 1a). While in addition, samples were stirred at 200 rpm for 1 min and the case of cultured solution, substantial algae removal then at 45 rpm for 10 min. Afterwards, samples were was observed with alum coagulation alone in whole allowed to settle quiescently for 20 min. Thereafter, the range of alum dosages (Fig. 1b). Thus the difference of upper 100 ml of the water sample was siphoned 1 cm algae removal between the case of coagulation alone and below the water surface and taken for determination of that with ferrate preoxidation was not very obvious for residual algae cell concentration. In the case of lake cultured water. It is worth noting that there was a sharp water, the settled samples were further filtered with filter increase of algae removal efficiency when the alum paper (1–2 mm pore size), and the residual algal dosages were between 50–60 mg/l in lake water and 40– concentration after filtration was also determined. 50 mg/l in cultured solution at the case with alum only. Residual algal concentration after coagulation test was This indicates an optimum dosage range of alum for determined by microscope counting of cells. effective coagulation of algal cells, due to the concei- vable isoelectric point between the alum and algal cells. 2.4. Scanning electron microscopy The figure shows that ferrate preoxidation has obvious effect on the coagulation of algae in either lake A blob of treated algae solution by potassium ferrate water or cultured solution. At any coagulant dosages oxidation and the control algae solution without ferrate adopted in the tests the algae removal of settled oxidation were dried for 2 h in the drying table, and they samples pretreated with ferrate is higher than that were gold-coated to a calculated coating thickness of without ferrate pretreatment. Even if ferrate dosage is 150 nm by Eiko IB-3 ion emitting apparatus. Then, they only 1 mg/l, an obvious effect of algae reduction can be were examined in a Hitachi S-520 scanning electron observed. Meanwhile, the removal efficiency increased microscope operated at 15 KV. gradually with the increase of alum dosage when pretreated by ferrate without an obvious isoelectric 2.5. UVA at 254 nm and UVA scanning point especially in cultured solution, and this lead to a relative wider optimum coagulant dosage range for the The scanning of ultraviolet absorbance was per- removal of algae. Pretreatment with ferrate remarkably formed at the wavelength ranging from 200 nm to enhanced the algae removal, so that the alum dosage 320 nm of untreated and treated cultured samples after required for achieving a certain algae removal efficiency filtered with a 0.45 mm cellulose acetate membrane filter. can be reduced. With the continuing increase of ferrate The absorbance of ultraviolet absorbance at 254 nm was dosages, the residual algae removal increased further. also determined. The absorbance of ultraviolet at Algal biocolloids carry negative surface charges at 254 nm by natural waters is a semi-quantitative indicator most pH levels [18], and the basic mechanism of iron- or 中国科技论文在线 http://www.paper.edu.cn

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Fig. 2. Variation of algae removal of settled samples with extension of preoxidation time: (a) Lake water (raw water Fig. 1. Effect of ferrate preoxidation on the removal of algae by quality: turbidity 10–30 NTU; algal concentration 8 Â 106– coagulation. Preoxidation time: 5 min. (a) Lake water (raw 2 Â 107 cells/l; pH 7.5–7.7; temperature 15–181C). (b) Cultured water quality: turbidity 10–30 NTU; algal concentration solution (raw water quality: turbidity 20–40 NTU; algal 6 7 8 Â 10 –2 Â 10 cells/l; pH 7.5–7.7; temperature 15–181C). (b) concentration 3.5 Â 108–4.2 Â 108 cells/l; pH 7.1 (adjusted); Cultured solution (raw water quality: turbidity 20–40 NTU; temperature 15711C). algal concentration 3.5 Â 108–4.2 Â 108 cells/l; pH 7.1 (adjusted); temperature 15711C). of ferrate preoxidation on algae removal in the case of lake water is more significant than that in cultured aluminium-hydroxide coagulation consists in mutual solution. It is believed that these effects are attributed attraction and neutralization of the charge by the to ferrate preoxidation, in which ferrate act as an aid to positively-charged hydroxide coagulant. While, it has coagulation processes [19]. been previously demonstrated that the NOM have a The effect of various pretreatment times on algae very strong influence on coagulation effectiveness. In removal by coagulation was tested (see Fig. 2). Algae the presence of NOM, the coagulant reacts first with the removal efficiency increased obviously even in a short free natural organic acids, e.g. humic acids and fulvic preoxidation time (as 1 min), and the removal efficiency acids in waters, and only when the coagulant dosages are further increases with the continuing extension of the high enough to neutralize the surface charges of the contacting time. Due to the slight increase of removal organic materials can the coagulant take part in efficiency with the extension of pretreatment times the electro-neutralization and bridging process [22]. (longer than 1 min), it was suggested that pretreatment Those observations lead to the conclusion that the with ferrate can influence the surface characteristics of NOM contained in surface water result in different algal cells in a very short time of oxidation and thus algae removal between lake water and cultured solution cause an enhanced coagulation. which is lack of natural organic materials, when treated In addition, the residual algae removal rate preox- with alum alone (see Fig. 1). Meanwhile, the effects idized with ferrate is further increased after filtered with 中国科技论文在线 http://www.paper.edu.cn

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1–2 mm pore size filter papers, as shown in Fig. 3, indicating that the filtration process accentuated the effects of ferrate preoxidation on algae removal. The algal cells after ferrate preoxidation and alum coagulation are easily intercepted by the filter, which otherwise did not precipitated within the sedimentation stage.

3.2. Eﬀects of ferrate preoxidation on algal cell surface architecture

The scanning electron microscopy (SEM) was used to provide additional information that was essential to better describe the underlying process mechanisms. Typical appearance of Scenedesmus (Fig. 4a) under our cultivation condition showed four cells coenobia with two spines arising from the ends of each terminal cells. All four cells have approximately the same dimensions. The cells were enclosed by a sheath (here referred to as the reticulate layer) [20]. Randomly scattered warts on the cell surface were clearly seen, which is the typical surface characteristic of Scenedesmus. Two striations as the longitudinal axis can be seen on the surface of each of the inner cells. Opposite spines regularly distribute around the spherical cell of living algae Chlorococoum (Fig. 4b). All two species of the above algal cells appeared in the pictures shrink a little compared to the living cells, because of the drying process during pretreatment needed for SEM tests. Fig. 4. Untreated cells (not subjected to ferrate preoxidation): Results of the comparison of SEM micrographs (a) four-cell coenobia with two long, straight spines arising between algal cells before and after pretreatment with from the ends of the terminal cells of Scenedesmus, Â 4000; ferrate demonstrated that the ferrate preoxidation (b) opposite spines regularly distribute around the spherical cell induced a number of clearly discernible effects on algal of Chlorococoum, Â 8000. behavior and cell architecture. The treatment caused the release of intracellular component into the surrounding medium (Figs. 5a and 6a). This phenomenon possibly caused by ferrate stimulation on algal cell or cleaved sheath by ferrate oxidation. Algae may release organic compounds into water, that are species-specific and growth phase-specific. It is reported that extracellular organic matters (EOM) from cultures of green and blue- green algae and diatoms behave like anionic and non- ionic polyelectrolytes [21]. Therefore, it is suggested that algal biopolymers secreted in response to ferrate oxidation behave as a coagulant aid. Another result of ferrate’s effect on algal cells was the intense sheath convolutions. SEM micrographs (Figs. 5b and 6a) showed that the cell surface architecture was eminent damaged, row organization of warts of Scenedesmus were not remained and the spines fell off from the Chlorococoum. What is special in potassium ferrate preoxidation is the formation of ferric hydroxide [(Fe(OH) ] colloids Fig. 3. Effects of ferrate preoxidation on algae removal of 3 settled and filtered samples of lake water. AFalgae removal after it is decomposed. Fig. 5c shows that Fe(OH)3 after filtration; BFalgae removal after sedimentation. Raw possibly precipitates on the algae surface. These water quality: turbidity 10–30 NTU; algal concentration precipitates can obviously change algal surface proper- 8 Â 106–2 Â 107 cells/l; pH 7.5–7.7; temperature 15–181C. ties. Once it is attached to the algal surface, the weights 中国科技论文在线 http://www.paper.edu.cn

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Fig. 6. Treated cells of Chlorococoum by preoxidation with ferrate: (a) extravasation of intracellular components out of the cells, Â 7000; (b) agglomeration of cells, Â 4000.

preoxidation with ferrate enhanced the coagulation (Fig. 1) through the modiﬁcation of algae envelope and their behavior as well, thus reducing the stability of algae colloids.

3.3. Eﬀects of ferrate preoxidation on UVA

Ultraviolet absorbance was used to indicate the variation of concentration and the chemical changes of DOM before and after treatment, because DOM is one of the most important factors of water quality that Fig. 5. Treated cells of Scenedesmus by preoxidation with aﬀects coagulation. Fig. 7 shows the comparison of ferrate: (a) extravasation of intracellular components out of UVA at 254 nm with and without alum coagulation in the cells, Â 4000; (b) intense sheath convolution, Â 4000; (c) various preoxidation time. UVA at 254 nm of the possible precipitation of Fe(OH)3 colloids on the algae surface, samples increased after ferrate oxidation at a very short Â 6000. contacting time (as 1 min, similar to Fig. 2), and varied smoothly with the continuing extension of oxidation time. However, UV absorbance decreased after follow- of the algal cells is increased and the algae settling ing coagulation-sedimentation process, and it also character is improved, which have been observed in the varied slightly with the extension of oxidation time. It process of coagulation. In addition, the Fe(OH)3 means that ferrate preoxidation possibly increased the colloids increased the concentration of particles in dissolved organic concentration of cultured solution or water, which is too low to cause eﬀective coagulation. changed the chemical structure of DOM. This result also Conglomeration of algal cells was also observed after suggests that the increment can be removed by follow- pretreatment with ferrate (Fig. 6b). It was suggested that ing alum coagulation. Residual DOM concentration 中国科技论文在线 http://www.paper.edu.cn

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Fig. 9. Effect of ferrate preoxidation on algae removal and Fig. 7. Effect of ferrate preoxidation on UVA at 254 nm of UVA at 254 nm of cultured solution. A-UV254 after filtration, cultured solution after filtration (0.45 mm pore size membrane treated with ferrate only; B-UV254 after filtration, ferrate filter). AFtreated with ferrate only; BFferrate preoxidation preoxidation followed by alum coagulation; C-algae removal followed by alum coagulation. Raw water quality: turbidity 20– after sedimentation, ferrate preoxidation followed alum coagu- 40 NTU; algal concentration 3.5 Â 108–4.2 Â 108 cells/l; pH 7.1 lation. Raw water quality: turbidity 20–40 NTU; algal con- 8 8 (adjusted); temperature 15711C. centration 3.5 Â 10 –4.2 Â 10 cells/l; pH 7.1 (adjusted); temperature 15711C.

of three curves occurred during the first 1 min of preoxidation time corresponding to the phenomenon that ferrate decomposed while its characteristic violet disappeared soon after injection. During the prolonged preoxidation time (e.g. longer than 1 min), algae removal increased gradually comparing to the smoothly variation of UVA at 254 nm. It demonstrated that ferrate preoxidation inactivated the algal cells in a very short contacting time and then the algae architecture was destroyed (Fig. 5b), in consequence, the cellar components were released to act as coagulant aid (Figs. 5a and 6a), which largely enhanced the following coagulation. Induced coagulant aid and the ferric hydroxide [Fe(OH)3] colloids derived from the decomposition of ferrate caused the conglomeration of algal cells (Fig. 6b) in prolonged preoxidation time, leading to the primary Fig. 8. Effect of ferrate preoxidation on scanning of UVA of cultured solution after filtration (0.45 mm pore size membrane coagulation of algae, which also enhanced the following filter). Treatment condition: alum 60 mg/l; ferrate 5 mg/l; algae coagulation by alum (Fig. 2). preoxidation time 1 h. Raw water quality: turbidity 20– 40 NTU; algal concentration 3.5 Â 108–4.2 Â 108 cells/l; pH 7.1 (adjusted); temperature 15711C. 4. Conclusions

Laboratory studies using algae-bearing lake water and preoxidized with ferrate followed by alum coagulation is cultured algae solution demonstrated that pretreatment lower than that without ferrate pretreatment. Corre- with potassium ferrate obviously enhanced the algae sponding to Fig. 7, scanning of UVA from 200 nm to removal by coagulation-sedimentation process with 320 nm shows the same information (see Fig. 8). Figs. 7 alum [Al2(SO4)3 Á 18H2O]. Algae removal efficiency and 8 together with Figs. 5a and 6a possibly support increased remarkably even at a short period of that algal biopolymers secreted in response to ferrate preoxidation time, and the efficiency was further preoxidation, which may behave as a coagulant aid. increased at a prolonged contact time. To achieve a Fig. 9 shows the effects of ferrate preoxidation on certain extent of algae removal, pretreatment with algae removal and the variation of UVA at 254 nm ferrate can reduce the dosage of alum required to cause of filtered samples. It can be seen that the large change an efficient coagulation and filtration. 中国科技论文在线 http://www.paper.edu.cn

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Efficient removal of algae caused by potassium ferrate waters-Laboratory case studies. J Water SRT-Aqua 1997; preoxidation and coagulation with alum is suggested to 46:1–11. be a consequence of several process mechanisms. Ferrate [8] Ma J, Li GB. Laboratory and full-scale plant studies of preoxidation inactivated algae, and also induced coagu- permanganate oxidation as an aid to the coagulation. lant aid secreted by algal cells. Meanwhile, ferric Water Sci Technol 1993;27:47–54. [9] Ma J, Li G, Chen ZL, Xu XG, Cai GQ. Enhanced hydroxide derived from the decomposition of ferrate coagulation of surface waters with high organic content improved the coagulation condition by increasing by permanganate preoxidation. IAWQ/IWSA Joint particle concentration in water. In addition, ferrate Specialized Conference. Particle Removal from Dams & preoxidation caused algae agglomerate formation before Reservoirs. 15–18, February, 2000, Durban, South Africa, the addition of coagulant, and the subsequent applica- 2000. tion of alum resulted in further coagulation. [10] Sukenik A, Teltch B, Wachs AW, Shelef G, Nir I, Levanon D. Effect of oxidants on microalgal coagulation. Water Res 1987;21:533–9. Acknowledgements [11] Wood RH. The heat, free energy and entropy of the ferrate(VI) ion. J Am Chem Soc 1958;80:2038–41. [12] Gilbert MB, Waite TD, Hare C. An investigation of the This work was supported by the National Natural applicability of ferrate ion for disinfection. J Am Water Science Foundation of China under the scheme of Works Assoc 1976;68:495–7. National Science Fund for Distinguished Young Scho- [13] Schink T, Waite TD. Inactivation of f2 virus with lars (Project number 59825106 ). ferrate(VI). Water Res 1980;14:1705–17. [14] Fagan J, Waite TD. Biofouling control with ferrate(VI). Environ Sci Technol 1983;7:23–125. References [15] Decula SJ, Chao AC, Smallwood C. Ames test of ferrate treated water. J Environ Eng 1983;109:1159–67. [1] Elder JF, Horne AJ. Copper cycles and copper sulphate [16] Goff H, Murmann RK. Studies on the mechanism of algicidal capacity in two Californian lakes. Envir Mgmt isotopic oxygen exchange and education of ferrate(VI) ion 2À 1978;2:17–30. (FeO4 ). J Am Chem Soc 1971;93:6058–65. [2] Fitzgerald GP. Laboratory evaluation of potassium [17] Owen DM, Amy GL, Chowdhury ZK, Paode R, McCoy permanganate as an algicide for water reservoirs. J South G, Viscosil K. NOM characterization and treatability. West Water Works Assn 1964;45:16–25. J Am Water Works Assoc 1995;87:46–63. [3] Kemp HT, Fuller RG, Davidson RS. Potassium perman- [18] Ives KJ. Electrokinetic phenomena of planktonic algae. ganate as an algicide. J Am Water Works Assoc Proc Soc Water Treat Exam 1956;5:41–53. 1966;58:255–63. [19] Ma J, Liu W. Enhanced coagulation of low temperature [4] Bare WFR, Jones NB, Middleebrooka EJ. Algae removal and low turbidity water by ferrate composite chemicals. using dissolved air flotation. J Water Pollut Control Fed Water Wastewater Eng 1997;23:9–11 (in Chinese). 1975;47:153–69. [20] Staehlin LA, Pickett-Heaps JD. The ultra-structure [5] Borchardt JA, O’melia CR. Sand filtration of algae of Scenedesmus (chlorophydeae). I. Species with the suspension. J Am Water Works Assoc 1961;53:1493–508. ‘‘reticulate’’ or ‘‘wary’’ type of ornamented layer. J Phycol [6] Fetrusevski B, Van Breemen AN, Alaerts G. Effect of 1975;11:163–85. permanganate pre-treatment and coagulation with dual [21] Bernhart H, Clasen J. Flocculation of micro-organisms. coagulants on algae removal in direct filtration. J Water J Water SRT-Aqua 1991;40:76–87. SRT-Aqua 1996;45:316–26. [22] Narkis N, Rebhun M. The mechanism of flocculation [7] Ma J, Graham N, Li GB. Effectiveness of permanganate processes in the presence of humic substances. J Am Water preoxidation in enhancing the coagulation of surface Works Assoc 1975;67:101–8.