Improved coagulation performance using preformed polymeric iron chloride (PICl)

Colloids and Surfaces A: Physicochem. Eng. Aspects 339 (2009) 192–198

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Improved coagulation performance using preformed polymeric iron chloride (PICl) Ta-Kang Liu a,∗ , Ching-Ju Monica Chin b a b

Institute of Ocean Technology and Marine Affairs, National Cheng Kung University, 1 University Road, Tainan City 70101, Taiwan Graduate Institute of Environmental Engineering, National Central University, 300 Jhongda Rd., Jhongli City 32001, Taiwan

a r t i c l e

i n f o

Article history: Received 1 August 2008 Received in revised form 21 January 2009 Accepted 16 February 2009 Available online 25 February 2009 Keywords: Coagulation Polymeric iron chloride (PICl) Restabilization Charge reversal

a b s t r a c t Jar tests were conducted using synthetic waters containing model colloids and organics to evaluate the coagulation performance of simple FeCl3 and polymeric iron chloride (PICl) having various polymer yield. Coagulation of synthetic model waters with PICls of different hydrolysis ratios was compared to simple ferric chloride under varying conditions of pH and model water concentrations. The use of PICls seemed to produce similar reduction in turbidity and TOC when compared with using FeCl3 as a coagulant under typical range for coagulation; however, treatable region was broadened to lower pHs for coagulation of both model waters. By eliminating the region of restabilization of turbidity at lower pHs, an additional region for coagulation using PICls at pH 5–6 was observed. A conceptual surface charge distribution when using different coagulants was established to explain the restablization observed in this study. The different coagulation behaviors of PICls when compared to simple FeCl3 are probably due to their larger sizes and bearing lower charge density. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Aluminum and iron salts are commonly used as coagulants in the treatment of water for public supply. Inexpensive compared to organic polymeric coagulants, these metal salts are used in large quantity as the primary coagulants in drinking water treatment processes. Coagulation includes all the reactions such as in situ coagulant formation, chemical particle destabilization and physical interparticle contacts while flocculation is a transport phenomenon that involves particle collisions [1]. Both physical transport and chemical destabilization are important in coagulation and flocculation processes; however, physics controls particle aggregation only when the chemistry is favorable [2]. A major drawback of using these metal-based coagulants directly is that the actual coagulant species are formed in situ by dilution under the prevailing raw water conditions and in competition with other reaction [3]. Therefore, it is crucial to control the chemistry of the coagulant in order to improve their effectiveness and optimize the coagulation and flocculation. Partial neutralization of metal salts prior to their application is a technique that can ensure optimum solution conditions and avoid the interference of competitive conditions. By partially neutralizing the metal salts, the desirable highly charged cationic coagulant species can be obtained prior to their application. Polyaluminum

∗ Corresponding author. Tel.: +886 6 2757575x31146; fax: +886 6 2753364. E-mail address: [email protected] (T.-K. Liu). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.02.029

chloride (PACl) has been developed by partially neutralizing AlCl3 and its use in practice has been continuously spreading [4,5]. It is believed that most of these commercial PACl coagulants contain substantial amount of tridecamer Al13 [6]. Coagulant species prepared by partially neutralizing the metal salts can be more effective due to their larger size and bearing higher positive charges, which makes them more strongly adsorbed on negatively charged surface of natural colloids. In connection with interests in improving the coagulation processes, numerous researchers have applied PACl for water and wastewater treatment and a considerable amount of literatures have been generated since the 1980s. PACl coagulants were found to be superior to simple alum in their wider operating pH range, lower dose required and less sludge produced, better performance in colder conditions, more effectively precipitating organic substances, and lower residual aluminum in treated water [7–11]. However, Shi et al. [12] found that PACl was less effective than conventional aluminum salt in removing humic acid with large molecular and hydrophobic properties, possibly due to the decomposition of Al13 during the coagulation processes. In recent years, public concerns about the potential connection between the residual aluminum in drinking water and the suspected adverse health effects, e.g., Alzheimer’s disease, have been brought to intense discussion [13,14]. As a result, the use of ironbased coagulants has gradually gained in popularity. The polymeric iron chloride (PICl), however, is still in a developing stage and its application is limited in contrast to the successful commercialization of PACl coagulants to improve coagulation processes [15]. One of the main problems seems to be the low polymer yield in the pre-

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Table 1 Speciations of the preformed coagulants PICl.

PICl-1.0 PICl-2.0

Hydrolysis ratio

pH

1.0 2.0

2.6 2.7

Unpolymerized fraction 58% 21%

Polymerized fraction 42% 79%

Precipitated fraction 0 0

formed PICl solutions. The polymer yield of PICl reported by Gray and Wang was ∼8% and ∼3%, respectively [3,15]. In order to increase the polymer yield, other inorganic anions such as silicate and sulfate were also used to prepare polymeric iron species [5,16]. However, Wang and Tang [5] found that the polymer yield in polyferric silicate (PFSi) is between 2% and 10%. Our previous study showed that “B rate”, i.e., the rate of change of hydrolysis ratio during preparation, has a dramatic effect on the speciation of PICl solution [17]. High yield PICl polymers of 79% were produced in our laboratory under a specific B rate and were stable without precipitation of ferric hydroxide microcolloids after aging for several weeks. The main objective of this study is to investigate the potential advantages of using this high yield PICl polymer as a coagulant for water treatment. In particular, jar test experiments were conducted using carefully controlled model waters in an attempt to identify the optimum treatment conditions for the preformed PICl coagulants in comparison to that of simple ferric chloride. 2. Materials and methods

Fig. 2. Residual turbidity of jar tests under different dose of PICl-1.0 and pH using model turbidity of 10 NTU. Number shown under each dot is residual turbidity after sedimentation.

2.1. Synthetic model waters Kaolin clay was selected for this study to simulate natural colloids since it is one of the most abundantly occurring clay minerals. The kaolin (Fisher Scientific Corp., Pittsburgh, PA) used in this study is certified ACS grade and pre-acidwash to remove organic or carbonate impurities. Twenty-five grams of kaolin was dispersed in 1 L of distilled water by an intense mechanical mixing. The resulting slurry of kaolin clay was then poured into a large plastic container and diluted to a total volume of 15 L. It was again thoroughly agitated to ensure that a uniform suspension was created. After settling for 48 h, the supernatant was siphoned off and served as the “seed” of colloids. Synthetic waters of desired turbidity were prepared by diluting the seed stock solution with distilled water. Since the synthetic model colloids were pre-settled for 48 h, the model sys-

Fig. 1. Residual turbidity of jar test under different dose of simple FeCl3 and pH using model turbidity of 10 NTU. Number shown under each dot is residual turbidity after sedimentation.

tems should have fine colloidal particles whose size is similar to those found in the pre-sedimentation reservoir of a water treatment plant. Using Stoke’s equation, the particle size can be roughly estimated to be less than ∼1 ␮m. In order to simulate natural particulates, concentrated humic substance from Suwannee River, GA, was added to the kaolin colloidal suspension to obtain a TOC concentration of 5 mg/L in the model waters. It should be noted that a 24-h period was allowed to equilibrate humic substances adsorbing onto the kaolin particles prior to its being used for coagulation study. 2.2. Preparation of coagulant solutions The solutions of crystallized FeCl3 ·6H2 O (Fisher Scientific Corp., Pittsburgh, PA) were prepared in 0.1 M and were only made when needed to avoid aging. The PICl solutions were obtained by the addi-

Fig. 3. Residual turbidity of jar tests under different dose of PICl-2.0 and pH using model turbidity of 10 NTU. Number shown under each dot is residual turbidity after sedimentation.

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Fig. 4. Comparison of residual turbidity under different type of coagulants using model turbidity of 30 NTU.

tion of 1.0 M NaHCO3 into 50 mL freshly prepared ferric chloride solutions using a Dosimat Model 665 autotitrator (Metrohm Ltd., Herisau, Switzerland) at addition rates of 100 ␮L/s with vigorous stirring. The preparation of PICl solutions was at 20 ◦ C using a temperature controlled water jacket and the ferric chloride solutions were partially neutralized at hydrolysis ratios (B = [OH]/FeT ) of 1.0 and 2.0 (referred as PICl-1.0 and PICl-2.0). Due to the dilution of the base added, the concentration of Fe(III) in PICl-1.0 and PICl-2.0 were 0.091 M and 0.083 M, respectively. The PICl solutions were aged for 24 h prior to the jar test experiments. 2.3. Coagulation study The first series of jar tests were conducted at room temperature using two model turbidity systems of 10 NTU and 30 NTU. The second series of jar tests were performed using model water having a turbidity of 10 NTU and TOC of 5 mg/L. The performance of PICl-1.0 and PICl-2.0 in comparison to that of simple ferric chloride (B = 0) to treat model turbidity under various treatment conditions was then evaluated. Jar tests were conducted at different concentrations of Fe(III), i.e., 0.015 mmol, 0.025 mmol, 0.05 mmol, 0.075 mmol, 0.1 mmol, and 0.2 mmol Fe/L. For each Fe(III) concentration, different amounts of the 0.2N sodium bicarbonate or sulfuric acid were added to the synthetic waters before coagulation to adjust alkalinity and consequently the final pH of the test waters. Trial and errors were necessary for adding NaHCO3 or H2 SO4 in a way such that the final pH can be well spread over a range from 3.0 to 10.0. After 1 min of rapid mixing, 20 min of flocculation and 30 min of sedimentation, 100 mL of the supernatant were drawn for pH, turbidity, and electrophoretic mobility (EPM) measurements. For the second series of jar tests, the supernatant of the jars in which coagulation occurred was filtered through a 0.7-␮m glass fiber and the

filtrate was measured for TOC. A three-dimensional dataset then can be generated with the three variables, pH, coagulant dose, and the final residual turbidity or TOC. This set of data was plotted on a log–log scale sheet with the thermodynamic boundaries of Fe(III) on it and used for optimizing the pH and coagulant dosages. This approach attempted to elucidate a pattern of response over a wide range of values of experimental parameters. 2.4. Analytical measurements The turbidity of the solutions in the coagulation study was measured using a Hach Model 18900 Ratio Turbidimeter (Loveland, CO). The EPM of the samples was measured using a Brookhavan ZetaPlus (Brookhavan Instruments Corp., Holtsville, NY). A Dohrmann model DC-180 carbon analyzer (Rosemount Analytical Div., Santa Clara, CA) was used for TOC analysis. The PICl solutions are operationally classified into unpolymerized, polymerized, and precipitated fractions. The speciations of the prepared polymeric coagulant of PICl-1.0 and PICl-2.0 using this classification is presented in Table 1. The polymerized and precipitated fractions were determined using a Brookhaven BI-9000 photon correlation spectrometer (PCS, Holtsville, NY). The unpolymerized fraction, having a size close to 1 nm, was determined by the colorimetric ferron method [17]. 3. Results and discussions 3.1. Comparison of coagulation using model colloids Figs. 1–3 show the first series jar test results of coagulation using model colloids of 10 NTU with three types of coagulants, i.e., simple

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FeCl3 , PICl-1.0 and PICl-2.0, respectively. The residual turbidity was plotted on the solubility diagram (or coagulation diagram) of Fe(III) with thermodynamic boundaries of Fe3+ , FeOH2+ , and Fe(OH)4 − indicated, following the methods developed by Amirtharajah and Johnson [18]. Regions of restabilization, 90% and 50% removal of turbidity are also shown. Each dot on the plot corresponds to a jar test under a specific final pH of the mixed solution and coagulant dose applied. The residual turbidity is shown under each data point. Regions of 90% and 50% turbidity removal were also depicted by interpolating the data set at residual turbidity of 1 NTU and 5 NTU, respectively. Amirtharajah and Johnson [18] have defined specific regions on the iron coagulation diagram according to types of coagulation mechanism. It is typically found that charge neutralization mechanism occurs around pH 5–6, while sweep coagulation normally occurs for pH > 6. At acidic pH, restabilization happens due to charge reversal. In Figs. 1 and 2, a restabilization region can be found on the left-hand side of the boundary where coagulation does not occur. The boundary of this restabilization region was similar to that defined by Amirtharajah and Johnson [18]. In this region, no visible flocs were observed and sometimes the residual turbidity was even higher than the initial turbidity. In Fig. 2 when PICl-1.0 solution was used as a coagulant, this pattern (i.e., region of restabilization and coagulation) was found to be similar to that of using simple FeCl3 as a coagulant (Fig. 1). The region of 90% removal of turbidity occurs when pH and Fe(III) concentration were greater than ∼6 mM and ∼0.05 mM, respectively. The region of 50% removal of turbidity falls in the same range of pH and a Fe(III) concentration of greater than ∼0.015 mM. Nonetheless, the maximum pH at which restabilization occurs is ∼5.5 in Fig. 2, which is approximately one-half of a pH unit smaller when compared to that of using simple FeCl3 coagulant. The maximum pH of jar tests investigated was ∼10 since values above this pH are not practical for drinking water treatment. In Fig. 3, where PICl-2.0 was used as a coagulant, the region of restabilization was, however, not observed at lower pHs. Filterable pin-point flocs were seen instead even at a very low pH around 3 for various coagulant doses applied in this study. Coagulation occurs for pH greater than ∼3 at all concentrations of coagulant tested, i.e., 0.015–0.2 mM. The region of 90% removal of turbidity has extended to a pH around 5 for Fe(III) concentration greater than 0.1 mM. However, the performance of these three coagulants are fairly close in the pH range of 6.0–9.0. The first series of jar tests were also conducted using model waters with turbidity of 30 NTU under otherwise the same conditions. Comparison of residual turbidity at different coagulant concentrations treating the 30-NTU model colloids was plotted in Fig. 4. In Fig. 4(a) where lower concentration of coagulants was tested, the reduction of turbidity was not very noticeable. For concentration of Fe(III) greater than 0.05 mM shown in Fig. 4(b)–(d), one can find that the most significant difference among these three coagulants is their performance below pH 6. Two observations can be made in this region of pH < 6: (1) a sharp increase in residual turbidity occurs at pH ∼6 for both simple FeCl3 and PICl-1.0; (2) When PICl-2.0 is used, the residual turbidity increases somewhat gradually as pH decreases. The optimum pH range is similar for these three coagulants, approximately pH 7–9 for Fe(III) concentration greater than 0.05 mM. 3.2. Effect of model colloid concentrations Comparison of the different boundaries on the coagulation diagram from two model colloids is presented in Fig. 5. In Fig. 5(a) for using simple FeCl3 , two observations can be made for the model colloids having a higher turbidity: (1) the region of restabilization decreases; and (2) the region of 90% turbidity removal increases. The increase of colloid concentration will increase the total surface

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Fig. 5. Comparison of different boundaries on the coagulation diagram from two model water systems using different coagulant. (a) Ferric chloride as a coagulant and (b) PICl-2.0 as a coagulant.

area available for adsorption, and thus increase the amount of coagulant required to destabilize these colloids. As a result, the region of restabilization will decrease for waters having higher concentration of colloids since it needs more coagulant for charge reversal to occur. The second observation shows an inverse relationship between colloid concentration and optimum removal of turbidity, implying a removal mechanism of sweep coagulation. In Fig. 5(b) for using PICl-2.0 solution, one can find that the boundary for 90% turbidity removal increases for the model colloids having higher turbidity throughout the pH range examined, especially in the acidic pH range. The improved quality is probably due to the increased collision efficiency at higher concentration of colloids. Upon inspection of Fig. 5, it can be found that using model colloids of 30 NTU is close to the previous tests using 10 NTU model water; however, the optimum coagulant dose is lower for the model colloids of 30 NTU. PICl-2.0 consistently showed better performance under pH 6 where restabilization occurred when the other two coagulants were used. Hence, the improved performance of PICl-2.0 when the concentration of model colloids increased may be an indirect evidence that PICl-2.0 can increase collision efficiency during flocculation. The iron polymers typically carries 0.5–0.75 charge per iron atom whereas a free ferric ion carries three charges [19]. Due to their larger sizes and bearing lower charge density than free ferric ions, it is believed that PICls are less hydrated and can more strongly adsorb on negatively charged surface of natural colloids. Non-DLVO forces such as the hydration force can be much stronger than either of the

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Fig. 6. Comparison of electrophoretic mobility (EPM) under different coagulant dose and pH using model turbidity of 30 NTU.

two DLVO forces at small separations when two surfaces approach closer than a few nanometers. They are particularly important for determining the magnitude of the adhesion between two surfaces in contact [20]. Contaminant colloids destabilized by polymers are considered less hydrated and smaller hydration force will be experienced during an event of collision. As a result, improved performance of using PICl-2.0 was observed for system having higher model turbidity. 3.3. Characteristics of electrophoretic mobility Results of the EPM measurements for these series of jar tests using model waters of 30 NTU are presented in Fig. 6. These data were obtained on settled, unfiltered supernatants. In Fig. 6(a) where coagulants were underdosed at 0.025 mM, the flocs remain negatively charged except when pH is approximately below 5 for three coagulants used. For concentration of Fe(III) greater than 0.05 mM as presented in Fig. 6(b)–(d), one can find that the colloids destabilized by PICl-2.0 have the lowest EPM especially for pH < 6. In most cases, the change of EPM for jar tests using PICl-2.0 is more gradual than that of using FeCl3 or PICl-1.0 solutions. From inspection of the slope near the point of zero charge, it also can be found that the slope of EPM curve using PICl-2.0 is relatively smaller. In general, the colloids destabilized by PICl-2.0 was less positive in pH 4–6 and less negative in pH 8–10 in comparison to that by FeCl3 and PICl-1.0. The optimum pH range, i.e., pH 7–9 at 90% removal of turbidity corresponds approximately to the EPM range of −1 to 0 ␮m cm/V s for all Fe(III) concentrations tested. For jar tests using model waters of 10 NTU, measurements of EPM were fairly simi-

lar to that when using model waters of 30 NTU. In both cases, the optimum coagulation conditions corresponding to EPM of −1 to 0.5 ␮m cm/V s and the point of zero charge occurred at pH 6–7 for the three coagulants. In general, when PICl-2.0 was used with a concentration greater than 0.025 mM Fe(III), the colloids were less positively charged around pH 4–6 and also less negatively charged around pH 8–10. Theoretically, coagulation occurs when repulsive electrical forces between particles are at a minimum. The magnitude of EPM corresponds to the zeta potential and thus the repulsive forces between particles. Therefore, one may expect maximum coagulation when EPM or zeta potential is close to zero. In this study, the optimum coagulation occurs at pH 6–8 for different type of coagulants and different initial turbidity, and it corresponds to colloidal surfaces of zero to slightly positively or negatively charged. 3.4. Coagulation of model colloids/NOM system The second series of jar tests were performed on model waters that have TOC of 5 mg/L and turbidity of 10 NTU, using simple FeCl3 and PICl-2.0 coagulants. Boundaries of 90% and 30% turbidity removal can be seen in Fig. 7. The percentage removal of TOC was also labeled beneath the data points. Different boundaries are found for both coagulants used. Since experiments were conducted using different type of coagulants and under otherwise the same conditions, the observed differences can be attributed to the type of coagulants used. When PICl-2.0 was used, additional region of organic removal near the thermodynamic boundaries around pH 3–4 was also observed. The reason for additional removal on the

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tive disinfectant. The pKa of hypochlorous acid is around 7.5, which implies less chlorine is required if coagulation is in acidic pH where hypochlorous acid is the dominant species. The use of PICl have a potential of enabling water treatment plant to conduct coagulation in a more acidic pH range that will result in a cascade of benefits for water treatment, such as less amount of alkali and chlorine used, thus less THMs in treated water, reduced sludge production, longer filter runs, and a smaller decrease in operating costs. 4. Conclusion

Fig. 7. Percentage TOC reduction using model NOM of 5 mg/L C. Boundaries shown are coagulation area for 90% turbidity reduction (dash line) and 30% turbidity reduction (solid line). TOC measurements were only conducted for jars that coagulation occurs.

left-hand region is probably the same for coagulating model colloids. For pH < 6, the PICl-2.0 produced coagulated particles that were less positive than that of using FeCl3 , similar to the EPM pattern indicated in Fig. 6. As a result, restabilization was prevented in this region. However, overdosing still can cause restabilization around pH 5 for coagulating model NOM/colloids systems. The partial neutralization of PICl-2.0 solution was used in comparison with simple FeCl3 for coagulation of model NOM systems in the hope of demonstrating if species in PICl-2.0 were unique in nature so that more reduction of organics can be achieved. In Fig. 7, a very close organic removal of ∼80% was observed for using both PICl-2.0 and FeCl3 . Nonetheless, PICl-2.0 can achieve ∼80% removal at a lower dose than using FeCl3 (0.2 mM vs. 0.3 mM). 3.5. Engineering significance Due to the restabilization at an acidic pH range when FeCl3 is used, a conventional water treatment plant is usually operated at a pH close to neutral or even higher pH to avoid restabilization. However, a sudden change of raw water quality such as during a storm event may sometimes shift the treatment conditions and results in a lower pH and therefore the unfavorable restabilization and shutdown of plant. In addition, operating at a higher pH may require more alkali and thus produce more sludge. Prechlorination is a practical treatment alternative in water treatment for algae and pathogen control. Chlorine reacts with water and forms hypochlorous acid that is the most effective disinfectant among all the aqueous species of chlorine. However, hypochlorous acid dissociates into hypochlorite ion at higher pH that is a less effec-

In this work, the feasibility of using a preformed iron(III)-based polymeric coagulant for application in drinking water treatment was explored. In order to evaluate the potential of utilizing this polymeric coagulant, jar tests were performed using model waters. It was found that the polymeric coagulant have broadened the treatable region of coagulation in the jar test experiments treating model colloids. The optimum pH extends to pH ∼5 when PICl-2.0 was used as a coagulant in comparison to pH ∼6 when FeCl3 is used. For coagulating model NOM/colloids system, treatable region was also broadened and similar TOC removal can be achieved at low coagulant dosage when PICL-2.0 was used. In additions, restabilization was not observed when PICl-2.0 was used, whereas it occured for pH < ∼6 when FeCl3 was used. Based on the charges each iron atom carries, the charge density of polymers is less than low-molecular-weight oligomeric species in FeCl3 solution. Colloidal particles are usually less negatively charged in acidic pH range and over-adsorption of coagulant causes charge reversal on the surface and thus the restabilization of the colloidal suspension. The higher charge density of oligomeric species may explain the restabilization observed in acidic conditions. When colloids are destabilized by polymeric coagulant in acidic pH, the lower charge density of the polymer makes the surface potential less positive than that destabilized by oligomeric species. Therefore, restabilization of colloids is then prevented as a result. Furthermore, the iron polymers are less hydrated due to their larger size and bearing low charge density than free ferric ions, model colloids destabilized by polymers are thus considered less hydrated and smaller hydration force will be experienced during an event of collision. Improved performance of using PICl-2.0 was therefore observed when the concentrations of model colloids increased, which may be an indirect evidence of an improved effectiveness upon particle collision. Acknowledgements The authors wish to thank Dr. Edward Chian from Georgia Institute of Technology for his help in providing the concentrated NOMs from Suwannee River. During the revision of this paper, the kind comments by the anonymous reviewers are greatly acknowledged. References [1] A.A. Amirtharajah, C.R. O’Melia, Coagulation processes: mixing, destabilization and flocculation, in: F.W. Pontius (Ed.), Water Quality and Treatment, 4th ed., McGraw-Hill, New York, 1990, pp. 269–365. [2] J.E. Tobiason, C.R. O’Melia, Physicochemical aspects of particle removal in depth filtration, J. Am. Water Work Assoc. 80 (1988) 54–62. [3] K.A. Gray, C. Yao, C.R. O’Melia, Inorganic metal polymers: preparation and characterization, J. Am. Water Work Assoc. 87 (1995) 136–146. [4] H.X. Tang, W. Stumm, The coagulation behaviors of Fe(III) polymeric species-I. Preformed polymers by base addition, Water Res. 21 (1987) 115–121. [5] D.S. Wang, H.X. Tang, Modified inorganic polymer flocculant-PFSi: its preparation, characterization and coagulation behavior, Water Res. 35 (2001) 3418–3428. [6] J.M. Duan, J. Gregory, Coagulation by hydrolysing metel salts, Adv. Colloids Interf. Sci. 100–102 (2003) 475–502.

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[7] B.A. Dempsey, R.M. Ganho, C.R. O’Melia, The coagulation of humic substances by means of aluminum salts, J. Am. Water Works Assoc. 76 (4) (1984) 141–150. [8] B.A. Dempsey, H. Sheu, T.M.T. Ahmed, J. Mentink, Polyaluminum chloride and alum coagulation of clay–fulvic acid suspensions, J. Am. Water Works Assoc. 77 (3) (1985) 74–80. [9] B.A. Dempsey, Removal of naturally occurring compounds by coagulation and sedimentation, CRC Crit. Rev. Environ. Control 14 (4) (1984) 311. [10] J.G. Janssens, A. Buekens, Theoretical analysis and practical application of the kinetic model of flocculation in the interpretation of jar tests, Aqua 2 (1987) 91–97. [11] T.R. Hundt, C.R. O’Melia, Aluminum–fulvic acid interaction: mechanisms and application, J. Am. Water Work Assoc. 80 (1988) 176–182. [12] B.Y. Shi, Q.S. Wei, D.S. Wang, Z. Zhu, H.X. Tang, Coagulation of humic acid: the performance of preformed and non-preformed Al species, Colloids Surf. A: Physiochem. Eng. Aspects 296 (2007) 141–148. [13] G.F. Nordberg, Human health effects of metals in drinking water: relationship to cultural acidification, Environ. Toxicol. Chem. 9 (1990) 887–894.

[14] R.F. Packham, D.D. Ratnayaka, Water clarification with aluminum coagulants in the UK, Water Suppl. 10 (1992) 35–47. [15] D.S. Wang, Z.K. Kuan, H.X. Tang, Differences in coagulation efficiencies between PACl and PICl, J. Am. Water Works Assoc. 95 (2003) 79–86. [16] J.Q. Jiang, N.J.D. Graham, Observation of the comparative hydrolysis precipitation behaviour of polyferric sulfate and ferric sulfate, Water Res. 323 (1998) 930–935. [17] T.K. Liu, E.S.K. Chian, Effect of base addition rate on the preparation of partially neutralized ferric chloride solutions, J. Colloid Interface Sci. 284 (2005) 542–547. [18] P.N. Johnson, A.A. Amirtharajah, Ferric chloride and alum as single and dual coagulants, J. Am. Water Work Assoc. 74 (1983) 232–239. [19] T.G. Spiro, S.E. Allerton, J. Renner, A. Terzis, R. Bils, P.J. Saltman, Hydrolytic polymerization of iron(III), J. Am. Chem. Soc. 88 (1966) 2721–2726. [20] P. Raveendran, A.A. Amirtharajah, Role of short-range forces in particle detachment during filter backwashing, J. Environ. Eng. ASCE 121 (1995) 860–868.