Environ. Sci. Technol. 2002, 36, 1815-1820
Relative Importance of Charge Neutralization and Precipitation on Coagulation of Kaolin with PACl: Effect of Sulfate Ion DONGSHENG WANG,† H O N G X I A O T A N G , * ,† A N D JOHN GREGORY‡ State Key Lab of Environmental Aquatic Chemistry, RCEES, Chinese Academy of Sciences, Box 2871, Beijing 100085, China, and Department of Civil and Environmental Engineering, University College London, Gower Street, London WC1E 6BT, U.K.
The effect of the sulfate ion on coagulation with polyaluminum chloride (PACl) was investigated by using an optical monitoring technique together with the conventional jar test procedure and electrophoretic mobility (EM) measurements. The effect of the SO42-/Al ratio, dosage, and pH were examined in detail. The experimental results show that sulfate has a significantly different effect on PACl coagulation as a result of preformed hydrolysis products, where charge neutralization and precipitation play different parts in the coagulation process. The increased rate of coagulation with increasing SO42-/Al ratio can be partially explained by charge neutralization effects, through increased adsorption and complexation of sulfate, thus giving increased particle collision efficiency. Different PACl samples were prepared with different values of B ([OH]/ [Al]). For B ) 0 (i.e., AlCl3) with mainly monomers, hydroxide precipitation tends to be accelerated in the presence of sulfate, giving significant turbidity removal. The high charge neutralization ability remains for samples with B ) 1.5 and 2.0, with large proportions of preformed oligomers and polymers. Sulfate promotes aggregation of hydrolyzed species for B ) 2.5, causing significantly improved coagulation efficiency through an electrostatic patch effect. The results illustrate further that particle charge plays a less important role in coagulation after reaching a certain value, while precipitate formation improves coagulation significantly.
Introduction In water and wastewater treatment, coagulation is an important unit process by which various aquatic particles aggregate into larger flocs, which are then removed by subsequent processes, such as sedimentation, flotation, and filtration. Traditional coagulants, such as iron and aluminum salts, are widely applied to promote the aggregation of particles. Extensive previous studies have paid most attention to the behavior of traditional coagulants. Recently, there has been growing interest in the use of alternative additives. * Corresponding author phone: +86 10 62923541; fax: +86 10 62923543; e-mail:
[email protected]. † Chinese Academy of Sciences. ‡ University College London. 10.1021/es001936a CCC: $22.00 Published on Web 03/07/2002
2002 American Chemical Society
Among them, inorganic polymer flocculants (IPFs) have undergone rapid progress and are being applied on a large scale in China, Russia, Japan, and Western Europe (1-3). Although there is growing recognition of the significantly different behavior of these preformed coagulants, most discussions on their mode of action have focused on charge neutralization as a destabilization mechanism (4-7). The relative importance of precipitation in their action, especially in the presence of common anions, is not yet entirely clear (8). After intensive debates early in the 1960s, it is generally accepted that coagulation by aluminum and iron salts is through the hydrolyzed species rather than the aqueous ions themselves (9-11). Two main mechanisms can be distinguished as adsorptive charge neutralization and bridging enmeshment involving precipitation of amorphous hydroxides (12). A comprehensive theory has emerged by combining these mechanisms with concentration versus pH stability diagrams (13). Through detailed analysis of the coagulation process, a precipitation charge neutralization (PCN) model was developed by Dentel (14). This suggests that coagulation by charge neutralization can be explained as partial coverage of negatively charged particle surface by positively charged aluminum hydroxide. The adsorption of polymeric species becomes less distinguishable from the precipitation of amAl(OH)3(s) at the mineral surface with increasing aluminum adsorption and surface coverage. However, the behavior of IPFs with preformed polymer species being present prior to dosing can lead to significantly different mechanisms. A parallel study on the coagulation feature of various PACls has shown that different species in the primary coagulant solution exhibit different roles during coagulation (15). The polymer species retain a positive charge and can, hence, restabilize particles up to higher pH values (9.0-10.0) than traditional coagulants, which have mostly monomeric species and tend to precipitate in the neutral pH region. It is well-known that the presence of certain anions can have profound effects on coagulation, depending on their nature. The relative significance of different anions is found in the following sequence, ranging from the most effective to the least effective: phosphate, silicate, sulfate, fluoride, bicarbonate, and nitrate (16). As discussed in detail by Letterman and Vanderbrook from the aspects of solution chemistry, sulfate has a rather moderate effect on the aluminum hydroxide system (17). It is known to weakly adsorb on the aluminum hydroxide and to alter precipitation kinetics. However, the detailed effect of sulfate on the action of PACl remains to be investigated. After our study on the action of different forms of PACl (15), the purpose of this investigation is to illustrate the effect of sulfate on coagulation of kaolin suspensions with the same PACl samples and to examine the relative importance of the charge neutralization and precipitation mechanisms. The dynamic aggregation of particles was monitored by a simple technique, which involves the measurement of transmitted light (turbidity) fluctuations in a flowing suspension, and is very sensitive to the state of aggregation of particles (18, 19). Measurements of electrophoretic mobility (EM) and residual turbidity (RT) provide valuable insights into the different mechanisms of coagulation by different aluminum species (13, 14, 17).
Experimental Section The experimental methods applied here are similar to those in the previous study (15) and are outlined here briefly. VOL. 36, NO. 8, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Speciation Distribution and pH Value of PACl Samples no
B
Ala (%)
Alb (%)
Alc (%)
pH
PACl0 PACl15 PACl20 PACl25
0.0 1.5 2.0 2.5
94.8 44.6 25.6 9.3
5.2 50.9 70.5 68.1
0.0 4.5 3.9 22.6
3.06 3.71 3.88 4.98
Materials. The various PACl samples were prepared by using a slow base titration method at room temperature. A solution of AlCl3‚6H2O (Fisons, Leicestershire, U.K.) was titrated slowly with NaOH, using a syringe pump (ColeParmer, London, U.K.) under rapid stirring conditions. The amount of NaOH added varied with the target [OH]/[Al] ratio (B values). The chosen B values were 0, 1.5, 2.0, and 2.5, and the resulting samples were denoted respectively as PACl0, PACl15, PACl20, and PACl25. The final concentration of aluminum was 0.1 M. The samples, after aging for 1 week, were analyzed by an assay procedure using ferron reagent (Sigma, Dorset, U.K.). The ferron assay method was the same as that applied by Hsu and Cao (20). The speciation results and the corresponding pH values are shown in Table 1. It needs to be pointed out that the species distribution of PACls depend largely on the preparation methods and conditions (i.e., the concentration of primary materials, titration speed, mixing conditions, etc.) With the aforesaid conditions established as shown in this study, the species distribution of PACls can easily be repeated. A stock suspension of purified kaolin (Imerys, St. Austell, U.K.) was prepared in deionized water (Elgastat option 3 system) to a concentration of 100 g/L. The particle size distribution of the kaolin suspension was determined by an Elzone 280 PC particle counter (Coulter Electronics Ltd., Bedfordshire, U.K.). The particles were mostly below about 5 µm in size, with a mean size of about 2 µm. The electrophoretic mobility (EM) of the particles was measured by a particle microelectrophoresis apparatus (Rank Brothers, Apparatus Mark II, Cambridge, U.K.). The particles were all negatively charged at all of the pH values examined. It is known that kaolin particles undergo self-aggregation when the solution pH is decreased below about 4. Working suspensions were prepared by diluting the stock suspension in a test solution containing 5 × 10-4 M NaNO3 and NaHCO3, respectively, to provide fixed concentrations of electrolyte and alkalinity. Methods. For the coagulation tests, a modified jar test procedure was applied by using a Flocculator 90 apparatus (Kemira, Helsingborg, Sweden). This allows for different stirring-times and speeds (rapid for mixing coagulant and slow for coagulation) to be preset. A total of 800 mL of the aforesaid test solution was transferred to a 1 L beaker. Under rapid stirring conditions, 0.40 mL of the stock kaolin suspension was added by using a micropipet, giving a clay concentration of 50 mg/L. The coagulation experiments were conducted at room temperature (23∼26 °C). Prior to the addition of coagulants, the target pH was adjusted by adding a predetermined amount of NaOH or HCl into the kaolin suspension, with rapid stirring. To investigate the effect of sulfate, the required amount of a 1 M Na2SO4 solution (Fisons) was added. The coagulant was added using a micropipet. After being dosed, 1 min of rapid mixing at 250 rpm was applied, followed by 15 min of slow stirring at 40 rpm. The flocs were allowed to settle for 10 min, and the residual turbidity (RT) was measured using a Hach Ratio/XR turbidimeter (Hach, Loveland, CO). The pH of the supernatant was also measured. A small sample was taken immediately after the 1 min rapid mix period for the determination of electrophoretic mobility. 1816
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Throughout the mixing and coagulation periods, the suspension was continuously sampled by peristaltic pump and monitored by a photometric dispersion analyzer (PDA 2000; Rank Brothers Ltd.). From 30 s before the addition of coagulant, monitoring was initiated to establish a baseline reading and then continued for a further 16 min to follow the progress of coagulation. The principle of this monitoring technique has been described previously (21), and it has been applied in many studies (8, 18, 19). Essentially, fluctuations in light transmission through a flowing suspension are used to derive a semiempirical index related to the state of aggregation of particles. The transmitted light intensity is monitored by a photodiode, the output of which consists of a large average (dc) component and a much smaller fluctuating (ac) part. The root-mean-square (rms) value of the fluctuating component is derived and divided by the dc value to give a dimensionless term R ) rms/dc. With the use of this ratio, one can avoid the effects of optical surface fouling and electronic drift. It can be shown (18) that, for a heterodisperse suspension, the ratio R can be expressed as
R ) (L/A)1/2(
∑N C
2 1/2
i i
)
(1)
where L is the optical path length, A is the effective crosssectional area of the light beam, and Ni and Ci are the number concentration and scattering cross section of particles of size i, respectively. This equation demonstrates that the fluctuating signal depends on the square root of the particle concentration and on the first power of the scattering section. The latter is highly dependent on the size of suspended particles. An analysis of the term (∑NiCi2)1/2 in eq 1 reveals that smaller particles have a negligible effect on R and that, in a coagulating suspension, the larger aggregates are gradually formed and have a dramatic influence on R (18). This means that as coagulation progresses, the value of R increases. Although the ratio R does not provide quantitative information on aggregate size, the relative increase in the R value is a useful indicator of the degree of coagulation. For a given suspension, it can be assumed that the larger R values imply a larger aggregate size. In the meantime, the rapid rise of the ratio R (the larger slope of the ratio curve) has been found to be strongly correlated with the residual turbidity from a conventional jar test procedure (8). In the following results, this ratio value is called a flocculation index (FI).
Results and Discussion Coagulation at Constant pH. At constant pH 6.0 and set dose of PACl (i.e., 8 × 10-5 M as Al), the effect of the SO42-/Al ratio on EM and RT is shown in Figure 1. The dosage chosen here is just in the middle of the restabilization zone for the various PACl samples in the absence of sulfate, as shown previously (15). In this region, the negative charge of the clay particles is reversed by the adsorption of excess cationic hydrolysis products so that the suspension is restabilized for all the coagulants. When sulfate is present, it has a large effect on coagulation with PACl. Different PACl samples exhibit different behavior depending on the [OH]/[Al] ratio. With a small increase of sulfate concentration, the positive charge of particles is greatly reduced, although not reversed, as shown in Figure 1a. It is indicative of the moderate interaction of sulfate with aluminum hydroxide (17). The further increase of sulfate concentration is not able to reverse the particle charge. For the prehydrolyzed samples, that with the highest [OH]/[Al] ratio (B ) 2.5) gives the largest reduction in EM. The EM values for this and the AlCl3 sample (B ) 0) are very close, whereas for B values of 1.5 and 2.0, the EM is significantly more positive when sulfate is present. The interaction of sulfate seems stronger with PACl0 and PACl25
FIGURE 1. Effect of the SO42-/Al ratio on coagulation of 50 mg/L kaolin suspensions at pH 6.0 by PACl coagulants with [OH]/[Al] ratios B ) 0-2.5 at concentration of 80 µM as Al: (a) electrophoretic mobilities, (b) residual turbidities. than with the other PACl samples. As the SO42-/Al ratio increases, the decrease of charge levels off to a nearly constant value. The EM then has a low positive value, and it is more difficult to distinguish between the various PACl samples. The residual turbidities measured after coagulation and sedimentation with the aforementioned system are shown in Figure 1b. Corresponding with the rapid drop of EM, significant turbidity removal can be observed for both PACl0 and PACl25 at low values of the SO42-/Al ratio. As this ratio is increased above about 5, there is limited restabilization (increased turbidity) for PACl0 but no significant restabilization for PACl25. Slightly higher sulfate levels are needed for PACl15 and PACl20 to reach a limited degree of destabilization of particles. For PACl15, there is an optimum SO42-/Al ratio of about 5 (as for PACl0, but a much higher RT). For PACl20, there is no significant restabilization after a SO42-/Al ratio of about 5 is reached, but there is only a very limited removal of turbidity under these conditions. Clearly, the preformed species react differently with sulfate and give very different coagulation behavior, even though the differences in the EM values are not marked. This indicates that EM becomes less important as an indication of colloid destabilization after reaching a certain value. The better turbidity removal can be explained by precipitation formation, as becomes more evident from the dynamic monitoring results. The change of FI with time for the coagulation experiments discussed previously is presented in Figure 2. The parameter shown is the ratio of SO42-/Al applied. It can be seen that the FI value, measured continuously by optical monitoring during the coagulation process, is very sensitive to the state of aggregation. The data presented in Figure 2 show the significant effect of sulfate on coagulation with different PACl samples. At low values of the SO42-/Al ratio, there is a significant lag time before the FI value starts to rise. The lag time is greatly reduced with an increase of the SO42-/Al ratio. In many cases, a steep rise in FI begins shortly after the 60 s rapid mix phase has ended. Under slow stirring conditions, the low shear conditions allow for quite large flocs to grow. Different PACl samples show quite different FI curves. The
rate of rise (slope) and the maximum FI value reached decrease significantly with an increase of the [OH]/[Al] ratio from B ) 0 to B ) 1.5 and 2.0. However, for B ) 2.5, a different behavior is found; the maximum FI value rises again. The rapid rise of the FI curves of PACl25 and, in the case of PACl0, at a low SO42-/Al ratio correspond well with the good turbidity removal, as shown in Figure 1b. For PACl15, a rapid rise of FI is still observable at a SO42-/Al ratio around 5, and the final FI value reached is larger than those of PACl20. Accordingly, it is in good correlation with the residual turbidity, as shown in Figure 1b. It is well-known that the rate of aggregation of particles depends on several factors, such as primary particle concentration, collision efficiency, and mode of particle transport (11). With the system investigated (i.e., the same concentration of primary particles and mixing conditions), the kinetics of aggregation should thus depend mainly on the collision efficiency. The increased rate with an increased SO42-/Al ratio can be partially explained by the lower particle charge through increased adsorption and complexation of sulfate and, hence, increased particle collision efficiency. However, accelerated precipitate formation in the presence of sulfate can also be from another cause, as shown from the lag phase and different scale of FI curves for different PACl samples. At the applied dosage of PACl at pH 6, the solution should be supersaturated with respect to amorphous Al(OH)3(s). For PACl0 at a low SO42-/Al ratio, the precipitation is gradually formed and is very small. The particles remain highly positively charged, giving a low collision efficiency in the initial stage. When the precipitated hydroxide particles reach a certain size and number, bulk or surface precipitation begins and causes rapid aggregation, as shown in the rapid rise of FI. With increased sulfate concentration, precipitate formation is accelerated, and the lag phase is reduced markedly. It is noteworthy that the maximum FI reached decreases gradually with an increase in sulfate concentration. As shown from the RT curves in Figure 1b, there exists an optimum region of the SO42-/Al ratio. This optimum ratio corresponds well with the shape of the FI curves (i.e., the lag phase and larger FI value). It seems that, in the presence of an optimum amount of sulfate ions, initially precipitated hydroxide particles can aggregate rapidly to form large particles. The aggregated precipitate might form positively charged patches on the negatively charged surfaces of the clay particles. As a result, a significant “patch coagulation” could occur, as suggested from coagulation with cationic polymers (22). Attractive forces between a patch and an oppositely charged clay surface as particles collide might account for the rapid rise of the FI curves. As the [OH]/[Al] ratio of PACl increases, the species components change significantly. The solubility based on the equilibrium with am-Al(OH)3(s) is thus greatly altered. No significant bulk precipitation occurs. The aggregation of particles depends mainly on the surface charge. At a low SO42-/Al ratio, particles are restabilized by the adsorption of excess cationic polymer species. With increasing sulfate concentration, although the charge of particles can be significantly reduced, the aggregation rate is relatively slow because of the low concentration of primary particles. With PACl25, sulfate induces coagulation of the colloidal aluminum species. An increased precipitation occurs, causing much more rapid coagulation aggregation, as shown by the steep rise of FI. The aggregation of species results in a loss of surface charge through complexation with sulfate and further hydrolysis, which corresponds well with the EM changes as shown in Figure 1a. However, a large quantity of polymers preformed in PACl25 will still be available to maintain its higher charge neutralization ability than PACl0, which results in no significant restabilization in residue turbidity, as shown in Figure 1b. VOL. 36, NO. 8, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Effect of the SO42-/Al ratio on the flocculation index for the same systems as in Figure 1: (a) PACl0, (b) PACl15, (c) PACl20, (d) PACl25. As observed here, sulfate has a significant effect on coagulation with PACl, depending on the SO42-/Al ratio and aluminum speciation. An increase in the SO42-/Al ratio above a certain value gives little further effect. A concentration of 10-3 M sulfate was then selected to examine further the effect of different species in PACl. The concentration adopted here is also close to the value in the study of Letterman and Vanderbrook on the effect of solution chemistry (17). Figure 3 shows the change of EM and RT with increasing coagulant dosage. From Figure 3a, it can be seen that EM increases, first rapidly with the increase of dosage and then tends to a plateau. The curves are not greatly different, but the higher the [OH]/[Al] ratio, the better charge neutralization ability. However, for samples with B ) 1.5 and 2.0, the positive mobility at the plateau is higher than that for B ) 2.5. For B ) 0 (AlCl3), the lowest positive mobility is observed. The RT curves exhibit more pronounced differences, as shown in Figure 3b. Clearly, different PACls show very different turbidity removals in the presence of sulfate. At a low concentration of Al(III), RT drops rapidly with an increase in coagulant dose. The higher the [OH]/[Al] ratio, the lower the dosage at which the decrease occurs. With a further increase in dosage, no restabilization is found for PACl0 and PACl25, while restabilization can still be observed for PACl15 and, especially, for PACl20. Apart from a rather later onset of destabilization, PACl0 behaves quite similarly to PACl25. An explanation for these results is that sulfate reacts quite differently with PACl on account of the different species being present: (i) complexing with monomer and promoting precipitation so that the formation of polymers with high charge neutralization ability is prevented, (ii) complexing and promoting aggregation of polymer species, and (iii) coagulating the colloidal aluminum hydroxide as a result of sulfate complexation decreasing the surface charge. In PACl0, the main components are monomers (i.e., 95% Ala), as shown in Table 1. At a low SO42-/Al ratio, the hydrolysis of Al(III) might still form certain polymer species. While at a high SO42-/Al ratio, sulfate complexes strongly with monomers, preventing the formation of polymer species such as Al13. This has also been shown by Kerven et al. (23). In the 1818
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FIGURE 3. Effect of coagulant dosage on (a) electrophoretic mobility and (b) residual turbidity with different PACl samples in the presence of sulfate (1 mM). meantime, the formation of precipitate can still be favorable and is accelerated through sulfate bridging (24). For PACl15, monomers remain one of the main species, 45% Ala. Therefore, the formation of precipitate can still be favored. However, with increasing amounts of polymer species, there is a high charge neutralization ability. This becomes more evident with PACl20, which contains mainly polymer species. Because the aggregation of polymer species still takes place
FIGURE 4. Effect of pH on coagulation with PACl (80 µM as Al) in the presence of sulfate (1 mM): (a) electrophoretic mobilities, (b) residual turbidities. in the presence of sulfate, which forms small crystal precipitation as shown by Bersillon et al. (25), a certain degree of destabilization occurs, as shown clearly in Figure 3b. In PACl25, the main species are polymer and colloidal particles. After being dosed, sulfate causes significant coagulation of these species, and a rapid precipitation occurs. The lower EM observed indicates the stronger interaction of sulfate with PACl25. The results indicate further that charge neutralization and precipitation give different coagulation behavior.
Coagulation at Constant SO42-/Al Ratio and Dosage: Effect of pH. The change of EM and RT as a function of pH at a fixed SO42-/Al ratio and dosage is presented in Figure 4. The sulfate and Al concentrations are 1 mM and 80 µM, respectively. Obviously, pH has a profound effect on the behavior of PACl in the presence of sulfate. As shown in Figure 4a, EM decreases monotonically with an increase in pH, depending on the value of the [OH]/[Al] ratio. The sample with B ) 2.0 gives the highest EM throughout the pH range, followed by B ) 1.5, 2.5, and 0. The pH at the isoelectric point (IEP) follows a similar trend, with the highest IEP (about 8.5) being that for B ) 2.0. Although sulfate interacts only moderately with hydrolyzed aluminum species, a significant decrease of EM can be observed as compared to those in the absence of sulfate, as shown previously (15). The effect of pH on RT is shown in Figure 4b, and there are marked differences between the various PACl samples. PACl15 and PACl20 all show rather poor turbidity removal but with some improvement at higher pH values. Some restabilization is also evident when the pH exceeds about 9.5. These observations are broadly in line with the EM data in Figure 4a. It is also evident that coagulation with PACl25 is much more effective, because significant turbidity removal can be achieved throughout the pH range. There is no clear correlation with the EM data in this case, indicating that precipitation probably plays a major role. It is likely that sulfate promotes the aggregation of the hydrolyzed species. However, the efficiency deteriorates with a further increase of pH, and the turbidity at pH values higher than about 9.5 is rather similar to that of the other PACl samples. The behavior of AlCl3 (B ) 0) is quite different. This coagulant gives moderate turbidity removal but only up to about pH 7.5, above which the performance deteriorates rapidly. The IEP in this case is about 7.2, and it appears that restabilization occurs when the EM becomes more negative than about -0.5 µ ms-1/v cm-1. It is likely that both precipitation and charge neutralization are important. The effect of pH is also apparent from the dynamic monitoring results shown in Figure 5. The parameter shown indicates the pH value. From the FI curves in Figure 5a, it can be seen that the aggregation rate with PACl0 increases markedly as the pH is raised. A similar trend can be observed
FIGURE 5. Effect of pH on flocculation index for the same systems as in Figure 4: (a) PACl0, (b) PACl15, (c) PACl20, (d) PACl25. VOL. 36, NO. 8, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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for a PACl15 and PACl20 increase in pH of up to about 9.5. However, in the high pH region, the decrease of aggregation rate is not so sharp as with PACl0. A much slower transition occurs, corresponding well with the change of EM. An interesting feature that is apparent with PACl25 is the decrease of aggregation rate with increasing pH. It indicates that the promoted aggregation of hydrolyzed species is significantly different from the accelerated precipitation, as in PACl0. Different species result in precipitates with quite a different nature. In PACl25, the aggregates coagulated with sulfate remain relatively highly charged, which induces possibly an electrostatic patch effect as evidenced by the rapid rise of FI compared with the other PACl (15). With an increase in pH, the positive charge of the aggregates becomes gradually neutralized. Then, the effect of patch coagulation decreases accordingly. Practical Implications. During the passed several decades, the IPFs have undergone rapid development and have the trend of becoming the main water and wastewater treatment reagents instead of the traditional low molecular metal salts. However, the exact mechanism for their superperformance remains to be explored. The experimental results show that the different species being preformed exhibit quite a different role on coagulation. In the presence of specific anions, the reaction pathways can be largely altered. The polymer species, once preformed, exhibit quite a stable nature, as shown in PACl15 and PACl20, and function mainly through charge neutralization, which would be more effective for source waters containing a large quantity of highly negatively charged particulates and DOC. The coexisting anions promote the precipitation of monomers and coagulation of colloidal species, which significantly accelerates the aggregation kinetics. Within a certain range, the particles formed in the presence of anions can act as electrostatic patches resulting in efficient coagulation. It indicates that the particle size distribution in IPFs is another important factor contributing to their coagulation efficiency. Therefore, to introduce the bridging function into IPFs would further promote their coagulation performance. Furthermore, the physicochemical property of water quantity can vary from time to time and water to water. The IPFs can then be tailormade to fit for the fluctuations in source water quality to achieve maximum coagulation results.
Acknowledgments D.W. is grateful to the kind help of Mr. Muhammad Saleem during the experiments at University College London (UCL). The scientific cooperation and discussions with Dr. I. M. Solomentseva during her visit to UCL are also appreciated. The kind advice from Dr. Rajat K. Chakraborti at State University of New York at Buffalo is greatly appreciated. We
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are very grateful for the kind comments and suggestions of the anonymous referees. This work is supported by grants from China (NSF 29807004 and 59778019). Partial funding was provided by UCL.
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Received for review December 5, 2000. Revised manuscript received December 11, 2001. Accepted December 12, 2001. ES001936A
