Environ. Sci. Technol. 2010, 44, 6371–6376
Breakage and Regrowth of Al-Humic Flocs - Effect of Additional Coagulant Dosage W E N - Z H E N G Y U , †,‡ J O H N G R E G O R Y , * ,† AND LUIZA CAMPOS† Department of Civil, Environmental and Geomatic Engineering, University College London, Gower Street, London, WC1E 6BT, U.K., and State Key Laboratory of Urban Water Resource and Environment (SKLUWRE), School of Municipal & Environmental Engineering, Harbin Institute of Technology, PO 73, Huanghe Road, Nangang District, Harbin, 150090, China
Received March 9, 2010. Revised manuscript received May 16, 2010. Accepted July 9, 2010.
The growth, breakage and regrowth of flocs formed by aluminum sulfate (alum) with humic acid (HA) in water at neutral pH was investigated by jar testing with continuous optical monitoring. Various initial dosages of alum and different breakage shears were investigated to compare the floc strengths and to explore the growth of flocs and regrowth of broken flocs. In all cases there was significant irreversibility of floc breakage when no additional coagulant was added. On the other hand, when a small additional dosage of alum was added to the suspension during floc breakage, the size of regrown flocs was higher than that before breakage. The result did not change with the variation of the initial dosage of alum, and the intensity and duration of floc breakage, provided that the additional coagulant was added shortly before the end of the breakage process. It seems that aluminum hydroxide is better able to form flocs, when newly precipitated, rather than after an extended period of high shear.
can adsorb on the precipitate and hence be removed by physical separation processes such as sedimentation and filtration. All of the experiments in the present work have been conducted at pH 7 where the second mechanism is much more important. Physical properties of hydroxide flocs, such as size, density, and strength have been investigated in recent years (3-5), and these may influence the regrowth ability of broken flocs. Floc regrowth after exposure to high shear was limited for all flocs under investigation, including Fe precipitate, FeNOM, and Al-NOM (3). Wang et al (5) found that humic flocs formed with AlCl3 and polyaluminum chloride (PACl) were difficult to regrow when the flocs had been exposed to highspeed stirring. Xu et al. (6) considered that there was a distinct irreversibility of the break-up process of flocs formed by nanoAl13 polymer and PACl. The flocs formed in natural water with alum and PACl showed the same behavior as in earlier studies (7). The result with kaolin flocs was similar with that for NOM flocs (8-10). The aim of the present work is to investigate factors which may influence the regrowth of broken hydroxide flocs, in order to give a better understanding of the process. In a stirred vessel, flocs break when the stirring speed is increased and (partially) regrow when the stirring speed is reduced. Both the breakage time and stirrer speed significantly affect floc regrowth, and these factors have been systematically examined for the alum-humic acid system. A major effect on floc regrowth has been found when a very small additional dosage of coagulant is introduced during the floc breakage period. In this case flocs may regrow completely after breakage, whereas for a 1-shot coagulant addition regrowth is limited. This effect has not been previously reported and could have great practical significance. This paper is thus focused on the growth, breakage, and regrowth of alumhumic flocs, including the effect of a second coagulant addition at the floc breakage stage. An improved understanding of this process should be an important addition to knowledge in the field of NOM coagulation and removal.
Materials and Methods Introduction Coagulation and flocculation remains the most common process for water treatment, normally for the removal of turbidity and natural organic matter (NOM). Even with microfiltration and ultrafiltration, there is a need for pretreatment by coagulation and flocculation to reduce membrane fouling, especially in the presence of NOM. Although the removal of pollutants by coagulation and flocculation has been studied for many years, knowledge of the floc growth mechanism remains limited. Furthermore, the regrowth of broken flocs is of both fundamental and practical significance. For removal of humic substances by hydrolyzing coagulants such as aluminum and ferric salts, two main mechanisms have been proposed (1, 2): (a) Binding of cationic metal species to anionic sites of the humic material, thus neutralizing their charge and leading to precipitation. This is more significant at fairly low pH, around 5 or less. (b) At higher pH, precipitation of amorphous metal hydroxide occurs quite rapidly and it is likely that many dissolved organic substances * Corresponding author phone: +44 20 7679 7818; e-mail:
[email protected]. † University College London. ‡ Harbin Institute of Technology. 10.1021/es1007627
2010 American Chemical Society
Published on Web 07/26/2010
Stock Humic Acid Solution and Coagulant. Four grams of humic acid, sodium salt (HA, Aldrich, Cat: H1, 675-2), was dissolved in deionized (DI) water, with pH adjusted to 7, and mixed by a magnetic stirrer for 24 h. The solution was diluted to 500 mL in a measuring flask and was stored in the dark. Aluminum sulfate hydrate (Al2(SO4)3 16H2O; Fisons, UK, >96%) “alum” was used. Stock alum solutions were prepared at a concentration of 0.1 M in DI water. Jar Test. For the flocculation tests, the stock humic acid was diluted in DI water (800 mL), in a 1 L beaker, with 5 mM NaHCO3 to give test solution with a HA concentration of 10 mg/L. During the jar test, the pH of final solution was maintained at 7.0 by prior addition of a predetermined amount of 0.1 M HCl. The temperature was maintained at 25 ( 1 °C. The solution was stirred at 50 rpm for 60s and then a certain dosage of alum was added, with a simultaneous increase of stirring speed to 200 rpm. The equipment used was a Flocculator 2000 (Kemira Kemi, Helsingborg, Sweden), which enables mixing speeds and times to be preset. The rapid mix speed of 200 rpm was maintained for 1 min, and then reduced to 50 rpm for 10 min or more to allow floc growth to occur. The speed was then increased to 200 rpm (or 80, 100, 150, 300, and 400 rpm) for 1 min to break the flocs, and then back to 50 rpm for 10 min for the flocs to VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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regrow. The relationship of mean velocity gradient, or shear rate (G) to the stirring speed can be calculated following Hermawan et al (10). For stirring rates 50-400 rpm, the shear rate varies between 23 and 520 s-1. In the case of a second coagulant addition, the additional dosage of alum was added into the stirred suspension during the floc breakage phase. Zeta Potential Measurement. Samples were taken of flocs after dosing and rapid mixing of coagulant and zeta potentials were determined by a Zeta Plus instrument (Brookhaven Instrument Corporation, U.S.). Measurements were also made on samples immediately after floc breakage. Floc Monitoring. During the entire process of floc formation, breakage and regrowth, sample was siphoned from the stirred jar at 20 mL/min through a 3 mm transparent plastic tube and continuously monitored by a device based on the “turbidity fluctuation” technique (11). The same principle is used in the photometric dispersion analyzer (PDA 2000, Rank Brothers, UK). After passing through the monitor, sample was returned to the stirred jar by a peristaltic pump. The experimental procedure was similar to that of Yukselen and Gregory (8). The average transmitted light intensity (dc value) through the flowing sample and the root-mean-square value (rms) of the fluctuating component are monitored. The ratio (rms/dc) provides a sensitive measure of particle aggregation and it is often called the Flocculation Index (FI). The FI value is strongly correlated with floc size and always increases as flocs grow larger and so provides a sensitive measure of particle aggregation (11). It significantly increases as aggregation occurs, and decreases when aggregates are broken. In this work, after the rms value reached an initial steady value (at 60 s), coagulant was added into the solution and the rms and dc values were recorded through a data acquisition card and logged on a computer at 1 s intervals. The FI results presented below are the absolute ratio (rms/dc) values (expressed as a percentage), taking into account the gain applied to the fluctuating signal. (In several earlier publications, FI values are given in arbitrary units). FI values depend essentially on the number concentration of particles (flocs) and their extinction (or scattering) cross sections. Hydroxide flocs are aggregates of very small primary particles and are of low density and hence of low effective refractive index. It follows that their extinction cross sections can be much less than their projected area. In other words, measurements by light transmission methods may give apparent floc sizes significantly smaller than the true values. Measurements based on forward light scattering may also give misleading results (12). When clay suspensions are dosed with hydrolyzing coagulants, the resulting flocs are composed of amorphous hydroxide precipitate with included clay particles. In such cases, the clay particles can give the dominant contribution to the optical properties of the flocs and the extinction cross section may be considerably increased, even though the actual floc size may be little changed (13). For the same reason, the FI value can decrease with increasing coagulant/particle ratio, for example, for a fixed particle concentration and increasing coagulant dosage. Essentially, increasing coagulant gives more flocs, but each floc has less particles and a lower extinction cross-section. The net effect is a decrease in FI value. In such cases the FI value does not give an unambiguous indication of floc size. It should be stressed that this effect is specific to hydrolyzing coagulants in the sweep coagulation regime. Also, it should affect other optical sizing methods, as well as the “turbidity fluctuation” technique used here. In the case of alum/humic flocs similar behavior would be expected, although the effect of humic acid on the extinction cross-section of alum flocs is not as great as with clay particles. Adsorbed organic material probably influences light absorption as well as light scattering by flocs, both effects contributing to the extinction coefficient. For alum/humic 6372
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FIGURE 1. Zeta potentials of alum-humic flocs, as a function of coagulant dosage, before and after breakage. Ten mg/L humic acid at pH 7.0. flocs FI values are quite low, typically at least 10× lower than for comparable alum flocs with clay particles. Nevertheless, they are easily measurable and increase strongly as flocs grow. The initial low FI value (about 0.005%) is due to noise in the opto-electronic circuit and to stray impurity particles in the sample.
Results and Discussion The Effect of Initial Dosage. Humic acid solutions (10 mg/L) were coagulated over a range of alum concentrations at a final pH of 7.0. Immediately after the 1 min rapid mix period, samples were taken for zeta potential measurements. Results in Figure 1 show that the alum-humic flocs are negatively charged at low alum dosage and that charge reversal occurs at around 0.2 mM Al. At higher dosages the zeta potential increases only slightly and does not exceed 5 mV. These values are typical of aluminum hydroxide flocs at pH 7. Also shown in Figure 1 are zeta potentials for flocs after a 1 min breakage period and it is clear that there is no significant difference between the two sets of values. In this respect, the surfaces exposed by floc breakage are indistinguishable from those of the original flocs. Results from continuous monitoring of humic acid coagulation with different alum concentrations are shown in Figure 2. At the lowest dosage (0.1 mM Al), no change of flocculation index was observed (Figure 2a). At a slightly higher dosage (0.13 mM) the FI begins to rise significantly only after a lag time of about 3 min, but for 0.15 mM Al and higher dosages a rapid rise in FI occurs soon after the end of the rapid mix phase. In these cases, the FI reaches a plateau value after a few minutes. Comparing these results with the zeta potential data in Figure 1, the onset of coagulation corresponds with dosages where the zeta potential has risen to about -8 mV. At a dosage of 0.15 mM Al, the zeta potential is about -3 mV and for higher dosages there is no appreciable change in the onset or rate of rise of the FI value. These results are typical of the behavior of hydrolyzing coagulants at moderate dosages and neutral pH, where a hydroxide precipitate is formed (9). There is a variable lag time, depending on coagulant dosage, during which colloidal precipitate particles are formed, which then aggregate to form flocs. During the lag phase, the aggregates are not yet large enough to give a measurable increase in FI value. The plateau FI values represent limiting floc sizes, determined by the effective shear rate in the stirred vessel and the strength of the flocs. When the zeta potential is more negative than about -10 mV, formation of flocs is considerably hindered. As the alum dosage increases above about 0.2 mM Al, there is a tendency for the plateau FI value to decrease slightly. This effect arises from a decreased extinction cross-section of the flocs, as explained above, and does not necessarily
FIGURE 2. The effect of initial dosage of alum (mM Al) on the formation, breakage and regrowth of flocs, as measured by the Flocculation Index. (a) without additional coagulant dosage and (b) with additional 0.03 mM Al added half way through the breakage period. Arrows show the beginning of a 60 s period of stirring at 200 rpm. Coagulant addition was at the start of a 60 s period of rapid mixing at 200 rpm and followed by slow stirring at 50 rpm.
FIGURE 3. Floc growth, breakage, and regrowth with 0.15 mM Al at different breakage stirring speeds (a) without additional dosing; (b) with additional 0.03 mM Al added half way through breakage period. Arrows indicate beginning of 60 s breakage period. imply a decreasing floc size with increasing coagulant dosage. For a given dosage, the FI value provides very useful comparative information on the growth, breakage, and regrowth of flocs. After a steady FI value had been attained, the stirring speed was increased from 50 to 200 rpm for 60 s. This caused an immediate and rapid decrease in FI, indicating significant floc breakage. The minimum FI value during the high speed stirring is related to the size of the broken flocs, which depends on floc strength and on the applied shear during breakage. When the stirring rate was returned to 50 rpm, the flocs began to regrow; but only to a fraction of their previous FI value regardless of the initial alum dosage. Since the zeta potential of broken flocs is the same as that before breakage (Figure 1), this effect cannot be due to increased electrical repulsion. In a further series of tests (Figure 2b), the four higher coagulant dosages (0.15-0.4 mM Al) were used with the same stirring conditions, but an additional low dosage of alum (0.03 mM) was added half way through the breakage period (i.e., 30 s after the increase of stirring speed to 200 rpm). After the floc breakage with additional low alum dosage, the FI value increased quickly, and the rate of increase was nearly the same as that during the initial floc growth. The humic acid flocs could regrow regardless of the different initial dosage of alum. Furthermore, the final FI values were higher than the corresponding values before floc breakage. It is clear from Figure 2a and b that an additional low dosage of alum can markedly improve the regrowth of broken flocs, probably as a result of the freshly precipitated aluminum hydroxide.
The Effect of Breakage Stirring Speed. The stirring speed during floc breakage is expected to have a major influence on the size of broken flocs and this is clearly apparent from the results in Figure 3. These tests were carried out just as before, except that, after the first plateau FI value had been reached (at 660 s) the stirring speed was increased to different values, from 80 to 400 rpm. Only one initial alum dosage was used -0.15 mM Al. Up to the point of floc breakage, the results were very similar to the corresponding FI values in Figure 1, but the minimum FI reached after breakage was strongly affected by the breakage stirring speed. It is well-known that higher stirring speeds give smaller floc sizes. For stirring speed of 80 rpm, or, especially, higher than 100 rpm, there was a considerable drop in the FI value immediately after the onset of increased shear. These results are in agreement with Serra et al. (14), who observed that, for breakage of latex flocs, floc sizes reduced with increasing shear rates (G > 30 s-1). Figure 3 shows the regrowth of flocs without and with additional low dosage of alum (0.03 mM Al). The regrowth ability of broken flocs without additional alum dosage (Figure 3a) was only about 50% and this value was not greatly affected by the breakage stirring speed. (More quantitative data will be given later in Table 1). However, if a low additional dosage of alum was added when the flocs were exposed to high stirring speed, there was a significant reversibility of the breakage process. In fact, the regrowth factors of flocs with different breakage stirring speeds were higher than 100%, that is, the FI values of flocs after regrowth were higher than those before breakage, for all breakage speeds (Figure 3b). Furthermore, the final FI VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Breakage and Re-Growth Factors (%) of Flocs Formed with Alum and Humic Acid for Different Breakage Stirring Speeds* breakage speed (rpm)
50
80
F1 72.7 F2 (without) 56.3 F2 (with) 161 V3/V1 (without) 97 86 119 122 V3/V1 (with)
100
150
200
300
50.6 54.5 141 79 120
34.6 54.6 119 72 115
27.0 53.0 121 65 116
19.5 48.0 118 57 114
200 400 (10 min) 16.7 23.1 47.1 32.3 114 113 56 49 111 110
* Initial alum dosage ) 0.15 mM Al. Results shown for cases without and with additional dosage of alum (0.03 mM Al) applied 30 s before end of breakage period. Breakage for 60 s, except for results in last column.
FIGURE 4. Formation, breakage, and regrowth of flocs with 0.15 mM Al, with and without additional alum dosage (0.03 mM Al). Breakage stirring speed 200 rpm. Breakage time 600 s, started at 720 s. Arrows 1 and 2 indicate times of additional alum dosing, at 30 and 570 s into the breakage period. value for the regrown flocs was nearly independent of breakage speed. Comparing Figure 3a and b, additional low dosage of alum greatly improved the regrowth ability. The Effect of Breakage Duration. In order to further investigate the regrowth of broken flocs and possible mechanisms, the flocs were exposed to high stirring speed (200 rpm) for a much longer time (10 min). The initial alum dosage was 0.15 mM Al and the results are shown in Figure 4. For the flocs without additional alum dosage, a significant drop in the FI value was observed immediately the shear rate increased to 200 rpm and then gradually decreased, similar to the observations of Jarvis et al. (15). The FI value began to increase again when the stirring speed was restored to 50 rpm, but the regrown flocs showed a noticeably lower FI value than in the case of the short breakage time. More quantitative information is given later (Table 1). Thus a longer exposure to high stirring speed reduced the ability of broken flocs to regrow. These results are very similar to those of Wang et al. (5) and Jarvis et al. (3), but the reason for this behavior is not yet clear. Additional low dosage of alum (0.03 mM Al) was introduced to further investigate the regrowth of flocs after a long breakage period. The second dosage was added at 0.5 or 9.5 min within the 10 min breakage period. With additional dosing at 0.5 min, the FI value slightly increased, and then decreased to a value similar to that obtained without additional alum. When the shear was reduced, the FI value of regrown flocs was only slightly higher than that without additional alum dosage. This is in marked contrast to the results in Figure 2b, where the breakage time was only 1 min and the broken flocs could fully regrow. Thus, during the long breakage period, the beneficial effects of a second alum dosage and fresh precipitate were lost. On the other hand, 6374
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when additional low dosage of alum was added after 9.5 min of breakage time, the FI value of regrown flocs was higher than that of flocs before breakage, indicating a complete reversibility of floc breakage in this case. From the results in Figures 2b, 3b, and 4, it is evident that when the second alum dosage was added 30 s before the end of high-speed stirring, flocs could fully regrow after the stirring speed was reduced. This applies to both long and short breakage times. In the case of dosing near the beginning of the long breakage period, precipitated aluminum hydroxide ages for several minutes, and during this time floc growth is restricted by the high shear rate. By the time that the lowshear conditions are re-established, the floc fragments, even with the additional coagulant, are no longer able to fully regrow. Discussion. The removal of NOM is believed to be dominated by precipitation of metal hydroxides at pH > 6 (1). After coagulant dosing precipitation begins almost immediately, initially to form very small (a few nm) particles which then aggregate to form an amorphous precipitate. Typically, the growth of the amorphous precipitate takes place over a period of several minutes in the slow stirring phase of the jar test, during which flocs grow to a limiting size (of the order of 100 µm or larger), depending on the floc properties and on the stirring conditions. Organic matter becomes incorporated in the growing flocs and the extent of NOM removal in subsequent solid-liquid separation processes is greatly dependent on the floc size and strength. The process was investigated through floc breakage and regrowth, and by adding additional low dosage of alum during the breakage period. In this work we have confirmed that the breakage of Alhumic flocs at high shear rates is not fully reversible, as has previously been found for alum-kaolin flocs in several previous studies, (e.g., ref 9). It is convenient to quantify the breakage and regrowth of flocs in terms of the following factors (9): V2 (breakage factor) V1
(1)
V 3 - V2 (re-growth factor) V 1 - V2
(2)
F1 ) F2 )
Where V1 is the maximum FI value before floc breakage; V2 is the minimum FI value when flocs are broken, and V3 is the maximum FI value after regrowth of broken flocs. Breakage and regrowth factors of flocs for breakage at different stirring rates are summarized in Table 1. It can be seen that breakage factors (F1) decreased with increasing shear, as expected. For a 60 s breakage period, the regrowth factors (F2) of flocs without additional dosage of alum are around 50% for all breakage stirring rates, but showed a slow decrease from 80 to 400 rpm. After 10 min of breakage at 200 rpm, the breakage factor (F1) was only slightly lower than that after 1 min at the same speed, showing that most of the floc breakage occurs quite quickly. However, without additional coagulant, the regrowth of flocs after a 10 min breakage at 200 rpm is appreciably less than that after only 1 min. With additional coagulant dosage, the regrowth factors are always greater than 100%, showing that the regrown flocs were larger than those before breakage. This is especially so for breakage at low stirring speeds. The differences in the regrowth factors with breakage speed are rather misleading, since the FI values for the regrown flocs do not show much variation with breakage speed. The differences in F2 reflect the great reduction in the FI values for broken flocs with increasing shear. The results in the last two rows of Table 1 are for the ratio of FI values of regrown flocs to those before
breakage (V3/V1). Without additional alum dosage these values show a steady decline with increasing breakage speed, reflecting a decrease in size of the regrown flocs. The lowest value (49%) is for flocs broken at 200 rpm for 10 min. When a second coagulant dosage is applied the V3/V1 values are nearly independent of breakage speed. It is especially interesting that, even when the flocs are not exposed to a higher stirring speed and so do not show breakage, additional alum dosage still gives an increase in FI and V3/V1 is about the same (119%) as for the broken flocs after regrowth. It is likely that the observed effects of additional alum dosing on the regrowth of broken flocs are predominantly due to the nature of the amorphous hydroxide precipitate and that the included impurities (suspended particles or NOM) play only a minor role. Very recent, unpublished data for the kaolin-alum system show similar effects of additional coagulant dosing on floc regrowth after breakage. The previous results suggest that the observed irreversible floc breakage has to do with the aging of the precipitated hydroxide, especially when exposed to high shear. Flocs were broken 10 min after coagulant dosing and so the hydroxide precipitation would have been complete. Breakage of flocs at this time gave fragments which only regrew to a limited extent. Additional coagulant added during the breakage would give further hydroxide precipitation and it is likely that the new precipitate could bind the floc fragments together, perhaps by a mechanism similar to heterocoagulation. Under these conditions reflocculation of broken flocs readily occurred. It is not easy to find an explanation in terms of particle charge since the zeta potentials of broken flocs are the same as those before breakage (Figure 1). There are some previous studies that show the effect of aging on aluminum hydroxide precipitates. For instance Batchelor et al (16) found that older precipitates showed significantly lower rates of reaction with Ferron reagent. A pronounced slowing of the reaction rate was observed for precipitate aged for periods between about 0.5 and 30 min. Since the Ferron complexation is with soluble Al species, these results show that aging of the hydroxide precipitate reduces the rate of dissolution under the conditions of the Ferron test. Rather similar observations were made by Duffy and vanLoon (17). Batchelor et al (16) also showed that aged aluminum hydroxide showed reduced ability to coagulate clay suspensions. The significant finding of the present work is that fresh hydroxide precipitate is able to promote the regrowth of broken flocs, provided that the second coagulant addition is made shortly before the end of the breakage period. When the additional coagulant is dosed at the beginning of a long breakage period, no benefit is found (Figure 4). By contrast, addition near the end of the breakage period gives better than 100% regrowth of broken flocs. Soon after coagulant addition, the newly formed precipitate particles bind together and eventually aggregate to form amorphous flocs, which grow to a limiting size under given shear conditions. This size depends on the shear rate and on the strength of the flocs, which is dependent on the binding between constituent particles. The fact that broken flocs do not fully regrow when the original low shear rate is restored means that the binding between particles is weaker. This effect is much more evident with hydroxide flocs than in other cases. For instance, clay particles flocculated with a cationic polyelectrolyte show almost completely reversible floc breakage, at least for quite short breakage periods (8). So the irreversible nature of hydroxide floc breakage must be related to the specific nature of the binding between particles and the way in which this changes, as a result of breakage or simply as the precipitate ages. From the present results it seems that freshly precipitated hydroxide is able to form stronger flocs than precipitate that has aged for several
minutes. The evidence for this is that a low additional dosage of coagulant is able to more than fully restore the regrowth ability of broken flocs. Even when flocs are not subject to breakage at high stirring speed, additional alum still leads to larger flocs (Figure 3b). During slow stirring, it is believed that the limiting floc size represents a steady state condition in a system where flocs are subject to breakage and regrowth. This is likely in a stirred vessel, such as in the jar test, where there are widely varying shear rates. Flocs would break in the regions of high shear near the tips of the stirrer blades and reform in more quiescent regions. After some minutes it may be that the broken flocs are less able to regrow. Thus an additional small dosage of coagulant could cause some increase in floc size because of the introduction of fresh precipitate several minutes after the initial coagulant dosing. Note that higher dosages of alum added initially (Figure 2a) give FI values appreciably lower than those achieved by a small additional dosage (Figure 3b), even though the final alum concentration in the latter case is only 0.18 mM Al. The effect of breakage stirring speed on the regrowth of broken flocs (Figure 3a) shows that aging time is not the only factor. Flocs broken at higher stirring rates are less able to regrow, although the time of exposure to low and high stirring speeds is the same in all cases. This reason for this effect is still not clear, but it may be related to collisions between floc fragments, which are more frequent at higher stirring rates. Erosion of small particles from broken flocs may also play a part. At all breakage speeds a small additional alum dosage gives floc regrowth to nearly the same size (Figure 3b), showing again the major effect of fresh hydroxide precipitate. With a very long breakage period, additional coagulant added too early has no effect (Figure 4). It is possible that stronger binding of hydroxide flocs occurs when precipitation is still in progress, that is, when there are still soluble Al species present. Newly precipitated material may help to cement aggregating particles together. This is a rather speculative suggestion and would need to be investigated in more detail. Although we still do not have a complete picture of the processes of hydroxide floc breakage and regrowth, the present work provides valuable information and points the way to further systematic experiments.
Acknowledgments W.-Z.Y.’s research at University College London was made possible by funding from the China Scholarship Council and a Simon Li Scholarship from UCL.
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(14) Serra, T.; Colomer, J.; Logan, B. E. Efficiency of different shear devices on flocculation. Water Res. 2008, 42, 1113–1121. (15) Jarvis, P.; Jefferson, B. J.; Parsons, S. A. How the natural organic matter to coagulant ratio impacts on floc structural properties. Environ. Sci. Technol. 2005, 39, 8919–8924. (16) Batchelor, B.; McEwan, J. B.; Perry, R. Kinetics of aluminum hydrolysis: measurement and characterization of hydrolysis products. Environ. Sci. Technol. 1986, 20, 891–894. (17) Duffy, S. J.; vanLoon, G. W. Characterization of amorphous aluminum hydroxide by the Ferron method. Environ. Sci. Technol. 1994, 28, 1950–1956.
ES1007627
