Influence of pH on Flocs Formation, Breakage and Fractal Properties ... · * Corresponding to: [email protected] Influence of pH on Flocs Formation, Breakage and Fractal Properties

* Corresponding to: [email protected]

Influence of pH on Flocs Formation, Breakage and Fractal

Properties — The Role of Al13 Polymer

Weiying Xu, Baoyu Gao*, Qinyan Yue, Xiaowen Bo Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and

Engineering, Shandong University, Ji’ nan 250100, China

ABSTRACT The Al13 ([Al13O4 (OH) 24(H2O) 12] 7+) polymer and polyaluminum chloride (PACl) were comparatively investigated for the coagulation of humic acid (HA). The growth, breakage, and fractal nature of flocs under different pH condi-tions were investigated using a laser diffraction particle-sizing device. The results showed that the floc properties were essentially dependent on pH. The flocs were small but compact under acidic conditions; as the pH increased, large flocs were produced with loose structures. Compared to PACl, Al13 polymer displayed better stability and functioned more rapidly after addition; and the flocs formed by Al13 were denser across the pH values investigated. It was also found that flocs formed at pH 5.50 were the strongest for both coagulants in the pH range investigated. Additionally, the increased shear rate could improve the flocs structures by conducing higher fractal dimension flocs. Key words: Al13 polymer; Floc growth; Floc breakage; Fractal dimension 1.0 INTRODUCTION Particle aggregation is an important process affecting the fate of flocs in coagulation and flocculation units in water treatment works (WTW). The properties of aggregated par-ticles, including particle size, shape and strength, substantially affect their removal by gravity sedimentation (O’Melia, 1998). Boller and Blaser claimed that small particles gener-ally have lower removal efficiencies (1998). Smaller particles would pose a challenge to the solid/liquid separation processes if the pre-formed flocs tend to be broken by small

increases in shear rate during water works unit processes (Jarvis et al., 2005a). Thus, flocs size and the ability to withstand the increased shear are important operational parameters for the efficient removal of aggregated particles. Besides, fractal structure of flocs is another particularly critical parameter that deserves special attention. Fractal theories for particle aggregates provide the detailed method for describing the structure of particles aggregates in various water systems (Bushell et al., 2002; Guan et al., 1998). Mass fractals may be summarized by the relationship between their mass M, a characteristic measure of size L, and the mass fractal dimension Df:

fDM L∝ (1)

Journal of Water Sustainability, Volume 1, Issue 1, June 2011, 45–57 © University of Technology Sydney & Xi’an University of Architecture and Technology

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Generally, compact aggregates have higher Df, while aggregates with loose structures have lower Df values.

Polyaluminum chloride (PACl), partially hydrolyzed products of Al(Ⅲ), has been widely used in coagulation process in water treatment to remove contaminants. PACl has been claimed by many investigators to be su-perior to the conventional Al-based coagulants (e.g. AlCl3 and alum) for particulate and/or organic matter removal in water treatment, because of its less sludge production and less dependence on temperature and pH (Matsui et al., 2003; Lin et al., 2008a). It has been recog-nized that the performance of PACl depends largely on the Al speciation characteristics and the best known Al species is Al13 polycation ([Al13O4 (OH) 24(H2O) 12]7+), which could be obtained by polymerization of Al in AlCl3 so-lution under controlled basic conditions (Akitt and Farthing, 1981; Bottero et al., 1980). Pre-vious studies stated that the diameter of Al13 molecule is a few nanometers; the lengths of their aggregates vary in the range of 300-400 nm. Thus the Al13 species is commonly called nano-Al13 (Bottero et al., 1990; Wang et al., 2000). It is commonly believed that Al13 is the most active species in PACl composition re-sponsible for coagulation or precipitation (Bottero et al., 1980; Hu et al., 2006; Lin et al., 2008b).

Traditionally, the study of Al-based coagu-lants focused on particle surface charge and solid/liquid separation efficiency. Far fewer studies of suspensions destabilized using alu-minum coagulants have provided insights into particle characteristics. In addition, although there has been growing interest in the use of Al13 polymer with emphasis on its coagulation behaviors and removal efficiency (Kazpard et al., 2006; Liu et al., 2009), the effect of Al13 polymer on fundamental floc parameters, such as floc size, strength and fractal characteristics has not yet been fully explored. Previous stu-dies showed that the formation and decompo-sition of Al13 depend critically on pH (Lin et

al., 2008a; Hu et al., 2006; Liu et al., 2009). Al13 remains stable at acidic pH, while it ag-gregates at pH above 6 and precipitates form in alkaline condition (Furrer et al., 1999). Hence, pH can significantly affect the coagu-lation behaviors of Al13 and subsequently af-fect the aggregation of flocs.

The aim of this paper is to compare the ef-fect of pH on the formation, breakage and structural properties of flocs formed by Al13

and traditional PACl. Humic acid (HA) solu-tion was used in experiments as HA is one of the main constituents of natural organic matter (NOM) in surface waters (Suffet and Mac-Carthy, 1989). This work also investigated changes of floc fractal structures with the var-iation of shear rate. 2.0 MATERIALS AND METHODS 2.1 Suspension Humic acid was obtained as a commercial reagent grade solid (Shanghai, China). The HA stock solution was prepared as follows: 1.0 g of HA was dissolved in deionized water that contained 4.2 g of NaHCO3 and diluted the solution with deionized water to 1 L.

The synthetic test water was prepared by dissolving 5.0 ml of HA stock solution in deionized water and diluting the solution to 500 ml. The properties of the synthetic test water used were as follows: UV254 =0.204 ± 0.02, TOC =4.77 ± 0.03 mg L−1, pH = 7.9 ± 0.2. 2.2 Coagulants Preparation and Charac-

terization All the reagents used to prepare each coagu-lant were of analytical grade and deionized water was used to make all solutions. The procedures of preparing each coagulant can be described as follows: PACl was synthesized by adding pre-determined amount of Na2CO3 slowly into AlCl3 solution under intense agita-


tion. The temperature was kept at 75.0±0.5°C using a recycling water bath. The target basic-ity (OH/Al molar ratio) of the PACl was 2.0. Al13 polymer was separated and purified from the PACl by ethanol-acetone method based on the different organic solvent solubility of dif-ferent Al species. Specifically, ten milliliter of PACl solution was transferred into a 1 L glass beaker, and then 50, 100 and 50 ml of etha-nol-acetone solution was introduced in se-quence under gentle agitation. Samples were filtered using 0.45 μm millipore membranes at the end of each addition of ethanol-acetone solution. The precipitates obtained by the second filtration were dried at room tempera-ture (Xu et al., 2011), which was the expected Al13 species. All the coagulants were stored at 5 oC in a refrigerator and both PACl and the solid Al13 remained stable within two months. The Al13 precipitate was diluted with deio-nized water to release Al13 from solids into solution when used. No obvious turbidity ap-peared in the coagulant solutions during the study.

Total Al concentrations (AlT) were deter-mined using ICP-AES (PerkinElmer, Optima 2000, UK). The Al species in PACl and Al13 polymer were analyzed by 27Al nuclear mag-netic resonance (NMR) spectroscopy with 27Al NMR spectra obtained from a Varian UNITY INOVA (500 MHz). Details of the oper-ating methods and parameters of the apparatus can be found in other literature (Liu et al., 2003). There are three signals in the NMR spectra: the signal near 0.0 ppm represents the monomeric and dimeric aluminum species (denoted as Alm); the signal at 62.5 ppm represents the Al13 species; and the signal at 80.0 ppm indicates the formation of Al(OH)4

- (the internal standard). Based on the contents of Alm and Al13 species, the other Al species (denoted as Alother) can be calculated by the following equation:

Alother = AlT −Alm −Al13 (2) The properties of coagulants used in this

study were summarized in Table 1.

2.3 Coagulation Optimization Initially, coagulation optimization tests were performed to ascertain the optimum coagulant dosage for HA removal at different pH. The initial pH of the raw water was adjusted to 3.50, 5.50, 6.50, 7.50 and 8.50 respectively using 0.1 mol L-1 HCl and 0.1 mol L-1 NaOH solutions. The coagulants doses ranged from 1.0 to 12.0 mg L-1 as Al2O3. Coagulation ex-periments were carried out by variable speed jar tester (ZR4-6, Zhongrun Water Industry Technology Development Co. Ltd., China) with 50*40 mm flat paddle impellers and 1 L cylindrical beakers. Each sample (1 L) was rapidly mixed at 200 revolutions per minute (rpm) for 1.5 min, slowly mixed at 40 rpm for 15 min, and then settled for 30 min. Coagu-lants and pH adjustment chemicals were add-ed at the start of the rapid mix. An unfiltered sample was obtained after the rapid mix for zeta potential measurement with a Zetasizer 3000HSa (Malvern Instruments, UK). At the end of each jar test, the supernatant sample was withdrawn by syringe from about 3 cm below the water surface and passed through a 0.45 μm glass filter paper for analysis. UV254 absorbance at 254 nm wavelength was tested using an UV-754 UV/VIS spectrophotometer (Precision Scientific Instrument Co. Ltd., Shanghai, China).

In order to investigate the relation of UV254 and TOC of the tested HA, different concen-trations of HA solution (4~24 mg L-1) were tested. The result showed that the UV254 ab-sorbance was linear with TOC (Figure 1) and consequently, the removal of UV254 absor-bance was used to evaluate HA removal effi-ciency in this study.

2.4 Floc Formation Processes

Floc growth experiments in pH range of 3.50~8.50 were conducted on a jar tester as before. Briefly, all the jar tests were carried out and repeated three times as follows: a


rapid mix at 200 rpm for 1.5 min, followed by a slow stir phase at 40 rpm for 15 min. A con-tinuous laser diffraction instrument (Master-sizer 2000, Malvern, U.K.) was used to meas-ure dynamic flocs size as the coagulation pro-ceeded. More details have been reported in other paper (Wang et al., 2009). Size mea-

surements were taken every 30 s for the dura-tion of the jar test. The size data were ex-pressed as an equivalent volumetric diameter, and d50 was selected as representative floc size in this paper (Wei et al., 2010), which referred to the 50 percentile floc size.

Table 1 The properties of coagulants

Coagulants AlT/mol·L-1 Alm/% Al13/% Alother/% PACl 1.02 23.35 35.87 40.78 Al13 0.092 2.97 95.38 1.65

0.0 0.1 0.2 0.3 0.4 0.5

2

4

6

8

10

12

14

R2=0.95

TOC mg L-1

UV254

2.5 Floc Breakage Gregory (2003) states that when comparing different flocs, the size for a given shear rate indicates floc strength. However, this is the case for the given shear condition under which the flocs were formed, it dose not give an in-dication of how flocs will behave under ex-posure to an increased shear rate, as could occur at a water treatment work when flocs are transferred from flocculators to a higher shear treatment process such as dissolved air

flotation or high rate filtration. In this paper, a floc breakage mode was used to evaluate the floc strength. After the slow stir phase the suspension was exposed to increased shear for a further 5 min. Separate experiments were carried out and repeated 3 times at increased shear of 40, 50, 75, 100, 150 and 200 rpm re-spectively (the corresponding G values for the apparatus in this study were 10.1, 13.6, 23.5, 34.6, 59.7 and 87.8 s-1) for 5 min. Particle size was monitored after exposure to each level of shear. The rate at which a floc suspension de-

Figure 1 Relationship between UV254 and TOC of the tested HA solution with different con-centration.


cays under exposure to shear is indicative of the strength of the flocs within the system. Previous studies revealed the relation on a log–log scale between the average velocity gradients G in the flocculation and the floc size of the suspension in equilibrium (Fran-cois,1987; Yeung and Pelton, 1996):

lg d =lg C － γ lg G (3)

where d is the floc diameter; C is the floc strength constant that strongly depends on the method used for particle size measurement; G is the average velocity gradient and γ is the stable floc size exponent dependent upon floc break-up mode. The slope of the line (γ) gives an indication of the rate of degradation. A larger γ value is indicative of floc that is more prone to break into smaller size under an in-creasing shear force and smaller γ implies the stronger flocs. 2.6 Fractal Dimension Light scattering experiments are an important tool for determining the fractal dimensions of small aggregates. The method is briefly given as follows: the scattered intensity I as a func-tion of the magnitude of the scattering wave vector Q is measured (Bushell et al., 2002; Guan et al., 1998), where,

Q =λθπ )2/sin(4 n (4)

n, θ and λ are the refractive index of the me-dium, the scattered angle, and the wavelength of radiation in vacuum, respectively. For in-dependently scattering aggregates, I is related to Q and the fractal dimension Df (Lin et al., 1989):

fDI Q−∝ (5)

So, on a log–log scale if there is a straight line, the slope of which is Df.

In order to investigate the effect of shear rate on the floc Df, floc re-growth tests were

carried out as follows: after the flocs forma-tion aforementioned, the suspension was ex-posed to 5 min of high shear (200 rpm) fol-lowed by a slow stir (40 rpm) as initially. Par-ticle Df was evaluated at different stages of the coagulation process. 3.0 RESULTS AND DISCUSSION 3.1 Coagulant Dose Optimization under

Different ph Conditions The optimal HA removal efficiency at differ-ent pH and the corresponding coagulant dose were shown in Figure 2. It was observed that the optimum HA removal efficiency by Al13 and PACl were similar at most pH values with the exception of pH 3.5, where Al13 gave rise to an appreciably better efficiency of about 83 % while the efficiency for PACl was 76.89 %. Both Al13 and PACl could achieve an excellent efficiency of above 90% when pH was 5.5, 6.5, 7.5 and 8.5. Additionally, Al13 could marginally reduce coagulant con-sumption. For Al13 polymer, the optimum dose at pH 5.5~8.5 was around 5.60 mg/L and at pH 3.5 was 7.25 mg/L; while for PACl, the corresponding dose was around 6.50 mg/L and 9.50 mg/L, respectively. In order to re-duce the influence of coagulant concentration on the behaviors of Al13 and PACl at different pH, the doses of Al13 and PACl in the follow-ing studies were fixed at 5.60 and 6.50 mg/L, respectively.

Figure 3 presented the zeta potential of the flocs and the final pH of the solution after coagulation at different pH. As the pH in-creased, zeta potential of flocs increased from the negative values to zero and reached a turning point at initial pH 6.5, where the final pH was actually 7.21 for Al13 and 7.06 for PACl coagulation. When pH was above 6.5, the zeta potentials decreased to the negative side again with pH increasing. It indicated that coagulants displayed better charge neutraliza-tion abilities at acidic pH than at alkaline pH.


It could also be found that zeta potential for Al13 was always higher than that for PACl, implying the stronger neutralization abilities of Al13 polymer. 3.2 Effect of ph on Floc Growth The profiles of floc growth for HA coagulated

with PACl and Al13 polymer at different pH were shown in Figure 4. It was observed that higher pH tended to produce larger flocs, no matter which coagulant was used. For Al13 coagulation, the flocs could reach the peak size within shorter time, while PACl tended to yield larger flocs with longer time.

50

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Coa

gula

nts dos

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g/L

as A

l 2O3)

8.57.56.55.5

HA r

emov

al effi

cien

cy(%

)

Optimum dose of Al13 for HA removal

Optimum dose of PAC for HA removal

3.5

Optimal HA removal efficiency for

Al13 coagulation

PACl coagulation

Figure 2 The optimal HA removal efficiency and the corresponding coagulant dose at different pH

values.

3 4 5 6 7 8 9

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Final pH

Zeta potential Al13

Zeta potential PACl

Zeta potential

pH

Final pH Al13

Final pH PACl

Figure 3 Zeta potential of flocs and the final pH of the treated water under different pH conditions with

Al13 dose of 5.60 mg/L and PACl dose of 6.50 mg/L.


0 200 400 600 800 1000

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Floc size d

0.5(um)

Time(s)

Al13 (5.60 mg/L )

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pH 6.5

pH 7.5

pH 8.5

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0 200 400 600 800 1000

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Floc size d0.5(um)

Time(s)

PACl (6.50 mg/L)

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pH 6.5

pH 7.5

pH 8.5

b

Figure 4 The changes of flocs size versus time for HA flocs formation exposed to a 15 min

shear at 10.1 s-1 at different pH (a) Al13; (b) PACl.

Generally, floc formation process can be roughly divided into two steps. Firstly, par-ticles destabilize and come into contact, which leads to a rapid growth rate in the mean di-ameter of the particles. During this period, a high shear rate is employed to facilitate the particles collision. Then, a lower velocity gra-dient is introduced in the slow-mixing process, where aggregates become larger and have more tenuous and fragile structures that are susceptible to breakup by fluid shear. Finally, a pseudo-steady state of constant floc size is reached after the slow stirring for a few mi-nutes. This stage is generally regarded as re-flecting a balance between floc growth and breakage under a given shear rate. In detail, the first period consists of two specific regions, the lag region and fast-growth region. In the lag region, coagulants come to contact and react with colloids, but floc growth is so small that floc size does not increase much in this region. The fast-growth region of coagulation curves illustrates the stage where the floc size increases significantly.

Four parameters were measured in this study to specifically analyze the process, in-cluding lag time, growth time, growth rate in growth region, and floc size in steady-state region. The lag time and the growth time (in-cluding the rapid growth time and the time of size increase in the second stage) can be di-rectly obtained from the coagulation curves.

The growth rate can be obtained by calculat-ing the data in the growth region, which can be determined as:

Growth rate = sizetimeΔΔ

(6)

The floc size was the mean value of floc size obtained over the steady state region.

The details of these parameters were pre-sented in Table 2. It was observed that both the lag time and growth time first decreased and then increased with solution pH. For PACl coagulation, the particle aggregation could achieve the steady-state in the shortest time at pH 6.5 (around 450 s); for Al13 polymer, the least time required was only around 330 s, which also occurred at pH 6.5. In alkaline conditions, the coagulation and flocculation rate significantly slowed down with a longer lag time and floc growth time. This could be attributed to the different mechanisms in-volved in the coagulation processes at differ-ent pH. Generally, the coagulation mechan-isms induced by aluminum coagulants favor charge neutralization at low pH (Liu et al., 2009), which agrees with the results as shown in Figure 3 in this paper. As pH increased, Alm species in PACl was transformed to amorph-ous Al(OH)3 precipitates and the adsorption became the dominant mechanisms, which would take more time. For Al13, when pH was above 6, Al13 could be decomposed by depro-tonation reactions that released the protons in


pairs to yield the Al13 with less positive charges such as Al13

5+, Al133+ and Al13

+. Con-sequently, the repulsions between Al13 became weaker, which facilitated the self-aggregation of Al13 polymer (Furrer et al., 1992). In addi-

tion, Al13 was also transformed to amorphous Al(OH)3 precipitates at alkaline pH. And both the aggregation and precipitation consumed coagulation time.

Table 2 The properties of HA flocs formed by Al13 and PACl at different pH conditions in

floc formation processes.

pH Coagulants Lag time (s) Grow time (s) Grow rate (μm·s -1)

Floc size (μm)

3.5 Al13 63.48 326.45 0.59 212.25 PACl 100.08 471.38 0.41 198.58

5.5 Al13 58.38 300.96 0.68 232.26 PACl 86.90 459.65 0.59 289.44

6.5 Al13 31.38 304.39 0.90 299.93 PACl 43.92 406.69 0.79 346.50

7.5 Al13 63.00 327.58 0.84 306.94 PACl 99.31 480.00 0.67 356.79

8.5 Al13 92.87 387.07 0.85 365.50 PACl 124.48 522.07 0.81 460.67

Additionally, the lag time and growth time

for Al13 polymer were significantly shorter than those for PACl, which proved that Al13 polymer could react more quickly after being dosed in suspension. This was because the preformed Al13 polymer was more stable than other species and could play an active part by way of charge neutralization or complexation immediately (Wang et al., 2009). 3.3 Effect of pH on Floc Breakage The effect of hydraulic gradient (G) on floc size at different pH was shown in Figure 5. It was observed that the flocs presented conti-nuous reduction in size with the increasing shear rate, which agreed with other researcher (Bache et al., 1999). The floc size after 5 min of elevated shear was plotted against G and a straight line could be drawn through the data on a log-log scale. The γ could be calculated according to Equation 3. A similar trend be-tween slope (γ) and pH was observed for both coagulants, which displayed a decrease and

then an increase. For Al13 coagulation, γ reached the lowest value of 0.53 at pH 5.5, and the corresponding value for PACl coagu-lation was also obtained at pH 5.5 (0.49). From an overall perspective, the γ values at acidic pH were lower than those at neutral and alkaline pH. It is also worth noticing that Al13 gave rise to higher γ than PACl did at acidic pH. This could be attributed to the complexa-tion reactions between HA and Al species, which played a significant role during coagu-lation processes at acidic pH (Gregor et al., 1997). It was claimed that the Alm species in PACl could react with HA to form octahe-drally coordinated complexes. This complex was thought as the initial flocs in HA-Alm flocs formation process, which would join with larger flocs by charge neutralization (Hi-radate and Yamaguchi, 2003). Likewise, Al13 polymer could react with HA to form HA–Al13 complex, which could be regarded as the initial flocs in Al13 polymer treatment. However, the HA-Al m complex formed in PACl coagulation process was much stronger


than HA-Al13 complex (Hiradate and Yama-guchi, 2003). Thus, flocs formed with PACl were stronger at acidic pH. At alkaline pH, the adsorption and enmeshment of particles onto Al hydroxide precipitates produced much larger flocs. Additionally, the flocs formed by PACl were much larger than those formed by Al13 polymer based on the results shown in Figure 4 and Table 2. During breakage, the larger flocs were more exposed to micro scale energy-dissipating eddies that gave rise to floc breakage, which is so called “fragmentation breakage mode”. However, smaller flocs were

more likely to become entrained within these eddies rather than being broken by them (Boller and Blaser, 1998), which could be classified as “erosion” (Jarvis et al., 2005b). Additionally, the bridging of Al13 aggregates presented that pH above 6.0 could also help improve the floc strength. Consequently, the floc size in PACl coagulation presented a steeper decrease with enhanced shear at alka-line pH. Bache and Rasool (Bache and Rasool, 2001) had the same findings that the value of γ increased when the water alkalinity was high.

Figure 5 The relationship between the changes in floc size and an increase shear rate at dif-

ferent pH values (a) Al13; (b) PACl. 3.4 Floc structural analysis The derivation of floc fractal property at dif-ferent pH from the scattered light intensity (I) as a function of wavenumber (Q) was shown in Figure 6 for HA flocs after 15 min of slow stir.

There was good linear correlation for all of the data sets with R2 values (R-correlation coefficient) exceeding 0.97 and all of the Df were between 2.00 and 2.52. It could be found that Df decreased with pH increase no matter which coagulant was used. Under the same pH condition, Al13 flocs presented higher Df

than PACl flocs. It was due to the higher posi-tive charge of Al13 polymer as shown in Fig-ure 3, which greatly weakened the repulsive forces between particles within the aggregates and hence led to a high degree of compaction.

Curves similar to those shown in Figure 6 were formed for the coagulating suspensions as they grew, broken, and subsequently re-grew (Figure 7). The results demonstrated how the degree of floc compaction changed with the variation of shear rate in the system. For Al13 polymer, the changes of Df exhibited a similar trend at different pH. Prior to brea-kage, HA flocs had a Df of 2.49, 2.45, 2.48,


2.38 and 2.09 at pH 3.50, 5.50, 6.50, 7.50 and 8.50. Whilst under exposure to high shear rate, the flocs had higher compaction degree with the corresponding Df of 2.52, 2.54, 2.55, 2.42 and 2.11. Also, the flocs for PACl appeared to display higher Df after being broken across the pH range investigated. This was because the flocs were broken into smaller aggregates on exposure to high shear rate and the primary particles re-arranged themselves into stable and more compact floc structures (Spicer et al., 1998). However, the increase of flocs com-paction degree was not considerable, espe-cially when pH was 8.50. An explanation for this inconsistence was that the flocs formed mainly by adsorption and sweep flocculation were not completely neutralized, and physical bonds played an important role within aggre-gates rather than chemical bonds that general-ly existed at acidic pH. When chemical bonds were broken, the flocs were rearranged into more compact structures, and bonding sites that were previously available became un-available, as they re-formed within the broken

aggregates. When physical bonds were broken, flocs reformation also occurred but the pre-vious and new bonds were all caused by van der Waals forces and hence, producing similar flocs during re-growth (Jarvis et al., 2005c).

It was surprising that the Df of PACl flocs decreased once the initial slow shear rate was reintroduced at pH 5.50. At pH 5.50, the Df of PACl floc increased from 2.40 to 2.44 after breakage and then reduced to 2.41 during re-growth. It seems likely that the alternation shear rate should provide undetectable break of HA-Alm, leading to differing degree of compaction. Also, the Al species transforma-tion could have an effect on the flocs struc-tures. Previous study proved that the Alm spe-cies in PACl could rapidly transform to Al13 species at pH 5.50 (Zhao et al., 2008). Then, the unstable HA-Al13 complexes (Jarvis et al., 2005b) could be generated, which made the flocs not as compact as before. Yet, there has not been a satisfactory explanation and further work is required in this field to make ade-quately quantitive assessment.

Figure 6 Relationship between the scattered light intensity (I) and the wavenumber (Q) on a

log-log scale and the determination of the fractal dimension after the initial growth phase for HA flocs coagulated with Al13 polymer and PACl at different pH values


Figure 7 Change in fractal dimension of flocs with time for Al13 polymer (5.60 mg/L) and

PACl (6.50 mg/L) at different pH after growth (40 rpm) and breakage (200 rpm) followed by a return to the initial 40 rpm for re-growth

4.0 CONCLUSIONS The effect of pH on HA floc growth, breakage and Df values in Al13 polymer and PACl coa-gulation were comparatively investigated in this paper.

At pH 6.50, the particle aggregation could achieve the steady-state in the shortest time. Compared to PACl, Al13 could react with contaminants more immediately. For the giv-en optimum coagulants dosages, both Al13 polymer and PACl gave rise to small but strong flocs at acidic pH than at higher pH. Compared to PACl, Al13 induced stronger flocs in neutral and alkaline conditions, whilst the results obtained at acidic pH were the re-verse. Higher pH generated flocs with loosely bound structures while flocs formed at acidic pH were more compressed. Under the same pH condition, Al13 polymer produced flocs with higher Df than PACl. Additionally, the exposure to a high shear rate contributed to a slight increase of floc compaction degree for both Al13 polymer and PACl.

ACKNOWLEDGEMENT

The research was supported by Specialized Research Fund for the Doctoral Program of

Higher Education (No 20070422019), the Na-tional Natural Sciences Foundation of China (No.50678095, 50808114), the Key Projects in the National Science & Technology Pillar Program in the Eleventh Five-year Plan Pe-riod (No. 2006BAJ08B05) and the National Major Special Technological Programmes Concerning Water Pollution Control and Management in the Eleventh Five-year Plan Period (No.2008ZX07422-003-02). The kind suggestions from the anonymous reviewers are highly appreciated.

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