Summary

The major Al species of PACl-C are monomeric and colloidal Al, while the majority of Al species in PACl-Eand PACl-Al13 is Alb (i.e., Al13), especially for PACl-Al13 that contains more than 95% Al13 of total Al concentration. It is found that Al13 can be roughly represented by Alb. PACl-C responds to pH differs from PACl-E as well as PACl-Al13. Large amounts of Alc form at neutral pH for PACl-C, while the pre-hydrolyzed Alb in PACl-E as well as PACl-Al13 remains more stable with varying pH except alkaline pH. The flocs formation by PACl-C coagulation at neutral pH relies on Al(OH)3 precipitates regardless of the dosage applied. In addition, either electrostatic patch or interparticle bridging induced by Al13 aggregates may be responsible for particle destabilization and aggregation for PACl-E and PACl-Al13 coagulation at alkaline pH, depending on the dosage. At low dosage, the partially neutralized particles by electrostatic patch are easier to occur for PACl-E and PACl-Al13 coagulation at alkaline pH. Contrary to low dosage, the plateau of most zero zeta potential occurs even though high dosage is applied. At such condition, more and larger Al13 aggregates could form at such condition and adsorb onto the surface of kaolin particles, which could increase the opportunities of interparticle bridging. Accordingly, the interparticle bridging may play more important role in particle destabilization and aggregation when sufficient and large Al13 aggregates form.

CHAPTER V

FORMATION AND STRUCTURE OF FRACTAL FLOCS INDUCED BY VARIOUS DESTABILIZATION MECHANISMS

Coagulation mechanisms could significantly affect the dynamics of particle aggregation and dominate the aggregates size and structure. The interactions of the hydrolyzed Al species with particles during coagulation in water are contributed by different coagulation mechanisms, which thereafter affect the aggregation dynamics of particles and the structure of flocs (Van Benschoten and Edzwald, 1990; Licskó, 1997). Research has demonstrated that charge neutralization induced by PACl can bring more rapid particle aggregation than enmeshment induced by alum (Matsui et al., 1998), and the flocs induced by charge neutralization become more compact that resist shear stress (McCurdy et al., 2004). Coagulation dynamics of various destabilization mechanisms and the properties of resulting flocs such as size and fractal dimension were investigated by SASLS. The morphology of flocs induced by various destabilization mechanisms were observed by WSEM. On the other hand, the morphology of Al13 aggregates molecules was scanned by TM-AFM in liquid environment. The proposed destabilization mechanisms and particles aggregation modes for coagulation by various PACl coagulants are also presented in this section.

5.1 Dynamic Growth of Al-Flocs

The growth profile of the flocs for PACl-C coagulation at neutral pH, PACl-E and PACl-Al13 coagulation at alkaline pH, as shown in Fig. 5.1, where the floc size is presented by the median equivalent diameter (d50). There marked differences between the growth trends of PACl-C flocs (sweep flocs), PACl-E and PACl-Al13

flocs. As shown in Fig. 5.1 (a), the growth rate of the PACl-C flocs raises with dosage. More particles are destabilized throughout efficient collisions when the dosage is increased, which leads to abrupt increase of the aggregation rate at the initial aggregation. The most dramatic observation is that, at high dosage (8 mg/L) the floc size sharply increases at the initial aggregation and then reaches a maximum, but decreases gradually over time, which could be explained by the breakage of flocs.

It has been accepted that the growth of flocs depends on a balance between the formation and breakage of flocs (Biggs and Lant, 2000; Ducoste and Clark, 1998).

Moreover, the aggregates size during coagulation is limited by local physical-chemical conditions such as dosage of coagulant or shear-induced mixing.

During the rapid initial formation of flocs, the flocculation of primary flocs dominates the evolution of particles size, then the influence of breakage increases as floc size increases until a steady-state size is approached. As the largest flocs form, the number of the primary particles will decrease to a minimum before the steady-state flocs is reached, which results in the significant decrease of collision rate as well as the increase in breakage rate. With PACl-C coagulation, the more dosage is applied, the larger the sweep flocs that contain large porosity and then are easily broken by surface erosion of eddies splitting (i.e., large-scale fragment) that is dependent of flocs structure (Jarvis et al., 2005). Consequently, the breakage rate of the sweep flocs at high PACl dosage is faster. Because the breakage of flocs is relatively

obvious at high dosage where the d50 size of flocs decrease from 450 µm to 350 µm, the possible breakage mode is surface erosion resulting in an increase in the small particle size ranges.

For PACl-E coagulation, the size of flocs also increases with dosage, and the equilibrium d50 size of flocs coagulated with various dosages remains 300 µm to 400 µm, as illustrated in Fig. 5.1 (b). At the same low dosage (1 mg/L), the d50 size of flocs coagulated by PACl-E is much larger than that by PACl-C in response to electrostatic patch induced with Al13 aggregates. Meanwhile, since PACl-C coagulation favors enmeshment, larger sweep flocs formed at high dosage are observed as a result of the occurrence of abundant Al(OH)3 precipitates. However, the initial growth of PACl-E flocs is more rapid than that of sweep flocs at various dosages studied. Enmeshment is generally slower than other coagulation mechanisms because Al(OH)3 precipitates require a few seconds to form in coagulation (Letterman et al., 1973). Therefore, PACl-E coagulation that favors charge neutralization (or electrostatic patch) can give faster particle coagulation. In addition, the size of PACl-E flocs formed at 8 mg/L also decreases over time, but the breakage rate of them is slower than that of sweep flocs. This suggests that PACl-E flocs possess a stronger resistance to shear stress due to stronger attractive forces between particles within PACl-E flocs under charge neutralization. Out of the minor decrease in floc size for PACl-E coagulation at high dosage, the breakage mode is inferred to be large-scale fragmentation that is the cleavage of flocs into several pieces of a size similar to parent flocs (Jarvis et al., 2005).

In the case of PACl-Al13 coagulation, the d50 of PACl-Al13 flocs increases with increasing dosage and ranges from 200 µm to 250 µm with dosage used, as presented in Fig. 5.1 (c). The d50 size of PACl-Al13 flocs formed at the 8 mg/L slowly

decreases with the flocculation time, similar to PACl-E coagulation. However, the PACl-Al13 coagulation has a higher breakage rate of flocs at such condition, which implies that PACl-E flocs are stronger than PACl-Al13 flocs.

The results of coagulation dynamics show that the dosage of PACl strongly affects the balance between efficient collision and the breakage of the flocs. The magnitude of flocs breakage during coagulation is well correlated with destabilization mechanisms. Therefore, the structure of flocs will be dependent of the dosage of coagulants as well as the predominant destabilization mechanisms.

(a) PACl-1(a) PACl-C

0 200 400 600 800 1000 1200 1400

0

5.2 Fractal Structure of Al-Flocs

The concept of fractal geometry has been applied successfully in the illustration of colloidal aggregation dynamics. Flocs formed by coagulation in water and wastewater treatment are considered to be the fractal structure (Thomas et al., 1999).

Fractal structure of flocs is subject to the process of flocs growth, involving particle aggregation and the breakage of flocs, which results in the change of fractal structure.

Many studies have proved that the structure of alum-kaolin flocs is correlated to floc size (Francois, 1987; Francois, 1988; Chakraborti et al., 2003). The following experiments were performed to understand the relationship between floc size and fractal structure for coagulation by various PACl coagulants.

5.2.1 Effects of Dynamic Growth of Floc on Fractal Structure

The mean size and fractal dimension (Ds) of the flocs formed by PACl-C, PACl-E and PACl-Al13 coagulation at various dosages are presented in Fig. 5.2. For PACl-C coagulation, the mean size and Ds of sweep flocs increase with dosage except for at the high dosage (8 mg/L) where a drop of mean size occurs, as shown in Fig. 5.2 (a). In theory, fractal dimensions do not change as a function of floc size. However, the virtual relationship between fractal dimensions and floc size tremendously hinges on the mode of particle coagulation at various conditions (Chakraborti et al., 2000; Jiang and Logan, 1996). Particle aggregation is dictated by aggregation limiting regimes, including diffusion-limited aggregation (DLA) when there is no repulsive force between the colloidal particles and reaction-limited aggregation (RLA) where additional repulsive forces caused by electrostatic forces or steric interaction prevents the particles from aggregation (Elimelech et al., 1995). The variation of the fractal dimension of kaolin flocs with the change in slow mixing time also can reflect the

regimes of coagulation kinetics (Berka and Rice, 2005).

As shown in Fig. 4.4 (a) and Fig. 5.1 (a), the charge reversal of particles occurs when the dosage is higher than 1 mg/L for PACl-C coagulation, but the efficient aggregation and turbidity removal still occurs. Because the shear stress induced by mixing overcomes the repulsion among particles during slow mixing, the particles can aggregate after encounter each other over time, suggesting the PACl-C coagulation at high dosage is governed by RLA. The higher dosage the stronger is the repulsion between particles, which causes low collision efficiency. As a result, particles or clusters need to collide many times before sticking occurs as the increase in dosage, ranging from 2 mg/L to 8 mg/L. This gives more opportunities to explore other configurations, and then particles are able to penetrate into a cluster before encountering another particle and sticking (Lin et al., 1989); as a consequence, more compact flocs are obtained. Furthermore, the breakage of flocs and an increase in the Ds of the weep flocs coagulated by PACl-C at 8 mg/L simultaneously occur with slow mixing time, as illustrated in Fig. 5.2 (a) and Table 5.1, indicating the flocs are restructuring during flocs breakage process. Similar studies have reported that a restructuring of primary particles within flocs due to breakage and re-aggregation that occurs in response to ambient shear (Clark and Flora, 1991; Oles, 1992). In this research, the rearrangement of primary particles within flocs can be caused by the breakage of flocs through a longer flocculation time and then are restructured throughout the new balance between the reformation and breakage of flocs with time.

This process possibly makes flocs into a denser and more compact structure.

Therefore, the Ds values of sweep flocs sharply increase from 2.2 to 2.33 when the breakage of flocs occurs. Contrary to high dosage, the change of the Ds values of sweep flocs formed at 1 mg/L is not obvious with mixing time because restructuring

caused by the breakage of flocs does not happen.

Similar tendency of changes in mean size and fractal dimension of flocs is also found for PACl-E coagulation, as illustrated in Fig. 5.2 (b). PACl-E coagulation favors electrostatic patch at low dosage and charge neutralization at high dosage.

With increasing dosage, the more efficient particle coagulation is obtained due to the weaker repulsion among particles that leads to the shorter distance between particles within the flocs and the flocs of higher Ds are thus formed. At high dosage, although the PACl-E flocs experience breakage, as shown in Fig. 5.1 (b), the almost invariant Ds of PACl-E flocs are observed to be in the range from 2.2 to 2.21 during flocs breakage. The change in fractal dimension of PACl-E flocs formed at low dosage is similar to that at high dosage. This demonstrates that a stable structure of flocs is formed because the breakage of PACl-E flocs does not result in the occurrence of interflocs restructuring. It is likely that the PACl-E coagulation at 8 mg/L is induced by charge neutralization at which the nearly zero charged particles can aggregate by DLA. In general, cluster-cluster flocs have fractal dimension values of around 1.8 at DLA and 2.1 at RLA, respectively (Lin et al., 1989). However, the sweep flocs and PACl-E flocs formed at RLA and DLA have higher Ds values around 2.3 and 2.2, which could be assigned to the breakage of flocs resulting particle-cluster aggregation that creates a denser internal structure within flocs.

By contrast, the different observation on the trends of change in mean size and Ds

value of PACl-Al13 flocs is found when dosage is gradually increased. The mean size and Ds values of the flocs after the addition of PACl-Al13 at various dosages are presented in Fig. 5.2 (c). The mean size of PACl-Al13 flocs slowly increases with dosage, while the Ds value of PACl-Al13 flocs decreases with increasing dosage from 2.18 to 2.03. Evidently, the change in flocs structure with increasing dosage for

PACl-Al13 coagulation is contrary to that for PACl-C and PACl-E coagulation. At alkaline pH, although the trends in particle destabilization by PACl-Al13 with increasing dosage is similar to PACl-E coagulation, as shown in Fig. 4.4, the resulting floc structure is markedly different, meaning that PACl-E and PACl-Al13 coagulation at alkaline pH and high dosage are dictated by different destabilization mechanisms.

This can be attributed to the potential effect of Al13 aggregates on particle coagulation.

Because other Al species such as monomeric Al and colloidal Al involves in PACl-E in addition to Al13, the sufficient large Al13 aggregates could not generate easily at alkaline pH and high dosage as result of mutual influence between various Al species.

At the same condition, because PACl-Al13 contains the extremely high content of Al13, which facilitates the aggregation of Al13, the formation of larger Al13 aggregates are more rapidly occurred to cause interparticle bridging. As shown in Table 5.1, at low dosage, the Ds increases slightly with slow mixing time, while the almost invariable Ds ranging from 2.04 to 2.05 at high dosage where the zero zeta potential is found, which is analogous to that for PACl-E coagulation at high dosage. Studies have suggested that the invariable fractal dimension of flocs represents cluster-cluster aggregation induced by DLA with no reconstruction and rearrangement in shear coagulation by interparticle bridging with polymers (Biggs et al., 2000; Yu et al., 2006). The results suggest that the larger Al13 aggregates formed by PACl-Al13

coagulation at high dosage could promote particle aggregation via interparticle bridging rather than charge neutralization, even though the zeta potential of destabilized particles are close to almost zero.

(b) PACl-E

Table 5.1 Fractal dimension (Ds) of various flocs during coagulation

Fractal dimension (Ds)

Coagulants Time (min)

1 mg/L 8 mg/L

10 2.09 2.28

15 2.11 2.30

PACl-C

20 2.11 2.33

10 2.06 2.20

15 2.07 2.21

PACl-E

20 2.08 2.21

10 2.11 2.04

15 2.12 2.04

PACl-Al13

20 2.14 2.05

Ds: Mean values of scattering exponent measured by SASLS assay

5.2.2 In-situ Observation on the Morphology of Flocs

In this research, the WSEM is used to observe the morphology and principal structure of primary flocs in the moist condition for the first time in order to further understand the intereactions of various Al species and kaolin particles under different coagulation mechanisms induced by PACl-C, PACl-E and PACl-Al13. The WSEM images of flocs coagulated by various PACl coagulants at low and high dosage are given in Fig. 5.3. There are marked differences between the sweep flocs formed coagulated by PACl-C at low and high dosage. At low dosage, sweep flocs have some spherical nuclei, but contain many amorphous matters at high dosage, as shown in Fig. 5.3 (a) and Fig. 5.3 (b). Because the PACl-C coagulation at neutral pH undergoes enmeshment or sweep flocculation mostly, the particle can be aggregated as sweep flocs by both surface precipitation and adsorption of aluminum hydroxide precipitates on the surface of particles (heterocoagulation) (Chowdhury and Amy, 1991). In enmeshment or sweep flocculation, there are external particles (aluminum hydroxide precipitates), which cloud the surface where the nucleation and growth occurs. Less amorphous hydroxide precipitates can only diffuse onto the particles by shear when low dosage is applied, which results in insufficient nucleation or precipitation charge neutralization induced by colloidal Al(OH)3(s). By contrast, more amorphous hydroxide precipitates are formed at high dosage and cover the surface of particles, which causes the formation of larger sweep flocs with a rough and ragged contour. However, the opportunities may increase for particle interpenetration during floc breakage because more floc conformations are exposed, which can influence the aggregation modes. The occurrence of particle-cluster aggregation will become relatively easier when flocs suffer breakage. As a result, the principal structure of sweep flocs formed by PACl-C coagulation at 8 mg/L is

more compact than that at 1 mg/L.

As shown in Fig. 5.3 (c) and Fig. 5.3 (d), the flocs formed by PACl-E coagulation at 8 mg/L are larger and more compact than that at 1 mg/L. At high dosage, since DLA induced by charge neutralization predominately governs the PACl-E coagulation without structuring of flocs, where the particles attach permanently to other particles at the first contact, PACl-E flocs are possibly formed by cluster-cluster aggregation, as reported by Torres et al. (1991). When the breakage of flocs occurs, the particle-cluster aggregation is likely formed, which results that the edge of the PACl-E flocs is loosely associated but the interior is so densely packed that primary particles is not easily observed. As a result, a larger and higher fractal dimensional PACl-E floc with a smooth and glossy contour is observed at 8 mg/L. Below overdose, since the breakage of PACl-E flocs does not occur, they have a more open structure induced by electrostatic patch in response to cluster-cluster aggregation.

The WSEM micrographics of PACl-Al13 flocs formed at low and high dosage are shown in Fig. 5.3 (e) and Fig. 5.3 (f). A distinct difference between flocs formed at low and high dosage is found. The flocs formed at low dosage are more compact and smaller, while the flocs formed at high dosage have a looser structure and are larger. At low dosage, weakly charged Al13 only can partially neutralize the negatively charged particles by adsorption, which results that many incomplete fully neutralized surfaces of particles are exposed. As a result, the flocs possess a higher fractal dimensional structure potentially due to electrostatic patch. At high dosage, the surface of kaolin particles is saturated with adsorbed Al13 aggregates that extend to attach other particles in the interior of PACl-Al13 flocs, which leads to looser structure of flocs caused by interparticle bridging. These results of WSEM imaging are in accordance with the observation from SASLS.

Fig. 5.3 The WSEM micrographics of flocs coagulated at various coagulants and dosages under 1 min rapid mixing followed by 20 min slow mixing.

PACl-C: (a) 1mg/L and (b) 8 mg/L; PACl-E: (c) 1 mg/L and (d) 8 mg/L;

PACl-Al13: (e) 1 mg/L and (f) 8 mg/L.

2 µm

(c)

(a) (b)

(d)

3 µm

5 µm 5 µm

1 µm 1 µm

(e) (f)

Kaolin Al13 aggregates Kaolin

5.3 In-situ Observation on the Morphology of Al13 Aggregates

In fact, it is difficult to quantify the Al13 aggregates in coagulation due to the variety of Al13 aggregates size. Study has indicated that the size of Al13 aggregates increase gradually from 570 nm at pH 4.35 to1200 nm at round pH 6.5 at a given concentration of Al13 (Dubbin and Sposito, 2005). In the past, Al13 molecule of various PACl coagulants has been observed by Hu et al. (2005). TM-AFM was used to observe the morphology of Al13 adsorbed onto the mica in air. The Al13 molecule, however, will condense at such condition. In this study, in-situ observation of the morphology of Al13 aggregates was performed by liquid TM-AFM to provide more evidences for the role of Al13 aggregates in the PACl-Al13 coagulation. The morphology of Al13 aggregates generated during PACl-Al13 coagulation at high dosage (8 mg/L) without suspended kaolin particles in reactor was observed by

In fact, it is difficult to quantify the Al13 aggregates in coagulation due to the variety of Al13 aggregates size. Study has indicated that the size of Al13 aggregates increase gradually from 570 nm at pH 4.35 to1200 nm at round pH 6.5 at a given concentration of Al13 (Dubbin and Sposito, 2005). In the past, Al13 molecule of various PACl coagulants has been observed by Hu et al. (2005). TM-AFM was used to observe the morphology of Al13 adsorbed onto the mica in air. The Al13 molecule, however, will condense at such condition. In this study, in-situ observation of the morphology of Al13 aggregates was performed by liquid TM-AFM to provide more evidences for the role of Al13 aggregates in the PACl-Al13 coagulation. The morphology of Al13 aggregates generated during PACl-Al13 coagulation at high dosage (8 mg/L) without suspended kaolin particles in reactor was observed by