Predominant Destabilization Mechanisms Model

Hydrolyzed Al(OH)3 precipitates are generally considered to destabilize particles by surface precipitation or precipitate charge neutralization through adsorption.

However, the size of amorphous aluminum hydroxide formed at such condition depends on dosage, which affects the destabilization efficiency. The schematic representation of the predominant coagulation mechanisms of PACl-C at neutral pH is presented in Fig. 5.6. Because the majority of PACl-C is monomeric and colloidal Al(OH)3(s), sweep flocs are formed at neutral pH by both surface precipitation and adsorption of aluminum hydroxide precipitates on the surface of particles. At neutral pH, as dosage of PACl-C exceeds solubility product, there are external aluminum hydroxide precipitates (Al(OH)3(am) and colloidal Al(OH)3(s)) that cloud the surface of particles and then the nucleation occurs to entrap particles, resulting in an larger amorphous precipitate that can sufficiently sweep particles from water. At low dosage, smaller and less positively charged aluminum hydroxide precipitates form and are attracted to larger negatively charged particles by precipitation charge neutralization (i.e., heterocoagulation), leading to the formation of smaller flocs that is unfavorable for particle removal through precipitation.

On the other hand, the coagulation mechanisms of cationic polymers in coagulation of colloidal particles are well known, including charge neutralization, interparticle bridging and electrostatic patch (Gregory, 1973). These mechanisms can happen concurrently and are frequently competing with each other. The coagulant dosage also has influences on the conformation of polymer and consequently the adsorption on the surface of particles, which causes different mechanisms of coagulation (Zhou and Franks, 2006). As a result, the Al13

conformation on the surface of particles will change at different PACl-Al13 dosage,

inducing different coagulation mechanisms.

For coagulation by PACl-E and PACl-Al13 at high alkaline pH, since increasing the dosage accompanies the formation of larger Al13 aggregates, the mechanisms between the reaction of Al13 aggregates and particles will change correspondingly.

However, PACl-E coagulation responds to dosage differs from PACl-Al13 coagulation at alkaline pH. The schematic representation of the predominant mechanisms of Al13

aggregates is presented in Fig. 5.7. For PACl-Al13 coagulation, because the larger Al13 aggregates could not be easily generated at low dosage via Al13 self-assembling, the weakly positively charged Al13 may adsorb onto the surface as flattened configuration and then cluster in a region to perform the particle coagulation by electrostatic patch where the partial charge neutralization of particles occurs, which contributes to patched flocs of high Ds. At high dosage, linear Al13 aggregates with a given dimension or other dimensional Al13 aggregates with neutralized charge can form rapidly via the collisions of coiled Al13 aggregates and then adsorb onto the surface of kaolin particle as extended configuration when the surfaces of particles are clouded. Therefore, the extended Al13 aggregates adsorbed on particle surface can assemble particles by interparticle bridging, which results in bridged flocs of low Ds. On the other hand, since PACl-E contains certain monomeric and colloidal Al except for Al13, the aggregation of Al13 could be postponed by other Al species at alkaline pH, which limits the formation of larger Al13 aggregates. Thus, the insufficient large Al13

aggregates can not cause interparticle bridging, even though high dosage is applied.

As such, PACl-E coagulation at alkaline pH is favorable for electrostatic patch or charge neutralization, which is dependent of dosage applied.

(a) Precipitation Charge Neutralization

particles dosing and mixing flocculation flocs

(b) Enmeshment

particles dosing and mixing flocculation sweep flocs

Fig. 5.6 Schematic representation of the destabilization mechanism induced by PACl-C coagulation at neutral pH. (a) Precipitation Charge Neutralization (b) Enmeshment.

(a) Electrostatic Patch

particles dosing and mixing flocculation patched flocs

: Al13 polycation

(b) Interparticle Bridging

particles dosing and mixing flocculation bridged flocs

: Al13 aggregates

Fig. 5.7 Schematic representation of the destabilization mechanism induced by Al13

aggregates. (a) Electrostatic Patch (b) Interparticle Bridging.

5.5 Summary

At neutral pH, the PACl-C with monomeric and colloidal Al species favors enmeshment or sweep flocculation in response to the occurrence of abundant Al(OH)3

precipitates at high dosage, while precipitate charge neutralization become the major destabilization mechanism at low dosage. By contrast, the PACl-E containing a high percentage of Al13 causes either electrostatic patch or charge neutralization mechanisms with Al13 aggregates at pH 10. The breakage of flocs induced by enmeshment (or sweep flocculation) and electrostatic patch occur at overdosing.

The breakage rate of sweep flocs is faster than that of PACl-E flocs. The structure of sweep flocs formed at RLA becomes denser with dosage, in which the breakage of sweep flocs can cause the increase in fractal dimension of flocs. At DLA, PACl-E flocs formed by electrostatic patch become more compact with dosage and have a stronger resistance to shear stress during coagulation.

PACl-Al13 coagulation induced by electrostatic patch is responsible for particle destabilization at low dosage and pH 10, which results in the higher fractal dimensional flocs. Interparticle bridging becomes the major mechanism when the sufficient high dosage is applied due to the formation of Al13 aggregates with nearly zero charge, which leads to flocs of low fractal dimension. The PACl-Al13 flocs formed by interparticle bridging are more easily broken during coagulation processes by shear stress. On the other hand, enmeshment or sweep flocculation causes sweep flocs with a rough and ragged contour, while electrostatic patch or charge neutralization induces flocs with a smooth and glossy surface. Interparticle bridging brings about PACl-Al13 flocs of a looser structure with a fluffy contour. At such condition, some larger linear Al13 aggregates composed of a chain of coiled Al13 and several coiled Al13 aggregates with different dimensions are found.

CHAPTER VI

SURFACE Al COMPOSITION OF Al(OH)3-RICH AND Al13-AGGREGATE FLOCS

For coagulation by Al-based coagulants, it is essential to realize the distribution of various Al species on the surface of flocs, which can reflect the reactions between predominant hydrolyzed Al species and particles. By using an X-ray photoelectron spectroscopy (XPS) that has been applied to quantify surface Al composition of various solid materials (Duong et al., 2005; Kloprogge et al., 1999), the interactions between various hydrolyzed Al species and particles during PACl coagulation can be further identified. Although Al13 molecules can aggregate and eventually transform into Al(OH)3 precipitates with time, the composition and structure of Al13 aggregates differs from Al(OH)3. The following experiments were carried out to differentiate the properties of Al(OH)3-rich and Al13-aggregate flocs to further understand the difference of Al(OH)3 precipitates and Al13 aggregates.

Based on the results of previous study, the flocs formed by PACl-C coagulation at neutral pH and PACl-Al13 coagulation at pH 10 under 8 mg/L are considered to be Al(OH)3-rich and Al13-aggregate flocs. To realize the predominant Al species on the surface of these flocs, their surface characteristics were examined by various surface analysis technology. The SEM as well as HR-TEM was used to observe the morphology of various PACl-kaolin flocs. The HR-XRD was also employed to examine the crystalline structure of these flocs. The WSEM was employed to inspect the formation of Al(OH)3 precipitates. In addition, the XPS was applied to identify the quantity of octahedral Al (Al^VI) and tetrahedral Al (Al^IV) on the surface of various PACl-kaolin flocs.

6.1 Structure of Al-Flocs

In coagulation, the structure of flocs provides the advantageous information for the understanding of the interaction between coagulant and particles. In general, coagulated flocs have a multilevel structure (Wu et al., 2002). In fact, coagulation by different Al-based coagulants results in different structures of Al-flocs due to the effect of hydrolyzed Al species on the surface structure of Al-flocs. In addition, the crystalline structure of Al-flocs also can reflect the characteristics of reactive species in coagulation. In this study, the Al(OH)3-rich flocs are formed by various Al species, while the crystalline structure of Al13-aggregate flocs is under investigation.

The following observation on surface and crystalline structure of these Al-flocs were performed by SEM as well as HR-XRD to further explore their structure.

6.1.1 Surface structure of Flocs

Surface structure of flocs is related to their morphology. Fig. 6.1 exhibits that the morphology of Al(OH)3-rich and Al13-aggregate flocs, respectively. As shown in Fig. 6.1 (a), the larger Al(OH)3-rich flocs are clearly observed. Their inner structure are compact but the porous and looser structure is visible in their edge where several colloids with the size of around 2 µm are found, as seen in Fig. 6.1 (b), which is assigned to a large number of colloidal Al in PACl-C. The results of EDX analysis indicate that these colloids are rich in Al element, suggesting Al(OH)3(s) are certainly adsorbed onto the particles. This also strongly substantiates the occurrence of particle-cluster aggregation for PACl-C coagulation at neutral pH. By contrast, the Al13-aggregate flocs are smaller, as shown in Fig. 6.1 (c). Close-up examination of this image reveals a network of strips which are composed of NaCl crystal, as illustrated in Fig. 6.1 (d). Furthermore, the nano-scale Al(OH)3(am) surrounding the

edge of Al(OH)3-rich flocs is found through HR-TEM observation, as illustrated in Fig. 6.2 (a), which proves that surface precipitation or the adsorption of hydrolyzed Al(OH)3(am) precipitates onto the surface of the kaolin particles occurs for PACl-C coagulation at neutral pH and high dosage. There has no other apparent crystalline structure within the Al13-aggregate flocs, as shown in Fig. 6.2 (b). Because PACl-Al13 coagulation was conducted at high dosage and alkaline pH, the Al13

aggregates most likely cloud the surface of kaolin so that the polygonal crystals (i.e., kaolin crystals) are not observed. The SEM and HR-TEM observation of flocs indicate that the surface morphology of Al(OH)3-rich and Al13-aggregate flocs are significantly different, suggesting the different crystalline structure exist in these flocs.

Fig. 6.1 SEM micrographics of Al(OH)3-rich and Al13-aggregate flocs formed by PACl-C and PACl-Al13 coagulation at 8 mg/L as Al.

(a

(a) (b)

colloidal Al(OH)3

(d) (d)

(c) (d)

NaCl

Fig. 6.2 HR-TEM images of (a) Al(OH)3-rich and (b) Al13-aggregate flocs formed by PACl-C and PACl-Al13 coagulation at 8 mg/L as Al.

Al(OH)3(am)

(a)

(b)

6.1.2 Crystalline Structure of Flocs

To further verify the possibility that other specific crystal Al species adsorbed on the surface of the Al(OH)3-rich and Al13-aggregate flocs, these flocs were examined by HR-XRD. The HR-XRD patterns of Al(OH)3-rich and Al13-aggregate flocs are depicted in Fig. 6.3. To discriminate the Al crystal from kaolin particle, the kaolin powder was also determined by XRD. Fig. 6.3 (a) shows two extensive peaks occur at about 12º and 24º for the kaolin samples. By contrast, Al(OH)3-rich flocs do not have well-formed crystalline structures compare to the kaolin crystals, as shown in Fig. 6.3 (b). For Al(OH)3-rich flocs, the signal of Al(OH)3(am) or colloidal Al(OH)3(s) is broad and ranges from 18º to 25º, which is in good agreement with the characteristic spectrum of aluminum hydroxide in the Joint Committee on Powder Diffraction Standards (JCPDS) database. On the other hand, because the kaolin suspension contains certain amount of NaOCl4, the presence of square or threadlike NaCl crystals on the surface of floc, as seen in Fig. 6.1 (d), results in the crystalline signal between 20º and 30º (Linnow et al., 2007). Likewise, the peak of NaCl crystals is also acquired for the Al13-aggregate flocs, as shown in Fig. 6.3 (c), in addition to the occurrence of a weak signal at around 8º, which can be assigned to Al13-like crystalline structure (Cheng and Chou, 2000). Out of high stability of internal tetrahedral Al (AlO4) within Al13 (Phillips et al., 2000), the Al13 aggregates formed through the bridging of deprotonated octahedral Al may keep Al13-like structure (Akitt, 1989). The results of HR-XRD scanning suggest that the crystalline structure of Al13 aggregates differs from that of Al(OH)3.

(a) Kaolin

6.2 In-situ Observation of Al(OH)3 Precipitates

Al(OH)3 precipitates possess various types and shapes in liquid with aging.

Because PACl-C coagulation favors the formation of Al(OH)3 precipitates, presumably the surface Al composition of Al(OH)3-rich flocs depends on the types of the Al(OH)3 precipitates adsorbed on the surface of kaolin particles. As shown in Fig. 6.1 (b) and Fig. 6.2 (a), two types of Al(OH)3 precipitates are markedly observed. In order to in-situ identify the Al(OH)3 precipitates clearly, the PACl-C coagulation was carried out at neutral pH and high dosage (8 mg/L) without adding kaolin particles to form abundant Al(OH)3 precipitates. The WSEM micrographics of Al(OH)3 precipitates are shown in Fig. 6.4. As shown in Fig. 6.4 (a), a considerable number of Al(OH)3(am) form and precipitate after the addition of PACl-C, which is attributed to an enormous amount of colloidal Al(OH)3(s) that are intrinsic to PACl-C. The size of these precipitates is more than 1 µm, and they are obviously sphere-like in shape and rigged, as illustrated in Fig. 6.4 (b). These colloidal Al(OH)3(s) precipitates have a pit on their surface, indicating the colloidal Al(OH)3(s) has a sunken surface. On the other hand, several smaller amorphous Al(OH)3 precipitates were observed on the surface of the colloidal Al(OH)3(s). In general, the formation of colloidal Al(OH)3(s) is slower, which increases the opportunity to grow larger nuclei, while Al(OH)3(am) is prone to in-situ formed rapidly by the hydrolysis of Al(III) resulting the smaller and irregular precipitates.

The result substantiates that in-situ formed and preformed Al(OH)3 precipitates are divergent in morphology, which implies the different Al composition are among Al(OH)3(am) and colloidal Al(OH)3(s).

Fig. 6.4 WSEM micrographics of Al(OH)3 precipitates formed after 1 min rapid mixing at 200 rpm: (a) (
3,750); (b) (
50,000).

3 µm

1 µm (a)

(b)

pit

Al(OH)3(am)

Al(OH)3(am)

colloidal Al(OH)3(s)

6.3 Surface Composition of Al-Flocs

The Al(OH)3 precipitates and Al13 possess a specific Al composition, and they keep a particular ratio between tetrahedral Al (Al^IV) and octahedral Al (Al^VI). To further understand the Al composition on the surfaces of Al(OH)3-rich and Al13-aggregate flocs, the quantitative analysis of the various Al species was identified by XPS. The high resolution XPS scans of the aluminum are presented in Fig. 6.5. The aluminum high resolution scans show the single Al 2p transitions with binding energies of 74 eV for kaolin powder, while two overlapping bands associated with two different Al 2p transitions with binding energies of 72 and 74 eV, corresponding to Al^IV and Al^VI (Duong et al., 2005). In general, tetrahedrally coordinated aluminum has a lower binding energy than octahedrally coordinated aluminum.

For Al(OH)3-rich flocs, the Al 2p with lower binding energy around 72 eV is assigned to Al^IV, while the Al 2p with higher binding energy at 74 eV is assigned to Al^VI, as illustrated in Fig. 6.5 (b). The ratio between Al^IV and Al^VI is 1:1.63, which implies theses Al(OH)3 precipitates on the surface of particles are intrinsically different. Because the kaolin only contains single Al^IV, as proved in Fig. 6.5 (a), it is unlikely that the presence of abundant Al^IV originates from kaolin powder. In addition, Al(OH)3 mainly possesses a Al^VI center (Masion et al., 2000; Isobe et al., 2003). Therefore, it is clear that there has a large number of Al(OH)3 with a Al^IV center on the Al(OH)3-rich flocs, which is similar to previous ²⁷Al-NMR study of amorphous Al(OH)3 that contains tetrahedral and pentahedral coordinations of Al(O)4 and Al(O)5 as well as octahedral coordination of Al(O)6 (Isobe et al., 2003).

This result can be explained by the condensation of in-situ formed Al13. In this study, there are only two species in aqueous solutions that have Al(O)4 (Al^IV): the

monomeric Al(OH)4

-(aq) and Al13(aq). For PACl-C coagulation at neutral pH, Al13

represents much more of the total Al(O)4 than Al(OH)4-. The monomeric Al of PACl-C transforms into metastable Al13 rapidly during initial coagulation where the in-situ formed Al13 can become solid aluminum hydroxide via the condensation of Al13 (Sposito, 1996). Although other study has proposed that the metastable Al13

will undergo directly structure rearrangement to form gibbsite with sufficient aging (Hsu, 1988), Furrer et al. (2002) have proved the speciation of Al(OH)3(am) flocs in natural rivers are originated from the aggregation of Keggin Al13 polycation as a result of the rapid mixing and episodic reaction in which they probably remain a Al(O)4 center. This phenomenon is accentuated particularly for this study. On the other hand, the condensation of outer Al-OH groups may accompany the formation of partial Al-O-Al strained linkage, leading to the occurrence of five-coordinate Al(O)5 and six-coordinate Al(O)6 in aluminum hydroxide (Piedra and Fitzgerald, 1996). As evidenced in Fig. 4.3, the ratio between Alb (Al13) and Alc (Al(OH)3) is about 1:1.6, which is in accordance with the results of XPS survey. As a result, though the PACl-C flocs involves abundant Al(OH)3 precipitates that mainly possess Al^VI, parts of them still remain Al^IV centers potentially due to rapid mixing for a short period.

A distinct change in the characteristic peak of tetrahedrally coordinated aluminum is apparent in the XPS survey data of Al13-aggregate flocs. The characteristic peak of Al 2p with a binding energy of 73 eV is obvious, as shown in Fig. 6.5 (c). This may be due to the changes in binding energy between the Al13

aggregates and the surface of particles. From the data of XPS survey for Al13-aggregate flocs, it is found that the Si/Al is about 0.6 that is higher than that for Al(OH)3-rich flocs, indicating the existence of more Si-O units on the surface of

Al13-aggregate flocs. For aluminum in oxide environments, the higher Si/Al ratio can progressively forces the Al-O bond to be more ionic and thus the Al 2p binding energy increases (Barr et al., 1997). This demonstrates that the binding energy of Al^IV-O on the surface of the Al13-aggregate flocs is higher than that on the surface of Al(OH)3-rich flocs. For Al13 molecules, the proportion of Al^IV to Al^VI should be 1:12 in theory (Johansson, 1960). On the surface of Al13-aggregate flocs, however, the Al^IV/Al^VI ratio is about 1:9.9, suggesting the existence of Al13 aggregates.

During the formation of Al13 aggregates, Al13 molecules condense and aggregate result from the decomposition of the outer octahedral structure. Accordingly, the Al^VI/Al^IV of Al13 aggregates is below 12.

The results of XPS survey suggest that the surface Al composition of flocs also could be affected by the mixing and reaction time other than predominant hydrolyzed Al species during coagulation. The hydrolyzed Al(OH)3(am) precipitates contain certain tetrahedral Al centers, while the Al composition of Al13 aggregates is significantly different from that of Al13 molecules.

(a) Kaolin

6.4 Summary

The Al(OH)3-rich flocs are larger and have a ragged and fuzzy exterior with an looser structure, while the Al13-aggregate flocs are smaller with smooth contour. The Al(OH)3-rich flocs do not possess well-formed crystalline structure, while the Al13

aggregates with Al13-like structure is found within the Al13-aggregate flocs. The preformed colloidal Al(OH)3(s) in PACl-C is spherical-like and has a sunken surface.

On the surface of the Al(OH)3-rich flocs, there are multitude of amorphous Al(OH)3

precipitates that involve either Al(O)4 or Al(O)6 center, where the Al^IV/Al^VI ratio is almost equal to 2:3, similar to the ratio of Alb/Alc during coagulation measured by Ferron method. In addition, the Al^IV/Al^VI ratio of the Al13 aggregates observed on the surface of Al13-aggregate flocs is higher than that of Al13 molecules potentially due to condensation of Al-OH groups.

CHAPTER VII

CONCLUSIONS AND RECOMMENDATIONS

Based on the results of this study, the following conclusions could be drawn:

7.1 Conclusions

1. The PACl-C with monomeric and colloidal Al species favors enmeshment or sweep flocculation at neutral pH in response to the occurrence of abundant Al(OH)3 precipitates, including Al(OH)3(am) and colloidal Al(OH)3(s). By contrast, the PACl-E containing 60% Al13 of total Al concentration causes either electrostatic patch or charge neutralization mechanisms with Al13 aggregates at pH

1. The PACl-C with monomeric and colloidal Al species favors enmeshment or sweep flocculation at neutral pH in response to the occurrence of abundant Al(OH)3 precipitates, including Al(OH)3(am) and colloidal Al(OH)3(s). By contrast, the PACl-E containing 60% Al13 of total Al concentration causes either electrostatic patch or charge neutralization mechanisms with Al13 aggregates at pH