The origin of Al(OH)3-rich and Al13-aggregate flocs composition in PACl coagulation

The composition of hydrolyzed Al species is essential for the understanding of coagulation with Al-based coagulants. Surface characteristics of flocs formed by coagulation with two distinct polyaluminum chloride (PACl) coagulants were identified. One commercial coagulant (PACl-C) with voluminous monomeric Al and colloidal Al(OH)3 and a custom-made PACl (PACl-Al13) containing high Al13 content were applied to destabilize kaolin particles. The flocs formed by PACl-C and PACl-Al13 at neutral and alkaline pH ranges, respectively, were observed by FE-SEM and HR-TEM. In addition, the Al composition of these flocs was characterized by XPS and HR-XRD, and the imaging of Al(OH)3 precipitates and Al13 aggregates were conducted by SEM as well as tapping mode AFM in liquid system. The observations of flocs indicate that the morphology of Al(OH)3-rich flocs are fluffy and porous around the edge of flocs, while the Al13-aggregate flocs have a glossy contour and irregular structure. Both Al(OH)3-rich and Al13-aggregate flocs do not possess well-formed crystalline structure except for the Al13-like crystal exists in the Al13-aggregate flocs. Among Al(OH)3 precipitates, colloidal Al(OH)3 is micro-scale in size, while amorphous Al(OH)3 is nano-scale. During the formation of Al13 aggregates, some coiled and clustered Al13 aggregates with smoother surface were observed. The XPS study on floc surface showed that tetrahedral (AlIV) /octahedral (AlVI) Al ratio on the surfaces of PACl-C and PACl-Al13 flocs is 1:1.6 and 1:9.9, respectively. Of the in situ formed Al13, almost half of Al-hydroxide precipitates on the surface of Al(OH)3-rich flocs possess the AlIV center. It also found that the irregularly aggregated Al13 with a similar Al13 crystalline structure subsists on the surface of Al13-aggregate flocs.