Distinct mechanisms of particle aggregation induced by alum and PACl: Floc structure and DLVO evaluation

Abstract

a b s t r a c tInteractions of silica microspheres were examined by light scattering in presence of alum and polyaluminum chloride (PACl) with various OH/Al ratios. The coagulation behaviors were investigated using different coagulant dosages at constant pH (6.5) and salt concentration (0.01 mol/L). Based on the measurement of size distribution and zeta potential, charge neutralization was proposed to be the primary coagulation step for all the coagulants while other distinct coagulation steps were also involved depending on in situ formed or preformed hydrolyzing products. Precipitate coverage and sweep flocculation were induced for alum, contrasted to polycation patch and bridge aggregation for PACl. Based on simplified DLVO theory, particle aggregation was explained in terms of interaction forces. It was demonstrated that PACl outperformed alum in particle agglomeration at lower concentration (1-2 mol/L), owing mainly to the considerable energy barrier reduction and deeper secondary minimum at longer distance induced by high positive polycations. The role of secondary minimum was found to be significant when secondary depth exceeded 12 kT. The interaction energy calculations coincided well with coagulation result for PACl, especially PACl22 with high content Al 13 . However, other interaction forces prevailed over the electrical repulsion for alum and PACl containing other aluminum species.

Poly aluminum chloride (PAC) enhanced formation of aerobic granules: Coupling process between physicochemical–biochemical effects

“…Moreover, visible granules appeared in both LTF SBR and STF SBR even the cell surface charge still low. These results can be likely explained by the neutralization and electrostatic patch coagulation of PAC [14]. In general, after dosing of PAC, the preformed Al species adsorbed to surface of flocs quickly, and the negative charge can be neutralized.…”

“…In contrast to Al salts, more attention was appealed by PAC, which was more effective and lower costing. Moreover, inherent difference in the coagulation behavior and particle removal mechanism between alum and PAC was also reported by Wu et al [14]. Compared to the numerous studies regarding the efficiency of PAC addition to wastewater in the context of coagulation-flocculation, reports dealt with enhancement of granules formation is scarce.…”

“…In general, after dosing of PAC, the preformed Al species adsorbed to surface of flocs quickly, and the negative charge can be neutralized. Moreover, the adsorbed species could aggregate, rearrange and finally form 'electrostatic patches' over portions of the surface, after then, the collision frequency of oppositely charged patches and surfaces of other floc would be considerably improved [14]. Meanwhile, bridge-connection might be also included since the SVI in PACfed system continuously dropped even the zeta potential turned to positive (Fig.…”

Controlling Coagulation Process: From Zeta Potential to Streaming Potential

“…Wu et al [50] studied the interactions of silica microspheres by light scattering in presence of alum and polyaluminium chloride (PACl) with various OH/Al ratios. They examined coagulation behaviors using different coagulant dosages at constant pH (6.5) and salt concentration (0.01 mol/L).…”

“…Depending on the measurement of size distribution and Zeta potential, charge neutralization was suggested to be the basic coagulation step for all the coagulants while other distinct coagulation steps were also implicated based on in situ formed or preformed hydrolyzing products. Precipitate coverage and sweep flocculation were called for alum, contrasted to polycation patch and bridge aggregation for PACl [50].…”