Alkalinity effect of coagulation with polyaluminum chlorides: Role of electrostatic patch

The coagulation behavior of polyaluminum chloride (PACl) with various basicities (B = OH/Al values) was investigated under different alkalinities. It is aimed to get better insights into the coagulation mechanisms involving interactions between hydrolyzed Al(III) products and particles. Jar tests were used to evaluate the coagulation efficiencies, including zeta potentials, residual turbidities (RT) and pH values. An optical monitoring technique of photometric dispersion analyzer (PDA) was utilized to observe the coagulation dynamics. The experimental results show that the traditional coagulant such as alum evolves a rapid hydrolysis after dosing and the in situ formed hydrolysis products can destabilize the kaolin particles by precipitation charge neutralization (PCN). The preformed polymeric species in PACls exhibit a relatively high stability after dosing and can form “electrostatic patches” on clay particle surfaces. These patches play a crucial role in “electrostatic patch coagulation” (EPC). Increasing alkalinity extends both PCN and EPC zones. Under the low alkalinity, EPC with high Alb contained in PACls works better than PCN coagulation; increased alkalinity improves the efficiency of traditional coagulant due to sweep flocculation. Under the higher alkalinity, more coagulant is required to achieve complete charge neutralization. The stoichiometric relationships between the dosage and alkalinity are different depending on the B values of PACls.