Coagulationmechanisms of polyaluminumchloride (PACl) at various dosageswere studied using a conventional jar test at different final and initial pH values during treating kaolin suspension. The optimal final pH and dosages for PACl were obtained based on residual turbidity and zeta potential of flocs. The coagulation zones at various PACl dosages and solution pH values were developed and compared with those of alum. It is found that the optimal mechanism under acidic condition is charge neutralization

while alkaline condition will facilitate the coagulation of PACl. Both charge neutralization coagulation and sweep coagulation can achieve high coagulation efficiency under the alkaline condition ranging from final pH 7.0 to 10.0. Stabilization

charge neutralization destabilization

restabilization and sweep zones occur successively with increasing PACl dosages with the final pH values fixed at 7.0 and 8.0

but restabilization zone disappears at final pH 10.0. When the final pH is not controlled and consequently decreases with increasing PACl dosage

no typical sweep zone can be observed and the coagulant efficiency decreases at high PACl dosage. It seems that the final pH is more meaningful than the initial pH for coagulation. Charge neutralization coagulation efficiency is dominated by zeta potential of flocs and PACl precipitates. The charge neutralization and sweep coagulation zones of PACl are broader in the ranges of coagulant dosage and pH than those of alum. The results are helpful for us to treat water andwastewater using PACl and to understand the coagulation process of PACl.