Preparation and evaluation of polyaluminum chloride sulfate (PACS) as a coagulant to remove natural organic matter from water

Abstract

A series of polyaluminum chloride sulfate (PACS) coagulants, which have different SO42–/Al3+ and OH/Al (γ) mole ratio, has been successfully developed using AlCl3·6H2O, Al2(SO4)3·18H2O and Na2CO3 as raw materials. The coagulation performance of PACS for removing natural organic matter (NOM) from surface water was evaluated, and the effect of SO42–/Al3+ mole ratio and γ value in coagulants PACS on DOC and UV254 removal was determined. Furthermore, the influence of pH and dosage of the selected PACS with a SO42–/Al3+ ratio of 0.0664 and a γ value of 2.0, which achieved the best coagulation performance for the removal of DOC and UV254 of all PACS coagulants, on the removal of DOC and UV254 and residual aluminum concentration in treated water was investigated. The results were compared with the ones of polyaluminum chloride (PAC) with γ value of 2.0. The experimental data show that the performance of PACS as a coagulant was highly dependent on SO42–/Al3+ mole ratio and γ value. Both for the selected PACS and for PAC, the best DOC and UV254 removal results were obtained in the range of pH from 5.0 to 8.2 and at the coagulation dose of 5.0 mg/L as Al. Under the optimum coagulation conditions, the selected PACS gave higher DOC and UV254 removal efficiencies, and lower residual aluminum concentrations in the treated water than PAC. The maximum removal of DOC and UV254 for PACS was approximately 88.0% and 93.0%, respectively. At the optimum coagulant dose and pH 6.5, the concentration of residual aluminum in treated water by both selected PACS and PAC can comply with the regulated limits. The major mechanisms of NOM removal by PACS and PAC coagulation involve complexation-charge-neutralization-precipitation.