Mechanism of natural organic matter removal by polyaluminum chloride: Effect of coagulant particle size and hydrolysis kinetics

摘要

The mechanism of natural organic matter (NOM) removal by AlCl_3 and polyaluminum chloride (PACl) was investigated through bench-scale tests. The fraction distributions of NOM and residual Al after coagulation in solution, colloid and sediment were analyzed as changes of coagulant dosage and pH. The influence of NOM, coagulant dose and pH on coagulation kinetics of AlCl_3 was investigated using photometric dispersion analyzer compared with PACl. Monomeric Al species (Al_a) shows high ability to satisfy some unsaturated coordinate bonds of NOM to facilitate particle and NOM removal, while most of the floes formed by Al_a are small and difficult to settle. Medium polymerized Al species (Alb) can destabilize particle and NOM efficiently, while some floes formed by Al_b are not large and not easy to precipitate as compared to those forrned by colloidal or solid Al species (Al_c). Thus, Al_c could adsorb and remove NOM efficiently.- The removal of contaminant by species of Al_a, Al_b and Al_c follows mechanisms of complexation, neutralization and adsorption, respectively. Unlike preformed Al_b in PACl, in-situ-formed Al_b can remove NOM and particle more efficiently via the mechanism of further hydrolysis and transfer into Al_c during coagulation. While the presence of NOM would reduce Al_b formed in-situ due to the complexation of NOM and Al_a.