Synthesis of Polyaluminum Chloride with a Membrane Reactor:  Operating Parameter Effects and Reaction Pathways

Abstract

Polyaluminum chloride (PAC) is an important flocculant in potable water and wastewater treatment. Al 13 or Al b is regarded as the most efficient species of PAC. A low base addition rate, small base drop sizes, and sufficient mixing usually favor the formation of PAC with higher Al b contents. In this paper, PAC was originally synthesized with a membrane reactor in which NaOH solution permeated through the micropores of an ultrafiltration membrane into AlCl 3 solution gradually to reduce the NaOH droplets size to nanoscale, about 10 6 times smaller than that of conventional methods, resulting in a great reduction in the local supersaturation and precipitates generated and, subsequently, in an increase in the Al b content. The effects of the membrane molecular weight cutoff (MWCO) and reactant concentration on the species distribution were investigated. It was found that Al b increases with decreasing MWCO and reactant concentration under the experimental conditions used. A new plausible reaction pathway is also proposed that assumes that the hydrolysis and polymerization of Al 3+ constitute a complex net of consecutive and parallel reactions and that the species distribution is mainly determined by the reactions kinetics.