Molecular weight distribution effects of polyacrylamide flocculants on clay aggregate formation

Abstract

This article investigates the effect of polyacrylamide dispersity (broad and narrow molecular weight distributions) on their flocculation performance in a system of kaolinite clay suspended in water. A series of uniform chain length (dispersity, Đ = 1.04 – 1.35) polyacrylamide flocculants were synthesized using reversible addition-fragmentation chain-transfer polymerization. These uniform polyacrylamides were used individually, as low dispersity flocculants, and in blends, as high dispersity flocculants. Flocculation of the kaolinite suspension was monitored using a focused beam reflectance measurement system where the chord-length distribution of the clay aggregates could be recorded in real-time. Polyacrylamide flocculants with lower dispersities made larger clay aggregates than those with lower dispersities at equal doses. In the polymer blends, the fraction of the high molecular weight component was the best indicator of aggregate size formation; the low molecular weight fraction influenced less the final aggregate size. These findings suggest that polymers with narrow molecular weight distributions can be used in lower dosages than standard high dispersity flocculants with a similar concentration of high molecular weight polymer chains.