Influence of pH and Humic Acid on Coagulation Kinetics of Kaolinite: A Dynamic Light Scattering Study

Abstract

Time-resolved dynamic light scattering was used to investigate the influence of pH and adsorbed humic acid on the coagulation kinetics of kaolinite clay colloids. Stability ratios, defined as the experimental fast coagulation rate divided by the coagulation rate of the sample under consideration, were determined from the initial increases in average hydrodynamic radius with time. Pure kaolinite exhibited fast coagulation (stability ratioW= 1) at all ionic strengths (0.001 to 1MNaClO4) when suspension pH was below 5.8. Above pH 5.8, coagulation rates were significantly reduced (W> 1) andWexhibited pronounced ionic strength dependence. The observed behavior is explained by edge-to-face heterocoagulation by electrostatic attractive forces at low pH and electrostatic stabilization at high pH. The transition in coagulation behavior occurs at the point of zero charge (PZC) of the kaolinite edge surfaces near pH 5.8. Small additions of humic acid at pH 4 resulted in edge charge reversal from positive to negative and substantially reduced coagulation rates (W> 1); similar behavior was observed at pH 6. Relating the observed stability ratiosWto zeta potentials suggests that at low to medium ionic strength (≤0.01MNaClO4) the stabilizing effect of adsorbed humic acid is purely electrostatic. However, at high ionic strength (≥0.1M) steric stabilization may also play a role.