INFLUENCE OF PH AND HUMIC-ACID ON COAGULATION KINETICS OF KAOLINITE -A DYNAMIC LIGHT-SCATTERING STUDY

Time-resolved dynamic light scattering was used to investigate the inf luence of pH and adsorbed humic acid on the coagulation kinetics of ka olinite clay colloids. Stability ratios, defined as the experimental f ast coagulation rate divided by the coagulation rate of the sample und er consideration, were determined from the initial increases in averag e hydrodynamic radius with time. Pure kaolinite exhibited fast coagula tion (stability ratio W = 1) at all ionic strengths (0.001 to 1 M NaCl O4) when suspension pH was below 5.8. Above pH 5.8, coagulation rates were significantly reduced (W > 1) and W exhibited pronounced ionic st rength dependence. The observed behavior is explained by edge-to-face heterocoagulation by electrostatic attractive forces at low pH and ele ctrostatic stabilization at high pH. The transition in coagulation beh avior occurs at the point of zero charge (PZC) of the kaolinite edge s urfaces near pH 5.8. Small additions of humic acid at pH 4 resulted in edge charge reversal from positive to negative and substantially redu ced coagulation rates (W > 1); similar behavior was observed at pH 6. Relating the observed stability ratios W to zeta potentials suggests t hat at low to medium ionic strength (less than or equal to 0.01 M NaCl O4) the stabilizing effect of adsorbed humic acid is purely electrosta tic. However, at high ionic strength (greater than or equal to 0.1 M) steric stabilization may also play a role. (C) 1998 Academic Press.