Aggregation kinetics of kaolinite-fulvic acid colloids as affected by the sorption of Cu and Pb

Aggregation kinetics of kaolinite-fulvic acid colloids, as influenced by two strongly sorbing trace metal cations,Cu(II) and Pb(II), was investigated by time-resolved dynamic laser light scattering experiments. The effects of Cu and Pb on the aggregation rate and electrophoretic mobility were compared with that of Ca, another major divalent metal cation which is less strongly adsorbed. Kaolinite-fulvic acid suspensions (in 0.01 M NaNO3 at pH 4 and pH 6) were spiked with solutions containing Cu, Pb, or Ca to give total divalent cation concentrations between 10(-5) and 8 x 10(-3) M. The concentration of kaolinite was varied between 25 and 200 mg L-1, while the concentration of fulvic acid ranged from 0.15 to 1.2 mg L-1. The mass ratio of kaolinite to fulvic acid was kept constant at 500:3 in all experiments. Relative aggregation rates, expressed as attachment efficiency alpha, were determined from linear increases in average hydrodynamic radius with time during the first 5-8 min of the aggregation experiments, always starting with a well-dispersed suspension at time zero. The corresponding slope for fast aggregation (alpha = 1) was measured for pure kaolinite suspended in 0.01 M NaNO3 at pH 4. Addition of fulvic acid to the suspensions completely inhibited kaolinite aggregation at pH 4 and pH 6. Additions of Cu, Pb, and Ca resulted in strongly increased aggregation rates of the kaolinite-fulvic acid particles. The potential of the three cations to enhance aggregation of the kaolinite-fulvic acid colloids increased in the order Ca < Cu less than or equal to Pb. At pH 4, the relationship between particle electrophoretic mobility and aggregation rate was the same for all three divalent metal cations. In the presence of Ca, an increase in pH from 4 to 6 resulted in decreased aggregation rates. However, in the presence of Cu or Pb, the opposite trend was observed and the relationship between electrophoretic mobility and aggregation rate was different than at pH 4. The effects of Cu, Pb, and Ca on the aggregation rates of kaolinite-fulvic acid colloids are explained by the different sorption behavior of the three divalent metal cations.