| Abstract
| - Aggregation kinetics of kaolinite−fulvic acid colloids, asinfluenced by two strongly sorbing trace metal cations, Cu(II)and Pb(II), was investigated by time-resolved dynamiclaser light scattering experiments. The effects of Cu andPb on the aggregation rate and electrophoretic mobility werecompared with that of Ca, another major divalent metalcation which is less strongly adsorbed. Kaolinite−fulvic acidsuspensions (in 0.01 M NaNO3 at pH 4 and pH 6) werespiked with solutions containing Cu, Pb, or Ca to give totaldivalent cation concentrations between 10-5 and 8 ×10-3 M. The concentration of kaolinite was varied between25 and 200 mg L-1, while the concentration of fulvic acidranged from 0.15 to 1.2 mg L-1. The mass ratio of kaoliniteto fulvic acid was kept constant at 500:3 in all experiments.Relative aggregation rates, expressed as attachmentefficiency α, were determined from linear increases inaverage hydrodynamic radius with time during the first 5−8min of the aggregation experiments, always starting witha well-dispersed suspension at time zero. The correspondingslope for fast aggregation (α = 1) was measured forpure kaolinite suspended in 0.01 M NaNO3 at pH 4. Additionof fulvic acid to the suspensions completely inhibitedkaolinite aggregation at pH 4 and pH 6. Additions of Cu,Pb, and Ca resulted in strongly increased aggregation ratesof the kaolinite−fulvic acid particles. The potential ofthe three cations to enhance aggregation of the kaolinite−fulvic acid colloids increased in the order Ca < Cu ≤Pb. At pH 4, the relationship between particle electrophoreticmobility and aggregation rate was the same for all threedivalent metal cations. In the presence of Ca, an increasein pH from 4 to 6 resulted in decreased aggregationrates. However, in the presence of Cu or Pb, the oppositetrend was observed and the relationship betweenelectrophoretic mobility and aggregation rate was differentthan at pH 4. The effects of Cu, Pb, and Ca on theaggregation rates of kaolinite−fulvic acid colloids areexplained by the different sorption behavior of the threedivalent metal cations.
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