COMPARISON OF HEMATITE COAGULATION BY CHARGE SCREENING AND PHOSPHATE ADSORPTION - DIFFERENCES IN AGGREGATE STRUCTURE

The formation and structure of hematite aggregates were examined by dy namic and static light scattering techniques. A large range in coagula tion kinetics was studied by varying either indifferent electrolyte (K CI) concentration or surface complexing anion (H2PO4-) concentration, P-T, at pH 6.0 +/- 0.1. Diffusion limited aggregation (DLA) was induce d by counterion screening at [KCl] > 80 mM or by surface charge neutra lization at P-T = 31 mu M (and ionic strength = 1.0 mM). In DLA, the f ractal dimension, d(f), of aggregates formed by either surface charge neutralization or counterion screening was 1.7 +/- 0.1. A reduction in the rate of coagulation in KCl for [KCI] < critical coagulation conce ntration (CCC) produced an increase in d(f) to 2.1 +/- 0.1. For aggreg ation induced by phosphate adsorption at constant ionic strength, ther e was no apparent trend in d(f) with coagulation rate. The value of d, was consistently less than 1.8 when reaction limited aggregation (RLA ) resulted from surface charge neutralization rather than counterion s creening. TEM observations of aggregates formed in the presence or abs ence of phosphate confirm that, when RLA is induced by phosphate adsor ption, resulting aggregates we much looser in structure than those for med by counterion screening. The results suggest that the high-affinit y binding of phosphate to hematite may result in a nonrandom distribut ion of surface charge that facilitates the coalescence of positive and negative charge crystal faces.