Fragmentation of kaolinite aggregates induced by ion-exchange reactions within adsorbed humic acid layers

Mixing of humic acids and kaolinite clay suspended in aqueous solution containing aluminum ions promotes aggregation of the clay instantaneously. To this fast clay destabilization succeed reversible fragmentation processes which display varying kinetic ranges for which the changes in the average masses of fragments are determined. We show the rate of formation of fragments to decay with the mass i as i(-2) and the maximal rate of aggregate breakup to increase with the fragment mass as i(lambda), where lambda lies between 0.35 and 0.40. At short terms the rate of variation of the average masses of the fragments is lower than expected from the theory. Aggregate fragmentation is attributed to slow progressive modifications in the positive and negative charge distributions within the amphoteric humic acid adsorbed layer which are induced by the high density of positive charge existing on the clay surface after aluminum ion adsorption. Kaolinite-aluminum ion-humic acid complexes aggregate by forming initially linear small portions of stacked platelets. These portions are connected together to form larger agglomerates of fractal dimension close to 2.5. (C) 2000 Academic Press.