Size and charge constraints in microaggregation: Model experiments with mineral particle size fractions

In natural soil systems the mass ratio of clay minerals and Fe oxides is decisive for aggregate formation and stability but little information exists on the importance of size constrains in microaggregation. Therefore, we determined the aggregation kinetics of mono- and dual-mineral mixtures at pH 6 by dynamic light scattering, using three size fractions of model minerals (goethite and illite). Aggregates were characterized by scanning electron microscopy (SEM), and measured for zeta potential (zeta), surface chemical composition by X-ray photoelectron spectroscopy (XPS), and specific surface area (SSA). Destabilization experiments using sonication revealed the stability against dispersion. Aggregation occurred in all homoaggregation experiments, particularly for fine illite (< 0.2 mu m). Illite-goethite heteroaggregation experiments revealed a broad range of aggregation with increasing goethite additions (0-90 %) for fine illite and a narrow one for medium and coarse illite (0-15 %). Maximal aggregation typically occurred at the point of zero charge (pzc) of respective mineral mixtures. Surface chemical analysis, N-2 gas adsorption, and SEM showed that the illite surfaces were largely covered by goethite layers and clusters. Illite-goethite heteroaggregates were most stable at their pzc, whereas homoaggregates were generally less stable. Our study suggests a primary role of the < 0.2-mu m clay fraction in the formation of stable clay-oxide microaggregates.