Behaviour of humic-bentonite aggregates in diluted suspensions

Abstract

Processes of aggregate formation and disaggregation are common in both waters and soils and are widely used in different industries. Aggregates composed of clays and humic substances (HSs) are, either directly or indirectly, associated with almost all biological, chemical, and physical phenomena in waters and soils. Formation and disaggregation of micron-size aggregates in a diluted suspension made up of HSs and bentonite (B) were studied by tracing distribution of aggregate sizes and their counts in freshly prepared and aged suspensions, and at high (10 000) and low (1.0) [HS]/[B] ratios. Diluted HSB suspensions are unstable over time with respect to number of aggregates, and distribution of aggregate sizes at particular [HS]/ [B] ratios. The observed disaggregation to aggregation and aggregation to disaggregation events are probably a consequence of exposure of fresh clay surface upon disaggregation of HSB aggregates, partitioning of adsorbed humic substances between old and freshly exposed clay surfaces, adsorption of dissolved HSs on fresh bentonite surface, and decomposition of large units of HSs into smaller ones, a process probably catalysed by clays. Successive aggregation to disaggregation or disaggregation to aggregation events yield both qualitative and quantitative transformations in HSB aggregates, which thus may affect availability, transport and fate of adsorbed chemicals in both aquatic and terrestrial systems.