The role of reactive surface sites and complexation by humic acids in the interaction of clay mineral and iron oxide particles

Aggregation and dispersion of mineral particles spontaneously take place under changing environmental conditions in natural systems. The structure of particle network in soils, the retardation or release of colloidal particles, and their mobility and transport are inherently influenced by natural organic matter bound to the mineral matrix. Since the surface properties of clay mineral and metal oxide particles, and the electrified mineral–water interfaces play a major role in formation, structure and strength of aggregates, any surface modification, especially by polyanionic organic complexants such as humic substances, has a significant affect on particle interaction in a mineral assemblage. The permanently and/or conditionally charged clay minerals (montmorillonite and kaolinite) and iron oxides (hematite and magnetite), as known major mineral components in natural waters, were selected for studying their surface charge characteristics and pH dependent interactions. We discuss how the surface charge correlates with particle aggregation through some characteristic examples for homo and heterocoagulation of similar and dissimilar mineral particles under acidic condition (at pH ∼4) in the dilute and concentrated systems studied by means of light scattering and rheology, respectively. The adsorption of a brown coal derived humic acid, and its influence on the surface charge character and particle aggregation of clay and iron oxide particles were also studied in dilute and concentrated suspensions. Humic acids can be bound to the most reactive surface sites of clay and oxide particles, i.e. to Al -OH mainly at the edges of clay lamellae, and to Fe -OH on iron oxides, in surface complexation reaction, therefore their role in particle aggregation is specific. Relations between surface complexation, surface charge modification, and particle aggregation in pure and mixed montmorillonite–iron oxide systems are explained.