Heteroaggregation of humic acid with montmorillonite in divalent electrolytes: effects of humic acid content and ionic concentration.

Purpose: Clay-humic substance complexes play a major role in controlling the mobility of elements and colloids in natural soils. The purpose of the present study is to explore the different reaction mechanisms induced by cations on the humic acid (HA) and montmorillonite (Mt) heteroaggregation and to analyze the binding mechanism of HA-Mt. Materials and methods: HA is extracted from soil and colloidal Mt are prepared as K⁺-saturated. The aggregation kinetics of HA-Mt composite nanoparticles in Ca(NO₃)₂, Mg(NO₃)₂, and Cu(NO₃)₂ solutions were detected by dynamic light scattering. Furthermore, FT-IR spectroscopy was used to characterize the reactive sites involved in interaction with the metal ions. Results and discussion: The results revealed that the order of reaction of these three metal cations with the HA-Mt composite was Cu²⁺ > Ca²⁺ > Mg²⁺, as evident from the total average aggregation rate, critical coagulation concentration, and activation energy. The heteroaggregation process was sensitive to the 1% mass percentage of HA; however, more HA (4% mass percentage) did not significantly affect this process compared with 1% HA. Higher cation concentration and higher HA content (10% mass percentage) were two necessary conditions for promoting HA-Mt heteroaggregation. The vibration peak intensities of the carboxyl group C-O bonds and hydroxyl group O-H bonds of HA were affected by the formation of coordinate bonds with different metal ions. Conclusions: Metal cations were preferentially complexed by the carboxyl groups of HA, and due to its polarization-induced and electric field–enhanced oxidizing properties, Cu²⁺ has the strongest aggregation ability for HA-Mt, followed by Ca²⁺ and Mg²⁺. The HA-Mt heteroaggregation is partially reversible by adjusting electrostatic repulsion. The results of this study improve our understanding of the roles of cations and HA in clay-humic substance interactions. [ABSTRACT FROM AUTHOR]