Quantitative characterization of non-DLVO factors in the aggregation of black soil colloids

The variable role and fate of soil colloids under different environmental conditions are derived from their dispersion and aggregation properties. In this work, dynamic and static light scattering were used to characterize the original size, aggregation kinetics of natural black soil colloids (BSCs) and structural features of aggregates in electrolytes with different cations (K⁺, Mg²⁺, Ca²⁺), respectively. For these three cations, the aggregation kinetics followed the trend of Ca²⁺ > Mg²⁺ > K⁺ and the critical coagulation concentration (CCC) followed the sequence: K⁺ (134.30 mmol L^-1) > Mg²⁺ (13.27 mmol L^-1) > Ca²⁺ (4.19 mmol L^-1). The results indicated that the aggregation behavior in different valence cation systems followed the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) model qualitatively. However, the quantitative differences of CCC suggest the existence of ion-specific effects. The effective ionic charge coefficient 1.31, 2.20, and 2.78 of K⁺, Mg²⁺ and Ca²⁺ were proposed to consider of all the non-DLVO factors, which were obtained by forming a relationship based on mathematic between the electrostatic repulsion and the van der Waals attractive interaction at the CCC. The non-classical polarization of cations in a strong soil electric field is a primary mechanism of cation effects on soil colloid interactions, causing the difference in colloid interaction energy and further affecting soil colloid aggregation. This result is crucial for enriching the theory of charged colloidal interactions.