Observation of specific ion effects in humus aggregation process

Abstract

Specific ion effects play a vital role in a variety of colloidal and interfacial processes. However, few studies have reported the specific ion effects in the humus aggregation process, which strongly influence the transport and fate of environmental pollutants. In this study, soil humus colloids were prepared and characterized, and the specific ion effects on humus aggregation in electrolyte solutions were investigated at a variety of concentrations and pH values using dynamic light scattering methods. Activation energy (ΔE), which is known to reflect the dynamics and stability of a colloidal system, was used to quantitatively characterize the specific ion effects. The results showed that given ΔE value of 2.48 × 103 J mol−1 at pH 3.0, the electrolyte concentrations were 91.6, 58.2, 3.8, and 0.8 mmol L−1 for Na+, K+, Mg2+, and Ca2+, respectively, thus indicating significant specific ion effects in the humus aggregation process. Most importantly, decreasing the electrolyte concentrations increased the differences in the ΔE value between two cation species with the same valence (i.e., ΔENa – ΔEK and ΔEMg – ΔECa), while increasing the pH increased the magnitude of ΔEMg – ΔECa. However, the classic Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory and the double layer theory, as well as the currently widely used ionic hydration and dispersion effects, failed to predict the experimentally observed increase in the specific ion effects with decreasing electrolyte concentrations in a quantitative sense. These results have implications for the necessity of involving specific ion effects for a better understanding of humus aggregation and interactions in aqueous and soil systems.