Differential effects of Cu ²⁺ and Zn ²⁺ on soil rheological properties: Possible contributions of cationic interfacial polarization

Abstract

Heavy metal contamination can significantly affect the migration and transformation of nutrients and pollutants in soil solution, while soil structural stability is a key factor in regulating the environmental behavior of pollutants. However, there is still a lack of systematic understanding regarding the effects of heavy metal ions on soil rheological properties and the mechanisms underlying these effects. Strengthening research in this field will help deepen our understanding of the patterns of substance migration in contaminated soil and the associated biogeochemical cycling processes. Therefore, this study uses Cu ²⁺ and Zn ²⁺ as representative heavy metal ions to systematically investigate their effects on soil rheological properties and the underlying mechanisms. The results indicate that, in both the Cu ²⁺ and Zn ²⁺ ion systems, all soil rheological parameters generally increase as ion concentration rises. Under same electrolyte concentrations, Cu ²⁺ ‐treated soils generally exhibited higher shear strength and viscoelasticity than Zn ²⁺ ‐treated soils, although the magnitude and significance of these differences were strongly dependent on soil water content. As water content increased, the rheological parameters of both ion systems gradually decreased. This response characteristic indicates that electrolyte concentration enhances soil structural stability by regulating the interaction between the interfacial electric field and particles. Furthermore, the polarization effect arising from differences in electronic configurations between Cu ²⁺ and Zn ²⁺ ions may amplify their differential influence on soil rheological behavior, particularly under relatively low water conditions. In contrast, increased water content weakens interparticle interactions through enhanced water‐film lubrication and dilution of electrolyte concentration, resulting in a looser soil structure. This study reveals that Cu ²⁺ and Zn ²⁺ exert distinct effects on soil structural stability by altering particle interactions potentially through interfacial electric fields and ion‐specific polarization behavior, with Cu ²⁺ inducing stronger increases in soil resistance to deformation than Zn ²⁺ under same electrolyte conditions. These findings provide new insights into how heavy metal ions regulate soil mechanical stability under contaminated conditions.