Multi-Phase Sorption Dynamics of Pb2+ / Cu2+ at the Soil-Water Interface: Deciphering pH-Dependent Binding Mechanisms via Molecular-Scale Probing

Abstract Many regions in China are facing varying degrees of soil pollution due to the presence of copper and lead. Therefore, it is necessary to study their mobility and bioavailability behavior in soil. In the present study, the operating pH levels were maintained at 2.0 and 4.0. Through the applications of isothermal and pseudo-second-order kinetic models, it could be observed consistently that the adsorption capacity for Pb²⁺ exceeded that for Cu²⁺, with the optimal adsorption of both metal ions occurring at a pH of 4.0 within all the considered three systems. This phenomenon could be attributed to the presence of some functional groups in the soil, particularly N-H (1636 cm⁻¹) in amino acids, which exhibits a strong affinity for copper ions. At lower pH levels, the protonation of amino groups led to electrostatic repulsion between -NH³⁺ and Cu²⁺, as well as between ≡Fe-OH²⁺ and Pb²⁺/Cu²⁺. Furthermore, the order of hydrated ion radius (Cu²⁺ > Pb²⁺) indicated that the metal ions with smaller hydrated ion radii and lower hydration free energies were adsorbed more readily. These findings not only enhanced our understanding about Pb²⁺/Cu²⁺ adsorption behavior in soil under varying environmental conditions but will also contribute to a more comprehensive risk assessment of metal ion contamination.