Influence of Electrostatic Field from Soil Particle Surfaces on Ion Adsorption–Diffusion

Abstract

Despite the fact that soil is electrically neutral at a macroscopic scale, the potential gradient in the electric double layer (EDL) adjacent to the solid–liquid interface is not zero. The driving forces, therefore, must include the concentration gradient as well as the potential gradient for the adsorption or diffusion of charged ions in the soil. The purpose of this study was to theoretically and experimentally demonstrate that the electrostatic field in the EDL originating from surface charges will strongly influence cation adsorption and diffusion. The generalized linear theory for describing nonlinear diffusion was applied to a miscible displacement experiment in this study. Both theoretical analysis and experimental data showed that the electric field in the EDL exerted a significant influence on ion adsorption and diffusion. Calculation based on the generalized linear theory clearly demonstrated how the electric field from surface charges of soil particles affected the kinetics of adsorption. The experimental results of Mg 2+ adsorption in soil and pure clay samples showed that the experimental data can be well represented by the new diffusion model that accounts for the influence of the electrostatic field from a charged soil particle surface. In contrast, the models established based on Fick's diffusion laws failed to give a theoretical interpretation for why ion distribution in the soil should obey Boltzmann distribution when equilibrium is reached and why a great discrepancy existed between Fick's laws and the experimental data. This study indicated that the exchange and diffusion processes occurring in the external electric field of the EDL are essentially the same process.