Migration mechanism of lead ions in unsaturated loess under temperature gradient

Based on the theory of porous media and the continuum thermoelasticity, the migration mechanism of heavy metal ions in unsaturated soils under temperature gradient is studied. Firstly, the adsorption model for the lead ion by soil particles is determined through the non-isothermal adsorption tests. Secondly, based on the mass conservation, energy conservation and momentum conservation equations for various components in the three-phase system of solid, liquid and gas, a multi-physical field coupling mathematical model characterizing the migration of pollutants in unsaturated soils under temperature gradient is established. The model takes into account the adsorption effects of heavy metal ions, the migration of pore fluid, the compressibility of phase change soil skeleton and pore fluid phase and the deformation of soils regarding the temperature, porosity, pore water pressure, pore gas pressure, heavy metal ion concentration, adsorption concentration and soil skeleton displacement as the basic unknowns, which comprehensively reflect the characteristics of heat and mass migration and deformation in unsaturated soils. Finally, a soil column test on the migration of lead ions in unsaturated loess is carried out to verify the effectiveness of the theoretical model and the numerical results, and the influences of various factors on the migration laws of lead ions are analyzed. The results show that the Langmuir nonlinear adsorption model is more consistent with the adsorption characteristics of unsaturated loess for lead ions, and the maximum adsorption capacity decreases with the increase of temperature. The temperature gradient can obviously promote the migration of pollutants in unsaturated loess. While the pollutants migrate in unsaturated loess soil column, the whole soil column is in tensile deformation state. The migration of pollutants decreases with time. The adsorption is a long-term process, and the effects of time on it are not significant.