Multi-ion Diffusion Model In Clay Cushion Under Non-ideal System

At present,various types of landfills and dredged sediment yards use compacted clay as a cushioning system in China.In the transporting process of pollutants in leachate at the bottom of the yard through the isolation barrier to the surrounding environment,ionic diffusion occupies the dominant position in the migration process,due to the low permeability of the compacted clay.How to reveal the behavior and influence mechanism of ion diffusion accurately has been a hot topic in the field of environmental geotechnical engineering.This paper introduces the modified Guntelberg activity coefficient to show the deviation of ion concentration between ideal and actual solution.Based on the generalized linear diffusion flux equation which using the chemical potential as the diffusion driving force,and considering the influence mechanism of diffusion potential and clay semi-permeable membrane effect on the ion diffusion behavior,the multi-ion diffusion model in the clay cushion is established.The finite element software COMSOL Multiphysics is used to conduct a series of numerical simulations.The main results are as follows:(1)Based on non-equilibrium thermodynamics,the linear diffusion model is deduced,which characterizes the diffusion driving force by chemical potential.Based on this,the diffusion potential and the semi-permeable membrane effect of clay are considered to establish the multi-ion diffusion model.(2)The finite element software COMSOL Multiphysics is used to solve the model,the effectiveness of the proposed model is verified by comparing with the experiment results and the existing model results.(3)Under the same boundary conditions and parameter values,the numerical calculation of the ion diffusion law under the single factor of the non-ideal of the solution,diffusion potential and semipermeable membrane effect of clay was carried out.The results show that the non-ideal of the solution has significant retardation effect on ion diffusion.When the ion valence state is fixed,the higher the initial concentration of ions,the more obvious the retardation effect on diffusion.When the concentration of contaminated source keeps constant,the greater retardation the retardation effect on diffusion.Due to the principle of electrical neutrality in the ion diffusion process,the diffusion potential is considered to promote the cation diffusion process in this model.The semipermeable membrane effect of clay is considered when the ion diffuses in the clay medium,it has a significant retardation effect on ion diffusion.(4)The numerical simulation of ion diffusion over time and space under of twofactor and three-factor combined action carried out.The results show that the combined effect of the two factors and the three factors on ions diffusion is not the result of the simple superposition effect.