Monte Carlo Simulation Of The Microcosmic Mechanism Of Ion Diffusion And Adsorption/Desorption In Soil

The soil is a system that a variety of external force fields coexist. The diffusion and adsorption/desorption of soil ions are important physical chemistry phenomena, which influence the retentivity, accumulation, transformation and migration of the nutrient and pollution elements between soil and the bulk solution. To study ion diffusion and adsorption/desorption in soil is of both theoretical and practical meanings in discovering the adsorption mechanism, explaining relevant physical and chemical properties of soil, and maintaining the sustainable development of the biogeochemical behaviors.For a longtime, lots of researches about soil ion diffusion and adsorption/desorption have been done and great achievements have been made by soil scientists. However, the Fick Law is still being applied directly to describe soil ion diffusion; the kinetics model of soil adsorption is a simple following of the classic chemical kinetic model and empirical model:all these models and theories ignore the effect of soil surface electric field and fail to reveal the true face of soil ion exchange, which let their accuracy and veracity under suspicion. Researchers have overlooked the influence of soil electric field on microcosmic processes, in the study of which computer stimulation is a very effective method and means. The present paper stimulates the process of ion diffusion and adsorption/desorption on soil surface using Monte Carlo method, explains the dynamic law, the selectivity, coverage and total adsorption of different ions and discusses the effect of ion size, charge and concentration on the exchange process under the consideration of soil electric field. Moreover, according to the simulation experiments, this paper verifies the mechanism models of ion diffusion and adsorption/desorption. The main achievements of these researches are as follows:(1)The only driving force of ion diffusion is the ion concentration gradient from bulk solution to double electric layer while neglecting soil electric field. The stimulation results validate the point and show that the ions are evenly distributed in bulk solution and the double electric layer after equilibrium. The results are in accordance with the Fick Law. Apparently, it is not the true picture of ion distribution in real soil for the reason that it follows Boltzmann distribution. Thus the classic Fick Law cannot be used to describe ion diffusion without the consideration of soil electric field.(2)The Monte Carlo simulation experiments show that the electric field accelerates the ion diffusion in double electric layer and the ions distribution follows Boltzmann Equation when it reaches equilibrium. So the electric field must be taken into consideration in soil ion diffusion.(3)Through the Monte Carlo simulation, it is found that the process of ion diffusion from bulk solution to double electric layer obeys the first-order kinetics, which is in accord with the theoretical model of ion adsorption dynamics under the consideration of soil electric field and also accords with the adsorption kinetic experiments. The Monte Carlo simulation confirms that the essence of soil ion adsorption/desorption is ion diffusion drived by soil electric field from the micro-mechanism and the ion adsorption equation under the effect of the electric force must obey the first-order kinetics.(4)Through the Monte Carlo simulation of soil ion adsorption/desorption based on the miscible displacement and the batch technique, the dynamic distribution curve is in accord with the theoretical results in the micro process. This shows that the Monte Carlo simulation and the theoretical models is verified its correctness for each other.lt is also another proof of the importance of electric force on soil ion adsorption/desorption.(5)The simulation experiments show that when the exchanging ions enter into the DDL layer from the bulk solution, there must be a certain amount of another ion entering into the bulk solution. The exchanging ions will bound in the DDL layer and form double electric layer together with the rest other ions as the process proceeds. When the equilibrium is reached, the ion concentration with larger selectivity would be higher than that of the low-selectivity ions. For example, in the K/Na system, the concentration of K would be higher than that of Na. This further confirms that ion adsorption and diffusion is the same process, which is a mutual diffusion.(6)The selectivity of different ions on the soil surface will change as the exchange proceeds and that of the exchanging ion will be larger. The difference in ion size has little to do with the selectivity coefficient. Through the relationship between ion selectivity and exchange time obtained from Monte Carlo simulation, the change of ion coverage to time can be calculated. The larger the selectivity coefficient is, the bigger the coverage percentage will be. When the exchange reaches equilibrium, the coverage remains unchanged.(7)The difference of the ion concentration in bulk solution results in the difference of total adsorption when it reaches equilibrium. When the concentration ratio reaches to a certain value, the total adsorption of two ions will be equal. As the ratio increases or decreases, the total adsorption of one ion will be larger than that of the other one. The computer simulation results fit the experiment data.In conclusion, based on the Monte Carlo stimulation of the soil diffusion and adsorption/desorption, the visualization and analyzability of the processes are achieved. The formation mechanism of the DDL layer under soil electric field and the influence on ion diffusion and adsorption/desorption are discussed through the obtained data. It is the first time to verify that soil electric field is the main reason in DDL formations using Monte Carlo method, which is also the main driving force for diffusion and soil ion diffusion and adsorption/desorption are the same process. What’s more, in researches about soil ion diffusion and adsorption/desorption, the surface electric field of soil particles must be taken into consideration for correct results.