Influence Of Soil Surface Potential On Ion Diffusion/Adsorption Dynamics

At present, a great number of researches related to the ion diffusion/adsorption kinetics in soil have been carried out, and most researchers accept the opinion that ion diffusion and ion exchange via electrostatic attractive forces occurring in DDL were two different and separate processes, and the reaction rate is limited by ion diffusion; Furthermore, Most of them directly employed Fick's first and second laws to describe ionic diffusion in the soil, and the electric potential from charged clay surface was neglected. The main target of this research is putting forward a new equation for describing cation exchange kinetics on the basis of the theory of "field", then determining the ion diffusion/adsorption rate and related parameters, so as to evaluate quantitatively the influence on ion diffusion/adsorption dynamics of soil surface potential, and at last to identify the leading position of the soil surface potential in ion diffusion/adsorption in the soil.In order to fulfill this task, firstly, the dynamic distribution equations of ion in DDL in both of adsorption and desorption process in the experiment of the miscible displacement and the batch technique have been established theoretically in this paper. Those equations can give us the clear pictures how the concentration of ions changes with time at different position both in DDL and in bulk solution when the adsorption and desorption are progressing. The results showed that, when the equilibrium of adsorption or desorption to be reached, the dynamic distribution will naturally change to the equilibrium distribution:the Boltzmann distribution. Thus, a new concept of ion diffusion/adsorption in DDL is introduced, that is, the process of ion exchange occurring in the diffuse double layer will be essentially a mutual diffusion driven by the gradient of the apparent concentration.Then, starting with the linear equation transformed from non-linear Fokker-Planck differential equation which the soil surface potential considered, and based on the instantaneous relaxation approximation in the ion diffusion process, we established the theoretical models that well describe the exchange process of ions in the soil under steady flow conditions, both theoretical analysis and experimental data in this paper demonstrated that ion exchange and ion diffusion in the electric field of DDL are not two different processes, they are one, the obtained results showed that the electric field in EDL exerted a significant influence on the ion exchange/diffusion.Based on the theoretical analysis, we also advanced the rate equations of ion diffusion/adsorption kinetics in different intensity soil surface field. For further reveal the influence of electric field from soil particle surface on cation exchange, in this study, three types of permanently charged materials, the K-saturated neutral purple soil and the K-saturated montmorillonite and the Ca-saturated montmorillonite, and K-saturated yellow soil with variable charge were used. The results showed that when strong forces adsorption (strong electrostatic adsorption or chemical bond adsorption) exist on soil particle surface, in the initial stage of diffusion/adsorption, the diffusion/adsorption process will appear the zero-order kinetics, and then transfer to the first-order kinetics, and the turning point from the zero-order to the first-order was very sharp for each system. Subsequently, By comparing the rate coefficient of ion as the electric field from particle surface not to be considered and the rate coefficient as the electric field to be considered, showing that, the electric field from soil particle surface strongly increases the diffusion/adsorption rate, and thus further explains that the soil surface potential is the main driving force of ion diffusion/adsorption process in soil, the exchange and the diffusion processes occurring in external electric field of DDL are really essentially the one process, that is ion exchange occurring in DDL via electrostatic attractive forces may be essentially a mutual diffusion process driven by the activity gradient of the ion (including the concentration gradient and the potential gradient), the opinion that ion diffusion and ion exchange via electrostatic attractive forces occurring in DDL were taken as two different and separate processes was absolutely mistake.By the comparative study of different ion exchange kinetics process on soil particle electric field at solid-liquid interface, Mg²⁺, Ca²⁺ adsorption kinetic in different particle surface potential was also studied. In this study, some important parameters such as ion diffusion/adsorption rate coefficient in different electrolyte solutions, the adsorption quantities, the surface coverage of the adsorbed ions and the average activity coefficients as the ion diffusion/adsorption equilibrium reached were studied. The experimental data showed that, in the initial stage of experiment, the adsorption process will appear the zero-order kinetics for the strong force adsorption, and then transfer to the first-order kinetics of the weak force adsorption, and the turning point from the zero-order to the first-order was very sharp for each system. When the supporting electrolyte concentration is equal, the adsorption rate of Ca²⁺ is obviously faster than Mg²⁺, the duration of the zero-order kinetics and the first-order kinetics process of Ca²⁺ is much longer than Mg²⁺, and the equilibrium adsorption capacity of Ca²⁺ is more than Mg²⁺, the average activity coefficients of Ca²⁺ on soil solid particle surface is also lower than Mg²⁺. By the analysis of the soil surface charge characters, we also found that in this paper, the differences of the relative effective charge coefficient and the surface electrochemical properties are the basic reason why the Ca²⁺,Mg²⁺ adsorption kinetics process different. At the same time, based on the theory and method in this research, we also can figure out some important properties for the adsorbed ions of the different adsorption forces respectively, such as the average activity coefficients, the rate coefficients, the adsorption quantities, the surface coverage of the adsorbed ions and the distributed space in the fixed liquid film of the adsorbed ions. These parameters make us possible in future to evaluate quantitatively the effects of different colloid surface potential on the ion diffusion/adsorption kinetics theoretically. The results once again makes clear the leading position of the soil surface potential in ion diffusion/adsorption in the soil from the different ion diffusion/adsorption kinetics process, prove the exchange and the diffusion processes occurring in external electric field of DDL are really essentially the one process, that is ion exchange occurring in DDL via electrostatic attractive forces may be essentially a mutual diffusion process driven by the activity gradient of the ion (including the concentration gradient and the potential gradient).In conclusion, this paper for the first time can evaluate quantitatively the effects of different colloid surface potential on the ion diffusion/adsorption kinetics theoretically from many ways such as the average activity coefficients, the rate coefficients, the adsorption quantities, the surface coverage of the adsorbed ions, etc. It is further concluded that ion exchange occurring in DDL via electrostatic attractive forces may be essentially a mutual diffusion process driven by the activity gradient of the ion.