Impact Of Electric Field Strength From Soil Particle Surface On Adsorption/desorption Kinetics Of Zn²⁺

Heavy metals adsorption/desorption by soils plays a key role in controlling the availability of heavy metals for plants. The current researches showed that, many processes, e.g. diffusion, adsorption and desorption of metal ions in soil system, are strongly influenced by the electric properties of soil particle surface. Therefore,in studying the diffusion, adsorption and desorption mechanism of heavy metals in soil, the evaluation of the influence of electric field strength from soil particle surface on heavy metal behaviors would be an important issue.However, all of these researches bear the same limitation that the effect of the electric field from soil particle surface on ion diffusion was not considered. Whereas there is a magnitude electric-field in soil system, it is necessary to consider the influence of electric-field in the research of ion diffusion in soil. The establishment of an easy approach to measure the surface potential and a simple linear theory for describing ionic diffusion providing the theory and method bases in the research of ion diffusion in soil as considering the effect of electric-field from soil particle surface. The kinetic method has been used in this experimental study to determine the surface charge properties. Neutral purple soil and yellow soil were taken as the experimental materials. The charge quantity, the surface potential, the surface charge density, the surface electric field strength and SSA was measured. The experimental result has compared between the two different soils. We also used the kinetic method to research Zn diffusion, adsorption and desorption in the two soils under the same condition as we determine the surface charge properties. The ion adsorption and desorption kinetics in two soils has been discussed. The experimental result as follows:Firstly, the kinetic method has been used to determine the electrochemical properties on solid/liquid interface in 2:1 and 1:1 type electrolyte system. The Surface Potential, surface charge density and the surface electric field strength of Magnesium saturated and K saturated soil samples whit 10% (Quality percentage) Montmorillonite are all higher than the soil samples whit no Montmorillonite. The surface potential, surface charge density and the surface electric field strength in 1:1 electrolyte system are all higher than in 2:1eletrolyte system. Secondly, in different supporting electrolyte systems, the adsorption quantities of Zn²⁺ are not equal. For the same type of supporting electrolyte system, high concentration(0.1 mol/L), the surface soil samples of the absorption of zinc ions are low, this part of the zinc ions are mainly specific absorbed, yellow soil of the specific adsorption more than the purple soil. Low concentration(10^-4mol/L),zinc ions are mainly electrostatic adsorption, the experiment reflects the electrostatic adsorption quantities of zinc ions in neutral purple soil is higher than in yellow soil.Thirdly, we discussed Zn²⁺ adsorption /desorption kinetics. The results showed that the Zn²⁺adsorption process, negative first-order kinetic process in the initial stage of diffusion (0-40min),then adsorption first-order kinetic process exists in yellow soil. Because the SSA in neutral purple soil is larger than in yellow soil ,but both of the liquid film volume differ not greatly, the diffusion path in neutral purple soil is shorter, so negative first-order kinetic process did not appear. The Zn²⁺ desorption process, desorption first-order kinetic process exists in neutral purple soil.Fourthly, we found that Zn²⁺ adsorption kinetic:(a)The same sample, in the same type background electrolyte system, the lower the concentration of background electrolyte, the greater the adsorption rate constant of Zn²⁺,the greater the surface electric field strength.(b)The same sample, in the same concentration of background electrolyte, the electric field strength and the adsorption rate constant of Zn²⁺ in 2:1 electrolyte system is lower than in 1:1 electrolyte system. And, all the figs show that ,samples with 10% (Quality percentage) Montmorillonite, as the surface electric field strength increases, the rate of absorption of Zn²⁺speed up, the adsorption rate constant greater.Fifthly, we also found that Zn²⁺ desorption kinetic:(a)The same sample, in the same type background electrolyte system, the higher the concentration of background electrolyte, as the lower the surface electric field strength, the greater the desorption rate constant of Zn²⁺.(b)The same sample, in the same concentration of background electrolyte, As the electric field strength in 2:1 electrolyte system is lower than in 1:1 electrolyte system. The desorption rate constant of Zn²⁺ in 2:1 electrolyte system is higher than in 1:1 electrolyte system. And, all the figs show that, samples with 10% (Quality percentage) Montmorillonite, as the surface electric field strength increases, the rate of desorption of Zn²⁺ reduces, the desorption rate constant lower.The results show that the soil surface electric charge and surface electric field strength strongly influence the ion adsorption / desorption, so the study of metal ions in the soil adsorption / desorption, the electrical properties of the surface must be as important factor to consider.