Effect Of Ion-interface Reaction On Soil Erosion Intensity

Soil erosion is a seriously threat to ecology and environment. It directly changes earth surface configuration, decreases soil fertility, destroys vegetation, accelerates the oxygenolysis of organic matter to release CO2, CH4, and oxynitride into atmosphere, and teansports pesticide and nutrient elements such as N and P into water bodies and results in the eutrophication of water bodies. Therefore soil erosion directly relates to the safety of production and living of human being, and it is important and significant to develop the theory and technology of prevention and control of soil erosion and soil and water conservation.Soil aggregate breakdown is the first step and the most crucial step in the process of soil erosion. The research on the mechanism of soil aggregate breakdown is necessary to understand soil erosion mechanism and to develop a technology of soil and water conservation. Although the current mechanisms of aggregate breakdown based on raindrop impact, differential swelling of aggregates, compression of trapped air and physico-chemical dispersion can explain the aggregate dispersion. However, they are only the apparent description for aggregate breakdown. From the viewpoint of colloid and interface science, soil charges produce a strong electric field near particle surface. When two adjacent particles approach each other and their electric fields overlap, an electrostatic repulsion force produces between the two particles. The electrostatic repulsion force, together with hydration force and long-range van der Waals attractive force, dominates the coacervation and dispersion of colloid particles, determines the stability and breakdown of soil aggregates, and further influence the strength of soil erosion during rainfall.In classical diffuse double layer theory and Derjaguin-Landau-Verwey-Overbeek(DLVO) theory, the electrolyte concentration and type are the main factors in influencing soil electric field. That is, the concentration and valence of counterions are regarded as the key parameters to determine the electrochemistry property(surface potential and electric field strength, etc.) of particles and the interaction between particles. But this theory cannot predict and explain the difference of particle interaction and aggregate stability among honovalent counterions. In other words, it only focuses on the valence and concentration of counterion, but neglect the ion specificity.Specific ion effects was found in the experiments of protein coacervation by Hofmeister in 1888. In the last ten years, scientists attribute the specific ion effects to the effects of ion size, hydration effect, and dispersion effects. Recently, it found that the specific ion effects is the result of change of energy of ionic non-valence electrons in the interaction between ion and surface. Different ion have different configuration and energy of electrons and present different ionic quantum fluctuation. Generally, the difference between ions are small because of the bondage of atomic nucleus. But in diffuse double layer, the ions near surface can be strongly polarized(non-classical polarization) by the electric field of surface, and thereby the difference between ions are amplified. At the same time, the ions that are polarized produce negative feedback to electric field and decrease it. The ionic non-classical polarization that results from the coupling effect of electric field- quantum fluctuation will influence the surface property of soil particle and the interaction between particles, and further influence the aggregate stability and soil erosion.Based on the above analysis, this study took Purple soil(constant charge soil) and Yellow soil(variable charge soil) as research object, calculated the surface property of soil particle and particle interaction in the cases of with and without specific ion effects. Through rainfall simulation, we determined the soil erosion intensity at different ionic concentration. It found that the specific ion effects significantly presented in particle interaction and aggregate stability. This study employed ionic non-classical polarization to successfully explain the change regulation of soil erosion intensity with specific cation/anion effects, and further revealed the electric field mechanism based ionic non-classical polarization of soil erosion during rainfall. The details of results and conclusions are as follows.(1) In the case of no specific ion effects, the effects of monovalent cations on the properties of soil particle surface and particle interaction were the same. The surface potential, electric field strength, and electrostatic repulsion force of soil particle among monovalent cation systems increased with the decrease of electrolyte concentration. It found that, when water wetted “dried” soil aggregate to dilute soil electrolyte, the soil electric field sharply increased, and the hydration force and electrostatic repulsion force immediately produced. The hydration force, together with the electrostatic repulsion force, firstly separated particles to distance ~1.5 nm to swell aggregate, then the hydration force disapeard. After that, the electrostatic repulsion force continued to separate particles thoroughly, which resulted in aggregate breakdown. With the impact and disturbance of raindrop and runoff, the micro-aggregates and primary particles that released from aggregates entered in surface water to form suspension and then transported with runoff, which resulted in soil erosion. The electrostatic repulsion force that originated from soil electric field determined the intensity of aggregate breakdown and soil erosion, the soil electric field, therefore, was the essential reason for aggregate breakdown and soil erosion during rainfall. In the case of specific ion effects, using the ionic absolute effective charge number(γ) to characterize the degree of ionic non-classical polarization, it found that the surface potential, electric field strength, and particle interaction among Li+(γ=1), Na+(γ=1.110), K+(γ=1.699), and Cs+(γ=2.506) systems were different and decreased in the order of Li+ > Na+ > K+ > Cs+, which presented strong specific cation effects. It analyzed that the difference came from ionic non-classical polarization. Furthermore, the difference in net particle interaction among the four cation systems predicted the different aggregate stability and erosion intensity during rainfall, and the ionic non-classical polarization would be responsible for it.(2) During rainfall simulation, there were large difference in particle transport and erosion intensity among Na+, K+, and Cs+ systems, which showed strong specific cation effects on soil erosion. Na+ system produced stable and persistent particle transport in the wide range of 0.0001–1 mol L-1; K+ system maintained stable particle transport only at concentration of <0.01 mol L-1, and the transport decreased with time at high concentration; For Cs+ system, at all concentration of 0.0001–1 mol L-1, its particle transport amounts decreased. Further analysis found that the difference in erosion intensity were more significant at low concentration than that at high concentration, indicating that specific cation effects mainly presented in diluted electrolyte solution. Therefore, the ion size, hydration effect, and dispersion effect would not be the reason of specific cation effects. With ionic absolute effective charge number characterizing ionic non-classical polarization, we recalculated the surface potential of soil particle. It found that the fitting curve between surface potential and erosion intensity in Na+ system predicted the erosion intensity of K+ and Cs+ systems very well, indicating that the erosion intensity uniformly varied with the surface potential of soil particle rather than electrolyte concentraction. Therefore soil electric field which was affected by the ionic non-classical polarization reasonably explained the experimental results of erosion intensity, and the ionic non-classical polarization was proved to be the origin of specific ion effects.(3) Although the repulsion force of soil negative electric field against anions decreases the possibility of anion-surface interaction, it found that, among HPO42-, H2PO4-, Cl-, and NO3- systems, their aggregate breakdown intensities and erosion intensities were significant different. In Cl- and NO3- systems, both aggregate breakdown intensity and erosion intensity were 0 g m-2 min-1 at concentration of 0.0001–1 mol L-1. But in HPO42- and H2PO4- systems, their aggregate breakdown intensity and erosion intensity increased with the decrease of electrolyte concentration. The zeta potential of the four anion systems increased with the decrease of concentration. Among the four anion systems, the zeta potential at the same concentration were different, and the biggest difference in zeta potential also presented at low electrolyte concentration rather than at high electrolyte concentration, indicating that electrostatic adsorption, specific adsorption, and dispersion force adsorption were not available to explain the increasing zeta potential among the four systems by anion adsorption. We calculated the repulsion energy between HPO42- and surface and the needed minimum ionic polar moment for overcoming the repulsion energy. By comparison, it found that the polar moment of HPO42- with non-classical polarization was bigger than the needed minimum ionic polar moment. That is, HPO42- could adsorb on surface by non-classical induction adsorption, and further increased the charge density and electric field strength of soil particles. Because of different non-classical induction adsorption of anions, the abilities of anions increasing soil electric field were different. As a result, the difference in non-classical polarization between anions resulted in different erosion intensities. Moreover, at low concentration, the strong soil electric field enlarged the difference in polarizability of anions. Therefore the difference in aggregate breakdown intensity and erosion intensity presented at low electrolyte concentration rather than at high concentration. Therefore, anionic non-classical induction adsorption which originated from anion non-classical polarization resulted in the specific anion effects in erosion of variable charge soil, and further revealed the erosion mechanism of variable charge soil with specific anion effects.(4) For constant charge soil, there were also difference in particle transport and erosion intensity among HPO42-, H2PO4-, Cl-, and NO3- systems, which indicted strong specific anion effects. But by comparison with variable charge soil, the particle transport and erosion intensity in constant charge soil were much different. First, the particle transport and erosion intensity were stronger in constant charge soil than that in variable charge soil, which may be the result of iron and aluminum oxides. Second, among the four anion systems, the sequence of their erosion intensity was differ from that in variable charge soil. The erosion intensity sequence among the four anion systems changed from HPO42- > H2PO4- > Cl-/NO3- in variable charge soil to NO3- > Cl- > HPO42- > H2PO4- in constant charge soil. The erosion intensity in NO3- and Clsystems significantly increased from 0 g m-2 min-1 in variable charge soil to 16.5 and 11.3 g m-2 min-1 in constant charge soil, respectively, whereas in HPO42- and H2PO4-systems, the erosin intensity increased a little. The difference between constant charge soil and variable charge soil may originated from the effects of iron and aluminum oxides and anion-surface interaction, but the essential reason needed further research.Baseed on the above results, we obtainsthree conclusions.(1) Soil electric field is the essential reason for aggregate breakdown and soil erosion. After water wets the aggregate and electrolyte concentration decreases, soil electric field sharply increases to produce hydration force and electrostatic repulsion force between particles, and further overcome van der Waals force to separate particles and break down aggregates. Then with the disturbance of the external effect such as raindrop impact and runoff shear, micro-aggregates and paimary particles enter surface water and transport with runoff, which induces soil erosion.(2) In classical theory, soil charge density and electrolyte type and concentration determine the strength of soil electric field. However, we discover that the ionic non-classical polarization in external electric field can strongly influence electric field strength, and further change particle interaction, aggregate stability, and erosion intensity.(3) Cations with non-classical polarization can decrease soil electric field, and anions with non-classical induction adsorption can increase soil electric field. The coupling effect of electric field and quantum fluctuation basing on ionic non-classical polarization will deeply influence particle interacrion, and aggregate stability. Thus this paper reveals the mechanism of aggregate breakdown and soil erosion during rainfall as the results of soil electric field under specific ion effects. It provides a possible way to prevent and control soil erosion basing on an “internal control” technology.