| The charges on the soil surface account for a series of physical, chemical and physiochemical properties of the soil.As is known to all, soil particles has to adopt a certain amount of counter-ion to counteract the surface charges and make itself electrically neutral.However, this kind of adsorption is easily influenced by the external conditions.When the external conditions change, the ions inside the diffusion layer and the bulk solution will be redistributed, named after ion exchange.The ion exchange property of soil can reflect the fertility level,fertilizer retaining capacity and shock-absorbing capacity very well and indicates the physical and chemical properties of soil comprehensively. It is of great significance to the study of soil fertility and many relative fields such as environment science, ecology and so on, and it becomes one of the hot spots.There are two main approaches to deal with the ion-exchange in soil at present:a. all the adsorbed counter-ions by the soil particles can be seen as point charges on the surface of the particles. Ion exchange is regarded as the direct chemical reaction between ions in the solution and the point charges on the surface;b. when the electric double layer is compressed tightly the adsorbed ions in EDL are condensed within a narrow zone on the soil particles surface which makes the concentration-distributing curve steep. The imaginary surface on which the curve exists parallelling to the surface could be seen as the Donnan membrane, so we can use the Donnan equilibrium theory to build a ion exchange model.However, obviously, the application of the two models has ignored an important issue-the electric field formed by the surface charges on the soil, which indeed has a big influence on the process of ion exchange.Contrary to people' expectation, the distribution of counter-ions in EDL follow the Boltzman law within a certain spatial region on soil surface (soil surface electric fields range), instead of locating on a imaginary electrical panel paralleling to the soil surface and having a one by one corresponding relationship to the soil surface charges.So we analyze the nature of the ion exchange in soil as considering the soil surface electric field in the perspective of field theory and apply the new ion exchange model to the combined determination of the surface properties of soil particles.(1) Based on the field theory, we estimate the equilibrium condition of ion-exchange in soil is the activity field gradient is zero everywhere, which involves electric field gradient and concentration gradient in soil. The essence of the ion-exchange process is the diffusion under the activity gradient.Then, we obtain the expression of ion activity on soil surface, selective coefficient, surface coverage of ions and the ion distribution between the solid/liquid phase.The formula shows that ion activity on soil surface and distribution between the solid/liquid phase are affected strongly by the surface potential and the relative effective charge coefficient. The introduction of relative effective charge coefficient is to characterize the difference of competitive power between different types of ions caused by ion hydration.It has been noticed that different types of ions with same valence express different competitiveness in ion exchange study and the order has been arranged qualitatively. What's more, they are aware of this deviation comes from hydration radius; unfortunately none of quantitative comparison of this sequence has been carried out yet.The relative effective charge coefficient bwteen Na/K and Na/Ca ions were obtained through the experiment. When apply the new ion-exchange model basing on the field theory, considering the relative effective charge coefficient or not are discussed respectively. The result shows that, the new model can reflect the ion-exchange in soil actually only after taking into account of the relative effective coefficient and the surface potential on soil particles has a great influence on ion exhange.(2) Based upon the new ion exchange model, a new determination of surface potential is proposed. Select one type of ion that will not be adopted chemically on soil surface as indicatior ion,then the determination of surface potential will be converted to the determination of concentration of indicator ion in the bulk solution and EDL.In this paper,the swelling method is used to verify the new determiantion method in solution with different kinds of 1:1 type electrolyte.Data, specific surface area, cation exchange capacity and zeta potential value in 10-4mol/LNaCl solution of different montmorillonite, used in the study were from PhilipF.Low 1981. Firstly, according to the constant charge character of montmorillonite, the surface potential values obtained from the new method and the charge density method are very close, but far greater than zeta potential value, even up to the zeta potential value 5 times.For the further verification the surface potential values obtained from the new method and the zeta potential method were brought into swelling pressure expression, and then two curves indicating swelling pressure and distance between particles could be obtained. The results showed that:the curve calculated from the surface potential value obtained by the new method has good agreement with Philip Low's experimental curve, while the curve calculated from the zeta potential is far from the experimental curve.The surface potential could be exactly determined by the new method, and the zeta potential can not substitute the surface potential. The new method not only makes the direct determination of surface potential come true, but also can determine the surface potential in different pH solution.According to the obained experimental equation of the relative effective charge coefficient for Na+/H+ and Ca2+/H+, the surface potential of montmorillonite in NaCl or CaCl2 suspension under different pH is calculated for known surface area.The result shows that:the absolute value of surface potential in the CaCl2 solution is lower than that of NaCl solution, indicating that Ca2+ has larger screening effect on surface potential than Na+. Meanwhile, the result gives that the probability of appearance of hydrogen ion on material surface is high because of the strong competitive of hydrogen ion, which leads to larger absolute value of surface potential for the weak shielding effect on the surface potential of the small size of hydrogen ions.(3) According to the ion exchange model based on the field theory, a combined determination of the surface electrochemistry properties has been developed. In order to obtain the surface potential, surface charge number, surface charge density, specific surface area and surface field strength, the experimental studies were carried out by determining the concentration of two non-specific indicator cation in the initial status and the balance status of the bulk solution and bringing them into the formula.Compared with the method discussed earlier, the new method can determine the surface electrochemistry properties under different pH, different types of electrolyte and different temperature condition without knowing the specific surface area advance. What's more, the new method has the potential to achieve device operation through the introduction of iterative algorithms.Firstly, the combined method was applied into the neutral Na/Ca and Na/K systems. The results showed that:the surface electrochemistry properties could exactly be determined by the combined method. The result of specific surface area and surface charge number of the same sample in different systems were similarly.The absolute value of the surface potential in Na/K system is higher than that of Na/Ca. But the combined method could not describe the electrochemistry properties very well in low pH of Na/K and Na/Ca systems.The reasons can be conclued as follows:on the one hand,the accuracy of the experimental equation of the relative effective charge coefficient of Na+,K+,H+ and Na+,Ca2+,H+ is not high, especially for Na+,K+,H+ system;on the other hand, the less Na+ adorption may cause increase of measurement error. |
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