Equilibrium Adsorption Method For The Determination Of Surface Potential On Soil Particles

Surface potential has broad application in many fields as well as in soil chemistry. Now there are mainly three methods for the determination of surface potential on soil particles-methods of Zeta potential, surface charge density and negative adsorption. It is generally accepted that Zeta potential can replace surface potential since the shear plane that Zeta potential represents is close to the Stem plane. However, many academician are beginning to realize that in the case of low electrolyte concentration the value of Zeta potential will much lower than that of surface potential, which means that Zeta potential can't replace surface potential. Theoretically, we can also obtain surface potential by introducing surface charge density into Gouy-Chapman equation; however this can only be done for permanent charged soils which surface charge density doesn't change along with the variety of electrolyte style, electrolyte concentration and the value of pH of solution. What's more, the value of SSA and total charge quantity vary from their determination methods and condition. Therefor, it is unreasonable to introduce the value of SSA and total charge quantity into the same equation to obtain the value of surface charge density and surface potential. Zhang, Sparks and Scrivne had put forward a method to measure the surface potential through the number of same sign ions negatively adsorbed by colloid particles. The result lead them to find that: it is doubtful that Zeta potential can replace surface potential. What has been mensioned above shows that again the rationality that Zeta potential replaces surface potential needs farther study. The greatest weakness of this method is that because of the exponential relationship between the number of ions negatively adsorbed by colloid particles and the value of surface potential little error in data will bring about great warp.This paper bases on math equation between surface potential on soil particles and average ion concentration in double electricity layer Li Hang and other people built. That helps us transform the problem of surface potential determination into the problem of average ion concentration in double electricity layer determination under the very solution concentration. On account of limitlessness of the equation from other conditions beyond DLVO theory the determination method basing on it can be used to get the value of surface potential in any electrolyte system.We introduce the data about Zeta potential, SSA, CEC of montmorillonite from Philip F Low's experiment(1980) into the equation Li Hang and other people built and the value of surface potential we get is much bigger than that of Zeta potential. Due to the character of permanent charged surface that montmorillonite has and the experimental curve between swelling pressure and distance between two particles Philip F Low found in his experiment we can safely draw the conclusion that value of surface potential we get is reliable. Then we adopt batch method , that is, when solution is in equilibrium condition, determine the electrolyte concentration in bulk solution and total charge quantity in double layer, to determine the value of surface potential on Neutral purple soil and latosol. Further, the experimental result has compared between the two different soils. The effect of electrolyte concentration, temperature, electrolyte type and pH on the surface charge properties was studied, too. The experimental result are as follows: Firstly, the surface potential range of the neutral purple soil in the 1:1 electrolyte type and 2:1 electrolyte type are -230~-280mv,-120~-160mv respectively. The result from kinetic method Yang Xinglun et al had used in 2004 is that the surface potential range of the neutral purple soil in the 1:1 electrolyte type and 2:1 electrolyte type are -320~-390mv, -160~-190mv respectively. Seen easily the results differs from methods. The reason may be variety in CEC, the electrolyte concentration in bulk solution and the method of SSA determination. Anyway, both results are much more than traditional result, decades mv. The result of surface potential in 2:1 electrolyte type is lower than the one in 1:1 type, and the former is about 50% of the latter which is accord with kinetic method. The result on latosol in the 1:1 electrolyte type and 2:1 electrolyte type are-170~-210mv,-90~-110mv respectively. It's also accord with kinetic method. The phenomenon that the thickness of double layer declines with the increase in concentration in bulk solution and rises with the increase in temperature; total charge quantity rises with the increase in concentration in bulk solution and declines with the increase in temperature can be found in both neutral purple soil and latosol. When talks about surface potential, it can be found in neutral purple soil that absolute value of surface potential declines with the increase in concentration in bulk solution and rises with the increase in temperature. The absolute value of surface potential on latosol also declines with the increase in concentration in bulk solution. But when temperature changes the law is not as obvious as in neutral purple soil. The absolute value of surface potential on latosol declines after it rises with the increase in temperature. The key is that latosol is variable charge soil. When temperature rises especially from 288K to 298K, total ion in DDL declines greatly, the trend that the absolute value of surface potential declines which brings about will be superiorer than that the absolute value of surface potential rises whcih little bigger cubage change in thickness of DDL with the increase in temperature brings about. When temperature rises from 298K to 308K, the reason is reverse.