Effect Of Chloride On The Vadose Zone Soil Adsorption And Migration Of Cadmium

Because of the shortage of water resources, in Tianjin area, people have used sewage for agriculture irrigation for a long time, and this lead to cadmium pollution. Cadmium has the quite strong activity in alkaline salinized soil. Irrigation, rainfall and any other external effect can make a part of the cadmium be active, and then migrate to the deep soil through the unsaturated zone, unsaturated zone can storage and transmission pollutants to groundwater, then increase the risk of cadmium pollution. It is necessary to study the adsorption and the migration behavior of cadmium in soil unsaturated zone, studies have showed that the soil pH value, organic matter content and other factors can effect cadmium adsorption and migration, and other research shows that inorganic salt especially chloride can desorption the cadmium in soil, and be helpful for cadmium migration. There is a chemical reaction between Cl-and Cd2+form CdCl+. this formation has the same biological activity with CdCl+. In this study we chooses sodium chloride as a typical inorganic salt chloride for static test and dynamic soil column test, study on the effect of chloride on the transport and transformation characteristic of cadmium in unsaturated soil. The main results are as follows:This study analysis the collected soil sample, Da Nanhe is in south sewage river irrigation area of Tianjin, the content of cadmium in this area is higher and the residue state relative content is higher too, according to the Tianjin soil environmental quality grading standards for the provisions of the cadmium content, surface for class V, the soil is seriously polluted, that may linked to the high soil pH and the higher organic matter content. Zhang Langzhuang is in Beijing sewage river irrigation area, the soil is clay, and the cadmium pollution is also serious, so long-term sewage irrigation caused serious influence on soil environment.The adsorption of cadmium investigated that, with chloride, adsorption process of cadmium is quick too, but then desorption is the main chemical behavior, until the chemical reaction balanced. The amount of adsorption is linked to the concentration of chloride, higher concentration of chloride lead to low adsorption of cadmium, also cause the reaction difficult to achieve balance. The large amount of Na+in solution may cause competitive adsorption with Cd2+. At the same time, Cl-and Cd2+format CdCl+, the ionic radius are bigger and not easy to be adsorbed, the adsorption quantity is also reduced. The initial concentration of cadmium can affect the adsorption rate and adsorption result too, when the initial concentration is lager the curve of adsorption is obvious. The effect of chloride on soil Da Nanhe is relatively alleviation, maybe because of the high content of organic matter. The organic matter has higher adsorption capacity than soil mineral colloid, and the complex relatively stable. And the effect of chloride on soil Zhang Langzhuang is more significant, maybe the clay mineral adsorption is easily affected by chloride. The pseudo-second-order model fitting that chloride can increase the adsorption rate, also reduced the equilibrium adsorption.This study used column test to investigate the transport of cadmium. The adsorption of cadmium in soil, can block downward migration rate. Add chloride to leaching solution can reduce the soil adsorption ability. Thomas model can describe the first layer’s leaching curve. That chloride makes the cadmium migration rate increased slightly, but the maximum adsorption decreased, and it is obvious when the content of chloride increased. That chloride can also influence the content and chemical form. The competitive adsorption of Na+and the formation of CdCl+can reduce the adsorption of cadmium. The relative content of exchange state is increased significantly, the relative content of residue state is greatly reduced, other three kinds of states change slightly.Therefore, the existence of chloride can make the cadmium adsorption reduced, the amount of cadmium that has been adsorbed may be desorbed, and the exchange state of cadmium in soil vadose zone can increase. Active cadmium in the unsaturated zone downward migration with water and may cause shallow groundwater pollution.