Adsorption And Desorption Characteristics Of Cadmium In Calcareous Soils On Karst Area

In recent years, Cadmium contanmination in calcareous soils has become more and more serious in China karst area. Studies on the Cadmium adsorption-desorption characteristics and mechanisms in soils are very useful to identify the specific chemical behaviors of Cadmium in environment, to offer the reference frame for risk assessment of Cadmium. Consequently, these researchs have important theoretical and practical significance. In this study, three typical calcareous soils were collected, including brown rendzina, rendizina and terra rossa. The characteristics and mechanisms of Cadmium adsorption-desorption in calcareous soils were studied detailedly by using the method of batch test. The main conclusions were as follows:1) The isothermal adsorption curves of Cd²⁺ in calcareous soils show nonlinear growth with the increase of the equilibrium concentration. Isothermal adsorption process can be divided into fast and slow adsorption phase. Langmuir and Freundlich equation can well describe the isothermal adsorption of Cd²⁺ process on three types of calcareous soil, including the Freundlich equation presenting the optimal fitting effect. The kinetics of Cd²⁺ on calcareous soils are consistent with pseudo-second order kinetics,and the variation of adsorption with reaction time has been found to follow three stages,i.e.,first stage with rapid increasing adsorption amount, second stage with slow increasing adsorption amount and three stage with adsorption reaching equilibrium.To compare the adsorption properties of three calcareous soils synthetically, it was found that rendizina has the biggest saturated adsorption quantity and the strongest adsorption binding energy, followed by brown rendzina, terra rossa is the worst.2) Cd²⁺ desorption capacity is opposite to its adsorption capacity. The stronger the adsorption capacity, the desorption capacity is getting worse. The desorption capacity of Cadmium in studied soils followed the order, terra rossa > brown rendzina > rendizina.As initial concentration of Cd²⁺ increased,the desorption rate in calcareous soils decreased at first and then increase,which reflected the changes of soils and Cd²⁺ bonding strength. The desorption isotherms of Cd²⁺ from brown rendzina and rendizina significantly deviated from the adsorption isotherms indicating hysteresis phenomenon, and the hysteresis indexs were 1.14 and 1.10 respectively.However, the adsorption/desorption isotherms from terra rossa were similar, showing that hysteresis was not obvious. The desorption kinetic of Cd²⁺ characterized by an initial fast release rate followed by a slower release stage and a model of pseudo-second-order reaction adequately describes the Cd²⁺ desorption from calcareous soils.3) The adsorption mechanism of Cd²⁺ in calcareous soil varies from one type of soil to another. The binding mechanisms of Cd²⁺ in brown rendzina and rendizina are mainly chemical bond adsorption and chemical precipitation adsorption, and the adsorption mechanism of terra rossa is mainly based on electrostatic adsorption. The desorption mechanisms of Cd²⁺ in calcareous soil are closely related to its adsorption mechanism, and its desorption mechanisms are mainly thermal diffusion and ion exchange.