Cd2+adsorption characteristics of typical soils in karst areas

The problem of cadmium (as Cd and Cd2+)pollution is prominent in karst areas. This paper studies it adsorption property to reveal the migration of Cd2+in the soil environment, thus providing a scientific basis for the environmental risk assessment of the soils. Adsorption of Cd2+by calcareous soils (brown calcareous soil、black calcareous soil) in the karst areas was investigated via batch sorption experiments. The results show a higher sorption capacity of calcareous soil to the heavy metal Cd, with the mean adsorption rate ranging from 89.84 to 98.84, of which black calcareous soil is greater than that of brown calcareous soil. The isothermal adsorption process of the both calcareous soils can be satisfactorily described by Langmuir and Freundlich models, while simulative result of the latter is better than that of the former. In the meantime, the Elovich,two-coefficient,W-M and first-order models are used to simulate the soil Cd2+sorption kinetics. On the one hand, the Elovich and two-coefficient equayions represent the best models to simulate the adsorption kinetics of Cd2+in brown calcareous soil with the correlation coefficients in excess of 0.9.On the other hand, the adsorption kinetics of Cd2+in black calcareous soils only fits the Elovich,two-coefficient and W-M equations with the significance level of above 0.8. The simulations for the black calcareous are processed with an initial Cd concentration of 100 mg/L, where the Elovich model is proved to be the best one. Moreover, the soil property analyses show that the contents of organic matters, calcium carbonate and CEC have a significant impact on the adsorption capacities of the soils studied, while the contents of iron, aluminum and silicon oxides in the soils show a minor influence on their Cd absorption capacities.