Study of Cadmium Adsorption and Desorption by Simulant Acid Rain on Different Silicate Clay Minerals in Soil

The cadmium adsorption and desorption by simulant acid rain on different clay minerals in soil have been discussed. When the pH value is 7, adsorption time is 1 h, and the medium is 0.05 mol.L^-1 KNO₃, the saturated adsorption capacity of cadmium on montmorillonite, illite, and sepiolite is found to be 2.88,1.08,2.82 mg/g respectively, but on kaolinite nothing is found. A correlation analysis shows that the saturated adsorption capacity of cadmium on silicate clay minerals are related to their physical and chemical natures and the correlation between the saturated adsorption capacity of cadmium and the amount of manganese oxide is extremely significant. Both pH value and ionic strength of simulant acid rain have significant effects on desorption of cadmium. The desorption rate of cadmium increases with the decline of the pH value and the increase of ion strength. The desorption reaction is fast, and in addition the cadmium desorption rate has little change with the delay of desorption time and the increase of desorption times. Montmorillonite, illite and sepiolite have high adsorption rates for cadmium, and the desorption rates are low under the simulant acid rain conditions. The strong adsorption of cadmium on montmorillonite, illite and sepiolite is an important reason for cadmium retention in soil.