Soil colloids, as the most subtle and active part of the soil, play a major role in the accumulation of heavy metals in the soil. Study on the effect of different soil colloids on the adsorption-desorption behavior and the transformation of chemical forms of cadmium will help to understand the environmental chemical behavior of cadmium, which has a great significance to the remediation of cadmium pollution in regional farmland. In this study, smectite, kaolin, goethite, aluminum hydroxide and humic acid which colloids are commonly found in soil were collected to explore the adsorption characteristics of cadmium on different soil colloid. The following conclusions were obtained: (1) The adsorption of the five colloids to cadmium belongs to monomolecular adsorption. Smectite can quickly absorb a large amount of cadmium, but the adsorption of cadmium is unstable and the desorption rate is the highest. The cadmium desorption rates of humic acid and goethite are lower. Adsorption-desorption experiment results show the retained adsorption capacity: humic acid (41.38 mg/g) > goethite (24.95 mg/g) > kaolin (19.43 mg/g) > smectite (15.06 mg/g) > aluminum hydroxide (10.84 mg/g). (2) The species analysis results show that the cadmium adsorbed by smectite and kaolin mostly exists in exchangeable state, and the cadmium adsorbed by goethite and humic acid is relatively stable. Except for goethite, the residual state content is relatively low. The species analysis results are consistent with the results of adsorption-desorption experiment. (3) The degree of influence of pH on cadmium adsorption by five colloid is: humic acid > goethite > kaolin > smectite > aluminum hydroxide.
