Abstract: The risk and degree of harm of exogenous thorium to soil environment are depended on the adsorption behavior of thorium on soil. The adsorption of exogenous thorium on soil samples, collected from rare earth industrial park in Baotou, China, and mineral components(kaolin, montmorillonite, CaCO3, hydrous iron and manganese oxides, humic acid) were investigated by the static adsorption manner, and the adsorption behaviors were characterized by SEM, EDS, XRD and FTIR. The results show that the strong adsorption ability of exogenous thorium(10-4 mol/L) on soil samples was observed by high adsorption ratio of over 97%, however, the adsorption rate of soil mineral components to exogenous thorium(1 g/L) is in the range of 28%—46%. According to the results of SEM and EDS, exogenous thorium is adsorbed onto soil through working with soil mineral components, and likely formed the stable calcic thorium carbonate and thorium phosphate; and based on the results of FTIR, exogenous thorium is adsorbed onto soil by reacting with active adsorption sites of soil mineral components, which is the physical adsorption and chemical adsorption processes; and the results of XRD show that soil mineral components are not almost changed before and after adsorption of thorium on soil, but crystal phase states changes of soil minerals occurred. Owing to the different adsorption manners of soil mimeral components, the adsorbed thorium exists in soil as different fractions.

Key words: Soil, Thorium, Adsorption behavior