Sorption of U(VI) onto natural soils and different mineral compositions: The batch method and spectroscopy analysis

Highlights: • Undersurface soil and surface soil have different mineral compositions, which lead to the difference in uranium adsorption. • U(VI) is complexed with aluminol edge sites, silanol edge sites and Fe(III)-O groups when adsorbed onto natural soils. • Under acidic conditions, U(VI) is mainly adsorbed on Fe(III)-oxyhydroxide/oxide groups. • Under alkaline conditions, Ca²⁺ in the solution will decrease the adsorption of uranium onto the soils. - Abstract: This research studied the sorption behavior of uranium(VI) onto two different kinds of soils: surface soil and undersurface soil that taken from the depth of 30 m undersurface. The soil samples were collected from a low and medium-level radioactive waste disposal site in the southwest of China. The effects of pH, solid-liquid ratio and contact time on the adsorption behavior were studied by batch adsorption method. The experiment results show that the mineral composition of soil and the speciation of U in natural groundwater are two main influencing factors. Muscovite and clinochlore, two of the main minerals of soil samples, dominate the sorption behavior of uranium onto natural soils at weak acidic and near neutral pH range. Under neutral and weak alkaline conditions, the thermodynamic calculation results show that Ca²⁺ and CO₃²⁻ have significant influence on the species of uranium in aqueous solution. The U sorption reduced sharply due to the formation of the CaUO₂(CO₃)₃²⁻ (aq) complex. This work provides a better insight of the sorption behavior of uranium onto natural soils, and gives an in-depth understanding about the influence of aqueous and surface speciation.