Incorporation of radioactive metals and trace metals into phosphate 'anhydrous' autunite:thermodynamic evaluation

摘要

The prediction of heavy metal solubility in the presence of phosphate is the key for designing a successful environmental remediation strategy or for the performance assessment of a nuclear waste repository.In order to evaluate the possibility for solubilities and incorporation of U and its fission products,such as Cs,Np,Pu,it is important to have the Gibbs free energies of rormation of possible end-member phases.In this paper,we use a linear free energy relationship to calculate the Gibbs free energies of formation of divalent cation "anhydrous" autunite phases [M~(2+)(UO_2)_2(PO_4)_2] and monovalent cation "anhydrous" autunite phases [M~+_2(UO_2)_2(PO_4)_2].Based on predicted Gibbs free energies of formation for autunite phases,an equilibrium calculation indicates that it is feasible to incorporate NpO_2~+ into the autunite phases.If concentration of Cs~+ is high,Cs~+ will be incorporated into the autunite phases.However,it is not feasible to incorporate low concentration Cs~+ released from spent fuel into theuranyl phosphate phases with layered structure and interlayer water (such as autunite phases,boltwoodite phases,and uranophane phases) as previously proposed.The predicted data can be considered as a first order approximation.