Thermochemical studies of X(NpO2)(PO4)(H2O)3 (X = K+, Rb+), neptunium analogs of the autunite/meta-autunite group

Actinide phosphate materials with autunite or meta-autunite type sheets consisting of vertex-sharing actinyl square bipyramids and phosphate tetrahedra have low aqueous solubility and potential applications for actinide immobilization in the environment. Neptunyl phosphates, X(NpO2)(PO4)(H2O)(3), X = K+ and Rb+, have been synthesized by hydrothermal reactions and characterized for their structures and purities. Direct thermochemical studies were performed using high temperature oxide melt drop solution calorimetry. After correcting for the presence of residual fluorapatite from the synthesis, the formation enthalpies of K(NpO2)(PO4)(H2O)(3) and Rb(NpO2)(PO4)(H2O)(3) are determined to be -3235 +/- 21 kJ/mol and -3270 +/- 11 kJ/mol, respectively. The thermodynamic data presented here may be important for understanding neptunium mobility after disposal of nuclear waste in a geological repository.