Np(V) solubility, speciation and solid phase formation in alkaline CaCl₂ solutions. Part II: Thermodynamics and implications for source term estimations of nuclear waste disposal

Abstract

The results of comprehensive solubility experiments with Np(V) in dilute to concentrated CaCl₂ solutions which included the spectroscopic investigation of the predominant aqueous Np(V) species and the thorough analysis of the solubility controlling Np(V) solid phases [1](Fellhauer, D., Rothe, J., Altmaier, M., Neck, V., Runke, J., Wiss, T., Fanghänel, Th., Np(V) solubility, speciation and solid phase formation in alkaline CaCl₂ solutions. Part I: Experimental solubility (Radiochim. Acta, DOI 10.1515/ract-2015-2489), in the following referred to as “Part I”), showed that the concentration of Np(V) in these systems is limited by equilibrium reactions between previously unknown (qua)ternary solid compounds, CaNpO₂(OH)_2.6Cl_0.4 · 2H₂O(s) (I), Ca_0.5NpO₂(OH)₂ · 1.3H₂O(s) (II) and Ca_0.5NpO₂(OH)₂(s) (III), and ternary aqueous complexes of Np(V) with the general formulae Ca_x[NpO₂(OH)₂]^2x–1, Ca_y[NpO₂(OH)₅]^2y–4 with y ≈ 2.4 ± 1.5 (EXAFS result) and innersphere Np(V) chloro complexes, Ca_z[NpO₂Cl]^2z+ with z = 0 and 1 (EXAFS result). A systematic thermodynamic evaluation of the experimental solubility data based on the specific ion interaction theory (SIT) and the Pitzer approach is performed. The stoichiometries of the ternary hydrolysis complexes are assessed as Ca[NpO₂(OH)₂]⁺ and Ca₃[NpO₂(OH)₅]²⁺. The corresponding thermodynamic solubility and complex formation constants at I = 0 as well as the ion interaction parameters (SIT and Pitzer) were derived yielding a comprehensive geochemical model for the Np(V) solubility behavior in pH neutral and alkaline CaCl₂ solutions over a large range of ionic strengths. This significantly improves source term estimations for scenarios with calcium dominated aquatic systems. The principally different behavior of Np(V) in NaCl and CaCl₂ solutions is highlighted, and the impact of the new findings on geochemical modelling of systems relevant for nuclear waste disposal is discussed.