Chemistry of niobium(V) in alkaline aquatic carbonated systems containing Ca: Solubility phenomena and formation of ternary Ca-Nb(V)-O/OH solid phases

Comprehensive solubility experiments with niobium(V) were conducted from oversaturation conditions inweakly alk aline, carbonated solutions equilibrated with calcite and carbonated cement. These conditions are expected in advanced degradation stages of cement, and are representative of cementitious environments as those foreseen in underground and near-surface repositories for short-lived low- and intermediate-level radioactive wastes (L/ILW-SL). Moderate niobium concentrations ([Nb]aq ≈ 10 6 M) were quantified at short contact times (t < 3 days), matching thermodynamic calculations with Nb2O5(am) as solubility-controlling solid phase. At longer equilibration times (t > 10 days), a further decrease in solubility ([Nb]aq ≤ 10 8 M) is observed, which evidences the formation of a sparingly soluble secondary phase transformation. Comparison of solubility data determined in this work with previous studies in carbonate-free systems support that Nb(V)-carbonate complexes can be neglected in the investigated range of carbonate concentrations (≈10 3–10 4 M). Systematic characterization of the resulting solid phases by means of Raman spectroscopy and powder X-ray diffraction unequivocally point towards the predominance of hydrated Ca-Nb(V) solid phase/s of rather amorphous character and small (sub-nm) particle size. ... mehrA conclusive assignment of the stoichiometry of this/these solid phase/s is not feasible at this stage, although available evidence points towards a Ca: Nb ratio clearly below 1.