Impact of sulfate on the solubility of Tc(IV) in acidic to hyperalkaline aqueous reducing systems

Abstract

Abstract The solubility of 99Tc(IV) was investigated from undersaturation conditions in NaCl–Na2SO4 (0.3 M ≤ I ≤ 5.0 M), MgCl2–MgSO4 (I = 13.5 M) and CaCl2–CaSO4 (I = 13.5 M) systems with 0.001 M ≤ [SO4 2−]tot ≤ 1.0 M and 1 ≤ pH m  ≤ 12 (with pH m  = −log[H+], in molal units). Reducing conditions were set by either Sn(II) or Fe(0). Special efforts were dedicated to accurately characterize the correction factors A m required for the determination of pH m from the experimentally measured pH values in the mixed salt systems investigated, with pH m  = pHexp + A m . The combination of (pe + pH m ) measurements with Pourbaix diagrams of Tc suggests that technetium is present in its +IV redox state. This hypothesis is confirmed by XANES, which unambiguously shows the predominance of Tc(IV) both in the aqueous and solid phases of selected solubility samples. XRD and SEM–EDS support the amorphous character of the solid phase controlling the solubility of Tc(IV). EXAFS data confirm the predominance of TcO2(am, hyd) at pH m  > 1.5, whereas the formation of a Tc(IV)–O–Cl solid phase is hinted at lower pHm values in concentrated NaCl–Na2SO4 systems with ≈5 M NaCl. Solubility data collected in sulfate-containing systems are generally in good agreement with previous solubility studies conducted in sulfate-free NaCl, MgCl2 and CaCl2 solutions of analogous ionic strength. Although the complexation of Tc(IV) with sulfate cannot be completely ruled out, these results strongly support that, if occurring, complexation must be weak and has no significant impact on the solubility of Tc(IV) in dilute up to highly saline media. Solubility upper-limits determined in this work can be used for source term estimations including the effect of sulfate in a variety of geochemical conditions relevant in the context of nuclear waste disposal.