Systematic XAS study on the reduction and uptake of Tc by magnetite and mackinawite

The mechanisms for the reduction and uptake of Tc by magnetite (Fe₃O₄) and mackinawite (FeS) are investigated using X-ray absorption spectroscopy (XANES and EXAFS), in combination with thermodynamic calculations of the Tc/Fe systems and accurate characterization of the solution properties (pH_m, pe, [Tc]). Batch sorption experiments were performed under strictly anoxic conditions using freshly prepared magnetite and mackinawite in 0.1 M NaCl solutions with varying initial Tc(vii) concentrations (2 × 10^-5 and 2 × 10^-4 M) and Tc loadings (400-900 ppm). XANES confirms the complete reduction of Tc(vii) to Tc(iv) in all investigated systems, as predicted from experimental (pH_m + pe) measurements and thermodynamic calculations. Two Tc endmember species are identified by EXAFS in the magnetite system, Tc substituting for Fe in the magnetite structure and Tc-Tc dimers sorbed to the magnetite {111} faces through a triple bond. The sorption endmember is favoured at higher [Tc], whereas incorporation prevails at low [Tc] and less alkaline pH conditions. The key role of pH in the uptake mechanism is interpreted in terms of magnetite solubility, with higher [Fe] and greater recrystallization rates occurring at lower pH values. A TcS_x-like phase is predominant in all investigated mackinawite systems, although the contribution of up to 20% of TcO₂·xH₂O(s) (likely as surface precipitate) is observed for the highest investigated loadings (900 ppm). These results provide key inputs for an accurate mechanistic interpretation of the Tc uptake by magnetite and mackinawite, so far controversially discussed in the literature, and represent a highly relevant contribution to the investigation of Tc retention processes in the context of nuclear waste disposal.