An X-ray absorption spectroscopy study of the coprecipitation of Tc and Re with mackinawite (FeS)

Abstract

X-ray absorption spectroscopy (XAS) has been used to define the local chemical environments of Tc and its proposed chemical analogue Re in precipitates of mackinawite (tetragonal FeS) and its oxidation products. Pertechnetate, [TcO 4] −, is reduced to Tc(IV) on coprecipitation with FeS, while Tc(IV) undergoes no redox change. A TcS 2-like phase is formed in both cases. On oxidation of the host lattice, Tc remains in oxidation state IV and forms a phase similar to TcO 2. Perrhenate, [ReO 4] −, behaves in a similar way to [TcO 4] − on coprecipitation although there is evidence that Re–S–Fe phases form. On reoxidation, the host FeS forms goethite and the Re may occupy channels in the goethite structure. Re(IV) is anomalous in that it forms a ReO 2−like phase on coprecipitation with FeS, perhaps due to hydrolysis before precipitation of the FeS. These results suggest that the Tc/Re analogy is only partially sound and the elements are not identical. The failure of Tc held in FeS to reform [TcO 4] − on reoxidation of the FeS suggests that its mobility may be substantially reduced by natural anoxic environments and coprecipitation might be considered as a method of reducing Tc waste discharges.