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

X-ray absorption spectroscopy (XAS) has been used to define the local chemi cal environments of Tc and its proposed chemical analogue Re in precipitate s of mackinawite (tetragonal FeS) and its oxidation products. Pertechnetate , [TcO4](-), is reduced to Tc(IV) on coprecipitation with FeS, while Tc(IV) undergoes no redox change. A TcS2-like phase is formed in both cases. On o xidation of the host lattice, Tc remains in oxidation state IV and forms a phase similar to TcO2. Perrhenate, [ReO4](-), behaves in a similar way to [ TcO4](-) on coprecipitation although there is evidence that Re-S-Fe phases form. On reoxidation, the host FeS forms goethite and the Re may occupy cha nnels 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 o nly partially sound and the elements are not identical. The failure of Tc h eld in FeS to reform [TcO4](-) on reoxidation of the FeS suggests that its mobility may be substantially reduced by natural anoxic environments and co precipitation might be considered as a method of reducing Tc waste discharg es. (C) 1999 Elsevier Science Ltd. All rights reserved.