Chemical speciation studies of radionuclides by XAFS

The local structures of U(VI) oxide precipitates produced as a function of LiOH concentration have been studied by XAFS. Curve-fitting analyses indicate that the solid formed in 0.1 M LiOH is similar to a schoepite-like phase where distinct linear uranyl ions (UO22+) are bridged along the equator by OH. ions. At higher LiOH concentrations, the uranyl groups become elongated and the precipitate structure resembles that of a uranate salt. We have studied the speciation of Tc in cement waste forms, where it is desirable to stabilize the highly soluble form of Tc(VII), TcO4-, by in situ reduction to a less mobile form, i.e. Tc(IV). Using the XANES to measure the oxidation state, our results show that significant reduction takes place in the presence of the cement additives blast furnace slag (BFS) and Na2S. In addition, EXAFS data show that in situ reduction leads to formation of Tc clusters possessing S and Tc coordination.