PNNL

Abstract

Technetium speciation, solubility and sorption behavior is strongly dependent on its valence state. Under oxic conditions, Tc exists as the soluble, weakly-sorbing pertechnetate [TcO4-] anion. The reduced form of technetium, Tc(IV), is stable in anoxic environments and is sparingly soluble as TcO2·xH2O(s). Here we investigate the heterogeneous reduction of Tc(VII) by Fe(II) sorbed on Al (hydr)oxides [diaspore (a-AlOOH) and corundum (a-Al2O3)]. Experiments were performed to study the kinetics of Tc(VII) reduction, examine changes in Fe surface speciation during Tc(VII) reduction (Mössbauer spectroscopy), and identify the nature of Tc(IV)-containing reaction products (X-ray absorption spectroscopy). We found that Tc(VII) was completely reduced by adsorbed Fe(II) within 11d (diaspore suspension) and 4d (corundum suspension). Mössbauer measurements revealed that the Fe(II) signal became less intense with Tc(VII) reduction, and was accompanied by increase in Fe(III) doublet and magnetically-ordered Fe(III) sextet signals, with latter parameters close to those for hematite. Formation of magnetically ordered Fe(III) did not depend on the oxidant nature, as both Tc(VII) or O2 lead to the formation of a virtually identical hematite-like phase. The Fe(II) doublet displayed no differences in Mössbauer parameters before and after Tc(VII) reduction, likely due to Fe(II) adsorption to similar sites and no Fe(II) sorption to or precipitation within solid phases formed. Tc-EXAFS spectroscopy revealed that the final heterogeneous redox product on corundum was similar to Tc(IV) oxyhydroxide, TcO2?xH2O. The formation of precursor polymeric TcnOy (4n-2y)+ chains prior to TcO2·xH2O precipitation might explain the formation of the separate TcO2-like phase on corundum without coprecipitated Fe.