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Reductive Sequestration of Pertechnetate (99TcO4) by Nano Zerovalent Iron (nZVI) Transformed by Abiotic Sulfide

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journal contribution
posted on 16.12.2015, 22:52 by Dimin Fan, Roberto P. Anitori, Bradley M. Tebo, Paul G. Tratnyek, Juan S. Lezama Pacheco, Ravi K. Kukkadapu, Mark H. Engelhard, Mark E. Bowden, Libor Kovarik, Bruce W. Arey
Under anoxic conditions, soluble pertechnetate (99TcO4) can be reduced to less soluble TcO2·nH2O, but the oxide is highly susceptible to reoxidation. Here we investigate an alternative strategy for remediation of Tc-contaminated groundwater whereby sequestration as Tc sulfide is favored by sulfidic conditions stimulated by nano zerovalent iron (nZVI). nZVI was pre-exposed to increasing concentrations of sulfide in simulated Hanford groundwater for 24 h to mimic the onset of aquifer biotic sulfate reduction. Solid-phase characterizations of the sulfidated nZVI confirmed the formation of nanocrystalline FeS phases, but higher S/Fe ratios (>0.112) did not result in the formation of significantly more FeS. The kinetics of Tc sequestration by these materials showed faster Tc removal rates with increasing S/Fe between 0 and 0.056, but decreasing Tc removal rates with S/Fe > 0.224. The more favorable Tc removal kinetics at low S/Fe could be due to a higher affinity of TcO4 for FeS than iron oxides, and electron microscopy confirmed that the majority of the Tc was associated with FeS phases. The inhibition of Tc removal at high S/Fe appears to have been caused by excess HS. X-ray absorption spectroscopy revealed that as S/Fe increased, the pathway for Tc­(IV) formation shifted from TcO2·nH2O to Tc sulfide phases. The most substantial change of Tc speciation occurred at low S/Fe, coinciding with the rapid increase in Tc removal rate. This agreement further confirms the importance of FeS in Tc sequestration.