title: Reductive sequestration of pertechnetate (⁹⁹TcO₄⁻) by nano zerovalent iron (nZVI) transformed by abiotic sulfide.
abstract: 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·nH2₂ 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.
authors.affiliation: Division of Environmental and Biomolecular Systems, Oregon Health & Science University, 20000 NW Walker Road, Beaverton, Oregon 97006, United States.
authors.forename: Dimin
authors.initials: D
authors.lastname: Fan
authors.forename: Roberto P
authors.initials: RP
authors.lastname: Anitori
authors.forename: Bradley M
authors.initials: BM
authors.lastname: Tebo
authors.forename: Paul G
authors.initials: PG
authors.lastname: Tratnyek
authors.forename: Juan S
authors.initials: JS
authors.lastname: Lezama Pacheco
authors.forename: Ravi K
authors.initials: RK
authors.lastname: Kukkadapu
authors.forename: Mark H
authors.initials: MH
authors.lastname: Engelhard
authors.forename: Mark E
authors.initials: ME
authors.lastname: Bowden
authors.forename: Libor
authors.initials: L
authors.lastname: Kovarik
authors.forename: Bruce W
authors.initials: BW
authors.lastname: Arey
mesh_terms: D007501:Iron / Q000737:chemistry*; D053768:Metal Nanoparticles / Q000737:chemistry*; D046529:Microscopy, Electron, Transmission; D010084:Oxidation-Reduction; D013670:Sodium Pertechnetate Tc 99m / Q000737:chemistry*; D013440:Sulfides / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D013440:Sulfides; D013670:Sodium Pertechnetate Tc 99m; D007501:Iron