Oxidative remobilization of technetium sequestered by sulfide-transformed nano zerovalent iron

Our previous study showed that formation of TcS₂-like phases is favored over TcO₂ under sulfidic conditions stimulated by nano zerovalent iron. This study further investigates the stability of Tc(IV) sulfide upon reoxidation by solution chemistry, solid phase characterization, and X-ray absorption spectroscopy. Tc dissolution data showed that Tc(VII) reduced by sulfide-transformed nZVI has substantially slower reoxidation kinetics than Tc(VII) reduced by nZVI only. The initial inhibition of Tc(IV) dissolution at S/Fe = 0.112 is due to the redox buffer capacity of FeS, which is evidenced by the parallel trends in oxidation-reduction potentials (ORP) and Tc dissolution kinetics. The role of FeS in inhibiting Tc oxidation is further supported by the Mössbauer spectroscopy and micro X-ray diffraction data at S/Fe = 0.112, showing persistence of FeS after 24-h oxidation but complete oxidation after 120-h oxidation. X-ray absorption spectroscopy data for S/Fe = 0.011 showed significantly increasing percentages of TcS₂ in the solid phase after 24-h oxidation, indicating stronger resistance of TcS₂ to oxidation. At S/Fe = 0.112, the XAS results revealed significant transformation of Tc speciation from TcS₂ to TcO₂ after 120-h oxidation. Given that no apparent Tc dissolution occurred during this period, the speciation transformation might play a secondary role in hindering Tc oxidation. Collectively, the results indicate that sequestrating Tc as TcS₂ under stimulated sulfate reduction is a promising strategy to improve the long-term stability of reduced Tc in subsurface remediation.