Reductive immobilization of 99Tc(VII) by FeS2: The effect of marcasite.

Reductive immobilization of 99Tc by a synthetic FeS 2 mixture, i.e. marcasite-pyrite 60:40, was studied by a combined approach of batch experiments and powder X-ray diffraction, X-ray photoelectron spectroscopy as well as Raman microscopy. It was found that the FeS 2 mixture removes 100% of Tc from the suspension after 7 days in contact at 6.0 < pH ≤ 9.0. The retention outside that pH range was slower and incomplete. Spectroscopic analysis showed that the redox active species at pH 6.0 is Fe2+ as expected from previous works with pyrite. However, at pH 10.0 the surprising oxidation of S2− to SO 4 2− was found responsible for Tc immobilization. This was explained by the high reactivity of marcasite that is easily oxidized to produce H 2 SO 4. Our work provides new molecular insights into the reductive mobilization of Tc(VII) by oxidative formation of sulfate. The assigned molecular reactions may also be relevant for the assessment of other redox reactive contaminants. Technetium re-oxidation experiments showed that the fast oxidation of marcasite is associated to the reduction of the remaining Tc(VII) in solution, which gives marcasite the potential of Tc natural remediation since it delays the re-oxidation of Tc(IV). [Display omitted] • Marcasite-pyrite mixture removes 100% of Tc(VII) at 6 < pH ≤ 9 after 7 days. • Retention is due to the reduction from Tc(VII) to Tc(IV). • Main redox sensitive species of the FeS 2 mixture are Fe2+ at pH 6 and S2− at pH 10. • Tc(IV) started to re-oxidize only after two months under ambient atmosphere. •Tc(VII) and solid elemental sulfur are final products of the re-oxidation. [ABSTRACT FROM AUTHOR]