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

Reductive immobilization of ⁹⁹Tc by a synthetic FeS₂ 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 FeS₂ 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 S^2- to SO₄^2- was found responsible for Tc immobilization. This was explained by the high reactivity of marcasite that is easily oxidized to produce H₂SO₄. 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).