New insights into 99Tc(VII) removal by pyrite: a spectroscopic approach

99Tc(VII) uptake by synthetic pure pyrite was studied in a wide pH range from 3.5 to 10.5 using batch experiments at 21°C combined with scanning electron microscopy (SEM), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) and Raman microscopy. We found that pyrite removes Tc quantitatively from solution (log Kd = 5.0 ± 0.1) within one day at pH ≥ 5.5. At pH < 5.5 the uptake process is slower, leading to 98% Tc removal (log Kd = 4.5 ± 0.1) after 35 days. The slower Tc uptake was explained by higher pyrite solubility under acidic conditions. After two months in contact with oxygen at pH 6 and 10, Tc was neither re-oxidized nor re-dissolved. XAS showed that the uptake mechanism involves the reduction from Tc(VII) to Tc(IV) and subsequent inner-sphere complexation of Tc(IV)-Tc(IV) dimers onto a Fe oxide like hematite at pH 6, and Tc(IV) incorporation into magnetite via Fe(III) substitution at pH 10. Calculations of Fe speciation under the experimental conditions predict the formation...