Temperature effects on the reduction of Se(VI) by natural pyrite

Abstract

⁷⁹Se is a key radionuclide concerned in the geological disposal of high-level radioactive waste. Unlike lower valent Se(IV), previous studies have reported that the reduction of aqueous Se(VI) into insoluble Se⁰ or FeSe₂ by pyrite is challenging under ambient temperatures. Considering the thermal environment of radioactive waste repositories and the widespread presence of pyrite in the host rocks, this study investigated Se(VI) reduction by natural arsenic-scarce and arsenic-rich pyrites, namely Py@TL and Py@LY, respectively, under conditions of pH ∼2.0 to ∼9.0 and temperatures ranging from 25 °C to 85 °C. The results reveal that elevated temperatures and acidic conditions significantly enhance Se(VI) reduction via a Se(IV) intermediate to form insoluble nanoscale Se⁰ clusters along with Se-S compounds, following a pseudo-zero-order kinetic model. The apparent activation energy (E_a) values were determined to be 40.5 ± 2.4 and 59.1 ± 3.0 kJ⋅mol⁻¹ for Se(VI) reduction by the arsenic-scarce Py@TL, and 51.0 ± 1.8 and 63.2 ± 12.6 kJ⋅mol⁻¹ for the arsenic-rich Py@LY, at pH ∼2.0 and ∼2.5, respectively. These E_a values highlight surface chemical reactions as the rate-determining step, elucidating the inertness of Se(VI) on pyrite surfaces at ambient temperatures. Notably, arsenic impurities in Py@LY appear to enhance the reactivity by modifying the local structure of pyrite, but excessive Fe²⁺, As³⁺ competition, and solid layers of As⁰ and S⁰ impede the reduction process and elevate the apparent E_a. These findings provide novel insights into Se(VI) reduction mechanisms under complex geochemical conditions.