Kinetics of FeSe2 oxidation by ferric iron and its reactivity compared with FeS2

The mobility and bioavailability of selenium is a major health and environmental issue and a main concern for geological disposal of high-level radioactive waste. Chemically and/or microbially mediated oxidation of insoluble Se-bearing particulate, such as iron selenides, to dissolved and mobile phases controls the transport and distribution of Se in the environment. The oxidation of ferroselite (FeSe₂) by ferric iron was investigated in anoxic conditions. The redox reaction can be represented by: FeSe₂ + 2Fe³⁺ = 2Se⁰ + 3Fe²⁺. Kinetic studies indicated that the reaction can be described by second-order rate law, with rate constants of 0.49±0.01, 0.85±0.02, 1.84±0.04, and 3.29±0.13 L mol^?1 s^?1 at pH 1.62, 1.87, 2.23, and 2.49, respectively. The positive correlation between reaction rate and pH implies that diffusion of Fe³⁺ oxidant to the mineral surface is the rate-determining step. The strong reactivity of FeSe₂ towards Fe³⁺ suggests that ferric iron may play a significant role in FeSe₂ oxidation process (e.g., by Se⁴⁺, O₂, etc.) and Se⁰ should be the first reaction product. Also, it was shown that the reduction rate of Fe³⁺ or Se⁴⁺ by pyrite (FeS₂) can be significantly increased in the presence of FeSe₂, suggesting a stronger reactivity of FeSe₂ compared with pyrite. The results obtained extend our knowledge about the subtle interaction between Se, pyrite and iron selenides in the environment, and give insight into the transfer of selenium from iron selenides to bio-available selenium (i.e., selenite and selenate) in the Se-rich environment.