Tunable production of elemental Se vs. H2Se through photocatalytic reduction of selenate in synthetic mine impacted brine: engineering a recoverable Se product

Abstract

In this paper, we investigate the tunability of Se reduction products (Se⁰_(s)vs. H₂Se_(g)) during the photocatalytic reduction of selenate over TiO₂, using formic acid as an electron hole scavenger, in synthetic mine-impacted brines (SMIB). Photocatalytic reduction can effectively remove Se from SMIB to <2 μg L⁻¹ from an initial Se concentration of >3300 μg L⁻¹ in under 10 × 10¹⁹ photons cm⁻². An increase in solution temperature led to a marked increase in selenate removal kinetics and an increase in selectivity towards H₂Se_(g), while increasing the concentration of formic acid led to an increase in selenate removal kinetics and a decrease in the selectivity towards H₂Se_(g). A bivariate response surface analysis was used to elucidate the mechanism behind the production of >99% gaseous H₂Se or >85% solid Se⁰, under varying reaction conditions. Finally, a two-pronged electron transfer model is proposed to explain the selectivity towards Se⁰_(s)vs. H₂Se_(g) under varying conditions: (i) Se⁰_(s) is produced through direct reduction of selenate by TiO₂ conduction band electrons and (ii) H₂Se gas is produced through electrons transferred into Se⁰, followed by an autocatalytic reduction of Se⁰ to H₂Se or through a direct reduction by CO₂˙⁻.