Photocatalytic nanoparticles in flow-through annular photoreactors for continuous selenate reduction in industrial wastewater

Abstract

The adverse effect of elevated selenium (Se) levels on aquatic ecosystems continues to drive the development of innovative technologies for treatment of Se-laden industrial effluents to sub-ppb levels. Photocatalysis is a promising water treatment technology that can selectively remove Se from industrial wastewater with minimal residuals. However, despite the reported advantages of photocatalysis across the many studied water treatment applications, the technology sees minimal industrial adoption due to a lack of work done on photoreactor engineering and process scale-up. The work presented herein demonstrates the heterogeneous photocatalytic reduction of selenate (SeO₄²⁻) from a synthetic mine-influenced brine (SMIB) using nanoscale TiO₂ (n-TiO₂) in a continuous annular photoreactor. A Box–Behnken design (BBD) is used to evaluate key parameters on SeO₄²⁻ removal efficiency and electrical energy per order (E_EO), including suspended n-TiO₂ loading, concentration of formic acid hole scavenger, reactor annulus gap, and axial flow velocity. The three-level BBD elucidated that the most critical factors influencing SeO₄²⁻ removal efficiency from SMIB were the TiO₂ photocatalyst concentration and axial velocity. On the other hand, the TiO₂ concentration and annulus gap were most impactful on the UV electrical energy per order (E_EO,UV) response. Se removal from SMIB down to 20 μg L⁻¹ was demonstrated using the continuous annular photoreactor, requiring 7.75 kWh m⁻³ of UV input. This work serves as the first proof of concept for continuous photocatalytic Se treatment and encourages further development of photocatalytic systems towards scale-up and real-world application.