Ambient solar-driven selenate reduction and removal from industrial brine using TiO₂-based buoyant photocatalysts

Selenium (Se), released from mining, power generation, and agriculture, is an environmentally and ecologically concerning contaminant due to its toxicity at elevated concentrations. Se oxyanions are highly soluble and mobile in aquatic ecosystems, and have a strong tendency to bioaccumulate and biomagnify, leading to acute and chronic toxicity in animals and humans. Photocatalysis presents a promising sustainable Se treatment solution and has successfully reduced and removed Se from mining-influenced matrices using UV-powered slurry photoreactor systems. Despite its potential, active photocatalytic water treatment faces significant challenges, particularly high operating costs and limited development of scalable deployment strategies, which hinder its real-world application. Herein, we adapt photocatalytic Se reduction and removal towards an easily deployable solar-driven semi-passive application using TiO₂-based buoyant photocatalysts (BPCs). The results demonstrate successful semi-passive photocatalytic selenium reduction and removal using BPCs under ambient solar conditions, achieving up to 99.6 % removal of Se from an industrial brine matrix containing ∼3.5 mg/L of Se, despite challenges from dissolved oxygen and reactive oxygen species (ROS). We advance the mechanistic understanding of the ambient Se reduction pathway, successfully identifying Se(IV) as a reduction intermediate, and uncovering the role formic acid plays in suppressing the oxidative effects of ROS, enabling the complete reduction of Se. We address the major challenges to both Se treatment and environmental photocatalysis and highlight the potential of sustainable solar-driven and semi-passive photocatalytic processes to address environmental challenges like selenium.