Photocatalytic reduction of chlorite in water using bismuth vanadate (BiVO4): effect of irradiance conditions and presence of oxalate on the reactivity and by-product selectivity

Abstract

Chlorite (ClO₂⁻) is a disinfection byproduct formed during drinking water treatment when source waters with a high oxidant demand are disinfected with chlorine dioxide. This study investigated the reduction of ClO₂⁻ using a visible light (vis) active photocatalyst, bismuth vanadate (BiVO₄). The effect of direct photolysis, the presence of oxalate, light source, and fluence on the reaction rate and by-product selectivity was evaluated. A commercial titanium dioxide (TiO₂) was used as a UV-benchmark comparison. Photolysis was an effective reductant of ClO₂⁻, but it produced the undesirable byproduct chlorate (ClO₃⁻) and required ultraviolet light (UV) less than 360 nm. BiVO₄ and TiO₂ reduced ClO₂⁻ using irradiation greater than 360 nm, but ClO₃⁻ was also produced. For BiVO₄, the addition of oxalate as a hole scavenger and dissolved organic carbon (DOC) analog increased the reaction rate and eliminated ClO₃⁻ formation, but it decreased the reaction rate for TiO₂ and ClO₃⁻ was still detected. In situ vibrational spectroscopy was used to link the reduced rate of TiO₂ to the chemisorption behavior between oxalate and TiO₂. Analyzing the reaction rates as a function of photon fluence instead of time showed BiVO₄ was more efficient at using vis than UV. A higher required energy dose for the 2-log removal of ClO₂⁻ was observed for BiVO₄ (185 J cm⁻²) compared to TiO₂ (5.11 J cm⁻²), indicating the poor ability of BiVO₄ to separate and use photogenerated charge carriers. The addition of silver to enhance charge separation improved the efficiency and reduced the required dose (45.5 J cm⁻²). From an operational standpoint, this study strengthens the argument that all photocatalysis treatment studies should report results as a function of fluence as well as time. Though BiVO₄ used more energy than TiO₂ to achieve the same removal, its ability to avoid ClO₃⁻ is attractive. Further, vis sources are more cost and energy efficient with longer lifetimes compared to UV sources, and advanced materials engineering can improve reaction efficiencies, allowing vis photocatalysts like BiVO₄ to be an attractive option for treatment of ClO₂⁻ and other critical oxidized contaminants.