Photocatalytic reduction of chlorite in water using bismuth vanadate (BiVO 4 ): Effect of irradiance conditions and presence of oxalate on the reactivity and by-product selectivity

Chlorite (ClO 2 - ) 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... Chlorite (ClO 2 - ) 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 ClO 2 - using a visible light (Vis) active photocatalyst, bismuth vanadate (BiVO 4 ). 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 (TiO 2 ) was used as a UV-benchmark comparison. Photolysis was an effective reductant of ClO 2 - , but it produced the undesirable byproduct chlorate (ClO 3 - ) and required ultraviolet light (UV) less than 360 nm. BiVO 4 and TiO 2 reduced ClO 2 - using irradiation greater than 360 nm, but ClO 3 - was also produced. For BiVO 4 , the addition of oxalate as a hole scavenger and dissolved organic carbon (DOC) analog increased the reaction rate and eliminated ClO 3 - formation, but it decreased the reaction rate for TiO 2 and ClO 3 - was still detected. In-situ vibrational spectroscopy was used to link the reduced rate of TiO 2 to the chemisorption behavior between oxalate and TiO 2 . Analyzing the reaction rates as a function of photon fluence instead of time showed BiVO 4 was more efficient at using Vis than UV. A higher required energy dose for the 2-log removal of ClO 2 - was observed for BiVO 4 (185 J/cm 2 ) compared to TiO 2 (5.11 J/cm 2 ), indicating the poor ability of BiVO 4 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 BiVO 4 used more energy than TiO 2 to achieve the same removal, its ability to avoid ClO 3 - 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 BiVO 4 to be an attractive option for treatment of ClO 2 - and other critical oxidized contaminants.