Defect-enhanced photocatalytic removal of dimethylarsinic acid over mixed-phase mesoporous TiO₂

Much attention has been paid to the pollutant dimethylarsenic acid (DMA), because of its high toxicity even at very low doses. Although TiO₂ photocatalytic oxidation (PCO) is one of the few effective methods for treating DMA-containing water, the efficient decomposition of DMA and simultaneous removal of toxic arsenic species remains a significant but challenging task. Here, defective mesoporous TiO₂ with mixed-phase structure was synthesized and used as both photocatalyst and adsorbent for DMA removal. Due to the reduced band-gap and enhanced separation of photogenerated charge carriers, the oxygen-deficient TiO₂ nanostructures exhibited 4.2 times higher PCO efficiency than commercial TiO₂ (P25). More importantly, the high surface area of the mesoporous TiO₂ provided sufficient active sites for in-situ adsorption and reaction, resulting in the efficient removal of as-formed As(V). Combining the experimental and characterization results, the different roles of reactive species during PCO reactions were clarified. In the presence of hole (h⁺) as the dominant oxidation species, DMA was demethylated and transformed into MMA. Thereafter, MMA was subsequently reduced to As(III) by photo-generated electrons. Superoxide radicals (O₂^•-) played a significant role in oxidizing As(III) into As(V), which was finally adsorptively removed by the mesoporous TiO₂.