Role of the Sn-TiO₂/Ti-SnO₂ Heterojunction in Enhancing the Photocatalytic Oxidation of Arsenite (As^III) through the Promotion of Charge Carrier Lifetime

High Resolution Image Download MS PowerPoint Slide This study proposes the heterojunction photocatalyst, Sn-doped TiO 2 /Ti-doped SnO 2 (herein named Sn 1 Ti 1 O 2 ), as a promising alternative to pure TiO 2 . Sn 1 Ti 1 O 2 demonstrates improved light harvesting efficiency over TiO 2 by generating longer-lived electron–hole (e CB – ―h VB + ) pairs, while also displaying a smaller band gap compared to pure TiO 2 . Consequently, we show that it is a promising candidate for the photocatalytic oxidation (PCO) of As III to the less toxic and more readily removable form As V . Transient absorption spectroscopy (TAS) shows increased e CB – ―h VB + recombination half-lives from ∼0.5 ms in TiO 2 to ∼1 ms in Sn 1 Ti 1 O 2 . The initial transient absorption signal for Sn 1 Ti 1 O 2 is twice that of pure TiO 2, suggesting early time scale (pre-μs) suppression of (e CB – ―h VB + ) recombination. Moreover, TAS showed that Sn 1 Ti 1 O 2 possesses more reactive charge carriers than TiO 2 under reactions with chemical scavengers. For the first time, TAS experiments were conducted using both a colorimetric indicator (molybdate) and As III to determine the PCO kinetics from As III to As V . The TAS molybdate─As III experiment results indicate that the oxidation process occurs on the sub-microsecond time scale, with a notable increase in absorption at ∼700 nm, providing evidence of the formation of the As V ─molybdate blue complex. PCO experiments showed that • OH radicals played the predominant role during PCO, followed by superoxide radicals (O 2 •─ ). • OH scavengers including isopropanol, rebamipide anhydrous, and dimethyl sulfoxide (DMSO) reduce the PCO yield of As III to 21, 30, and 23%, respectively. While O 2 •─ scavengers including superoxide dismutase (SOD) and p -benzoquinone suppressed the PCO yield of As III to a lesser degree, with yields of 35 and 49% seen, respectively. The effects of irradiance intensity, salinity, pH, As III concentration, and photocatalyst mass on both the quantum efficiency (QE) and PCO kinetics were investigated. The Sn 1 Ti 1 O 2 catalyst exhibited effective recyclability, validating its economical reusability. Overall, the study demonstrates the potential of the Sn 1 Ti 1 O 2 heterojunction photocatalyst for the PCO of As III to the less toxic As V in water treatment, showing faster oxidation kinetics and improved charge separation compared to pure TiO 2 as proven by TAS.