Formation of hydroperoxo (-OOH) species on the surface of self-doped Bi2.15WO6: reactivity towards As(iii) oxidation

Arsenite oxidation and adsorptive arsenic removal from contaminated water: a review

Arsenic poisoning of groundwater is one of the most critical environmental hazards on Earth. Therefore, the practical and proper treatment of arsenic in water requires more attention to ensure safe drinking water. The World Health Organization (WHO) sets guidelines for 10 μg/L of arsenic in drinking water, and direct long-term exposure to arsenic in drinking water beyond this value causes severe health hazards to individuals. Numerous studies have confirmed the adverse effects of arsenic after long-term consumption of arsenic-contaminated water. Here, technologies for the remediation of arsenic from water are highlighted for the purpose of understanding the need for a single-point solution for the treatment of As(III)-contaminated water. As(III) species are neutral at neutral pH; the solution requires transformation technology for its complete removal. In this critical review, emphasis was placed on single-step technologies with multiple functions to remediate arsenic from water.

Photocatalytic Generation of H2O2 Via a Hydrogen-Abstraction Pathway by Bi2.15WO6 under Visible Light

Photocatalytic technology is a popular research area for converting solar energy into environmentally friendly chemicals and is considered the greenest approach for producing H2O2. However, the corresponding reactive oxygen species (ROS) and pathway involved in the photocatalytic generation of H2O2 by the Bi2.15WO6-glucose system are still not clear. Quenching experiments have established that neither •OH nor h+ contribute to the formation of H2O2, and show that the formed surface superoxo (≡Bi-OO•) and peroxo (≡Bi-OOH) species are the predominant ROS in H2O2 generation. In addition, various characterizations indicate the enhanced electron-transfer on the surface of Bi2.15WO6 with increasing contents of glucose via the ligand-to-metal charge transfer pathway, confirming H-transfer from glucose to ≡Bi-OO• or ≡Bi-OOH. The increased production of H2O2 with decreasing bond dissociation energy (BDEO-H) values of various phenolic compounds again supports the H-transfer mechanism from phenolic compounds to ≡Bi-OO• and then to ≡Bi-OOH. DFT calculations further reveal that on the Bi2.15WO6 surface, oxygen is sequentially reduced to ≡Bi-OO• and ≡Bi-OOH, while H-transfer from H2O or glucose to ≡Bi-OO• and ≡Bi-OOH, resulting in the production of H2O2. The lower energy barrier of H-transfer from adsorbed glucose (0.636 eV) than that from H2O (1.157 eV) indicates that H-transfer is more favorable from adsorbed glucose. This work gives new insight into the photocatalytic generation of H2O2 by Bi2.15WO6 in the presence of glucose/phenolic compounds via the H-abstraction pathway.

Importance of the ligand-to-metal charge transfer (LMCT) pathway in the photocatalytic oxidation of arsenite by TiO2

Photooxidation of As(III) by TiO₂ is a complicated process in which the oxidation mechanisms are always controversial. In this study, the enhanced photooxidation rates of As(III) with increasing pH values from 8.0 to 11.0 indicate the high photocatalytic reactivity of TiO₂ under alkaline conditions. Moreover, As(III) improves the production of H₂O₂, indicating H-abstraction from As(III) (soluble or adsorbed) for H₂O₂ production. Although O₂˙^-, h⁺, ˙OH and -OOH are always regarded as the reactive oxygen species in the UV-TiO₂ system, the superoxo and peroxo species formed on the surface of TiO₂ also contribute to As(III) oxidation. The As(III)-O-Ti(IV) surface complexes formed on TiO₂, as well as the decreased bandgaps of TiO₂ with increasing concentrations of As(III) indicate that the ligand-to-metal charge transfer (LMCT) pathway also contributes to the oxidation of As(III) under alkaline conditions. Electrochemical analyses further reveal that As(III) enhances the electron density on the surface of TiO₂, thereby improving the catalytic reactivity of TiO₂. We therefore suggest that H-abstraction from As(III) or H₂O to the formed superoxo and peroxo species results in the formation of H₂O₂, accompanied by the oxidation of As(III). This enriches our knowledge on the oxidation of As(III), as well as other contaminants rich in -OH groups during the photocatalytic oxidation processes.

Photocatalytic reduction of Cr(VI) by Bi2.15WO6 complexed with polydopamine: Contribution of the ligand-to-metal charge transfer path

Photocatalytic reduction of Cr(VI) in water environments attracts more attention; however, the mechanisms involved in this process have not been clearly elucidated yet. In this study, the photocatalytic reduction of Cr(VI) by polydopamine modified Bi_2.15WO₆ (PDA/BWO) under visible light was conducted. Kinetics results show that PDA apparently accelerates the reduction of Cr(VI). The quasi-first-order kinetic constant of Cr(VI) reduction by 5PDA/BWO is 70.0 times that of the original BWO, reaching 0.070 min^-1. X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and Raman analyses confirm the formation of ligand-to-metal charge transfer (LMCT) complex [Bi(III)OC] between PDA and BWO. The formed Bi(III)OC complex enhances visible light response and narrows the bandgap of PDA/BWO. The photoelectrochemical and photoluminescent characterization further reveals that the formed Bi(III)OC complex inhibits the recombination of carriers, thus enhancing the photocatalytic reactivity of PDA/BWO. Electrons, are derived from three paths, including dye sensitization, LMCT and bandgap excitation, contribute to Cr(VI) reduction by PDA/BWO. This study provides new insights on the paths of Cr(VI) reduction by PDA/BWO under visible light.