Charge transfer and synergistic mechanism of polydopamine-modified oxygen-vacancy-containing TiO2 in light-driven reduction of hexavalent chromium

Hexavalent chromium (Cr(VI)) is a widespread heavy metal pollutant in industrial wastewater that is highly carcinogenic and deleterious due to bioaccumulation. Detoxifying Cr(VI) to the less-toxic Cr(III) requires a synergistic adsorption–reduction mechanism. In this study, titanium dioxide (TiO 2 )-based composite photocatalysts (X-PNT) with polydopamine (PDA) and engineered oxygen vacancies (OVs) were developed to explore interfacial processes. Characterization results showed that PDA formed a bridge structure between TiO 2 particles, thereby creating charge transfer pathways. The band gap width of the photocatalyst after incorporating PDA narrowed by 0.16 eV compared to pure TiO 2 , and the photocurrent was enhanced by 2.43 times. Density functional theory calculations showed that PDA modification improved the electronic structure and adsorption capacity of TiO 2 . In photocatalytic experiments, the 0.1PNT composite achieved a Cr(VI) removal efficiency of 68.88 %, with light-driven reduction contributing 60.68 %. Quenching experiments confirmed direct electron transfer as the primary reduction pathway. Two-dimensional correlation spectroscopy (2DCOS) revealed a sequential reaction pathway, suggesting that Cr(VI) removal is a multistep synergistic process. This study provides valuable insights into PDA functionalization and OV engineering, facilitating the design and development of highly efficient polymer–metal oxide composites for Cr(VI) detoxification.