Simultaneous removal of colorless micropollutants and hexavalent chromium by pristine TiO₂ under visible light: An electron transfer mechanism

It is well known that pristine titanium dioxide (TiO₂) is unable to absorb and utilize visible light to degrade organics and reduce hexavalent chromium (Cr(VI)). However, for the first time, this research found that when some organic compounds were mixed with hexavalent chromium, titanium dioxide could not only degrade organic compounds but also reduce hexavalent chromium synchronously under the drive of visible light. The pristine TiO₂ exhibited excellent simultaneity removal of bisphenol A (BPA) and Cr(VI) under visible light irradiation, and almost 100% BPA (50 mg/L) and 100% Cr(VI) (10 mg/L) were removed just within 60 min. Photoluminescence (PL), electrochemical characterization and chemical quenching studies suggested that the reaction mechanism is similar to dye sensitization and electron transfer is responsible for the superior recation. Influences of different reaction conditions were systematically investigated, such as the initial concentration of BPA, the crystal structure of TiO₂, the catalyst dosage, the initial pH value, and the common inorganic anions. Compared to the rutile, anatase exhibited the most remarkable performance for BPA degradation and Cr(VI) reduction under visible light irradiation. In addition, many other typical refractory organics, such as acetaminophen and tetracycline, have been successfully applied in this photocatalytic system of TiO₂ under visible light irradiation (TiO₂/Vis system) to synergistically remove self-contamination and Cr (VI) pollution. The findings of the present study provide a valuable insight for sewage treatment, especially the composite wastewater containing organic pollutants and the hexavalent chromium.