Enhanced photocatalytic reduction of aqueous Re(VII) in ambient air by amorphous TiO2/g-C3N4 photocatalysts: Implications for Tc(VII) elimination

[Display omitted] •A simple amorphous TiO2/g-C3N4 composite photocatalyst was fabricated.•This photocatalyst has the excellent specific surface area and narrow band gap.•XPS and XAFS results suggest that Re(VII) should be mainly reduced to Re(IV).•It exhibits the extremely slow re-dissolution of Re(IV) when compared with P25.•This photocatalyst containing amorphous TiO2 can be used for Tc elimination. Although effective photocatalytic removal for aqueous ReO4− (as a chemical analogue of TcO4−) have been established, photocatalysts are not satisfactory for further applications in term of low light harvesting efficiency and fast re-dissolution of ReO2·nH2O. Herein, the amorphous TiO2/g-C3N4 composites are fabricated by a simple hydrolysis approach, producing the amorphous TiO2/30 wt% g-C3N4 (TCN-3) composite with larger specific surface area (~456 m2 g−1), narrow band gap (~2.67 eV) and excellent light-capturing ability when compared with commercial P25. Lower recombination efficiency of photo-generated electron-hole based on the Z-scheme mechanism between amorphous TiO2 and g-C3N4 was demonstrated by electron paramagnetic resonance and photoelectrochemical analysis. In addition, the as-synthesized TCN-3 exhibits an excellent photocatalytic reduction of Re(VII) in ambient air with the highest removal percentage of ~90% as compared with crystalline TiO2/g-C3N4, and can be regenerated even after eight recycles. X-ray absorption spectroscopy confirmed that the extremely slow re-dissolution of Re(IV) resulted from both shortening of ReO2·nH2O chains (decrease in coordination number of Re-Re) and association of Re(IV) with amorphous TiO2 in TCN-3 (formation of Re-Ti bond). The overall results further facilitate the photocatalytic reduction and elimination for rhenium/technetium, and open the wide-range applications for photocatalysis in the disposal of some radioactive elements.