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

• A simple amorphous TiO 2 /g-C 3 N 4 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 TiO 2 can be used for Tc elimination. Although effective photocatalytic removal for aqueous ReO 4 − (as a chemical analogue of TcO 4 −) have been established, photocatalysts are not satisfactory for further applications in term of low light harvesting efficiency and fast re-dissolution of ReO 2 · n H 2 O. Herein, the amorphous TiO 2 /g-C 3 N 4 composites are fabricated by a simple hydrolysis approach, producing the amorphous TiO 2 /30 wt% g-C 3 N 4 (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 TiO 2 and g-C 3 N 4 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 TiO 2 /g-C 3 N 4 , 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 ReO 2 · n H 2 O chains (decrease in coordination number of Re-Re) and association of Re(IV) with amorphous TiO 2 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. [ABSTRACT FROM AUTHOR]