Electronic structure-regulated graphitic carbon nitride/titanium dioxide Z-scheme heterojunction for boosting photocatalytic reduction of rhenium(VII)/technetium(VII) from acidic nuclear wastewater

The disposal of pertechnetate anion ( 99 TcO 4 − ) in wastewater has been an important issue due to the high mobility, radioactive and chemical toxicity of 99 TcO 4 − . In this study, a facile and feasible hydrothermal approach was proposed to prepare graphitic carbon nitride/titanium dioxide (g-C 3 N 4 /TiO 2 ) heterojunctions, aiming at the reduction of rhenium (VII) (Re(VII), a surrogate for technetium(VII)) by photocatalysis. Compared to the individual components of g-C 3 N 4 or TiO 2 , the photocatalytic reduction performance for perrhenate anion (ReO 4 − , 10 mg/L) over CNT-20 (the mass ratio of g-C 3 N 4 to TiO 2 is 20 wt%, 40 mg) is significantly improved, achieving ∼93 % within 10 min under an air atmosphere and UV–vis light irradiation. Furthermore, the obtained g-C 3 N 4 /TiO 2 demonstrates excellent stability and maintains a high activity of ∼82 % after five cycles. Importantly, in the presence of various coexisting ions, the reduction efficiency remained at about 85–90 %. However, the reduction efficiency for Re(VII) declined to approximately 40–50 % when organic pollutants (10 ppm MO or HA) were introduced. The effective combination between g-C 3 N 4 and TiO 2 brings out a Z-scheme type g-C 3 N 4 /TiO 2 heterojunction, which promotes spatial separation and synergistic utilization of photogenerated carriers. Besides, electron paramagnetic resonance (EPR) characterization indicates that the carbon dioxide radical anion (•CO 2 − ) generated from the oxidation of formic acid/formate anion contribute to the fast reduction removal of ReO 4 − . This work provides a practical and user-friendly hydrothermal technique for rapidly and efficiently reducing nonradioactive Re(VII), which may offer a potential way to dispose of the radionuclide technetium(VII) ( 99 Tc(VII)).