Xuefeng Hua,∗, Huanhuan Ji a, Fei Chang b, Yongming Luoa,∗

Abstract

In this study, a rapid reduction of Cr(VI) and degradation of 2,4,6-trichlorophenol (2,4,6-TCP) in a simultaneous manner was reported through the catalysis of g-C₃N₄ under visible light (λ > 420 nm) irradiation. The effects of initial concentration of reactants, dissolved O2 and pH value were investigated systematically. It indicated that, under the optimized concentration, the Cr(VI) reduction and 2,4,6-TCP oxidation could be accomplished in couple of hours in the presence of g-C₃N₄. And also, the O₂ involvement and low pH value were able to significantly improve the removal rate of Cr(VI) and 2,4,6-TCP. In addition, the reaction mechanism was investigated through monitoring the reduced states of Cr(VI) and active oxygen intermediates formed during photoreaction by ESR and XPS, as well as determining the degradation products of 2,4,6-TCP by HPLC–MS. The results supported that the redox reactions of Cr(VI) and 2,4,6-TCP can be performed simultaneously via a synergistic oxidation–reduction mechanism in the presence of g-C₃N₄ under visible light irradiation.

Experimental

Photocatalytic experiments

Cr(VI)/2,4,6-TCP reaction solution was prepared by diluting certain volume of 2,4,6-TCP and Cr(VI) stock solution to 30 mL with deionized water in a 50 mL serum bottle. The finally concentration of Cr(VI) and 2,4,6-TCP were respectively 2 × 10−4 M and 1 × 10−4 M unless otherwise noted. Before irradiation, 30 mg gC₃N₄ was added to the solution and magnetically stirred for 0.5 h in dark to obtain adsorption/desorption equilibrium. The light irradiation system was equipped with a 300W Xe lamp (CEL-HXF300, Beijing Aulight Co., Ltd.) and a cut-off filter to ensure the wavelength of irradiation light is above 420 nm. The light intensity impinging on the suspension was 150 mW/cm2 as measured with a radiometer (CEL-NP2000, Beijing Aulight Co., Ltd.). During photoreaction, 3 mL aliquots were sampled at every 0.5 h interval for subsequent analysis after centrifugation and filtration.

The synergistic effect of Cr(VI) reduction and 2,4,6-TCP degradation over g-C3N4 under visible light irradiation: (A) Cr(VI) reduction and (B) 2,4,6-TCP degradation.

The effects of (A) 2,4,6-TCP initial concentration on Cr(VI) (2 × 10−4 M) reduction and (B) Cr(VI) initial concentration on 2,4,6-TCP (1 × 10−4 M) degradation.

Conclusion

The synergistic effect of Cr(VI) reduction and 2,4,6-TCP degradation was observed over g-C₃N₄ under visible light irradiation. Various conditions were performed to optimize the synergistic effect and investigate the reaction mechanism. The results indicated that acidic environment was beneficial for the reduction of Cr(VI) and oxidation of 2,4,6-TCP. The presence of oxygen could improve the reaction rate for both Cr(VI) reduction and 2,4,6- TCP oxidation. The initial concentration of substrates will affect the reaction rate in a certain concentration range. Mechanistically understanding, both CB electrons and O₂•− responsible for the reduction of Cr(VI), and most the reduced Cr(III) was adsorbed on the g-C₃N₄ surface. Coupling products was generated in the presence of Cr(VI) during 2,4,6-TCP photodegradation. There are still some Cl-containing intermediates exist in the solution after 2,4,6- TCP degrade completely. We should take care the potential risk of the Cl-containing intermediates. Hole of VB, O₂•− and ·OH derived from the reaction between reduced states of chromium and H₂O₂enhanced the degradation rate of 2,4,6-TCP.