Boosting solar-driven metal-free activation of sulfites by biochar-based photocatalyst for organic pollutants degradation: in-situ precise regulation and the enhancement mechanism

Abstract The photocatalytic activation of sulfites, a common by-product in industries, is a green and sustainable technology with great promise for the treatment of refractory pollutants in water. In this study, N vacancies and N doping were constructed at precise sites in graphitic carbon nitride (CN), following the combination with biochar (BC), synthesizing the BVCN with excellent photocatalytic activation of sulfites under solar light. When the BC was 5wt% (5BVCN), the reaction rate constant of reactive red 120 (RR120) in SO 3 2− -containing solution reached 0.0247 min −1 , which was 5.49 times of CN and 15.43 times of 5BVCN in SO 3 2− -free solution. Characterizations and density functional theory (DFT) calculations revealed that N vacancies could trap electrons, while N doping regulated the electronic structure, forming mid-gap states to enhance the separation of carriers. In BVCN, BC rich in pyridinic N serves as both electron transfer channel and electron storage medium, having π-π interaction with structurally regulated CN (VCN). BVCN has narrower band gap and low recombination rate of photogenerated carriers, responds well to visible light, and is easy to firmly associated with SO 3 2− , enhancing the electron transfer from SO 3 2− to BVCN. In the SO 3 2− -containing system, the primary active species were identified as SO 3 •− , •O 2 − and h + . Moreover, BVCN exhibited good stability and recyclability. The system shows potential for treating wastewater containing sulfites, realizing resource utilization. Graphical Abstract