Industrial wastewater, particularly from sectors like paper, printing, and dyeing, presents a significant environmental challenge due to its high toxicity and poor biodegradability. Addressing these challenges, a recent study published in BiocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More by Yani Zang, Jie Ding, Jiayi Wang, Chengxin Chen, Hanjun Sun, Jiwei Pang, Luyan Zhang, Nanqi Ren, Lan Ding, and Shanshan Yang, introduces a novel metal-free photocatalyst. Their research focuses on synthesizing a biochar-based photocatalyst (BVCN) for solar-driven sulfite activation to degrade organic pollutants. The core innovation involves precisely engineering nitrogen vacancies and nitrogen doping within graphitic carbon nitride (CN) and then combining it with biochar (BC).

The researchers synthesized BVCN with varying biochar concentrations. They found that a 5wt% biochar concentration (5BVCN) yielded exceptional results. In a solution containing sulfites, the reaction rate constant for degrading reactive red 120 (RR120), a common azo dye, reached an impressive 0.0247 min⁻¹. This rate was 5.49 times higher than that achieved by CN alone and a remarkable 15.43 times higher than 5BVCN in a sulfite-free solution. Furthermore, the 5BVCN system achieved a 92.0% degradation rate for RR120 within 90 minutes, with a corresponding total organic carbon (TOC) mineralization rate of 52.1%. The photocatalyst also demonstrated good universality, effectively degrading four different azo dyes, with Congo red almost completely degraded in 45 minutes.

The enhanced performance of BVCN is attributed to a sophisticated interplay of structural and electronic properties. Characterization and density functional theory (DFT) calculations revealed that the engineered nitrogen vacancies act as electron traps, while nitrogen doping regulates the electronic structure, leading to the formation of “mid-gap states”. These mid-gap states are crucial as they enhance the separation of photogenerated charge carriers (electrons and holes), preventing their recombination and thus improving photocatalytic efficiency.

Biochar plays a multifaceted role in this system. Rich in pyridinic nitrogen, BC serves as both an electron transfer channel and an electron storage medium, exhibiting a π-π interaction with the structurally regulated CN. This interaction promotes long-range π-electron delocalization, further facilitating the migration and separation of photogenerated carriers. The presence of biochar also contributes to a narrower band gap in BVCN, enabling better responsiveness to visible light. Moreover, biochar enhances the adsorption of RR120 molecules. The introduction of sodium sulfite significantly increased the adsorption rates of VCN and 5BVCN to RR120, up to 6.1% and 29% respectively. DFT calculations showed that sulfites preferentially adsorb to the pyridinic nitrogen sites on biochar, strengthening the interaction between BVCN and sulfite ions and enhancing electron transfer from sulfite to the photocatalyst.

In the sulfite-containing system, radical scavenging experiments identified sulfite radicals , superoxide radicals ,and holes as the primary active species responsible for RR120 degradation. The mechanism involves the photocatalyst generating electron-hole pairs under solar light. Electrons react with oxygen to form superoxide radicals, while holes directly oxidize sulfite ions to generate highly reactive sulfite radicals. These active species then efficiently degrade the organic pollutants.

Beyond its impressive degradation efficiency, BVCN also demonstrated good stability and recyclability, maintaining 70.49% degradation efficiency after 5 cycles. The structural integrity of the photocatalyst remained largely unchanged after repeated use, indicating its robustness for practical applications in wastewater treatment. This innovative biochar-based photocatalyst offers a sustainable and cost-effective solution for addressing challenging industrial wastewater, aligning with principles of resource utilization and cleaner production.

Source: Zang, Y., Ding, J., Wang, J., Chen, C., Sun, H., Pang, J., Zhang, L., Ren, N., Ding, L., & Yang, S. (2025). Boosting solar-driven metal-free activation of sulfites by biochar-based photocatalyst for organic pollutants degradation: in-situ precise regulation and the enhancement mechanism. Biochar, 7(76), 1–18