Antibiotic residues in water are an emerging environmental concern because conventional treatment processes may not remove them completely. Now, researchers have shown that carefully tuning the balance of nitrogen and boron in biochar made from waste sawdust can greatly improve its ability to activate an oxidant and break down the widely used antibiotic sulfamethoxazole.
The study, published in Biochar, provides new insight into how the chemical composition of biochar controls its catalytic behavior and offers a strategy for designing more effective carbon-based materials for water purification. The researchers prepared nitrogen and boron co-doped biochars with different nitrogen-to-boron, or N:B, ratios and tested them as activators of peroxymonosulfate, commonly known as PMS.
"Our results show that simply adding nitrogen and boron is not enough. Their relative proportion determines how the biochar interacts with PMS and how efficiently pollutants are degraded," said Hongna Li, a corresponding author of the study. "By adjusting this ratio, we can control the surface structure, active sites and electron-transfer behavior of the material."
The difference was striking. Lowering the N:B ratio from 4 to 0.25 increased the sulfamethoxazole degradation rate constant from 0.20 to 0.47 min⁻¹, a 2.35-fold increase. The researchers also found that the underlying removal mechanism changed as the composition was adjusted.
At higher N:B ratios, pollutant removal depended more strongly on adsorption, meaning sulfamethoxazole was primarily captured on the biochar surface. At lower ratios, particularly between 1 and 0.25, PMS activation became much stronger and catalytic degradation played a greater role.
Detailed material characterization revealed why. Lower N:B ratios increased the biochar's surface area, pore accessibility, structural defects and graphitic nitrogen content. These features improved electron transfer between the biochar and PMS and created favorable sites for oxidant activation.
The team further identified singlet oxygen, a reactive but nonradical form of oxygen, as the major oxidizing species responsible for sulfamethoxazole degradation. It accounted for approximately 73 to 82 percent of degradation in most of the tested systems. Experiments using chemical scavengers and electron paramagnetic resonance spectroscopy supported this mechanism, while density functional theory calculations showed how defect-rich, nitrogen-containing carbon sites could promote PMS adsorption and activation.
The optimized material also performed well under conditions closer to real wastewater treatment. The N:B = 0.25 system maintained more than 91.8 percent removal efficiency across pH 3 to 9 and achieved over 70 percent removal within 20 minutes in drinking water, river water and secondary effluent. In a continuous-flow reactor, biochar immobilized on cotton achieved approximately 99.6 percent sulfamethoxazole removal after 200 minutes of operation at a flow rate of 3 mL per minute.
Beyond performance, the study highlights a broader design principle. Rather than treating heteroatom doping as a simple additive modification, the researchers show that the ratio between dopants can determine whether a carbon material mainly adsorbs contaminants or actively catalyzes their destruction.
By converting waste sawdust into a tunable catalyst, the findings could help guide the development of efficient biochar-based technologies for removing antibiotic residues and other organic contaminants from water.
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Journal Reference: Zhang, J., Gao, Y., Wang, S. et al. Tailored co-doped biochar by varying the N:B ratio as peroxymonosulfate activator for sulfamethoxazole degradation. Biochar 8, 138 (2026).
https://doi.org/10.1007/s42773-026-00647-3
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About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.
