Aqueous transformation of phenolic pollutants on biochar via dissolved oxygen-driven polymerization

Phenolic compounds are toxic organic pollutants with widespread occurrence in aquatic environments. While their transformation mechanisms have been well studied in advanced oxidation processes with abundant reactive species, the oxidation processes on biochar surfaces remain poorly understood under oxidant-free conditions. Here, we demonstrate that 2,4-dimethylphenol (2,4-DMP) undergoes surface-mediated mineralization and polymerization on high-temperature biochar in the presence of O₂ under ambient conditions. Biochar shows strong affinity for 2,4-DMP, effectively concentrating it at the surface and facilitating subsequent oxidative process. Both adsorption and catalytic performance improved with increasing pyrolysis temperature, with PB900 exhibiting the highest activity, promoting the formation of dimers, trimers, and tetramers via C-C or C-O-C bonds. Mechanistic analysis revealed that limited ·OH, generated through O₂ activation facilitated by delocalized π-electrons at edge defects, drives the oxidative coupling of 2,4-DMP, while surface C-OH/C=O groups facilitate electron transfer. Toxicity predictions indicated that the low-molecular-weight products exhibited reduced biological toxicity relative to the parent compound, whereas the polymerization products posed a higher toxicity risk. This work highlights the potential of carbon-based materials to drive oxidative polymerization under ambient conditions, revealing a previously overlooked transformation pathway for phenolic pollutants in natural and engineered environments.