Unravelling the structure-performance relationship and adsorption-electron transfer synergy of biochar in catalytic...

Valorization of biomass wastes into carbon-based catalysts for water treatment is a clear paradigm of sustainability. This study systematically investigates the catalytic ozonation performance, structure-performance relationship and reaction mechanism of a series of biochar catalysts prepared via temperature-controlled pyrolysis of five waste biomasses, including coffee residue, pomelo peel, coconut shell, corn cob and mango kernel. The coffee-residue-derived biochar at 900 °C is identified as the best catalyst, showing rapid and 100% removal of oxalic acid (OA) and various other micropollutants, high effectiveness under various conditions, including different pH values, reaction temperatures, dosages of catalyst and ozone (O₃), water matrices, and the presence of different anions, good stability under flow operation, as well as easy regeneration with nearly complete performance recovery by thermal treatment at 300 °C. Linear relationships between the key properties of the biochar catalysts (density of phenolic hydroxyl (OH) group, density of carbonyl (CO) groups and graphitization degree) and their performance parameters (O₃ adsorption capacity, O₃ decomposition rate, H₂O₂ formation rate and OA removal rate) are disclosed. Experimental and theoretical calculation results reveal a novel adsorption-electron transfer synergistic mechanism toward high performance. The phenolic OH and CO groups are the major and supplemental active sites, respectively, for O₃ activation, while a high graphitization degree is essential to enable efficient electron transfer, cooperatively promoting O₃ decomposition to produce rich hydroxyl radicals (^•OH) for rapid pollutant degradation.