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Key Role of Persistent Free Radicals in Hydrogen Peroxide Activation by Biochar: Implications to Organic Contaminant Degradation

Fang, Guodong, Gao, Juan, Liu, Cun, Dionysiou, Dionysios D., Wang, Yu, Zhou, Dongmei
Environmental Science & Technology 2014 v.48 no.3 pp. 1902-1910
biochar, conifer needles, corn straw, correlation, electron paramagnetic resonance spectroscopy, graphene, hexane, hydrogen peroxide, hydroxyl radicals, methanol, methylene chloride, salicylic acid, solvents, temperature, toluene, wheat
We investigated the activation of hydrogen peroxide (H₂O₂) by biochars (produced from pine needles, wheat, and maize straw) for 2-chlorobiphenyl (2-CB) degradation in the present study. It was found that H₂O₂ can be effectively activated by biochar, which produces hydroxyl radical (•OH) to degrade 2-CB. Furthermore, the activation mechanism was elucidated by electron paramagnetic resonance (EPR) and salicylic acid (SA) trapping techniques. The results showed that biochar contains persistent free radicals (PFRs), typically ∼10¹⁸ unpaired spins/gram. Higher trapped [•OH] concentrations were observed with larger decreases in PFRs concentration, when H₂O₂ was added to biochar, indicating that PFRs were the main contributor to the formation of •OH. This hypothesis was supported by the linear correlations between PFRs concentration and trapped [•OH], as well as kₒbₛ of 2-CB degradation. The correlation coefficients (R²) were 0.723 and 0.668 for PFRs concentration vs trapped [•OH], and PFRs concentration vs kₒbₛ, respectively, when all biochars pyrolyzed at different temperatures were included. For the same biochar washed by different organic solvents (methanol, hexane, dichloromethane, and toluene), the correlation coefficients markedly increased to 0.818–0.907. Single-electron transfer from PFRs to H₂O₂ was a possible mechanism for H₂O₂ activation by biochars, which was supported by free radical quenching studies. The findings of this study provide a new pathway for biochar implication and insight into the mechanism of H₂O₂ activation by carbonaceous materials (e.g., activated carbon and graphite).