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

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)-O-center dot) 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)-O-center dot] 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)-O-center dot. This hypothesis was supported by the linear correlations between PFRs concentration and trapped [(OH)-O-center dot], as well as k(obs) of 2-CB degradation. The correlation coefficients (R-2) were 0.723 and 0.668 for PFRs concentration vs trapped [(OH)-O-center dot], and PFRs concentration vs k(obs)., 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).