This paper investigated hydroxyl radical ((OH)-O-center dot) generation from biochar suspensions for diethyl phthalate (DEP) degradation in the presence of oxygen. Electron paramagnetic resonance (EPR) coupled with a salicylic acid trapping method were used to detect free radicals in biochar and verify (OH)-O-center dot generation from biochar suspensions. Free radicals (FRs) in biochar could induce (OH)-O-center dot generation, and approximate to 12 spins of FRs were consumed to produce one trapped [OH] molecule. The proposed mechanism of (OH)-O-center dot generation was that FRs in biochar transferred electrons to O-2 to produce the superoxide radical anion and hydrogen peroxide, which reacted further with FRs to produce (OH)-O-center dot. Free radical-quenching studies utilizing superoxide dismutase, catalase, and deferoxamine as scavengers were used to testify this mechanism. Furthermore, (OH)-O-center dot generated from biochar suspensions could degrade DEP efficiently. These findings of this study provide new insights into the physicochemical properties and environmental implications of biochar. (C) 2014 Elsevier Ltd. All rights reserved.