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Three reduced graphene oxide (rGO) with different defect contents were prepared at 250, 600 and 1000 °C, respectively, using thermal reduction method, and their effects on the oxidative coupling processes of 4-methoxyphenol (MOP) in aqueous solution were investigated. In the presence of rGOs, the oxidation of MOP was dramatically accelerated. At the same surface area dose, the catalytic efficiencies of three rGOs followed an ascending order of rGO 250 ≤ rGO 600 < rGO 1000 . Using liquid chromatography-mass spectrometry and molecular modeling computation, we identified the coupling products (dimer, trimer, and tetramer), and revealed that ortho C O and ortho-ortho C C coupling between two MOP radicals were the main reaction pathways. Dissolved oxygen and structural defects of rGOs played important roles in the oxidation of MOP. It is proposed that dissolved oxygen reacted with the defect sites of rGO to produce surface-bound superoxide species, which captured H atom from the phenolic hydroxyl group of MOP to generate MOP radical and H 2 O 2 . A portion of H 2 O 2 can be catalyzed further by rGO to hydroxyl radical, which continued to react with MOP to produce MOP radical and H 2 O. [ABSTRACT FROM AUTHOR]

• rGO promoted the oxidative coupling of BPs in water in the dark. • HO-PBDEs and HO-PBBs were the main reaction products. • Dissolved oxygen was the oxidant of BPs oxidation. • Dissolved oxygen oxidized BPs by forming of reactive oxygen species. The widespread use of bromophenols in industrial products has led to their emergence in natural water environments. This study investigated the oxidative coupling reactions of 2,6-dibromophenol (2,6-diBP), 2,4-dibromophenol (2,4-diBP) and 3,5-dibromophenol (3,5-diBP) during the water purification by reduced graphene oxide (rGO). Results showed that rGO markedly promoted the oxidation of three bromophenols (BPs). The oxidation efficiencies of three BPs were related to their homolytic bond dissociation energies and decreased following the order of 2,6-diBP > 2,4-diBP > 3,5-diBP. Mass spectrometry and density functional theory calculation revealed that C O and C C coupling among BPs radicals were the main reaction pathways, with the hydroxylated polybrominated diphenyl ethers (HO-PBDEs) and the hydroxylated polybrominated biphenyls (HO-PBBs) being the main reaction products. The oxidant of the reaction was dissolved oxygen, which was first transformed to the surface-bound reactive oxygen species on graphene defects and then oxidized BPs into HO-PBDEs and HO-PBBs. To our knowledge, this is the first report indicating that HO-PBDEs and HO-PBBs could be generated during the adsorption of BPs by rGO. Given the enhanced toxicity of HO-PBDEs and HO-PBBs, caution should be taken when treating BPs-containing water with rGO. [ABSTRACT FROM AUTHOR]