Formation of hydroxylated polybrominated diphenyl ethers and hydroxylated polybrominated biphenyls during the adsorption of bromophenols by reduced graphene oxide

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 (HOPBDEs) 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.