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Reduced graphene oxide (rGO) is one of the most widely used carbon nanomaterials. When it is released into the environment, rGO can markedly affect the transformation of many pollutants, and change their fate and risk. In this work, the synergetic effects of rGO and Cu(II) on the oxidation of 2-naphthol were examined in water in the dark. It was found that the coexistence of rGO and Cu(II) significantly promoted the oxidation of 2-naphthol. Corresponding products were identified as the coupling oligomers of 2-naphthol (dimer, trimer and tetramer) and hydroxylated compounds (OH-2-naphthol, OH-dimer, di–OH–dimer and naphthoquinone derivatives). In the oxidation reaction, rGO played dual roles, i.e. adsorbent and electron-transfer mediator. rGO firstly adsorbed Cu(II) and 2-naphthol on its surface, and then transferred electrons from 2-naphthol to Cu(II) to yield 2-naphthol radicals and Cu(I). 2-Naphthol radicals coupled to each other to form different oligomers of 2-naphthol. Cu(I) was re-oxidized back to Cu(II) by dissolved oxygen, which sustained the continuous oxidation of 2-naphthol. During the autoxidation of Cu(I), reactive oxygen species were generated, which further reacted with 2-naphthol to form hydroxylated products. These findings provide new insights into the risk assessment of rGO and 2-naphthol in aquatic environments. Image 1 • rGO and Cu(II) synergistically promoted the oxidation of 2-naphthol. • Products were the coupling oligomer and hydroxylated compounds of 2-naphthol. • 2-Naphthol transferred electron to Cu(II) to yield 2-naphthol radical and Cu(I). • Cu(I) was re-oxidized back to Cu(II) by dissolved oxygen. • Reactive oxygen species were formed during the autoxidation of Cu(I). rGO and Cu(II) synergistically promoted the oxidation of 2-naphthol to form the corresponding coupling oligomers and hydroxylated compounds. [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]

Substituent effects of 4-nitrophenol (4-NP), phenol (PE), 4-methylpheol (4-MP), and 4-methoxyphenol (4-MOP) on their oxidative coupling reactions mediated by reduced graphene oxide (rGO) were examined using a batch method. The reaction products were identified as the oligomers of their individual parent compound, and C C and C O coupling between phenolic radicals were the main reaction pathways. The oxidative kinetics were well described with pseudo-first order model. The rate constants ( k obs ) of 4-NP, PE, 4-MP and 4-MOP were 0.47 × 10 −3 , 1.00 × 10 −3 , 2.03 × 10 −3 , and 3.53 × 10 −3  h −1 , respectively, following the order 4-NP < PE < 4-MP < 4-MOP. The k obs of four phenolic compounds was negative linear correlation with their homolytic bond dissociation energies (BDE(O H)), suggesting that BDE(O H) was an effective indicator to predict the oxidation susceptibility of phenolic compounds in rGO system. Dissolved oxygen played a critical role in the oxidation reactions through the formation of superoxide anion (O 2 − ) on rGO. These O 2 − further cleaved the O H bond of phenolic compounds to produce phenolic radicals, which interacted each other and led to the following oxidative coupling reactions. The finding of this study is important for predicting the environmental fate of rGO and phenolic compounds. [ABSTRACT FROM AUTHOR]