Oxidation of bromophenols by carbon nanotube activated peroxymonosulfate (PMS) and formation of brominated products: Comparison to peroxydisulfate (PDS)

Recently, we have examined the oxidation kinetics and products of bromophenols (BrPs) by nonradical activation of peroxydisulfate (PDS) in the presence of carbon nanotubes (CNT). In this work, the transformation of BrPs by CNT activated peroxymonosulfate (PMS) was comparatively investigated. It was found that both radical (i.e., sulfate radical (SO4−) and hydroxyl radical (OH)) and nonradical species (i.e., singlet oxygen (1O2) and reactive PMS-CNT complexes) contributed to BrPs degradation by the PMS/CNT system. Their relative contributions to BrPs degradation were very close irrespective of solution pH, while high temperature strengthened the contribution of radicals due to thermo activation of PMS. The effects of chloride ion, bicarbonate and natural organic matter (NOM) on BrPs transformation were mainly attributed to their scavenging effects for SO4− and OH. Additionally, NOM could hinder the interaction of PMS with CNT active sites, thus inhibiting BrPs degradation. The analysis by electrospray ionization-triple quadrupole mass spectrometry showed the formation of brominated polymeric products (i.e., hydroxylated polybrominated diphenyl ethers and hydroxylated polybrominated biphenyls) on CNT surface rather than in aqueous solution when BrPs were treated by the PMS/CNT system, similar to the case of PDS. Meanwhile, inorganic Br− was released but its potential oxidation by SO4− and OH leading to bromate formation was rather negligible, probably due to the fact that CNT as a reducing agent could rapidly convert reactive bromine intermediates back to Br−. Therefore, both PMS/CNT and PDS/CNT systems are promising strategies for effectively removing BrPs with alleviation of toxic byproducts in treated water effluents, while the former involving both nonradical and radical species is more advantageous in treatment of diverse pollutants (e.g., electron-rich moieties containing organics and refractory compounds).