Gallic acid accelerated BDE47 degradation in PMS/Fe(III) system: Oxidation intermediates autocatalyzed redox cycling of iron

Abstract

Fe(II)-catalyzed peroxymonosulfate (PMS) activation process was able to degrade persistent organic pollutants, such as polybrominated diphenyl ethers (PBDEs), but the slow transformation rate from the generated Fe(III) to Fe(II) restricted the efficiency of this process. In this study, we found that the addition of small quantities of gallic acid (GA), a model compound of natural polyphenols, in the Fe(III)/PMS process (namely GA/Fe(III)/PMS process) could exert a long-term influence on Fe(II) recycle and accelerated the degradation of 2,2′,4,4′-tetrabromodiphenyl ether (BDE47) over 72 h. Under conditions of 20.3 µM GA, 13.6 µM Fe(III) and 400 µM PMS, the degradation efficiency of BDE47 reached 85%, which was 9.4 times higher than that in the Fe(III)/PMS process. The degradation kinetics can be divided into an initial “fast stage” (kobs1 = 0.298 h−1) and a second “slow stage” (kobs2 = 0.021 h−1). Aromatic radicals such as hydroxycyclohexadienyl radical (poly-HCD) produced by the attack of SO4−/HO on GA was proposed to be responsible for Fe(II) recycle in the first stage, while ring-opened products following SO4−/HO attack of GA-quinone mainly initiated Fe(III) reduction in the second stage. Owing to the multiple Fe(III) reduction pathways, PMS would be continuously activated by Fe(II) to form SO4− and HO, which were the dominant reactive species for BDE47 degradation. Finally, natural polyphenols extracted from green tea were proven effective in enhancing BDE47 degradation in Fe(III)/PMS process. This study not only provides a new way to propagate Fe(II)-activated persulfate chain reactions for the degradation of refractory organic contaminants, like PBDEs, but also sheds new insight into the reactivity of organic byproducts toward Fe in PMS-based oxidation system.