Peroxymonosulfate activation by iron self-doped sludge-derived biochar for degradation of perfluorooctanoic acid: A singlet oxygen-dominated nonradical pathway

The harmful effects on environment and human health caused by the widespread use of perfluorooctanoic acid (PFOA) have appealed much attention. However, traditional advanced oxidation processes (AOPs) are usually inefficient for PFOA degradation. In this study, iron species self-doped biochar derived from municipal sludge (ISBC) was synthesized by a simple method of EDTA-citric acid (CA) leaching/pyrolysis, exploring an efficient activation method of peroxymonosulfate (PMS) for PFOA degradation. Results showed that most of heavy metals were recovered from the sludge via the EDTA-CA pretreatment, at the same time, the special coral-like porous structure were formed in ISBC. A larger specific surface area and pore volume, as well as uniform Fe and N elements distribution of ISBC facilitated the reaction of surface PMS activation, which contributed to the enhancement in PFOA removal. Notably, it was found that pyridinic nitrogen and ketonic and quinone groups were key active sites, and their mediated singlet oxygen (1O2) was confirmed to be the dominated reactive species for PFOA removal in ISBC/PMS system, instead of the radicals mediated by iron species. Furthermore, a possible PFOA degradation pathway by the nonradical 1O2 oxidation was studied by combining density functional theory (DFT) calculation with experimental results. This study proposes a potential method to recover sludge heavy metals and activate PMS for PFOA and other organics simultaneously, which provides a valuable process for the water treatment and high value utilization of excess sludge.