Identification of ∼200 transformation products of polyhalogenated compounds to characterize their transformation pathways in sludges

Abstract

Sewage sludge adsorbs a large amount of harmful organic pollutants, particularly the persistent and hydrophobic polyhalogenated compounds (PHCs). PHCs have been subjected to biological and chemical oxidation treatments during wastewater treatment processes; however, the species and concentrations of their transformation products (TPs) in sludge remain unknown, and the transformation pathways are unclear. In this study, 234 TPs of PHCs, including 77 TPs of chlorinated paraffins (CPs-TPs), 102 TPs of organochlorine pesticides (OCPs-TPs), 45 TPs of dechlorane plus (DPs-TPs), and 10 TPs of brominated flame retardants (BFRs-TPs), were identified in sludge through Ph4PCl-enhanced ionization coupled with ultra-performance liquid chromatography–Orbitrap–mass spectrometry. Based on the chemical structures of the identified TPs, we identified three major transformation pathways: dehalogenation-hydroxylation, carbon chain decomposition, and desulfurization. Approximately 97 TPs were newly discovered through the pathways. Carbon chain decomposition products of OCPs and DPs were detected for the first time at relatively high abundances. More hydroxylation products of DPs and hexabromocyclododecane (HBCD) and multi-dehalogenation products of heptachlor, toxaphene, DPs and HBCDs were detected at relative intensities higher than those of the known TPs. The oxidation treatment of sludge achieved up to 13 %–94 % of PHCs to be removed, with dehalogenation-hydroxylation as the main transformation pathway. Advanced treatment technologies are needed for degradation of both PHCs and their TPs.