Formation and Transformation of Halogenated Organic Compounds during Full-Scale Shale Gas Wastewater Treatment

This study investigated the formation and transformation of halogenated organic compounds (HOCs) in a shale gas wastewater (SGW) treatment plant using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. SGW, characterized by high concentrations of halogens and dissolved organic matter (DOM) with low molecular weight, oxidation state, and aromaticity, was found to contain chlorinated HOCs (Cl-HOCs) as the predominant species, suggesting their primary downhole origins. Differential formation and transformation patterns of HOCs across treatment units revealed a clear selectivity for a specific DOM structure. The aerobic tank primarily transformed highly aromatic lipids, lignins/carboxylic-rich alicyclic molecule (CRAM)-like structures, and polyphenols, producing predominantly Cl-HOCs. In contrast, the membrane bioreactor (MBR) facilitated the formation of numerous iodinated HOCs (I-HOCs), primarily derived from lignins/CRAM-like structures, polyphenols, and highly unsaturated aliphatic compounds. Mass difference network analysis further elucidated the microbial-mediated formation pathways of I-HOCs in the MBR, identifying key iodinated reactions, such as + I–H, + IO–H, and + HIO. Our results provide critical insights into the formation and transformation of HOCs during SGW treatment, highlighting the significance of iodination reactions in the MBR.