Molecular insights into transformation of dissolved organic matter in treating shale gas wastewater by the UV/H₂O₂-ultrafiltration-reverse osmosis combined process

The integration of oxidation and membrane technologies demonstrates significant potential in shale gas wastewater (SGW) treatment. However, the molecular-level transformation mechanisms of the complex dissolved organic matter (DOM) in SGW during this coupled process remain unclear. This study systematically assessed the performance of UV/H₂O₂-ultrafiltration (UF)-reverse osmosis (RO) combined process in removing contaminations from SGW and used Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to elucidate the transformation characteristics and mechanisms of DOM during treatment. At an optimal H₂O₂ dosage (100 mg/L), UV/H₂O₂ oxidation removed 16.76 % of UV₂₅₄ and 12.96 % of fluorescent dissolved organic matter, while achieving only 4.50 % removal of dissolved organic carbon, indicating that structural transformations of DOM were predominant over mineralization. FT-ICR MS analysis showed that UV/H₂O₂ significantly increased DOM saturation and oxidation, effectively degrading highly aromatic compounds such as polyphenols and polycyclic aromatic hydrocarbons, with a preferential removal of reduced organic compounds. Mass difference analysis, based on 19 transformation reactions, identified dealkylation, oxygenation, dehydrogenation, dehydration, and decarboxylation as common reactions during UV/H₂O₂ treatment, with demethylation being the most dominant and CHNOS compounds exhibiting the highest reactivity. UF treatment had minimal impact on DOM molecular structure, whereas RO effectively rejected most DOM. Due to steric hindrance and electrostatic repulsion, RO preferentially removed organic compounds with high m/z, high O/C, and high unsaturation. Additionally, linear alkyl benzene sulfonates showed a relatively high abundance in RO effluent due to its high ionization efficiency, while non-mineralized DOM was concentrated in the RO concentrate. This study provides crucial insights into the optimization of oxidation-membrane combined processes for SGW treatment at the molecular level.