Transformation and degradation processes of dissolved organic matter in shale oil fracturing flowback water: Implications for environmental management

Shale oil extraction yields shale oil fracturing flowback water (SOFFW) rich in recalcitrant soluble organic compounds, the mismanagement of which can cause ecological harm. The current lack of research on dissolved organic matter (DOM) in SOFFW hampers the informed selection and optimization of oxidative methodologies. This study employed diverse characterizations to investigate the degradation and transformation mechanisms of DOM in SOFFW during Fenton oxidation. It identified three primary components in SOFFW, predominantly humic-like substances, the intensity of which was considerably reduced through oxidation. 2D-COS-FTIR analysis revealed that during oxidation, phenolic organic compounds showed preferential reactivity, followed by compounds containing double bonds and, finally, aliphatic compounds. Fenton oxidation effectively degraded highly unsaturated phenolics and polycyclic aromatic hydrocarbons, simultaneously generating an abundance of aliphatic and oxygen-poor unsaturated phenolic compounds. Through carboxylation, demethylation, and hydrogenation reactions, the proportion of large organic molecules (400-700 Da) decreased, leading to the formation of smaller molecules in the 200-400 Da range. Our findings demonstrate that hydroxyl radicals in the Fenton oxidation process react rapidly with electron-rich compounds, such as highly oxygenated unsaturated phenolics, while showing lower reactivity toward aliphatic substances. These findings substantially contribute to the efficient treatment of SOFFW and offer valuable insights for practical applications.