Molecular investigation into the transformation of recalcitrant dissolved organic sulfur in refinery sour water during stripping process

Refinery sour water primarily originates from the tops of towers in various units and coker condensate, and cannot be discharged directly to a wastewater treatment plant due to high levels of chemical oxygen demand (COD) and organic sulfur contents. Even after the recovery of H₂S from the sour water by the stripping process, the effluent still contains a high concentration of dissolved organic sulfur (DOS), which can have a huge bad influence. While chemical composition of dissolved organic matter (DOM) in refinery wastewater has been extensively studied, the investigation of recalcitrant DOS from sour waters remains unclear. In the present study, chemical composition of sour water DOMs (especially DOS) was investigated using fluorescence spectroscopy (excitation-emission matrix, EEM) and mass spectrometry, including gas chromatography-mass spectrometry (GC-MS) and high-resolution Orbitrap MS. The GC-MS and EEM results showed that volatile and low-aromaticity compounds were effectively removed during the stripping process, while compounds with high hydrophilicity and humification degree were found to be more recalcitrant. The Orbitrap MS results showed that weak-polar oxygenated sulfur compounds were easier to be removed than oxygenated compounds. However, the effluent still contained significant amounts of sulfur-containing compounds with multiple sulfur atoms, particularly in the form of highly unsaturated and aromatic compounds. The Orbitrap MS/MS results of CHOS-containing compounds from the effluent indicate that the sulfur atoms may exist as sulfonates, disulfide bonds, thioethers. Understanding the composition and structure of sour water DOS is crucial for the development of effective treatment processes that can target polysulfide compounds and minimize their impact on the environment.