Characterization of non-volatile organic contaminants in coking wastewater using non-target screening: Dominance of nitrogen, sulfur, and oxygen-containing compounds in biological effluents.

While abundant volatile compounds (VOCs) have been identified in coking wastewater, the structures and occurrence of non-volatile organic compounds (non-VOCs) have remained unknown. In this study, 3966 non-VOCs belonging to 24 groups were tentatively identified for the first time in wastewater from four biological coking wastewater treatment systems in northern China using a non-target screening technique. A total of 227 compounds with CHNO, CHO, CHOS, and CHNOS elemental compositions were assigned with level 2 identification confidence, and 19 of them were confirmed with authentic standards, with 9-methyl-9H-carbazole-3-carbaldehyde (1706.3-2032.7 μg/L) and 3-Indolyl acetic acid monomethyl terephthalate (773.7-1449.9 μg/L) as the top two compounds in the influents, and 9-methyl-9H-carbazole-3-carbaldehyde (31.8-130.1 μg/L) and monomethyl terephthalate (13.9-196.6 μg/L) as the top two in the effluents. The four groups of substances accounted for 93.4% and 71.5% of the total responses of tentatively identified compounds in the influents and biological effluents, respectively, and were estimated to contribute 32.3-48.9% of the chemical oxygen demand in the biological effluents. In comparison with those in the influent, abundant S-containing compounds (CHOS and CHNOS, 35.2% of the total responses) were observed in the biological effluents, suggesting their highly bio-refractory characteristics. The advanced treatment process using synchronized oxidation-adsorption could almost completely remove the CHOS and CHNOS compounds from the biological effluents.