Compositional characteristics of dissolved organic matter in pharmaceutical wastewater effluent during ozonation

Highlights: • Ozone was prone to react with HOA of DOM in pharmaceutical wastewater effluent. • Ozone preferentially decreased hydrophobic humic-like compounds. • Molecular-level insight of DOM changes was obtained during ozonation. • Ozone effectively removed the CHO, CHON, CHOS, and condensed aromatics compounds. • Acute toxicity was positively correlated with unsaturated compounds. The compositional characteristics of dissolved organic matter (DOM) in pharmaceutical wastewater effluent can affect the further improvement and application of the ozone treatment process. The present study investigated the changes of chemical structures, molecular weight (MW) distribution, hydrophobicity/hydrophilicity distribution, fluorescence properties and the molecular composition of DOM in pharmaceutical wastewater effluent during ozonation. Besides, the toxicity change of pharmaceutical wastewater effluent during ozonation was estimated. The results show that ozone is prone to attack high MW fractions, which contributes the most to the UV₂₅₄ value and could improve the biodegradability of refractory DOM in pharmaceutical wastewater effluent. Hydrophobic acid contained the most aromatic and unsaturated bonded organic matter, and was more readily oxidized under ozonation. In fluorescent components, ozonation significantly decreased humic-like acid compounds, and hydrophobic humic-like compounds exhibited the highest removal through parallel factor analysis. At the molecular level, the main organics removed by ozone were compounds with high H/C and low O/C, especially compounds where H/C >1.5. The CHO, CHON and CHOS compounds exhibited high removal under ozonation in formula classes. Lignin compounds, condensed aromatics compounds, and unsaturated hydrocarbons were effectively removed by ozone in compound classes. After ozonation, the number of lipid and sugar compounds increased. In addition, O/Cwa (the intensity-weighted average parameters of O/C) and NOSCwa (nominal oxidation state of carbon) were significantly positively correlated with acute toxicity on the luminescence. With the increase of ozone dose, the acute toxicity of pharmaceutical wastewater effluent after ozonation first decreased and then increased.