Influence of sulfamethoxazole on anaerobic digestion: Methanogenesis, degradation mechanism and toxicity evolution.

Sulfamethoxazole (SMX), one of the most widely used sulfonamides antibiotics, is frequently detected in the livestock wastewater. Currently, the focus needs to shift from performance effects to understanding of mechanisms and intermediate toxicity analysis. Our study found that SMX (0.5, 1, and 2 mg/L) stimulated methane production by promoting the process of acetogenesis and homo-acetogenesis. Since 1 mg/L SMX could inhibit the transformation of butyric acid, thus, the stimulation of methane was weak under this condition. Under anaerobic conditions, acetate kinase (AK) and cytochrome P450 enzymes (CYP450) continued to participate in SMX degradation. The increase in SMX concentration affected the release of metabolic enzymes, causing changes in SMX degradation pathways. Based on the main biotransformation products, five biotransformation pathways were proposed, the major transformation reactions including hydroxylation, hydrogenation, acetylation, deamination, oxidation, the elimination of oxygen atoms on sulfonyl, isoxazole ring and NS bond cleavage. Toxicity prediction analysis showed that the toxicities of most SMX transformation products were lower than that of SMX.