Redox-dependent biotransformation of sulfonamide antibiotics exceeds sorption and mineralization: Evidence from incubation of sediments from a reclaimed water-affected river

Trace levels of sulfonamide antibiotics are ubiquitous in reclaimed water, yet environmental pathways to completely remove those chemicals are not well understood when such water is used to restore flows in dried rivers. This study investigated sulfonamide sorption-desorption, biodegradation, and mineralization processes with seven sediments from a reclaimed water-dominant river. Batch experiments were conducted under oxic and anoxic (nitrate-reducing) conditions, and each removal process of sulfamethazine, sulfadiazine, and sulfamethoxazole (SMX) was evaluated individually at environmentally relevant concentrations (≤ 10 μg/L). Over 28 days, 44 ± 32% of sulfonamides were biodegraded, while the full mineralization to carbon dioxide was < 1%. Around 5% of sulfonamides were removed via sediment sorption, with a positive correlation with sediment organic contents. Detailed investigation of SMX biodegradation revealed that although its transformation appeared to be faster in anoxic than oxic tests by day 2, it reversed over 28 days with a longer apparent half-life in anoxic tests (69 ± 25 days) than that in oxic tests (12 ± 11 days). This is attributed to the formation of reversible metabolites at denitrifying conditions, such as DesAmino-SMX of which the production was affected by nitrite concentrations. Despite measurements of three frequently reported metabolites, > 70% biotransformation products remained unknown in this study. The findings highlight the persistency of sulfonamides and their derivatives, with research needed to further elucidate degradation mechanisms and to perform risk assessment of reclaimed water reuse.