Sequential treatment of UV254 photodegradation and microalgal bioremediation in aquaculture effluent: Florfenicol toxicity mitigation and nutrient recovery.

Aquaculture effluent contains multiple pollutants, including the antibiotic florfenicol (FF) and nitrogen compounds, which pose challenges for simultaneous removal. This study investigated the effects of FF on Chlorella sorokiniana and evaluated UV254 irradiation as a pretreatment method for FF decomposition. C. sorokiniana exhibited strong tolerance to FF, with 54.9 % growth inhibition at 100 mg/L, but showed limited FF removal capacity. Furthermore, FF completely inhibited nitrate utilization by the C. sorokiniana. Molecular docking simulations, enzyme activity assays, and substrate removal experiments revealed that FF binds to the residues of microalgal nitrate reductase, thereby reducing enzyme activity and leading to impaired nitrate assimilation. Direct UV254 irradiation was found to be an effective approach for FF removal, achieving over 98 % removal within 60 min. The degradation efficiency was dependent on FF concentration and solution matrix, with the highest degradation rate constant of 0.1 min-1 observed at 5 mg/L FF in a seawater matrix. During the photodegradation process, eight intermediate products (P1-P8) were detected. Through the identification of active species and theoretical calculations, four potential degradation pathways were proposed, involving •H-driven reactions such as C-C bond cleavage, defluorination, dechlorination, and desulfonation. Toxicity assessment software and microalgal cultivation experiments with degradation products indicated that UV254 pretreatment reduced the overall ecological toxicity of FF. In conclusion, this study provides a highly effective and cost-efficient coupled method for the treatment of emerging pollutants and resource recovery of conventional pollutants in aquaculture effluent.