Chlorination Kinetics, Transformation Pathways and Risks of Sulfacetamide and Sulfathiazole

Sulfonamides (SAs) are widely applied synthetic antibacterial agents, and their overuse leads to widespread aquatic residues ranging from ng·L−1 to μg·L−1, posing severe ecological hazards. In this study, the chlorination reaction kinetics, transformation mechanisms and corresponding ecological risks of two typical aquaculture sulfonamides, sulfacetamide (SFA) and sulfathiazole (STZ), were systematically explored under controlled aquatic conditions. All batch degradation tests, product identification experiments and water matrix simulation trials were conducted in biological triplicate (n = 3), and kinetic fitting achieved a high coefficient of determination R2 > 0.985, indicating excellent statistical correlation and reliability of the experimental data. Under constant conditions of 20.0 ± 1 °C and pH 5.0, the apparent second-order rate constants (kapp) of SFA and STZ were quantitatively determined as 84.82 M−1·s−1 and 103.68 M−1·s−1, respectively; reaction rates peaked at neutral pH 7.0 and exhibited a positive linear correlation with temperature. Ammonium ions exerted prominent inhibitory effects on SA elimination: at an NH4+ concentration of 1.0 mg·L−1, the removal efficiencies of both SAs dropped sharply from approximately 95% to 15%. Multiple toxic chlorinated transformation products (CTPs), including chlorophenols and chloroanilines, that could continuously accumulate in water systems were detected via GC-MS. The practical implications of this work are as follows: high ammonium loads in aquaculture effluents will severely weaken the disinfection and removal capacity of conventional chlorination for residual SAs, and the generated persistent chlorinated aromatic intermediates create overlooked hidden risks to aquatic biota and human consumers. This quantitative kinetic dataset can provide parameter support for optimizing chlorine dosing strategies in aquaculture water treatment and evaluating the ecological safety of breeding wastewater discharge.