Ammonium is associated with enhanced haloacetamide formation, cytotoxicity, and aryl hydrocarbon receptor activation during sulfate radical-based oxidation of acetaminophen.

Acetaminophen (APAP), a widely used pharmaceutical, is frequently detected in surface waters and is a known precursor to toxic nitrogenous disinfection byproducts such as haloacetamides (HAMs). This study evaluated the performance of a sulfate radical-based advanced oxidation process (SR-AOP), employing Fe3O4@GO and peroxydisulfate (S2O82-), for APAP removal under real river water conditions across dry and wet seasons, characterized by seasonal ammonium (NH4+) variation. While APAP removal in deionized water reached 41.6 % at 15 min and 90.8 % at 4 h, the corresponding values in dry-season river water were reduced to 1.9 % and 82.9 %, respectively, indicating strong matrix interference during short reaction times. To assess potential health risks, cytotoxicity and cell proliferation were assessed using human foreskin fibroblast (HF) cells at consistent APAP concentrations. Experiments were conducted using river water samples from different seasons. Cell viability was lowest after SR-AOP, followed by SR-AOP plus chlorination, and chlorination alone. Aryl hydrocarbon receptor (AhR) activity in HepG2 cells, a key indicator of biological response to environmental pollutants, also increased (p < 0.001) after treatment, indicating the formation of biologically active byproducts. The treatment sequence influenced toxicity: SR-AOP plus chlorination and SR-AOP alone exhibited the greatest effects, followed by chlorination alone and then APAP contamination. Z-score analysis revealed that elevated NH4+ concentrations (up to 10.4 mg/L) were associated with enhanced TCAM formation and biological responses, particularly under SR-AOP-based treatments. These findings underscore the importance of combining chemical and biological evaluations, providing sufficient reaction time, and controlling NH4+ levels to minimize harmful byproduct formation in AOP-based water reuse.