pH significantly affects removal of trace antibiotics in chlorination of municipal wastewater

► Slight pH values fluctuation markedly affect the removal efficiencies of antibiotics. ► Both free chlorine species and antibiotic species affected the overall reaction rate. ► Anionic antibiotic species were much more reactive than cationic species toward FAC. ► Chlorination of SDZ, SMX, CIP, NOR, and TMP with OCl− became significant at pH > 7.5. ► pH effect varied greatly for different kinds of antibiotics belonging to same class.