The degradation of ciprofloxacin in the UV/NH2Cl process: kinetics, mechanism, pathways and DBP formation

Abstract

In this work, UV/NH₂Cl was utilized to remove ciprofloxacin (CIP) from water and the formation of disinfection by-products (DBPs) following disinfection was investigated to determine its applicability and safety. It was demonstrated that the degradation of CIP in UV/NH₂Cl was significantly more effective than that in NH₂Cl oxidation alone or UV alone. The presence of NOM (0–8 mg L⁻¹), CO₃²⁻ (0–8 mM), and NO₃⁻ (0–8 mM) inhibited CIP degradation with k_obs,CIP from 0.030 to 0.012, 0.017, and 0.020 min⁻¹, respectively. In contrast, the NH₂Cl concentration (0.25–2 mM) and Cl⁻ (0–8 mM) promoted the degradation of CIP with k_obs,CIP increasing from 0.022–0.054 min⁻¹ and 0.030–0.034 min⁻¹. HCO₃⁻ had dual effects on the degradation of CIP. pH slightly affected the degradation of CIP by UV/NH₂Cl in near neutral pH range but greatly inhibited it under strong acidic and alkaline conditions. The reaction rate constant of CIP and HO˙ was calculated to be k_HO˙,CIP = 3.0 × 10⁹ M⁻¹ s⁻¹ and the contribution of the reactive chlorine species and reactive nitrogen species to CIP degradation was much greater than that of HO˙. Intermediate products of CIP were identified and analyzed to propose 3 possible degradation pathways. The formation of DBPs with NH₂Cl disinfection was lower than that with NaClO disinfection after the oxidation of UV/NH₂Cl. Briefly, this study provides much useful information for a better understanding of the UV/NH₂Cl process for water treatment.