Removal of norfloxacin using coupled synthesized nanoscale zero-valent iron (nZVI) with H2O2 system: Optimization of operating conditions and degradation pathway

In this study, nanoscale zero-valent iron (nZVI) was synthesized in the laboratory by the borohydride reduction method. nZVI was characterized by transmission electron microscopy (TEM), scan electronic microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). In addition, the degradation performance of norfloxacin (NOR) in nZVI/H2O2 Fenton-like system was investigated in detail under various experimental conditions. It was found that the removal of NOR depended on its initial concentration, initial pH, temperature, H2O2 concentration and nZVI dose. At the ambient temperature, the degradation rate of NOR by nZVI/H2O2 Fenton-like reaction under the acidic medium (pH = 3-4) was more than 95% within 40 min of the reaction time, and the mineralization degree was around 50% in terms of total organic carbon (TOC) measurement. Optimal temperature for NOR removal was 308 K, and higher temperature would cause useless decomposition of H2O2 and consequently reduction of NOR degradation. The degradation rate and total removal efficiency of NOR were favored by increasing the nZVI and H2O2 dosages. The optimal conditions for NOR degradation were: initial pH = 4.0, H2O2 concentration = 20 mmol/L, nZVI dose = 100 mg/L, temperature = 308 K. The rate constants (K-obs) of NOR degradation could be fitted well with a pseudo-second-order kinetic equation. Several degradation intermediates of NOR were detected and confirmed, including C15H20FN30 (m/z 278), C(9)Fi(71)FN(2) (m/z 167), C7H7FN2 (m/z 139), and C7H8FN (m/z 126), respectively. Based on the identified intermediates and the Gaussian quantum statistics molecular bonding energy calculation, the possible pathway for NOR degradation under nZVI/H2O2 Fenton-like reaction was discussed. (C) 2016 Elsevier B.V. All rights reserved.