Strong enhancement on diclofenac degradation in Cu(II)/H2O2 system by adding ascorbic acid: Efficiency, mechanism and influencing factors

Utilization of trace Cu(II) (at mu M level) inherently existing in wastewater to activate hydrogen peroxide (H2O2) has exhibited significant potential to remove aqueous organic contaminants. Nevertheless, the Cu(II)/H2O2 system was only efficient under alkaline conditions. Herein, ascorbic acid (H2A), an environmentally benign and natural reductant, was introduced to broaden pH range of Cu(II)/H2O2 process by expediting the Cu(II)/Cu(I) cycle. The H2A/Cu(II)/H2O2 system performed well in degrading diclofenac across a wide pH range from 4.0 to 9.0, and its kinetic rate constant of diclofenac elimination was 247 times greater than its counterpart without H2A at the optimal pH of 6.0, outperforming the performance of most previously reported reductant-enhanced Cu(II)/H2O2 system. Electron paramagnetic resonance analysis, periodate colorimetric method, and in situ Raman spectroscopy tests indicated that hydroxyl radical (HO center dot), rather than Cu(III), was the dominant reactive species of diclofenac degradation. Thirteen degradation products of diclofenac were identified by LC-Q-TOF-MS, and five different elimination pathways were proposed. Furthermore, the presence of SO4 2-exerted an insignificant influence on diclofenac degradation, while the addition of Cl-, HCO3 -and humic acid slightly suppressed the abatement of diclofenac. H2A/Cu(II)/H2O2 system was still highly efficient in degrading diclofenac in actual water matrix (i.e., reservoir water and lake water). Overall, this study provided an eco-friendly approach to enhance the oxidation capacity of the Cu(II)/H2O2 system over a wide pH range, which might be a potential technology for degrading refractory organic pollutants in water treatment.