Degradation of carbamazepine in reverse osmosis concentrate of the secondary effluent by electrochlorination and its reaction mechanism

Abstract

Reverse osmosis concentrate (ROC) from the secondary effluent in municipal wastewater reclamation contains emerging pollutants (EPs) that require further treatment, as these EPs may otherwise pose risks to human health and ecological balance. Since ROC has relatively high salinity, Electrochlorination (E-Cl) could exactly be a promising technology for its EPs removal. However, a comprehensive and systematic investigation has yet to be carried out. In this study, targeted at E-Cl process for the ROC treatment, a series of conscientiously designed experiments were conducted to evaluate the removal performance of carbamazepine (CBZ), investigate the effects of operating conditions and water matrix on the CBZ removal, and reveal its reaction mechanisms. An excellent CBZ removal efficiency of 99.8 % was achieved within 30 min at 2 mA cm⁻² in 5 mM NaCl solution. Among water matrix components, SO₄^2-, NO₃^–, PO₄^3- and NH₄⁺ showed minimal effects, whereas HCO₃^– and humic acid had significant influences on the CBZ removal. The roles of ClO• and Cl₂•^- radicals in the E-Cl process were clarified through quenching experiments combined with electron paramagnetic resonance and radical probe tests. The steady-state concentrations of ClO• (88.95 × 10^-14 M) and Cl₂•^- (493.02 × 10^-14 M) were much higher than those of HO• (6.91 × 10^-14 M). Both ClO• and Cl₂•^- played key roles in CBZ degradation in the E-Cl process. Probable degradation pathways of CBZ were proposed based on the intermediates identification and density function theory (DFT) calculations. This study demonstrates the potential of E-Cl systems for effective EPs removal in ROC, providing mechanistic insights for practical applications.