Differentiating reactive chlorine species for micropollutant abatement in chloride containing water by electrochemical oxidation process

Abstract

Electrochemical oxidation process (EOP) is promising for micropollutant degradation in water treatment, where chloride ions (Cl⁻) are inevitable in aqueous systems, leading to the EOP/Cl⁻ system. The oxidation of Cl^– at anodes generates reactive chlorine species (RCS), including heterogeneous chlorine species (Cl_hetero), homogeneous free available chlorine (FAC), chlorine dioxide (ClO₂), and chlorine radicals (CRs). This study developed a method to differentiate various RCS responsible for the removal of carbamazepine in EOP/Cl⁻ using the RuO₂/IrO₂-Ti anode. Compared to EOP, the formation of RCS significantly enhanced the degradation of carbamazepine in EOP/Cl⁻, primarily through heterogeneous Cl_hetero, homogeneous molecular chlorine (Cl₂), and CRs. The relative contribution of specific RCS to carbamazepine degradation significantly varied at different pHs, Cl^– concentrations, and current densities. As pH increased from 5.3 to 10.0 with 10 mM Cl^–, the relative contributions of Cl_hetero and CRs decreased, while Cl_hetero dominated carbamazepine degradation at pH 7.0 and 10.0. Cl₂ was the dominant species for carbamazepine degradation at pH 5.3, while its role significantly decreased at higher pH. The increase of Cl^– concentrations enhanced the relative contributions of Cl_hetero, Cl₂, and CRs at pH 5.3 and 18 mA/cm². The rise of current density from 18 to 39 mA/cm² significantly promoted the relative contributions of Cl_hetero and CRs at pH 7.0 and 10 mM Cl^–. This study elucidated the specific roles of reactive species for micropollutant degradation in EOP/Cl⁻, highlighting the significance of heterogeneous Cl_hetero and homogeneous CRs and Cl₂.