Effect of pH and Cl- concentration on the electrochemical oxidation of pyridine in low-salinity reverse osmosis concentrate: Kinetics, mechanism, and toxicity assessment

To reduce the pressure of salt separation crystallization and contaminants removal, we systematically investigated the electrochemical oxidation of pyridine (Pyr) in low-salinity reverse osmosis concentrate (ROC) using a Ti/RuO2-IrO2 anode with a high-performance chlorine evolution reaction. We mainly focused on the pH- and Cl- concentration-dependent reaction kinetics, degradation mechanisms, and toxicity assessment. Specifically, the Pyr degradation followed a pseudo-first-order kinetic model. A pH-dependent reaction kinetics study showed that reactive chlorine species (RCS) and & BULL;OH were the dominant species that reacted with Pyr. Furthermore, the degradation efficiency of Pyr reached its maximum value at pH 2, owing to the formation of more RCS. Additionally, Pyr degradation depends on the contribution of RCS in the presence of Cl-. Under low and high Cl- concentrations, the secondary effects were free available chlorine (FAC) and & BULL;OH, respectively. The concentration of trichloromethane (TCM) increased to 9.96 mu M first and then decreased to 5.42 mu M within 90 min. The variation trend in its concentration is consistent with that of the RCS contribution under different pH and Cl- concentration. The active sites of Pyr were calculated using frontier electron density (FED) and Laplacian bond order (LBO) calculations. The degradation performance and mechanisms of Pyr, wherein ring cleavage on the 6 N atom of Pyr was the main mechanism by electrochemical treatment, were explored through theoretical calculations and gas chromatography-mass spectrometry (GC-MS) analysis. The generation of transformation products (TPs) increased the toxicity of the Pyr water after electrochemical treatment. The overall findings provide new insights into the treatment of ROC, offering significant reference information for improving water utilization and near-zero liquid discharge (near-ZLD) of real wastewater.