High-valence ruthenium species alter the pathway of sodium hypochlorite in removing pollutants: Investigation of oxidation kinetics and mechanisms

Sodium hypochlorite (NaClO) is widely used as an oxidizing and disinfecting agent in water treatment. However, studies on the catalytic degradation of pollutants by NaClO using metal ions remain limited. In this study, Ru (III) was identified as the most effective catalyst among 14 tested metal ions. A systematic investigation was conducted on the effects of pH (2−12), Ru (III) concentration (0–20 μM), NaClO concentration (0–100 μM), and coexisting ions (0–10 mM) on the degradation kinetics of CBZ. The results demonstrated that the Ru (III)/NaClO system achieved 100 % CBZ removal within 5 min at low doses. The apparent second-order rate constant (k_app) was proportional to the Ru(III) concentration (k_app = (66.33 ± 2.05) (μM⁻¹·M⁻¹·s⁻¹) × [Ru(III)] (μM) + (66.56 ± 15.88) M⁻¹·s⁻¹, R² = 0.996) and to the square of the NaClO concentration (k_app = (0.0841 ± 0.0017) (μM⁻²·M⁻¹·s⁻¹) × [NaClO]² (μM²) + (0.8958 ± 7.9582) M⁻¹·s⁻¹, R² = 0.998). EPR, isotopic labeling, XANES, UV–Vis, and quenching experiments indicated that high-valent Ru-oxo (HVRO) species are the primary reactive oxidants. HVRO altered the oxidation pathway of NaClO, thereby reducing the formation risk of chlorinated by-products. Common coexisting ions (e.g., Cl⁻, CO₃²⁻, PO₄³⁻) slightly promoted the oxidative performance. This study offers new insights into NaClO-based advanced oxidation processes.