Quenching residual H₂O₂ from UV/H₂O₂ with granular activated carbon: A significant impact of bicarbonate

UV/H₂O₂ has been used as an advanced oxidation process to remove organic micropollutants from drinking water. It is essential to quench residual H₂O₂ to prevent increased chlorine demand during chlorination/chloramination and within distribution systems. Granular activated carbon (GAC) filter can quench the residual oxidant and eliminate some of the dissolved organic matter. However, knowledge on the kinetics and governing factors of GAC quenching of residual H₂O₂ from UV/H₂O₂ and the mechanism underlying the enhancement of the process by HCO₃^- is limited. Therefore, this study aimed to analyse the kinetics and influential factors, particularly the significant impact of bicarbonate (HCO₃^-). H₂O₂ decomposition by GAC followed first-order kinetics, and the rate constants normalised by the GAC dosage (k_n) were steady (1.6 × 10^-3 L g^-1 min^-1) with variations in the GAC dosage and initial H₂O₂ concentration. Alkaline conditions favour H₂O₂ quenching. The content of basic groups exhibited a stronger correlation with the efficiency of GAC in quenching H₂O₂ than did the acidic groups， with their specific k_n values being 8.9 and 2.4 min^-1 M^-1, respectively. The presence of chloride, sulfate, nitrate, and dissolved organic matter inhibited H₂O₂ quenching, while HCO₃^- promoted it. The interfacial hydroxyl radical (HO^•) zones were visualised on the GAC surface, and HCO₃^- addition increased the HO^• concentration. HCO₃^- increased the concentration of persistent free radicals (PFRs) on the GAC surface, which mainly contributed to HO^• generation. A significant enhancement of HCO₃^- on H₂O₂ quenching by GAC was also verified in real water. This study revealed the synergistic mechanism of HCO₃^- and GAC on H₂O₂ quenching and presents the potential applications of residual H₂O₂ in the H₂O₂-based oxidation processes.