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

UV/HO has been used as an advanced oxidation process to remove organic micropollutants from drinking water. It is essential to quench residual HO 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 HO from UV/HO and the mechanism underlying the enhancement of the process by HCO is limited. Therefore, this study aimed to analyse the kinetics and influential factors, particularly the significant impact of bicarbonate (HCO). HO decomposition by GAC followed first-order kinetics, and the rate constants normalised by the GAC dosage (k) were steady (1.6 × 10 L g min) with variations in the GAC dosage and initial HO concentration. Alkaline conditions favour HO 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 values being 8.9 and 2.4 min M, respectively. The presence of chloride, sulfate, nitrate, and dissolved organic matter inhibited HO quenching, while HCO promoted it. The interfacial hydroxyl radical (HO) zones were visualised on the GAC surface, and HCO addition increased the HO concentration. HCO increased the concentration of persistent free radicals (PFRs) on the GAC surface, which mainly contributed to HO generation. A significant enhancement of HCO on HO quenching by GAC was also verified in real water. This study revealed the synergistic mechanism of HCO and GAC on HO quenching and presents the potential applications of residual HO in the HO-based oxidation processes.