Effect of C/Fe Molar Ratio on H2O2 and •OH Production during Oxygenation of Fe(II)-Humic Acid Coexisting Systems

Hydrogen peroxide (H2O2) and hydroxyl radical (•OH) production during oxygenation of reduced iron (Fe(II)) and natural organic matter (NOM) in the subsurface has been increasingly discovered, whereas the effect of the C/Fe molar ratio in Fe(II) and NOM coexisting systems remains poorly understood. In this study, aqueous Fe(II) and reduced humic acid (HAred) mixture at different C/Fe molar ratios (0-20) were oxygenated. Results show that both H2O2 and •OH accumulation increased almost linearly with the increase in the C/Fe ratio, with a more prominent increase in •OH accumulation at high C/Fe molar ratios. At low C/Fe molar ratios (C/Fe ≤ 1.6), electrons mainly transferred from dissolved inorganic Fe(II), surface-adsorbed Fe(II), and a low proportion of HA-complexed Fe(II) to O2; with the increase in the C/Fe ratio to a high level (C/Fe ≥ 5), the main electron source turned to HA-complexed Fe(II) and free HAred. The changes in the electron source and electron transfer pathway with the increase in the C/Fe ratio increased the yield of •OH relative to H2O2. This study highlights the important role of the C/Fe ratio in controlling H2O2 and •OH production and therefore in accurately evaluating the associated environmental impacts.