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.