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

Hydrogen peroxide (HO) 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 (HA) mixture at different C/Fe molar ratios (0-20) were oxygenated. Results show that both HO 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 O; 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 HA. 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 HO. This study highlights the important role of the C/Fe ratio in controlling HO and OH production and therefore in accurately evaluating the associated environmental impacts.