Effect of C/Fe Molar Ratio on H 2 O 2 and • OH Production during Oxygenation of Fe(II)-Humic Acid Coexisting Systems.

Hydrogen peroxide (H 2 O 2 ) 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 red ) mixture at different C/Fe molar ratios (0-20) were oxygenated. Results show that both H 2 O 2 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 2 ; 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 red . 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 H 2 O 2 . This study highlights the important role of the C/Fe ratio in controlling H 2 O 2 and • OH production and therefore in accurately evaluating the associated environmental impacts.