Complexation by Particulate Organic Matter Alters Iron Redox Behavior

The role of iron (Fe) in environmental redox processes, such as microbial respiration and pollutant turnover, is influenced by its association with organic matter, particularly in redox-dynamic systems such as wetlands. While the association between Fe and dissolved organic matter (DOM) has been studied extensively, the association between Fe and particulate organic matter (POM), which differs in size and composition from DOM, is poorly understood. In this work, we investigated the complexation of aqueous Fe(II) by mineral-free POM over a full redox cycle using wet chemical and spectroscopic (X-ray absorption and Mössbauer spectroscopy) techniques. The mass of Fe(II) complexed by POM under anoxic conditions ranged from 18.9 ± 1.2 mg Fe·g–1 POM at pH 4.5 to 37.6 ± 1.5 mg Fe·g–1 POM at pH 7. Part of the complexed Fe(II) was oxidized to Fe(III) (21–46%) under anoxic conditions, indicating that complexation by POM altered the Fe(II) redox stability. We then exposed POM-complexed Fe to O2 at pH 5.5 and 7 to simulate oxidizing conditions. Upon exposure to aqueous O2, the complexed Fe(II) was rapidly and completely oxidized at pH 5.5 and pH 7, faster than uncomplexed Fe(II), suggesting that complexation by POM promoted abiotic Fe(II) oxidation. The resulting Fe(III)-POM consisted of Fe(III)-organic phases and poorly crystalline Fe(III) (oxyhydr)oxides. The results of this work demonstrate that POM acts as a major complexant of Fe and alters Fe redox behavior, thereby affecting the role of Fe in environmental redox processes.