Mn Oxide Transformation and Alteration in Contaminant Oxidation Rate in the Presence of Fe(II).

Iron and manganese oxides are ubiquitous minerals in soils that play significant roles in contaminant fate and transport. Due to the complex structure and biogeochemistry of soil matrices, constituent concentrations do not conform to thermodynamic constraints leading to the presence of Fe(II) in the presence of Mn oxides. Manganese oxides have been shown to catalyze the abiotic oxidation of Fe(II) under suboxic and anoxic conditions (Lovley and Philips; Myers and Nealson), forming freshly-precipitated Fe(III) (oxyhydr)oxides that can serve as additional binding sites effectively increasing contaminant sorption capacity (Ehlert et al., 2016; He and Hering, 2009). In this way, the interactions between Fe and Mn oxides act as a naturally regenerative As filtration system. However, the rate of contaminant oxidation by Mn oxides can decrease under reducing conditions due to surface passivation by reaction products such as Mn(II), Fe(II), and microbial metabolites (Lafferty et al., 2010; Ehlert et al., 2014; Ying et al., 2011; Ying et al., 2013). Here, we provide insight into the effect of Fe(III) oxide precipitation in the presence of birnessite on As(III) oxidation rate and the concomitant Mn and Fe oxide transformations taking place during Fe(II) and As(III) oxidation. Using a combination of aqueous and solid phases analysis techniques including hydride generation-ICP, SEM, XPS, XANES, and EXAFS, our results show that upon Fe(II) addition, lepidocrocite and ferrihydrite are precipitated as the reduction of birnessite leads to formation of hausmannite. Upon As(III) addition, As(V) is produced as hausmannite undergoes reductive dissolution and is completely removed, as birnessite is the only detectable form of Mn oxide in the system after 92 hours of reaction. Our study demonstrates the impact of Fe(II) on the oxidative potential of Mn oxides in soil aggregates and the effect of the surface passivation on redox cycling and mobilization of redox active trace metal and metalloid contaminants.