Manganese­(III/IV) oxides are naturally occurring oxidants of arsenic (As) and can transform the more mobile and toxic arsenite [As­(III)] to the less mobile and less toxic arsenate [As­(V)]. However, physical heterogeneity of soils contribute to the formation of redox transition zones which can host the interaction of Fe­(II) with Mn­(III/IV) oxides, leading to altered Mn­(III/IV) oxide reactivity. In the current study, we use a diffusion-controlled reactor to simulate such a redox interface to determine how As­(III) oxidation by the Mn­(III/IV) oxide birnessite is affected by Fe­(II) within transport-limited environments. Our results show that Fe­(II) oxidation by birnessite in diffusion-limited systems forms Fe­(III) (oxyhydr)­oxides with a range of crystallinities, with ferrihydrite being the dominant phase even in the presence of high Fe­(II) concentrations. Fe­(III) (oxyhydr)­oxide formation is concomitant with birnessite transformation and release of Mn­(II), which leads to a decrease in Mn AOS without significant alteration to Mn mineralogy. Using X-ray photoelectron spectroscopy depth-profiling analysis and scanning electron microscopy imaging, we found that even as Fe­(II) is gradually introduced to birnessite, Fe­(III) (oxyhydr)­oxide precipitates did not coat the birnessite surface uniformly upon oxidation but instead formed discrete and unevenly dispersed surface-associated phases, leaving birnessite surfaces exposed to the surrounding solution. Average oxidation state of Mn in birnessite decreased rapidly after exposure to Fe­(II) coincident with a fraction of solids transforming from hexagonal to triclinic birnessite. When As­(III) was added to the diffusion-limited Fe–Mn oxide system, our results showed that pre-exposure of birnessite to high Fe­(II) concentrations leads to a delay in the appearance of As­(V) in solution as compared to oxidation by birnessite exposed to lower Fe­(II). Additionally, the maximum steady state concentration of As­(V)aq was suppressed in the high Fe system. Taken together, these findings show that though pre-exposure of birnessite to high concentrations of Fe­(II) inhibited As­(III) oxidation by Mn oxides within these systems, the precipitation of higher total mass of Fe­(III) (oxyhydr)­oxides in the high Fe system leads to greater retention of As.

Exposure to soils contaminated with heavy metals can pose human health risk to children through ingestion of contaminated soil. Soil properties such as soil pH, reactive Fe and Al oxide content, clay content, soil organic matter (SOM), and cation exchange capacity (CEC) can reduce contaminant bio-accessibility and exposure. In vitro bio-accessibility (%IVBA) of As and Pb in 19 soils was determined using U.S. EPA Method 1340. Soil properties reduced the bio-accessibility of As by 17–96.5% and 1.3–38.9% for Pb. For both As and Pb, bio-accessibility decreased with increasing Al and Fe oxide content. Al oxides were found to be the primary driver of As and Pb bio-accessibility. Multiple regressions with AlOx, soil pH, %clay and/or FeOx predicted %IVBA As (p < 0.001). The multiple regression including log (FeOx + AlOx) and %clay explained 63% of the variability in %IVBA Pb (p < 0.01). Fe and Al oxides were found to be important drivers of As and Pb bio-accessibility, regardless of in vitro method. These findings suggested soil pH should be used in addition to reactive oxides to predict bio-accessible As. Risk-based adjustments using soil properties for exposure via incidental ingestion should be considered for soils contaminated with As and/or Pb.

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