Iron-containing clay minerals are ubiquitous in soils, sediments, and water and provide a significant source of redox-active Fe for reduction of soluble contaminants such as uranium(VI). While biological reduction of clay minerals has been known for some time, it has only recently been shown that Fe(II) can abiotically reduce Fe(III) in clay minerals. Here we used Mössbauer spectroscopy to show that Fe(II) reduces an extensive amount of Fe(III) in a low Fe clay mineral (Wyoming montmorillonite SWy-2, 2.3 wt% Fe). The extent of reduction ranges from 12 to 78% over a pH range of 4.0 to 7.5 and Fe(II) concentration from 0.4 to 2.2 mM and increases as the amount of sorbed Fe(II) increases until about half of the mineral is reduced. The significant amount of Fe(III) reduction in SWy-2 stands in contrast to the spatially isolated nature of Fe atoms in SWy-2, and along with extraction data, suggests Fe(II)-Fe(III) electron transfer occurs through the basal planes. Furthermore, we found no evidence for Fe(II)-Fe(III) electron hopping between Fe atoms in SWy-2 using Mössbauer spectroscopy. When we reacted SWy-2 reduced by Fe(II) or by dithionite with U(VI) we found that reduced U species formed. For solids loadings up to 40 g/L SWy-2 a mixed valence solid containing U(V) and U(VI) was produced, and at the high solids loading of 100 g/L UIVO2 was formed. In contrast, NAu-1 and NAu-2 reduce U(VI) to UIVO2 at much lower loadings. Our results from experiments with Fe(II) and U(VI) suggest that the electron hopping mechanism may be important for the reactivity of clay minerals toward U(VI).

Fe(II)-Fe(III) electron transfer in a low Fe clay mineral and its implications for U(VI) reduction

253rd National Meeting (2017)

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