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Electron Transfer and Dissolution of Clay Minerals Influence Green Rust Precipitation at Iron-reducing conditions
Citation:
Betts, A., T. Sowers, M. Siebecker, J. Wang, E. Elzinga, T. Luxton, K. Scheckel, A. Thompson, AND D. Sparks. Electron Transfer and Dissolution of Clay Minerals Influence Green Rust Precipitation at Iron-reducing conditions. Geological Society of America, Cincinnati, OH, October 09 - 12, 2022.
Impact/Purpose:
Iron geochemistry is critical to the fate and transport of contaminants in the near surface environment. Green rusts are mixed-valence Iron hydroxides that are potent sorbents and redox reactants in anoxic conditions. However, influences on green rust reactivity in soils and sediments are still being understood. The results of this work show that soil parent material and clay mineral content will influence green rusts by weathering and electron transfer. The implication of this study is that green rusts should be expected to have very different composition and reactivity depending on clay mineral types in a soil or sediment. Interested parties are academics in surface geochemistry and regional partners dealing with metal and/or organic pollution at the interface of surface and groundwater soils and sediments.
Description:
Flooded soil environments like wetland and riparian areas are transformers of trace elements and nutrients in large part because of anoxic redox cycling of Fe. Under common flooded soil conditions, biogeochemical processes solubilize Fe by reductive dissolution and release Fe(II)aq which can resorb or precipitate quickly. Even if metastable, the secondary precipitates of Fe(II) can be highly reactive in redox and sorption reactions. Fe can precipitate mixed layered hydroxides such as the Fe(II)-Fe(III) hydroxide green rust (GR), but is likely to coprecipitate with elements from clay mineral dissolution (e.g. Mg, Al, Si) which will affect its stability and behavior. The influences of electron transfer, clay mineral dissolution and aqueous conditions on secondary Fe(II) precipitates on natural clay minerals were studied in a series of experiments. Clay fractions were studied under conditions simulating flooding and active reductive dissolution of Fe(III)oxides. The source of clay minerals was the Bt horizon of a Delaware Matapeake silt loam with weatherable aluminosilicates (i.e. hydroxy-interlayered vermiculite, muscovite). Anoxic sorption of mM Fe(II) to these clay minerals were studied as a function of near-neutral pH (6.5- 7.5), Fe(II)aq concentration and reaction time (up to 55 days) in batch reactors. Sorption products were characterized by selective extraction and multiple spectroscopic techniques such as Fe K-edge XAS, X-ray diffraction (XRD), Fe57 Mössbauer spectroscopy, and Scanning Transmission X-ray Microscopy. We found that Fe(II) precipitated more slowly and to a lesser extent when electron transfer was limited by dithionite pre-treatment of the clay. The sorbed Fe coprecipitated as XRD-amorphous green rust and the paired reduction of the structural Fe in the clay. This precipitate formed after 1 day of reaction and was observable by Fe EXAFS for the duration of the experiment (135 days). The role of elements from dissolved aluminosilicates will be discussed. Our findings have the potential to advance the understanding of Fe(II) cycling and interaction with clay minerals in soils and sediments.