ELECTRON TRANSFER AND DISSOLUTION OF CLAY MINERALS INFLUENCE GREEN RUST PRECIPITATION AT IRON-REDUCING CONDITIONS

Flooded soil and sediments, like wetland and riparian areas, are transformers of trace elements and nutrients primarily through anoxic redox cycling of Iron (Fe). Biogeochemical processes can solubilize Fe via reductive dissolution and release Fe(II) which readily precipitates the mixed Fe(II)-Fe(III) hydroxide green rust (GR). In natural environments, this important step of the Fe redox cycle is poorly understood but these results provide detail on governing mechanisms. Green rust is likely to coprecipitate with elements from clay mineral dissolution (e.g. Mg, Al, Si) with changes to natural green rust composition and reactivity. To examine this, the influences of electron transfer, clay mineral dissolution and aqueous conditions on secondary Fe(II) precipitates on natural clay minerals were studied.

Clay fractions were studied under conditions simulating active reductive dissolution of Fe(III)oxides. The source of clay minerals was the Bt soil horizon of a Delaware Matapeake silt loam with weatherable aluminosilicates (i.e. hydroxy-interlayered vermiculite, mica, and chlorite) and 2.6 wt% Fe. Anoxic sorption of mM Fe(II) to these clay minerals was studied in batch reactors as a function of pH (6.5 – 7.5), Fe(II)aq concentration (0.5-3 mM), and reaction time (up to 55 days). Sorption products were characterized by selective extraction and multiple techniques such as Fe K-edge XAS, X-ray diffraction (XRD), Fe⁵⁷ Mössbauer spectroscopy, and Scanning Transmission X-ray Microscopy. The sorbed Fe(II) coprecipitated as a mixture of green rust and Fe(II)-Al Layered hydroxide (30% and 15%, respectively) coupled with changes to the clay minerals including the reduction of structural Fe(III) and dissolution of Al and Si. This precipitate formed after 1 day of reaction and was observable by Fe EXAFS for the duration of the experiment (135 days). Our findings indicate that clay minerals in soils and sediments influence secondary Fe(II) precipitates by coupled electron transfer and dissolution/coprecipitation. Clay minerals will change with geographic location and so will green rust composition and its ability to attenuate pollutants during iron reducing conditions in flooded soils and sediments.