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Nanophase Fe-oxyhydroxides in freshwater ferromanganese nodules (FFN) from Green Bay, Lake Michigan, and adsorbed arsenate have been investigated by X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Z-contrast imaging, and ab initio calculations using the density functional theory (DFT). The samples from northern Green Bay can be divided into two types: Fe-Mn nodules and Fe-rich nodules. The manganese-bearing phases are todorokite, birnessite, and buserite. The iron-bearing phases are feroxyhyte, nanophase goethite, two-line ferrihydrite, and nanophase FeOOH with guyanaite structure. Z-contrast images of the Fe-oxyhydroxides show ordered FeOOH nano-domains with guyanaite structure intergrown with nanophase goethite. The FeOOH nanophase is a precursor to the goethite. Henceforth, we will refer to it as “proto-goethite.” DFT calculations indicate that goethite is more stable than proto-goethite. Our results suggest that ordering between Fe and vacancies in octahedral sites result in the transformation from feroxyhyte to goethite through a proto-goethite intermediate phase. Combining Z-contrast images and TEM-EDS reveals that arsenate (A  s  O    4   3  −${\rm{AsO}}_4^{3 - }$   ) tetrahedra are preferentially adsorbed on the proto-goethite (001) surface via tridentate adsorption. Our study directly shows the atomic positions of Fe-oxyhydroxides with associated trace elements. The methods can be applied for identifying structures of nano-phases and adsorbed trace elements and heavy metals.

american mineralogist

Study on nanophase iron oxyhydroxides in freshwater ferromanganese nodules from Green Bay, Lake Michigan, with implications for the adsorption of As and heavy metals