The goal of this study is to identify the effects of Al incorporation and aging under aerobic conditions on the physicochemical properties of ferrihydrite (Fh). Fh is an environmentally pervasive Fe(III) mineral and plays an active role in several geochemical processes such as the mobility and transport of contaminants in aqueous and environmental systems. Fh often co-precipitates in the presence of impurities such as Al and can incorporate up to 20 to 30 mole % Al (%-Alfh). However, the sites of Al incorporation into Fh’s structure remain uncertain. We hypothesize that Al3+ atoms substitute into the Fe1 octahedral position based on the Michel Fh structural model. Two sets of approx. 28 and 16-month old samples of 0, 12, and 24%-AlFh were characterized. The surface charge was indirectly probed using flow microcalorimetry (FMC). The heats of exchange (Qexch in mJ/mg) of two anionic probes, chloride (Cl-) and nitrate (NO3-), were obtained. Phase transformation products and their associated properties were investigated using qualitative and semi-quantitative X-ray diffraction (SQXRD) analysis, X-ray absorption spectroscopy (XAS), and X-ray magnetic dichroism spectroscopy (XMCD). The Qexch measured immediately after synthesis (iQexch), was iQexch 0%-Alfh &lt; iQexch 12%-Alfh &lt; iQexch 24%-Alfh. After aging, the aQexch for the 12%-AlFh was greater than the 24%-AlFh. Goethite (GH) and hematite (HT) were the transformation products identified by XRD. In all instances, the presence of Al inhibits the crystallization of Fh with time. SQXRD data for the 28-month old 0%-AlFh samples showed an increase in the ratio of HT to GT relative to the 16-month 0%-AlFh. Both 12%-AlFh samples did not show any phase transformation, while both 24%-AlFh samples did appear to have undergone phase transformation. Data from XAS and XMCD showed that Al incorporation increased the ratio of tetrahedrally coordinated Fe3+ to octahedral Fe3+. This is possibly an indication that Al3+ is substituting into the surface octahedral Fe1 position, which would cause an increase in the octahedral to tetrahedral Fe3+ ratio. Taken all together, the results of this work showcase how the combined effects of aging and Al-incorporation affects the overall reactivity of Fh and AlFh, and provide experimental constraints to modeling efforts to predict contaminant transport behavior under environmentally relevant conditions.

Effects of Al content and aging under aerobic conditions on the physicochemical properties of ferrihydrite

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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