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Iron-(hydr)oxide minerals are among the most prominent groups of minerals in soils, sediments, and anthropogenic environments such as tailings and slag piles (Cornell and Schwertmann 1996). Their large surface area and reactivity makes them effective sorbents for many contaminants in the environment. This is especially the case with respect to the retention of arsenic by Fe-(hydr)oxides. Arsenic is highly toxic and its occurrence in sulfide-bearing soils, sediments, and tailings results in environmental and health problems on the global scale (Vaughan 2006). The oxyanion (AsO4)3− has a high affinity to sorb on Fe-O surface terminations and its mobility in soils, sediments, and tailings is thus often controlled by the presence of Fe-(hydr)oxides (e.g., Morin and Calas 1996). Not surprisingly, there have been many studies on the adsorption behavior of (AsO4)3− on Fe-(hydr)oxide surfaces in different environments and under different pH and Eh settings. The most common analytical tools to study the distribution and crystal-chemical environments of adsorbed arsenate species on Fe-(hydr)oxides have been synchrotron-based methods such as X-ray absorption spectroscopy, X-ray fluorescence spectroscopy, and X-ray diffraction. These methods allow the characterization of the distribution and speciation of arsenic on Fe-hydroxide surfaces at the lower to upper micrometer scale (e.g., Singer et al. 2013). Rather than following this more or less conventional approach, Lee et al. (2016) studied the adsorption of arsenate ions on complex Fe-rich nodules with HRTEM and chemical modeling. The results of their study are groundbreaking in terms of how we study adsorption and transformation processes involving mineral surfaces and contaminants in aqueous solution. The authors could show that areas composed of Fe-hydroxides are not simply composed of the common phases ferrihydrite and goethite but are instead a mixture of multiple low-crystalline nano-size Fe-hydroxide phases. Their detailed study on the occurrence, chemical composition, and intergrowth of Fe-(hydr)oxide phases in the nodules enabled them to (1) develop a model for their transformation pathway and (2) resolve the spatial relationship between their abundance and the chemical distribution of arsenate. Additionally, impressive modified high-resolution TEM images (so called Z-contrast images) in combination with chemical modeling allowed the development of an atomic model for the adsorption of the arsenate ion on one of the Fe-hydroxide modifications. Although the visualization of adsorbed ions on solid surfaces with HRTEM is not new (e.g., Wang et al. 2004), it has been never shown for contaminants adsorbed on mineral surfaces in the environment.

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GeoRef, Copyright 2018, American Geological Institute.<br/>2016-096409