On April 14, 2010, when meltwaters from the Eyjafjallajökull glacier mixed with hot magma, an explosive eruption sent unusually fine-grained ash into the jet stream. It quickly dispersed over Europe. Previous airplane encounters with ash resulted in sandblasted windows and particles melted inside jet engines, causing them to fail. Therefore, air traffic was grounded for several days. Concerns also arose about health risks from fallout, because ash can transport acids as well as toxic compounds, such as fluoride, aluminum, and arsenic. Studies on ash are usually made on material collected far from the source, where it could have mixed with other atmospheric particles, or after exposure to water as rain or fog, which would alter surface composition. For this study, a unique set of dry ash samples was collected immediately after the explosive event and compared with fresh ash from a later, more typical eruption. Using nanotechniques, custom-designed for studying natural materials, we explored the physical and chemical nature of the ash to determine if fears about health and safety were justified and we developed a protocol that will serve for assessing risks during a future event. On single particles, we identified the composition of nanometer scale salt coatings and measured the mass of adsorbed salts with picogram resolution. The particles of explosive ash that reached Europe in the jet stream were especially sharp and abrasive over their entire size range, from submillimeter to tens of nanometers. Edges remained sharp even after a couple of weeks of abrasion in stirred water suspensions.

The retention of Am(III) by coprecipitation with or adsorption onto preformed magnetite was investigated by X-ray diffraction (XRD), solution chemistry, and X-ray absorption spectroscopy (XAS). In the coprecipitation experiment, XAS data indicated the presence of seven O atoms at 2.44(1) Å, and can be explained by an Am incorporation at Fe structural sites at the magnetite surface. Next-nearest Fe were detected at distances suggesting that Am and Fe polyhedra share corners in geometries ranging from bent to close to linear Am-O-Fe bonds. After aging for two years, the coordination number and the distance to the first O shell significantly decreased, and atomic shells were detected at higher distances. These data suggest a structural reorganization and an increase in structural order around sorbed Am. Upon contact with preformed FeO, Am(III) forms surface complexes with cosorbed Fe at the surface of magnetite, a possible consequence of the high concentration of dissolved Fe. In a separate experiment, chloride green rust (GR) was synthesized in the presence of Am(III), and subsequently converted to Fe(OH)(s) intermixed with magnetite. XAS data indicated that the actinide is successively located first at octahedral brucite-like sites in the GR precursor, then in Fe(OH)(s), an environment markedly distinct from that of Am(III) in FeO. The findings indicate that the magnetite formation pathway dictates the magnitude of Am(III) incorporation within this solid.

Green rust is a naturally occurring layered mixed-valent ferrous-ferric hydroxide, which can react with a range of redox-active compounds. Sulfate-bearing green rust is generally thought to have interlayers composed of sulfate and water. Here, we provide evidence that the interlayers also contain monovalent cations, using X-ray photoelectron spectroscopy and synchrotron X-ray scattering. For material synthesized with Na(+), K(+), Rb(+), or Cs(+), interlayer thickness derived from basal plane spacings correlates with the radius of the monovalent cation. In addition, sequential washing of the materials with water showed that Na(+) and K(+) were structurally fixed in the interlayer, whereas Rb(+) and Cs(+) could be removed, resulting in a decrease in the basal layer spacing. The incorporation of cations in the interlayer opens up new possibilities for the use of sulfate green rust for exchange reactions with both anions and cations: e.g., radioactive Cs.

Fe-oxides are ubiquitous in soils and sediments and form during Fe(0) corrosion. Depending on redox conditions and solution composition, Fe-oxides such as ferrihydrite, goethite, magnetite, and green rust (GR) may form. These phases typically have high surface area and large affinity for adsorption of trace components. Further, Fe(II)-Fe(III) (hydr)oxides are redox active. Cerium, a member of the lanthanide family, can be used as an analogue for the tri- and tetra-valent actinides found in radioactive waste, expected to be stored in subsurface repositories. In experiments with ferrihydrite, Ce(III) was effectively scavenged from Fe(II)-bearing solutions within 5 min at pH 7. During transformation of ferrihydrite to green rust, however, all Ce(III) was released to solution. By varying initial solution Fe(II):Fe(III) ratio, magnetite and goethite formed together with GR(Na,SO(4)), resulting in decreased Ce(III) release. X-ray photoelectron spectroscopy revealed Ce(III) adsorbed on magnetite. When Fe-oxides were synthesized by air oxidation of Fe(II) solutions at pH 7, GR(Na,SO(4)) played a catalytic role in the oxidation of Ce(III) to Ce(IV) by O(2), removing more than 90% of the dissolved Ce. Transmission electron microscopy revealed that it formed discrete nanocrystals of CeO(2(s)). These results demonstrate that Fe-oxide interaction with radionuclides is likely to depend strongly on the local redox conditions. By analogy with Ce, the trivalent actinides are not expected to be sequestered by preformed GR in anoxic environments. Our results also suggest that trivalent actinides and lanthanides are released when dissimilatory iron reduction of Fe(III)-oxides leads to GR formation However, under oxidizing conditions, GR may influence radionuclide mobility by catalyzing their transformation to a higher oxidation state.

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Green rust / Redox / Radioactive waste / Neptunium / Selenium / ChromiumSummary. Green rust is an Fe(II)-Fe(III) layered double hydroxide that can be present in groundwater in rock formations where long term nuclear waste repositories are being planned. Green rust (GR) is redox-active, making data for its behaviour a necessity in safety assessment modeling because the mobility of many elements, including radionuclides, is controlled by redox reactions. In this paper, we compare the influence of geometry and charge of three redox active ions on their reaction with colloidal size particles of green rust sodium sulfate (GR Na,SO 4 ). Pentavalent neptunyl, selenite and chromate all have different geometries and valence states. Ions with shape and charge similar to SO 4 2− , the GR Na,SO 4 interlayer anion such as, for example, CrO 4 2− , diffuse into the GR interlayer where electrons are readily transferred, and the element may become incorporated into the final oxidised product. More often, the geometry and/or charge of the ion differ significantly from the interlayer anion, such as is the case for SeO 3 2− and NpO 2 + . These are reduced at the exterior of the flat hexagonal GR crystals, primarily at the edges. Se(IV) is reduced to Se(0) and precipitates as a separate phase, while Np(V) is reduced to Np(IV) and precipitates possibly as NpO 2 or as a substitute in an iron oxide. Whether it adsorbs to pore walls in the groundwater flowpath, or it travels as a colloid, green rust can have a significant effect on radionuclide mobility in the near and far field of repositories.

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Fougerite (IMA 2003-057) was accepted by the Commission on New Minerals and Mineral Names of the International Mineralogical Association (IMA) in 2004 as a new mineral to represent the green rust (GR) family. The data on which it was approved, however, are inconsistent. X-ray diffraction patterns from the Fougères soils contain no peaks that could meaningfully be attributed to a GR phase. The sequential dissolution procedure used to identify GR in the soils was not rigorously tested for selectivity. If indeed it is selective, the results indicate the presence of 40À78% Fe in minerals other than GR. Other Fe-bearing phases were not included in the interpretation of the spectroscopy data that were presented. The data are consistent with the presence of Fe-bearing clays and other silicate minerals. In light of the ambiguous and conflicting data, we recommend that the case for fougerite as a mineral be re-evaluated by the IMA.