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Abstract: The coprecipitation of organic carbon with iron minerals is important for its preservation in soils and sediments, but the mechanisms for carbon-iron interactions and thus the controls on organic carbon cycling are far from understood. Here we coprecipitate carboxylic acids with iron (oxyhydr)oxide ferrihydrite and use near-edge X-ray absorption fine structure spectroscopy and wet chemical treatments to determine the relationship between sequestration mechanism and organic carbon stability against its release and chemical oxidative remineralisation. We show that organic carbon sequestration, stabilisation and persistence increase with an increasing number of carboxyl functional groups. We suggest that carboxyl-richness provides an important control on organic carbon preservation in the natural environment. Our work offers a mechanistic basis for understanding the stability and persistence of organic carbon in soils and sediments, which might be used to develop an overarching relationship between organic functional group-richness, mineral interactions and organic carbon preservation in the Earth system.

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Abstract: Thanks to its insolubility, mineral dust is considered a stable proxy in polar ice cores. With this study we show that the Talos Dome ice core (TALDICE, Ross Sea sector of East Antarctica) displays evident and progressive signs of post-depositional processes affecting the mineral dust record below 1000 m deep. We apply a suite of established and cutting-edge techniques to investigate the properties of dust in TALDICE, ranging from concentration and grain size to elemental composition and Fe mineralogy. Results show that through acidic/oxidative weathering, the conditions of deep ice at Talos Dome promote the dissolution of specific minerals and the englacial formation of others, affecting primitive dust features. The expulsion of acidic atmospheric species from ice grains and their concentration in localized environments is likely the main process responsible for englacial reactions. Deep ice can be seen as a “geochemical reactor” capable of fostering complex reactions which involve both soluble and insoluble impurities. Fe-bearing minerals can efficiently help in exploring such transformations.

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Abstract: The physical properties of clays and micas can be controlled by exchanging ions in the crystal lattice. Atomically thin materials can have superior properties in a range of membrane applications, yet the ion-exchange process itself remains largely unexplored in few-layer crystals. Here we use atomic-resolution scanning transmission electron microscopy to study the dynamics of ion exchange and reveal individual ion binding sites in atomically thin and artificially restacked clays and micas. We find that the ion diffusion coefficient for the interlayer space of atomically thin samples is up to 104 times larger than in bulk crystals and approaches its value in free water. Samples where no bulk exchange is expected display fast exchange at restacked interfaces, where the exchanged ions arrange in islands with dimensions controlled by the moiré superlattice dimensions. We attribute the fast ion diffusion to enhanced interlayer expandability resulting from weaker interlayer binding forces in both atomically thin and restacked materials. This work provides atomic scale insights into ion diffusion in highly confined spaces and suggests strategies to design exfoliated clay membranes with enhanced performance.

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Abstract: Tation keywords: Rare earth elements; Critical metals; Clay minerals; Resources he Rare Earth Elements (REE) are essential for high-tech industries. Most of the world’s heavy REE come from clay deposits formed by weathering of granites in China, and the extraction of rare earths is often performed with little care for the environment. A lack of understanding of how these clay deposits form and how REE are held in them hampers the search to identify alternative resources outside China. Many metals are loosely stuck (‘adsorbed’) to the outer surfaces of clay crystals. An international team of researchers led by the Universities of St Andrews and Brighton used synchrotron techniques to determine where the REE were in the deposit and what surrounded them. They investigated deposits from China and a prospective site in Madagascar. Using Diamond Light Source’s Microfocus beamline (I18) allowed them to study the samples micron by micron. Combining X-ray Absorption Spectroscopy (XAS) with Scanning X-ray Fluorescence (SXRF) element mapping showed what surrounded the metals and their distribution in the sample. Although the rock samples from the two areas are different, the REE in both stick to the clay surfaces identically. At the atomic level, the Madagascan clay deposits are the same as those currently exploited in China. Understanding how the metals stick to the clay crystals will allow us to develop easier, more environmentally friendly ways to extract them. These results hint that deposits like those currently found in China may be more widespread than we thought. The hunt is now on to find other rare earth deposits to provide alternative supplies for the technologies that underpin a carbon-free future.

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Abstract: Coal fly ash is an industrially useful by-product of coal combustion, with millions of tonnes held in repositories globally. This fine and cheaply-available waste material, is proven to sometimes be enriched in valuable elements/compounds, such as rare earth element (REE) minerals and actinides, alongside heavy toxic elements such as Cr, Pb, As and Cd, making coal fly ash both a potential unconventional source of valuable minerals and a hazardous material depending on the elemental trace chemistry of the parent coal body. The importance of REE and actinides for use in advanced electronics technologies and the nuclear industry, alongside concerns over geopolitics and instability in the global supply market, means there is now a renewed incentive for countries to secure native sources of economically-sustainable rare earth elements through research and development efforts. Unconventional sources of REE, such as coal fly ash, represent a potentially interesting value proposition, especially for developing countries. The work presented in this thesis provides a study of rare earth elements and actinides in Nigerian simulant coal fly ash from the perspective of resource recovery and environmental protection. A suite of advanced analytical laboratory techniques and novel synchrotron radiation techniques, alongside micromanipulation methods, were used in this study to understand the amounts and mineralogical association of REE and actinides in 3 different Nigerian coal deposits. Bulk and micro mineralogical analyses were first performed on the simulant coal fly ash using x-ray diffraction and scanning electron microscopy / with energy dispersive x-ray fluorescence and spectroscopy used for elemental analysis. It was observed that the Nigerian simulant coal fly ash samples were less complex in mineralogy than conventional rare earth ores (with quartz, mullite, haematite and cristobalite as the major mineral phases), composed of discrete micron-scale particles of rare earth mineral monazite ([Ce,La,Nd,Th]PO4), xenotime (YPO4) and zircon (ZrSiO4), alongside uraninite (UO2) and thorite (ThSiO4). Subsequent bulk elemental analysis performed using inductively-coupled plasma mass spectrometry, showed the simulant coal fly ash to be enriched in the rare earth elements, being more enriched in the higher-valued critical rare earth elements Nd, Eu, Tb, Dy, Y and Er, compared to conventional rare earth ores. Sequential extraction analysis of the simulant fly ash materials showed significant recoverability of rare earth elements in the acid-soluble fraction using cheap and environmentally-friendly ethanoic acid; the toxic heavy metals Cd and As were also significantly recovered in the acid-soluble fraction. Synchrotron radiation analysis of individual micro-particles of monazite and uraninite using micro-x-ray fluorescence mapping, micro-x-ray fluorescence tomography, micro-x-ray absorption near edge spectroscopy and micro-x-ray diffraction showed a zonation pattern in the monazite particles, with the rims rich in the rare earth elements, and the core rich in U and Th but depleted in the rare earth elements. In the uraninite particles, U was homogeneously-distributed, existing in the chemically reduced IV oxidation state. Gamma-ray spectrometry analysis of the bulk coal and simulant coal fly ash was also performed using a high-purity Ge gamma-ray detector. Compared to World mean levels, the radioactivity and associated hazard associated with the parent coal samples were insignificantly-low. However, the equivalent activity concentration and radiological hazard levels associated with the simulant coal fly ashes were significantly-high, above United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) World mean and recommended levels.