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Abstract: Fly ash represents a promising alternative source of rare earth elements (REE). However, information on REE containing mineral phases and their association with other fly ash components, vital for REE recovery from fly ash, is currently lacking. Herein, the mass fraction, distribution, crystallography and solid-state chemistry of REE, U and Th in Nigerian simulated fly ash samples were characterised using a range of laboratory and synchrotron x-ray based analytical techniques to underpin future extraction methodologies. Inductively coupled plasma mass spectrometry following full-acid digest of forty-five samples revealed recoverable average total REE content which ranged between 442 mgkg−1and 625 mgkg−1, comprising over 30 wt% of the critical REE Nd, Eu, Tb, Dy, Y and Er. These REE within the fly ash samples were found to be most frequently associated with discrete monazite, xenotime and Y-bearing zircon mineral particles, with the former the most detected, which could be beneficiated through gravity separation. Analysis of monazite particles isolated from the composite samples through a complimentary suite of analytical synchrotron radiation techniques revealed a core-shell pattern, with the shell rich in colocalised Ce, Nd and La, and the core enrich in both U and Th. Ce in monazite was found to exist in a mixed trivalent and tetravalent oxidation state, with the monazite structure amorphized due to the high temperature combustion process. Such results demonstrate the strong co-association and physical distribution of REE, U and Th within monazite in fly ash; knowledge of which can subsequently be used to optimise or develop a more selective, cost-effective and environmentally friendly solvent extraction methodology, by targeting the strongly colocalised and surface bound REE in fly ash monazite particles.

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Abstract: The complex tribochemical nature of lubricated tribological contacts is inaccessible in real time without altering their initial state. To overcome this issue, a new design of a pin-ondisc tribological apparatus was developed and combined with synchrotron X-ray absorption spectroscopy (XAS). Using the designed apparatus it is possible to study in-situ the transient decomposition reactions of various oil additives on different surfaces under a wide range of realistic operating conditions of contact pressure (1.0 - 3.0 GPa), temperature (25 - 120 oC) and sliding speed (30 - 3000 rpm or 0.15 - 15 m/s). To test the apparatus, several tribological tests were performed at different shearing times ranging from 2.5 to 60 minutes. These tests were carried out under Helium atmosphere at a temperature of 80 oC, contact pressure of 2.2 GPa and sliding speed of 50 rpm. The XAS experiments showed that the zinc dialkyldithiophosphate (ZDDP) antiwear additive decomposes in the oil to form a tribofilm on the iron surface at different reaction kinetics from the ones of the thermal film. This confirmed the findings of several previous studies that the formation of the tribofilm is a thermally activated mechanically assisted process, which is faster than the one involved the thermal film that is only thermally activated. Furthermore, the results indicated that the sulfur of the formed film, whether a tribofilm or a thermal film, appears initially in the form of sulfate, with some sulfide, which under heat or shear is reduced into mainly sulfide.

Open Access

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Abstract: Methane (CH4) represents one of the key compounds in the global carbon (C) cycle. A major proportion of the CH4 in the Earth system is produced in marine sediments by methanogenesis, the final step in the gradual fermentation of organic matter deposited on the seafloor. Once emitted to the atmosphere, CH4 acts as a powerful greenhouse gas. Despite high rates of methanogenesis in continental shelf and slope environments, the ocean today contributes only a relatively small amount of this potent greenhouse gas to the global atmospheric budget. The low atmospheric CH4 efflux from the ocean is largely due to the effective biological removal of dissolved CH4 through anaerobic oxidation with sulfate (SO42-) in marine sediments. This removal of pore water CH4 occurs within a distinct sulfate/methane transition zone (SMTZ), preventing the large amounts of CH4 generated in marine sediments from escaping to the water column. The relevant pathways and the environmental factors that control the rates of CH4 oxidation in marine sediments are, however, still incompletely understood. In some settings, for example, pore water CH4 is found throughout the SO42--bearing zone, pointing towards an inefficient CH4 oxidation by SO42- in certain marine environments. Other more recent findings further indicate that nitrate and nitrite, as well as metal oxides (e.g. manganese and iron (Fe) oxides) can enhance the conversion of CH4 to CO2 in the absence of oxygen. Sulfate may thus not be the only electron acceptor used by microorganisms to oxidize CH4 in anoxic sediments, but knowledge about the significance of additional electron acceptors for the global CH4 cycle is still lacking. In addition, little is known about how CH4 oxidation may impact the marine cycling of Fe and phosphorus (P), both key nutrients for oceanic phytoplankton. This thesis aims to refine our understanding of the potential impacts and controls of CH4 oxidation in marine sediments. In particular, it discusses how anaerobic oxidation of CH4 affects the sequestration and sedimentary records of Fe and P, using a wide range of geochemical tools. The results presented here also demonstrate that climate change and anthropogenic nutrient loading may alter the position and efficiency of the CH4 oxidation barrier in coastal sediments, which in turn could lead to increased atmospheric CH4 emissions from the coastal ocean.

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Abstract: Initiatives to support the sustainable development of the nanotechnology sector have led to rapid growth in research on the environmental fate, hazards and risk of engineered nanoparticles (ENP). As the field has matured over the last 10 years, a detailed picture of the best methods to track potential forms of exposure, their uptake routes and best methods to identify and track internal fate and distributions following assimilation into organisms has begun to emerge. Here we summarise the current state of the field, focussing particularly on metal and metal oxide ENPs. Studies to date have shown that ENPs undergo a range of physical and chemical transformations in the environment to the extent that exposures to pristine well dispersed materials will occur only rarely in nature. Methods to track assimilation and internal distributions must, therefore, be capable of detecting these modified forms. The uptake mechanisms involved in ENP assimilation may include a range of trans-cellular trafficking and distribution pathways, which can be followed by passage to intracellular compartments. To trace toxicokinetics and distributions, analytical and imaging approaches are available to determine rates, states and forms. When used hierarchically, these tools can map ENP distributions to specific target organs, cell types and organelles, such as endosomes, caveolae and lysosomes and assess speciation states. The first decade of ENP ecotoxicology research, thus, points to an emerging paradigm where exposure is to transformed materials transported into tissues and cells via passive and active pathways within which they can be assimilated and therein identified using a tiered analytical and imaging approach.

Open Access

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Abstract: The neutral, distorted octahedral complex [TiCl4(SenBu2)2] (1), prepared from the reaction of TiCl4 with the neutral SenBu2 in a 1:2 ratio and characterized by IR and multinuclear (1H, 13C{1H}, 77Se{1H}) NMR spectroscopy and microanalysis, serves as an efficient single-source precursor for low-pressure chemical vapor deposition (LPCVD) of titanium diselenide, TiSe2, films onto SiO2 and TiN substrates. X-ray diffraction patterns on the deposited films are consistent with single-phase, hexagonal 1T-TiSe2 (P3̅m1), with evidence of some preferred orientation of the crystallites in thicker films. The composition and structural morphology was confirmed by scanning electron microscopy (SEM), energy dispersive X-ray, and Raman spectroscopy. SEM imaging shows hexagonal plate crystallites growing perpendicular to the substrate, but these tend to align parallel to the surface when the quantity of reagent is reduced. The resistivity of the crystalline TiSe2 films is 3.36 ± 0.05 × 10–3 Ω·cm with a carrier density of 1 × 1022 cm–3. Very highly selective film growth from the reagent was observed onto photolithographically patterned substrates, with film growth strongly preferred onto the conducting TiN surfaces of SiO2/TiN patterned substrates. TiSe2 is selectively deposited within the smallest 2 μm diameter TiN holes of the patterned TiN/SiO2 substrates. The variation in crystallite size with different diameter holes is determined by microfocus X-ray diffraction and SEM, revealing that the dimensions increase with the hole size, but that the thickness of the crystals stops increasing above ∼20 μm hole size, whereas their lengths/widths continue to increase.