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Abstract: In this work, we present the results from multi-length-scale studies of a Mn-Na-W/SiO2 and a La-promoted Mn-Na-W/SiO2 catalyst during the oxidative coupling of methane reaction. The catalysts were investigated from the reactor level (mm scale) down to the single catalyst particle level (μm scale) with different synchrotron X-ray chemical computed tomography techniques (multi-modal chemical CT experiments). These operando spatially-resolved studies performed with XRD-CT (catalytic reactor) and multi-modal μ-XRF/XRD/absorption CT (single catalyst particle) revealed the multiple roles of the La promoter and how it provides the enhancement in catalyst performance. It is also shown that non-crystalline Mn species are part of the active catalyst component rather than crystalline Mn2O3/Mn7SiO12 or MnWO4.

Open Access

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Abstract: Bacteria acquire phosphate (Pi) by maintaining a periplasmic concentration below environmental levels. We recently described an extracellular Pi buffer which appears to counteract the gradient required for Pi diffusion. Here, we demonstrate that various treatments to outer membrane (OM) constituents do not affect the buffered Pi because bacteria accumulate Pi in the periplasm, from which it can be removed hypo-osmotically. The periplasmic Pi can be gradually imported into the cytoplasm by ATP-powered transport, however, the proton motive force (PMF) is not required to keep Pi in the periplasm. In contrast, the accumulation of Pi into the periplasm across the OM is PMF-dependent and can be enhanced by light energy. Because the conventional mechanism of Pi-specific transport cannot explain Pi accumulation in the periplasm we propose that periplasmic Pi anions pair with chemiosmotic cations of the PMF and millions of accumulated Pi pairs could influence the periplasmic osmolarity of marine bacteria.

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Abstract: Magmatic systems are dominated by five volatiles, namely H2O, CO2, F, Cl, and S (the igneous quintet). Multiple studies have measured partitioning of four out of these five volatiles (H2O, CO2, F, and Cl) between nominally volatile-free minerals and melts, whereas the partitioning of sulfur is poorly known. To better constrain the behavior of sulfur in igneous systems we measured the partitioning of sulfur between clinopyroxene and silicate melts over a range of pressure, temperature, and melt composition from 0.8 to 1.2 GPa, 1000 to 1240 °C, and 49 to 66 wt% SiO2 (13 measurements). Additionally, we determined the crystal-melt partitioning of sulfur for plagioclase (6 measurements), orthopyroxene (2 measurements), amphibole (2 measurements), and olivine (1 measurement) in some of these same run products. Experiments were performed at high and low oxygen fugacities, where sulfur in the melt is expected to be dominantly present as an S6+ or an S2– species, respectively. When the partition coefficient is calculated as the total sulfur in the crystal divided by the total sulfur in the melt, the partition coefficient varies from 0.017 to 0.075 for clinopyroxene, from 0.036 to 0.229 for plagioclase, and is a maximum of 0.001 for olivine and of 0.003 for orthopyroxene. The variation in the total sulfur partition coefficient positively correlates with cation-oxygen bond lengths in the crystals; the measured partition coefficients increase in the order: olivine < orthopyroxene < clinopyroxene ≤ amphibole and plagioclase. At high oxygen fugacities in hydrous experiments, the clinopyroxene/melt partition coefficients for total sulfur are only approximately one-third of those measured in low oxygen fugacity, anhydrous experiments. However when the partition coefficient is calculated as total sulfur in the crystal divided by S2– in the melt, the clinopyroxene/melt partition coefficients for experiments with melts between ~51 and 66 wt% SiO2 can be described by a single mean value of 0.063 ± 0.010 (1σ standard deviation about the mean). These two observations support the hypothesis that sulfur, as S2–, replaces oxygen in the crystal structure. The results of hydrous experiments at low oxygen fugacity and anhydrous experiments at high oxygen fugacity suggest that oxygen fugacity has a greater effect on sulfur partitioning than water. Although the total sulfur clinopyroxene-melt partition coefficients are affected by the Mg/(Mg+Fe) ratio of the crystal, partition coefficients calculated using S2– in the melt display no clear dependence upon the Mg# of the clinopyroxene. Both the bulk and the S 2– partition coefficients appear unaffected by IVAl in the clinopyroxene structure. No effect of anorthite content nor of iron concentration in the crystal was seen in the data for plagioclase-melt partitioning. The data obtained for orthopyroxene and olivine were too few to establish any trends. The partition coefficients of total sulfur and S 2– between the crystals studied and silicate melts are typically lower than those of fluorine, higher than those of carbon, and similar to those of chlorine and hydrogen. These sulfur partition coefficients can be combined with analyses of volatiles in nominally volatile-free minerals and previously published partition coefficients of H2O, C, F, and Cl to constrain the concentration of the igneous quintet, the five major volatiles in magmatic systems.

Open Access

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Abstract: Understanding the long-term fate, stability, and bioavailability of uranium (U) in the environment is important for the management of nuclear legacy sites and radioactive wastes. Analysis of U behavior at natural analogue sites permits evaluation of U biogeochemistry under conditions more representative of long-term equilibrium. Here, we have used bulk geochemical and microbial community analysis of soils, coupled with X-ray absorption spectroscopy and μ-focus X-ray fluorescence mapping, to gain a mechanistic understanding of the fate of U transported into an organic-rich soil from a pitchblende vein at the UK Needle's Eye Natural Analogue site. U is highly enriched in the Needle's Eye soils (∼1600 mg kg−1). We show that this enrichment is largely controlled by U(VI) complexation with soil organic matter and not U(VI) bioreduction. Instead, organic-associated U(VI) seems to remain stable under microbially-mediated Fe(III)-reducing conditions. U(IV) (as non-crystalline U(IV)) was only observed at greater depths at the site (>25 cm); the soil here was comparatively mineral-rich, organic-poor, and sulfate-reducing/methanogenic. Furthermore, nanocrystalline UO2, an alternative product of U(VI) reduction in soils, was not observed at the site, and U did not appear to be associated with Fe-bearing minerals. Organic-rich soils appear to have the potential to impede U groundwater transport, irrespective of ambient redox conditions.

Open Access

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Abstract: Iron is an essential element, and cornflake-style cereals are typically fortified with iron to a level up to 14 mg iron per 100 g. Even single cornflakes exhibit magnetic behaviour. We extracted iron microparticles from samples of two own-brand supermarket cornflakes using a strong permanent magnet. Synchrotron iron K-edge X-ray absorption near-edge spectroscopic data were consistent with identification as metallic iron, and X-ray diffraction studies provided unequivocal identification of the extracted iron as body-centred cubic (BCC) α-iron. Magnetometry measurements were also consistent with ca. 14 mg per 100 g BCC iron. These findings emphasise that attention must be paid to the speciation of trace elements, in relation to their bioavailability. To mimic conditions in the stomach, we suspended the iron extract in dilute HCl (pH 1.0–2.0) at 310 K (body temperature) and found by ICP-MS that over a period of 5 hours, up to 13% of the iron dissolved. This implies that despite its metallic form in the cornflakes, the iron is potentially bioavailable for oxidation and absorption into the body.