Fatty acids produced by H
2
-metabolizing bacteria are sometimes observed to be more D-depleted than those of photoautotrophic organisms, a trait that has been suggested as diagnostic for ...chemoautotrophic bacteria. The biochemical reasons for such a depletion are not known, but are often assumed to involve the strong D-depletion of H
2
. Here, we cultivated the bacterium
Cupriavidus necator
H16 (formerly
Ralstonia eutropha
H16) under aerobic, H
2
-consuming, chemoautotrophic conditions and measured the isotopic compositions of its fatty acids. In parallel with the wild type, two mutants of this strain, each lacking one of two key hydrogenase enzymes, were also grown and measured. In all three strains, fractionations between fatty acids and water ranged from -173‰ to -235‰, and averaged -217‰, -196‰, and -226‰, respectively, for the wild type, SH
-
mutant, and MBH
-
mutant. There was a modest increase in δD as a result of loss of the soluble hydrogenase enzyme. Fractionation curves for all three strains were constructed by growing parallel cultures in waters with δD
water
values of approximately -25‰, 520‰, and 1100‰. These curves indicate that at least 90% of the hydrogen in fatty acids is derived from water, not H
2
. Published details of the biochemistry of the soluble and membrane-bound hydrogenases confirm that these enzymes transfer electrons rather than intact hydride (H
-
) ions, providing no direct mechanism to connect the isotopic composition of H
2
to that of lipids. Multiple lines of evidence thus agree that in this organism, and presumably others like it, environmental H
2
plays little or no direct role in controlling lipid δD values. The observed fractionations must instead result from isotope effects in the reduction of NAD(P)H by reductases with flavin prosthetic groups, which transfer two electrons and acquire H
+
(or D
+
) from solution. Parallels to NADPH reduction in photosynthesis may explain why D/H fractionations in
C. necator
are nearly identical to those in many photoautotrophic algae and bacteria. We conclude that strong D-depletion is not a diagnostic feature of chemoautotrophy.
Chemical durability of silicate glasses is among the most intriguing topics in the glass community, as multi-component silicate glasses form the basis for a wide range of modern functional glasses, ...and their behavior in the surrounding environment must be well-predicted for practical use. For instance, many industrial glasses, including nuclear waste glass and optical fibers, require inherently high chemical durability as they are designed to present in the environment long, while the applications of some other advanced glass materials and processing techniques, such as bioactive glass and glass etching, rely on the careful control of the glass dissolution behavior. Therefore, the underlying science that governs the dissolution mechanisms and kinetics of multicomponent silicate glasses in different environments, especially relating to the impact of common constituent oxides, must be well-understood to prompt the application and future development of advanced glassy materials.Despite a considerable effort to understand this topic, the existing studies often fall short in resolving actual technical challenges that are encountered during the design and processing (e.g., chemical etching) of modern functional silicate glasses, as the majority of them focus on simple glasses comprising only traditional glass components (e.g., Na2O, SiO2 and B2O3), or are tailored to a specific application. The specialty glasses, however, comprises various framework and non-framework components – especially high field-strength cations (HFSCs) – in addition to the traditional glass formers, with each component contributing its own part to the glass structure and resulting chemical durability. As a result, the current knowledge of dissolution mechanisms, which was acquired from glasses with low compositional complexity, cannot fully serve the broad spectrum of functional glasses. In this context, the present work aims to decipher the composition – structure – solubility relationship in aluminoborosilicate system, with particular focus on elucidating the effect of HFSCs on glass durability and mechanisms that lead to this effect. Accordingly, alkali/alkaline-earth aluminoborosilicate glasses containing different HFSCs, namely Nb5+, Zr4+, Ti4+, La3+ and V5+, have been selected as the subject of this research. Under extreme scenario (i.e., pH = 2 or 13), dissolution studies of aluminoborosilicate glasses indicate a dissolution – precipitation mechanisms, with Si or Ca precipitation governing the formation of passivating layer, whereas in neutral media, a partial hydrolysis – in-situ recondensation dissolution mechanism is observed, followed by conditional precipitation as the reaction proceeds. Combining the dissolution studies in different pH regions, it can be seen that glass dissolution behavior is not governed by one single equilibrium, but by a collection of the various physio-chemical interactions occurring simultaneously at the glass-water interface and may evolve with reaction progress. While the presence of HFSCs impacts the glass dissolution behavior, the actual effect is not solely determined by the structural role of HFSCs in the glass. The solubility of HFSCs in the targeted environment, as well as the potential interactions between HFSCs and other glass/solution components, also matter. It is thus important to consider every compositional/structural driver from the glass, as well as all the environment factors, in the modelling of glass dissolution. Future work recommended for this topic include: (i) In-depth investigation on the structure of chemically diverse HFSCs in aluminoborosilicate glass, as well as exploring their interactions with common glass components and dissolution media, to narrow the uncertainty in predicting the dissolution behavior of glasses containing these HFSCs, (ii) Correlate the glass dissolution kinetics with the variation of pH and solution chemistry through systematic investigation, and (iii) utilize in-situ/high resolution characterization and modelling techniques to investigate the fundamental science that governs glass dissolution.
The ocean is a dynamic environment for methane’s biogeochemical processes. Three major processes take center stage: biological methanogenesis, anaerobic methane oxidation, and aerobic methane ...oxidation. These three key methane biogeochemical processes are intricately balanced within the oceanic environment, resulting in minimal methane escaping into the atmosphere. This dissertation aims to integrate fieldwork with laboratory research, combining investigative tools such as chemical measurements, radiotracer incubations, anaerobic cultivation, and stable isotope probing. The objective is to probe the behaviors of aerobic methane oxidation in relation to the ocean’s oxygen availability and isotopic signatures during methanogenesis from methyl-based compounds.Chapters 1 and 2 of my dissertation delve into the effectiveness and efficiency of aerobic methane oxidation in the ocean’s water column. The Santa Barbara Basin (SBB) is known for experiencing seasonal deoxygenation and reoxygenation cycles, resulting in fluctuating methane concentrations in the deep water column. In chapter 1, my study comprehensively investigated this seasonal cycle through a nine-month period of repeated sampling and measurement of key parameters associated with methane biogeochemistry in the deep waters of the SBB. These parameters include oxygen, nitrate, methane concentrations, and the rate of aerobic methane oxidation. My findings revealed a sequential pattern. First, a decline in oxygen concentration was observed to precede a decrease in nitrate concentration. Second, the accumulation of methane followed, with a marked decline in both oxygen and nitrate levels. Finally, changes in the methane oxidation rate, which reflects the activity of the methanotroph community, occurred subsequently, albeit with a slight time lag. I also discovered that the rate of methane oxidation is primarily dependent on the availability of methane within the water column. Furthermore, my research uncovered that the transient methane pulse accompanying the observed oxygen depletion in the SBB triggered the development of a persistent methanotrophic community, even after methane concentrations had returned to normal levels – an ecological memory effect. In Chapter 2, beyond the analysis of observed trends, I utilized methane concentrations, methane oxidation rates, and vertical methane diffusion to calculate the minimum methane source required for the deep water column. This computation was conducted across a range of contrasting environmental conditions, ultimately revealing that anoxic conditions demand a greater influx of methane into the water column. A comparison to data collected during a 2023 oceanic research expedition revealed an even greater demand for methane input, in the presence of well-established and persistent anoxic conditions. Chapter 3 of my dissertation is dedicated to investigating mechanistic underpinnings of stable carbon isotope fractionations during methylotrophic methanogenesis by marine archaea. Understanding these mechanistic underpinnings enables the inclusion of methylotrophic methanogenesis in isotopically informed biogeochemical reaction networks for anaerobic environments. This study included wild type methylotrophic methanogens as well as a mutant strain in which the reduction of methylated substrates is coupled to hydrogen oxidation or acetate oxidation, providing further mechanistic insight of isotopic variations originating at the reaction branch point.
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•Discarded bamboo chips were used as the raw material.•PAB is prepared by mid-high temperature activation and low-temperature air oxidation.•Air oxidation markedly increases ...oxygen-containing functional groups on the surface.•Air oxidation leads to a larger mesoporous volume and higher mesoporous ratio.•Oxidized porous activated biochar shows dramatic increases in specific capacitance.
Activated biochar prepared by a single pyrolysis process often has poor pore structure and insufficient surface oxygen-containing groups, which severely inhibit its application in supercapacitors. Herein, we synthesized bamboo-based porous activated biochar (PAB) via coordinated regulation including carbonization at 450 °C, ZnCl2 activation at 400 ∼ 800 °C, and air oxidation at 200 ∼ 350 °C. The results showed that the coordinated regulation could efficiently improve the physicochemical structure and electrochemical properties of PAB. Air oxidation exhibited an obvious improving effect on the activated carbon prepared with mid-temperature activation (optimal at 600 °C), which was strengthened with increasing oxidation temperature from 200 °C to 350 °C. Air oxidation following 600 °C activation could improve the mesoporous rate by etching the pore, and simultaneously introduce more oxygen-containing groups such as carbonyl (C=O) and carboxyl (–COOH) groups, which could enhance the wettability of PAB. The optimal synergistic temperature of ZnCl2 activation and air oxidation was 600 °C and 350 °C (PAB-600-350), respectively. PAB-600-350 is a wonderful supercapacitor electrode material with a higher surface oxygen content (20.74%) and a superior mesoporous rate (35%). At 1 A/g, PAB-600-350 showed the highest capacitance of 256 F/g, which was 2-fold that of PAB-600 (128 F/g). PAB-600-350 capacitors also supplied an excellent energy density of 12.54 Wh/kg at the power density of 225 W/kg in 1 M Na2SO4 electrolyte. Furthermore, a density functional theory (DFT) analysis was performed to investigate the interaction between PAB and electrolyte ions. The results showed that oxygen-containing functional groups on the surface can increase the dipole moment of the material and enhance the adsorption of PAB for electrolyte ions (K+).
Achieving thinner and higher performance display/substrate glasses and transparent glass-ceramics with tunable properties requires a precise control of acid-etching process, thus necessitating a ...comprehensive understanding of glass composition–structure–dissolution behavior relationships in acidic medium. Unfortunately, the literature on this subject has been focused only on a narrow set of glass chemistries. Therefore, consensus on the mechanisms that govern the acidic dissolution of multicomponent silicate glasses over a broad compositional space is still lacking. The present work employs a suite of state-of-the-art spectroscopic techniques, including 1D and 2D NMR, TEM-EELS, ICP-OES, and XPS, to provide an insight into the mechanism and kinetics of corrosion of alkali/alkaline-earth aluminoborosilicate glasses (comprising high field strength cations – HFSCs, i.e., La3+, Ti4+, Zr4+ and Nb5+) in acidic media (HCl; pH = 2). Incorporating the HFSCs into the glasses induces significant structural changes in their network, thus, impacting the forward rate dissolution kinetics. Based on the results, we hypothesize that the glasses dissolve at pH = 2 through an ‘interfacial dissolution – re-precipitation mechanism (IDPM)’ and ‘in-situ recondensation’ coupled pattern, wherein the IDPM results in a Si-rich alteration layer, followed by local recondensation occurring due to limited kinetics near the interfacial solution between the uncorroded glass surface and the outer alteration layer.
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Abstract One major factor impeding the design of nuclear waste glasses with enhanced waste loadings is our insufficient understanding of their composition–structure–durability relationships, ...specifically in the environments the waste form is expected to encounter in a geological repository. In particular, the high field‐strength cations (HFSCs) are an integral component of most waste streams. However, their impact on the long‐term performance of the glassy waste form remains mostly undeciphered. In this context, the present study aims to understand the impact of some HFSCs (i.e., Nb 5+ , Zr 4+ , Ti 4+ , and La 3+ ) on the dissolution behavior of alkali/alkaline‐earth aluminoborosilicate‐based model nuclear waste glasses in hyper‐alkaline media. At pH = 13, the studied glasses dissolve through the dissolution–reprecipitation mechanism, with Ca precipitation being the most vital step to passivation. In Ca‐free glasses, although the HFSCs slow down the forward rate, they do not seem to impact the residual rate behavior of glasses. The presence of Ca 2+ , however, initiates the rapid precipitation of network polymerizing HFSCs (i.e., Nb 5+ , Zr 4+ , and Ti 4+ ) into a Ca 2+ /HFSCs‐based passivating layer, thus suggesting a synergy between Ca 2+ and HFSCs that contributes to the enhanced long‐term durability of the glasses. Such synergy is not strongly evident for La 3+ , but instead, a potential La/Si affinity is observed upon the formation of the alteration layer.
Most of the methane input to the world’s oceans is intercepted by microorganisms in sediment and the overlying water column and oxidized before it has an opportunity to reach the atmosphere, where it ...acts as a greenhouse gas. The factors controlling methane consumption in the ocean are not well established and its biogeochemistry in dynamic marine environments is understudied in-part because of challenges in capturing spatial and temporal variability. Our study focused on the factors that structure methane’s biogeochemistry in a dynamic marine environment, the Santa Barbara Basin. The deep-water column of the Santa Barbara Basin experiences seasonal oxygen loss and episodic replenishment which we found to be major factors in structuring the accumulation of methane and the rate at which microorganisms consumed that methane. We found the gradual decline in oxygen that commonly occurs through the summer culminated with a pronounced accumulation of methane in the water column during the fall. Rates of methane oxidation remained low in summer, increased with the buildup of methane in fall, and remained elevated into spring, even after methane concentration had declined. However, results from methane oxidation kinetics experiments revealed a zero-order kinetic dependence on oxygen concentration, indicating that oxygen’s effect on methanotrophy at the ecosystem scale is likely indirect. We also captured an apparent mixing event during fall that drove spatial and temporal variability in oxygen, nitrate and methane concentrations in the Santa Barbara Basin, with stark variations at the investigated timescale of 8 days and along isobaths at a spatial scale of 7 km. Collectively, these results indicate the seasonal development and attenuation of a methanotrophic community associated with restricted circulation, but also of a spatiotemporal variability not previously appreciated for this environment.
Graphite/SiC composites with excellent tribological properties and chemical stability are ideal bearing materials for use in water-lubricated conditions. The composites can be fabricated rapidly by ...reactive melt infiltration (RMI) with near-net advantages. However, maintaining an adequate carbon content in the matrix after the violent reaction between carbon and liquid silicon is the critical requirement for wear rate and more stable friction coefficient. In this study, a carbon-composite-powder (PFC@G) constructed of graphite particles and glassy carbon derived from phenolic resin, was used as the main carbon source with a larger size (∼200 μm) to improve the carbon content of the graphite/SiC composites. The tribological properties of the composites were measured by a block-on-ring tribometer under water-lubricated conditions. The weight ratio of phenolic resin to graphite particles was adjusted to manipulate the microstructure of PFC@G, which can affect the carbon content of the composites in the course of RMI. The composites with high carbon content (34.47 vol%) were successfully fabricated when the weight ratio is 0.8, which exhibited the most outstanding tribological properties with low friction coefficient (∼0.014) and wear rate (∼10−6 mm3 N−1m−1) at a load of 15 N.
The overarching goal of the present multiscale investigation is to unearth the kinetics and mechanisms of corrosion of borosilicate glasses in hyper-alkaline (pH = 13) environments as a function of ...their chemical composition. Accordingly, a series of 3- to 6-component borosilicate glasses have been designed starting from Na2O–B2O3–SiO2 ternary, wherein the compositional complexity has been added in a systematic tiered approach, finally resulting in the composition of the well-known international simple glass (ISG). Tetramethylammonium hydroxide (TMAH), one of the most widely used alkaline etchant in the glass and electronics industry, has been used as the corrosion media. A series of state-of-the-art characterization techniques including magic angle spinning nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, inductively coupled plasma optical emission spectroscopy, elastic recoil detection analysis, and high-resolution transmission electron microscopy have been employed to unearth the compositional dependence of glass corrosion in hyper-alkaline environments. The glass compositions underwent congruent corrosion in the forward rate regime, whereas the controlling mechanism of corrosion in the residual rate regime depends on the presence/absence of Ca in the surrounding environment and can be explained on the basis of the dissolution–reprecipitation model. The dependence of corrosion kinetics and the chemistry of alteration products (in the residual rate regime) on the glass composition have been discussed. The results presented in this contribution will ultimately supplement the scientific literature attempting to understand the fundamental science governing the aqueous corrosion of silicate-based glass chemistries and add to the growing database required to develop nonempirical predictive models for designing glasses with controlled dissolution rates.
![Minimal Influence of [NiFe]...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/1664-302X/small "Minimal Influence of [NiFe] Hydrogenase on Hydrogen Isotope Fractionation in H2-Oxidizing Cupriavidus necator")
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