Oxygen Isotope Exchange Research Articles

Time-Dependence of Surface Composition, Transport Properties Degradation, and Thermodynamic Consideration of La0.6Sr0.4Co0.2Fe0.8O3-δ under Chromium Poisoning

In order to investigate the long-term degradation of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathodes due to chromium (Cr) poisoning, the transport properties were investigated by two different approaches: one is electrochemical impedance spectroscopy (EIS) on porous LSCF electrodes, the other is the oxygen isotope exchange and SIMS analyses on LSCF pellets. The oxygen surface exchange coefficient (k*) determined from EIS decreased with increasing poisoning time. For LSCF pellets, the k* values decreased linearly with poisoning time (the first stage) and then became nearly constant (the second stage). The first degradation of k* was attributed to the formation of SrCrO4 and CoFe2O4 within the initial 7∼8 h poisoning time, whereas in the second stage, LaCrO3 formation was detected on the surface of LSCF. Those chemical behaviors were compared with thermodynamic calculations where the k* was observed to decrease in the first stage corresponding to the precipitation of SrCrO4 and CoFe2O4, and LaCrO3 is formed in the second stage. Those considerations lead to the conclusions that the k* decrease is dominated by the decrease in available sites such as Sr, Co or oxygen vacancies on the LSCF surface, whereas the constant k* values may be attributed to the remaining/formed LaCrO3 component of the reacted LSCF.

Oxygen diffusion and surface exchange kinetics for the mixed-conducting oxide La0.6Sr0.4Co0.8Fe0.2O3-δ

Studies of oxygen surface exchange kinetics for La0.6Sr0.4Co0.8Fe0.2O3-δ oxide were performed using the technique of isotopic exchange of molecular oxygen with analysis of gas phase isotopic composition in a static circulation system at the temperatures of 600-800 °С in the oxygen pressure range of 0.27-2.13 kPa. The values of interphase exchange rate and oxygen diffusion coefficient were determined. The effective activation energies for oxygen exchange and diffusion processes as well as the exponents in the dependence of these values versus oxygen pressure in the double logarithmic coordinates were calculated. The process of oxygen dissociative adsorption at the surface of La0.6Sr0.4Co0.8Fe0.2O3-δ oxide was found to be the rate-determining stage.

Short-term impacts of forest clear-cut on P accessibility in soil microaggregates: An oxygen isotope study

Forest clear-cuts may have severe effects on the soil structure and related nutrient cycling, though with yet unknown consequences for nutrient pools such as phosphorus (P) within microaggregates. We sampled the bulk mineral topsoil prior to clear cut as well as 1 and 2years thereafter from the experimental forest site Wüstebach, Germany, and we assessed the degree of oxygen isotope exchange in HCl-extractable soil phosphate of two microaggregate size fractions (<20μm, 20–250μm) after incubating soil with 18O-labeled water. We found that after the clear-cut, microaggregate phosphates exchanged significantly more oxygen with the incubation water than before clear cut. One and two years after clear cut, the respective δ18O values of soil phosphates (δ18OP,HCl) were elevated by 16 and 38% (<20μm) and by 43 and 53% (20–250μm) than before the clear-cut, respectively, indicating that additional microaggregate P had been made available to biological P cycling. The degree of oxygen exchange after the clear-cut was significantly greater in larger soil microaggregates than in the smaller sized ones, reflecting that also at microaggregate level size controlled the increase in the bioavailability of P with changes in land management.

Degradation kinetics of LSM–YSZ cathode materials for SOFC

Solid oxide fuel cells are perspective electrochemical devices for hydrogen energy applications and require long time performance. The paper studies the influence of the processes of particles coarsening and cation interdiffusion on the degradation kinetics of the LSM–YSZ cathode material catalytic activity as related to the interaction processes with oxygen from the gas phase. The exchange kinetics with the gas phase oxygen was studied by the oxygen isotope exchange method with the gas phase equilibration at the temperature of 850 °С and Po2 = 10−2 atm. The samples were exposed for 1000 h. The coefficient of gas phase and surface oxygen exchange was found to decrease at the exposure. The paper discusses the physical reasons, which lead to the oxygen surface exchange coefficient decrease in LSM–YSZ electrodes and degradation of the polarization conductivity measured by electrochemical impedance spectroscopy.

Ceria: Recent Results on Dopant-Induced Surface Phenomena

Redox studies on dense zirconia-doped ceria pellets were carried out by thermogravimetric investigations and dilatometry. Up to 1600 K reduction parameters determined by both methods correspond to each other. At higher temperatures, however, thermogravimetry overestimates the degree of reduction since mass loss is not only due to oxygen exsolution but also to selective evaporation of CeO2 whose vapour pressure is considerably higher than that of ZrO2. As a consequence surface segregation of zirconia occurs in (Ce,Zr)O2−δ pellets leading to a porous surface zone of Ce2Zr2O7 pyrochlore which gradually grows in thickness. Surface enrichment of zirconia is detrimental for splitting CO2 or H2O since re-oxidation temperatures of (Ce,Zr)O2−δ are known to be shifted towards lower temperatures with increasing ZrO2 content. Thus, very harsh reduction conditions should be avoided for the (Ce,Zr)O2−δ redox system. The kinetics investigations comprised the high temperature reduction step (T ≅ 1600 K) and the “low” temperature oxidation reaction with a carbon dioxide atmosphere (T ≅ 1000 K). The reduction kinetics (at around 1600 K and an oxygen activity of 7 × 10−4 in the gas phase) directly yield the (reduction) equilibrium exchange rate of oxygen in the order of 10−7 mol·O/(cm3·s) as the kinetics are surface controlled. The oxidation step at around 1000 K, however, occurs in the mixed control or in the diffusion control regime, respectively. From oxygen isotope exchange in combination with SIMS depth profiling oxygen exchange coefficients, K, and oxygen diffusivities, D, were determined for so-called equilibrium experiments as well as for non-equilibrium measurements. From the obtained values for K and D the (oxidation) equilibrium exchange rates for differently doped ceria samples were determined. Their dependency on the oxygen activity and the nature and the concentrations of a tetravalent dopant (Zr) and trivalent dopants (La, Y, Sm) could be semi-quantitatively rationalised on the basis of a master equation for the equilibrium surface exchange rate.

The effect of prior hydrothermal alteration on the melting behaviour during rhyolite formation in Yellowstone, and its importance in the generation of low-δ18O magmas

Constraining the contribution of crustal lithologies to silicic magmas has important implications for understanding the dynamics of these potentially highly explosive systems. Low-δ18O rhyolite lavas erupted after caldera-forming events in Yellowstone have been interpreted as the products of bulk crustal melting of previously deposited and hydrothermally altered rhyolitic material in the down-dropped caldera roof. For lack of compositional data, the “self-cannibalisation bulk melting”-theory relies on the assumption that hydrothermally altered materials are near-cotectic and hydrous (>3 wt% H2O) and will therefore readily melt at temperatures below 850 °C. In this study, we examine the drillcores Y2, Y9 and Y13 from a USGS drilling campaign in Yellowstone in order to characterise the hydrothermally altered material in terms of major and trace elements, oxygen isotopes and water contents. Rhyolite δ18O values can decrease from “normal” (+5.8 to +6.1‰) on the surface to as low as −5‰ at depths of 100–160 m and probably lower as a function of increasing temperature with depth. While material in the drillcores is variably altered and silicified, oxygen isotope exchange in these samples is not accompanied by systematic changes in major and trace element composition and is independent of uptake of water. More than 75% of the drillcore samples have <0.5 wt% H2O, making water the most limiting factor during melting. Modelled melting curves using rhyolite-MELTS suggest a maximum of 35% melt can be created at 850 °C, and that bulk melting would require extremely high temperatures >1100 °C. Therefore, large-scale bulk melting is unrealistic and low-δ18O rhyolite magmas more likely result from assimilation of <30% partially melted altered crust with low δ18O into a normal-δ18O rhyolite magma from the main reservoir. This mechanism is supported by isotopic mass-balance models as well as thermal and volumetric constraints, and may be similarly applicable to other low-δ18O settings worldwide.

Effect of Mg and Al on manganese oxides as catalysts for VOC oxidation

Manganese mixed oxides with the compositions Mn-Mg-Al-O and Mn-O were synthesized by the self-combustion method. The effects of Mg and Al incorporation on the structural, textural, redox and catalytic properties of the manganese oxides were evaluated. The catalysts were characterized by X-ray diffraction (XRD), N2 adsorption, temperature-programed desorption of oxygen (O2-TPD), temperature-programmed reduction (H2-TPR), oxygen storage capacity (OSC), 18O/16O oxygen isotope exchange, and their catalytic performance was evaluated in the oxidation of toluene, ethyl acetate and 2-propanol. The analyses showed that the use of a glycine/nitrate ratio equal to 0.56 in the self-combustion favors the formation of the Mn3O4 phase and the incorporation of Mg and Al into the structure stabilizes this phase after a thermal process. In addition, the surface area increases, the particle size decreases, and the presence of electrophilic oxygens is favored, which increases the adsorption sites, exposes the active centers and facilitates the occurrence of redox cycles, leading to a better catalytic performance in the total conversion of the oxygenated compounds on the Mn-Mg-Al-O mixed oxide. In the oxide that only contains manganese, the presence of nucleophilic oxygens with great mobility was observed, leading to its good activity in the oxidation of the VOCs.The catalytic tests showed that a high oxygen exchange capacity leads to lower temperatures of conversion of toluene, which suggests that, for the oxidation of aromatic compounds, the lattice oxygen plays a fundamental role in the catalytic activity. In contrast, in the oxidation of oxygenated compounds that require lower temperatures of conversion, the existence of surface oxygens such as O2− and O− favors the catalytic process and therefore ensures superior activity in the transformation of ethyl acetate and 2-propanol on the Mn-Mg-Al-O mixed oxide.

Decoding fjord water contribution and geochemical processes in the Aysen thermal springs (Southern Patagonia, Chile)

The Aysen region, located in Southern Patagonia, Chile (43°38′S to 49°16′S) has optimum conditions for the formation of geothermal systems: Magmatic processes, abundant rainfall and active faults systems. In fact, several thermal springs emerge in coastal and inland areas of the Aysen region. Thermal springs in the coastal areas are spatially related to the Liquiñe-Ofqui Fault System (LOFS), a major strike-slip fault system that runs along the southern segment of the Southern Volcanic Zone, and could be controlling groundwater circulation. Despite the existence of literature regarding the composition of thermal springs, they do not elucidate the origin of thermal waters, the source of their chemical components nor have their hydrogeochemical processes been investigated. This knowledge will provide a useful tool for exploration of geothermal resources in the Aysen region.Sixteen thermal waters, three fjord waters, and three meteoric water samples were collected and analysed for major-minor ions, some trace elements (B, Li) and stable isotopes of δ2H and δ18O. Classic geochemical tools (i.e.: ratio of elements, ion vs chloride content), Hierarchical Cluster Analysis (HCA) and Factorial Analysis (FA) suggest there are three water groups (G1, G2 and G3). G1 and G3 are NaCl type coastal thermal springs with electrical conductivity (EC) above 1000μS/cm and mostly neutral pH (6.4 to 8.4). G1 has the highest average concentrations of Cl−, SO42−, Na+, Ca2+ while G3 has the highest values T°, SiO2, HCO3−, Li, B. The G2 samples are from Aysen inland areas, have EC below 1000μS/cm, slightly alkaline pH (7.9 to 9.6) and are Na-Cl-HCO3 type with higher average values of T°, SiO2, HCO3− and Li than G1.Factorial analysis indicates that two factors explain 82.1% of the total dataset variance. Factor 1 is formed by Cl−, SO42−, Na+, Ca2+, Li and B, which we associated with fjord water mixing processes. The factor 2 formed by T°, SiO2, HCO3−, Li and B might be interpreted as: i) silicate weathering from the North Patagonian Batholith and ii) volatile elements transported through high temperature vapours from a volcanic degassing source. These two components which could interact and overlap are address as “magmatic-hydrothermal” in this work. The factorial scores distribution is consistent with the HCA suggesting that fjord water mixing is the dominant geochemical process for G1 samples while G2 samples are influenced by magmatic-hydrothermal fluids. The G3 thermal waters show both processes with a greater influence of magmatic-hydrothermal fluids. The calculated saline factor (3.4% to 42.3%) supports fjord water mixing processes as one of the dominant processes affecting the coastal thermal spring's composition.The stable isotopic data (δ2H and δ18O) fall in the local meteoric water line (LMWL), suggesting current recharge and limited oxygen isotopic exchange during rock-dissolution processes. This could indicate short residence times and/or abundant contribution of actual meteoric water at shallow depths.

Dislocations Accelerate Oxygen Ion Diffusion in La0.8Sr0.2MnO3 Epitaxial Thin Films.

Revealing whether dislocations accelerate oxygen ion transport is important for providing abilities in tuning the ionic conductivity of ceramic materials. In this study, we report how dislocations affect oxygen ion diffusion in Sr-doped LaMnO3 (LSM), a model perovskite oxide that serves in energy conversion technologies. LSM epitaxial thin films with thicknesses ranging from 10 nm to more than 100 nm were prepared by pulsed laser deposition on single-crystal LaAlO3 and SrTiO3 substrates. The lattice mismatch between the film and substrates induces compressive or tensile in-plane strain in the LSM layers. This lattice strain is partially reduced by dislocations, especially in the LSM films on LaAlO3. Oxygen isotope exchange measured by secondary ion mass spectrometry revealed the existence of at least two very different diffusion coefficients in the LSM films on LaAlO3. The diffusion profiles can be quantitatively explained by the existence of fast oxygen ion diffusion along threading dislocations that is faster by up to 3 orders of magnitude compared to that in LSM bulk.

Modeling of biotic and abiotic processes affecting phosphate oxygen isotope ratios in a mineral-water-biota system

Abiotic and biotic reactions operate side by side in the cycling of phosphorus (P) in the environment, but the relative roles of these two reactions vary both spatially and temporally. In biotic reactions, the uptake and release of P are catalyzed by enzymes and thus change phosphate oxygen isotope ratios, while in abiotic reactions, the absence of hydrolysis-condensation reactions results in no apparent changes in isotope composition, except short-term kinetic isotope effect due solely to preferential ion exchange. Therefore, isotope method could be a promising tool to differentiate relative roles of these two reactions in the environment but the relationship of the dynamic concentration and isotope exchange at the biota-water interface is largely unknown. In this study, we aimed to develop a process-based isotope model underpinning the competition of abiotic (sorption, desorption, and ion exchange) and biotic (uptake, metabolism, and release) reactions during uptake and recycling of ferrihydrite-bound P by E. coli. Our model comprises equations describing the partitioning relationship among different P pools and their corresponding oxygen isotope compositions and is based exclusively on oxygen isotope exchange at multiple sites including mineral surface, aqueous phase, and bacterial cells. The process-based model adequately reproduced the measured concentration and isotope compositions over time. Furthermore, parametric and sensitivity analyses using the model indicated that the rate of biological uptake of P was the major factor controlling the changes of phosphate isotope composition. In conclusion, our model provides new insights into a mechanistic aspect of isotope exchange and could be potentially useful for future efforts to understand the interplay of biotic and abiotic factors on phosphorus cycling in natural environments.

Post-mortem oxygen isotope exchange within cultured diatom silica.

RationalePotential post‐mortem alteration to the oxygen isotope composition of biogenic silica is critical to the validity of palaeoclimate reconstructions based on oxygen isotope ratios (δ18O values) from sedimentary silica. We calculate the degree of oxygen isotope alteration within freshly cultured diatom biogenic silica in response to heating and storing in the laboratory.MethodsThe experiments used freshly cultured diatom silica. Silica samples were either stored in water or dried at temperatures between 20 °C and 80 °C. The mass of affected oxygen and the associated silica‐water isotope fractionation during alteration were calculated by conducting parallel experiments using endmember waters with δ18O values of −6.3 to −5.9 ‰ and −36.3 to −35.0 ‰. Dehydroxylation and subsequent oxygen liberation were achieved by stepwise fluorination with BrF5. The 18O/16O ratios were measured using a ThermoFinnigan MAT 253 isotope ratio mass spectrometer.ResultsSignificant alterations in silica δ18O values were observed, most notably an increase in the δ18O values following drying at 40–80 °C. Storage in water for 7 days between 20 and 80 °C also led to significant alteration in δ18O values. Mass balance calculations suggest that the amount of affected oxygen is positively correlated with temperature. The estimated oxygen isotope fractionation during alteration is an inverse function of temperature, consistent with the extrapolation of models for high‐temperature silica‐water oxygen isotope fractionation.ConclusionsRoutinely used preparatory methods may impart significant alterations to the δ18O values of biogenic silica, particularly when dealing with modern cultured or field‐collected material. The significance of such processes within natural aquatic environments is uncertain; however, there is potential that similar processes also affect sedimentary diatoms, with implications for the interpretation of biogenic silica‐hosted δ18O palaeoclimate records.

Investigation of the ozone formation reaction pathway: Comparisons of full configuration interaction quantum Monte Carlo and fixed-node diffusion Monte Carlo with contracted and uncontracted MRCI.

The association/dissociation reaction path for ozone (O2 + O ↔ O3) is notoriously difficult to describe accurately using ab initio electronic structure theory, due to the importance of both strong and dynamic electron correlations. Experimentally, spectroscopic studies of the highest lying recorded vibrational states combined with the observed negative temperature dependence of the kinetics of oxygen isotope exchange reactions confirm that the reaction is barrierless, consistent with the latest potential energy surfaces. Previously reported potentials based on Davidson-corrected internally contracted multireference configuration interaction (MRCI) suffer from a spurious reef feature in the entrance channel even when extrapolated towards the complete basis set limit. Here, we report an analysis of comparisons between a variety of electronic structure methods including internally contracted and uncontracted MRCI (with and without Davidson corrections), as well as full configuration interaction quantum Monte Carlo, fixed-node diffusion Monte Carlo, and density matrix renormalization group.

Correlation of Structure and Surface Exchange Kinetics on Bismuth-Based Oxides

High temperature cubic phase bismuth oxide has the highest ionic conductivity because of the highly defective fluorite structure, with near unity ionic transference number. However, bismuth oxide has multiple crystallographic polymorphs at different temperature domains and the conductivity of bismuth oxide can vary by orders of magnitude depending on the phase. Although cubic phase bismuth oxide can be stabilized by doping, doped bismuth oxides, such as erbium stabilized bismuth oxide (ESB), undergo an anion ordering transition near 600 °C, leading to a decrease in ionic conductivity. The phase transition and anion ordering in stabilized bismuth oxides provides a unique opportunity to directly correlate crystal structure, conductivity and surface exchange properties on ionically conducting oxides. Here, gas phase isotopic oxygen exchange was performed on Bi2O3 and ESB as a function of exchange time at different temperatures, to determine the structure effect on surface exchange kinetics. Our results suggest that bismuth-based oxides have intrinsic catalytic activity toward oxygen exchange and the doping of erbium affects the surface exchange rate. The relationship between diffusion coefficient (D) and surface exchange coefficient (k) on bismuth-based oxides is explored and possible rate-limiting steps are proposed.

Zhamanshin astrobleme provides evidence for carbonaceous chondrite and post-impact exchange between ejecta and Earth\u2019s atmosphere

Chemical fingerprints of impacts are usually compromised by extreme conditions in the impact plume, and the contribution of projectile matter to impactites does not often exceed a fraction of per cent. Here we use chromium and oxygen isotopes to identify the impactor and impact-plume processes for Zhamanshin astrobleme, Kazakhstan. ε54Cr values up to 1.54 in irghizites, part of the fallback ejecta, represent the 54Cr-rich extremity of the Solar System range and suggest a CI-like chondrite impactor. Δ17O values as low as −0.22‰ in irghizites, however, are incompatible with a CI-like impactor. We suggest that the observed 17O depletion in irghizites relative to the terrestrial range is caused by partial isotope exchange with atmospheric oxygen (Δ17O = −0.47‰) following material ejection. In contrast, combined Δ17O–ε54Cr data for central European tektites (distal ejecta) fall into the terrestrial range and neither impactor fingerprint nor oxygen isotope exchange with the atmosphere are indicated.

Oxygen isotope exchange between the gas-phase and the electrochemical cell O2, Pt | YSZ | Pt, O2 under conditions of applied potential difference

The kinetics of oxygen isotope exchange between gas-phase oxygen and the electrochemical cell O2, Pt | ZrO2 + 10 mol % Y2O3 (YSZ) | Pt, O2 with applied potential difference (ΔU = ±1.2 V) is studied in the temperature range of 600–800°С and the oxygen pressure interval of 3–13 kPa. An original design of a vacuum electrochemical cell with the separated gas space is put forward for studying how the potential difference on the electrochemical cell influences the kinetics of interaction of gas-phase oxygen with the gas electrode O2, Pt | YSZ in the electrochemical cell. It is shown that the oxygen interphase exchange rate is the higher the more negative the charge on the electrode studied; moreover, the mechanism of gas-phase oxygen exchange with the gas electrode O2, Pt | YSZ in the electrochemical cell depends fundamentally on the electrode charge sign. The possible reasons for the revealed differences are discussed; the corresponding models are proposed.

Influence of exchangeable oxygen on biogenic silica oxygen isotope data

Application of biogenic opal oxygen isotope ratios from fossil and subfossil diatoms to paleoceanographic problems has been hampered by analytical and calibration issues in concert with a lack of experimental data to test fundamental assumptions about sample processing techniques. Here we present experiments where we react purified sediment trap and sediment core diatom samples to waters of different oxygen isotopic composition to quantify oxygen isotope exchange between laboratory processing solutions and δ18Odiatom values. We generate δ18Odiatom data using a microfluorination technique, and present FTIR data for samples both before and after vacuum dehydroxylation in order to investigate the mineralogic behavior of biogenic silica after dehydroxylation occurs. Our data demonstrate that exposure of diatoms to different δ18Oequil. water solutions during sample preparation alters final δ18Odiatom values and this alteration occurs during sample dehydroxylation prior to IRMS analysis. In addition, we present data that show structural diatom hydroxyl is persistent in sediment core samples, but the degree of equilibration with surrounding water decreases with sample age and/or core depth increase. Based on FTIR data, we propose that OH– loss from biogenic silica occurs post depositionally and may occur after heating in the laboratory, producing molecular scale reorganization of the silica tetrahedra within the biogenic opaline structure. Finally, we provide preliminary estimates of the time necessary for complete dehydroxylation of diatom silica, which would result in fully mature biogenic opal. These data suggest that high latitude marine sediments are slow to mature, and that the rate of exchangeable oxygen reduction varies exponentially at the locations we examined. Maturation time estimates require millions of years, and likely vary depending on core location.

Fluid Inclusions and Hydrogen, Oxygen, and Lead Isotopes of the Antuoling Molybdenum Deposit, Northern Taihang Mountains, China

AbstractThe Antuoling Mo deposit is a major porphyry‐type deposit in the polymetallic metallogenic belt of the northern Taihang Mountains, China. The processes of mineralization in this deposit can be divided into three stages: an early quartz–pyrite stage, a middle quartz–polymetallic sulfide stage, and a late quartz–carbonate stage. Four types of primary fluid inclusions are found in the deposit: two‐phase aqueous inclusions, daughter‐mineral‐bearing multiphase inclusions, CO2–H2O inclusions, and pure CO2 inclusions. From the early to the late ore‐forming stages, the homogenization temperatures of the fluid inclusions are 300 to &gt;500°C, 270–425°C, and 195–330°C, respectively, with salinities of up to 50.2 wt%, 5.3–47.3 wt%, and 2.2–10.4 wt% NaCl equivalent, revealing that the ore‐forming fluids changed from high temperature and high salinity to lower temperature and lower salinity. Moreover, based on the laser Raman spectra, the compositions of the fluid inclusions evolved from the NaCl–CO2–H2O to the NaCl–H2O system. The δ18OH2O and δD values of quartz in the deposit range from +3.9‰ to +7.0‰ and −117.5‰ to −134.2‰, respectively, reflecting the δD of local meteoric water after oxygen isotopic exchange with host rocks. The Pb isotope values of the sulfides (208Pb/204Pb, 36.320–37.428; 207Pb/204Pb, 15.210–15.495; 206Pb/204Pb, 16.366–17.822) indicate that the ore‐forming materials originated from a mixed upper mantle–lower crust source.

Oxygen isotope fractionation in the CaCO3-DIC-H2O system

The oxygen isotope ratio (δ18O) of inorganic and biogenic carbonates is widely used to reconstruct past environments. However, the oxygen isotope exchange between CaCO3 and H2O rarely reaches equilibrium and kinetic isotope effects (KIE) commonly complicate paleoclimate reconstructions. We present a comprehensive model of kinetic and equilibrium oxygen isotope fractionation between CaCO3 and water (αc/w) that accounts for fractionation between both (a) CaCO3 and the CO32− pool (αc/CO32-), and (b) CO32− and water (αCO32-/w), as a function of temperature, pH, salinity, calcite saturation state (Ω), the residence time of the dissolved inorganic carbon (DIC) in solution, and the activity of the enzyme carbonic anhydrase. The model results suggest that: (1) The equilibrium αc/w is only approached in solutions with low Ω (i.e. close to 1) and low ionic strength such as in the cave system of Devils Hole, Nevada. (2) The sensitivity of αc/w to the solution pH and/or the mineral growth rate depends on the level of isotopic equilibration between the CO32− pool and water. When the CO32− pool approaches isotopic equilibrium with water, small negative pH and/or growth rate effects on αc/w of about 1–2‰ occur where these parameters covary with Ω. In contrast, isotopic disequilibrium between CO32− and water leads to strong (>2‰) positive or negative pH and growth rate effects on αCO32-/w (and αc/w) due to the isotopic imprint of oxygen atoms derived from HCO3−, CO2, H2O and/or OH−. (3) The temperature sensitivity of αc/w originates from the negative effect of temperature on αCO32-/w and is expected to deviate from the commonly accepted value (−0.22±0.02‰/°C between 0 and 30°C; Kim and O’Neil, 1997) when the CO32− pool is not in isotopic equilibrium with water. (4) The model suggests that the δ18O of planktic and benthic foraminifers reflects a quantitative precipitation of DIC in isotopic equilibrium with a high-pH calcifying fluid, leading to a relatively constant foraminifer calcite δ18O-temperature relationship (−0.21±0.01‰/°C). The lower average coral δ18O data relative to foraminifers and other calcifiers is best explained by the precipitation of internal DIC derived from hydrated CO2 in a high-pH calcifying fluid and minimal subsequent DIC-H2O isotopic equilibration. This leads to a reduced and variable coral aragonite δ18O-temperature relationship (−0.11 to −0.22‰/°C). Together, the model presented here reconciles observations of oxygen isotope fractionation over a range of CaCO3-DIC-H2O systems.

From in-Situ to in-Operando Evaluation of SOFC Cathodes for Enhanced ORR Activity and Durability

To study the SOFC cathode oxygen reduction reaction (ORR) mechanism and the effects of ambient gas composition on ORR, we used in-situ 18O isotope exchange techniques to probe the exchange of H2O and CO2 with two of the most common SOFC cathode materials, (La0.8Sr0.2)0.95MnO3-x (LSM) and La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF). A series of temperature programmed isotope exchange measurements were performed to comprehensively study the interaction of H2O and CO2 with the cathode surface as a function of temperature, oxygen partial pressure, and contaminant gas concentration. The experimental data are summarized in a Temperature-PO2 diagram to visualize the dominant reactions at each temperature and PO2 for the two cathode materials. We further report on the development of a novel in-operando 18O isotope exchange technique by modifying our heterogeneous catalysis system for operation and measurement under electrochemical polarization. The system allows ORR investigations under real time, real life, operating conditions. We will present our studies on the effect of cathodic polarization on LSM and LSCF. The studies show the effects of operating voltage, temperature, and oxygen partial pressure on the oxygen exchange coefficient. We will also present our model for the understanding and analysis of the isotope oxygen exchange profiles under in-operando conditions.

The influence of oxygen isotope exchange between CO2 and H2O in natural CO2-rich spring waters: Implications for geothermometry

Oxygen isotope ratio (δ18O) value deviations from the Meteoric Water Line with no significant change in the hydrogen isotope (δ2H) composition have been reported in naturally occurring CO2-rich waters from around the world. Here we review the effects of oxygen isotope exchange with CO2, high temperature equilibration with bedrock minerals and mineral dissolution and precipitation reactions on the CO2-rich water isotopic composition. We present two case studies from Daylesford (Australia) and Pah Tempe (Utah, USA) mineral springs, where we use a numerical geochemical modelling approach to resolve the influence of low temperature water-rock interactions and CO2 equilibration to the oxygen isotope ranges observed in the mineral waters. In both cases, we find that mineral dissolution – precipitation reactions are unlikely to have a significant effect on the groundwater isotopic compositions, and that the observed δ18O values in natural CO2 springs can be simply explained by equilibrium fractionation between water and free phase CO2. Traditionally, the interaction of CO2 and water in a natural CO2-rich groundwater setting has only been associated with water 18O depletion and this is the first study to consider 18O enrichment. We establish that in a natural setting, CO2 and water equilibration can result in water 18O depletion or enrichment, and that the change in the oxygen isotope composition ultimately depends on the initial CO2 and water δ18O values. Our new conceptual model therefore provides a mechanism to explain water 18O enrichment at ambient temperatures. This finding is critical for the use of δ18O in groundwater geothermometry and for the interpretation of natural water circulation depths: we argue that in some cases, natural waters previously interpreted as geothermal based on their oxygen isotope composition may actually have acquired their isotopic signature through interaction with CO2 at ambient temperatures.