Rate Of Oxygen Isotope Exchange Research Articles

An experimental simulation of oxygen isotope exchange reaction between amorphous silicate dust and carbon monoxide gas in the early Solar System

The reaction mechanism and kinetics of oxygen isotope exchange between tens of nanometer-sized amorphous silicate grains with forsterite composition (amorphous forsterite) and low-pressure carbon monoxide (CO) gas (PCO) of 0.05–1 Pa at 643–883 K were examined to investigate oxygen isotopic evolution in the protosolar disk that led to the mass-independent oxygen isotopic variation of planetary materials. Both CO gas supply- and diffusion-controlled isotope exchange reactions were observed. At 753–883 K and PCO of 0.05–1 Pa, the supply of CO gas controls the isotope exchange reaction, and its rate is 2–3 orders of magnitude smaller than that of the H2O supply-controlled isotope exchange reaction. The diffusion-controlled isotope exchange occurred at 643–703 K and PCO of 0.3 Pa, and the reaction rate of D (m2/s) = (3.1 ± 2.3) × 10−23 exp[−41.7 ± 9.6 (kJ mol−1) R−1 (1/T − 1/1200)] was obtained.We found that the oxygen isotope exchange rates of amorphous forsterite with CO and H2O gases are larger than those of gaseous isotope exchange between CO and H2O gases at a wide range of temperatures, wherein amorphous forsterite crystallization does not precede the isotope exchange reaction of amorphous forsterite with these gases. The most sluggish isotope exchange rate between H2O and CO in the gas phase suggests that amorphous forsterite would play a role in accelerating gaseous isotopic equilibrium through the isotope exchange of amorphous forsterite with both CO and H2O. We found that the oxygen isotopic equilibrium between 0.1 μm-sized amorphous forsterite, CO, and H2O would be accomplished through the isotope exchange of amorphous forsterite at temperatures as low as ∼600–700 K in the dynamically accreting protosolar disk, which is significantly lower than expected for the case of gaseous isotope exchange (>∼800 K).

Rates of carbon and oxygen isotope exchange between calcite and fluid at chemical equilibrium

The isotopic composition of carbonate minerals provides a record of historical geochemical and environmental conditions, but the ability to interpret these compositions as paleo-proxies hinges on their preservation over thousand to million year timescales. At chemical equilibrium, alteration of initial isotopic compositions of calcite can occur in the presence of a fluid without visible changes in morphology at the submicron scale, complicating the interpretation of stable isotope compositions of carbonates. However, the rates and mechanisms of isotope exchange at chemical equilibrium are poorly understood. To evaluate the rates and processes by which C and O isotopes are exchanged between calcite and fluid, batch reactor experiments were conducted at chemical equilibrium between calcite and a fluid enriched in 13C and 18O relative to the solid at 25 °C. Both natural and synthetic calcite of different grain sizes were investigated to evaluate the impact of mineral surface area and size on C and O isotope exchange rates. Our experimental results indicate that rapid exchange of both C and O isotopes occurs within 72 h for all calcite grain sizes studied, likely indicative of exchange of surface species in combination with a backward reaction during dissolution and Ostwald ripening of high energy surface sites. After 72 h, C and O isotope exchange rates were slower but near constant for timescales of thousands of hours. Surface-area normalized C and O isotope exchange rates were similar for all calcite grain sizes studied, and O and C were exchanged in a ∼3:1 ratio consistent with exchange of CO32–. The rates of C and O exchange were ∼4 orders of magnitude lower than far-from-equilibrium calcite dissolution rates, suggesting exchange was controlled either by dissolution-precipitation of pre-existing reactive sites alone, or a combination of dissolution-precipitation and solid state/aqueous mediated diffusion. Overall, the results of this study suggest alteration of O and C isotope compositions of calcite at ambient temperatures can proceed readily over short time scales, though the extent to which this process continues to operate over geologic time scales is difficult to predict at present. The results of this study further highlight the importance of generating a mechanistic understanding of the process of isotope exchange at chemical equilibrium, and represent a step towards this understanding.

An experimental study on oxygen isotope exchange reaction between CAI melt and low-pressure water vapor under simulated Solar nebular conditions

Calcium-aluminum-rich inclusions (CAIs) are known as the oldest high-temperature mineral assemblages of the Solar System. The CAIs record thermal events that occurred during the earliest epochs of the Solar System formation in the form of heterogeneous oxygen isotopic distributions between and within their constituent minerals. Here, we explored the kinetics of oxygen isotope exchange during partial melting events of CAIs by conducting oxygen isotope exchange experiments between type B CAI-like silicate melt and 18O-enriched water vapor (PH2O = 5 × 10−2 Pa) at 1420 °C. We found that the oxygen isotope exchange between CAI melt and water vapor proceeds at competing rates with surface isotope exchange and self-diffusion of oxygen in the melt under the experimental conditions. The 18O concentration profiles were well fitted with the three-dimensional spherical diffusion model with a time-dependent surface concentration. We determined the self-diffusion coefficient of oxygen to be ∼1.62 × 10−11 m2 s−1, and the oxygen isotope exchange efficiency on the melt surface was found to be ∼0.28 in colliding water molecules. These kinetic parameters suggest that oxygen isotope exchange rate between cm-sized CAI melt droplets and water vapor is dominantly controlled by the supply of water molecules to the melt surface at PH2O < ∼10−2 Pa and by self-diffusion of oxygen in the melt at PH2O > ∼1 Pa at temperatures above the melilite liquidus (1420–1540 °C). To form type B CAIs containing 16O-poor melilite by oxygen isotope exchange between CAI melt and disk water vapor, the CAIs should have been heated for at least a few days at PH2O > 10−2 Pa above temperatures of the melilite liquidus in the protosolar disk. The larger timescale of oxygen isotopic equilibrium between CAI melt and H2O compared to that between H2O and CO in the gas phase suggests that the bulk oxygen isotopic compositions of ambient gas at ∼1400 °C in the type B CAI-forming region is preserved in the oxygen isotopic compositions of type B CAI melilite. Based on the observed oxygen isotopic composition, we suggest that a typical type B1 CAI (TS34) from Allende was cooled at a rate of ∼0.1–0.5 K h−1 during fassaite crystallization.

Oxygen isotope exchange rates between phosphate and water catalyzed by inorganic pyrophosphatase: Implications for the biogeochemical cycle of phosphorus

The oxygen isotope composition of phosphate (δ18OPO4) has been widely used as (paleo)temperature and bio-signature proxies and a tracer of biogeochemical cycling of phosphorus (P). In natural aqueous systems, reactive inorganic phosphorus is dominantly present as dissolved inorganic phosphate (DIP), and reactions involving DIP are primarily carried out by microorganisms and catalyzed by enzymes. One enzyme suggested to dominate P-cycling in nature is inorganic pyrophosphatase (PPase), a ubiquitous intracellular enzyme that catalyzes oxygen (O) isotope exchange between DIP and water. Herein, we determined PPase-catalyzed O-isotope exchange rates between DIP and water over the range of typical Earth surface temperatures (3 - 37°C) using isotope ratio mass spectrometry (IRMS). Exchange rates determined at pH 7.4 and between 3 and 37°C, were described by first-order reaction kinetics (rate constant k = 1.51E-05 to 3.13E-04 sec−1; half-life of reaction t1/2 = 37 to 765 minutes), and strongly dependent on temperature. The temperature dependence of the exchange reaction was fitted by the Arrhenius equation, with an activation energy of 61.8 kJ/mole. The rate of PPase-catalyzed O-isotope exchange is ca. 7 - 8 orders of magnitude faster than the rate of abiotic reactions (pH 5) at 37°C calculated by the extrapolation of high-temperature rate data. The results of this study can be used to improve the interpretation of measured δ18O values of phosphate preserved in the rock record or during biogeochemical reactions (e.g., deconvolution of simultaneous abiotic and enzymatic/microbial processes).

Oxygen isotope exchange between water and carbon dioxide in soils is controlled by pH, nitrate and microbial biomass through links to carbonic anhydrase activity

Abstract. The oxygen isotope composition of atmospheric carbon dioxide (CO2) is intimately linked to large-scale variations in the cycling of CO2 and water across the Earth's surface. Understanding the role the biosphere plays in modifying the oxygen isotope composition of atmospheric CO2 is particularly important as this isotopic tracer has the potential to constrain estimates of important processes such as gross primary production at large scales. However, constraining the atmospheric mass budget for the oxygen isotope composition of CO2 also requires that we understand better the contribution of soil communities and how they influence the rate of oxygen isotope exchange between soil water and CO2 (kiso) across a wide range of soil types and climatic zones. As the carbonic anhydrases (CAs) group of enzymes enhances the rate of CO2 hydration within the water-filled pore spaces of soils, it is important to develop understanding of how environmental drivers can impact kiso through changes in their activity. Here we estimate kiso and measure associated soil properties in laboratory incubation experiments using 44 soils sampled from sites across western Eurasia and north-eastern Australia. Observed values for kiso always exceeded theoretically derived uncatalysed rates, indicating a significant influence of CAs on the variability of kiso across the soils studied. We identify soil pH as the principal source of variation, with greater kiso under alkaline conditions suggesting that shifts in microbial community composition or intra–extra-cellular dissolved inorganic carbon gradients induce the expression of more or higher activity forms of CAs. We also show for the first time in soils that the presence of nitrate under naturally acidic conditions reduces kiso, potentially reflecting a direct or indirect inhibition of CAs. This effect appears to be supported by a supplementary ammonium nitrate fertilisation experiment conducted on a subset of the soils. Greater microbial biomass also increased kiso under a given set of chemical conditions, highlighting a putative link between CA expression and the abundance of soil microbes. These data provide the most extensive analysis of spatial variations in soil kiso to date and indicate the key soil trait datasets required to predict variations in kiso at large spatial scales, a necessary next step to constrain the important role of soil communities in the atmospheric mass budget of the oxygen isotope composition of CO2.

Pervasive Hydrothermal Events Associated with Large Igneous Provinces Documented by the Columbia River Basaltic Province

The degree and extent of crustal hydrothermal alteration related to the eruption of large igneous provinces is poorly known and not easily recognizable in the field. We here report a new δ18O dataset for dikes and lavas from the Columbia River Basalt Group (16–15 Ma) in the western USA, and document that dikes on average are 1–2‰ more depleted in δ18O than basalt flows. We show that this observation is best explained with the involvement of heated meteoric waters during their cooling in the crust. The largest 6–8‰ depletion is found around and inside a 10 m-thick feeder dike that intruded the 125 Ma Wallowa tonalitic batholith. This dike likely operated as a magma conduit for 4–7 years, based on the extent of heating and melting its host rocks. We show that this dike also created a hydrothermal system around its contacts extending up to 100 m into the surrounding bedrock. A model that considers (a) hydrothermal circulation around the dike, (b) magma flow and (c) oxygen isotope exchange rates, suggests that the hydrothermal system operated for ~150 years after the cessation of magma flow. In agreement with a previously published (U-Th)/He thermochronology profile, our model shows that rocks 100 m away from such a dike can be hydrothermally altered. Collectively, our sample set is the first documentation of the widespread hydrothermal alteration of the shallow crust caused by the intrusion of dikes and sills of the Columbia River Basalt Province. It is estimated that heating and hydrothermal alteration of sediments rich in organic matter and carbonates around the dikes and sills releases 18 Gt of greenhouse gases (CH4 and CO2). Furthermore, hydrothermal δ18O depletion of rocks around dikes covers 500–600 km3, which, when scaled to the total CRB province constitutes 31,000 km3 of low-δ18O rocks. These volumes of crust depleted in δ18O are sufficient to explain the abundant low-δ18O magmas in eastern Oregon and western Idaho. This work also demonstrates that the width and magnitude of δ18O depletion around dikes can identify them as feeders. Given this, we here interpret Paleoproterozoic dikes in Karelia with the world’s lowest δ18O depletions (−27.8‰) as feeders to the coeval large igneous province aged 2.2–2.4 Ga that operated under the Snowball Earth glaciation conditions.

Modeling of MAl12 Keggin Heteroatom Reactivity by Anion Adsorption

Heteroatom-substituted Keggins, of general formula MAl12 (M = Al, Ga, Ge), are a class of nanoclusters whose properties are sensitive to changes in composition and intermolecular interactions. Previous studies have shown that they display significant differences in oxygen-isotope exchange rates, depending on the identity of heteroatom M. By exploring the intermolecular interactions of these nanoclusters with a series of anions using density functional theory (DFT) calculations, we find bond length changes and adsorption energy values that track with experimentally measured oxygen exchange reactivity trends: Ga < Al < Ge. We compare elongations in μ4O–Alo bond lengths to known heteroatom reactivity trends and anion pKa properties, suggesting a window for producing isolable products. To yield insights into the atomistic interactions that dictate the crystallization process, we investigate trends in adsorption energy, DFT-optimized geometries, and vibrational modes and calculate the distribution of charge in...

An “On-Line” Method for Oxygen Isotope Exchange Between Gas-Phase CO2 and Water

An “on-line” mixing system has been developed and evaluated for continuous oxygen isotope exchange between gas-phase CO2 and liquid water. The system is composed of three basic parts: equipment and materials used to introduce water and gas into a mixing reservoir, the mixing and exchange reservoir, and a vessel used to separate gas and water phases exiting the system. A series of experiments were performed to monitor the isotope exchange process over a range of temperatures (5–40 °C) and CO2 partial pressures (202–15,200 Pa). Isotopic exchange was evaluated using CO2 having δ18O values of 30.4 and 37.8 ‰ and waters of two distinct oxygen isotope compositions (−6.5 to −5 and 6 to 7.5 ‰). Isotope ratios were determined by isotope ratio mass spectrometry and cavity ring-down spectroscopy. CO2 did not reach oxygen isotope equilibrium under the conditions described here. However, oxygen isotope exchange rate constants were determined at different temperatures and regressed to yield the expression k (h−1) = 0.020 × T (°C) + 0.28. Using this expression, the residence time required to reach oxygen isotope equilibrium may be estimated for a given set of environmental conditions (e.g., δ18O value of water, temperature). System parameters can be modified to achieve a specific δ18O value for CO2. Consequently, the exchange system described here has the ability to deliver a constant flow of CO2 at a desired oxygen isotope composition. This ability is attractive for a variety of applications such as experiments that utilize flow-through reactors and environmental chambers or require static chemical conditions.

Isotopic evidence of the pivotal role of sulfite oxidation in shaping the oxygen isotope signature of sulfate

The oxygen isotope composition of sulfate serves as an archive of oxidative sulfur cycling. Studies on the aerobic oxidation of reduced sulfur compounds showed discrepancies in the relative incorporation of oxygen from dissolved molecular oxygen (O2) and water (H2O) into newly formed sulfate, which likely result from slight differences in the production and consumption rate of sulfoxy intermediates that exchange oxygen isotopes with water. Sulfite is often considered the final sulfoxy intermediate in the oxidation of reduced sulfur compounds to sulfate and its residence time strongly affects the oxygen isotope signature of produced sulfate. However, data on the oxygen isotope signature of sulfate derived from sulfite oxidation are scarce.We determined the oxygen isotope effects of abiotic oxidation of sulfite with O2 or ferric iron (Fe3+) under different pH conditions (pH1, 4.9 and 13.3). These parameters impact the relative contribution of oxygen from H2O and O2 to the produced sulfate, and control the competition between the rates of oxygen isotope exchange between sulfite and water and the sulfite oxidation. There is a striking overlap in the range of oxygen isotope offsets between sulfate and water from our experiments at different chemical conditions (Δ18OSO4−H2O from 5.9‰ for anaerobic oxidation with Fe3+ up to 17.6‰ for oxidation at low pH with O2 as sole oxidant, respectively) with the variations in the oxygen isotope composition of sulfate derived from oxidative processes in the environment. This implies that oxygen isotope effects during sulfite oxidation largely control the isotope signature of sulfate derived from the oxidation of sulfur compounds. However, our results also show that preexisting non-equilibrium isotope signatures of sulfite are likely partially preserved in the final sulfate product under most environmental conditions. Our study furthermore provides a mechanistic explanation for positive isotope offsets between the oxygen isotope composition of sulfate and water observed in anoxic pyrite oxidation experiments with Fe3+ as the sole oxidizing agent. This apparently inverse isotope effect is caused by the interplay between sulfite-water oxygen exchange and normal kinetic isotope fractionation effects during sulfite oxidation, the former driving the isotope composition towards the isotope equilibrium fractionation between sulfite and water, inducing a positive offset whereas the latter induces a negative offset.

The rate of oxygen isotope exchange between nitrate and water

The oxygen isotope exchange rate between nitrate and water was measured at a temperature of 50–80°C and pH −0.6 to 1.1. Oxygen isotope exchange is a first-order reaction, with the exchange rate being strongly affected by both reaction temperature and pH, with increased rates of isotope exchange at higher temperatures and lower pH values. The rate of oxygen isotope exchange under natural conditions is extremely slow, with an estimated half-life for isotope exchange of 5.5×109years at 25°C and pH 7. The extremely slow rate of oxygen isotope exchange between nitrate and water under typical environmental conditions illustrates that nitrate-δ18O signatures (and also nitrate δ17O and Δ17O signatures) associated with various nitrate sources, as well as isotope compositions produced by biogeochemical processes, will be preserved. Hence, it is valid to use the value of nitrate-δ18O to investigate the sources and biogeochemical behavior of nitrate, in a similar manner to the use of sulfate-δ18O signatures to study the sources and biogeochemical behavior of sulfate.Equilibrium oxygen isotope fractionation factors have been determined, although quantification of the nitrate–water equilibrium fractionation factor is not possible due to the presence of nitrate as both protonated (i.e. HNO3) and unprotonated forms (i.e. NO3-) under the experimental conditions, and the difficulty in accurately calculating nitrate speciation in low pH, high ionic strength solutions.

Influence of dissolved ions on determination of oxygen isotope composition of aqueous solutions using the CO2‐H2O equilibration method

Stable isotope compositions of natural waters, such as seawater, glaciers and basinal brines, can provide valuable information about Earth's hydrological cycle and its evolutionary history. However, a high concentration of dissolved ions in some natural waters hinders an accurate analysis of their oxygen isotope composition. A laboratory study was carried out in order to provide guidelines on how to resolve this analytical difficulty. CO(2) gas was equilibrated with saline aqueous solutions of various chemical compositions at 25 °C. Subsequently, the oxygen isotope composition of the CO(2) was determined at different equilibration times using a dual-inlet isotope ratio mass spectrometer in order to evaluate the oxygen isotope salt effect and the rate of oxygen isotope exchange between CO(2) and the saline solution. Using the experimentally determined oxygen isotope salt effects of aqueous chloride and sulfate solutions, an empirical method for the prediction of the oxygen isotope salt effect of a 1.0 molal chloride or sulfate solution was proposed. The rates of oxygen isotope exchange between CO(2) and saline solutions were also examined. Our experimental data indicates that the sequence of the oxygen isotope exchange time is as: MgSO(4) > CaCl(2) ≈ Na(2)SO(4) > NaCl > MgCl(2) > KCl > H(2)O. The isotope salt effect and the kinetics of isotope exchange must be taken into account when the oxygen isotope composition of a saline aqueous solution is determined using the CO(2)-H(2)O equilibration method. Our experimental data and the proposed prediction method provide essential guidelines for the accurate δ(18)O analysis of saline aqueous solutions.

Real-time monitoring of the overall exchange of oxygen isotopes between aqueous [formula omitted] and H2O by Raman spectroscopy

In this contribution we demonstrate that in situ Raman spectroscopy is a powerful tool to study the exchange kinetics of oxygen isotopes between aqueous oxo-anions and water using the CO32-–H2O system as an example. In situ exchange experiments have been carried out using a 1M Na2CO3 solution at 45, 60, 75, and 100°C in a closed system using an in-house-made Teflon©-based fluid cell. The solution was prepared with H2O enriched with 97at.% 18O. At the given pH from 10.92 to 9.65 at 45–100°C, respectively, CO32- is the dominant DIC species (>99% CO32-). At the beginning of the reaction the Raman spectrum of the solution is characterised by an intense band near 1067cm−1 that has been assigned to the ν1(CO3) symmetric stretching vibration of the CO32- molecule. With increasing reaction time three, well-separated bands successively appear near 1046, 1026, and 1006cm−1. These bands result from mass-related (isotopic) splitting of the ν1(CO3) mode and reflect the symmetric stretching vibration of the oxygen-related isotopologues C16O3-n18On2- (n=1, 2, and 3). The relative integrated intensities of these bands are related to the amount of 18O present in the aqueous CO32- molecule, which allowed monitoring the time-dependent distribution of the four isotopologues at each temperature. The measured distributions as a function of time were found to agree well with those modelled on the basis of the overall reactions ΣC18On16O3-n+3H218O⇌ΣC18On+116O3-n-1+3H216O, where the sum sign represents the sum of all carbonate species in solution and n=0, 1, and 2. Moreover, the measured equilibrium fractions of 18O agree well with those expected from mass balance and published fractionation factors. After correcting the observed exchange rates for the dependence of the exchange kinetics on OH− and CO2(aq) activities, the observed overall logarithmic oxygen isotope exchange rates describe a linear relationship with the inverse temperature along with published exchange data that were obtained by conventional means. From this relationship, we obtained an activation energy for the oxygen isotope exchange process of 63.4±1.4kJ/mol, which agrees very well with the activation energy for the CO2(aq)+OH-⇌HCO3- reaction, further verifying that the formation of bicarbonate from CO2 and OH− is the rate-determining step for the oxygen isotope exchange in strongly alkaline solutions. Accordingly, the application of Raman spectroscopy is a highly promising tool for deciphering the isotope exchange reaction kinetics in water–oxo-anion systems.

Rate of Oxygen Isotope Exchange between Selenate and Water

The rate of oxygen isotope exchange between selenate and water was investigated at conditions of 10 to 80 °C and pH -0.6 to 4.4. Oxygen isotope exchange proceeds as a first-order reaction, and the exchange rate is strongly affected by reaction temperature and pH, with increased rates of isotope exchange at higher temperature and lower pH. Selenate speciation (HSeO(4)(-) vs SeO(4)(2-)) also has a significant effect on the rate of isotope exchange. The half-life for isotope exchange at example natural conditions (25 °C and pH 7) is estimated to be significantly in excess of 10(6) years. The very slow rate of oxygen isotope exchange between selenate and water under most environmental conditions demonstrates that selenate-δ(18)O signatures produced by biogeochemical processes will be preserved and hence that it will be possible to use the value of selenate-δ(18)O to investigate the biogeochemical behavior of selenate, in an analogous fashion to the use of sulfate-δ(18)O to study the biogeochemical behavior of sulfate.

17O NMR and Computational Study of a Tetrasiliconiobate Ion, [H2+xSi4Nb16O56](14−x)−

Rates of oxygen-isotope exchange were measured in the tetrasiliconiobate ion [H(2+x)Si(4)Nb(16)O(56)]((14-x)-) to better understand how large oxide ions interact with water. The molecule has 19 nonequivalent oxygen sites and is sufficiently complex to evaluate hypotheses derived from our previous work on smaller clusters. We want to examine the extent to which individual oxygen atoms react independently with particular attention given to the order of protonation of the various oxygen sites as the pH decreases from 13 to 6. As in our previous work, we find that the set of oxygen sites reacts at rates that vary over approximately 10(4) across the molecule at 6<pH<13 but with similar pH dependencies. There is NMR evidence of an intra- or intermolecular reaction at pH∼7, where new peaks began to slowly form without losing the (17)O isotopic tag, and at pH ≤ 6 these new peaks formed rapidly. The oxygen atoms bonded to silicon atoms began to isotopically exchange at pH 9 and below. The (17)O NMR peak positions also vary considerably with pH for some, but not all, nonequivalent oxygen sites. This variation could be only partly accounted by electronic calculations, which indicate that oxygen atoms should shift similarly upon protonation. Instead, we see that some sites change enormously with pH, whereas other, similarly coordinated oxygen atoms are less affected, suggesting that either some protons are exchanging so rapidly that the oxygen sites are seeing an averaged charge, or that counterions are modulating the effect of the coordinated protons.

Oxygen-Isotope Exchange between CO2 and Solid Ti18O2

The light–induced oxygen-isotope exchange between gaseous CO2 and solid Ti18O2 (anatase) and the spontaneous thermal isotope exchange that takes place between the vacuum-calcined solid Ti18O2 and CO2 were studied by gas-phase high-resolution Fourier transform infrared absorption spectroscopy over a period of several days. The absorption ro-vibrational spectra of all the measured carbon dioxide isotopologues were assigned and served as the quantification of the time-dependent isotope exchange between the oxygen atoms from the Ti18O2 solid and the oxygen related to the gaseous CO2. The C18O2 was formed as the dominating final product with a minor content of C16O18O. The rate of oxygen-isotope exchange is highly sensitive to the conditions of the titania pretreatment; vacuum-annealed Ti18O2 at 450 °C exhibited a very high spontaneous oxygen exchange activity with gaseous C16O2. A mechanism for the 18O/16O exchange process is discussed at the molecular level. The photocatalytic formation of methane, acetylene...

Diffusion and interface growth in hafnium oxide and silicate ultrathin films on Si(001)

Medium energy ion scattering has been used in combination with $^{16}\mathrm{O}$ and $^{18}\mathrm{O}$ isotope tracing to determine elemental depth distributions and elucidate oxygen transport in 2--5 nm thick HfO${}_{2}$ and HfSiO${}_{x}$ films grown by atomic layer deposition on Si(001). Both the oxygen isotope exchange rate in the dielectric as well as the interfacial silicon oxide growth rates were examined as a function of time, temperature, film stoichiometry (HfO${}_{2}$, HfSiO${}_{x}$, and HfSiO${}_{x}$N${}_{y}$), and crystallinity. The amount of exchanged oxygen in the oxide was found to decrease with increasing SiO${}_{2}$ content. When the SiO${}_{2}$ to HfO${}_{2}$ ratio reaches 1:1 in HfSiO${}_{x}$ an almost full suppression of the oxygen exchange is observed. The activation barrier for the SiO${}_{2}$ growth at the HfO${}_{2}$/Si and HfSiO${}_{x}$/Si interfaces was found to be much lower than that in the SiO${}_{2}$/Si and SiO${}_{x}$N${}_{y}$/Si cases, which is attributed to distinctly different oxygen incorporation mechanisms. The primary route for oxygen delivery to the interface responsible for the SiO${}_{2}$ growth is via exchange, however, direct oxidation by molecular oxygen cannot be discounted completely. In the presence of an interfacial nitride layer the $^{18}\mathrm{O}$-$^{16}\mathrm{O}$ exchange is replaced by the $^{18}\mathrm{O}$-N exchange, which slows diffusion and reduces the oxidation rate.

Oxygen-Isotope Exchange Rates for Three Isostructural Polyoxometalate Ions

We compare oxygen-isotope exchange rates for all structural oxygens in three polyoxoniobate ions that differ by systematic metal substitutions of Ti(IV) --> Nb(V). The [H(x)Nb(10)O(28)]((6-x)-), [H(x)TiNb(9)O(28)]((7-x)-), and [H(x)Ti(2)Nb(8)O(28)]((8-x)-) ions are all isostructural yet have different Brønsted properties. Rates for sites within a particular molecule in the series differ by at least approximately 10(4), but the relative reactivities of the oxygen sites rank in nearly the same relative order for all ions in the series. Within a single ion, most structural oxygens exhibit rates of isotopic exchange that vary similarly with pH, indicating that each structure responds as a whole to changes in pH. Across the series of molecules, however, the pH dependencies for isotope exchanges and dissociation are distinctly different, reflecting different contributions from proton- or base-enhanced pathways. The proton-enhanced pathway for isotope exchange dominates at most pH conditions for the [H(x)Ti(2)Nb(8)O(28)]((8-x)-) ion, but the base-enhanced pathways are increasingly important for the [H(x)TiNb(9)O(28)]((7-x)-) and [H(x)Nb(10)O(28)]((6-x)-) structures at higher pH. The local effect of Ti(IV) substitution could be assessed by comparing rates for structurally similar oxygens on each side of the [H(x)TiNb(9)O(28)]((7-x)-) ion and is surprisingly small. Interestingly, these nanometer-size structures seem to manifest the same general averaged amphoteric chemistry that is familiar for other reactions affecting oxides in water, including interface dissolution by proton- and hydroxyl-enhanced pathways.

Isotope-Exchange Dynamics in Isostructural Decametalates with Profound Differences in Reactivity

Rates of oxygen-isotope exchange at all structural sites in two isostructural polyoxometalates, [H(x)Nb(10)O(28)]((6-x)-) and [H(x)Ti(2)Nb(8)O(28)]((8-x)-), show that small changes in structure have surprising and profound effects: a single-site substitution of Ti(IV) for Nb(V) inverts the pH dependencies for rates throughout the structures. Within a given structure, all oxygens exhibit similar pH dependencies although they react over a range greater than approximately 10(4), indicating that pathways involve concerted motions of the entire lattices. Profound sensitivity to changes in structure and composition suggests reaction pathways in polyoxometalate ions will be highly variable even within structural classes. The results also require new thinking about how ab initio simulations are used to understand reaction pathways involving extended structures, like the mineral-water interface. Our data indicate that reactions proceed via metastable intermediates and that the simulations must be structurally faithful or will miss the essential chemistry.

Rates of Oxygen-Isotope Exchange between Sites in the [HxTa6O19](8-x)−(aq) Lindqvist Ion and Aqueous Solutions: Comparisons to [HxNb6O19](8-x)−(aq)

Rates of steady oxygen-isotope exchange differ in interesting ways for two sets of structural oxygens in the [HxTa6O19](8-x)-(aq) Lindqvist ion when compared to published data on the [HxNb6O19]8-x(aq) version. Because of the lanthanide contraction, the [HxTa6O19](8-x)-(aq) and [HxNb6O19](8-x)-(aq) ions are virtually isostructural and differ primarily in a full core (Kr vs Xe) and the 4f14 electrons in the [HxTa6O19](8-x)-(aq) ion. For both molecules, both pH-dependent and -independent pathways are evident in isotopic exchange of the 12 mu2-O(H) and 6 eta=O sites. Rate parameters for eta=O exchange at conditions where there is no pH dependence are, for the Ta(V) and Nb(V) versions respectively, K(298)(0) = 2.72 x 10(-5) s(-1) and 9.7 x 10(-6) s(-1), DeltaH = 83.6 +/- 3.2 and 89.4 kJ.mol(-1), and DeltaS = -51.0 +/- 10.6 and -42.9 J.mol(-1).K-1. For the mu2-O sites, K(298)(0) = 1.23 x 10(-6) s(-1), DeltaH = 70.3 +/- 9.7 and 88.0 kJ.mol(-1), and DeltaS = -116.1 +/- 32.7 and -29.4 J.mol(-1).K-1. Protonation of the 6 eta=O sites is energetically unfavored relative to the 12 mu2-O bridges in both molecules, although not equally so. Experimentally, protonation labilizes both the mu2-O(H) and eta=O sites to isotopic exchange in both molecules. Density-functional electronic-structure calculations indicate that proton affinities of structural oxygens in the two molecules differ with the [HxTa6O19](8-x)-(aq) anion having a smaller affinity to protonate than the [HxNb6O19]8-x(aq) ion. This difference in proton affinities is evident in the solution chemistry as pKa = 11.5 for the [HTa6O19]7-(aq) ion and pKa = 13.6 for the [HNb6O19]7-(aq) ion. Most striking is the observation that eta=O sites isotopically equilibrate faster than the mu2-O sites for the [HxTa6O19](8-x)-(aq) Lindqvist ion but slower for the [HxNb6O19](8-x)-(aq) ion, indicating that predictions about site reactivities in complicated structures, such as the interface of aqueous solutions and oxide solids, should be approached with great caution.

A model for oxygen and sulfur isotope fractionation in sulfate during bacterial sulfate reduction processes

We present a model of bacterial sulfate reduction that includes equations describing the fractionation relationship between the sulfur and the oxygen isotope composition of residual sulfate (δ 34S SO4_residual, δ 18O SO4_residual) and the amount of residual sulfate. The model is based exclusively on oxygen isotope exchange between cell-internal sulfur compounds and ambient water as the dominating mechanism controlling oxygen isotope fractionation processes. We show that our model explains δ 34S SO4_residual vs. δ 18O SO4_residual patterns observed from natural environments and from laboratory experiments, whereas other models, favoring kinetic isotope fractionation processes as dominant process, fail to explain many (but not all) observed δ 34S SO4_residual vs. δ 18O SO4_residual patterns. Moreover, we show that a “typical” δ 34S SO4_residual vs. δ 18O SO4_residual slope does not exist. We postulate that measurements of δ 34S SO4_residual and δ 18O SO4_residual can be used as a tool to determine cell-specific sulfate reduction rates, oxygen isotope exchange rates, and equilibrium oxygen isotope exchange factors. Data from culture experiments are used to determine the range of sulfur isotope fractionation factors in which a simplified set of equations can be used. Numerical examples demonstrate the application of the equations. We postulate that, during denitrification, the oxygen isotope effects in residual nitrate are also the result of oxygen isotope exchange with ambient water. Consequently, the equations for the relationship between δ 34S SO4_residual, δ 18O SO4_residual, and the amount of residual sulfate could be modified and used to calculate the fractionation-relationship between δ 15N NO3_residual, δ 18O NO3_residual, and the amount of residual nitrate during denitrification.