Kinetics Of Isotopic 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).

Kinetics of Oxygen Exchange and N2O Decomposition Reaction over MeOx/CeO2 (Me = Fe, Co, Ni) Catalysts.

MeOx/CeO2 (Me = Fe, Co, Ni) samples were tested in an 18O2 temperature-programmed isotope exchange and N2O decomposition (deN2O). A decrease in the rate of deN2O in the presence of oxygen evidences the competitive adsorption of N2O and O2 on the same sites. A study of isotope oxygen exchange revealed dissociative oxygen adsorption with the subsequent formation of surface oxygen species. The same species, more probably, result from N2O adsorption and the following N2 evolution to the gas phase. We supposed the same mechanism of O2 formation from surface oxygen species in both reactions, including the stages responsible for its mobility. A detailed analysis of the kinetics of isotope exchange has been performed, and the rates of one-atom (RI) and two-atom (RII) types of exchange were evaluated. The rate of the stage characterizing the mobility of surface oxygen was calculated, supposing the same two-step mechanism was relevant for both types of exchange. The effect of oxygen mobility on the kinetics of deN2O was estimated. An analysis of the possible pathways of isotope transfer from MeOx to CeOx showed that direct oxygen exchange on the Me-Ce interface makes a valuable contribution to the rate of this reaction. The principal role of the Me-Ce interface in deN2O was confirmed with independent experiments on FeOx/CeO2 samples with a different iron content.

Experimental determination of hydrogen isotope exchange rates between methane and water under hydrothermal conditions

The hydrogen isotopic composition of methane (CH4) is used as a fingerprint of gas origins. Exchange of hydrogen isotopes between CH4 and liquid water has been proposed to occur in both low- and high-temperature settings. However, despite environmental evidence for hydrogen isotope exchange between CH4 and liquid water, there are few experimental constraints on the kinetics of this process. We present results from hydrothermal experiments conducted to constrain the kinetics of hydrogen isotope exchange between CH4 and supercritical water. Seven isothermal experiments were performed over a temperature range of 376–420 °C in which deuterium-enriched water and CH4 were reacted in flexible gold reaction cell systems. Rates of exchange were determined by measuring the change in the δD of CH4 over the time course of an experiment. Regression of derived second order rate constants (kr) vs. 1000/T (i.e., an Arrhenius plot) yields the following equation: ln(kr) = −17.32 (±4.08, 1 s.e.) × 1000/T + 3.19 (±6.01, 1 s.e.) (units of kr of sec−1 [mol/L]−1), equivalent to an activation energy of 144.0 ± 33.9 kJ/mol (1 s.e.). These results indicate that without catalysts, CH4 will not exchange hydrogen isotopes with liquid water on a timescale shorter than the age of the Earth (i.e., billions of years) at temperatures below 100–125 °C. Exchange at or below these temperatures is thought to occur due to the activity of life, and thus hydrogen isotopic equilibrium between methane and water may be a biosignature at low temperatures on Earth (in the present or the past) and on other planetary bodies. At temperatures ranging from 125 to 200 °C, hydrogen isotope exchange between CH4 and liquid water can occur on timescales of millions to hundreds of thousands of years, indicating that in thermogenic natural gas systems CH4 may isotopically equilibrate with water and achieve equilibrium isotopic compositions. Finally, the kinetics indicate that in deep-sea hydrothermal systems, the hydrogen (and thus clumped) isotopic composition of CH4 is likely set by formation and/or storage conditions isolated from the active flow regime. The determined kinetics indicate that once methane is entrained in circulating fluids, the expected time-temperature pathways are insufficient for measurable hydrogen isotope exchange between CH4 and water to occur.

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 Isotopic Exchange between Amorphous Silicate and Water Vapor and Its Implications for Oxygen Isotopic Evolution in the Early Solar System

Abstract Meteoritic evidence suggests that oxygen isotopic exchange between 16O-rich amorphous silicate dust and 16O-poor water vapor occurred in the early solar system. In this study, we experimentally investigated the kinetics of oxygen isotopic exchange between submicron-sized amorphous forsterite grains and water vapor at protoplanetary disk-like low pressures of water vapor. The isotopic exchange reaction rate is controlled either by diffusive isotopic exchange in the amorphous structure or by the supply of water molecules from the vapor phase. The diffusive oxygen isotopic exchange occurred with a rate constant D (m2 s−1) = (1.5 ± 1.0) × 10−19 exp[−(161.5 ± 14.1 (kJ mol−1))R −1(1/T−1/1200)] at temperatures below ∼800–900 K, and the supply of water molecules from the vapor phase could determine the rate of oxygen isotopic exchange at higher temperatures in the protosolar disk. On the other hand, the oxygen isotopic exchange rate dramatically decreases if the crystallization of amorphous forsterite precedes the oxygen isotopic exchange reaction with amorphous forsterite. According to the kinetics for oxygen isotopic exchange in protoplanetary disks, original isotopic compositions of amorphous forsterite dust could be preserved only if the dust was kept at temperatures below 500–600 K in the early solar system. The 16O-poor signatures for the most pristine silicate dust observed in cometary materials implies that the cometary silicate dust experienced oxygen isotopic exchange with 16O-poor water vapor through thermal annealing at temperatures higher than 500–600 K prior to their accretion into comets in the solar system.

Tracing Mineral Reactions Using Confocal Raman Spectroscopy

Raman spectroscopy is a powerful tool used to identify mineral phases, study aqueous solutions and gas inclusions as well as providing crystallinity, crystallographic orientation and chemistry of mineral phases. When united with isotopic tracers, the information gained from Raman spectroscopy can be expanded and includes kinetic information on isotope substitution and replacement mechanisms. This review will examine the research to date that utilizes Raman spectroscopy and isotopic tracers. Beginning with the Raman effect and its use in mineralogy, the review will show how the kinetics of isotope exchange between an oxyanion and isotopically enriched water can be determined in situ. Moreover, we show how isotope tracers can help to unravel the mechanisms of mineral replacement that occur at the nanoscale and how they lead to the formation of pseudomorphs. Finally, the use of isotopic tracers as an in situ clock for mineral replacement processes will be discussed as well as where this area of research can potentially be applied in the future.

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.

Lead isotope exchange between dissolved and fluvial particulate matter: a laboratory study from the Johor River estuary.

Atmospheric aerosols are the dominant source of Pb to the modern marine environment, and as a result, in most regions of the ocean the Pb isotopic composition of dissolved Pb in the surface ocean (and in corals) matches that of the regional aerosols. In the Singapore Strait, however, there is a large offset between seawater dissolved and coral Pb isotopes and that of the regional aerosols. We propose that this difference results from isotope exchange between dissolved Pb supplied by anthropogenic aerosol deposition and adsorbed natural crustal Pb on weathered particles delivered to the ocean by coastal rivers. To investigate this issue, Pb isotope exchange was assessed through a closed-system exchange experiment using estuarine waters collected at the Johor River mouth (which discharges to the Singapore Strait). During the experiment, a known amount of dissolved Pb with the isotopic composition of NBS-981 (206Pb/207Pb = 1.093) was spiked into the unfiltered Johor water (dissolved and particulate 206Pb/207Pb = 1.199) and the changing isotopic composition of the dissolved Pb was monitored. The mixing ratio of the estuarine and spike Pb should have produced a dissolved 206Pb/207Pb isotopic composition of 1.161, but within a week, the 206Pb/207Pb in the water increased to 1.190 and continued to increase to 1.197 during the next two months without significant changes of the dissolved Pb concentration. The kinetics of isotope exchange was assessed using a simple Kd model, which assumes multiple sub-reservoirs within the particulate matter with different exchange rate constants. The Kd model reproduced 56% of the observed Pb isotope variance. Both the closed-system experiment and field measurements imply that isotope exchange can be an important mechanism for controlling Pb and Pb isotopes in coastal waters. A similar process may occur for other trace elements.This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.

Kinetics of Hydrogen Isotope Exchange in β-Phase Pd–H–D

Hydrogen isotope gas exchange within palladium powders is examined using a batch-type reactor coupled to a residual gas analyzer (RGA). Exchange rates in both directions (H2 + PdD and D2 + PdH) are measured in the temperature range 178–323 K for the samples with different particle sizes. The results show this batch-type exchange is closely approximated as a first-order kinetic process with a rate directly proportional to the surface area of the powder particles. An exchange rate constant of 1.40 ± 0.24 μmol H2/atm cm2 s is found for H2 + PdD at 298 K, 1.4 times higher than that for D2 + PdH, with an activation energy of 25.0 ± 3.2 kJ/mol H for both exchange directions. A comparison of exchange measurement techniques shows these coefficients, and the fundamental exchange probabilities are in good agreement with those obtained by NMR and flow techniques.

Electride support boosts nitrogen dissociation over ruthenium catalyst and shifts the bottleneck in ammonia synthesis.

Novel approaches to efficient ammonia synthesis at an ambient pressure are actively sought out so as to reduce the cost of ammonia production and to allow for compact production facilities. It is accepted that the key is the development of a high-performance catalyst that significantly enhances dissociation of the nitrogen–nitrogen triple bond, which is generally considered a rate-determining step. Here we examine kinetics of nitrogen and hydrogen isotope exchange and hydrogen adsorption/desorption reactions for a recently discovered efficient catalyst for ammonia synthesis—ruthenium-loaded 12CaO·7Al2O3 electride (Ru/C12A7:e−)—and find that the rate controlling step of ammonia synthesis over Ru/C12A7:e− is not dissociation of the nitrogen–nitrogen triple bond but the subsequent formation of N–Hn species. A mechanism of ammonia synthesis involving reversible storage and release of hydrogen atoms on the Ru/C12A7:e− surface is proposed on the basis of observed hydrogen absorption/desorption kinetics.

Determination of water-soluble phosphate content of soil using heterogeneous exchange reaction with 32P radioactive tracer

The nutrient uptake by plants is significantly influenced by the quantity of phosphate in the soil solution. Optimal fertilization rate demands the determination of water soluble phosphate quantity, the rate of desorption and phosphate exchange between the soil and soil solution at steady state. The aim of this paper was to determine the ratio of water-soluble/exchangeable to mineralized/organic phosphate after incubation of the soil with different phosphate quantities. The sorption process of phosphate on soil was studied by heterogeneous isotope exchange using a radioactive isotope of phosphorous. Using the correct mathematical analysis of the kinetics of heterogeneous isotope exchange, the rate of phosphate exchange was established under steady state condition. The distribution of radioactivity in equilibrium provided information on the reversibility or irreversibility of phosphate sorption. The results revealed that during a relatively short incubation period, the water-soluble/exchangeable phosphorous quantity, namely the portion of phosphate which can easily be taken up by plants, was proportional to the added phosphorous quantity. The tightly sorbed phosphorous quantity, however, reached a limit, suggesting that the reactants leading to tight sorption are consumed. The exchange rate of phosphorous between the soil and the solution was found to be proportional to the phosphorous quantity in the soil solution, showing that the rate-determining step of the heterogeneous isotope exchange process was the diffusion of phosphate ions.

Kinetics of CO2(g)–H2O(1) isotopic exchange, including mass 47 isotopologues

The analysis of mass 47 isotopologues of CO2 (mainly 13C18O16O) is established as a constraint on sources and sinks of environmental CO2, complementary to δ13C and δ18O constraints, and forms the basis of the carbonate clumped isotope thermometer. This measurement is commonly reported using the Δ47 value — a measure of the enrichment of doubly substituted CO2 relative to a stochastic isotopic distribution. Values of Δ47 for thermodynamically equilibrated CO2 approach 0 (a random distribution) at high temperatures (≥ several hundred degrees C), and increase with decreasing temperature, to ≈0.9% at 25°C. While the thermodynamic properties of doubly substituted isotopologues of CO2 (and, similarly, carbonate species) are relatively well understood, there are few published constraints on their kinetics of isotopic exchange. This issue is relevant to understanding both natural processes (e.g., photosynthesis, respiration, air–sea or air–groundwater exchange, CO2 degassing from aqueous solutions, and possibly gas–sorbate exchange on cold planetary surfaces like Mars), and laboratory handling of CO2 samples for Δ47 analysis (e.g., re-equilibration in the presence of liquid water, water ice or water adsorbed on glass or metal surfaces). We present the results of an experimental study of the kinetics of isotopic exchange, including changes in Δ47 value, of CO2 exposed to liquid water between 5 and 37°C. Aliquots of CO2 gas were first heated to reach a nearly random distribution of its isotopologues and then exposed at low pressure for controlled periods of time to large excesses of liquid water in sealed glass containers. Containers were held at 5, 25 and 37°C and durations of exchange ranging from 5min to 7days. To avoid the formation of a boundary layer that might slow exchange, the tubes were vigorously shaken during the period of exchange. At the end of each experiment, reaction vessels were flash frozen in liquid nitrogen. CO2 gas was then recovered from the head space of the reaction vessel, purified and analyzed for its Δ47, δ13C and δ18O by gas source isotope ratio mass spectrometry. Equilibrium was reached for both δ18O and Δ47 after durations of a few hours to tens of hours. δ18O values at equilibrium were consistent with known fractionation factors for the CO2–H2O system. The evolution of δ18O and Δ47 with experiment duration were consistent with first-order reactions, with rate constants equal to each other (within error), averaging 0.19h−1 at 5°C, 0.38h−1 at 25°C and 0.65h−1 at 37°C. We calculate an activation energy for this isotopic exchange reaction of 26.2kJ/mol. By comparison, Mills and Urey (1940) measured the rate of 18O exchange between CO2(aq) and water to have a rate of 11h−1 at 25°C and an activation energy of 71.7kJ/mol. Our finding of a slower rate and lower activation energy is consistent with the rate limiting step of our experiment being the CO2(g)–CO2(aq) exchange, even when samples are shaken during the partial equilibration. Our results broadly resemble those from the study of (Affek, 2013), though this prior study found a lower rate constant for Δ47. We propose that the difference is due to analytical uncertainties and explore the theoretical consequences of unequal reaction rates between 12C18O16O and 13C18O16O with a forward model.

Isotope Exchange between &lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;2&lt;/sub&gt; Gas and Mechanoactivated Oxides of the Family Rare Earth – Manganese – Oxygen

The results of measurements of the bulk diffusion of tracer oxygen atoms in the oxides LnMnO3+δ (Ln = La, Nd, Sm) in the temperature range 400 – 750°С are presented. The measurements were carried out on micro-and nanopowders. Nanoscale powders were prepared by mechanical activation. A method based on the study of the kinetics of oxygen isotope exchange between the powder and gaseous oxygen enriched with 18O isotope was used to obtain data on the diffusion coefficients. The average concentration of 18O isotope in the powders was measured using NRA technique. The obtained diffusion coefficients lay in the range of 10-21 - 10-24 m2/s, the diffusion activation energy for all the oxides have been close to 1 eV. These results suggest that the migration of tracer oxygen in oxides LаMnO3+δ, NdMnO3+δ and SmMnO3+δ at low temperatures is realized via structural defects. As for the oxide LaMnO3+δ, its oxygen diffusion coefficients at low temperatures have been lower than the values extrapolated from high temperatures. Such behavior of diffusion properties has not been previously observed in other metal oxides. In this regard, the vacancy formation energy in the rare earth manganites has been supposed to increase with decreasing temperature.

Mechanical Activation of Mn-O Oxides: Structural Phase Transitions, Magnetism and Oxygen Isotope Exchange

The mechanically activated oxides MnO2, Mn2O3, Mn3O4 and MnO were studied under ambient and non-ambient temperature conditions. Regimes of mechanical treatment were found that allowed one to preserve the single-phase state without an essential change of oxide chemical composition. New data about the temperatures and sequence of phase transitions in mechanically activated manganese oxides as well as about the kinetics of isotope exchange were obtained. Magnetic properties of the treated oxides were measured in the temperature range of 4-300К. It is shown that mechano-activation essentially influenced the temperature and field dependences of magnetization, temperatures of magnetic phase transitions and leads to the appearance of additional magnetic phases. New data on the rates of surface reactions during isotope exchange and oxygen self-diffusion coefficients are obtained.

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.

Stable iron isotope fractionation between aqueous Fe(II) and model Archean ocean Fe–Si coprecipitates and implications for iron isotope variations in the ancient rock record

Iron isotope fractionation between aqueous Fe(II) (Fe(II)aq) and two amorphous Fe(III) oxide–Si coprecipitates was investigated in an aqueous medium that simulated Archean marine conditions, including saturated amorphous silica, low sulfate, and zero dissolved oxygen. The equilibrium isotope fractionation (in 56Fe/54Fe) between Fe(II)aq and Fe(III)-Si coprecipitates at circum-neutral pH, as inferred by the three-isotope method, was −3.51±0.20 (2σ)‰ and −3.99±0.17 (2σ)‰ for coprecipitates that had Fe:Si molar ratios of 1:2 and 1:3, respectively. These results, when combined with earlier work, indicate that the equilibrium isotope fractionation factor between Fe(II)aq and Fe(III)–Si coprecipitates changes as a function of Fe:Si ratio of the solid. Isotopic fractionation was least negative when Fe:Si=1:1 and most negative when Fe:Si=1:3. This change corresponds with changes in the local structure of iron, as revealed by prior spectroscopic studies. The kinetics of isotopic exchange was controlled by movement of Fe(II) and Si, where sorption of Fe(II) from aqueous to solid phase facilitated atom exchange, but sorption of Si hindered isotopic exchange through blockage of reactive surface sites. Although Fe(II)–Fe(III) isotopic exchange rates were a function of solid and solution compositions in the current study, in all cases they were much higher than that determined in previous studies of aqueous Fe(III) and ferrihydrite interaction, highlighting the importance of electron exchange in promoting Fe atom exchange. When compared to analogous microbial reduction experiments of overlapping Fe(II) to Fe(III) ratios, isotopic exchange rates were faster in the biological experiments, likely due to promotion of atom exchange by the solid-phase Fe(II) produced in the biological experiments. These results provide constraints for interpreting the relatively large range of Fe isotope compositions in Precambrian marine sedimentary rocks, and highlight important differences between modern and ancient marine environments due to the absence or presence of dissolved silica. Evidence can be found in the Fe isotope compositions of the ancient rock record for both abiologic and biologic processes, distinction of which becomes apparent when sedimentological and diagenetic processes are fully explored, as well as Fe mass balance.

Temperature Dependence of Ion and Water Diffusion in Crown Ether Loaded Nafion Matrix

Temperature dependence study of the self-diffusion coefficient of Cs(+) ion in dibenzo-18-crown-6 (DB18C6) modified Nafion-117 (Cs-Naf-CR) was carried out in the temperature range of 50-65 °C. Temperature dependence of water diffusion in Cs-Naf-CR was also studied to understand the mechanism of cation and water transport in the membrane. Because of the very slow kinetics of isotopic exchange, self-diffusion measurement of Na(+) in Na-Naf-CR was carried out only at 60 °C. The result indicates that self-diffusion behavior is governed by the nature of the cation in which the crown ether was loaded in the membrane matrix. The activation energy of diffusion for Cs(+) ion and water in Cs-Naf-CR was found to be much higher than that in the pure Cs(+) form of Nafion (Cs-Naf). Water uptake of the membrane was also found to have reduced compared to Cs/Na-Naf. The results point to the binding of the ions by DB18C6 and the destruction of the water channels in the crown ether loaded membrane. The differential scanning calorimetry (DSC) data supports these observations.

Fe isotope exchange between Fe(II) aq and nanoparticulate mackinawite (FeS m) during nanoparticle growth

We detail the results of an experimental study on the kinetics of Fe isotope exchange between aqueous Fe(II) aq and nanoparticulate mackinawite (FeS m) at 25 °C and 2 °C over a one month period. The rate of isotopic exchange decreases synchronously with the growth of FeS m nanoparticles. 100% isotopic exchange between bulk FeS m and the solution is never reached and the extent of isotope exchange asymptotes to a maximum of ~ 75%. We demonstrate that particle growth driven by Ostwald ripening would produce much faster isotopic exchange than observed and would be limited by the extent of dissolution–recrystallisation. We show that Fe isotope exchange kinetics are consistent with i) FeS m nanoparticles that have a core–shell structure, in which Fe isotope mobility is restricted to exchange between the surface shell and the solution and ii) a nanoparticle growth via an aggregation–growth mechanism. We argue that because of the structure of FeS m nanoparticles, the approach to isotopic equilibrium is kinetically restricted at low temperatures. FeS m is a reactive component in diagenetic pyrite forming systems since FeS m dissolves and reacts to form pyrite. Isotopic mobility and potential equilibration between FeS m and Fe(II) aq thus have direct implications for the ultimate Fe isotope signature recorded in sedimentary pyrite.

Brønsted acidity study of fiberglass materials by H/D-exchange

The kinetics of hydrogen isotopic exchange over fiberglass materials and reference samples (SiO 2, H 3PO 4/SiO 2, and HZSM-5) was studied at 500 °C under the dynamic adsorption–desorption equilibrium. The concentrations and H/D-exchange rates of different Brønsted acid sites (BAS) were determined using the numerical modeling of isotope exchange dynamics. The correlation between H/D-exchange rate of reference samples and their acidity determined by FTIR spectroscopy of adsorbed CO was established. Based on this correlation, the strength of BAS in fiberglass materials was shown to be comparable to that of HZSM-5 zeolite.

Kinetics of isotopic exchange between calcium molybdate and molybdate ions in aqueous solution

The heterogeneous isotopic anion exchange kinetics and equilibria between calcium molybdate and sodium molybdate solutions have been studied by using 99Mo as tracer in batch experiments. The values of exchange ratio lower than unity suggest that rate-limiting step is particle diffusion process and the effect of re-crystallization can be neglected. The self-diffusion coefficients calculated using both Paterson´s and Nernst–Plank approximations are increased by the temperature. The observed values for isotope exchange characteristics such as exchange fractions, exchanging amounts and fractional attainment of equilibrium are consistent with those of their calculated values. Activation energy and thermodynamic parameters calculated based on transition state theory indicate the existence of both energy and entropy barrier in the system.