Values Of Fractionation Factors Research Articles

Molecular simulations of kinetic stable calcium isotope fractionation at the calcite-aqueous interface

The measurement of calcium stable isotope fractionation during crystal growth can provide insights into the dominant pathways and mechanisms of crystal growth, but these interpretations require a detailed understanding of mass dependent reaction rates and free energy states at the molecular scale. Here we report the results of free energy calculations used to determine the rate-limiting step controlling kinetic stable isotope fractionation during calcium ion attachment to active growth sites – kink sites – at the calcite surface. The free energy profile for attachment is combined with ion coordination states to reconstruct the multi-step molecular pathway for the ion attachment process. Dissociative desolvation of a water molecule coordinating the calcium ion approaching the kink is identified as the rate-limiting step, with a free energy barrier of ∼18kJ/mol, which is equal to the energetic cost of water molecule leaving Ca2+ coordination shell in the aqueous phase. Molecular dynamics simulations were used to investigate the dependence of this desolvation step on calcium ion mass. Both brute force MD and an analysis of mass dependent water exchange coefficients (kwex) give identical kinetic stable isotope fractionation factor values αf44/40=0.992, which corresponds to Δ44/40Ca of ∼ -8‰, originating from a mass-dependent transition frequency for the Ca-bound solvation water at the kink. This value is significantly higher than the fractionation factor associated with water exchange dynamics around Ca2+ in bulk aqueous solution determined here (αf44/40=0.999). Our results confirm that the growing calcite surface should be depleted in 44Ca with respect to the aqueous phase due to kinetically preferred desolvation of the lighter isotope. We conclude that the maximum kinetic stable isotope fractionation associated with calcium attachment to surfaces is not constant but can vary due to changes in the interfacial environment that affect local solvent dynamics during desolvation.

Hydrogen isotope fractionation variations of n-alkanes and fatty acids in algae and submerged plants from Tibetan Plateau lakes: Implications for palaeoclimatic reconstruction

The hydrogen isotope compositions (δD) of n-alkanes and fatty acids (FAs) are widely applied in palaeoclimatic reconstructions, and the determinations of their hydrogen isotope fractionation factor values (ε) are vital for quantitatively reconstructing past precipitation variations. Currently, studies on n-alkane and FA ε values focus on terrestrial plants, which, however, show large uncertainties because of the influence of evapotranspiration. Therefore, in this study, we analysed the ε values of algae and submerged plants immersed in lakes, which are not affected by evapotranspiration, to understand the hydrogen isotope fractionation of plant lipid synthesis. By investigating the δD values of lipids (n-alkanes and FAs) in algae and submerged plants and the δD values of co-existing water (including lake bottom water, surface sediment water, and leaf water of algae and submerged plants) from five Tibetan Plateau lakes, we find that the n-alkane ε values of algae and submerged plants show narrow changes, ranging from −176 to −159‰ and −167 to −142‰, respectively. The FA ε values of algae and submerged plants also show small variations, ranging from −160 to −121‰ (except Chara) and −161 to −138‰, respectively. Therefore, the average biosynthetic hydrogen isotope fractionation of these plants is −162‰ for n-alkanes and −145‰ for FAs, and the small ε differences between FAs and n-alkanes can be related to the different magnitudes of FA utilization in n-alkane synthesis. Finally, we find that the biosynthetic hydrogen isotope fractionation factors of aquatic plants are close to those of terrestrial grasses but slightly more negative than those of terrestrial woody plants. Thus, our results are helpful for understanding the hydrogen isotope fractionation variations in terrestrial plant lipids, which is beneficial for palaeohydrological reconstructions.

Evaluating Methane Oxidation Efficiencies in Experimental Landfill Biocovers by Mass Balance and Carbon Stable Isotopes

Biocovers are an alternative for mitigating fugitive and residual emissions of methane from landfills. In this study, we evaluated the performance of two experimental passive methane oxidation biocovers (PMOBs) constructed within the existing final cover of the St-Nicéphore landfill (Quebec, Canada). The biocovers were fed in a controlled manner with raw biogas and surface fluxes were obtained using static chambers. This enabled calculating mass balances of CH4 and oxidation efficiencies (f o_MB). Most of the time, f o_MB ≥ 92 % were obtained for loadings as high as 818 g CH4 m−2 day−1 (PMOB-2) and 290 g CH4 m−2 day−1 (PMOB-3B). The lowest efficiencies (f o_MB = 45.5 % and 34.0 %, respectively) were obtained during cold days (air temperature ~0 °C). Efficiencies were also calculated using stable isotopes (f o_SI); the highest f o_SI were 66.4 % for PMOB-2 and 87.3 % for PMOB-3B; whereas the lowest were 18.8 % and 23.1 %, respectively. However, f o_SI values reflect CH4 oxidation up to a depth of 0.10 m, which may partly explain the difference in regards to mass balance-derived efficiencies. Indeed, it is expected that a significant fraction of the total CH4 oxidation occurs within the zone near the surface, where there is greater O2 availability. The influence of the values of fractionation factors α ox and α trans were also evaluated in this paper.

Carbon and oxygen isotope fractionations between aragonite and calcite of shells from modern molluscs

Carbon and oxygen isotope fractionations between calcite and aragonite were investigated by analyzing marine gastropods and bivalves that lived under temperate to tropical climates. Species that secrete both aragonite and calcite layers were studied to ensure similitude of changes in the isotopic composition of water, in diet, and in metabolic activity during shell growth. Aragonite and calcite layers from the adult parts of the shell were identified and mapped by using Raman spectroscopy. Powder samples were obtained by micro‐drilling under a stereo microscope. Thirty‐six pairs of aragonite–calcite samples were obtained from ten gastropod and five bivalve species. Biogenic aragonite is 13C‐enriched by 0.95±0.81‰ and 18O‐enriched by 0.37±0.65‰ relative to co-existing biogenic calcite. Direction and magnitude of the carbon isotope fractionation are compatible with those already determined by using low‐temperature experimental approaches. The observed oxygen isotope difference between biogenic aragonite and calcite is assigned to the difference (≈0.4‰) in the acid fractionation factor values that must be taken into account during digestion of carbonate polymorphs at 90°C. It is concluded that biogenic calcium carbonate polymorphs precipitate close to, but not in isotopic equilibrium with seawater. Therefore, empirical oxygen isotope fractionation equations that were established on the basis of modern mollusc shells and ambient waters should be preferred for the calculation of aquatic paleotemperatures.

Vapor Pressure Isotope Effects of Water Studied by Molecular Orbital Calculations

H/D and 16O/18O fractionation factors between liquid water and water vapor in the temperature range of 0 to 100°C were calculated using the reduced partition function ratios of small water clusters, (H2O) n with n=1 to 10, obtained at the HF/6-31G(d) and B3LYP/6-311G(d) levels of theory. The calculated fractionation factor values were heavily dependent on the choice of the MO theory and the basis set. Both the calculation levels showed that the formation of hydrogen bonds in liquid water caused the heavier isotopes of hydrogen and oxygen to be preferentially fractionated into the liquid water phase. While the HF/6-31G(d) level calculations yielded better results of the absolute values of the H/D and 16O/18O fractionation factors, the B3LYP/6-311G(d) level calculations reproduced the slopes of their temperature dependence better than the HF/6-31G(d) level calculations. As a whole, the quantitative agreements between the experiment and the present calculations were not satisfactory. It was indicated that a proper choice of MO theory, the basis set and the number of hydrogen bonds per molecule in liquid water may reproduce well the experimental vapor pressure isotope effect data of water.

Vapor Pressure Isotope Effects of Water Studied by Molecular Orbital Calculations

H/D and 16O/18O fractionation factors between liquid water and water vapor in the temperature range of 0 to 100°C were calculated using the reduced partition function ratios of small water clusters, (H2O) n with n=1 to 10, obtained at the HF/6-31G(d) and B3LYP/6-311G(d) levels of theory. The calculated fractionation factor values were heavily dependent on the choice of the MO theory and the basis set. Both the calculation levels showed that the formation of hydrogen bonds in liquid water caused the heavier isotopes of hydrogen and oxygen to be preferentially fractionated into the liquid water phase. While the HF/6-31G(d) level calculations yielded better results of the absolute values of the H/D and 16O/18O fractionation factors, the B3LYP/6-311G(d) level calculations reproduced the slopes of their temperature dependence better than the HF/6-31G(d) level calculations. As a whole, the quantitative agreements between the experiment and the present calculations were not satisfactory. It was indicated that a proper choice of MO theory, the basis set and the number of hydrogen bonds per molecule in liquid water may reproduce well the experimental vapor pressure isotope effect data of water.

A Mixed Quantum-Classical Approach for Computing Effects of Intramolecular Motion on the Low-Barrier Hydrogen Bond

The fractionation factor is defined as the equilibrium constant for the reaction: R − H + DOH ↔ R − D + HOH. Of interest are values of fractionation factors for reactions where reactants and/or products form intramolecular low-barrier hydrogen bonds. Experimentally measured isotopic fractionation factors are usually interpreted via a one-dimensional potential energy surface along the intrinsic proton hydrogen bond coordinate. Such a one-dimensional picture cannot be completely correct. Intramolecular motions, such as vibrations and librations, can modulate the underlying potential energy surface along the hydrogen bond coordinate and thus affect the isotopic fractionation factor. We have recently generated a picture of the motion of the proton in a low-barrier hydrogen bond as taking place in an effective single-dimensional potential, which we term the potential of mean force (PMF). In this paper, we compute the PMF for a molecule with an intramolecular hydrogen bond in order to quantify the effect of intramolecular motions on the fractionation factor. The PMF and isotopic fractionation factor are computed with a combination of high-level density functional theory and molecular dynamics simulations.

Proton spin-lattice relaxation of colloidal silica suspensions in H2O/D2O mixtures

The proton spin-lattice relaxation rates of amorphous colloidal suspensions are reported over a wide Larmor frequency range for varying degrees of isotopic dilution. At low magnetic fields, the amplitude of the relaxation dispersion reveals a non-linear dependence upon deuteration. This behaviour is primarily attributed to isotopic fractionation effects of slow exchanging hydrogens situated at solvent-shielded sites on the silica surface, but which are involved in dipolar interactions with readily solvent accessible proton species. A theoretical spin relaxation model is proposed which accounts for the isotope dilution effects in a multiple-phase system composed of the abundant slow-relaxing bulk spins and spin environments in mutual dipolar interaction at the oxide interface. The broadness of the observed relaxation dispersions is treated as a superposition of a discrete number of Lorentzian-like power spectra. At least two relaxation components are resolved from the present dispersion data which obviously reflect the heterogeneity of the proton sites at the silica surface. Each component seems to consist of a water and a pH dependent silanol contribution. The exchange of the observed proton classes has been estimated to occur on the microsecond time scale. An overall quantitative analysis of our relaxation dispersion data at different H/D ratios reveals considerable isotope fractionation between some slow exchanging surface hydrogens and the water solvent. Fractionation factor values <0.5 were obtained.

A theoretical study of intra-molecular vibrational effects on fractionation factors for molecules containing intra-molecular low-barrier hydrogen bonds

The fractionation factor is defined as the equilibrium constant for the reaction; R–H+DOH ⇌ R–D+HOH. Of interest are values of fractionation factors that obtain for reactions where reactants and/or products form intra-molecular low-barrier hydrogen bonds. Experimentally measured isotopic fractionation factors are usually interpreted via a one-dimensional potential energy surface along the intrinsic proton hydrogen bond coordinate. Here we show that coupling the intrinsic proton coordinate to an intra-molecular vibrational mode has large effects on the values of isotopic fractionation factors and offer a picture for the fractionation factor for low-barrier hydrogen bonds based on the one-dimensional potential of mean force on the intrinsic proton coordinate.

Sulfur isotopic variations during seawater evaporation with fractional crystallization

Seawater was evaporated, stepwise isothermally at 23.5°C, for 73 days, up to a degree of evaporation of 138 × by H2O weight. At various stages of evaporation the precipitate was totally removed from the brine and the latter was allowed to evaporate further. The sulfur isotopic compositions of the precipitates and related brines show the following characteristics: The initial δ24S of the original seawater is +20‰. The δ34S of both precipitates and associated brines decrease gradually in the gypsum field up to the end of the halite field, where δ34Sprecipitate = + 19.09‰andδ34Sbrine = + 18.40‰. The precipitates are always enriched in 34S relative to the associated brines in these fields, but the enrichment becomes smaller towards the end of the halite field. A crossover. where the δ34S of the brines becomes higher than those of the precipitates, occurs at the beginning of the Mg-sulfate field. The δ34Sprecipitate increases from + 19.09‰ at the end of the halite field through +19.35‰ in the Mg-sulfate field to + 19.85‰ in the K-Mg-sulfate field, whereas the δ34Sbrine increased from +18.40‰, through +20.91‰ to +20.94‰, respectively. This evolution implies different values of fractionation factors (α) for the minerals precipitated at the late halite, Mg-sulfate and K-Mg-sulfate fields, other than that for gypsum (1.00165). The value of αprecipitate-residual brine would then be very slightly >1 in the late halite field and <1 in the two later fields.The experimental pattern of evolution of the δ34S-values of the precipitates is in good agreement with data for natural anhydrites interbedded in halites, where δ34S-values are lower relative to basal gypsum (and secondary anhydrite), and of primary minerals of the Mg- and K-Mg-sulfate facies, reported in evaporitic sequences, such as those of the Delaware (U.S.A.) and of the Zechstein (Germany) basins. Thus, these results shed new light on observations of natural evaporitic sequences and suggest that the compositional trend of δ34S under the present experimental conditions may simulate and explain natural evaporitic processes.