Comparative carbonate and hydroxide complexation of cations in seawater

Comparative concentrations of carbonate and hydroxide complexes in natural solutions can be expressed in terms of reactions with bicarbonate that have no explicit pH dependence (). Stability constants for this reaction with n = 1 were determined using conventional formation constant data expressed in terms of hydroxide and carbonate. Available data indicate that stability constants appropriate to seawater at 25 °C expressed in the form are on the order of 10^4.2 for a wide range of cations (M^z+) with z = +1, +2 and +3. Φ₁ is sufficiently large that species appear to substantially dominate MOH^z−1 species in seawater. Evaluations of comparative stepwise carbonate and hydroxide stability constant behavior leading to the formation of n = 2 and n = 3 complexes suggest that carbonate complexes generally dominate hydroxide complexes in seawater, even for cations whose inorganic speciation schemes in seawater are currently presumed to be strongly dominated by hydrolyzed forms (). Calculated stability constants, and , indicate that the importance of carbonate complexation is sufficiently large that carbonate and hydroxide complexes would be generally comparable even if calculated Φ₂ and Φ₃ values are overestimated by two or more orders of magnitude. Inclusion of mixed ligand species in carbonate-hydroxide speciation models allows cation complexation intensities (M_T/[M^z+]) to be expressed in the following form:

     

Effect of the formation of EDTA complexes on the diffusion of metal ions in water

The diffusion coefficients of aquo metal ions (M^z+) and their EDTA complexes (M-EDTA^(z−4)+) were compared to understand the effect of EDTA complexation on the diffusion of metal ions by the diffusion cell method for Co²⁺, Ga³⁺, Rb⁺, Sr²⁺, Ag⁺, Cd²⁺, Cs⁺, Th⁴⁺, , and trivalent lanthanides. Most studies about ionic diffusion in water have dealt with free ion (hydrated ion). In many cases, however, polyvalent ions are dissolved as complexed species in natural waters. Hence, we need to study the diffusion behavior of complexes in order to understand the diffusion phenomenon in natural aquifer and to measure speciation by diffusive gradient in thin films (DGT), which requires the diffusion coefficients of the species examined. For many ions, the diffusion coefficients of M-EDTA^(z−4)+ are smaller than those of hydrated ions, but the diffusion coefficients of M-EDTA^(z−4)+ are larger than those of hydrated ions for ions with high ionic potentials (Ga³⁺ and Th⁴⁺). As a result, the diffusion coefficients of EDTA complexes are similar among various metal ions. In other words, the diffusion of each ion loses its characteristics by the complexation with EDTA. Although the difference is subtle, it was also found that the diffusion coefficients of EDTA complexes increase as the ionic potential increases, which can be explained by the size of the EDTA complex of each metal ion.     

A surface structural model for ferrihydrite II: Adsorption of uranyl and carbonate

The adsorption of uranyl (UO₂²⁺) on ferrihydrite has been evaluated with the charge distribution (CD) model for systems covering a very large range of conditions, i.e. pH, ionic strength, CO₂ pressure, U(VI) concentration, and loading. Modeling suggests that uranyl forms bidentate inner sphere complexes at sites that do not react chemically with carbonate ions. Uranyl is bound by singly-coordinated surface groups present at particular edges of Fe-octahedra of ferrihydrite while another set of singly-coordinated surface groups may form double-corner bidentate complexes with carbonate ions. The uranyl surface speciation strongly changes in the presence of carbonate due to the specific adsorption of carbonate ions as well as the formation of ternary uranyl-carbonate surface complexes. Data analysis with the CD model suggests that a uranyl tris-carbonato surface complex, i.e. (UO₂)(CO₃)₃⁴⁻, is formed. This species is most abundant in systems with a high pH and carbonate concentration. This finding differs significantly from previous interpretations made in the literature. At high pH and low carbonate concentrations, as can be prepared in CO₂-closed systems, the model suggests the additional presence of a ternary uranyl-monocarbonato complex. The binding mode (type A or type B complex) is uncertain. At high uranyl concentrations, uranyl polymerizes at the surface of ferrihydrite giving, for instance, tris-uranyl surface complexes with and without carbonate. The similarities and differences between U(VI) adsorption by goethite and ferrihydrite are discussed from a surface structural point of view.     

The origin of Zn isotope fractionation in sulfides

Isotope fractionation of Zn between aqueous sulfide, chloride, and carbonate species (Zn²⁺, Zn(HS)₂, , , ZnS(HS)⁻, ZnCl⁺, ZnCl₂, , and ZnCO₃) was investigated using ab initio methods. Only little fractionation is found between the sulfide species, whereas carbonates are up to 1‰ heavier than the parent solution. At pH > 3 and under atmospheric-like CO₂ pressures, isotope fractionation of Zn sulfides precipitated from sulfidic solutions is affected by aqueous sulfide species and the δ⁶⁶Zn of sulfides reflect these in the parent solutions. Under high P_CO2 conditions, carbonate species become abundant. In high P_CO2 conditions of hydrothermal solutions, Zn precipitated as sulfides is isotopically nearly unfractionated with respect to a low-pH parent fluid. In contrast, negative δ⁶⁶Zn down to at least −0.6‰ can be expected in sulfides precipitated from solutions with pH > 9. Zinc isotopes in sulfides and rocks therefore represent a potential indicator of mid to high pH in ancient hydrothermal fluids.     

Sorption of yttrium and rare earth elements by amorphous ferric hydroxide: Influence of solution complexation with carbonate

The influence of solution complexation on the sorption of yttrium and the rare earth elements (YREEs) by amorphous ferric hydroxide was investigated at 25 °C over a range of pH (4.0-7.1) and carbonate concentrations . Distribution coefficients, defined as , where [MS_i]_T is the total concentration of sorbed YREE, M_T is the total YREE concentration in solution, and [S_i] is the concentration of amorphous ferric hydroxide, initially increased in magnitude with increasing carbonate concentration, and then decreased. The initial increase of is due to sorption of YREE carbonate complexes , in addition to sorption of free YREE ions (M³⁺). The subsequent decrease of , which is more extensive for the heavy REEs, is due to the increasing intensity of YREE solution complexation by carbonate ions. The competition for YREEs between solution complexation and surface complexation was modeled via the equation:

     

Kinetic isotopic fractionation during diffusion of ionic species in water

Experiments specifically designed to measure the ratio of the diffusivities of ions dissolved in water were used to determine . The measured ratio of the diffusion coefficients for Li and K in water (D_Li/D_K = 0.6) is in good agreement with published data, providing evidence that the experimental design being used resolves the relative mobility of ions with adequate precision to also be used for determining the fractionation of isotopes by diffusion in water. In the case of Li, we found measurable isotopic fractionation associated with the diffusion of dissolved LiCl (D_⁷Li/D_⁶Li=0.99772±0.00026). This difference in the diffusion coefficient of ⁷Li compared to ⁶Li is significantly less than that reported in an earlier study, a difference we attribute to the fact that in the earlier study Li diffused through a membrane separating the water reservoirs. Our experiments involving Mg diffusing in water found no measurable isotopic fractionation (D_²⁵Mg/D_²⁴Mg=1.00003±0.00006). Cl isotopes were fractionated during diffusion in water (D_³⁷Cl/D_³⁵Cl=0.99857±0.00080) whether or not the co-diffuser (Li or Mg) was isotopically fractionated. The isotopic fractionation associated with the diffusion of ions in water is much smaller than values we found previously for the isotopic fractionation of Li and Ca isotopes by diffusion in molten silicate liquids. A major distinction between water and silicate liquids is that water surrounds dissolved ions with hydration shells, which very likely play an important but still poorly understood role in limiting the isotopic fractionation associated with diffusion.     

Boron isotopes as pH proxy: A new look at boron speciation in deep-sea corals using B MAS NMR and EELS

Dissolved boron in modern seawater occurs in the form of two species, trigonal boric acid B(OH)₃ and tetrahedral borate ion . One of the key assumption in the use of boron isotopic compositions of carbonates as pH proxy is that only borate ions, , are incorporated into the carbonate. Here, we investigate the speciation of boron in deep-sea coral microstructures (Lophelia pertusa specimen) by using high field magic angle spinning nuclear magnetic resonance (¹¹B MAS NMR) and electron energy-loss spectroscopy (EELS). We observe both boron coordination species, but in different proportions depending on the coral microstructure, i.e. centres of calcification versus fibres. These results suggest that careful sampling is necessary before performing boron isotopic measurements in deep-sea corals. By combining the proportions of B(OH)₃ and determined by NMR and our previous ion microprobe boron isotope measurements, we propose a new equation for the relation between seawater pH and boron isotopic composition in deep-sea corals.     

Modeling the effects of bond environment on equilibrium iron isotope fractionation in ferric aquo-chloro complexes

Four or five sets of ab initio models, including Unrestricted Hartree Fock (UHF) and hybrid Density Functional Theory (DFT) are calculated for each species in a series of aqueous ferric aquo-chloro complexes: , , , FeCl₃(H₂O)₃, FeCl₃(H₂O)₂, , FeCl₅H₂O²⁻, , ) in order to determine the relative isotopic fractionation among the complexes, to compare the results of different models for the same complexes, to examine factors that influence the magnitude of the isotopic fractionation, and to compare bond-partner-driven fractionation with redox-driven fractionation.Relative to , all models show a nearly linear decrease in ⁵⁶Fe/⁵⁴Fe as the number of Cl⁻ ions per Fe³⁺ ion increases, with slopes of −0.8‰ to −1.0‰ per Cl⁻ at 20 °C. At 20 °C, 1000 ln β (β = ⁵⁶Fe/⁵⁴Fe reduced partition function ratio relative to a dissociated Fe atom) values range from 8.93‰ to 9.73‰ for , 8.04-9.12‰ for , 7.61-8.73‰ for , 7.14-8.25‰ for , and 3.09-4.41‰ for . The fractionation between and ranges from 1.5‰ to 2.6‰, depending on the model; this is comparable in magnitude to fractionation effects due to Fe³⁺/Fe²⁺ redox reactions. β values from the UHF models are consistently higher than those from the hybrid DFT models.Isotopic fractionation is shown to be sensitive to differences in ligand bond stiffness (above), coordination number, bond length, and the frequency of the asymmetric Fe-X stretching vibrational mode, as predicted by previous theoretical studies. Complexes with smaller coordination numbers have higher 1000 ln β (7.46‰, 5.25‰, and 3.48‰ for , ,, respectively, from the B3LYP/6-31G(d) model). Species with the same number of chlorides but fewer waters also show the effect of coordination number on 1000 ln β: (7.46‰ vs. 7.05‰ for FeCl₃(H₂O)₂ vs. FeCl₃(H₂O)₃ and 5.25‰ vs. 4.94‰ for vs. FeCl₅H₂O²⁻ with the B3LYP/6-31G(d) model). As more Fe-Cl bonds substitute for Fe-OH₂ bonds (with a resulting decrease in β), the lengths of the Fe-Cl bonds and the Fe-O bonds increase.Preliminary modeling of shows an Fe³⁺/Fe²⁺ fractionation of 3.2‰ for the B3LYP/6-31G(d) model, in agreement with previous studies. The addition of an explicit outer hydration sphere of 12 H₂O molecules to models of improves agreement with measured vibrational frequencies and bond lengths; 1000 ln β increases by 0.8-1.0‰. An additional hydration sphere around increases 1000 ln β by only 0.1‰.Isotopic fractionations predicted for this simple system imply that ligands present in an aqueous iron environment are potentially important drivers of fractionation, and suggest that significant fractionation effects are likely in other aqueous systems containing sulfides or organic ligands. Fractionation effects due to both speciation and redox must be considered when interpreting iron isotope fractionations in the geological record.     

Isotopic fractionations associated with phosphoric acid digestion of carbonate minerals: Insights from first-principles theoretical modeling and clumped isotope measurements

Phosphoric acid digestion has been used for oxygen- and carbon-isotope analysis of carbonate minerals since 1950, and was recently established as a method for carbonate ‘clumped isotope’ analysis. The CO₂ recovered from this reaction has an oxygen isotope composition substantially different from reactant carbonate, by an amount that varies with temperature of reaction and carbonate chemistry. Here, we present a theoretical model of the kinetic isotope effects associated with phosphoric acid digestion of carbonates, based on structural arguments that the key step in the reaction is disproportionation of H₂CO₃ reaction intermediary. We test that model against previous experimental constraints on the magnitudes and temperature dependences of these oxygen isotope fractionations, and against new experimental determinations of the fractionation of ¹³C-¹⁸O-containing isotopologues (‘clumped’ isotopic species). Our model predicts that the isotope fractionations associated with phosphoric acid digestion of carbonates at 25 °C are 10.72‰, 0.220‰, 0.137‰, 0.593‰ for, respectively, ¹⁸O/¹⁶O ratios (1000 lnα^∗) and three indices that measure proportions of multiply-substituted isotopologues . We also predict that oxygen isotope fractionations follow the mass dependence exponent, λ of 0.5281 (where ). These predictions compare favorably to independent experimental constraints for phosphoric acid digestion of calcite, including our new data for fractionations of ¹³C-¹⁸O bonds (the measured change in Δ₄₇ = 0.23‰) during phosphoric acid digestion of calcite at 25 °C.We have also attempted to evaluate the effect of carbonate cation compositions on phosphoric acid digestion fractionations using cluster models in which disproportionating H₂CO₃ interacts with adjacent cations. These models underestimate the magnitude of isotope fractionations and so must be regarded as unsucsessful, but do reproduce the general trend of variations and temperature dependences of oxygen isotope acid digestion fractionations among different carbonate minerals. We suggest these results present a useful starting point for future, more sophisticated models of the reacting carbonate/acid interface. Examinations of these theoretical predictions and available experimental data suggest cation radius is the most important factor governing the variations of isotope fractionation among different carbonate minerals. We predict a negative correlation between acid digestion fractionation of oxygen isotopes and of ¹³C-¹⁸O doubly-substituted isotopologues, and use this relationship to estimate the acid digestion fractionation of for different carbonate minerals. Combined with previous theoretical evaluations of ¹³C-¹⁸O clumping effects in carbonate minerals, this enables us to predict the temperature calibration relationship for different carbonate clumped isotope thermometers (witherite, calcite, aragonite, dolomite and magnesite), and to compare these predictions with available experimental determinations. The success of our models in capturing several of the features of isotope fractionation during acid digestion supports our hypothesis that phosphoric acid digestion of carbonate minerals involves disproportionation of transition state structures containing H₂CO₃.     

A potentiometric study of the stability of aqueous yttrium-acetate complexes from 25 to 175 °C and 1-1000 bar

The stability of yttrium-acetate (Y-Ac) complexes in aqueous solution was determined potentiometrically at temperatures 25-175 °C (at P_s) and pressures 1-1000 bar (at 25 and 75 °C). Measurements were performed using glass H⁺-selective electrodes in potentiometric cells with a liquid junction. The species YAc²⁺ and were found to dominate yttrium aqueous speciation in experimental solutions at 25-100 °C (log [Ac⁻] < −1.5, pH < 5.2), whereas at 125, 150 and 175 °C introduction of into the Y-Ac speciation model was necessary. The overall stability constants β_n were determined for the reaction

     

Hydroxamate siderophore-promoted reactions between iron(II) and nitroaromatic groundwater contaminants

Recent studies show that ferrous iron (Fe^II), which is often abundant in anaerobic soil and groundwater, is capable of abiotically reducing many subsurface contaminants. However, studies also demonstrate that Fe^II redox reactivity in geochemical systems is heavily dependent upon metal speciation. This contribution examines the influence of hydroxamate ligands, including the trihydroxamate siderophore desferrioxamine B (DFOB), on Fe^II reactions with nitroaromatic groundwater contaminants (NACs). Experimental results demonstrate that ring-substituted NACs are reduced to the corresponding aniline products in aqueous solutions containing Fe^II complexes with DFOB and two monohydroxamate ligands (acetohydroxamic acid and salicylhydroxamic acid). Reaction rates are heavily dependent upon solution conditions and the identities of both the Fe^II-complexing hydroxamate ligand and the target NAC. Trends in the observed pseudo-first-order rate constants for reduction of 4-chloronitrobenzene (k_obs, s⁻¹) are quantitatively linked to the formation of Fe^II species with standard one-electron reduction potentials, (Fe^III/Fe^II), below −0.3 V. Linear free energy relationships correlate reaction rates with the (Fe^III/Fe^II) values of different electron-donating Fe^II complexes and with the apparent one-electron reduction potentials of different electron-accepting NACs, (ArNO₂). Experiments describing a redox auto-decomposition mechanism for Fe^II-DFOB complexes that occurs at neutral pH and has implications for the stability of hydroxamate siderophores in anaerobic environments are also presented. Results from this study indicate that hydroxamates and other Fe^III-stabilizing organic ligands can form highly redox-active Fe^II complexes that may contribute to the natural attenuation and remediation of subsurface contaminants.     

Boric acid adsorption on humic acids: Ab initio calculation of structures, stabilities, B NMR and B,B isotopic fractionations of surface complexes

Boric acid, B(OH)₃, forms complexes in aqueous solution with a number of bidentate O-containing ligands, HL⁻, where H₂L is C₂O₄H₂ (oxalic acid), C₃O₄H₄ (malonic acid), C₂H₆O₂ (ethylene glycol), C₆H₆O₂ (catechol), C₁₀H₈O₂ (dioxynaphthalene) and C₂O₃H₄ (glycolic acid). McElligott and Byrne [McElligott, S., Byrne, R.H., 1998. Interaction of and in seawater: Formation of . Aquat. Geochem.3, 345-356.] have also found B(OH)₃ to form an aqueous complex with . Recently Lemarchand et al. [Lemarchand, E., Schott, J., Gaillardeet, J., 2005. Boron isotopic fractionation related to boron sorption on humic acid and the structure of surface complexes formed. Geochim. Cosmochim. Acta69, 3519-3533] have studied the formation of surface complexes of B(OH)₃ on humic acid, determining ¹¹B NMR shifts and fitted values of formation constants, and ¹¹B, ¹⁰B isotope fractionations for a number of surface complexation models. Their work helps to clarify both the nature of the interaction of boric acid with the functional groups in humic acid and the nature of some of these coordinating sites on the humic acid. The determination of isotope fractionations may be seen as a form of vibrational spectroscopy, using the fractionating element as a local probe of the vibrational spectrum. We have calculated quantum mechanically the structures, stabilities, vibrational spectra, ¹¹B NMR spectra and ¹¹B,¹⁰B isotope fractionations of a number of complexes B(OH)₂L⁻ formed by reactions of the type:

     

The ionic strength dependence of lead (II) carbonate complexation in perchlorate media

Lead speciation in many aqueous geochemical systems is dominated by carbonate complexation. However, direct observations of Pb²⁺ complexation by carbonate ions are few in number. This work represents the first investigation of the equilibrium over a range of ionic strength. Through spectrophotometric observations of formation at 25 °C in NaHCO₃-NaClO₄ solutions, formation constants of the form were determined between 0.001 and 5.0 molal ionic strength. Formation constant results were well represented by the equation:

     

Carbon dioxide in silicate melts: A molecular dynamics simulation study

We have performed a series of molecular dynamics simulations aimed at the evaluation of the solubility of CO₂ in silicate melts of natural composition (from felsic to ultramafic). In making in contact within the simulation cell a supercritical CO₂ phase with a silicate melt of a given composition, we have been able to evaluate (i) the solubility of CO₂ in the P-T range 1473-2273 K and 20-150 kbar, (ii) the density change experienced by the CO₂-bearing melt, (iii) the respective concentrations of CO₂ and species in the melt, (iv) the lifetime and the diffusivity of these species and (v) the structure of the melt around the carbonate groups. The main results are the following:(1) The solubility of CO₂ increases markedly with the pressure in the three investigated melts (a rhyolite, a mid-ocean ridge basalt and a kimberlite) from about ∼2 wt% CO₂ at 20 kbar to ∼25 wt% at 100 kbar and 2273 K. The solubility is found to be weakly dependent on the melt composition (as far as the present compositions are concerned) and it is only at very high pressure (above ∼100 kbar) that a clear hierarchy between solubilities occurs (rhyolite < MORB < kimberlite). Furthermore at a given pressure the calculated solubility is negatively correlated with the temperature.(2) In CO₂-saturated melts, the proportion of carbonate ions is positively correlated with the pressure at isothermal condition and is negatively correlated with the temperature at isobaric condition (and vice versa for molecular CO₂). Furthermore, at fixed (P, T) conditions the proportion of carbonate ions is higher in CO₂-undersaturated melts than in the CO₂-saturated melt. Although the proportion of molecular CO₂ decreases when the degree of depolymerization of the melt increases, it is still significant in CO₂-saturated basic and ultrabasic compositions at high temperatures. This finding is at variance with experimental data on CO₂-bearing glasses which show no evidence of molecular CO₂ as soon as the degree of depolymerization of the melt is high (e.g. basalt). These conflicting results can be reconciled with each other by noticing that a simple low temperature extrapolation of the simulation data predicts that the proportion of molecular CO₂ in basaltic melts might be negligible in the glass at room temperature.(3) The carbonate ions are found to be transient species in the liquid phase, with a lifetime increasing exponentially with the inverse of the temperature. Contrarily to a usual assumption, the diffusivity of carbonate ions into the liquid silicate is not vanishingly small with respect to that of CO₂ molecules: in MORB they differ from each other by a factor of ∼6 at 1473 K and only a factor of ∼2 at 2273 K. Although the bulk diffusivity of CO₂ is governed primarily by the diffusivity of CO₂ molecules, the carbonate ions contribute significantly to the diffusivity of CO₂ in depolymerized melts.(4) Concerning the structure of the CO₂-bearing silicate melt, the carbonate ions are found to be preferentially associated with NBO’s of the melt, with an affinity for NBOs which exceeds that for BOs by almost one order of magnitude. This result explains why the concentration in carbonate ions is positively correlated with the degree of depolymerization of the melt and diminishes drastically in fully polymerized melts where the number of NBO’s is close to zero. Furthermore, the network modifier cations are not randomly distributed in the close vicinity of carbonate groups but exhibit a preferential ordering which depends at once on the nature of the cation and on the melt composition. However at the high temperatures investigated here, there is no evidence of long lived complexes between carbonate groups and metal cations.     

An in situ XAS study of copper(I) transport as hydrosulfide complexes in hydrothermal solutions (25-592 °C, 180-600 bar): Speciation and solubility in vapor and liquid phases

Chloride and hydrosulfide are the principal ligands assumed to govern transport of copper in hydrothermal fluids. Existing solubility experiments suggest that Cu(I)-hydrosulfide complexes are dominant compared to chloride complexes at low salinities in alkaline solutions (H₂S_(aq)/HS⁻ pH buffer), and may be important in transporting Cu in low density magmatic vapors, potentially controlling the liquid-vapor partitioning of Cu. This study provides the first in situ evidence of the solubility of copper sulfides and the nature and structure of the predominant Cu species in sulfur-containing fluids at temperatures up to 592 °C and pressures of 180-600 bar. XANES and EXAFS data show that at elevated T (?200 °C), Cu solubility occurs via a linear Cu complex. At 428 °C in alkaline solutions, Cu is coordinated by two sulfur atoms in a distorted linear coordination (angle ∼150-160°). This geometry is consistent with the species predicted by earlier solubility studies. In addition, in situ measurements of the solubility of chalcocite in 2 m NaHS solutions performed in this study are in remarkably good agreement with the solubilities calculated using available thermodynamic data for Cu(I)-hydrosulfide complexes, also supporting the interpretation of speciation in these studies and validating the extrapolation of low-T thermodynamic properties for to high P-T. Data on phase separation for the 2 m NaHS solution show that while significant amounts of copper can be partitioned into the vapor phase, there is no indication for preferential partitioning of Cu into the vapor. This is consistent with recent partitioning experiments conducted in autoclaves by Pokrovski et al. (2008a) and Simon et al. (2006). XANES data suggest that the species present in the low density phase is very similar to that present in the high density liquid, i.e., , although Cu(HS)(H₂S)⁰ cannot be excluded on the basis of XAS data.     

Diffusion of ionic tracers in the Callovo-Oxfordian clay-rock using the Donnan equilibrium model and the formation factor

The transient diffusion of cationic and anionic tracers through clay-rocks is usually modeled with parameters like porosity, tortuosity (and/or constrictivity), sorption coefficients, and anionic exclusion. Recently, a new pore scale model has been developed by Revil and Linde [Revil A. and Linde N. (2006) Chemico-electromechanical coupling in microporous media. J. Colloid Interface Sci.302, 682-694]. This model is based on a volume-averaging approach of the Nernst-Planck equation. The influence of the electrical diffuse layer is accounted for by a generalized Donnan equilibrium model through the whole connected pore space that is valid for a multicomponent electrolyte. This new model can be used to determine the composition of the pore water of the Callovo-Oxfordian clay-rock, the osmotic efficiency of bentonite as a function of salinity, the osmotic pressure, and the streaming potential coupling coefficient of clay-rocks. This pore scale model is used here to model the transient diffusion of ionic tracers (²²Na⁺, ³⁶Cl⁻, and ) through the Callovo-Oxfordian clay-rock. Speciation of shows that ∼1/3 of the SO₄ is tied-up in different complexes. Some of these complexes are neutral and are therefore only influence by the tortuosity of the pore space. Using experimental data from the literature, we show that all the parameters required to model the flux of ionic tracers (especially the mean electrical potential of the pore space and the formation factor) are in agreement with independent evaluations of these parameters using the osmotic pressure determined from in situ pressure measurements and HTO diffusion experiments.     

First-principles estimates of equilibrium magnesium isotope fractionation in silicate, oxide, carbonate and hexaaquamagnesium(2+) crystals

Equilibrium mass-dependent magnesium isotope fractionation factors are estimated for a range of crystalline compounds including oxides, silicates, carbonates, and salts containing the solvation complex. Fractionation factors for the gas-phase species Mg and MgO are also presented. Fractionation factors are calculated with density functional perturbation theory (DFPT), using norm-conserving pseudopotentials. The results suggest that there will be substantial inter-mineral fractionation, particularly between tetrahedrally coordinated Mg²⁺ in spinel (MgAl₂O₄) and the more common octahedrally coordinated Mg²⁺-sites in silicate and carbonate minerals. Isotope fractionations calculated for Mg²⁺ in hexaaquamagnesium(2+) salts are in good agreement with previous fractionation models of based on large molecular clusters (Black et al., 2007), but show possibly more significant disagreement with a more recent study (Rustad et al., 2010). These models further suggest that solvated , in the form of , will have higher ²⁶Mg/²⁴Mg than coexisting magnesite and dolomite. Calculated fractionations are consistent with Mg-isotope fractionations observed in peridotite mineral separates and inorganic carbonate precipitates. Predicted large, temperature-sensitive spinel-silicate fractionations, in particular, may find use in determining equilibration temperatures of peridotites and other high-temperature rock types.     

Isotopic fractionation of Mg(aq), Ca(aq), and Fe(aq) with carbonate minerals

Density-functional electronic structure calculations are used to compute the equilibrium constants for ²⁶Mg/²⁴Mg and ⁴⁴Ca/⁴⁰Ca isotope exchange between carbonate minerals and uncomplexed divalent aquo ions. The most reliable calculations at the B3LYP/6-311++G(2d,2p) level predict equilibrium constants K, reported as 10³ln (K) at 25 °C, of −5.3, −1.1, and +1.2 for ²⁶Mg/²⁴Mg exchange between calcite (CaCO₃), magnesite (MgCO₃), and dolomite (Ca_0.5Mg_0.5CO₃), respectively, and Mg²⁺(aq), with positive values indicating enrichment of the heavy isotope in the mineral phase. For ⁴⁴Ca/⁴⁰Ca exchange between calcite and Ca²⁺(aq) at 25 °C, the calculations predict values of +1.5 for Ca²⁺(aq) in 6-fold coordination and +4.1 for Ca²⁺(aq) in 7-fold coordination. We find that the reduced partition function ratios can be reliably computed from systems as small as and embedded in a set of fixed atoms representing the second-shell (and greater) coordination environment. We find that the aqueous cluster representing the aquo ion is much more sensitive to improvements in the basis set than the calculations on the mineral systems, and that fractionation factors should be computed using the best possible basis set for the aquo complex, even if the reduced partition function ratio calculated with the same basis set is not available for the mineral system. The new calculations show that the previous discrepancies between theory and experiment for Fe³⁺-hematite and Fe²⁺-siderite fractionations arise from an insufficiently accurate reduced partition function ratio for the Fe³⁺(aq) and Fe²⁺(aq) species.     

Complexation of Cu in Hydrothermal NaCl Brines: Ab initio molecular dynamics and energetics

Chloride complexation of Cu⁺ controls the solubility of copper(I) oxide and sulfide ore minerals in hydrothermal and diagenetic fluids. Solubility measurements and optical spectra of high temperature CuCl solutions have been interpreted as indicating the formation of CuCl, , and complexes. However, no other monovalent cation forms tri- and tetrachloro complexes. EXAFS spectra of high temperature Cu-Cl solutions, moreover, appear to show only CuCl and complexes at T > 100 °C. To reconcile these results, I investigated the nature and stability of Cu-Cl complexes using ab initio cluster calculations and ab initio (Car-Parrinello) molecular dynamics simulations for CuCl-NaCl-H₂O systems at 25 to 450 °C. Ab initio molecular dynamic simulations of 1 m CuCl in a 4 m Cl solution give a stable complex at 25 °C over 4 ps but show that the third Cl⁻ is weakly bound. When the temperature is increased along the liquid-vapour saturation curve to 125 °C, the complex dissociates into and Cl⁻; only forms at 325 °C and 1 kbar. Even in a 15.6 m Cl brine at 450 °C, only the complex forms over a 4 ps simulation run.Cluster calculations with a static dielectric continuum solvation field (COSMO) were used in an attempt directly estimate free energies of complex formation in aqueous solution. Consistent with the MD simulations, the complex is slightly stable at 25 °C but decreases in stability with decreasing dielectric constant (ε). The complex is predicted to be unstable at 25 °C and becomes increasingly unstable with decreasing dielectric constant. In hydrothermal fluids (ε < 30) both the and complexes are unstable to dissociation into and Cl⁻.The results obtained here are at odds with recent equations of state that predict and complexes are the predominant species in hydrothermal brines. In contrast, I predict that only complexes will be significant at T > 125 °C, even in NaCl-saturated brines. The high-temperature (T > 125 °C) optical spectra of CuCl solutions and solubility measurements of Cu minerals in Cl-brines need to be reinterpreted in terms of only the CuCl and complexes.