Ab-initio structure, energy and stable Fe isotope equilibrium fractionation of some geochemically relevant H-O-Fe complexes

The hexa-aqua complexes [Fe(H₂O)_6−m−n(OH)_n]⁽²⁻ⁿ⁾⁺n = 0 → 3, m = 0 → 6 − n; [Fe(H₂O)_6−m−n(OH)_n]⁽³⁻ⁿ⁾⁺n = 0 → 4, m = 0 → 6 − n were investigated by ab-initio methods with the aim of determining their ground-state geometries, total energies and vibrational properties by treating their inner solvation shell as part of their gaseous precursor¹ (or “hybrid approach”). After a gas-phase energy optimization within the Density Functional Theory (DFT), the molecules were surrounded by a dielectric representing the Reaction Field through an implicit Polarized Continuum Model (PCM). The exploration of several structural ligand arrangements allowed us to quantify the relative stabilities of the various ionic species and the role of the various forms of energy (solute-solvent electronic interaction, cavitation, dispersion, repulsion, liberation free energy) that contribute to stabilize the aqueous complexes. A comparison with experimental thermochemistries showed that ab-initio gas-phase + solvation energies are quite consistent with experimental evidence and allow the depiction of the most stable form in solution and the eventual configurational disorder of water/hydroxyl species around central cations. A vibrational analysis performed on the ⁵⁴Fe, ⁵⁶Fe, ⁵⁷Fe and ⁵⁸Fe isotopomers indicated important separative effects systematically affected by the extent of deprotonation. The role of the system’s redox state (fO₂) and acidity (pH) on the isotopic imprinting of the aqueous species in solution was investigated by coupling the separative effects with speciation calculations. The observed systematics provided a tool of general utility in the interpretation of the iron isotopic signature of natural waters. Applications to the interpretation of isotopic fractionation in solution dictated by redox equilibria and to the significance of the Fe-isotopic imprinting of Banded Iron Formations are given.     

Molecular modelling of rare earth element complexation in subduction zone fluids

Complexation of (trace) elements in fluids plays a critical role in determining element mobility in subduction zones, but to date, the atomic-scale processes controlling elemental solubilities are poorly understood. As a first step towards computer simulation of element complexation in subduction zone fluids, a thermodynamic cycle was developed to investigate the hydration environment and energetics of lanthanide complexes using density functional theory. The first solvation shell is explicitly defined and the remaining part of the aqueous fluid is modelled using a polarisable continuum model, which allows extrapolation to a broad pressure and temperature range.We illustrate our method by comparing solvation of lanthanide series elements in H₂O in the presence of fluoride or chloride for conditions relevant to subduction zones. The energetics of lanthanide- and lanthanide-fluoride/chloride hydration complexes were determined computationally. Calculated hydration free energies for trivalent lanthanides with explicit eight- and nine-fold coordinated first hydration shells show good agreement with literature data at room pressure and temperature. The hydration free energy is more negative for smaller complexes (heavy lanthanides) relative to larger complexes (light lanthanides), with the difference between La and Lu in water amounting to 361 kJ mol⁻¹. The hydration free energy of all lanthanide ions becomes less negative with increasing pressure (p) and temperature (T) up to 2.5 GPa and 1000 K (typical conditions in the upper part of subducting slabs). The free energy difference between light- and heavy-lanthanides remains essentially unchanged at elevated (p, T) conditions. There are minor geometrical differences in local hydration environment between light lanthanide-chloride (La-Nd) and heavy lanthanide-chloride (Pm-Lu) hydrated complexes, without a distinguishable energy difference. Complexation with fluoride is energetically more favourable than with chloride by 206 ± 4 kJ mol⁻¹ across the entire lanthanide series. The association of fluoride-water and chloride-water fragments with lanthanide-water complexes is energetically more favourable for aqueous lanthanide complexes surrounded by fewer first hydration shell water molecules.The methods developed in this study, in conjunction with simulation of the energetics of trace element incorporation into minerals, open the possibility to use molecular modelling to constrain elemental behaviour in subduction zones.     

Experimental superheating of water and aqueous solutions

The metastable superheated solutions are liquids in transitory thermodynamic equilibrium inside the stability domain of their vapor (whatever the temperature is). Some natural contexts should allow the superheating of natural aqueous solutions, like the soil capillarity (low T superheating), certain continental and submarine geysers (high T superheating), or even the water state in very arid environments like the Mars subsurface (low T) or the deep crustal rocks (high T). The present paper reports experimental measurements on the superheating range of aqueous solutions contained in quartz as fluid inclusions (Synthetic Fluid Inclusion Technique, SFIT) and brought to superheating state by isochoric cooling. About 40 samples were synthetized at 0.75 GPa and 530-700 °C with internally-heated autoclaves. Nine hundred and sixty-seven inclusions were studied by micro-thermometry, including measuring the temperatures of homogenization (Th: L + V → L) and vapor bubbles nucleation (Tn: L → L + V). The Th-Tn difference corresponds to the intensity of superheating that the trapped liquid can undergo and can be translated into liquid pressure (existing just before nucleation occurs at Tn) by an equation of state. Pure water (840-935 kg m⁻³), dilute NaOH solutions (0.1 and 0.5 mol kg⁻¹), NaCl, CaCl₂ and CsCl solutions (1 and 5 mol kg⁻¹) demonstrated a surprising ability to undergo tensile stress. The highest tension ever recorded to the best of our knowledge (−146 MPa, 100 °C) is attained in a 5 m CaCl₂ inclusion trapped in quartz matrix, while CsCl solutions qualitatively show still better superheating efficiency. These observations are discussed with regards to the quality of the inner surface of inclusion surfaces (high P-T synthesis conditions) and to the intrinsic cohesion of liquids (thermodynamic and kinetic spinodal). This study demonstrates that natural solutions can reach high levels of superheating, that are accompanied by strong changes of their physico-chemical properties.     

Kinetics of crystal nucleation in ionic solutions: Electrostatics and hydration forces

The heat of precipitation, the mean crystal size and the broadness of crystal size distribution of barium sulfate precipitating in aqueous solutions of different background electrolytes (KCl, NaCl, LiCl, NaBr or NaF), was shown to vary at constant thermodynamic driving force (supersaturation) and constant ionic strength depending on the salt present in solution. The relative inversion in the effect of respective background ions on the characteristics of barite precipitate was observed between two studied supersaturation (Ω) and ionic strength (IS) conditions. The crystal size variance (β²) increased in the presence of background electrolytes in the order LiCl < NaCl < KCl at Ω = 10^3.33 and IS = 0.03 M and KCl < NaCl < LiCl at Ω = 10^3.77 and IS = 0.09 M. At a given Ω and IS the respective size of barite crystals decreased with increasing β² in chloride salts of different cations and remained constant in sodium salts of different anions.We suggest that ionic salts affect the kinetics of barite nucleation and growth due to their influence on water of solvation and bulk solvent structure. This idea is consistent with the hypothesis that the kinetic barrier for barium sulfate nucleation depends on the frequency of water exchange around respective building units that can be modified by additives present in solution. In electrolyte solution the relative switchover between long range electrostatic interactions and short range hydration forces, which influence the dynamics of solvent exchange between an ion solvation shell and bulk fluid, results in the observed inversion in the effect of differently hydrated salts on nucleation rates and the resulting precipitate characteristics.     

Oxygen stable isotope fractionation behavior of cerussite and hydrocerussite: New results and reconciliation of the recent literature

Recent empirical and theoretical calculations of the temperature-dependant oxygen stable isotope fractionation behavior of cerussite have highlighted potential problems with earlier work on this topic. The synthetic cerussite which was used earlier by the lead author to determine fractionation factors was re-examined using energy dispersive X-ray analysis, and found to be internally contaminated with inclusions of the phase hydrocerussite at levels of 5-10% by volume. The volume of hydrocerussite present within the samples is not sufficient to explain the entire discrepancy between this work and the empirical and theoretical calculations made earlier by the second author of this paper. Regardless of the exact causes of experimental failure or kinetic effects, the hydrocerussite contamination and the difficulty of demonstrating that these experiments reached isotopic equilibrium suggest that the use of cerussite oxygen isotope fractionation factors determined by slow precipitation experiments be discontinued in favor of the empirically calibrated fractionation factor 1000 ln α_{cerussite-water} = 2.29(10⁶/T²) − 3.56. In addition, we have determined that the oxygen isotope fractionation factor between hydrocerussite and water at 20 °C is 1.0232.     

A method for activity calculations in saline and mixed solvent solutions at elevated temperature and pressure: A framework for geological phase equilibria calculations

Quantitative thermodynamic calculations that involve aqueous fluids have proved difficult because of the complexity of the interactions that occur within the fluids. Existing thermodynamic models are difficult to apply to mixed solvent or highly saline solutions at P > 0.3 GPa and T > 300 °C. This work constructs a method for activity-composition calculations in saline, mixed solvent, supercritical aqueous solutions. Mixing is formulated on a mole-fraction scale in terms of a set of independent end-members that describe composition and speciation within the solution. The ideal mixing term takes speciation into account and avoids problems with the common ion effect. Non-ideal interactions are represented by an activity coefficient term that combines a limited form of Debye-Hückel and a van Laar formulation. This approach, referred to as the DH-ASF model, is thermodynamically valid over a wide range of P, T and fluid composition. The value of the model lies in its broad applicability, and small number of calibration parameters. Experimental data from the literature for the systems NaCl-H₂O, KCl-H₂O, H₂O-SiO₂-CO₂, H₂O-NaCl-CO₂, H₂O-NaCl-SiO₂ and for H₂O-albite melts have been used to calibrate the DH-ASF model. Calculations were performed using Thermocalc, computer software that calculates equilibria for mineral-based chemical systems.¹ The model represents the data to within experimental error in most cases. Conditions modelled include pressures between 0.2 and 1.4 GPa, temperatures between 500 and 900 °C, and x_H2O from 0.1 to 1. Calibrated parameters are consistent with expectations based on the conceptual model for the fluid, and are relatively insensitive to changes in pressure and temperature for most examples. The DH-ASF model is thermodynamically valid for a range of P-T conditions that includes pressures from 0.1 to 2 GPa and temperatures from 200 to 1000 °C. A lack of experimental data restricts calibration of the model for many end-members. However, it may be possible to neglect parameters associated with end-members present in small amount. In this case, or with new experimental data for calibrations, the DH-ASF model allows previously inaccessible geological systems and processes to be modelled.     

Inner-Helmholtz potential development at the hematite (α-Fe2O3) (0 0 1) surface

Electric potentials of the (0 0 1) surface of hematite were measured as a function of pH and ionic strength in solutions of sodium nitrate and oxalic acid using the single-crystal electrode approach. The surface is predominantly charge-neutral in the pH 4-14 range, and develops a positive surface potential below pH 4 due to protonation of μ-OH⁰ sites (pK_1,1,0,int = −1.32). This site is resilient to deprotonation up to at least pH 14 (−pK_−1,1,0,int ? 19). The associated Stern layer capacitance of 0.31-0.73 F/m² is smaller than typical values of powders, and possibly arises from a lower degree of surface solvation. Acid-promoted dissolution under elevated concentrations of HNO₃ etches the (0 0 1) surface, yielding a convoluted surface populated by sites. The resulting surface potential was therefore larger under these conditions than in the absence of dissolution. Oxalate ions also promoted (0 0 1) dissolution. Associated electric potentials were strongly negative, with values as large as −0.5 V, possibly from metal-bonded interactions with oxalate. The hematite surface can also acquire negative potentials in the pH 7-11 range due to surface complexation and/or precipitation of iron species (0.0038 Fe/nm²) produced from acidic conditions. Oxalate-bearing systems also result in negative potentials in the same pH range, and may include ferric-oxalate surface complexes and/or surface precipitates. All measurements can be modeled by a thermodynamic model that can be used to predict inner-Helmholtz potentials of hematite surfaces.     

Solubility products of amorphous ferric arsenate and crystalline scorodite (FeAsO4 · 2H2O) and their application to arsenic behavior in buried mine tailings

Published solubility data for amorphous ferric arsenate and scorodite have been reevaluated using the geochemical code PHREEQC with a modified thermodynamic database for the arsenic species. Solubility product calculations have emphasized measurements obtained under conditions of congruent dissolution of ferric arsenate (pH < 3), and have taken into account ion activity coefficients, and ferric hydroxide, ferric sulfate, and ferric arsenate complexes which have association constants of 10^4.04 (FeH₂AsO₄²⁺), 10^9.86 (FeHAsO₄⁺), and 10^18.9 (FeAsO₄). Derived solubility products of amorphous ferric arsenate and crystalline scorodite (as log K_sp) are −23.0 ± 0.3 and −25.83 ± 0.07, respectively, at 25 °C and 1 bar pressure. In an application of the solubility results, acid raffinate solutions (molar Fe/As = 3.6) from the JEB uranium mill at McClean Lake in northern Saskatchewan were neutralized with lime to pH 2-8. Poorly crystalline scorodite precipitated below pH 3, removing perhaps 98% of the As(V) from solution, with ferric oxyhydroxide (FO) phases precipitated starting between pH 2 and 3. Between pH 2.18 and 7.37, the apparent log K_sp of ferric arsenate decreased from −22.80 to −24.67, while that of FO (as Fe(OH)₃) increased from −39.49 to −33.5. Adsorption of As(V) by FO can also explain the decrease in the small amounts of As(V)(aq) that remain in solution above pH 2-3. The same general As(V) behavior is observed in the pore waters of neutralized tailings buried for 5 yr at depths of up to 32 m in the JEB tailings management facility (TMF), where arsenic in the pore water decreases to 1-2 mg/L with increasing age and depth. In the TMF, average apparent log K_sp values for ferric arsenate and ferric hydroxide are −25.74 ± 0.88 and −37.03 ± 0.58, respectively. In the laboratory tests and in the TMF, the increasing crystallinity of scorodite and the amorphous character of the coexisting FO phase increases the stability field of scorodite relative to that of the FO to near-neutral pH values. The kinetic inability of amorphous FO to crystallize probably results from the presence of high concentrations of sulfate and arsenate.     

LA-MC-ICPMS Pb-Pb dating of rutile from slowly cooled granulites: Confirmation of the high closure temperature for Pb diffusion in rutile

Rapid Pb-Pb dating of natural rutile crystals by laser ablation multiple-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) is investigated as a tool for constraining geological temperature-time histories. LA-MC-ICPMS was used to analyse Pb isotopes in rutile from granulite-facies rocks from the Reynolds Range, Northern Territory, Australia. The resultant ages were compared with previous U-Pb zircon and monazite age determinations and new mica (muscovite, phlogopite, and biotite) Rb-Sr ages from the same metamorphic terrane. Rutile crystals ranging in size from 3.5 to 0.05 mm with ?20 ppm Pb were ablated with a 300-25 μm diameter laser beam. Crystals larger than 0.5 mm yielded sufficiently precise ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios to correct for the presence of common Pb, and individual rutile crystals often exhibited sufficient Pb isotopic heterogeneity to allow isochron calculations to be performed on replicate analyses of a single crystal. The mean of 12 isochron ages is 1544 ± 8 Ma (2 SD), with isochron ages for single crystals having uncertainties as low as ±1.3 Myr (2 SD). The ²⁰⁷Pb-²⁰⁶Pb ages calculated without correction for common Pb are typically <0.5% higher than the common-Pb-corrected isochron ages reflecting the very minor amounts of common Pb present in the rutile. The LA-MC-ICPMS method described samples only the outer 0.1-0.2 mm of the rutile crystals, resulting in a grain size-independent apparent closure temperature (T_c) for Pb diffusion in rutile that is less than the T_c of monazite ?0.1 mm in diameter, but significantly higher than the Rb-Sr system in muscovite (550 °C), phlogopite (435 °C) and biotite (400 °C). Even small rutile crystals are extremely resistant to isotopic resetting. For the established slow cooling rate of ca. 3 °C/Myr, the T_c for Pb diffusion in the analysed rutile is ca. 630 °C. This is in excellent agreement with recent experimental results that indicate that rutile has a higher T_c than previously thought (ca. 600-640 °C for rutile 0.1-0.2 mm diameter cooled at 3 °C/Myr; near 600 °C [Cherniak D.J., 2000. Pb diffusion in rutile. Contrib. Mineral. Petrol. 139, 198-207], versus 400 °C [Mezger, K., Hanson G.N., Bohlen S.R., 1989a. High precision U-Pb ages of metamorphic rutile: applications to the cooling history of high-grade terranes. Earth Planet. Sci. Lett. 96, 106-118.] for 1 °C/Myr), and with current T_c estimates for monazite and other high temperature geochronometers, which have been revised upwards in recent years. The new rutile ages, together with the other geochronological data from the region, support the interpretation that the Reynolds Range underwent prolonged slow cooling on a conductive geotherm, under nearly steady-state conditions. Slow cooling at ca. 3 °C/Myr persisted for at least 40 Myr followed the peak of high-T/low-P metamorphism to granulite-facies conditions, and probably continued at ca. 2-3 °C/Myr for ca. 200 Myr overall.     

Quantum chemical study of the Fe(III)-desferrioxamine B siderophore complex—Electronic structure, vibrational frequencies, and equilibrium Fe-isotope fractionation

This study presents molecular orbital/density functional theory (MO/DFT) calculations of the electronic structure, vibrational frequencies, and equilibrium isotope fractionation factors for iron desferrioxamine B (Fe-DFO-B) complexes in aqueous solution. In general, there was good agreement between the predicted properties of Fe(III)-DFO-B and previously published experimental and theoretical results. The predicted fractionation factor for equilibrium between Fe(III)-DFO-B and Fe(III)-catecholate at 22 °C, 0.68 ± 0.25‰, was in good agreement with a previously measured isotopic difference between bacterial cells and solution during the bacterial-mediated dissolution of hornblende [Brantley S. L., Liermann L. and Bullen T. D. (2001) Fractionation of Fe isotopes by soil microbes and organic acids. Geology29, 535-538]. Conceptually, this agreement is consistent with the notion that Fe is first removed from mineral surfaces via complexation with small organic acids (e.g., oxalate), subsequently sequestered by DFO-B in solution, and ultimately delivered to bacterial cells by Fe(III)-DFO-B complexes. The ability of DFO-B to discriminate between Fe(III) and Fe(II)/Al(III) was investigated with Natural Bond Orbital (NBO) analysis and geometry calculations of each metal-DFO-B complex. The results indicated that higher affinity for Fe(III) is not strictly a function of bond length but also the degree of Fe-O covalent bonding.     

Ab-initio structure, energy and stable Cr isotopes equilibrium fractionation of some geochemically relevant H-O-Cr-Cl complexes

Twelve Cr(III) molecules and six Cr(VI) molecules of geochemical interest in the system Cr-H-O-Cl were investigated by ab initio Density Functional Theory in the aim of determining stationary state geometries, electronic energies and vibrational properties. The vibrational analysis conducted on the ⁵⁰Cr, ⁵²Cr, ⁵³Cr and ⁵⁴Cr isotopomers indicates important separative effects which result in mass spectrometric measurable fractionation factors in a wide thermal range. The effect of the central cation on the computed vibrational modes is interpreted in terms of Redlich’s product rule with the introduction of an isotopic ergodicity factor related to the effect of bulk molecular masses and momenta of inertia about the principal rotational axes. After the initial gas phase optimization, some relevant molecules were subjected to the effects of reaction field through Onsager’s Model, Tomasi’s continuum Model and Conductor-like Screening Model procedures. Structures and vibrational properties prove in all cases not to be severely affected by solvation, although the effect of the reaction field is to reduce somewhat the isotopic fractionation in Cr(III)-Cr(VI) aqueous redox equilibria, with respect to the gaseous state. Some of the computed separative effects are applicable to geochemical investigations.     

The influence of Fe(II) competition on the sorption and migration of Ni(II) in MX-80 bentonite

The results from batch sorption experiments on montmorillonite systems have demonstrated that bivalent transition metals compete with one another for sorption sites. For safety analysis studies of high level radioactive waste repositories with compacted bentonite near fields, the importance of competitive sorption on the migration of radionuclides needs to be evaluated. Under reducing conditions, the bentonite porewater chosen has a Fe(II) concentration of ∼5.3 × 10⁻⁵ M through saturation with siderite. The purpose of this paper is to assess the influence of such high Fe(II) concentrations on the transport of Ni(II) through compacted bentonite, Ni(II) was chosen as an example of a bivalent transition metal. The one-dimensional calculations were carried out at different Ni(II) equilibrium concentrations at the boundary (Ni(II)_EQBM) with the reactive transport code MCOTAC incorporating the two site protolysis non electrostatic surface complexation/cation exchange sorption model, MCOTAC-sorb. At a Ni(II)_EQBM level of 10⁻⁷ M without Fe(II) competition, the reactive transport calculations using a constant K_d approach and the MCOTAC-sorb calculation yielded the same breakthrough curves. At higher Ni(II)_EQBM (10⁻⁵ M), the model calculations with MCOTAC-sorb indicated a breakthrough which was shifted to later times by a factor of ∼5 compared with the use of the constant K_d approach.When sorption competition was included in the calculations, the magnitude of the influence depended on the sorption characteristics of the two competing sorbates and their respective concentrations. At background Fe(II) concentrations of 5.3 × 10⁻⁵ M, and a Ni(II)_EQBM level of 10⁻⁷ M, the Ni(II) breakthrough time was ∼15 times earlier than in the absence of competition. At such Fe(II) concentrations the Ni(II) breakthrough curves at all source concentrations less than 3.5 × 10⁻⁵ M (fixed by the NiCO_3,S solubility limit) are the same i.e. Ni(II) exhibits linear (low) sorption.Competitive sorption effects can have significant influences on the transport of radionuclides through compacted bentonite i.e. reduce the migration rates. Since, for the case considered here, the Fe(II) concentration in the near field of a high-level radioactive waste repository may change in time and space, the transport of bivalent transition metal radionuclides can only be properly modelled using a multi-species reactive transport code which includes a sorption model.     

Theoretical calculation of oxygen isotope fractionation factors in carbonate systems

Using established methods of statistical mechanical calculation and a recent compilation of vibrational frequency data, we have computed oxygen isotope reduced partition function ratios (β values) for a large number of carbonate minerals. The oxygen isotope β values of carbonates are inversely correlated to both the mass and radius of the cation bonded to the carbonate anion but neither correlation is good enough to be used as a precise and accurate predictor of β values. There is an approximately 0.6% relative increase in the β values of aragonite per 10 kbar increase in pressure. These estimates of the pressure effect on β values are broadly similar to those deduced previously for calcite using the methods of mineral physics. In comparing the β values of our study with those derived recently from first-principles lattice dynamics calculations, we find near-perfect agreement for calcite and witherite (<0.3% deviation), reasonable agreement for dolomite (<0.9% deviation) and somewhat poorer agreement for aragonite and magnesite (1.5-2% deviation). In the system for which we have the most robust constraints, CO₂-calcite, there is excellent agreement between our calculations and experimental data over a broad range of temperatures (0-900 °C). Similarly, there is good to excellent correspondence between calculation and experiment for most other low to moderate atomic mass carbonate minerals (aragonite to strontianite). The agreement is not as good for high atomic mass carbonates (witherite, cerussite, otavite). In the case of witherite and cerussite, the discrepancy may be due, in part, to our calculation methodology, which does not account for the effect of cation mass on the magnitude of vibrational frequency shifts associated with heavy isotope substitution. However, the calculations also reveal an incompatibility between the high- and low-temperature experimental datasets for witherite and cerussite. Specifically, the shapes of fractionation factor versus 1/T² curves in the calcite-witherite and calcite-cerussite systems do not conform to the robust constraints on the basic shape of these curves provided by theory. This suggests that either the high- or low-temperature datasets for both minerals is in error. Dolomite-calcite fractionation factors derived from our calculations fall within the wide range of fractionations for this system given by previous experimental and natural sample studies. However, our compilation of available low-temperature (25-80 °C) experimental data reveal an unusual temperature dependence of fractionations in this system; namely, the data indicate an increase in the magnitude of fractionations between dolomite (or proto-dolomite) and calcite with increasing temperature. Such a trend is incompatible with theory, which stipulates that fractionations between carbonate minerals must decrease monotonically with increasing temperature. We propose that the anomalous temperature dependence seen in the low-temperature experimental data reflect changes in the crystallinity and degree of cation ordering of the dolomite phase over this temperature interval and the effect these changes have on the vibrational frequencies of dolomite. Similar effects may be present in natural systems at low-temperature and must be considered in applying experimental or theoretical fractionation data to these systems. In nearly all cases, carbonate mineral-calcite fractionation factors given by the present calculations are in as good or better agreement with experimental data than are fractionations derived from semi-empirical bond strength methods.     

Kinetics of iron (II) oxidation determined in the field

This paper presents the results of extensive field trials measuring rates of Fe(II) oxidation at a number of Fe-bearing mine drainage discharges in the UK. Batch experiments were carried out with samples taken at regular intervals and Fe(II) concentration determined spectrophotometrically using 2′2-bipyridyl as the complexing agent. Initial concentrations for Fe(II) were 5.65-76.5 mg/L. Temperature, pH and dissolved O₂ (DO) were logged every 10 s, with pH at the start of the experiments in the range 5.64-6.95 and alkalinity ranging from 73 to 741 mg/L CaCO₃ equivalent. A numerical model based on a fourth order Runge-Kutta method was developed to calculate values for k₁, the rate constant for homogeneous oxidation, from the experimental data. The measured values of pH, temperature, [Fe(II)] and DO were input into the model with resulting values for k₁ found to be in the range 2.7 × 10¹⁴-2.7 × 10¹⁶ M⁻² atm⁻¹ min⁻¹. These values for k₁ are 1-3 orders of magnitude higher than previously reported for laboratory studies at a similar pH. Comparison of the observed Fe(II) oxidation rates to data published by other authors show a good correlation with heterogenous oxidation rates and may indicate the importance of autocatalysis in these systems. These higher than expected rates of Fe oxidation could have a significant impact on the design of treatment schemes for the remediation of mine drainage and other Fe-bearing ground waters in the future.     

Solvation processes in steam: Ab initio calculations of ion-solvent structures and clustering equilibria

Reports of the high ion content of steam and low-density supercritical fluids date back to the work of Carlon [Carlon H. R. (1980) Ion content of air humidified by boiling water.J. Appl.Phys.51, 171-173], who invoked ion and neutral-water clustering as mechanism to explain why ions partition into the low-density aqueous phase. Mass spectrometric, vibrational spectroscopic measurements and quantum chemical calculations have refined this concept by proposing strongly bound ion-solvent aggregates and water clusters such as Eigen- and Zundel-type proton clusters H₃O⁺·(H₂O)_m and the more weakly bound water oligomers (H₂O)_m. The extent to which these clusters affect fluid chemistry is determined by their abundance, however, little is known regarding the stability of such moieties in natural low-density high-temperature fluids. Here we report results from quantum chemical calculations using chemical-accuracy multi-level G3 (Curtiss-Pople) and CBS-Q theory (Peterson) to address this question. In particular, we have investigated the cluster structures and clustering equilibria for the ions and H₃S⁺·(H₂O)_m(H₂S)_n, where m ? 6 and n ? 4, at 300-1000 K and 1 bar as well as under vapor-liquid equilibrium conditions between 300 and 646 K. We find that incremental hydration enthalpies and entropies derived from van’t Hoff analyses for the attachment of H₂O and H₂S onto H₃O⁺, and H₃S⁺ are in excellent agreement with experimental values and that the addition of water to all three ions is energetically more favorable than solvation by H₂S. As clusters grow in size, the energetic trends of cluster hydration begin to reflect those for bulk H₂O liquids, i.e. calculated hydration enthalpies and entropies approach values characteristic of the condensation of bulk water (ΔH^o = −44.0 kJ mol⁻¹, ΔS^o = −118.8 J K mol⁻¹). Water and hydrogen sulfide cluster calculations at higher temperatures indicate that a significant fraction of H₃O⁺, and H₃S⁺ ions exists as solvated moieties.     

Placing constraints on phase equilibria and thermophysical properties in the system MgO-SiO2 by a thermodynamically consistent vibrational method

We use a lattice vibrational technique to derive thermophysical and thermochemical properties and phase equilibria in the system MgO-SiO₂ at pressures and temperatures relevant to Earth’s mantle. The technique is based on an extension of Kieffer’s model to incorporate details of the phonon spectrum, and it includes treatment of intrinsic anharmonicity. We use a least squares inversion technique applied to available experimental data, and show that it results in an accurate representation of thermodynamic properties and sound wave velocities of high-pressure phases in the system MgSiO₃. The vibrational method has been validated against laboratory experimental data in the temperature range between 0 and 2500 K and at pressures between 1 bar and 30 GPa. The technique results in a phase diagram consistent with the majority of thermophysical and thermochemical data. It is shown that intrinsic anharmonicity affects significantly slopes and positions of the phase boundaries. Our analysis indicates inconsistencies in a number of data sets of thermophysical properties for stishovite, majorite and ortho-enstatite necessitating new measurements. For akimotoite elasticity data at high-pressure and high-temperature conditions and 1 bar heat capacity measurements are needed. For stishovite elasticity measurements are necessary to reconcile elasticity data with V-P-T measurements. Additionally V-P-T measurements at pressures higher than 10 GPa are needed, which should be reconciled with V-P-T data at lower pressures. Raman and infrared spectroscopic data are necessary for both clino-enstatite and majorite. Additionally structural data are needed to resolve the discrepancy between values for the degree of disorder in majorite. Volume expansion data for ortho-enstatite are needed and effects causing differences in measured volume expansion should be elucidated.     

Iron isotopes in natural carbonate minerals determined by MC-ICP-MS with a Fe-Fe double spike

We have developed a method for iron isotope analysis by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) using a ⁵⁸Fe-⁵⁴Fe double spike. A 20 min analysis produces mass-bias-corrected iron isotope data with an external reproducibility of ±0.05 (2 SD) on δ⁵⁶Fe, which represents a decrease in analysis time compared to sample-standard bracketing techniques. The estimation of external reproducibility is based on replicate analysis of the ETH hematite in-house standard. The double spike method has two advantages. First, matrix effects during MC-ICP-MS analysis are decreased with tests showing that accurate iron isotope data can, in some cases, be obtained even when matrix levels exceed iron concentration (Na/Fe, Mg/Fe, and Ca/Fe up to 5, 2, and 0.1, respectively). Because chemical separation reduces matrix/Fe to levels more than three orders of magnitude lower than this, measured Fe isotope compositions are unlikely to be compromised by matrix effects. Second, it is possible to spike samples before chemical purification, which enables any isotopic fractionation effect because of incomplete recovery of iron from a sample to be accounted for. This may be important where obtaining quantitative iron yields from samples is difficult, such as the extraction of dissolved iron from water samples. Fe isotope data on a set of standard reference materials (igneous rocks, ferromanganese nodules, sedimentary rocks, and ores) are presented, which are in agreement with previously published data considering analytical uncertainties. Mantle-derived standard rock samples that are the source of iron for surficial, (bio)geochemical cycling yield a mean δ⁵⁶Fe of 0.041 ± 0.11‰ (n = 8; 2 SD) with reference to IRMM-14. Hydrothermal and metamorphic calcium carbonate rocks with a relatively low iron content (100-4000 ppm) have δ⁵⁶Fe = −1.25 to −0.07‰. Structural Fe(II) in hydrothermal calcites has δ⁵⁶Fe = −1.25 to −0.27‰. The light iron in this range of carbonate minerals may reflect the iron isotope composition of the hydrothermal fluids from which the carbonate precipitated, or the presence of Fe(III) and/or organic material in the hydrothermal fluids during calcite precipitation.     

Enhancement and inhibition of iron photoreduction by individual ligands in open ocean seawater

In laboratory experiments, we investigated the effect of five individual Fe-binding ligands: phaeophytin, ferrichrome, desferrioxamine B (DFOB), inositol hexaphosphate (phytic acid), and protoporphyrin IX (PPIX) on the Fe(II) photoproduction using seawater of the open Southern Ocean. Addition of 10-100 nM Fe(III) to open Southern Ocean seawater without the model ligands and containing; 1.1 nM dissolved Fe(III), 1.75 ± 0.28 equivalents of nM Fe of natural ligands with a conditional stability constant (log K′) of 21.75 ± 0.34 and a concentration DOC of 86.8 ± 1.13 μM C leads to the formation of amorphous Fe(III) hydroxides. These amorphous Fe(III) hydroxides are the major source for the photoproduction of Fe(II). The addition of the model ligands changed the Fe(II) photoproduction considerably and in various ways. Phaeophytin showed higher Fe(II) photoproduction than ferrichrome and the control, i.e., amorphous Fe(III) hydroxides. Additions of phytic acid between 65 and 105 nM increased the concentration of photoproduced Fe(II) with 0.16 nM Fe(II) per nM phytic acid, presumably due to the co-aggregation of Fe(III) and phytic acid leading via an increasing colloidal surface to an increasing photoreducible Fe(III) fraction. DFOB and PPIX strongly decreased the photoproduced Fe(II) concentration. The low Fe(II) photoproduction with DFOB confirmed reported observations that Fe(III) complexed to DFOB is photo-stable. The PPIX hardly binds Fe(III) in the open Southern Ocean seawater but decreased the photoproduced Fe(II) concentration by complexing the Fe(II) with a binding rate constant of k_Fe(II)PPIX = 1.04 × 10⁻⁴ ± 1.53 × 10⁻⁵ s⁻¹ nM⁻¹ PPIX. Subsequently, PPIX is suggested to act as a photosensitizing producer of superoxide, thus increasing the dark reduction of Fe(III) to Fe(II). Our research shows that the photochemistry of Fe(III) and the resulting photoproduced Fe(II) concentration is strongly depending on the identity of the Fe-binding organic ligands and that a translation to natural conditions is not possible without further characterization of the natural occurring ligands.     

The Fe-rich liquidus in the Fe-FeS system from 1 bar to 10 GPa

The composition and evolution of a metallic planetary core is determined by the behavior with pressure of the eutectic and the liquidus on the Fe-rich side of the Fe-FeS eutectic. New experiments at 6 GPa presented here, along with existing experimental data, inform a thermodynamic model for this liquidus from 1 bar to at least 10 GPa. Fe-FeS has a eutectic that becomes more Fe-rich but remains constant in T up to 6 GPa. The 1 bar, 3 GPa, and 6 GPa liquidi all cross at a pivot point at 1640 ± 5 K and FeS_37 ± 0.5. This liquid/crystalline metal equilibrium is T-x-fixed and pressure independent through 6 GPa. Models of the 1 bar through 10 GPa experimental liquidi show that with increasing P there is an increase in the T separation between the liquidus and the crest of the metastable two-liquid solvus. The solvus crest decreases in T with increasing P. The model accurately reproduces all the experimental liquidi from 1 bar to 10 GPa, as well as reproducing the 0-6 GPa pivot point. The 14 GPa experimental liquidus ( [Chen et al., 2008a] and Chen et al., 2008b) deviates sharply from the lower pressure trends indicating that the 0-10 GPa model no longer applies to this 14 GPa data.