Standard Partial Molal Thermodynamic Properties Research Articles

A spectrophotometric study of samarium (III) speciation in chloride solutions at elevated temperatures

The speciation of samarium (III) in chloride-bearing solutions was investigated spectrophotometrically at temperatures of 100–250 °C and a pressure of 100 bars. The simple hydrated ion, Sm 3+, is predominant at ambient temperature, but chloride complexes are the dominant species at elevated temperatures. Cumulative formation constants for samarium chloride species were calculated for the following reactions: Sm 3 + + Cl - = SmCl 2 + β 1 Sm 3 + + 2 Cl - = SmCl 2 + β 2 Within experimental error, the values for the first formation constant ( β 1), are identical to the values predicted by Haas et al. [Haas J. R., Shock E. L. and Sassani D. C. (1995) Rare earth elements in hydrothermal systems: estimates of standard partial molal thermodynamic properties of aqueous complexes of the rare earth elements at high pressures and temperatures. Geochim. Cosmochim. Acta, 59, 4329–4350]. The values for the second formation constant ( β 2) at 200 and 250 °C are in fair agreement with those of Haas et al. (1995) and Gammons et al. [Gammons C. H., Wood S. A. and Li Y. (2002) Complexation of the rare earth elements with aqueous chloride at 200 ° C and 300 ° C and saturated water vapor pressure. Special Publication—The Geochemical Society, (Water–Rock Interactions, Ore Deposits, and Environmental Geochemistry), pp. 191–207]. Calculations of monazite solubility indicate that Sm is less mobile in chloride-bearing solutions than Nd, which may indicate that the HREE are less mobile than the LREE.

An experimental study of the solubility and speciation of neodymium (III) fluoride in F-bearing aqueous solutions

The solubility of neodymium (III) fluoride was investigated at temperatures of 150, 200 and 250 °C, saturated water vapor pressure, and a total fluoride concentration (HF° aq + F −) ranging from 2.0 × 10 −3 to 0.23 mol/l. The results of the experiments show that Nd 3+ and NdF 2+ are the dominant species in solution at the temperatures investigated and were used to derive formation constants for NdF 2+ and a solubility product for NdF 3. The solubility product of NdF 3 ( log K sp = log a Nd 3 + + 3 log a F - ) is −24.4 ± 0.2, −22.8 ± 0.1, and −21.5 ± 0.2 at 250, 200 and 150 °C, respectively, and the formation constant of NdF 2 + ( log β = log a NdF 2 + - log a Nd 3 + - log a F - ) is 6.8 ± 0.1, 6.2 ± 0.1, and 5.5 ± 0.2 at 250, 200 and 150 °C, respectively. The results of this study show that published theoretical predictions significantly overestimate the stability of NdF 2+ and the solubility of NdF 3. The potential impact of the results on natural systems was evaluated for a hypothetical fluid with a composition similar to that responsible for REE mineralization in the Capitan pluton, New Mexico. In contrast to results obtained using the theoretical predictions of Haas [Haas J. R., Shock E. L., and Sassani D. C. (1995) Rare earth elements in hydrothermal systems: estimates of standard partial molal thermodynamic properties of aqueous complexes of the rare earth elements at high pressures and temperatures. Geochim. Cosmochim. Acta 59, 4329–4350.], which indicate that NdF 2+ is the dominant species in solution, calculations employing the data presented in this paper and previously published experimental data for chloride and sulfate species [Migdisov A. A., and Williams-Jones A. E. (2002) A spectrophotometric study of neodymium(III) complexation in chloride solutions. Geochim. Cosmochim. Acta 66, 4311–4323; Migdisov A. A., Reukov V. V., and Williams-Jones A. E. (2006) A spectrophotometric study of neodymium(III) complexation in sulfate solutions at elevated temperatures. Geochim. Cosmochim. Acta 70, 983–992.] show that neodymium chloride species predominate and that neodymium fluoride species are relatively unimportant. This suggests that accepted models for REE deposits that invoke fluoride complexation as the method of hydrothermal REE transport may need to be re-evaluated.

A spectrophotometric study of erbium (III) speciation in chloride solutions at elevated temperatures

The speciation of erbium in chloride-bearing solutions was investigated spectrophotometrically at temperatures of 100 to 250 °C and a pressure of 100 bars. The hydrated ion, Er 3+, is predominant at ambient temperature, but chloride complexes are dominant at elevated temperature. Formation constants were calculated for the following reactions: ( β 1) Er 3+ + Cl − = ErCl 2+ ( β 2) Er 3+ + 2 Cl − = ErCl 2 + The values obtained for the first formation constant ( β 1) were 0.88 ± 0.11, 1.59 ± 0.12, 2.34 ± 0.11, and 3.09 ± 0.14 at 100, 150, 200 and 250 °C, respectively. Values of the second formation constant could only be determined at 200 and 250 °C, and were 2.95 ± 0.34 and 4.12 ± 0.12, respectively. The values for the first formation constant ( β 1), are identical within experimental error to the values predicted by Haas et al. [Haas, J. R., Shock, E. L., and Sassani, D. C. (1995). Rare Earth Elements in hydrothermal systems: estimates of standard partial molal thermodynamic properties of aqueous complexes of the Rare Earth Elements at high pressures and temperatures. Geochim. Cosmochim. Acta, 59, 4329–50.], whereas the values for the second formation constant ( β 2) agree relatively well with those predicted by Haas et al. (1995) and determined experimentally by Gammons et al. [Gammons, C.H., Wood, S.A., and Li, Y. (2002). Complexation of the Rare Earth Elements with aqueous chloride at 200 °C and 300 °C and saturated water vapor pressure. Special Publication — The Geochemical Society, (Water–Rock Interactions, Ore Deposits, and Environmental Geochemistry), 191–207.].

A spectrophotometric study of neodymium(III) complexation in sulfate solutions at elevated temperatures

The formation constants of neodymium complexes in sulfate solutions have been determined spectrophotometrically at temperatures of 30–250 °C and a pressure of 100 bars. The dominant species in the solution are NdSO 4 + and Nd(SO 4) 2 −, with the latter complex being more important at higher temperature. Equilibrium constants were calculated for the following reactions: Nd 3 + + SO 4 2 - = NdSO 4 + , β 1 ; Nd 3 + + 2 · SO 4 2 - = Nd ( SO 4 ) 2 - , β 2 ; NdSO 4 + + SO 4 2 - = Nd ( SO 4 ) 2 - , K s . The values of β 1 and β 2, were determined for 30 and 100 °C, whereas for higher temperatures it was only possible to determine the stepwise formation constant K s. The values of the formation constants obtained in this study for 30 and 100 °C are in excellent agreement with those predicted theoretically by Wood [Wood, S.A., 1990b. The aqueous geochemistry of the rare-earth elements and yttrium. 2. Theoretical predictions of speciation in hydrothermal solutions to 350 °C at saturation water vapor pressure. Chem. Geol. 88 (1–2), 99–125] and Haas et al. [Haas, J.R., Shock, E.L., Sassani, D.C., 1995. Rare earth elements in hydrothermal sysytems: estimates of standard partial molal thermodynamic properties of aqueous complexes of the rare earth elements at high pressures and temperatures. Geochim. Cosmochim. Acta 59 (21), 4329–4350], and those for the stepwise formation constant ( K s) agree reasonably well with the predictions of Wood (1990b).

Thiols in hydrothermal solution: standard partial molal properties and their role in the organic geochemistry of hydrothermal environments

In order to assess the potential role of organic sulfur compounds in hydrothermal systems, we use recent advances in theoretical geochemistry to estimate the standard partial molal thermodynamic properties and parameters for the revised Helgeson-Kirkham-Flowers equation of state for aqueous straight-chain alkyl thiols. Thermodynamic properties are used to determine the potential for thiol formation in both modern and Archean hydrothermal systems. We conclude that organic sulfur compounds may hold the key to the organic chemistry that led to the origin of life in hydrothermal settings. These results may also explain the presence of sulfur in a number of biomolecules present in ancient thermophilic microorganisms.

Thermodynamic properties of surface species at the mineral–water interface under hydrothermal conditions: a Gibbs energy minimization single-site 2p KA triple-layer model of rutile in NaCl electrolyte to 250°C

Standard partial molal thermodynamic properties of surface species in a single-site 2p K A triple-layer model (TLM) for rutile in NaCl electrolyte, consistent with the SUPCRT92-98 data, were estimated at temperatures T = 0 to 250°C and saturated vapor pressure P SAT using a Gibbs energy minimization (GEM) approach (Kulik, 1999) and hydrothermal potentiometric titration data (Machesky et al., 1998a). Evaluation strategy was based on conventional setting to zero of standard state entropy and heat capacity effects of the “surface hydronium desorption” reaction >O 0.5H 2 + = >O 0.5H 0 + H aq +, with one-term temperature extrapolation appropriate. The standard state properties of the “surface water dissociation” reaction >O 0.5H 0 = >O 0.5 − + H aq + in a three-term extrapolation, with Cp ° 298(>O 0.5 −) assumed equal to Cp ° 298(OH aq −) and Δ S ° 298 estimated from enthalpy of rutile surface protonation in water Δ H ° prot,298 , yielded a very good match of predicted pristine point of zero charge (pH PPZC) and measured point of zero charge (pH PZC) on rutile to T = 250°C. Correlation of Δ H ° prot,298 for various oxides versus pH PPZC suggests that the entropy Δ S ° prot,298 = 25 ± 4 J K −1 mol −1 may be constant for all oxide surfaces, pH PPZC( T) be hence a function of pH PPZC at T 0 = 25°C only: pH PPZC,T = −29.134 + T 0/ T(pH PPZC,T0 + 3.2385) + 4.545ln T. The standard partial molal properties of Na + and Cl − outer-sphere surface complexes on rutile were estimated from GEM TLM fits of potentiometric titration data at different T and m NaCl. Adjustable parameters were: standard partial molal Gibbs energy g 0(>O 0.5 −Na +), interdependent with inner capacitance density C 1 (increasing from 1.6 F · m −2 at 25°C to 2.3 F · m −2 at 250°C), and maximum site density Γ max (>O 0.5H 2 +Cl −). No surface activity term (SAT) ln Ξ = f(Γ max ) corrections were needed for >O 0.5 −Na + species, i.e., Na + adsorption on rutile occurs in the “TLM ideal region” up to 300°C and 1 m NaCl. From the GEM fits, parameters of three-term temperature extrapolations were found for a reaction >O 0.5H 0 + Na aq + = >O 0.5 −Na + + H aq +. Directly fitted g 0 (>O 0.5H 2 +Cl −) values were less certain because the SAT with low Γ max(>O 0.5H 2 +Cl −) = 0.9 ± 0.1 sites nm −2 had to be involved. Hence, symmetric electrolyte adsorption (pH PZSE = pH PPZC) has been assumed, from which the standard partial molal properties of >O 0.5H 2 +Cl − species were found using a zero-term extrapolation of an isocoulombic reaction >O 0.5H 2 +Cl − + >O 0.5 − + Na aq + = >O 0.5 −Na + + >O 0.5H 2 + + Cl aq −. This balanced like-charge reaction can serve as a very useful constraint reducing the set of adjustable TLM parameters at given T and pH PZC to C 1, p K Na and Γ max (>O 0.5H 2 +Cl −), optimal values of which could have been found from GEM fits of the rutile potentiometric titration data at different T and m NaCl.

Iron(III) solubility and speciation in aqueous solutions. experimental study and modelling: part 1. hematite solubility from 60 to 300°C in NaOH–NaCl solutions and thermodynamic properties of Fe(OH)4−(aq)

The solubility of natural and synthetic hematite (α-Fe2O3) was measured in NaOH–NaCl solutions (0.007 ≤ m(NaOH) ≤ 2.0) between 60 and 300°C at saturated water vapour pressure and under excess oxygen. Solubility constants determined in the present study and by Yishan et al. (1986) at 300°C were combined with the thermodynamic properties of hematite (Hemingway, 1990) and water (SUPCRT92, Johnson et al., 1992) to generate within the framework of the revised Helgeson–Kirkham–Flowers (HKF) model the standard partial molal thermodynamic properties at 25°C and 1 bar, and the revised HKF equations of state parameters of Fe(OH)4−. The extrapolated value for the Gibbs energy of formation for Fe(OH)4− at 25°C is −201.97 kcal/mol. Thermodynamic calculations show that Fe(OH)4− exhibits a chemical behaviour different from that of Ga(OH)4− and Al(OH)4−.

Calculation of the standard partial molal thermodynamic properties and dissociation constants of aqueous HCl 0 and HBr 0 at temperatures to 1000°C and pressures to 5 kbar

Dissociation constants of aqueous ion pairs HCl 0 and HBr 0 derived in the literature from vapor pressure and supercritical conductance measurements (Quist and Marshall, 1968b; Frantz and Marshall, 1984) were used to calculate the standard partial molal thermodynamic properties of the species at 25°C and 1 bar. Regression of the data with the aid of revised Helgeson-Kirkham-Flowers equations of state (Helgeson et al., 1981; Tanger and Helgeson, 1988; Shock et al., 1989) resulted in a set of equations-of-state parameters that permits accurate calculation of the thermodynamic properties of the species at high temperatures and pressures. These properties and parameters reproduce generally within 0.1 log unit (with observed maximum deviation of 0.23 log unit) the log K values for HBr 0 and HCl 0 given by Quist and Marshall (1968b) and Frantz and Marshall (1984), respectively, at temperatures to 800°C and pressures to 5 kbar.

Experimental determination of aqueous sodium-acetate dissociation constants at temperatures from 20 to 240°C

Dissociation constants of sodium acetate ion pair (NaCH 3COO 0) were determined at the liquid–vapor saturation pressure by Raman spectroscopy at temperatures from 20 to 240°C and by potentiometry at temperatures from 25 to 172°C. Large differences were found between these two experimentally determined data sets; dissociation constants generated from Raman spectroscopic data were found to be ∼1.5 orders of magnitude higher than corresponding constants obtained using potentiometric measurements. Values generated from Raman spectroscopic data are consistent with the high temperature NaCH 3COO 0 dissociation constants reported by [Oscarson, J.L., Gillespie, S.E., Christensen, J.J., Izatt, R.M., Brown, P.R., 1988. Thermodynamic quantities for the interaction of H + and Na + with C 2H 3O 2 − and Cl − in aqueous solution from 275 to 320°C. J. Soln. Chem., Vol. 17, pp. 865-885], whereas those obtained by potentiometry are consistent with the low temperature NaCH 3COO 0 dissociation constants measured by [Archer, D.W., Monk, C.B., 1964. Ion-association constants of some acetates by pH (glass electrode) measurements. J. Chem. Soc., pp. 3117–3122.] and [Daniele, P.G., De Robertis, A., De Stefano, C., Sammartano, S., Rigano, C., 1985. On the possibility of determining the thermodynamic parameters for the formation of weak complexes using a simple model for the dependence on the ionic strength of activity coefficients: Na +, K +, and Ca 2+ complexes of low molecular weight ligands in aqueous solution. J. Chem. Soc. Dalton, pp. 2353–2361.], as well as the predictions of [Shock, E.L., Koretsky C.M., 1993. Metal–organic complexes in geochemical processes: Calculation of standard partial molal thermodynamic properties of aqueous acetate complexes at high pressures and temperatures. Geochim. Cosmochim. Acta, Vol. 57, pp. 4899–4922.] and [Bénézeth, P., Castet, S., Dandurand, J.L., Gout, R., Schott, J., 1994. Experimental study of aluminum acetate complexing between 60 and 200°C. Geochim. Cosmochim. Acta, Vol. 58, pp. 4561–4571]. The origin of these differences likely stems in the species being sampled by different experimental techniques; Raman spectroscopy only identifies inner sphere sodium-acetate complexes as associated species whereas potentiometric measurements identify both inner and outer sphere sodium-acetate complexes as associated species. Regression of sodium acetate ion pair dissociation constants obtained from Raman spectroscopy and potentiometry enable characterization of true dissociation constants for both inner and outer sphere complexes. These latter parameters are used to assess the formation of these complexes at temperatures and solution compositions typical of natural fluids.

Experimental investigation of aluminum-silica aqueous complexing at 300°C

Equilibrium constants for aqueous Al–Si complexes at 300°C and saturated water vapour pressure (86 bar) were generated experimentally from the difference between the solubility of boehmite in silica-free and silica-bearing solutions. At strongly acidic (pH<2) and near-neutral to basic (pH>4.5) conditions aluminum silica aqueous complexes form according to Al 3++H 4SiO 4 0=AlH 3SiO 4 2++H + and Al(OH) 4 −+H 4SiO 4 0=Al(OH) 3H 3SiO 4 −+H 2O for which the logarithm of the equilibrium constants was found to be 3.23±0.25 and 2.32±0.20, respectively. No solubility increase following addition of aqueous silica was observed in the slightly acidic range (2.5<pH<4), indicating that Si complexation with Al(OH) 2+, Al(OH) 2 +, and Al(OH) 3 0, is negligible. The equilibrium constant obtained for AlH 3SiO 4 2+ formation at 300°C was combined with published values for this complex at 25 to 150°C [Pokrovski, G.S., Schott, J., Harrichoury, J.-C., Sergeyev, A.S., 1996. The stability of aluminum silicate complexes in acidic solutions from 25 to 150°C. Geochim. Cosmochim. Acta 60, 2495–2501.] to generate standard partial molal thermodynamic properties at 25°C and 1 bar and HKF (revised Helgeson–Kirkham–Flowers model; [Tanger, J.C., Helgeson, H.C., 1988. Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: revised equations of state for the standard partial molal properties of ions and electrolytes. Am. J. Sci., 288, 19–98; Shock, E.L., Helgeson, H.C., 1988. Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: correlations algorithms for ionic species and equation of state predictions to 5 kbar and 1000°C. Geochim. Cosmochim. Acta, 52, 2009–2036; Shock, E.L., Oelkers, E.H., Johnson, J.W., Sverjensky, D.A., Helgeson, H.C., 1992. Calculation of the thermodynamic properties of aqueous species at high pressures and temperatures. J. Chem. Faraday Trans. 88 (6), 803–826.] equation of state parameters for this complex. Calculations indicate that Al–Si complexing can increase Al solubility of up to 1/2 an order of magnitude at 500°C and 1 kbar. As a consequence, Al–Si complexes should be taken into account to accurately model chemical equilibria in hydrothermal and metamorphic fluids.

Standard partial molal properties of aqueous alkylphenols at high pressures and temperatures

Potential influences on the distribution of alkylphenols in oilfield waters include simple partitioning between oil, water, and solid phases, generation of alkylphenols during kerogen maturation in source rocks, and reactions at the oil-water interface in the oil reservoir. Assessment of whether reactions at the oil-water interface influence phenol distributions in basinal aqueous waters requires thermodynamic properties of the aqueous alkylphenols. In this study we have calculated standard partial molal thermodynamic properties of aqueous phenol and cresols at 25°C and estimated standard partial molal thermodynamic properties of aqueous xylenols at 25°C. These thermodynamic properties (Gibbs free energy, enthalpy, entropy, heat capacity, and volume) together with estimated coefficients for the revised Helgeson-Kirkham-Flowers (HKF) equations of state allow prediction of thermodynamic properties of reactions involving these species at high temperatures and pressures. Computed logarithmic equilibrium constants (log K) for hydration reactions involving alkylphenol are in close agreement with log K values calculated from published experimental data (Dohnal and Fenclova, 1995). Resulting thermodynamic properties may be used together with analytical results, to assess whether metastable equilibrium is a major influence on the abundance and distribution of alkvlvhenols in vetroleum and associated water.

Calculation of the standard partial molal thermodynamic properties of KCl 0 and activity coefficients of aqueous KCl at temperatures and pressures to 1000°C and 5 kbar

Regression of experimental activity coefficient and dissociation constant data reported in the literature with the Hückel (1925) and Setchénow (1892) equations (Helgeson et al., 1981) and the revised HKF equations of state (Tanger and Helgeson, 1988; Shock et al., 1989, 1992) generated parameters and thermodynamic properties of dissociated KCI and KCl 0 at 25°C and 1 bar that can be used to calculate the standard partial molal thermodynamic properties of KCl 0 and the activity coefficients of KCl at temperatures and pressures to 1000°C and 5 kbar.

Thermodynamic properties of aqueous species and the solubilities of minerals at high pressures and temperatures: the system Al 2O 3H 2OKOH

Experimental solubilities of gibbsite and corundum in aqueous KOH solutions at elevated temperatures and pressures reported by Fricke and Jucaitis (1930), Wesolowski (1992), and Pascal and Anderson (1989) were used together with the Hückel (1925) and Setchénow (1892) equations for activity coefficients of aqueous species and the thermodynamic properties of the aluminate ion AlO 2 − to retrieve dissociation constants for the potassium aluminate ion pair KAlO 2 0. The calculations were carried out using values of the Hückel extended-term parameter for aqueous KOH and dissociation constants for KOH 0 generated in the present study. Supercritical solubilities of corundum reported in the literature were regressed with the aid of a speciation model which explicitly provides for the formation in solution of triple ion clusters (Oelkers and Helgeson, 1990). Values of the dissociation constant for KAlO 2 0 generated from these data were subsequently regressed with the revised Helgeson-Kirkham-Flowers (HKF) equations of state (Tanger and Helgeson, 1988; Shock et al., 1989) to obtain HKF equations of state coefficients and the standard partial molal thermodynamic properties at 25°C and 1 bar of KAlO 2 0, which can be used to calculate the standard partial molal thermodynamic properties of the species at temperatures to 1000°C and pressures to 5 kbar. Combining these parameters and properties with those for AlO 2 − (Pokrovskii and Helgeson, 1995) permits calculation to within 0.05 log units of the experimental solubilities of corundum in KOH solutions reported by Barns et al. (1963) and Anderson and Burnham (1967) at temperatures to 800°C and pressures to 5 kbar. Similar calculations carried out for temperatures from 80 to 300°C at P SAT ( P SAT refers to 1 bar at temperatures < 100°C and to the equilibrium pressure for coexisting liquid and vapor H 2O at higher temperatures) indicate that addition of KCl to dilute KOH solutions increases the solubilities of gibbsite, boehmite, and diaspore in response to increasing formation of KAlO 2 0 and the decrease in the activity coefficient of AlO 2 −. At temperatures ∼ ≤ 80°C, the two factors contribute comparably to the solubility of gibbsite in solutions with KCl concentrations up to 5 molal.

Gallium speciation in aqueous solution. Experimental study and modelling: Part 2. Solubility of α-GaOOH in acidic solutions from 150 to 250°C and hydrolysis constants of gallium (III) to 300°C

The solubility of α-GaOOH was measured in low ionic strength (0.025 M) HClNaCl solutions over the pH range 1.6–4.1 at 150, 200, and 250°C. A least squares linear regression of these data was used to determine the dissociation constants ( K s0 ∗ to K s3 ∗ ) of the mineral from which hydrolysiss constants of aqueous gallium species were derived. The dissociation constants generated in this study were used together with the thermodynamic properties of α-GaOOH (Pokrovski et al., 1997) to calculate the standard partial molal thermodynamic properties at 25°C, 1 bar and the equations of state parameters for Ga 3+, Ga(OH) 2+, Ga(OH) 2 +, and Ga(OH) 3 0 within the framework of the revised HKF model (Tanger and Helgeson, 1988; Shock and Helgeson, 1988; Shock et al., 1992). These thermodynamic properties enabled calculation of gallium speciation as a function of pH and temperature. Our results indicate that the hydrolysis of Ga occurs at low pH and its speciation is strongly dominated by the negatively-charged species Ga(OH) 4 − in the pH range of natural solutions at 25°C. At increasing temperature, the amount of Ga(OH) 2+ and Ga(OH) 3 0 becomes significant. These data are used to compare the geochemical behavior of Ga and Al in natural fluids.

Gallium speciation in aqueous solution. Experimental study and modelling: Part 1. Thermodynamic properties of Ga(OH) 4− to 300°C

The solubility of pure synthetic α-GaOOH was measured in dilute aqueous solutions at pH values ranging from 5 to 12.5, temperatures from 25 to 250°C, and at saturated water vapor pressures. Experimentally determined solubility constants were combined with the thermodynamic properties of α-GaOOH (Pokrovski et al., 1996) to generate, within the framework of the revised Helgeson-Kirkham-Flowers model (Tanger and Helgeson, 1988; Shock and Helgeson, 1988; Shock et al., 1992), the standard partial molal thermodynamic properties at 25°C and 1 bar, and revised equations of state parameters for Ga(OH) 4 −. Thermodynamic calculations indicate that Ga(OH) 4 − exhibits a chemical behavior very similar to that of Al(OH) 4 −.

Inorganic species in geologic fluids: Correlations among standard molal thermodynamic properties of aqueous ions and hydroxide complexes

Correlations among experimentally determined standard partial molal thermodynamic properties of inorganic aqueous species at 25°C and 1 bar allow estimates of these properties for numerous monatomic cations and anions, polyatomic anions, oxyanions, acid oxyanions, neutral oxy-acid species, dissolved gases, and hydroxide complexes of metal cations. Combined with correlations among parameters in the revised Helgeson-Kirkham-Flowers (HKF) equation of state (Shock et al., 1992), these estimates permit predictions of standard partial molal volumes ( V° ), heat capacities ( C° P ), and entropies ( S° ), as well as apparent standard partial molal enthalpies ( Δ H° P,T ) and Gibbs free energies (ΔḠ° P,T) of formation to 1000°C and 5 kb for hundreds of inorganic aqueous species of interest in geochemistry. Data and parameters for more than 300 inorganic aqueous species are presented. Close agreement between calculated and experimentally determined equilibrium constants for acid dissociation reactions and cation hydrolysis reactions supports the generality and validity of these predictive methods. These data facilitate the calculation of the speciation of major, minor, and trace elements in hydrothermal and metamorphic fluids throughout most of the crust of the Earth.

Rare earth elements in hydrothermal systems: Estimates of standard partial molal thermodynamic properties of aqueous complexes of the rare earth elements at high pressures and temperatures

Standard partial molal thermodynamic properties including association constants for 246 inorganic aqueous rare earth element (REE) complexes with chloride, fluoride, hydroxide, carbonate, sulfate, bicarbonate, nitrate, and orthophosphate can be calculated at pressures from 1 to 5000 bars and temperatures from 0 to 1000°C, using experimental data from the literature and correlation algorithms. Predicted association constants for REE complexes are used to calculate the speciation of the REEs in simulated and natural fluid compositions over ranges of pH, temperature, and pressure. Our results demonstrate that in a generalized chloride-rich hydrothermal fluid, REE transport may be facilitated by formation of chloride, fluoride, and hydroxide complexes at acidic, neutral, and basic pH conditions, respectively. The HREEs (GdLu) are complexed more strongly by fluoride, and less strongly by chloride, than the LREEs (LaSm), whereas at basic pH conditions HREEs and LREEs strongly associate with hydroxide to an equivalent degree. Estimates of REE speciation in natural hydrothermal solutions that differ in composition and geologic setting demonstrate that different REE-complexes predominate in different environments. It follows that ad hoc assumptions about the identity of complexes which are responsible for REE mobility in a given geologic setting are not necessary.

Metal-organic complexes in geochemical processes: Estimation of standard partial molal thermodynamic properties of aqueous complexes between metal cations and monovalent organic acid ligands at high pressures and temperatures

Regression of standard state equilibrium constants with the revised Helgeson-Kirkham-Flowers (HKF) equation of state allows evaluation of standard partial molal entropies ( S o) of aqueous metal-organic complexes involving monovalent organic acid ligands. These values of S o provide the basis for correlations that can be used, together with correlation algorithms among standard partial molal properties of aqueous complexes and equation-of-state parameters, to estimate thermodynamic properties including equilibrium constants for complexes between aqueous metals and several monovalent organic acid ligands at the elevated pressures and temperatures of many geochemical processes which involve aqueous solutions. Data, parameters, and estimates are given for 270 formate, propanoate, n-butanoate, n-pentanoate, glycolate, lactate, glycinate, and alanate complexes, and a consistent algorithm is provided for making other estimates. Standard partial molal entropies of association ( Δ − S r o ) for metal-monovalent organic acid ligand complexes fall into at least two groups dependent upon the type of functional groups present in the ligand. It is shown that isothermal correlations among equilibrium constants for complex formation are consistent with one another and with similar correlations for inorganic metal-ligand complexes. Additional correlations allow estimates of standard partial molal Gibbs free energies of association at 25°C and 1 bar which can be used in cases where no experimentally derived values are available.

Metal-organic complexes in geochemical processes: Calculation of standard partial molal thermodynamic properties of aqueous acetate complexes at high pressures and temperatures

Estimates of standard partial molal properties at high temperatures and pressures for aqueous acetate complexes of major and trace elements in geologic fluids were made with the aid of experimental data from the literature and correlation algorithms. A system of correlation expressions allows the incorporation of all available experimental data, but also allows estimates in the absence of any measurements. Likely uncertainties for each type of estimate were assessed. Thermodynamic data and equation of state parameters permit calculation of standard partial molal properties including dissociation constants for 114 acetate complexes. Incorporation of many of these dissociation constants with those for hydroxide, chloride, and sulfate complexes demonstrate that acetate complexes are ineffectual in transporting major rock forming elements or trace metals in sedimentary basin brines. In contrast, these complexes could be considerably more efficient in metal transport in low salinity groundwater solutions with elevated concentrations of organic solutes. Calculations indicate that up to 40% of the acetate may be present as sodium and calcium complexes in basin brines with total salinities around 1.0 molal.

Aldehydes in hydrothermal solution: Standard partial molal thermodynamic properties and relative stabilities at high temperatures and pressures

Aldehydes are common in a variety of geologic environments and are derived from a number of sources, both natural and anthropogenic. Experimental data for aqueous aldehydes were taken from the literature and used, along with parameters for the revised Helgeson-Kirkham-Flowers (HKF) equations of state, to estimate standard partial molal thermodynamic data for aqueous straight-chain alkyl aldehydes at high temperatures and pressures. Examples of calculations involving aldehydes in geological environments are given, and the stability of aldehydes relative to carboxylic acids is evaluated. These calculations indicate that aldehydes may be intermediates in the formation of carboxylic acids from hydrocarbons in sedimentary basin brines and hydrothermal systems like they are in the atmosphere. The data and parameters summarized here allow evaluation of the role of aldehydes in the formation of prebiotic precursors, such as amino acids and hydroxy acids on the early Earth and in carbonaceous chondrite parent bodies.