David J. Wesolowski

LIQUID-VAPOR FRACTIONATION OF OXYGEN AND HYDROGEN ISOTOPES OF WATER FROM THE FREEZING TO THE CRITICAL TEMPERATURE

The equilibrium fractionation factors of oxygen and hydrogen isotopes between liquid water and water vapor have been precisely determined from 25 to 350°C on the VSMOW-SLAP scale, using three different types of apparatus with static or dynamic techniques for the sampling of water vapor. Our results for both oxygen and hydrogen isotope fractionation factors between 25 and 100°C are in excellent agreement with the literature (e.g., Majoube, 1971). Our results for the hydrogen isotope fractionation factor above 100°C also agree quantitatively with the literature values of Merlivat et al. (1963) and Bottinga (1968). The results for the hydrogen isotope fractionation factor obtained in this study and from most of the literature were regressed to the equation, 103Inα1−v(D) = 1158.8(T3109) −1620.1 (T2106) + 794.84(T103) −161.04 + 2.9992(109T3), from 0°C to the critical temperature of water (374.1°C) within ± 1.2(1σ) (n = 157); T(K). The cross- over temperature is 229 ± 13°C (1σ). Our values for the oxygen isotope fractionation factor between liquid water and water vapor are, however, at notable variance with the only dataset available above 100°C in the literature (Bottinga, 1968), which is systematically higher (av. + 0.15 in 103 In α1−v(18O)) with large errors (± 0.23 in 1σ). Our results and most of the literature data below 100°C were regressed to the equation, 103 In α1−v(18O) = −7.685 + 6.7123(103T) − 1.6664(106T2) + 0.35041 (109T3), from 0 to 374.1°C within ± 0.11 (1σ)(n = 112); T(K). A third water-steam isotope geothermometer, using the ratio of ΔδD/Δδ18O given by water and steam samples, is readily obtained from the above equations. This geothermometer is less affected by incomplete separation of water and steam, and partial condensation of steam than those employing the oxygen and hydrogen isotopic compositions alone.

Aluminum speciation and equilibria in aqueous solution: V. Gibbsite solubility at 50°C and pH 3–9 in 0.1 molal NaCl solutions (a general model for aluminum speciation; analytical methods)

This study reports 184 new measurements of the solubility of gibbsite at 50°C and 0.1 molal ionic strength in NaCl solutions of acetate, bistris, and tris buffers with hydrogen ion concentrations ranging from 10−3to 10−9molal. Samples collected at 35, 63, 66, 120 and 144 days show no detectable difference in the total aluminum at similar pH values. Correction of the measured solubilities for com- plexation reactions involving Al3+ with acetate and Al(OH)4− with bistris gives the solubility curve due to Al(OH)y3−y species alone, which is smooth and continuous, with a minimum near 10−8 molal (0.3 ppb) and pH 5.5. The corrected solubilities are shown to be in complete agreement with measurements of the same material in more strongly acidic (palmer and wesolowski, 1992) and basic solutions (wesolowski, 1992) and with the formation constant for Al(OH)2+determined potentiometrically by palmer and wesolowski (1993). An additional species, Al(OH)+2, was introduced in order to explain the solubility at pH values around 5.5, and the molal formation quotient for the reaction Al(OH)3,cr + H+ ⇆ Al(OH)2+ + H2O was determined to be 10−3.04 ± 0.05 at 50°C and 0.1 molal ionic strength. The results of this study were combined with our previous results and the new boehmite solubility data of castet et al. (1993) to provide a consistent model for the distribution of monomeric aluminum hydrolysis species and the solubility of gibbsite in 0–5 molal NaCl brines in the 0 to 100°C range. Salinity is shown to be a major factor controlling the solubility of aluminum minerals in solutions 1 to 2 units more acidic than the neutral pH at temperatures of 0 to 100°C. Acetate complexation is modeled from the results of this study and palmer and bell (1994), and is shown to enhance the solubility of gibbsite by more than an order of magnitude in mildly acidic brines containing a few thousand parts per million total acetate, in the absence of competition by other metal ions. A model is also presented for the aluminum hydrolysis constants at higher temperatures at infinite dilution which is quantitatively consistent with the low temperature data. Detailed aluminum analysis techniques employing ion chromatography are discussed in the Appendix.

The activity-composition relationship of oxygen and hydrogen isotopes in aqueous salt solutions: III. Vapor-liquid water equilibration of NaCl solutions to 350°C

Abstract The effect of dissolved NaCl on equilibrium oxygen and hydrogen isotope fractionation factors between liquid water and water vapor was precisely determined in the temperature range from 130–350°C, using two different types of apparatus with static or dynamic sampling techniques of the vapor phase. The magnitude of the oxygen and hydrogen isotope effects of NaCl is proportional to the molality of liquid NaCl solutions at a given temperature. Dissolved NaCl lowers appreciably the hydrogen isotope fractionation factor between liquid water and water vapor over the entire temperature range. NaCl has little effect on the oxygen isotope fractionation factor at temperatures below about 200°C, with the magnitude of the salt effect gradually increasing from 200–350°C. Our results are at notable variance with those of Truesdell (1974) and Kazahaya (1986), who reported large oxygen and hydrogen isotope effects of NaCl with very complex dependencies on temperature and NaCl molality. Our high-temperature results have been regressed along with our previous results between 50 and 100°C (Horita et al., 1993a) and the low-temperature literature data to simple equations which are valid for NaCl solutions from 0 to at least 5 molal NaCl in the temperature range from 10–350°C. Our preliminary results of oxygen isotope fractionation in the system CaCO3-water ± NaCl at 300°C and 1 kbar are consistent with those obtained from the liquid-vapor equilibration experiments, suggesting that the isotope salt effects are common to systems involving brines and any other coexisting phases or species (gases, minerals, dissolved species, etc.). The observed NaCl isotope effects at elevated temperatures should be taken into account in the interpretation of isotopic data of brine-dominated natural systems.

The activity-composition relationship of oxygen and hydrogen isotopes in aqueous salt solutions: I. Vapor-liquid water equilibration of single salt solutions from 50 to 100°C

The differences between oxygen and hydrogen isotope activity and composition ratios of water in single salt solutions (NaCl, KCl, MgCl2, CaCl2, Na2SO4, and MgSO4) were determined by means of a vapor-liquid water equilibration method over the temperature range of 50 to 100°C. A parallel equilibration technique of pure water and salt solutions with the same isotopic composition at the same experimental conditions enabled the precise determination of the isotope salt effects. Hydrogen isotope activity ratios of all of the salt solutions studied were appreciably higher than composition ratios. That is, DH ratio of water vapor in isotope equilibrium with a solution increases as salt is added to the solution. Magnitudes of the hydrogen isotope effects are in the order CaCl2 ≥ MgCl2 > MgSO4 > KCl ≈ NaCl > Na2SO4 at the same molality. Except for KCl solutions at 50°C, oxygen isotope activity ratios in the solutions were lower than, or very close to, the composition ratios. The isotope effects observed are all linear with the molalities of the salt solutions, and either decrease with temperature or are almost constant over the temperature range. Salt solutions of divalent cations (Ca and Mg) exhibited oxygen isotope effects much larger than those of monovalent cations (Na and K). Magnitudes of the oxygen isotope effects in NaCl solutions, and of the hydrogen isotope effects in Na2SO4 and MgSO4 solutions, may increase from 50 to 100°C. Our results agree with most of those from the literature near room temperature, but are at notable variance with those by Truesdell (1974) around 100°C. The results in this study and the literature data near room temperature were satisfactorily fitted to simple equations as a function of concentration of the salt solutions and temperature.

Nafion ∕ TiO2 Proton Conductive Composite Membranes for PEMFCs Operating at Elevated Temperature and Reduced Relative Humidity

Nafion/TiO 2 composite membranes with different TiO 2 contents were studied in an H 2 /O 2 proton exchange membrane fuel cell (PEMFC) over a wide range of relative humidity (RH) values from 26 to 100% at temperatures of 80and 120°C. The composite membranes, which were prepared using a recast procedure, showed a pronounced improvement over unmodified Nafion membranes when operated at 120°C and reduced RH. For instance, at 50% RH, the Nafion/20% TiO 2 membrane demonstrated a performance identical to that of an unmodified Nafion membrane operated at 100% RH. This performance level was comparable to that of a bare Nafion membrane at 80°C. The high performance of the Nafion/TiO 2 composite membranes at low RH was attributed to improved water retention due to the presence of absorbed water species in the electrical double layer on the TiO 2 surface. The zeta potential and thickness of the hydrodynamically immobile water layer at the TiO 2 /water interface were discussed as parameters influencing the water balance in the membranes. The obtained experimental PEMFC performance data were fitted using an analytical equation, and calculated parameters were analyzed as functions of RH and TiO 2 content in the composite membranes.

Aluminum speciation and equilibria in aqueous solution: III. Potentiometric determination of the first hydrolysis constant of aluminum(III) in sodium chloride solutions to 125°C

Abstract The first molal hydrolysis quotient of aluminum(III) was measured potentiometrically from 25 to 125°C at 25° intervals at ionic strengths of 0.1, 0.3, 1.0 and 5 mol · kg−1 with sodium chloride as the supporting electrolyte. The experimental method involved using a hydrogen-electrode concentration cell modified to compensate for any intrinsic potential offset between the two electrodes. The initial concentration of Al3+ was varied to test for the presence of multinuclear aluminum species while being kept to a maximum of 10−3 mol · kg−1 to minimize their occurrence. Similarly, the maximum degree of hydrolysis of Al3+ reached in each titration was ca. 30%, after which polymerization and/or precipitation became apparent. The equilibrium quotients obtained in this study and selected values from the literature were fitted by an empirical equation incorporating a linear dependence of log K1,1 on the reciprocal temperature (Kelvins) over the range 10–200°C and three ionic-strength-dependent parameters. Comparisons are made between the results of this study and the literature values.

The activity-composition relationship of oxygen and hydrogen isotopes in aqueous salt solutions: II. Vapor-liquid water equilibration of mixed salt solutions from 50 to 100°C and geochemical implications

The difference between oxygen and hydrogen isotope activity and composition ratios of water in mixed salt solutions in the system Na-K-Mg-Ca-Cl-SO4-H2O was determined by means of a vaporliquid water equilibration method over the temperature range of 50 to 100°C. The observed isotope salt effects in complex mixed salt solutions to very high ionic strengths agree quantitatively with calculations based on the assumption of a simple additive property of the isotope salt effects of the individual salts in the solutions. Sofer and Gat (1972, 1975) and Horita and Gat (1989) also observed that this simple mixing rule applies to synthetic and natural chloride-mixed salt solutions at room temperature. Equations to convert between the isotope activity and composition scales for brines and fractionation factors between brines and other substances are presented. For most geochemical interactions between brines and other phases (vapor, gases, minerals) such as evaporation/boiling, mineral precipitation, and mineral/rock alteration, the isotope activity scale should be used. The isotope composition scale, on the other hand, is most useful for studies of mixing of different brines and formation of brines by mineral dissolution. Misusage of the two isotopic scales of brines will, and probably in the literature has, lead to incorrect conclusions in many isotopic studies of brine-dominated systems (origin of brines, temperature of mineral formation, isotope ratios of fossil fluids).

Aqueous high-temperature solubility studies. I. The solubility of boehmite as functions of ionic strength (to 5 molal, NaCl), temperature (100 -290°C), and pH as determined by in situ measurements

Abstract The solubility of boehmite, γ-AlOOH, was measured in a modified hydrogen-electrode concentration cell, which provided continuous in situ measurements of hydrogen ion molality over the range of ionic strengths from 0.1 to 5.0 mol · kg−1 (NaCl) at temperatures from 100 to 290°C. A series of conventional solubility measurements was also carried out in acidic solutions over the same temperature range (i.e., pH was not monitored, but rather calculated from mass balance). The combined results yielded the molal solubility quotients, Qs0 and Qs4 for the equilibria: AlOOH(cr) + 3H+ ⇄ Al3+ + 2H2O Qs0 = [Al3+]/[H+]3 AlOOH(cr) + 2H2O ⇄ Al(OH)4− + H+ Qs4 = [Al(OH)4−][H+]In the regression of each isothermal data set, the values for the first hydrolysis quotient, Al3+ + H2O ⇄ Al(OH)2+ + H+ Q1,1 = [Al(OH)2+][H+]/[Al3+]were fixed according to a previous potentiometric study (Palmer and Wesolowski, 1993) . Moreover, for one series of titrations at 0.1 mol · kg−1 ionic strength at 150°C, the remaining two solubility quotients, Qs2 and Qs3, were determined simultaneously from the regression. However, at all other conditions, the values of Qs2 and Qs3 were also fixed at values consistent with the corresponding 0.03 mol · kg−1 ionic strength results (Benezeth et al., 2001) by invoking the isocoulombic assumption (i.e., an assumption of minimal ionic strength dependence for reactions with no net change in charge). The stability field with respect to pH of the Al(OH)2+ and Al(OH)30 aqueous species were found to be very narrow, and hence assumptions concerning their stabilities had little effect on the predicted shape of the solubility profiles at high ionic strength. Global fits were made of the log Qs0 and log Qs4 values as functions of temperature and ionic strength after combining with corresponding values from an analogous study at 0.03 mol · kg−1 ionic strength (Benezeth et al., 2001) , as well as appropriate constants taken from the literature. The fits were further constrained by inclusion of gibbsite solubility data ( Wesolowski 1992 , Palmer and Wesolowski 1992 after adjustment for the relative free energies of formation of gibbsite and boehmite. This treatment ensured that a continuous empirical model exists for Al3+ and Al(OH)4− from ambient conditions to 300°C and infinite dilution to five mol · kg−1 ionic strength.

Aqueous proton transfer across single-layer graphene

Proton transfer across single-layer graphene proceeds with large computed energy barriers and is therefore thought to be unfavourable at room temperature unless nanoscale holes or dopants are introduced, or a potential bias is applied. Here we subject single-layer graphene supported on fused silica to cycles of high and low pH, and show that protons transfer reversibly from the aqueous phase through the graphene to the other side where they undergo acid–base chemistry with the silica hydroxyl groups. After ruling out diffusion through macroscopic pinholes, the protons are found to transfer through rare, naturally occurring atomic defects. Computer simulations reveal low energy barriers of 0.61–0.75 eV for aqueous proton transfer across hydroxyl-terminated atomic defects that participate in a Grotthuss-type relay, while pyrylium-like ether terminations shut down proton exchange. Unfavourable energy barriers to helium and hydrogen transfer indicate the process is selective for aqueous protons.

Magnetite surface charge studies to 290°C from in situ pH titrations

The proton-induced surface charge of magnetite was investigated in 0.03 and 0.30 molal sodium trifluoromethanesulfonate solutions from 25°C to 290°C by potentiometric titrations using a stirred hydrogen electrode concentration cell. Pure magnetite with excellent crystallinity was produced by reaction with the Ni/NiO/H2O hydrogen fugacity buffer at 500°C. Inflection points in the 0.03 molal proton sorption isotherms (pHinfl) at 6.50, 6.24, 5.65, 5.47, 5.31 and 5.55 at temperatures of 50°C, 100°C, 150°C, 200°C, 250°C and 290°C, respectively, were used as estimates of the pristine point of zero charge (pHppzc) for modeling purposes. These pHinfl values parallel 1/2 pKw and agree within the assigned uncertainty (±0.3 pH units) at all temperatures with independent estimates of the pHppzc calculated from an extension of 88the revised MUSIC model. The surface charging can be adequately described by a one-pK model with a surface protonation constant fitted to the pHinfl values, and giving the standard state thermodynamic properties log KH,298=7.00, ΔH298°=−32.4±0.8 kJ/mol and constant ΔCp=128±16 J K−1 mol−1, with ΔS298° assumed to be equal to that of rutile protonation (25.5±3.4 J K−1 mol−1. The 0.03 and 0.30 molal proton sorption isotherms also exhibit pHs of common intersection (pHcip) at 6.33, 5.78, 5.37, 4.82, 4.62 and 4.90 at 50°C, 100°C, 150°C, 200°C, 250°C and 290°C, respectively. The difference between the pHcip and pHppzc≅pHinfl values can be related to specific binding of Na+ on the negatively charged surface, which increases with increasing temperature, although the pHcip values may also be affected by dissolution of the solid. The electrical double layer model includes a basic Stern layer capacitance, with specific cation and anion binding at the Stern layer, and a fixed diffuse layer capacitance computed from Guoy–Chapman theory. To fit the steepness and asymmetry of the charging curves above the pHppzc, an additional cation binding constant was invoked, which allows the cation to experience the surface potential. Significant kinetically controlled dissolution of magnetite was observed below the pHppzc, which may be a result of leaching of Fe2+ from the surface, to produce a magnetite+hematite assemblage, despite the high hydrogen partial pressures (ca. 10 bars) used in these experiments.