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
Introduction
Aluminum is the third most abundant element in the earth’s crust as well as one of the most important industrial metals, and therefore it is not surprising that so much attention has been given to its aqueous chemistry. There have appeared numerous compilations of thermodynamic data involving aqueous aluminum chemistry (e.g., Apps and Neil 1990, Pokrovskii and Helgeson 1995, Shock et al 1997), but problems persist in the application of these databases to existing geological models. These disparities cause marked differences in the predicted absolute and relative stabilities of dissolved aluminum species and solid aluminum-containing phases. The common problem with the application of present codes is the uncertainty in the extrapolation of solubility quotients to high temperatures at high ionic strengths, because the complex Pitzer model (Pitzer, 1973) is not defined above 100°C and other less rigorous models are by the necessity of being general are too simplistic. Thus, there is a need for direct experimental data in this region where specific electrolyte interactions are strong.
Detailed studies of the solubility of gibbsite, Al(OH)3(cr), in NaCl brines have been carried out over the past decade in this laboratory Wesolowski 1992, Palmer and Wesolowski 1992, Wesolowski and Palmer 1994. These experiments covered a range of conditions, namely, temperatures (6–80°C), pH values from weakly acidic to strongly basic, and ionic strengths (NaCl) as high as 5 mol · kg−1. The results of these studies, in combination with appropriate data already available in the literature, have established the thermodynamics of Al3+ and its mononuclear hydrolysis products over this entire range of conditions. Despite the fact that two recent independent studies of boehmite, γ-AlOOH(cr) Bourcier et al 1993, Castet et al 1993 have extended the temperature range of aqueous aluminum thermodynamics to 350°C, discrepancies still exist among the predicted stabilities of aluminum species in solution above 100°C. Apart from extensive research on the aluminate anion in strongly basic solutions, to our knowledge no solubility measurements are available for more acidic solutions at high ionic strengths.
The solubility equilibria pertinent to the dissolution of boehmite are: These equilibria are represented by the general equation (n = 0–4): All of the previous studies of boehmite solubility in acidic to near neutral pH were carried out in dilute solutions, such that extrapolation of the solubility quotients to the infinite dilution standard state usually involved simple application of the Debye-Hückel limiting law to represent the activity coefficient ratio, (γ{Al(OH)n3−n})/γ{H+})3−n, and the activity of water was assumed to be unity. On the contrary, in basic solutions there have been innumerable experimental studies extending to high ionic strengths in concentrated NaOH and KOH (see Pokrovskii and Helgeson, 1995) solutions, presumably motivated by the importance of this system to the aluminum industry. However, under these conditions the formation of aluminate dimers and sodium aluminate ion pairs must be considered explicitly in any speciated thermodynamic description of boehmite or gibbsite solubility in basic solutions. The results at low ionic strength (0.03 mol · kg−1, NaCl) obtained in our laboratory are presented by Bénézeth et al. (2001), who define the solubility constants at infinite dilution (Ks0–Ks4) to 290°C and compare them with the corresponding values in the existing literature.
In the present study, the ionic strength was extended to 5 mol · kg−1 (NaCl) requiring an empirical model of the activity coefficient ratio in addition to the Debye-Hückel term and the activity of water must also be defined. In acidic solutions, the concentration of HCl, and hence Al3+, was too low to observe the formation of Al2(OH)24+, Al3(OH)45+, and Al13O4(OH)247+ postulated by Baes and Mesmer (1976) to exist at ambient conditions, noting that high temperatures should further destabilize polynuclear, highly charged species. The sodium aluminate ion pair undoubtedly exists in basic solutions at high NaCl concentrations (see Pokrovskii and Helgeson, 1995 and Diakonov et al., 1996 for summaries of ion-pair formation quotients), but in the stoichiometric treatment described here, these ion pairs are not treated explicitly and therefore are accounted for by the activity coefficient ratio, such that the fitting equation for Qs4 provides a total measure of free and ion-paired aluminate ions. The key variable to the equilibria in , , , , is pH, which in the present study was measured for the most part directly at temperature.
Hydrogen-electrode concentration cells (HECC) have been used for nearly 30 yr in this laboratory, and to a limited extent elsewhere, to study a wide range of homogeneous aqueous reactions (cf. Mesmer et al 1995, Wesolowski et al 1995), including the first hydrolysis quotient of Al3+ (Palmer and Wesolowski, 1993), and the stabilities of aluminum polynuclear species Mesmer 1971, Macdonald et al 1973. The electrochemical potential of this cell is derived from the difference in hydrogen ion concentrations (molality in this case) between that of a known reference solution and the “unknown” test solution. Consequently, the derived pH of this test solution is defined as the negative logarithm of the hydrogen ion molality, pHm. Advantages in using this definition, which is in keeping with the definitions of other species in solution particularly at high temperatures, have been discussed by Mesmer and Holmes (1992). This treatment ignores HCl ion-pair formation, which becomes more appreciable with increasing temperature and reduces the calculated acidity of the reference solution. Currently, uncertainties in the HCl ion-pair formation constant and activity coefficient of H+ in NaCl solutions are too large to make meaningful corrections to the stoichiometric hydrogen ion molality approach adopted here.
Section snippets
Synthesis and characterization of boehmite
Boehmite was synthesized hydrothermally (Castet et al., 1993) from acid-washed gibbsite (Wesolowski, 1992) and was the same material used and described by Bénézeth et al. (2001). The results of X-ray diffraction, SEM, surface area, and thermogravimetric analyses of the pristine boehmite and samples taken at the completion of the solubility measurements established that no phase changes had occurred with only a small degree of recrystallization taking place during the experiments. The details of
Measurement of hydrogen ion molality
The initial cell composition in a typical experiment with an acidic reference is as follows: To minimize activity coefficient differences and liquid junction potentials, ratios of m1 : m2; and (m1 or m3) : m4 were <0.1, and m2 ≈ m4. The hydrogen ion concentration in the test compartment is determined relative to the known concentration in the reference compartment from the Nernst equation:
Solubility results
The isothermal titration experiments carried out with the HECC are summarized in Table 1, Table 2. Note at ionic strengths > 0.1 mol · kg−1, the presence of high salt concentrations created a matrix interference effect with the ion chromatographic analyses at low aluminum concentrations (<10−6 mol · kg−1) (Wesolowski and Palmer, 1994) that precluded measurements in this range. Hence, it was generally impossible to obtain reliable solubility data near the minimum of the solubility profile. In
Future research
The solubility results for gibbsite and boehmite obtained in this research program over a wide temperature range to high ionic strengths will next be combined with isopiestic data from this laboratory to provide an ion-interaction model for the system Na+-Al(OH)4−-OH−-Cl− from ambient temperatures to 300°C. These results will be of particular value to the aluminum industry related to hydrothermal treatment of bauxite and recovery of gibbsite, and to geothermal energy production where
Acknowledgements
This research was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences and the Office of Geothermal and Wind Technologies, U.S. Department of Energy, under contract DE-AC05 to 00OR22725, Oak Ridge National Laboratory, managed by and operated by UT-Battelle, LLC. The authors wish to acknowledge the able assistance of Dr. L. Anovitz, who coordinated the analytical measurements of the boehmite samples and advised us of their significance.
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