The aqueous chemistry of aluminum. A new approach to high-temperature solubility measurements

Abstract

Predictions of changes in geothermal reservoir permeability and porosity during exploitation and reinjection, as well as aluminosilicate scale formation in wells and plant equipment, are currently limited by inaccuracies and discrepancies in our knowledge of the aqueous speciation of aluminum and the solubilities of aluminosilicates in high-temperature brines. To address this problem, the solubility of pure synthetic boehmite (AlOOH) has been measured in noncomplexing solutions over a wide range of pH (2–10), temperature (100–290°C), and ionic strength (0.03–1 mol·kg −1 NaCl) in a hydrogen-electrode concentration cell (HECC) that provided continuous, in situ measurement of hydrogen ion molality. This represents the first such study ever reported of a pH-dependent mineral solubility profile across the entire pH range of natural waters at temperatures above 100°C. Samples of the solution were withdrawn after the pH reading stabilized for analysis of total aluminum content by ion chromatography. Acidic or basic titrants could then be metered into the cell to affect a change in the pH of the solution. The direction of approach to the equilibrium saturation state could be readily varied to ensure that the system was reversible thermodynamically. A least-squares regression of the results obtained at low ionic strength was used to determine the molal solubility products ( Q s0to Q s4) of boehmite, which allowed comparison with those obtained from two recently-reported high-temperature studies of boehmite solubility, which relied on the conventional batch technique. Comparisons are also made with the low-temperature (<90°C) hydrolysis constants for aluminum obtained from solubility measurements with gibbsite as the stable phase. Based on these results, it is possible to draw some general conclusions concerning the relative importance of the aluminum species in solution and to reduce significantly the number of experiments needed to define this complex system. Finally, the application of this new technique to the study of the kinetics and thermodynamics of the dissolution and formation of more complex aluminosilicate minerals is discussed.