Solubility and Reaction Rates of Aluminum Solid Phases under Geothermal Conditions

Experimental studies involving equilibrium solubility and dissolution/precipitation rates were initiated on aluminum hydroxide phases prevalent under geothermal reservoir conditions. A large capacity, hydrogen-electrode concentration cell (HECC) was constructed specifically for this purpose. This cell provides continuous, accurate in situ pH measurements of solid/solution mixtures to 295C with provision for either removing solution samples for analysis of the metal content, or adding either of two titrants. The cell was first used to measure the solubility of pure synthetic boehmite (AlOOH) in NaCl brines over a wide range of pH (2-10), temperature (100 to 290C), and ionic strength (0.03-5 molal NaCl). This represents the first such study ever reported of mineral solubility profiles across the entire pH range of natural waters at temperature above 100C, including direct pH monitoring. A least-squares regression of the results obtained at 0.03 ionic strength was used to determine the molal solubility products (Qs0 to Qs4) of boehmite, which allows comparison with those constants obtained from two recent high-temperature studies of boehmite solubility that relied on the conventional batch technique. The ability to perturb pH isothermally by addition of acidic or basic titrant opens the door for studies of the kinetics of dissolution/precipitation, even for relatively fast reactions. Therefore studies of the dissolution/precipitation rates of boehmite (from 100 to 290C) and gibbsite (50C) were initiated in neutral to basic solutions at 0.1 molal ionic strength. The results of these experiments and their significance in natural and industrial systems will be discussed. We have extended this work to include studies of gibbsite at different temperatures (30-70C), ionic strengths (0.03-1 molal NaCl) and pH’s (acidic and basic media), as well as the study of kaolinite and more complex aluminosilicate phases.