Influence of pH and Temperature on Basaluminite Dissolution Rates

Abstract

The processes, rates, and controlling factors of basaluminite (Al4(SO4)(OH)10·4H2O) dissolution were assessed using batch dissolution experiments in both H2SO4 and HCl at pHs of 2.4, 2.9-3.1, 3.5-3.6 and 4.0-4.1, and temperatures of c. 279, 293, 303 and 312 K. Basaluminite dissolution is incongruent over most of the studied pH range, giving generally a lower Al/S ratio in solution than in the pristine basaluminite sample. The lower Al/S ratio may be at least partially explained by the preferential release of sulfate compared to Al from the dissolving basaluminite. The dissolution rates range between 10–7.6 and 10–9.1 mol·m−2 ·s −1 . At 291-293K, the slowest rates were observed at pH 4.1 in H2SO4 solutions, while at pH 3.0, the slowest rates were observed at 279 K in HCl solutions. Decreases in pH and increases in temperature increase dissolution rates. The influence of pH and temperature on the basaluminite dissolution rate, expressed as Al release, can be described by the following expression: 𝑟𝑎𝑡𝑒𝐴𝑙 = 107.3±0.5𝑎𝐻+ 0.64±0.04𝑒 −78±3⁄𝑅𝑇 Where rateAl is the basaluminite dissolution rate, based on the rate of Al release from dissolving basaluminite (in mol·m−2 ·s−1 ); aH+ is the activity of hydrogen ions in solution; R is the Universal gas constant (in kJ·mol−1 ·K−1 ) and T is temperature (in K). In light of the calculated value for the activation energy (78±3 kJ·mol−1 ), basaluminite dissolution appears to be surface-controlled. The reaction for basaluminite dissolution under the experimental conditions is proposed to be Al4(SO4)(OH)10·4H2O + 10 H +→ 4 Al3+ + SO4 2- + 14 H2O.