Influence of pH and Temperature on Basaluminite Dissolution Rates
ACS Earth and Space Chemistry
American Chemical Society
Copyright © 2018 American Chemical Society
Reason for embargo
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.
We acknowledge funding for this study provided by the EC Marie Curie Intra-European Fellowship program (Project entitled ‘Reactivity of Aluminum Sulfate Minerals in Mine Wastes’; RASMIM) through a fellowship to P.A. We thank Gareth Tarbuck (UCL, London, UK) for help with the geochemical analyses and mineralogical determinations and their interpretation. We also thank the reviewers whose comments significantly improved the manuscript.
This is the author accepted manuscript. The final version is available from the American Chemical Society via the DOI in this record.