Organic and inorganic ligand effects on magnesite dissolution at 100 °C and pH=5 to 10

The mechanisms and kinetics of magnesite dissolution were investigated at 100 degrees C, 0.1 M NaCl, and pH from 5 to 10 as a function of organic (acetate, oxalate, citrate, succinate, EDTA) and inorganic (sulphate, phosphate, borate) ligand concentration by hydrothermal atomic force microscopy (HAFM) and mixed-flow reactor solution analysis. At circum-neutral pH, the experiments showed a most pronounced effect in citrate- and EDTA-bearing solutions. For citrate, the modification of the reactivity of a distinct kink-site could be detected. More pronounced than the effect of organic ligands was the effect of 0.01 M NaHCO3 (at pH around 8) which caused a general decrease in reactivity at far from equilibrium conditions. The ligands phosphate, oxalate, citrate, and EDTA were found to weaken but not to fully compensate the retardation of the reactivity generated by NaHCO3. The macroscopic, mixed-flow reactor rates are generally lower than those assessed in the HAFM reactor. This difference can be explained by the different normalization (geometric versus BET) of metal flux and by the specific surface sensitivity of the HAFM-data. There is a positive correlation (R-2 similar to 0.7)between the results of the two rate measurement methods. Macroscopic rates normalized to the geometric surface areas show agreement to microscopic rates within the uncertainty of experimental measurements. Results obtained in this study demonstrate that magnesite reactivity at the conditions pertinent to CO2 geological sequestering sites is not appreciably affected by the ligands that are likely to be present in deep carbonate aquifers (acetate, oxalate, citrate, succinate, sulphate, phosphate) in concentrations less than 1-10 mM. C 2007 Elsevier B.V. All rights reserved.