Jordan, Guntram; Pokrovsky, Oleg S.; Guichet, Xavier; Schmahl, Wolfgang W.
Organic and inorganic ligand effects on magnesite dissolution at 100.
In: Chemical Geology, Vol. 242, Nr. 3-4: S. 484-496
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The mechanisms and kinetics of magnesite dissolution were investigatedat 100 degrees C, 0.1 M NaCl, and pH from 5 to 10 as a function oforganic (acetate, oxalate, citrate, succinate, EDTA) and inorganic(sulphate, phosphate, borate) ligand concentration by hydrothermalatomic force microscopy (HAFM) and mixed-flow reactor solution analysis.At circum-neutral pH, the experiments showed a most pronounced effect incitrate- and EDTA-bearing solutions. For citrate, the modification ofthe reactivity of a distinct kink-site could be detected. Morepronounced than the effect of organic ligands was the effect of 0.01 MNaHCO3 (at pH around 8) which caused a general decrease in reactivity atfar from equilibrium conditions. The ligands phosphate, oxalate,citrate, and EDTA were found to weaken but not to fully compensate theretardation of the reactivity generated by NaHCO3. The macroscopic,mixed-flow reactor rates are generally lower than those assessed in theHAFM reactor. This difference can be explained by the differentnormalization (geometric versus BET) of metal flux and by the specificsurface sensitivity of the HAFM-data. There is a positive correlation(R-2 similar to 0.7)between the results of the two rate measurementmethods. Macroscopic rates normalized to the geometric surface areasshow agreement to microscopic rates within the uncertainty ofexperimental measurements. Results obtained in this study demonstratethat magnesite reactivity at the conditions pertinent to CO2 geologicalsequestering sites is not appreciably affected by the ligands that arelikely to be present in deep carbonate aquifers (acetate, oxalate,citrate, succinate, sulphate, phosphate) in concentrations less than1-10 mM.