Effect of organic and inorganic ligands on calcite and magnesite dissolution rates at 60 °C and 30 atm pCO2
Chemical Geology 265(1-2): 33-43
Calcite and magnesite dissolution rates were measured at 60 °C, 30 atm pCO2, 0.1 M NaCl, and pH from 4.95 ± 0.05 to 5.60 ± 0.05 as a function of organic (acetate, oxalate, malonate, succinate, phthalate, citrate, EDTA) and inorganic (sulphate, phosphate, borate, silicate) ligand concentration in the range of 10- 5 to 10- 2 M. These conditions can be considered as boundary model environments for sedimentary oil-field basins of underground CO2 storage. Experiments on dissolution of magnesite powders (100-200 µm) and calcite crystal planes were performed in a batch reactor with in-situ pH measurements and under controlled hydrodynamic conditions using the rotating disk technique. At 60 °C in circumneutral solutions in the presence of 0.02 M NaHCO3 and 30 atm pCO2 (pH = 4.95), calcite dissolution is weakly affected by the presence of ligands: the rates increase at the maximum by a factor of 2 and, at 0.01 M ligand concentration in solution, the order is: silicate < citrate < NaCl ~ borate < malonate < EDTA < sulphate < acetate. The order of ligand effects on calcite dissolution at pH = 5.55 (0.1 M NaHCO3, 30 atm pCO2) is: phosphate < NaCl < citrate < acetate < succinate < malonate < phthalate < EDTA. Magnesite dissolution rates at 60 °C, 30 atm pCO2 and 0.02 M NaHCO3 (pH = 4.95) were weakly affected by the presence of acetate, silicate, borate and NaCl but increase in the presence of sulphate, EDTA, citrate and oxalate. These ligand-affected rates were rationalized using a phenomenological equation which postulates the Langmuirian adsorption of a negatively-charged or neutral ligand on rate-controlling surface sites, presumably > MeOH2+ (Me = Ca, Mg). Proposed equations of rate-ligand concentration dependencies can be directly incorporated into reaction transport codes. Results obtained in this study demonstrate that both magnesite and calcite reactivity is not appreciably affected by acetate, oxalate, citrate, succinate, sulphate, and phosphate that are most likely present in deep carbonate aquifers at the physical and chemical conditions pertinent to CO2 geological sequestering sites. The concentration of ligands necessary to increase the rates by a factor of 3 to 10 is on the order of 0.01 M. Such a high concentration is unlikely to be encountered in deep sedimentary basins. Therefore, as a first approximation, reactive transport modelling of dissolution induced by CO2 injection in carbonate rocks does not require to explicitly account for the effect of dissolved organics.