The Effects Of Dilute Organic Acids On Calcite Dissolution At 22 Degrees And 10^5 PA Total Pressure

Abstract

The role of dilute organic acids on calcite dissolution is important given the abundance of calcite and its importance in controlling surface and ground water geochemistry. The dissolution of calcite in the presence of three organic ligands, citrate, oxalate, and phthalate, has been investigated at 22° C and 10^5 Pa total pressure in a stirred, flow-through reactor. The input fluids were low salinity (I ranged from 0.001 to 0.02), Na-Ca-HCO3-CI solutions having an initial pH of 7.8. The dissolution rate of calcite in a ligand-free solution is essentially linear with respect to log undersaturation. In a purely inorganic solution, the dissolution rate equals 5.95 x 10·13 mol cm^-2 sec^-1 at log (IAP/K) of -1.25 and a surface area of 1890 cm^2/g. Concentrations of the ligands range from 10 to 5000 µm/1. Citrate at a concentration of 50 µm/1 appears to have no effect on calcite dissolution rates. Oxalate, at concentrations of 50 and 100 µm/l, decreases the rate of calcite dissolution. The dissolution rate of calcite in a solution containing 50 µM oxalate at log (IAP/K) -1.25 is approximately 2.60 x 10^-13 mol cm^-2 sec^-1. Phthalate decreases calcite dissolution at low concentrations, i.e., up to 50 µm/l. In some cases, the calcite dissolution rates were decreased to less than 10% of the rates found in the purely inorganic solution. For example, at a concentration of 50 µM phthalate, the calcite dissolution rate was approximately 0.49 x 10^-13 mol cm^-2 sec^-1. The decrease in calcite dissolution at low concentrations is believed due to the ligand complexing with surface dissolution sites. Higher concentrations of phthalate, 100 to 5000 µm/1, actually increase the rate of dissolution relative to changes due to simple IAP/K parameters. At a concentration of 100 µm/l phthalate, the calcite dissolution rate is 3.92 mol cm^-2 sec^-1 x 10^-13 at log (IAP/K) of -1.25. This increase in calcite dissolution is thought to be caused by the destabilization of surface sites. Calcite surfaces were examined by SEM at magnifications up to 4000x. Different organic ligands produce different etch and dissolution features.