Submitted to: Geochimica et Cosmochimica Acta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 1, 1997
Publication Date: N/A
Interpretive Summary: Prediction of major ion chemistry beneath irrigated lands is required for proper management of salinity and ion nutrition, as well as possible need for reclamation. Accurate prediction of calcite precipitation kinetics is essential for prediction of major ion chemistry. In this paper we determine the calcite precipitation rate as a function of dissolved organic carbon (DOC) and partial pressure of carbon dioxide. We develop a detailed equation relating crystal growth and nucleation to degree of supersaturation with respect to calcite, DOC and carbon dioxide concentration. The equation will be incorporated into chemical models for irrigation management and reclamation.
Technical Abstract: This study was conducted to develop a model for the precipitation rate of calcite under varying CO2 partial pressures and concentrations of dissolved organic carbon (DOC). Precipitation rates of calcite were measured in solutions with supersaturation values (omega) between 1 and 20 and in the presence of 2 m2L-1 of calcite.Experiments were run at partial pressures of CO2 (PCO2) in the range of 0.035 to 10 kPa and DOC concentrations in the range of 0.02-0.94 mM. We found an increase in precipitation rate (at constant omega) when PCO2 increased. For constant omega, we also found a linear relationship between calcite precipitation rate and activity of CaHCO3+, indicating that CaHCO3+ species have an active role in the mechanism of calcite precipitation. These findings suggest that the increase in the precipitation rate with higher PCO2 levels is likely caused by the increase in the negative charge on the calcite surface together with an increase in the activity of CaHCO3+ species in solution. The mechanism of inhibition of calcite crystal growth by organic ligands has been shown to be surface coating of the crystals by DOC. The amount of DOC adsorbed on the calcite surface crystals follows a Langmuir isotherm for all the PCO2 levels studied, however the DOC amount necessary to inhibit calcite precipitation increased when PCO2 increased. Solution pH and ionic strength affect the structure and degree of dissociation of the organic matter functional groups, which in turn affects the inhibition of crystal growth and heterogeneous nucleation.