KINETICS AND MECHANISMS OF PRECIPITATION OF CALCITE AS AFFECTED BY P-CO2 AND ORGANIC-LIGANDS AT 25-DEGREES-C

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 (DOG). Precipitation rates of calcite were me asured in solutions with supersaturation values (Omega) between 1 and 20 and in the presence of 2 m(2)L(-1) of calcite. Experiments were run at partial pressures of CO2 (P-CO2) in the range of 0.035-10 kPa and DOC concentrations in the range of 0.02-3.50 mM. The effects of these two variables were quantified separately for the precipitation mechani sms of crystal growth and heterogeneous nucleation. We found an increa se in precipitation rate (at constant Omega) when P-CO2 increased. For constant Omega, we also found a linear relationship between calcite p recipitation rate and activity of CaHCO3+, indicating that CaHCO3+ spe cies have an active role in the mechanism of calcite precipitation. Th ese findings suggest that the increase in the precipitation rate with higher P-CO2 levels is likely caused by the increase in the negative c harge on the calcite surface together with an increase in the activity of CaHCO3+ species in solution. The mechanism of inhibition of calcit e crystal growth by organic ligands has been shown to be surface coati ng of the crystals by DOG. The amount of DOC adsorbed on the surface o f the calcite crystals follows a Langmuir isotherm for all the P-CO2 l evels studied; however, the amount of DOC necessary to inhibit calcite precipitation increased. With increasing P-CO2, the negative charge o n the crystal increases, which affects crystal growth, but also these increases in P-CO2 cause a decrease in the solution pH and increase in the ionic strength for constant Omega. Solution pH and ionic strength affect the structure and degree of dissociation of the organic functi onal groups, which in turn affects the and DOC concentration on the in hibition of crystal growth and heterogeneous nucleation. The effect of P-CO2 and DOC concentration on the precipitation rate of calcite is e xpressed in a precipitation rate model which reflects the contribution s of crystal growth and heterogeneous nucleation. Copyright (C) 1998 E lsevier Science Ltd.