KINETICS AND MECHANISMS OF PRECIPITATION OF CALCITE AS AFFECTED BY PCO2 AND ORGANIC LIGANDS AT 25OC - II. COMPARATIVE STRUCTURAL CHEMISTRY OF FULVIC ACID

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 (Ω) 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 0.035-10 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 mechanisms of crystal growth and heterogeneous nucleation. We found an increase in precipitation rate (at constant Ω) when PCO2 increased. For constant Ω, 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 surface of the calcite crystals follows a Langmuir isotherm for all the PCO2 levels studied; however, the amount of DOC necessary to inhibit calcite precipitation increased. With increasing PCO2, the negative charge on the crystal increases, which affects crystal growth, but also these increases in PCO2 cause a decrease in the solution pH and increase in the ionic strength for constant Ω. Solution pH and ionic strength affect the structure and degree of dissociation of the organic functional groups, which in turn affects the inhibition of crystal growth and heterogeneous nucleation. The effect of PCO2 and DOC concentration on the precipitation rate of calcite is expressed in a precipitation rate model which reflects the contributions of crystal growth and heterogeneous nucleation.