An Experimental Study of the Effect of Dissolved Acetate ion on Calcite Precipitation Kinetics and its Implications for Subsurface CO2 Storage

Abstract

Abstract The influence of acetate ion on the calcite precipitation kinetics has been studied using a continuously stirred mixed flow reactor supplied with calcium carbonate supersaturated solution seeded by Iceland spar calcite at pH of 9.0. Two types of liquid solutions were injected into the reactor with similar flow rates to induce calcite precipitation: 1) a mixture of sodium carbonate/sodium bicarbonate solution, and 2) calcium chloride solution. 0-20 mM acetate was added to the second solution. The saturation state and the composition of each sample were calculated using the computer code PHREEQC. Findings reveal that the addition of small amount of acetate significantly reduces the crystal growth rate of calcite. When all other experimental conditions were kept constant, 0.015 mol/kg concentration of acetate reduced the steady state calcite precipitation rate by half. Presumably, the measured inhibition could be explained by the adsorption of acetate ions at the active growth sites of calcite. Acetate ion did not decrease the degree of supersaturation of the solution in the reactor by the complex formation with calcium ion in the solution phase. Rather, the inhibition effect was explained by a Langmuir type adsorption kinetic model. This model was used to fit the calcite precipitation data obtained from the steady state calcite precipitation rate measurements in a linear fashion. Furthermore, SEM observations showed that the presence of acetate ion disturbs the crystal growth of calcite and results in the discontinuous appearance onto the seed surface. TEM investigation of the calcium carbonate precipitates showed that both calcite and aragonite had formed. In the base case, without acetate, the matrix crystal and the precipitate had different crystallographic orientations in the TEM SAED pattern. This implies that the calcite crystal growth did not take place layer by layer. The acetate ion reduces calcite precipitation rate; it might also affect the mineral carbonation process by slowing down the uptake of CO2 to form calcite in basaltic aquifers. The calcite precipitation growth rate data obtained in the presence of acetate can probably be extended to simulate the inhibitory effect of acetate to the calcite group minerals such as rhodochrosite, siderite, magnesite, and smithsonite since all have the same crystal structure as calcite.