Polymorphism and morphology of calcium carbonate minerals precipitated in the presence of low-molecular-weight organic acids with different numbers of carboxyl group at 40–80 °C

Abstract

Low-molecular-weight organic acids are the predominant biological molecules released by microbes in the earth's surface environments. To elucidate the effect of the molecules on the formation and polymorphism of calcium carbonate (CaCO₃) minerals in hot water environments, we studied the formation of CaCO₃ minerals in systems containing citric acid, malic acid, or acetic acid at 40, 60, and 80 °C for 24 h. Each system contained 5.0 mmol/L Ca²⁺, 20.0 mmol/L total carbonate ions, and 0.0, 0.01, 0.1, 1.0, 2.0, and 5.0 mmol/L organic acids. Our results demonstrated that citric acid considerably suppressed aragonite formation and promoted rhombohedral calcite formation with increasing citric acid concentration at temperatures up to 80 °C. Malic acid showed a similar effect on CaCO₃ polymorphism to a slightly lesser extent than citric acid, whereas acetic acid exhibited a much lower effect than the other two organic acids. Moreover, the rhombohedral calcite crystals changed to a polyhedral morphology, and then to polyhedral crystals elongated along the c-axis with increasing citric acid concentration at 40 °C. However, very little or no significant effect on the calcite morphology was observed in the systems containing malic acid and acetic acid. The greater effect of citric acid on the CaCO₃ polymorphism is likely due to the stronger adsorption affinity of citric acid for the aragonite surface compared to that of malic acid and acetic acid. In the morphology of calcite, citric acid is likely to preferentially adsorb on the {hk0} faces such as {110} and {100} of calcite crystals at 40 °C, resulting in the inhibition of crystal growth in the direction perpendicular to the c-axis and promotion of growth in the direction parallel to the c-axis.