Influence of low–molecular–weight dicarboxylic acids on the formation of calcium carbonate minerals in solutions with Mg²⁺ ions

抄録

Organic acids are biological molecules that present abundantly in the earth’s surface environments. Low–molecular–weight dicarboxylic acids are reactive organic acids containing two carboxyl groups in the molecular structure. To evaluate the effect of these dicarboxylic acids on the formation of CaCO₃ minerals, formation experiments were performed by the batch method using 100 ml solutions containing 5.0 mM Ca²⁺ and Mg²⁺, 20.0 mM HCO₃⁻, and 0.0, 0.1, 0.5, 1.0, 2.0, or 5.0 mM oxalic acid, malonic acid, or glutaric acid at 25 °C for 10 days. In addition, adsorption experiments with the dicarboxylic acids on the surfaces of calcite and aragonite were conducted to reveal the adsorption affinity for the surfaces of CaCO₃ minerals. The results confirmed that the dicarboxylic acids inhibited significantly the formation of aragonite and favored the formation of calcite depending on the molecular structure in the following order: oxalic acid > malonic acid > glutaric acid. Notably, oxalic acid had much greater effect on the CaCO₃ formation at lower concentrations, while glutaric acid showed no clear effect even at higher concentrations. The adsorption experiments revealed that the dicarboxylic acids exhibited much higher adsorption affinity for the surface of aragonite than for the calcite surface and showed following order of adsorption affinity for both aragonite and calcite surfaces: oxalic acid > malonic acid > glutaric acid. This is consistent with their effectiveness in the CaCO₃ formation. Therefore, the adsorption affinity of the dicarboxylic acids for the surface of CaCO₃ minerals contributed to the inhibition of aragonite formation and also the resultant formation of calcite by favorable adsorption of the molecules on the surface of aragonite over that of calcite.