2018 年 22 巻 1 号 p. 19-27
Uronic acids are common organic molecules that are released from microbes and plants in the earth’s surface environments. To evaluate the effect of uronic acids on the precipitation rate and polymorphism of calcium carbonate (CaCO3) minerals, precipitation experiments of CaCO3 minerals were performed in systems containing glucuronic acid and galacturonic acid using the batch method with a 100 mL solution at 25°C. Each solution contained 5.0 mM Ca2+ and Mg2+, and 20.0 mM HCO3− ions with 0.00, 0.01, 0.05, 0.1, 0.5, and 1.0 mM of glucuronic acid or galacturonic acid. Additionally, the adsorption experiments with these uronic acids were performed on the surface of calcite and aragonite to confirm their adsorption affinity for the surfaces of CaCO3 minerals. The results showed that both uronic acids inhibited the precipitation of CaCO3 minerals with increasing the concentrations. The inhibition effect of galacturonic acid was much greater than that of glucuronic acid. In addition, only aragonite was formed as a stable polymorph in solutions containing no uronic acid due to the effect of Mg2+ ions. However, galacturonic acid inhibited the formation of aragonite and enhanced the formation of calcite as a stable polymorph of CaCO3 minerals, whereas glucuronic acid showed no significant effect on the polymorphism at concentrations below 1.0 mM. These adsorption experiments revealed that galacturonic acid exhibited much greater adsorption on both surfaces of aragonite and calcite compared to glucuronic acid. Consequently, the extent of the effect on the precipitation rate and polymorphism is consistent with the adsorption affinity of uronic acids for the surfaces of CaCO3 minerals.
