POLYMORPHIC CHANGES OF CALCIUM CARBONATE MINERALS IN THE PRESENCE OF GALACTURONIC ACID AND GLUCURONIC ACID AT 40–60°C

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Galacturonic acid and glucuronic acid are important constituents of polysaccharides produced by microbes and plants. To demonstrate the effect of such uronic acids on the polymorphism of calcium carbonate (CaCO₃) minerals in hot spring environments, we studied the formation of CaCO₃ minerals in the systems containing galacturonic acid or glucuronic acid at 40, 50, and 60°C for 24 h. Each system contained 5.0 mmol/L Ca²⁺ and 20.0 mmol/L total carbonate ions with 0.0, 0.01, 0.1, 1.0, 2.0, and 5.0 mmol/L galacturonic acid or glucuronic acid. Our results revealed that needle-like shaped aragonite crystals were predominantly formed in the absence of uronic acids. In the presence of uronic acids, galacturonic acid strongly inhibited aragonite formation and favored rhombohedral calcite formation as galacturonic acid concentration increased. Glucuronic acid demonstrated a similar effect on the CaCO₃ polymorphism, but the effectivity was relatively lower than that of galacturonic acid. Moreover, the aspect ratio of aragonite crystals tended to decrease more with galacturonic acid concentration than with glucuronic acid. These effects of the uronic acids on the CaCO₃ polymorphism were considered to be attributed to the higher adsorption affinity of galacturonic acid to the aragonite surface than the calcite surface. These results indicate that microbial uronic acids and organic polymers containing uronic acid residues are closely involved in the CaCO₃ polymorphism in hot spring environments.