Effect of acidic microbial polysaccharides on calcium carbonate polymorph formation in hot water at 40-80 °C

抄録

Microbial organic molecules are widely involved in the formation of calcium carbonate (CaCO₃) minerals in various environments on the earth’s surface including hot springs. To evaluate the effect of acidic microbial polysaccharides on the CaCO₃ polymorph formation in hot water environments, batch experiments on the CaCO₃ formation were performed in the systems containing alginic acid (A system) or gellan gum (G system) at 40, 60, and 80 °C for 24 h. Each system contained 5.0 mmol/L Ca²⁺ and 20.0 mmol/L total carbonate ions with 0.0, 1.0, 5.0, 10, 50, and 100 mg/L alginic acid or gellan gum. Results showed that needle- or rod-like shaped aragonite crystals were formed as the predominant polymorph in the absence of polysaccharides at 40-80 °C. In the presence of polysaccharides, alginic acid greatly inhibited the aragonite formation and favored the rhombohedral calcite formation as the predominant polymorph with increasing alginic acid concentration at 40 °C. The effect of alginic acid on the polymorphs slightly decreased as temperature increased but was maintained up to 80 °C. Gellan gum also exhibited a similar effect to a lesser extent than alginic acid at the same concentrations and temperatures. Thus, acidic microbial polysaccharides exhibited a significant inhibitory effect on the aragonite formation and favored the calcite formation depending on their molecular structures in hot water environments. This effect of polysaccharides on the CaCO₃ polymorphs is attributed to the high adsorption affinity of the polymer molecules for the aragonite surface.