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

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 degrees 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 degrees 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 degrees C. The effect of alginic acid on the polymorphs slightly decreased as temperature increased but was maintained up to 80 degrees C. Gellan gum also exhibited a similar effect to a lesser extent than alginic acid at the same concen-trations 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.