Roles of microbial acidic polysaccharides in precipitation rate and polymorph of calcium carbonate minerals

Organic molecules secreted by bacterial cells are capable of influencing dissolution and precipitation rates of various minerals including calcium carbonate (CaCO3) minerals. To evaluate the effects of polysaccharides on the precipitation rates and polymorph of CaCO3 minerals, precipitation experiments were performed in systems containing alginic acid or gellan gum by the batch method using 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, 0.2, and 0.5 mg/ml of alginic acid (A systems: A1–A5) or gellan gum (G systems: G1–G5). Results showed that both alginic acid and gellan gum significantly inhibited the precipitation of CaCO3 minerals with increasing concentrations of the polysaccharides. Notably, it was found that the inhibition effect of alginic acid was much greater than that of gellan gum. In addition, only aragonite was formed in solutions containing no polysaccharides, owing to the effect of Mg2+ ions. However, the dominance of aragonite as a polymorph decreased and that of calcite increased with increasing concentrations of both polysaccharides, and this effect on the polymorph was much greater for alginic acid than for gellan gum. These effects on precipitation rates and polymorph are likely caused by the adsorption of both polysaccharides on the surfaces of calcite and aragonite. However the higher charge density of alginic acid may have contributed to the stronger inhibition effects on precipitation rates, and relatively higher adsorption affinity of the aragonite surfaces with the polysaccharides also inhibits growth of aragonite resulting in formation of calcite as a dominant polymorph.