SYNTHESIS OF CALCITE AND ARAGONITE, AND QUANTITATIVE EVALUATION OF AMINO ACID ADSORPTION ON THEIR SURFACES

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  • KAWANO MOTOHARU
    Graduate School of Science and Engineering, Kagoshima University
  • TOKONAMI MISAKI
    Department of Earth and Environmental Sciences, Faculty of Science, Kagoshima University

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Experiments on adsorption of amino acids on the surfaces of synthetic calcite and aragonite were performed to evaluate the adsorption affinity of amino acids for the CaCO_3 mineral surfaces. Calcite was synthesized using a solution containing 20 mM CaCl_2 and 40 mM NaHCO_3 by stirring at 25℃ for 5 days. Similarly, aragonite was obtained from a solution containing the same concentrations of CaCl_2 and NaHCO_3 with 40mM MgCl_2. Adsorption experiments were conducted by the batch method using 10.0 ml polypropylene tubes with 0.2 g of calcite or aragonite and 5.0 ml of solution containing 10 μM of acidic (Asp, Glu), neutral (Asn, Gln, Gly, Ala), or basic (His, Lys, Arg) amino acids. Five tubes of each amino acid with pH range of approximately 8 to 11 were prepared and sharken for 1 hr, after which the solution pH and concentrations of dissolved amino acids were measured. To evaluate the adsorption affinity of Asp and Arg, adsorption isotherms on the calcite and aragonite surfaces were measured using Asp and Arg solutions with different concentrations up to 100 μM at a constant solution pH of 9.0. The results indicated that calcite tended to significantly adsorb basic amino acids more than acidic and neutral amino acids, whereas aragonite adsorbed much greater amounts of acidic amino acids than basic and neutral amino acids. Adsorption isotherms for Asp and Arg on the calcite and aragonite surfaces confirmed that the calcite adsorbed Arg over 30 times more than Asp, whereas the aragonite adsorbed approximately twice as much Asp as Arg. These different adsorption behaviors of amino acids on the surfaces of calcite and aragonite are likely to be caused by the different charged site densities on the surfaces of these minerals.

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