To clarify possible origins of irreversible adsorption/desorption of cesium on clay minerals, we performed first principles molecular dynamics simulations for vermiculite including water molecules along with Mg^<2+>, K^+, and Cs^+ in its interlayers. Our close examination on the structural and dynamical properties of interlayer water included in vermiculite together with the cations shows that Mg^<2+> and Cs^+ form respectively outer- and inner-sphere surface complexes in the interlayers, while K^+ forms both types of complexes. The structure of electrical double layers formed in the interlayers is found to depend significantly on the types of surface complexes of the interlayer cations. The reorientational motions of H_2O molecules are suggested to be enhanced by Cs^+ in contrast to a minor influence of Mg^<2+> and K^+. Our observed enhanced diffusion of interlayer H_2O molecules along with their rapid reorientation indicates that Cs^+ ions work as structure-breaking ions even in confined 2-dimensional space of the interlayers.

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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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Large amounts of radiocesium were released into the environment by the Fukushima Daiichi nuclear power plant accident in Japan in March, 2011. In this study, the vertical distribution of radiocesium in the side bar deposits of two rivers that originate in Fukushima Prefecture, the Utsushi and Agano Rivers, was determined and the transportation of radiocesium was discussed. The side bar deposits comprised sandy and muddy layers; the latter layer tended to have higher radiocesium concentrations. The concentration of radiocesium per clay fraction was calculated by dividing the concentration of radiocesium per fine earth fraction (particle diameter &le; 2 mm) by the clay content. The concentration of radiocesium per clay fraction was high in the lower layers of the side bar that appears to have been deposited shortly after the nuclear power plant accident. Subsequently, the concentration of radiocesium per clay fraction tended to decrease with the deposition of sediment. In addition, in the lower layer of the Utsushi River side bar, significant radioactivity was detected in the weathered biotite particles of the sand-size fraction.

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