Journal of the Clay Science Society of Japan (in Japanese)

The desorption behaviors of Cs from Cs bearing smectite by major cations (Na^+, K^+, Mg^<2+>, Ca^<2+> and NH_4^+) were systematically examined. The suspension of the Cs bearing smectite was prepared by reaction of 1g/L smectite with 75 nM Cs^+ solution in 0.02 M NaCl solutions. The desorption behaviors of Cs were examined by adding the major cations of which concentrations ranged from 10^<-3> to 10^<-1>N to the smectite suspensions. All cations except for K^+ lead to the desorption of Cs from smectite when the concentrations of the added cations increased. On the other hand, K^+ plays a role for inhibition of Cs desorption from smectite. The order of the ability for the desorption of Cs from smectite by the major cations was summarized as follow: Ca^<2+> ≒ Mg^<2+> > NH_4^+ > Na^+ > K^+. The selectivity coefficients based on Gaines-Thomas convention were estimated from the observed desorption behaviors.

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When altered granite containing halloysite was mixed with Sekishu roof tile clay mined from the Tsunozu formation in Shimane prefecture, the deformation ratio during drying of these specimens was larger than that for the roof tile clay alone. To clarify the reason for this, three different roof tile clay mixtures were prepared. The first contained altered granite, the second contained altered granite with ground aggregated halloysite, and the third contained separated halloysite (halloysite sieved from altered granite). The three mixtures, together with unmixed roof tile clay, were then extruded using a vacuum kneading machine through an extrusion die that was shifted upward from the center of the machine. The physical properties of the specimens before drying were estimated by consolidation tests based on the Japanese Industrial Standards (JIS). It was found that the specimen containing altered granite with ground aggregated halloysite exhibited a lower expansion index than the specimen containing altered granite alone. The specimen containing separated halloysite, with short acicular halloysite particles in the clay fraction (<5μm), exhibited the lowest expansion index of the three mixtures. The deformation ratio depended on the expansion index, which was affected by swelling of the aggregated halloysite particles in the silt fraction (5-75μm). On the basis of these results, the amount of deformation that is likely to occur during drying of roof tile products can be easily estimated by measuring the expansion index during consolidation tests.

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The use of magnesium oxide (MgO) as a stabilizing agent for inorganic soil pollutants including heavy metals, arsenic, boron, fluoride and selenium is reviewed. On mixing MgO with soils it reacts with soil water to form magnesium hydroxide and supplies magnesium and hydroxide ions. Magnesium ion forms sparingly soluble compounds with fluoride and arsenic but their solubilities are not low enough to meet the water quality and soil quality standards in Japan. Similarly, the solubilities of hydroxides of cadmium and lead are not necessarily low enough. Experimental studies suggest that the major stabilization mechanism is the incorporation into the magnesium hydroxide structure for fluoride and surface complexation at surface hydroxyl groups for oxyanions. For cationic heavy metals, surface complexation reactions at hydroxylated surfaces of soil minerals are highly probable but they do not seem to have been adequately assessed. MgO is known to react with various soil minerals to form layer silicates and Mg-Al double hydroxides. Stabilization of pollutants by thus formed secondary minerals is also probable but it has not received due attention.

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