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The solubility of U(VI) was measured in highly basic solutions (pOHc = -logfOH"] ranges from 3 down to 0) at the ionic strength I = 0.5-2.0 Μ (Μ: mol dm -3 ) over a wide range of carbonate ion concentrations C = IX 10~3-0.5 Μ by both oversaturation and undersaturation methods. In oversaturation experiments, the solubility of U(VI) decreased with increasing equilibration time from 1 week to 1 year, suggesting an increase in the crystallinity of the solid phase with aging. The solid phase was identified as Na 2 U 2 0 7 · χ H 2 0 (x = 3-5) by X-ray diffraction. Undersaturation experiments conducted for 1 month with this solid phase indicated a rapid attainment of equilibrium. These data were well interpreted by considering the formation of [U0 2 (0H) 3 ]", [U0 2 (0H) 4 ] 2 " and [U0 2 (C0 3 ) 3 ] 4 ", and the apparent equilibrium constants for [UO 2 (0H) 3 ]~ and [U0 2 (0H) 4 ] 2 " were determined with fairly small deviations. Using the evaluated solubility product of Na 2 U 2 0 7 · χ H 2 0 (χ = 3-5), the stability constants for the above species were evaluated. The upper limit values of the stability constants of the carbonatohydroxodioxouranium(VI) complexes were also evaluated.

In a study of the adsorption behavior of cations onto quartz, the distribution coefficient of a variety of cations was determined using the batch method, and using the titration method, the surface charge densities of quartz in a number of electrolyte solutions. The two values thus determined were analyzed applying the electrical double-layer model, from which optimum parameter values were derived for double-layer electrostatics and intrinsic adsorption equilibrium constants. Based on these parameter values, the mechanism of cation adsorption is discussed: A key factor governing this mechanism proved to be the hydration behavior of cations. Consideration of the Coulomb interaction between the adsorbate ions and adsorbent surface led to the finding of a simple rule governing in common the adsorption equilibrium constants of different metal ions.

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The concentration of ions in plant cells and tissues is an essential factor in determining physiological function. In the present study, we established that concentration gradients of mobile ions exist in both xylem exudates and tissues within a barley (Hordeum vulgare) primary leaf. For K + and NO3 -, ion concentrations generally decreased from the leaf base to the tip in both xylem exudates and tissues. Ion gradients were also found for Pi and Cl -in the xylem. The hydathode strongly absorbed Pi and re-translocated it to the rest of the plant, whereas Cl -was extruded. The ion concentration gradients developed early during leaf growth, increased as the tissue aged and remained under both high and low transpiration conditions. Measurement of the expression profiles of Pi, K + and NO3 -transporters along the longitudinal axis of the leaf revealed that some transporters are more expressed at the hydathode, but for most transporters, there was no significant variation along the leaf. The mechanisms by which longitudinal ion gradients develop in leaves and their physiological functions are discussed.

79 Se is a major dose-determining redox-sensitive nuclide in safety analysis of radioactive waste disposal sites. In aqueous solutions, selenium forms soluble anionic species (Se IV O 3 2and Se VI O 4 2-) that hardly sorb on negatively charged surfaces of common host-rock minerals. However, Se is known to have a strong affinity with sulphides and interacts with pyrite, a common minor mineral of argillaceous rocks being considered as host formations for radioactive waste repositories. In this study, we present micro-and bulk X-ray spectroscopy data (l-XRF, l-XANES, and EXAFS) showing that, under nearly anoxic conditions, dissolved SeO 3 2and SeO 4 2sorb directly onto the pyrite surface and are subsequently reduced to Se 0 with increasing ageing time (up to 8 months). These results suggest that the mobility of 79 Se IV released from radioactive waste could greatly decrease through uptake on the pyrite surface followed by transformation into a sparingly soluble reduced form.