Difference in the stable isotopic fractionations of Ce, Nd, and Sm during adsorption on iron and manganese oxides and its interpretation based on their local structures

Many elements have become targets for studies of stable isotopic fractionation with the development of various analytical techniques. Although several chemical factors that control the isotopic fractionation of heavy elements have been proposed, it remains controversial which properties are most important for the isotopic fractionation of elements. In this study, the stable isotopic fractionation of neodymium (Nd) and samarium (Sm) during adsorption on ferrihydrite and delta-MnO2 was examined. This examination was combined with speciation analyses of these ions adsorbed on the solid phases by extended X-ray absorption fine structure (EXAFS) spectroscopy. Neodymium isotope ratios for Nd on ferrihydrite and delta-MnO2 systems were, on average, 0.166 parts per thousand and 0.410 parts per thousand heavier than those of the liquid phase, which correspond to mean isotopic fractionation factors between the liquid and solid phases (alpha(Lq-So)) of Nd on ferrihydrite and delta-MnO2 of 0.999834 (2 sigma = +/- 0.000048) and 0.999590 (2 sigma = +/- 0.000106), respectively. Similarly, averaged Sm isotope ratios on ferrihydrite and delta-MnO2 were 0.206 parts per thousand and 0.424 parts per thousand heavier than those of the liquid phase and the corresponding alpha(Lq-So) values were 0.999794 (+/- 0.000041) and 0.999576 (+/- 0.000134), respectively. These results indicate that the directions of isotopic fractionation in the Nd and Sm systems are in contrast with that recently found for Ce(III) systems despite the similar chemical characteristics of rare earth elements. EXAFS analyses suggest that the bond length of the first coordination sphere (REE-O bond) of Nd and Sm adsorbed on delta-MnO2 is shorter than that of their aqua ions, although this was not clear for the ferrihydrite systems. The shorter bond length relative to the aqua ion is indicative of a stronger bond, suggesting that the equilibrium isotopic fractionation for the Nd and Sm systems can be governed by bond strength as has often been discussed for isotopic fractionation in solid-water adsorption systems. Meanwhile, EXAFS analyses of the Ce/ferrihydrite system showed a distorted structure for the first coordination sphere that was not observed for Ce3+ aqua ions. Such distortion was also observed for La adsorption on ferrihydrite and delta-MnO2. In addition, previous studies have suggested a high stability of the hydrated state for La and Ce in terms of Gibbs free energy change. Thus, we suggest here that the difference in the stable isotopic fractionation for Ce (and predicted for La) vs. Nd and Sm can be explained by (i) the shorter bond lengths of adsorbed relative to dissolved species for Nd and Sm and (ii) the distorted structure of adsorbed Ce (and La) species and high stability of the aqua Ce ion. (c) 2013 Elsevier Ltd. All rights reserved.