Polarized EXAFS characterization of the sorption mechanism of yttrium on hectorite

摘要

Yttrium uptake in 2g/L hectorite suspensions at high ionic strength (0.5 M NaCl) as a function of pH (6.5 and 7) and total Y concentration (10 and 50 mu M) was investigated by solution chemistry and Polarized EXAFS (P-EXAFS) spectroscopy. At pH 6.5, Y uptake readily occurred, but equilibrium was not attained within 96 h of reaction. A greater fraction of Y was sorbed at pH 7, and near-equilibrium conditions were attained after 48 h of reaction. Polarized EXAFS data indicated that the environments of Y sorbed at the two pH values were structurally similar. Yttrium was surrounded by a split first coordination sphere of four O atoms at 2.27-2.28 angstrom and four O at 2.43 angstrom. The average d(Y-O) distance agrees with eight-fold coordination of Y. Neighboring cationic shells of Mg atoms at 3.16-3.18 angstrom oriented at the magic angle and out-of-plane Si atoms at 3.48-3.51 angstrom were also detected. These distances and orientations are consistent with Y forming inner-sphere surface complexes at layer edges of hectorite by sharing edges with Mg octahedra and corner with Si tetrahedra. The observed angular dependence further suggests that Y is tilted away from the Mg-containing median planes of hectorite platelets.