Polarized EXAFS characterization of the sorption mechanism of yttrium on hectorite

Yttrium uptake in 2 g/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 μM) 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 Å and four O at 2.43 Å. The averaged(Y-O) distance agrees with eight-fold coordination of Y. Neighboring cationic shells of Mg atoms at 3.16−3.18 Å oriented at the magic angle and out-of-plane Si atoms at 3.48−3.51 Å 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.