A proton NMR study on the specific sorption of yttrium(III) and europium(III) on gamma-alumina [γ-Al2O3]

The specific sorption of Eu(III) and Y(III) on gamma-alumina was investigated with solid-state H-1 NMR. Solution pH was shown to influence the recorded 1H NMR spectra of gamma-alumina, and thus, metal ion-containing samples were prepared under constant pH conditions, 8.00 +/- 0.05. The metal ion concentration in the samples was varied between 6.58 x 10(-7) M-3.95 x 10(-4) M in case of Y3+ and 6.58 x 10(-8) M-1.32 x 10(-4) M in case of Eu3+. The mineral concentration was kept constant at 4 g/I. After separation of the liquid phase, the samples were dried under vacuum to remove physisorbed water from the mineral surface. However, even after 48 h of drying at 150 degrees C and 20 mTorr, water was still detected in the proton spectra as two distinct peaks with chemical shifts at 1.3 and 0.9 ppm. The europium addition to the gamma-alumina samples induced significant spectral changes in comparison with yttrium-containing samples. These changes were attributed to the paramagnetism of europium rather than to complexation reactions occurring on the mineral surface. Proton spectra obtained for yttrium samples were therefore used to detect the spectral changes induced by the sorption reaction itself. The results revealed a large distribution of protons being removed from the mineral surface upon yttrium complexation. Removed protons were assigned to both bridging surface hydroxyls such as (Al-VI)(2)-OH as well as to terminal hydroxyls, e.g., of type Al-VI-OH. Acidic protons belonging to (Al-VI)(3)-OH groups were not observed to participate in the surface reaction. (C) 2011 Elsevier Inc. All rights reserved.