A 1H and 27Al NMR investigation of yttrium(III) and europium(III) interaction with kaolinite

Sorption of the trivalent metals Eu3+ and Y3+ on a natural kaolinite mineral was investigated with solid-state H-1 and Al-27 NMR. The metal ion concentration in the samples was varied between 4 x 10(-6) M and 8 x 10(-5) M, while the kaolinite concentration and ionic strength with respect to Na+ were kept constant at 5 g/1 and 1 mM, respectively. The aqueous samples were equilibrated for at least six days at pH 8.0 +/- 0.1 before the mineral was separated from the supernatant, recovered, and dried. Metal ion addition to the kaolinite samples had no influence on recorded Al-27 spectra, which show a center peak at 6.15 ppm corresponding to octahedrally coordinated aluminum in the gibbsite-like sheets of kaolinite. The Al-27 measurements were, however, imperative to perform as potential mineral dehydroxylation was monitored from the Al-27 spectra before and after drying treatment and normalization of H-1 spectra was based on the acquired Al-27 data. The attachment of Y3+/Eu3+ on the kaolinite surface was shown to reduce H-1 resonances with chemical shifts at 23-3.5 ppm, 1.6, 1.3, 0.9 and 0.7-0.3 ppm. The loss of proton signal could be attributed to metal ion attachment to isolated Al-OH groups on the kaolinite edge surfaces and possibly to bridging Al-OH-Al hydroxyls on the gibbsite-like basal plane of the mineral. (C) 2013 Elsevier B.V. All rights reserved.