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 1H and 27Al NMR. The metal ion concentration in the samples was varied between 4 × 10− 6 M and 8 × 10− 5 M, while the kaolinite concentration and ionic strength with respect to Na+ were kept constant at 5 g/l 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 27Al spectra, which show a center peak at 6.15 ppm corresponding to octahedrally coordinated aluminum in the gibbsite-like sheets of kaolinite. The 27Al measurements were, however, imperative to perform as potential mineral dehydroxylation was monitored from the 27Al spectra before and after drying treatment and normalization of 1H spectra was based on the acquired 27Al data. The attachment of Y3 +/Eu3 + on the kaolinite surface was shown to reduce 1H resonances with chemical shifts at 2.3–3.5 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.