STRUCTURE OF URANIUM(VI) SURFACE COMPLEXES ON KAOLINITE

The sorption of U(VI) onto the reference kaolinite KGa-1b has been investigated both by batch experiments and EXAFS measurements. Sorption samples were prepared in 0.1 mol/L NaClO4 at total U concentrations ranging from 1 µmol/L to 20 µmol/L, 4 g/L kaolinite, pH 3.0 to 10.5, presence and absence of ambient CO2, and 60-h equilibration. The sorption curves for 1 µM U(VI) obtained in presence and absence of CO2 show that the adsorption edge occurs at pH 5.5. The uptake of U(VI) by kaolinite strongly increased above pH 4.0 and reached its sorption maximum (100 %) in the pH range from 6.0 to 8.0. Above pH 8.0, the amount of U(VI) sorbed onto kaolinite de-creased due to the formation of the U(VI) car-bonato species in aqueous solution, to 0 % at pH 10.5. In the closed system without CO2 the sorption of U(VI) was 100 % in the pH range 6.0 to 10.5. At 10 µM total U(VI), the sorption edge is shifted slightly higher to pH 6.0. The uranium uptake also decreased above pH 8.0 in the presence of CO2 and remains at 100 % in the CO2-free system. Similar results were found by Sekine, Redden and coworkers [1,2]. The aim of our study was to combine batch experiments with EXAFS spectroscopy to study the speciation of U(VI) at the kaolinite surface in more detail. The samples for the EXAFS measurements were prepared as shown in Table 1. The highest uranium loading of the EXAFS samples was 1200 ppm and the lowest loading was less than 100 ppm, i.e., significantly less than in the previous EXAFS study of Thompson et al. [3]. The uranium L3-edge EXAFS spectra were recorded in fluores-cence mode at room temperature at the Rossendorf Beamline (ROBL) at the Europea