Kinetics of uranyl ions sorption on heterogeneous smectite structure at pH 4 and 6 using a continuous stirred flow-through reactor

Abstract

The <2μm clay fractions of three smectite samples (BA1, PS2 and PS3) were submitted to UO22+ sorption experiments at pH4 (1×10−4) and pH6 (2.0×10−1) in a continuous stirred flow-through reactor. The aim was to evaluate the influence of heterogeneous structure of smectite during the adsorption and desorption processes, where the breakthrough curves (BTCs) were used to model the mass balance. Kinetic models were applied to describe the adsorption – desorption kinetics for a liquid-solid system. Sample PS2 (K-Illite/beidellite/Na,Ca-montmorillonite) adsorbed at pH4 more 34% and 138% of UO22+ than samples BA1 (beidellite/Ca,Na-montmorillonite) and PS3 (K-illite/Na,Ca-montmorillonite) where the dominant mechanism was the ion-exchange (outer-sphere complexation). The UO22+ amount adsorbed at pH6 decreased significantly, indicating changes in the sorption mechanism. The desorption experiments at pH4 suggested a complete reversibility of the UO22+ previously adsorbed, whereas lower reversibility of the UO22+ previously adsorbed and a stronger complexation occurred at pH6. The kinetics was highly influenced by the structural properties of heterogeneous smectite and the surface complexation mechanism involved. Samples submitted to sorption experiments at pH4 and 6 were analysed by X-ray photoelectron spectrometry (XPS). Two binding energies were identified at 380.8±0.3eV and 382.4±0.3eV, before and after desorption processes at pH4. The proportion of the higher binding energy component decreased after desorption process, suggesting that the UO22+ weakly adsorbed was previously desorbed. The results obtained at pH6 show two different binding energies at 380.3±0.3eV and 381.8±0.3eV, whose proportions remained unchanged before and after desorption process, indicating the higher retention of UO22+ on smectite.