Influence of pH, layer charge location and crystal thickness distribution on U(VI) sorption onto heterogeneous dioctahedral smectite

The UO22+ adsorption on smectite (samples BA1, PS2 and PS3) with a heterogeneous structure was investigated at pH 4 (I = 0.02 M) and pH 6 (I = 0.2 M) in batch experiments, with the aim to evaluate the influence of pH, layer charge location and crystal thickness distribution. Mean crystal thickness distribution of smectite crystallite used in sorption experiments range from 4.8 nm (sample PS2), to 5.1 nm (sample PS3) and, to 7.4 nm (sample BA1). Smaller crystallites have higher total surface area and sorption capacity. Octahedral charge location favor higher sorption capacity. The sorption isotherms of Freundlich, Langmuir and SIPS were used to model the sorption experiments. The surface complexation and cation exchange reactions were modeled using PHREEQC-code to describe the UO22+ sorption on smectite. The amount of UO22+ adsorbed on smectite samples decreased significantly at pH 6 and higher ionic strength, where the sorption mechanism was restricted to the edge sites of smectite. Two binding energy components at 380.8 +/- 0.3 and 382.2 +/- 0.3 eV, assigned to hydrated UO22+ adsorbed by cation exchange and by inner-sphere complexation on the external sites at pH 4, were identified after the U4f(7/2) peak deconvolution by X-photoelectron spectroscopy. Also, two new binding energy components at 380.3 +/- 03 and 381.8 +/- 0.3 eV assigned to AlOUO2+ and SiOUO2+ surface species were observed at pH 6. (C) 2016 Elsevier B.V. All rights reserved.