External surface adsorption of uranyl–hydroxo complexes on zeolite particles in relation to the double-layer potential

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Abstract
The natural zeolites used in this study and identified by X-ray diffraction were scolecite, chabazite, heulandite and stilbite. Thermal analysis (TG and DSC) and water adsorption isotherms were also used as characterization techniques. The zeta potential was measured by a microelectrophoretic technique on zeolite suspensions containing dilute uranyl solutions. The external adsorption of the uranyl species started at pH 4 and the highest values were reached between pH 6 and 7, when the adsorption could be ascribed to single (UO2)3(OH)+ 5 species. The zeta potential of zeolite particles in uranyl-free suspensions became more negative with increasing pH, explained by hydroxylation of the zeolite surface, resulting in a negative electrical double layer. For heulandite and stilbite a similar zeta potential vs. pH curve was obtained in the presence of uranyl species. The curves obtained in uranyl-containing solutions for scolecite and chabazite, which adsorb the greatest amount of uranyl species, showed a spectacular increase of the zeta potential to positive values between pH 4 and 5 but at higher pH the zeta-potential decreased to negative values. This was probably due to complicated electrostatic interactions between the external surface of the zeolite and the uranyl species in solution.