Sorption Of U(Ⅵ) And Th(Ⅳ) On Silica

The sorption of U(â…¥) and Th(â…£) on SiO₂ was studied by using a batch technique.Sorption of U(â…¥) and Th(â…£) on SiO₂ as functions of solid-to-liquid ratio(m/V),pH,ionic strength,U(â…¥) and Th(â…£) concentrations,PO₄^3- and FA ligands was studied.It was found that sorption kinetics of U(â…¥) is relatively rapid and 1 h is sufficient to reach sorption steady state.The sorption of U(â…¥) increases from about 0 to 100%with pH over the range of 2.8 to 5.8.Sorption isotherms were collected at pH 4.3 and three ionic strengths(0.01,0.1 and 0.5 mol/L NaCl).It was found that the sorption of U(â…¥) increases with decreasing ionic strength at pH 4.3 and relatively low U(â…¥) concentrations.The sorption of U(â…¥) with variable m/V at pH 4.2 and initial U(â…¥) concentration of 1.03×10^-4 mol/L indicated that the U(â…¥) distribution coefficient increases slightly with the increase of m/V over the range of 1 to 100 g/L. In addition,it was found that sorption/desorption is reversible with respect to aqueous pH values,whereas a hysteresis was found for desorption isotherms.All equilibrium data were fitted with a surface complexation model.A mononuclear inner-sphere surface complex≡SOUO₂⁺ and a polynuclear surface complex≡XO(UO₂)₃(OH)₅ were used to quantitatively interpret sorption of U(â…¥) on silica. Moreover,it was found that the presence of phosphate promotes U(â…¥) sorption.The presence of 20 mg/L FA has little effect on the sorption of U(â…¥) on SiO₂.The sorption of Th(â…£) on SiO₂ is strongly affected by solid-to-liquid ratio(m/V). It was found that the sorption rate of Th(â…£) was relatively slow,10 h is need to reach sorption steady state.Sorption of Th(â…£) on SiO₂ increases from 0%to 95% over the pH range of 2.50—4.0.The pH sorption edge shifts to right when the concentration of Th(â…£) increases.The sorption and desorption isotherms of Th(â…£) can be fitted by the Freundlich equation.The sorption of Th(â…£) is weakly dependent on the ionic strengths.The sorption of Th(â…£) in NaNO₃ solution is higher than that in NaCl solution.In addition,the positive effect of phosphate on the sorption of Th(â…£) onto SiO₂ was demonstrated obviously.The presence of 20 mg/L FA has little effect on the sorption of Yh(â…£) on SiO₂.