Sorption of U(VI) and phosphate on γ-alumina: Binary and ternary sorption systems

https://doi.org/10.1016/j.colsurfa.2008.11.032Get rights and content

Abstract

The sorption of U(VI) and phosphate on γ-alumina was investigated in binary (phosphate/γ-alumina; U(VI)/γ-alumina) and ternary (U(VI)/phosphate/γ-alumina) systems as functions of contact time, pH, ionic strength, solid-to-liquid ratio and U(VI) and/or phosphate concentrations by using a batch experimental method. It was found that the sorption of phosphate on γ-alumina increases with pH from 2.5 to 5.2 and then decreases with pH from 5.2 to 9.4. The sorption of phosphate on γ-alumina is insensitive to ionic strength. On the other hand, the sorption of U(VI) on γ-alumina increases with increasing pH over the range of 4–6. The sorption of U(VI) on γ-alumina increases slightly with decreasing ionic strength. In the ternary sorption system, it was found that the presence of phosphate increases the sorption of U(VI), whereas the presence of U(VI) has little effect on the sorption of phosphate. The sorption of U(VI) and phosphate in binary and ternary systems were interpreted in terms of surface complexation models. The effects of γ-alumina dissolution and CO2 in the sorption systems were considered in modeling calculations. Four surface complexes of phosphate, triple bondXOHAlHPO4+, triple bondXH2PO4, triple bondXHPO4 and triple bondXPO42−, and two surface complexes of U(VI), triple bondXOUO2+ and triple bondXOUO2(OH)2, were respectively used to reproduce the sorption of phosphate in phosphate/γ-alumina system and U(VI) sorption in U(VI)/γ-alumina system. The co-sorption of U(VI) and phosphate in the ternary sorption system was interpreted by a model which combines the surface complexation models for the binary sorption systems together in addition to considering the formation of a ternary surface complex, triple bondXOUO2HPO4.

Introduction

The transport of uranium(VI) in the subsurface is a continuous concern with respect to the uranium-contaminated sites and to the storage and disposal of nuclear waste. Uranium(VI) sorption on clay minerals including oxides of Si, Fe and Al is an important process strongly affecting uranium(VI) transport. Both organic and inorganic ligands in subsurface environment have significant effects on speciation of uranium(VI), thus they strongly influence uranium(VI) sorption and transport. Phosphate is one of such ligands governing the behavior of U(VI), yet a much less studied one [1], [2], [3].

Surface complexation models have been used to describe sorption of both cations and anions on clay minerals. The sorption of phosphate on clay minerals has been generally considered as an inner-sphere complexation, i.e., ligand exchange reactions [4], [5]. Laiti et al. [6] used a constant-capacitance model to interpret the interaction of orthophosphate and aged γ-alumina by considering three ligand exchange reactions between surface hydroxyl groups and aqueous phosphate species. He et al. [7] studied the effect of ionic strength upon phosphate adsorption on γ-alumina by using a triple-layer model. Both outer-sphere and inner-sphere surface complexes were tested to fit the experimental data. They found that inner-sphere complexation results in a better fit. Inner-sphere complexation was also considered to be the mechanism of phosphate sorption onto amorphous Fe(III) oxides and goethite [1].

As a result of ligand exchange reactions, the sorption of phosphate usually leads to an increase of negative surface charge on a clay mineral, so that an enhanced sorption of cations might be expected because of electrostatic attraction. On the other hand, potential inner-sphere complexes of cations could be simultaneously reduced because of the occupation of sites by phosphate [8].

In our previous paper [3], a promotion effect of phosphate on U(VI) sorption onto γ-alumina was observed. However, very limited sorption data of phosphate at equilibrium were collected. As a result, it is impossible to set up a reasonable surface complexation model to quantitatively interpret the sorption of U(VI) and phosphate in the co-sorption system. Thus further investigations are needed.

In this study, we will try to quantitatively describe the sorption of phosphate and U(VI) in the binary sorption systems in terms of surface complexation models based on the sorption data of U(VI) and phosphate collected. Then these models will be combined together to interpret the co-sorption of U(VI) and phosphate in the ternary sorption system. The objectives are (1) to examine the applicability of surface complexation models developed from single sorbate systems to co-sorption in a more complicated double sorbates system and (2) to quantitatively interpret the effects of phosphate on U(VI) sorption onto γ-alumina.

Section snippets

Materials

A chromatography γ-alumina (99%, Shanghai Chemical Reagent Co., Ltd.) was used. The γ-alumina was washed with ultra-pure water (18 MΩ) until the electric conductivity of the supernatant reached stable, then separated and dried at 40 °C. The B.E.T. specific surface area (A, m2/g) of the γ-alumina, determined from N2 adsorption/desorption isotherms, was found to be 130.7 m2/g. The distribution of particle-size of the γ-alumina specimen dispersed in water, analyzed by a Mastersizer 2000 meter

Phosphate/γ-alumina system

Phosphate sorption percentage as a function of contact time at [P(V)]TOT = 8.00E−4 mol/L, [NaCl] = 0.1 mol/L and m/V = 2 g/L is shown in Fig. 1. The sorption of phosphate and the pH value of the aqueous phase increase with contact time and reach steady values after 4 days. The increase of pH during sorption was also observed in our previous study [3]. The increasing pH implies that ligand exchange should be the mechanism of phosphate sorption on γ-alumina. In practice, 5 days was selected as the contact

Conclusions

U(VI) and phosphate sorption onto γ-alumina in binary systems (phosphate/γ-alumina; U(VI)/γ-alumina) were studied. Double-layer models were used to interpret the equilibrium sorption data in these sorption systems. The influences of the γ-alumina dissolution and CO2 in the sorption systems were considered in modeling calculations. For the phosphate/γ-alumina sorption system, four surface complexes, triple bondXOHAlHPO4+, triple bondXH2PO4, triple bondXHPO4 and triple bondXPO42− were used to interpret sorption equilibration of phosphate

Acknowledgments

The financial support by the National Natural Science Foundation of China (Nos. 20501010 and J0630962) is gratefully appreciated.

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