Sorption properties of Th(IV) on the raw diatomite—Effects of contact time, pH, ionic strength and temperature

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Abstract

Diatomite has a number of unique physicochemical properties and has diversified industrial uses. Natural diatomite has been tested as a potential sorbent for the removal of Th(IV) from aqueous solutions. The results indicate that sorption of Th(IV) is strongly dependent on ionic strength at pH<3, and is independent of ionic strength at pH>3. Outer-sphere complexation or ion exchange may be the main sorption mechanism of Th(IV) to diatomite at low pH values, whereas the sorption of Th(IV) at pH>3 is mainly dominated by inner-sphere complexation or precipitation. The competition for Th(IV) between aqueous or surface adsorbed anions (e.g., herein ClO4, NO3 and Cl) and surface functional groups of diatomite is important for Th(IV) sorption. The thermodynamic data (ΔH0, ΔS0, ΔG0) are calculated from the temperature-dependent sorption isotherms. The results suggest that sorption process of Th(IV) on diatomite is spontaneous and endothermic.

Introduction

Diatomite is a pale-colored, soft, lightweight sedimentary rock composed principally of silica microfossils of aquatic unicellular algae. Diatomite's highly porous structure, good sorption ability, chemical inertness, low density and high surface area result in a number of industrial applications as filtration media for various organic and inorganic chemicals as well as an adsorbent (Al-Degs et al., 2001). It is insoluble in water, and is extremely stable in the ambient conditions. A number of laboratory-scale studies were performed to investigate the potential application of diatomite as adsorbent for the removal of uranium (Aytas et al., 1999) and heavy metal ions (Al-Degs et al., 2000; Khraisheh et al., 2004) and textile dyes (Al-Ghouti et al., 2003) from wastewater. The results indicated that diatomite is a very suitable material in wastewater management. However, the mechanism of metal ion sorption on diatomite is still ambiguous. Only macroscopic data of metal ion sorption on diatomite are available.

Thorium is only stable at its valence +IV state in solution, and is usually used as a chemical analogue of tetravalent radionuclides such as Zr, Hf, Np, U and Pu, which are difficult to study and to keep in the tetravalent form (Choppin, 1999). In recent decades, sorptions of Th(IV) on different sorbents as a function of pH, ionic strength and humic substances have been studied extensively (Reiller et al., 2005; Chen and Wang, 2007a, Chen and Wang, 2007b; Tan et al., 2007a, Tan et al., 2007b; Xu et al., 2006). The results indicated that sorption of Th(IV) mainly occurs at pH<4, and the sorption is strongly dependent on pH values. The sorption of Th(IV) on clay minerals and oxides is mainly dominated by surface complexation and precipitation, especially at high pH values. To the best of our knowledge, the applications of diatomite in removal of radionuclides from large volume of solutions are still few (Osmanlioglu, 2007). Detailed research reports of Th(IV) sorption on natural diatomite are not available.

In this work, sorption of Th(IV) on natural diatomite is studied as a function of pH, ionic strength and temperature. The influence of competitive cations and anions on Th(IV) sorption is also investigated. The thermodynamic data (ΔH0, ΔS0, ΔG0) are calculated from temperature-dependent sorption isotherms and are used to evaluate the sorption properties of Th(IV) on natural diatomite. The sorption mechanism of Th(IV) is also discussed in detail.

Section snippets

Chemicals

All chemicals used in the experiments were purchased as analytically pure, and no further purification was done. The natural diatomite sample was derived from Chengzhou county (Zhejiang province, China) and was characterized in detail. Elemental analysis was carried out by plasma emission spectroscopy, the main chemical components of diatomite are SiO2 (65%), Al2O3 (17.5%), Fe2O3 (4.8%), Na2O (0.5%) and CaO (1.1%), which are quite similar to those of Jordanian diatomite reported by Al-Degs et

Characterization of diatomite

Fig. 1 shows the X-ray powder diffraction (XRD) pattern of natural diatomite. The XRD analysis was performed with CuKα radiation (λ=0.154 nm) with a Rigaku X-ray diffractometer. Patterns were identified by comparison to the JCPD standards. The diffraction spectrogram indicates that the diatomite consists mainly of silica (SiO2) with small amounts of Al2O3, Fe2O3, CaO and Na2O. The amorphous band shown in Fig. 1 is probably due to the glass formation of SiO2, peaks at 19°, 21°, 26° and 35° are

Conclusions

From the results of this paper, one can draw the following conclusions: (1) Sorption of Th(IV) to diatomite is quick and can be fitted by a pseudo-second-order rate model very well. (2) Sorption of Th(IV) is strongly dependent on pH values. The sorption is dependent on ionic strength at pH<3, and independent of ionic strength at pH>3. (3) The presence of anions influences the sorption of Th(IV) obviously under our experimental conditions. (4) Outer-sphere complexation or ion exchange may be the

Acknowledgement

Financial support from National Natural Science Foundation of China (20501019, 20677058) is acknowledged.

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