Elsevier

Chemical Engineering Journal

Volume 156, Issue 2, 15 January 2010, Pages 313-320
Chemical Engineering Journal

Adsorption behavior of methylene blue onto titanate nanotubes

https://doi.org/10.1016/j.cej.2009.10.023Get rights and content

Abstract

Calcined titanate nanotubes were synthesized with hydrothermal treatment of the commercial TiO2 (Degussa P25) followed by calcination. The morphology and structures of as-prepared samples were investigated by transmission electron microscopy, X-ray diffraction and N2 adsorption/desorption. The samples exhibited a tubular structure and a high surface area of 157.9 m2/g. The adsorption of methylene blue onto calcined titanate nanotubes was studied. The adsorption kinetics was evaluated by the pseudo-first-order, pseudo-second-order and Weber's intraparticle diffusion model. The pseudo-second-order model was the best to describe the adsorption kinetics, and intraparticle diffusion was not the rate-limiting step. The equilibrium adsorption data were analyzed with three isotherm models (Langmuir model, Freundlich model and Temkin model). The best agreement was achieved by the Langmuir isotherm with correlation coefficient of 0.993, corresponding to maximum adsorption capacity of 133.33 mg/g. The adsorption mechanism was primarily attributed to chemical sorption involving the formation of methylene blue-calcined titanate nanotubes nanocomposite, associated with electrostatic attraction in the initial bulk diffusion.

Introduction

Titanium dioxide (TiO2) has been widely used in heterogeneous photocatalysis due to its stable physico-chemical property and high photocatalytic activity. Nanotubular TiO2-derived materials are particularly interesting since the discovery of carbon nanotubes in 1991 [1] and first fabrication of titania nanotubes in 1998 [2]. Among these, one-dimensional titanate nanotubes have attracted extensive attention for their particular morphology and unique physical and chemical properties. Such nanotubes can be synthesized via hydrothermal reaction between TiO2 and NaOH [3], [4], [5]. The obtained nanotubes exhibit multi-walled scroll-type open-ended structures and have large internal and external surfaces and interlayer spaces. These peculiar microstructures make titanate nanotubes have great potential for applications as adsorbents and photocatalysts. Several works [6], [7] have reported that titanate nanotubes with calcination post-treatment exhibit favorable photocatalytic activity.

It has been demonstrated that photocatalytic degradation rate depends on photocatalyst–pollutant molecule interaction and good adsorption of pollutant molecule can improve the efficiency of photocatalytic degradation [8], [9], [10]. The adsorption of pollutants is also of interest as it concerns other experimental studies, including photocatalyst surface modification [11], dye-sensitized photodegradation of organics [12], and so on. Obviously, it is of great importance to investigate the adsorption process of organic pollutants on the surface of titanate nanotubes to clarify the mechanism of photocatalytic reactions and facilitate the applications in contaminant destruction. Unfortunately, previous studies of titanate nanotubes focused on preparation, structure analysis and photocatalytic efficiency evaluation, and little attention were paid to identifying characteristic of organics adsorption onto titanate nanotubes. Furthermore, very few studies have been done with the focus on adsorption mechanism of organic pollutants on the surface of titanate nanotubes.

In the present work, titanate nanotubes (labeled as TNTs) were prepared by a hydrothermal reaction, whereafter the as-prepared nanotubes were calcined at a constant temperature. One of the familiar basic dyes, methylene blue (MB, Fig. 1), was employed as the model pollutant, and the adsorption of MB onto calcined titanate nanotubes (labeled as CTNTs) was systematically investigated. Specifically, the zeta potential at different pH for CTNTs suspension and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra of MB adsorption onto CTNTs were measured, and adsorption mechanism of MB onto CTNTs was discussed.

Section snippets

Chemicals

MB (purity ≥98.5%, Beijing Chemical Reagents Company, Beijing, China) and TiO2 (P25, Degussa, Frankfurt, Germany) were used as received. According to the manufacturer, Degussa P25 contains approximately 90% anatase and 10% rutile. A 3000 mg/L stock MB solution was first prepared in deionized water. Other chemicals such as sodium hydroxide and ethanol were purchased as analytical reagent. All the solutions used in the experiment were prepared with deionized water.

CTNTs preparation

Titanate nanotubes were

Morphology and structures of titanate nanotubes

Fig. 2 showed the TEM images of the as-prepared products. One can see that a large amount of randomly tangled nanotubes was obtained. The hollow and open-ended characteristics of the TNTs can be observed from the TEM micrograph as shown in Fig. 2(a). The HRTEM image (see Fig. 2(b)) indicated that the prepared nanotubes had uniform inner (ca. 4.5 nm) and outer diameters (ca. 9 nm) along the length. Moreover, the as-prepared nanotubes possessed multi-walled tubular structures and generally

Conclusions

CTNTs were prepared through an alkaline hydrothermal treatment of TiO2 (Degussa P25) with 10 M NaOH aqueous solution followed by calcination in the air at 400 °C for 2 h. The morphology and structures of the nanotubes were well-preserved after calcination, keeping a scrolled multi-walled tubular configuration. The present study showed that the CTNTs, with the high surface area of 157.9 m2/g, had great adsorption capacities of MB. The equilibrium adsorption was attained in nearly 60 min. It was

Acknowledgements

The authors would like to acknowledge financial support from the Research Fund for the Doctoral Program of Higher Education (Grant No. 20070001045). Thanks are also to S.F. Weng from College of Chemistry and Molecular Engineering, Peking University for his assistance in ATR-FTIR analysis. The two anonymous reviewers are also gratefully acknowledged for their constructive comments and suggestions.

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