Adsorption behavior of multiwall carbon nanotube/iron oxide magnetic composites for Ni(II) and Sr(II)

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Abstract

Multiwall carbon nanotube (MWCNT)/iron oxide magnetic composites were prepared, and were characterized by scan electron microscopy using a field emission scanning electron microscope, X-ray diffraction and vibrating sample magnetometer. The adsorptions of Ni(II) and Sr(II) onto MWCNT/iron oxide magnetic composites were studied as a function of pH and ionic strength. The results show that the adsorptions of Ni(II) and Sr(II) on the magnetic composites is strongly dependent on pH and ionic strength. The adsorption capacity of the magnetic composites is much higher than that of MWCNTs and iron oxides. The solid magnetic composites can be separated from the solution by a magnetic process. The Langmuir model fits the adsorption isotherm data of Ni(II) better than the Freundlich model. Results of desorption study shows that Ni(II) adsorbed onto the magnetic composites can be easily desorbed at pH < 2.0. MWCNT/iron oxide magnetic composites may be a promising candidate for pre-concentration and solidification of heavy metal ions and radionuclides from large volumes of aqueous solution, as required for remediation purposes.

Introduction

Carbon nanotubes (CNTs) [1] have come under intense multidisciplinary study because of their unique physical and chemical properties. CNTs include single-wall (SWCNTs) and multiwall (MWCNTs) depending on the number of layers comprising them. They have been used to adsorbents for hydrogen and other gases due to their highly porous and hollow structure, large specific surface area, light mass density and strong interaction between carbon and hydrogen molecules [2], [3], [4], [5], [6], [7]. CNTs have been found as efficient adsorbents for dioxin [8]. The adsorption capacity of CNTs was superior to that of activated carbon attributed to the stronger interactions between dioxins and CNTs. Liu et al. [9] reported that CNTs filled with gallium could be used as highly sensitive metal vapor sensors and absorbents, and copper vapor not only could deposit into open CNTs but also get into closed CNTs. Yang et al. [10] reported adsorption of polycyclic aromatic hydrocarbons by C60 fullerene, SWCNTs and MWCNTs. Yan et al. [11] reported adsorption of microcystins by CNTs. There are still a few reports on CNTs application for heavy metal ion and radionuclide treatment [12], [13], [14], [15], [16], [17], [18], [19], [20]. The earlier studies indicated that CNTs may be a promising adsorption material used in the waste treatment.

Because of their relatively large specific area, CNTs can be used as supports for catalyst and adsorption materials [21], [22], [23], [24], [25], [26]. The application of magnetic particle technology to solve environmental problems has received considerable attention in recent years. Magnetic particles can be used to adsorb contaminants from aqueous or gaseous effluents and, after adsorption, can be separated from the medium by a simple magnetic process. Examples of this technology are the use of magnetite particles to accelerate the coagulation of sewage [27], a magnetite-coated functionalized polymer such as a resin to remove radionuclides from milk [28], the magnetic particles coated poly(oxy-2,6-dimethyl-1,4-phenylene) for the adsorption of organic dyes [29] and polymer-coated magnetic particles for oil spill remediation [30]: an experiment in environmental technology. However, all these materials have the drawback of a small surface area or a small adsorption capacity, which limits their application.

Removals of heavy metal ions and radionuclides from waste solutions are an important environmental concern in waste management. The objectives of this study are: (1) to prepare and characterize MWCNT/iron oxide magnetic composites, and (2) to investigate the adsorption behavior of Ni(II) and Sr(II) on the magnetic composites as a function of pH and ionic strength. Nickel is a toxic metal ion present in wastewater. More than 40% of nickel produced is used in steel factories, in nickel batteries and in the production of some alloys, which causes an increase in the Ni(II) burden on the ecosystem and deterioration water quality. Moreover, 63Ni is a decay product of fission reactor facilities. Strontium is chosen as a divalent fission product, susceptible to being present in nuclear repositories.

Section snippets

Preparation and characterization of MWCNT/iron oxide magnetic composites

MWCNTs were synthesized by using chemical vapor deposition (CVD) of acetylene in hydrogen flow at 760 °C using Ni–Fe nanoparticles as catalysts. (Fe(NO3)2 and Ni(NO3)2 were treated by sol–gel process and calcinations to get FeO and NiO, and then deoxidized doubly by H2 to get Fe and Ni [17], [18]. Oxidized MWCNTs were prepared by oxidization with 3 M HNO3 [17], [18]. Briefly, 400 mL 3 M HNO3 including 2 g of MWCNTs was ultrasonically stirred for 24 h, filtrated, rinsed with doubly distilled water

Results of characterization of magnetic composites

After the preparation, a test with the Nd–Fe–B magnet showed that the whole material is magnetic and completely attracted to the magnet. The magnetization measurements show that the specific saturation magnetization σs is 29.2 emu g−1 (magnetic field = ±20 kOe) (Fig. 1). The X-ray diffraction patterns (Fig. 2) of the magnetic composites indicate that two peaks corresponding to the structure of MWCNTs also exist in the XRD pattern of the magnetite composites and the XRD pattern of the magnetic

Conclusion

The magnetic composites can be prepared with a high adsorption capacity MWCNTs. XRD characterization suggests that the magnetic phase formed is maghemite or magnetite. SEM image shows an entangled network of MWCNTs with clusters of iron oxides attached to them and suggests the formation of MWCNT/iron oxide magnetic composites. Ni(II) adsorption on the magnetic composites is pH and ionic strength dependent. The Langmuir model fitted the adsorption isotherm data of Ni(II) better than the

Acknowledgements

Financial support from the Natural Science Foundation of China (20677058) and Ministry of Science and Technology of China (2007CB936602) is acknowledged.

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