Porous structure and Cr(VI) removal abilities of Fe3O4 prepared from Fe–urea complex

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Abstract

In this work, the porous structure and the Cr(VI) removal abilities of the magnetite (Fe3O4) nanopowder prepared by one-pot solvothermal reaction of Fe–urea complex ([Fe(CON2H4)6](NO3)3) were investigated. Low-angle X-ray diffraction (XRD), nitrogen adsorption–desorption measurements, and transmission electron micrograph (TEM) observations show that the Fe3O4 has a mesoporous structure with an average pore size of 5.3 nm and BET surface area of 120 m2/g. The removal of gaseous species including CON2H4, NO, NO2 and O2 during the formation of the Fe3O4 are considered to be the most probable reason for the formation of mesoporous structure. The adsorption behavior of Cr(VI) on the mesoporous Fe3O4 can be well described by Langmuir adsorption model, and the maximum adsorption capacity for Cr(VI) is estimated to be 15.4 mg/g.

Highlights

► Solvothermal reaction of [Fe(CON2H4)6](NO3)3 can produce mesoporous Fe3O4. ► The mesoporous Fe3O4 shows high removal ability for Cr(VI). ► The adsorption of Cr(VI) on the Fe3O4 can be well described by Langmuir model.

Introduction

As a functional material, Fe3O4 has attracted much attention not only from academic viewpoint but also from various industrial uses [1]. It is a ferrimagnetic and nontoxic material, and it has the highest saturation magnetization of 92.0 emu/g in magnetic iron oxides. Furthermore, it can be prepared via various methods with a very low cost [2]. These excellent properties make it possible to be used in many applications such as ferro-fluids, magnetic recording, magnetic separation, and biotechnology [3], [4], [5], [6], [7]. When the particle size of Fe3O4 decreased to nanometer scale, it exhibits a superparamagnetic property; and, this nanosize effect could further expand its potential applications in many fields, e.g., increasing applications for catalysis and bio-medical field [8], [9], [10], [11], [12]. Recently, Fe3O4 nanopowder attracts much attention from environmental applications in which it is used as a magnetic adsorbent as well as a magnetic support of photocatalysts [13], [14], [15], [16]. In this case, it is used as a bi-functional material, i.e., it is used as an adsorbent as well as a magnetic support material. The most important advantage of using this material is that it can be easily separated from reaction system with an external magnetic field, i.e., its recovery from reaction system can be readily achieved.

For all of these applications, high quality Fe3O4 nanopowder with improved properties is required to satisfy increasing demands. For this, various synthesis methods of Fe3O4 have been developed in the past decades, such as precipitation method, microemulsion, hydrothermal method, sonochemical approach, mechanochemical approach, and thermal decomposition method, [17], [18], [19], [20], [21], [22]. However, the development of an easy preparation method of Fe3O4 nanopowder with low cost is still a challenging research subject in this area. In the case of application for environment, the Fe3O4 is required to have desirable surface properties including high surface area and many surface exchange sites. Yavuz et al. recently reported that Fe3O4 nanopowder had excellent adsorption capacity for arsenic in water [23]. At the same time, the authors showed that the Fe3O4 particle size had a dramatic effect on the adsorption of arsenic, i.e., the adsorption capacity for arsenic dramatically increased with the decrease of Fe3O4 particle size. This implies that the surface area of Fe3O4 nanopowder plays an important role in the sorption process. Owing to its high surface area, the Fe3O4 with mesoporous structure can be expected to have high potential uses for environment, e.g., water treatment. However, there are relatively few reports in which mesoporous Fe3O4 is used for the removal of heavy metals or organic pollutants from waste water. This may be due to the difficulties in the preparation of mesoporous Fe3O4 with high surface area. In 2006, Jiao et al. reported the synthesis of ordered mesoporous Fe3O4 for the first time [24]. In that case, an ordered mesoporous α-Fe2O3 was firstly synthesized using ordered mesoporous silica (KIT-6) as a hard template, then the ordered mesoporous Fe3O4 was obtained by reducing the ordered mesoporous α-Fe2O3 with hydrogen. Since then still little reports have been found on the preparation of mesoporous Fe3O4, probably due to the difficulties in synthesis of the material.

We have recently developed a new method for the preparation of Fe3O4 nanopowder by a simple solvothermal reaction of an Fe–urea complex ([Fe(CON2H4)6](NO3)3) [25]. Using this method, pure Fe3O4 nanopowder with superparamagnetic properties can be prepared in a single step. Another advantage of this method is that [Fe(CON2H4)6](NO3)3 can be easily synthesized from readily available Fe(NO3)3·9H2O and CON2H4. These advantages mean that the method does not need an expensive and complicated unit to produce Fe3O4 nanopowder. In the present study, the porous structure of the Fe3O4 nanopowder as well as its adsorption abilities for Cr(VI) have been investigated in view of application to water treatment. The Fe3O4 nanopowder prepared by this method has been found to possess a mesoporous structure with a high surface area of 120 m2/g and a large average pore size of 5.3 nm. This is an interesting finding because the mesoporous Fe3O4 can be directly prepared by this method without using any templates.

Section snippets

Materials

Fe(NO3)3·9H2O (99%, Tianjin Fengchuan Chemical Reagents Company), CON2H4 (99%, Tianjin ShengAo Chemical Reagents Company), absolute ethanol (99%, Beijing Chemical Reagents Company), potassium dichromate (99.8% K2Cr2O7, Shanghai Fujiang Chemicals Company) were used as received.

Synthesis

The Fe3O4 nanopowder was prepared by the method reported in our previous study [25]. Firstly, [Fe(CON2H4)6](NO3)3 was synthesized through a one-step method as follows: 20.0 g of Fe(NO3)3·9H2O was dissolved in 40 mL ethanol

Characterization of Fe3O4

Fig. 1 shows the low-angle XRD of the sample prepared by the solvothermal reaction of [Fe(CON2H4)6](NO3)3 in ethanol at 200 °C. The inset shows the high-angle XRD of the sample, which reveals the formation of Fe3O4 as reported in the previous study [25]. In the low-angle XRD, a relatively strong and broad peak clearly appeared at 2θ = 1.22° (d = 7.2 nm). It is known that the appearance of a low-angle XRD peak at 2θ < 2° indicate the formation of a mesoporous framework [26], [27]. From half-width of the

Conclusions

In conclusion, this work demonstrates that the Fe3O4 nanopowder prepared by solvothermal reaction of [Fe(CON2H4)6](NO3)3 has a mesoporous structure with relatively high BET surface area. The result suggests that mesoporous Fe3O4 nanopowder can be obtained by this one-pot solvothermal method without using any templates. It is highly probable that the mesoporous structure of the Fe3O4 is formed via the extraction of gaseous species that are produced in the synthesis process. Due to its high

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (Grant No. 20861006) and Natural Science Foundation of Inner Mongolia (Grant No. MS0806).

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