Non-thermal plasma synthesis of sea-urchin like α-FeOOH for the catalytic oxidation of Orange II in aqueous solution

https://doi.org/10.1016/j.apcatb.2015.03.053Get rights and content

Highlights

  • Sea-urchin like goethite was synthesized by using gliding arc plasma in humid air.

  • The as synthesized material was mesoporous with high surface area.

  • The plasma goethite was active in Fenton-like bleaching of Orange II dye.

Abstract

In this study, a template-free synthesis of iron oxyhydroxide nanostructures by gliding arc plasma at atmospheric pressure was evaluated. The results showed that exposure of a Mohr’s salt solution to the plasma discharge induces a rapid oxidation–precipitation of iron(II) into a non-porous and amorphous iron(III) (hydr) oxide. After ageing in temporal post-discharge for three hours, the amorphous iron (hydr) oxide was transformed into crystalline goethite (α-FeOOH). The presence of goethite was confirmed by FTIR, Raman spectroscopy and thermogravimetric analysis. Textural analyses showed that the material is mesoporous with a BET surface area of 134 m2 g−1. SEM pictures revealed that the plasma-synthesized goethite particles consist of sea-urchin like hollow spheres. The catalytic activity of such goethite in the Fenton degradation of Orange II (organic dye) showed that this material can be used as heterogeneous catalyst for effective removal of organic pollutants from wastewater. This study establishes that the plasma discharge of gliding arc type can be used as a green and cheap efficient route for the synthesis of porous metal oxide nanostructures.

Introduction

During the last decade, iron (hydr) oxides have drawn significant interest for their potential applications in the field of wastewater treatment because of their demonstrated excellent adsorption and catalytic capacities, and their environmentally benign nature [1], [2], [3], [4], [5], [6]. Among them, iron (hydr) oxides with three dimensional nanostructures composed of hierarchically assembled nanosized building blocks have several advantages for adsorption and catalysis, such as their high surface area and their easy separation [7], [8]. However, the conventional methods used for the production of iron (hydr) oxides with hierarchical nanostructures are quite expensive because they need the use of templates such as ethylene glycol, and other chemicals including urea and tetrabutylammonium bromide [7]. Another disadvantage of these methods is that they are not environmentally friendly. To overcome the above disadvantages, Faria et al. have recently developed a new green method for the synthesis of iron oxyhydroxide with nanosized particles and high surface area [9]. Briefly, they mixed an alcoholic solution of sodium hydroxide with iron(II) solution, and then, they added hydrogen peroxide to the mixture to form a δ-FeOOH precipitate. During this process, sodium hydroxide and hydrogen peroxide were used as precipitating and oxidizing reagents, respectively. Based on this previous work, we developed a new surfactant-free method to prepare iron oxyhydroxide by using gliding arc plasma with humid air as feeding gas. Our hypothesis is that the hydroxyl radicals created by the plasma discharge can simultaneously act as oxidizing and precipitating reagents for the fabrication of iron oxyhydroxide. Indeed, Depenyou et al. highlighted the formation of lepidocrocite (γ-FeOOH) while treating a carbon steel by gliding arc plasma [10]. The plasma discharges in humid air are known to induce acidifying and oxidizing effects in an aqueous target solution. In such plasmas, a part of the thermal energy carried on by the arc is transferred to the surrounding “parent species” of the feeding gas (i.e. O2, N2 and H2O) and thus favours the cleavage of Hsingle bondOH and Odouble bondO bonds. This feature requires less energy than for Ntriple bondN bond breaking and allows rising gaseous moieties from their fundamental energy level to some excited state. Thus, the NOradical dot and HOradical dot radicals mainly formed in the arc by electron impact as shown in Eqs. (1)–(4) were identified and quantified by emission spectroscopy [11].H2O + e  Hradical dot + HOradical dot + eH2O + e  H+ + HOradical dot + 2eO2 + e  2Oradical dot + eN2 + Oradical dot  NOradical dot + Nradical dot

The first aim of our study is to investigate whether the gliding arc plasma species (mostly HOradical dot radicals) can be used as reagents for the synthesis of hierarchical iron oxyhydroxide nanostructures. The most important merit of this process as compared with other methods consists in the fact that it only requests mild conditions and a short processing time. Moreover, this process can be considered as a green and cheap route, since the only chemical used to produce the reactive species is a water saturated air which is non-pollutant, available and renewable. The second objective of our work is to evaluate the catalytic performance of the plasma-synthesized material in the heterogeneous Fenton removal of organic pollutants from water.

The plasma-synthesized materials were characterised by X-ray powder diffraction (XRD), Fourier transform-infrared spectroscopy (FTIR), Raman spectroscopy, thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and nitrogen physisorption. The catalytic performances were evaluated for the bleaching of an Orange II dye solution. Orange II is a toxic and non-biodegradable azoïc dye often present in textile and food industrial wastewater.

Section snippets

Preparation of α-FeOOH nanostructures

The design of the reactor used for the preparation of iron oxyhydroxide was described in our previous works [12], [13], [14] and consists of a pair of aluminium electrodes symmetrically disposed on both sides of an atomizing nozzle (diameter = 1 mm) and connected to an AC 220 V/10kV-1A high voltage transformer which delivers a mean current of 160 mA (600 V) in operating conditions (Fig. 1). The selected feeding gas is water saturated air. Air is provided by a compressor and then saturated by water by

Preparation and characterization of iron oxyhydroxide

Fig. 3 shows the evolution of the solution pH as a function of the exposure time of the iron(II) solution to the plasma discharge. The initial solution pH was 6.5. When the reaction proceeds, the pH of the solution decreases drastically during the first 30 min. It then stabilizes around 2.2 after 1 h. This decrease in pH is a consequence of the acidifying properties of humid air plasma as described in previous works [15], [16]. Such properties are mainly due to the transformation of NOradical dot radicals

Conclusion

The aim of this work was (i) to synthesize porous iron oxyhydroxides by using a gliding arc plasma process and (ii) to evaluate the catalytic performance of the as synthesized material in the heterogeneous Fenton degradation of Orange II. Results showed that the exposure of iron(II) solution to the plasma discharge induces the formation of amorphous and non-porous iron(III) (hydr) oxide. Ageing of this amorphous material resulted in the formation of sea-urchin like goethite. During the ageing

Acknowledgements

The authors thank the “Université catholique de Louvain” (Belgium) for the grant awarded to A. Tiya-Djowe in the frame of the fellowship “Coopération au développement” program. They are also grateful to Professor J.-L Brisset of “Université de Rouen” for the gliding arc plasma reactor support.

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