Non-thermal plasma synthesis of sea-urchin like α-FeOOH for the catalytic oxidation of Orange II in aqueous solution
Graphical abstract
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 HOH and OO bonds. This feature requires less energy than for NN bond breaking and allows rising gaseous moieties from their fundamental energy level to some excited state. Thus, the NO and HO 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− → H + HO + e−H2O + e− → H+ + HO + 2e−O2 + e− → 2O + e−N2 + O → NO + N
The first aim of our study is to investigate whether the gliding arc plasma species (mostly HO 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 NO 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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