Compositional, Structural, Surface Characterizations of Natural Magnetite from Air Massif (Niger) in Relation to Its Catalytic Activity

Ferrimagnetic materials such as natural magnetite are used for practical ap-plications because of their electronic, magnetic and catalytic properties in the degradation of organic compounds. In order to determine its physicochemi-cal properties in relation to its catalytic activity, the natural magnetite of Ofoud Mount (Niger) is characterized by X-ray florescence (XRF), X-ray diffraction (DRX), specific surface area (BET) and Fourier transformed infrared (FTIR). The result shows an iron content of 97.09% and a specific surface area of 69.742 m 2 /g. The crystal structure of magnetite is cubic with lattice parameters α = β = γ = 90˚, a (Å) = b (Å) = c (Å) = 8.3740. The results of this study suggest that the natural magnetite of Ofoud Mount can be used as iron source in various fields of science despite the presence of a few impurities that can improve its catalytic activity.

their stability [4] [5] [6]. According to H. He et al. [7], natural magnetite exhibits catalytic performance superior to the pure synthetic magnetite. It also contains ferrous and ferric ions which improve its catalytic activity more than other ferromagnetic materials such as hematite and goethite which contain only ferric ion [8]. Due to its redox properties, magnetite offers high activity in the oxidation process of many non-biodegradable organic compounds (pentachorophenol, phenol, polycyclic aromatic compounds PAHs) [9] [10]. Magnetics are ideal for treating soil contaminating with hydrocarbons through the heterogeneous Fenton reaction [11]. Many studies have demonstrated that the capacity of raw rocks rich in magnetite to adsorb and degrade pollutants is largely dependent on their mineral structure and composition [12]. It specifically depends on parameters such as the presence of impurities, the specific surface area, the oxidation state. The chemical composition of magnetite also varies from one iron ore to another, which gives it variable chemical properties [13]. The characterization of each raw rock is therefore essential for a better understanding of its properties and its use. Previous studies on general characterization have been carried out on raw rocks rich in magnetite [14], on purified materials [15] and synthesized magnetite [6]. The aim of this study was to characterize the iron mineral rich in magnetite collected from the Air massif at Ofoud Mount in relation to its catalytic activities.

Material and Methods
The natural materials rich in magnetite used in this study was collected from the Air massif at Ofoud Mount [5]. These materials have been characterized after grinding and sieved through a mesh to obtain an average size of 0.05 mm [16].
The mineral composition of the raw magnetite was generated by X-ray fluorescence (XRF), the mineralogical analysis was done with the X-ray diffraction method (XRD) and the specific surface area by the standard BET method.

Mineral Composition
The raw rock content (Table 1)

Structural Properties
The physical and chemical properties of natural magnetite depend also on its structure. The crystal structure and unit parameters of magnetite were studied at room temperature using an Empyreal X-ray diffractometer. The Mount Ofoud magnetite diffractogram is shown in Figure 1.
The XRD pattern is characterized by several reflections (Figure 1 [17]. According to Zhong et al. [21] with the increase of titanium content in the magnetite, the average particle size decreases and the size distribution becomes narrow. The low intensity peak at the position [˚2Th.] = 18.336 can thus be linked to the presence of titanium in the magnetite. The crystal structure of magnetite is cubic with lattice parameters α = β = γ = 90˚, a (Å) = b (Å) = c (Å) = 8.3740. The only magnetite phase detected crystalize in the Fd-3m with space number 227 [22]. The fit was satisfactory for the Fe 3 O 4 phase after introducing the atom coordinates (Table 1). Its density of 5.24 g/cm 3 was appropriate with previous research [23]. The result of the DRX analysis shows the chemical purity of this natural magnetite.

Specific Surface
In the heterogeneous Fenton process, the reaction between ferrous or ferric ions and hydrogen peroxide the active sites are located on the surface of the catalyst [24] [25]. The specific surface of the magnetite was determined by N2-BET multipoint analysis using a Novantin kantachrome type surface analyzer.
As can be seen in Table 3, eight methods were used to determine the surface area of the magnetite. Among these analyses, BET is the most popular which has been successfully proved in evaluating the specific area of materials. The main surface area was determined to be 69.742 m 2 /g with Multi Points BET ( Figure   2). The relatively high value obtained by adsorption of nitrogen on the magnetite powder is similar to other researches Gorski et al. [22] for a synthetic magnetite.
Compared to the specific surface of pure magnetite obtained by several authors, this surface is relatively high [26] [27]. The high value may be related to the presence of titanium in the magnetite because it is approximately equal to the titanomagnetite surface area obtained by Zhong et al. [21]. During the heteroge-    neous Fenton process, the reaction between ferrous and ferric ions with hydrogen peroxide occurs on the surface of the catalyst [25]. The high specific surface of the crushed magnetite can thus favor the oxidation of the adsorbed compounds. The cumulative pore size distribution for magnetite is shown in Figure   3. It shows that the micropores vary from 1.6 to 6.0 nm. The average pore diameter estimated from the peak position is around 2.8 nm [8]. The most of the pore volume is in the mesoporous range (2 to 10 nm), showing the strong porous structure of the magnetite. The BET surface area and the average pores diameter values show that it can be used for pollutants adsorption.

Conclusion
This study demonstrates that the natural magnetite from Ofoud mount is highly pure materials which can be used in various fields of science. The XRD analysis showed that magnetite is the only phase detected despite the presence of traces elements (generated by XRF) that can improve its catalytic activities.