This works aims at preparing at room stable Na2FeO4 and tracking its degradation over time. The synthetic, during this step, was carried out by electrochemical method. The latter was given maximum focus because of its simplicity and the high degree of purity of the resulting product with respect to wet and dry method. This paper reviews the development of the electrochemical method applied to the synthesis of stable at room Na2FeO4, optimizing the parameters impacting the performance of the oxidation of iron(II) in to iron(VI) in alkaline NaOH, saturated at a temperature of 61°C and a current density of 1 . 4 A/dm2, in order to simplify the synthesis process, to minimize the cost and to improve the production of iron(VI) to meet the growing demand of ferrate(VI) useful for water treatment. The supervision of the degradation of synthesized Na2FeO4 shows its stability over a period of 10 months, which makes storage and transport easier. The phases obtained were characterized by IR spectrometry, X-ray, M?ssbauer, spectroscopy and thermogravimeric analysis.
Iron compounds in the oxidation state (VI) have the advantage of being powerful antioxidants and bactericides, which explains their particular interest in water treatment. Moreover, their results in reduction of ferric hydroxide, their flocculating power contributes to the elimination of organic pollutants, minerals (hydrocarbons, heavy metals, radioactive isotopes...) and industrial effluents.
The synthesis of ferrate(VI) appears to be very delicate, because of the instability that gives them their high oxidizing power. Although the existence of alkaline ferrate has been testified for a century [1,2]. Several efforts have been made to synthesize the solid sodium ferrate [3-7]. Difficulties were encountered in isolating the solid product from each of the resulting solutions and stabilizing it.
The Na2FeO4 phase was synthesized by electrochemical way. Its oxidizing power makes it possible to use it as an oxidizing agent and a disinfectant in water treatment. It first reacts as iron(VI) causing an oxidation process during which the iron(VI) is reduced to Fe(III) that is itself used in the treatment of waste water to precipitate phosphate. The induced oxidation is not accompanied by unwanted byproducts. The aim of our work is to synthesize compounds based on iron(VI) stable at room temperature and replace potassium hydroxide by sodium hydroxide to obtain a significant reduction in the cost of synthesis of these ferrates. The electrochemical method used can provide a useful way for the industrial mass production of iron(VI).
The first electrochemical synthesis of ferrate(VI) is due to Poggendorff (Poggendorff, 1841) [
Next the various tests made by G. Grube [9,10], in concentrated NaOH environment by varying the Current density and temperature and the electrolysis time. During these tests, we found out that the performance of the iron oxidation varies depending on the current density, the temperature and the electrolysis time, according to
The increase in temperature leads to higher yields but with a maximum at T = 61˚C
In fact, from
The reactions involved are:
Anode Simultaneous oxidation of iron and solvent according
to the reaction:
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To obtain sufficiently high concentration ferrate, we use another source of Fe(III), that is the ferric salt FeCl3, 6H2O according to the following reaction:
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Formation process Trivalent iron reacts with the OH− to form an oxohydroxyl complex de FexOy·nH2O type which will then be oxidized electrochemically in the presence of ferrate halide (NaCl).
Cathode Only the reduction in hydrogen of the solvent was held.
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Measures by a radiation ray diffractometer CuK of a compound of ferrate powder Na2FeO4
The spectrum obtained on Na2FeO4 bears a strong similarity with that of isomorphous compounds. There is a splitting of the lines corresponding to planes (102), (202), (013), (200), (002), (004) [14-16].
The appearance of an infrared spectrum is related to the symmetry of the molecule or study group. We expected for, with tetrahedral structure, to find: first, the fundamental bands characteristic of a symmetry 2d: either bands and from the two degenerate modes of vibration: the symmetric stretching and angular deformation within the tetrahedron resulting in inactive modes in infrared absorption, bands and must be absent from the spectra [
(elongation of the tetrahedron) have led W. Griffith [
IR spectroscopy is a quantitative method for the determination of iron(VI) compounds in the ferrate. The shape of the spectra is related to the symmetry of the molecule or groups (tetrahedral structure) the IR spectrum of Na2FeO4.
The IR results support those obtained by XRD.
In general, two stages of decomposition were obtained in the TGA curve up to 500˚C
The two stages of decomposition were accompanied by endothermic heat effects as measured by TGA.
The Mössbauer effect highlights the absorption of apho-
ton Y by a nucleus of 57Fe (present at a rate of about 2.6 percent in the natural iron): we varied gradually the energy of the photon emitted by variation of the speed of the source (57Co. When it reaches a value equal to the difference between the energy level of the core in its ground state (l = 1/2) and its level in the excited state (l = 3/2) the photon is absorbed. This phenomenon results in a peak on the spectrum.
Mössbauer spectroscopy also helped to highlight the existence of a magnetic order at low temperature [23-27]. Na2FeO4 characterization by Mössbauer spectroscopy, after ten months of storage at room temperature, reveals a degradation of iron(VI) over time according to the Mössbauer spectrum
This allows us to visualize the oxidation of iron and therefore control the rate of iron(VI) over time. This degradation is manifested on the spectrum by the isomer shift of the peaks to 0.22 mm/s for sextuplets 1 and 0.34 for sextuplets 2 (see
This isomer shift is due to degradation of Fe(VI) to iron(III) because of moisture.
From the results, it’s possible to synthesize at room stable sodium ferrate Na2FeO4 electrochemically at a temperature of 61˚C and a current density of I = 1.4 A/dm2 in an alkaline medium saturated NaOH.
Infrared spectroscopy shows that we are dealing with a compound containing the group.
Mössbauer spectroscopy of iron allowed us to visualize the oxidation of iron and therefore to control the rate of iron(VI), and track its degradation in iron(III) over time.
The XR spectrum of Na2FeO4 is isomorphic to that given by K2FeO4 literature.
The spectrum shows a peak ATG at 100˚C corresponding to the release of water and a peak at 295˚C corresponding to the decomposition of Na2FeO4.