Investigation of electrochemical synthesis of ferrate, Part I: Electrochemical behavior of iron and its several alloys in concentrated alkaline solutions

Abstract

In recent years, considerable attention has been paid to various applications of Fe(VI) due to its unique properties such as oxidizing power, selective reactivity, stability of the salt, and non-toxic decomposition by-products of ferric ion. In environmental remediation processes, Fe(VI) has been proposed as green oxidant, coagulant, disinfectant, and antifoulant. Therefore, it is considered as a promising multi-purpose water treatment chemical. Fe(VI) has also potential applications in electrochemical energy source, as 'green cathode'. The effectiveness of ferrate as a powerful oxidant in the entire pH range, and its use in environmental applications for the removal of wide range of contaminants has been well documented by several researchers. There is scientific evidence that ferrate can effectively remove arsenic, algae, viruses, pharmaceutical waste, and other toxic heavy metals. Although Fe(VI) was first discovered in early eighteen century, detailed studies on physical and chemical properties of Fe(VI) had to wait until efficient synthetic and analytical methods of Fe(VI) were developed by Schreyer et al. in the 1950s. Actually, there have been developed three ways for the preparation of Fe(VI) compounds : the wet oxidation of Fe(II) and Fe(III) compounds, the dry oxidation of the same, and the electrochemistry method, mainly based on the trans passive oxidation of iron. High purity ferrates Fe(VI) can be generated when electrode of the pure iron metal or its alloys are anodized in concentrated alkaline solution. It is known that the efficiency of electrochemical process of Fe(VI) production depends on many factors such as current density, composition of anode material, types of electrolyte etc. In this paper, the electrochemical synthesis of ferrate(VI) solution by the anodic dissolution of iron and its alloys in concentrated water solution of NaOH and KOH is investigated. The process of transpassive dissolution of iron to ferrate(VI) was studied by cyclic voltammetry, galvanostatic and potentiostatic pulse method. Cyclic voltammetry gave useful data on potential regions where ferrate(VI) formation is to be expected in the course of transpassive anodic oxidation of iron and some of its alloys, and its stability in the electrolytes of different composition. In addition, step-wise oxidation of iron in anodic oxidation is confirmed. Galvanostatic pulse experiments confirmed the character of successive anodic oxidation of iron, as the three-step process of ferrate(VI) formation is clearly observed. In the cathodic pulse complex reduction of ferrate (VI), firstly to Fe(III) species and then to mixed Fe(II) and Fe(III) compounds and finally to elementary iron is confirmed. The significant difference between the mechanisms of anodic oxidation of pure iron and low carbon steel at the one side and electrical ferrous-silicon steel at the other is observed. The influence of material chemical composition on the electrochemical behavior of electrode in course of anodic polarization in strong alkaline solutions is discussed in terms of composition of passivating layer formed on the electrode. On the base of the experimental data, efficient synthesis of ferrate(VI) can be expected in the region of anodic potentials between + 0,55 and + 0,75 V against Hg|HgO reference electrode in the same solution, depending on the anode materials composition, in the alkaline electrolytes concentration between 10 and 15 M.