The influence of electrolyte composition on electrochemical ferrate(VI) synthesis. Part II: anodic dissolution kinetics of a steel anode rich in silicon

The anolyte composition and process temperature could improve the kinetics of iron anode dissolution and subsequent ferrate(VI) production significantly. This also holds for the anode composition. Silicon-rich steel (SRS) was employed as the anode material to produce ferrate(VI), and the characteristics observed were compared with those of the pure iron anode obtained during our previous study. Using anolytes 14 M NaOH, 14 M KOH and mixtures thereof, the systems were studied by means of potentiodynamic methods, electrochemical impedance spectroscopy and batch electrolysis experiments. In addition, scanning electron microscopy and metallographic images of the material surface were taken to identify changes in the phase composition of the material, caused by anodic polarization in strongly alkaline solutions. The dissolution kinetics increases with increasing temperature and, at 60 °C, also with increasing K⁺ content in the anolyte. Compared to iron, SRS easily dissolves into ferrate(VI), even at 20 °C in pure NaOH, indicating the lower inferior protective properties of oxy-hydroxide surface layers. The current efficiency achieved was almost 55% under these conditions. In the other anolytes, a maximum current efficiency of ca. 40% was obtained at 60 °C. The authors conclude that, at 60 °C, the efficiency is lowered by intensified oxygen evolution. This causes intensive solution convection, disturbing the surface conditions supporting ferrate(VI) formation.     

The influence of electrolyte composition on electrochemical ferrate(VI) synthesis. Part I: anodic dissolution kinetics of pure iron

The influence of anolyte composition and temperature on the anode dissolution kinetics of pure iron and subsequent ferrate(VI) production was studied by means of potentiodynamic voltammetry and electrochemical impedance spectroscopy. The results obtained were verified by batch electrolyses. Pure NaOH, KOH, and mixtures thereof were used as an anolyte. The motivation for this study is to understand in more detail the electrolysis process at which ferrate(VI) is electrochemically produced in situ in the solid form which is more suitable for practical utilization. A significant impact of the anolyte composition on the system behavior was indicated. It is related to the solubility of the anode dissolution products in the anolyte. It was concluded that the fast reaction kinetics in the transpassive potential region is connected with a deterioration of the ferrate(VI) synthesis efficiency. This is explained by the kinetic enhancement corresponding to the intensification of oxygen evolution as a parasitic reaction.     

The rapid electrochemical preparation of dissolved ferrate(VI): Effects of various operating parameters

The preparation of ferrate(VI) by the anodic dissolution of an iron wire gauze in concentrated NaOH solution is described. An anolyte of 0.35-0.48 M Na₂FeO₄ can be produced during 3-6 h electrolysis in initial 16 M NaOH solution at 35 °C. The experimental results indicate that the Fe(VI) concentration variation rate during electrolysis is close related to the factors such as current density, alkaline concentration, the ratio of effective surface area to anolyte volume, the passivation of iron anode and the decomposition of ferrate(VI), etc., and the relevant empirical equation is given. The alkalinity of anolyte has large effect on the electrogeneration of ferrate(VI), especially during an interval electrolysis.     

Influence of anode material composition on the stability of electrochemically‐prepared ferrate(VI) solutions

The stability of electrochemically‐prepared ferrate(VI) solution in 14 M NaOH solution was studied. White cast iron and pure iron were used as anode materials and the anodic current density during the ferrate(VI) preparation was varied in the range 4.4 to 42.8 mA cm⁻². The solution temperature ranged from 20 to 50 °C. The ferrate(VI) decomposition rate was found to depend strongly on solution temperature, anodic current density and also on the anode material composition. The decomposition rate was higher for white cast iron ie the anode material with greater iron carbide content. © 1999 Society of Chemical Industry     

The cyclic voltammetric study of ferrate(VI) formation in a molten Na/K hydroxide mixture

The electrochemical synthesis of ferrate(VI) was studied in a molten salt environment as a function of temperature (170–200 °C). A eutectic NaOH–KOH melt was selected as the most appropriate system for this purpose. Cyclic voltammetry was used to characterize the processes taking place on the stationary iron electrode. The anodic current peaks corresponding to the ferrate(VI) production as well as cathodic current peaks corresponding to the ferrate(VI) reduction appeared to be strongly dependent on the temperature at which the experiment was performed. Ferrate(VI) formation by anodic oxidation of the iron electrode in the molten electrolyte during the batch electrolysis was confirmed for the first time by visible and Mössbauer spectroscopy. The maximum current efficiency of Fe(VI) formation reached during galvanostatic batch electrolysis has attained 72%. It was reached at the temperature of 170 °C.     

The electrochemical generation of ferrate at pressed iron powder electrodes: effect of various operating parameters

The effect of various parameters on the yield for the electrochemical generation of ferrate was investigated for pressed pellet iron electrodes. An optimum yield was observed for a NaOH concentration of 14-16 M of the anolyte. The rate of iron dissolution and generation of ferrate increased when the temperature of the electrochemical cell is raised from room temperature to 50 °C. The pressure applied to the iron powder during the formation of the pellet electrode did not have a strong influence on ferrate generation, at least in the range investigated in this work (5-8 ton/cm²). On the other hand, the purity, particle size and packing density of the powders are important factors in determining the current yield for ferrate generation and the maximum yield was not obtained with the smallest particles investigated (<10 μm). An optimum yield for ferrate generation of 60% was observed for a 2 h electrolysis. The surface of the pressed pellet iron electrode was also analyzed following the electrolysis in 14 M NaOH by X-ray diffraction and X-ray photoelectron spectroscopies. The electrogenerated ferrate was not detected at the electrode surface but the presence of various iron oxides and sodium carbonate was evidenced by these techniques.     

Electrochemical generation of ferrate Part I: Dissolution of an iron wool bed anode

The preparation of ferrate by the anodic dissolution of iron in 10m NaOH using a membrane cell with an iron wool anode is described. It is shown that the current efficiency drops from an initial value of 45–60% to 25% during a 2–3 h electrolysis. This is shown to be due to a change in the iron anode surface, probably the composition, structure and/or thickness of surface films. The influence of cell current as well as NaOH concentration and temperature on the current efficiency is described. The kinetics of the reactions of ferrate with water, alcohols and phenol have also been investigated and it is shown that some organics (methanol, ethanediol and phenol) undergo complete oxidation to CO₂ and H₂O even at room temperature.     

The electrochemical generation of ferrate at pressed iron powder electrode: comparison with a foil electrode

In this work, Fe(VI) (or ferrate) was generated by electrochemical oxidation of iron electrodes, made by pressing iron (99.5%) powder, in 14 M NaOH at room temperature. For comparison purpose, an iron foil electrode was also used to evaluate the effect of porosity on the yield for Fe(VI) generation. The cyclic voltammograms (CVs) of pure iron pellet revealed a broad anodic wave between −1 and 0.5 V versus Hg/HgO corresponding to iron dissolution and passivation of the electrode. These processes are followed by an irreversible oxidation wave attributed to the oxygen evolution reaction (OER) and the generation of ferrate. On the return scan, the cathodic wave at about 0 V versus Hg/HgO is associated with the reduction of electrochemically generated ferrate. The electrochemical generation of ferrate occurred with higher concentration and yield at an iron pellet electrode than at a foil electrode due to the porous structure of the pellet electrode, which favors iron dissolution. However, the electrochemically active thickness of the pellet electrode was estimated to be only about 1% of the actual pellet thickness.     

Influence of anode material on current yield during ferrate(vi) production by anodic iron dissolution: Part III: Current efficiency during anodic dissolution of pure iron to ferrate(vi) in concentrated alkali hydroxide solutions

The dependence of the current efficiency for the oxidation of a pure iron (99.95%) anode to ferrate(vi) ions in 14m NaOH was measured in the region of bubble induced convection for the temperature range 20 to 50°C. The highest current yield obtained after 180min electrolysis was 60% at a current density of 2.3mAcm–2 and a temperature of 30°C. The same current yield was found at a current density of 4.5mAcm–2 and a temperature of 40°C. The dependence of the ferrate(vi) current yields on the NaOH concentration in the electrolyte solution was studied in the range 12 to 17m. The optimum concentration was found to be 16M. The quasistationary anodic polarization curve of pure iron in the transpassive potential region was measured. Apparent oxygen evolution starts at a potential 110mV higher than for white and grey cast iron anodes.     

Solubility of potassium ferrate in 12 M alkaline solutions between 20°C and 60°C

The solubility of potassium ferrate (K₂FeO₄) was measured in aqueous solutions of NaOH and KOH of total concentration 12 M containing various molar ratios of KOH:NaOH in the range 12:0 to 3:9. Several analytical methods were tested for the determination of ferrate concentration. The final method chosen consisted of potentiometric titration of the ferrate sample with an alkaline solution of As₂O₃. The assumption was made that ferrate dissociates in concentrated KOH solutions predominantly to KFeO₄⁻. The solubility constant, S, defined as the product of the molar concentration of the potassium ion, K⁺, and the ferrate anion, KFeO₄⁻, was found to be 0·044 ± 0·006 mol² dm⁻⁶ for 20°C, 0·093 ± 0·004 mol² dm⁻⁶ for 40°C and 0·15 ± 0·09 mol² dm⁻⁶ for 60°C. From these results the heat of dissolution of K₂FeO₄ was calculated as −14·3 kJ mol⁻¹. At 60°C the enhanced decomposition of the ferrate at the higher temperature led to a greater deviation in solubility values compared with data for either 20°C or 40°C.     

The electrochemical generation of ferrate at porous magnetite electrode

In this work, ferrate(VI) was generated by the electrochemical oxidation of porous magnetite electrodes, made by melting pure magnetite grains. Pretreatment of the anode by cathodic polarization was necessary for ferrate(VI) generation and the achievement of high current efficiency. A electrolyte composition was found to be 16 M NaOH. In this electrolyte, the effect of anode current density J on Fe(VI) synthesis rate, current efficiency, and internal cell temperature were studied. An optimum result was obtained at J=3.3 mA cm⁻², 30 °C in 16 M NaOH for 5 h.     

Corrosion behavior of alumina-based ceramic core materials in caustic alkali solutions

Corrosion behaviors of the porous alumina-based ceramic core materials in KOH and NaOH solution were investigated. Corrosion tests were carried out at 100°C, 150°C, and 200°C, and the concentration of KOH and NaOH was 50, 67, and 75 wt%, respectively. The results indicated that the optimal concentration was 67 wt% for KOH solution and 50 wt% for NaOH solution, respectively. Increasing corrosion temperature and prolonging corrosion time were helpful to enhance the corrosion effect, and temperature played an extra important role during the whole corrosion process. NaOH solution was better than KOH solution for corrosion at the same temperature and concentration.     

Research progress in the electrochemical synthesis of ferrate(VI)

There is renewed interest in the +6 oxidation state of iron, ferrate (VI) (Fe^VIO₄²⁻), because of its potential as a benign oxidant for organic synthesis, as a chemical in developing cleaner (“greener”) technology for remediation processes, and as an alternative for environment-friendly battery cathodes. This interest has led many researchers to focus their attention on the synthesis of ferrate(VI). Of the three synthesis methods, electrochemical, wet chemical and thermal, electrochemical synthesis has received the most attention due to its ease and the high purity of the product. Moreover, electrochemical processes use an electron as a so-called clean chemical, thus avoiding the use of any harmful chemicals to oxidize iron to the +6 oxidation state. This paper reviews the development of electrochemical methods to synthesize ferrate(VI). The approaches chosen by different laboratories to overcome some of the difficulties associated with the electrochemical synthesis of ferrate(VI) are summarized. Special attention is paid to parameters such as temperature, anolyte, and anode material composition. Spectroscopic work to understand the mechanism of ferrate(VI) synthesis is included. Recent advances in two new approaches, the use of an inert electrode and molten hydroxide salts, in the synthesis of ferrate(VI) are also reviewed. Progress made in the commercialization of ferrate(VI) continuous production is briefly discussed as well.     

Electrochemical production of ferrate(vi) using sinusoidal alternating current superimposed on direct current. Pure iron electrode

The current yield for the anodic oxidation of a pure iron (99.95%) electrode to ferrate(VI) ions in 14 M NaOH between 30 and 60 °C using a sinusoidal alternating current (a.c.) at amplitudes in the range 38–88 mA cm–2 and frequencies in the range 0.5 mHz to 5 kHz superimposed on direct current (d.c.) of 16 mAcm–2 was measured under conditions of bubble induced convection in a batch cell. The current yield for ferrate(VI) synthesis exhibited a complex dependence on temperature and a.c. frequency, but generally a maximum was observed in a frequency range 2–50Hz depending on the a.c. amplitude. A global maximum current yield after 180 min of electrolysis of 33% was reached at the following conditions: a.c. amplitude of 88 mA cm–2, a.c. frequency of 50 Hz and temperature of 40 °C. At the optimum conditions the highest d.c. electrolysis yield was 23%. Thus, operation with the a.c. component leads to an increase in the yield by 43% with respect to d.c. electrolysis alone.     

Current efficiency during anodic dissolution of iron to ferrate(vi) in concentrated alkali hydroxide solutions

The dependence of the current efficiency for oxidation of an iron anode to ferrate(vi) ions in 14m NaOH was measured in the region of free convection. The highest current yield of 40% was obtained at a current density of 2.1 mA cm^–2 and temperature of 30°C. The iron anode was activated by cathodic prepolarization. The iron concentration in low oxidation states in solution was determined as 0.13 ± 0.1 and 0.29 ± 0.25 g Fe dm^–3 at 20 and 30°C, respectively. The steady state anodic polarization curves of iron in the transpassive potential region are shifted to lower potential values with increasing NaOH concentration from 11 to 171 m. At 40°C all the curves show a limiting current density around 660 mV vs Hg/HgO, namely 9 and 23 mA cm^–2 at NaOH concentrations of 11 and 17 m, respectively.     

Influence of anode material on current yield during ferrate(vi) production by anodic iron dissolution Part II: Current efficiency during anodic dissolution of white cast iron to ferrate(vi) in concentrated alkali hydroxide solutions

The dependence of the current efficiency for the oxidation of a white cast iron anode to ferrate(vi) ions in 14 m NaOH was measured in the region of bubble induced convection for the temperature range 10 to 50 °C. Using a white cast iron anode the highest current yield obtained after 3 h of electrolysis was 55% at a current density of 8 mA cm^–2 and a temperature of 20°C. The anode was activated by cathodic prepolarization. The iron concentration in low oxidation states contained in the post electrolysis solution increased with temperature over the whole temperature range studied. This concentration was almost 10 times higher than in the case of grey cast iron reported earlier. The dependence of ferrate(vi) current yield on the hydroxide concentration was studied in the range 5 to 16 m. The optimum concentration was found to be 14 m. The quasisteady state anodic polarization curve of white cast iron anode in the transpassive potential region was measured. This shows the start of oxygen evolution occurring at higher potentials compared to grey cast iron.     

Influence of anode material on current yields during ferrate(vi) production by anodic iron dissolution Part I: Current efficiency during anodic dissolution of grey cast iron to ferrate(vi) in concentrated alkali hydroxide solutions

Current efficiency for the oxidation of a grey cast iron anode to ferrate(vi) in 14 m NaOH was measured in the region of bubble induced convection. The maximum current yield, obtained after 180 min of electrolysis, had a value of 11% at a current density of 32 mA cm^–2 and a temperature of 20°C. The ferrate(vi) current yield increased throughout the whole current density range under study (from 1 to 36 mA cm^–2). The iron anode was activated by cathodic prepolarization. The iron concentration in lower oxidation states in the solution after electrolysis was constant within experimental error over the whole temperature range under study. The quasisteady state anodic polarization curve for grey cast iron in the transpassive potential region was measured. This anodic polarization curve was shifted to more negative potential, at the same current density, compared to the potential of the mild steel anode.     

Removal of phosphate from agricultural soil by electrokinetic remediation with iron electrode

Phosphorus is considered the limiting nutrient in eutrophication of agricultural soil in Korea. This study evaluates the coupled application of electrokinetic process by iron and titanium electrodes for removal of phosphate from agricultural soils. Experiments were conducted to evaluate phosphate removal under the following conditions: (I) control; (II) 1% starch addition in the soil without EK; (III) 1% starch addition at the anolyte using a cast iron anode and a carbon cathode; (IV) no starch addition using a cast iron anode and a carbon cathode, and; (V) 1% starch at the anolyte using a titanium anode and a carbon cathode. When an iron anode was used under 0.5, 1.0 and 2.0 V/cm, the removal of phosphate was significant at 2 V/cm. The addition of starch also helps to remove nitrate significantly using an iron electrode. The results reveal that iron electrodes result in significantly more removal compared to titanium electrodes.     

Electrochemical behaviour of pure iron in concentrated sodium hydroxide solutions at different temperatures: a triangular potential sweep voltammetric study

The passivation and dissolution of pure iron in NaOH solutions (1 M–10M) have been studied at 30°, 60° and 80° C by triangular potential sweep voltammetry. The variation of peak height and peak potential with sweep rates for the three anodic peaks in the forward direction and two cathodic peaks in the backward direction when polarized from –1.30 V in 1.0 M NaOH solution, suggest that a monolayer adsorption model holds good. The appearance of limiting currents at higher concentrations of NaOH has been explained in terms of the chemical dissolution of two oxides formed by successive oxidation. The formation of different oxides, hydroxides and solution soluble species under transient conditions has been discussed.     

多孔圆筒铸铁阳极电解制备高铁酸盐的研究

采用双阴极室隔膜电解槽，以多孔圆筒铸铁为阳极电解制备水处理剂高铁酸盐。实验表明，所用电解槽与平板铸铁阳极电解槽相比可得到更高的电流效率和更高浓度的高铁酸盐。在14mol／L的氢氧化钠溶液中加入0．1％（质量分数，下同）氯化钠，30℃下以30mA／cm^2的电流密度电解6h，电流效率高出36．2％，高铁酸根离子浓度达0．07mol／L。