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.     

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     

Anodic behaviour of arsenopyrite and cathodic reduction of ferrate(VI) and oxygen in alkaline solutions

This paper presents the results of an electrochemical study of the anodic characteristics of arsenopyrite in strongly alkaline solutions and of the cathodic reduction of ferrate(VI) and of dissolved oxygen at an arsenopyrite surface at potentials which are relevant to the oxidation reactions. Cyclic voltammetry at both arsenopyrite disc and arsenopyrite disc/platinum ring electrodes has shown that arsenic(III) is the main product of the anodic process at potentials in the region of the rest potential during oxidation by either ferrate(VI) or oxygen. Evidence for partial passivation of both the anodic and cathodic reactions has been obtained from potentiostatic current–time transients. The initial stage of oxidation by ferrate(VI) has been shown to be mass-transport controlled and this is also true of the oxidation by oxygen in dilute solutions of sodium hydroxide.     

Degradation of some micro-pollutants from aqueous solutions using ferrate (VI): Physico-chemical studies

The aim of this communication is to investigate various physico-chemical parametric studies in the oxidative degradation of bisphenol A (BPA) and diclofenac (DCF) in aqueous solutions using the ferrate (VI). A wide range of pH (7.0–12.0) and concentrations of BPA/or DCF (0.03–0.5 mmol/L) is studied at a constant concentration of ferrate (VI) 0.1 mmol/L. Apparent rate constant was found to be 8.35 × 10² and 7.62 × 10² L/mol/min, respectively, for BPA and DCF degradation by the ferrate (VI). Further, decreasing the pH from 12.0 to 7.0, the corresponding increase is percentage degradation of BPA and DCF is found to be from 23 to 87% for BPA and from 14 to 41% for DCF, respectively, at the ferrate (VI) to micro-pollutant molar ratio 1:1. Total organic carbon data showed that partial mineralization of BPA/or DCF is achieved at a single operation of ferrate (VI) treatment. Moreover, decreasing the micro-pollutant concentration from 0.5 to 0.03 mmol/L has caused to increase the percentage TOC removal from 15 to 45% (for BPA) and from 10 to 38% (for DCF), respectively, at pH 7.0. The presence of NaCl, NaNO_3, and Na₂HPO₄ electrolytes could not affect significantly the oxidation of BPA and DCF by ferrate (VI). However, the presence of NaNO₂ and Na₂SO₃ co-existing ions hampered significantly the degradation of BPA and DCF using ferrate (VI).     

Antimony Remediation Using Ferrate(VI)

This research investigates a remediation technique for antimony involving the adsorption and co-precipitation of aqueous antimony by in-situ formed ferric hydroxide. Flocculation is initiated by the oxidation of iron(II) with potassium ferrate(VI), K₂FeO₄, along with oxidation of the more toxic Sb(III) to Sb(V). A 3/1 mole ratio of Fe(II)/Fe(VI) and a total Fe/Sb mole ratio of 300/1 was needed to achieve total antimony concentrations below the maximum contaminant levels for drinking water (6 μg/l).     

Oxidation of caffeine by acid‐activated ferrate(VI): Effect of ions and natural organic matter

Caffeine (CAF) is the most commonly consumed stimulant and frequently detected emerging pollutant in influents and effluents of wastewater treatment plants (WWTP) and surface waters. Acid‐activated ferrate(VI) (Fe^VI , Fe(VI)) oxidizes CAF in water in seconds to minutes at three times lower molar ratio of Fe(VI) to CAF than oxidative transformation observed in hours by nonactivated Fe(VI) (8.0 vs. 25.0). CAF oxidation by acid‐activated Fe(VI) is not affected by ionic constituents of water. Organic components of natural organic matter (NOM) and secondary effluent wastewater (SE) decrease efficiency of CAF transformation. However, acid‐activated Fe(VI) could mineralize other organics present in both NOM and SE as indicated by the dissolved organic carbon (DOC) removal. Comparatively, no mineralization was seen without activation of Fe(VI). Four oxidized products of CAF were identified by a liquid chromatography high resolution mass spectrometry technique. The reaction pathways of the oxidation of CAF by activated Fe(VI) have been proposed. © 2017 American Institute of Chemical Engineers AIChE J, 2017     

The role of potassium ferrate(VI) in the inactivation of Escherichia coli and in the reduction of COD for water remediation

The use of potassium ferrate(VI) as an alternative water remediation chemical has been studied and is reported in this paper. The water remediation performance of potassium ferrate(VI) was evaluated in comparison with sodium hypochlorite, ferric sulphate (FS) and aluminium sulphate (AS). The effects of the dosages of ferrate(VI), hypochlorite and FS/AS and solution pH were investigated. A model water with Escherichia coli (E. coli) number concentration of 3.2×10⁸/100 ml was used to examine the comparative disinfection performance and a real sewage sample was used to assess the comparative coagulation performance. The study demonstrated that the potassium ferrate(VI) performed superior to sodium hypochlorite in the inactivation of E. coli; less ferrate(VI) dose and contact time were required to achieve the same E. coli killing efficiency, the disinfection performance was less affected by the solution pH, and the disinfection rate of the ferrate(VI) was faster than that with sodium hypochlorite. In sewage treatment, ferrate(VI) performed superiorly as a oxidant and a coagulant; it can reduce 30% more COD, and kill 3 log more bacteria in comparison with AS and FS at a similar or even smaller dose.     

Decolorization of azo dye Orange II by ferrate(VI)-hypochlorite liquid mixture, potassium ferrate(VI) and potassium permanganate

Ferrate(VI)-hypochlorite liquid mixture was prepared using hypochlorite, an industrial by-product, via wet oxidation method. Its oxidizing ability was investigated by decolorizing azo dye Orange II in batch experiments, and compared with potassium ferrate(VI) and potassium permanganate. Effects of the oxidant concentration, dye concentration, initial pH of dye solutions and UV 254 nm irradiation were examined. The color removal by potassium permanganate, potassium ferrate(VI) and the ferrate(VI)-hypochlorite liquid mixture at 30 min reached 17.7%, 62.0% and 95.2%, respectively. The ferrate(VI)-hypochlorite liquid mixture maintained a high decolorization efficiency over a wide pH range from 3.0 to 11.0, indicating that the initial solution pH had little impact on its oxidizing power. However, the decolorization efficiency by potassium permanganate was proved to be highly pH dependent and the lowest efficiency was observed at neutral pH. UV 254 nm irradiation did not enhance the decolorization efficiencies significantly for both the ferrate(VI)-hypochlorite liquid mixture and potassium permanganate over a wide pH range.     

Heterogeneous photocatalytic reduction of Fe(VI) in UV-irradiated titania suspensions: effect of ammonia

Results of the heterogeneous photocatalytic reduction of Fe(VI) in UV-irradiated TiO₂ suspensions in the presence of ammonia are presented. The initial rate of Fe(VI) reduction, R, may be expressed as R = k _Fe(VI)[Fe(VI)]^1.25 where k _Fe(VI) = a[Ammonia]+b), a = 6.0 × 10³ μm ^0.25 s and b = 4.1 × 10⁶ μm ^−1.25s⁻¹. The rate constant, k _Fe(VI), increases with the ammonia concentration. The photocatalytic oxidation of ammonia is enhanced in the presence of Fe(VI). A mechanism involving Fe(V) as a reactive intermediate is presented which explains the faster photocatalytic oxidation of ammonia in the presence of Fe(VI).     

Advances in electrochemical Fe(VI) synthesis and analysis

Hexavalent iron species (Fe(VI)) have been known for over a century, and have long-time been investigated as the oxidant for water purification, as the catalysts in organic synthesis and more recently as cathodic charge storage materials. Conventional Fe(VI) syntheses include solution phase oxidation (by hyphchlorite) of Fe(III), and the synthesis of less soluble super-irons by dissolution of FeO₄ ²⁻, and precipitation with alternate cations. This paper reviews a new electrochemical Fe(VI) synthesis route including both in situ and ex situ syntheses of Fe(VI) salts. The optimized electrolysis conditions for electrochemical Fe(VI) synthesis are summarized. Direct electrochemical synthesis of Fe(VI) compounds has several advantages of shorter synthesis time, simplicity, reduced costs (no chemical oxidant is required) and providing a possible pathway towards more electro-active and thermal stable Fe(VI) compounds. Fe(VI) analytical methodologies summarized in this paper are a range of electrochemical techniques. Fe(VI) compounds have been explored as energy storage cathode materials in both aqueous and non-aqueous phase in “super-iron” battery configurations. In this paper, electrochemical synthesis of reversible Fe(VI/III) thin film towards a rechargeable super-iron cathode is also summarized.     

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.     

无机盐高铁酸钾在水处理中的应用

高铁酸钾具有强氧化性，可有效灭活微生物，氧化降解无机和有机污染物，去除悬浮颗粒物；其还原产物三价铁离子具有一定絮凝作用。近年来其合成及其在水处理中的应用取得了显著进展。综述了高铁酸钾在饮用水水质的改善及废（污）水处理中的研究与实践所取得的成果，探讨了其在取代氯氧化剂、源水有机污染物控制、提高污（废）水处理效能方面应用的可行性，指出了其发展中尚待解决的问题，以期为高铁酸钾在水处理中应用研究的深入提供借鉴作用。     

Ultrasonic-assisted convenient chemical synthesis of battery grade potassium ferrate(VI)

An ultrasound-assisted convenient method was developed for the synthesis of battery grade potassium ferrate (K₂FeO₄) with high yield (53-59%). The purity of the synthesized salt was determined by chromite method to be 95-96.8%. It was found that sample of the solid potassium ferrate has a tetrahedral structure with a space group of D_2h (Pnma) from X-ray diffraction (XRD) spectrum. From the scanning electronic microscopy (SEM), the K₂FeO₄ powders were crystallized polyhedron-shaped stick, and the particles had dimensions on the order of 25-200 μm in length and 1-10 μm in width. The electrochemical performance of the K₂FeO₄ electrodes was studied by using cyclic voltammetry and galvanostatic discharge methods in 10 mol/L KOH aqueous electrolyte. The synthesized product possesses a capacity of 302.4 mAh/g and coulombic efficiency of 74.5%.     

Studies of adsorption behavior of crosslinked chitosan for Cr(VI), Se(VI)

In this work, it was found that crosslinked chitosan (CCTS) had strong adsorption ability for some anions under certain conditions. Cr(VI) and Se(VI) existed in anion forms in aqueous solution, and their adsorption rates by CCTS were 97% for Cr(VI) at pH 3.0 and 95% for Se(VI) at pH 4.0. In addition, the adsorption balance time and isotherm of CCTS for Cr(VI) and Se (VI) were discussed and adsorption mechanism was explained. This research will be useful for designing CCTS‐based adsorption for metallic toxin removal and preconcentrating Cr(VI) and Se(VI) in their trace analysis. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 3216–3219, 2000     

Potassium ferrate(VI) and decomposed K2FeO4 assisted methanol electro-oxidation in alkaline media

The cyclic voltammograms (CVs) and current-time curves of methanol electro-oxidation, assisted by K₂FeO₄, decomposed K₂FeO₄ and Fe(NO₃)₃ added in strong alkaline solution, were investigated on a Pt electrode. The results show that Fe(NO₃)₃ and K₂FeO₄, especially decomposed K₂FeO₄, can promote the activity and poisoning tolerance of a Pt electrocatalyst for methanol oxidation in alkaline solution. We propose that iron(III)-containing compounds such as Fe(OH)₃ and FeOOH in a strong alkaline solution tend to encourage the formation of OH adsorption (OH_ads) species, which are necessary for the promotion of the CO-like intermediates of oxidation on the Pt electrode.     

Mechanisms of Cr(VI) removal from water by various types of activated carbons

The removal mechanisms of Cr(VI) from water using different types of activated carbons, produced from coconut shell, wood and dust coal, were investigated in this project. Different types of activated carbons have different surface characteristics. The coconut shell and dust coal activated carbons have protonated hydroxyl groups on the surface (H‐type carbons), while the surface of the wood‐based activated carbon has ionised hydroxyl groups (L‐type carbons). The adsorption kinetics of chromium onto the activated carbons at pH values ranging from 2 to 6 were investigated. It was found that the optimum pH to remove total chromium was 2 for wood‐based activated carbon, while for coconut shell and dust coal activated carbons, the optimum pH was around 3–4. The difference in the optimum pH for different activated carbons to remove Cr(VI) from water can be explained by the different surface characteristics and capacity of the activated carbons to reduce Cr(VI) to Cr(III). © 1999 Society of Chemical Industry     

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.     

A simple mathematical analysis on the effect of sand in Cr(VI) reduction using zero valent iron

A simple mathematical model was proposed to analyze the enhancement of Cr(VI) reduction when sand materials are added to the zero valent iron (ZVI). Natural decay of Cr(VI) in a control experiment was analyzed by using a zero-order decay reaction. Adsorption kinetics of Cr(VI) to sand was modeled as a first-order reversible process, and the reduction rate by ZVI was treated as a first-order reaction. Natural decay of Cr(VI) was also included in other experiments, i.e., the adsorption to sand, the reduction by ZVI, and both adsorption and reduction when sand and ZVI are present together. The model parameters were estimated by fitting the solution of each model to the corresponding experimental data. To observe the effect of sand addition to ZVI, both adsorption and reduction rate models were considered simultaneously including the natural decay. The solution of the combined model was fitted to the experimental data to determine the first-order adsorption and reduction rate constants when sand as well as ZVI is present. The first-order reduction rate constant in the presence of sand was about 35 times higher than that with ZVI only.     

New method in synthesis of nano uranyl(VI) Schiff base complexes: Characterization and electrochemical studies

This study presents a new method in synthesis of nano uranyl Schiff base complexes. In this method slow addition of dilute uranyl(VI) acetate solution to dilute Schiff base solution following the reflux for about 24 h, yields nano uranyl(VI) Schiff base complexes. Characterization of Schiff base ligands and nano uranyl complexes has been done using ¹H NMR, IR, UV–vis spectroscopy, elemental analysis. Schiff base ligands were synthesized by the condensation of one mole 3,4-diaminobenzophenone and two moles salicylaldehyde or substituted salicylaldehyde (3-OMe, 4-OMe, 5-OMe, 5-Br, 5-Cl). The electrochemical properties of the uranyl(IV) complexes were investigated by cyclic voltammetry. A good correlation was observed between the oxidation potentials and the electron withdrawing character of the substituents on the Schiff base ligands, according to the following trend: 5-MeO < H < 5-Br ≈ 5-Cl. Also the effect of the position of the substituted groups of Schiff base on the anodic potentials is as follows: 5-OMe < 3-OMe < 4-OMe.     

催化动力学光度法测定废水中痕量铬(VI)

在pH值为4.0的缓冲溶液中,磷酸和棉红在100℃下反应生成天蓝色物质(λmax=675nm),该物质在微量铬(VI)的催化下被溴酸钾氧化褪色,建立了测定痕量铬(VI)的催化动力学新方法,方法的检出限是0.76ng·mL-1,线性范围0～0.06μg·mL-1,用于测定电镀废水中铬(VI),结果令人满意。