Ferrate Species Research Articles

New insights into the Ferrate-Sulfite system for the degradation of polycyclic aromatic Hydrocarbons: A dual role for sulfite

The presence of polycyclic aromatic hydrocarbons (PAHs) in water is a significant environmental concern due to their potential health risks. While sulfite activation of ferrate (Fe(VI)) has shown promise in enhancing the degradation of antibiotics and pesticides in water systems, its applicability to the degradation of PAHs remains largely unexplored. Here, we investigated the efficiency and mechanisms of PAH degradation via sulfite-activated ferrate. The ferrate-sulfite system significantly accelerated PAH degradation, achieving a 1.53 to 3.13-fold rate increase compared to ferrate-only treatment. Key active species involved in PAHs degradation were Fe(V), Fe(IV), and SO4●−. Sulfite accelerated the degradation of PAHs by undergoing a swift reaction with Fe(VI), leading to the formation of Fe(V) and SO4●−. Sulfite accelerated the degradation of PAHs by undergoing a swift reaction with Fe(VI) resulting in the production of Fe(V) and SO4●−. Through the reduction of ferrate species by sulfite, Fe(III) were produced and accelerated the decomposition of residual Fe(VI) into Fe(IV). Sulfite accelerates the degradation of PAHs by ferrate through these two processes. It promotes Fe(V) and Fe(IV) production from ferrate in a different way. Importantly, the toxicity of PAHs degradation intermediates is significantly reduced. Overall, these findings indicate that the ferrate-sulfite combination has significant potential to address environmental and health concerns associated with PAHs in water sources.

Degradation of mineral-immobilized pyrene by ferrate oxidation: Role of mineral type and intermediate oxidative iron species

Ferrate (Fe(VI)) salts like K2FeO4 are efficient green oxidants to remediate organic contaminants in water treatment. Minerals are efficient sorbents of contaminants and also excellent solid heterogeneous catalysts which might affect Fe(VI) remediation processes. By targeting the typical polycyclic aromatic hydrocarbon compound - pyrene, the application of Fe(VI) for oxidation of pyrene immobilized on three minerals, i.e., montmorillonite, kaolinite and goethite was studied for the first time. Pyrene immobilized on the three minerals was efficiently oxidized by Fe(VI), with 87%–99% of pyrene (10 μM) being degraded at pH 9.0 in the presence of a 50-fold molar excess Fe(VI). Different minerals favored different pH optima for pyrene degradation, with pH optima from neutral to alkaline following the order of montmorillonite (pH 7.0), kaolinite (pH 8.0), and goethite (pH 9.0). Although goethite revealed the highest catalytic activity on pyrene degradation by Fe(VI), the greater noneffective loss of the oxidative species by ready self-decay in the goethite system resulted in lower degradation efficiency compared to montmorillonite. Protonation and Lewis acid on montmorillonite and goethite assisted Fe(VI) oxidation of pyrene. The intermediate ferrate species (Fe(V)/Fe(IV)) were the dominant oxidative species accountable for pyrene oxidation, while the contribution of Fe(VI) species was negligible. Hydroxyl radical was involved in mineral-immobilized pyrene degradation and contributed to 11.5%–27.4% of the pyrene degradation in montmorillonite system, followed by kaolinite (10.8%–21.4%) and goethite (5.1%–12.2%) according to the hydroxyl radical quenching experiments. Cations abundant in the matrix and dissolved humic acid hampered pyrene degradation. Finally, two different degradation pathways both producing phthalic acid were identified. This study demonstrates efficient Fe(VI) oxidation of pyrene immobilized on minerals and contributes to the development of efficient environmentally friendly Fe(VI) based remediation techniques.

Reinvestigation of the oxidation of organic contaminants by Fe(VI): Kinetics and effects of water matrix constituents

Since previous studies mostly ignored the contributions of Fe(IV) and Fe(V) during the determination of reaction rate constants of ferrate (Fe(VI)) with trace organic contaminants (TrOCs), the intrinsic oxidation ability of Fe(VI) was overestimated. For the first time, this study systemically evaluated the reactivity of Fe(VI) towards four kinds of TrOCs by blocking Fe(IV)/Fe(V) over the TrOCs degradation, and evaluated the effects of coexisting water matrix constituents. Results revealed that Fe(VI) exhibited superior reactivity towards phenolic compounds. Different from other tested TrOCs, phenolic compounds were mainly degraded by Fe(VI) rather than Fe(IV)/Fe(V). Taking bisphenol A (BPA) as the target TrOC, we found that the coexisting constituents can not only affect the reactivity of different ferrate species (i.e., Fe(IV), Fe(V), and Fe(VI)), but also alter the concentrations of ferrates. HPO42- inhibited the reaction between Fe(VI) and H2O2, while Ca2+, Mg2+, and NH4+ promoted the generation of Fe(IV)/Fe(V) from Fe(VI). Besides, humic acid could increase the contribution of Fe(IV)/Fe(V) to the oxidation of BPA. These findings were validated in real water samples. Taken together, this study provides a new perspective regarding the intrinsic oxidation reactivity of Fe(VI), thereby urging reconsideration of the proper strategies for utilization of high-valent Fe species in practices.

Synthesis and characterization of a sterically encumbered homoleptic tetraalkyliron(III) ferrate complex

Homoleptic iron-alkyl complexes have been implicated as key intermediates in iron-catalyzed cross-coupling with simple iron salts. Tetraalkyliron(III) ferrate species have been shown to be accessible with either methyl or benzyl ligands, where the former complex is S = 3/2 and distorted square planar while the latter is a S = 5/2 distorted tetrahedral species. In the current study, a new tetraalkyliron(III) complex is synthesized containing modified methylene substituents that incorporate large trimethylsilyl (TMS) groups to further probe steric and electronic ligand effects in tetraalkyliron(III) complexes by increasing the electron-donating ability of the ligand while retaining steric bulk. Detailed structural and DFT studies provide insight into electronic structure and bonding of the four-coordinate trimethylsilylmethyl iron(III) complex compared to the previously reported analogs containing methyl and benzyl ligands.

The N-Methylpyrrolidone (NMP) Effect in Iron-Catalyzed Cross-Coupling with Simple Ferric Salts and MeMgBr.

The use of N-methylpyrrolidone (NMP) as a co-solvent in ferric salt catalyzed cross-coupling reactions is crucial for achieving the highly selective, preparative scale formation of cross-coupled product in reactions utilizing alkyl Grignard reagents. Despite the critical importance of NMP, the molecular level effect of NMP on in situ formed and reactive iron species that enables effective catalysis remains undefined. Herein, we report the isolation and characterization of a novel trimethyliron(II) ferrate species, [Mg(NMP)6 ][FeMe3 ]2 (1), which forms as the major iron species in situ in reactions of Fe(acac)3 and MeMgBr under catalytically relevant conditions where NMP is employed as a co-solvent. Importantly, combined GC analysis and 57 Fe Mössbauer spectroscopic studies identified 1 as a highly reactive iron species for the selective formation generating cross-coupled product. These studies demonstrate that NMP does not directly interact with iron as a ligand in catalysis but, alternatively, interacts with the magnesium cations to preferentially stabilize the formation of 1 over [Fe8 Me12 ]- cluster generation, which occurs in the absence of NMP.

The N‐Methylpyrrolidone (NMP) Effect in Iron‐Catalyzed Cross‐Coupling with Simple Ferric Salts and MeMgBr

AbstractThe use of N‐methylpyrrolidone (NMP) as a co‐solvent in ferric salt catalyzed cross‐coupling reactions is crucial for achieving the highly selective, preparative scale formation of cross‐coupled product in reactions utilizing alkyl Grignard reagents. Despite the critical importance of NMP, the molecular level effect of NMP on in situ formed and reactive iron species that enables effective catalysis remains undefined. Herein, we report the isolation and characterization of a novel trimethyliron(II) ferrate species, [Mg(NMP)6][FeMe3]2 (1), which forms as the major iron species in situ in reactions of Fe(acac)3 and MeMgBr under catalytically relevant conditions where NMP is employed as a co‐solvent. Importantly, combined GC analysis and 57Fe Mössbauer spectroscopic studies identified 1 as a highly reactive iron species for the selective formation generating cross‐coupled product. These studies demonstrate that NMP does not directly interact with iron as a ligand in catalysis but, alternatively, interacts with the magnesium cations to preferentially stabilize the formation of 1 over [Fe8Me12]− cluster generation, which occurs in the absence of NMP.

Electrosynthesis of Near-Neutral Ferrate Species for Drinking Water Treatment Using a Recirculating Batch Reactor

High oxidation state iron species of ferrates (e.g., Fe (VI), Fe (V) and Fe (IV)) are very promising oxidants and are in particular excellent candidates for water treatment applications. Their advantages include, for instance, a higher redox potential (+2.2 V vs. SHE) than the commonly used ozone oxidant (+2.07 V vs. SHE), no production of carcinogenic and harmful disinfection by-products (DBPs – produced with disinfectants such as chlorine) and decomposition into benign ferric hydroxides which form excellent coagulants. Ferrates are known to remove many different types of pollutants, including organic matter such as biofilms, algae, and coliforms, and inorganic matter such as cyanide, heavy metals, and radionuclides, and bio-contaminants like viruses and chlorine-resistant bacteria. One approach in production of ferrates is oxidation of Fe3+-containing solutions using an inert anode electrode. The oxidation reaction can be expressed by: Fe3+ + 4H2O → FeO4 2- + 8H+ + 3e- E0 = -2.2 V vs. SHE On-site production of ferrates for drinking water treatment by electrochemical means avoids a number of challenges with respect to transport and storage of chemical ferrates and also allows for direct integration of the electrochemical process with the treatment chain. In earlier papers on electrochemical ferrates, we have reported on a novel analytical method of ferrate quantification, degradation of ferrate and production of ferrates in small batch-type reactors [1-3]. Here we present a semi-batch (2 L) reactor which includes recirculation, temperature control, lower cell resistance, excellent mixing, provision for the use of ion exchange membrane, and in-situ pH control. The reactor uses boron doped diamond (BDD), Fe (III) solution and can be operated with or without an ion exchange membrane. The impact of several variables including current density (5 - 15 mA cm-2), pH (5 - 9), concentration of the dissolved iron salts on the production of ferrates temperature (15 - 30 oC) and operation with/without membrane have been studied and will be presented. Current efficiencies of as high as 90% were reached for the first 20 minutes of operation using a 30 mM of Fe (III) feed and a current density of 45 mA cm-2. The recirculating reactor results were successfully interpreted by a simple model which considered first order kinetics for Fe (VI) generation.

Degradation of ferrate species produced electrochemically for use in drinking water treatment applications

Stability of ferrate species, produced electrochemically for on‐site treatment of drinking water, has been thoroughly studied for a number of important conditions including: pH, temperature, initial concentration effects, and the presence of impurities, in order to assess the degradation process quantitatively. The limited number of ferrate degradation studies reported to this date have all focused on the chemically‐produced ferrate species and have examined conditions which are unsuitable for drinking water treatment. Degradation of electrochemical ferrates over a wide temperature range of 5 °C to 80 °C shows an Arrhenius‐type reaction behaviour with an activation energy of 27.3 ± 0.6 kJ mol−1. Initial rate analysis of ferrate degradation reveals a reaction order of about 1. The impacts of potential water impurities, such as salts and natural organic matter, and the source water (e.g. tap, deionized, and natural lake) have been studied and are presented. Degradation of ferrates in the pH range of 5 to 13 appears to have a first‐order kinetic behaviour with rate constants ranging from 1.02 × 10−3 s−1 to 6.67 × 10−5 s−1. Understanding the stability properties is important for further development and deployment of on‐site generated ferrates for the purposes of drinking water treatment.

Electrochemical Ferrates for Drinking Water Treatment: Quantification, Synthesis and Degradation Studies

Ferrate ions are highly oxidizing and unstable species which are challenging to quantify and analyze. They are, however, becoming increasingly recognized as an excellent candidate for a number of applications such as water treatment. Development of ferrate-based technologies for drinking water treatment in remote area requires: a) an accurate assay of the produced ferrates, b) better understanding of the stability properties and c) in-situ electrochemical generation at neutral pH conditions. In order to investigate the possibility of using on-site generated ferrates for treatment of drinking water, we conducted a thorough research program covering the following topics: 1) Quantification of Electrochemically generated Ferrates. Four analytical methods (existing and new) for quantification of ferrates were investigated and compared. These methods include titrimetric analysis, and spectrometric techniques such as direct colorimetric, measurements of ABTS or NaI colorimetric. In terms of accuracy, cost, simplicity and time required the modified indirect UV-Vis/NaI method is shown to be the most effective of all the four methods investigated. 2) Electrochemical ferrate stability. Stability of ferrate species, produced electrochemically for on-site treatment of drinking water, was studied for a number of conditions including: pH, temperature, initial concentration effects, and presence of impurities in order to assess the degradation process quantitatively. Degradation of ferrates for the pH range of 5 to 13 appears to have a first order kinetics behavior. Degradation of ferrates over a temperature range of 5 ºC to 60 °C shows an Arrhenius-type behavior with an activation energy of 348 kJ mol-1. Initial rate analysis of degradation reveals a reaction order of about 1. Impact of potential impurities, such as salts and natural organic matter (NOM), source of water (e.g., tap, deionized and water from natural lake) were also studied and results will be presnted. 3) Electrochemical generation of ferrate species at neutral conditions. Undivided batch (100 mL) cell. An undivided batch cell was used to study the electrochemical generation of ferrate species at neutral conditions (pH ~ 7) using boron-doped diamond (BDD) electrodes and iron (III) salts for applications in drinking water treatment. The impact of several relevant variables, including current density (5-55 mA cm-2), pH (5-9), type and concentration of the dissolved iron salts on the production of ferrates were examined. In addition, linear sweep voltammetry (LSV) studies were conducted using buffer electrolytes with and without the presence of iron (III) to decouple the parasitic oxygen evolution reaction. The LSV measurements in the presence of iron (III) and with a neutral electrolyte exhibit oxidation peaks centered ~ 2.0 V (vs. SHE), indicating the production of ferrates at this pH. The rate of ferrate generation is not strongly affected by the pH condition; however, current density and the source of iron were found to have a higher impact on the production rate of ferrates. The efficacy of the process was higher using FeCl3 compared to other salts such as Fe2O4 and FeO(OH).Semi-batch (2 L) reactor. This approach provided a number of improvements over the batch cell, including: recirculation, proper temperature control, lower cell resistance, excellent mixing, provision for the use of ion exchange membrane, and in-situ pH control. The preliminary results from this benchtop semi-batch reactor showed an improvement in the current effiency from 8% (batch cell) to 75% (semi-batch cell). Overall, about 8 mmol of ferrates was generated during a 2 hour electrosynthesis period exceeding the water treatment targets of about 14 µmol of ferrates. Figure 1

Ferrates: greener oxidants with multimodal action in water treatment technologies.

CONSPECTUS: One of the biggest challenges for humanity in the 21st century is easy access to purified and potable water. The presence of pathogens and toxins in water causes more than two million deaths annually, mostly among children under the age of five. Identifying and deploying effective and sustainable water treatment technologies is critical to meet the urgent need for clean water globally. Among the various agents used in the purification and treatment of water, iron-based materials have garnered particular attention in view of their special attributes such as their earth-abundant and environmentally friendly nature. In recent years, higher-valent tetraoxy iron(VI) (Fe(VI)O4(2-), Fe(VI)), commonly termed, ferrate, is being explored for a broad portfolio of applications, including a greener oxidant in synthetic organic transformations, a water oxidation catalyst, and an efficient agent for abatement of pollutants in water. The use of Fe(VI) as an oxidant/disinfectant and further utilization of the ensuing iron(III) oxides/hydroxide as coagulants are other additional attributes of ferrate for water treatment. This multimodal action and environmentally benign character of Fe(VI) are key advantages over other commonly used oxidants (e.g., chlorine, chlorine dioxide, permanganate, hydrogen peroxide, and ozone). This Account discusses current state-of-the-art applications of Fe(VI) and the associated unique chemistry of these high-valence states of iron. The main focus centers around the description and salient properties of ferrate species involving various electron transfer and oxygen-atom transfer pathways in terms of presently accepted mechanisms. The mechanisms derive the number of electron equivalents per Fe(VI) (i.e., oxidation capacity) in treating various contaminants. The role of pH in the kinetics of the reactions and in determining the removal efficiency of pollutants is highlighted; the rates of competing reactions of Fe(VI) with itself, water, and the contaminants, which are highly pH dependent, determine the optimum pH range of maximum efficacy. The main emphasis of this account is placed on cases where various modes of ferrate action are utilized, including the treatment of nitrogen- and sulfur-containing waste products, antibiotics, viruses, bacteria, arsenic, and heavy metals. For example, the oxidative degradation of N- and S-bearing contaminants by Fe(VI) yields either Fe(II) or Fe(III) via the intermediacy of Fe(IV) and Fe(V) species, respectively (e.g., Fe(VI) → Fe(IV) → Fe(II) and Fe(VI) → Fe(V) → Fe(III)). Oxidative transformations of antibiotics such as trimethoprim by Fe(VI) generate products with no residual antibiotic activity. Disinfection and inactivation of bacteria and viruses can easily be achieved by Fe(VI). Advanced applications involve the use of ferrate for the degradation of cyanobacteria and microcystin originating from algal blooms and for covalently embedding arsenic and heavy metals into the structure of formed magnetic iron(III) oxides, therefore preventing their leaching. Applications of state-of-the-art analytical techniques, namely, in situ Mössbauer spectroscopy, rapid-freeze electron paramagnetic resonance, nuclear forward scattering of synchrotron radiation, and mass spectrometry will enhance the mechanistic understanding of ferrate species. This will make it possible to unlock the true potential of ferrates for degrading emerging toxins and pollutants, and in the sustainable production and use of nanomaterials in an energy-conserving environment.

Ferryl and Ferrate Species: Mössbauer Spectroscopy Investigation

Ferryl and Ferrate Species: Mössbauer Spectroscopy Investigation

Oxidation of Microcystin-LR by Ferrate(VI): Kinetics, Degradation Pathways, and Toxicity Assessments

The presence of the potent cyanotoxin, microcystin-LR (MC-LR), in drinking water sources poses a serious risk to public health. The kinetics of the reactivity of ferrate(VI) (Fe(VI)O4(2-), Fe(VI)) with MC-LR and model compounds (sorbic acid, sorbic alcohol, and glycine anhydride) are reported over a range of solution pH. The degradation of MC-LR followed second-order kinetics with the bimolecular rate constant (kMCLR+Fe(VI)) decreasing from 1.3 ± 0.1 × 10(2) M(-1) s(-1) at pH 7.5 to 8.1 ± 0.08 M(-1) s(-1) at pH 10.0. The specific rate constants for the individual ferrate species were determined and compared with a number of common chemical oxidants employed for water treatment. Detailed product studies using liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) indicated the oxidized products (OPs) were primarily the result of hydroxylation of the aromatic ring, double bond of the methyldehydroalanine (Mdha) amino acid residue, and diene functionality. Products studies also indicate fragmentation of the cyclic MC-LR structure occurs under the reaction conditions. The analysis of protein phosphatase (PP1) activity suggested that the degradation byproducts of MC-LR did not possess significant biological toxicity. Fe(VI) was effective for the degradation MC-LR in water containing carbonate ions and fulvic acid (FA) and in lake water samples, but higher Fe(VI) dosages would be needed to completely remove MC-LR in lake water compared to deionized water.

Comparative study of oxidative degradation of sodium diatrizoate in aqueous solution by H2O2/Fe2+, H2O2/Fe3+, Fe (VI) and UV, H2O2/UV, K2S2O8/UV

This study centers on Advanced Oxidation Processes (AOPs) based on the use of iron salts and ultraviolet (UV) radiation for the removal of diatrizoate (DTZ) contrast medium, selected as a model compound resistant to all conventional natural and waste water treatment methods. We studied the effectiveness of Fenton’s reagent, Iron (III) and Iron (VI) salts, and oxidation processes based on the use of UV radiation, UV/H2O2 and UV/K2S2O8, for the degradation of DTZ in aqueous medium. With Fenton’s reagent, the percentage DTZ removal was independent of the initial Fe2+ concentration and was most influenced by the initial hydrogen peroxide concentration. In contrast, the Fenton-like system was affected by both initial Fe3+ and H2O2 concentration. The effects of ferrate salt-bases methods were highly dependent on the solution pH, and a higher rate constant (1.74×102M−1s−1) was obtained when dihydrogen ferrate species predominated in the medium. The UV/H2O2 system was strongly dependent on the pH and initial H2O2 concentration, showing a favorable synergic effect at H2O2 concentrations <10.0mM. The UV/K2S2O8 system proved to be more effective than the UV/H2O2 system, with higher rate constants, which were 24.0% higher for pH=6.5 and [DTZ]0=25mgL−1. Study of the influence of the matrix on these AOPs revealed a markedly higher percentage DTZ removal and reaction rate in surface water with the UV radiation-based systems than with the iron salt-based systems. According to the total organic carbon values obtained, a high degree of mineralization was only attained with the UV/K2S2O8 system.

Ferrate(VI): In situ generation and water treatment – A review

Over the past few years, the higher oxidation states of iron (ferrate, Fe(VI)) are of interest because of their involvement in reactions of environmental, industrial, and biological importance. New ferrate chemistry is still being developed and new analytical techniques are used to characterise the ferrate species. Applications of ferrate to treat common pollutants and emerging contaminants such as arsenic, estrogens, pharmaceuticals, and personal-care products are being explored. Ferrate is emerging as a green chemical for organic synthesis and for treating toxins in water. This review paper aims to discuss the potential of generating ferrate(VI) in situ and using it for water treatment. The first part provides a short review of recent advances in Fe(VI) synthesis and introduces its in situ electrochemical synthesis. The second part is devoted to application of Fe(VI) in the treatment of water as an oxidant, disinfectant and coagulant. Since iron is required as a growth factor by humans, normally innocuous to the environment with no by-products dangerous to human and environmental health, and less toxic than aluminium, ferrate(VI) may be considered as a green chemical for water treatment.