Generation of Ferryl Species through Dioxygen Activation in Iron/EDTA Systems: A Computational Study

Redox property of coordinated iron ion enables activation of O2 via in-situ generated H2O2 and additionally added H2O2 in EDTA-chelated Fenton reaction

The Fenton system was a generation system of reactive oxygen species via the chain reactions, which employed H2O2 and O2 as radical precursors and Fe2+/Fe3+ as electron-donor/acceptor for triggering or terminating the generation of radicals. Recent work mainly emphasized the Fe2+- activated H2O2 and the application of in-situ generated •OH, while neglecting other side-reactions. In this work, EDTA (Ethylene diamine tetraacetic acid) was employed as a chelating agent of iron ions, which simultaneously changed the redox property of coordinated iron. The Fe2+-EDTA complexes in the presence of dissolved oxygen enabled the two-electron transfer from Fe2+ to O2 and the in-situ production of H2O2, which further activate H2O2 for yielding •OH. Meanwhile, coordinated Fe3+ exhibited non-negligible reactivity toward H2O2, which was higher than that of free Fe3+ in the traditional Fenton system. The complexation of EDTA with Fe3+ could enhance the Fe2+ generation reaction by the H2O2, accompanied by the O2•- formation. The enhancement of O2•- formation and Fe2+-EDTA regeneration induced the subsequent H2O2 activation by Fe2+-EDTA, thus accelerating the Fe3+-EDTA/Fe2+-EDTA cycle for simultaneously producing O2•- and •OH. To sum up, the EDTA-chelated Fenton system extended the applicable pH range to circumneutral/alkaline level and tuned the redox property of coordinated iron for diversifying the •OH production routes. The research reinterpreted the chain reactions in the Fenton system, revealing another way to enhance the radical production or other property of the Fenton/Fenton-like system.

Generation of Reactive Oxygen Species and Degradation of Pollutants in the Fe2+/O2/Tripolyphosphate System: Regulated by the Concentration Ratio of Fe2+ and Tripolyphosphate

Tripolyphosphate (TPP) has many advantages as a ligand for the optimization of the Fe²⁺/O₂ system in environmental remediation applications. However, the relationship between remediation performance and the Fe²⁺/TPP ratio in the system has not been previously described. In this study, we report that the degradation mechanism of p-nitrophenol (PNP) in Fe²⁺/O₂ systems is regulated by the Fe²⁺/TPP ratio under neutral conditions. The results showed that although PNP was effectively degraded at different Fe²⁺/TPP ratios, the results of specific reactive oxygen species (ROS) scavenging experiments and the determination of PNP degradation products showed that the mechanism of PNP degradation varies with the Fe²⁺/TPP ratio. When C_Fe²⁺ ≥ C_TPP, the initially formed O₂^•- is converted to •OH and the •OH degrades PNP by oxidation. However, when C_Fe²⁺ < C_TPP, the O₂^•- persists long enough to degrade PNP by reduction. Density functional theory (DFT) calculations revealed that the main reactive species of Fe²⁺ in the system include [Fe(TPP)(H₂O)₃]^- and [Fe(TPP)₂]^4-, whose content in the solution is the key to achieve system regulation. Consequently, by controlling the Fe²⁺/TPP ratio in the solution, the degradation pathways of PNP can be selected. Our study proposed a new strategy to regulate the oxidation/reduction removal of pollutants by simply varying the Fe²⁺/TPP ratio of the Fe²⁺/O₂ system.

Catalytic properties of the ferryl ion in the solid state: a computational review

This review summarises the last findings in the emerging field of heterogeneous catalytic oxidation of light alkanes by ferryl species supported on solid-state systems such as the conversion of methane into methanol by FeO-MOF74.

Oxidative degradation of phenol by sulfidated zero valent iron under aerobic conditions: The effect of oxalate and tripolyphosphate ligands

After adding either organic or inorganic ligands, sulfidated nano-zero-valent iron (SnZVI) was used for aerobic degradation of phenol, and the effect of the ligand species on oxidation performance was investigated. We found that SnZVI hardly degraded phenol in the absence of ligand addition. Ligands initiated and promoted the degradation of pollutants by SnZVI. The data herein show that a characteristic inorganic ligand, tripolyphosphate (TPP), is more effective in enhancing oxidation than a characteristic organic ligand oxalate. In addition to the scavenging of reactive oxidants by the organic ligand, more ferrous ion (Fe(II)) dissolution from SnZVI in the TPP system is another cause for the superior enhancement by the inorganic ligand. In the oxalate system, as the sulfur content of SnZVI increased, the oxidation efficiency increased because FeS shell promoted the transfer of electrons to produce more reactive oxygen species (ROS). In TPP system, the effect of sulfur content on oxidation performance is more complex. The SnZVI with low sulfur content showed poor oxidation performance compared with that of nZVI. Further experiments proved that sulfidation might weaken the complexation of TPP with surface bound Fe, which would slow down the ionic Fe(II) dissolution rate. Therefore, sulfidation has the dual effects of enhancing electron transfer and inhibiting the complexation of inorganic ligands. In addition, the mechanisms of ROS generation in different ligand systems were investigated herein. Results showed that the critical ROS in both the oxalate and TPP systems are hydroxyl radicals, and that they are produced via one-electron activation of O₂.

Accelerated oxidation of microcystin-LR by Fe(II)-tetrapolyphosphate/oxygen in the presence of magnesium and calcium ions

Fe(II)-tetrapolyphosphate complexes are known to activate molecular oxygen (Fe(II)-TPP/O₂) to produce reactive oxidants (most likely, Fe(IV)-TPP complexes) that are capable of degrading refractory organic contaminants in water. This study found that magnesium and calcium ions (Mg²⁺ and Ca²⁺) accelerate the degradation of micfrocystin-LR (MC-LR), the most toxic and abundant cyanotoxin, by the Fe(II)-TPP/O₂ system. The addition of Mg²⁺ and Ca²⁺ increased the observed rate constant of MC-LR degradation by up to 4.3 and 14.8 folds, respectively. Mg²⁺ and Ca²⁺ accelerated the MC-LR degradation in the entire pH range, except for the alkaline region with pH > ca. 10. The addition of Mg²⁺ and Ca²⁺ also reshaped the pH-dependency of the MC-LR degradation, greatly increasing the rate of MC-LR degradation at neutral pH. It was found that Mg²⁺ and Ca²⁺ accelerate the reaction of Fe(II)-TPP complexes with oxygen, resulting in faster production of reactive oxidants. The findings from cyclic voltammetry and potentiometric titration suggest that Mg²⁺ and Ca²⁺ form ternary complexes with Fe(II)-TPP, which exhibit higher reactivity with oxygen. Due to the effects of Mg²⁺ and Ca²⁺, the rate of MC-LR degradation by the Fe(II)-TPP/O₂ system was even higher in natural water than in deionized water.

Natural iron ligands promote a metal-based oxidation mechanism for the Fenton reaction in water environments

The Fenton reaction is an effective advanced oxidation process occurring in nature and applied in engineering processes toward the degradation of harmful substances, including contaminants of emerging concern. The traditional Fenton application can be remarkably improved by using iron complexes with organic ligands, which allow for the degradation of contaminants at near-neutral pH and for the reduction of sludge production. This work discusses the mechanisms involved both in the classic Fenton process and in the presence of ligands that coordinate iron. Cyclohexane was selected as mechanistic probe, by following the formation of the relevant products, namely, cyclohexanol (A) and cyclohexanone (K). As expected, the classic Fenton process was associated with an A/K ratio of approximately 1, evidence of a dominant free radical behavior. Significantly, the presence of widely common natural and synthetic carboxyl ligands selectively produced mostly the alcoholic species in the first oxidation step. A ferryl-based mechanism was thus preferred when iron complexes were formed. Common iron ligands are here proven to direct the reaction pathway towards a selective metal-based catalysis. Such a system may be more easily engineered than a free radical-based one to safely remove hazardous contaminants from water and minimize the production of harmful intermediates.

Density-functional theory models of Fe(iv)O reactivity in metal-organic frameworks: self-interaction error, spin delocalisation and the role of hybrid exchange

We study the reactivity of Fe(iv)O moieties supported by a metal-organic framework (MOF-74) in the oxidation reaction of methane to methanol using all-electron, periodic density-functional theory calculations. We compare results concerning the electronic properties and reactivity obtained using two hybrid (B3LYP and sc-BLYP) and two standard generalised gradient corrected (PBE and BLYP) semi-local density functional approximations. The semi-local functionals are unable to reproduce the expected reaction profiles and yield a qualitatively incorrect representation of the reactivity. Non-local hybrid functionals provide a substantially more reliable description and predict relatively modest (ca. 60 kJ mol^-1) reaction energy barriers for the H-atom abstraction reaction from CH₄ molecules. We examine the origin of these differences and we highlight potential means to overcome the limitations of standard semi-local functionals in reactivity calculations in solid-state systems.

Ligand effects on arsenite removal by zero-valent iron/O2: Dissolution, corrosion, oxidation and coprecipitation

Ligands may increase the yields of reactive oxygen species (ROS) in zero-valent iron (ZVI)/O₂ systems. To clarify the relationship between the properties of ligands and their effects on the oxidative removal of contaminants, five common ligands (formate, acetate, oxalate, ethylenediaminetetraacetic acid (EDTA), and phosphate) as well as acetylacetone (AA) were investigated with arsenite (As(III)) as the target contaminant at three initial pH values (3.0, 5.0, and 7.0). The addition of these ligands to the ZVI/O₂ system resulted in quite different effects on As(III) removal. EDTA enhanced the oxidation of As(III) to arsenate (As(V)) but inhibited the removal of As(V). Oxalate was the only ligand in this work that accelerated both the removal of As(III) and As(V). By analyzing the ligand effects from the four aspects: dissolution of surface iron (hydr)oxides, corrosion of ZVI, reaction with ROS, and interference with precipitation, the following properties of ligands were believed to be important: ability to provide dissociable protons, complexation ability with iron, and reactivity with ROS. The complexation ability is a double-edged sword. It could enhance the generation of ROS by reducing the reduction potential of the Fe(III)/Fe(II) redox couple, but also could inhibit the removal of arsenic by coprecipitation. The elucidated relationship between the key property parameters of ligands and their effects on the ZVI/O₂ system is helpful for the rational design of effective ZVI/ligand/O₂ systems.

Advancing Fenton and photo-Fenton water treatment through the catalyst design

The review is devoted to modern Fenton, photo-Fenton, as well as Fenton-like and photo-Fenton-like reactions with participation of iron species in liquid phase and as heterogeneous catalysts. Mechanisms of these reactions were considered that include hydroxyl radical and oxoferryl species as the reactive intermediates. The barriers in the way of application of these reactions to wastewater treatment were discussed. The following fundamental problems need further research efforts: inclusion of more mechanism steps and quantum calculations of all rate constants lacking in the literature, checking the outer sphere electron transfer contribution, determination of the causes for the key changes in the homogeneous Fenton reaction mechanism with a change in the reagents concentration. The key advances for Fenton reactions implementation for the water treatment are related to tremendous hydrodynamical effects on the catalytic activity, design of ligands for high rate and completeness of mineralization in short time, and design of highly active heterogeneous catalysts. While both homogeneous and heterogeneous Fenton and photo-Fenton systems are open for further improvements, heterogeneous photo-Fenton systems are most promising for practical applications because of the inherent higher catalyst stability. Modern methods of quantum chemistry are expected to play a continuously increasing role in development of such catalysts.

A critical review of the application of chelating agents to enable Fenton and Fenton-like reactions at high pH values

To overcome the drawback of low pH requirement of the classical Fenton reaction, researchers have applied chelating agents to form complexes with Fe and enable Fenton reaction at high pHs, which is reviewed in this article. The chelating agents reviewed include humic substances, polycarboxylates, aminopolycarboxylic acids, and polyoxometalates. Ligands affect the reactivity of Fe-complexes by changing their redox potentials, promoting their reaction with H₂O₂, and competing with target contaminants for the oxidative species. Fe(III)-complexes are reduced to Fe(II)-complexes by O₂^- not H₂O₂, as indicated by their redox potentials. The stability constants of Fe-complexes increase with increasing pK_a values of the corresponding ligands and also with increasing charge density of the metal ions. A higher stability constant of Fe(III)-complex indicates higher reaction rate of corresponding Fe(II)-complex with H₂O₂ and lower reduction rate of Fe(III)-complex to Fe(II)-complex. OH, O₂^-, and ferryl species were reported to be the reactive species on the contaminant removal in the chelate-modified Fenton process. The generation of these species depends on the chelating agents and reaction conditions. The process is very efficient in degrading contaminants, indicating a potential treatment approach for the pollution remediation at natural pH.

Synchronized methylene blue removal using Fenton-like reaction induced by phosphorous oxoanion and submerged plasma irradiation process

In this study, a combination of phosphorus (PP) oxoanions in a submerged plasma irradiation (SPI) system was used to enhance the removal efficiency of dyes from wastewater. The SPI system showed synergistic methylene blue removal efficiency, due to the plasma irradiation and Fenton-like oxidation. The ferrous ions released from the iron electrode in the SPI system under plasmonic conditions form complexes with the PP anions, which can then react with dissolved oxygen (O₂) or hydrogen peroxide (H₂O₂) via Fenton-like reactions. The experimental results revealed that a sodium triphosphate (TPP) combined SPI system has a higher dye removal efficiency than a tetrasodium pyrophosphate (DP) or a sodium hexametaphosphate (HMP) combined SPI system under similar dissolved iron ion concentrations. To confirm the accuracy of the proposed removal mechanism via Fenton-like oxidation, it was compared to SPI systems under an oxygen environment (TPP/SPI/O₂ (k = 0.0182 s^-1)) and a nitrogen environment (TPP/SPI/N₂ (k = 0.0062 s^-1)). The results indicate that the hydroxyl radical (OH) in the TPP/SPI/O₂ system is the major oxidant in methylene blue removal, because the dye degradation rates dramatically decreased with the addition of radical scavengers such as tert-butanol (k = 0.0023 s^-1) and methanol (k = 0.0021 s^-1). On the other hand, no change was observed in the methylene blue removal efficiency of the TPP/SPI/O₂ system when it was subjected to a wide range of pHs (3-9). In addition, it was proved that this system could be used to eliminate six different commercial dyes. The results of this study indicated that the TPP/SPI/O₂ system is a promising advanced oxidation approach for dye wastewater treatment.

Synergistic degradation of antibiotic norfloxacin in a novel heterogeneous sonochemical Fe0/tetraphosphate Fenton-like system

In this study, synergistic degradation of antibiotic norfloxacin (NOR) was obtained in a novel sonochemical ultrasound/zero-valent iron/tetraphosphate system (US/ZVI/TPP). Compared to three common organic ligands (EDTA, EDDS, and DTPA), TPP could perform more excellently in activation of O₂ to produce reactive oxidative species (ROS) and lead to efficient Fenton-like oxidative degradation of NOR in the sonochemical in situ chemical oxidation (ISCO) system. An optimized initial condition was obtained as 10mg/L NOR, 0.3mM TPP, 1g/L ZVI and initial pH 7, and the US/ZVI/TPP system would effectively degrade NOR with relative low dosage of ZVI and ligand as well as broad pH work range 3-9. It was found that three ROS (OH, O₂^- and H₂O₂) instead of OH only would participate in the NOR degradation, while the in situ generation of H₂O₂ during the series of Fe-TPP reactions should be more critical. Fourteen organic intermediates and four inorganic products were detected during the NOR decomposition, suggesting that two main degradation pathways would occur under OH oxidation via cleavage of the piperazine ring and defluorination of the benzene ring, respectively. Finally, an integrated reaction mechanism in the US/ZVI/TPP system was proposed including solid-liquid interfacial iron corrosion as well as bulk homogenous oxygen activation and Fenton reactions, wherein US would play mechanically and chemically promotional roles. Besides, triple-repeated treatments suggested the relative long-term re-usage of ZVI particles and low effluent dissolved iron (<0.6mg/L).

Polyphosphate-enhanced production of reactive oxidants by nanoparticulate zero-valent iron and ferrous ion in the presence of oxygen: Yield and nature of oxidants

The production of reactive oxidants from nanoparticulate zero-valent iron (nZVI) and ferrous ion (Fe(II)) in the presence of oxygen was greatly enhanced by the addition of tetrapolyphosphate (TPP) as an iron-chelating agent. Compared to other ligands, TPP exhibited superior activity in improving the oxidant yields. The nZVI/TPP/O2 and the Fe(II)/TPP/O2 systems showed similar oxidant yields with respect to the iron consumed, indicating that nZVI only serves as a source of Fe(II). The degradation efficacies of selected organic compounds were also similar in the two systems. It appeared that both hydroxyl radical (OH) and ferryl ion (Fe(IV)) are produced, and OH dominates at acidic pH. However, at pH > 6, little occurrence of hydroxylated oxidation products suggests that Fe(IV) is a dominant oxidant. The degradation rates of selected organic compounds by the Fe(II)/TPP/O2 system had two optimum points at pH 6 and 9, and these pH-dependent trends are likely attributed to the speciation of Fe(IV) with different reactivities.

Hydroxylation catalysis by mononuclear and dinuclear iron oxo catalysts: a methane monooxygenase model system versus the Fenton reagent Fe(IV)O(H2O)5(2+)

Hydroxylation of aliphatic C-H bonds is a chemically and biologically important reaction, which is catalyzed by the oxidoiron group FeO(2+) in both mononuclear (heme and nonheme) and dinuclear complexes. We investigate the similarities and dissimilarities of the action of the FeO(2+) group in these two configurations, using the Fenton-type reagent [FeO(2+) in a water solution, FeO(H(2)O)(5)(2+)] and a model system for the methane monooxygenase (MMO) enzyme as representatives. The high-valent iron oxo intermediate MMOH(Q) (compound Q) is regarded as the active species in methane oxidation. We show that the electronic structure of compound Q can be understood as a dimer of two Fe(IV)O(2+) units. This implies that the insights from the past years in the oxidative action of this ubiquitous moiety in oxidation catalysis can be applied immediately to MMOH(Q). Electronically the dinuclear system is not fundamentally different from the mononuclear system. However, there is an important difference of MMOH(Q) from FeO(H(2)O)(5)(2+): the largest contribution to the transition state (TS) barrier in the case of MMOH(Q) is not the activation strain (which is in this case the energy for the C-H bond lengthening to the TS value), but it is the steric hindrance of the incoming CH(4) with the ligands representing glutamate residues. The importance of the steric factor in the dinuclear system suggests that it may be exploited, through variation in the ligand framework, to build a synthetic oxidation catalyst with the desired selectivity for the methane substrate.