Oxidative Degradation of Dimethyl Phthalate (DMP) by the Fe(VI)/H2O2 Process

This study investigates the degradation of dimethyl phthalate (DMP) with hydrogen peroxide and ferrate (Fe(VI)) under various reaction conditions. The results showed that the optimum conditions for dimethyl phthalate removal from water were as follows: (a) pH 7.0 and (b) the original molar ratio of [Fe(VI)]/[H₂O₂]/[DMP] equal to 10:2:1. Under the optimum conditions, the degradation rate of DMP can reach 89.7% in 360 min. Furthermore, 2,5-dihydroxybenzaldehyde, isophthalic acid, 2-ethylhexanol, oxalic acid, 2,6-dihydroxybenzoic acid, 2,6-dihydroxybenzaldehyde, 2,5-dihydroxybenzoic acid, and monomethyl phthalate were identified as the degradation intermediates, and degradation pathways were proposed.

Removal of sulfapyridine by ferrate(VI): efficiency, influencing factors and oxidation pathway

The removal of sulfapyridine (SPY) by ferrate(VI) in aqueous solutions under a broad range of reaction conditions, including ferrate(VI) dosage, solution pH, natural organic matter and coexisting inorganic ions, was systematically investigated and the oxidation pathway of SPY by ferrate(VI) was deeply explored. Results showed that 500 µg/L of SPY was efficiently decomposed (86%) within 1 min by 5 mg/L ferrate(VI) at pH 5.6. The increment in ferrate(VI) dosage was surely favorable for SPY removal. The best SPY degradation was achieved at pH 5.6, under or above which the removal decreased. The introduction of inorganic ions in solutions retarded SPY removal. Ten oxidation intermediates were identified and the cleavages of C-S and S-N bonds might be the primary ways of SPY oxidation by ferrate(VI).

Oxidation of Sulfonamide Antibiotics of Six-Membered Heterocyclic Moiety by Ferrate(VI): Kinetics and Mechanistic Insight into SO2 Extrusion

This work presents ferrate(VI) (Fe^VIO₄^2-, Fe^VI) oxidation of a wide range of sulfonamide antibiotics (SAs) containing five- and six-membered heterocyclic moieties ( R) in their molecular structures. Kinetics measurements of the reactions between Fe^VI and SAs at different pH (6.5-10.0) give species-specific second-order rate constants, k₅ and k₆ of the reactions of protonated Fe^VI (HFeO₄^-) and unprotonated Fe^VI (Fe^VIO₄^2-) with protonated SAs (HX), respectively. The values of k₅ varied from (1.2 ± 0.1) × 10³ to (2.2 ± 0.2) × 10⁴ M^-1 s^-1, while the range of k₆ was from (1.1 ± 0.1) × 10² to (1.0 ± 0.1) × 10³ M^-1 s^-1 for different SAs. The transformation products of reaction between Fe^VI and sulfadiazine (SDZ, contains a six-membered R) include SO₂ extrusion oxidized products (OPs) and aniline hydroxylated products. Comparatively, oxidation of sulfisoxazole (SIZ, a five-membered R) by Fe^VI has OPs that have no SO₂ extrusion in their structures. Density functional theory calculations are performed to demonstrate SO₂ extrusion in oxidation of SDZ by Fe^VI. The detailed mechanisms of oxidation are proposed to describe the differences in the oxidation of six- and five-membered heterocyclic moieties ( R) containing SAs (i.e., SDZ versus SIZ) by Fe^VI.

Ferrate(vi) initiated oxidative degradation mechanisms clarified by DFT calculations: a case for sulfamethoxazole

Ferrate(vi) is an efficient and environmentally friendly oxidant for the degradation of organic micropollutants. However, the related mechanism for the degradation is ambiguous and can hardly be elucidated empirically due to the rapid oxidation process and unstable intermediates for experimental trapping. Herein we performed density function theory (DFT) calculations to unveil the mechanism of ferrate(vi)-mediated degradation, taking sulfamethoxazole as a model compound. The results show that nucleophilic attack (rather than electrophilic attack) of HFeO₄^- on the isoxazole moiety of sulfamethoxazole initiates the subsequent degradations, and ferrate(vi) rather than the water molecule provides O atoms for the oxidation of the nitroso group and isoxazole moiety. Electron delocalization from the Fe atom to the isoxazole moiety is crucial for the ring-opening of isoxazole, and organometallic intermediates suggested previously are not the necessary ones in the oxidation of sulfamethoxazole by HFeO₄^-. Thus, this study has theoretically clarified the ferrate(vi) oxidation mechanisms for a representative sulfonamide, which were also partially corroborated by the intermediates and products observed in the previous experimental studies for phosphite and tryptophan. This study provides an exemplification on the application of quantum chemical calculations to clarify the degradation pathways of organic micropollutants, which is important for the prediction of degradation products needed in their engineering design.

Kinetics and mechanism of oxidation of tryptophan by ferrate(VI)

Kinetics of the oxidation of tryptophan (Trp) and kynurenine (Kyn), precursors of nitrogenous disinfection byproducts (N-DBP), by ferrate(VI) (Fe(VI)O4(2-), Fe(VI)) were investigated over the acidic to basic pH range. The second-order rate constants decreased with increase in pH, which could be described by the speciation of Fe(VI) and Trp (or Kyn). The trend of pH dependence of rates for Trp (i.e., aromatic α-amino acid) differs from that for glycine (i.e., aliphatic α-amino acid). A nonlinear relationship between transformation of Trp and the added amount of Fe(VI) was found. This suggests that the formed intermediate oxidized products (OPs), identified by LC-PDA and LC-MS techniques, could possibly compete with Trp to react with Fe(VI). N-Formylkynurenine (NFK) at pH 7.0 and 4-hydroxyquinoline (4-OH Q) and kynurenic acid (Kyn-A) at pH 9.0 were the major OPs. Tryptophan radical formation during the reaction was confirmed by the rapid-freeze quench EPR experiments. The oxygen atom transfer from Fe(VI) to NFK was demonstrated by reacting Fe(18)O4(2-) ion with Trp. A proposed mechanism explains the identified OPs at both neutral and alkaline pH. Kinetics and OPs by Fe(VI) were compared with other oxidants (chlorine, ClO2(•), O3, and (•)OH).

Oxidation of trimethoprim by ferrate(VI): kinetics, products, and antibacterial activity

Kinetics, stoichiometry, and products of the oxidation of trimethoprim (TMP), one of the most commonly detected antibacterial agents in surface waters and municipal wastewaters, by ferrate(VI) (Fe(VI)) were determined. The pH dependent second-order rate constants of the reactions of Fe(VI) with TMP were examined using acid-base properties of Fe(VI) and TMP. The kinetics of reactions of diaminopyrimidine (DAP) and trimethoxytoluene (TMT) with Fe(VI) were also determined to understand the reactivity of Fe(VI) with TMP. Oxidation products of the reactions of Fe(VI) with TMP and DAP were identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Reaction pathways of oxidation of TMP by Fe(VI) are proposed to demonstrate the cleavage of the TMP molecule to ultimately result in 3,4,5,-trimethoxybenzaldehyde and 2,4-dinitropyrimidine as among the final identified products. The oxidized products mixture exhibited no antibacterial activity against E. coli after complete consumption of TMP. Removal of TMP in the secondary effluent by Fe(VI) was achieved.

Oxidation of inorganic contaminants by ferrates (VI, V, and IV)--kinetics and mechanisms: a review

Inorganic contaminants are found in water, wastewaters, and industrial effluents and their oxidation using iron based oxidants is of great interest because such oxidants possess multi-functional properties and are environmentally benign. This review makes a critical assessment of the kinetics and mechanisms of oxidation reactions by ferrates (Fe(VI)O(4)(2-), Fe(V)O(4)(3-), and Fe(IV)). The rate constants (k, M(-1) s(-1)) for a series of inorganic compounds by ferrates are correlated with thermodynamic oxidation potentials. Correlations agree with the mechanisms of oxidation involving both one-electron and two-electron transfer processes to yield intermediates and products of the reactions. Case studies are presented which demonstrate that inorganic contaminants can be degraded in seconds to minutes by ferrate(VI) with the formation of non-toxic products.

Mechanisms of oxidation of organosulfur compounds by ferrate(VI)

The oxidation of organosulfur compounds requires the transfer of oxygen atoms and is important in decontamination of chemical warfare agents, desulfurization of fossil fuel for high quality, deodorization of wastewater and sludge, and remediation of industrial effluents. The kinetics of the oxidation of organosulfur compounds (sulfur-containing amino acids, aliphatic and aromatic thiols, and mercaptans) by the environmentally-friendly oxidant, ferrate(VI) FeO(4)(2-), was quantitatively examined in this study using a kinetic model considering possible reactions among the species of ferrate(VI) and organosulfur compounds. The ratios of ferrate(VI) to the various organosulfur compounds for the one oxygen-atom transfer were 0.50 and 0.67 for Fe(II) and Fe(III) as final products, respectively. The second-order rate constants for the oxidation of organosulfur compounds by protonated ferrate(VI) HFeO(4)(-) ion were correlated with thermodynamic 1-e(-) and 2-e(-) reduction potentials in order to understand the mechanisms of the reactions. The oxidation of the compounds involved a 1-e(-) transfer step from Fe(VI) to Fe(V), followed by 2-e(-) transfer to Fe(III) as the reduced product (Fe(VI)→Fe(V)→Fe(III)). The 2-e(-) transfer steps resulted in the formation of Fe(II) (Fe(VI)→Fe(IV)→Fe(II)). Conclusions drawn from the correlations are consistent with the experimentally determined stoichiometries and products of the reactions. The calculated half-lives for the oxidation were in the range of ms to s at a dose of 10mg K(2)FeO(4)L(-1) and hence ferrate(VI) has a great potential in treating organosulfur compounds present in water and wastewater.

Oxidation of inorganic compounds by Ferrate(VI) and Ferrate(V): one-electron and two-electron transfer steps

Ferrate(VI) (Fe(VI)O(4)(-), Fe(VI)) and ferrate(V) (Fe(V)O(4)(3-), Fe(V)) have a high oxidizing power and upon decomposition form a nontoxic byproduct, Fe(III), which makes them environmentally friendly oxidants in water and wastewater treatment. The kinetics of the reaction between Fe(VI) and I(-) was determined using a stopped-flow technique. The second-order rate constants (k, M(-1) s(-1)) for the oxidation of I(-) and other inorganic compounds by protonated ferrate(VI) (HFe(VI)O(4)(-)) and ferrate(V) (Fe(V)O(4)(3-)) ions were correlated with thermodynamic reduction potentials to understand the reaction mechanisms. A linear relationship was found between log k and 1-e(-) reduction potentials for iodide, cyanides, and superoxide while oxy-compounds of nitrogen, sulfur, selenium, and arsenic demonstrated a linear relationship for 2-e(-) reduction potentials. The estimated limits for the reduction potentials of the couple are E(o)(Fe(VI)/Fe(V)) >or= 0.76 V and E(o)(Fe(VI)/Fe(IV)) >or= -0.18 V. Conclusions drawn from the correlations are consistent with the mechanisms postulated from stoichiometries, intermediates, and products of the reactions. Implication of the kinetic results in treatment using ferrate(VI) is briefly discussed.

Oxidation of nitrogen-containing pollutants by novel ferrate(VI) technology: a review

Nitrogen-containing pollutants have been found in surface waters and industrial wastewaters due to their presence in pesticides, dyes, proteins, and humic substances. Treatment of these compounds by conventional oxidants produces disinfection by-products (DBP). Ferrate(VI) (Fe(VI)O(4)(2-), Fe(VI)) is a strong oxidizing agent and produces a non-toxic by-product Fe(III), which acts as a coagulant. Ferrate(VI) is also an efficient disinfectant and can inactivate chlorine resistant microorganisms. A novel ferrate(VI) technology can thus treat a wide range of pollutants and microorganisms in water and wastewater. The aim of this paper is to review the kinetics and products of the oxidation of nitrogen-containing inorganic (ammonia, hydroxylamine, hydrazine, and azide) and organic (amines, amino acids, anilines, sulfonamides, macrolides, and dyes) compounds by ferrate(VI) in order to demonstrate the feasibility of ferrate(VI) treatment of polluted waters of various origins. Several of the compounds can degraded in seconds to minutes by ferrate(VI) with the formation of non-hazardous products. The mechanism of oxidation involves either one-electron or two-electrons processes to yield oxidation products. Future research directions critical for the implementation of the ferrate(VI)-based technology for wastewater and industrial effluents treatment are recommended.