Nanoscale Fe0/Cu0 bimetallic catalysts for Fenton-like oxidation of the mixture of nuclear-grade cationic and anionic exchange resins

Fenton-Like Reaction: Recent Advances and New Trends

The Fenton reaction refers to the reaction in which ferrous ions (Fe2+) produce hydroxyl radicals and other reactive oxidizing substances by decomposing hydrogen peroxide (H2O2). This paper reviews the mechanism, application system, and materials employed in the Fenton reaction including conventional homogeneous and non-homogeneous Fenton reactions as well as photo-, electrically-, ultrasonically-, and piezoelectrically-triggered Fenton reactions, and summarizes the applications in the degradation of soil oil pollutions, landfill leachate, textile wastewater, and antibiotics from a practical point of view. The mineralization paths of typical pollutant are elucidated with relevant case studies. The paper concludes with a summary and outlook of the further development of Fenton-like reactions.

Amino acids modified nanoscale zero-valent iron: Density functional theory calculations, experimental synthesis and application in the Fenton-like degradation of organic solvents

To improve the adsorption and catalytic performance of heterogeneous Fenton-like catalysts for oil wastes, amino acids were used to modify nanoscale zero-valent iron (AA@Fe0), which were applied in the Fenton-like degradation of organic solvents (tributyl phosphate and n-dodecane, named TBP and DD). Twelve amino acids, i.e., glycine (Gly), alanine (Ala), leucine (Leu), proline (Pro), phenylalanine (Phe), methionine (Met), cysteine (Cys), asparagine (Asn), serine (Ser), glutamic acid (Glu), lysine (Lys) and arginine (Arg), were selected and calculated by density functional theory (DFT). The optimized structure, charge distribution, the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), interaction region indicator (IRI) isosurface map and adsorption energy of AA@Fe0, AA@Fe0-TBP and AA@Fe0-DD were studied, which indicated that Fe is more likely to approach and charge transfer with -COO and -NH3 on the α-carbon of amino acids. There is strong attraction between Fe and -COO, and Van der Waals force between Fe and -NH3, respectively. In the interaction of AA@Fe0 with TBP and DD, Van der Waal force plays an important role. AA@Fe0 was synthesized in laboratory and characterized to investigate physicochemical properties. In Fenton-like degradation of organic solvents, the change of COD in water phase during the degradation process as well as the volume of the organic phase after the reaction were investigated. The results of calculations combined with experiments showed that Ser-modified Fe0 performed the best in these amino acids, with 98% removal of organic solvents. A possible catalytic mechanism was proposed in which amino acids acted a linking role between Fe and organic solvents, activating H2O2 to generate hydroxyl radicals for the degradation of organic solvents.

Decomposition mechanisms of nuclear-grade cationic exchange resin by advanced oxidation processes: Statistical molecular fragmentation model and DFT calculations

The treatment and disposal of radioactive waste are presently facing great challenges. Spent ion exchange resins have become a focus of attention due to their high production and serious environmental risks. In this paper, a simplified model of cationic exchange resin is proposed, and the degradation processes of cationic resin monomer initiated by hydroxyl radicals (·OH) are clarified by combining statistical molecular fragmentation (SMF) model and density functional theory (DFT) calculations. The prediction of active sites indicates that the S-O bonds and the C-S bond of the sulfonic group are more likely to react during the degradation. The meta-position of the sulfonic group on the benzene ring is the most active site, and the benzene ring without the sulfonic group has a certain reactivity. The C11-C14 and C17-C20 bonds, on the carbon skeleton, are the most easily broken. It is also found that dihydroxy addition and elimination reactions play a major role in the process of desulfonation, carbon skeleton cleavage and benzene ring separation. The decomposition mechanisms found through the combination of physical models and chemical calculations, provide theoretical guidance for the treatment of complex polycyclic aromatic hydrocarbons.

Performance and degradation pathway of florfenicol antibiotic by nitrogen-doped biochar supported zero-valent iron and zero-valent copper: A combined experimental and DFT study

Fluorinated compounds are a class of organic substances resistant to degradation. Although zero-valent iron (Fe0) has a promising reducing capability, it still fails to degrade fluorine-containing antibiotics (i.e., florfenicol) efficiently. In this study, we applied a simple one-pot pyrolytic approach to synthesize nitrogen-doped biochar supported Fe0 and zero-valent copper (Cu0) composite (Fe/Cu@NBC) and investigated its performance on florfenicol removal. The results clearly showed that approximately 91.4% of florfenicol in the deionized water was removed by Fe/Cu@NBC within 8 h. As the reaction time was extended to 15 d, the total degradation rate of florfenicol reached 96.6%, in which the defluorination and dechlorination rates were 73.2% and 82.1%, respectively. Both experimental results and density functional theory calculation suggested that ∙OH and ·O2- triggered β-fluorine elimination, resulting in defluorination prior to dechlorination. This new finding was distinct from previous viewpoints that defluorination was more difficult to occur than dechlorination. Fe/Cu@NBC also had a favorable performance for removal of florfenicol in surface water. This study provides a new insight into the degradation mechanism and pathway of florfenicol removal in the Fe/Cu@NBC system, which can be a promising alternative for remediation of fluorinated organic compounds in the environment.

Effects of oxygen content on gaseous and solid products during molten salt oxidation of cation exchange resins

Molten salt oxidation (MSO) is an advanced method for waste resins treatment; nevertheless, the research about gas product variations of resins under different stoichiometric air feed coefficient (α) is rare. The optimal working condition of hazardous waste disposal is obtained through thermodynamic equilibrium calculation, and the method to improve the treatment efficiency is found to guide the optimization of the actual experiment. In this paper, Fact Sage was used to calculate the oxidation products of cation exchange resins (CERs) at different temperatures and α, focusing on the similarities and differences through the contents of CO, CH₄, CO₂, and SO₂ during the oxidation of CERs, the MSO of CERs, and the theoretical calculation. The results indicated that the gas products of the calculation and reality of the oxidation process of CERs are quite different, while the CO contents of CERs during MSO are close to the calculated values. The main reason for this consequence is that in the oxidation process of CERs, the S in the sulfonic acid group will form thermally stable C-S with the styrene-divinylbenzene skeleton. Moreover, the introduction of carbonate can promote the destruction of C-S and absorb SO2 as sulfate, weakening the influence of C-S on the oxidation products of CERs. The gas chromatograph results indicated that the SO₂ content is reduced from 0.66% in the process of CERs oxidation to 0.28% in MSO of CERs. When 1.25 times stoichiometric air feed coefficient is fed, the sulfate content in the carbonate is the highest at 900 °C, which is 23.4%.

In-situ formed nanoscale Fe0 for fenton-like oxidation of thermosetting unsaturated polyester resin composites: Nondestructively recycle carbon fiber

Thermosetting unsaturated polyester resin (UPR) composites were found widespread industrial applications. However, the numerous stable carbon-carbon bonds in cross-linked networks made them intractable for degradation, causing the large-scale composite wastes. Here a nanoscale Fe⁰ catalyst in-situ forming strategy was exploited to nondestructively recycle carbon fiber (CF) from UPR composites via Fenton-like reaction. The nano-Fe⁰ catalyst employed in this strategy activated H₂O₂ for removing UPR, featuring mild conditions and efficient degradation ability. Aiming at facile growth of the catalyst, a porous UPR was achieved by the hydrolysis of alkalic system. The nanoscale Fe⁰ catalyst was subsequently formed in-situ on the surface of hydrolyzed resin by borohydride reduction. Benefiting from fast mass transfer, the in-situ grown nano-Fe⁰ showed more efficient degradation ability than added nano-Fe⁰ or Fe²⁺ catalyst during Fenton-like reaction. The experiments indicated that hydrolyzed resin could be degraded more than 90% within 80 min, 80 °C. GC-MS, FT-IR analysis and Density functional theory (DFT) calculation were conducted to explained the fracture processes of carbon skeleton in hydrolyzed resin. Especially, a remarkable recovery process of CF from composites was observed, with a 100 percent elimination of resin. The recycled CF cloth exhibited a 99% strength retention and maintained the textile structure, microtopography, chemical structure, resulting in the nondestructive reclaim of CF. This in-situ formed nanoscale Fe⁰ catalytic degradation strategy may provide a promising practical application for nondestructively recycle CF from UPR composites.

Degradation of the mixed nuclear-grade cationic and anionic exchange resins using Fe2+/H+ homogeneous Fenton oxidation

To further improve the treatment capacity of actual wastes, H⁺ was introduced into the homogeneous Fenton system as a co-catalyst for dissolution and degradation of the mixed nuclear-grade cationic and anionic exchange resins. The effects of acid type and concentration, catalyst type and concentration, H₂O₂ dosage, initial temperature, antifoaming agent and resin ratio were studied. The concentration of inorganic acid, type and concentration of catalyst had significant influence on the decomposition of mixed resins. The experimental results showed that when the mixing ratio of resins was 1:1, the initial temperature was 96 ± 1 °C, the amount of H₂O₂ was 200 mL, and the concentration of H⁺/Fe²⁺ was 1 M/0.1 M, complete dissolution and 79% weight reduction of mixed resins were obtained. Combined with density functional theory (DFT) calculations, cationic exchange resin and anionic exchange resin showed different reactivity in the experiment. Hydroxyl radicals (•OH) tended to attack -SO₃^- groups with more negative charges, and the barrier energy of -SO₃^- ion dissociation was 8.2 kcal/mol, which caused the cationic exchange resin to be easily destroyed. According to the characterization results, the characteristic intermediates were determined, indicating that desulfonation, valence change of nitrogen atom, and cleavage of long-chain carbon skeleton existed during the reaction, but incomplete oxidation still remained.

Cyclohexane oxidation using advanced oxidation processes with metals and metal oxides as catalysts: a review

Selective oxidation of cyclohexane has gained substantial interest in the field of research due to the prominence of its products in industrial processes. Particularly, advanced oxidation processes (AOPs) constitute a positive technology for the oxidation of cyclohexane owing to their high oxidation potentials and environmental benign properties. This review entails to address the progress made in advanced oxidation of cyclohexane over nanostructured metals and metal oxides catalysts. The main focus is directed toward the photocatalysis, Fenton oxidation and ozonation as advanced oxidation processes. Mainly, the fundamental principles, prime factors of the AOPs in conjunction with metal and metal oxide catalysts and the mechanistic insight toward the oxidation of cyclohexane are highlighted. The affirmative effects of the metals and metal oxide catalysts mainly focusing on particle size, structure and elemental composition is stressed. Lastly, the advantages and disadvantages of the AOPs and the strategic approaches to counter the disadvantages are also clearly elucidated.

Effects of Temperature and Sulfuric Acid and Iron (II) Concentrations on the Efficacy of Decontamination of Spent Ion-Exchange Resins Containing Hematite

The effect of H2SO4 and FeSO4 concentrations and temperature on the efficacy of decontamination of spent ion-exchange resins was estimated. The study was performed on model spent ion-exchange resins purposefully contaminated with hematite and Co-57 radionuclide. It was found that an increase in solution temperature up to 50 °C and the addition of FeSO4 increases the efficacy of decontamination of spent ion-exchange resins by 1 M and 2 M H2SO4 solutions by 1–2 orders of magnitude, whereas the decontamination factor value here is >103. Since under static conditions, the secondary adsorption of Co-57 was observed, the extra washing of ion-exchange resins by 3 M solution of NaNO3 is required. Decontamination under dynamic conditions excludes the secondary adsorption of Co-57, so that the necessity of the extra stage of washing can be skipped. Under dynamic conditions, the consumption of a solution of the composition H2SO4 (1 mol/L) + FeSO4 (0.2 mol/L) is 1.5-fold lower in comparison with the 2M solution of H2SO4 at compatible values of the decontamination factor. Such an approach enables reduction in the volume of secondary waste and the equipment corrosion due to the decrease in H2SO4 concentration.

Degradation of the mixed organic solvents of tributyl phosphate and n-dodecane by heterogeneous Fenton-like oxidation using nanoscale zero-valent iron as the catalyst

The treatment and disposal of spent radioactive organic solvents, i.e., tributyl phosphate (TBP) and diluent (such as kerosene, n-dodecane, etc.), produced in the reprocessing of spent fuel in the closed cycle are crucial for the sustainable development of the nuclear industry. In this study, the synthesized nanoscale zero-valent iron (nZVI) was used as the heterogeneous Fenton-like catalyst to promote the generation of hydroxyl radicals (•OH) by reacting with H₂O₂ to degrade the mixed organic solvents of TBP and n-dodecane. nZVI was characterized by scanning electron microscopy (SEM), nitrogen adsorption/desorption isotherms, and X-ray photoelectron spectroscopy (XPS) to investigate the micro-morphology, nano-particle size, and surface valence state. The change of Fe²⁺/Fe³⁺ concentration ratio during the reaction was measured to clarify the performance of nZVI. The effects of temperature, catalyst dosage, H₂O₂ dosage, and acidifier concentration on the degradation of TBP and n-dodecane were studied, and the results were complemented by the COD of the aqueous solution and the volume reduction rate of the organic phase. The mixed organic solvents of TBP and n-dodecane containing Co²⁺ were used to simulate the spent radioactive organic solvents and to study the distribution of nuclides after the reaction. The results showed that most of the radioactivity was in the residual solution, and the condensate contained almost no radioactivity. The degradation of TBP and n-dodecane was carried out separately, which showed that n-dodecane was more difficult to degrade. Density functional theory (DFT) calculations were applied to determine the adsorption energy of organic solvents and nZVI. According to the Fourier transform infrared spectra (FTIR) and their corresponding DFT calculations, liquid chromatography-mass spectrometry (LC-MS) and gas chromatography-mass spectrometry (GC-MS) of the sample after the reaction, the possible intermediates were analyzed, and the degradation mechanism was speculated.

Fulvic acid degradation in Fenton-like system with bimetallic magnetic carbon aerogel Cu-Fe@CS as catalyst: Response surface optimization, kinetic and mechanism

In this study, Cu-Fe bimetallic magnetic chitosan carbon aerogel catalyst (Cu-Fe@CS) was prepared by the sol-gel method to degrade Fulvic acid (FA) in Fenton-like system. Degradation experiment results showed bimetallic catalyst Cu-Fe@CS can degrade more FA than monometallic catalysts (Cu@CS and Fe@CS) due to the synergistic effect between the copper and iron. Plackett Buiman (PB) design showed that pH and temperature exhibited significant influence on FA degradation. The significant factors were optimized by Central Composite Design (CCD), the results revealed that the maximum FA removal reached 96.59% under the conditions of pH 4.07 and temperature 93.77 °C, the corresponding TOC removal reached 77.7%. The kinetic analysis implied that the reaction followed pseudo-first order kinetic with correlation coefficient (R²) = 0.9939. The Arrhenius fitting analysis revealed that Cu-Fe@CS had a lower activation energy (Ea) than Cu@CS and Fe@CS, meaning that reaction was easier to occur in Fenten-like system with Cu-Fe@CS. Catalyst still remained the higher FA and TOC removals of 96.28% and 77.33% after six runs, respectively. The FA removal was reduced by 65.53% with 12 mmol tertiary butanol (TBA) as scavenger, indicating that •OH played an important role in FA degradation. Finally, the catalytic degradation mechanism was proposed.

Revealing the enhancing mechanisms of Fe-Cu bimetallic catalysts for the Fenton-like degradation of phenol

To develop a heterogeneous Fenton-like catalyst with desirable activity and reusability remains a great challenge for the practical degradation of environmental remediation. Herein, we demonstrate a dendritic Fe-Cu bimetallic catalyst consisted of a Cu/Fe₃O₄ shell and a FeCu core (E100). In comparisons of single Cu, Fe and Fe₃O₄, E100 performs far better performance for the Fenton-like degradation of phenol, and its dominant Fenton-like active centers are Fe species under acidic pH or Cu species under neutral pH. Particularly, Cu-based Fenton-like reactions are greatly accelerated by galvanic micro-cells effects that come from the special co-existence of Cu/Fe₃O₄ shell, and subsequently, owing to the Cu leaching from the shell, the inner FeCu core of E100 is able to be exposed and further strengthen Fe-based Fenton-like reactions. Overall, the appropriate synergistic effects endow E100 with superior catalytic activity and reusability than other catalysts. Our work pushes forward a step for understanding the catalytic mechanism of Fe-Cu bimetallic catalysts and provides new sights for fabricating efficient Fenton-like catalysts for environmental remediation.

Engineering atomically dispersed single Cu–N3 catalytic sites for highly selective oxidation of benzene to phenol

A single-atom Cu catalyst with a CuN3 structure is prepared by a facile and practical strategy, the pyrolysis method, showing desirable conversion and selectivity for the oxidation reaction of benzene to phenol.

Cu-MOF/hemin: a bionic enzyme with excellent dispersity for the determination of hydrogen peroxide released from living cells

The stability, repeatability and sensitivity of an electrochemical biosensor material are closely connected with the dispersibility of metal organic frameworks (MOFs) in aqueous media. Herein, a nanocomposite based on Cu-MOF/hemin, which is not only highly water-soluble but also simple and efficient in synthesis, was used for the construction of a non-enzymatic sensor to detect hydrogen peroxide (H₂O₂). The Cu-MOF/hemin was characterized via scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS)-mapping, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and thermal gravimetric analysis (TGA), which indicate that hemin and the Cu-MOF were successfully combined. As a H₂O₂ electrochemical biomimetic enzyme, the Cu-MOF/hemin exhibited excellent electrocatalytic performance, which was confirmed by the electrochemical experiments and chromogenic reactions, and the possible mechanism of the reactions has been deduced. The electrochemical sensor based on the biomimetic enzyme exhibited an extended linear detection range from 0.01-5.0 mM (R = 0.998), low detection limit of 4.14 μM, and high selectivity and stability under the optimized conditions. More importantly, the practical application ability of the sensor was verified by the test of H₂O₂ in human serum samples and it could be used for the real-time detection of H₂O₂ released from living cells with satisfactory results. Therefore, this novel nanocomposite has certain potential in preparing electrochemical sensing platforms for nonenzymatic biosensing and provides a new method for clinical diagnosis and real-time monitoring.