Graft copolymerization to cellulose by the metallic ion–hydrogen peroxide initiator system

The decomposition of hydrogen peroxide and the graft copolymerization of methyl methacrylate has been investigated by use of cellulose samples adsorbing various metallicions. Metallic ions generally accelerate the decomposition of hydrogen peroxide, increase the number of grafts, and lower the average molecular weight. However their effects are much influenced by the range of pH values. It is clear that the amount of grafts formed is not necessarily proportional to the amount of decomposed hydrogen peroxide and is dependent upon a function peculiar to each metallic ion. The effective metallic ions in the neutral system were Cu²⁺, Ag⁺, Fe²⁺, Co²⁺, Cr³⁺, and Zn²⁺. The effects of Ce³⁺, Mg²⁺, Hg³⁺, Cd²⁺, Ni²⁺ and Mn²⁺ were either negligible or negative. Comparative studies on various conditions confirmed that Fe²⁺ in the neutral system gives graft copolymer having a minimum average molecular weight and the greatest number of grafts.     

Photometric and Gas-Chromatographic Determination of Hydrogen Peroxide and Peroxybutanoic Acid in Oxidized Butanoic Acid

Procedures were developed for determining hydrogen peroxide and peroxy acids mixed with peroxide compounds of other classes in the oxidation products of butanoic acid with atmospheric oxygen and hydrogen peroxide. Conditions were found for the selective decomposition of hydrogen peroxide with catalase in the presence of an excess of the carboxylic acid deactivating the enzyme. The errors introduced by the acylation of hydrogen peroxide with the carboxylic acid in the course of sample treatment with the enzyme were eliminated by adding diphenyl sulfide or dimethyl sulfoxide, which selectively reduced the peroxy acids. The concentrations of hydrogen peroxide and the peroxy acid were found from the difference between the total concentration of the peroxide compounds before and after treating a sample with catalase and a sulfur-containing reagent by the photometric method using a reagent containing Fe²⁺ ions and N, N-dimethyl-p-phenylenediamine. Peroxy acids were determined by GLC from the yields of the oxidation products of diphenyl sulfide with the peroxy acid (diphenyl sulfoxide and diphenyl sulfones).     

The nature of active centers of a polymeric catalase analog

The decomposition of hydrogen peroxide by a polymer complex of Fe³⁺ and poly(acrylic acid) partially amidated by diethylenetriamine has been studied. The molecular mechanism of catalysis is discussed. The rate constant of the reaction is k = 6.9 × 10⁷ exp {?2.300/RT}. Decomposition of hydrogen peroxide proceeds via activation by the diethylenetriamine which is a cofactor.     

Redox Reactions of Quercetin and Quercetin-5'-sulfonic Acid with Fe3 + and Cu2 + Ions and with H2O2

Redox reactions of quercetin and quercetin-5'-sulfonic acid with Fe^{3 +} and Cu^{2 +} ions and with H₂O₂ were studied spectrophotometrically. Oxidation of the flavonoids occurs at the 3-OH and 4-OH groups. The redox reactions are largely influenced by pH. With Fe^{3 +} ions, oxidation occurs in strongly acidic (pH 1-2), and with Cu^{2 +} ions, in weakly acidic (pH 4-5) solutions. Oxidation of quercetin and quercetin-5'-sulfonic acid with Fe^{3 +} and Cu^{2 +} ions is accompanied by complexation. Hydrogen peroxide oxidizes the flavonoids at pH 1-3.5, and at pH > 4 oxidation is insignificant.     

Proton Transfer Accompanied by the Oxidation of Adenosine

Despite numerous experimental and theoretical studies, the proton transfer accompanying the oxidation of 2′-deoxyadenosine 5′-monophosphate 2’-deoxyadenosine 5’-monophosphate (5’-dAMP, A ) is still under debate. To address this issue, we have investigated the oxidation of A in acidic and neutral solutions by using transient absorption (TA) and time-resolved resonance Raman (TR³) spectroscopic methods in combination with pulse radiolysis. The steady-state Raman signal of A was significantly affected by the solution pH, but not by the concentration of adenosine (2–50 mm ). More specifically, the A in acidic and neutral solutions exists in its protonated ( A H⁺(N1+H⁺)) and neutral ( A ) forms, respectively. On the one hand, the TA spectral changes observed at neutral pH revealed that the radical cation ( A ^.+) generated by pulse radiolysis is rapidly converted into A ^.(N6−H) through the loss of an imino proton from N6. In contrast, at acidic pH (<4), A H^.2+(N1+H⁺) generated by pulse radiolysis of A H⁺(N1+H⁺) does not undergo the deprotonation process owing to the pK_a value of A H^.2+(N1+H⁺), which is higher than the solution pH. Furthermore, the results presented in this study have demonstrated that A , A H⁺(N1+H⁺), and their radical species exist as monomers in the concentration range of 2–50 mm . Compared with the Raman bands of A H⁺(N1+H⁺), the TR³ bands of A H^.2+(N1+H⁺) are significantly down-shifted, indicating a decrease in the bond order of the pyrimidine and imidazole rings due to the resonance structure of A H^.2+(N1+H⁺). Meanwhile, A ^.(N6−H) does not show a Raman band corresponding to the pyrimidine+NH₂ scissoring vibration due to diprotonation at the N6 position. These results support the final products generated by the oxidation of adenosine in acidic and neutral solutions being A H^.2+(N1+H⁺) and A ^.(N6−H), respectively.     

Hydrogen Peroxide Formation in Aqueous Solutions under UV-C Radiation

The formation of hydrogen peroxide in bidistilled water under the influence of UV-C radiation from a DKB-9 low-pressure mercury lamp has been studied. The yield of hydrogen peroxide was (1 ± 0.2) × 10^?7 mol (L s)^?1. The wavelengths of radiation under the influence of which the formation of H₂O₂ is possible have been estimated. It has been assumed that the intermediate product of the reaction is the HO₂^./O₂^.-radical. To identify it, oxidation–reduction reactions in aqueous solutions containing Fe²⁺, Fe³⁺, and I^? ions at pH values from 0.8 to 8.1 have been studied. The quantum yield of HO₂^. radicals in an acidic medium under the influence of radiation from the mercury lamp is 0.015 ± 0.005.     

Mechanistic Details of the Fe+-Mediated C?C and C?H Bond Activations in Propane: A Theoretical Investigation

Quantum-chemical calculations employing a density-functional theory/Hartree-Fock hybrid method (B3LYP) have been used to explore the mechanistic details of the C? C and C? H bond-activation processes in propane mediated by a bare Fe⁺ ion. While the theoretically predicted results are in complete accord with all available experimental data, they give rise to a different mechanistic picture than envisaged previously. In contrast to earlier speculation, the activation barriers for the initial insertion steps of Fe⁺ into a C? H or C? C bond are found to be significantly below the Fe⁺ + C₃H₈ channel. The rate-determining steps for both, the C? C and the C? H bond activation branches of the [FeC₃H₈]⁺ potential-energy surface rather occur late on the respective reaction coordinates and are connected with saddle points of concerted rearrangement processes. The C? C bond activation, which leads to the exothermic reductive elimination of methane, occurs via the C? C inserted species and not as a side channel originating from a C? H inserted ion, as assumed hitherto. For the C? H bond-activation processes, which finally results in the exothermic expulsion of molecular hydrogen, two energetically similar reaction channels for an [1,2]-elimination exist. The results clearly show, that an [1,3]-H₂-elimination mechanism cannot compete with the [1,2]-elimination paths, in line with the experimental findings. Overall, a lower energy demand for the reductive elimination of methane compared to the loss of H₂ is obtained, straightforwardly explaining the preference of the former process observed experimentally.     

Beneficial Effect of Acetic Acid on the Formation of FeIII(OOH) Species and on the Catalytic Activity of Bioinspired Nonheme FeII Complexes.

Three new amine/pyridine Fe^II complexes bearing pentadentate ligand with one, two or three electron enriched 4-methoxy-3,5-dimethylpyridine were used as catalysts for the oxidation of small organic molecules by hydrogen peroxide. The distribution of products formed suggests that these ligands are not enough electron donating to promote the O−O heterolytic cleavage of the oxidant in order to generate selective Fe^V(O) species. Using acetic acid in the reaction mixtures results in a significant increase of the efficiency of these catalytic systems. Our investigations show that the use of AcOH leads to the protonation/dissociation of a pyridyl moiety and the formation of (N₄)Fe^II(OAc)(OH) species. These complexes readily react with excess hydrogen peroxide to yield (N₄)Fe^III(OAc)(OOH) intermediates. These latter intermediates are proposed to evolve into (N₄)Fe^IV(OAc)(O), which are more efficient than the usual (N₄)Fe^IV(O) and (N₅)Fe^IV(O).     

Reaction of Hydroxyurea with Iron(III): Products and the Stoichiometry of the Redox Reaction

Products and reaction stoichiometry of the interaction between iron(III) and hydroxyurea (HU) in aqueous solutions were studied. Under condition of an excess of iron(III) over hydroxyurea, initially formed mono(hydroxyureato)iron(III) complex decomposed through redox processes, forming two moles of Fe^II, one mole of NH₄⁺, one mole of CO₂, and 0.5 mole of N₂O per 1 mole of initially present HU. Hydroxyurea radical, H₂N‐CO‐NHO^?, was detected in the course of the reaction. The possible mechanism of the formation of the products is discussed and compared with that obtained for horseradish peroxidase mediated oxidation of hydroxyurea with hydrogen peroxide. The possibility of direct reaction of hydroxyurea with the iron center of protein R2 of the ribonucleotide reductase is proposed. The reaction examined may serve as a simple model for testing the suggested hydroxyurea like activity for chemical substances.     

Factors That Affect the Nature of the Final Oxidation Products in “Peroxo‐Shunt” Reactions of Iron–Porphyrin Complexes

The present study focuses on the oxidation of the water‐soluble and water‐insoluble iron(III)–porphyrin complexes [Fe^III(TMPS)] and [Fe^III(TMP)] (TMPS=meso‐tetrakis(2,4,6‐trimethyl‐3‐sulfonatophenyl)porphyrinato, TMP=meso‐tetrakis(2,4,6‐trimethylphenyl)porphyrinato), respectively, by meta‐chloroperoxybenzoic acid (m‐CPBA) in aqueous methanol and aqueous acetonitrile solutions of varying acidity. With the application of a low‐temperature rapid‐scan UV/Vis spectroscopic technique, the complete spectral changes that accompany the formation and decomposition of the primary product of O? O bond cleavage in the acylperoxoiron(III)–porphyrin intermediate [(P)Fe^III? OOX] (P=porphyrin) were successfully recorded and characterized. The results clearly indicate that the O? O bond in m‐CPBA is heterolytically cleaved by the studied iron(III)–porphyrin complexes independent of the acidity of the reaction medium. The existence of two different oxidation products under acidic and basic conditions is suggested not to be the result of a mechanistic changeover in the mode of O? O bond cleavage on going from low to high pH values, but rather the effect of environmental changes on the actual product of the O? O bond cleavage in [(P)Fe^III? OOX]. The oxoiron(IV)–porphyrin cation radical formed as a primary oxidation product over the entire pH range can undergo a one‐ or two‐electron reduction depending on the selected reaction conditions. The present study provides valuable information for the interpretation and improved understanding of results obtained in product‐analysis experiments.     

Radiation oxidation of phenol in the presence of petrochemical wastewater components

Radiolytical decomposition of phenol was investigated at⁶⁰Co gamma irradiation (1–2 Gy·s^–1, 10 kGy) of pre- and continuously aerated aqueous solutions at concentrations of phenol 1–100 mg· ·dm^–3 and in the presence of sodium hydroxide, sulphuric acid, sodium and ferrous sulphate, formaldehyde, 2-propanol,n-hexane, xylene, benzene, and commercial gasoline. From the decomposition rate at doses 50–400 Gy, a phenomenological model of linear relation between the dose acquired for 37% decomposition (D ₃₇), initial concentration (g·m^–3) of phenol (p ₀) and of an admixture (s ₀) was confirmed in the formD ₃₇=52f _tr(p ₀+f _eq s ₀), wheref's are constants which can be attributed to the relative transformation resistance of phenol towards the OH radicals in given matrix (f _tr, for pure waterf _tr=1) and relative acceptor capacity of competing substrate (f _eq). In real wastewaters, the efficient decrease of phenols content may be substantially lower than that in model solutions, obviously due to radiation oxidation of aromates, as proved by irradiation of aqueous solutions of benzene. Technical and economical feasibility of the process is discussed.     

Über das Auftreten Höherer Selenandisulfonate im Verlauf der Reduktion von Selenit mit Sulfit bzw. Selenotrithionat

Sulfite and SeS₂O ₆ ²⁻ reduce SeO ₃ ²⁻ in acidic solutions to selenium. Selenandisulfonates are observed as intermediate products. They are separated by high voltage paper ionophoresis in acidic basis electrolytes and identified on the paper by application of³⁵S- and⁷⁵Se-labelled compounds. The formation of selenandisulfonates depends on thepH of the solution. In neutral systems species exist with up to 4 Seatoms in the chain, in acidic solutions species with up to 7 Se-atoms in the chain are found. Se₇S₂O ₆ ²⁻ has been detected for the first time. Selenium precipitates by the decomposition of the selenandisulfonates with the longest chains.      

Effect of the Addition of Zero Valent Iron (Fe0) on the Batch Biological Sulphate Reduction Using Grass Cellulose as Carbon Source

Mineral mining generates acidic, saline, metal-rich mine waters, often referred to as acid mine drainage (AMD). Treatment of AMD and recovering saleable products during the treatment process are a necessity since water is, especially in South Africa, a scarce commodity. The aim of the study presented here was to investigate the effect of zero valent iron (Fe⁰) on the biological removal of sulphate from AMD in batch reactors. The performance of the reactors was assessed by means of sulphate reduction, chemical oxygen demand (COD), volatile fatty acid (VFA) utilisation and volatile suspended solids (VSS) concentration. To this end, three batch reactors, A, B and C (volume 2.5 L), were operated similarly with the exception of the addition of grass cuttings and iron filings. Reactors A and B received twice as much grass (100 g) as C (50 g). Reactor A received no iron filings to act as a control, while reactors B and C received 50-g iron filings for the experimental duration. The results showed that Fe⁰ appears to provide sustained sulphate removal when sufficient grass substrate is available. In reactors A and C, sulphate removal efficiency was higher when the COD concentration was lower due to utilisation. In reactor B, sulphate removal efficiency was accompanied by an accumulation of COD as hydrogen (H₂) provided by the Fe⁰ was utilised for sulphate reduction. Furthermore, these results showed the potential of Fe⁰ to enhance the participation of microorganisms in sulphate reduction.     

Direct Synthesis in the Undergraduate Laboratory: Synthesis of Copper Complexes from Zero-Valent Metals as a Demonstration of Base-Catalyzed Direct Synthesis

Direct synthesis is an important and active research field for scientists and technologists involved with the use of elemental metals. An undergraduate laboratory demonstration is presented that exposes students to this important synthetic technique. The direct synthesis of [Cu(NH₃)₄]²⁺ and [Cu(en)₂]²⁺ complexes in aqueous solution from zero-valent Cu metal is employed as an experiment illustrating the oxidizing properties of alkaline hydrogen peroxide solutions. The experiment also shows the decomposition of hydrogen peroxide catalyzed by the copper complexes. Finally, students can learn that the direct oxidation of metallic copper by alkaline hydrogen peroxide solution is an efficient and novel alternative approach to synthesize these and other copper complexes.     

Acidity and Hydrogen Exchange Dynamics of Iron(II)‐Bound Nitroxyl in Aqueous Solution

Nitroxyl‐iron(II) (HNO‐Fe^II) complexes are often unstable in aqueous solution, thus making them very difficult to study. Consequently, many fundamental chemical properties of Fe^II‐bound HNO have remained unknown. Using a comprehensive multinuclear (¹H, ¹⁵N, ¹⁷O) NMR approach, the acidity of the Fe^II‐bound HNO in [Fe(CN)₅(HNO)]³⁻ was investigated and its pK_a value was determined to be greater than 11. Additionally, HNO undergoes rapid hydrogen exchange with water in aqueous solution and this exchange process is catalyzed by both acid and base. The hydrogen exchange dynamics for the Fe^II‐bound HNO have been characterized and the obtained benchmark values, when combined with the literature data on proteins, reveal that the rate of hydrogen exchange for the Fe^II‐bound HNO in the interior of globin proteins is reduced by a factor of 10⁶.     

The aromatic peroxygenase from Marasmius rutola—a new enzyme for biosensor applications

The aromatic peroxygenase (APO; EC 1.11.2.1) from the agraric basidomycete Marasmius rotula (MroAPO) immobilized at the chitosan-capped gold-nanoparticle-modified glassy carbon electrode displayed a pair of redox peaks with a midpoint potential of −278.5 mV vs. AgCl/AgCl (1 M KCl) for the Fe²⁺/Fe³⁺ redox couple of the heme-thiolate-containing protein. MroAPO oxidizes aromatic substrates such as aniline, p-aminophenol, hydroquinone, resorcinol, catechol, and paracetamol by means of hydrogen peroxide. The substrate spectrum overlaps with those of cytochrome P450s and plant peroxidases which are relevant in environmental analysis and drug monitoring. In M. rotula peroxygenase-based enzyme electrodes, the signal is generated by the reduction of electrode-active reaction products (e.g., p-benzoquinone and p-quinoneimine) with electro-enzymatic recycling of the analyte. In these enzyme electrodes, the signal reflects the conversion of all substrates thus representing an overall parameter in complex media. The performance of these sensors and their further development are discussed.     

Reexamination of reduced pteridine cofactors behaviour by proton nuclear magnetic resonance spectroscopy and mass spectrometry

In aqueous solutions, the autoxidation by air of 2-amino-4-hydroxy-6,7-dimethyl-5,6,7,8-tetra-hydropteridine, a hydroxylase cofactor, leads to the corresponding 7,8-dihydro derivate. Oxidation by hydrogen peroxide and Horseradish peroxidase does not give a stable quinonoid form as previously claimed but affords two metabolic products. Kinetics of the two pathways and structures of the different final compounds were determined by ¹H nmr and mass spectrometry.     

Decomposition of oxalate by hydrogen peroxide in aqueous solution

The decomposition rate of oxalate by hydrogen peroxide has been investigated by a KMnO₄ titration method. The rate equation for decomposition of hydrogen peroxide in the aqueous phase is 1n([H₂O₂]/[H₂O₂]₀)=?k₁·t, where k₁=0.2, for [H⁺]<2M, k₁=0.2+0.34([H⁺]?2), for [H⁺]>2M. As the acidity increases over 2M, an acid catalysis effect appeard. The new rate equation proposed for the decomposition of oxalate by hydrogen peroxide is $$ - \frac{d}{{dt}}X_{[OX]} = k_2 [H_2 O_2 ]_0 (1 - X_{[OX]} )(e^{ - k_1 t} - \frac{{[OX]_0 }}{{[H_2 O_2 ]_0 }}X_{[OX]} )$$ The rate constant for decomposition of oxalate, k₂, increased with nitric acid concentration and the effect of hydrogen ion concentration was expressed as k₂=a[H⁺]ⁿ, where the values fora andn were a=1.54, n=0.3 at [H⁺]<2M, a=0.31, n=2.5 at [H⁺]>2M, respectively.     

Factorial design analysis of the catalytic activity of di‐imine copper(II) complexes in the decomposition of hydrogen peroxide

Factorial design analysis was applied to the study of the catalytic activity of di‐imine copper(II) complexes, in the decomposition of hydrogen peroxide. The studied complexes show a tridentate imine ligand (apip), derived from 2‐acetylpyridine and 2‐(2‐aminoethyl)pyridine, and a hydroxo or an imidazole group at the fourth coordination site of the copper ion. The factorial design models for both [Cu(apip)imH]²⁺ and [Cu(apip)OH]⁺ were similar. Increasing the peroxide concentration from 3.2 × 10^?3 to 8.1 × 10^?3 mol L^?1 resulted in increased oxygen formation. Increasing the pH from 7 to 11 also increased oxygen formation and had an effect about twice as large as the peroxide one. Both complexes also had an important interaction effect between peroxide concentration and pH. However, increasing the catalyst concentration led to a decrease in total oxygen formation. The obtained results were corroborated by further data, achieved by using the usual univariate method, and helped to elucidate equilibrium steps occurring in the studied systems. In very alkaline solutions, the studied [Cu(apip)imH]²⁺ complex can form the corresponding dinuclear species, [Cu₂(apip)₂im]³⁺. While the mononuclear complex proved to be an efficient catalyst in hydrogen peroxide decomposition, the corresponding dinuclear compound seemed to be able to coordinate with the dioxygen molecule, inhibiting its observed release. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 472–479, 2001     

Electrosynthesis of Н<Subscript>2</Subscript>О<Subscript>2</Subscript> from О<Subscript>2</Subscript> in a Gas-Diffusion Electrode Based on Mesostructured Carbon CMK-3

Mesostructured carbon CMK-3 (Carbon Mesostructured by KAIST) synthesized by the template method is studied as the electrocatalyst for electrosynthesis of Н₂О₂ from О₂ in a gas-diffusion electrode (GDE) in alkaline and acidic solutions. The texture characteristics of the original material and its mixture with hydrophobizer (polytetrafluoroethylene) are studied by the method of low-temperature nitrogen adsorption. The rate constants for hydrogen peroxide decomposition on these materials in alkaline and acidic solutions are calculated. Kinetic parameters of oxygen reduction in alkaline and acidic solutions are determined as well as the capacitance of gas-diffusion electrodes based on mesocarbon. The selectivity of the electrocatalyst is estimated by finding the current fracture γ consumed in oxygen reduction to hydrogen peroxide. Data on the kinetics of hydrogen peroxide accumulation during electrosynthesis of Н₂О₂ from О₂ are obtained. The acidic solution of hydrogen peroxide with the concentration more than 3 M is obtained with the current efficiency higher than 80%.