OH Radical Attack Research Articles

The mechanism of the sonochemical degradation of benzoic acid in aqueous solutions

The sonolytic degradation of benzoic acid in aqueous solution was investigated at an ultrasonic frequency of 355 kHz. The degradation rate was found to be dependent upon the solution pH and the surface activity of the solute. The degradation rate was favoured at a solution pH lower than the pK a of benzoic acid. At pH < pK a, HPLC, GC and ESMS analysis showed that benzoic acid could be degraded both inside the bubble by pyrolysis and at the bubble/solution interface by the reaction with OH radicals. At higher pH (> pK a) benzoic acid could only react with OH radicals in the bulk solution. During the sonolytic degradation of benzoic acid, mono-hydroxy substituted intermediates were observed as initial products. Further OH radical attack on the mono-hydroxy intermediates led to the formation of di-hydroxy derivatives. Continuous hydroxylation of the intermediates led to ring opening followed by complete mineralization. Mineralization of benzoic acid occurred at a rate of < 40μM/h.

Iron(III)-photoinduced degradation of 4-chloroaniline in aqueous solution

The degradation of a prototypical halogenoaromatic pollutant, 4-chloroaniline (4-CA), photoinduced by Fe(III) species, has been studied in acidic aqueous solutions (pH 2–4) of Fe(ClO4)3 by means of product analysis and nanosecond transient absorption spectroscopy. The degradation process is initiated by the attack of OH radicals on 4-CA, leading to radical cations, 4-CA+ as the major transient species. At high photon fluences, the decay of 4-CA+ is dominated by second-order reactions; an accelerating effect of Fe(III) could be evidenced at low fluence. A large number of minor reaction intermediates could be detected by product analysis following continuous irradiation at λexc=365nm of aerated solutions, arising from ring substitution, ring opening and oligomerization processes. At higher concentration of 4-CA, the specificity of the degradation process increases; the oligomerization pathway becomes predominant and leads to three major oligomeric products, one of which could be unequivocally identified, while probable structures were proposed for the other two. The chemical nature of these products, as well as the observation of the p-benzoquinone/hydroquinone couple as the major photoproduct in deaerated solution, led to the proposal that p-benzoquinone monoimine is a key intermediate in the degradation of 4-CA. Direct oxidation of 4-CA+ by Fe(III) is put forward as a possible explanation for its formation. Prolonged irradiation at λexc=365nm in the presence of oxygen leads to complete mineralization; the process is, however, complex due to the formation of light-absorbing aromatic intermediates on the one hand, and to the behavior of the Fe(III)/Fe(II) redox couple during irradiation on the other. Fe(III)-photoinduced oxidation of 4-CA is a fairly efficient process as long as the photoinducing Fe(III) species are not depleted. The further progress of the reaction requires the reoxidation of Fe(II) into Fe(III), which is accomplished in processes requiring both light and oxygen, probably by reactions of Fe(II) with photoinduced oxidizing radicals.

Kinetic and chemical assessment of the UV/H 2O 2 treatment of antiepileptic drug carbamazepine

The UV/H 2O 2-induced degradation of carbamazepine, a worldwide used antiepileptic drug, recently found as contaminant in many municipal sewage treatment plant (STP) effluents and other aquatic environments, is investigated. The oxidation treatment caused an effective removal of the drug. At complete abatement of the substrate after 4 min treatment, a 35% value of removed total organic carbon (TOC) was obtained. A kinetic constant of (2.05 ± 0.14) × 10 9 l mol −1 s −1 was determined for OH radical attack to carbamazepine in the UV/H 2O 2 process. Preparative TLC of the reaction mixture led to the isolation of acridine-9-carboxaldehyde as a reaction intermediate. HPLC and GC/MS analysis indicated formation of small amounts of acridine, salicylic acid, catechol and anthranilic acid among the reaction products. Under the same reaction conditions, synthetically prepared 10,11-epoxycarbamazepine was easily degraded to acridine as main product, suggesting that this epoxide is a likely intermediate in the oxidative conversion of carbamazepine to acridine. Under sunlight irradiation, carbamazepine in water underwent slow degradation to afford likewise acridine as main product. In view of the mutagenic properties of acridine, these results would raise important issues concerning the possible environmental impact of carbamazepine release through domestic wastewaters and support the importance of prolonged oxidation treatments to ensure complete degradation of aromatic intermediates.

Electron-beam initiated crosslinking in poly( N-isopropylacrylamide) aqueous solution

The reactions between the OH, H and e aq − transients of water radiolysis and a linear poly( N-isopropylacrylamide) were identified by the spectral and kinetic properties of intermediates. The radical responsible for crosslink formation is the isopropyl-centered radical that forms mainly in OH radical attack on the polymer. Below pH 3 this radical undergoes reversible protonation with p K a≈2.1. The radical decay is composed of fast and slow parts. The initial fast decay is attributed to intramolecular reactions of radicals on the same chain (loop formation), the following slow decay to competition between further intramolecular and intermolecular (H-type crosslinks) termination processes. The differences in the network formation during irradiation of aqueous monomer and polymer solutions are discussed.

Photodegradation of chlortoluron sensitised by iron(III) aquacomplexes

The degradation of chlortoluron photoinduced by Fe(III) aquacomplexes has been investigated under continuous irradiation at 365 nm. The determined quantum yields of chlortoluron disappearance give evidence for the involvement of the most photoactive species Fe(OH) 2+, which leads to the formation of hydroxyl radicals and Fe(II); the importance of oxygen in the degradation process was also demonstrated. According to the nature of the obtained photoproducts, the attack of OH radicals on two sites of chlortoluron are favoured: the aromatic ring and the methyl group of the urea function. A homogenous photocatalytic process based on the oxidation of Fe(II) into Fe(III) is responsible for the continuous production of hydroxyl radicals in such a system. This photocatalytic cycle allows the total mineralisation of compounds like chlortoluron using conditions compatible with a safe aquatic environment.

Epicatechin-copper 9II) complexes: Damage of small intestinal epithelium

Abstract Four epicatechins [(−)-epicatechin (EC), (−)-epicatechin gallate (ECg), (−)-epigallocatechin (EGC), (−)-epigallocatechin gallate (EGCg)] and their corresponding copper complexes were compared with regard to their effect on the viability of Caco-2 colon cancer cells in vitro, measured by 3-(4,5-dimethylthyazol-2-yl) 2,5-diphenyltetrazolium bromide (MTT) assay. The viability of Caco-2 cells exposed to EC (1 mM), ECg (1 mM) or EGC (1mM) respectively, for 30 min, was comparable to that of the saline control group, while EGCg (1 mM) apparently enhanced cellular activity. in contrast, the cells treated with epicatechin-copper complexes were killed. Bivalent copper 91 mM), in similar conditions, did not affect the cells. No cell leakage or other histological differences were observed, implying a rapid cell death. The suggested mechanism of killing is by OH radical attack, produced in the presence of epicatechin-copper complexes, but not in the presence of either of the epicatechins or copper alone. The reaction sites are discussed.

Ab Initio and Variational Transition-State Theory Study of the CF3CF2OCH3+ OH Reaction Using Integrated Methods: Mechanism and Kinetics

The reaction of the OH radical with the fluorinated ether CF 3 CF 2 OCH 3 was studied by integrated ab initio electronic structure methods and variational transition-state theory with multidimensional tunneling. The OH radical attacks the three hydrogens of the methyl group, with two of these approaches being symmetric. In each hydrogen abstraction reaction, five stationary points (reactants, reactant complex, saddle point, product complex, and products) were located and characterized. A similarity was found between the three approaches of the OH radical, with energy differences of 0.1 kcal mol - 1 for the barrier heights, and this allows one to assume that the dynamics for these hydrogen abstraction reactions will be very similar. Thus, the intrinsic reaction path was constructed for just one approach of the OH radical using the IMOMO integrated method, and the rate constant was calculated for the whole reaction as being thrice the rate constant for this approach for the temperature range 250-500 K using variational transition-state theory with multidimensional tunneling effect. The integrated method reproduced the behavior of the model reaction (CH 4 + OH) at the higher level, with substantial savings in computational cost. Variational effects and tunneling were found to be important, a behavior already known for the model system. The theoretical rate constants underestimated the experimental values, even though high ab initio electronic levels were used. This is a general problem in computational chemistry, associated with the one-particle and n-particle basis set limitations, and is not directly related to the integrated method used here.

Kinetic study of the reactions of OH with the simple alkyl iodides: CH 3I, C 2H 5I, 1-C 3H 7I and 2-C 3H 7I

The atmospheric fate of biogenic species such as the alkyl iodides is controlled mainly by photolysis, and by oxidation initiated by attack of Cl atoms and/or OH radicals. In order to assess the contribution made by OH attack on the simpler alkyl iodides, the absolute second-order rate coefficients for the reactions (1) OH+ CH 3 I→ products, (2) OH+ C 2 H 5 I→ products, (3) OH+ 1-C 3 H 7 I→ products, (4) OH+ 2-C 3 H 7 I→ products. were determined. These reactions were studied at P=1.5 and 5 Torr and at room temperature (298±2 K) using a fast-flow discharge system with resonance-fluorescence detection of OH radicals. The absolute second-order rate coefficients are k 1=(0.99±0.20)×10 −13 cm 3 molecule −1 s −1, k 2=(7.7±1.0)×10 −13 cm 3 molecule −1 s −1, k 3=(25.0±3.0)×10 −13 cm 3 molecule −1 s −1and k 4=(16.0±2.0)×10 −13 cm 3 molecule −1 s −1. The reactions were pressure independent over the range measured. The major atmospheric global degradation pathway is photolysis, with OH-initiated oxidation gaining importance for the propyl iodides. The effects of variable [Cl] on a local scale are discussed.

Degradation of supramolecular complexes photoinduced by iron(III): the influence of the host on the reactivity of the guest.

Fe(III)-photoinduced degradation of two different supramolecular host-guest complexes, naphthalene with alpha-cyclodextrin and 4-chloroaniline with p-sulfonic-calix[4]arene; was studied in aqueous solution. The stoichiometries, equilibrium constants and conformations of these complexes were characterized. In both cases, the degradation of the guest could not be accounted for by simple competition kinetics of OH radical attack on the guest and host molecules, respectively. This observation was explained by the occurrence of subsequent radical reactions of transients resulting from an OH attack onto the host molecule, which are assumed to be able to induce additional degradation reactions of the guest. An inhibiting influence of the calix[4]arene host on the abundance of oligomeric degradation intermediates of 4-chloroaniline was observed.

Degradation of a biorecalcitrant dye precursor present in industrial wastewaters by a new integrated iron(III) photoassisted–biological treatment

This study focuses on the mineralization of 5-amino-6-methyl-2-benzimidazolone (AMBI), an important precursor in the industrial production of dyes, through an integrated Fe(III) photoassisted–biological system without addition of other electron acceptor than O2. The iron photoassisted process produces a biocompatible solution, removing 100% of the initial biorecalcitrant compound and 40% of the Dissolved Organic Carbon (DOC), and then the complete mineralization was achieved in the biological treatment. The transformation of AMBI photoinduced by only Fe(III) is mainly attributed to three simultaneous process: direct photolysis of the [Fe3+–AMBI] complex, the attack of the complex by OH radicals generated by the photolysis of Fe(OH)2+, and by the attack of supplementary OH radicals generated by the Fenton and photo-Fenton like reactions, which are induced by the H2O2 that have been formed “in situ”. The following topics are also studied in this paper: (a) the UV-Vis spectroscopic characterization of Fe(III), AMBI, and its mixture in aqueous solution, (b) the involvement of OH radicals and oxygen in the photodegradation of AMBI, (c) the comparison between O2 and H2O2 as electron acceptors in the iron photoassisted pretreatment, and their performances as a pretreatment step in the coupled photochemical biological reactor. Some field experiments under direct solar radiation were carried out using a compound parabolic collector (CPC) erected at the Swiss Federal Institute of Technology (EPFL), Lausanne, Switzerland. The obtained results indicate that a coupled solar–biological treatment system at pilot scale is a possible way to achieve the complete mineralization of biorecalcitrant pollutants.

Acoustic Cavitation-assisted Decomposition of 4-Nonylphenol in Water

Abstract Acoustic cavitation-assisted decomposition of 4-nonylphenol in water takes place through OH radical-attack and pyrolysis at the cavitation bubble interface and in the bulk solution. The cavitation-induced physical and chemical effects by addition of Fe2+ and Fe3+ play an important role in the enhancement of 4-nonylphenol decomposition.

Photocatalytic degradation mechanism for heterocyclic derivatives of triazolidine and triazole

In an attempt to improve the understanding of the basic mechanisms on the degradation of pollutants in water by TiO 2 photocatalysis, we discussed the primary degradation mechanism of three triazolidine derivatives, such as 1,2,4-triazolidine-3,5-dione (TRIANE), 4-hydroxy-1,2,4-triazolidine-3,5-dione (OH-TRIANE) and 4-phenyl-1,2,4-triazolidine-3,5-dione (Ph-TRIANE) and one triazole derivative of the 4-phenyl-1,2,4-triazole-2,5-dione (Ph-TRIOLE), on the basis of the experimental results together with molecular orbital (MO) calculation of frontier electron density and partial charge. The above four heterocycles were selected as molecular probe, principally because the highest frontier electron density was situated at different places of the molecule, while their structures were very similar, two major pathways have been revealed by LC/MS analysis for each heterocyclic compound. The pathway (a) corresponded to the hydroxylation of the atom bearing the highest electron density, via oxidation on nitrogen atom of azo group with respect to the photodegradation of TRIANE, OH-TRIANE and Ph-TRIOLE and opening of aromatic ring when Ph-TRIANE was used. This initial attack occurred with the preferential electrophilic attack of OH radicals. The pathway (b) was caused by the attack of active species on to carbon atom of carbonyl group. Considering the calculation of the relative electrophilic density of this carbon atom for the four heterocyclics and the evolution of large amount of N 2 gas at the initial degradation step, it has been suggested that this step (b) was favored by adsorption of carbonyl group on TiO 2 surface as theoretically determined by partial charge and confirmed by IR analysis. The participation of hole (h +) to form R–CO + was envisaged in this step. The presence of 1,2,4,5-tetrazixane-3,6-dione also agrees with the participation of TiO 2 surface. The nitrogen inorganic analysis (N 2, NH 4 + and NO 3 −) determined by gas chromatography and ion liquid chromatography, show that the hydrazo group were photoconverted mainly into N 2 gas and partially to NH 4 + ions. No nitrate ions were observed at the beginning of the degradation even when a OH substituent was present on one nitrogen atom (case of OH-TRIANE). However, the presence of –OH group increased the NO 3 −/NH 4 + ratio observed after a few hours of irradiation. The presence of OH also improved the total mineralization of carbon atom into CO 2, which was attributed to mesomeric effect of –OH group.

Environmental photocatalytic processes with POM. The photodecomposition of atrazine and photoreduction of metal ions from aqueous solutions

Polyoxometalate anions (POM), resulting from the condensation of tungstate anions in strongly acidified solutions, can absorb efficiently light in the UV‐near Vis region.The produced excited state is a very powerful oxidative reagent which can, mainly via OH radical attack, oxidize and mineralize a great variety of organic pollutants in aqueous solutions, while the photocatalytic circle is closed by reoxidation of the reduced POM, mainly, by O2. Metal ions can also serve as oxidants and close the photocatalytic cycle. In the process metal ions are reduced, precipitated and removed from the solution. Thus photocatalytic treatment for decontamination of waste waters from both organic and inorganic pollutants (metal ions) can be, in principle, achieved. Atrazine, a widely used herbicide with s‐triazine structure, is photodecomposed to cyanuric acid in presence of in aqueous solutions. However, no precipitation of metal ions is obtained when atrazine is the sole organic substrate in aqueous solutions. The low solubility of atrazine produces insufficient concentration of reduced POM to cause precipitation.

Heterogeneous photodegradation of aqueous hydroxy butanedioic acid by microporous polyoxometalates

A new type of heterogeneous photocatalytic materials, microporous polyoxotungstates such as H3PW12O40/SiO2, H4SiW12O40/SiO2 and Na4W10O32/SiO2, were synthesized by incorporating the polyoxotungstates into the silica matrix via a sol–gel technique, and some physico-chemical properties of as-synthesized composites were measured. The photocatalytic activity of the composites was tested via degradation of the model molecule, hydroxy-butanedioic acid (malic acid, (MA)). After 90 or 180min irradiation in the presence of Na4W10O32/SiO2 or H3PW12O40/SiO2 (H4SiW12O40/SiO2), an aqueous MA (100mg/l) was totally degraded into the intermediates such as oxalic acid, glyceric acid, tartaric acid, butenedioic acid, acetic acid, formic acid and so on. After subsequent 90 or 60min irradiation under the same conditions, these intermediates were totally mineralized into CO2 and H2O. Disappearance of MA (in the range of 0–200mg/l) followed Langmuir–Hinshelwood first-order kinetics, and the reaction rate constant and adsorption equilibrium constant were measured. Most of the intermediates, identified during the process of H4SiW12O40/SiO2-photocatalyzed MA degradation, are the same with those detected during the process of TiO2-photocatalyzed MA degradation. The mechanism studies showed that the desolubilized polyoxotungstates appears to photo-oxidative degradation of MA by both OH radical attack and excited state polyoxotungstate direct oxidation.

Micellar electrophoretic capillary chromatographic analysis of the products produced in the radiolytic oxidation of toluene and phenol

In this paper, we consider the effect of CH 3 and OH substitution on a benzene ring in directing the site of OH radical attack in oxidative processes initiated by the gamma radiolysis of aqueous solutions of phenol and toluene. The analytical method used was micellar electrophoretic capillary chromatography. This approach permits the resolution of the radiolytic products resulting from the oxidation of the hydroxycyclohexadienyl radicals initially produced by addition of OH to the aromatic compounds and allows one to obtain a complete mechanistic picture of the radiation chemical processes.

Fingerprinting of polysaccharides attacked by hydroxyl radicals in vitro and in the cell walls of ripening pear fruit

Hydroxyl radicals (•OH) may cause non-enzymic scission of polysaccharides in vivo, e.g. in plant cell walls and mammalian connective tissues. To provide a method for detecting the action of endogenous •OH in vivo, we investigated the products formed when polysaccharides were treated with •OH (generated in situ by ascorbate-H2O2-Cu2+ mixtures) followed by NaB3H4. Treatment with •OH increased the number of NaB3H4-reacting groups present in citrus pectin, homogalacturonan and tamarind xyloglucan. This increase is attributed partly to the formation of glycosulose and glycosulosuronic acid residues, which are then reduced back to the original (but radioactive) sugar residues and their epimers by NaB3H4. The glycosulose and glycosulosuronic acid residues were stable for &amp;gt; 16h at 20°C in ethanol or buffer (pH4.7), but were destroyed in alkali. Driselase-digestion of the radiolabelled polysaccharides yielded characteristic patterns of 3H-products, which included galactose and galacturonate from pectin, and isoprimeverose, galactose, glucose and arabinose from xyloglucan. Pectin yielded at least eight 3H-labelled anionic products, separable by electrophoresis at pH3.5. The patterns of radioactive products form useful ‘fingerprints’ by which •OH-attacked polysaccharides may be recognized. Applied to the cell walls of ripening pear (Pyrus communis) fruit, the method gave evidence for progressive •OH radical attack on polysaccharides during the softening process.

Fingerprinting of polysaccharides attacked by hydroxyl radicals in vitro and in the cell walls of ripening pear fruit.

Hydroxyl radicals (*OH) may cause non-enzymic scission of polysaccharides in vivo, e.g. in plant cell walls and mammalian connective tissues. To provide a method for detecting the action of endogenous *OH in vivo, we investigated the products formed when polysaccharides were treated with *OH (generated in situ by ascorbate-H(2)O(2)-Cu(2+) mixtures) followed by NaB(3)H(4). Treatment with *OH increased the number of NaB(3)H(4)-reacting groups present in citrus pectin, homogalacturonan and tamarind xyloglucan. This increase is attributed partly to the formation of glycosulose and glycosulosuronic acid residues, which are then reduced back to the original (but radioactive) sugar residues and their epimers by NaB(3)H(4). The glycosulose and glycosulosuronic acid residues were stable for >16 h at 20 degrees C in ethanol or buffer (pH 4.7), but were destroyed in alkali. Driselase-digestion of the radiolabelled polysaccharides yielded characteristic patterns of (3)H-products, which included galactose and galacturonate from pectin, and isoprimeverose, galactose, glucose and arabinose from xyloglucan. Pectin yielded at least eight (3)H-labelled anionic products, separable by electrophoresis at pH 3.5. The patterns of radioactive products form useful 'fingerprints' by which *OH-attacked polysaccharides may be recognized. Applied to the cell walls of ripening pear (Pyrus communis) fruit, the method gave evidence for progressive *OH radical attack on polysaccharides during the softening process.

Highly concentrated phenolic wastewater treatment by the Photo-Fenton reaction, mechanism study by FTIR-ATR

Phenol degradation by Photo-Fenton reaction has been studied in highly concentrated wastewaters and most intermediate species have been identified by Fourier Transform IR-Spectroscopy with ATR device. During the photodegradation of highly concentrated phenol solutions, the formation of dissolved and precipitate tannin has been observed. The possibility of a Fe 3+-Pyrogallol complex formation, previous to the tannin formation, has been proposed too. The complex formation involving Fe 3+ ions could be related to the observed Photo-Fenton activity decrease. Tannin formation inhibits the complete mineralization of phenol because OH radicals attack will produce further condensation steps and the polymer size increase. This fact limits the applicability of the process for highly concentrated phenolic wastes mineralization. However, the tannin precipitation allows its separation from the solution by conventional filtration, and reduction of the corresponding dissolved organic carbon. These observations have been proved from the identification of primary degradation products, catechol and hydroquinone. Catechol is considered to be the first step for the formation of tannins. Degradation process for phenol, catechol and hydroquinone have been monitored by total organic carbon (TOC) measurements along the reaction time span. From these results, a global mechanism for the Photo-Fenton degradation of phenol is proposed.

Theoretical Prediction of EPR Coupling Constants for the Determination of the Selectivity in the OH Addition to Toluene

High-level theoretical methods based on density functional and Hartree−Fock−Moller−Plesset theories have been employed to study the selectivity of the OH radical attack on toluene and to predict expected hyperfine coupling constants of the isomeric addition adducts. In recent work, we have found that the adduct with OH• added to the methyl-substituted carbon atom (ipso addition) of the ring may be especially stable both in toluene and in the xylenes. However, since this kind of adduct does not form a phenol, it is not directly identified in the products. The calculations indicate that quite different values should be observed for the EPR signals of the methyl hydrogen atoms of the four isomers, suggesting that electron paramagnetic resonance measurements could provide decisive information on the selectivity of the OH• addition to toluene. The calculated values of the hyperfine coupling constants are compared with the experimental values reported by Jolibois et al. for thymine.

Radiation chemical oxidation of aniline derivatives

The reactions of ˙OH, O˙− and N3˙ with chloro- and hydroxy-anilines were studied by pulse radiolysis. The rates of the OH radical reaction are higher (k ∼ 5 × 109 dm3 mol−1 s−1) than those found for the O˙− and N3˙ reactions (k ∼ 2 × 109 dm3 mol−1 s−1). Neither the position of the substituent nor the introduction of an additional Cl to monochloroanilines has any significant effect on the rates of the ˙OH reaction. The intermediates formed in all the aniline derivatives studied herein have λmax values around 310–320 and 350–380 nm. The OH radical reacts both by addition and direct H abstraction giving rise to OH adducts (350–380 nm) and anilino radicals (310–320 nm). The extent of these two reactions depends on the position of the substituent, the former being more predominant in the meta than in the ortho and para isomers. The initially-formed OH adducts subsequently undergo dehydration, leading to anilino radicals in the case of chloroanilines and phenoxyl radicals with hydroxyanilines. The OH attack at the carbon bonded to Cl in all three monochloroanilines is not significant (≤15%). Semi-empirical quantum calculations using the PM3 method were carried out to evaluate the possible sites of the OH radical attack. The charge distribution and the heats of formation data reveal that the ˙OH attack extends over more than one carbon center. The relative stabilities of the isomeric OH adducts formed from the attack at the unsubstituted carbons of chloro- and hydroxy-anilines are nearly the same, their respective heats of formation being approximately −70 and −230 kJ mol−1.