S. Solar

Degradation of monochlorophenols by γ-irradiation

Abstract The degradation of aqueous 5 × 10 −4 mol dm −3 2-, 3- and 4-chlorophenol (2-CIP, 3-CIP, 4-CIP) under reducing and oxidizing conditions has been studied by steady state γ-radiolysis. The reaction with e aq − leads to reductive dechlorination which equals the degradation of the chlorophenols. Solely phenol is the major organic compound formed. In N 2 O saturated solutions the dehalogenation is much less effective. In the case of 4-CIP 40% and in that of 2- and 3-CIP only 20% of the decomposed substrates are resulting in chlorine free organic compounds. From these pyrocatechol for 2-CIP and hydroquinone for 4-CIP could be identified in minor amounts. Main products are the hydroxylated substrates originating from the primarily formed ortho- and para OH-adducts. These are for 2-CIP: chlorohydroquinone and 3-chlorocatechol, for 3-CIP: chlorohydroquinone, 3- and 4-chlorocatechol and for 4-CIP: 4-chlorocatechol. In the presence of oxygen, where the OH-adducts are converted into peroxyl radicals, the degradation yield of the substrates corresponds to that of OH radicals up to doses of 0.5 kGy. The dechlorination process in this dose range is 50% for 4-CIP, and 33% for 2- and 3-CIP. The position of chlorine on the aromatic ring strongly influences the dehalogenation and degradation process. A pronounced dependency of the dose/yield curves on the oxygen concentration was observed. This indicates that for an efficient degradation process of chlorophenols oxygen supply is necessary during irradiation.

Degradation of 2,4-dichlorophenoxyacetic acid by ionizing radiation: influence of oxygen concentration.

Ionizing radiation has been proved as a promising method for the degradation of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). The gamma-radiolytic decomposition and chloride formation follows an apparent first order kinetic, kde/kCl = 3. For complete removal of 500 microM 2,4-D, a dose of 4 kGy is required. Phenolic intermediates (2,4-dichlorophenol, the isomers 2-chloro-4-hydroxy- and 4-chloro-2-hydroxy phenoxyacetic acid and three hydroxylation products of 2,4-D) are formed. Using oxygen saturation during irradiation, they are removed again with a dose of 4 kGy. For the formation of the main organic product acetic acid a reaction mechanism is discussed. Oxygen concentration enhances strongly fragmentation and mineralization. A reduction of 36% TOC could be achieved.

Identification of phenolic components in dried spices and influence of irradiation.

LC-ESI-MS analysis was used for identification of phenolic compounds in the methanolic extracts of commercially available dried oregano, sage and thyme. Rosmarinic acid, apigenin-glucuronide, luteolin-glucuronide, as well as quinic acid were present in all three spices. Whereas in thyme and sage only derivatives from flavonoid compounds were identified, in oregano also the aglycons eriodictyol, naringenin, hispidulin, apigenin and luteolin could be found. Some constituents, especially glucosides and glucuronides, were observed for the first time in the methanol extracts of these spices. To ascertain an irradiation influence (dose: 10kGy) the MS peak area counts of twenty constituents, among them 10 glycosides/glucuronides, were compared. Although the majority of those compounds exhibited a slight decrease with irradiation, the changes were not significant.

Radiolysis and pulse radiolysis of chlorinated phenols in aqueous solutions

Abstract The radiation induced decomposition of 2-, 3- and 4-chlorophenols was studied under steady-state conditions. The rate constants for the reactions of H, OH and e - ag with 2-Cl-phenol were determined. Absorptions spectra of the H- and OH- adducts are presented.

Radiation induced decomposition of chlorinated phenols in water

Abstract Experiments with 4-Cl-phenol as a model compound for pesticides were performed under steady-state conditions using deoxygenated solutions as well as such saturated with air, oxygen or oxygen mixed with ozone. The yield of Cl - ions serviced as an indicator for the degradation process. As main products of the first step of decomposition were identified: polyhydroxybenzenes, aldehydes and acids. The yield of aldehydes was studied as a function of the absorbed dose and substrate concentration. In the presence of ozone a chain-reaction of the oxidative pollutant degradation takes place. Transient absorption spectra and kinetics obtained by preliminary pulse radiolysis studies of 4-Cl-phenol in the presence of oxygen as well as probable reaction mechanisms are also presented.

Reaction of OH with phenylalanine in neutral aqueous solution

Abstract The reactivity of OH towards phenylalanine (phala) in aqueous solutions, p H 7–8, was studied by pulse radiolysis. The site specificity of OH as well as the kinetic and spectroscopic data of the different OH adducts could be resolved by means of a semilinear optimization alogrithm. As main transients (50%) the OH adducts on ortho positions of the aromatic ring ( R 1 , K = (3.5 ± 0.4) × 10 9 dm 3 mol −1 s −1 , e 305 = 380 ± 40 m 2 mol −1 , decay: 2 k = (1.0 ± 0.2) × 10 9 dm 3 mol −1 s −1 ) are formed, followed from 30% para isomers ( R 3 , k = (2.2 ± 0.3) X 10 9 dm 3 mol −1 s −1 , e 320 = 410 ± 50 m 2 mol −1 , decay: 2 k = (1.5 ± 0.3) × 10 9 dm 3 mol −1 s −1 ). The meta isomers ( R 2 ) are formed to 14% ( k = (1.0 ± 0.2) × 10 9 dm 3 mol −1 s −1 , e 315 = 350 ± 40 m 2 mol −1 , decay: 2 k = (1.3 ± 0.2) × 10 9 dm 3 mol −1 s −1 . The formation of benzyl-type radicals by direct abstraction of an H atom from the aniline moiety was not significant. This process together with a possible addition of OH to the ipso position of the aromatic ring was found to be 6%. MNDO calculations demonstrated that replacing CH 3 by alanine group on the benzene ring no change takes place in the electronic charge distribution on the ring atoms as well as on the energetic stabilities of the cyclohexadienyl radical isomers.

Resolved multisite OH-attack on aqueous aniline studied by pulse radiolysis

Abstract The individual formation and decay kinetics as well as the absorption characteristics of the simultaneously formed primary species by OH attack on aniline in aqueous (pH 8–9.6), saturated with N2O, have been determined by pulse radiolysis combined with a computer optimization procedure. It was established that 36% OH radicals react directly with the -NH2 moiety (k = (5.0 ± 0.6) × 109dm3mol-1s-1 resulting in anilino radical, C6H5NH (ϵ300 = 2500 ± 100, ϵ355 = 950 ± 50, ϵ420 = 420 ± 40 dm3 mol-1cm-1)), which decays by a second order reaction with 2k = (1.8 × 0.2) × 109dm3mol-1s-1. The main primary process (54%) is the formation of the ortho-directed OH-adduct (k = (7.5 ± 0.8) × 109dm3mol-1s-1, ϵ300 = 1800±100, ϵ355 =3300±170dm3mol-1cm-1), which subsequently decays by water splitting resulting again in C6H5NH. The remaining 10% OH attack most probably the para position of the aniline ring (k = (1.5±0.2) × 109dm3mol-1s-1) forming the corresponding OH-adduct (ϵ300 = 1700±100, ϵ355 = 3600±200 dm3mol-1cm-1), which disappears with 2k = (1.1±0.3) × 109dm3mol-1cm-1; reactions with meta- and ipso-positions, however, cannot be excluded. H-atoms react with the neutral form of aniline with k =(1.9 ± 0.1) × 109dm3mol-1cm-1, forming H-adducts. Further the rate constant of e-aq with aniline was determined to (3.0 ± 0.1) × 107dm3mol-1cm-1. Qualitative analysis of final products were also performed.

Impact of gamma-irradiation on the antioxidative properties of sage, thyme, and oregano

Ionizing radiation is known to stimulate the generation of oxygen radicals which destabilize organic molecules resulting in a decrease of the systems antioxidant potential. The Salmonella typhimurium (TA102) reverse mutation assay, co-incubated with t-BOOH and H(2)O(2), was used to assess the effects of gamma-irradiation (dose: 10kGy) on the antioxidant properties of sage, thyme, and oregano in chloroform and methanol extracts as well as in their mixture. In addition, measurements of the trolox equivalent antioxidative capacity (TEAC), total polyphenol content (TPC), and tocopherol equivalent (TE) were performed. For the methanol and mixed extracts, antioxidative properties in the S. typhimurium reverse mutation assay were only found when using an exogenous metabolic activation system. Generally, the greatest inhibition of mutagenicity was observed in the chloroform fractions of irradiated and of non-irradiated herbs. The relative antioxidant activities for the different herbs were as follows: TA102/t-BOOH, TEAC, TPC: thyme approximately oregano>sage. TA102/H(2)O(2): thyme approximately oregano approximately sage. TE: sage>thyme>oregano. For the majority of the investigated samples the impact of irradiation was insignificant. Therefore, gamma-irradiation at the doses tested seems to have little, if any, effect on the antioxidative capacity of the tested herbs.

Pulse radiolysis of pyridine and methylpyridines in aqueous solutions

Abstract The radicals formed from pyridine, 3-methylpyridine, 3,5-dimethylpyridine, 2,6-dimethylpyridine and 2,4,6-trimethylpyridine by attack of H, e-aq, OH and O•- in acqueous solutions were investigated by pulse radiolysisin the pH-range 1–13.8. The UV-vis. absorption spectra as well as the formation and decay kinetics for the protonated and unprotonated forms of the methylpyridine radicals studied are presented. The pKa-values for the OH-adducts were determined.

Pulse radiolysis of methyl viologen in aqueous solutions

Pulse radiolysis of air-free aqueous methyl viologen (MV2+) solutions was carried out at various pH. The attack of e–aq on MV2+, with k(e–aq+ MV2+)= 7.5 × 1010 dm3 mol–1 s–1, leads to the formation of the long-lived radical cation (MV˙+), which possesses two absorption maxima at 392.5 nm (Iµ392.5= 4200 m2 mol–1) and 600 nm (Iµ600= 1450 m2 mol–1). The H-atoms react with MV2+ at pH 1 forming two species, which have superimposed absorption bands. By means of a computer simulation they are resolved in the absorptions belonging to: (1) a protonated form of the radical cation (MV˙+H+), which is produced with k(H + MV2+)=(3.5 ± 0.2)× 108 dm3 mol–1 s–1, has 2 absorption maxima at 390 nm (Iµ390= 1700 m2 mol–1) and 595 nm (Iµ595= 760 m2 mol–1) and decays by second-order kinetics with k= 3.5 × 109 dm3 mol–1 s–1; (2) an H-adduct (MV˙2+H) on the ring carbon, which is formed with k(H + MV2+)= 2.5 × 108 dm3 mol–1 s–1, absorbs at 310 nm (Iµ310= 900 m2 mol–1) and 470 nm (Iµ470= 630 m2 mol–1) and decays by conversion into MV˙+H+ in a first-order process with k= 6 × 103 s–1. For the equilibrium MV˙+H+⇌ MV˙++ H+ pK= 2.9 ± 0.1 was determined. The presented data explain, at least partly, the instability of MV2+ when used as an electron acceptor in various devices for the utilization of solar energy.