Mechanism of photodegradation of aqueous organic pollutants. 2. Measurement of the primary rate constants for reaction of hydroxyl radicals with benzene and some halobenzenes using an EPR spin-trapping method following the photolysis of hydrogen peroxide (original) (raw)
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Environmental Science and Technology
The technique of spin-trapping, with EPR detection of spin adducts, has been applied to the study of photodegradation of benzene, chlorobenzene, bromobenzene, iodobenzene, o-dichlorobenzene, m-dichlorobenzene sensitized by the photolysis of H2O2 in aqueous solution. By employing a competition kinetic scheme and relative initial slopes of signal amplitudes, plus published rate constants for the reaction of OH radicals with the spin trap (DMPO), it has been possible to obtain rate constants for the reaction of OH radicals with benzene and its halo derivatives. Rate constants obtained by this method at neutral and acidic solutions are similar to published methods using pulse radiolysis, where available. Significant differences were found for measurements carried out in basic solutions. The rate constants do not vary much among the halobenzene derivatives studied, and an average rate constant of ~5.0 x 109 M-1 s-1 could be used for any of these compounds.
The Journal of Physical Chemistry, 1991
A technique for the determination of the rate constants for the reaction of 'OH radicals with organic substrate is described. This technique is based on the use of spin trapping with electron paramagnetic resonance (EPR) detection of spin adducts. The desired rate constants are obtained by measuring either the initial rate of production of the EPR signal of the spin adduct or its amplitude after a fixed time, as a function of the concentration of the substrate. By using the known rate constant for the reaction of 'OH radicals with the spin trap (DMPO), rate constants for the reaction of *OH radicals with a given substrate can be obtained within a competition kinetic scheme. The method has been validated by determining the rate constants for the reaction of 'OH radicals with phenol and with formate, two reactions well studied by using pulse radiolysis. Our results agree with literature values within experimental error. The method is then applied to several chlorophenols. The rate constants for 3-chloro-substituted phenols are significantly less than for 4-chloro or 2-chloro-substituted phenols. Some rate constants are significantly larger than the diffusionantrolled rate constant. This is explained by proposing a Grotthus-type mechanism for the movement of 'OH radicals through water. (16) Bolton, J. R.; Clayton, R. K.; Red, D. W. Phorochem. Phorobiol. 1969, 9, 209. (17) Moa, P. N.; Fytianos, K.; Samanidou, V.; Korte, F. Bull. Enuiron. (18) Neta, P.; Steenken, S.; Janzen, E. G.; Shetty, R. V.
Journal of Physical Chemistry, 1991
A technique for the determination of the rate constants for the reaction of 'OH radicals with organic substrate is described. This technique is based on the use of spin trapping with electron paramagnetic resonance (EPR) detection of spin adducts. The desired rate constants are obtained by measuring either the initial rate of production of the EPR signal of the spin adduct or its amplitude after a fixed time, as a function of the concentration of the substrate. By using the known rate constant for the reaction of 'OH radicals with the spin trap (DMPO), rate constants for the reaction of *OH radicals with a given substrate can be obtained within a competition kinetic scheme. The method has been validated by determining the rate constants for the reaction of 'OH radicals with phenol and with formate, two reactions well studied by using pulse radiolysis. Our results agree with literature values within experimental error. The method is then applied to several chlorophenols. The rate constants for 3-chloro-substituted phenols are significantly less than for 4-chloro or 2-chloro-substituted phenols. Some rate constants are significantly larger than the diffusionantrolled rate constant. This is explained by proposing a Grotthus-type mechanism for the movement of 'OH radicals through water. (16) Bolton, J. R.; Clayton, R. K.; Red, D. W. Phorochem. Phorobiol. 1969, 9, 209. (17) Moa, P. N.; Fytianos, K.; Samanidou, V.; Korte, F. Bull. Enuiron. (18) Neta, P.; Steenken, S.; Janzen, E. G.; Shetty, R. V.
Sensitized photo-oxidation of dihydroxybenzenes and chlorinated derivatives. A kinetic study
Journal of Photochemistry and Photobiology A: Chemistry, 1991
The dye-sensitized photo-oxidation of dihydroxybenzenes and chlorinated derivatives was studied as a function of solvent polarity. Rate constants for the disappearance of singlet molecular oxygen (O,('A,)), determined by time-resolved phosphorescence at A > 1050 nm, are in the range 5 X l@ to 3 X 10' M-' s-' depending on the solvent polarity and substitution pattern of the reactant. The ratio between the rate constant for reactive quenching k, and the total quenching constant k, is also strongly dependent on substitution pattern and solvent. The values of k, indicate that the O#A,)-mediated photo-oxidation of the dihydroxybenzenes is a viable possibility for their degradation in an aqueous environment. A charge-transfer mechanism is suggested for the photo-oxygenation. Dihydroxybenzenes are fairly good O,('AJ self-sensitizers of photo-oxidation on irradiation at 308 nm, with Oz('A,) quantum yields of approximately 0.2 in the most favourable case. For hydroquinones, this presumably occurs through the excitation of the corresponding quinone derivatives which are always present in traces in the aerated solutions.
Kinetics of the dye-sensitized photooxidation of trihydroxybenzenes
Journal of Photochemistry and Photobiology A: Chemistry, 2000
The kinetics of the singlet molecular oxygen [O 2 ( 1 g )]-mediated photooxidation of the three isomeric trihydroxybenzenes (THBs), compounds of potential environmental significance as aquatic contaminants, has been studied in water solution as a function of pH and ionic strength, as well as in benzene and acetonitrile. Rate constants for chemical and overall interactions with O 2 ( 1 g ), determined by time-resolved IR phosphorescence detection and polarographic methods, are in the range 0.05×10 7 -24.0×10 7 M −1 s −1 , depending on the medium and the particular compound. In water at pH 2, the photooxidation quantum efficiencies of 1,2,3-THB, 1,2,4-THB and 1,3,5-THB are 0.07, 0.07 and 0.19, respectively. The photooxidative processes are highly favored by the ionization of the OH groups, the increase of solvent polarity, and the presence of salts in the medium, strongly suggesting the participation of a polar encounter complex of the type [THB-O 2 ( 1 g )]. This kinetic behavior is similar to that observed in simple phenols and dihydroxybenzenes, although THBs are much easier photooxidizable, even in non-ionized form. These results possess environmental relevance, because they demonstrate that any THB in aqueous media undergoes spontaneous and fast solar-promoted photooxidation under practically any field condition.
Applied Catalysis B: Environmental, 2011
The role of hydroxyl radical in different solar photocatalytic processes, namely photo-Fenton, titanium dioxide-based oxidation and organic photocatalysis (triphenylpyrylium) has been investigated. Using a methodology based on the flash photolytic generation of OH • from N-hydroxypyridine-2(1H)-thione, followed by t-stilbene trapping, the reaction rate constants of different pesticides (dimethoate, methidathion, alachlor and pyrimethanyl) with this reactive oxygen species in deaerated acetonitrile have been determined. The results obtained under photo-Fenton conditions are in reasonable agreement with the estimated rate constants; hence the assumption that the photo-Fenton reaction mainly involves participation of the hydroxyl radical seems plausible. The oxidation pattern using titanium dioxide was also investigated; however, under these conditions no clear correlation could be found due to participation of an alternative electron transfer mechanism.
SCOPE OF THE JOURNAL, 2004
The degradation of benzene, monochlorobenzene and 1,2-dichlorobenzene in aqueous solution by gamma irradiation was investigated. The effect of the irradiated solution composition was studied. The results showed that benzene is more resistant to destruction than chlorinated benzenes. The presence of oxidizing and reducing reactive species and the rapid reaction rates with halogenated benzenes increased the degradation rate of the pollutants. Dechlorination of CB and 1,2-DCB was observed. High performance liquid chromatography (HPLC) and spectroscopy (UV-Vis) were used to monitor changes in the radiation solutions. The final aqueous irradiation products were shown to be a complex mixture of by-products. The addition of scavengers such as methanol and ethanol required larger doses to decompose the pollutants when compared to those solutions with no additives.
Journal of Photochemistry and Photobiology A: Chemistry, 1993
The photolysis of 4-bromophenol in aqueous solution in the presence of the spin trap 5,5-dimethyl-pyrroline-N-oxide (DMPO) results in the formation of four distinct electron paramagnetic resonance (EPR) spectra. Three of these have been identified as arising from the DMPO spin adducts of an aryl radical (probably the 4-hydroxyphenyl radical, spectrum A), a hydrated electron plus later protonation (spectrum B) and the hydroxyl radical (spectrum C). Spectrum D is that of the p-benzosemiquinone anion. Spectra A and B appear immediately on initiation of photolysis in both deoxygenated and oxygenated solutions. Spectra C and D arise from secondary products and require the presence of oxygen. The component EPR spectra do not depend on the pH value in the pH range 7.0 −10.5, but the overall appearance of the spectra changes with increasing pH, showing a larger contribution of both adducts A and B compared with adduct C. The results are compared with those of previous EPR investigations of 4-chlorophenol and of photochemical studies of 4-substituted halogenophenols using conventional sources of irradiation and the flash photolysis technique. The mechanism of aqueous photolysis of the compounds is discussed.