Heinz-Peter Schuchmann

OH-radical formation by ultrasound in aqueous solution--Part II: Terephthalate and Fricke dosimetry and the influence of various conditions on the sonolytic yield.

Terephthalate and Fricke dosimetry have been carried out to determine the sonolytic energy yields of the OH free radical and of its recombination product H2O2 in aqueous solutions under various operating conditions (nature of operating gas, power, frequency, temperature). For example, in the sonolysis of Ar-saturated terephthalate solutions at room temperature, a frequency of 321 kHz, and a power of 170 W kg-1, the total yield [G(.OH) + 2 G(H2O2)], equals 16 x 10(-10) mol J-1. This represents the total of .OH that reach the liquid phase from gas phase of the cavitating bubble. The higher the solute concentration, the lower the H2O2 production as more of the OH free radicals are scavenged, in competition with their recombination. Fricke dosimetry, in the absence and presence of Cu2+ ions, shows that the yield of H atom reaching the liquid phase is much lower, with G(H.) of the order of 3 x 10(-10) mol J-1. These sonolytic yields are smaller in solutions that are at the point of gas saturation, and increase to an optimum as the initial sonication-induced degassing and effervescence subsides. The probing of the sonic field has shown that the rate of sonolytic free-radical formation may vary across the sonicated volume depending on frequency and power input.

The photochemistry of aqueous nitrate ion revisited

Aqueous nitrate solutions were photolysed at 254 nm in the absence of oxidizable additives, in the presence of methanol or propan-2-ol and oxygen and in the presence of cyclopentane under anaerobic conditions. The main nitrogen-containing products are nitrite and peroxynitrite. The quantum yields depend on the pH, nitrate concentration, nature of the additive and the light intensity. The intrinsic nitrite yield in alkaline solutions could not be determined directly because, under the conditions of the nitrite assay, the accompanying peroxynitrite decomposes to form nitrite and nitrate; it is smaller than the apparent nitrite yield. In the acidic (pH 4–7) range, the intrinsic nitrite quantum yield is equal to the apparent nitrite yield because there is no buildup of peroxynitrite under these conditions. The apparent nitrite quantum yield increases from 0.01 (no oxidizable additive) to approximately 0.03 (cyclopentane (millimolar range), oxygen free) to 0.06 (methanol (millimolar range), air saturated). At pH 13 and in the absence of oxidizable additives, the apparent nitrite quantum yield increases to about 0.1, whereas from material balance considerations the intrinsic nitrite quantum yield is estimated to be 0.06, twice the oxygen quantum yield of 0.03. Spectrophotometrically, peroxynitrite is detected in the alkaline range only, because its protonated form is unstable. In the absence of oxidizable additives, the quantum yield of peroxynitrite is about 0.1, i.e. only about two-thirds of the quantum yield in the presence of oxidizable additives. Mechanistic considerations on the basis of the pH dependence of the quantum yields of the products nitrite, peroxynitrite and oxygen, as well as their dependence on the kind of additive, indicate that the decisive factor of photolysis in the absence of additives is the formation of the nitric oxide peroxyl radical, ONOO, formed by reaction of peroxynitrite with the primarily generated OH radical. The decay of ONOO is the source of O2 in this system. Nitric oxide, NO, the other fragment of this decay reaction, reacts with nitrogen dioxide, which is one of the primarily formed intermediates. The latter reaction is one of the pathways to the product nitrite, particularly in the alkaline range. The formation of NO during photolysis has been verified by electron spin resonance (ESR) spectroscopic detection of the nitroxide 1,1,3,3-tetramethyl-isoindolin-2-oxyl, the NO adduct to 7,7,8,8-tetramethyl-o-quinodimethane. Of the three primary processes discussed in the literature, we conclude that reactions (1) and (2) occur with quantum yields of approximately 0.09 and 0.1 respectively NO3−+hv→NO2+O−(O−+H2O→OH+OH−)(1)NO3−+hv→ONOO−(2) It appears that none of the peroxynitrite anion is formed in a cage reaction through the recombination of the primary fragments from reaction (1). The primary process shown in reaction (3) is of relatively minor importance, with a quantum yield of no more than 0.001 NO3−+hv→NO2−+O(3) In the presence of methanol (or propan-2-ol) and oxygen under acidic conditions, formaldehyde (or acetone) is formed in an amount equivalent to nitrite via peroxyl radical reactions (quantum yield of approximately 0.06 for both alcohols). In the alkaline range, the apparent formaldehyde quantum yield decreases with increasing pH, while formic acid is produced in increasing amounts. The formation of formic acid is ascribed to the reaction of peroxynitrite anion with photolytically generated formaldehyde. The acetone quantum yield does not decrease with increasing pH over the whole alkaline pH range. In the presence of cyclopentane under oxygen-free conditions, apart from nitrite (and peroxynitrite when alkaline), the compounds nitrocyclopentane, cyclopentyl nitrate, cyclopentene, cyclopentanol and cyclopentanone are produced. The formation of the organic nitrogen compounds leads to an increase in the pH as photolysis proceeds. This pH shift is particularly pronounced in the neutral range.

The photolysis of potassium peroxodisulphate in aqueous solution in the presence of tert-butanol : a simple actinometer for 254 nm radiation

Aqueous potassium peroxodisulphate (0.01 mol dm−3) together with tert-butanol (0.1 mol dm−3) was photolysed with 254 nm light at 20 °C. The peroxodisulphate ion is cleaved, giving rise to sulphate radicals SO4−. These attack the tert-butanol under hydrogen abstraction and hydrogen sulphate formation. In deoxygenated solutions, Φ(H+) = 1.4, Φ(SO42−) = 1.4 and Φ((HOC(CH3)2CH2)2) is approximately 0.7; in the presence of oxygen, Φ(H+) = 1.8. The enhancement of Φ(H+) by oxygen comes about through the production of superoxide via one of the bimolecular decay channels of the tert-butanol-derived peroxyl radical. The superoxide radical reduces S2O82− under liberation of further SO4−. The oxygenated peroxodisulphate-tert-butanol system is shown to constitute a straightforward actinometer for 254 nm light. Some data and observations regarding the tert-butanol-free peroxodisulphate system are also presented.

The fate of peroxyl radicals in aqueous solution

The reactions of peroxyl radicals occupy a central role in oxidative degradation. Under the term Advanced Oxidation Processes in drinking-water and wastewater processing, procedures are summarized that are based on the formation and high reactivity of the OH radical. These react with organic matter (DOC). With O 2 , the resulting carbon-centered radicals O 2 give rise to the corresponding peroxyl radicals. This reaction is irreversible in most cases. An exception is hydroxycyclohexadienyl radicals which are formed from aromatic compounds, where reversibility is observed even at room temperature. Peroxyl radicals with strongly electron-donating substituents eliminate O 2 .− , those with an OH-group in a-position HO 2 . . Otherwise organic peroxyl radicals decay bimolecularly. The tetroxides formed in the first step are very short-lived intermediates and decay by various pathways, leading to molecular products (alcohols, ketones, esters and acids, depending on the precursor), or to oxyl radicals, which either fragment by scission of a neighbouring C-C bond or, when they carry an a-hydrogen, undergo a (water-assisted) 1,2-H-shift.

Sonolysis of aqueous 4-nitrophenol at low and high pH.

The sonolysis of 4-nitrophenol in argon-saturated aqueous solution has been studied at 321 kHz. In order to evaluate separately the effect of OH radicals that are formed in the cavitational bubble and part of which react in the aqueous phase with this substrate, radiolytic studies in N2O-saturated solutions were carried out for comparison. A detailed product study of the sonolysis of 4-nitrophenol solutions shows that at pH 10, where 4-nitrophenol is deprotonated (pKa = 7.1), its sonolytic degradation is fully accounted for by OH-radical-induced reactions in the aqueous phase. At this pH, the sonolytic yield of H2O2 resulting from OH radical recombination in the solution, measured as a function of the 4-nitrophenol concentration, is reduced in line with the scavenging capacity of the 4-nitrophenolate. In contrast, at pH 4 the formation of H2O2 is already fully suppressed when the solution is 7 x 10(-4) mol dm-3 in 4-nitrophenol, and oxidative-pyrolytic degradation predominates, as exemplified by the large yields of CO and CO2 which are accompanied by a large H2 yield. The basis of this difference in behavior is a hydrophobic enrichment of 4-nitrophenol (which is undissociated at pH 4) at the interface of the cavitational bubble by a factor of about 80. The pH dependence of the yields of the pyrolytic products reflects the hydrolytic equilibrium concentration of 4-nitrophenol. The paper also demonstrates that the complexity of this sonochemical system precludes its use a gauge to determine the temperature in the interior of the cavitational bubble.

Sonolysis of tert-butyl alcohol in aqueous solution

A product study of the sonolysis of the volatile substrate t-butanol in aqueous solution indicates that substrate decomposition is practically completely determined, even at concentrations as low as millimolar, by oxidative pyrolysis going on in the gas phase within the collapsing cavitational bubble. OH-Radical-induced reactions in solution are insignificant since the volatility of this substrate, its gas-phase concentration within the bubble enhanced by a certain degree of hydrophobicity, causes OH radicals generated thermolytically from water vapour to be intercepted before they can reach the aqueous phase. The nature of the products, as well as the t-butanol-concentration dependence of the product yields, can be qualitatively explained on the basis of the t-butanol-pyrolysis mechanism. Kinetic considerations involving the relative yields of the pyrolysis products ethane, ethylene and acetylene lead to an estimate of a value of 3600 K for the average pyrolysis temperature at a t-butanol bulk concentration of 10–3 molar.

The SO4(.-)-induced chain reaction of 1,3-dimethyluracil with peroxodisulphate.

The sulphate radical SO4(.-) reacts with 1,3-dimethyluracil (1,3-DMU) (k = 5 X 10(9) dm3 mol-1 s-1) thereby forming with greater than or equal to 90 per cent yield the 1,3-DMU C(5)-OH adduct radical 4 as evidenced by its absorption spectrum and its reactivity toward tetranitromethane. Pulse-conductometric experiments have shown that a 1,3-DMU-SO4(.-) aduct 3 as well as the 1,3-DMU radical cation 1, if formed, must be very short-lived (t1/2 less than or equal to 1 microsecond). The 1,3-DMU C(5)-OH adduct 4 reacts slowly with peroxodisulphate (k = 2.1 X 10(5) dm3 mol-1 s-1). It is suggested that the observed new species is the 1,3-DMU-5-OH-6-SO4(.-) radical 7. At low dose rates a chain reaction is observed. The product of this chain reaction is the cis-5,6-dihydro-5,6-dihydroxy-1,3-dimethyluracil 2. At a dose rate of 2.8 X 10(-3) Gys-1 a G value of approximately 200 was observed ([1,3-DMU] = 5 X 10(-3) mol dm-3; [S2O8(2-)] = 10(-2) mol dm-3; [t-butanol] = 10(-2) mol dm-3). The peculiarities of this chain reaction (strong effect of [1,3-DMU], smaller effect of [S2O(2-)8]) is explained by 7 being an important chain carrier. It is proposed that 7 reacts with 1,3-DMU by electron transfer, albeit more slowly (k approximately 1.2 X 10(4) dm3 mol-1 s-1) than does SO4(.-). The resulting sulphate 6 is considered to hydrolyse into 2 and sulphuric acid which is formed in amounts equivalent to those of 2. Computer simulations provide support for the proposed mechanism. The results of some SCF calculations on the electron distribution in the radical cations derived from uracil and 1-methyluracil are also presented.

Superoxide Radical Reactions in Aqueous Solutions of Pyrogallol and N-propyl Gallate: The Involvement of Phenoxyl Radicals. A Pulse Radiolysis Study

The reactions of O2-. in aqueous solutions of pyrogallol 1 and the antioxidant n-propyl gallate 2 have been studied. In both cases the initial reaction gives hydrogen peroxide and the corresponding phenoxyl radical (k(1 + O2-.) = 3.4 x 10(5), k(2 + O2-.) = 2.6 x 10(5) dm3 mol-1S-1). These phenoxyl radicals have been produced independently by reacting 1 and 2 with Br2-. and their spectra and first pKa values measured (pKa(phenoxyl radical from 1) = 5.1, pKa(phenoxyl radical from 2) = 4.1). It is necessary to correct the observed spectra for the contribution of the H-adducts, formed by the reaction of radiolytically produced H atoms with the substrates (k(1 + H) = 2.5 x 10(9), k(2 + H) = 3.8 x 10(9) dm3 mol-1 S-1). The H-adduct spectra are given. In the reactions of O2-. with the substrates the initial transient absorbances are characteristic of the phenoxyl radicals; however at longer times a new transient absorbing around 500 nm (epsilon congruent to 10(4) dm3 mol-1 cm-1) appears. This is believed to be the deprotonated hydroxy-orthoquinone, formed by the reaction of phenoxyl radicals with O2-. (k congruent to 1.5 x 10(8) dm3 mol-1 S-1, from kinetic curve-fitting). The absorbance due to the hydroxy-orthoquinones decays by first-order kinetics (1.6 x 10(2) in the case of 1 and 1.1 x 10(2) s-1 in the case of 2). This is thought to be mainly the result of the conversion of the hydroxy-orthoquinone into its hydrate. Similar experiments were carried out with catechol and ethyl protocatechuate. The chemistry appears to be similar to that of the pyrogallol derivatives. The rate constant for reaction of these compounds with O2-. is, however, only less than or equal to x 10(4) dm3 mol-1 s-1.

[1] Pulse radiolysis

Publisher Summary Among the fast-kinetics methods in chemistry, electron-pulse radiolysis stands out for its ability to deliver reproducibly a short burst of energy (nanoseconds to microseconds) that induces ionization and excitation. Electron-pulse radiolysis is mainly used on dilute aqueous samples where the energy is practically exclusively spent in the production of reactive species derived from the solvent water, namely, the hydrated electron e–(aq), the hydroxyl radical •OH, and the hydrogen atom H. This is in contrast to flash-photolytic free radical generation where the energy is absorbed by a free radical-forming initiator; this procedure must fail in principle whenever the substrate absorbs more strongly than the initiator. Thus, the application of pulse radiolysis is independent of the optical properties of the medium. The solvated electron and the •OH radical are among the most reactive radicals known. In the energy-absorption process, these radicals are formed in clusters called “spurs.” These are submicroscopically small regions where initially a high radical density prevails, and reactions within the spur that give rise to some H2O2 and H2 are over at times no greater than 10–8 sec. Afterward, the distribution of the radical species is homogeneous.

Photolysis at 185 nm of dimethyl ether in aqueous solution: involvement of the hydroxymethyl radical

Abstract In the 185 nm photolysis of aqueous solutions of dimethyl ether (saturated at atmospheric pressure; 1.1 M) the following products are formed (quantum yields in parentheses): methane (0.06); hydrogen (0.03); methanol (0.02); 1,2-dimethoxyethane (0.014); formaldehyde (0.012); 2-methoxyethanol (0.012); ethylene glycol (0.009); ethanol (0.005); methyl ethyl ether (0.001). These products are explained by three preliminary processes (reactions (i) – (iii)), the rearrangement process (reaction (iv)) known to be undergone by alkoxyl radicals in aqueous solution and subsequent free-radical reactions. In aqueous solutions the quantum yield of primary processes leading to products is smaller by about an order of magnitude than those in cyclohexane solutions or those previously found with similar ethers as pure liquids. This apparently means that water as a solvent has a quenching effect. In aqueous solutions there is an excited species which is reactive towards nitrous oxide and a proton, leading to the formation of nitrogen and hydrogen respectively. Free hydrated electrons generated by photoionization do not appear to be involved in these reactions.