Degradation of perfluorinated compounds by sulfate radicals - New mechanistic aspects and economical considerations.

Perfluorinated organic compounds (PFC) are an important group of pollutants, which are difficult to be degraded in conventional water treatment. Even hydroxyl radical based processes are not capable to degrade these compounds. Sulfate radicals can oxidize a group of PFC, i.e., perfluorinated carboxylic (PFCAs) acids. However, information in literature on kinetics and reaction mechanism is largely based on model simulations which are prone to errors. The present study provides mechanistic insights based on product formation, material balances, competition kinetics experiments and quantum chemical calculations. Furthermore, energy requirements for sulfate radical based degradation of PFCA is evaluated in the present study. PFCAs can be partly mineralized in chain reactions initiated by sulfate radicals (SO4─). The perfluorinated acetic acid (TFA), propionic acid, and butanoic acid are largely degraded in a primary reaction with sulfate radicals. In case of PFCA with a chain length of > 4 carbons low yields of PFCA products were observed. Regarding reaction kinetics sulfate radicals react very slow with PFCAs (≈ 104 M-1 s-1). Thus, the energy demand required for generation of SO4─ by photolysis of S2O82─ (UV/S2O82-) is very high. A 90% degradation of a PFCA by UV/S2O82- was estimated to be 55 kW h m-3 in pure water.

Reaction of 2-propanol with ozone in aqueous media.

This paper deals with reactions occurring in the aqueous system of 2-propanol/ozone. The considered reactions are discussed from thermodynamic and kinetic points of view. The major finding refers to the fact that 2-propanol reacts with O3 mainly via hydride transfer: (HO)(H3C)2CH + O3 â†’ [(HO)(H3C)2C+ + HO3-]cage â†’ (HO)(H3C)2C+ + HO3- â†’ (H3C)2CO + H2O + O2 Arguments supporting this proposed mechanism are: high exergonicity of reaction (ΔG = -234 kJ mol-1 for the first two steps), low HO yield - (2.4 ± 0.5)% and high acetone yield - (87.2 ± 1.5)%. Other oxidation products detected within the system are acetaldehyde - (1.4 ± 0.1)%, formaldehyde - (4.0 ± 0.1)%, acetic acid - (2.8 ± 0.2)%, formic acid - (0.6 ± 0.2)% and hydrogen peroxide - (1.5 ± 0.1)%. The temperature dependence of the second order rate constant for the reaction 2-propanol + O3 â†’ products is ln kII = 29.64-8500 × T-1. The activation energy and pre-exponential factor derived from this relationship are (71 ± 3) kJ mol-1 and (7.5 ± 6.4) × 1012 M-1 s-1, respectively. At 23 °C, the second order rate constant is kII = (2.7 ± 0.1) M-1 s-1. The low reaction rate can be explained by the transfer of one hydride ion from 2-propanol to electrophilic ozone.

Hydroxyl radical yields in the Fenton process under various pH, ligand concentrations and hydrogen peroxide/Fe(II) ratios.

The Fenton process, one of several advanced oxidation processes, describes the reaction of Fe(II) with hydrogen peroxide. Fe(II) is oxidized to Fe(III) that reacts with hydrogen peroxide to Fe(II) and again initiates the Fenton reaction. In the course of the reactions reactive species, e.g. hydroxyl radicals, are formed. Conditions such as pH, ligand concentrations and the hydrogen peroxide/Fe(II) ratio may influence the OH radical yield. It could be shown that at pH < 2.7 and >3.5 the OH radical yield decreases significantly. Two ligands were investigated, pyrophosphate and sulfate. It was found that pyrophosphate forms a complex with Fe(III) that does not react with hydrogen peroxide and thus, the Fenton reaction is terminated and the OH radical yields do not further increase. The influence of sulfate is not as strong as that of pyrophosphate. The OH radical yield is decreased when sulfate is added but even at higher concentrations the Fenton reaction is not terminated.

Ozonation of anilines: Kinetics, stoichiometry, product identification and elucidation of pathways.

Anilines as archetypes for aromatic amines, which play an important role in the production of, e.g., dyestuffs, plastics, pesticides or pharmaceuticals were investigated in their reaction with ozone. Due to their high reactivity towards ozone (1.2 × 10(5)-2.4 × 10(6) M(-1) s(-1)) the investigated aniline bearing different substituents are readily degraded in ozonation. However, around 4 to 5 molecules of ozone are needed to yield a successful transformation of aniline, most likely due to a chain reaction that decomposes ozone without compound degradation. This is inferred from OH radical scavenging experiments, in which compound transformation per ozone consumed is increased. Mechanistic considerations based on product formation indicate that addition to the aromatic ring is the preferential reaction in the case of aniline, p-chloroaniline and p-nitroaniline (high amounts of o-hydroxyaniline, p-hydroxyaniline, chloride, nitrite and nitrate, respectively were found). For aniline an addition to the nitrogen happens but to a small extent, since nitroso- and nitrobenzene were observed as well. In the case of N-methylaniline and N,N-dimethylaniline, an electron transfer reaction from nitrogen to ozone was proven due to the formation of formaldehyde. In contrast, for p-methylaniline and p-methoxyaniline the formation of formaldehyde may result from an electron transfer reaction at the aromatic ring. Additional oxidation pathways for all of the anilines under study are reactions of hydroxyl radicals formed in the electron transfer of ozone with the anilines (OH radical yields = 34-59%). These reactions may form aminyl radicals which in the case of aniline can terminate in bimolecular reactions with other compounds such as the determined o-hydroxyaniline by yielding the detected 2-amino-5-anilino-benzochinon-anil.

A New Reaction Pathway for Bromite to Bromate in the Ozonation of Bromide.

Ozone is often used in the treatment of drinking water. This may cause problems if the water to be treated contains bromide as its reaction with ozone leads to the formation of bromate, which is considered to be carcinogenic. Bromate formation is a multistep process resulting from the reaction of ozone with bromite. Although this process seemed to be established, it has been shown that ozone reacts with bromite not by the previously assumed mechanism via O transfer but via electron transfer. Besides bromate, the electron-transfer reaction also yields O3(•-), the precursor of OH radicals. The experiments were set up in such a way that OH radicals are not produced from ozone self-decomposition but solely by the electron-transfer reaction. This study shows that hydroxyl radicals are indeed generated by using tBuOH as the OH radical scavenger and measuring its product, formaldehyde. HOBr and bromate yields were measured in systems with and without tBuOH. As OH radicals contribute to bromate formation, higher bromate and HOBr yields were observed in the absence of tBuOH than in its presence, where all OH radicals are scavenged. On the basis of the results presented here, a pathway from bromide to bromate, revised in the last step, was suggested.

Degradation of chlorotriazine pesticides by sulfate radicals and the influence of organic matter.

Atrazine, propazine, and terbuthylazine are chlorotriazine herbicides that have been frequently used in agriculture and thus are potential drinking water contaminants. Hydroxyl radicals produced by advanced oxidation processes can degrade these persistent compounds. These herbicides are also very reactive with sulfate radicals (2.2-3.5 × 10(9) M(-1) s(-1)). However, the dealkylated products of chlorotriazine pesticides are less reactive toward sulfate radicals (e.g., desethyl-desisopropyl-atrazine (DEDIA; 1.5 × 10(8) M(-1) s(-1))). The high reactivity of the herbicides is largely due to the ethyl or isopropyl group. For example, desisopropyl-atrazine (DIA) reacts quickly (k = 2 × 10(9) M(-1) s(-1)), whereas desethyl-atrazine (DEA) reacts more slowly (k = 9.6 × 10(8) M(-1) s(-1)). The tert-butyl group does not have a strong effect on reaction rate, as shown by the similar second order reaction rates between desethyl-terbuthylazine (DET; k = 3.6 × 10(8) M(-1) s(-1)) and DEDIA. Sulfate radicals degrade a significant proportion of atrazine (63%) via dealkylation, in which deethylation significantly dominates over deisopropylation (10:1). Sulfate and hydroxyl radicals react at an equally fast rate with atrazine (k (hydroxyl radical + atrazine) = 3 × 10(9) M(-1) s(-1)). However, sulfate and hydroxyl radicals differ considerably in their reaction rates with humic acids (k (sulfate radical + humic acids) = 6.8 × 10(3) L mgC(-1) s(-1) (mgC = mg carbon); k (hydroxyl radical + humic acids) = 1.4 × 10(4) L mgC(-1) s(-1)). Thus, in the presence of humic acids, atrazine is degraded more efficiently by sulfate radicals than by hydroxyl radicals.

Carbohydrate radicals: from ethylene glycol to DNA strand breakage.

Radiation-induced DNA strand breakage results from the reactions of radicals formed at the sugar moiety of DNA. In order to elucidate the mechanism of this reaction investigations were first performed on low molecular weight model systems. Results from studies on deoxygenated aqueous solutions of ethylene glycol, 2-deoxy-d-ribose and other carbohydrates and, more relevantly, of d-ribose-5-phosphate have shown that substituents can be eliminated from the ß-position of the radical site either proton and base-assisted (as in the case of the OH substituent), or spontaneously (as in the case of the phosphate substituent). In DNA the C(4') radical undergoes strand breakage via this type of reaction. In the presence of oxygen the carbon-centred radicals are rapidly converted into the corresponding peroxyl radicals. Again, low molecular weights models have been investigated to elucidate the key reactions. A typical reaction of DNA peroxyl radicals is the fragmentation of the C(4')-C(S') bond, a reaction not observed in the absence of oxygen. Although OH radicals may be the important direct precursors of the sugar radicals of DNA, results obtained with poly(U) indicate that base radicals may well be of even greater importance. The base radicals, formed by addition of the water radicals (H and OH) to the bases would in their turn attack the sugar moiety to produce sugar radicals which then give rise to strand breakage and base release. For a better understanding of strand break formation it is therefore necessary to investigate in more detail the reactions of the base radicals. For a start, the radiolysis of uracil in oxygenated solutions has been reinvestigated, and it has been shown that the major peroxyl radical in this system undergoes base-catalysed elimination of [Formula: see text], a reaction that involves the proton at N(l). In the nucleic acids the pyrimidines are bound at N(l) to the sugar moiety and this type of reaction can no longer occur. Therefore, with respect to the nucleic acids, pyrimidines are good models only in acid solutions where the [Formula: see text] elimination reaction is too slow to compete with the bimolecular reactions of the peroxyl radicals. Moreover, the long lifetime of the radical sites on the nucleic acid strand may allow reactions to occur which are kinetically of first order, and which cannot be studied in model systems at ordinary dose rates. It is therefore suggested to extend model system studies to low dose rates and to oligonucleo-tides. Such studies might eventually reveal the key reactions in radical-induced DNA degradation.

Formation of bromate in sulfate radical based oxidation: mechanistic aspects and suppression by dissolved organic matter.

Sulfate radical based oxidation is discussed being a potential alternative to hydroxyl radical based oxidation for pollutant control in water treatment. However, formation of undesired by-products, has hardly been addressed in the current literature, which is an issue in other oxidative processes such as bromate formation in ozonation of bromide containing water (US-EPA and EU drinking water standard of bromate: 10 µg L(-1)). Sulfate radicals react fast with bromide (k = 3.5 × 10(9) M(-1) s(-1)) which could also yield bromate as final product. The mechanism of bromate formation in aqueous solution in presence of sulfate radicals has been investigated in the present paper. Further experiments were performed in presence of humic acids and in surface water for investigating the relevance of bromate formation in context of pollutant control. The formation of bromate by sulfate radicals resembles the well described mechanism of the hydroxyl radical based bromate formation. In both cases hypobromous acid is a requisite intermediate. In presence of organic matter formation of bromate is effectively suppressed. That can be explained by formation of superoxide formed in the reaction of sulfate radicals plus aromatic moieties of organic matter, since superoxide reduces hypobromous acid yielding bromine atoms and bromide. Hence formation of bromate can be neglected in sulfate radical based oxidation at typical conditions of water treatment.

Reaction of gadolinium chelates with ozone and hydroxyl radicals.

Gadolinium chelates are used in increasing amounts as contrast agents in magnetic resonance imaging, and their fate in wastewater treatment has recently become the focus of research. Oxidative processes, in particular the application of ozone, are currently discussed or even implemented for advanced wastewater treatment. However, reactions of the gadolinium chelates with ozone are not yet characterized. In this study, therefore, rate constants with ozone were determined for the three commonly used chelates Gd-DTPA, Gd-DTPA-BMA, and Gd-BT-DO3A, which were found to be 4.8 ± 0.88, 46 ± 2.5, and 24 ± 1.5 M(-1) s(-1), respectively. These low rate constants indicate that a direct reaction with ozone in wastewater is negligible. However, application of ozone in wastewater leads to substantial yields of (•)OH. Different methods have been applied and compared for determination of k((•)OH+Gd chelate). From rate constants determined by pulse radiolysis experiments (k((•)OH+Gd-DTPA) = 2.6 ± 0.2 × 10(9) M(-1) s(-1), k((•)OH+Gd-DTPA-BMA) = 1.9 ± 0.7 × 10(9) M(-1) s(-1), k((•)OH+Gd-BT-DO3A) = 4.3 ± 0.2 × 10(9) M(-1) s(-1)), it is concluded that a reaction in wastewater via (•)OH radicals is feasible. Toxicity has been tested for educt and product mixtures of both reactions. Cytotoxicity (MTT test) and genotoxicity (micronuclei assay) were not detectable.

The (•)OH radical yield in the H2O2 + O3 (peroxone) reaction.

The peroxone process is one of the AOPs that lead to (•)OH. Hitherto, it has been generally assumed that the (•)OH yield is unity with respect to O3 consumption. Here, experimental data are presented that suggest that it must be near 0.5. The first evidence is derived from competition experiments. The consumption of 4-chlorobenzoic acid (4-CBA), 4-nitrobenzoic acid (4-NBA) and atrazine present in trace amounts (1 µM) has been followed as a function of the O3 concentration in a solution containing H2O2 (1 mM) and tertiary butanol (tBuOH, 0.5 mM) in excess over the trace compounds. With authentic (•)OH generated by γ-radiolysis such a competition can be adequately fitted by known (•)OH rate constants. Fitting the peroxone data, however, the consumption of the trace indicators can only be rationalized if the (•)OH yield is near 0.5 (4-CBA: 0.56, 4-NBA: 0.49, atrazine: 0.6). Additional information for an (•)OH yield much below unity has been obtained by a product analysis of the reactions of tBuOH with (•)OH and dimethyl sulfoxide with (•)OH. The mechanistic interpretation for the low (•)OH yield is as follows (Merényi et al. Environ. Sci. Technol. 2010, 44, 3505-3507). In the reaction of O3 with HO2(-) an adduct (HO5(-)) is formed that decomposes into O3(•-) and HO2(•) in competition with 2 O2 + OH(-). The latter process reduces the free-radical yield.

Standard Gibbs free energies of reactions of ozone with free radicals in aqueous solution: quantum-chemical calculations.

Free radicals are common intermediates in the chemistry of ozone in aqueous solution. Their reactions with ozone have been probed by calculating the standard Gibbs free energies of such reactions using density functional theory (Jaguar 7.6 program). O(2) reacts fast and irreversibly only with simple carbon-centered radicals. In contrast, ozone also reacts irreversibly with conjugated carbon-centered radicals such as bisallylic (hydroxycylohexadienyl) radicals, with conjugated carbon/oxygen-centered radicals such as phenoxyl radicals, and even with nitrogen- oxygen-, sulfur-, and halogen-centered radicals. In these reactions, further ozone-reactive radicals are generated. Chain reactions may destroy ozone without giving rise to products other than O(2). This may be of importance when ozonation is used in pollution control, and reactions of free radicals with ozone have to be taken into account in modeling such processes.

Degradation of ozone-refractory organic phosphates in wastewater by ozone and ozone/hydrogen peroxide (peroxone): the role of ozone consumption by dissolved organic matter.

Ozonation is very effective in eliminating micropollutants that react fast with ozone (k > 10(3) M(-1) s(-1)), but there are also ozone-refractory (k < 10 M(-1) s(-1)) micropollutants such as X-ray contrast media, organic phosphates, and others. Yet, they are degraded upon ozonation to some extent, and this is due to (•)OH radicals generated in the reaction of ozone with organic matter in wastewater (DOM, determined as DOC). The elimination of tri-n-butyl phosphate (TnBP) and tris-2-chloroisopropyl phosphate (TCPP), added to wastewater in trace amounts, was studied as a function of the ozone dose and found to follow first-order kinetics. TnBP and TCPP concentrations are halved at ozone to DOC ratios of ∼0.25 and ∼1.0, respectively. The (•)OH rate constant of TCPP was estimated at (7 ± 2) × 10(8) M(-1) s(-1) by pulse radiolysis. Addition of 1 mg H(2)O(2)/L for increasing the (•)OH yield had very little effect. This is due to the low rate of reaction of H(2)O(2) with ozone at wastewater conditions (pH 8) that competes unfavorably with the reaction of ozone with wastewater DOC. Simulations based on the reported (Nöthe et al., ES&T 2009, 43, 5990-5995) (•)OH yield (13%) and (•)OH scavenger capacity of wastewater (3.2 × 10(4) (mgC/L)(-1) s(-1)) confirm the experimental data. Based on a typically applied molar ratio of ozone and H(2)O(2) of 2, the contribution of H(2)O(2) addition on the (•)OH yield is shown to become important only at high ozone doses.

The reaction of ozone with the hydroxide ion: mechanistic considerations based on thermokinetic and quantum chemical calculations and the role of HO4- in superoxide dismutation.

The reaction of OH(-) with O(3) eventually leads to the formation of *OH radicals. In the original mechanistic concept (J. Staehelin, J. Hoigné, Environ. Sci. Technol. 1982, 16, 676-681), it was suggested that the first step occurred by O transfer: OH(-)+O(3)-->HO(2)(-)+O(2) and that *OH was generated in the subsequent reaction(s) of HO(2)(-) with O(3) (the peroxone process). This mechanistic concept has now been revised on the basis of thermokinetic and quantum chemical calculations. A one-step O transfer such as that mentioned above would require the release of O(2) in its excited singlet state ((1)O(2), O(2)((1)Delta(g))); this state lies 95.5 kJ mol(-1) above the triplet ground state ((3)O(2), O(2)((3)Sigma(g)(-))). The low experimental rate constant of 70 M(-1) s(-1) is not incompatible with such a reaction. However, according to our calculations, the reaction of OH(-) with O(3) to form an adduct (OH(-)+O(3)-->HO(4)(-); DeltaG=3.5 kJ mol(-1)) is a much better candidate for the rate-determining step as compared with the significantly more endergonic O transfer (DeltaG=26.7 kJ mol(-1)). Hence, we favor this reaction; all the more so as numerous precedents of similar ozone adduct formation are known in the literature. Three potential decay routes of the adduct HO(4)(-) have been probed: HO(4)(-)-->HO(2)(-)+(1)O(2) is spin allowed, but markedly endergonic (DeltaG=23.2 kJ mol(-1)). HO(4)(-)-->HO(2)(-)+(3)O(2) is spin forbidden (DeltaG=-73.3 kJ mol(-1)). The decay into radicals, HO(4)(-)-->HO(2)*+O(2)(*-), is spin allowed and less endergonic (DeltaG=14.8 kJ mol(-1)) than HO(4)(-)-->HO(2)(-)+(1)O(2). It is thus HO(4)(-)-->HO(2)*+O(2)(*-) by which HO(4)(-) decays. It is noted that a large contribution of the reverse of this reaction, HO(2)*+O(2)(*-)-->HO(4)(-), followed by HO(4)(-)-->HO(2)(-)+(3)O(2), now explains why the measured rate of the bimolecular decay of HO(2)* and O(2)(*-) into HO(2)(-)+O(2) (k=1 x 10(8) M(-1) s(-1)) is below diffusion controlled. Because k for the process HO(4)(-)-->HO(2)*+O(2)(*-) is much larger than k for the reverse of OH(-)+O(3)-->HO(4)(-), the forward reaction OH(-)+O(3)-->HO(4)(-) is practically irreversible.

Ozonation of wastewater: rate of ozone consumption and hydroxyl radical yield.

For the prediction of the elimination efficiency of micropollutants from wastewater by ozone, the ozone rate constants of the micropollutants and the kinetics of the reaction of ozone with wastewater must be known. The latter is multiphasic with k = 0.071 (mg DOC)(-1) s(-1) for the first mg/L ozone (at a DOC of 7.2 mg/L) followed by 0.011 (mg DOC)(-1) s(-1) the next 5 mg/L ozone and the k = 0.0019 (mg DOC)(-1) s(-1) for subsequent 4 mg/L ozone as determined by stopped-flow and batch-quench methods. An analysis of gel permeation and UV absorption data indicates that the wastewater DOC is largely polymeric, and at 12 mg/L the concentration of its subunits must be near 100 microM with epsilon(254 nm) approximately 3000 M(-1) cm(-1). The *OH radical yield as determined by the tertiary butanol assay is approximately 13%. From its dose dependence, it follows that new *OH-generating sites are formed during ozonation. The *OH scavenging capacity of the wastewater DOC has been determined at 3 x 10(4) (mg DOC)(-1) s(-1). The contribution of bicarbonate to the OH scavenging capacity is small in comparison, approximately 10% of DOC. Simulations indicate that at 5 mg/L ozone only the most reactive (k > 3 x 10(30 M(-1) s(-1)) micropollutants are fully eliminated but at 10 mg/L ozone the slow ozone decay starts to contribute and even the much less reactive ones (k = 300 M(-1) s(-1)) are oxidized (25% remaining).

Ozonolysis of lignin models in aqueous solution: anisole, 1,2-dimethoxybenzene, 1,4-dimethoxybenzene, and 1,3,5-trimethoxybenzene.

The lignin models anisole, 1,2-dimethoxybenzene, 1,4-dimethoxybenzene, and 1,3,5-trimethoxybenzene were reacted with ozone in aqueous solution, and major products were identified and quantified with respect to ozone consumption when reference material was available. Hydroxylation products in yields equivalent to those of singlet oxygen and muconic products (in analogy to the Criegee mechanism) dominate. The formation of quinones points to the release of methanol. Hydroxyl radicals (*OH, quantified, main precursor: O3*-), singlet oxygen (quantified), O2*- radicals (quantified), and as counterparts of the *OH radicals radical cations of these methoxybenzenes must each play an important role as intermediates. In the case of 1,4-dimethoxybenzene, for example, the following products were identified (yields in parentheses when quantified): methyl(2Z,4E-4-methoxy-6-oxo-hexa-2,4-dienoate 5 (52%), hydroquinone 6 (2%), 1,4-benzoquinone 7 (8%), 2,5-dimethoxyhydroquinone 8,2,5-dimethoxy-1,4-benzoquinone 9, singlet oxygen (6%), hydrogen peroxide (56%), *OH (approximately 17%), O2*- (< or = 9%). Gibbs energies for the various potential reaction pathways were calculated with the help of the Jaguar 7.5 program.

Oxidation of some typical wastewater contaminants (tributyltin, clarithromycin, metoprolol and diclofenac) by ozone.

The rate constants of the reactions of O(3) with some typical wastewater pollutants (tributyltin, the macrolide antibiotic clarithromycin, the beta blocker metoprolol and the analgesic diclofenac) were determined and some mechanistic aspects were elucidated. Except for tributyltin compounds that react only slowly with O(3) (k=4-7 M(-1) s(-1)), the compounds react fast (k>10(4) M(-1) s(-1)) and can be eliminated at low O(3) doses. Clarithromycin reacts at its dimethylamino group and yields the corresponding N-oxide that is no longer biologically active. The nitrogen is also the major site of O(3) attack in diclofenac and in metoprolol. This gives rise to *OH radicals and these are the precursors of hydroxylated products and markedly contribute to chloride release in diclofenac.

Photoprocesses of chloro-substituted p-benzoquinones.

The photochemistry of chloro-(ClBQ), dichloro-(2,5- and 2,6-Cl 2BQ), and trichloro-1,4-benzoquinone (Cl 3BQ) was studied in aqueous solution and/or in mixtures with acetonitrile. Final products are the corresponding hydroquinones (QH 2s) and 2-hydroxy-1,4-benzoquinones (QOHs). Three transients were detected by UV-vis absorption spectroscopy. The triplet state appears within the 20 ns 248 nm pulse and is converted within 0.1-1 micros into a photohydrate (HI aq). HI aq which is spectroscopically and kinetically separated from the triplet state decays within 5 ms, whereas the anion of the hydroxyquinone (QO (-)) grows in at ca. 500 nm in the 0.1-1 s time range. The proton formation and decay kinetics within 0.1-10 micros were observed by transient conductivity in the course of the reaction of the triplet state with water en route to HI aq at pH 4-9. Formation of QO (-) results in a permanent conductance. The efficient photoconversion of Cl n BQs at low concentrations (<0.2 mM) into QH 2s and HOQs is due to a redox reaction of Q with rearranged HI aq. The quantum yield of photoconversion at lambda irr = 254 nm is 0.8-1.2 for ClBQ or Cl 2BQs in aqueous acetonitrile and smaller (0.4) for Cl 3BQ. The yield of semiquinone radical ( (*)QH/Q (*-)) of Cl n BQs ( n = 1-4) in acetonitrile-water (1:1) is low (<20%) at low substrate concentration but is significantly increased upon addition of an H-atom donor, for example, 2-propanol. Other mechanisms involving (*)QH/Q (*-) radicals, such as quenching of the triplet state at enhanced Cl n BQ concentrations and H-atom abstraction from an organic solvent in mixtures with water, have also to be considered.

Oxidation of diclofenac with ozone in aqueous solution.

Ozonation of diclofenac in aqueous solution in the presence and absence of an *OH scavenger, tertiary butanol (t-BuOH), was studied, and the most important reaction intermediates and products were identified. The second-order O3 rate constantwas determined by competition with buten-3-ol and was found to be 6.8 x 10(5) M(-1) s(-1) at 20 degrees C. From this high rate constant, it has been concluded that O3 must initially add on the amino nitrogen. Decomposition of the adduct results in the formation of O3*- (--> *OH) and aminyl radical precursors. A free *OH yield of 30% was estimated based on the HCHO yields generated upon reaction of *OH with 0.01 M t-BuOH. Almost all diclofenac reacted when the molar ratio of O3/diclofenac was approximately 5:1 in the presence of t-BuOH and approximately 8:1 in its absence. As primary reaction products (maximum yield), diclofenac-2,5-iminoquinone (32%), 5-hydroxydiclofenac (7%), and 2,6-dichloroaniline (19%) were detected with respect to reacted diclofenac in the presence of t-BuOH. These primary products degraded into secondary ones when the O3 dose was increased. In the *OH-mediated reaction (absence of t-BuOH) small yields of 5-hydroxydiclofenac (4.5%), diclofenac-2,5-iminoquinone (2.7%), and 2,6-dichloroaniline (6%) resulted. Practically all Cl- (95%) was released in the absence of t-BuOH but only about 45% in the presence of t-BuOH at an O3/diclofenac molar ratio of 10: 1. Based on the reaction products, mechanisms that may account for the high O3 consumption during ozonation of diclofenac are suggested. For technical applications, adequate supply of O3 is needed not only to eliminate diclofenac, but also for the degradation of its potentially toxic products like diclofenac-2,5-iminoquinone and 5-hydroxydiclofenac.

The energetics of rearrangement and water elimination reactions in the radiolysis of the DNA bases in aqueous solution (eaq- and *OH attack): DFT calculations.

DFT calculations on the relative stability of various nucleobase radicals induced by e(aq)(-) and (*)OH have been carried out for assessing the energetics of rearrangements and water elimination reactions, taking the solvent effect of water into account. Uracil and thymine radical anions are protonated fast at O2 and O4, whereby the O2-protonated anions are higher in energy (50 kJ mol(-1), equivalent to a 9-unit lower pK(a)). The experimentally observed pK(a)=7 is thus that of the O4-protonated species. Thermodynamically favored protonation occurs slowly at C6 (driving force, thymine: 49 kJ mol(-1), uracil: 29 kJ mol(-1)). The cytosine radical anion is rapidly protonated by water at N3. Final protonation at C6 is disfavored here. The kinetically favored pyrimidine C5 (*)OH adducts rearrange into the thermodynamically favored C6 (*)OH adducts (driving force, thymine: 42 kJ mol(-1)). Very similar in energy is a water elimination that leads to the Ura-5-methyl radical. Purine (*)OH adducts at C4 and C5 (plus C2 in guanine) eliminate water in exothermic reactions, while water elimination from the C8 (*)OH adducts is endothermic. The latter open the ring en route to the FAPY products, an H transfer from the C8(*)OH to N9 being the most likely process.