Photodegradation of Triclosan on the Kaolinite Surface: Kinetic, Mechanistic, and Molecular Modeling Approach.

The photodegradation of triclosan (TCS) was investigated on the kaolinite surface. The quantum yield was evaluated, and the photoproducts were identified by HPLC/MS (LC/Q-TOF), showing that the phototransformation is completely different from that reported in aqueous solutions. In particular, the formation of dioxin derivatives was fostered and occurred with a higher efficiency when compared to aqueous solutions. This suggests that TCS has specific interactions with the clay that clearly modifies its photochemical behavior. Moreover, it has also been shown that higher concentrations of TCS, namely, higher than 1.0 µmol g-1 of kaolinite, lead to a significant decrease of the photodegradation rate constant and enhance the formation yield of dimer-type photoproducts. This suggests that the distribution of TCS is clearly not homogeneous at the clay surface and the formation of aggregates is more likely occurring. To get a better insight into this specific interaction, a molecular dynamic modeling of TCS adsorption at the surface of kaolinite was carried out. This clearly shows that when equilibrium is reached, TCS binds to the kaolinite surface by hydrogen bonds involving the phenol function of TCS and the hydroxyl groups of the kaolinite surface. Such behavior confers a particular conformation to the adsorbed TCS that is different from that obtained in water and which could be a key step to partially explain the specific photochemical reactivity in both media. In addition, several TCS molecules appear to interact with each other through the π-stacking (aromatic stacking) process while retaining this hydrogen bond with the kaolinite surface. This is clearly in favor of cluster formation on the clay surface and promotes dimer-type photoproducts.

Environmental photodegradation of emerging contaminants: A re-examination of the importance of triplet-sensitised processes, based on the use of 4-carboxybenzophenone as proxy for the chromophoric dissolved organic matter.

The photoreactions sensitised by the excited triplet states of chromophoric dissolved organic matter (3CDOM*) are very important in the photochemical attenuation of emerging contaminants in natural waters. Until quite recently, anthraquinone-2-sulphonate (AQ2S) was the only available CDOM proxy molecule to estimate the contaminant reaction kinetics with 3CDOM*, under steady-state irradiation conditions. Unfortunately, the AQ2S triplet state (3AQ2S*) is considerably more reactive than average 3CDOM*. We have recently developed an alternative protocol based on 4-carboxybenzophenone (CBBP), the triplet state of which (3CBBP*) is less reactive compared to 3AQ2S*. Here we show that in the case of ibuprofen (IBP), paracetamol (APAP) and clofibric acid (CLO), the reaction rate constants with 3CBBP* are more reasonable as 3CDOM* reactivity estimates than those obtained by using AQ2S. In contrast, similar rate constants are measured for the reaction of atrazine (ATZ) with either 3AQ2S* or 3CBBP*. Moreover, the reactivity of ATZ with both 3AQ2S* and 3CBBP* is very similar to that with 3CDOM*, available through a literature estimate. The possibility to validate the ATZ-3CBBP* reactivity estimate against the 3CDOM* data, and to accurately predict the reported IBP and CLO field lifetime, support the suitability of CBBP as CDOM proxy. The replacement of AQ2S with CBBP as proxy molecule does not reverse the qualitative prediction, according to which 3CDOM* would be the main process involved in the photodegradation of the studied contaminants in waters with high dissolved organic carbon (DOC). However, the CBBP-based data prompt for an important reconsideration of the estimated lifetimes at high DOC.

Degradation of Methyl 2-Aminobenzoate (Methyl Anthranilate) by H2O2/UV: Effect of Inorganic Anions and Derived Radicals.

This study shows that methyl 2-aminobenzoate (also known as methyl anthranilate, hereafter MA) undergoes direct photolysis under UVC and UVB irradiation and that its photodegradation is further accelerated in the presence of H2O2. Hydrogen peroxide acts as a source of hydroxyl radicals (·OH) under photochemical conditions and yields MA hydroxyderivatives. The trend of MA photodegradation rate vs. H2O2 concentration reaches a plateau because of the combined effects of H2O2 absorption saturation and ·OH scavenging by H2O2. The addition of chloride ions causes scavenging of ·OH, yielding Cl2·- as the most likely reactive species, and it increases the MA photodegradation rate at high H2O2 concentration values. The reaction between Cl2·- and MA, which has second-order rate constant k C l 2 • - + M A = (4.0 ± 0.3) × 108 M-1·s-1 (determined by laser flash photolysis), appears to be more selective than the ·OH process in the presence of H2O2, because Cl2·- undergoes more limited scavenging by H2O2 compared to ·OH. While the addition of carbonate causes ·OH scavenging to produce CO3·- ( k C O 3 • - + M A = (3.1 ± 0.2) × 108 M-1·s-1), carbonate considerably inhibits the photodegradation of MA. A possible explanation is that the elevated pH values of the carbonate solutions make H2O2 to partially occur as HO2-, which reacts very quickly with either ·OH or CO3·- to produce O2·-. The superoxide anion could reduce partially oxidised MA back to the initial substrate, with consequent inhibition of MA photodegradation. Fast MA photodegradation is also observed in the presence of persulphate/UV, which yields SO4·- that reacts effectively with MA ( k S O 4 • - + M A = (5.6 ± 0.4) × 108 M-1·s-1). Irradiated H2O2 is effective in photodegrading MA, but the resulting MA hydroxyderivatives are predicted to be about as toxic as the parent compound for aquatic organisms (most notably, fish and crustaceans).

Transformation of avobenzone in conditions of aquatic chlorination and UV-irradiation.

Emerging contaminants represent a wide group of the most different compounds. They appear in the environment at trace levels due to human activity. Most of these compounds are not yet regulated. Sunscreen UV-filters play an important role among these emerging contaminants. In the present research the reactions of 4-tert-butyl-4'-methoxydibenzoylmethane (avobenzone), the most common UV filter in the formulation of sunscreens, were studied under the combined influence of active chlorine and UV-irradiation. Twenty five compounds were identified by GC/MS as transformation products of avobenzone in reactions of aquatic UV-irradiation and chlorination with sodium hypochlorite. A complete scheme of transformation of avobenzone covering all the semivolatile products is proposed. The identification of the two primary chlorination products (2-chloro-1-(4-tert-butylphenyl)-3-(4-methoxyphenyl)-1,3-propanedione and 2,2-dichloro-1-(4-tert-butylphenyl)-3-(4-methoxyphenyl)-1,3-propanedione) was confirmed by their synthesis and GC/MS and NMR analysis. Although the toxicities of the majority of these products remain unknown substituted chlorinated phenols and acetophenones are known to be rather toxic. Combined action of active chlorine and UV-irradiation results in the formation of some products (chloroanhydrides, chlorophenols) not forming in conditions of separate application of these disinfection methods. Therefore caring for people «well-being¼ it is of great importance to apply the most appropriate disinfection method. Since the primary transformation products partially resist powerful UV-C irradiation they may be treated as stable and persistent pollutants.

Elucidation of oxidized products of Chloridazon in advanced oxidation processes using a liquid chromatography/negative electrospray ionization tandem mass spectrometric technique.

RATIONALE: 5-Amino-4-chloro-2-phenyl-3(2H)-pyridazone (Chloridazon) is an important systemic herbicide; its transformation by different Advanced Oxidation Processes could result in the formation of different and complex products. These products, that need to be identified, may present a more toxic effect than the parent compound. METHODS: The exact and unequivocal structural characterization of the detected by-products of Chloridazon was performed by in-depth analysis of the liquid chromatography/negative electrospray ionization tandem mass spectrometric (LC/ESI+/MS(2) ) fragmentation. Such a technique is a powerful and sensitive analytical tool to study environmental issues. The reactive radicals that induce the degradation of Chloridazon were generated by selective excitation of titanium dioxide as photocatalyst and also by UV photolysis of hydrogen peroxide. RESULTS: Several processes are initiated by hydroxyl radical reaction with Chloridazon. Among them (i) hydroxylation of the phenyl group at the ortho, meta and para positions; (ii) a second hydroxylation process as a secondary reaction is also obtained within the phenyl moiety; (iii) scission of the phenyl-N bond through a hydrolysis process; and (iv) a substitution of the chlorine atom by a hydroxyl group. CONCLUSIONS: LC/ESI-Q-TOFMS appeared to be a valuable and precise tool for structural elucidation of the unknown by-products that were generated during hydroxyl radical reactions with Chloridazon. Several hydroxylated and dihydroxylated isomers were identified together with dechlorinated and bridge opening products.

Photochemical fate and photocatalysis of 3,5,6-trichloro-2-pyridinol, degradation product of chlorpyrifos.

In this study we have focused on 3,5,6-trichloro-2-pyridinol (TCP), degradation product of chlorpyrifos. Photolysis experiments were conducted in order to elucidate its degradation mechanism. Identification of products was performed using the LC-MS technique. To evaluate the mineralization efficiency, TiO2 photocatalytic study was performed. Under photolytic experimental conditions, the concentration of TCP after 120 min of irradiation reached 5.9 ± 1.5% of the initial concentration, while chloride concentration reached approximately 73% of total chloride concentration. The TOC measurements after 120 min of photocatalytic degradation experiment revealed high mineralization rate, i.e. 53.6 ± 1.9%, while chloride concentration reached 26.6 mg L(-1) what means almost quantitative transformation of organic chlorine into chloride. TIC chromatogram (ESI, negative ion mode) of the reaction mixture after 30 min of irradiation revealed the presence of several peaks. One of them has already been reported previously. Two other products have been identified in this study for the first time. They have been formed by radical attack of the reactive OH(•) species on the carbonyl group followed by the corresponding N-C or C-C bond cleavages and recyclization with formation of the pyrrol structures substituted with carboxylic groups. Both deprotonated molecules easily lose CO2 in ESI conditions.

Photochemical processes induced by the irradiation of 4-hydroxybenzophenone in different solvents.

The singlet and triplet excited states of 4-hydroxybenzophenone (4BPOH) undergo deprotonation in the presence of water to produce the anionic ground-state, causing fluorescence quenching and photoactivity inhibition. The same process does not take place in an aprotic solvent such as acetonitrile. In acetonitrile, 4BPOH is fluorescent (interestingly, one of its fluorescence peaks overlaps with peak C of humic substances), it yields singlet oxygen upon irradiation and induces the triplet-sensitised transformation of phenol (with a rate constant of (6.6 ± 0.3) × 10(7) M(-1) s(-1) (µ ± σ) between phenol itself and a triplet 4BPOH). The 4BPOH shows an intermediate behaviour in a partially protic solvent such as 2-propanol, where some deprotonation of the excited states is observed. In acetonitrile/2-propanol mixtures (at least up to 50% of 2-propanol) there is also some evidence of alcohol oxidation by the 4BPOH triplet state, while the experimental data are silent concerning such a possibility in pure 2-propanol. Considering that benzophenones are important components of chromophoric dissolved organic matter (CDOM) in surface waters, the present findings could have significance for the photoactivity of the hydrophilic surface layers vs. the hydrophobic cores of CDOM.

Application of liquid chromatography/electrospray ionization tandem mass spectrometry for the elucidation of hydroxyl radical oxidation of metsulfuron methyl and related sulfonylurea pesticide products: evidence for the triazine skeleton scission.

RATIONALE: Sulfonylureas are among the most important class of antidiabetic and herbicides. Solar light excitation and Advanced Oxidation Processes may result in the formation of a wide array of products owing to the relative complex structure. These products, that should be identified, may present a more toxic effect than the parent compound. METHODS: Liquid chromatography/electrospray ionization quadrupole time-of-flight mass spectrometry (LC/ESI-QTOFMS) with accurate mass determination emerges as a valuable technique for the precise elucidation of all possible byproducts. The hydroxyl radical was generated by excitation of the iron(III) aquacomplex [Fe(H(2)O)OH](2+) and hydrogen peroxide at pH 3.5. Three different sulfonylureas were studied: metsulfuron methyl, cinosulfuron and thifensulfuron methyl. RESULTS: Several products owing to the reactivity of hydroxyl radicals with sulfonylurea were obtained. They arise from scission of the sulfonylurea bridge, hydroxylation of the aromatic ring, demethylation of the methoxy group and more importantly and unequivocally from the rupture of the triazine skeleton. To reach such scission, a primary demethylation of the methoxy group on the triazine moiety seems to act as a precursor process. Such a process was observed with the three studied sulfonylurea compounds. CONCLUSIONS: The reported results demonstrated the usefulness of accurate mass measurements undertaken by LC/ESI-QTOFMS for structural elucidation of the unknown byproducts that were generated during hydroxyl radical reactions with some sulfonylureas. It has been possible herein to identify the structures of products arising from the opening of the recalcitrant triazine structure via hydrolysis processes in acidic solutions.

Heterogeneous photocatalytic degradation of pesticides using decatungstate intercalated macroporous layered double hydroxides.

Decatungstate W10O32(4-) was efficiently intercalated between the layers of three-dimensionally ordered macroporous Mg2Al-layered double hydroxide. The structural and textural properties of as-prepared intercalated compound were characterized using different solid-state characterization techniques such as X-ray powder diffraction, FTIR and Raman spectroscopies and electronic microscopy. The photocatalytic properties of immobilized W10O32 (4-) within Mg2Al structure were investigated using 2-(1-naphthyl) acetamide (NAD) as a model of pesticide. The influence of different parameters such as amount of catalyst, pH and oxygen concentration were investigated. An optimal NAD degradation was obtained for a photocatalyst concentration of 60 mg l(-1). Under our experimental conditions, this heterogeneous photocatalyst induces photodegradation of 60 % of NAD after 17 h of irradiation at 365 nm and at pH 6.6. Interestingly, pesticide photodegradation leads to the mineralization of substrates to H2O and CO2 and the photocatalyst can be recycled and reused without any loss of activity over four cycles.

Assessing the photochemical transformation pathways of acetaminophen relevant to surface waters: transformation kinetics, intermediates, and modelling.

This work shows that the main photochemical pathways of acetaminophen (APAP) transformation in surface waters would be direct photolysis (with quantum yield of (4.57 ± 0.17)⋅10(-2)), reaction with CO3(-·) (most significant at pH > 7, with second-order rate constant of (3.8 ± 1.1)⋅10(8) M(-1) s(-1)) and possibly, for dissolved organic carbon higher than 5 mg C L(-1), reaction with the triplet states of chromophoric dissolved organic matter ((3)CDOM*). The modelled photochemical half-life time of APAP in environmental waters would range from days to few weeks in summertime, which suggests that the importance of phototransformation might be comparable to biodegradation. APAP transformation by the main photochemical pathways yields hydroxylated derivatives, ring-opening compounds as well as dimers and trimers (at elevated concentration levels). In the case of (3)CDOM* (for which the triplet state of anthraquinone-2-sulphonate was used as proxy), ring rearrangement is also hypothesised. Photochemistry would produce different transformation products (TPs) of APAP than microbial biodegradation or human metabolism, thus the relevant TPs might be used as markers of APAP photochemical reaction pathways in environmental waters.

Transparent organosilica photocatalysts activated by visible light: photophysical and oxidative properties at the gas-solid interface.

The photophysical properties of several photosensitizers (PSs) included or grafted in silica monoliths were compared to their properties in solution. The effects of the solid support on their steady-state and transient absorption spectra, on their quantum yields of singlet oxygen ((1)O2) production, and on their ability to photoinduce the oxidation of dimethylsulfide (DMS) were investigated. Two cyanoanthracene derivatives (9,14-dicyanobenzo[b]triphenylene, DBTP, and 9,10-dicyanoanthracene, DCA), as well as three phenothiazine dyes (methylene blue, MB(+), new methylene blue, NMB(+), methylene violet, MV), were encapsulated in silica, analyzed and compared to two reference PSs (perinaphthenone, PN and rose bengal, RB). A DBTP derivative (3-[N-(N″-triethoxysilylpropyl-N'-hexylurea)]carboxamido-9,14-dicyanobenzo[b]triphenylene, 3) was also prepared and grafted onto silica. Thanks to the transparency and the free-standing shape of the monoliths, the complete spectroscopic characterization of the supported PSs was carried out directly at the gas-solid interface. The influence of the silica network, the PS, and the adsorption/grafting link between the PS and silica was investigated. The effects of PS concentration, gaseous atmosphere, humidity, and hydrophobicity on the production of (1)O2 were analyzed. With all PSs, (1)O2 production was very efficient (quantum yields of (1)O2 production, relative to PN, between 0.6 and 1), and this species was the only one involved in the pollutant photooxidation. The influence of the matrix on the PSs' photophysics could be considered as negligible. In contrast, the matrix effect on DMS photooxidation was extremely important: the gas diffusion inside the porous structure, and thus, the photoactivity of the materials, strictly depended on silica's surface area and porosity. Our results highlight the suitability of these silica structures as inert supports for the study of the photosensitizing properties at the gas-solid interface. Moreover, thanks to the adsorption properties of the matrix, the synthesized materials can be used as microphotoreactor for the (1)O2-mediated oxidation of volatile pollutants.

Comparison of the photophysical properties of three phenothiazine derivatives: transient detection and singlet oxygen production.

New methylene blue (NMB+) and methylene violet (MV) are known for their photosensitizing properties for singlet oxygen ((1)O2) generation upon visible-light irradiation, and various examples of their use in the photodynamic inactivation of microorganisms and for photomedicinal purposes have been reported. However, their photophysical properties have never been extensively and systematically analyzed and compared. In the current work, we studied their absorption and fluorescence behavior relative to their parent compound, methylene blue (MB+), detected the transient species generated upon excitation of the photosensitizers and determined their quantum yields of singlet oxygen production. We could measure very high quantum yields of singlet oxygen production for all the studied compounds. NMB+ appeared similar to MB+, even though it produces (1)O2 much more efficiently, and was slightly influenced by the solvent. MV, in contrast, was much more sensitive to the chemical environment, and the transient species formed upon irradiation were different in methanol and acetonitrile. It appeared to be a very good singlet oxygen sensitizer, but the influence of the chemical environment should be carefully considered for any application. The comparative characterization of these sensitizers will represent a support for the determination and the understanding of the photochemical mechanisms occurring by using these phenothiazine dyes for various photobiological applications.

Efficient cyanoaromatic photosensitizers for singlet oxygen production: synthesis and characterization of the transient reactive species.

In order to graft cyanoaromatic molecules onto various inert supports, we designed two new cyanoanthracene derivatives of benzo[b]triphenylene-9,14-dicarbonitrile (DBTP, 1), which already demonstrated good photosensitizing properties. We synthesized 3-(N-hydroxypropyl)carboxamido-9,14-dicyanobenzo[b]triphenylene, 3 and 3-(N-N0-Boc-aminohexyl)carboxamido-9,14-dicyanobenzo[b]triphenylene, 4 and compared their photophysical properties in acetonitrile relative to those of the parent compound 1 and its carboxylic derivative 9,14-dicyanobenzo[b]triphenylene-3-carboxylic acid, 2. The transient species were analysed and the quantum yields of singlet oxygen production (ΦΔ) determined in acetonitrile. The effect of chemical functionalization can be considered negligible, since absorption spectra, fluorescence emission spectra and fluorescence lifetimes do not significantly change with the substituent. The triplet-triplet absorption spectra and the triplet excited state lifetimes are similar for the whole series. For compounds 1-4 high values of ΦΔ, close to that of the standard sensitizer 1H-phenalen-1-one (PN, ΦΔ ≈ 1), and higher than that of the well-known photosensitizer 9,10-dicyanoanthracene (DCA), are due to very efficient intersystem crossing from the singlet to the triplet excited state and subsequent energy transfer to ground state oxygen ((3)O2). They belong to a class of very efficient photosensitizers, absorbing visible light and stable under irradiation, they may be functionalized without significant changes to their photophysical behaviour, and grafted onto various supports.

Photochemical processes involving the UV absorber benzophenone-4 (2-hydroxy-4-methoxybenzophenone-5-sulphonic acid) in aqueous solution: reaction pathways and implications for surface waters.

The sunlight filter benzophenone-4 (BP-4) is present in surface waters as two prevailing forms, the singly deprotonated (HA-) and the doubly deprotonated one (A(2-)), with pKa2 = 7.30 ± 0.14 (µ ± σ, by dissociation of the phenolic group). In freshwater environments, BP-4 would mainly undergo degradation by reaction with ·OH and direct photolysis. The form HA(-) has a second-order reaction rate constant with ·OH (k(·OH)) of (1.87 ± 0.31)·10(10) M(-1) s(-1) and direct photolysis quantum yield Φ equal to (3.2 ± 0.6)·10(-5). The form A(2-) has (8.46 ± 0.24)·10(9) M(-1) s(-1) as the reaction rate constant with ·OH and (7.0 ± 1.3)·10(-5) as the photolysis quantum yield. The direct photolysis of HA(-) likely proceeds via homolytic breaking of the O-H bond of the phenolic group to give the corresponding phenoxy radical, as suggested by laser flash photolysis experiments. Photochemical modelling shows that because of more efficient direct photolysis (due to both higher sunlight absorption and higher photolysis quantum yield), the A(2-) form can be degraded up to 3 times faster than HA(-) in surface waters. An exception is represented by low-DOC (dissolved organic carbon) conditions, where the ·OH reaction dominates degradation and the transformation kinetics of HA(-) is faster compared to A(2-). The half-life time of BP-4 in mid-latitude summertime would be in the range of days to weeks, depending on the environmental conditions. BP-4 also reacts with Br2(·-), and a rate constant k(Br2(·-),BP-4) = (8.05 ± 1.33)·10(8) M(-1) s(-1) was measured at pH 7.5. Model results show that reaction with Br2(·-) could be a potentially important transformation pathway of BP-4 in bromide-rich (e.g. seawater) and DOM-rich environments.

Combined theoretical and experimental study of the photophysics of asulam.

The photophysics of the neutral molecular form of the herbicide asulam has been described in a joint experimental and theoretical, at the CASPT2 level, study. The unique π → π* aromatic electronic transition (f, ca. 0.5) shows a weak red-shift as the polarity of the solvent is increased, whereas the fluorescence band undergoes larger red-shifts. Solvatochromic data point to higher dipole moment in the excited state than in the ground state (µ(g) < µ(e)). The observed increase in pKa in the excited state (pKa* - pKa, ca. 3) is consistent with the results of the Kamlet-Abboud-Taft and Catalán et al. multiparametric approaches. Fluorescence quantum yield varies with the solvent, higher in water (ϕ(f) = 0.16) and lower in methanol and 1-propanol (approx. 0.02). Room temperature fluorescence lifetime in aqueous solution is (1.0 ± 0.2) ns, whereas the phosphorescence lifetime in glassy EtOH at 77 K and the corresponding quantum yield are (1.1 ± 0.1) s and 0.36, respectively. The lack of mirror image symmetry between modified absorption and fluorescence spectra reflects different nuclear configurations in the absorbing and emitting states. The low value measured for the fluorescence quantum yield is justified by an efficient nonradiative decay channel, related with the presence of an easily accessible conical intersection between the initially populated singlet bright (1)(L(a) ππ*) state and the ground state (gs/ππ*)(CI). Along the main decay path of the (1)(L(a) ππ*) state the system undergoes an internal conversion process that switches part of the population from the bright (1)(L(a) ππ*) to the dark (1)(L(b) ππ*) state, which is responsible for the fluorescence. Additionally, singlet-triplet crossing regions have been found, a fact that can explain the phosphorescent emission detected. An intersystem crossing region between the phosphorescent state (3)(L(a) ππ*) and the ground state has been characterized, which contributes to the nonradiative deactivation of the excitation energy.

Photochemical degradation of the plant growth regulator 2-(1-naphthyl) acetamide in aqueous solution upon UV irradiation.

The photochemical degradation of 2-(1-naphthyl) acetamide (NAD) in aqueous solution using simulated sunlight excitation as well as UV light within the 254-300 nm range was investigated to obtain an insight into the transformation mechanism that could occur under environmental conditions. Several photoproducts were identified using HPLC/MS/MS techniques. The degradation quantum yield was found to be independent of the excitation wavelength, but showed a dependence of oxygen concentration. This increased by a factor of approximately 3 from aerated to oxygen-free solutions. There is a clear involvement of both triplet and singlet excited states in NAD photoreactivity. The participation of singlet oxygen as a significant route in NAD degradation was ruled out by comparison with the behavior using Rose Bengal as a photosensitizer. A mechanistic pathway implying hydroxylation process through NAD radical cation species as well as an oxidation reaction by molecular oxygen is proposed. The photochemical behavior of NAD appears to mainly involve the aromatic moieties without any participation of the amide side chain. Toxicity tests clearly show that the generated primary photoproducts are responsible for a significant increase in the toxicity. However, upon prolonged irradiation this toxicity tends to decrease.

Simulated solar light phototransformation of organophosphorus azinphos methyl at the surface of clays and goethite.

The photochemical behavior of the pesticide azinphos methyl at the surface of clays (kaolinite, bentonite) and goethite was studied using Suntest setup (λ > 300 nm). The quantum yield on the clays was found to be roughly three times lower than that in aqueous solution. However, the photochemical efficiency was much higher at the surface of goethite owing to its photocatalytic activity through the hydroxyl radical production. The added humic substances on kaolonite show an inhibition of azinphos methyl degradation while the incorporation of iron(III) aquacomplexes leads to an important increase of the disappearance together with the formation of iron(II). Hydroxyl radical species were found to be formed either by excitation of goethite or clays. The goethite support acts as a more efficient catalyst for the formation of these reactive oxygen species. The photodecomposition reactions observed were (i) hydrolysis process leading to the formation of benzotriazone and the oxidation of the P = S bond giving rise to the formation of the oxon derivative, and (ii) homolytic cleavage of the N-C and C-S bonds of the organophosphorus bridge leading to the formation of dimers that appear to be specific to the irradiation at the surface of solid supports since they were not observed when the irradiation was performed in aqueous media: a statement that is related to the presence of aggregates at the surface of solid supports.

Assessing the occurrence of the dibromide radical (Br2⁻•) in natural waters: measures of triplet-sensitised formation, reactivity, and modelling.

The triplet state of anthraquinone-2-sulphonate (AQ2S) is able to oxidise bromide to Br(•)/Br(2)(-•), with rate constant (2-4)⋅10(9)M(-1)s(-1) that depends on the pH. Similar processes are expected to take place between bromide and the triplet states of naturally occurring chromophoric dissolved organic matter ((3)CDOM*). The brominating agent Br(2)(-•) could thus be formed in natural waters upon oxidation of bromide by both (•)OH and (3)CDOM*. Br(2)(-•) would be consumed by disproportionation into bromide and bromine, as well as upon reaction with nitrite and most notably with dissolved organic matter (DOM). By using the laser flash photolysis technique, and phenol as model organic molecule, a second-order reaction rate constant of ~3⋅10(2)L(mg C)(-1)s(-1) was measured between Br(2)(-•) and DOM. It was thus possible to model the formation and reactivity of Br(2)(-•) in natural waters, assessing the steady-state [Br(2)(-•)]≈10(-13)-10(-12)M. It is concluded that bromide oxidation by (3)CDOM* would be significant compared to oxidation by (•)OH. The (3)CDOM*-mediated process would prevail in DOM-rich and bromide-rich environments, the latter because elevated bromide would completely scavenge (•)OH. Under such conditions, (•)OH-assisted formation of Br(2)(-•) would be limited by the formation rate of the hydroxyl radical. In contrast, the formation rate of (3)CDOM* is much higher compared to that of (•)OH in most surface waters and would provide a large (3)CDOM* reservoir for bromide to react with. A further issue is that nitrite oxidation by Br(2)(-•) could be an important source of the nitrating agent (•)NO(2) in bromide-rich, nitrite-rich and DOM-poor environments. Such a process could possibly account for significant aromatic photonitration observed in irradiated seawater and in sunlit brackish lagoons.

Photochemical and photosensitised reactions involving 1-nitronaphthalene and nitrite in aqueous solution.

The excited triplet state of 1-nitronaphthalene, 1NN, ((3)1NN) is able to oxidise nitrite to ˙NO(2), with a second-order rate constant that varies from (3.56 ± 0.11) × 10(8) M(-1) s(-1) (µ±σ) at pH 2.0 to (3.36 ± 0.28) × 10(9) M(-1) s(-1) at pH 6.5. The polychromatic quantum yield of ˙NO(2) photogeneration by 1NN in neutral solution is Φ(˙NO(2))(1NN)≥ (5.7 ± 1.5) × 10(7)× [NO(2)(-)]/{(3.4 ± 0.3) × 10(9)× [NO(2)(-)] + 6.0 × 10(5)} in the wavelength interval of 300-440 nm. Irradiated 1NN is also able to produce ˙OH, with a polychromatic quantum yield Φ(˙OH)(1NN) = (3.42 ± 0.42) × 10(-4). In the presence of 1NN and NO(2)(-)/HNO(2) under irradiation, excited 1NN (probably its triplet state) would react with ˙NO(2) to yield two dinitronaphthalene isomers, 15DNN and 18DNN. The photonitration of 1NN is maximum around pH 3.5. At higher pH the formation rate of ˙NO(2) by photolysis of NO(2)(-)/HNO(2) would be lower, because the photolysis of nitrite is less efficient than that of HNO(2). At lower pH, the reaction between (3)1NN and ˙NO(2) is probably replaced by other processes (involving e.g.(3)1NN-H(+)) that do not yield the dinitronaphthalenes.

Stability of pesticides' residues under ultraviolet germicidal irradiation.

Legislation for food safety is limited mostly to pesticides monitoring and no attention is paid to the presence and toxicity of by-products formed after pesticide application. Stability studies of three selected transformation products: IMP - 2-isopropyl-6-methyl-4-pyrimidinol (diazinon hydrolysis product), TCP - 3,5,6-trichloro-2-pyridinol (chlorpyrifos transformation product), and 6CNA - 6-chloronicotinic acid (imidacloprid and acetamiprid transformation product) were performed under exposure to sunlight at room temperature (22 °C) and in the dark at 4 °C over 90 days. The results showed slight change in concentration with samples under refrigeration in darkness.Alternatively, an aqueous solution of TCP exposed to sunlight resulted in a high decrease of initial concentration within time. The toxicity assessment was performed using luminescent bacteria Vibrio fischeri and the results expressed low toxicity in the case of IMP and 6CNA. However, for TCP the calculated EC50 value after 30 minutes of exposure equated to roughly 15.1 mg L-1. Stability of the selected transformation products upon 245 nm irradiation indicated little decrease in concentration for IMP and 6CNA in deoxygenated and oxygenated aqueous solutions. In the case of TCP, the photochemical behaviour appeared to depend on oxygen concentration in the medium. For more detailed comparison, the degradation quantum yields were calculated.