Contrary to common expectations, the hydroxyl scavengers, carbonate and bicarbonate, are able to enhance the phototransformation by nitrate of a number of substituted phenols. Carbonate and bicarbonate, in addition to modifying the solution pH, are also able to induce a considerable formation of the carbonate radicals upon nitrate photolysis. The higher availability of less-reactive species than the hydroxyl radical would contribute to substantially enhance the photodegradation of the phenols/phenolates that are sufficiently reactive toward the carbonate radical. This phenomenon has a potentially important impact on the fate of the relevant compounds in surface waters. In contrast, the degradation of compounds that are not sufficiently reactive toward CO(3)(-*) is inhibited by carbonate and bicarbonate because of the scavenging of *OH.

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This works shows that the addition of phenol and 2-propanol as model organic compounds significantly decreases the direct photolysis quantum yield of 4-chloro-2-methylphenoxyacetic acid (MCPA) upon UVB irradiation in aqueous solution. Laser flash photolysis data suggest that 2-propanol is able to decrease the formation of the MCPA excited states under irradiation. A decrease from 0.54 to 0.34 of the photolysis quantum yield of the anionic form of MCPA (which prevails over the undissociated one in surface waters) could have a considerable impact on the MCPA lifetime in ecosystems where the direct photolysis is the main phototransformation pathway. In surface water bodies where the direct photolysis has comparable kinetics as the reaction with • OH, a decrease of the quantum yield would enhance the relative importance of the • OH pathway, which yields considerably less toxic intermediates than the direct photolysis.

Bicarbonate enhances the transformation of phenol upon irradiation of hematite, and phenol nitration upon irradiation of both nitrate and nitrite. Hematite under irradiation is able to oxidise the carbonate ion to the CO3-. radical, which in turn oxidises phenol to the phenoxyl radical faster compared to the direct photo-oxidation of phenol by hematite. The formation of CO3-. from hematite and carbonate under irradiation is supported by the detection of 3,3'-dityrosine from tyrosine, added as a probe for CO3-.. It is shown that Fe(III) might be an important photochemical source of CO3-. in Fe-rich waters, e.g. waters that contain more than 1 mg L(-1) Fe. The enhancement by bicarbonate of phenol nitration upon nitrate irradiation is probably accounted for by an increased photogeneration rate of nitrogen dioxide. The process could lead to enhanced phenol photonitration by nitrate in waters rich of inorganic carbon (>10 mM bicarbonate). Bicarbonate also increases the transformation and nitration rates of phenol upon nitrite photolysis. The effect is due to the combination of basification that enhances phenol nitrosation and nitration, and of peculiar bicarbonate chemistry. It is shown that bicarbonate-enhanced phenol nitration upon nitrite photolysis could be a significant photonitration pathway, leading to the generation of toxic nitrated compounds in natural waters in which the scavenging of hydroxyl radicals by nitrite is competitive with that of Dissolved Organic Matter (DOM).

Photobromination of phenol takes place upon UV/Vis irradiation of FeIII and bromide under acidic conditions, and most likely involves the brominating agent Br2(-*). Bromination is also observed in the presence of nitrate and bromide under UV irradiation, most likely involving Br2(-*) formed upon oxidation of bromide by *OH. Moreover, quantitative bromination of phenol is observed in the dark in the presence of hydrogen peroxide and bromide. This process is strongly favored under acidic conditions, but a residual, pH-independent bromination pathway is also present. The rates and yields of bromination (up to 100%) are considerably higher than those reported for chlorination under comparable conditions, suggesting that the higher activity of bromine species could compensate for the lower concentration of bromide ions in aerosol compared to chlorides. The reported processes are potent tial sources of reactive bromine species (Br2(-*), HBrO) and aromatic bromo derivatives in atmospheric aerosols, in particular after the acidification process linked with aerosol aging.

Here we show that the photolysis of FeCl 2+ upon UV irradiation of Fe(III) at pH 0.5, yielding Cl • and then Cl 2 -• , upon further reaction with Cl -, induces phenol degradation. The photolysis of FeCl 2+ can be highlighted and studied as the huge interference by FeOH 2+ can be avoided under such conditions. Our data allowed the assessment of a photolysis quantum yield for FeCl 2+ of 5.8 · 10 -4 under UVA irradiation, much lower compared to the literature value of 0.5. The discrepancy can be explained if the photolysis process is efficient but photoformed Fe 2+ and Cl • undergo recombination inside the solvent cage.

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: The potential of Ñy ash procured form coal-Ðred thermal power plants was studied as a heterogeneous catalyst in the oxidation of aqueous sodium sulÐde solutions with hydrogen peroxide in the temperature range of 303È323 K. The e †ects of various parameters (source of Ñy ash, Ñy ash loading, initial concentrations of sodium sulÐde and hydrogen peroxide, electrolyte and deactivation of catalytic e †ect of Ñy ash) were studied. For an initial sodium sulÐde and hydrogen peroxide concentration of 26É98 ] 10~2 kmol m~3 and 24É28 ] 10~2 kmol m~3 respectively, only 4% (w/v) Ñy ash loading intensiÐed the rate of oxidation by a factor of 4É52 over that without Ñy ash at 303 K. The deactivation of the catalytic e †ect of Ñy ash was found to be less than 20% even after six repeated uses. The kinetics of aqueous phase decomposition of hydrogen peroxide was also studied in the presence of Ñy ash in alkaline medium.