Valter Maurino

Photochemical reactions in the tropospheric aqueous phase and on particulate matter

This paper is a tutorial review in the field of atmospheric chemistry. It describes some recent developments in tropospheric photochemistry in the aqueous phase and on particulate matter. The main focus is regarding the transformation processes that photochemical reactions induce on organic compounds. The relevant reactions can take place both on the surface of dispersed particles and within liquid droplets (e.g. cloud, fog, mist, dew). Direct and sensitised photolysis and the photogeneration of radical species are the main processes involved. Direct photolysis can be very important in the transformation of particle-adsorbed compounds. The significance of direct photolysis depends on the substrate under consideration and on the colour of the particle: dark carbonaceous material shields light, therefore protecting the adsorbed molecules from photodegradation, while a much lower protection is afforded for the light-shaded mineral fraction of particulate. Particulate matter is also rich in photosensitisers (e.g. quinones and aromatic carbonyls), partially derived from PAH photodegradation. These compounds can induce degradation of other molecules upon radiation absorption. Interestingly, substrates such as methoxyphenols, major constituents of wood-smoke aerosol, can also enhance the degradation of some sensitisers. Photosensitised processes in the tropospheric aqueous phase have been much less studied: it will be interesting to assess the photochemical properties of Humic-Like Substances (HULIS) that are major components of liquid droplets. The main photochemical sources of reactive radical species in aqueous solution and on particulate matter are hydrogen peroxide, nitrate, nitrite, and Fe(iii) compounds and oxides. The photogeneration of hydroxyl radicals can be important in polluted areas, while their transfer from the gas phase and dark generation are usually prevailing on an average continental scale. The reactions involving hydroxyl radicals can induce very fast transformation of compounds reacting with (*)OH at a diffusion-controlled rate (10(10) M(-1) s(-1)), with time scales of an hour or less. The hydroxyl-induced reactivity in solution can be faster than in the gas phase, influencing the degradation kinetics of water-soluble compounds. Moreover, photochemical processes in fog and cloudwater can be important sources of secondary pollutants such as nitro-, nitroso-, and chloro-derivatives.

Indirect Photochemistry in Sunlit Surface Waters: Photoinduced Production of Reactive Transient Species

This paper gives an overview of the main reactive transient species that are produced in surface waters by sunlight illumination of photoactive molecules (photosensitizers), such as nitrate, nitrite, and chromophoric dissolved organic matter (CDOM). The main transients (˙OH, CO3(-˙) , (1)O2, and CDOM triplet states) are involved in the indirect phototransformation of a very wide range of persistent organic pollutants in surface waters.

The role of colloidal particles in the photodegradation of organic compounds of environmental concern in aquatic systems

Abstract Aquatic components which can be active in the photochemically induced transformation of organic compounds include colloidal and sedimentary oxides and Sulfides, and organic colloids, either xenobiotic or naturally occurring. Particular attention is devoted to the degradation processes taking place at the semiconductor/water interface. The possible application of these photocatalytic degradation processes to water purification is outlined.

The fate of organic nitrogen under photocatalytic conditions: degradation of nitrophenols and aminophenols on irradiated TiO2

The photocatalytic transformation of 2-, 3- and 4-nitrophenol (NP) and 2-, 3- and 4-aminophenol (AP) over irradiated titanium dioxide was investigated in aerated aqueous solution. At pH < 6, extensive mineralization (more than 90%) of the substituted phenols occurs, whereas, at pH 11, organic carbon is persistently observed. Photocatalysis converts 80% of the nitro group in nitrophenols to NO3− through the intermediate formation of NO2− ions; the remaining 20% is transformed into NH4+ ions. This implies that, even in aerated solutions, reductive pathways are significant, as confirmed by the detection of aminophenol. By contrast, photocatalytic degradation of aminophenols converts the NH2 group into about 60%–70% NH4+ ions and only about 10% NO3−, when about 20% of the organic carbon remaining. A slow transformation of the ammonium ions into NO3− ions is observed at longer irradiation times.

Effect of Fluorination on the Surface Properties of Titania P25 Powder: An FTIR Study

A study was carried out on the consequences of the -OH(surf)/F(-) exchange occurring at the surface of TiO(2) P25 when suspended in HF/F(-) solutions. The maximum extent of fluorination was reached at pH 3.0, resulting in the fixation on the surface of ca. 2.5 F(-)/nm(2). The surface features of fluorinated samples under two selected conditions were investigated by IR spectroscopy, in comparison with pristine TiO(2). The collected data suggested that bridged -OH(surf), likely located on regular facets, was more resistant to exchange with F(-). Combined high resolution transmission electron microscopy (HRTEM), inductively coupled plasma mass spectrometry (ICP-MS) and IR measurements indicated that the fluorination performed in the adopted condition did not induce any etching of TiO(2) particles, and the -OH(surf)/F(-) exchange appeared reversible by treatment in concentrated basic solutions. Furthermore, fluorination resulted in an increase of the Lewis acid strength of surface Ti(4+) sites, which, as a consequence, retained adsorbed water molecules even after outgassing at 423 K. Such an effect involved the overwhelming majority of cations exposed on regular facets.

Photochemical Fate of Carbamazepine in Surface Freshwaters: Laboratory Measures and Modeling

It is shown here that carbamazepine (CBZ) would undergo direct photolysis and reaction with (•)OH as the main phototransformation pathways in surface waters. Environmental lifetimes are expected to vary from a few weeks to several months, and predictions are in good agreement with available field data. Acridine (I) and 10,11-dihydro-10,11-trans-dihydroxy-CBZ (V) are the main quantified phototransformation intermediates upon direct photolysis and (•)OH reaction, respectively. The photochemical yield of mutagenic I from CBZ is in the 3-3.5% range, and it is similar for both direct photolysis and (•)OH reaction: it would undergo limited variation with environmental conditions. In contrast, the yield of V would vary in the 4-8.5% range depending on the conditions, because V is formed from CBZ by (•)OH (9.0% yield) more effectively than upon direct photolysis (1.4% yield). Other important photointermediates, mostly formed from CBZ upon (•)OH reaction, are an aromatic-ring-dihydroxylated CBZ (VI) and N,N-bis(2-carboxyphenyl)urea (VII). Compounds VI and VII are formed by photochemistry and are not reported as human metabolites; thus, they could be used as tracers of CBZ phototransformation in surface waters. Interestingly, VI has recently been detected in river water.

Light-assisted 1,4-dioxane degradation

Abstract The oxidative degradation induced by light of 1,4dioxane been investigated in homogeneous solution in the presence of inorganic peroxides (either hydrogen peroxide or peroxydisulfate) and under heterogeneous photocatalytic conditions in the presence of titanium dioxide. The effect of photon energy spectral distribution, pH, O2 and scavengers has been examined. The degradation mechanism involves common intermediates. Ethylene glycol diformate was observed as the main intermediate. In the presence of UV light peroxydisulfate showed high degradation efficiency. A tentative chain radical oxidation mechanism is postulated. The distinctive features of heterogeneous photocatalytic treatment were the good efficiency under simulated solar spectrum and the absence of inhibition of the degradation rate by HCO3−. The presence of peroxydisulfate enhances the TiO2 photocatalytic degradation rate.

Modelling the photochemical fate of ibuprofen in surface waters.

We show that the main photochemical processes involved in the phototransformation of anionic ibuprofen (IBP) in surface waters are the reaction with (•)OH, the direct photolysis and possibly the reaction with the triplet states of chromophoric dissolved organic matter ((3)CDOM). These conclusions were derived by use of a model of surface water photochemistry, which adopted measured parameters of photochemical reactivity as input data. The relevant parameters are the polychromatic UVB photolysis quantum yield (Φ(IBP) = 0.33 ± 0.05, μ±σ), the reaction rate constant with (•)OH (k(IBP,•OH)=(1.0 ± 0.3)⋅10(10) M(-1) s(-1)), the (1)O(2) rate constant (k(IBP,)( ¹O₂)= (6.0 ± 0.6)⋅10(4) M(-1) s(-1)), while the reaction with CO(3)(-•) can be neglected. We adopted anthraquinone-2-sulphonate (AQ2S) and riboflavin (Ri) as CDOM proxies and the reaction rate constants with the respective triplet states were k(IBP,3AQ2S)=(9.7 ± 0.2)⋅10(9) M(-1) s(-1) and k(IBP,3Ri) = 4.5⋅10(7) M(-1) s(-1). The reaction with (3)CDOM can be an important IBP sink if its rate constant is comparable to that of (3)AQ2S, while it is unimportant if the rate constant is similar to the (3)Ri* one. The photochemical pathways mainly lead to the transformation (oxidation and/or shortening) of the propanoic lateral chain of IBP, which appears to be significantly more reactive than the isobutyl one. Interestingly, none of the detected intermediates was produced by substitution on the aromatic ring.

Inhibition vs. enhancement of the nitrate-induced phototransformation of organic substrates by the •OH scavengers bicarbonate and carbonate

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.

Photocatalytic degradation of bentazon by TiO2 particles

The degradation of 3-isopropy1–2,1,3-benzothiadiazin-4-one-2,2-dioxide, the herbicide bentazon, by AMI simulated sunlight has been investigated in the presence of aqueous suspensions of TiO2. The half-life of the degradation process is ca. 30 minutes at pH=2 and ca. 4 minutes at pH=7 in the presence of 0.5 g L−1 of TiO2. The near quantitative recovery of sulfate ions and CO2 as well as the variation with time of dissolved and adsorbed organic carbon suggest that the process leads to complete mineralization of the herbicide. The process is extremely efficient even at very low concentration: in fact irradiation of 20 ppb of bentazon in the presence of 50 ppm TiO2 allows to recover < 0.1 ppb after 10 minutes.