Claudio Minero

Photocatalytic degradation of phenol in aqueous titanium dioxide dispersions

The results of a study of photocatalytic degradation of phenol using aqueous oxygenated TiO2 (anatase) suspensions in a batch Pyrex photoreactor are reported. The influence on the photodegradation rate of various parameters as pH, phenol and TiO2 content, oxygen partial pressure, anions present in the dispersions was investigated. A complete oxidation of phenol was observed. Intermediate compounds, catechol and quinone, were detected. It was observed that the photodegradation also proceeded with sunlight radiation. A mechanistic and kinetic model, which accounts for the results obtained, is given. Likely reasons for inactivity of the rutile modification for this reaction are also given.

Chemical degradation of chlorophenols with Fenton's reagent (Fe2+ + H2O2)

Aqueous solutions of Fentons reagent (Fe2+ + H2O2) have been used to effect the total decomposition of the chlorophenols: 2-chlorophenol, 3-chlorophenol, 4-chlorophenol, 3,4-dichlorophenol and 2,4,5-trichlorophenol. The mineralization of these chlorinated aromatic substrates to CO2 and free Cl− has been studied as a function of [Fe2+] and [HClO4]. Increasing the concentration of Fe2+ enhances the decomposition process, while an increase in the concentration of HClO4, inhibits the reaction. The presence of Fe3+ alone (without any Fe2+) with H2O2 has no effect on the degradation of the chlorophenols. In all cases, the stoichiometric quantity of free Cl− was obtained at the completion of the decomposition reaction; but the rates of disappaearance of the chlorophenol and of the formation of the Cl− are not similar. This suggests that some chlorinated aliphatic species may be formed as possible intemediates.

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.

Photodegradation of dichlorophenols and trichlorophenols in TiO2 aqueous suspensions: kinetic effects of the positions of the Cl atoms and identification of the intermediates

The photocatalytic degradation of six dichlorophenols (DCPs) and three trichlorophenols (TCPs) in TiO2 aqueous suspensions was studied. The apparent first-order rate constants kapp of disappearance of these CPs and monochlorophenols (MCPs) were found to correlate with their Hammett constants λ and their 1-octanol-water partition coefficients Kow: kaw (h−1)= −10σ+5.2 log Kow−7.5 under the conditions used (correlation coefficient, 0.987, if 2,4,6-TCP was omitted). The aromatic intermediates identified by their UV spectra (high performance liquid chromatography (HPLC) separation) and mass spectra (gas chromatography (GC) separation) corresponded to the hydroxylation of the aromatic nucleus with the following order of reactivity para> ortho>meta. Para-benzoquinones with 0–2 Cl atoms and 0 or 1 OH group and intermediates with two aromatic or quinone rings were also detected. The temporal variations of some intermediates were determined. All of these compounds, in which the C6 ring subsisted, were very unstable, so that their maximum concentration was very low compared with the initial CP concentration. The release of chloride and the evolution of CO2 also followed apparent first- order kinetics within the first part of the degradation. The effects of the number and position of Cl atoms on these fkinetics are discussed.

Large solar plant photocatalytic water decontamination: Degradation of pentachlorophenol

Abstract The photocatalytic degradation of pentachlorophenol in the large solar plant built at the Plataforma Solar de Almeria is presented. Degradation of cubic meters per hour of pentachlorophenol at a concentration of mg L−1 is demonstrated and is easily achieved in short residence times. Details of the plant based on parabolic solar concentrators, together with the examination of the main factors affecting total efficiency are discussed in comparison with small scale laboratory experiments. Experiments carried out with addition of peroxydisulphate show a noticeable increment in the degradation efficiency and are suggested for the treatment of concentrated or refractory pollutants.

Kinetic analysis of photoinduced reactions at the water semiconductor interface

Abstract A reduced kinetic model for the initial steps of the photocatalytic process is presented with the aim to analytically solve the resulting kinetic system. Several possible kinetic models have been explored. Attention was paid to obtain equations with physical meaning and reduced complexity. An analytical equation is obtained for the rate and the quantum yield, which retains the principal features of the photocatalytic process, namely the light induced charge separation and recombination, the oxidative and reductive electron transfers, the formation of a stable oxidized intermediate, and, unlike the previous kinetic models, also the back reaction of the oxidized substrate. Compared to the two-parameter Langmuir–Hinshelwood equation, all the previous features and the dependence on the light intensity are described with only three parameters, which collect all the kinetic constants, and account for experimental concentrations of substrate and electron scavenger, light intensity and catalytic system characteristics. The kinetic behavior of photocatalytic systems under all the possible values of experimental parameters can be graphically presented. The analysis of the obtained rate equation shows that the best utilization of photons is attainable at low light intensities, suggesting that preconcentration of solar light is unnecessary.

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.

Large solar plant photocatalytic water decontamination: Effect of operational parameters

Abstract The effect of the concentration of the photocatalyst, the organic substrate and the light intensity is presented for the photocatalytic degradation of pentachlorophenol in the large solar plant built at the Plataforma Solar de Almeria. The degradation of pentachlorophenol is easily achieved in short residence times. The main factors affecting the total efficiency are discussed in view of a simple model for the primary events in the photocatalytic system. The derived approximated equations are able to fit the experimental data well. The optimization of the degradation and mineralization rates is strongly dependent on the organic substrate and predominantly depends on kinetic factors. The partition or competition of the substrate and its degradation products on the surface of the photocatalyst seems unimportant.

Large solar plant photocatalytic water decontamination: Degradation of atrazine

Photocatalytic degradation of atrazine was successfully and efficiently carried out in a large modular flow trough system under solar light. The degradation leads to complete mineralization of the alkyl substituents of s-triazine, giving trihydroxy-s-triazine as a final product. An efficient detoxification can be attained. The presence of 1 × 10−3m of peroxydisulphate, kept constant during the treatment, is sufficient to remove 98% of atrazine with very short residence times (less than 6 min), and to convert the parent triazine to more than 90% of trihydroxy-s-triazine in less than 2 h. The peroxydisulphate is particularly advantageous when large amounts of polluted ground water have to be treated. The factors affecting the total efficiency are discussed in view of a simple model for the primary events in the photocatalytic system.

Sonochemical oxidation of phenol and three of its intermediate products in aqueous media : catechol, hydroquinone, and benzoquinone. Kinetic and mechanistic aspects

The sonochemical oxidation of phenol has been examined in airequilibrated aqueous media at various pH’s and at various insonation powers. Its disappearance follows zero-order kinetics at [phenol]initial ~ 30 to 70 μM Three principal intermediate species formed at pH 3: catechol (CC), hydroquinone (HQ), and p-benzoquinone (BQ); at natural pH (5.4–5.7) only catechol and hydroquinone formed. No intermediate species were detected at pH 12 under the conditions used. The sonochemical fate of CC, HQ, and BQ was also examined at pH 3 and at natural pH’s. At pH 3, BQ is the major species formed during insonation of HQ, while HQ is produced during insonation of BQ. In both cases, an additional intermediate formed in trace quantities that is identified as hydroxy-p-benzoquinone. These same intermediate species have been identified in the heterogeneous photocatalyzed oxidation of phenol in irradiated titania suspensions. The present results confirm the important role of ’OH radicals in degradation processes. Although CO2 is the ultimate product in heterogenous photocatalysis, irradiation of a phenolic aqueous solution by ultrasounds showed no loss of total organic carbon (TOC) after several hours, even though the aromatic substrate and the intermediates had degraded. A simple kinetic model/scheme is described to account for the events in the conversion of the substrates to products. It is concluded that the hydrophobic benzoquinone reacts with ¹OH and H¹ radicals at the hydrophobic gas bubble/liquid interface, while the hydrophilic species (phenol, CC, and HQ) react, to a large extent, with the ¹ OH radicals in the solution bulk.