André M. Braun

Singlet oxygen in surface waters — Part I: Furfuryl alcohol as a trapping agent

The photo-oxygenation of furfuryl alcohol (FFA) in water sensitized by naturally occurring dissolved organic materials under visible light is shown to proceed almost exclusively by a singlet oxygen mechanism. FFA does not physically quench1O2 and its β-value was determined to be(2.3±0.2) × 10−3M (k(1O2+FFA) = 1.2 × 108M−1s−1). It is thus recommended as a highly soluble, efficient trapping agent for1O2 determinations in natural waters.

Antioxidant activity of flavonoids: Efficiency of singlet oxygen (1Δg) quenching

Flavonoids, polyphenolic pigments widely present in plants, have been reported to act as scavengers of various oxidizing species. However, most often an overall antioxidant effect was measured. In this paper we report the results of a systematic study of the reactivity of 13 selected flavonoids (from the flavonol, flavone, flavanone and flavane families) with singlet oxygen (1O2(1 delta g)) in order to establish a structure-activity relationship. The rate constants of the chemical reaction of these flavonoids with 1O2(k r) and their rate constants of 1O2 physical quenching (kq) have been determined by kinetic measurements and near-IR singlet oxygen luminescence. The efficiency of the physical quenching is mainly controlled by the presence of a catechol moiety on ring B, whereas the structure of ring C (particularly the presence of a hydroxyl group activating the double bond) is the main factor determining the efficiency of the chemical reactivity of these compounds with 1O2. The total reactivity factor determining the efficiency of the chemical reactivity of these compounds with 1O2. The total reactivity scale is dominated by kq, which is in general higher than kr. (+)-Catechin is the most efficient quencher of the series.

Industrial waste water treatment: large scale development of a light-enhanced Fenton reaction

The feasibility of a large scale application of the light-enhanced Fenton reaction has been investigated for the treatment of a highly contaminated industrial waste water containing toxic aromatic amines (dimethyl anilines or xylidines) as the main pollutants. The Fenton reagent, a combination of hydrogen peroxide and a ferrous salt, is a potent oxidizing agent of organic compounds in acidic aqueous solution, and UV/visible irradiation may significantly enhance the degradation rates. Preliminary laboratory tests on the model compounds, 2,4- and 3,4-xylidine have been performed for selecting appropriate experimental conditions. Subsequent experimentation on the industrial waste water at a large scale level (3000 mg C l−1, 500 l) has been carried out using an experimental design methodology for the simulation and the evaluation of the effects of the two critical factors, hydrogen peroxide and ferrous ion concentrations. The results indicate that the light-enhanced Fenton reaction is a most effective treatment process under acidic conditions and is a realistic alternative to adsorption of xylidines on activated carbon as used at present.

Glossary of terms used in photocatalysis and radiation catalysis (IUPAC Recommendations 2011)

This glossary of terms covers phenomena considered under the very wide terms photocatalysis and radiation catalysis. A clear distinction is made between phenomena related to either photochemistry and photocatalysis or radiation chemistry and radiation catalysis. The term “radiation” is used here as embracing electromagnetic radiation of all wavelengths, but in general excluding fast-moving particles. Consistent definitions are given of terms in the areas mentioned above, as well as definitions of the most important parameters used for the quantitative description of the phenomena. Terms related to the up-scaling of photocatalytic processes for industrial applications have been included. This Glossary should be used together with the Glossary of terms used in photochemistry, 3rd edition, IUPAC Recommendations 2006: (doi:10.1351/pac200779030293) as well as with the IUPAC Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”, 2006– doi:10.1351/goldbook) because many terms used in photocatalysis are defined in these documents.

Singlet oxygen in surface waters — Part II: Quantum yields of its production by some natural humic materials as a function of wavelength

Quantum yields for production of singlet oxygen for some natural dissolved organic materials (DOM) in water showed a decrease with increasing wavelength, indicating that calculations assuming a constant quantum yield measured at a short wavelength can overestimate the1O2 concentration in sunlit surface waters by more than a factor of 2. Quantum yields ranged from 0.005 to 0.026; they correspond to steady state surface1O2 concentrations in midday, Swiss, spring or fall sunlight (800 W/m2) of(1–5)×10−14M per mg/L DOM and halflives of reactive organics of a few days to a few hours.

Fluorescence of pterin, 6-formylpterin, 6-carboxypterin and folic acid in aqueous solution: pH effects

Steady-state and time-resolved studies have been performed on four compounds of the pterin family (pterin, 6-carboxypterin, 6-formylpterin and folic acid) in aqueous solution, using the single photon counting technique. The fluorescence characteristics (spectra, quantum yields, lifetimes) of these compounds and their dependence on the pH have been investigated. Most pterins can exist in two acid-base forms over the pH range between 3 and 13. Emission spectra and excitation spectra were obtained for both forms of each compound studied. Fluorescence quantum yields (phi(F)) in acidic and basic media were measured. The phi(F) of folic acid (< 0.005 in both media) is very low compared to those of pterin (0.27 in basic media and 0.33 in acidic media), 6-carboxypterin (0.18 in basic media and 0.28 in acidic media) and 6-formylpterin (0.07 in basic media and 0.12 in acidic media). The variation in integrated fluorescence intensity and fluorescence lifetimes (tau(F)) was analysed as a function of pH. Dynamic quenching by OH- was observed and the corresponding bimolecular rate constants for quenching of fluorescence (k(q)) were calculated. The reported values for k(q) (M(-1) s(-1)) are 3.6 x 10(9), 1.9 x 10(9) and 1.1 x 10(10) M(-1) s(-1) for pterin, 6-carboxypterin and 6-formylpterin, respectively.

VOC photodegradation at the gas–solid interface of a TiO2 photocatalyst: Part I: 1-butanol and 1-butylamine

Abstract The gas–solid heterogeneous photocatalytic oxidation of 1-butanol and 1-butylamine in air was investigated using supported TiO2 as a catalyst. The supported catalyst was prepared using a sol–gel method and irradiated employing two different light sources, a medium pressure mercury lamp or a xenon-chloride excimer lamp. The experimental set-up was especially designed for generating a gas stream containing stable and defined concentrations of the model pollutants. The gas stream at the reactor exit was analyzed on line by gas chromatography and the structures of the intermediates were established by gas chromatography coupled with mass spectrometry. Six major intermediates (butanal, butanoic acid, 1-propanol, propanal, ethanol and ethanal) were identified in the case of the photocatalytic degradation of 1-butanol. 1-Butylamine was less efficiently adsorbed on the catalyst and its degradation was slower. Three intermediates could be identified in this case (N-butylidene-1-butylamine, N-ethylidene-1-butylamine and N–butylformamide). Based on these results, a degradation mechanism is proposed for both compounds. Mineralization could be achieved under various conditions of concentrations and flow rates and was confirmed by infrared spectroscopy.

Laser flash photolysis of dissolved aquatic humic material and the sensitized production of singlet oxygen

Aqueous solutions of isolated humic substances were investigated by laser flash photolysis. Kinetics of the decay of excited states were measured in emission and absorption. From the absorption data at least two transients with different lifetimes (order of and s) could be deduced. Quenching effects were determined for oxygen and Ni(II), Co(II), and Cd(I1). The quenching efficiency of oxygen is similar to the efficiency of the paramagnetic metal ions. The quantum yield of the singlet oxygen production sensitized by humics was determined to be 1-370 with the furfuryl alcohol method. The results are discussed with respect to the photochemistry of aquatic ecosys tems.

Lanthanide oxide-doped titanium dioxide : Effective photocatalysts for the degradation of organic pollutants

A series of lanthanide oxide-doped titanium dioxide photocatalysts Ln2O3/TiO2 (Ln3+ = Eu3+, Pr3+, Yb3+) were prepared. The photocatalysts reveal a substantially enhanced activity for the degradation of organic pollutants, as compared to undoped TiO2. The photodegradation processes of p-nitrobenzoic acid, (1), p-chlorophenoxyacetic acid, (2), aniline, (3), salicylic acid, (4), and trans-cinnamic acid, (5), with the different photocatalysts was examined. The photodegradation of (1)–(5) is significantly faster with Ln2O3/TiO2 photocatalysts and leads to complete mineralization of the organic compounds. The high activity of the Ln2O3/TiO2 photocatalysts is attributed to the enhanced association of the functionalized organic pollutants to lanthanide-ion surface sites.

Singlet oxygen (1Δg) production by pterin derivatives in aqueous solutions

Six compounds of the pterin family (pterin, 6-carboxypterin, 6-formylpterin, folic acid, biopterin and neopterin) have been investigated for their efficiencies of singlet oxygen (O2(1Δg)) production and quenching in aqueous solutions. The quantum yields of 1O2 production (ΦΔ) have been determined by measurements of the 1O2 luminescence in the near-infrared (1270 nm) upon continuous excitation of the sensitizer. Under our experimental conditions, all studied compounds (except folic acid) are relatively efficient 1O2 sensitizers with ΦΔ values of up to 0.47. Results show that the nature of the substituent at position 6 on the pterin moiety, as well as the pH, affect considerably the capacity of pterins to produce 1O2. All compounds investigated are efficient 1O2 quenchers: the rate constant of 1O2 total quenching (kt) by folic acid (3.0(± 0.3) × 107 M−1 s−1) is one order of magnitude larger than those for the other pterin derivatives investigated (1.4(± 0.1) × 106 M−1 s−1to 2.9(± 0.3) × 106 M−1 s−1).