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PubMed2005

Oxidation of acetovanillone by photochemical processes and hydroxyl radicals.

Acetovanillone [Ethanone, 1-(4-hydroxy-3-metoxyphenyl)] is one of the major pollutants that is present in the wastewater produced during the boiling of raw material in the cork industry. The oxidation of its aqueous solutions by monochromatic UV radiation alone and combined with hydrogen peroxide, Fenton's reagent and the photo-Fenton system has been investigated. In the single UV radiation process, the apparent rate constants and the quantum yields are determined, and in the UV/H2O2 combination, the additional efficiency in the oxidation process due to the presence of hydrogen peroxide is established. The influence of some operating variables, such as initial concentrations of H2O2 and Fe(II), as well as the pH, is discussed in the Fenton and photo-Fenton systems, and the partial contribution of the radical pathways to the global oxidation rates are evaluated. The rate constant for the reaction of acetovanillone with hydroxyl radicals is also determined by means of a competition kinetics model, its value being 5.62 x 10(9) M(-1)s(-1). Finally, chemical oxidation experiments of wastewaters generated in this industry were carried out by using the same advanced oxidation processes. Specifically, the elimination of acetovanillone in these effluents was determined, and the removal of the global organic pollutant content was also evaluated.

Acetophenones↗
PubMed2005

Probing transient molecular structures in photochemical processes using laser-initiated time-resolved X-ray absorption spectroscopy.

Molecular structures during chemical processes are crucial for predicting molecular reactivity and reaction mechanisms. Using a laser pulse as an internal clock for starting fundamental chemical processes, molecular structural dynamics can be characterized by coherent vibrational motions and by incoherent transitions between different intermediate states. Recent developments in pulsed X-ray facilities allow structural determination of discrete excited states and reaction intermediates using laser-initiated time-resolved X-ray absorption spectroscopy (LITR-XAS). Moreover, femtosecond X-ray sources have begun making significant contributions in monitoring coherent molecular motions. This review summarizes recent developments in the field, including technical and scientific challenges as well as several examples involving excited state molecular structure and electronic configuration determinations. Future applications of this technique with high time resolution will enable visualization of fundamental chemical events in many systems and further our understanding in photochemistry.

Absorptiometry, Photon↗
PubMed2000

A laser flash photolysis and pulse radiolysis study of primary photochemical processes of flumequine.

The 355 nm laser flash photolysis of argon-saturated pH 8 phosphate buffer solutions of the fluoroquinolone antibiotic flumequine produces a transient triplet state with a maximum absorbance at 575 nm where the molar absorptivity is 14,000 M(-1) cm(-1). The quantum yield of triplet formation is 0.9. The transient triplet state is quenched by various Type-1 photodynamic substrates such as tryptophan (TrpH), tyrosine, N-acetylcysteine and 2-deoxyguanosine leading to the formation of the semireduced flumequine species. This semireduced form has been readily identified by pulse radiolysis of argon-saturated pH 8 buffered aqueous solutions by reaction of the hydrated electrons and the CO2*- radicals with flumequine. The absorption maximum of the transient semireduced species is found at 570 nm with a molar absorptivity of 2,500 M(-1) cm(-1). In argon-saturated buffered solutions, the semireduced flumequine species formed by the reaction of the flumequine triplet with TrpH stoichiometrically reduces ferricytochrome C (Cyt Fe3+) under steady state irradiation with ultraviolet-A light. In the presence of oxygen, O2*- is formed but the photoreduction of Cyt Fe3+ by O2*- competes with an oxidizing pathway which involves photo-oxidation products of TrpH.

Anti-Infective Agents↗
PubMed2004

[Preliminary study on HOx photochemical processes in urban atmosphere of Guangzhou City].

Simultaneous measurements of atmospheric hydroxyl radicals (OH) and other pollutants in Guangzhou city were carried out in July, 2000. The quantitative analysis of HOx reaction cycle during daytime in summer was made. The calculation results indicated that the total production rates of OH and HO2 were about 4.5 x 10(8) OH/(cm3 x s) and 3.8 x 10(8) HO2/(cm3 x s), respectively. The primary OH source in urban atmosphere was the photolysis of HONO, while the main OH sinks were the reactions of OH with VOCs, HCHO, NO2 and CO. The HOx chemistry in urban atmosphere is quite different from that in the remote clean atmosphere.

Air Pollutants↗
PubMedDate not supplied

[Control of the stability of conditions in photochemical processing of radiographic films. Ministry of Health and Medical Industry of the Russian Federation].

To develop general requirements for evaluating and controlling the operating parameters of X-ray equipment at the units (rooms) of X-ray diagnosis, the International Electric Engineering Commission (IEEC) has published a number of guidelines concerning control methods. Of the most importance are the IEEC Guidelines 1223-2-1-93, 1223-2-2-93, and 1223-2-3-93 for controlling the premises and technical equipment of X-ray photo laboratories, which have translated into Russian, adjusted to our conditions, approved by the Ministry of Health and Medical Industry of the Russian Federation and published as 3 guidelines P 42-502-95, P 42-503-95, P 42-504-95 to be introduced on August 1, 1995. The publication totally reflects the main points of the guidelines, which enables them to be used under practical conditions.

Humans↗
PubMed2006

Dynamics of vibrational overtone excitations of H2SO4, H2SO4-H2O: hydrogen-hopping and photodissociation processes.

Photochemical processes of sulfuric acid (H2SO4) and sulfuric acid monohydrate (H2SO4-H2O) following overtone excitation of the OH stretching mode are studied by classical trajectory simulations using the semiempirical PM3 potential suface in "on the fly" calculations. The main results are the following: (1) In the excitation of H2SO4 to the fifth OH-stretch overtone, hopping of the H atom between oxygen atoms is found to take place in 22% of the trajectories, only once during simulations of 400 ps. (2) All the trajectories for H2SO4 show a rapid cis-trans isomerization. (3) The photolysis of H2SO4 into SO3 + H2O takes place in 5% of the trajectories on a time scale of approximately 9 ps. (4) Only low overtone levels of H2SO4-H2O have sufficiently long lifetimes to be spectroscopically relevant. Excitation to these OH stretching overtones is found to result in the dissociation of the cluster. H hopping or dissociation of H2SO4 does not take place.

Journal Article↗
PubMed2004

Decolorizing of lignin wastewater using the photochemical UV/TiO2 process.

Studies on applying the photochemical UV/TiO2 oxidation process to treat the lignin-containing wastewater for dissolved organic carbon (DOC), color and reducing A254 (the absorption at the wavelength of 254 nm) have been carried out. The data obtained in this study demonstrate that the UV/TiO2 process is effective in oxidizing the lignin thus reducing the color and DOC of the wastewater treated. The combined UV/TiO2 treatment can achieve better removal of DOC and color than the UV treatment alone. Color removal, based on American Dye Manufacture Index (ADMI) measurement, is greater than 99% if the pH is maintained at 3.0 with the addition of 1 g l(-1) TiO2. When 10 g l(-1) TiO2 is applied, the oxidation reduction potential (ORP) value is reached to result in an 88% removal of both DOC and color. A model was developed based on the variation of ORP during the photochemical reaction to simulate the decoloring process. The proposed model can be used to predict the color removal efficiency of the UV/TiO2 process.

Color↗
PubMed2006

Evaluation of chemical and photochemical oxidation processes for degradation of phosmet on lowbush blueberries (Vaccinium angustifolium).

Chemical and photochemical oxidation processes were evaluated for their ability to degrade residual phosmet on lowbush blueberries and for their role in the conversion of phosmet to phosmet oxon--a toxic metabolite of phosmet. Chemical processes included 1 ppm of aqueous ozone, 1% hydrogen peroxide, 100 ppm of chlorine, and UV, whereas photochemical processes included hydrogen peroxide/UV, chlorine/UV, and ozone/hydrogen peroxide/UV. Phosmet applied as Imidan 2.5EC under laboratory conditions resulted in a mean residual concentration of 44.4 ppm, which was significantly degraded (p < 0.05) by ozone and chlorine, yielding reductions of 57.7 and 46%, respectively. Interaction between phosmet (Imidan 2.5EC) and any chemical or photochemical treatment did not result in conversion to phosmet oxon. Residual analysis of commercially grown blueberries revealed mean phosmet (Imidan 70W) levels of 10.65 ppm and phosmet oxon levels of 12.49 ppm. Treatment of commercial blueberries resulted in significant reductions in phosmet regardless of treatment type; however, only UV, hydrogen peroxide/UV, and ozone treatments degraded phosmet (Imidan 70W) to less toxic metabolites and reduced phosmet oxon levels. Treatment-induced conversion of phosmet to phosmet oxon was noticeably influenced by variations between phosmet formulations. Acceleration of photochemical degradation by UV was not observed. Selective oxidation by ozone represents a significant postharvest process for degrading residual phosmet on lowbush blueberries.

Blueberry Plants↗
PubMed2006

A micellar model system for the role of zeaxanthin in the non-photochemical quenching process of photosynthesis--chlorophyll fluorescence quenching by the xanthophylls.

To get an insight to the mechanism of the zeaxanthin-dependent non-photochemical quenching in photosystem II of photosynthesis, we probed the interaction of some xanthophylls with excited chlorophyll-a by trapping both pigments in micelles of triton X-100. Optimal distribution of pigments among micelles was obtained by proper control of the micelle concentration, using formamide in the reaction mixture, which varies the micellar aggregation number over three orders of magnitude. The optimal reaction mixture was obtained around 40% (v/v) formamide in 0.2-0.4% (v/v) triton X-100 in water. Zeaxanthin in the micellar solution exhibited initially absorption and circular dichroism spectral features corresponding to a J-type aggregate. The spectrum was transformed over time (half-time values vary-an average characteristic figure is roughly 20 min) to give features representing an H-type aggregate. The isosbestic point in the series of spectral curves favors the supposition of a rather simple reaction between two pure J and H-types dimeric species. Violaxanthin exhibited immediately stable spectral features corresponding to a mixture of J-type and more predominately H-type dimers. Lutein, neoxanthin and beta-carotene did not show any aggregated spectral forms in micelles. The spectral features in micelles were compared to spectra in aqueous acetone, where the assignment to various aggregated types was established previously. The specific tendency of zeaxanthin to form the J-type dimer (or aggregate) could be important for its function in photosynthesis. The abilities of five carotenoids (zeaxanthin, violaxanthin, lutein, neoxanthin and beta-carotene) to quench chlorophyll-a fluorescence were compared. Zeaxanthin, in its two micellar dimeric forms, and beta-carotene were comparable good quenchers of chlorophyll-a fluorescence. Violaxanthin was a much weaker quencher, if at all. Lutein and neoxanthin rather enhanced the fluorescence. The implications to non-photochemical quenching process in photosynthesis are discussed.

Carotenoids↗
PubMed2005

Coarse-grained model of the interaction of light with polymeric material: onset of ablation.

A coarse-grained model has been developed for molecular dynamics simulations of the interaction of light with polymeric materials. The photon energy can result in a vibrational excitation (photothermal process) or disruption of a chemical bond (photochemical process) in a polymer. In the latter case, the formation of active radial sites and the occurrence of chemical reactions have to be taken into consideration. The novel feature of this model is the incorporation of chemical reactions into the united atom approximate representation of the polymer structure, which permits the study of laser ablation, degradation, or the effect of various chemical reactions on large time and length scales. The chemical reactions are included in the model in a probabilistic manner as in the kinetic Monte Carlo method. This model adopts physically and experimentally known quantities such as enthalpies and probabilities of reactions. Properties such as laser irradiation time, laser fluence, and wavelength are explicitly included. Moreover, no chemically correct interaction potential is required to incorporate the effects of chemical reactions on the dynamics of the system after energy deposition. We find that the model provides a plausible description of the essential processes. The laser-induced pressure relaxation is the main mechanism responsible for the onset of polymer ablation. Since the pressure relaxation processes are slow, there is a delay in the onset of ablation after the end of the laser pulse as is observed experimentally. The vaporization processes are not efficient for material removal, and the effect is minimal for both photochemical and photothermal processes. A lower fluence is needed for the onset of ablation with photochemical processes than photothermal processes.

Light↗
PubMed1999

Evolution of interstellar ices.

Infrared observations, combined with realistic laboratory simulations, have revolutionized our understanding of interstellar ice and dust, the building blocks of comets. Ices in molecular clouds are dominated by the very simple molecules H2O, CH3OH, NH3, CO, CO2, and probably H2CO and H2. More complex species including nitriles, ketones, and esters are also present, but at lower concentrations. The evidence for these, as well as the abundant, carbon-rich, interstellar, polycyclic aromatic hydrocarbons (PAHs) is reviewed. Other possible contributors to the interstellar/pre-cometary ice composition include accretion of gas-phase molecules and in situ photochemical processing. By virtue of their low abundance, accretion of simple gas-phase species is shown to be the least important of the processes considered in determining ice composition. On the other hand, photochemical processing does play an important role in driving dust evolution and the composition of minor species. Ultraviolet photolysis of realistic laboratory analogs readily produces H2, H2CO, CO2, CO, CH4, HCO, and the moderately complex organic molecules: CH3CH2OH (ethanol), HC(=O)NH2 (formamide), CH3C(=O)NH2 (acetamide), R-CN (nitriles), and hexamethylenetetramine (HMT, C6H12N4), as well as more complex species including amides, ketones, and polyoxymethylenes (POMs). Inclusion of PAHs in the ices produces many species similar to those found in meteorites including aromatic alcohols, quinones and ethers. Photon assisted PAH-ice deuterium exchange also occurs. All of these species are readily formed and are therefore likely cometary constituents.

Carbon Monoxide↗
PubMed2001

Visible light-dependent degradation of long-chain alkenes in killed cells of Emiliania huxleyi and Nannochloropsis salina.

Photosensitized degradation rates of phytoplanktonic n-alkenes under visible light exposure were determined in laboratory experiments. Killed cells of Emiliania huxleyi and Nannochloropsis salina were used as source of biogenic alkenes. In E. huxleyi killed cells, minor C31 and C33 n-alkenes were strongly photodegraded, while the major C37 and C38 n-alkenes appeared particularly recalcitrant towards photochemical processes. This particular photochemical recalcitrance has been attributed to the chemical structure and localization of these hydrocarbons in the cells. Most of the n-alkenes of N. salina were strongly photodegraded in killed cells. The photodegradation of phytoplanktonic alkenes showed apparent second-order kinetics with respect to light exposure, and the half-life doses obtained logically decrease with increasing number of double bonds in these compounds. These results strongly suggest that significant amounts of phytoplanktonic n-alkenes must be photodegraded in the euphotic zone of the oceans during senescence.

Alkenes↗