堀越 智

Novel light sources based on microwave discharge electrodeless lamps (MDEL) are examined as potential light sources to drive photochemical processes, in particular advanced oxidation processes (AOPs) of which various applications are described. The MDELs possess several features that make them attractive as possible environmental remediation lamps and as light sources to activate metal oxide photocatalysts in environmental remediation processes. Accordingly, the article describes some of the many features, albeit non-exhaustively, of MDEL devices and reports some photoreactors that incorporate these MDELs. Fundamental issues of MDEL light sources in remediation processing of actual pollutants are introduced. Examples are taken from the oxidative destruction of volatile organic compounds (VOCs; e. g. acetaldehyde and toluene), from the oxidative degradation of contaminants present in wastewaters (e.g. the 2,4-D herbicide and the endocrine disruptor bisphenol-A) and from the remediation of dioxin-contaminated fly-ash, along with an actual industrial wastewater sample from the manufacturing of cement.

A metallic condensing cone that concentrates microwave radiation (equivalent to an optical lens) has been developed and used as part of a system to activate a microwave discharge electrodeless lamp (MDEL) in the oxidative treatment of wastewaters by aiding the novel self-ignition of the lamp on irradiation at low microwave power levels. This approach to self-ignition can potentially lead to considerable energy savings in such treatments. System performance was examined for the ignition power of microwaves of such MDEL devices in water, whose usefulness was assessed by investigating the photolytic transformation of aqueous solutions of representatives of three classes of contaminants: chlorinated phenols, herbicides and endocrine disruptors, specifically 4-chlorophenol (4-CP), 2,4-dichlorophenoxyacetic acid (2,4-D) and 4,4'-isopropylidenediphenol (bisphenol-A; BPA), respectively, taken as model wastewaters in air-equilibrated, in oxygen-saturated and in TiO(2)-containing aqueous media. The results are discussed in terms of the dynamics of the photo-induced degradation processes.

The photoassisted degradation (HPLC-UV absorption), dehalogenation (HPLC-IC) and mineralization (TOC decay) of the flame retardants tetrabromobisphenol-A (TBBPA) and tetrachlorobisphenol-A (TCBPA) were examined in UV-irradiated alkaline aqueous TiO(2) dispersions (pH 12), and for comparison the parent bisphenol-A (BPA, an endocrine disruptor) in pH 4-12 aqueous media to assess which factor impact most on the photodegradative process. Complete degradation (2.7-2.8 x 10(-2) min(-1)) and dehalogenation (1.8 x 10(-2) min(-1)) of TBBPA and TCBPA occurred within 2 h of UV irradiation, whereas only 45-60% mineralization (2.3-2.7 x 10(-3) min(-1)) was complete within 5 h for the flame retardants at pH 12 and ca. 80% for the parent BPA. Factors examined in the pH range 4-12 that impact the degradation of BPA were the point of zero charge of TiO(2) particles (pH(pzc): electrophoretic method), particle or aggregate sizes of TiO(2) (light scattering), and the relative number of(center dot)H radicals(as DMPO-(center dot)OH adducts: ESR spectroscopy) produced in the UV-irradiated dispersion. Dynamics of BPA degradation (2.0-2.4 x 10(-2) min(-1)) were pH-independent and independent of particle/aggregate size, but did correlate with the number of(center dot)OH radicals, at least at pHs 4 to 8-9, after which the rates decreased somewhat at pH > 9 with decreasing adsorption owing to Coulombic repulsive forces between the very negative TiO(2) surface and the anionic forms of BPA (pK(a)s ca. 9.6-11.3), even though the number of(center dot)OH radicals continued to increase at the higher pHs. (C) 2008 Elsevier B.V. All rights reserved.

Characteristic microwave effects on the TiO2 behavior in photoassisted processes have been examined for surface affinity, electric surface charge, size of aggregated TiO2 particles, and not least the number of (OH)-O-center dot radicals formed using in situ observation techniques. Novel microwave and UV radiation components were assembled to investigate the microwave effects. Microwave radiation can influence analytical instruments. Hence, the assembled components were designed so as to microwave irradiate only the sample solution containing the TiO2 photomediator particles. Peculiar microwave effects implicated in photoassisted reactions involve both thermal and non-thermal factors. (C) 2007 Elsevier B.V. All rights reserved.

Early events occurring when p-halophenols (p-XPhOH; X = F, Cl, Br) and phenol (PhOH) interact with a metal-oxide surface (e.g. TiO2) through adsorption and/or photoinduced adsorption have been examined by FT-IR diffuse reflectance (DRIFT) spectroscopy. Exposure of p-XPhOH/TiO2 specimens (X = H, F, Cl, Br) to very weak UV light (290-390 nm; <= 1.8 mu W cm(-2)) led to significant changes in the C-O stretching band, v(C-O), in the spectral range 1100-1350 cm(-1). A new band emerged at higher frequency with isosbestic points clearly observable, inferring formation of a single intermediate product in all four cases, namely the 4-halocatechols (catechol in the case of phenol). This inference is supported by theoretical MOPAC calculations of point charges and radical frontier electron densities of all the atoms in the original phenolic structures. The TiO2 photo-assisted degradation (UV irradiance similar to 0.80 mW cm(-2); UV-vis spectroscopy) and mineralization (evolution of CO2) of the phenols was also examined in UV-irradiated aqueous dispersions, together with debalogenation (HPLC ion chromatography). Analyses of process dynamics indicate little variations in the rates of photodegradation (slightly faster for the p-BrPhOH/TiO2 specimen). Photoinduced dehalogenation was nearly complete after ca. 3 h of irradiation with k(X) varying as p-FPhOH >> p-ChPhOH > p-BrPhOH. About 35% of the F- ions could not be accounted for, as they remained strongly adsorbed on the TiO2 surface. Evolution of CO2 was also nearly quantitative but slower than dehalogenation. Within experimental error, k(CO2) followed the trend p-FPhOH >= p-CIPhOH >= PhOH >= p-BrPhOH. The influence of F-, Cl- and Br- ions on the dynamics of the TiO2-assisted photodegradation of phenol was also examined. Results revealed little variation except for the presence of Br- ions, which caused phenol to photodegrade three times slower. Either the Br- ions blocked surface active sites or else the bromide ion and the phenol competed for the oxidizing (OH)-O-center dot radicals and/or the photogenerated valence band holes. (C) 2007 Elsevier B.V. All rights reserved.

The microwave-enhanced synthesis of 3-cyclohexyl-1-phenyl-1-butanone (CPB) from crotonophenone and iodocyclohexane in the presence of t-BuOOH and triethylborane was examined at ambient temperature and under reflux conditions in bulk solution using no less than four different synthetic protocols. Chemical yields of CPB (93 and 95%, respectively) obtained after 60 and 120 min under microwave dielectric heating coupled to a cooling system (MW/Cool protocol) for a reaction temperature of 20-24 degrees C exceeded the yields from the reaction occurring at room temperature (24 degrees C; no microwaves; 40% and 72%) for the same reaction times and temperature. The competitive radical reaction to produce CPB and a small amount of by-products generated from the thermally-induced synthesis by the non-selective radical reaction was restricted solely to the process occurring at ambient temperature (RT protocol), under microwave dielectric heating to reflux (MW protocol) and heating with an oil-bath also to reflux (Oil-bath protocol). By contrast, no by-products were generated or detected for the reaction taking place by the MW/Cool method. Comparison of product yields under conditions of identical temperature conditions must involve some specific microwave effect in the synthesis of the title compound.

The Urushibara Ni (U-Ni) hydrogenation catalyst and some modified forms, and for comparison Raney-Ni, were subjected to conventional (oil bath) and MW heating, and subsequently characterized by electron dispersive X-ray analysis (EDX), by BET surface area, and by scanning electron microscopy (SEM); yields of the catalyzed hydrogenation of acetophenone to 1-phenylethanol in 2-propanol by one of the modified forms (U-NiB) were greatly improved (from 68% to 95%).

Exposure to low doses of the xenoestrogen bisphenol A (BPA) and to the hormonal 2,4- dichlorophenoxyacetic acid (2,4-D) herbicide, an environmental endocrine disruptor, can have serious health consequences such as the induction of mammary gland ductal hyperplasias and carcinoma (LaChapelle et al., Reprod. Toxicol., 2007, 23, 20; Murray et al., Reprod. Toxicol., 2007, 23, 383). To the extent that these toxins are present in wastewaters ( Donald et al., Sci. Total Environ. 1999, 231, 173; Brotons et al., Environ. Health Perspect. 1994, 103, 608; Olea et al., Environ. Health Perspect. 1996, 104, 298; Biles et al., J. Agric. Food Chem. 1997, 45, 3541; Markey et al., J. Steroid Biochem. Mol. Biol., 2003, 83, 235), we examined their oxidative destruction in aqueous media by a novel light source. A tungsten-triggered microwave discharge electrodeless lamp (W-MDEL) was fabricated for possible use in wastewater treatment using vacuum UV-transparent quartz in which a tungsten trigger, also embedded in quartz, was attached to the MDEL to aid in the self-ignition of the lamp on irradiation at low microwave power levels. The quantity of mercury gas in the W-MDEL was optimized by monitoring the continuous radiation and peak intensities of the emitted light in the vacuum UV (VUV) and UVC regions. The usefulness of the W-MDEL device was assessed through the degradation of 2,4- D and BPA in air-equilibrated aqueous media and in oxygen-saturated aqueous media. Enhanced degradation of these two xenoestrogenic toxins was achieved by increasing the number of W-MDEL devices while keeping constant the microwave radiation feeding each W-MDEL lamp. This novel lamp provides an additional light source in the photooxidation of environmental contaminants without the need for a metal-oxide photocatalyst. Under our conditions, process dynamics using the W-MDEL light source are greater than with the more conventional photochemical methods that employ low-pressure Hg arc electrode lamps in synthetic quartz to degrade these two toxic contaminants.

A microwave discharge electrodeless lamp (MDEL) filled with low pressure mercury gas has been developed for the photocatalytic treatment of organic air pollutants over anatase TiO2 pellets. The benefit of the combination of MDEL and TiO2 pellets under microwave radiation was assessed by examining the degradation of acetaldehyde in humidified synthetic air. The effect of microwave irradiation was quantified by the decomposition rate of acetaldehyde and the desorption rate of acetaldehyde on the TiO2 surface. It is shown that acetaldehyde is readily transformed at the surface of the TiO2 pellets by the combined action of UV light and microwave irradiation coupling homogeneous and heterogeneous photoreactions. (C) 2007 Elsevier B.V. All rights reserved.

The microwave-enhanced synthesis of phenylacetylene bromide (bromoethynylbenzene) from phenylacetylene and N-bromosuccinimide NBS) in the presence of AgNO3 was examined under non-thermal conditions in bulk solution with a microwave reactor coupled to a refrigeration unit. Generation of by-products from the thermal-induced synthesis of phenylacetylene bromide was suppressed under microwave (MW) irradiation when the temperature was maintained at near-ambient conditions. In the absence of MW radiation, formation of phenylacetylene bromide was an inefficient process under ambient conditions alone. Product yields were as low as 12% after 15 min under microwave irradiation at 157 degrees C (superheating of solvent DMF) and as high as 62% when temperature was maintained at ca. 18 degrees C under microwave radiation, compared to 14% under ambient conditions (no microwaves) for the same reaction time. (C) 2007 Elsevier B.V. All rights reserved.

A novel microwave discharge electrodeless lamp (MDEL) has been developed for wastewater treatment with Advanced Oxidation Processes (AOPs) using environment risk-free gases (e.g., xenon, nitrogen, helium, oxygen, hydrogen and argon alone or a mixture thereof) that provide the heeded light plasma source when microwave irradiated. The MDEL was optimized through an examination of the light intensity of the emitted radiation in the UV-visible spectral region at controlled pressures and gas-mixture ratios and to test whether the gases self-ignite on irradiation with microwaves. The usefulness of the MDEL was assessed by examining the degradation of aqueous 2,4-dichlorophenoxyacetic acid (2,4-D) in the absence and presence of TiO2 particles irradiated,simultaneously by both microwave (MW) and by UV radiation emitted from the microwave-triggered (2.45 GHz) electrodeless lamp. The decomposition efficiencies for the disposal of the 2,4-D herbicide are compared with the MDEL in close contact, (inside) with the 2,4-D solution or the 2,4-D/TiO2 dispersion and with the MDEL located outside the reactor. Degradation of 2,4-D with the MDEL was monitored spectroscopically and by the loss of total organic carbon (TOC) using no less than seven different protocols, namely (i) MW irradiation alone, (ii) MDEL (outside), (iii) Hg lamp/TiO2, (iv) Hg lamp/TiO2, (v) MDEL (outside)/TiO2, (vi) MDEL (inside), and (vii) MDEL (inside)/TiO2. Most efficient in the degradation of 2,4-D were the MDEL/TiO2 systems with the MDEL lamp inside the reactor in contact with the 2,4-D/TiO2 aqueous dispersion followed closely by the MDEL alone (no TiO2) also in contact with the 2,4-D solution. (C) 2007 Elsevier B.V. All rights reserved.

The extent of photoreduction of a metal ion, such as Ag+, occurring on the TiO2 particle surface was monitored in real time in the presence of hole scavengers methanol and ethanol by the quartz crystal microbalance (QCM) technique. Silver cluster deposition on metal-oxide particles that accompanies electron transfer from TiO2 to Ag+(ads) depends on the surface charge of the particles, which also influences the close approach of Ag+ ions to the particle surface. Formaldehyde and acetaldehyde produced, respectively, as intermediates during silver deposition in the presence of the alcohols contributed negligibly to the mass of silver deposited on the TiO2 surface. The QCM technique provides a means to monitor photoadsorption. (C) 2006 Elsevier B.V. All rights reserved.

The microwave-assisted photodegradation of bisphenol-A (BPA) in the presence of TiO2 was examined at ambient temperature to unravel some details on the importance of the microwave non-thermal effect. Normally, bisphenol-A photodegrades under microwave irradiation as a result of a thermal effect emanating from the microwave radiation which sees increase in temperature in the aqueous TiO2 dispersion. However, on cooling the dispersion with circulating silicone oil at -20 degrees C, the temperature of the dispersion could be maintained constant at 21 degrees C. Under the latter irradiation conditions, the microwave-assisted photodegradation of BPA was shown to be not only due to a microwave thermal effect, but also to a significant non-thermal effect that might implicate hot-spots on the TiO2 particle surface leading to enhanced photodegradation under constant ambient temperature. Other origins of the microwave non-thermal effect may include additional formation of charge carriers on the photomediator TiO2 and formation of additional trap sites that may otherwise prolong the lifetimes of the charge carriers through diminished recombination and ultimately increase process kinetics. (c) 2006 Elsevier B. V. All rights reserved.

The photodegradation of acetaldehyde as a model air-pollutant was examined over anatase TiO2 pellets to assess the characteristics of a microwave discharge electrodeless lamp (MDEL), i.e., a microwave-powered (2.45 GHz) low pressure mercury (with a small quantity of argon) lamp. The TiO2 photocatalyst was irradiated simultaneously by both microwave and UV irradiation. This system was effective for the photocatalytic degradation of acetaldehyde. The degradation mechanism of acetaldehyde in this system includes pyrolysis, photolysis, photocatalytic degradation, and oxidation by ozone. The effects of microwave irradiation on the degradation process are described.

The photocatalytic degradations of benzoic acid, phenol and salicylic acid were examined subsequent to in situ adsorption/desorption of the organic substrates on a TiO2-coated quartz crystal microbalance (QCM). The amount of substrate adsorbed from the bulk aqueous solution on the TiO2-coated QCM sensor was investigated at different pH under dark conditions. The order of adsorption was salicylic acid > phenol similar to benzoic acid oil the acidic side; phenol > benzoic acid > salicylic acid under neutral conditions; benzoic acid > phenol similar to salicylic acid in alkaline media. Salicylic acid was ail intermediate in the photodegradation of benzoic acid. (c) 2005 Elsevier B.V. All rights reserved.

Some of advanced oxidation processes (AOP) are characterised by a special chemical feature: the ability to use the high reactivity of *OH radicals in driving oxidation processes. These radicals are suitable for achieving the complete abatement, even including the mineralization of less reactive pollutants. In this study, a photocatalytic process is used to degrade one herbicide of the imidazolinone family, Imazapyr. It was shown to be photodegraded rapidly and extensively in an aqueous solution. The decline of Imazapyr concentration in the solution followed a first-order kinetics. The apparent first order rate constant was found equal to 0.19 min(-1) in distilled water at natural pH 3.8. The smaller activities found at acidic and basic pH were explained by considering the ionisation state of Imazapyr and the charge density of TiO2. The present work dealt with the influence of metal ions like Ni2+ and Cu2+ which are frequently present in agricultural wastewater on the photocatalytic efficiency of TiO2 in the elimination of Imazapyr. A detrimental effect of the presence of metallic species was observed only with samples containing amount of copper and nickel in the presence of TiO2. Several hypotheses were proposed to explain this phenomenon, passivation of TiO2 surface by adsorption Of Cu2O and/or Cu-0, formation of a complex or recombinaison of the e(-)/h(+) pairs. At higher concentrations of metallic species like Cu2+ and Ni2+, a plateau was reached which could be explained by the photo-Fenton like reaction. In an attempt to understand the basic mechanisms of the degradation of Imazapyr in water by TiO2 photocatalysis, we discussed the primary degradation mechanism on the basis of the experimental results together with molecular orbital calculation of frontier electron density and partial charge. (c) 2005 Elsevier B.V. All rights reserved.

Photoactive TiO2 specimens extracted from commercial sunscreen lotions had been shown earlier [R. Dunford, A. Salinaro. L. Cai, N. Serpone, S. Horikoshi, H. Hidaka, J. Knowland, FEBS Lett. 418 (1997) 87] to cause damage to both DNA plasmids in vitro and to whole human skin cells in cultures by photoproduced (OH)-O-center dot radicals. This article reports on the effects of TiO2 specimens, whose particle surface was modified by a thermally assisted procedure to produce TiO2 specimens of considerably reduced photo-activity. Deactivation of the photocatalytic activity of TiO2 impacts on the kinetics of photooxidation of phenol and has a significant effect in diminishing, if not suppressing completely, damage caused to DNA plasmids, to human cells, and to yeast cells compared to non-modified specimens exposed to UVB/UVA simulated solar radiation. Photo-inactive TiO2 could be beneficial in sunscreen formulations and in polymer blends. since they also completely retain their UVB/UVA absorption/scattering (screening) characteristics. Synergistic effects of titania specimens with an organic sunscreen active agent (Padimate-O) on DNA plasmids and the survival of yeast cells in the presence of titanium dioxide, and in the presence of such UV filters as Padimate-O and Parsol 1789 (i.e., avobenzone) under UV irradiation are reported. (c) 2005 Elsevier B.V. All rights reserved.

The photocatalyzed degradation of the 4-chlorophenol toxin (4-CP) in aqueous naked TiO2 and platinized TiO2 suspensions simultaneously subjected to UV light and microwave radiation was revisited to examine the fate of this toxin in the microwave-assisted photocatalytic process by monitoring loss of total organic carbon (TOC; mineralization), formation of chloride ions (dechlorination of 4-CP), and identification of intermediates using HPLC and electrospray mass spectral (LC-MSD) techniques. Attempts are made to delineate microwave thermal and nonthermal factors that impinge on the degradation by comparing experimental results front microwave-generated heat versus results from a conventional (externally heated) thermally-assisted process, and from results in which the thermal factors were minimized by examining the degradative process at constant ambient temperature (25 degrees C). Possible microwave radiation effects on the Pt co-catalyst supported on TiO2 were also probed through comparison of the degradation of 4-CP occurring on Pt/TiO2 and on naked TiO2 photocatalysts. Results suggest that. in a microwave radiation field, naked TiO2 and Pt/TiO2 particle surfaces interact with the microwaves. The degradation pathway exhibited characteristics of hydrolysis of reactants and intermediates. Nonthermal microwave effects play a role in the overall degradative process occurring in platinized TiO2 dispersions. The possible nature of these unusual microwave effects is briefly discussed. (c) 2005 Published by Elsevier B.V.

Attempts to mineralize a variety of s-triazine herbicides and dyes containing the triazine ring by the photocatalytic TiO2 Method or by the Fenton process have typically terminated at cyanuric acid as the end product or at precursors such as ammelide, rather than at CO2 for the carbons and NH4+ and/or NO3- ions for the nitrogens, when such triazines are subjected to (OH)-O-center dot radical (free or otherwise) oxidation. A recent study on the fate of triazoles (e.g. amitrole) in UV-illuminated titania dispersions [N. Watanabe, S. Horikoshi, A. Kawasaki, H. Hidaka, N. Serpone, Environ. Sci. Technol. 39 (2005) 2320] produced ring-expanded triazine intermediates that ultimately also terminated at cyanuric acid despite efforts to totally mineralize the triazole substrates. Another study [Y.-C. Oh, W.S. Jenks, J. Photochem. Photobiol. A: Chem. 162 (2004) 323] reported that the cyanuric acid ring is degraded by free (OH)-O-center dot radicals produced from the Fenton reagent and from UV-illuminated TiO2 aqueous dispersions at various pHs (2-12) whose particle surface was fully fluorinated [C. Minero, G. Mariella, V. Maurino, E. Pelizzetti, Langmuir 16 (2000) 2632]. Our own studies have been unable to degrade cyanuric acid in Fenton solutions, in UV-irradiated naked TiO2 and fluorinated TiO2 dispersions, except in a hydrothermal/supercritical aqueous medium [S. Horikoshi, Y. Wada, N. Watanbe, H. Hidaka, N. Serpone, New J. Chem. 27 (2003) 1216] and as reported earlier by others on titania modified by metal complexes. Using various analytical methods (e.g. ESI-MS spectrometry, HPLC-ion chromatography, and UV/visible spectroscopy), we failed to detect any intermediates and mineralized end products that might have resulted from the degradation of this recalcitrant species. We also report ESR and electrochemical results for the characterization of naked TiO2 and fluorinated TiO2 specimens. Clearly, cyanuric acid remains a recalcitrant species where (OH)-O-center dot radicals are the oxidizing agents. The number of (OH)-O-center dot radicals produced from UV-irradiated TiO2 (naked) and fluorinated TiO2 depend on the incident light irradiance; at low light irradiance a greater number of (OH)-O-center dot radicals are formed on naked TiO2, whereas at high light irradiance a greater number of (OH)-O-center dot radicals seems to form in fluorinated TiO2 (F/TiO2) than on naked TiO2 dispersions owing to a rapid degradation of the DMPO spin trap by surface-bound (OH)-O-center dot radicals taking place on the naked particle surface. (c) 2005 Elsevier B.V. All rights reserved.

Amitrole (ATz, 3-amino-lH-1,2,4-triazole) is a widely employed herbicide with strong estrogenic activity that can lead to abnormalities of the thyroid gland and can cause mutations. The photocatalytic transformation of ATz was carried out at the UV-irradiated TiO2/H2O interface, along with the triazole derivatives Tz(1H-1,2,4-triazole) and DaTz (3,5-diamino-1H-1,2,4-triazole) to assess the decomposition of these herbicides, to identify intermediates, and to elucidate some mechanistic details of the ATz degradation. Conversion of the nitrogens of these triazoles to NH4 and/ or NO3- ions occurs competitively and depends on the number of amine functions on the five-membered triazole rings. Photomineralization of the substrates in terms of loss of nitrogen to NH4+/NO3- was rather low (ca. 25-40%) for each of the triazoles, whereas evolution Of CO2 (loss of TOC) was more significant (60-70%), indicating considerable retention of nitrogen in the intermediate products. UV-Vis spectroscopy, TOC assays, FT-IR spectroscopy, proton NMR spectrometry, electrospray LC-MS, and molecular orbital calculations were brought to bear in assessing the temporal course of the photocatalyzed process(es). Results show that after cleavage of the triazole ring, the various intermediate fragments recombine to yield ring-expanded six-membered triazine intermediates, which slowly degrade to give the refractory cyanuric acid under the conditions used.

The photocatalyzed degradation and mineralization of branched C4 and C5 acids {2-methylpropanoic acid (2-MPA), 2-methylbutanoic acid (2-MBA), 3-methylbutanoic acid (3-MBA), and trimethylacetic acid (TMA)} and oxidized C4 and C5 carboxylic acids {e.g., 2-hydroxybutanoic acid (2-HBA), 2-hydroxy-2-methylbutanoic acid (2-OH-2-MBA), 2-hydroxyvaleric acid (2-HVA), and 2-ketovaleric acid (2-KVA)} were examined in UV-irradiated air-equilibrated aqueous TiO2 dispersions to assess: (a) the extent of (dark) adsorption; (b) the relationship between the extent of adsorption and degradation dynamics; and (c) the dynamics of mineralization (loss of total organic carbon (TOC)). Branching had little, if any impact on the adsorption process by comparison with analogous linear acids reported earlier. Complete degradation of the branched acids occurred in relatively short time (less than or equal to 2 h), whereas mineralization required longer irradiation times (3-4.5 h). The trends correlate with C-H bond strengths. Branched acids degrade at rates slightly greater than unbranched acids with degradation dynamics decreasing with increase in chain length. The 2-HVA and 2-KVA acids degraded at nearly identical rates. Methylation of 2-HBA had no effect on the dynamics of degradation. However, oxidized carboxylic acids (2-HVA and 2-KVA) degraded five-fold faster than the corresponding linear valeric acid; similarly for 2-OH-2-MBA, which degraded four-fold faster than 2-MBA. These observations explain the difficulty in detecting primary hydroxylated intermediates in (OH)-O-.-radical oxidized linear carboxylic acids. (C) 2004 Elsevier B.V. All rights reserved.

Thermal and nonthermal effects originating when a system is subjected to a microwave radiation field in the TiO2-photocatalyzed transformation of model substances containing various functional groups (e.g., benzoic acid, phthalic acid, o-formylbenzoic acid, phthalaldehyde, succinic acid, dimethyl phthalate, diethyl phthalate, and phenol) have been examined under simultaneous irradiation by ultraviolet (UV) and microwave (MW) radiations. Characteristics of the microwave effects and the fate of each substrate during the microwave-assisted photocatalytic process were monitored by UV absorption spectroscopy, HPLC methods, total organic carbon assays, and identification of intermediates using electrospray mass spectral techniques. Microwave thermal and nonthermal effects were delineated by comparing results from MW-generated internal heat versus conventional external heating, and at constant ambient temperature under a microwave field. Factors involved in the nonthermal component of the microwave radiation were inferred for the initial adsorption of the substrate and its subsequent degradation occurring on the surface of TiO2 particles. Microwave effects bear on the mechanism through which a model substrate undergoes oxidative degradation. A characteristic feature of these effects was briefly examined by considering the behavior of polar (dipole moments) substrates in a microwave radiation field.

The photodegradation of the endocrine disruptor 4,4'-isopropyldiphenol (bisphenol-A, BPA; 0.1 mM) was examined in aqueous TiO2 dispersions to assess thermal versus non-thermal effects of the microwave radiation assisted photocatalytic process driven by UV radiation. The effects were examined by comparing the results of the photodegradative process from the hybrid UV/microwave irradiation (PD/MW) with a UV/conventional heating method (PD/TH) and with UV illumination alone (PD). Mechanistic considerations followed from monitoring the process by UV/Vis absorption spectroscopy (cleavage of benzene rings), total organic carbon (TOC) assays, identification of intermediates by electrospray mass spectral techniques, and from calculations of frontier electron densities and partial charges of all non-hydrogen atoms in the BPA skeleton. Near-quantitative mineralization of BPA occurred after only 90 min of irradiation by the PD/MW and PD/TH routes, contrary to the PD route, which accounts for only 67% mineralization under identical conditions. Microwave radiation alone on a solution of BPA (no TiO2) had no effect on the TOC, except for an increase of the pH of the solution (6.7-7.2) and formation of a small quantity of formic acid (ca. 0.005 mM) originating from oxidation of the isopropyl methyl groups after a 30 min irradiation period. For the other three routes, other intermediates were identified in addition to acetic acid and formic acids: 4-hydroxyacetophenone (PD/MW, PD/TH, PD), 4-hydroxybenzaldehyde (PD/MW, PD/TH), 4-hydroxyphenylisopropanol (PD/TH, PD), phenol and hydroquinone (PD/TH), and 3-hydroxy-1,3,5-hexatriene (PD/MW). Mechanistic differences are attributed to adsorption mode variations of the BPA substrate on the TiO2 particle surface. (C) 2004 Elsevier B.V. All rights reserved.

A commercial detergent whose major components are an anionic surfactant and a fluorescent whitening agent can be photodegraded in aqueous TiO(2) dispersions under irradiation with concentrated sunlight in the presence of air. The degradation process followed apparent first-order kinetics in terms of the total sunlight energy impinging on the photoreactive system. The effects of (a) TiO(2) loading, (b) circulation flow rate, and (c) pH of the reactant solution on the kinetics of decomposition of the detergent were examined. Under the prevailing conditions, the optimal operational parameters for this detergent were, respectively: TiO(2) loading, 6 gl(-1); circulation flow rate, 4.9 1 min(-1); and pH, 4.9. The rate of increase of the surface tension was greater than the rate of decrease of the concentration of the detergent. This study adds to our knowledge base in the effective use of sunlight irradiation to detoxify wastewaters containing undesirable detergents. (C) 2004 Elsevier Ltd. All rights reserved.

The photocatalyzed degradation (PD) of the cationic rhodamine-B (RhB) dye was examined in aqueous TiO(2) dispersions using UV/Vis illumination assisted by microwave radiation (PD/MW). The initial degradation by the PD/MW method is compared to the PD method and to the thermally assisted PD method using conventional heating (PD/TH). Total organic carbon (TOC) assays show that the efficiency of complete mineralization of the dye follows PD/MW > PD/TH > PD > MW. In all cases, microwave radiation alone had no effect on the loss of TOC. The degradation involving microwave radiation was especially efficient when coupled to UV irradiation. By contrast, the extent of degradation of RhB involving suitable excited states through visible irradiation of the dye was rather inefficient when coupled to microwave radiation. Contact angle measurements on the TiO(2) photocatalyst particles indicate that microwave radiation also causes an increase in the hydrophobic character of the TiO(2) surface, with consequences on the adsorption mode of the dye substrate and thus on the overall mechanism of degradation. Deethylated RhB intermediates were identified by an electrospray ESI ionization mass spectral technique in the positive ion mode and subsequently confirmed by HPLC/ absorption spectroscopy. Computer simulations led to estimates of frontier electron densities of all atoms of the RhB structure, affording inferences as to the position of radical attack on RhB. The nitrogen atoms of the dye were all converted to NH(4)(+) ions. The major difference between the thermally assisted PD/TH method and the microwave-assisted PD/MW method showed that nonthermal effects from the microwave radiation impact significantly on the nature of the photocatalyst surface. These effects led to a more efficient photodegradation and mineralization of the dye substrate.

Bisphenol A (BPA) can be decomposed photocatalytically under UV-illumination in aqueous TiO(2) dispersion. The two methyl groups in BPA were initially attacked with (.)OH and/or (.)OOH radicals having strong oxidizing power, followed by the cleavage of the two phenyl moieties. Finally, the photomineralization to CO(2) gas occurred via oxidative processes involving carboxylic acids and aldehydes. The decomposition of structurally similar substances of 4,4'-ethylidenebisphenol (EBP) and 4,4'-methylenebisphenol (MBP) was compared. The decomposition of BPA gave more kinds of intermediates such as 4-isopropylphenol, 4-ethylphenol, etc. On the other hand, that of EBP gave mainly 4-isopropylphenol and that of MBP gave a predominant product of 4-hydroxybenzaldehyde. These photooxidative pathways were proposed on the base of the evidence of oxidative intermediate formation. (C) 2003 Published by Elsevier Science Ltd.

The sodium dodecylbenzene sulfonate (DBS) surfactant photodegrades in aqueous titanium dioxide (TiO2) dispersions under highly concentrated solar light exposure (ca. 70 suns) via apparent first-order kinetics with accumulated light energy as the experimental variable. The choice of optimal experimental parameters for the photodegradation of DBS with a concentrated solar illumination device is explored: optimal circulation flow rate, 8.31 min(-1); pH, 5.0; and TiO2 loading, 5 g l(-1). Foaming action of the DBS-TiO2 suspension increased with increase in increments of the flow rate. The foam produced at the higher TiO2 loadings exhibited greater stability than in the absence of TiO2. The increase in surface tension is faster than the temporal decrease of the concentration of DBS, which followed the typical. Langmuir-Hinshelwood (LH) model: LH apparent rate constant, k(LH) = 0.184 mM kJ(-1); LH adsorption constant, K-LH = 0.527 mM(-1). The relative photonic efficiency, E,I; against phenol; k(app) of DBS is 0.53. The rate of removal of total organic carbon (TOC) of DBS was improved by a combination of TiO2 loading and added K2S2O8 additive; i.e. at pH 5.0 and TiO2 loading of 0.2 g l(-1), or pH 8.4 and TiO2 loading of 5.0 g l(-1). However, at pH 5.0 and TiO2 loading 5.0 g l(-1), the rate of TOC removal was not enhanced in the presence of K2S2O8 as the oxidizing agent and electron scavenger. The results are explained by contributions from the photochemical oxidation with K2S2O8 and from the K2S2O8-assisted photocatalytic oxidation of DBS. (C) 2002 Elsevier Science B.V. All rights reserved.

Refractory atrazine and cyanuric acid were degraded under hydrothermal and supercritical aqueous media (HY-SC) conditions, as well as in the presence of ozone (HY-SC/O-3) and UV-illuminated ozone (HY-SC/UV/O-3) to assess whether the efficacy of the decomposition process could be enhanced using a single-pass flow-through treatment device under a constant pressure of 23 MPa. The progress of the degradation was evidenced by the extent of removal of total organic carbon (TOC) in solution, by UV absorption spectroscopy (opening of the atrazine and cyanuric acid heterorings), and by the extent of deamination (formation of NH4+) and dechlorination (release of Cl- ions) of the two compounds. Loss of atrazine was confirmed by LC-MSD techniques in the positive ion mode. Formation of various intermediates from the degradation of atrazine was substantiated by positive and negative ion mode MSD analyses. Dechlorination of atrazine occurred around 100 degreesC under hydrothermal conditions for the HY-SC and HY-SC/O-3 processes but not for HY-SC/UV/O-3; it did increase rapidly at higher temperatures (beginning at ca. 220-230 degreesC) for all three methods: HY-SC through HY-SC/UV/O-3. Deamination, removal of TOC, and loss of atrazine mass spectral features began around 260-280 degreesC in hydrothermal aqueous media. Degradation of cyanuric acid showed a similar behavior. For the treated effluent solution in the collector reservoir, ozonation enhanced somewhat both dechlorination and mineralization, but had no significant effect on the deamination of either atrazine or cyanuric acid.

The photostabilities of a random set of commercially available sunscreen lotions and their active ingredients are examined spectroscopically subsequent to simulated sunlight UV exposure. Loss of filtering efficacy can occur because of possible photochemical modifications of the sunscreen active agents. Changes in absorption of UVA/ UVB sunlight by agents in sunscreen lotions also leads to a reduction of the expected photoprotection of human skin and DNA against the harmful UV radiation. The active ingredients were investigated in aqueous media and in organic solvents of various polarities (methanol, acetonitrile, and n-hexane) under aerobic and anaerobic conditions. The UV absorption features are affected by the nature of the solvents with properties closely related to oil-in-water (o/w) or water-in-oil (w/o) emulsions actually used in sunscreen formulations, and by the presence of molecular oxygen. The photo stabilities of two combined chemical ingredients (oxybenzone and octyl methoxycinnamate) and the combination oxybenzone/titanium dioxide were also explored. In the latter case, oxybenzone undergoes significant photodegradation in the presence of the physical filter TiO2.

In this brief article we examined the characteristics of a novel UV-VIS-microwave integrated irradiation device and explore its usefulness using the now well-established photo-degradation of a cationic dye (rhodamine-B; RhB) as a test substrate. The device consisted of a double quartz cylindrical plasma photoreactor (DQCPP) equipped with a microwave (MW) powered electrodeless Hg/Ne irradiation source. The latter is activated only by the MW radiation. The integrated MW-UV-VIS radiations so produced and incident on the DQCPP reactor significantly enhanced the decomposition of the RhB dye through simultaneous photocatalytic and photosensitization processes. (C) 2002 Elsevier Science B.V. All rights reserved.

Simulated wastewaters that contain methylene blue (MB) were bleached in a photocatalytic aqueous TiO2 dispersion illuminated by concentrated sunlight using a parabolic round concentrator reactor (PRCR). The kinetic analysis was carried out well when the temporal concentration variation was a function of the concentrated light energy irradiated. The photocatalyzed N-demethylation of NIB takes place concurrently with photocatalytic decomposition of MB by pseudo-first-order kinetics. The dependence of the photo-decomposition kinetics on the initial concentration of MB is consistent with the Langmuir-Hinshelwood model. Elimination of TOC (total organic carbon) also occurs by pseudo-first-order kinetics prior to full bleaching of the aqueous TiO2 dispersion, after which the TOC level decreases only slightly. Also, compared with the open to air, the photodegradation of MB is not influenced by molecular oxygen bubbling continuously through the reactant suspensions during illumination. TiO2 loadings and flow rates markedly affect the degradation of MB. Under concentrated sunlight, the relative photonic efficiency of MB photodegradation is zeta(rel) = 0.49 (relative to phenol). The efficiency zeta(rel) for the degradation of MB is independent of photoreactor geometry (cylindrical bottle reactor versus round-bottomed flask), of light sources (solar light concentrator versus a Hg lamp) and of the operating mode used (flow versus batch operation). (C) 2002 Elsevier Science B.V. All rights reserved.

In an attempt to improve the understanding of the basic mechanisms on the degradation of pollutants in water by TiO(2) photocatalysis, we discussed the primary degradation mechanism of three triazolidine derivatives, such as 1,2,4-triazolidine-3,5-dione (TRIANE), 4-hydroxy-1,2,4-triazolidine-3,5-dione (OH-TRIANE) and 4-phenyl-1,2,4-triazolidine-3,5-dione (Ph-TRIANE) and one triazole derivative of the 4-phenyl-1,2,4-triazole-2,5-dione (Ph-TRIOLE), on the basis of the experimental results together with molecular orbital (MO) calculation of frontier electron density and partial charge. The above four heterocycles were selected as molecular probe, principally because the highest frontier electron density was situated at different places of the molecule, while their structures were very similar, two major pathways have been revealed by LC/MS analysis for each heterocyclic compound. The pathway (a) corresponded to the hydroxylation of the atom bearing the highest electron density, via oxidation on nitrogen atom of azo group with respect to the photodegradation of TRIANE, OH-TRIANE and Ph-TRIOLE and opening of aromatic ring when Ph-TRIANE was used. This initial attack occurred with the preferential electrophilic attack of OH(circle) radicals. The pathway (b) was caused by the attack of active species on to carbon atom of carbonyl group. Considering the calculation of the relative electrophilic density of this carbon atom for the four heterocyclics and the evolution of large amount of N(2) gas at the initial degradation step, it has been suggested that this step (b) was favored by adsorption of carbonyl group on TiO(2) surface as theoretically determined by partial charge and confirmed by IR analysis. The participation of hole (h(+)) to form R-C=O(+circle) was envisaged in this step. The presence of 1,2,4,5-tetrazixane-3,6-dione also agrees with the participation of TiO(2) surface. The nitrogen inorganic analysis (N(2), NH(4)(+) and NO(3)(-)) determined by gas chromatography and ion liquid chromatography, show that the hydrazo group were photoconverted mainly into N(2) gas and partially to NH(4)(+) ions. No nitrate ions were observed at the beginning of the degradation even when a OH substituent was present on one nitrogen atom (case of OH-TRIANE). However, the presence of -OH group increased the NO(3)(-) /NH(4)(+) ratio observed after a few hours of irradiation. The presence of OH also improved the total mineralization of carbon atom into CO(2), which was attributed to mesomeric effect of -OH group. (C) 2002 Elsevier Science B.V. All rights reserved.