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UBC Published Paper eSource Optics 185nm Filter
UBC Published Paper eSource Optics 185nm Filter
May 29, 2019
Photochemistry and Photobiology, 2014, 90: 238–240
Research Note
Novel Collimated Beam Setup to Study the Kinetics of VUV-Induced
Reactions
Clara Duca, Gustavo Imoberdorf and Madjid Mohseni*
Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, BC, Canada Received 26 February 2013, accepted 1 August 2013, DOI: 10.1111/php.12157
ABSTRACT
Vacuum UV (VUV) process is an incipient advanced oxida- tion process, which can be used for water treatment. This process relies on the formation of hydroxyl radicals through the VUV-induced photolysis of water. In particular, the use of ozone-generating mercury vapor lamps, which emit 10% of the radiation at 185 nm and 90% at 254 nm, is showing very promising results for the degradation of micropollu- tants. The kinetics of VUV process has been studied in batch- and flow-through reactors, but the effect of 254 and 185 nm photons cannot be isolated, mass transfer resistances can take place and the interpretation of the results is complex. In this technical note, a new VUV collimated beam to conduct kinetic tests is presented, which offers several advantages: (1) it allows the irradiation of samples with 185, 254 nm pho- tons, or both, (2) the concentration of reagents is uniform in the reaction volume and (3) it allows to change the fluence rate by changing the distance between the lamp and the pho- toreactor. Details of the geometry are presented, as well as an analysis of the collimation and uniformity of the radiation of the new VUV-collimated beam setup.
INTRODUCTION
Conventional water treatments are very efficient for the removal of particulates, degradation of contaminants and disinfection. However, these processes are not very effective for the degrada- tion of micropollutants. In recent years, Vacuum UV (VUV) pro- cess, among other advanced oxidation processes (AOPs), has shown great promises and high efficacy at degrading organic contaminants. VUV process, like other AOPs, is based on the formation of hydroxyl radicals (HO), which are very strong non- specific oxidizing species, capable of degrading wide range of micropollutants. Radiation of 185 nm is able to produce HO rad- icals in the water. These radicals are able to degrade most of the organics. On the other hand, 254 nm radiation alone (without the addition of any catalysts) is not as effective as VUV in the deg- radation of micropollutants because no HO radicals are created in the system. The degradation in this case occurs just thorough the micropollutant’s photolysis which has several disadvantages: (1) only chromophoric compounds can be treated, (2) mineraliza- tion cannot be achieved with the dose that is normally used in
*Corresponding author email: madjid.mohseni@ubc.ca (Madjid Mohseni) © 2013 The American Society of Photobiology
water treatments and (3) the effectiveness is low as most organ- ics have a low molar absorbtion coefficient at these wavelengths. The significant advantage of VUV over other AOPs comes from the fact that it does not require addition of chemicals, has a high efficiency and has relatively low-energy requirements. Because of their low price and extensive availability, ozone-generating mercury (Hg) vapor lamps are commonly used. They emit 10% of the radiation at 185 nm and 90% at 254 nm.
The kinetics of the photochemical reactions associated with different AOPs, such as UV photolysis, UV/H2O2 and photo- Fenton process, have been studied in details with collimated beams (1,2). When properly designed, collimated beam can pro- vide quasi-collimated and uniform radiation, which allows obtaining valuable kinetic data that can be easily analyzed. In addition, a small volume of the reacting solution is required for each test and, if the solution is thoroughly mixed, the concentra- tion of reagents in the reaction vessel is uniform.
In spite of the advantages of collimated beams, there had not been any setup specifically built to study the kinetics of VUV- induced reactions. To the best of our knowledge, VUV process had only been studied in batch- and flow-through laboratory scale reactors. In such cases, the concentration of micropollutants and the local incident radiation may not be uniform in the reactor volume, which represent a clear obstacle to interpret the kinetic data obtained.
In this technical report, a novel VUV-collimated beam is pre- sented. An ozone-generating amalgam lamp was used as a source of radiation. The collimated beam is equipped with different optical filters which make possible to irradiate samples with 254, 185 nm, or both wavelengths.
DESCRIPTION OF THE VUV-COLLIMATED BEAM
The VUV-collimated beam (Fig. 1) comprised an ozone-generat- ing amalgam Hg lamp (Light Sources GPHVA357T5VH/4W) placed in a T-shape polyvinyl chloride (PVC) enclosure, which is continuously purged with nitrogen to remove oxygen present in air. The flux without purging nitrogen was measured to be 1.07 9 10!5 W m!2 which is too low for the degradation to take place at measurable rate. After purging the system with nitrogen the flux was much higher, 3.0 9 10!5 W m!2. Accord- ing to this result, nitrogen purging allows increasing the VUV flux reaching the sample by 75%. Two Teflon cylinders were placed around the quartz envelope to cover the ends of the lamp to improve the collimation of the radiation. The top of the T-shaped enclosure is sealed by a PVC head with a Suprasil quartz and/or an optical filter, which allows studying of a spe- cific wavelength during the batch kinetics studies. The quartz and filter are held by three Teflon O’rings, which seal the enclo- sure. Two different combinations of quartz windows and filters are used. To conduct experiments with only 185 nm radiation, a special optical filter is used to block 254 nm. This filter is formed by a MgF2 flat quartz coated with a metal–dielectric– metal aluminum thin film (eSource Optics). Its peak transmission wavelength is at 184.9 " 2.5 nm and its diameter and thickness are 50.8 " 0.25 and 4.2 mm, respectively. An analytical quality flat UV grade–fused silica window was placed under the 185 nm filter to protect the metallic coating from ozone that could be formed inside the enclosure. To expose samples to both 254 and 185 nm photons, another head with an analytical quality flat UV
grade–fused silica can be used. When only 254 nm is required, a regular germicidal Hg lamp (Light Sources GPHVA357T5L/4W) is used instead of the ozone-generating Hg lamp.
To irradiate water samples, two special cylindrical reaction vessels were built. The first vessel is an open top container made with regular quartz, except the bottom part made of Suprasil quartz to allow 185 and 254 nm radiation to be transmitted. The second vessel is similar as the first one, but with the top part closed, to make it possible to work with volatile compounds. The diameter and height of both vessels are 4.8 and 1.5 cm, respectively. A stirrer is mounted on top of the setup to mix the solution during the irradiation.
Ideally, a collimated beam setup should be able to provide uniform and collimated radiation to the solution. That is, the incident radiation at the window of the reaction vessel should be uniform and the direction all the photons should be normal to the window of the vessel. However, in real setups, these two requirements cannot be completely satisfied, and quasi-collimated radiation with small changes in the incident radiation is achieved. To analyze the uniformity and extent of collimation of the radia- tion reaching the reaction vessel, a radiation model based on the Monte Carlo technique was developed. The approach and hypotheses considered in the model were the same as presented by previous studies (3). Figure 2 shows the normalized incident radiation at the bottom of the vessel. A fairly uniform distribu- tion of the radiation was obtained and the difference between the maximum incident radiation at the center of the vessel and the minimum at the borders was lower than 5%. Similarly, modeling results show a small divergence of the radiation (Fig. 3) and 90% of the beams are deviated <20° from the normal. The dis- persion of radiation was reduced by the two Teflon tubes sur- rounding the quartz sleeve (Fig. 1), which cut the beams with a high divergence of radiation. Modeling results show that the developed collimated beam can provide uniform and quasi- collimated radiation.
Finally, the fluence rate, a fundamental parameter for photo- chemical studies, was measured using a radiometer and an acti- nometric technique. In the first case, a research radiometer was
used (research tungsten halogen/CS769 radiometer from Inter- national Light Technologies). The sensor (185 nm Au Cathode Vacuum phototube with quartz window) was placed on top of the optical filter and the fluence rate value was determined. Methanol actinometry (4) was also used. Samples were prepared with a relatively high concentration of methanol (300 ppm) in MilliQ water. Samples were irradiated during different periods of time and the concentration of methanol was quantified with an enzymatic method (5). The fluence obtained in both cases (radio- meter and actinometry) was 0.03 mW cm!2.
The degradation of atrazine was measured with this setup. First, a set of experiments were conducted to follow the degrada- tion of atrazine with only 254 nm radiation. After that, the same experiments were performed using only 185 nm radiation. It is clear (Fig. 4) that 185 nm radiation is much more efficient to degrade atrazine because of the generation of HO radicals, partic- ularly taking into account that the fluence rate at 185 was 10 times lower than at 254 nm. These data can be obtained using the proposed collimated beam setup, which is an option that allows studying the effect of radiation at 254 and 185 nm sepa- rately. The degradation obtained, hence, would be the combina- tion of the two wavelengths.
The VUV-collimated beam presented in this technical note is a very useful tool to study the degradation of pollutants using VUV radiation and other research involving VUV-induced photoreactions.
Acknowledgements—The authors are grateful to RES’EAU-WaterNET Strategic Network and to Natural Sciences and Engineering Research Council of Canada (NSERC) for their financial support.
REFERENCES
1. Kruithof, J. C., P. C. Kamp and B. J. Martijn (2007) UV/H2O2 treat- ment: A practical solution for organic contaminant control and pri- mary disinfection. Ozone-Sci. Eng. 29, 273–280.
2. Shemer, H., Y. K. Kunukcu and K. G. Linden (2006) Degradation of the pharmaceutical Metronidazole via UV, Fenton and photo-Fenton processes. Chemosphere 63, 269–276.
3. Imoberdorf, G. E., F. Taghipour and M. Mohseni (2008) Radiation field modeling of multi-lamp, homogeneous photoreactors. J. Photoch. Photobiol. A 198, 169.
4. Heit, G., A. Neuner, P. Y. Saugy and A. M. Braun (1998) Vacuum- UV (172 nm) actinometry. The quantum yield of the photolysis of water. J. Phys. Chem. A 102, 5551–5561.
5. Klavons, J. A. and R. D. Bennett (1986) Determination of methanol using alcohol oxidase and its application to methyl ester content of pectins. J. Agric. Food Chem. 34, 597–599.
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