Development of DBD Plasma-based UV-C Excimer Light Sources for Health and Environmental Applications
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Date
15-07-2024
Researcher
Kiran
Supervisor
Prakash, Ram
Journal Title
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Volume Title
Publisher
Indian Institute of Tehcnology, Jodhpur
Abstract
To render unwanted or harmful microorganism’s incapable of reproducing, the ultraviolet light is very useful in our daily life. Ultraviolet (UV) light is a spectrum of light just below the range visible to the human eye. The UV light is divided into four distinct spectral areas, and they are Vacuum Ultraviolet (VUV) (100–200 nm), UV-C (200–280 nm), UV-B (280–315 nm) and UV-A (315–400 nm). These spectral areas are very specific. The UV-C radiation from 200–280 nm is highly useful to inactivate a wide range of microorganisms based on DNA absorption capacity in this range. This spectral range can be further utilized in advanced oxidation processes (AOPs) to generate reactive species like hydroxyl radicals (•OH) to oxidize organic and inorganic contaminants in water and wastewater. When we look for the real-life problems, wastewater discharged from the textile industry contains approximately 15% unfixed dyes, predominantly 60–70% azo dyes, which pose significant environmental and health risks due to their persistence and potential toxicity. Also, organic micro-pollutants (OMPs) have become common causes of pollution and have attracted considerable attention in recent years due to their extensive environmental consequences. To mitigate the issue, in the recent decades, dielectric barrier discharge (DBD) plasma-based excimer/exciplex sources are being researched in generating UV-C (230–280 nm) and far UV-C (200–230 nm) light radiations. This thesis work is focused on the design, development and optimization of cylindrical and planar excilamps emitting different wavelengths in UV regions for health and environmental applications. In the present work, DBD plasma-based far UV-C (KrCl*) and an advanced UV-C (XeI*) excilamps with a very narrow and intense spectrum peaking at a wavelength of 222 nm and a wide band 253 nm, respectively, have been developed and used for the health and environmental applications. Two types of DBD geometries, i.e., planar and coaxial, have been worked out. In this context, the optimization of high-voltage electrodes has been carried out to minimize lamp heating without any external cooling. The main reason for the controlled heating is the homogenous discharge at low gas pressure and the helical moulded copper wire electrode with optimized pitch. These lamps are optimized for higher efficiency. Due to the confined gas gap and bi-polar pulse power arrangements, the spectra obtained from far UV-C (KrCl*) are very narrow, having a full width half maximum (FWHM) of 1.7–1.9 nm. The electrical to optical conversion efficiency for this lamp is found to be ~12.5% and complete inactivation of S. aureus and E. coli bacteria with initial control ~ 107 CFU/ml is achieved at a UV dose of 3 mJ/cm2 and 12 mJ/cm2, respectively. In furtherance to it, the optimized XeI* and KrCl* excilamps are utilized for the treatment of wastewater, mainly containing azo dyes and OMPs by direct photolysis or in combination with AOPs (i.e., excilamp/TiO2 and excilamp/H2O2). An advanced process is developed in which one electrode of XeI* excilamp is coated with the photocatalyst, improving the Reactive Black 5 (RB5) mineralization efficiency. The result confirms 13 times faster degradation in XeI*-excimer/H2O2 than XeI*-excimer/TiO2, attributed to an abundance of •OH generated by the modified XeI*- excimer/H2O2. A maximum energy yield of 5712 mg/kWh is reported in the case of XeI*- excimer/H2O2. Compared with 254 nm, RB5 shows 1.26 times higher molar absorption at 222 nm. The obtained energy yield (6565 mg/kWh) for excimer-222/H2O2 demonstrates that the process is efficient in terms of energy consumption. In one of the studies, non-thermal plasma (NTP) has been integrated with KrCl* excilamp to eliminate the use of chemicals and simultaneously enhance OMP degradation. Plasmaproduced nitrates (𝑁𝑂3-) and hydrogen peroxide (H2O2) have been utilized for the generation of •OH when exposed to KrCl* excilamp. The degradation rate of different OMPs is found to be several times higher under plasma + KrCl* excilamp compared to plasma + LPUV due to the generation of the abundance of •OH (46.7 × 10-8 M s-1) under KrCl* from 𝑁𝑂3- and H2O2 present in the water matrix. This is due to the higher molar absorption of 𝑁𝑂3- and H2O2 at 222 nm than 254 nm. This plasma coupling with KrCl* excilamp outperformed all the other methods regarding OMP degradation. Development and demonstration of efficient far UV-C radiation (222 nm) sources in this thesis for different applications at the lab scale is a significant achievement with promising future scope. More efforts are needed to implement far UV-C radiation sources in the actual conditions, with potential advantages for environmental sustainability, public health, and technological innovation.
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Citation
Kiran (2019). Development of DBD Plasma-based UV-C Excimer Light Sources for Health and Environmental Applications (Doctor's thesis). Indian Institute of Tehcnology, Jodhpur