Date of Award
5-2026
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Environmental Engineering and Earth Science
Committee Chair/Advisor
Dr. Ezra L. Cates
Committee Member
Dr. Tanju Karanfil
Committee Member
Dr. David Ladner
Committee Member
Dr. Brain Powell
Abstract
PFAS are persistent synthetic chemicals, with unique properties that enable them to easily enter and accumulate in water sources. Their public health impacts have raised concern and highlighted the need for effective treatment. Numerous techniques for PFAS removal from water have been studied, few have proven the ability of breaking down PFAS rather than separating and concentrating it. Many of these methods are chain-length dependent, showing slower degradation rate of short-chain PFAS and requiring substantial amounts of energy, and chemical agents that may produce byproducts.
Among available PFAS destruction methods, direct photolysis by high-energy UV with wavelength below 200 nm (VUV) is a simple, chain-length independent method. During the literature review, I recognized that this method relies on different light sources; some commercially available, such as low-pressure mercury (LP-Hg) lamps, which exhibit efficiency limitations and variations, and other less commercially available, such as corona discharge xenon excimer (Xe2*) lamp, which have not been thoroughly tested on PFAS. These findings indicate strong potential for process enhancement and optimization, either as a standalone or add-on PFAS destruction tools.
Therefore, this dissertation aims to address these limitations through the following objectives: (i) assess LP-Hg lamp components and determine key hardware considerations for the effective PFAS photolytic destruction, (ii) explore PFAS photodegradation using an unconventional VUV source – Xe2* lamp, (iii) experimentally verify the significantly faster photocatalytic degradation involved in perfluorooctanesulfonate (PFOS) under enhanced UV/VUV source with hexagonal-Boron Nitride (hBN). These objectives are addressed through the following studies.
The first study focused on direct photolysis by 185 nm photons emitted by LP-Hg lamps. Since LP-Hg lamps have been used mainly in ultraviolet-C applications, output data and operational guidelines specific to VUV are limited. In this study, I assessed perfluorobutanoic (PFBA) photodegradation using LP-Hg lamp systems from various manufacturers to evaluate its output variability and isolate key design and operational aspects. Results indicated wide variation in VUV output, with over a 90% reduction in electrical energy per order destruction (EE/O) between the least and most effective combinations of lamp, ballast, and sleeves, due to physical design differences. Furthermore, output efficiency was affected by the steady-state temperature achieved when submerged in the photoreactor, relative to the lamp’s intended operating temperature. Consequently, VUV intensity comparisons performed in an N2-purged collimated beam with a VUV radiometer were poor predictors of performance when submerged in water. The results also suggest that many past laboratory studies of VUV photolytic treatment likely used sub-optimal lamp configurations, resulting in misleading results with respect to VUV energy efficiency.
In the second study, I explored Xe2* lamp, which has high wall-efficiency emission centered at 172 nm with a full width at half maximum (FWHM) of ±7 nm and a range of 155-191 nm, under various treatment conditions, mixing speeds, and power supply settings. Despite the ~50× higher VUV output and generation efficiency of the Xe2* source, degradable perfluorocarboxylic acids (PFCAs) showed lower degradation efficiency under Xe2* lamp than under LP-Hg lamp. The PFBA showed greater sensitivity to Xe2* lamp irradiation than other PFCAs of both longer and shorter chain lengths, while PFSAs showed minimal sensitivity. Results revealed up to ~70% degradation of PFBA at 50 ppb at pH 7, and up to 25% of fluoride recovery within 2 hours. Solution pH had a mild impact on PFBA photolysis using Xe2* source between pH 4 and 10, with a significant decrease at pH 2.5. Mixing speed and nitrogen purging showed no enhancement in PFBA photolysis. However, power supply settings notably influenced PFBA photolysis under 155-191 nm irradiation, suggesting performance dependencies that merit further investigation for process optimization. These results combined support a new hypothesis related to solution conditions within the very thin Xe2* irradiation zone. High solution absorbance and water splitting quantum yield below 175 nm likely create a fluctuating, self-attenuating layer that reduces 175-191 nm transmittance and limits PFAS mass transfer.
In the third study, I systematically verified the mechanisms behind enhanced PFAS degradation under optimized 185/254 nm irradiation with hBN photocatalyst. This work built upon the optimum LP-Hg lamp configuration identified in my first study, later integrated into a separate team study1, which showed significant PFOS degradation.1 In my study, a controllable 172 nm-Xe2* excimer source was employed to elucidate the photochemical mechanisms. Results revealed a two-photon mechanism involving combined VUV/UVC irradiation on PFOS-hBN system, rather than a simple photocatalytic effect. VUV radiation dominated PFOS accelerated photocatalytic degradation, while direct VUV photolysis alone was ineffective. Understanding this complex mechanism contributes to broader applications of VUV-enhanced photocatalysis, especially where conventional UVC photocatalysis is less effective.
Recommended Citation
Alhamdan, Eman Zuhair, "Investigation of Low-Pressure Mercury Lamp System Configurations and Xenon Excimer Lamp System Development as Vacuum Ultraviolet Sources for Optimal Photolytic Degradation of Poly-/Perfluoroalkyl Substances" (2026). All Dissertations. 4218.
https://open.clemson.edu/all_dissertations/4218