School of Civil and Environmental Engineering
Ph.D. Thesis Defense Announcement
Advanced Treatment Processes for Destruction of Per- and Polyfluoroalkyl Substances (PFAS) in Water Treatment
By Xiaoyue Xin
Advisor:
Dr. Ching-Hua Huang
Committee Members: Dr. Yongsheng Chen (CEE), Dr. Sotira Yiacoumi (CEE), Dr. Shane Snyder (CEE), Dr. Daniel Ashley (Spelman College)
Date and Time: July, 18, 2025. 9:00 – 11:00 AM EST
Location: Ford ES&T 3229
Teams Meeting ID: 220 473 631 260 7; Passcode: QQ3DW6wK
Per- and polyfluoroalkyl substances (PFAS) are a class of persistent anthropogenic
contaminants recognized for their widespread environmental occurrence,
bioaccumulation, and significant health implications. Their exceptional resistance
to conventional water treatment methods underscores the critical need for
innovative remediation technologies. Conventional UV lamps, emitting primarily at
254 nm or higher wavelengths, are useful in advanced water treatment processes;
however, these lamps present some limitations, including mercury use, limited
photon energy, and low efficiency in generating reactive species for contaminant
degradation. This study investigates novel far-UVC irradiation at 222 nm, a mercuryfree,
higher photon energy UV technology with the potential of enhancing reactive
species generation and improving contaminant degradation efficiency. This
research comprehensively evaluates the effectiveness of far-UVC 222 nm as an
advanced treatment strategy for PFAS-contaminated waters through direct
photolysis, advanced reduction processes (ARPs), and advanced oxidation
processes (AOPs).
First, this study investigated the susceptibility of 19 representative PFAS to direct
photolysis and defluorination under far-UVC 222-nm irradiation. Enhanced
photolysis occurred for perfluorocarboxylic acids (PFCAs), fluorotelomer
unsaturated carboxylic acids (FTUCAs) and GenX, compared to that at conventional
254-nm irradiation on a similar fluence basis. In contrast, other PFAS, including
PFSAs, 6:2 diPAP, 5:3 FTCA, 6:2 FTS, FOSA and FHxSA, showed minimal decay by
photolysis under UV 222 nm irradiation. For degradable PFAS, up to 81% of parent
compound decay (photolysis rate constant (k222-nm) = 8.19-34.76 L·Einstein-1;
quantum yield (222-nm) = 0.031-0.158) and up to 31% of defluorination were
achieved within four hours, and the major transformation products were shorterchain
PFCAs. Solution pH, dissolved oxygen, carbonate, phosphate, chloride and
humic acids had mild impacts, while nitrate significantly affected PFAS
photolysis/defluorination at 222 nm. Decarboxylation is a crucial step of photolytic
decay. The slower degradation of short-chain PFCAs than long-chain ones are
related to molar absorptivity and may also be influenced by chain-length dependent
structural factors, such as differences in pKa, conformation, and perfluoroalkyl
radical stability. Meanwhile, the possible transformation pathway of PFCAs was
explored using density functional theory (DFT)-based theoretical calculations.
These new findings are among the first to demonstrate the capability of 222-nm
light to degrade PFAS and provide the basis for further development of far-UVC
technology for PFAS in water treatment.
Second, PFAS degradation by integrating far-UVC irradiation at 222 nm with sulfitebased
advanced reduction processes (ARPs) was investigated. The UV-based ARPs
have emerged as an effective strategy to degrade and defluorinate PFAS
contaminants in water. However, current studies have mainly focused on UV 254
nm irradiation, and the feasibility of treating PFAS with UV222/ARP remains
unknown. Comparative analysis on the fundamental photochemical properties of
UV222/sulfite systems and conventional UV254/sulfite systems revealed that 222-
nm irradiation significantly improves the performance by generation of more
hydrated electrons (eaq
-), the primary reactive species driving PFAS degradation, and
exhibits superior energy efficiency, characterized by lower electrical energy per
order (EEO). The higher efficiency of UV222/sulfite can be attributed to stronger light
absorption of sulfite and higher photon energy at 222 nm. Under optimized stepwise
sulfite dosing conditions, the UV222/sulfite ARP achieved high perfluorooctyl
sulfonic acid (PFOS) removal efficiency, nearly 85% reduction in parent compound
and 66% defluorination within a six-hour period, while the degradation of shorterchain
PFHxS and PFBS was slower. Real water matrix components can influence
treatment efficiency. The impacts of nitrate/nitrite were transient that diminished
after rapid photolysis at 222 nm, while dissolved organic matter (DOM) and
carbonates exerted strong reactive species scavenging effects. These findings
establish UV222/sulfite ARP as a promising strategy to enhance PFAS degradation.
Careful optimization of system parameters and water matrices will increase the
adaptability for PFAS environmental remediation.
Third, a pilot-scale investigation was conducted to assess the influence of ozonation
(O3) and ozone/hydrogen peroxide (O3/H2O2) advanced oxidation process (AOP),
respectively, on the fate of PFAS in a wastewater effluent subjected to reuse. The
objective was to assess the potential of PFAS transformation by AOP under real
water treatment conditions. The study evaluated 40 target PFAS and associated
precursors (based on the total oxidizable precursor (TOP) assay) under various
treatment conditions, including different ozone doses (1.0-4.0 mg·L-1), H2O2 doses
(0-0.20 mg·L-1), and contact time (0–20 min). Results indicated that short-chain (C3-
C7) PFAAs dominated in concentrations, while overall PFAA concentrations were
elevated by both oxidative treatment processes, particularly after high-dose
ozonation treatment. TOP assays revealed that there were considerable amounts of
PFAA precursors in the reuse wastewater and their concentrations were decreased
after the oxidative treatment with increase of some of the PFAAs. This pilot study
demonstrated that ozone and ozone-based AOP treatments can have a moderate
influence on the transformation of PFAS and increase of PFAA levels under practical
conditions.
Last, based on previous study suggesting the potential of ozone and AOP in the
transformation of PFAS precursors into terminal PFAAs, and growing research
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evidence supporting the potential of oxidizing PFAA precursors into terminal PFAAs
through UV/AOPs, mainly through the facilitation of generation of reactive radical
species, PFAS degradation by integrating far-UVC irradiation at 222 nm with
peroxydisulfate (PDS)-based AOPs was thus explored. Comparative analysis of
UV222/PDS and conventional UV254/PDS systems revealed that 222-nm irradiation
significantly enhanced the generation of reactive radical species, including hydroxyl
and sulfate radicals, thereby accelerating degradation kinetics of PFAS precursors.
Radical generation was highly influenced by reaction conditions, such as solution
pH, initial PDS dose, and UV fluence. PFAS precursors including 6:2 FTSA, 6:2 FTCA,
and FHxSA showed rapid and complete decay within a short irradiation time. A
unique advantage of UV222/PDS treatment was the capacity for continuous
degradation, as PFAS precursors transformed into terminal PFCAs, subsequently
undergoing direct photolysis, resulting in chain-shortening and moderate overall
defluorination. However, terminal PFAS such as PFOA demonstrated limited
enhancement under UV222/PDS conditions. GenX exhibited moderate
improvement, whereas Perfluoro(2-ethoxyethane)sulfonic acid (PFEESA) remained
resistant to degradation by UV222/PDS. Real water matrices notably reduced
UV222/PDS efficiency, slowing PFAS precursor degradation and increasing
intermediate byproduct formation. UV222/PDS systems present significant
promises for enhancing remediation of PFAS precursors, although addressing
limitations associated with terminal PFAS and complex water matrices remains
essential for broad environmental applicability.
Overall, this study demonstrates the exceptional potential and significant
advantages of far-UVC irradiation at 222 nm for PFAS remediation, presenting
detailed mechanistic insights and identifying both strengths and ongoing
challenges. The thorough exploration of water matrix effects enhances the practical
relevance of these findings, offering valuable guidance for optimizing treatment
strategies in real-world applications. This research substantially advances scientific
understanding and informs the future development of innovative, scalable, and
sustainable solutions to address global PFAS contamination challenges.