ORCID Identifier(s)

0000-0003-2827-5113

Graduation Semester and Year

2021

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Civil Engineering

Department

Civil Engineering

First Advisor

Hyeok Dr. Choi

Abstract

Per- and polyfluoroalkyl substances (PFAS) are anthropogenic compounds comprised of a perfluoroalkyl backbone and a terminal functional group. Their detrimental health effects of in humans, like other halogenated chemicals, have been well documented and thus the frequent occurrence of PFAS in the water environment is a recent global concern. After US Environmental Protection Agency (EPA) released a drinking water health advisory for the two most detected long chain PFAS (such as perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS)), industrial production and application of short chain PFAS continues to rise. Feasible and sensible treatment strategies are in dire need for environmental remediation and water treatment. No studies so far have demonstrated the reactivity of PFAS as a function of alkyl chain length and functional group in TiO2 photocatalysis, which is considered as one of the promising advanced oxidation technologies (AOTs) and the most cost-efficient technologies for PFAS remediation. This research is presented in two subsections, where decomposition of selected PFAS was evaluated under various TiO2 photocatalytic and photochemical conditions inducing AOTs. In the first study, we carefully selected 9 PFAS based on chain lengths, functional groups, and structural properties, namely 4 perfluorocarboxylic acids (PFCAs) including PFOA, 3 perfluorosulfonic acids (PFSAs) including PFOS, hexafluoropropylene oxide dimer (GenX), and 6:2 fluorotelomer sulfonate (6:2 FTS) and investigated dependency of the photocatalytic decomposition of PFAS on their properties. In addition, various chemical oxidants and reductants were introduced with titanium dioxide (TiO2) photocatalysis and elucidate the decomposition mechanism of both carboxylic and sulfonic PFAS. Some notable findings include long chain PFCAs and 6:2 FTS were removed in the TiO2/UVC system mostly via chemical decomposition while GenX, PFSAs, and short chain PFCAs were removed mostly via physical adsorption. Sulfate radicals (SRs, SO4•-) generated with PS played an important role in decomposing both long and short chain PFCAs, while these radicals were ineffective to PFSAs. Hydrated electron produced from activation of sulfite by UVC defluorinated both PFCAs and PFSAs significantly. Overall susceptibility of PFAS to the chemical reactions was explained with their properties and the reactivity of reactive species produced in each system. In the second study, lead-doped TiO2 coated with reduced graphene oxide (TiO2-Pb/rGO) was developed as a ternary photocatalyst to overcome the limitation of the poor reactivity of neat TiO2 photocatalysts caused by quick electron-hole recombination. Degradation kinetics of PFOA using this catalyst was compared to that of bare TiO2, TiO2-Pb, TiO2 /rGO, and TiO2-Pb/rGO under UVC. High photocatalytic activity of TiO2-Pb/rGO was observed, which is attributable to the effects of rGO and Pb addition on TiO2 bandgap energy, surface interfacial charge transfer mechanisms and oxygen diffusivity. The role of Pb and rGO in TiO2 was investigated, and the reactive species responsible for the reaction was identified. The radical quenching experiments implied the main roles of hole and oxygen on the highly efficient photodegradation of PFOA. Significant byproduct and fluoride release were observed upon decomposition of PFOA. This system showed great potential for in situ application of polyfluorinated compounds and long chain PFCAs remediation. The efficacy of these systems for a wider range of PFAS was evaluated. The reaction mechanism for the system was complex and future studies should identify an appropriate TiO2-metal/rGO pair capable of decomposing sulfonic PFAS. Here, first study compared photocatalytic a decomposition of 9 PFAS mainly PFCA and PFSA but future studies should explore other PFAS and compare reactivity to current study. In addition, it would be important to understand the effect of organic and inorganic contaminants present in water in photocatalytic and photochemical decomposition of PFAS. In second study, only long chain PFCA were successful degraded by TiO2-Pb/rGO under UVC. Follow up studies investigating various TiO2-Pb/rGO oxidant/reductant combinations could potentially decompose other PFAS groups.

Keywords

Photocatalysis, Photochemical decomposition, Photolysis, Per- and polyfluoroalkyl substances (PFAS), PFAS adsorption, Advanced oxidation, Lead and reduced graphene oxide modified titanium dioxide

Disciplines

Civil and Environmental Engineering | Civil Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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