- Title
- Experimental and Theoretical Investigation of the Thermal Decomposition of Per- and Poly-Fluoroalkyl Substances (PFAS)
- Creator
- Weber, Nathan
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2023
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Per- and poly-fluoroalkyl substances (PFAS) are an increasingly pressing contamination issue. One of the most effective technologies for removing and destroying PFAS from the environment is thermal treatment, which uses high temperatures to decompose PFAS. However, limited research has been conducted to determine if the common PFAS of perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) can be safely destroyed without releasing harmful emissions into the environment. An experimental methodology was developed to facilitate the study of the thermal decomposition of PFOS and PFOA under controlled reaction conditions using flow reactors. In support of the experimental studies, quantum chemical calculations were undertaken to elucidate and analyse the elementary reactions involved, thus creating a kinetic model that reflects the thermal decomposition of PFOS. The pyrolysis of PFOS in inert bath gases (helium) was studied in an α-alumina flow reactor between 400 and 1,000 °C. PFOS was confirmed (consistent with the primary reactions predicted from quantum chemical simulation) to decompose into the products of HF, SO2 and perfluorooctanoyl fluoride (C8F16O) at temperatures above 400 °C. However, as the reaction temperature increased above 500 °C, a new product (C2F4) was discovered alongside the expected HF, SO2 and C8F16O decomposition products. To understand the pyrolysis mechanism, additional quantum chemical simulations were conducted with the aim of developing a multistep chemical kinetic model describing the thermal decomposition of PFOS. A competing mechanism for the initial thermal decomposition of PFOS was discovered, in which a PFOS molecule fissions into C8F17 and HOSO2 radicals, and subsequently, C8F17 will fission into the singlet carbene CF2, which can combine to form C2F4. A reaction pathway found that C8F16O fissions into FCO and C7F15, and eventually into CF2. Water vapour (H2O(g)) was added to the helium bath gas, and the effect it has on PFOS decomposition was studied in the temperature range of 500 – 1,000 °C. Parallel experimental studies on the effect of H2O(g) (present with PFOS at 1,700 and 10,000 ppm) established that the primary decomposition products were HF, CO, SO2 and C2F4, similar to the products observed under inert conditions. However, as the temperature increased, the concentration of HF and CO increased while the total overall concentration of fluorocarbons products decreased. A chemical kinetic model, with the additional quantum chemical insights, provided crucial insights into understanding the impact H2O(g) and OH radicals have on the singlet carbene CF2 that are produced. Changing the bath gas from helium to compressed air (notably to examine the effect of O2), the thermal decomposition of PFOS was studied in an α-alumina reactor under reaction temperatures between 500 and 1,000 °C. The air bath gas was found to result in the formation of significant quantities of COF2. Quantum chemical calculations confirmed that O2 reacts directly with CF2 to form COF2 and an O atom. Adding 2,000 and 20,000 ppm of H2O(g) to the air bath gas proved to engender the mineralisation of PFOS into HF, CO2 and SO2. At temperatures above 850 °C and in the presence of 20,000 ppm of H2O(g), the only products detected were HF, CO2 and SO2. An elemental F balance determined that 99 ± 5% of the F atoms present in PFOS have been converted into HF, and approximately 100 ± 5% of C atoms were converted into CO2. The chemical kinetic model was expanded to include reactions with O2 and H2O(g), demonstrating that excess OH radicals were produced, ensuring all the CF2 decompose into HF. The impact of different reactor tube materials (quartz, stainless steel 316 and stainless steel 253 MA) on the thermal decomposition of PFOS was investigated between 400 and 1000 °C using helium as the bath gas. The quartz reactor, at temperatures above 700 °C, produced significant amounts of SiF4. The SS 316 reactor produced similar products to the α-alumina reactor until the temperature exceeded 800 °C. At temperatures above 800 °C, a reduction in the concentration of SO2 was observed, and the formation of a blue sulfur solid complex was observed on the surface of the reactor. Stainless steel 253 MA also generated considerable quantities of SiF4 without a notable decrease in HF concentration. The thermal decomposition of PFOA under inert pyrolysis conditions was studied in both α-alumina and SS reactor tubes between 400 and 1,000 °C. Between 400 and 600 °C PFOA primarily decomposes into perfluoroheptene (C7F14), CO2 and HF. This experimental observation is in disagreement with the primary products of HF, CO and C7F14O predicted from quantum chemical simulation of PFOA decomposition. Notwithstanding, quantum chemical calculations revealed that PFOA can be physisorbed onto an alumina nanocluster, subsequently releasing C7F14 and CO2 and reducing the rate of formation of CO and C7F14O. As the temperatures reached above 600 °C, C2F4 and C2F6 were the dominant products from the thermal decomposition of C7F14O.
- Subject
- PFAS; thermal; decomposition; kinetics
- Identifier
- http://hdl.handle.net/1959.13/1500383
- Identifier
- uon:54913
- Rights
- Copyright 2023 Nathan Weber
- Language
- eng
- Full Text
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| View Details Download | ATTACHMENT01 | Thesis | 17 MB | Adobe Acrobat PDF | View Details Download | ||
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