Towards upscaling the sonolysis of per- and poly-fluoroalkyl substances (PFAS)
Creators: name: Sidnell, Timothy Patrick
Publisher: name: University of Surrey
Year: 2023

description: Per- and polyfluoroalkyl substances (PFAS) comprise over 9,000 manmade chemicals with the CnF2n+1 moiety. Their hydro/lipophobicity and thermal/chemical inertness made PFAS invaluable in numerous applications, from food packaging to firefighting. Over many decades, PFAS have infiltrated global environments, causing increased cancer rates and many other illnesses. They are poorly metabolised and unaffected by conventional water treatment or environmental processes. Of many technologies trialled to date, PFAS degradation using ultrasound (sonolysis) is highly effective. However, limited knowledge of degradation mechanisms and scalable reactor designs have prevented its industrialisation. Hence, the optimisation of a scalable PFAS sonolytic reactor is presented here. Perfluorooctane sulfonic acid (PFOS) acted as a model PFAS, due to its environmental prevalence and slow sonolysis rate. <br>
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description: The reaction mechanism was shown to be consistent at frequencies of 100-1,000 kHz and is initiated via PFAS adsorption at the bubble interface, followed by cleavage of the tail-headgroup bond. However, the mechanism varies more significantly below 100 kHz. For the first time, complete stoichiometric equations for PFOS and PFOA sonolysis were derived which indicate H2 and H+ as products and CO as an intermediate. In agreement with prior literature, PFOS degraded fastest under 400 kHz ultrasound, within the range 44-1,000 kHz. More novel, rates reduced at ultrasonic power intensities exceeding 2.84 W cm-2, due to transducer-fluid decoupling. Hence, power was later optimised based on transducer area and multiple transducers thus enabled modular scale-up. This improved efficiency by maximising amplifier power utilisation. Temperatures of 20-30 °C hastened degradation, but increased cooling costs. In contrast with literature, liquid height scarcely affected rates when power density (W L-1) was controlled. Correlating degradation rates with other sonochemical metrics indicated that solvated electrons degrade PFOS, not radicals nor thermolysis. Likewise, flow rates which moderately perturb bubble surfaces enhanced degradation, likely by increasing PFOS-electron interactions. Treatment of PFAS in Ozofractionated landfill leachate and aqueous film forming foam required influent-specific pre-treatments (acidification and 10x dilution, respectively). 90 % PFAS removal was 100x more expensive in the concentrated foam, despite fast sonolysis rates, due to its high concentration.<br>
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description: After completion of this thesis, sonolysis remains a promising technology for cleansing the world of PFAS pollution. However, optimisation was done with limited resolution, and further tests or modelling approaches could clarify true optimum conditions. Further mechanistic steps in the PFAS sonolysis process also remain to be proven. Other routes to scale-up, such as single reactor multi-transducer/frequency or wider transducers could also be promising. Finally, incorporation of sonolysis with other pre- and post-sonolysis treatments (prior to environmental emission) are the last major research hurdles. Hence, PFAS sonolysis could be utilised industrially and in wastewater treatment plants within a few years. Meanwhile, modelling of efficient, and scalable designs for more general sonolysis reactors will likely continue to be researched for some time longer.<br>