Sonolysis of per- and poly fluoroalkyl substances (PFAS): A meta-analysis

Abstract

Human ingestion of per- and polyfluoroalkyl substances (PFAS) from contaminated food and water is linked to the development of several cancers, birth defects and other illnesses. The complete mineralisation of aqueous PFAS by ultrasound (sonolysis) into harmless inorganics has been demonstrated in many studies. However, the range and interconnected nature of reaction parameters (frequency, power, temperature etc.), and variety of reaction metrics used, limits understanding of degradation mechanisms and parametric trends. This work summarises the state-of-the-art for PFAS sonolysis, considering reaction mechanisms, kinetics, intermediates, products, rate limiting steps, reactant and product measurement techniques, and effects of co-contaminants. The meta-analysis showed that mid-high frequency (100 - 1,000 kHz) sonolysis mechanisms are similar, regardless of reaction conditions, while the low frequency (20 - 100 kHz) mechanisms are specific to oxidative species added, less well understood, and generally slower than mid-high frequency mechanisms. Arguments suggest that PFAS degradation occurs via adsorption (not absorption) at the bubble interface, followed by headgroup cleavage. Further mechanistic steps toward mineralisation remain to be proven. For the first time, complete stoichiometric reaction equations are derived for perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) sonolysis, which add H₂ as a reaction product and consider CO an intermediate. Fluorinated intermediate products are derived for common, and more novel PFAS, and a naming system proposed for novel perfluoroether carboxylates. The meta-analysis also revealed the transition between pseudo first and zero order PFOA/S kinetics commonly occurs at 15 - 40 µM. Optimum values of; ultrasonic frequency (300 - 500 kHz), concentration (>15 - 40 μM), temperature (≈20 °C), and pH range (3.2 - 4) for rapid PFOX degradation are derived by evaluation of prior works, while optimum values for the dilution factor applied to PFAS containing firefighting foams and applied power require further work. Rate limiting steps are debated and F^- is shown to be rate enhancing, while SO₄^2- and CO₂ by products are theorised to be rate limiting. Sonolysis was compared to other PFAS destructive technologies and shown to be the only treatment which fully mineralises PFAS, degrades different PFAS in order of decreasing hydrophobicity, is parametrically well studied, and has low-moderate energy requirements (several kWh g^-1 PFAS). It is concluded that sonolysis of PFAS in environmental samples would be well incorporated within a treatment train for improved efficiency.

Keywords: Literature review; Meta-analysis; PFAS; Parametric; Sonolysis; Ultrasonic degradation.

Graphical abstract

Fig. 1

Example of a C8 perfluorocarbon chain and four possible functional groups of PFAS, top to bottom: Perfluorononanoic acid (PFNA), Perfluorooctane sulfonic acid (PFOS), Perfluorooctyl phosphonic acid (PFOPA) and, 8:2 Fluorotelomer alcohol (8:2 FTA) .

Fig. 2

Position and relative collapse temperatures and pressures experienced by hydrophilic, semi-hydrophilic and hydrophobic surfactants, as well as relative anchoring and spacing of short and long chains at ultrasonic cavities. Not to scale.

Fig. 3

Competing PFAS sonolysis mechanisms at mid-high frequencies: PFAS adsorption to bubble interface under ultrasound (1), bubble growth to critical size (2) bubble collapse and reaction initiation via thermolysis (3a) vs solvated electron attack (3b), release of truncated perfluoro moiety and repeated oxidation-truncation (1-4a loop) vs sono-intermediate pyrolysis in bubble core (4b), formation of C1-C2 intermediates (5) and intermediate hydrolysis to end products in liquid bulk (6). Not to scale.

Fig. 4

Comparison of frequency effects on the relative reaction rate (scaled to the fastest rate observed in all three experiments) for two works on PFOA and PFOS (Campbell et al. 2009 [41]) and PFOS (Wood et al. 2020 [36]) sonolysis at constant applied power.

Fig. 5

Truncation mechanism for PFECs with removal of C₂F₄O group. Plausible and observed reaction given to demonstrate possible repeated truncation from Tridecafluoro-3,6,9-trioxadecanoic acid (TDFTODA) to NFDOHpA.

Fig. 6

Range and optimum temperatures tested in PFAS sonolytic works (note that the data for Shende et al. indicates the range of final temperatures achieved during the reaction, not the temperature during treatment).

Fig. 7

Optimum solution pH and pH ranges tested for all known PFAS sonolytic works.

Fig. 8

Levels of control and interconnected nature of sonolysis reaction parameters.