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Article

Molecular Dynamics Simulation of the Impact of Functional Head Groups and Chain Lengths of PFAS Degradation Using Ultrasound Technology

by
Bruno Bezerra de Souza
,
Jitendra A. Kewalramani
,
Richard W. Marsh
and
Jay Meegoda
*
Civil and Environmental Engineering, New Jersey Institute of Technology, University Heights, Newark, NJ 07102, USA
*
Author to whom correspondence should be addressed.
Water 2025, 17(7), 1025; https://doi.org/10.3390/w17071025
Submission received: 25 February 2025 / Revised: 20 March 2025 / Accepted: 24 March 2025 / Published: 31 March 2025
(This article belongs to the Section Wastewater Treatment and Reuse)
Browse Figures
Versions Notes

Abstract

:
PFASs, or per- and polyfluoroalkyl substances, comprise a diverse group of synthetic chemicals known for their widespread use, persistence, and potential environmental and health risks. The sonolytic treatment of PFASs is one of the technologies with the ability to complete destruction without harmful byproducts. This study aims to provide a theoretical explanation for the sonolytic treatment of PFAS. Combining insights from molecular dynamics simulations with experimental data, the influence of chain length and functional headgroups on the PFAS destruction mechanism was investigated. The findings revealed that the impact on functional head groups and chain length on PFAS degradation via sonolysis treatment is complex and multifaceted. The preliminary degradation step is attributed to be headgroup cleavage, while differences in degradation rates between perfluorocarboxylic acids (PFCAs) and perfluorosulfonic acids (PFSAs) are primarily influenced by adsorption at the air–water interface of micro/nanobubbles created by ultrasound and dictated by compound hydrophobicity characteristics. Moreover, longer-chain PFAS compounds tend to degrade faster than shorter-chain counterparts due to their enhanced hydrophobic characteristics, facilitating adsorption and subsequent mineralization. The sonolytic environment significantly influences PFAS degradation, with aqueous sonolysis proving the most effective compared to dry pyrolysis or thermal combustion, highlighting the importance of considering environmental factors in remediation strategies. These insights provide valuable guidance for designing effective PFAS remediation strategies, emphasizing the need to consider molecular structure and environmental conditions. Further research and technological innovation are essential for developing sustainable approaches to mitigate PFAS pollution’s adverse impacts on human health and the environment.

1. Introduction

Per- and polyfluoroalkyl substances (PFASs) are a diverse group of synthetic chemicals composed of an alkyl chain with at least one fully fluorinated carbon atom [1,2,3]. Due to the presence of the C-F bond, one of the strongest covalent bonds in nature, PFASs exhibit high thermal and chemical stability. For this reason, along with their ability to repel oil and water and act as a surfactant, this class of chemicals has been utilized since the 1950s in an extended array of consumer and industrial applications [1,2,3,4,5,6]. These include clothing, cosmetics, firefighting foams, oil production, mining, and food processing [7,8]. However, these same chemical properties cause PFAS to be bio-accumulative and toxic to both humans and the environment [5,8,9]. The release of PFAS from fire-training/response sites, wastewater treatment plants, landfills, and industrial sites has led to global contamination of soil and groundwater [2,5,6,10]. Perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) are the most widely produced and frequently identified PFAS compounds [11,12,13,14,15,16].
The majority of PFAS consumption in humans occurs through drinking water due to its solubility [17]. Repeated exposure can result in PFAS accumulation in major organs, such as the lungs, liver, and brain, where PFASs can interact with tissue and serum proteins [8,9,18,19,20,21,22]. Several studies have linked PFAS exposure to changes in kidney and thyroid activity, endocrine disturbance, cancer formation, and immunotoxicity [8,9,23,24,25]. The release of PFASs has raised concerns for regulatory agencies due to their persistence and resistance to degradation combined with evolving toxicology research on this class of contaminants [2,7]. This has led to the establishment of worldwide regulations and use restrictions, as well as an increased need for PFAS removal technologies [3].
Various separation and destruction techniques have been used in the past in order to attempt to remove PFASs from water and degrade them [3,17]. Several methods, such as anion exchange resins, granular activated carbon, nanofiltration, foam fractionation, reverse osmosis, and others, can successfully remove PFASs from water [3,17,26]. However, these methods do not destroy the PFAS molecules; they only separate them from the water and generate PFAS-concentrated waste [3]. Prior to 2020, incineration was considered an appropriate method for the destruction of concentrated PFAS waste, but this method’s effectiveness came into question when the USEPA published a brief concluding that the fate of incineration byproducts is not clearly understood [27,28]. However, several other methods have shown promise for PFAS destruction, such as sonolysis, electrochemical oxidation, photocatalysis, plasma, supercritical water oxidation, and others [3,29]. This study primarily focused on PFAS destruction via sonolysis, which uses ultrasound waves to mineralize the PFAS.
High-frequency, high-power ultrasound technology, also known as sonolysis, has proven to be effective in mineralizing PFASs at room temperature, eliminating the release of fluoro-organics in gaseous form and producing fluoride ions, carbon monoxide, and carbon dioxide [3,30,31,32]. Sonolytic PFAS destruction involves the following stages of acoustic cavitation: bubble formation, bubble growth, and implosive collapse [31,33,34,35]. The intense energy released during the collapse of cavitation bubbles leads to both sonophysical and sonochemical effects, such as pyrolysis and radical reactions [31,33,35]. These processes are the driving mechanism of sonolytic PFAS degradation. The breakdown of PFAS through sonolysis is suggested to be associated primarily with high-temperature pyrolysis taking place at the surface of imploding cavitational bubbles. This phenomenon is described in detail by Suslick [36,37,38,39,40], Sidnell et al. [33], and various other authors in the literature [30,41,42,43,44,45,46].
Numerous researchers have observed through spectral analysis that the temperature during a nanobubble collapse generally falls within the range from 4000 to 6000 K, with an average value of approximately 5000 K being the most commonly observed [36,38,40,41,47,48]. In Power of Ultrasonics, Lauterborn and Mettin explain acoustic cavitation, noting that highly spherical, single bubbles in a bubble trap (single-bubble) can reach temperatures as high as 20,000 K, based on light-emission spectra, while temperatures in multi-bubble systems do not exceed 5000–6000 K [49]. Since ultrasound technology produces multi-bubble systems, where millions of bubbles continuously form and collapse, the collapse temperature of nanobubbles is typically estimated to be around 5000 K.
Upon reaching a specific size, bubbles undergo violent bubble implosion, resulting in the disassociation/ionization of water, gases, and other species within the bubble. This phenomenon leads to high localized temperatures in and around cavitational bubbles. Aqueous PFASs exhibit surfactant behavior, suggesting that PFAS mineralization primarily occurs at the air–water interface of the bubble. Prior to bubble implosion, the hydrophilic anionic head of PFASs orientates toward the bulk liquid, while the hydrophobic carbon–fluorine tail is in the bubble gas phase (Figure 1A). It is postulated that preceding bubble collapse, PFASs effectively accumulate and concentrate at the bubble surface. Consequently, sorption to the bubble interface is deemed the first step for PFAS mineralization during sonolysis, with PFAS diffusion to the air–water interface of the bubble identified as the likely rate-limiting process. Figure 1 schematically illustrates the key stages of PFAS degradation via ultrasound.
PFAS exhibits variations in properties such as carbon chain length and functional headgroups, resulting in nuanced interactions during the application of ultrasound. The limited comprehensive understanding of the kinetics, toxicity, environmental fate, and associated human health risks across the vast array of over thousands of PFAS compounds necessitates further exploration [50,51,52]. The efficacy of destruction treatment technologies on these diverse PFAS compounds remains relatively unknown [3,53].
While numerous researchers have delved into the sonolytic degradation of PFAS, much of the current understanding of ultrasound mechanisms is based on theoretical assumptions. This is largely due to the complex nature of the process, which involves extreme temperatures and rapid collapses, making it challenging to analyze each step in detail. Although previous studies have provided valuable insights, a complete real-time mapping of these stages using both experimental and computational approaches remains an ongoing effort.
The existing literature predominantly focuses on the final step (Figure 1C), leaving the adsorption (bubble formation) and collapse moments (Figure 1A and Figure 1B respectively) unexplored. Several studies have explored a numerical simulation approach to simulate bubble dynamics (part A) and contaminants and bubble surface interactions. The intricate nature of sonolysis, characterized by high temperatures (≈5000 K), localized bubble collapse, and short durations (nano/picosecond range), renders the assessment of these steps challenging. Consequently, the mechanistic understanding of PFAS destruction through ultrasound remains uncertain. This manuscript seeks to address these knowledge gaps by combining insights from molecular dynamics simulations with ultrasound experimental data to investigate the influence of chain length and functional headgroups on the PFAS destruction mechanism. Similar work has been performed by other authors, where they have compared different aspects of PFAS remediation (adsorption, destruction, and bioaccumulation) via laboratory experiments and virtual analysis [54].
This study leverages computational simulations to enhance our understanding of the mechanisms involved in PFAS degradation during ultrasound treatment. Although prior research has suggested the potential role of hydrophobicity, direct evidence supporting this idea has been limited. By systematically analyzing each stage of the process and integrating simulation data with experimental findings, this work provides further clarity on the influence of hydrophobicity in PFAS degradation. Moreover, it highlights how molecular properties—such as chain length and head group characteristics—affect degradation pathways. These findings build upon earlier hypotheses and contribute to a more detailed understanding of the fundamental factors governing PFAS breakdown through ultrasound technology.
This study attempts to bridge the information gap surrounding the potential byproducts arising from PFAS destruction under sonolytic treatment. It establishes a novel correlation between experimental and simulation data, laying the foundation for advancing PFAS research by enabling the broader use of simulation tools to better understand the use of ultrasound technology to treat PFAS and mitigate its societal risks. The potential emergence of poorly understood intermediates or carbon–fluorine end-products introduces new health risks that demand careful consideration. A comprehensive elucidation of the PFAS degradation mechanism via ultrasound is essential for informed management and mitigation of associated risks.

2. Materials and Methods

This research performed simulations to assess the degradation of five types of PFAS molecules: PFOA, perfluorohexane sulfonic acid (PFHxS), perfluorohexanoic acid (PFHxA), perfluorobutane sulfonic acid (PFBS), and perfluorobutanoic acid (PFBA). As illustrated in Figure 2, non-polymeric, perfluorinated PFAS molecules consist of a hydrophobic fluorinated tail with a variable length connected to a hydrophilic polar head—Functional Head Group. PFASs can be categorized based on their terminal functional groups, known as head groups, with two major types: carboxylic groups and sulfonic groups [55,56]. Those containing carboxylic groups are termed perfluoroalkyl carboxylic acids (PFCAs), while those with sulfonic groups are called perfluoroalkyl sulfonic acids (PFSAs). PFOA [56], as an example from the PFCA group, is shown in Figure 2.
Additionally, PFASs can be classified by their carbon chain length, indicating the number of fluorinated carbon atoms in the tail structure. The two commonly described categories are short-chain, n < 6 for PFSAs and n < 7 for PFCAs; and long-chain, n ≥ 6 for PFSAs and n ≥ 7 for PFCAs, where n refers to the number of carbons in the molecule [55,56,57]. In this research, a total of two PFSAs and three PFCAs compounds were chosen, or five compounds with two long-chain and three short-chain PFASs. Table 1 summarizes the PFAS molecules studied, reflecting a diverse selection of compounds, ensuring a balanced representation for a comprehensive analysis.
Please note that the Log Kow for PFHxS is not included in Table 1 due to the specific chemical source utilized for this study, which did not provide the Log Kow value for PFHxS [58]. Since these values are relatively similar to each other, the authors chose to maintain consistency by using a single source rather than combining data from multiple sources, which could potentially distort the comparison of hydrophobicities. Despite not being explicitly listed, it can be reasonably inferred that the Log Kow for PFHxS falls within the range from 3.71 to 5.11, as there appears to be a linear trend among the other four compounds, correlating with their perfluorinated carbon chain lengths.

2.1. Details of Simulations

The degradation of the five PFAS molecules listed was investigated through molecular dynamics simulations employing the ReaxFF force field under high temperatures [59]. ReaxFF is a force field that employs the bond order approach, allowing for the dynamic formation and dissociation of bonds in molecules. The force field considers bonded interactions, such as bonds, angles, and torsions, as well as non-bonded interactions, such as van der Waals and Coulomb forces [59]. The bond-order concept, linked to bond energies, enables atom interactions to change with distance and energy variations, facilitating the simulation of complex chemical reactions [59]. Electronic interactions are addressed through a combination of empirical parameters and quantum mechanical principles, with force field parameters tailored to experimental or quantum mechanical data; this ensures an accurate representation of electronic interactions between atoms [59].
This simulation study focused on essential atomic interactions, particularly the C–F bond, crucial for the stability of PFAS molecules, and incorporates thermal properties to simulate PFAS pyrolysis. Singh et al.’s parameters for the ReaxFF potential of the C–F bond were utilized, tested against density functional theory (DFT) data, and proven to represent bond-breaking and formation processes effectively [59,60].
All the simulations performed in this study were conducted with periodic boundary conditions applied across all three spatial dimensions. Each simulation started with an energy minimization step. This was followed by an equilibration phase within an isothermal–isobaric (NPT) ensemble, where the system was maintained at a temperature of 373.15 K and a pressure of 1 atm. After completing both minimization and equilibration, simulations were carried out using a canonical (NVT) ensemble. The system was simulated in each specific environment for 8 nanoseconds, spanning a temperature range from 373.15 K to 5000 K to replicate the nano/microbubble implosion step during the application of ultrasound. The simulated environments included water vapor (H2O), oxygen gas (O2), and nitrogen gas (N2). Each configuration of the PFAS molecules mentioned above was simulated with a total of 1000 molecules in each gas environment in order to replicate a realistic PFAS concentration. To account for the instantaneous implosion of nanobubbles and the computational limitation, the NVT simulations were constrained to 8 nanoseconds.
All the molecular dynamics (MD) simulations were performed using the large-scale atomic/molecular massively parallel simulator (LAMMPS, 29 August 2024), a specialized software for molecular dynamics simulations [61,62]. Avogadro software (Avogadro 1.2.0) was employed to create a digital representation of molecules with accurate bond lengths and angles, while OVITO software(OVITO 3.11.0) was utilized to visualize the simulations and generate the renderings [63,64]. The simulations involved 10 molecules of PFOA, 13 each of PFHxS and PFHxA, and 20 molecules each of PFBS and PFBA. The selection of these quantities aimed at ensuring an approximately equal number of carbon atoms in each simulation.

2.2. Physical Experimental Materials

The heptafluorobutyric acid (PFBA) (≥98.0%), nonafluorobutane-1-sulfonic acid (PFBS) (≥98.0%), tridecafluorohexane-1-sulfonic acid (PFHxS) (≥98.0%), total ionic strength adjustment buffer (TISAB II), fluoride standard with TISAB II, and HPLC-grade methanol were purchased from Thermo Fisher Scientific (Portland). The undecafluorohexanoic acid (PFHxA) and PFOA (>98.0%) were purchased from TCI America, and USEPA 533 PFAS standards were obtained from Wellington Laboratories, Overland Park, KS, USA. For all the tests in this study, Agilent Milli-Q water (121 Hartwell Avenue Lexington, MA 02421) (>18 MΩ·cm) was used. PFAS salts were diluted in Milli-Q water to prepare the PFAS waste solution used in this study.

2.3. Apparatus and Measurements

The concentrations of all PFASs included in this study were determined using liquid chromatography with tandem mass spectrometry Agilent 6470 Triple Quadrupole LC/MS System—LC-QQQ (Agilent Technologies, Santa Clara, CA, USA) equipped with a Zorbax Eclipse Plus C18 Column (Agilent technologies, Santa Clara, CA, USA). The inorganic fluoride concentration was measured using a fluoride-ion selective electrode (F-ISE) (Thermo-Scientific, Waltham, MA, USA), which measures the activity of fluoride ions as a voltage response.

2.4. Ultrasound Reactor Tank

A 10 L capacity ultrasonic reactor with multiple ultrasound frequencies was custom-designed in collaboration with PCT Systems, Inc. (San Jose, CA, USA). The reactor features two single-frequency transducer plates, each comprising six piezo elements. In this study, one transducer plate operated at 890 kHz and was affixed to the bottom of the reactor, while the other transducer plate operated at 950 kHz and was attached to a side wall, following the configuration and frequency optimization detailed in Kewalramani et al. [30,31].
To control the heat generated during sonication for the treatment of PFAS waste, a cooling water system was utilized to circulate through the reactor cooling coil. The sonication process for the PFAS waste was conducted in cycles lasting fifteen minutes each, with a total treatment duration of six hours. At intervals of thirty minutes, samples of 5 mL were extracted and placed in polypropylene vials for later analysis of PFAS and fluoride ion content. These collected samples were then stored in a refrigerator at 4 °C until the analysis was performed.

2.5. Chemical Analysis

Water samples for PFAS analysis were diluted with methanol to ensure that the final PFAS concentration remained below 100 ppb. Prior to analysis, the samples were filtered through a 0.25 μm polyether sulfone (PES) syringe filter. The analysis for targeted PFAS was conducted using a modified USEPA 8327 method on an Agilent 6470 Triple Quadrupole LC MS System. The detection limit for the method was 0.0001 mg/L. For assessing inorganic fluoride concentration, a fluoride-ion selective electrode (F-ISE) from Thermo-Scientific was employed. The water samples were mixed with TISAB II in a 1:1 ratio, according to a detailed analytical method described in Kewalramani et al. [30,31].

2.6. Correlation Between Simulation and Experimental Data

While the experimental data provide the overall results of the ultrasound treatment system stages (A) + (B) + (C) (Figure 1A–C), the virtual data utilized in this study exclusively present results from the secondary stage of pyrolysis/combustion of ultrasound treatment system stages (B) + (C) (Figure 1A,B). Therefore, we employed the percentage of defluorination as a metric for correlation of the simulated and experimental data, given that, as shown in Figure 1, during phase (A)—adsorption—no destruction is observed. Consequently, the simulated data focused on the second stage remain valid for correlation and comparison with the experimental data, as it is the moment that all the destructive action takes place.
For the defluorination analysis of the experimental setup, the following equation was used, which accounts for the amount of fluoride released during the treatment period and has been used by other authors within the field of PFAS destructions [65,66]:
D e f l u o r i n a t i o n % = [ F − ] [ T O F o ] × 100
For the experimental setup, TOF0 represents the Total Organic Fluorine at the initial time (t = 0 min), calculated based on the initial concentrations of PFASs. The formation of fluoride ions [F−] during treatment was determined using a fluoride-ion selective electrode, as mentioned earlier. Regarding the simulated data, since a closed system was used, the authors were able to determine the initial quantity of fluorine atoms attached to the carbon atoms in the PFAS molecule, which represents the simulated TOF0. This allowed them to measure the release of fluorine from its strong bond with carbon, transforming into fluoride ions [F−].
Comparing simulation with experimental data is challenging due to the assumptions and simplifications made in the virtual study. Although the simulation operates on a much smaller scale and system size than typical lab experiments, a meaningful connection can still be drawn. In our previous work, we determined the rate constant, k, across various temperatures using a linear fitting approach [35]. This k value for each temperature was then applied in the Arrhenius equation to calculate the activation energy, which was subsequently compared with other experimental results from the literature under similar conditions as for ultrasound technology. Both the simulated and experimental data demonstrated a strong correlation, showing substantial agreement between the findings. Further details on the kinetic analysis and comparison between virtual and experimental results are provided in the manuscript by Bezerra de Souza et al., 2023 [35].

3. Results and Discussion

3.1. Impact of Functional Head Group and Chain Length

Numerous scholars suggested that the initial stage of PFAS degradation would likely involve headgroup cleavage, a more plausible occurrence than defluorination [33,35,58,67,68,69]. This proposition gains support from the higher bond energies of C-F bonds (≈485 kJ mol−1), surpassing those of C–C bonds (≈346 kJ mol−1) or C-S bonds (≈301–355 kJ mol−1) existing between the perfluoroalkyl chain and the functional headgroup [70]. Computational studies, including recent investigation by authors into the computational mineralization of PFOA and PFOS under ultrasound, corroborate this perspective by indicating headgroup cleavage as the preliminary degradation step [35]. Temperatures in the range of 5000 K were achieved inside bubbles during mid-sonication, creating ideal conditions for potential C-F cleavage. However, it is crucial to note that these extreme conditions persist for only a short duration of a few nano- or pico-seconds, with a cooling rate of approximately 109 K s−1 for single-bubble sonication [36,37,38]. Prior research of authors has undertaken a detailed examination of the C-F cleavage mechanism under elevated temperatures through computational analysis [35].
Additionally, various authors have observed that PFCAs tend to degrade at a faster rate than PFSAs of equivalent chain lengths [30,69,71]. For instance, PFOA has been documented to degrade more rapidly than PFOS, and similar trends have been reported by others, such as the faster degradation of PFHxA compared to PFHxS [30,69,71]. The conventional hypothesis attributes these observations to the higher bubble collapse temperature required for cleaving the C–S bond in the C–SO3H group of PFSAs, compared to the C–C bond in the C–COOH group of PFCAs [33]. It can also be attributed to the higher number of fluorine atoms present in PFSAs when compared to PFCAs for the same number of perfluorinated carbon atoms, resulting in a higher bond energy requirement. Nevertheless, the lack of measurements pertaining to C–C and C–S bond strengths at the required bubble collapse temperature leaves room for uncertainty. An alternative hypothesis posits that the larger size of the sulfonate head group, in comparison to the carboxyl group, confers greater protection to the headgroup–tail bond against thermal, radical, or electron attacks [33]. Also, Psillakis et al.’s (2009) experimental study suggested that there are confirmational restrictions to the interfacial enrichment of PFSAs compared to PFCAs at the water–air interface of microdroplets when the number of perfluorinated carbons is above four [72]. These hypotheses are yet to be confirmed or denied due to the difficult nature of quantifying each sonolytic step.
In this study, a combination of experimental and computational data facilitated an investigation into the factors contributing to the faster degradation of PFCAs in contrast to PFSAs during sonolysis treatment. All five compounds were subjected to identical humid pyrolytic temperatures (≈5000 K in water) and durations (8 ns) mimicking a violent bubble implosion moment. Figure 3 shows the outcomes of a simulation, demonstrating minimal distinctions between the compounds when subjected to bubble implosion collapse (stage B—Figure 1B). This result aligns well with the aforementioned bond energies for the C–C bond in the C–COOH group of PFCAs (≈348 kJ mol−1), and C–S bond (≈301–355 kJ mol−1) for the PFSAs group. As expected, the simulations revealed that PFSAs exhibit a slightly higher defluorination rate than PFCAs due to the lower energy requirement of their function head group, but overall, all compounds had similar responses to the bubble implosion, suggesting that the C–S bond energy in the C–SO3H group of PFSAs is in the narrow range < 348 kJ mol−1 close to the C–COOH group. Therefore, the difference in bond energy between C–C in PFCAs and C–S in PFSAs had no direct impact on the PFAS degradation via sonolysis.
On the other hand, Figure 3 reveals fluctuations in the degradation rate observed by physical experiments, opening up room for an intriguing question: what is the specific step in sonolysis that will influence the degradation rates for PFAS compounds? If the extreme implosion conditions shown in Figure 1B do not exert significant impact, then it is conclusive that the initial step during bubble formation and PFAS adsorption, as illustrated in Figure 1A (stage A), plays a significant role in determining the differences in PFAS degradation via sonolysis. During this step, PFAS molecules diffuse to the air–water interface of the bubble, with the hydrophilic head remaining in the liquid phase while the hydrophobic tail penetrates the bubble surface [33]. Consequently, the hydrophobic characteristics of PFAS compounds emerge as the primary determinant limiting rate of PFAS degradation through ultrasonic treatment.
These findings can be further justified by individually investigating the hydrophobicity of each PFAS compound. Hydrophobicity refers to the tendency of substances to repel in water [73,74]. The octanol/water partition coefficient (Kow), typically expressed logarithmically as Log Kow, is commonly used to indicate relative hydrophobicities, with higher values indicating greater hydrophobicity [73,74]. Table 1 shows the Log Kow values (representing hydrophobicity) for the PFAS compounds utilized in this study. As observed, the hydrophobicity increases with the length of the perfluorinated carbon chain. The hydrophobicity data presented in Table 1 closely align with the findings shown in Figure 3, where PFSA compounds exhibit equal or higher defluorination rates compared to PFCAs at similar perfluorinated carbon lengths. The unusually low degradation rate of PFBA is likely attributed to its formation as a product from PFOA and PFHxA mineralization.
The computational result, along with the experimental data and hydrophobicity knowledge of PFAS compounds presented in this manuscript, supports that the rate-limiting step toward PFAS degradation in ultrasonic treatment is the adsorption of PFAS at the air–water interface, as a larger number of molecules attached to the bubble surface would produce higher PFAS degradation. These findings elucidate that the differences in degradation rates between PFSAs and PFCAs are primarily due to the adsorption step (stage A—Figure 1A), which is directly associated with the hydrophobicity of the fluorinated compounds, as confirmed by the Log Kow values. Once the PFASs are absorbed to the air–water interface, PFAS destruction is independent of the functional head group, as suggested by computational results and confirmed by the bond energies. Given that there is no difference in the humid pyrolysis/radical attack during bubble implosion (Figure 1B) between PFSAs and PFCAs, the adsorption stage (Figure 1A) concludes as the rate-limiting factor. This highlights the critical role of hydrophobicity in either enhancing or inhibiting PFAS degradation in bulk liquid via bubble implosion during sonolysis.

3.2. Impact of Chain Length

PFAS compounds can be effectively classified based on the length of their central carbon chain, a structural characteristic that exerts significant influence on their properties and behaviors across diverse environments. The length of the carbon chain in PFAS molecules correlates with an escalated risk of bioaccumulation due to the prevalence of more C-F covalent bonds, recognized as being among the strongest bonds in nature [9,56,75]. Short-chain PFASs, on the other hand, display a reduced propensity for bioaccumulation yet present greater resistance to degradation than long-chain PFAS in part due to their mobility [9,19,55,56,76]. The persistence of these short-chain variants and their precursors raises environmental concerns, introducing complexity to the understanding of their ecological impact. Furthermore, the distinct environmental occurrence of short-chain PFASs, primarily in the dissolved phase, accentuates their increased mobility in aquatic environments, facilitating long-range transport [56,57,76].
Numerous studies have investigated the correlation between PFAS sonolytic degradation rates and their respective chain lengths [30,67,71,77,78]. Specifically, PFOAs exhibit a higher capacity for mineralization compared to short-chain compounds, like PFHxA and PFBA, within their designated PFCA group. The rapid degradation of larger PFAS molecules has been correlated to their augmented adsorption to the bubble interface, facilitated by larger surfactant molecules and increased hydrophobicity [30,71,78], hypotheses confirmed in the previous section. While hydrophobicity is established as a pivotal factor in enhancing the PFAS degradation process via ultrasound, additional elements, including surfactant molecule size and electron withdrawing effects within the perfluoro chain, contribute to observed differences in degradation rates [33].
Figure 3 demonstrates that, in general, PFAS molecules with long chains degraded at a faster rate than their short-chain counterparts. Meanwhile, Figure 3 shows a consistent pattern in the simulated experiment under bubble implosions, regardless of the number of fluorinated carbon chains. This pattern is observed in both PFCA and PFSA molecules, with longer-chain PFSAs exhibiting an increased destruction rate, while short-chain PFSAs have a lower defluorination rate. Similarly, in the simulations of PFCAs, the defluorination rate appears constant for both long- and short-chain PFAS molecules (Figure 3).
This observed pattern is further explained by the ultrasound treatment process, where extreme collapse temperatures and pressures increase in proximity to the bubble core, leading to quicker mineralization through humid pyrolysis and radical attack [33,35]. The accumulation of PFAS compounds at the liquid–air interface of the bubble, with the hydrophobic tail penetrating the bubble core, results in longer PFAS chain molecules being present in critical regions before their short-chain counterparts. The rate of mineralization is dictated by how quickly the molecule reaches the bubble interface, aligning with previous research on surfactant sonolysis [30,33,71,78,79], suggesting that all parts of the surfactant experience approximately the same condition at the implosion temperature [33,80].
Campbell and Hoffman (2015), Shende et al. (2023), Fernandez et al. (2016), and Kewalramani (2023) observed an increase in the rate of defluorination for longer-chain PFASs when compared to small-chain PFASs [30,71,77,81]. These findings, combined with the results presented in Figure 3, are attributed to the increased hydrophobic characteristics of these long chain perfluoroalkyl surfactants [30,71,77,81]. In turn, the affinity of these molecules for adsorbing into the water–bubble interface was increased, resulting in enhanced humid pyrolytic degradation [30,71,77,78,81]. The consistent increase in hydrophobicity with chain length supports the conclusion that hydrophobicity is a key parameter influencing PFAS destruction via ultrasound technology. Hence, the addition of a positively charged surfactant, e.g., carumonam bromide (CTAB), that can interact with negatively charged PFASs and form a PFAS-CTAB complex [82,83] with higher hydrophobicity can enhance the adsorption and thereby PFAS degradation. The above findings can also be used to facilitate faster destruction of short-chain PFAS molecules by modifying the water chemistry of the original PFAS solution.
Hydrophobicity has proven to be crucial not only in ultrasound-based PFAS remediation but also in other remediation technologies, e.g., non-thermal plasma and electrochemical oxidation [84,85]. Previous reviews on PFAS remediation via adsorption into different materials have highlighted the importance of hydrophobic interactions, particularly in the removal of long-chain PFASs, such as PFOA and PFOS. Adsorbents with amine groups have demonstrated high adsorption capacity for these long-chain PFASs in comparison to short-chain PFASs, as observed in batch adsorption tests conducted on various adsorbent materials, both commercially available and synthesized.

3.3. Influence of the Sonolytic Environment

The following section presents an analysis based solely on simulations, as the experimental evaluation of the scenarios discussed herein would be highly complex and cost-prohibitive. Based on the agreement observed between the simulation and experimental data presented in this study, the authors proceeded to investigate the intricacies of sonolysis under different environments. Simulations were conducted under varied conditions, simulating environments saturated with oxygen and nitrogen molecules, mimicking thermal combustion/incineration and dry pyrolysis under simplified atmospheres, respectively. This investigation was conducted in order to understand the optimal conditions for the degradation of PFASs.
The results, as shown in Figure 4, revealed a reduced defluorination under these diverse conditions in comparison to the control conditions with a water vapor atmosphere (humid pyrolysis), as depicted in Figure 3. This corroborates the initial hypothesis that aqueous sonolysis stands as the most effective technology for PFAS destruction. The implications of these findings are quite significant, highlighting the distinctive mineralization mechanism associated with bubble implosion during ultrasound application under different environments. The observed lower defluorination percentages in the oxygen and nitrogen-saturated environments draws similar conclusions to those already discussed in the literature by [35,86], that the formation of known and unknown compounds during sonolysis introduces a competitive dynamic for interaction with the vicinity of the nanobubble thus, reducing the efficiency of the system. Narimani et al. (2022) developed a kinetic model showing PFAS degradation in the presence of O2 and H2O under three different thermal treatment conditions: pyrolysis, incineration, and humidity [87]. Results from these studies highlight the significance of high temperatures and the presence of excess hydrogen and hydroxyl (H/OH) radicals (from water) in the feed to achieve complete PFAS mineralization and suppress the formation of byproducts of incomplete combustion.
In a study conducted by Shende et al. (2021), various gases, including helium, nitrogen, argon, oxygen, and ozone, were introduced into a solution containing a blend of PFOAs and PFOSs during sonolysis, at an ultrasonic frequency of 575 kHz [86]. Their investigations revealed a decrease in the degradation kinetics of PFOAs and PFOSs upon gas sparging, pointing toward a notable interference in the degradation process [86]. This aligns with the current research, fortifying the hypothesis that gas composition and its interference with the sonolytic process significantly influence PFAS degradation kinetics. Therefore, confirming that the sonolysis degradation of PFAS in a water-based system will result in the most optimal conditions [35,86].

4. Discussion

Based on the results presented, it is evident that the impact of functional head group and chain length on the degradation of PFAS via sonolysis treatment is complex and multifaceted. While the higher bond energies of C-F bonds suggest headgroup cleavage as the preliminary degradation step, the differences in degradation rates between PFCAs and PFSAs are primarily attributed to the adsorption stage at the air–water interface, influenced by the hydrophobicity of these compounds. Computational simulations and experimental data suggest that once PFAS molecules are absorbed onto the air–water interface, their destruction via sonolysis becomes independent of the functional head group. Moreover, the hydrophobicity of PFAS compounds emerges as a critical factor in enhancing or inhibiting their degradation, highlighting the importance of understanding the interplay between molecular structure and environmental conditions in PFAS remediation efforts.
Furthermore, it is observed that longer-chain PFAS compounds tend to degrade at a faster rate than their short-chain counterparts. The explanation lies in the hydrophobic characteristics of longer-chain PFAS molecules, which facilitate their adsorption to the interface of micro/nanobubbles generated by ultrasound and subsequent mineralization. The consistent increase in hydrophobicity with chain length further supports the conclusion that hydrophobicity is a key parameter influencing PFAS destruction via ultrasound technology.
The influence of the sonolytic environment on the degradation of PFAS compounds adds another layer of understanding to the complexities of PFAS remediation processes. This study explores variations during sonolysis, simulating environments saturated with oxygen and nitrogen that are also similar to incineration conditions and simplified atmospheres, respectively. The findings reveal a notable reduction in defluorination under these diverse conditions compared to the control conditions in a water system, affirming aqueous sonolysis (humid pyrolysis—radical attack) as the most effective mechanism for PFAS destruction. This underscores the significance of the mineralization mechanism associated with bubble implosion during the application ultrasound under different environments. The presence of oxygen and nitrogen introduces competitive dynamics for interaction within the vicinity of the nanobubble, thereby reducing the efficiency of the system. These observations are consistent with the previous literature, such as the study conducted by Shende et al. [81] and Narimani et al. [87], which demonstrated a decrease in the degradation kinetics of PFAS compounds upon gas sparging.
Ultimately, this research underscores the importance of conducting sonolysis degradation of PFAS in a water-based system to achieve optimal conditions. These insights provide valuable guidance for designing and implementing effective PFAS remediation strategies, highlighting the need for careful consideration of environmental factors. In conclusion, the findings confirm the importance of considering molecular structure, particularly functional head groups and chain length, in understanding and optimizing PFAS remediation strategies. The relationship between hydrophobicity, adsorption behavior, and degradation kinetics elucidates the complexities involved in addressing PFAS contamination in environmental matrices.
This study focused exclusively on matrices composed of pure water, oxygen, or nitrogen. More complex matrices, such as wastewater, groundwater, leachate, and other environments where PFASs are present, were not considered due to the additional complexity required for molecular dynamics modeling. The presence of multiple contaminants in these matrices introduces significant challenges in validating the molecular dynamics system. Incorporating additional contaminants would necessitate the development of specific force fields for each contaminant to model their interactions within the system. Given the computational and methodological challenges associated with such an expansion, this aspect was not addressed in this current research. Future research and technological advancements will be crucial in incorporating different matrices in the molecular dynamics model and developing effective and sustainable strategies to mitigate the adverse impacts of PFAS pollution on human health and the environment.

5. Conclusions

In conclusion, the study highlights the intricate dynamics governing the degradation of PFAS via sonolysis treatment. It discusses the influence of molecular structure, including functional head groups and chain length, on ultrasound degradation. The findings emphasize the pivotal role of hydrophobicity in facilitating adsorption of PFAS molecules to the micro/nanobubble interface and subsequent mineralization, elucidating key factors driving PFAS destruction. Furthermore, the research demonstrates the significance of conducting sonolysis in water-based systems for optimal efficiency, elucidating the competitive dynamics introduced by oxygen and nitrogen in alternative environments. These insights provide valuable guidance for designing effective PFAS remediation strategies and underscore the importance of ongoing research and innovation in addressing PFAS contamination challenges. Ultimately, a comprehensive understanding of PFAS degradation mechanisms is crucial for mitigating their adverse impacts on human health and the environment.

Author Contributions

Conceptualization, B.B.d.S. and J.M.; methodology, B.B.d.S. and J.M.; validation, B.B.d.S. and J.M.; formal analysis, B.B.d.S. and J.M.; investigation, B.B.d.S. and J.M.; resources, J.M.; writing—original draft preparation, all; writing—review and editing, all; visualization, B.B.d.S. and J.M.; supervision, J.M.; project administration, J.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

No new data were created other than what is presented.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. High-temperature pyrolysis mechanism for PFAS destruction. Color code: blue—cool bulk liquid; yellow—warm interfacial region; red—hot bubble core; black elements—PFAS compounds. [33]. (A) During bubble formation, PFAS molecules are diffused to the air–water interface of the bubble, with the hydrophilic anionic head remaining in the liquid and the hydrophobic carbon–fluorine tail penetrating the air–water interface at the bubble surface. (B) Bubble implosion is associated with the pyrolytic destruction of PFAS and the ionization of water and compounds in the vicinity of the bubble. (C) Reaction products mark the final step of the sonolysis treatment, wherein PFAS molecules have been degraded into inorganic compounds (F− and SO42−), as well as carbon monoxide and carbon dioxide.
Figure 1. High-temperature pyrolysis mechanism for PFAS destruction. Color code: blue—cool bulk liquid; yellow—warm interfacial region; red—hot bubble core; black elements—PFAS compounds. [33]. (A) During bubble formation, PFAS molecules are diffused to the air–water interface of the bubble, with the hydrophilic anionic head remaining in the liquid and the hydrophobic carbon–fluorine tail penetrating the air–water interface at the bubble surface. (B) Bubble implosion is associated with the pyrolytic destruction of PFAS and the ionization of water and compounds in the vicinity of the bubble. (C) Reaction products mark the final step of the sonolysis treatment, wherein PFAS molecules have been degraded into inorganic compounds (F− and SO42−), as well as carbon monoxide and carbon dioxide.
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Figure 2. Molecule structure of PFOA highlighting the chain length and functional head group.
Figure 2. Molecule structure of PFOA highlighting the chain length and functional head group.
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Figure 3. PFAS mineralization: (left)—simulated data; (right)—experimental data.
Figure 3. PFAS mineralization: (left)—simulated data; (right)—experimental data.
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Figure 4. PFAS mineralization at different conditions: (left)—simulated in a O2 environment; (right)—simulated in a N2 environment.
Figure 4. PFAS mineralization at different conditions: (left)—simulated in a O2 environment; (right)—simulated in a N2 environment.
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Table 1. Nomenclature for perfluoroalkyl substance (PFAS) molecules included in study.
Table 1. Nomenclature for perfluoroalkyl substance (PFAS) molecules included in study.
Molecule NameChemical FormulaAcronymChain LengthPerfluorinated CarbonLog (Kow) *Functional GroupType
Perfluorooctanoic acidC8HF15O2PFOALong75.11CarboxylPFCAs
Perfluorohexanesulphonic acidC6HF13O3SPFHxSLong6-SulfonicPFSAs
Perfluorohexanoic acidC6HF11O2PFHxAShort53.71CarboxylPFCAs
Perfluorobutane sulfonateC4HF9O3SPFBSShort42.63SulfonicPFSAs
Perflurobutanoic acidC4HF7O2PFBAShort32.31CarboxylPFCAs
Note: * Estimated data obtained using MarvingSketch 188.11.0 [58].
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Bezerra de Souza, B.; Kewalramani, J.A.; Marsh, R.W.; Meegoda, J. Molecular Dynamics Simulation of the Impact of Functional Head Groups and Chain Lengths of PFAS Degradation Using Ultrasound Technology. Water 2025, 17, 1025. https://doi.org/10.3390/w17071025

AMA Style

Bezerra de Souza B, Kewalramani JA, Marsh RW, Meegoda J. Molecular Dynamics Simulation of the Impact of Functional Head Groups and Chain Lengths of PFAS Degradation Using Ultrasound Technology. Water. 2025; 17(7):1025. https://doi.org/10.3390/w17071025

Chicago/Turabian Style

Bezerra de Souza, Bruno, Jitendra A. Kewalramani, Richard W. Marsh, and Jay Meegoda. 2025. "Molecular Dynamics Simulation of the Impact of Functional Head Groups and Chain Lengths of PFAS Degradation Using Ultrasound Technology" Water 17, no. 7: 1025. https://doi.org/10.3390/w17071025

APA Style

Bezerra de Souza, B., Kewalramani, J. A., Marsh, R. W., & Meegoda, J. (2025). Molecular Dynamics Simulation of the Impact of Functional Head Groups and Chain Lengths of PFAS Degradation Using Ultrasound Technology. Water, 17(7), 1025. https://doi.org/10.3390/w17071025

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