ReviewPer- and poly-fluoroalkyl substance remediation from soil and sorbents: A review of adsorption behaviour and ultrasonic treatment
Introduction
Per and poly-fluoroalkyl substances (PFAS) are a class of anthropogenic chemical compounds, containing the perfluoroalkyl moiety CnF2n+1 (Buck et al., 2011). Polyfluorinated PFAS also contain non-fluorinated carbon chain regions, such as –CH2-CH2-, which are known as fluorotelomers (Buck et al., 2011). PFAS are amphiphilic, meaning they express both hydrophilic and hydrophobic properties due to the presence of a charged functional group and a perfluorinated alkyl chain, respectively (Fujii et al., 2007). These unique properties make PFAS useful for several commercial applications, such as in aqueous film fire-fighting foams (AFFFs), waterproof clothes, and non-stick frying pans (Zhang et al., 2019). PFAS have been manufactured and utilised for more than 60 years (Giesy and Kannan, 2002) and to date, over 4700 PFAS compounds have been developed (OECD, 2018); as a result, they have become ubiquitous in the environment, worldwide (Lau et al., 2007; Pelch et al., 2019; Ross et al., 2018a, 2018b). PFAS have even been found in snow samples taken from regions as remote as the Greenland Arctic (Butt et al., 2010). Only three PFAS and their precursors are currently subject to international regulations, with perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) being designated under the Stockholm Convention as persistent organic pollutants (POPs) (Stockholm Convention, 2009, Stockholm Convention, 2019) and perfluorohexanesulphonic acid (PFHxS) proposed for inclusion.
Perfluorinated PFAS resist biological and chemical degradation, and some bioaccumulate in mammal and plant species (Hale et al., 2017). Some PFAS are now understood to cause deterioration of health in humans, contributing to the development of several cancers, thyroid disease, hypertension, high cholesterol, reduced immunity and liver damage (Agency for Toxic Substances and Disease Registry, 2018). Human exposure to PFAS can occur by ingesting contaminated water, ingesting food grown using contaminated soil, and by using equipment produced using PFAS (U.S. EPA, 2018). The majority of Americans have been exposed to PFAS and have detectable levels of them in their blood (ASTDR, 2017). Hence, PFAS remediation for both soil and drinking water is of immediate concern.
PFOS and PFOA, types of perfluoroalkyl acids (PFAA) (Steenland et al., 2010), are the two main PFAS considered in the literature, since they bio-accumulate and are toxic to sea-life and mammals (Hale et al., 2017), and because they are amongst the most commonly found perfluorinated environmental contaminants (Zareitalabad et al., 2013). Production of PFOS and PFOA was phased out in the US by the Environmental Protection Agency (EPA) (Wang et al., 2014), and all PFAS are being phased out in the European Union (EU) by 2030, with action beginning by 2025 (Lerner, 2019). However, PFOA and PFOS are still manufactured outside the U.S, and products containing PFAS such as non-stick pans are still imported into the US (U.S. EPA, 2018) and the EU, with little control over small quantities and novel PFAS (KEMI (Swedish Chemicals Agency), 2015). Hence, pollution will likely continue until a worldwide ban is enacted. Attempts have been made to develop more environmentally friendly alternative PFAS, however these have unknown safety implications (Sunderland et al., 2019; Wang et al., 2014), and can act as precursors to short chain PFAS by breakdown in the environment (Cheremisinoff, 2016).
Remediation of PFAS-impacted matrices can require separation (removal of PFAS from the water or soil to another matrix) followed by destruction. Water treatment focuses on bringing PFAS levels below safe thresholds and soil remediation on PFAS removal or fixation (Arias Espana et al., 2015; Banks et al., 2020; Horst et al., 2018; Kucharzyk et al., 2017; Mahinroosta and Senevirathna, 2020; Merino et al., 2016; Rahman et al., 2014; Ross et al., 2018a, 2018b; Sörengård et al., 2021; Vecitis et al., 2009). Soil remediation approaches includes techniques such as incineration (Meng et al., 2017), use of electron beams, excavation and disposal to landfill, on site or in situ smouldering, thermal desorption, ball milling (Senevirathna et al., 2020), soil washing and soil stabilisation (Duchesne et al., 2018; Hale et al., 2017; Ross et al., 2018a, 2018b). To maintain the soil structure, soil washing is usually associated with adsorptive and destructive technologies to treat PFAS in wash water (Senevirathna et al., 2020). Separative technologies for wastewater streams include sorbents such as ion exchange (IX) resins (Appleman et al., 2014; Deng et al., 2010; Horst et al., 2018; Rahman et al., 2014; Ross et al., 2018a, 2018b), silicas (Jentoft, 2013; Johnson et al., 2007; Punyapalakul et al., 2013; Tang et al., 2010) and activated carbon (Chen et al., 2017; Hansen et al., 2010; Lee et al., 2013) [either granular (GAC) (Du et al., 2015; Senevirathna et al., 2010) or powdered (PAC) (Qu et al., 2009; Yu and Hu, 2011)], all of which must be regenerated or destroyed, and result in another PFAS waste stream to be treated (Eschauzier et al., 2012, Rahman et al., 2014, Yu et al., 2009, Yu et al., 2012, Zhang et al., 2019, Zhao et al., 2011). Similarly, other separation techniques such as foam fractionation (Horst et al., 2018; KEMI (Swedish Chemicals Agency), 2015), and membrane separation (Hopkins et al., 2018; Merino et al., 2016; Rahman et al., 2014), also produce a concentrated PFAS stream that requires a destructive stage. Many destructive PFAS remediation technologies for water/liquid PFAS solutions are reviewed elsewhere (Arias Espana et al., 2015; Banks et al., 2020; Horst et al., 2018; Kucharzyk et al., 2017; Merino et al., 2016; Rahman et al., 2014; Ross et al., 2018a, 2018b; Vecitis et al., 2009) however, few seem environmentally beneficial. Sonolysis (the use of ultrasonic waves to break down a substance) via high frequency ultrasound (>100 kHz) is a very promising technique (Cao et al., 2020; Ross et al., 2018a, 2018b) for complete PFAS mineralisation, without production of a short chain PFAS waste stream (Wood et al., 2020). Consideration of soil treatment or sorbent regeneration must be done in the context of a subsequent treatment technology and, if possible, concurrent treatment and destruction options.
Complete PFAS remediation is challenged by their strong carbon-fluorine bonds (3M, 1999; KEMI (Swedish Chemicals Agency), 2015) and proclivity to stick to surfaces (Lath et al., 2019; Tang et al., 2010). Little is known regarding the fundamental surface chemistry of PFAS and the effectiveness of solvent washing to regenerate solid matrices contaminated by PFAS. This work aims to give an overview of PFAS adsorption characteristics and what this means for separation and destructive treatment consisting of solvent washing, or alternatively, a combination of solvent washing and ultrasound technology.
Section snippets
Treatment of PFAS-impacted soils
Traditional non-destructive PFAS remediation methods from soil include soil stabilisation, thermal desorption and soil washing. Thermal desorption involves heating in- or ex-situ (Vidonish et al., 2016), and is effective from 70% up to 99% removal of multiple PFAS (including PFSA's, FOSA's, and PFCA's) from soil. However, it requires high temperatures ~450–550 °C, is energy-intensive and the vaporised contaminants require post-treatment (Sörengård et al., 2020). Soil stabilisation involves
Self-assembly of amphiphiles and adsorption
Amphiphiles like PFAS are capable of self-assembling to form colloid-like structures in solution and on surfaces in quantities exceeding the critical micelle concentration (CMC) (Krafft and Riess, 2015; Milovanovic et al., 2017) and hemi-micelle concentration respectively (Gladysz et al., 2004). These structures have a hydrophilic outer layer and a hydrophobic core. The diameter of the structures formed by hydrocarbon surfactants can vary from 5 to 100 nm (Milovanovic et al., 2017), depending
Fundamentals of sonochemistry
Ultrasonic cavitation utilises sound waves with frequencies above the audible range of human hearing (>18 kHz) to irradiate aqueous media, forming microbubbles (Mason and Lorimer, 2002). Through multiple compression and expansion pressure cycles, these bubbles reach a critical size of at least twice their initial radius, beyond which they collapse severely on compression, a phenomenon known as cavitation (Mason and Peters, 2002; Santos et al., 2009). Cavitation arises from gases trapped in
Conclusions
Some current PFAS soil remediation techniques destroy soil, and whilst a series of non-destructive techniques exist, methods that also mineralise PFAS are required. Adsorption of PFAS onto surfaces such as soils is relevant at environmental concentrations, so any potential treatment must disrupt this. The adsorption process for amphiphilic PFAS is thought to governed mainly by hydrophobic interactions between the adsorbent and the perfluoroalkyl chain on the PFAS molecule, with electrostatic
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was partially supported and funded by the Royal Academy of Engineering Industrial Fellowships Scheme - IFS1819\34 and Arcadis UK.
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