Introduction

It is recognised globally that per- and poly-fluoroalkyl substances (PFASs) will be a generational chemical of concern to human health and ecosystems [1, 2•, 3]. The high persistence and mobility of a large number of PFAS [4•, 5,6,7] has resulted in their widespread distribution and accumulation in the environment [8,9,10] and concern regarding potential human and environmental health effects [11,12,13,14,15,16,17,18,19].

Military, domestic and co-shared airport or runway facilities and sites from around the world have been shown to be contaminated with PFAS [6, 20,21,22,23]. Historically, PFAS as the main component of aqueous film-forming foams (AFFF) was released at sites in association with activities such as firefighting training, maintenance and testing of firefighting equipment, hangar deluge systems, deployments in emergencies or inadvertent release from spills or during storage [24]. There is growing evidence that AFFF-exposed asphalt and concrete pavements at airport facilities can become contaminated with PFAS and subsequently act as a short and long-term source for release into the environment [25,26,27]. It is therefore important to understand the spatial distribution of PFAS in asphalt and concrete at airport facilities to make informed management decisions and strategies for the protection of human health and the environment. This is important as PFAS-contaminated asphalt and concrete may remain in situ for many decades, removed where resurfacing operations such as asphalt runway re-grooving leads to new potentially contaminated surfaces or materials such as those originating from concrete fire training areas are removed for disposal or reuse/repurposing.

Physico-chemical remediation options for PFAS and associated contaminants in soils, sediments, surface waters and groundwaters have received considerable attention in the literature [28,29,30,31,32,33,34,35,36,37]. However, there is currently little published information available for the management or remediation of asphalt and concrete contaminated by PFAS. This study undertook a review of the literature to assess the feasibility of physico-chemical options for the management and/or remediation of PFAS-contaminated asphalt and concrete pavement/pads. The aims of the study were to.

  1. 1.

    Identify potential management and remediation options that may be applicable to PFAS-contaminated asphalt and concrete, both in situ (i.e. pavements and pads) and ex situ (i.e. waste materials from pavement replacement or renewal); and

  2. 2.

    Focus on sealants as a rapid response option for the management of PFAS-contaminated concrete for the protection of human health and the environment. Identification of existing or prospective management and mitigation sealant options for asphalt and concrete will underpin future laboratory and pilot- to full-scale field studies as a rapid response to the management of PFAS-contaminated asphalt and concrete at airport and fire training facilities.

As a basis for this review, information on remediation of environmental solids and solutes has been used to provide insights in the specific context of asphalt and concrete remediation. Remediation technologies related to the treatment of PFAS contained in waters, including sonication, photolysis, foam floatation, membrane filtration and mineral-based catalysis, were not included due to their limited transferability to solid materials. In addition, many of these techniques/technologies have only proven partially effective [38,39,40,41,42] reflecting factors including the suite of PFAS compounds extant and the inherent stability of the C–F bond as an enduring structural component of PFAS fluorocarbon chains.

Contrasting Properties of Asphalt and Concrete: Implications for Treatment Technology

In the evaluation of potential treatment options for PFAS-contaminated asphalt and concrete or new, uncontaminated pavements, and to assist in the identification of critical knowledge gaps, we have critically assessed a range of existing technologies. Importantly, the distinctly different composition, mineralogy, form, matrix and physico-chemical properties of concrete (cement minerals + aggregate + other potential additives including recycled concrete, high contained alkalinity, inherent hydrophilicity), versus asphalt (bitumen + aggregate + other potential additives such as plastics or polymeric viscosity modifiers, inherent hydrophobicity) creates a major challenge to evaluate efficacy for both pavement materials.

Performance testing would entail evaluation under a variety of expected physico-chemical conditions over the full life cycle. This is particularly the case for pavement-derived mixed asphalt and concrete wastes, presumably involving a range of particle sizes, to validate treatment regimes, application rates, performance (e.g. short- and long-term PFAS leachability), scalability and cost.

Management and Remediation Strategies for PFAS-Contaminated Asphalt and Concrete

There are no specific studies currently published that have examined the treatment or remediation of PFAS in asphalt and concrete materials or surfaces. There are however a small number of studies on the distribution and leachability of PFAS from a concrete slab [25, 26] and growing science on better analysis of PFAS in asphalt [27] and the potential for volatilisation of PFAS from paved and asphalt surfaces [43]. Potential management and remediation options in the literature that may be applied to PFAS-contaminated asphalt or concrete can be classified into five categories (Table 1):

  1. 1.

    Destructive: physico-chemical, thermal, and biological approaches to reduce the total and leachable PFAS load;

  2. 2.

    Co-amendment: addition of materials to bind PFAS (mineral- and/or carbon-based amendments and combined amendment blends) and reduce or eliminate leachability, but do not reduce the total PFAS load;

  3. 3.

    Washing: material washing adapted/modified (e.g. soils) for asphalt and concrete to reduce the total PFAS load;

  4. 4.

    Encapsulation: surface sealants acting as a barrier to impede or eliminate the ingress of water or PFAS/chemical agents and egress of PFAS, but do not reduce the total load; and

  5. 5.

    Impregnation: penetrative sealants that may bind to mineral surfaces and/or fill pores to prevent PFAS leachability due to the ingress of water or other solvents or solutes such as petroleum hydrocarbons in jet fuel, but do not reduce the total PFAS load.

The following provide details of the four categories in relation to management of PFAS-contaminated asphalt or concrete pavements/pads.

Sealants are discussed in significant depth in the “Destructive Techniques” section; the addition of co-amendments and washing is briefly discussed here as they;

  1. 1.

    may constitute adjunctive techniques used with sealants and

  2. 2.

    assist in placing the assessment of surface or penetrating sealants in context.

Destructive Techniques

Destructive techniques such as thermolysis/thermal oxidation of materials for removal of the total PFAS load in asphalt and concrete offer a potentially viable treatment option [44, 45] (Table 1). However, there will be significant capital equipment and operational costs and evaluation of the feasibility of effectively treating large volumes of waste often, if not crushed and screened, in a range of particle sizes, in a timely fashion. These methods may be considered primarily ex situ following the destruction/removal of PFAS-contaminated media. Consequently, the effects of preparation (e.g. crushing and constituent separation) of former pavement asphalt and concrete, volatilisation efficiency (extent of PFAS liberation), bitumen component volatilisation, degradation efficiency and the potential requirement to capture (scrub) and dispose of volatilised PFAS compounds need to be understood before field-based implementation [43, 46].

Similar to thermolysis/thermal oxidation, chemically based oxidation techniques will likely require material preparation to access more obliquely bound PFAS within asphalt and concrete and liquid residues that are generated and production of large volumes of liquid waste that will require additional treatment and disposal adding additional treatment challenges and costs (Table 1). There are also concerns over the effectiveness of some chemical-based oxidation approaches for different PFAS (e.g. sulfonate as opposed to carboxylate functionalised PFAS) and the potential for generation of short-chain perfluoroalkyl acids (PFAAs) [45]. Furthermore, oxidant consumption, given the presence of bitumen in asphalt, and acid/reagent consumption, given the need to operate at a low pH relative to the inherent alkalinity contained within concrete, may present significant operational and cost challenges.

Laboratory- and field-based studies of PFAS biodegradation attest to an inherent difficulty in promoting PFAS destruction [47, 48] (Table 1). In many cases, only a modification of the functional group is achieved with the fluorocarbon tail proving recalcitrant to a variety of microbial/enzymatic degradation pathways. As in the case of chemical oxidation approaches, there is also potential for generation of short-chain PFAAs [45, 47]. A recent review of PFAS bioremediation [49] attests to the significant challenges associated with this approach that include the limited number of promising microbial species identified, the transfer of laboratory results under ideal conditions to the field, slow growth rates that may limit broadscale application and tolerance to fluoride toxicity during defluorination.

The F–C bond is robust and therefore difficult to destroy, which leads to its environmental stability. A key limiting factor of for instance the PFAA structure is the steric environment that is not amenable to enzymatic attack [7]. Therefore, significant energy is required to catalyse the reaction and biologically this can be provided via oxidative or reductive processes. Indigenous bacterial species isolated from PFAS-contaminated environments have shown the ability to degrade PFAS. Two strains of Pseudomonas bacteria were able to breakdown 32 and 28% of PFAS, respectively, within 10 days of incubation under alkanotrophic conditions [50]. A decrease of approximately 32% in PFAS was reported during a 96 h incubation of Pseudomonas parafulva [51] and 67% decrease in PFAS over 48 h incubation of Pseudomonas aeruginosa [52]. It was found that Pseudomonas plecoglossicida isolated from a soil contaminated by waste from petrochemical production had a unique ability to use perfluorooctanesulfonic acid (PFOS) as a sole source of carbon and energy and capable of 75% degradation of added PFOS [53]. The potential of acidophilic anaerobic Fe(III)-dependent ammonia and/or hydrogen-oxidising bacteria (e.g. Acidimicrobium sp. strain A6) (Feammox bacteria) to reductively defluorinate PFAS (perfluorooctanoic acid (PFOA); PFOS) (60% reduction in 100 days) has been highlighted [54]. Nonetheless, biodegradation despite its current limitations (ability of microorganisms to cleave the C–F bond), if successful, promises to be a useful technology in for instance the long-term, passive degradation of the potential slow release of PFAS contained in concrete and asphalt in final repositories to reduce the inventory, or as a more passive PFAS degradation method within (mixed) waste heaps or millings akin to a commercial mining heap leach operation. Biochemical transformation could potentially play a part in a treatment train approach (e.g. wetlands [55]), as the transformation of long-chain PFASs into short-chain compounds may facilitate their removal through washing, for example.

Mineral- or Carbon-Based Amendments

A range of mineral- or carbon-based amendment materials, ranging from clays, zeolites, activated carbon and graphene, could potentially be used (individually or in combination) to minimise the leachability of PFAS residues from concrete and asphalt (Table 1). Mineral- and carbon-based amendments, and in particular blends including both experimental materials and proprietary products [6, 29, 30, 36], have been shown to be successful for the in situ stabilisation of PFAS-contaminated soils and, by extension, could be applicable to recycled, repurposed or existing, intact PFAS-contaminated concrete and asphalt (Table 1). Nonetheless, PFAS removal by granular activated carbon (GAC) may vary considerably in terms of both uptake capacity and selectivity depending on the type/source of GAC (e.g. [56]). The amendments could be applied to new concrete and asphalt pavements/pads, as an additional line of defence, to adsorb or lock in exposed contaminants such as legacy and next-generation PFAS, preventing their leachability and mobility into the environment.

The applicability of mineral- or carbon-based amendments for contaminated concrete or asphalt will rely on their incorporation within an enduring, likely polymeric sealant, substrate. Minerals and carbon-based additives are already in widespread use for instance in polymers, albeit for other purposes such as clay-polymer nanocomposite adhesives and sealants [57, 58] or as carbon reinforcement [59, 60].

Material Washing

Soil washing using water or water/chemical mixtures has been shown to be an efficient remediation option for a variety of PFAS-contaminated soil types; however, clay soils due to their finer grain size and physico-chemical mechanical cohesion constitute a greater challenge in terms of dispersal and PFAS leachability [31]. Similarly, in the case of concrete and asphalt, when for instance only surface contamination is present and such that infiltration/diffusion or more pervasive binding with the matrix has not occurred, then, surface washing (using one or more solvents or surfactants, heat or a combination thereof) may constitute a potential treatment option (Table 1). In the case of deeper infiltration into the material profile and PFAS adsorption/binding, crushing of concrete and asphalt may be required to effectively access binding/sorption sites to increase PFAS mass removal.

Given the suite of hydrophobic, hydrophilic and charged PFAS [61, 62], there may be a need for the addition of solvents [63] or surfactants [64] to enhance the washing or leaching of PFAS from concrete and asphalt materials. This, along with the generation of substantial quantities of PFAS-contaminated washings as particulate, colloidal and dissolved PFAS (and other components) may necessitate the use of additional treatment technologies to be factored into life cycle analysis and costing considerations.

Encapsulation and Impregnation Sealants

The use of sealants for encapsulation of PFAS-contaminated concrete and asphalt pavements/pads offers an immediate and potentially effective in situ intervention to reduce PFAS mobility (and potentially ex situ, e.g. waste piles) into the environment (Table 1). This is particularly so where a history of application of AFFF and surface infiltration (e.g. in a firefighting training area) may have led to PFAS contamination through the profile of an intact concrete- and asphalt-paved surface.

The function of a sealant for management of a PFAS-contaminated asphalt or concrete pavement/pad may be considered threefold: (i) to prevent egress of accumulated PFAS, (ii) to prevent PFAS (and other contaminant) ingress and (iii) as an adsorptive/binding substrate for accumulated PFAS. Sealants are largely used as a physical barrier to prevent PFAS and water ingress and PFAS egress; however, there is a potential for innovative adsorptive materials to be added into sealants to bind/lock up PFAS in concrete and asphalt pavements/pads. In the specific context of civilian and defence airfields which constitute major sites of PFAS-contaminated concrete and asphalt internationally, a major limitation on the use of a sealant will be its physico-chemical stability in response to functional requirements needed at sites that may include aircraft and ground traffic (physical wear challenges, jet engine heat), suitability for use on tarmac/taxiway/hangar floor surfaces (expansion/contraction, expansion joints, pitting, delamination, non-slip, and reflectivity) and stability in response to chemical agents (lubricants, hydraulic fluid, new AFFF formulations). Given the immediate need to reduce the environmental impact of PFAS-contaminated concrete and asphalt pavements, this review focuses on the potential of sealants and their use in encapsulation of PFAS in concrete and asphalt as remediation options.

Table 1 Summary of management strategies for PFAS-contaminated concrete and asphalt materials
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Sealants for Management of PFAS-Contaminated Concrete and Asphalt Pavements/Pads

Sealant Types and Functional Requirements

Concrete and asphalt pavements/pads such as those at airfields and training grounds, whether concrete (inflexible) or asphalt (flexible), constitute a demanding operational environment. Numerous physico-chemical stressors including heat, hydrocarbons, fire training/firefighting activities (AFFF/PFAS use) and road or aircraft traffic may impact on both the serviceability and lifetime of these pavements.

In recognition of the diversity of operational demands placed on concrete and asphalt pavements, surface treatment via the application of sealants may be a pragmatic method of controlling the ingress of various chemicals, such as hydrocarbons, PFASs and chloride, which may contribute to concrete deterioration or result in the concrete becoming a sink and/or source of contaminants. In addition, sealants may prevent the leachability of contaminants from materials by preventing water ingress and egress.

For the purposes of this review, the term “sealant” is used to describe those sprayed onto the surface and those considered as penetrative into the pavement profile. In the specific case of asphalt, sealant may also be used in terms of a material to seal contiguous cracking often formed due to thermal expansion [65, 66]. Surface and penetrative sealants for concrete and asphalt will have differing attributes and utility in the prevention of PFAS ingress into pavements, PFAS egress from contaminated pavements and as an in situ binding and/or pore-blocking agents.

The general requirements for potential sealants on airfield and training concrete and asphalt pavements/pads will include the following:

  • Surface coating—topical barrier to ingress

  • Penetrative coating—depth barrier

  • Water repellency

  • Absorbency/immobilisation

  • Wear resistance/trafficability—aircraft, vehicles

  • Heat resistance—aircraft auxiliary power units

  • Chemical resistance—PFAS, petroleum hydrocarbons

  • UV/weathering resistance

  • Reflectivity, particularly in hangar/floodlit applications

  • Non-slip surface

  • Ease of application, adhesion

  • Cost efficiency

Concrete and asphalt surface sealant treatments are often used for protection against deterioration and to prevent the ingress of aggressive [67, 68] or foreign substances (e.g. salts [69, 70]). However, application of surface sealants to airfield concrete pavements/pads has not been widely reported [71]. Surface treatments have been assessed in relation to processes that can impact airport concrete pavement (freeze–thaw, flame ablation and wear), and it was found silanes to perform better than epoxy and polyurethane sealants [72]. Surface sealant treatment of concrete has received more attention in the construction industry where it is used to minimise water and chloride ingress (i.e. bridges, polished concrete flooring).

In the case of asphalt pavements, polymer modified emulsion (PME) or surface-enriched sprayed treatment (SEST) has been applied to extend the lifetime of airfield flexible (asphalt) pavements, with PME as the common surface treatment in recent years [71]. Similar agents generically termed rejuvenator seal materials (RSM) have been investigated for asphalt pavements [73, 74]. These may range from topcoats of bituminous materials including emulsified asphalt, but may also include latex, styrene–butadiene–styrene (SBS) (e.g. [75]) or where a rejuvenator is added to RAP to restore its properties [76], often as softening agent including hydrocarbon- or vegetable-based oils, or blends thereof, before it is reused, including as a pavement topcoat to an existing asphalt or as RAP in its own right [77].

Nonetheless, in the case of asphalt, sealants are often focussed more on cracks rather than broader surface application and dominated by the application of other types of hydrocarbons, sometimes with additives including SBS and crumbed rubber [78, 79] due to the need for interfacial compatibility given the inherent hydrophobicity of asphalt pavements [80].

In general, there are four groups of surface sealant treatments for concrete or asphalt based on their primary function [67, 68, 81,82,83,84] with hydrophobic and sealant combinations and multifunctional types also possible (Fig. 1). These groups consist of the following:

1. Surface coating, which acts as a physical barrier, typically by forming a continuous polymer film on the surface of thickness 0.1 to 1 mm (i.e. organic polymers, polymer/nanocomposite, cementitious). Traditional polymer coatings (epoxy, acrylic, polyurethane) have been improved recently with the addition of nanoparticles (nanocomposite polymers [67, 68]). Cementitious (including polymer modified) and bituminous coatings for asphalt [76] form a low permeability layer on the surface. In concrete, the mortar is fine-grained and can be modified with polymers to increase bond strength and decrease permeability.

2. Impregnation or penetrative treatments, with partial or total capillary pore-filling effect (i.e. silicate-based solutions, polymers, biopolymers), act to increase hardness and impermeability of the concrete surface. These agents are designed to either actively penetrate the concrete or asphalt and block pores or act as water repellents to penetrate access into the matrix (coating the pore walls and rendering them hydrophobic). Repellents prevent moisture ingress but allow water vapour transmission. Blockers, on the other hand, prevent water and vapour migration which may adversely affect concrete durability.

3. Hydrophobic impregnation, as agents to make the concrete or asphalt water repellent, with no pore-filling effect and therefore vapour transfer through the pores, is not inhibited (i.e. silane- or siloxane-based water repellent products that penetrate concrete pores and react with the hydration products of concrete to form a hydrophobic lining).

4. Multifunctional surface treatments, a recent classification [67, 68] includes newly developed treatments that do not fit into the three groups above, but instead have at least two functions (i.e. layer and impregation), for example silane-clay nanocomposites, ethyl silicate and tetraethylorhosilicate (TEOS).

Fig. 1
figure 1

Clockwise from upper left, PFAS-contaminated concrete pavement: surface, pore lining and pore-blocking sealants

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Reviews for concrete [67, 68] and for asphalt [65, 66, 73, 76] have summarised the surface treatment mechanisms and performance of sealants. When sealants are applied to extend the lifetime of concrete surfaces, surface treatments typically need to be clean, dry and free of pore-blocking contaminants [85]. Similarly, asphalt surfaces also need to be clean, particularly where ravelling (aggregate loss) has occurred due to physico-chemical degradation leading to embrittlement of the asphalt binder [65]. In the case of PFAS-contaminated concrete or asphalt, there will be a requirement to also capture and manage any contaminated solids or solutes generated in the surface preparation. Nonetheless, surface coatings such as fog seals (a light application of slow setting asphalt emulsion used to restore asphalt flexibility and postpone other surface treatments) are not often used on asphalts as there is a potential to reduce the frictional characteristics of the pavement [73] in addition to the potential for delamination and the generation of foreign object debris (FOD) with the potential for uptake into jet engines [66].

Where silane/siloxane penetrative sealants are applied to aged concretes, a lime wash may be necessary to refortify both alkalinity and reactive Ca2+ within the upper concrete pore structure. Mechanical and physical properties of concrete include strength and abrasive resistance, whilst durability aspects of concrete include water and chloride permeability and resistance to carbon dioxide, chemicals, temperature or UV. Solar reflectance helps the pavement to stay cool. Inorganic materials with high UV/VIS/NIR reflectance (i.e. metal oxide nanoparticles or inorganic pigments such as TiO2) or even light-emitting concrete [86] have been developed, but their application as pavement coatings is still very limited [87].

The impact of sealant concrete and asphalt treatments on surface texture and friction can increase the risk of slipping for foot traffic in addition to hazards for aircraft as outlined above. This aspect is not typically addressed in the performance of surface treatments to extend the service life of concrete and asphalt. However, it is reported that surface treatments are likely to increase the risk of skid/slips as they typically reduce texture and friction. This is likely to be dependent of the nature of the surface, i.e. may not make a significant impact on the ‘skid risk’ of a surface that has ample texture and friction but may become an issue for surfaces with marginal texture [88]. Measures to increase surface roughness of surface sealants include the addition of particulates or physical texturing agents during or post-application to concrete or asphalt pavements/pads.

In general, organic surface treatments including a range of polymers such as polyurethane, epoxy and polyaspartic are more commonly used to preventing water ingress and contaminant leachability, whereas inorganic surface treatments are often added to increase durability [67]. Several knowledge gaps identified in relation to the performance of surface sealant treatments for concrete [67], but also relevant to asphalt pavement, as follows:

  • The effect of coating with organic polymers and hydrophobic impregnation on mechanical properties,

  • Effectiveness of surface coatings on fire-damaged pavements,

  • The effect of temperature and radiation on the performance of acrylic, epoxy and polyurethane coatings,

  • Identifying other factors that cause treatments to age, and

  • Effectiveness of newly developed treatments to primary function.

Sealants for Management of PFAS-Contaminated Concrete and Asphalt Pavements/Pads

In the specific context of civilian and defence airfield facilities/sites, sealants have the potential to.

1. Create a surface barrier (rain or washdown in operational or training areas) of contaminated pavement/pads preventing the ingress and egress of PFAS, and/or

2. Increase the sorption/fixing (binding, immobilisation) of PFAS within the concrete/asphalt matrix, thus reducing its leachability if water penetrates the surface.

A combination of properties of sealants may enhance PFAS binding/immobilisation in concrete and asphalt pavements/pads. For instance, using a combination of sealants, i.e. penetrative silane sealant with an acrylic (polymeric) topcoat [68].

Consideration of sealant properties that may enhance PFAS immobilisation in concrete and asphalt pavements/pads can be informed by soil amendment studies:

  • Immobilisation of PFAS can occur through the addition of an adsorbent, such as, GAC, graphene, biochar or modified clays [31, 36, 89].

  • Mesoporous carbon (2–50-nm pore diameter) has been reported to have larger adsorption capacity for hydrophobic PFAS, whilst micropore (< 2-nm pore diameter) surface area influences adsorption capacity of hydrophilic and marginally hydrophobic PFAS [62].

  • Modified clay at 10% (w/w) addition to soil has been reported to bind/immobilise PFAS [29].

  • γ-AlOOH functionalised with silane containing poly(ethylene glycol) and a perfluorinated region has been found to be effective in membrane treatment removing PFOA and PFOS for drinking water treatment. The C–F chain length was found to be the factor determining effectiveness with long-chain PFAS (> C8) found to be effectively removed [90]. An aqueous suspension of polyaluminium chloride (PACl) stabilised with polydiallyldimethylammonium chloride (polyDADMAC) was found to be effective in removing PFOA and PFOS during its evaluation for potential in situ treatment of groundwater [91].

  • Polyacrylic resins have been found to bind PFAS through anion exchange, rather than adsorption, and the resin may become irreversibly saturated [92].

The necessity for the concrete or asphalt surface to be clean prior to treatment to facilitate good sealant adhesion and the practicality of cleaning PFAS-contaminated surfaces prior to application of a surface treatment (with a potential for a PFAS-contaminated leachate to be generated) are important considerations in providing an effective surface coating. The performance of surface treatments on concrete in the literature is based on ‘new’ concrete surfaces and therefore performance assessment on ‘aged’ concrete are a knowledge gap that needs further research. In addition, there are likely to be two additional but important requirement placed on sealants for concrete and asphalt pavements/pads that (i) they can act as a joint and crack sealant and (ii) they can be used in widespread surficial/areal coverage.

Integrated Assessment of Sealants for Management of PFAS-Contaminated Concrete and Asphalt Pavements/Pads at Civilian and Defence Airfield Facilities/Sites

There were no studies identified in literature that specifically addressed the binding, immobilisation or sealing of PFAS-contaminated concrete or asphalt pavements. A summary of surface and penetrative concrete and asphalt sealant types and properties (negatives and positives) can be found in Table 2. Eleven major groups were identified encompassing a range of organic, inorganic and hybrid or multi-component materials as both surface and penetrative sealants (Table 2). Other novel concrete-specific sealants such as bacterially mediated in situ calcite precipitation [93] and self-healing concrete [94,95,96,97] were identified as being developed or proposed.

An integrated assessment (matrix) of the properties of the eleven sealant groups versus the range of properties encountered or required for concrete or asphalt pavement/pads at civilian and defence airfield facilities/sites is presented in Table 3. A number of sealants are proprietary formulations with often only limited data available such that many of the attributed performance characteristics are at present not independently quantified/verified.

Based on the integrated assessment, the most prospective surface sealants for PFAS-contaminated concrete at civilian and defence airfield facilities/sites, based on sealant properties and site requirements, are aspartic polymers and modified bitumens with the latter also considered the most relevant for asphalt (Table 3). These sealants include binders, such as asphalt blended with one or more of epoxy or polyurethane or other polymers, various aggregate types of different origin and composition and other structural components such as recycled tyre rubber or plastics [98]. Bitumen overlays over concrete [99,100,101] and asphalt [66, 76] are commonly used in a variety of applications. A more recent innovation is the use of a fibreglass-reinforced, polyurethane-based polymer concrete as a runway repair material [102].

The primary advantage of modified bitumens is their high water repellency and relatively lower cost, good adhesion, in particular to similarly hydrophobic asphalt surfaces, trafficability and a range of physico-chemical factors (Tables 2 and 3) and potential for one or more of the additives to be able to adsorb or otherwise chemically bind PFAS. It is this range of functional attributes that positions both aspartic polymers and modified bitumen’s as better surface sealants for concrete and asphalt pavements at civilian and defence airfield facilities/sites, ahead of unmodified bitumen and a range of other polymer types (acrylic, epoxy, polyurethane), particularly in terms of adhesion trafficability and heat resistance amongst other properties.

In the context of penetrative sealants, inorganic silicates, organic silicone/silane/siloxane sealants or a soybean-based biopolymer sealant appear promising for PFAS-contaminated concrete at civilian and defence airfield facilities/sites, albeit based on limited evidence available in the application of proprietary industry formulations (Table 3). The extent of interaction in terms of the extent of penetration and binding of the various silicates and biopolymers is at present unknown for asphalt. By virtue of these sealants’ migration into the concrete profile, however, they have the ability to act as pore-coating or pore-filling agents to restrict both ingress and egress of a variety of contaminants including PFAS. Other useful properties include a likely high degree of adhesion, trafficability and wear resistance (Table 3). As with surface sealants such as modified bitumens, some of the inorganic silicates or organic silicone-based compounds may also have the ability to bind/immobilise PFAS.

It is unlikely that any concrete or asphalt sealant in isolation will meet the wide range of functional requirements required for airfield facilities/sites in relation to either PFAS immobilisation, to prevent egress from extant contaminated concrete, and/or as a sufficient robust physico-chemical barrier to prevent PFAS ingress into new or existing pavements (Table 3). Moreover, it is likely that a combination of both surface and penetrative sealants will be required [60] to meet operational and management requirements with respect to new or existing AFFF/PFAS contamination in new or aged concrete and asphalt pavements. Where requirements of reflectivity or a non-slip surface are not met, by either surface or penetrative sealants, it is likely that the addition of materials such as synthetic TiO2 and or sand, respectively, may be added to achieve an acceptable outcome (Table 3).

Given the lack of direct testing of the potential uptake of PFAS compounds by either surface or penetrative sealants, this remains an area of potential functional enhancement by targeting the use, in particular, of polymeric materials as absorbents. This is particularly the case for acrylic, epoxy, polyurethane and organic silicate materials which may have the potential to bind anionic, cationic or zwitterionic PFAS. This binding may occur via interaction with reactive monomers, during polymerisation in the presence of solvents and accelerators, or following polymerisation (post-curing). If any of these molecular-scale processes are operative, this has the potential to significantly increase the functional utility of polymers per se or when used in combination with other materials such as PAC in a hybrid concrete or asphalt treatment.

Table 2 Summary of surface and penetrative concrete and asphalt sealant types and detailed or proposed attributes
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Table 3 Ranked* assessment of potential concrete and sealants versus functional requirements of surface and penetrative concrete sealant types. *White: positive, light grey: speculative, dark grey: negative
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Knowledge Gaps

Surface and penetrative sealants may offer an immediate solution for management of PFAS-contaminated pavements/pads in airfield facilities. Whilst a range of potential surface and penetrative sealants have been identified and assessed on their inherent properties and requirements, a comparative and independent testing of their characteristics for PFAS management is absent. On this basis, it is recommended that a series of laboratory trials be undertaken that evaluates.

  • The ability of individual polymers (acrylic, epoxy, polyurethane and organic silicate) to adsorb/bind PFAS,

  • The ability of modified bitumens to bind PFAS,

  • The degree of penetration of inorganic and organic silicate compounds in concrete and asphalt and concomitant ability to bind PFAS moieties during or following application,

  • The degree of water repellency of surface sealants, and

  • The extent of egress of PFAS following sealant application from already contaminated concrete and asphalt pavements/pads (e.g. using a Leaching Environmental Assessment Framework (LEAF) test [128]).

Collectively, such laboratory trials provide fundamental knowledge for larger-scale field trials that would assess characteristics such as PFAS leachability, water repellence, wear/trafficability, adhesion, ease of application and cost. It would also be important to assess sealant performance on new and aged concrete or asphalt. Longer-term trials established with concrete to assess the effects of factors such as natural weathering (temperature, UV, etc.) and usage due to traffic on PFAS leachability could also be considered given the range of operational demands likely to be experienced.

The knowledge gained through laboratory trials is also expected to underpin research and development of polymer-based sealant technologies for application to PFAS management in airfield facilities. Research and development are needed, in particular, on technologies/materials with an innate capacity to bind PFAS, possibly via the incorporation of other entities such as ion-exchange resins or carbon-based materials or as part of a physically robust, multi-layer systems.

Considering PFAS contamination has adversely impacted the reuse of concrete and asphalt wastes, research is needed to aid PFAS management in recycled, repurposed or new pavement surfaces. This could also focus on the further development of adsorbent materials with the capacity to adequately cover the spectrum of hydrophobic to hydrophilic (anionic, cationic and zwitterionic) PFAS. To achieve this, it is conceivable that an approach based on blending materials that has emerged over the past decade, technologies that are most appropriate for the levels of contamination and type of wastes, is desirable. This will rely to a large extent on concurrent advances in identification and quantification in terms of both total and potentially leachable fractions of PFAS coupled with further development in waste classification and associated trigger values as part of emerging management and legislative frameworks.

Innovation in amendment could involve the addition of other, potentially controlled-release co-amendments within PFAS-containing asphalt and/or concrete with the capacity to degrade in situ the chemically robust fluorocarbon chains, during their disposal at a landfill. In a similar vein, the combined use of complementary destructive technologies that perhaps use a suite of catalytic surfaces, oxidants and thermolysis to achieve more efficient, cost-effective PFAS degradation should also be considered. Degradation may not necessarily involve the destruction of the fluorocarbon chain per se, but potentially functional group conversion such that the PFAS are appropriately derivatised to enable more robust separation or otherwise long-term sequestration/disposal.

Conclusions

This review described here addresses the potential for management of PFAS contained in contaminated concrete and asphalt via the application of sealants to minimise both water ingress and egress. At present, there is no published information specifically related to the mitigation of PFAS loss or other management techniques for concrete and asphalt despite the existence of more than 10,000 contaminated airfield sites in the USA alone.

Any sealant will also have to address a range of functional requirements that may span areas as diverse as trafficability, UV, chemical and heat resistance to reflectivity and friction/non-slip surfaces. Crucially, the substantial differences in the composition and behaviour of concrete and asphalt pavements, in particular their hydrophilicity and hydrophobicity, respectively, may require the use of different sealants. In addition, there may be substantial differences in both new and aged, PFAS-contaminated concrete and asphalt. This may include abundant leachable alkalinity in new versus aged concrete and the occurrence of widespread thermal cracking and rivelling (aggregate loss) in asphalt.

A range of potentially useful sealants have been identified that range from bituminous or modified bituminous materials, a range of polymers, biopolymers, and inorganic and organic silicon-based materials. These sealants encompass a range of functionalities with some acting as surface sealants with others having pore-lining or pore-blocking characteristics.

Based on our review, it is unlikely that any single sealant class will be able to meet all the functional requirements for applications that may include concrete or asphalt pavements in one or more of active or legacy fire training areas, hangars, aprons or runways. Hence, it is likely that a combination of one or more sealants, in particular surface, and one of a pore-lining or pore-blocking sealant is required. In addition to the need to meet the diversity of field applications, additives such as GAC, fine sand or TiO2 pigment may be required to meet the specific functional requirements of PFAS adsorption, non-slip/friction and reflectivity, respectively.

Several knowledge gaps are apparent in the potential use of sealants as a method of managing PFASs in active and legacy contaminated concrete and asphalt pavements. These include the need for extensive laboratory trials on both contaminated and uncontaminated concrete and asphalt to assess the potential to solute, and hence, PFAS ingress and egress. In addition, a focus should be on the develop of innovative hybrid materials that not only mitigate PFAS mobility but also contain adsorbents that may actively bind labile PFAS. Once suitable materials are identified, these should then be subjected, along with other promising sealant types or combinations thereof, to extensive pilot-scale and field trials to determine both efficacy and longevity. This review serves as a starting point for further studies to evaluate their short or long-term effectiveness in immobilisation of PFAS residues in in situ or ex situ concrete and asphalt.