In recent years, a number of conventional and novel sorbents have been studied at the bench-scale to evaluate their potential to remove PFAS from water. Many of these studies were designed to support the use of the sorbents in ex situ packed-bed sorption processes. However, it is not always clear what data should be collected at the bench-scale to make accurate predictions of sorbent performance during full-scale applications. One common approach is the rapid small-scale column test (RSSCT); the RSSCT approach was developed decades ago and provides scaling equations to enable the design of a small-scale column that simulates the performance (i.e., breakthrough) of a full-scale packed-bed sorption process. The RSSCT scaling equations rely on knowledge of the sorption kinetics and affinity of a sorbent as a function of particle size and the mechanisms that control mass transfer of the sorbate to the binding sites on the sorbent. These scaling equations have previously been developed for activated carbon and for sorbates that exhibit specific mechanisms of mass transfer. It is unclear whether these scaling equations are useful when evaluating other conventional sorbents (i.e., ion exchange resins) or novel sorbents that exhibit unique sorption mechanisms. It is likewise unclear whether one set of scaling equations will adequately simulate the breakthrough of complex mixtures of PFAS that may have variable diffusion coefficients, or the extent to which background water constituents (e.g., natural organic carbon, anions) impact scale-up in the context of PFAS remediation. There is a critical need to evaluate existing experimental frameworks (i.e., RSSCTs) or to develop and validate novel experimental frameworks to simulate the performance of full-scale adsorption processes for PFAS.
The adsorption behavior of PFAS mixtures on commercially available sorbents, such as activated carbons and ion exchange resins, has received only limited attention in the scientific literature. Additionally, the effects of co-constituents (e.g., natural organic matter), solution properties (e.g., pH, dissolved salts), and co-occurring chemicals of concern (e.g., chlorinated organic compounds) on the adsorption characteristics of PFAS mixtures are largely unknown. To accurately predict competitive adsorption of PFAS mixtures, data are needed for a range of concentrations across a range of molar ratios that are representative of surface and groundwater impact encountered at DoD sites. Furthermore, there is a need to understand the effects of experimental parameters on PFAS desorption (release), adsorption and desorption kinetics (mass transfer), and non-ideal behavior (hysteresis). Data collected from these studies will support the development of multi-component mathematical models that accurately describe the adsorption-desorption behavior of PFAS mixtures on commercially available sorbents over a range of relevant concentrations and environmental conditions.
Development of technologies for treatment of vapor phase PFAS is also of interest. A number of in situ and ex situ treatment technologies that have been proposed to treat PFAS-impacted soil and groundwater are likely to generate vapor streams containing volatile PFAS species that will need to be captured to prevent release into the environment. Whereas vapor-phase GAC systems are likely to be used for off-gas treatment, the performance of vapor-phase GAC systems for PFAS is largely unknown. Furthermore, vapor-phase GAC systems may not effectively remove short-chain length PFAS that may be generated during the treatment process (e.g., in situ thermal treatment) nor non-PFAS fluorocarbon products of incomplete destruction. Therefore, there is a need to develop and test conventional or novel adsorbents that provide effective removal of volatile PFAS generated during remediation activities.
In addition to vapor phase PFAS, treatment of short chain or ultrashort chain may be of increasing concern as the regulatory landscape evolves and toxicity data become available for a wider range of PFAS. Further, as PFAS destruction technologies come online, short-chain and ultrashort-chain PFAS may be generated due to incomplete mineralization of target PFAS. To meet this need, existing adsorbents are likely to be modified or refined and novel adsorbents with broader specificity will continue to be developed. To ensure a detailed and unbiased understanding of the adsorptive capacity, efficacy, and performance of novel adsorbents, fundamental research will be critical to the advancement and adoption of new materials for treatment of PFAS-impacted waters.
Various absorptive soil amendments may also play a critical role in mitigating PFAS transport at impacted sites. In particular, secondary sources with lower PFAS soil concentrations that occupy relatively large footprints may be amenable to treatment via these amendments. The goal of any novel amendment would be to reduce PFAS mobility and bioavailability through enhanced stabilization and retention in surface soils.