by Ted Tyler, Kleinfelder
Per- and polyfluoroalkyl substances (PFAS) are chemicals that have been utilized in industry and manufacturing since the 1940s, have been identified in the environment, and can potentially bio accumulate in human beings and wildlife. The challenge in addressing PFAS in the environment is that there are thousands of PFAS chemicals with varying fate and transport characteristics. They are persistent in the environment, and vary in their response to particular treatment technologies. Moreover, PFAS are often encountered with co-contaminants such as fuels or chlorinated solvents that also require treatment and may impact the selection of treatment technologies for PFAS. While there are many technologies being evaluated for their potential in PFAS defluorination and degradation, the intent of this article is to focus on in-situ technologies that are implementable and hold promise for PFAS transformation or degradation.
PFAS source area treatment in the unsaturated zone, has largely involved excavation and disposal, often adding amendments to impacted soil that effectively reduces leachability and results in lower disposal costs. Treatment technologies for PFAS impacted groundwater have relied heavily on groundwater extraction followed by utilization of various ex-situ treatment technologies (i.e., carbon adsorption, ion exchange, etc.) to remove PFAS prior to disposal of the treated water. Prospects and advances in the in-situ treatment of PFAS impacted groundwater offer an opportunity to treat PFAS in-situ, which significantly reduces capital equipment expenditures, operation and maintenance (O&M), and waste management costs associated with ex-situ treatment. Alternatively, in-situ technologies offer opportunities to reduce the mass of PFAS in groundwater prior to extraction, which can help reduce the scale and cost of equipment and associated O&M and waste disposal requirements. Following is a brief discussion of advances and prospects for in-situ treatment technologies to address dissolved phase PFAS, and a discussion of combined remedies to provide a holistic site-specific solution.
Among in-situ alternatives, in-situ sequestration for groundwater treatment is perhaps the furthest along, and field case studies are available demonstrating successful application. This is in part attributable to their focus on adsorption of dissolved phase PFAS which, while still developing, is well understood. In-situ sequestration involves the injection of colloidal activated carbon (CAC), which if not combined with either an in-situ bio or in-situ chemical mechanism, seeks adsorption for long-term cost effective sequestration rather than seeking the transformation or degradation of PFAS compounds. The primary drawback for in-situ sequestration is that ultimately the PFAS-laden adsorbent must be removed and disposed, followed by reinjection. Moreover, whether or not all perfluoroalkyl acid (PFAA) precursors are sequestered or not is a subject of continued research. Chemical coagulants are also being evaluated to optimize in-situ sequestration.
Another technology that can be applied for in-situ sequestration, source treatment or permeable reactive barriers, are organoclays. Kleinfelder has demonstrated the improved adsorption effectiveness of organoclays as compared with granular activated carbon (GAC). Preliminary bench testing also indicates that to the extent that PFAS can be adsorbed into the interlayer structure of such organoclays, there is a viable potential for complete PFAS defluorination, including typical dead-end daughter PFAAs such as perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA). The mechanism for defluorination is a combination of (1) the establishment of stable hydrated electrons, and (2) the co-occurrence of such hydrated electrons in close proximity with PFAS, absent potential quenching of such hydrated electrons by light or oxygen, within the interlayers of the clay. Field testing is still required to demonstrate the effectiveness of this technology under site conditions, but preliminary evidence points to this being a viable cost-effective and green technology for in-situ, as well as ex-situ applications. Organoclays used alone, or in combination with other in-situ treatment technologies could potentially eliminate the need for ex-situ treatment.
Knowledge regarding the application of in-situ bioremediation (ISB), or bioaugmentation, for PFAS compounds is still developing. However, microbial transformation of PFAS precursors appears feasible under aerobic conditions, whereas end daughter PFAAs (such as PFOS and PFAS) appear to be resistant, at least based on a limited number of studies. Few detailed studies have been performed to evaluate the potential for microbial transformation under anaerobic conditions.
The use of fungi for in-situ remediation of PFAS has also shown promise at the bench-scale, though they are challenging to maintain under field conditions. Packaging of fungi-generated enzymes for bioaugmentation may prove to be a viable approach for PFAS bioaugmentation, but research continues. In addition, other strains of PFAS degrading bacteria are being studied for use in bioaugmentation.
Given the low action levels for PFAS compounds (parts per trillion levels), in-situ biotransformation may be best applied as part of a mass reduction strategy because low PFAS concentrations will not support a robust microbial population. Also, in-situ biotransformation appears to target PFAA precursors, but may not degrade the end daughter product PFAAs. Consequently, a combined remedy approach (e.g., in-situ biotransformation coupled with ex-situ granular activated carbon [GAC] adsorption) may be necessary to meet cleanup objectives.
Treatment of PFAS compounds through cometabolism may ultimately provide a means of degrading PFAS even at low concentrations. Cometabolism is the degradation of a target contaminant, or secondary substrate, as a result of the conditions established by the degradation of another primary substrate. Cometabolism does not rely on the concentration of the target contaminant (such as PFAS), which is fortuitously transformed or degraded. Research for cometabolic mechanisms is ongoing.
In-situ chemical oxidation
Knowledge regarding the use of chemical oxidants (in-situ chemical oxidation, or ISCO) for the degradation of PFAS is also developing. While common in-situ oxidants such potassium permanganate have been shown to be ineffective in the oxidation of PFAS, other oxidants or oxidant systems may be effective. As an example, heat (hydrogen peroxide) activated persulfate has been shown to oxidize PFAA precursors as well as polyfluoroalkyl carboxylic acids (PFCAs; such as PFOA), but appears to be ineffective in oxidizing end daughter polyfluoroalkyl sulfonic acids (PFSAs; such as PFOS). Another example is perozonation, whereby higher PFAS concentrations; including PFOA, PFOS and perfluorohexane sulfonic acid (PFHxS); may be transformed or degraded. An additional example would be PFAS oxidation through catalyzed hydrogen peroxide (CHP) or through potassium permanganate injection with the objective of yielding more powerful oxidants (e.g., superoxide radical) capable of oxidizing many PFAS compounds.
As with in-situ biotransformation, the in-situ use of oxidants or oxidant systems effective in treating PFAS would also likely be part of a mass reduction/combined remedy strategy whereby source area or elevated concentration PFAA precursors were oxidized, yielding PFCAs and/or PFSAs that would then be treated using a secondary technology such as extraction and adsorption with GAC.
In-situ chemical reduction
Preliminary studies have shown that in-situ chemical reduction (ISCR) utilizing zero valent metals/bimetals such as Pd/Fe and Pd/Mg with clay interlayers may also provide a viable treatment mechanism for PFAS. As an example, amino-clay coated zero valent iron (AC-ZVI) may be effective for the treatment of anionic PFAS molecules such as PFOA, PFOS and their precursors. As with ISB and ISCO, ISCR may also need to be coupled with other technologies to reach site cleanup objectives.
Combined Remedies for Site Solution
Various in-situ treatment technologies may be applied for the partial treatment of PFAS including precursors of PFOS and PFOA, but for a holistic solution at a particular site, multiple in-situ technologies and/or combinations of in-situ and ex-situ technologies may provide the best strategy. A first step in developing a strategy for managing or treating PFAS is to screen technologies for their ability to treat various PFAS compounds. Source PFAS typically degrade into PFAA precursors, and subsequently to dead end daughter PFAAs such as PFCAs and PFSAs. Depending on the technology being screened, it may have the potential for treatment or management of the precursors, may categorically target PFCAs or PFSAs, or may target shorter chained dead-end daughter PFAAs such as PFOA or PFAS (see chemical structures below). Table 1 presents the various PFAS categories for consideration, depending on the particular source composition (i.e., Legacy aqueous film forming foam [AFFF] will have a different composition as compared with more modern AFFF products).
Perfluorooctanesulfonic Acid (PFOS) in 3-D. An example PFAA in the category of PFSAs.
Author: Jynto (https://commons.wikimedia.org/wiki/User:Jynto)
Perfluorooctanesulfonic Acid (PFOS) in 2-D.
Author: Pngbot / Benjah-bmm27
Perfluorooctanoic Acid (PFOA) in 3-D. An example PFAA in the category of PFCAs.
Author: Manuel Almagro Rivas (https://commons.wikimedia.org/wiki/User:Malmriv)
Perfluorooctanoic Acid (PFOA) in 2-D
Author: Manuel Almagro Rivas (https://commons.wikimedia.org/wiki/User:Malmriv)
PFAS Treatment or Management Technology Screening
As an example application of Table 1 in developing a strategy for a hypothetical scenario, consider a site where the release of PFAS (Legacy PFOS AFFF) has resulted in source area (soil and groundwater) contamination and a dissolved plume. For this scenario, consider a strategy wherein the source area is treated through in-situ chemical oxidation using hydrogen peroxide (heat) activated persulfate in the source area.
As shown in Table 2, this strategy in the source area can be considered for the treatment of PFCAs, but will likely mobilize PFSAs. Now as part of a combined remedy strategy, and depending on site conditions and the ability for effective hydraulic capture, consider coupling source treatment by ISCO with pump and treat such as through GAC adsorption. As shown in Table 3, this combination of technologies could be considered as a strategy to address both source area and dissolved plume, PFSAs to meet remediation objectives. The objective of such a strategy is to reduce costs for ex-situ treatment through ISCO mass reduction in the source area such that the scale of ex-situ treatment may be reduced. Bench testing should be considered to establish the site-specific viability of such an approach in advance of field pilot testing or full-scale implementation.
Disclaimer: The information set forth in this communication should not be construed nor relied upon as legal or technical advice and is not intended as a substitute for legal, technical, or regulatory consultation.