At the start of this project, the primary approach to mitigate per- and polyfluoroalkyl substance (PFAS) transport and impact to drinking water was sorbents, which came with challenges to effectively capture shorter chain perfluorinated alkyl acids (PFAAs). Very little was known about potential technologies that could destroy PFAS, in particular perfluorooctanesulfonic acid (PFOS), which had been identified as the most recalcitrant PFAS to even abiotic technologies. What was known at the time was that microbial degradation of PFAS would lead to terminal PFAAs. Also, the abiotic technologies that had been evaluated at some level for destruction of PFOS and perfluorooctanoic acid (PFOA) were either unsuccessful, had questionable results, or were simply not amenable for in situ remediation of groundwater. The large PFAS-impacted plumes being generated due to the frequent use of aqueous film-forming foams (AFFFs) called for the need to identify in situ destructive remediation technologies to mitigate PFAS transport off-site. The primary goal of this project was to identify reductive technologies amenable for in situ destructive remediation of PFAS with PFOS as the initial focus.
A subset of zero valent metals/bimetal particles alone and associated with a carrier for ease of injection (e.g., synthesized within clay interlayers or onto granulated activated carbon (GAC)) and a metal-catalyzed Vitamin B12 to defluorinate PFAS (primarily PFOS) as well as a coupled reductive/oxidative (permanganate (PM)/persulfate) technology was investigated. To different extents, depending on success of preliminary experiments, the studies included:
- Quantifying the magnitude, rate, and effectiveness of a technique to defluorinate linear PFOS in aqueous batch reactions within an environmentally relevant PFOS concentration ranges with multiple lines of evidence (PFOS loss, generation of inorganic [fluoride and sulfite] and organic products).
- Characterizing intermediates resulting from incomplete defluorination.
- Quantifying matrix effects.
- Evaluating defluorination in column studies.
This research led to the development of a reductive technology using Ni0Fe0 nanoparticles synthesized onto activated carbon (AC). The technology was demonstrated in batch and 1-D column reactors to transform and mineralize linear (L-) and branched PFAS with fluoride and sometimes sulfite generation and organic intermediates. All PFAS transformed and mineralized to varying degrees. Overall, fluoride generation was greatest for the perfluoroalkyl sulfonic acids (PFSAs) with the highest transformation and mineralization observed for PFOS (> 94%).
For the PFAAs, transformation rates and % fluoride generated relative to moles of parent transformed decreased with decreasing perfluorocarbon chain length. In addition, the team exemplified that VB12 technology cannot be enhanced to degrade L-PFSAs due to steric hindrances and not activation energy; thus, addition of stronger reductants/catalysts will not help. The other bimetal nPdFe0 that initially showed promise, did not transform PFOS, but only sorbed and complexed PFOS; PFOS complexation with iron reduces quantifiable PFOS concentrations. The team also found that neither PM, persulfate or a combination of the two (ScisoR Technology®) can transform PFOS. Apparent PFOS loss appeared to be due to clustering and salting out, which leads to sampling artifacts, thus underestimating PFOS concentrations. Fluoride impurities in reagents used as well as peaks that interfere with proper fluoride quantitation led to wrongly assuming fluoride was being generated.
Additional column studies that best mimic how well this technology may work in the field either as an in situ permeable reactive barrier, above ground treatment (a reactive GAC rather than just a sorptive GAC), or in situ injection are needed to evaluate further transformation and defluorination at increased residence times. In addition, potential impacts of additional groundwater matrix parameters including non-PFAS co-occurring chemicals likely to be present at AFFF-impacted sites as well as regeneration potential of the nNi Fe -AC nanocomposite should be evaluated. Preliminary data indicated that there is potential for in situ regeneration.
Park, S., J. Zenobio*, and L.S. Lee. 2018. Perfluorooctane sulfonate (PFOS) loss with Pd /nFe nanoparticles: adsorption and Fe-complexation, not transformation, Journal of Hazardous Materials, 342:20-28. https://doi.org/10.1016/j.jhazmat.2017.08.001
Park, S., C. de Perre, and L.S. Lee. 2017. Alternate reductants with VB12 to transform C8 and C6 Perfluoroalkyl Sulfonates: Limitations and insights into isomer-specific transformation rates, products and pathways. Environmental Science & Technology, 51 (23):13869–13877. https://doi.org/10.1021/acs.est.7b03744
Park, S. L.S. Lee, V. F. Medina, A. Zull, and S. Waisner. 2016. Heat-activated persulfate oxidation of PFOA, 6:2 ﬂuorotelomer sulfonate, and PFOS under conditions suitable for in-situ groundwater remediation. Chemosphere, 145:376-383. https://doi.org/10.1016/j.chemosphere.2015.11.097