Selma Mededovic Thagard
Thagard received her Ph.D. in chemical engineering from Florida State University. Before coming to Clarkson, Thagard held post-doctoral appointments at Toyohashi University of Technology in Japan and at Colorado State University. Her area of expertise is in electrical discharge plasma processes with a focus on theoretical and experimental investigations of fundamental plasma chemistry in single and multiphase plasma environments and in plasma reactor design. Thagard has coauthored over 40 articles in refereed journals and has presented over 30 lectures as invited speaker. Thagard serves on the Editorial Board of Plasma Chemistry and Plasma Processing.
Rapid removal of poly- and perfluorinated compounds from investigation derived waste (IDW) in a pilot-scale plasma reactor
A pilot-scale plasma reactor was used to rapidly and effectively degrade poly- and perfluoroalkyl substances (PFAS) from liquid investigation derived waste (IDW) (i.e., purge water from monitoring wells) obtained from 13 different site investigations at Air Force installations. In the raw water, numerous PFAS were detected in a wide concentration range (~10 to 105 ng/L; total oxidizable precursors (TOP) ~102 to 105 ng/L). Plasma-based water treatment resulted in rapid perfluoroalkyl acids (PFAAs) removal from 4L individual IDW samples with faster rates for longer-chain perfluorocarboxylic acids (PFCAs) and perfluorosulfonates (PFSAs) (C ? 8 and ? 6, respectively) than for PFCAs and PFSAs of shorter chain length. In nine of the 13 IDW samples both PFOS and PFOA were removed to below HAL concentrations in < 1 minute whereas longer treatment times (up to 50 minutes) were required for the remaining four IDW samples due to either extremely high solution electrical conductivity which decreased the plasma-liquid contact area (one IDW sample) or high concentrations of PFAAs and/or their precursors; the latter were found to be converted to PFAAs during the treatment. Overall, 36 to 99% of the TOP present in the IDWs were removed. There was no effect of non-PFAS co-contaminants on the degradation efficiency. Overall, the results indicate that plasma-based water treatment is a viable technology for the treatment of PFAS-contaminated IDW.
Evaluation of Current Use C6 AFFF: What Do They Really Contain?
INTRODUCTION: Aqueous film forming foam (AFFF) is a highly efficient fire suppressant agent used to extinguish flammable liquids that has been widely used at industrial and military sites. AFFF contains large concentrations of fluorinated surfactants including per- and polyfluoroalkyl substances (PFAS) which are persistent in the environment and associated with adverse health effects. In an effort to decease their persistence and toxicity, current AFFF formulations are being advertised as containing only C6 and smaller perfluorinated surfactants and low molecular weight polymers. To evaluate this claim and better characterize a currently available AFFF solution, AFFF Ansulite 6% (AFC-6MS-C) was obtained and analyzed using mass spectroscopy before and after oxidation using the total oxidizable precursor assay (TOP).
METHODS: Twelve perfluoroalkyl acids (PFAAs) and 10 precursor concentrations were measured in negative ionization mode using a UPLC-QToF-HRMS (Xevo G2-XS, Waters Corp.). All samples were spiked with 2 ng of labeled internal standards to allow for quantification using C-13 isotopic dilution methods. A detailed description of the analytical methods, QA procedures and individual detection limits have been published previously (Singh et al., 2019). Total oxidizable precursors (TOP) were measured using a modified persulfate oxidation method developed by Houtz and Sedlak (2012). Specifically, significantly higher concentrations of persulfate were required to oxidize all the precursors present.
RESULTS AND DISCUSSION: Analysing AFFF for PFAAs and precursors is difficult and requires significant dilutions so as not to saturate the instrument detector. These dilutions raise detection limits into the mg/L range. In the sample analysed, PFBA had the highest concentration (270 mg/L) followed by PFHxA (130 mg/L) (Figure 1). Measurable concentrations of >C6 acids were also found including PFHpA, PFOA, PFNA, PFOS, PFDA, and PFDS. Of these, PFOA and PFOS had the highest concentrations (71 and 42 mg/L, respectively). 6:2 FTS was the only precursor identified (at 130 mg/L). The TOP assay as developed by Houtz and Sedlak (2012) was only partially effective at oxidizing the precursors so increased doses and reaction times were used. Even after doses of up to 240 mM persulfate and reaction times of 6 hours concentrations of PFAAs and 6:2 FTS continued to increase indicating that higher doses and longer reaction times are needed to completely oxidize the precursors present. Although 6:2 FTS was the only precursors found, its increased concentration upon persulfate addition indicated there are significant amounts of other precursors present that can be oxidized to 6:2 FTS. Concentrations of individual PFAAs and precursors produced by oxidation with persulfate generally increased with increasing dose and reaction time. For the highest dose and longest reaction time (240 mM and 6 hrs) PFHxA was found at highest concentration (1290 mg/L) followed by shorter chain PFAAs, PFBA and PFPeA (520 and 360 mg/L, respectively). PFOA concentrations approximately doubled to 130 mg/L after oxidation.
CONCLUSIONS: Current formulations of AFFF were found to contain measureable concentrations of >C6 PFAAs and PFAA precursors. We are continuing these experiments with other suppliers and exploring other oxidation techniques and plasma destruction technologies for AFFF treatment.