Abiotic Reduction of Perfluoroalkyl Acids by NiFe0-Activated Carbon
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In recent years, the presence of per- and polyfluoroalkyl substances (PFAS) in aquatic systems has led to research on their fate, effects and treatability. PFAS have been found in various environmental matrices including wastewater effluents, surface, ground, and drinking water. Perfluoroalkyl acids (PFAAs) are the class of PFAS most commonly tested due to their ability to migrate rapidly through groundwater and include perfluoroalkyl sulfonic acids (PFSAs) and perfluoroalkyl carboxylic acids (PFCAs). Of the globally distributed and persistent PFAAs, PFSAs are the most resistant to biological and oxidative chemical attack. This doctoral study focused on a reductive treatment approach with zero valent metals/bimetals nanoparticles (NPs) synthesized onto a carbon material to reduce NP aggregation. Initial work focused on exploring reactivity of different combinations of nano (n) Ni, nFe0 and activated carbon (AC) at 22 oC to 60 oC for transforming perfluorooctanesulfonate (PFOS) from which nNiFe0-AC at 60 oC led to transformation of both linear (L-) and branched (Br-) PFOS isomers. The remaining research focused on work with nNiFe0-AC at 60 oC in batch reactors including optimizing nNiFe0-AC preparation, quantifying PFOS transformation kinetics and evaluating the effects of PFAA chain length (C4, C6 and C8) and polar head group (PFSA versus PFCA) as well a groundwater matrix on transformation magnitude. Optimization of analytical methods to provide multiple lines of evidence of transformation including fluoride, sulfite and organic product generation was an ongoing throughout the research.
nNiFe0-AC prepared with a 3-h synthesis stirring time led to the highest PFOS transformation of 51.1 ± 2.1% with generation of ~ 1 mole of sulfite (measured as sulfate) and 12 moles of fluoride. Several poly/per-fluorinated intermediates with single and double bonds were identified using quadrupole time-of-flight mass spectrometry (QToF-MS) in negative electrospray ionization (ESI-) mode with MS/MS fragmentation confirmation as well as one and later two desulfonated products with QToF negative atmospheric pressure chemical ionization (APCI-). All organic transformation products were found in only particle extracts as well as most of the sulfite generated. PFOS transformation kinetics showed that generated fluoride concentrations increased for the first day whereas sulfate concentrations continued to increase during the 5-d reaction. The transformation products identified showed defluorination of single- and double-bond structures, formation of C8 to C4 PFCAs and paraffins from cleavage of the C-S bond.
The length of the perfluoroalkyl chain affected the length of time to achieve peak removal, but overall magnitude of transformation when reactions appeared complete were similar for both PFSAs and PFCAs. Like PFOS, PFOA transformation maxed in 1 d whereas shorter chains required more time to reach their peak removal, which is hypothesized to be due to lower sorption of the shorter chain PFAAs to the reactive surfaces. Measured F mass balance was higher for PFOS and PFOA (>90% F) compared to shorter chain PFAAs (~50-70% F). The Perfluorohexanesulfonate (PFHxS) and perfluorobutanesulfonate (PFBS) degradation products include single bond polyfluoroalkyl sulfonates and shorter-chain perfluoroalkyl carboxylates. For example, PFHxS transformation resulted in perfluorohexane carboxylic acid (PFHxA) and perfluorobutane carboxylic acid (PFBA). PFCA transformation products included per- & polyfluoroalkyl carboxylates with single bonds and alcohols with single and double bonds. The effect of inorganic matrix on transformation with nNiFe0-AC at 60 oC was explored using a contaminated groundwater collected at a former fire-training area in Massachusetts. Transformation appeared ‘generally’ lower than in the single-solute clean water systems, which may have been due to the presence of PFAS precursors that degraded to PFAAs and competitive adsorption between anionic PFAAs and inorganic ions onto the NP surface.
The research presented here demonstrates that nNiFe0-AC at 60 oC can mineralize PFAAs even in a typical groundwater matrix. Additional lab and pilot scale studies are needed to clarify the mechanisms leading to transformation as well as why transformation reactions plateau prior to all the parent compounds being transformed. The latter may be due to a poisoning phenomenon that can occur in closed systems, which may not occur in a flowing system more characteristic of an environmental scenario, as well as surface area and reactive site constraints or particle passivation.