<div>
<p>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, nFe<sup>0</sup> and activated carbon
(AC) at 22 <sup>o</sup>C to 60 <sup>o</sup>C for transforming perfluorooctanesulfonate
(PFOS) from which nNiFe<sup>0</sup>-AC at 60 <sup>o</sup>C led to transformation
of both linear (L-) and branched (Br-) PFOS isomers. The remaining research focused
on work with nNiFe<sup>0</sup>-AC at 60 <sup>o</sup>C in batch reactors including
optimizing nNiFe<sup>0</sup>-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.</p>
<p>nNiFe<sup>0</sup>-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.</p>
<p>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 nNiFe<sup>0</sup>-AC at 60 <sup>o</sup>C 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.</p><p>The research presented here demonstrates that
nNiFe<sup>0</sup>-AC at 60 <sup>o</sup>C 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.</p></div>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8046632 |
| Date | 14 May 2019 |
| Creators | Jenny E Zenobio Euribe (6638495) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Abiotic_Reduction_of_Perfluoroalkyl_Acids_by_NiFe_sup_0_sup_-Activated_Carbon/8046632 |
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