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Environmental Processes
Sorption of Perfluoroalkyl Acids to Fresh and Aged Nanoscale Zerovalent Iron Particles Yanyan Zhang, Yue Zhi, Jinxia Liu, and Subhasis Ghoshal Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00487 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 3, 2018
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Sorption of Perfluoroalkyl Acids to Fresh and Aged Nanoscale Zerovalent
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Iron Particles
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Yanyan Zhang, Yue Zhi, Jinxia Liu, Subhasis Ghoshal*
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Department of Civil Engineering, McGill University, Montreal, Quebec H3A 0C3, Canada
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*Corresponding Author: Phone: (1)-514-398-6867; fax: (1)-514-398-7361; e-mail:
[email protected] 8 9 10
Total word counting: 7991
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Main text: 5491
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Table: 1
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Figures: 4
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TOC/Abstract Art
25 nZVI Injection for TCE
Fuel fire AFFFs Iron oxy(hydr)oxides
log KF = 4.88
Fe0 Iron sulfides log KF = 4.82
Fe0
log KF = 3.96
H2O
Fe0
log KF = 5.00
HS–
Fe0
log KF = 4.84 log KF = 4.73
Fe3O4
γ-Fe2O3
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ABSTRACT
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The sorption of perfluoroalkyl acids (PFAAs), particularly perfluorooctanesulfonic acid (PFOS), to
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freshly-synthesized nanoscale zerovalent iron (nZVI) and aged (oxidized) and sulfidated nZVI, was
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investigated under anaerobic conditions. The of sorption of PFAAs to nZVI was 2–4 orders of
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magnitude higher than what has been reported for sediments, soils, and iron oxides. The
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hydrophobicity of the perfluorocarbon chain dominated the sorption, although FTIR spectra indicated
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specific interactions between sulfonate and carboxylate head groups and nZVI. The contributions from
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electrostatic interactions depended on the surface charge and pH. Humic acids influenced sorption
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only at concentrations above 50 mg/L. nZVI aged in deoxygenated water up to 95 days showed similar
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sorption isotherms for PFOS to fresh nZVI, because Fe(OH)2 was the predominant phase on the nZVI
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surface independent of aging time. Sulfidation of nZVI reduced sorption of PFOS by 1 log unit owing
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to the FeS deposited, but the sorption affinity was restored after aging because of formation of
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Fe(OH)2. Oxidation of nZVI by water and dissolved oxygen also resulted in similar sorption of PFOS
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as fresh nZVI at environmentally-relevant concentrations. The results suggest that injection of nZVI
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could reduce PFAA concentrations in groundwater despite changes to its surface chemistry with aging.
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INTRODUCTION
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Since its first development in 1960s, significant amounts of aqueous film-forming foams (AFFFs)
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have been applied during military training and fire-fighting activities.1,2 At many AFFF-impacted sites,
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this has resulted in groundwater contamination by per- and polyfluoroalkyl substances (PFASs),3
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which are the key fire extinguishing components in AFFFs. Many of these PFASs can chemically or
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biologically transform to perfluoroalkyl acids (PFAAs) in the natural environment.2,4–7 PFAAs are
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highly persistent, and have relatively high water solubility and thus migrate in groundwater.8 For
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example, up to milligrams per liter levels of PFAAs were detected in the groundwater at military bases
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where fire-fighting activities were conducted.3 Food web analyses have shown that long-chain PFAAs
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are bioaccumulative.9 Exposure to some PFAAs can cause adverse effects including hepatotoxicity,
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tumor induction, and endocrine disruption.10
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Chlorinated solvents such as trichloroethene are common pollutants in groundwater and are often
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encountered together with PFAAs at the AFFF-impacted sites because of their use in formulation of
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ignition fluids during fire-training exercises.11,12 Nanoscale zerovalent iron (nZVI) has been proposed
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as an in situ remediation agent for chlorinated solvent contaminated groundwater.13 It is necessary to
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investigate the interactions between PFAAs and nZVI at sites where nZVI-based remediation is
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attempted and how the treatments may influence the concentration and composition of PFAAs in
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groundwater.
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PFAAs are strong acids (pKa < 0) that dissociate in most natural environments, thus sorption of PFAAs
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to charged solids such as nZVI is expected to be influenced by the pH and ionic strength of the
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aqueous phase and surface charge.14 Long-chain PFAAs can overcome the electrostatic repulsion and
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sorb to negatively-charged mineral surfaces and natural organic matter because of their hydrophobicity.
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nZVI particles have a core-shell structure comprised of an Fe0 core and iron(hydr)oxide shell. Aging or
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prolonged exposure of nZVI in water leads to corrosion of Fe0, resulting in increased iron(hydr)oxides
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on the surface, change in particle morphology, and reduction in Fe0 content.15,16 nZVI can also interact
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with dissolved organic matter,17,18 and react with groundwater ions, in particular, bisulfide formed by
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microbial sulfate reduction, resulting in deposits of iron sulfides on the nZVI surface.19,20 This
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sulfidation process can significantly enhance the reactivity of nZVI towards chlorinated solvents and
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sulfidated nZVI is considered as a more feasible agent for in situ remediation.17,21–23 The change in
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composition and morphology of nZVI during aging in groundwater is expected to affect its 3
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interactions with PFAAs.
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In this study, the sorption of PFAAs to nZVI was characterized by developing sorption isotherms.
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FTIR was used to identify the specific interactions between PFAAs and nZVI. Sorption of
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perfluorooctanesulfonic acid (PFOS) to nZVI was also investigated at different solution pH and ionic
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strength, and in the presence of dissolved organic matter. PFOS is one of the most common PFAAs
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detected in groundwater,3 and is listed as a Persistent Organic Pollutant under the Stockholm
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Convention.24 The effect of nZVI aging on sorption of PFOS was further tested under anaerobic aging
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(oxidation of nZVI during reduction of water to H2), sulfidation, and aerobic aging (oxidation by water
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and dissolved oxygen) scenarios. Sorption isotherms of PFOS were also developed for the synthesized
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nanoscale iron species generated during aging and sulfidation of nZVI, including Fe(OH)2, γ-Fe2O3,
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Fe3O4, γ-FeOOH, and FeS. To our knowledge, this is the first study on sorption of PFAAs to nZVI.
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EXPERIMENTAL SECTION
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Materials
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PFOS potassium salt (PFOS-K, ≥ 98%), perfluorohexanesulfonic acid potassium salt (PFHxS-K, ≥
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98%), perfluorobutanesulfonic acid (PFBS, 97%), perfluorodecanoic acid (PFDA, 98%),
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perfluorononanoic acid (PFNA, 97%), and perfluorooctanoic acid (PFOA) (96%) used for sorption
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experiments were purchased from Sigma-Aldrich (Table S1). Standards of PFAAs and
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isotope-labelled PFAAs used for quantification were purchased from Wellington Laboratories.
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FeSO4·7H2O (> 99%) and FeCl3·6H2O (≥ 99%) were purchased from MP Biomedical, and Acros
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Organics, respectively. Carboxymethyl cellulose sodium salt (CMC, MW 700 000 g/mol), humic acid
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sodium salt (HA, technical grade), NaBH4 (99.99%), Na2S, and CaCl2 were purchased from
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Sigma-Aldrich. Ammonium hydroxide (30%, w/w), FeCl2·4H2O (99–102%), NaCl, and NaOH were
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purchased from Fisher Scientific. Plasma pure HCl (37%, w/w) and HNO3 (70%, w/w) were
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purchased from SCP Science. Acetonitrile and methanol were HPLC grade and were purchased from
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Fisher Scientific. ASTM Type I water was used in all experiments.
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nZVI Synthesis and Aging
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nZVI particles were synthesized by NaBH4 reduction23 in an anaerobic glove chamber (Coy
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Laboratories) containing high purity mix of 2–4% H2 in N2. Briefly, NaBH4 (0.75 g in 15 mL) was
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added dropwise to a continuously mixed solution of FeSO4·7H2O (2.0 g in 100 mL of 30% methanol)
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at 3 mL/min using a syringe pump, followed by a mixing time of 1 h. A glass-coated magnetic stir bar
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was used to achieve efficient stirring (700 rpm). The resulting nZVI particles were separated by a
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magnet, washed five times with water and three times with methanol, and dried inside the chamber.
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Water and solvents were deoxygenated by purging with N2 for 45 min before bringing into the
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chamber.
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Sulfidated nZVI (S-nZVI) particles were prepared inside the anaerobic chamber by adding a Na2S
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solution to an aqueous suspension of nZVI with a prescribed S/Fe molar ratio and then sonicating for
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10 min.23 The resulting S-nZVI particles were washed five times with water to remove any unbound
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bisulfide ions before use.
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Anaerobic aging of nZVI was conducted in 22 mL glass vials with 10 mL of 2.0 g/L nZVI suspension.
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The suspensions were prepared inside the anaerobic chamber with deoxygenated water, and the vials
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were fitted with butyl rubber stoppers and crimp sealed and shaken at 175 rpm at 25 °C for 20, 50, and
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95 days. The resulting particles were referred to as nZVI-20 d, nZVI-50 d, and nZVI-95 d, respectively.
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Aerobic aging of nZVI was conducted in a 120 mL glass vial with 100 mL of 8.0 g/L nZVI suspension;
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the suspension was stirred at 300 rpm for 72 h in open air. The resulting particles were referred to as
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O-nZVI-72 h. Synthesis methods for other nanosized iron species formed after aging, including
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Fe(OH)2, FeS, γ-Fe2O3, Fe3O4, and γ-FeOOH, are provided as Supporting Information (SI-2).
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Sorption Experiments
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All sorption experiments were conducted in triplicates inside the anaerobic chamber. Sorption
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isotherms were obtained by single-solute batch sorption experiments in 15 mL polypropylene tubes
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mounted on an end-over-end rotator for seven days at room temperature (~22 °C). Preliminary tests
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showed seven days was sufficient to reach sorption equilibrium (Figure S1). The freshly-synthesized
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nZVI and the other iron nanoparticles were dispersed by sonication in the sealed vial, and the
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suspension (2.0 g/L) was added into the tubes to a final concentration of 0.1 g/L using NaCl (5 mM) as
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background electrolyte.
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PFAA aqueous stock solution was spiked to obtain an initial concentration range of 1–20 000 µg/L in
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10 mL of test solution. pH was not adjusted and the pH of the suspension after sorption was close (
