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Selective and fast adsorption of perfluorooctane sulfonate from wastewater by magnetic fluorinated vermiculite Ziwen Du, Shubo Deng, Siyu Zhang, Wei Wang, Bin Wang, Jun Huang, Yujue Wang, Gang Yu, and Baoshan Xing Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 15 Jun 2017 Downloaded from http://pubs.acs.org on June 15, 2017
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Selective and fast adsorption of perfluorooctane sulfonate from wastewater
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by magnetic fluorinated vermiculite
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Ziwen Du†,‡, Shubo Deng†,*, Siyu Zhang§, Wei Wang†, Bin Wang†, Jun Huang†, Yujue
4
Wang†, Gang Yu†, Baoshan Xing‡
5
†
6
Beijing Key Laboratory for Emerging Organic Contaminants Control, School of Environment,
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Tsinghua University, Beijing 100084, China
8
‡
9
01003, USA
State Key Joint Laboratory of Environment Simulation and Pollution Control (SKLESPC),
Stockbridge School of Agriculture, University of Massachusetts, Amherst, Massachusetts,
10
§
11
Applied Ecology, Chinese Academy of Science, Shenyang 110016, China
12
*
13
E-mail:
[email protected] (S. Deng)
Key Laboratory of Pollution Ecology and Environmental Engineering, and Institute of
Corresponding author, Tel.: +86-10-62792165. Fax: +86-10-62794006.
14 15 16
A revised manuscript submitted to Environmental Science & Technology
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TOC
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ABSTRACT
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A novel magnetic fluorinated adsorbent with selective and fast adsorption of perfluorooctane
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sulfonate (PFOS) was synthesized via a simple ball milling of Fe3O4 and vermiculite loaded
40
with a cationic fluorinated surfactant. The loaded Fe3O4 nanoparticles increased the
41
dispersibility of fluorinated vermiculite (F-VT) in water and allowed the magnetic
42
separability. The nano-sized Fe3O4 was homogeneously embedded into the adsorbent
43
surfaces, improving the hydrophilicity of F-VT external surface, and this hybrid adsorbent
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still kept the hydrophobic fluorinated interlayer structure. With this unique property,
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Fe3O4-loaded F-VT has very fast and selective adsorption for PFOS in the presence of other
46
compounds, due to the fluorophilicity of C-F chains intercalated in the adsorbent interlayers.
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This novel adsorbent has a high sorption capacity for PFOS, exhibiting much higher PFOS
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removal from fire-fighting foam wastewater than powdered activated carbon and resin due to
49
its high selectivity for PFOS. The used Fe3O4-loaded F-VT was successfully regenerated by
50
methanol and reused five times without reduction in PFOS removal and magnetic
51
performance. The Fe3O4-loaded F-VT shows a promising application for PFOS removal from
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real wastewaters.
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INTRODUCTION
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Perfluorooctane sulfonate (PFOS) has been widely used as an additive in fluorinated
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polymer synthesis and aqueous film forming foams (AFFF), stain and water repellent, and
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chrome mist suppressant.1 Since PFOS is worldwide distributed, persistent, bioaccumulative
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and toxic, it has been listed as one of persistent organic pollutants (POPs) in the Stockholm
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Convention.1 However, PFOS is still involved in chrome plating and AFFF application in
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developing countries during the exemption period, and one of major contamination sources
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for PFOS in China is the release of PFOS in AFFF application.2 Although some fluorinated
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chemicals have been produced to replace PFOS, they are also considered as potential POPs,
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and some of them have been proved to be toxic, duo to the similar physicochemical properties
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to PFOS.3 PFOS or other hazardous fluorinated alternatives may need a long period of time to
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phase out. Application of PFOS-containing products or direct discharge of PFOS-associated
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wastewater can pollute nearby aquatic environments.3, 4 It is necessary to develop effective
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techniques to remove PFOS from wastewater and contaminated water.
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Adsorption using various adsorbents such as activated carbon, resins, aminated biomass
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and clays has been found to be an easy and effective method to remove perfluoroalkyl and
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polyfluoroalkyl substances (PFASs) from water,5 but the effectiveness of these adsorbents
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decreased greatly in the presence of coexisting organic matters in real wastewater.5-7 It was
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reported that 3M Cottage Grove Facility (USA) used the activated carbon filter to remove
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perfluorooctanoate (PFOA) and PFOS from groundwater to meet the drinking water criteria
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of the Minnesota Department of Health.8 Schaefer et al. (2006) reported that PFOS and 4
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PFOA could easily penetrate the carbon beds in German wastewater treatment plants.9
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Besides the competition of coexisting PFASs,10 coexisting hydrocarbons can also create
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competition for the adsorption sites, leading to the loss of PFOS sorption.7 In addition to
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organic matters, adsorption of PFOS on the aminated materials including anion-exchange
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resins is even reduced dramatically by competitive inorganic anions.11-13 For these reasons,
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we previously prepared a novel fluorinated alkyl chain modified montmorillonite for PFOS
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selective removal from water.10 Fluorinated alkyl chain is amphiphobic (hydrophobic and
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oleophobic), and thus hydrocarbon compounds are hardly to be adsorbed by C-F chain. Whereas,
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C-F chain on the fluorinated material can adsorb perfluorinated tail of PFOS molecule via
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fluorophilic interactions based on “like dissolves like theory”. However, the fluorinated
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montmorillonite powders easily form aggregates in solution, reducing its adsorption capacity
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and rate. This problem also exists on other types of organically modified clay powders.14, 15
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PFOS adsorption on porous adsorbents is very slow due to its slow intraparticle
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diffusion.5 Decreasing adsorbent size may enhance the adsorption rate of PFOS, but this
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would make the adsorbent separation difficult. To overcome these problems, we prepared a
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magnetic and surface-hydrophilic but interlayer-fluorinated clay adsorbent via a simple
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ball-milling method. The dispersibility and selectivity of this adsorbent were investigated,
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and its surface as well as interlayer properties were characterized. The removal of PFOS from
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AFFF-contaminated wastewater by this novel adsorbent was compared with the powdered
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activated carbon (PAC) and anion-exchange resin. The magnetic performance, stability and
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reusability of the prepared adsorbent were also evaluated. 5
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MATERIALS AND METHODS
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Chemicals and Materials. Vermiculite (VT, < 74 µm) with a cation exchange capacity
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(CEC) of 1.43 meq/g was obtained from Palabora Mining Co. (South Africa). The cationic
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surfactant, N,N,N-trimethyl-3-(perfluorooctyl sulfonamido) propan-1-aminium iodide, a
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polyfluoroalkyl quaternary ammonium (PFQA, C8F17SO2NH(CH2)3N(CH3)3I) was obtained
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from Silworld Chemical Co. (Wuhan, China). PFOS (≥ 98%), perfluorobutane sulfonate
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(PFBS, ≥ 98%), PFOA (≥ 96%), perfluorobutanoic acid (PFBA, ≥ 98%) and sodium
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1-octanesulfonate (OS, ≥ 98%) were purchased from Sigma-Aldrich (St. Loius, USA). Decyl
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polyglycoside (DPG) was purchased from Zhejiang Taizhou Tu-Poly Co. (China). Diethylene
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glycol butyl ether (DGBE) and tripolyphosphate used as the AFFF ingredients were
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purchased from Sinopharm Chemical Reagent Co. (China). All chemicals were of analytical
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grade. The AFFF wastewater was provided by a fire fighting manufacturing Co. (Taizhou,
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China), and it was collected from the fire fighting training for fuel fire. The TOC value of the
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AFFF wastewater was 2234 mg/L, and the major compositions were DPG (0.89 g/L) and
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DGBE (1.2 g/L). Other additives in AFFF and fuel hydrocarbons may be also present in the
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wastewater. PFOS was the major PFAS detected in the AFFF wastewater (pH=6.4), and its
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concentration was measured to be about 22.5 mg/L (Table S1). Various PFASs were also
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detected, but their concentrations were much lower, probably coming from the ingredients
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co-existed in the AFFF formulation16 and the transformation during the combustion process.
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Preparation of Magnetic Fluorinated Vermiculite. An amount of 10 g of vermiculite was
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added into 100 mL of 0.1 mol/L HNO3 solution, followed by stirring at 90 °C for 3 h. The 6
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obtained solid was washed with deionized (DI) water until neutral pH, and then added into
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100 mL of 8 g/L sodium carbonate solution, followed by stirring at 80 °C for 3 h. The
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obtained Na-Vermiculite (Na-VT) was washed by DI water until neutral pH. The Na-VT was
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separated and dried at 105 °C, and then ground and sieved through a 200-mesh sieve. The
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fluorinated vermiculite was prepared according to our previous method.10
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Magnetic fluorinated vermiculite was prepared by ball milling. An amount of 10 g of bulk
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Fe3O4 powder was added with stainless steel balls (diameter = 5.60 mm, 180 g) into a stainless
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steel vial (80 mL) in a planetary ball mill equipment, and then the equipment was operated at
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550 rpm for 2 h. The obtained Fe3O4 nanopowder and F-VT were mixed at different mass
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ratios of 1:49, 1:19, 1:9 and 1:4 (total 800 mg), and the mixture was milled by 50 g steel balls
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for 2 h using the above equipment. The as-prepared Fe3O4-loaded F-VTs were denoted as
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1/49-MF-VT, 1/19-MF-VT, 1/9-MF-VT and 1/4-MF-VT, respectively. For comparison, the
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F-VT was also milled without nano-Fe3O4 under the same conditions, and the product was
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named as BM-F-VT.
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Adsorbent Characterization. X-ray diffraction (XRD) patterns were collected using a
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D/MAX-RB (Rigaku) X-ray diffractometer equipped with a Cu−Kα radiation source to
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analyze the interlayer structure of different adsorbents. Adsorbent hydrodynamic size was
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measured by a laser particle analyzer (Mastersizer 2000, UK). The morphology of
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Fe3O4-loaded F-VT was observed by a field emission scanning electron microscope
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(MERLIN VP Compact, Carl Zeiss, Germany) equipped with an energy-dispersive X-ray
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analyzer (SEM-EDS). The magnetic strength of Fe3O4-loaded F-VT was measured by a 7
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vibrating sample magnetometer (VSM, Lakeshore 730T, USA). The Brunauer-Emmett-Teller
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(BET) surface areas of adsorbents were measured using N2 adsorption at 77 K by a gas
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adsorption instrument (Autosorb iQ, Quantachrome Corp., USA).
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Sorption Experiments. Sorption experiments were conducted at 25°C in an orbital shaker
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at 150 rpm for 48 h with 5 mg of adsorbent in 250 mL polypropylene flask containing 100
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mL of solution. In the investigation of sorption kinetics of different PFASs (PFBA, PFBS,
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PFOA and PFOS), their initial concentrations were 25 mg/L. The pseudo-second-order model
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was used to fit the kinetic data since it contains initial adsorption rate for comparison of
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different PFASs. To investigate the selectivity of adsorbents, DPG, DGBE and OS were
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selected as coexisting hydrocarbon compounds. DPG and DGBE are two typical hydrocarbon
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surfactants added in the AFFF.17-21 OS is the hydrocarbon analog of PFOS and selected to
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compare with PFOS. The initial concentrations of PFOS in simulated solutions were 25 mg/L
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(46.5 µmol/L), and the concentrations of coexisting organic compounds were set to be the
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same molar concentration in the single-solute experiments and dual-solute experiments. In
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the dual-solute solutions, PFOS was dissolved separately with various coexisting organic
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compounds at 46.5 µmol/L. All solutions were adjusted to pH 6 with HCl and NaOH. All
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sorption experiments were conducted in duplicate, and a parallel group of controls without
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adsorbent was set up. The controls showed little change during the sorption experiments.
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In the investigation of PFOS removal from AFFF wastewater, different doses (20
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mg/L-250 mg/L) of adsorbents were added respectively into 100 mL of AFFF wastewater
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and simulated PFOS solution (PFOS = 22.5 mg/L; pH =6.4). The initial PFOS concentrations 8
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in the simulated solutions used to study DPG and tripolyphosphate effects were 25 mg/L,
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while those of coexisting DPG and tripolyphosphate varied from 20 to 500 mg/L.
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Regeneration and Reuse Experiments. A dose of 75 mg/L Fe3O4-loaded F-VT was added
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into 200 mL of AFFF wastewater for 48 h sorption. The spent adsorbent was separated by a
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square magnet, and then put into 40 mL of methanol. The regeneration experiments were
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conducted in a shaker at 150 rpm for 12 h, and PFOS concentrations were measured to
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calculate the regeneration efficiencies. The adsorbent regenerated using methanol was reused
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in the next sorption cycle under the same sorption conditions, and five sorption-regeneration
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cycles were carried out to evaluate the adsorbent reusability.
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Analytical Methods. After adsorption, all samples were centrifuged at 6000 rpm for 10
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min, and the supernatant was separated for direct analysis. The high concentrations (above 1
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mg/L) of DPG, PFBA, PFBS, PFOA and PFOS in the simulated solutions were directly
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measured using a LC-10ADvp HPLC with a CDD-6A conductivity detector (HPLC-CDD)
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from Shimadzu (Japan). A high performance liquid chromatography-tandem mass
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spectrometry (HPLC-MS/MS) was used to measure the PFASs in the AFFF wastewater and
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the PFASs in the simulated solutions (below 1 mg/L) after dilutions with an UltiMate 3000
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HPLC (Dionex by Thermo Fisher Scientific Inc., MA, USA) equipped with an API 3200
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triple quadrupole mass spectrometer (AB SCIEX, ON, Canada). The preparation process of
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wastewater samples was described in the Supporting Information. The information about
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calibration curves for PFASs using HPLC-MS/MS was shown in Table S2. To guarantee the
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validity of the data, the accuracy, precision, and detection limit for each detection system were 9
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determined and shown in Table S1 and Table S3. The quality control measures were listed in
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Supporting Information. The detailed procedures of HPLC-CDD and HPLC-MS/MS analysis
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were described in our previous studies.3,
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(QP2010, Shimadzu, Kyoto, Japan) was used to determine DGBE after extraction by
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methylene chloride, dried by nitrogen blow-down and dissolved with methanol. The detailed
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procedure was described in the previous study.24,
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solutions were determined by the standard UV digestion-method.26 Total organic carbon (TOC)
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was analyzed by a TOC-VCHP analyzer (Shimadzu, Japan).
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RESULTS AND DISCUSSION
13, 22, 23
Gas chromatography-mass spectrometry
25
Total nitrogen concentrations in the
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Preparation and Characterization of Magnetic F-VT. The Fe3O4/F-VT mass ratio had an
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important effect on the sorption of PFOS and the hydrodynamic sizes of prepared adsorbents
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(Figure 1). With the increase of Fe3O4/F-VT ratios, the adsorbed amounts of PFOS first
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increased significantly (p < 0.05, N = 3) at the Fe3O4/F-VT ratios below 1/19, and then
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decreased rapidly from 346.2 mg/g to 38.4 mg/g (pure Fe3O4 nanoparticle) (Figure 1a). When
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the Fe3O4/F-VT ratios increased from 0 to 1/19, the median diameter (D50) values of adsorbent
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were sharply decreased from 13.8 to 1.5 µm. The size peak in the range of 1-100 µm for the
200
F-VT sample moved left slightly after the ball milling treatment (BM-F-VT) (Figure 1b). The
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SEM image of BM-F-VT shows that many particles were below 1 µm (Figure S1), indicating
202
that the nano-sized particles of BM-F-VT aggregated in water and thus large particles were
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obtained. By contrast, a new size peak below 1 µm appeared for the sample of 1/19-MF-VT (5%
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nano-Fe3O4 added), indicating the better dispersion of these particles in water. The specific 10
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surface areas of F-VT, BM-F-VT and 1/19-MF-VT were measured to be 12.2, 27.9 and 28.2
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m2/g, respectively. Ball milling treatment made the adsorbent particles smaller and thus
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possessed higher specific surface areas, resulting in the higher sorption of PFOS on the
208
BM-F-VT and 1/19-MF-VT. As shown in Figure 1c, the 1/19-MF-VT powders were dispersed
209
well in water after 30 minutes of shaking, while most BM-F-VT aggregated on the water surface.
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Since the loaded Fe3O4 nanoparticles on the adsorbent surfaces can increase the adsorbent
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dispersibility in water, more adsorption sites are available for PFOS, probably contributing to
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the higher adsorption on the 1/19-MF-VT. When the mass ratios of Fe3O4/F-VT further
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increased above 1/19, the PFOS sorption on the MF-VT decreased (Figure 1a). Since the
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nano-Fe3O4 had a lower adsorption for PFOS than F-VT, more Fe3O4 nanoparticles on the
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adsorbent surfaces would reduce the PFOS sorption on the MF-VTs.
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The added Fe3O4 amount also influenced the magnetic separation of the hybrid adsorbents
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(Figure 1d). When the Fe3O4/F-VT ratio was 1/49, the residual adsorbent powders made the
218
solution turbid within a magnet field, and 89.1% of the adsorbent was magnetically separated.
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With the further increase of Fe3O4 amounts, the magnetic separation was improved. The
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adsorbent prepared with above 5% Fe3O4 was able to be separated up to 99.4% by a magnet,
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and the clear solution was observed (Figure 1d). In consideration of PFOS sorption and
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magnetic separation, the 1/19-MF-VT was selected and used in the following experiments.
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300
9 200 6 100
3
8
Volume (%)
(a)
Median diameter (um)
(b)
6 4 2
0 4
0 0.01
0.1
1
10
100
1000
Particle size (µm)
BM
-F -V 1/ T 49 -M FVT 1/ 19 -M FVT 1/ 9M FVT 1/ 4M FN an VT oFe 3 O
0
100 (d) 80 60 40 20 0 1/ 49 -M FVT 1/ 19 -M FVT 1/ 9M FVT 1/ 4M FVT Na no -F e 3 O
Magnetic separation (%)
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F-VT BM-F-VT 1/19-MF-VT
224
4
PFOS sorbed (mg/g)
400
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Figure 1. Effects of Fe3O4/F-VT mass ratio on PFOS sorption (a), particle size distributions
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of different adsorbents (b), dispersion of 1/19-MF-VT and BM-F-VT in DI water after 30
227
minutes of shaking (c) and effects of Fe3O4/F-VT mass ratio on magnetic separation (d)
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The particle size and distribution of Fe3O4 loaded on the 1/19-MF-VT were observed by
229
SEM-EDS (Figure 2). Many small bright particles were widely distributed on vermiculite
230
(Figure 2a), and they were manifested as Fe by SEM-EDS mapping (Figure 2b, green dots),
231
indicating the well-distributed Fe3O4 nanoparticles on the adsorbent surfaces. The particle
232
size of Fe3O4 on 1/19-MF-VT was estimated to be around 200 nm in the enlarged image
233
(Figure 2c), consistent with the D50 (201 nm, Figure 1a) of pure Fe3O4 nanoparticles
234
obtained by ball milling. Figure 2d shows the elemental composition of the yellow cross 12
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marked area in the Figure 2c, and two peaks of Fe were observed, verifying the presence of
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Fe3O4. Besides Fe and O, other elements (C, F, Si, Al and Mg) coming from the F-VT were
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also detected at the marked point, indicating the embedment of Fe3O4 into the surface of
238
hybrid adsorbent. The homogeneously embedded nanosized Fe3O4 made the hybrid adsorbent
239
easily dispersed in water and magnetically separable.
240 241
Figure 2. SEM image of 1/19-MF-VT at a magnification of 5,000X (a) and its corresponding
242
Fe3O4 distribution (green image) (b), the enlarged image at 30,000X (c), and the EDS
243
mapping result of the yellow cross marked area (d)
244
Sorption kinetics and isotherm of PFOS. The sorption kinetics of PFOS on the F-VT,
245
BM-F-VT and 1/19-MF-VT are illustrated in Figure 3a, and the corresponding parameters of
246
pseudo-second-order model are listed in Table S4. PFOS sorption equilibrium on both
247
BM-F-VT and 1/19-MF-VT was almost achieved within 4 h, while the sorption equilibrium on
248
the F-VT was reached after about 12 h. According to the fitting results from the 13
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pseudo-second-order model (Table S4), the initial adsorption rates (v0) of PFOS on the
250
adsorbents decreased in the order of 1/19-MF-VT (3759.4 mg/g/h) > BM-F-VT (1774.8
251
mg/g/h) > F-VT (1154.6 mg/g/h), and the equilibrium adsorbed amount of PFOS on the
252
1/19-MF-VT was also much higher than those on the BM-F-VT and F-VT. The loaded
253
nano-Fe3O4 on the F-VT by ball milling enhanced the sorption rate of PFOS via improving
254
the adsorbent dispersion in solution. To compare with PAC and the best reported
255
anion-exchange resin, the sorption kinetics of PFOS on PAC and IRA67 under the same
256
experimental conditions in DI water were also investigated (Figure S2a). The v0 values
257
obtained from the pseudo-second-order model were 887.2 mg/g/h for the PAC and 37.1
258
mg/g/h for the IRA67, much lower than that of the 1/19-MF-VT. Although the IRA67 had a
259
higher equilibrium adsorbed amount for PFOS than the 1/19-MF-VT, it required more than 48
260
h to reach the sorption equilibrium. To fully compare 1/19-MF-VT with PAC and IRA67, the
261
adsorption kinetics of other PFASs (PFOA, PFBS and PFBA) in DI water was also
262
investigated (Figure S2b, S2c, S2d). Similarly, the IRA67 had the highest equilibrium
263
adsorbed amounts for PFOA, PFBS and PFBA, followed by the 1/19-MF-VT and PAC. The
264
1/19-MF-VT exhibited the fastest adsorption rate for the PFASs among the three adsorbents,
265
and the equilibrium adsorbed amounts of different PFASs on the 1/19-MF-VT increased in the
266
order of PFBA < PFBS < PFOA < PFOS.
267
The sorption isotherm of PFOS on the 1/19-MF-VT was also examined (Figure 3b), and the
268
maximum sorption capacity for PFOS was 1127 mg/g (2.26 mmol/g), much higher than most
269
of the reported adsorbents (Table S5). The adsorbents with amine groups, such as 14
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anion-exchange resins, chitosan and aminated biomass, possess higher PFOS adsorption than
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1/19-MF-VT, but they do not have selectivity for PFOS. The loaded PFQA in the 1/19-MF-VT
272
is responsible for PFOS adsorption, and the C-F chain of PFOS can adhere to the fluoro part of
273
PFQA.10 In addition, PFOS may form the micelles or hemi-micelles on the positive surfaces of
274
the loaded Fe3O4. 5, 27 The adsorption capacities of 1/19-MF-VT for short chain PFASs (PFOA,
275
PFBS and PFBA) increased in the order of PFBA (166 mg/g) < PFBS (262 mg/g) < PFOA (681
276
mg/g), consistent with the increase of their C-F chain lengths (Figure S3). 400
(a)
1200 900
qe (mg/g)
qt (mg/g)
300
200
100
277
600 300
1/19-MF-VT
0
(b)
0
4
8
BM-F-VT
12
16
F-VT
20
24
0 0
28
50
100
150
200
250
Ce (mg/L)
t (h)
278
Figure 3. Sorption kinetics of PFOS on F-VT, BM-F-VT and 1/19-MF-VT (a) and sorption
279
isotherm on 1/19-MF-VT (b)
280
Selective Sorption of PFOS. Three organic compounds including OS, DPG and DGBE
281
were selected as coexisting sorbates to investigate the selectivity of the MF-VT for PFOS.
282
The different properties of three hydrocarbon compounds are listed in Table S6. The
283
fluorinated vermiculites including 1/19-MF-VT, F-VT and BM-F-VT had much higher
284
adsorption for PFOS than other compounds, exhibiting excellent sorption selectivity for
285
PFOS (Figure 4a). Although OS has the same carbon numbers and functional groups as
286
PFOS, OS removal was much lower than PFOS, indicating the important role of C-F chains 15
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in the selective adsorption. According to our previous study,10 hydrophobic hydrocarbons
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could be sorbed by the hydrocarbon part of PFQA, but hardly adsorbed by the C-F chain of
289
PFQA due to the oleophobic property of C-F chain. PFOS can be adsorbed on the C-F chain
290
of PFQA via fluorophilic interactions. Hydrophobic DPG showed a relatively higher sorption
291
on the fluorinated vermiculite than DGBE and OS, but still much lower than PFOS. The low
292
adsorption of DGBE and OS onto the adsorbents demonstrates that hydrophobic
293
hydrocarbons were mainly adsorbed via hydrophobic interactions. Among the three
294
adsorbents, the adsorptive removal of PFOS on the 1/19-MF-VT was much higher than those
295
on both F-VT and BM-F-VT, while the removal of other compounds changed little,
296
indicating that the ball milling treatment did not decrease the selective ability for PFOS.
297
Moreover, the 1/19-MF-VT adsorbent still maintained the stable and high sorption for PFOS
298
in the presence of the above hydrocarbon compounds in the dual-solute solutions (Figure 4b).
299
The similar adsorbed amounts of PFOS in both the single-adsorbate and dual-adsorbate
300
solutions indicate little influence of coexisting hydrocarbon compounds on PFOS sorption. 80
1/19-MF-MT BM-F-VT F-VT
60
PFOS removal (%)
40 20 0
60 40 20
D E BG
PG D
N /A
E D BG
PG D
PF O
S
0 O S
301
(b)
O S
Removal (%)
80 (a)
302
Figure 4. PFOS removal by the 1/19-MF-VT, F-VT and BM-F-VT in single-sorbate solution
303
(a), and PFOS removal by 1/19-MF-VT in dual-sorbate solution in the presence of 16
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Different PFASs may exist in real water or wastewater, and the adsorption of PFBA,
306
PFBS, PFOA and PFOS on 1/19-MF-VT in the single-solute solution and mixed-solute
307
solution was compared (Figure S4). In both solutions, the adsorbed amounts increased in the
308
increasing order of PFBA < PFBS < PFOA < PFOS, consistent with the increase of C−F
309
chain length. When PFOS was mixed with other three PFASs, the adsorbed amount of PFOS
310
decreased slightly (3.7%), while other PFASs displayed a sharp decrease of their adsorbed
311
amounts, indicating the preferential adsorption of PFOS on the 1/19-MF-VT.
312
To further evaluate the selective sorption ability of Fe3O4-loaded F-VT in the actual
313
application, the 1/19-MF-VT was used to remove PFOS from real AFFF wastewater and
314
simulated PFOS solution, in comparison with PAC and the best anion-exchange resin IRA 67
315
reported. In the simulated PFOS solution, almost 100% of PFOS removal was achieved by
316
IRA67 at a low dose of 60 mg/L, while 100 mg/L 1/19-MF-VT or 200 mg/L PAC was
317
required to remove PFOS completely (Figure 5a). However, in the real AFFF wastewater, the
318
PFOS removal (less than 52%) by both PAC and IRA67 increased slowly within the dose of
319
250 mg/L, while the 1/19-MF-VT exhibited much higher PFOS removal than PAC and
320
IRA67, and PFOS removal was about 98% at the adsorbent dose of 150 mg/L (Figure 5b). At
321
the adsorbent dose of 60 mg/L, the PFOS removal from the AFFF wastewater by the
322
1/19-MF-VT decreased about 16% compared with PFOS solution, while those by PAC and
323
IRA67 decreased over 80%. The effects of coexisting DPG and tripolyphosphate in the AFFF
324
wastewater on PFOS removal from the simulated solution were further studied (Figure S5). 17
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With the increase of DPG concentrations, PFOS removal on the PAC and IRA67 decreased
326
more than 1/19-MF-VT. The IRA67 outperformed the 1/19-MF-VT at the DPG
327
concentrations below 200 mg/L, while PFOS removal by the 1/19-MF-VT turned to be higher
328
than that by the IRA67 after DPG concentrations above 500 mg/L. Since the real AFFF
329
wastewater contained high concentrations of DPG, other organic additives, and fuel
330
hydrocarbons, the 1/19-MF-VT showed much higher PFOS removal in the real wastewater
331
(Figure 5b). It indicates that wastewater with higher concentrations of TOC may benefit from
332
treatment with the 1/19-MF-VT, while waters with lower TOC will need to be evaluated based
333
on the type of coexisting pollutants to determine which adsorbent will work the best. In
334
contrast, inorganic tripolyphosphate anions enhanced the PFOS adsorption onto the PAC and
335
1/19-MF-VT but dramatically reduced that on the IRA 67 (Figure S5b). Coexisting inorganic
336
salts can enhance PFOS sorption through electrical double-layer compression and salting-out
337
effect,5 leading to the increased removal of PFOS on the PAC and 1/19-MF-VT. However,
338
inorganic tripolyphosphate anions can compete with anionic PFOS for exchange sites on the
339
IRA 67,13 resulting in the decrease of PFOS sorption and making it enormously inferior to
340
1/19-MF-VT at the higher salt concentrations (Figure S5b). This effect is consistent with other
341
anions reported, such as sulfate and Cr(VI).13 The composition of wastewater was complicated,
342
and various unknown coexisting compounds might cause PAC and IRA 67 to lose effectiveness
343
via competitive adsorption. Nevertheless, the 1/19-MF-VT still possessed relatively high
344
PFOS removal, indicating its excellent selective ability.
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100 80
PAC IRA67 1/19-MF-VT
60 40 20 0
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(a)
PFOS removal (%)
PFOS removal (%)
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0
50
100
150
200
100
Adsorbent dose (mg/L)
PAC IRA67 1/19-MF-VT
80 60 40 20 0
250
(b)
0
50
100
150
200
250
Adsorbent dose (mg/L)
346
Figure 5. Effect of adsorbent doses on the removal of PFOS from simulated solution (a) and
347
AFFF wastewater (b)
348
Sorption Mechanism. The form of PFQA intercalated between clay layers was analyzed
349
by XRD (Figure S6). The basal spacing (d001) of vermiculite is 1.42 nm according to the
350
Bragg equation (2dsinθ=λ),28 and it shifted to 1.44 nm after PFQA loading (F-VT). Besides
351
the peak of 1.44 nm, other two major responses of crystalline structure were appeared at
352
around 2.00 nm and 4.82 nm in the XRD pattern. After the ball milling of Fe3O4 and F-VT,
353
the layer structure of adsorbent was changed. The peak around 1.44 nm showed a very slight
354
change to 1.47 nm, while the peak of 2.00 nm disappeared and the peak of 4.28 nm decreased
355
evidently to 3.77 nm. Probably, the ball milling process might compress the layer structure,
356
resulting in the change of interlayer distance. Based on the interlayer distance and the PFQA
357
molecular size, the arrangement of PFQA within 1/19-MF-VT can be calculated. The
358
interlayer distance of organo-vermiculite is the difference between the value of d001 spacing
359
and the thickness of the tetrahedron–octahedron–tetrahedron (TOT) layer (0.96 nm).29 The
360
height of the quaternary ammonium group with three methyls is about 0.51 nm,30 and thus, 19
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361
plus the TOT layer (0.96 nm), the thickness of 1/19-MF-VT interlayer was approximately
362
1.47 nm, consistent with the value of d001 analyzed from XRD. It indicates a lateral
363
monolayer of PFQA molecules lying flat in the 1/19-MF-VT layer (Figure 6a). The reflection
364
at 3.77 nm can be considered as a paraffin-type bilayer of PFQA intercalated between TOT
365
layers,30 according to the spacing of TOT layer and the molecular length of PFQA
366
(approximately 1.9 nm) (Figure 6b). In this case, the outer fluorophilic C-F chain of PFQA
367
can expel hydrocarbons, but attract PFOS in water.
368
On the basis of adsorbent characteristics and PFOS sorption results, the enhanced
369
adsorption mechanism of PFOS on the Fe3O4-loaded F-VT prepared by ball milling is
370
illustrated in Figure 6c. The Na-VT after PFQA modification possessed not only
371
PFQA-intercalated interlayers but also the hydrophobic external surfaces, readily forming
372
aggregates in water. Ball milling process can embed Fe3O4 nanoparticles homogeneously in
373
the hybrid adsorbent, making the hybrid adsorbent disperse better in water due to the more
374
hydrophilic adsorbent surfaces, and then more sorption sites were available for PFOS
375
adsorption. The pHpzc value of nano-Fe3O4 was measured to be 6.8, higher than the pH value
376
of 6 used in the sorption experiments, and thus the positive nano-Fe3O4 surfaces could
377
electrostatically adsorb PFOS anions (Figure 6c). However, the PFOS sorption onto Fe3O4
378
was much lower than the F-VT (Figure 1a), and the addition amount of Fe3O4 was not high
379
(5%). Therefore, PFOS sorption should mainly occur on the fluorinated adsorbent surfaces.
380
The intercalated structure of PFQA was not destroyed after ball milling, and the C-F chain of
381
PFQA could selectively adsorb PFOS via fluorophilic interactions.10 20
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383
384 385
Figure 6. Schematic diagram for lateral monolayer (a) and paraffin-type bilayer (b)
386
arrangements of PFQA in the 1/19-MF-VT (PFQA: gray balls are C atoms; white balls are H
387
atoms; red balls are O atoms; blue balls are N atoms; smaller yellow balls are F atoms; bigger
388
yellow balls are S atoms), and the adsorbent preparation processes and the sorption
389
mechanism of PFOS on the 1/19-MF-VT prepared by ball milling (c) (A: fluorination process;
390
B: ball milling process with nano-Fe3O4)
391
Stability of 1/19-MF-VT. To evaluate the stability of PFQA on the 1/19-MF-VT, the 21
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total nitrogen (TN) concentrations in different solutions (DI water, tap water, salt solution
393
and AFFF wastewater) containing 1/19-MF-VT after 30 days of shaking were determined
394
(Figure S7). Since PFQA concentrations in solution had a good linear relationship with TN
395
concentrations (Figure S8), TN may reflect the amount of PFQA desorbed from the
396
1/19-MF-VT. In consideration of the possible degradation of PFQA into some
397
nitrogen-containing species during the long-term experiments, the measurement of TN in
398
solution is reasonable. According to our previous study, the loaded amount of PFQA on the
399
adsorbent was approximately equal to the CEC of vermiculite (1.43 mmol/g).10 The dose of
400
1/19-MF-VT added into the solution was 2 g/L, and thus the TN concentration would
401
increase to about 80 mg/L if all PFQA were desorbed. The TN concentrations in all solutions
402
after shaking with 1/19-MF-VT were almost the same as those in the blank samples
403
(no statistically significant difference, p > 0.05) (Figure S7), and the changes of TN in the
404
solutions were below 0.05 mg/L except that in the wastewater (2.3 mg/L), indicating almost
405
no degradation and desorption of PFQA from 1/19-MF-VT. Inorganic cations like Ca2+ and
406
Na+ commonly exist in aquatic environments and would affect the adsorbent stability. Almost
407
no PFQA was desorbed in the salt solution containing 20 mmol/L of CaCl2 and 20 mmol/L of
408
NaCl, possible due to the stronger exchange ability and high hydrophobicity of PFQA. Clays
409
with cation-exchange ability preferentially adsorb large organic ammonium over inorganic
410
positive ions, and the exchange reactions are considered to be essentially irreversible.14, 15, 31
411
Large organic ions are held more firmly by clays than inorganic cations,15 and the
412
hydrophobic alkane chains of PFQA should protect ammonium groups far away from 22
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hydrophilic cations. Consequently, 1/19-MF-VT could keep being stable in the above four
414
solutions. The 1/19-MF-VT also exhibited high stability at low solution pH and high
415
concentrations of CaCl2. Almost no PFQA was detected (no statistically significant difference
416
from the blank, p > 0.05) in solution at pH 1 and in 1 mol/L CaCl2 solution (Figure S9). High
417
concentration of CaCl2 may increase the stable stay of PFQA on the clay via salting-out
418
effects.5 It is reported that the biotransformation of PFQA was very slow with unobservable
419
change of the spiked mass in aerobic soil.32 In our study, PFQA molecules were mainly
420
intercalated within the nano-sized interlayers (below 3 nm) of the 1/19-MF-VT, and thus
421
bacteria hardly get access to the loaded PFQA, making biodegradation extremely difficult.
422
Furthermore, the fluorinated materials and quaternary ammonium salts are widely used as
423
antimicrobial materials/agents,33-35 implying that bacteria are not easy to grow on the
424
1/19-MF-VT and long adsorbent life can be achieved.
425
To further investigate the stability of 1/19-MF-VT, potassium permanganate (KMnO4)
426
was used to oxidize the 1/19-MF-VT. The TN concentrations were only detectable at the
427
concentration of KMnO4 above 50 mmol/L (Figure S10). When the concentration of KMnO4
428
was 300 mmol/L, the TN concentration in solution was 4.8 mg/L, accounting for about 6% of
429
the loaded PFQA in the 1/19-MF-VT. Therefore, the PFQA in the 1/19-MF-VT is stable, and
430
it would be degraded only under the severe oxidation conditions.
431
It should be pointed out that PFQA is normally synthesized via the reaction of
432
perfluorooctanesulfonyl fluoride and beta-diethylaminoethylamine in diethyl ether at room
433
temperature.35 To avoid the release of PFASs into the environment during the making of 23
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434
PFQA, the safeguards including preventing the volatilization of perfluorooctanesulfonyl
435
fluoride and incineration of fluorinated organic waste at above 1000°C should be taken.36
436
Regeneration and reuse of 1/19-MF-VT. The regeneration efficiency of the spent
437
1/19-MF-VT after the sorption of PFOS from AFFF wastewater by methanol and the removal
438
percentages of PFOS by the reused 1/19-MF-VT in five sorption cycles are shown in Figure
439
7a. The desorption percentages of PFOS were achieved nearly 100%, except for that in the
440
first elution. Correspondingly, the removal of PFOS dropped from 68.9% to 61.8% after the
441
first regeneration, and then kept relatively steady in the following four cycles. The lower
442
regeneration in the first cycle may be attributed to the electrostatic adsorption of PFOS on the
443
positively charged surface like Fe3O4, which cannot be desorbed by pure methanol.5, 13 In
444
addition, the magnetic properties of 1/19-MF-VT in five sorption cycles were also analyzed
445
(Figure 7b). Likewise, the highest magnetization of 1/19-MF-VT decreased from 5.25 emu/g
446
to 4.87 emu/g after the first regeneration, and then fluctuated between 4.75 emu/g and 4.93
447
emu/g in the following cycles, indicating the stability of Fe3O4 in the reuse process. The
448
regenerated 1/19-MF-VT by methanol exhibited the steady removal of PFOS from
449
wastewater and stable magnetic properties in five cycles, showing high reusability and
450
stability. For safe treatment of the fluorinated adsorbent waste, the ultimate spent
451
1/19-MF-VT should be incinerated at above 1000 ℃ to completely destroy PFQA.36
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80 60
40 40
20
20
1
2
3
4
5
0
Magnetization (emu/g)
100
60
0
452
6
(a)
Regeneration (%)
PFOS removal (%)
80
(b)
4
cycle 1 cycle 2 cycle 3 cycle 4 cycle 5
2 0 -2 -4 -6
-10000 -5000
Cycles
0
5000
10000
Magnetic field (Oe)
453
Figure 7. PFOS removal from wastewater by the 1/19-MF-VT (a) and the magnetic
454
properties of the 1/19-MF-VT (b) in five successive sorption cycles
455
The simple ball milling treatment can load Fe3O4 nanoparticles onto the surface of F-VT,
456
making it possess unique hydrophilic external surface and hydrophobic fluorinated interlayer
457
structure. The Fe3O4-loaded F-VT exhibited selective, high and fast sorption for PFOS, due to
458
its better dispersibility in water and the intercalated PFQA in interlayers. The coexisting
459
hydrocarbon compounds had little effect on PFOS sorption on the 1/19-MF-VT, and this
460
adsorbent had much higher removal of PFOS from AFFF wastewater than other reported
461
adsorbents, showing a promising application for selective removal of PFOS from real
462
PFAS-contaminated water or wastewater. The 1/19-MF-VT is a very stable hybrid adsorbent,
463
i.e. PFQA in the adsorbent was hardly released into waters or wastewaters. This adsorbent
464
with excellent selectivity for PFOS and separability can also be used for pretreatment of
465
water samples in solid-phase extraction and selective recovery of PFOS from wastewaters.
466
ASSOCIATED CONTENT
467
Supporting Information
468
Kinetic parameters of the pseudo-second-order model for PFOS sorption, properties of 25
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organic compounds used in the selective sorption experiments, SEM image of BM-F-VT,
470
XRD patterns of Na-VT, F-VT and 1/19-MF-VT, effect of DPG and tripolyphosphate on
471
PFOS removal, adsorption of different PFASs, stability of 1/19-MF-VT under different
472
conditions. These materials are available free of charge via the Internet at http://pubs.acs.org.
473
ACKNOWLEDGMENTS
474
We thank the National Nature Science Foundation of China (Project no. 21577074,
475
21177070), Tsinghua University Initiative Scientific Research Program (Project no.
476
20141081174), and Collaborative Innovation Center for Regional Environmental Quality for
477
financial support. Ziwen Du also thanks the China Scholarship Council to support his study at
478
the University of Massachusetts, Amherst for one year.
479
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