Selective and fast adsorption of perfluorooctane sulfonate from

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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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Environmental Science & Technology

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Selective and fast adsorption of perfluorooctane sulfonate from wastewater

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by magnetic fluorinated vermiculite

3

Ziwen Du†,‡, Shubo Deng†,*, Siyu Zhang§, Wei Wang†, Bin Wang†, Jun Huang†, Yujue

4

Wang†, Gang Yu†, Baoshan Xing‡

5

†

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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

§

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Applied Ecology, Chinese Academy of Science, Shenyang 110016, China

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*

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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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1

ACS Paragon Plus Environment


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TOC

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23 24 25 26 27 28 29 30 31 32 33 34 35 36 2


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ABSTRACT

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A novel magnetic fluorinated adsorbent with selective and fast adsorption of perfluorooctane

39

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

44

still kept the hydrophobic fluorinated interlayer structure. With this unique property,

45

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.

47

This novel adsorbent has a high sorption capacity for PFOS, exhibiting much higher PFOS

48

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.

53 54 55 56 57 3



58

INTRODUCTION

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Perfluorooctane sulfonate (PFOS) has been widely used as an additive in fluorinated

60

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

109

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

133

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.

135

Adsorbent Characterization. X-ray diffraction (XRD) patterns were collected using a

136

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

141

analyzer (SEM-EDS). The magnetic strength of Fe3O4-loaded F-VT was measured by a 7



142

vibrating sample magnetometer (VSM, Lakeshore 730T, USA). The Brunauer-Emmett-Teller

143

(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

147

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

149

was used to fit the kinetic data since it contains initial adsorption rate for comparison of

150

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

173

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

175

measured using a LC-10ADvp HPLC with a CDD-6A conductivity detector (HPLC-CDD)

176

from Shimadzu (Japan). A high performance liquid chromatography-tandem mass

177

spectrometry (HPLC-MS/MS) was used to measure the PFASs in the AFFF wastewater and

178

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

181

wastewater samples was described in the Supporting Information. The information about

182

calibration curves for PFASs using HPLC-MS/MS was shown in Table S2. To guarantee the

183

validity of the data, the accuracy, precision, and detection limit for each detection system were 9



184

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

186

were described in our previous studies.3,

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(QP2010, Shimadzu, Kyoto, Japan) was used to determine DGBE after extraction by

188

methylene chloride, dried by nitrogen blow-down and dissolved with methanol. The detailed

189

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)

191

was analyzed by a TOC-VCHP analyzer (Shimadzu, Japan).

192

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

194

important effect on the sorption of PFOS and the hydrodynamic sizes of prepared adsorbents

195

(Figure 1). With the increase of Fe3O4/F-VT ratios, the adsorbed amounts of PFOS first

196

increased significantly (p < 0.05, N = 3) at the Fe3O4/F-VT ratios below 1/19, and then

197

decreased rapidly from 346.2 mg/g to 38.4 mg/g (pure Fe3O4 nanoparticle) (Figure 1a). When

198

the Fe3O4/F-VT ratios increased from 0 to 1/19, the median diameter (D50) values of adsorbent

199

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

201

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

203

obtained. By contrast, a new size peak below 1 µm appeared for the sample of 1/19-MF-VT (5%

204

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

206

m2/g, respectively. Ball milling treatment made the adsorbent particles smaller and thus

207

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.

210

Since the loaded Fe3O4 nanoparticles on the adsorbent surfaces can increase the adsorbent

211

dispersibility in water, more adsorption sites are available for PFOS, probably contributing to

212

the higher adsorption on the 1/19-MF-VT. When the mass ratios of Fe3O4/F-VT further

213

increased above 1/19, the PFOS sorption on the MF-VT decreased (Figure 1a). Since the

214

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

217

(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.

11



15 12

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 (%)

223

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

226

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)

228

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

236

Fe3O4. Besides Fe and O, other elements (C, F, Si, Al and Mg) coming from the F-VT were

237

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



249

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

271

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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287

in the selective adsorption. According to our previous study,10 hydrophobic hydrocarbons

288

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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hydrocarbon compounds (b) (N/A: only PFOS solution without coexisting compounds)

305

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



325

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.

18


Page 18 of 31


100 80

PAC IRA67 1/19-MF-VT

60 40 20 0

345

(a)

PFOS removal (%)

PFOS removal (%)

Page 19 of 31

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



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


Page 20 of 31

Page 21 of 31


382

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



392

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


Page 22 of 31

Page 23 of 31


413

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



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

24


Page 24 of 31

Page 25 of 31


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



469

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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Selective and fast adsorption of perfluorooctane sulfonate from

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