Selective and High Sorption of Perfluorooctanesulfonate and

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Selective and High Sorption of Perfluorooctane Sulfonate and Perfluorooctanoate by Fluorinated Alkyl Chain Modified Montmorillonite Ziwen Du, Shubo Deng, Siyu Zhang, Bin Wang, Jun Huang, Yujue Wang, Gang Yu, and Baoshan Xing J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04757 • Publication Date (Web): 11 Jul 2016 Downloaded from http://pubs.acs.org on July 13, 2016

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The Journal of Physical Chemistry

Selective

and

High

Sorption

of

Perfluorooctane

Sulfonate

and

Perfluorooctanoate by Fluorinated Alkyl Chain Modified Montmorillonite Ziwen Du†,‡, Shubo Deng*,†, Siyu Zhang§, Bin Wang†, Jun Huang†, Yujue Wang†, Gang Yu†, Baoshan Xing*,‡ †

State Key Joint Laboratory of Environment Simulation and Pollution Control (SKLESPC),

Beijing Key Laboratory for Emerging Organic Contaminants Control, School of Environment, Tsinghua University, Beijing 100084, China ‡

Stockbridge School of Agriculture, University of Massachusetts, Amherst, Massachusetts,

01003, USA §

Key Laboratory of Pollution Ecology and Environmental Engineering, and Institute of

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

1

ACS Paragon Plus Environment


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ABSTRACT A novel fluorinated montmorillonite (F-MT) was synthesized via exchange of cationic fluorinated surfactant to selectively adsorb perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) from water. F-MT displayed fast and high sorption for PFOS and PFOA at concentrations below 10 µg/L, and performed better than the best activated carbon and resin previously reported. The spent F-MT can be completely regenerated by methanol solution and be reused five times without reduction in sorption capacity. Moreover, the F-MT possessed excellent selectivity for PFOS and PFOA in the presence of other organic pollutants. The coexisting phenol, pyridine, dodecylbenzene sulfonate (SDBS) and phenanthrene (PHE) exhibited no effect on PFOS and PFOA sorption. Due to the nano-scale interlayer structure and chemical nature of F-MT, macromolecular humic acid had little effect on PFOS and PFOA sorption. Also, F-MT adsorbed very little SDBS and PHE. The results of competitive sorption and density functional theory calculations verified that PFOS and PFOA were adsorbed on the C−F chain of F-MT, while PHE and SDBS sorption occurred on the hydrocarbon part of F-MT. The unique hydro-oleophobic C−F chain on the F-MT was responsible for selective sorption of perfluoroalkyl acids (PFAAs), providing a new mechanistic insight for the interactions between PFAAs and fluorinated adsorbents. In addition, F-MT offers a promising potential for removal of PFOS and PFOA from contaminated water.

2


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1. INTRODUCTION Perfluoroalkyl acids (PFAAs) have been widely used in various industrial materials such as fluoropolymer additives, stain and water repellents, fire-fighting foam ingredients, and chrome mist suppressants.1 Some PFAAs have been regarded as a group of potentially harmful contaminants which are ubiquitously distributed, persistent, and bioaccumulative.1-2 Perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) are the most typical PFAAs. PFOS was listed as one of persistent organic pollutants in the Stockholm Convention, while PFOA has also been limited/forbidden in the relevant regulations in America, Canada and Germany.3-4 Nevertheless, they are still produced and applied in many special fields such as fluoropolymer, semiconductor, and chrome plating industries. Therefore, it is necessary to develop effective techniques to remove them from wastewater or contaminated surface water and groundwater. Sorption has been found to be an easy and effective method to remove PFAAs. Many adsorbents such as activated carbon (AC), resins, biomass and clays have shown the removal of PFAAs from water or wastewater in recent years,4 but their effectiveness decreased greatly in the presence of coexisting organic matters in water.5-6 Even inorganic anions can reduce sorption of PFAAs on the anion-exchange resin.7 Therefore, it is attractive and challenging to prepare selective adsorbents for PFAAs removal from water. In previous studies, molecular imprinting technique was used to synthesize adsorbents with selective ability for PFAAs.8-10 Molecular imprinting adsorbents could preserve selectivity toward the template PFC molecules from other contaminants, but competitive compounds with similar molecule size, 3



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structure, and/or functional groups, such as some hydrocarbon surfactants, still could dramatically decrease the selective ability of these adsorbents. It has been reported that the graft of fluorinated alkyl silane on silica-based materials enhanced the sorption of PFAAs and these adsorbents were tested in solid-phase extraction process of sample pretreatment before PFAAs determination.11-13 These fluorinated adsorbents exhibited lower sorption capacities for PFAAs than AC and anion exchange resins reported in the literature, probably due to low loaded amount of fluorinated alkyl silane. The sorption mechanism of PFAAs on the fluorinated adsorbents remains unclear. It is well known that the nonpolar C−F chains possess special hydrophobic and oleophobic properties. Perfluoro-coated/lined materials are widely used as containers/caps for sorption experiments of hydrocarbon compounds, since few hydrocarbon compounds can be adsorbed by perfluorinated surfaces.14-17 Recently, it has been found that PFOS molecule with a long C−F chain may be unable to be adsorbed directly onto the hydrocarbon surface in water because of its oleophobicity.18 Therefore, adsorbents with perfluorinated/fluorinated surface should be effective for PFAAs sorption but ineffective for hydrocarbon compounds, i.e., highly selective for PFAAs. Boyd, Mortland and Chiou

19

pioneered hydrocarbon organic-modified clays to

effectively remove hydrocarbon compounds from water, and thereafter this type of adsorbent has been widely studied and applied. It was also found that organo-modified clay is a stable complex, i.e, the cationic surfactants intercalated in the clay are hardly desorbed.20 We used a similar idea to prepare a novel adsorbent using montmorillonite as a base material and 4


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modified it by fluorinated quaternary ammonium for PFAAs removal. The adsorbent was evaluated for sorption of PFOS and PFOA from aqueous solution in terms of sorption kinetics, capacity, selectivity, and regeneration. The selective sorption mechanisms of PFOS and PFOA on the fluorinated montmorillonite (F-MT) were investigated via the selective sorption experiments and density functional theory (DFT) calculations. The selective sorption mechanisms were proposed based on the unique amphiphobic property of C−F chain in PFOS/PFOA.

2. EXPERIMENTAL SECTION 2.1. Chemicals and materials. Na-saturated montmorillonite (Na-MT, < 74 µm) with a cation exchange capacity (CEC) of 0.99 ± 0.05 meq/g (mmol(+)/g) was obtained from Juhe Co.

(Chifeng,

China).

The

cationic

surfactant,

N,N,N-trimethyl-3-(perfluorooctyl

sulfonamido) propan-1-aminium iodide, a polyfluoroalkyl quaternary ammonium (PFQA, C8F17SO2NH(CH2)3N(CH3)3I) was obtained from Silworld Chemical Co. (Wuhan, China). The PFAAs including PFOA (≥ 96%), PFOS (≥ 98%), perfluorohexane sulfonate (PFHxS, ≥ 98%), perfluorohexanoic acid (PFHxA, ≥ 97%), perfluorobutanoic acid (PFBA, ≥ 98%), and perfluorobutane sulfonate (PFBS, ≥ 98%) were purchased from Sigma-Aldrich (St. Loius, MO, USA), and their properties are listed in Table S1.

14

C-labelled phenanthrene

(PHE) was purchased from ARC (St. Loius, MO, USA). Sodium dodecyl benzene sulfonate (SDBS), phenol and pyridine were purchased from Acros Organics Co. (NJ, USA). All chemicals were analytical grade. 2.2. Preparation of F-MT. The PFQA solutions were prepared by adding predetermined 5



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amounts (1.25, 3.75, 5, 7.5, 12.5 or 15 mmol) of PFQA into 50 mL of 30% ethanol solution, and then 5 g of Na-MT was sequentially added into the above solutions and stirred slightly to avoid the formation of spume in a water bath at 80°C for 12 h. The obtained solid materials were filtered and subsequently washed with methanol solution (8:2, v/v) to remove residual unbounded PFQA from the clay interlayer until no nitrogen content (determined by the classical UV digestion-method)21-22 was detected in the washing water after distilling off methanol. This indicates that the final obtained F-MT adsorbent is stable, i.e. greatly hydrophobic PFQA intercalated in the clay should not drop off in the water. Finally, the moist F-MT was dried at 60°C, ground and sieved through a 200 mesh (< 74 µm) sieve. 2.3. Characterization of F-MT. X-ray diffraction (XRD) patterns were collected using a D/MAX-RB (Rigaku) X-ray diffractometer equipped with a Cu−Kα radiation source (divergence slit between 1.5° and 20° (2θ) at a step size of 0.02°) to analyze the F-MT interlayer structure. The zeta potentials were analyzed with a Delsa Nano C zeta potential analyzer (Beckman Coulter, USA), and all data were determined four times. The amounts of PFQA loaded on the F-MTs were calculated according to the results from the thermo-gravimetric analysis (TGA) using a TA Instrument Q50 thermo-gravimetric analyzer (TA Instruments, USA) under nitrogen protection at a heating rate of 5 °C/min from 25°C to 1000°C. 2.4. Sorption Experiments. Sorption experiments were conducted at 25°C in an orbital shaker at 170 rpm for 48 h with adsorbent dose of 10 mg/L in 42 mL vessels containing 40 mL of adsorbate solution. Except for the experiments of sorption isotherm, the initial 6


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concentrations of PFAAs in solutions were 5 µmol/L. All solutions were adjusted to pH 5 with HCl and NaOH, except those in the investigation of pH effect (pH change < ± 0.2 after sorption experiments). All sorption experiments were run in duplicate, and a parallel set of controls without adsorbent was set up. The controls showed no significant loss of degradation or sorption by vessels. Sorption isotherm experiments were carried out with initial PFOS/PFOA concentrations ranging from 0.25 to 10 µmol/L. In the investigation of humic acid (HA) effects, HA concentrations were 1 and 10 mg/L, coexisting with PFOS/PFOA in solution. To investigate the selectivity of F-MT, SDBS, pyridine, PHE and phenol were selected as the coexisting compounds. In the single-solute experiments, the concentrations of PFOS/PFOA as well as PHE, SDBS, pyridine or phenol were all 5 µmol/L. In the dual-solute experiments, the competitive sorption of F-MTs for PFOS/PFOA was conducted in the presence of PHE, SDBS, pyridine or phenol at 5 µmol/L. 2.5. Computational method. To explore selective sorption mechanisms of PFAAs b\y PFQA, DFT calculations were performed to visualize sorption configurations and calculate binding energies (∆Ebinding). In order to simplify the calculations, PFQA molecule was taken as a model adsorbent in the water environment. Perfluorocarbon and hydrocarbon chains, most likely responsible for sorption of PFOA, PFOS, SDBS and PHE on PFQA, were considered in the calculations. Geometries of sorption complexes were optimized to stationary points at the B3LYP/6-31G level. Frequency analysis was performed at the same level to make sure that the stationary 7



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point is a local minimum (no imaginary frequency) and to obtain a zero-point vibrational energy (ZPVE). Electron energies (E0) of sorption complexes were calculated at the B3LYP/6-31+G(d,p)//B3LYP/6-31G level, and ∆Ebinding values were calculated by following equations. E = E0 + ZPVE

(1)

∆Ebinding = E(sorption complex) – E(PFQA) – E(Adsorbate)

(2)

The integral equation formalism polarized continuum model (IEFPCM) based on the self-consistent-reaction-field (SCRF) was used to mimic solvation effects in E0 calculations.23 All calculations were performed with a Gaussian 09 software package. 2.6. Analytical methods. All samples were centrifuged at 6000 rpm for 10 min before analysis. Samples containing

14

C-labelled PHE were quantitated using a liquid-scintillation

counter (LS-6500 Multi-Purpose Scintillation Counter, Beckman Instruments, USA). SDBS, pyridine and phenol were analyzed by a Perkin–Elmer LS45 fluorescence spectrometer (Perkin-Elmer Corp., USA). High performance liquid chromatography-tandem mass spectrometry was used to determine PFAAs in the solution with an UltiMate 3000 HPLC (Dionex by Thermo Fisher Scientific Inc., USA) equipped with an API 3200 triple quadrupole mass spectrometer (AB SCIEX, Canada). The detailed parameters of PFAAs analysis were described in our previous study.24

3. RESULTS AND DISCUSSION 3.1. Effect of PFQA-loaded amount. TGA curves of the pure PFQA chemical, Na-MT and different F-MTs prepared at different PFQA concentrations are shown in Figure S1. The 8


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weight loss of fluorinated-montmorillonite takes place in four steps: (1) water-desorption, (2) dehydration, (3) decomposition and evolution of loaded surfactant and (4) dehydroxylation of montmorillonite OH units.25 The weight loss of Na-MT and F-MTs (Figure S1b) before 180°C should be attributed to the dehydration of physically sorbed water. The surfactant PFQA thermally decomposes at 240 °C (Figure S1a), and thus the mass loss of F-MTs after 180°C came from the decomposition of PFQA and possible evolution of products associated with organic residues, besides dehydroxylation of OH units from pristine clay itself. Therefore, the amount of loaded PFQA should be the difference between the mass loss of F-MT and pristine Na-MT, but Na-MT and different F-MTs displayed different hygroscopic capacities. With the increase of PFQA reaction concentrations, the amounts of adsorbed water on the F-MTs decreased (Figure S1b). Thus, the weight loss from adsorbed water should be calculated separately, and the loaded amounts of PFQA were finally obtained from the following equation:

Floaded =

( Ft − Fw ) − ( N t − N w ) × (1 − Fw ) (1 − Ft ) × M PFSA

(3)

where Floaded (mmol/g) is PFQA-loaded amount on the montmorillonite; Ft (g/g) is the total mass loss of F-MT during the heating process of TGA; Fw (g/g) is the mass loss of sorbed water on F-MT; Nt (g/g) is the total mass loss of Na-MT during the heating process of TGA; Nw (g/g) is the mass loss of sorbed water of Na-MT; MPFQA (0.7259 g/mmol) is the molar mass of PFQA. According to the calculation, the F-MTs prepared at different PFQA concentrations had the different loaded PFQA amounts of 0.21, 0.57, 0.73, 0.83, 0.99, 1.00 mmol/g, respectively. 9



Since the CEC of Na-MT is about 0.99 mmol(+)/g (i.e., 0.99 mmol/g for ammonium salt), almost same as the largest loaded amount (1.00 mmol/g), indicating that almost all of exchange sites on the montmorillonite should be occupied by PFQA. The F-MTs were denoted as n-F-MT (n = 0.21, 0.58, 0.74, 0.84, 1.00 and 1.01), in which the prefix number represents the molar ratio of loaded PFQA to CEC of Na-MT. Figure 1 shows the effect of PFQA-loaded amount on PFOA sorption. The adsorbed amounts of PFOA increased substantially with the increase of PFQA-loaded amount, indicating that PFQA was the key component for PFOA sorption. The best 1.01-F-MT was used in the following sorption experiments. 120 100 80 60 40 20

T 0. 58 -F -M T 0. 74 -F -M T 0. 84 -F -M T 1. 00 -F -M T 1. 01 -F -M T

0

0. 2

1FM

PFOA adsorbed (μmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Effect of PFQA amount loaded in F-MT on PFOA sorption. 3.2. Sorption of PFOS and PFOA on F-MT. Figure 2a displays the sorption kinetics of PFOS and PFOA on F-MT, and the pseudo-second-order model was used to fit the kinetic data. The sorption equilibria of PFOS and PFOA were both almost achieved within 21 h, and most sorption was accomplished at initial several hours. The initial sorption rates (v0) of 10


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PFOS and PFOA were 263.2 and 225.7 µmol/g/h, respectively, much higher than the reported values of activated carbon (2.7 µmol/g/h for PFOA and 6.2 µmol/g/h for PFOS) and anion-exchange resin (8.1 µmol/g/h for PFOA and 37.2 µmol/g/h for PFOS),26 while the equilibrium adsorbed amount (qe) was 215.8 µmol/g (116.1 mg/g) for PFOS and 124.9 µmol/g (51.7 mg/g) for PFOA. Evidently, PFOS was adsorbed more readily by F-MT than PFOA. Compared with PFOA, PFOS molecule possesses one more C−F unit and may have stronger fluorophilicity to fluorinated surface, potentially making it easily adsorb on the same C−F chain length of PFQA on F-MT. 300

240

(a)

(b) PFOS 2 t/qt = 1/263.2 + t/215.8, R = 0.99

160 120

PFOA 2 t/qt = 1/225.7 + t/124.9, R = 0.91

80

qe (µmol/g)

250

200

qt (µmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


150

0 0

12

24

36

48

PFOS PFOA Freundlich Langmuir

100 50

40 0

200

60

0

2

4

6

8

Ce (µmol/L)

t (h)

Figure 2. Sorption kinetics of PFOS and PFOA on F-MT and fitting results using the pseudo-second-order equation (t/qt = 1/v0 + t/qe where qe represents the adsorbed amount at equilibrium, and v0 is the initial sorption rate) (a), as well as their sorption isotherms and modeling fits by the Freundlich and Langmuir equations (b) Sorption isotherms of PFOS and PFOA on F-MT are illustrated in Figure 2b, and the fitting results by the Freundlich and Langmuir models are presented in Table S2. The sorption isotherms were described better by the Freundlich model than the Langmuir one according to 11



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the obtained correlation coefficients (R2). Figure 2b clearly shows that F-MT has higher sorption affinity for PFOS than PFOA. Anion-exchange resin and AC generally have excellent performance for PFOS and PFOA sorption among the reported adsorbents.4,

27

Several studies have investigated the

sorption isotherms of PFOS and PFOA at µg/L level on the commercial adsorbents, and their comparison with F-MT is tabulated in Table S3.28-31 Sorption capacity of PFOS and PFOA on adsorbents changed greatly with their equilibrium concentrations. Commonly, the concentrations of PFOS and PFOA in surface waters are below 1 µg/L, while these in wastewater as well as polluted surface water associated with PFAAs industries or directly discharged wastewater can reach above 1 µg/L or even above 10 µg/L in severe cases.2, 24, 32-33

Therefore, 1, 10 and 100 µg/L were taken as the equilibrium concentrations in order to

compare the sorption capacity of F-MT with the reported materials. As presented in Table S3, the F-MT had the highest sorption amounts for PFOS and PFOA at the three concentrations among all sorbents, except that the anion-exchange resin had a little higher sorption of PFOS/PFOA at 100 µg/L. Therefore, the F-MT can be used as an effective adsorbent for the removal of PFOS/PFOA from water or wastewater at relatively low concentrations. The effect of solution pH on the adsorption of PFOS and PFOA on F-MT is presented in Figure 3a, and the adsorption of PFOS and PFOA decreased gradually with the increase of solution pH. The pKa value of PFOS is -3.27 (Table S1), and thereby PFOS molecules exist as anions in solution. In contrast, PFOA has a pKa of nearly 3.8. PFOA anions gradually changed into the neutral form with deceasing pH from 5 to 2. Since the pH at point of zero 12


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charge (pHpzc) of F-MT was about 2.5 (Figure 3b), the F-MT should have negative surfaces at pH above 2.5. The amount of surface negative charges increased with increasing pH (Figure 3b), leading to stronger electrostatic repulsion between F-MT and PFOS/PFOA anions and correspondingly lower adsorption. At pH below 2.5, the F-MT surfaces were positively charged, and electrostatic attraction could be involved in the adsorption of anionic PFOS. 10 400

(a) PFOS PFOA

300

Zeta potential (mV)

Adsorbed am ount(μ m ol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200 100

(b)

2.5

0 -10 -20 -30 -40

0

1

2

3

4

5

6

7

8

9

2

10

3

4

5

6

7

8

9

pH

pH

Figure 3. Effect of solution pH on PFOS and PFOA adsorption (a) as well as zeta potentials of F-MT at different pHs (b) 3.3. Sorption selectivity. Four common organic compounds including SDBS, PHE, phenol and pyridine were selected to study the sorption selectivity of F-MT for PFOS and PFOA. These competitive adsorbates are different in hydrophobic and ionic properties and molecular structure (Table S4). The F-MT had much higher adsorbed amounts for PFOS and PFOA than other compounds (Figure 4), exhibiting excellent sorption selectivity for PFOS and PFOA. In contrast, the Na-MT had the similar low sorption for all of six adsorbates, indicating the dominant role of PFQA in the selective sorption for PFOS and PFOA. Phenol and pyridine were barely adsorbed by the F-MT, probably due to their strong polar functional groups. In a previous study of selective ability of molecularly imprinted polymer (MIP) 13



adsorbent for PFOS, phenol and sodium pentachlorophenate (PCP) were taken as competitive compounds.8 The adsorbed amount of PFOS on the MIP was about 9-fold of that of phenol and only 4-fold of that of PCP, while the PFOS sorption amount on the F-MT was 67-fold of phenol and 466-fold of pyridine, indicating a better selective performance than MIP. Although the adsorbed amounts of SDBS and PHE on the F-MT were markedly increased after PFQA loading, the values were still much lower than PFOS and PFOA. Hydrophobic interaction might be involved in the uptake of SDBS and PHE on the F-MT, but the sorption of more hydrophobic PHE (log Kow = 4.53, Table S4) was lower than that of SDBS (log Kow = 1.96, Table S4). Thus, the selective sorption process is complicated and the oleophobic property of C−F chain in PFQA may also play an important role.

Adsorbed amount (µmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

300

F-MT Na-MT

250 200 150 100 50 0

Phenol PFOS PFOA Pyridine SDBS PHE

Figure 4. Sorption selectivity of PFOS and PFOA onto the F-MT and Na-MT in comparison with four organic compounds in single-sorbate solution. To further evaluate the sorption selectivity of F-MT and investigate the sorption mechanism, each competitive contaminant was added into PFOS or PFOA solution to determine their competitive effects. The F-MT adsorbent still maintained its stable and high 14


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sorption for PFOS and PFOA in the presence of other adsorbates (Figure 5). The same adsorbed amounts of PFOS in the single-solute and dual-solute solutions indicate no influence of coexisting organic compounds on PFOS sorption. The sorption of PFOA also kept nearly the same in the presence of compounds except for SDBS (Figure 5b). The concentrations of compounds in the dual-solute solutions after sorption experiments were determined, and their adsorbed amounts are shown in Figure S2. The adsorbed amounts of SDBS and PHE decreased in the dual-adsorbate solutions in comparison with these in the single-adsorbate solutions, whereas still very low amounts of pyridine and phenol were adsorbed by the F-MT in both solution systems. The adsorbed amounts of SDBS decreased significantly in the presence of PFOS/PFOA. Compared with SDBS, PHE sorption decreased a little in the presence of PFOS/PFOA, and the presence of PHE did not influence PFOS/PFOA sorption either (Figure 5), indicating that PHE and PFOS/PFOA had different sorption sites on the F-MT. Although the oleophobic C−F long chain of PFQA should repel hydrophobic PHE, PFQA also has a hydrocarbon part which is able to adsorb PHE/SDBS via hydrophobic interaction. PFOS was recently found to be hardly adsorbed directly onto the hydrocarbon surface via hydrophobic interaction due to its oleophobicity,18 but it can form aggregation/micelle structure by C−F chains close to each other to reduce the contact with water.4 Therefore, PFOS/PFOA molecules should be adsorbed on the F-MT via their C−F chains adsorbed on the C−F chain of PFQA, while the sorption of conventional organic compounds should occur on the hydrocarbon part of PFQA. For SDBS, the much more significant decrease of sorption in the presence of PFOS/PFOA (Figure S2) may be related to 15



their negatively charged property. The adsorbent surfaces after the sorption of anionic PFAAs carry negative charges,4 which could electrostatically repel the approaching SDBS anions. Conversely, the adsorbed SDBS was able to generate electrostatic repulsion for PFAAs, resulting in the decline of PFOA sorption (Figure 5b). However, the presence of SDBS had little influence on the PFOS sorption (Figure 5a) and almost no SDBS was adsorbed on the F-MT in the dual-sorbate solution containing PFOS (Figure S2), indicating much stronger

240


competitiveness and higher preference of PFOS during the sorption process. Adsorbed amount (µmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(a) PFOS

180 120 60 0

N/A

Phenol SDBS

PHE Pyridine

150

(b) PFOA

120 90 60 30 0

N/A

Phenol SDBS

PHE Pyridine

Figure 5. Sorption of PFOS (a) and PFOA (b) onto the F-MT in dual-sorbate solution in the presence of competitive compounds (N/A: PFAA blank solution without the presence of coexisting hydrocarbon). The effect of coexisting HA on PFOS and PFOA sorption was also investigated (Figure S3). When the concentration of HA was 1 mg/L, the adsorbed amounts of PFOS and PFOA onto the F-MT were almost the same as the value in the absence of HA, but a little decrease of PFOS and PFOA sorption appeared in the presence of 10 mg/L HA. Since the F-MT is highly hydrophobic and possesses nano-scale interlayers, it is hard for macromolecular HA with numerous surface hydrophilic groups to be adsorbed on the F-MT. Humic acid was 16


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reported to be able to adsorb PFAAs as a competitive adsorbent and make them mobile in aquatic environment,34-35 which should result in the decrease of uptake of PFOS and PFOA on the F-MT. The concentration of HA in natural water commonly cannot reach as high as 10 mg/L,36 but in some impacted waters with high HA concentrations, the effect of HA on the sorption of PFOS/PFOA should be considered. 3.4. Selective sorption mechanism. Sorption scenarios and relative binding energies of PFOA, PFOS, SDBS and PHE on the C−F chain and hydrocarbon chain of PFQA are illustrated in Figure 6. The ∆Ebinding values of PFOS and PFOA on the C−F chain of PFQA are 17 and 19 kJ/mol, respectively, much lower than these values (167 kJ/mol for PFOA and 37 kJ/mol for PFOS) on the hydrocarbon part of PFQA (Figure 6a-6d), indicating the primary sorption of PFOS/PFOA occurring on the C−F chain of PFQA. The hydrophobic C−F tails of PFOS and PFOA press close to the C−F chains of PFQA, which can minimize the contact with water and lower the ∆Ebinding (defined here as fluorophilicity). PFOS possesses one more C−F unit and is more hydrophobic and fluorophilic than PFOA, leading to a lower binding energy on the C−F chain of PFQA and consequently stronger sorption, consistent with their adsorbed amounts obtained in the sorption experiments. Perfluorocarbon chain is oleophobic and hard to be forthrightly adsorbed onto the hydrocarbon substances,18 resulting in higher ∆Ebinding when approaching to the hydrocarbon part of PFQA. Therefore, PFOS/PFOA would be preferentially adsorbed on the perfluorinated part of PFQA via fluorophilic interaction, rather than its hydrocarbon part due to the oleophobicity.

17



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Figure 6. Sorption configurations and calculated binding energies (∆Ebinding) of PFOA (a and b), PFOS (c and d), SDBS (e and f) and PHE (g and h) on C−F chain (a, c, e and g) and hydrocarbon part (b, d, f and h) of PFQA (Gray balls: C atom; Light gray balls: H atom; Red balls: O atom; Dark blue balls: N atom; Light blue balls: F atom; Green balls: Cl atom; Purple balls: K atom). 18


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In contrast, the hydrocarbon part of PFQA is favorable for sorption of SDBS and PHE. SDBS sorption on C−F chain of PFQA exhibited much higher ∆Ebinding than that on hydrocarbon part of PFQA (Figure 6e and 6f). Although the ∆Ebinding difference between PHE sorption on hydrocarbon part and C−F chain of PFQA is not as much as that of SDBS (Figure 6g and 6h), the interatomic distance measurements suggest that adsorbed PHE is closer to the hydrocarbon part of PFQA than its C−F chain. The minimum distance from the PHE plane to F atom of C−F chain of PFQA is about 3.2 Å, while its minimum distance to the nearest H atom is only 2.5 Å. Similar to PFOS/PFOA, the C−F chain of PFQA has the oleophobicity to repel the hydrocarbon compounds, making them hardly get close to the C−F chain, while SDBS and PHE are relatively easier to be adsorbed on the hydrocarbon part of PFQA via hydrophobic interaction. Overall, the sorption of PFOA, PFOS, SDBS and PHE obtained by DFT calculations is consistent with the results of sorption experiments. The layer structures of Na-MT and different F-MTs were analyzed by XRD, and the patterns are shown in Figure 7. The basal spacing (d001) of montmorillonite can be obtained from XRD data according to the Bragg equation (2dsinθ=λ),37 and the main d001 shifted from 1.37 nm (Na-MT) to 1.44~1.46 nm (F-MTs) after PFQA were loaded on the clay. Besides the peak of around 1.44 nm, another significant response of crystalline structure appeared in the XRD pattern with the further increase of PFQA loading, and the d001 value of this response was 2.72 nm for 0.82-F-MT and 2.85 nm for 1.01-F-MT. The interlayer distance of organo-montmorillonite is the difference between the value of d001 spacing and the thickness of the tetrahedron–octahedron–tetrahedron (TOT) layer (0.96 nm).38 Based on the interlayer 19



distance and the PFQA molecular size, the arrangement of PFQA within the F-MT can be illustrated. When PFQA molecule is lying flat, the height of the quaternary ammonium group with three methyls is 0.51 nm.38 In this case, plus the thickness of TOT layer (0.96 nm), the thickness of F-MT was about 1.47 nm which is consistent with the value of d001 obtained from XRD analysis. This indicates a lateral monolayer of PFQA arrangement in the F-MT interlayer, as illustrated in Figure 8a. The basal reflections at 2.72 nm and 2.85 nm occurred on the 0.84-F-MT and 1.01-F-MT, respectively, which can be considered as a paraffin-type monolayer arrangement.38 According to the spacing of TOT layer and the length of PFQA (approximately 1.9 nm), the angles (α) between the molecular chain and basal plane were calculated to be 68° and 84° for 0.84-F-MT and 1.01-F-MT, respectively (Figure 8b). 1.44nm 2.85nm 1.01-F-MT 1.44nm

Counts/sec.

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2.72nm 0.84-F-MT

1.45nm

0.58-F-MT 1.46nm 0.21-F-MT 1.37nm Na-MT 2

4

6

8

10

12

14

16

18

20

2θ(°) Figure 7. XRD patterns of Na-MT and F-MTs with different loaded amount of PFQA Based on the discussion of selective sorption mechanisms from DFT calculation and PFQA arrangement, a possible schematic diagram for PFOS and PHE (as a representative of 20


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hydrophobic hydrocarbons) sorption on the F-MT is illustrated in Figure 8c and 8d. When the arrangement of PFQA was a paraffin-type monolayer, PFOS should be distributed in a similar paraffin way with its C−F chain clinging to the perfluorinated part of PFQA (Figure 8d), due to the fluorophilic effect. In the F-MT interlayer where PFQA arranged in a lateral form, PFOS lies on the F-MT surface, close to the C−F chain of PFQA molecule (Figure 8c). PHE should be partitioned to the alkane of PFQA, and the form of PHE sorption is related to the arrangement of PFQA, as presented in Figure 8c and 8d, maximizing the contact with the hydrocarbon chains. Although alkane chains of bonded cationic surfactant molecules can vibrate and let adsorbates access to their preferred sorption sites,39 it should be noted that steric effect of molecules in clay interlayers is not included duo to the limitation of computational complexity, which needs to be further investigated.

(a)

(b)

(c)

(d)

Figure 8. Schematic diagram for lateral (a) and paraffin-type (b) monolayer arrangements of PFQA in the F-MT and selective sorption of PHE and PFOS in the corresponding lateral (c) 21



and paraffin (d) forms. To further support the sorption of PFOS on the long C−F chain of F-MT, the sorption of six different PFAAs on F-MT in the single-solute solution and mixed-solute solution was compared (Figure 9). In both solution systems, the sorption amounts increased regularly, in the order of PFBA < PFBS < PFHxA < PFHxS < PFOA < PFOS, with the increase of C−F chain length. When PFOS was mixed with other five PFAAs, the adsorbed amount of PFOS decreased a little, while other PFAAs displayed a sharp decrease (above 50%) of their adsorbed amounts in the single solute system. Obviously, different PFAAs shared the same sorption sites (long C−F chain) on the F-MT, and PFOS was preferentially adsorbed over other PFAAs.

250

250

(a) Single solute



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200 150 100 50 0

PFBS PFBA PFHxSPFHxA PFOS PFOA

(b) Mixed solute

200 150 100 50 0

PFBS PFBA PFHxS PFHxA PFOS PFOA

Figure 9. Sorption of different PFAAs on the F-MT in single-solute solution (a) and mixed-solute solution containing six different PFAAs (PFOA, PFOS, PFHxA, PFHxS, PFBA, PFBS) at 5 µmol/L (b). 3.5. Regeneration and reuse of F-MT. The experiments of F-MT regeneration and reuse were conducted to investigate the reusability of F-MT, and the experimental methods were described in the Supporting Information. The regeneration efficiency of the spent F-MT after 22


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the sorption of PFOS/PFOA by methanol and ethanol solutions is present in Figure S4, and the reuse result of F-MT in five sorption cycles is shown in Figure 10. It can be seen that the desorption percentage of both PFOS and PFOA increased with increasing methanol and ethanol concentrations, and nearly 100% was achieved after methanol concentration of above 80% (Figure S4). The ethanol solution showed lower desorption efficiency than methanol, possibly due to the lower solubility of PFOS/PFOA in ethanol solution. The F-MT regenerated by methanol solution exhibited steady sorption for PFOS and PFOA in five cycles (Figure 10), showing high reusability and stability.

Adsorbed amount (umol/g)

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PFOA

240

PFOS

180 120 60 0

1

2

3

4

5

Cycles

Figure 10. Adsorbed amounts of PFOS and PFOA on the F-MT in five successive sorption cycles

4. CONCLUSIONS The F-MT exhibited high and selective sorption as well as reusability for the removal of PFOS and PFOA from water. The coexisting hydrocarbon compounds almost had no effect on PFOS and PFOA sorption on the F-MT, showing a promising application for PFAAs 23



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removal from water or wastewater. We have demonstrated the important role of C−F chain of PFQA in the selective sorption process. The C−F long chains not only repel the coexisting hydrocarbon compounds but also adsorb PFAAs molecules via fluorophilic interaction, clarifying the misunderstanding of PFAAs sorption on hydrophobic adsorbents via hydrophobic interaction. This fluorinated adsorbent with unique selectivity for PFAAs may be used for pretreatment of water samples in solid-phase extraction and selective recovery of PFAAs from wastewater. Moreover, the new insight of hydro-oleophobic property of C−F chain could be useful to understand the interactions of PFAAs at water-solid interfaces, as well as transport and fate of PFAAs in aquatic environments.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the http://pubs.acs.org. Physical properties of PFAAs, fitting parameters of sorption isotherms for PFOS and PFOA, comparison of PFAAs sorption on different adsorbents, properties of hydrocarbon compounds, TGA patterns of PFQA and Na-MT and F-MTs, comparison of four hydrocarbon compounds sorption in the dual-solute and in single-solute solutions, effect of coexisting HA, regeneration and reuse experiments.

AUTHOR INFORMATION Corresponding Authors *E-mail:

[email protected]

(S.

Deng).

Phone:

24


+86-10-62792165.

Fax:

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+86-10-62794006. *E-mail: [email protected] (B. Xing).

ACKNOWLEDGMENTS We thank the National High-Tech Research and Development Program of China (Project no. 2013AA06A305), the National Nature Science Foundation of China (Project no. 21177070), Tsinghua University Initiative Scientific Research Program (Project no. 20141081174), program for Changjiang Scholars and Innovative Research Team in University (IRT1261), and Collaborative Innovation Center for Regional Environmental Quality for financial support. We also thank the China Scholarship Council for supporting Ziwen Du to do his research at UMass for one year.

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Graphic abstract


Selective and High Sorption of Perfluorooctanesulfonate and

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