Understanding the Adsorption of PFOA on MIL-101 (Cr)-Based

Article pubs.acs.org/est

Understanding the Adsorption of PFOA on MIL-101(Cr)-Based Anionic-Exchange Metal−Organic Frameworks: Comparing DFT Calculations with Aqueous Sorption Experiments Kai Liu, Siyu Zhang, Xiyue Hu, Kunyang Zhang, Ajay Roy, and Gang Yu* School of Environment, Beijing Key Laboratory for Emerging Organic Contaminants Control, State Key Joint Laboratory of Environment Simulation and Pollution Control (SKLESPC), Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: To examine the effects of different functionalization methods on adsorption behavior, anionic-exchange MIL101(Cr) metal−organic frameworks (MOFs) were synthesized using preassembled modification (PAM) and postsynthetic modification (PSM) methods. Perfluorooctanoic acid (PFOA) adsorption results indicated that the maximum PFOA adsorption capacity was 1.19 and 1.89 mmol g−1 for anionicexchange MIL-101(Cr) prepared by PAM and PSM, respectively. The sorption equilibrium was rapidly reached within 60 min. Our results indicated that PSM is a better modification technique for introducing functional groups onto MOFs for adsorptive removal because PAM places functional groups onto the aperture of the nanopore, which hinders the entrance of organic contaminants. Our experimental results and the results of complementary density functional theory calculations revealed that in addition to the anion-exchange mechanism, the major PFOA adsorption mechanism is a combination of Lewis acid/base complexation between PFOA and Cr(III) and electrostatic interaction between PFOA and the protonated carboxyl groups of the bdc (terephthalic acid) linker.

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INTRODUCTION Many contaminants of emerging concern exist in ionic form in the aqueous environment. Among them, perfluorinated compounds (PFCs) predominately exist in anionic form because they contain acidic functional groups such as carboxylates and sulfonates. They are ubiquitously detected1,2 and environmentally persistent.3 In general, PFC removal methods include adsorption,4 separation,5 and degradation.6,7 Benefitted by its high surface area, activated carbon (AC) has been demonstrated to be a cost-effective material for the adsorptive removal of PFCs in a laboratory setting.8 However, the adsorption capacity of PFCs on AC is low, and existing water treatment plants that use AC treatments are unable to remove influent PFCs.9,10 Recently, an ion-exchange resin has been demonstrated to have a high sorption capacity, albeit at much lower rate than AC. Therefore, the development of new adsorption materials that combine both high surface area and versatile functionality is needed for the fast and efficient removal of aqueous PFCs. Metal−organic frameworks (MOFs) are a class of inorganic− organic hybrid materials that have received considerable attention for environmental remediation purposes. The increasing interest in MOFs is derived primarily from their extensive porosity and ease of modification.11 Here, we focus on the adsorption of a single archetypical PFC, perfluorooctanoic acid (PFOA), onto a series of functionalized MIL© XXXX American Chemical Society

101(Cr) (MIL: Materials of Institute Lavoisier). MIL-101(Cr) belongs to a class of chromium(III) terephthalate MOFs discovered by Férey et al. in 2005. It possesses an extremely high specific surface area (SBET ≈ 4000 m2 g−1), and guest molecules can access its quasi-spherical cages (Ø ≈ 2.9 and 3.4 nm) via 1.2 and 1.6 nm apertures (Figure 1a).12 Benefiting from acid and base resistance and high thermal (up to 320 °C) and aqueous stability, MIL-101(Cr) can be used as a reference MOF for studying various guest−host interactions for aqueous organic contaminant removal. Thus, far, most studies in this field have investigated dye removal using pristine MOFs,12−14,16 with the enhanced adsorption capacity and kinetics of MOFs relative to those of ACs having been clearly demonstrated in these studies. Regarding gas adsorption onto MOFs, the functionalization of MOFs is a promising approach in tuning guest−host interactions.13 This tuning is achieved either via synthesis of MOFs using a functionalized organic linker (preassembled method, PAM) or via a postsynthetic method (PSM) (Figure 1b). Both methods impart distinctive steric and electronic properties to the material and hence affect its adsorption Received: February 12, 2015 Revised: May 16, 2015 Accepted: June 11, 2015

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DOI: 10.1021/acs.est.5b00802 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

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MATERIALS AND METHODS Preparation of Sorbents. MIL-101 was synthesized according to published HF-free hydrothermal methods.18,19 Briefly, bdc (0.82 g) and Cr(NO3)3·9H2O (2.0 g) were added to 24 mL of Milli-Q water. The resultant suspension was sonicated for 30 min at room temperature before being heated to 218 °C for 18 h in a 100 mL Teflon-lined autoclave. After cooling to room temperature, the crude product was collected by centrifugation at 8000 rpm for 10 min. The crude product was washed with dimethylformamide at 100 °C in an autoclave for 12 h and then washed with ethanol at 100 °C in an autoclave for 12 h. The resulting material was dried in a vacuum oven at ∼100 °C for 12 h. MIL-101-DMEN was prepared via a modified MIL-101-ED synthesis procedure.12 Briefly, MIL-101 was dehydrated at 150 °C in a vacuum oven overnight. N,N-Dimethylethylenediamine (0.75 mmol) was added to a MIL-101 (0.5 g) suspension in anhydrous toluene (30 mL). The mixture was stirred under reflux overnight. The crude was dried in vacuo to remove toluene and was subsequently washed with Milli-Q water and ethanol at 100 °C in an autoclave for 12 h. The resultant material was dried in a vacuum oven at ∼100 °C for 12 h to yield MIL-101-DMEN. MIL-101-QDMEN was synthesized by reacting a suspension of MIL-101-DMEN (0.5 g) in 6 mL of dry dichloromethane with methyl triflate (1.5 mmol). The mixture was stirred at room temperature for 12 h. The crude product was dried in vacuo to remove dichloromethane and unreacted methyl triflate and subsequently washed with Milli-Q water and ethanol at 100 °C in an autoclave for 12 h. The resultant material was dried in a vacuum oven at ∼100 °C for 12 h to yield MIL-101QDMEN. MIL-101-NH2 was synthesized using published HF-free hydrothermal methods.18,19 Briefly, 2-NH2-bdc (0.36 g) and Cr(NO3)3·9H2O (0.8 g) were added to 15 mL of Milli-Q water. The resulting suspension was sonicated for 30 min at room temperature before being heated to 150 °C for 12 h in a 50 mL Teflon-lined autoclave. After cooling to room temperature, the crude product was collected by centrifugation at 8000 rpm for 10 min and then washed with Milli-Q water and ethanol at 100 °C in an autoclave for 12 h. The resulting material was dried in a vacuum oven at ∼100 °C for 12 h. MIL-101-NMe3 was synthesized by reacting a suspension of MIL-101-NH2 (0.5 g) with methyl triflate (3.0 mmol) in 6 mL of dry dichloromethane. The mixture was stirred at room temperature for 12 h. The crude product was dried in vacuo to remove dichloromethane and unreacted methyl triflate and subsequently washed with Milli-Q water and ethanol at 100 °C in an autoclave for 12 h. The resulting material was dried in a vacuum oven at ∼100 °C for 12 h. Both quaternized MOFs (MIL-101-QDMEN and MIL-101NMe3) were acidified with 0.1 M HCl for 12 h at room temperature to yield the corresponding MOF anion exchangers. Acidified MOFs were collected by centrifuge at 8000 rpm for 10 min and then washed repeatedly with Milli-Q water until neutral pH. Corresponding MOF anion exchangers in Cl− form were subsequently obtained by drying the MOFs in a vacuum oven at ∼100 °C for 12 h. All MOFs were stored in a vacuum desiccator in the dark prior to use. Characterization. Powder X-ray diffraction (PXRD) patterns were collected using a D/MAX-RB (Rigaku) X-ray diffractometer equipped with a Cu−Kα radiation source to

Figure 1. Perspective view of (a) the quasi-spherical cage of MIL101(Cr); (b) CUS used for MIL-101(Cr) functionalization using PSM and scheme that represents the surface functionalization of the proposed anionic-exchanger MOFs: (c) MIL-101(Cr)-NMe3; (d) MIL-101(Cr)-QDMEN, prepared by PAM and PSM, respectively. Chromium trimers, framework carbon atoms, and oxygen atoms are shaded in green, white, and gray, respectively. Quaternary ammonium moieties are shaded in red.

behavior. Very recently, amine-grafted MOFs prepared via PSM were shown to improve dye and pharmaceutical and personal care products adsorption;14,15 unfortunately, the effects exerted by different functionalization methods on MOF adsorption behavior have not been compared. Furthermore, despite growing efforts in the research domain of aqueous OC (organic compounds) adsorption using MOFs, the mechanism, which is difficult to elucidate on the basis of experimental data alone, is not well understood. In recent years, density functional theory (DFT) calculations have been used to better understand the adsorption mechanism of gases.16,17 However, the aqueous adsorption of OC onto MOFs has not yet been investigated by DFT calculations. Taking the aforementioned factors into consideration, we herein describe our attempt to design and fabricate anionicexchanger MOFs based on MIL-101(Cr) functionalized by both PAM and PSM approaches (Figures 1c, 1d), where the anionic-exchange performance is enhanced by an additional quaternized amine functional group. The steric and electronic effects of the aforementioned functionalization methods on the adsorption performance of MIL-101(Cr) are experimentally examined using PFOA as a model compound. We have observed a remarkable enhancement in adsorption capacity that is comparable to that of ion-exchange resins. Additionally, the anionic-exchanger MOFs exhibited adsorption rates that are significantly faster than those of ACs. In addition, we performed quantum chemical calculations based on DFT to complement our experimental effort, not only to rationalize the difference in adsorption performance but also to interpret the adsorption mechanisms of both pristine and functionalized MOFs. Our results show that the high adsorption capacity and ultrafast adsorption rates demonstrated by ion-exchange MOFs render them promising materials for aqueous anionic contaminant removal. Our mechanistic study provides valuable insights into the development of efficient MOF-orientated adsorbents for environmental remediation. B


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absorbed onto MOFs was calculated on the basis of the difference in solution concentration before and after sorption. Computational Methods. The DFT calculations of PFOA adsorption on MOFs used in this study were focused primarily on MIL-101(Cr). Because the unit-cell structure of MIL-101 reported by Férey et al.20 was too large to allow all possible PFOA adsorption configurations to be evaluated, a cluster model composed of two Cr-trimers linked by a single bdc ligand (Figure 2a) was employed to save computational

confirm the MOF structure. Surface morphology was characterized by scanning electron microscopy (SEM) using a Hitachi S-4500. The presence of functional groups was confirmed by Fourier-transform infrared (FT-IR) spectra recorded on a Thermo Nicolet Nexus 870 FT-IR spectrometer system using samples pelletized with KBr. The composition of MOFs was determined using X-ray photoelectron spectrometry (XPS) (Thermo Scientific ESCALAB 250Xi equipped with an Al−Kα X-ray radiation source). The binding energy was calibrated using the C 1s peak at 284.8 eV. BET surface areas were determined using nitrogen adsorption and desorption isotherms measured using a gas adsorption instrument (Autosorb iQ, Quantachrome, U.S.). MOFs were prepared by purging with nitrogen gas at 120 °C for 6 h before analyses. Zeta potentials were analyzed with a zeta-potential analyzer (Delsa Nano C, Beckman Coulter, U.S.). MOF suspensions (0.1 wt %) were prepared by mixing MOFs with Milli-Q water at the desired pH values under ultrasonication; 2 mM NaH2PO4 was added as a pH buffer. Sorption Experiments. All sorption experiments (except for those investigating kinetics) were carried out using an orbital shaker at 150 rpm and at room temperature. Adsorbent (4 mg) was added to Nalgene 50 mL round-bottom polypropylene centrifuge tubes (Thermo Scientific) loaded with 40 mL of PFOA solution and 2 mM NaH2PO4 as a pH buffer. The initial solution pH was adjusted to 5.0 by adding 1 M NaOH or 1 M HCl. The pH was adjusted prior the addition of adsorbent, and no further adjustment was made. A 1 mL solution sample was collected at a predetermined time. A duration of 12 h was employed for the experiments. Sorption kinetic experiments were carried out with a magnetic stirrer at 150 rpm and at room temperature. Adsorbent (20 mg) was added to 250 mL polypropylene flasks containing 200 mL of PFOA solution with an initial pH adjusted to 5.0. The effect of pH on sorption was investigated by conducting sorption experiments at various initial pH values in the range of 3.0− 10.0, where the pH values were adjusted by adding 1 M NaOH or HCl solution. The final pH was recorded after the sorption experiments were completed. The ionic strength of added NaOH or HCl was negligible. All isotherm experiments were conducted in triplicate, kinetic and pH effect experiments were performed in duplicate, and average values were reported. Regeneration Experiments. Adsorbents (10 mg) was added into PFOA solution (100 mL) at pH 5.0. After 12 h adsorption, spent adsorbents were recovered by filtration with 0.22 μm nylon membrane and placed in 40 mL regeneration solution containing 1% NaCl/methanol (30/70, v/v), followed by overnight shaking on orbital shaker at 150 rpm at room temperature. The regenerated adsorbents were filtered by 0.22 μm nylon membrane and dried in a vacuum oven at ∼100 °C for 5 h before the next adsorption run. Duplicates were performed, and average value was reported. PFOA Analysis. One milliliter of solution sample was filtered through a 0.22 μm nylon syringe filter. Negligible sorption of PFOA onto nylon filters was determined in a control experiment. PFOA concentrations were determined using a Shimadzu (Japan) LC-20A HPLC fitted with a CDD10Avp conductivity detector. Sample volumes of 20 μL were injected into the HPLC system. A TC-C18 column (4.6 × 250 mm2; 5 μm) from Agilent Technologies (U.S.) was used, and methanol/0.03 M NaH2PO4 (75/25) was used as mobile phase at a flow rate of 0.8 mL min−1. Linear calibration curves (R2 > 0.99) were obtained for all experiments. The amount of PFOA

Figure 2. (a) Cluster model of MIL-101(Cr); (b) possible configuration of the protonated MIL-101(Cr); (c) protonated MIL101(Cr)-NH2; (d) MIL-101(Cr)-NMe3; (e) protonated MIL101(Cr)-DMEN; (f) MIL-101(Cr)-QDMEN.

resources. Cleaved bonds on Cr-trimers were saturated by H atoms to maintain the original hybridization. The adaptation of the current cluster model allows for the investigation of all potential PFOA adsorption sites, including CUS, coordinated H2O, bdc, and cationic sites, which reflect the known adsorption mechanisms of MOFs.22 Similar cluster models have been shown to adequately describe interactions between CO2 and MOFs.17 Because the isotherm adsorption experiments were conducted in aqueous solutions at pH 5, which is close to the isoelectric point of MIL-101(Cr) in buffered solution (Supporting Information Figure S7), an additional cluster model was constructed to reflect the most possible configuration of protonated MIL-101 (Figure 2b). A comparison of the MIL-101(Cr) model and its protonated forms helps rationalize the PFOA adsorption mechanism at different solution pH values. The effect of amine moieties on PFOA adsorption was investigated using cluster models of amine-functionalized MIL-101(Cr)s (Figures 2c, 2e) and their quaternary amine anion-exchanger forms (Figures 2d, 2f). It has to be noted that although the cluster model approach is successful in probing adsorption sites on MOFs,17,21 the bulk behavior of MOFs remains the same using supercell approach employing a complete unit cell, in which the abrupt termination of the periodic structure of MOFs is avoided. All DFT calculations were performed using the DMol3 code.23 Spin-polarized generalized gradient approximation with the Perdew−Wang 199124 exchange-correlation functional was used in the calculations. The double numeric polarization20 basis set was used for describing atomic orbitals. Because of the presence of transition metals, the DFT semicore pseudopots approximation25 was utilized. A real-space orbital global cutoff of 4.4 Å was applied. The convergence thresholds for optimization were 10−5 (energy), 2 × 10−3 (gradient), and 5 C


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Environmental Science & Technology × 10−3 (displacement). The calculations were performed in the conductor-like screening model (COSMO)26 with a dielectric constant of 78.54 for simulating solvent effects. Geometry optimization of the cluster models and their PFOA-adsorbed forms was achieved by reaching a minimal potential surface using the aforementioned methods. Binding energies (Ebinding) between PFOA and various adsorption sites on geometrically optimized MOFs were calculated according to eqs 1 and 2 For Cr sites: E binding = E(AC) − E(A′) − E(PFOA)

(1)

Figure 3. Sorption kinetics of PFOA on MIL-101(Cr) (▲), MIL101(Cr)-NH2 (■), MIL-101(Cr)-NMe3 (□), MIL-101(Cr)-DMEN (●), and MIL-101(Cr)-QDMEN (○). Results fitted using the pseudosecond-order model.

For non‐Cr sites: E binding = E(AC) − E(A) − E(PFOA) (2)

where A′ represents an adsorbent model with one CUS. The total energies (E) of MOF adsorbents (A), PFOA, and adsorption complexes (AC) were calculated using methods consistent with geometry optimization. For each adsorption site, two or more orientations of PFOA were considered to evaluate different orientations of PFOA attack.

with nonporous ion-exchange resins by which the diffusion limitation is absent, it is notable that the sorption rate of MOFs is orders of magnitude greater than that of resins.29 This can be explained by the double contributions of both adsorption and ion-exchange action occurring over a significantly greater surface area that is accessible to PFOA on MOFs. All sorption kinetics appear to follow pseudo-second-order models (t/qt = 1/ν0 + t/qe, where qe and qt are the amount of PFOA adsorbed at equilibrium and predetermined time t, and v0 is the initial sorption rate). The obtained parameters are shown in Table 1.

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RESULTS AND DISCUSSION Structural Characteristics. PXRD analysis was conducted to confirm the identities of the MOFs. PXRD patterns of all of the MOFs, including those of MIL-101(Cr)-QDMEN and MIL-101(Cr)-NMe3, are consistent with the patterns of crystalline MIL-101(Cr) reported in the literature (Supporting Information Figure S1). This similarity suggests that quaternization does not affect the integrity of the framework. FT-IR analysis was performed to confirm the successful functionalization of MIL-101(Cr); the IR results are presented in Supporting Information Figures S2 and S3. The asymmetrical and symmetrical vibration bands (3350−3500 cm−1) of primary aromatic amines are shown for MIL-101(Cr)-NH2. (Supporting Information Figure S2). The presence of the NH and CH vibration bands (2500−3000 cm−1) in MIL-101(Cr)DMEN and MIL-101(Cr)-QDMEN indicate the presence of DMEN and quaternized DMEN, respectively (Supporting Information Figure S3). Further analysis of the chemical compositions of MOFs is summarized in Supporting Information Figures S4 and S5. The binding energies of 400.1 and 401.9 eV were assigned to PhNMe3 and RNMe3 in MIL-101(Cr)-NMe3 and MIL-101(Cr)-QDMEN, respectively. The BET surface areas of all of the investigated MOFs were measured. Functionalization-induced decreases in the BET surface areas are clearly demonstrated (Supporting Information Table S1). SEM images of MIL-101(Cr), MIL-101(Cr)-DMEN, and MIL-101(Cr)-QDMEN show crystallites (approximately 1 μm) and aggregated particles (approximately 0.2 μm) for MIL101(Cr)-NH2 and MIL-101(Cr)-NMe3, respectively (Supporting Information Figure S6). Small nanoparticles (∼100 nm) observed for MIL-101(Cr)-NH2 and MIL-101(Cr)-NMe3 are consistent with the broad Bragg reflections observed in the PXRD spectra (Supporting Information Figure S1). Sorption Kinetics. The sorption kinetics of PFOA on tested MOFs are shown in Figure 3. In general, sorption equilibrium is reached within 60 min for all MOFs tested. In comparison to other porous adsorbents (PAC and GAC) tested under similar conditions,27,28 substantially enhanced sorption rates were clearly observed. These enhanced sorption rates imply that the uniform pores and cavities of MOFs facilitate the internal diffusion of PFOA inside their nanopores. Comparing

Table 1. Kinetic Parameters of the Pseudo-Second-Order Model for PFOA Adsorption on MOF Adsorbents pseudo-second-order parameters adsorbent MIL-101(Cr) MIL-101(Cr)NH2 MIL-101(Cr)NMe3 MIL-101(Cr)DMEN MIL-101(Cr)QDMEN

qe (mmol g−1)

ν0 (mmol−1 h−1)

κ2 (g mmol−1 h−1)

R2

1.16 0.90

4.58 3.00

3.40 3.70

0.967 0.995

1.21

5.94

3.93

0.965

1.43

5.62

2.75

0.990

1.91

19.33

5.30

0.985

A pseudo-second-order model is commonly used to describe sorption kinetics in which chemical sorption controls the sorption rate and in which the number of active sites on the sorbent determines the sorption capacity;30 high correlation coefficients (R2) imply possible chemical interactions between PFOA and MOFs. Given that PFOA predominately exists in its anionic form under the conditions used in the kinetics experiments (pH = 5), possible interactions may include electrostatic interaction between PFOA anion and cationic sites on MOFs, hydrogen bond between PFOA anions and H2O molecules coordinated to the Cr metal center, direct adsorption of PFOA anion onto Cr CUS, and π-PFOA anion interaction on the bdc ligand. The adsorption mechanism is discussed in detail in the Computational Results section. Sorption Isotherms. The sorption isotherms of PFOA on MIL-101(Cr) and its derivatives are shown in Figure 4. Both Langmuir and Freundlich models were used to model the experimental data; the results are presented in Table 2. High R2 values suggest that these plots exhibit classical Langmuir-type isotherms. The maximum sorption capacities (qm) of PFOA follow the order MIL-101(Cr)-QDMEN > MIL-101(Cr)DMEN > MIL-101(Cr)-NMe3 > MIL-101(Cr) > MILD


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adsorbed per surface area on anionic-exchange MOFs suggests that adsorbent surface area is not the dominant factor governing PFOA sorption on MOFs. Further examination of adsorption mechanism is necessary to identify the electronic and steric effects exerted by the added functional groups. To our best knowledge, the PFOA adsorption capacity of quaternized MIL-101(Cr) is among the highest of known porous adsorbents. For example, the adsorption capacity of PFOA on MIL-101(Cr)-QDMEN (1.82 mmol g−1) is nearly triple that of PAC (0.67 mmol g−1) under similar conditions.29 This difference indicates that large surface areas and pore volumes contribute to high adsorption capacities in porous materials. However, the PFOA adsorption capacity of MIL101(Cr) and its derivatives is lower than those of some nonporous aminated materials such as anionic-exchange resin AI400, which exhibits a PFOA adsorption capacity of 2.92 mmol g−1.29 Although the PFOA concentrations used throughout the sorption experiments were maintained well below the critical micelle concentration (cmc), PFOA molecules are known to agglomerate into hemimicelles at concentrations 0.001−0.01 times the cmc.31 Therefore, the PFOA hemimicelles formed inside the MOF nanopores are reasonably deduced to have denied further access to additional PFOA molecules, thereby leading to a lower sorption capacity compared to those of the aforementioned nonporous anionic exchanger adsorbents. Nevertheless, both sorption kinetics and capacities are critical for practical applications. Regeneration of Adsorbents. Industrial application often requires regeneration of spent adsorbent. In this work, we have selected MIL-101(Cr)-QDMEN, the anionic-exchange MOF expressing the highest PFOA adsorption capacity, to evaluate its regeneration potential. Our result indicates approximately 90% adsorption performance can be retained when reused for the third time (Supporting Information Figure S8). IR analysis clearly shows the NH and CH vibration bands (2500−3000 cm−1) in MIL-101(Cr)-QDMEN sample after the fourth adsorption cycle (Supporting Information Figure S9 ). This suggests the attached quaternary amine moiety is stable during repeated regeneration process. Effects of pH on Adsorption. The effects of pH on the adsorption of PFOA onto pristine, aminated, and quaternaryammonium-modified MIL-101(Cr)s were studied to investigate the contribution of electrostatic interaction vs nonelectrostatic interaction. MOFs (5.0 mg) were added to a PFOA solution (50 mL at 100 ppm) containing NaH2PO4 buffer (2 mM). The mixture was agitated on an orbital shaker at 150 PRM for 5 h. Solution samples were extracted for analysis of the PFOA concentration. The initial and final solution pH values recorded indicated that the solution pH change was negligible. Figure 5 shows that the sorption of PFOA onto pristine and functionalized MIL-101(Cr)s was significantly affected by the solution pH. Recently, Goss reported that the correct pKa of PFOA is approximately −0.5,32 suggesting that PFOA exists in predominately deprotonated form under the tested solution pH (pH 3−9). Therefore, PFOA adsorption may be affected by the surface charge of MOF at different solution pH values. Zeta potentials of MOFs were tested in the absence (Figure 6) and in the presence of NaH2PO4 buffer (2 mM) (Supporting Information Figure S10). In general, zeta potentials of quaternized and aminated MOFs were higher than that of pristine MIL-101(Cr), which accounts for the enhanced adsorption of anionic PFOA. However, the zeta potentials decreased significantly in the presence of sodium as a

Figure 4. Sorption isotherms of PFOA on MIL-101(Cr) (▲), MIL101(Cr)-NH2 (■), MIL-101(Cr)-NMe3 (□), MIL-101(Cr)-DMEN (●), and MIL-101(Cr)-QDMEN (○). Results fitted using both Langmuir and Freundlich models.

Table 2. Isotherm Parameters for the Adsorption of PFOA on MOFs Langmuir constants MOFs MIL101(Cr) MIL101(Cr)NH2 MIL101(Cr)NMe3 MIL101(Cr)DMEN MIL101(Cr)QDMEN

Freundlich constants

qm (mmol g−1)

b (L mmol−1)

R2

K

n−1

R2

1.11

8.08

0.982

1.26

0.41

0.968

0.70

3.90

0.978

0.63

0.47

0.939

1.19

20.27

0.935

1.24

0.16

0.720

1.29

7.66

0.979

1.53

0.44

0.954

1.82

9.41

0.977

2.01

0.39

0.918

101(Cr)-NH2. The enhanced adsorption capacity of PSMprepared MIL-101(Cr)-DMEN (1.29 mmol g−1) over MIL101(Cr) (1.11 mmol g−1) is attributed to the additional electrostatic interactions between the added amine moiety and PFOA. However, PAM-prepared aminated MOF (MIL101(Cr)-NH2) exhibited an adsorption capacity (0.70 mmol g−1) lower than that of MIL-101(Cr). This phenomenon is explained by the steric hindrance of the aromatic amine situated on the aperture of the nanopore. Regarding the entrance of PFOA, its micelles or hemimicelle forms are obstructed from entering the nanopores because of the narrower apertures. Therefore, for adsorptive removal of OCs, PSM is a better choice than PAM for MOF functionalization. In addition, both quaternized MOFs, MIL-101(Cr)-NMe3 and MIL-101(Cr)QDMEN, which are assisted by the additional ion-exchange sites, exhibit adsorption capacities (1.19 and 1.82 mmol g−1) significantly greater than those of aminated MOFs. The improvement in the adsorption capacity of MIL-101(Cr)NMe3 over MIL-101(Cr)-NH2 is explained by the increase in van der Waals radii for quaternary amine groups; i.e., more PFOA molecules are allowed to interact with the quaternary amines at the aperture. On the other hand, the introduction of functional groups to MIL-101(Cr) is accompanied by the reduction of BET surface area (Supporting Information Table S1). To exclude the surface area effect on adsorption capacity, the adsorption isotherms of MOFs were normalized to the corresponding BET surface area (Supporting Information Figure S7), and reversed order of adsorption performance is observed for anionicexchange MOFs. The great differences in the amount of PFOA E


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Table 3. Calculated Relative Binding Energies of MOFs Adsorption Complexes (ACs)

Figure 5. Effect of equilibrium solution pH on the adsorption of PFOA on MIL-101(Cr) (▲), MIL-101(Cr)-NH2 (■), MIL-101(Cr)NMe3 (□), MIL-101(Cr)-DMEN (●), and MIL-101(Cr)-QDMEN (○).

Figure 6. Zeta potentials of MIL-101(Cr) (▲), MIL-101(Cr)-NH2 (■), MIL-101(Cr)-NMe3 (□), MIL-101(Cr)-DMEN (●), and MIL101(Cr)-QDMEN (○).

counterion. For example, the point of zero charge (PZC) shifted from 9.2 to 5.0 and from 10 to 5.7 in the presence of sodium in the cases of pristine MIL-101(Cr) and MIL101(Cr)-QDMEN, respectively. Because approximately 75% of the PFOA adsorption capacity was retained for MIL-101(Cr) at the PZC in the presence of sodium, nonelectrostatic interaction may be the dominant force. This conclusion is strikingly different from the adsorption mechanism reported for the adsorption of organic dyes onto MIL-101(Cr) and PFCs onto AC and anionic-exchange resins, where electrostatic interaction was the major force. Similar results were observed for quaternary ammonium MOF (MIL-101(Cr)-QDMEN), which suggests that an alternative approach is needed to elucidate the predominant adsorption mechanism of PFOA onto MIL-101(Cr)-based MOFs.

scenario (@adsorption sites)

relative total energy

relative binding energy

MIL-101(Cr) MIL-101(Cr)′ PFOA H2O AC1 (@Cr) AC2 (@Cr) AC3 (@coordinated H2O, bdc) AC4 (@coordinated H2O) AC5 (@coordinated H2O) AC6 (@coordinated H2O) AC7 (@coordinated H2O, bdc) AC8 (@bdc) AC9 (@coordinated H2O, bdc) AC10 (@bdc) protonated MIL-101(Cr) protonated MIL-101(Cr)′ AC11 (@Cr) AC12 (@Cr) AC13 (@coordinated H2O) AC14 (@bdc) AC15 (@H+) AC16 (@H+) MIL-101(Cr)-NH3 MIL-101(Cr)-NH3′ AC17 (@N cation) AC18 (@N cation) MIL-101(Cr)-NMe3 MIL-101(Cr)-NMe3′ AC19 (@ N cation) AC20 (@ N cation) MIL-101(Cr)-DMEN AC21 (@N cation) AC22 (@N cation) MIL-101(Cr)-QDMEN AC23 (@N cation) AC24 (@N cation)

−3777.763 −3701.274 −1953.465 −76.450 −5654.796 −5654.803 −1953.465 −5731.278 −5731.266 −5731.263 −5731.268 −5731.266 −5731.249 −5731.252 −3778.150 −3701.658 −5655.187 −5655.213 −5731.652 −5731.639 −5731.693 −5731.697 −3785.479 −3708.977 −5738.994 −5738.990 −3903.396 −3826.902 −5856.904 −5856.912 −3971.004 −5924.522 −5924.518 −4010.318 −5963.814 −5963.821

− − − − −148 −167 −131 −110 −98 −92 −104 −99 −55 −63 − − −167 −236 −97 −61 −203 −214 − − −129 −120 − − −113 −133 − −139 −127 − −80 −99

3.638 Å, respectively). In addition to direct coordination, PFOA hydrogen bonds to Cr-coordinated water molecules in parallel (AC4), cross (AC5), and vertical (AC6) orientations, all of which produce similar relative binding energies (−100, −98, and −92, respectively) that are approximately 33% weaker than that between PFOA and Cr CUS. However, because of the high binding affinity between Cr CUS and PFOA over Crcoordinated H2O and PFOA, rapid ligand exchange between PFOA and H2O can be expected. For PFOA adsorption onto the bdc ligand, vertical attack of the PFOA carboxyl termini (AC8) is the favored position. The relative adsorption energy produced from this scenario (−99) is comparable to that of hydrogen bonding between PFOA and coordinated water. This result indicates that π-anion interaction between OCs and MIL101 was greatly underestimated in previous studies. Notably, scenarios involving collective interactions are favored over ones with a singular interaction. For example, in scenario AC3, the relative adsorption energy is −131, which is comparable to that of the most favored mechanisms (AC2). Interatomic distances indicate that two mechanisms apply: (1) π-CF interactions between bdc and the CF chain of PFOA

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COMPUTATIONAL RESULTS PFOA Adsorption onto Pristine MIL-101. To investigate the possible adsorption mechanisms of PFOA onto MIL101(Cr), 10 possible scenarios (Supporting Information Figure S11) were selected to reflect all possible mechanisms, including hydrogen bonding between PFOA and Cr-coordinated water molecules, π-anion interaction between the phenyl rings of the bdc ligand and PFOA, and direct coordination between PFOA and Lewis acid Cr CUS (upon removal of coordinated water) to form a Lewis acid/base complex. The relative binding energies are summarized in Table 3. In general, Cr CUS is the most favored adsorption site (scenario AC2), consistent with the results of a previous report of gas-phase ibuprofen adsorption onto MIL-101(Cr), where the major adsorption site was identified as the Cr center.33 The interatomic distance measurements suggest that two oxygen atoms from the PFOA carboxyl termini chelate to the Cr CUS (dO−Cr = 2.045 and F


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is slightly higher than that of MIL-101(Cr)-NH2 (Table 3). The experimental results, however, reveal that the adsorption capacity of MIL-101(Cr)-DMEN is nearly double that of MIL101(Cr)-NH2 (Table 2). This phenomenon indicates that the adsorption performance of MIL-101(Cr)-NH2 suffers from the steric hindrance caused by the amine group present at the nanopore aperture (Figure 1c). Therefore, PSM is a more effective strategy for introducing −NH2 groups onto MOFs, which enables adsorbates such as PFOA to enter the apertures of nanopores freely. Quaternization of Amine MOFs. Experimentally, we observed that a higher order of aminespecifically, quaternized amineexhibited favorable adsorption characteristics in aqueous solutions for both MIL-101(Cr)-NMe3 and MIL101(Cr)-QDMEN (Table 2). Given that we observed that the sterically hindered MIL-101(Cr)-NH2 significantly lowering the PFOA adsorption capacity, our observation of increased adsorption capacity for the even more sterically demanding MIL-101(Cr)-NMe3 appears counterintuitive. DFT calculations were performed on the aforementioned quaternized MOFs to investigate the relative PFOA binding energy of the respective quaternized amine. To our surprise, PFOA attacking from cross and vertical positions of the quaternized amine in MIL101(Cr)-NMe3 has a higher relative binding energy than PFOA attacking from MIL-101(Cr)-QDMEN (Table 3). Such a contradiction in the experimental results (Table 2) is attributed to the increased van der Waals radii in quaternized amines: because PFOA is hydrophobic and tends to form micelles and hemimicelles,29 more PFOA molecules interact with quaternized amines compared to primary and secondary amines. However, because the relative binding energy is reduced, the influences of other mechanisms are more pronounced, evidently as a consequence of the strong pH effect observed for both quaternized MOFs in the presence of sodium as a counterion (Figure 5). In summary, we have shown that quaternary-amine-functionalized MIL-101(Cr)s are highly effective for the adsorptive removal of aqueous PFOA. Fast adsorption rates and high adsorption capacities have been demonstrated. The effects of different functionalization sites have been evaluated experimentally on isostructural MOFs prepared via PAM and PSM. Our experimental results suggest that because PAM produces functional groups at the aperture that hinders the entrance of OCs into the nanopores, PSM is a suitable method for introducing functional groups onto MOFs for the removal of aqueous OCs. We have elucidated the adsorption mechanism of pristine and functionalized MIL-101(Cr) via DFT calculations. Our DFT calculation results indicate that the primary PFOA adsorption site on pristine MIL-101(Cr) is the Cr(III) metal center. For protonated MIL-101(Cr), however, the mechanism of electrostatic interaction between the PFOA and the protonated carboxyl groups of the bdc linker, plus the Lewis acid/base complex between PFOA and Cr(III), is collectively the dominant adsorption mechanism.

(dF−H = 3.03, 4.56, 4.46, and 4.01 Å) and (ii) hydrogen bonding between the PFOA carboxyl termini and coordinated water (dO−H = 1.59). In contrast, excessive collective interactions exerted on PFOA can cause structural distortion of the long CF skeletal chain, thus weakening the binding energy. For example, scenario AC9 is similar to AC3, with one additional adsorption site: (1) hydrogen bonding between the PFOA carboxyl termini and coordinated water (dO−H = 3.18), (2) π-CF interactions between bdc and the CF chain of PFOA (dF−H = 3.12, 4.39, 3.39, and 3.08 Å), and (3) hydrogen bonding between the PFOA’s CF3 termini and coordinated water (dO−H = 3.53). The resulting relative binding energy of AC9 (−55) is only approximately 40% of that of AC3. Protonation of MIL-101(Cr). Based on the results of zetapotential measurements (Figure 6), MIL-101(Cr) is surrounded by a layer of positive charges at pH < 5, which is close to the pH where the aqueous absorption experiments were carried out. Therefore, an investigation of the effect of protonation on the intermolecular interactions between PFOA and MIL-101 is necessary. In addition to the previously discussed adsorption mechanisms, protonated MIL-101 has an extra adsorption site on the protonated carboxyl group of the bdc ligand. The most stable configurations for protonated MIL101 were identified on the basis of the results of the DFT study (Figure 2b), where six configurations covering all possible adsorption sites were investigated (Supporting Information Figure S12). The relative PFOA adsorption energies for protonated MIL-101 are listed in Table 3. Because of the close proximity of the added proton to the Cr CUS, collective interaction between the two is favored. Scenario AC12, the most favored AC conformation with PFOA attacking from parallel positions, involves the following: (1) electrostatic interaction between the PFOA’s carboxyl termini and protonated carboxyl group (dO−H = 0.99) and (2) direct coordination of the same PFOA termini to the Cr CUS (dO−Cr = 4.01, 2.30). Compared to scenario AC2, where PFOA attacks pristine MIL-101(Cr) in a parallel manner, the relative binding energy increases to −236an increase of 41%. This amplification can be explained by the electron-withdrawing effect of the additional proton: the Lewis acid Cr CUS becomes more electron deficient compared to the protonated bdc and therefore forms a stronger complex with the Lewis base PFOA. The essential role played by H+ in the adsorption of PFOA leads to the interpretation of the pH-dependence of adsorption capacities (Figure 5) as follows: the amount of H+ surrounding the MIL-101 particles decreases with increasing pH, as indicated by the zeta potential measurement at the presence of phosphate buffer (Supporting Information Figure S10). In contrast, the relative binding energy of PFOA to bdc- or Crcoordinated H2O remains unaffected compared to the pristine MIL-101(Cr). Adding Amine Functional Groups. When amine groups were added onto bdc via PAM and -DMEN onto Cr CUS of the pristine MIL-101(Cr) via PSM, amine-functionalized MOFs, MIL-101(Cr)-NH2 and MIL-101(Cr)-DMEN, were obtained, respectively. Because amine groups are protonated in the aqueous solution, additional adsorption sites are available because PFOA preferentially attacks cationic groups. To compare the PFOA binding energy between the two amine groups, the two most favorable scenarios for each protonated amine MOFs, which involve the parallel and cross attack of PFOA, were analyzed (Supporting Information Figure S13). In both cases, the relative binding energy of MIL-101(Cr)-DMEN

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ASSOCIATED CONTENT

S Supporting Information *

N2 physisorption results, PXRD patterns, IR spectra, highresolution XPS spectra, SEM images of the MOFs, zetapotential of MOFs at the presence of buffer, regeneration results, and configurations of optimized PFOA/MOFs adsorption complex used for DFT calculation. The Supporting G


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Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b00802.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 6278 7137. Fax: +86 10 6279 4006. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the THU−VEOLIA Joint Research Center for Advanced Environmental Technology, the National High Technology Research and Development Program of China (2013AA06A305), and the Program for Changjiang Scholars and Innovative Research Team in University. We thank Dr. Michael Hoffmann from California Institute of Technology for providing valuable insight into PFCs adsorption mechanism, Dr. Li (Luke) Ke from University of Georgia for assistance on DFT calculation, and Dr. Oliver Hao from University of Maryland for providing general guidance in writing and reviewing the manuscript.

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Understanding the Adsorption of PFOA on MIL-101 (Cr)-Based

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