understanding the adsorption of pfoa on mil-101(cr)-based

Understanding the Adsorption of PFOA on MIL-101(Cr)-BasedAnionic-Exchange Metal−Organic Frameworks: Comparing DFTCalculations with Aqueous Sorption ExperimentsKai 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 ofEnvironment Simulation and Pollution Control (SKLESPC), Tsinghua University, Beijing 100084, China

*S Supporting Information

ABSTRACT: To examine the effects of different functionaliza-tion methods on adsorption behavior, anionic-exchange MIL-101(Cr) metal−organic frameworks (MOFs) were synthesizedusing preassembled modification (PAM) and postsyntheticmodification (PSM) methods. Perfluorooctanoic acid (PFOA)adsorption results indicated that the maximum PFOAadsorption capacity was 1.19 and 1.89 mmol g−1 for anionic-exchange MIL-101(Cr) prepared by PAM and PSM,respectively. The sorption equilibrium was rapidly reachedwithin 60 min. Our results indicated that PSM is a bettermodification technique for introducing functional groups ontoMOFs for adsorptive removal because PAM places functionalgroups onto the aperture of the nanopore, which hinders theentrance of organic contaminants. Our experimental results and the results of complementary density functional theorycalculations revealed that in addition to the anion-exchange mechanism, the major PFOA adsorption mechanism is acombination of Lewis acid/base complexation between PFOA and Cr(III) and electrostatic interaction between PFOA and theprotonated carboxyl groups of the bdc (terephthalic acid) linker.

■ INTRODUCTIONMany contaminants of emerging concern exist in ionic form inthe aqueous environment. Among them, perfluorinatedcompounds (PFCs) predominately exist in anionic formbecause they contain acidic functional groups such ascarboxylates and sulfonates. They are ubiquitously detected1,2

and environmentally persistent.3 In general, PFC removalmethods include adsorption,4 separation,5 and degradation.6,7

Benefitted by its high surface area, activated carbon (AC) hasbeen demonstrated to be a cost-effective material for theadsorptive removal of PFCs in a laboratory setting.8 However,the adsorption capacity of PFCs on AC is low, and existingwater treatment plants that use AC treatments are unable toremove influent PFCs.9,10 Recently, an ion-exchange resin hasbeen demonstrated to have a high sorption capacity, albeit atmuch lower rate than AC. Therefore, the development of newadsorption materials that combine both high surface area andversatile functionality is needed for the fast and efficientremoval of aqueous PFCs.Metal−organic frameworks (MOFs) are a class of inorganic−

organic hybrid materials that have received considerableattention for environmental remediation purposes. Theincreasing interest in MOFs is derived primarily from theirextensive porosity and ease of modification.11 Here, we focuson the adsorption of a single archetypical PFC, perfluor-ooctanoic acid (PFOA), onto a series of functionalized MIL-

101(Cr) (MIL: Materials of Institute Lavoisier). MIL-101(Cr)belongs to a class of chromium(III) terephthalate MOFsdiscovered by Ferey et al. in 2005. It possesses an extremelyhigh specific surface area (SBET ≈ 4000 m2 g−1), and guestmolecules can access its quasi-spherical cages (Ø ≈ 2.9 and 3.4nm) via 1.2 and 1.6 nm apertures (Figure 1a).12 Benefitingfrom acid and base resistance and high thermal (up to 320 °C)and aqueous stability, MIL-101(Cr) can be used as a referenceMOF for studying various guest−host interactions for aqueousorganic contaminant removal. Thus, far, most studies in thisfield have investigated dye removal using pristineMOFs,12−14,16 with the enhanced adsorption capacity andkinetics of MOFs relative to those of ACs having been clearlydemonstrated in these studies.Regarding gas adsorption onto MOFs, the functionalization

of MOFs is a promising approach in tuning guest−hostinteractions.13 This tuning is achieved either via synthesis ofMOFs using a functionalized organic linker (preassembledmethod, PAM) or via a postsynthetic method (PSM) (Figure1b). Both methods impart distinctive steric and electronicproperties to the material and hence affect its adsorption

Received: February 12, 2015Revised: May 16, 2015Accepted: June 11, 2015

Article

pubs.acs.org/est

© XXXX American Chemical Society A DOI: 10.1021/acs.est.5b00802Environ. Sci. Technol. XXXX, XXX, XXX−XXX

pubs.acs.org/est

http://dx.doi.org/10.1021/acs.est.5b00802

behavior. Very recently, amine-grafted MOFs prepared via PSMwere shown to improve dye and pharmaceutical and personalcare products adsorption;14,15 unfortunately, the effects exertedby different functionalization methods on MOF adsorptionbehavior have not been compared. Furthermore, despitegrowing efforts in the research domain of aqueous OC (organiccompounds) adsorption using MOFs, the mechanism, which isdifficult to elucidate on the basis of experimental data alone, isnot well understood. In recent years, density functional theory(DFT) calculations have been used to better understand theadsorption mechanism of gases.16,17 However, the aqueousadsorption of OC onto MOFs has not yet been investigated byDFT calculations.Taking the aforementioned factors into consideration, we

herein describe our attempt to design and fabricate anionic-exchanger MOFs based on MIL-101(Cr) functionalized byboth PAM and PSM approaches (Figures 1c, 1d), where theanionic-exchange performance is enhanced by an additionalquaternized amine functional group. The steric and electroniceffects of the aforementioned functionalization methods on theadsorption performance of MIL-101(Cr) are experimentallyexamined using PFOA as a model compound. We haveobserved a remarkable enhancement in adsorption capacity thatis comparable to that of ion-exchange resins. Additionally, theanionic-exchanger MOFs exhibited adsorption rates that aresignificantly faster than those of ACs. In addition, we performedquantum chemical calculations based on DFT to complementour experimental effort, not only to rationalize the difference inadsorption performance but also to interpret the adsorptionmechanisms of both pristine and functionalized MOFs. Ourresults show that the high adsorption capacity and ultrafastadsorption rates demonstrated by ion-exchange MOFs renderthem promising materials for aqueous anionic contaminantremoval. Our mechanistic study provides valuable insights intothe development of efficient MOF-orientated adsorbents forenvironmental remediation.

■ MATERIALS AND METHODS

Preparation of Sorbents. MIL-101 was synthesizedaccording to published HF-free hydrothermal methods.18,19

Briefly, bdc (0.82 g) and Cr(NO3)3·9H2O (2.0 g) were addedto 24 mL of Milli-Q water. The resultant suspension wassonicated for 30 min at room temperature before being heatedto 218 °C for 18 h in a 100 mL Teflon-lined autoclave. Aftercooling to room temperature, the crude product was collectedby centrifugation at 8000 rpm for 10 min. The crude productwas washed with dimethylformamide at 100 °C in an autoclavefor 12 h and then washed with ethanol at 100 °C in anautoclave for 12 h. The resulting material was dried in a vacuumoven 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 inanhydrous toluene (30 mL). The mixture was stirred underreflux overnight. The crude was dried in vacuo to removetoluene and was subsequently washed with Milli-Q water andethanol at 100 °C in an autoclave for 12 h. The resultantmaterial was dried in a vacuum oven at ∼100 °C for 12 h toyield MIL-101-DMEN.MIL-101-QDMEN was synthesized by reacting a suspension

of MIL-101-DMEN (0.5 g) in 6 mL of dry dichloromethanewith methyl triflate (1.5 mmol). The mixture was stirred atroom temperature for 12 h. The crude product was dried invacuo to remove dichloromethane and unreacted methyl triflateand subsequently washed with Milli-Q water and ethanol at 100°C in an autoclave for 12 h. The resultant material was dried ina vacuum oven at ∼100 °C for 12 h to yield MIL-101-QDMEN.MIL-101-NH2 was synthesized using published HF-free

hydrothermal methods.18,19 Briefly, 2-NH2-bdc (0.36 g) andCr(NO3)3·9H2O (0.8 g) were added to 15 mL of Milli-Q water.The resulting suspension was sonicated for 30 min at roomtemperature before being heated to 150 °C for 12 h in a 50 mLTeflon-lined autoclave. After cooling to room temperature, thecrude product was collected by centrifugation at 8000 rpm for10 min and then washed with Milli-Q water and ethanol at 100°C in an autoclave for 12 h. The resulting material was dried ina 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 mLof dry dichloromethane. The mixture was stirred at roomtemperature for 12 h. The crude product was dried in vacuo toremove dichloromethane and unreacted methyl triflate andsubsequently washed with Milli-Q water and ethanol at 100 °Cin an autoclave for 12 h. The resulting material was dried in avacuum oven at ∼100 °C for 12 h.Both quaternized MOFs (MIL-101-QDMEN and MIL-101-

NMe3) were acidified with 0.1 M HCl for 12 h at roomtemperature to yield the corresponding MOF anion exchangers.Acidified MOFs were collected by centrifuge at 8000 rpm for10 min and then washed repeatedly with Milli-Q water untilneutral pH. Corresponding MOF anion exchangers in Cl− formwere subsequently obtained by drying the MOFs in a vacuumoven at ∼100 °C for 12 h. All MOFs were stored in a vacuumdesiccator in the dark prior to use.

Characterization. Powder X-ray diffraction (PXRD)patterns were collected using a D/MAX-RB (Rigaku) X-raydiffractometer equipped with a Cu−Kα radiation source to

Figure 1. Perspective view of (a) the quasi-spherical cage of MIL-101(Cr); (b) CUS used for MIL-101(Cr) functionalization using PSMand scheme that represents the surface functionalization of theproposed 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 areshaded in green, white, and gray, respectively. Quaternary ammoniummoieties are shaded in red.

Environmental Science & Technology Article

DOI: 10.1021/acs.est.5b00802Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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confirm the MOF structure. Surface morphology wascharacterized by scanning electron microscopy (SEM) using aHitachi S-4500. The presence of functional groups wasconfirmed by Fourier-transform infrared (FT-IR) spectrarecorded on a Thermo Nicolet Nexus 870 FT-IR spectrometersystem using samples pelletized with KBr. The composition ofMOFs was determined using X-ray photoelectron spectrometry(XPS) (Thermo Scientific ESCALAB 250Xi equipped with anAl−Kα X-ray radiation source). The binding energy wascalibrated using the C 1s peak at 284.8 eV. BET surface areaswere determined using nitrogen adsorption and desorptionisotherms measured using a gas adsorption instrument(Autosorb iQ, Quantachrome, U.S.). MOFs were prepared bypurging 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 waterat the desired pH values under ultrasonication; 2 mMNaH2PO4 was added as a pH buffer.Sorption Experiments. All sorption experiments (except

for those investigating kinetics) were carried out using anorbital shaker at 150 rpm and at room temperature. Adsorbent(4 mg) was added to Nalgene 50 mL round-bottompolypropylene centrifuge tubes (Thermo Scientific) loadedwith 40 mL of PFOA solution and 2 mM NaH2PO4 as a pHbuffer. The initial solution pH was adjusted to 5.0 by adding 1M NaOH or 1 M HCl. The pH was adjusted prior the additionof adsorbent, and no further adjustment was made. A 1 mLsolution sample was collected at a predetermined time. Aduration of 12 h was employed for the experiments. Sorptionkinetic experiments were carried out with a magnetic stirrer at150 rpm and at room temperature. Adsorbent (20 mg) wasadded to 250 mL polypropylene flasks containing 200 mL ofPFOA solution with an initial pH adjusted to 5.0. The effect ofpH on sorption was investigated by conducting sorptionexperiments at various initial pH values in the range of 3.0−10.0, where the pH values were adjusted by adding 1 M NaOHor HCl solution. The final pH was recorded after the sorptionexperiments were completed. The ionic strength of addedNaOH or HCl was negligible. All isotherm experiments wereconducted in triplicate, kinetic and pH effect experiments wereperformed 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 hadsorption, spent adsorbents were recovered by filtration with0.22 μm nylon membrane and placed in 40 mL regenerationsolution containing 1% NaCl/methanol (30/70, v/v), followedby overnight shaking on orbital shaker at 150 rpm at roomtemperature. The regenerated adsorbents were filtered by 0.22μm nylon membrane and dried in a vacuum oven at ∼100 °Cfor 5 h before the next adsorption run. Duplicates wereperformed, and average value was reported.PFOA Analysis. One milliliter of solution sample was

filtered through a 0.22 μm nylon syringe filter. Negligiblesorption of PFOA onto nylon filters was determined in acontrol experiment. PFOA concentrations were determinedusing a Shimadzu (Japan) LC-20A HPLC fitted with a CDD-10Avp conductivity detector. Sample volumes of 20 μL wereinjected into the HPLC system. A TC-C18 column (4.6 × 250mm2; 5 μm) from Agilent Technologies (U.S.) was used, andmethanol/0.03 M NaH2PO4 (75/25) was used as mobile phaseat a flow rate of 0.8 mL min−1. Linear calibration curves (R2 >0.99) were obtained for all experiments. The amount of PFOA

absorbed onto MOFs was calculated on the basis of thedifference in solution concentration before and after sorption.

Computational Methods. The DFT calculations of PFOAadsorption on MOFs used in this study were focused primarilyon MIL-101(Cr). Because the unit-cell structure of MIL-101reported by Ferey et al.20 was too large to allow all possiblePFOA adsorption configurations to be evaluated, a clustermodel composed of two Cr-trimers linked by a single bdcligand (Figure 2a) was employed to save computational

resources. Cleaved bonds on Cr-trimers were saturated by Hatoms to maintain the original hybridization. The adaptation ofthe current cluster model allows for the investigation of allpotential PFOA adsorption sites, including CUS, coordinatedH2O, bdc, and cationic sites, which reflect the knownadsorption mechanisms of MOFs.22 Similar cluster modelshave been shown to adequately describe interactions betweenCO2 and MOFs.17 Because the isotherm adsorption experi-ments were conducted in aqueous solutions at pH 5, which isclose to the isoelectric point of MIL-101(Cr) in bufferedsolution (Supporting Information Figure S7), an additionalcluster model was constructed to reflect the most possibleconfiguration of protonated MIL-101 (Figure 2b). Acomparison of the MIL-101(Cr) model and its protonatedforms helps rationalize the PFOA adsorption mechanism atdifferent solution pH values. The effect of amine moieties onPFOA adsorption was investigated using cluster models ofamine-functionalized MIL-101(Cr)s (Figures 2c, 2e) and theirquaternary amine anion-exchanger forms (Figures 2d, 2f). Ithas to be noted that although the cluster model approach issuccessful in probing adsorption sites on MOFs,17,21 the bulkbehavior of MOFs remains the same using supercell approachemploying a complete unit cell, in which the abrupt terminationof the periodic structure of MOFs is avoided.All DFT calculations were performed using the DMol3

code.23 Spin-polarized generalized gradient approximationwith the Perdew−Wang 199124 exchange-correlation functionalwas used in the calculations. The double numeric polarization20

basis set was used for describing atomic orbitals. Because of thepresence of transition metals, the DFT semicore pseudopotsapproximation25 was utilized. A real-space orbital global cutoffof 4.4 Å was applied. The convergence thresholds foroptimization were 10−5 (energy), 2 × 10−3 (gradient), and 5

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



C


× 10−3 (displacement). The calculations were performed in theconductor-like screening model (COSMO)26 with a dielectricconstant of 78.54 for simulating solvent effects.Geometry optimization of the cluster models and their

PFOA-adsorbed forms was achieved by reaching a minimalpotential surface using the aforementioned methods. Bindingenergies (Ebinding) between PFOA and various adsorption siteson geometrically optimized MOFs were calculated according toeqs 1 and 2

= − ′ −E E E EFor Cr sites: (AC) (A ) (PFOA)binding (1)

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

(2)

where A′ represents an adsorbent model with one CUS. Thetotal energies (E) of MOF adsorbents (A), PFOA, andadsorption complexes (AC) were calculated using methodsconsistent with geometry optimization. For each adsorptionsite, two or more orientations of PFOA were considered toevaluate different orientations of PFOA attack.

■ RESULTS AND DISCUSSIONStructural Characteristics. PXRD analysis was conducted

to confirm the identities of the MOFs. PXRD patterns of all ofthe MOFs, including those of MIL-101(Cr)-QDMEN andMIL-101(Cr)-NMe3, are consistent with the patterns ofcrystalline MIL-101(Cr) reported in the literature (SupportingInformation Figure S1). This similarity suggests that quaterni-zation 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 presentedin Supporting Information Figures S2 and S3. The asym-metrical 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 NHand CH vibration bands (2500−3000 cm−1) in MIL-101(Cr)-DMEN and MIL-101(Cr)-QDMEN indicate the presence ofDMEN and quaternized DMEN, respectively (SupportingInformation Figure S3). Further analysis of the chemicalcompositions of MOFs is summarized in SupportingInformation Figures S4 and S5. The binding energies of400.1 and 401.9 eV were assigned to PhNMe3 and RNMe3 inMIL-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 BETsurface areas are clearly demonstrated (Supporting InformationTable 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 MIL-101(Cr)-NH2 and MIL-101(Cr)-NMe3, respectively (Support-ing Information Figure S6). Small nanoparticles (∼100 nm)observed for MIL-101(Cr)-NH2 and MIL-101(Cr)-NMe3 areconsistent with the broad Bragg reflections observed in thePXRD spectra (Supporting Information Figure S1).Sorption Kinetics. The sorption kinetics of PFOA on

tested MOFs are shown in Figure 3. In general, sorptionequilibrium is reached within 60 min for all MOFs tested. Incomparison to other porous adsorbents (PAC and GAC) testedunder similar conditions,27,28 substantially enhanced sorptionrates were clearly observed. These enhanced sorption ratesimply that the uniform pores and cavities of MOFs facilitate theinternal diffusion of PFOA inside their nanopores. Comparing

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

A pseudo-second-order model is commonly used to describesorption kinetics in which chemical sorption controls thesorption rate and in which the number of active sites on thesorbent determines the sorption capacity;30 high correlationcoefficients (R2) imply possible chemical interactions betweenPFOA and MOFs. Given that PFOA predominately exists in itsanionic form under the conditions used in the kineticsexperiments (pH = 5), possible interactions may includeelectrostatic interaction between PFOA anion and cationic siteson MOFs, hydrogen bond between PFOA anions and H2Omolecules coordinated to the Cr metal center, direct adsorptionof PFOA anion onto Cr CUS, and π-PFOA anion interactionon the bdc ligand. The adsorption mechanism is discussed indetail in the Computational Results section.

Sorption Isotherms. The sorption isotherms of PFOA onMIL-101(Cr) and its derivatives are shown in Figure 4. BothLangmuir and Freundlich models were used to model theexperimental data; the results are presented in Table 2. High R2

values suggest that these plots exhibit classical Langmuir-typeisotherms. The maximum sorption capacities (qm) of PFOAfollow the order MIL-101(Cr)-QDMEN > MIL-101(Cr)-DMEN > MIL-101(Cr)-NMe3 > MIL-101(Cr) > MIL-

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

Table 1. Kinetic Parameters of the Pseudo-Second-OrderModel for PFOA Adsorption on MOF Adsorbents

pseudo-second-order parameters

adsorbentqe

(mmol g−1)ν0

(mmol−1 h−1)κ2

(g mmol−1 h−1) R2

MIL-101(Cr) 1.16 4.58 3.40 0.967MIL-101(Cr)-NH2

0.90 3.00 3.70 0.995

MIL-101(Cr)-NMe3

1.21 5.94 3.93 0.965

MIL-101(Cr)-DMEN

1.43 5.62 2.75 0.990

MIL-101(Cr)-QDMEN

1.91 19.33 5.30 0.985



D


101(Cr)-NH2. The enhanced adsorption capacity of PSM-prepared MIL-101(Cr)-DMEN (1.29 mmol g−1) over MIL-101(Cr) (1.11 mmol g−1) is attributed to the additionalelectrostatic interactions between the added amine moiety andPFOA. However, PAM-prepared aminated MOF (MIL-101(Cr)-NH2) exhibited an adsorption capacity (0.70 mmolg−1) lower than that of MIL-101(Cr). This phenomenon isexplained by the steric hindrance of the aromatic amine situatedon the aperture of the nanopore. Regarding the entrance ofPFOA, its micelles or hemimicelle forms are obstructed fromentering the nanopores because of the narrower apertures.Therefore, for adsorptive removal of OCs, PSM is a betterchoice than PAM for MOF functionalization. In addition, bothquaternized MOFs, MIL-101(Cr)-NMe3 and MIL-101(Cr)-QDMEN, which are assisted by the additional ion-exchangesites, exhibit adsorption capacities (1.19 and 1.82 mmol g−1)significantly greater than those of aminated MOFs. Theimprovement in the adsorption capacity of MIL-101(Cr)-NMe3 over MIL-101(Cr)-NH2 is explained by the increase invan der Waals radii for quaternary amine groups; i.e., morePFOA molecules are allowed to interact with the quaternaryamines at the aperture.On the other hand, the introduction of functional groups to

MIL-101(Cr) is accompanied by the reduction of BET surfacearea (Supporting Information Table S1). To exclude thesurface area effect on adsorption capacity, the adsorptionisotherms of MOFs were normalized to the corresponding BETsurface area (Supporting Information Figure S7), and reversedorder of adsorption performance is observed for anionic-exchange MOFs. The great differences in the amount of PFOA

adsorbed per surface area on anionic-exchange MOFs suggeststhat adsorbent surface area is not the dominant factorgoverning PFOA sorption on MOFs. Further examination ofadsorption mechanism is necessary to identify the electronicand 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 knownporous adsorbents. For example, the adsorption capacity ofPFOA on MIL-101(Cr)-QDMEN (1.82 mmol g−1) is nearlytriple that of PAC (0.67 mmol g−1) under similar conditions.29

This difference indicates that large surface areas and porevolumes contribute to high adsorption capacities in porousmaterials. However, the PFOA adsorption capacity of MIL-101(Cr) and its derivatives is lower than those of somenonporous aminated materials such as anionic-exchange resinAI400, which exhibits a PFOA adsorption capacity of 2.92mmol g−1.29 Although the PFOA concentrations usedthroughout the sorption experiments were maintained wellbelow the critical micelle concentration (cmc), PFOAmolecules are known to agglomerate into hemimicelles atconcentrations 0.001−0.01 times the cmc.31 Therefore, thePFOA hemimicelles formed inside the MOF nanopores arereasonably deduced to have denied further access to additionalPFOA molecules, thereby leading to a lower sorption capacitycompared to those of the aforementioned nonporous anionicexchanger adsorbents. Nevertheless, both sorption kinetics andcapacities are critical for practical applications.

Regeneration of Adsorbents. Industrial application oftenrequires regeneration of spent adsorbent. In this work, we haveselected MIL-101(Cr)-QDMEN, the anionic-exchange MOFexpressing the highest PFOA adsorption capacity, to evaluateits regeneration potential. Our result indicates approximately90% adsorption performance can be retained when reused forthe third time (Supporting Information Figure S8). IR analysisclearly shows the NH and CH vibration bands (2500−3000cm−1) in MIL-101(Cr)-QDMEN sample after the fourthadsorption cycle (Supporting Information Figure S9 ). Thissuggests the attached quaternary amine moiety is stable duringrepeated regeneration process.

Effects of pH on Adsorption. The effects of pH on theadsorption of PFOA onto pristine, aminated, and quaternary-ammonium-modified MIL-101(Cr)s were studied to investigatethe contribution of electrostatic interaction vs nonelectrostaticinteraction. MOFs (5.0 mg) were added to a PFOA solution(50 mL at 100 ppm) containing NaH2PO4 buffer (2 mM). Themixture was agitated on an orbital shaker at 150 PRM for 5 h.Solution samples were extracted for analysis of the PFOAconcentration. The initial and final solution pH values recordedindicated that the solution pH change was negligible. Figure 5shows that the sorption of PFOA onto pristine andfunctionalized MIL-101(Cr)s was significantly affected by thesolution pH. Recently, Goss reported that the correct pKa ofPFOA is approximately −0.5,32 suggesting that PFOA exists inpredominately deprotonated form under the tested solution pH(pH 3−9). Therefore, PFOA adsorption may be affected by thesurface charge of MOF at different solution pH values. Zetapotentials of MOFs were tested in the absence (Figure 6) andin the presence of NaH2PO4 buffer (2 mM) (SupportingInformation Figure S10). In general, zeta potentials ofquaternized and aminated MOFs were higher than that ofpristine MIL-101(Cr), which accounts for the enhancedadsorption of anionic PFOA. However, the zeta potentialsdecreased significantly in the presence of sodium as a

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

Table 2. Isotherm Parameters for the Adsorption of PFOAon MOFs

Langmuir constants Freundlich constants

MOFsqm

(mmol g−1)b

(L mmol−1) R2 K n−1 R2

MIL-101(Cr)

1.11 8.08 0.982 1.26 0.41 0.968

MIL-101(Cr)-NH2

0.70 3.90 0.978 0.63 0.47 0.939

MIL-101(Cr)-NMe3

1.19 20.27 0.935 1.24 0.16 0.720

MIL-101(Cr)-DMEN

1.29 7.66 0.979 1.53 0.44 0.954

MIL-101(Cr)-QDMEN

1.82 9.41 0.977 2.01 0.39 0.918



E


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 ofsodium in the cases of pristine MIL-101(Cr) and MIL-101(Cr)-QDMEN, respectively. Because approximately 75% ofthe PFOA adsorption capacity was retained for MIL-101(Cr) atthe PZC in the presence of sodium, nonelectrostatic interactionmay be the dominant force. This conclusion is strikinglydifferent from the adsorption mechanism reported for theadsorption of organic dyes onto MIL-101(Cr) and PFCs ontoAC and anionic-exchange resins, where electrostatic interactionwas the major force. Similar results were observed forquaternary ammonium MOF (MIL-101(Cr)-QDMEN),which suggests that an alternative approach is needed toelucidate the predominant adsorption mechanism of PFOAonto MIL-101(Cr)-based MOFs.

■ COMPUTATIONAL RESULTSPFOA Adsorption onto Pristine MIL-101. To investigate

the possible adsorption mechanisms of PFOA onto MIL-101(Cr), 10 possible scenarios (Supporting Information FigureS11) were selected to reflect all possible mechanisms, includinghydrogen bonding between PFOA and Cr-coordinated watermolecules, π-anion interaction between the phenyl rings of thebdc ligand and PFOA, and direct coordination between PFOAand Lewis acid Cr CUS (upon removal of coordinated water)to form a Lewis acid/base complex. The relative bindingenergies are summarized in Table 3. In general, Cr CUS is themost favored adsorption site (scenario AC2), consistent withthe results of a previous report of gas-phase ibuprofenadsorption onto MIL-101(Cr), where the major adsorptionsite was identified as the Cr center.33 The interatomic distancemeasurements suggest that two oxygen atoms from the PFOAcarboxyl termini chelate to the Cr CUS (dO−Cr = 2.045 and

3.638 Å, respectively). In addition to direct coordination,PFOA hydrogen bonds to Cr-coordinated water molecules inparallel (AC4), cross (AC5), and vertical (AC6) orientations,all of which produce similar relative binding energies (−100,−98, and −92, respectively) that are approximately 33% weakerthan that between PFOA and Cr CUS. However, because of thehigh binding affinity between Cr CUS and PFOA over Cr-coordinated H2O and PFOA, rapid ligand exchange betweenPFOA and H2O can be expected. For PFOA adsorption ontothe bdc ligand, vertical attack of the PFOA carboxyl termini(AC8) is the favored position. The relative adsorption energyproduced from this scenario (−99) is comparable to that ofhydrogen bonding between PFOA and coordinated water. Thisresult indicates that π-anion interaction between OCs and MIL-101 was greatly underestimated in previous studies.Notably, scenarios involving collective interactions are

favored over ones with a singular interaction. For example, inscenario AC3, the relative adsorption energy is −131, which iscomparable 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

Figure 5. Effect of equilibrium solution pH on the adsorption ofPFOA 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 MIL-101(Cr)-QDMEN (○).

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

scenario (@adsorption sites)relative total

energyrelative binding

energy

MIL-101(Cr) −3777.763 −MIL-101(Cr)′ −3701.274 −

PFOA −1953.465 −H2O −76.450 −

AC1 (@Cr) −5654.796 −148AC2 (@Cr) −5654.803 −167

AC3 (@coordinated H2O, bdc) −1953.465 −131AC4 (@coordinated H2O) −5731.278 −110AC5 (@coordinated H2O) −5731.266 −98AC6 (@coordinated H2O) −5731.263 −92

AC7 (@coordinated H2O, bdc) −5731.268 −104AC8 (@bdc) −5731.266 −99

AC9 (@coordinated H2O, bdc) −5731.249 −55AC10 (@bdc) −5731.252 −63

protonated MIL-101(Cr) −3778.150 −protonated MIL-101(Cr)′ −3701.658 −

AC11 (@Cr) −5655.187 −167AC12 (@Cr) −5655.213 −236

AC13 (@coordinated H2O) −5731.652 −97AC14 (@bdc) −5731.639 −61AC15 (@H+) −5731.693 −203AC16 (@H+) −5731.697 −214

MIL-101(Cr)-NH3 −3785.479 −MIL-101(Cr)-NH3′ −3708.977 −AC17 (@N cation) −5738.994 −129AC18 (@N cation) −5738.990 −120MIL-101(Cr)-NMe3 −3903.396 −MIL-101(Cr)-NMe3′ −3826.902 −AC19 (@ N cation) −5856.904 −113AC20 (@ N cation) −5856.912 −133MIL-101(Cr)-DMEN −3971.004 −AC21 (@N cation) −5924.522 −139AC22 (@N cation) −5924.518 −127

MIL-101(Cr)-QDMEN −4010.318 −AC23 (@N cation) −5963.814 −80AC24 (@N cation) −5963.821 −99



F


(dF−H = 3.03, 4.56, 4.46, and 4.01 Å) and (ii) hydrogen bondingbetween the PFOA carboxyl termini and coordinated water(dO−H = 1.59). In contrast, excessive collective interactionsexerted on PFOA can cause structural distortion of the long CFskeletal chain, thus weakening the binding energy. For example,scenario AC9 is similar to AC3, with one additional adsorptionsite: (1) hydrogen bonding between the PFOA carboxyltermini and coordinated water (dO−H = 3.18), (2) π-CFinteractions between bdc and the CF chain of PFOA (dF−H =3.12, 4.39, 3.39, and 3.08 Å), and (3) hydrogen bondingbetween the PFOA’s CF3 termini and coordinated water (dO−H= 3.53). The resulting relative binding energy of AC9 (−55) isonly approximately 40% of that of AC3.Protonation of MIL-101(Cr). Based on the results of zeta-

potential measurements (Figure 6), MIL-101(Cr) is sur-rounded by a layer of positive charges at pH < 5, which isclose to the pH where the aqueous absorption experimentswere carried out. Therefore, an investigation of the effect ofprotonation on the intermolecular interactions between PFOAand MIL-101 is necessary. In addition to the previouslydiscussed adsorption mechanisms, protonated MIL-101 has anextra adsorption site on the protonated carboxyl group of thebdc ligand. The most stable configurations for protonated MIL-101 were identified on the basis of the results of the DFT study(Figure 2b), where six configurations covering all possibleadsorption sites were investigated (Supporting InformationFigure S12). The relative PFOA adsorption energies forprotonated MIL-101 are listed in Table 3. Because of theclose proximity of the added proton to the Cr CUS, collectiveinteraction between the two is favored. Scenario AC12, themost favored AC conformation with PFOA attacking fromparallel positions, involves the following: (1) electrostaticinteraction between the PFOA’s carboxyl termini andprotonated carboxyl group (dO−H = 0.99) and (2) directcoordination of the same PFOA termini to the Cr CUS (dO−Cr= 4.01, 2.30). Compared to scenario AC2, where PFOA attackspristine MIL-101(Cr) in a parallel manner, the relative bindingenergy increases to −236an increase of 41%. Thisamplification can be explained by the electron-withdrawingeffect of the additional proton: the Lewis acid Cr CUS becomesmore electron deficient compared to the protonated bdc andtherefore forms a stronger complex with the Lewis base PFOA.The essential role played by H+ in the adsorption of PFOAleads to the interpretation of the pH-dependence of adsorptioncapacities (Figure 5) as follows: the amount of H+ surroundingthe MIL-101 particles decreases with increasing pH, asindicated by the zeta potential measurement at the presenceof phosphate buffer (Supporting Information Figure S10). Incontrast, the relative binding energy of PFOA to bdc- or Cr-coordinated H2O remains unaffected compared to the pristineMIL-101(Cr).Adding Amine Functional Groups. When amine groups

were added onto bdc via PAM and -DMEN onto Cr CUS ofthe pristine MIL-101(Cr) via PSM, amine-functionalizedMOFs, MIL-101(Cr)-NH2 and MIL-101(Cr)-DMEN, wereobtained, respectively. Because amine groups are protonated inthe aqueous solution, additional adsorption sites are availablebecause PFOA preferentially attacks cationic groups. Tocompare the PFOA binding energy between the two aminegroups, the two most favorable scenarios for each protonatedamine MOFs, which involve the parallel and cross attack ofPFOA, were analyzed (Supporting Information Figure S13). Inboth cases, the relative binding energy of MIL-101(Cr)-DMEN

is slightly higher than that of MIL-101(Cr)-NH2 (Table 3).The experimental results, however, reveal that the adsorptioncapacity of MIL-101(Cr)-DMEN is nearly double that of MIL-101(Cr)-NH2 (Table 2). This phenomenon indicates that theadsorption performance of MIL-101(Cr)-NH2 suffers from thesteric hindrance caused by the amine group present at thenanopore aperture (Figure 1c). Therefore, PSM is a moreeffective strategy for introducing −NH2 groups onto MOFs,which enables adsorbates such as PFOA to enter the aperturesof nanopores freely.

Quaternization of Amine MOFs. Experimentally, weobserved that a higher order of aminespecifically, quaternizedamineexhibited favorable adsorption characteristics inaqueous solutions for both MIL-101(Cr)-NMe3 and MIL-101(Cr)-QDMEN (Table 2). Given that we observed that thesterically hindered MIL-101(Cr)-NH2 significantly lowering thePFOA adsorption capacity, our observation of increasedadsorption capacity for the even more sterically demandingMIL-101(Cr)-NMe3 appears counterintuitive. DFT calculationswere performed on the aforementioned quaternized MOFs toinvestigate the relative PFOA binding energy of the respectivequaternized amine. To our surprise, PFOA attacking from crossand vertical positions of the quaternized amine in MIL-101(Cr)-NMe3 has a higher relative binding energy than PFOAattacking from MIL-101(Cr)-QDMEN (Table 3). Such acontradiction in the experimental results (Table 2) is attributedto the increased van der Waals radii in quaternized amines:because PFOA is hydrophobic and tends to form micelles andhemimicelles,29 more PFOA molecules interact with quater-nized amines compared to primary and secondary amines.However, because the relative binding energy is reduced, theinfluences of other mechanisms are more pronounced,evidently as a consequence of the strong pH effect observedfor both quaternized MOFs in the presence of sodium as acounterion (Figure 5).In summary, we have shown that quaternary-amine-function-

alized MIL-101(Cr)s are highly effective for the adsorptiveremoval of aqueous PFOA. Fast adsorption rates and highadsorption capacities have been demonstrated. The effects ofdifferent functionalization sites have been evaluated exper-imentally on isostructural MOFs prepared via PAM and PSM.Our experimental results suggest that because PAM producesfunctional groups at the aperture that hinders the entrance ofOCs into the nanopores, PSM is a suitable method forintroducing functional groups onto MOFs for the removal ofaqueous OCs. We have elucidated the adsorption mechanismof pristine and functionalized MIL-101(Cr) via DFTcalculations. Our DFT calculation results indicate that theprimary PFOA adsorption site on pristine MIL-101(Cr) is theCr(III) metal center. For protonated MIL-101(Cr), however,the mechanism of electrostatic interaction between the PFOAand the protonated carboxyl groups of the bdc linker, plus theLewis acid/base complex between PFOA and Cr(III), iscollectively the dominant adsorption mechanism.

■ ASSOCIATED CONTENT

*S Supporting InformationN2 physisorption results, PXRD patterns, IR spectra, high-resolution XPS spectra, SEM images of the MOFs, zeta-potential of MOFs at the presence of buffer, regenerationresults, and configurations of optimized PFOA/MOFsadsorption complex used for DFT calculation. The Supporting



G


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

■ AUTHOR INFORMATION

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

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This study was supported by the THU−VEOLIA JointResearch Center for Advanced Environmental Technology,the National High Technology Research and DevelopmentProgram of China (2013AA06A305), and the Program forChangjiang Scholars and Innovative Research Team inUniversity. We thank Dr. Michael Hoffmann from CaliforniaInstitute of Technology for providing valuable insight intoPFCs adsorption mechanism, Dr. Li (Luke) Ke from Universityof Georgia for assistance on DFT calculation, and Dr. OliverHao from University of Maryland for providing generalguidance in writing and reviewing the manuscript.

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(33) Babarao, R.; Jiang, J. Unraveling the energetics and dynamics ofibuprofen in mesoporous metal−organic frameworks. J. Phys. Chem. C2009, 113 (42), 18287−18291.



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