Highly Effective Removal of Nonsteroidal Anti-inflammatory

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Highly Effective Removal of Nonsteroidal Anti-inflammatory Pharmaceuticals from Water by Zr(IV)-Based Metal−Organic Framework: Adsorption Performance and Mechanisms Shuo Lin,† Yufeng Zhao,† and Yeoung-Sang Yun* School of Chemical Engineering, Chonbuk National University, Jeonbuk 54896, Republic of Korea

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S Supporting Information *

ABSTRACT: Nonsteroidal anti-inflammatory pharmaceuticals are emerging organic micropollutants in surface water, groundwater, and wastewater, whose removal is very important yet challenging. As a new class of porous functional materials, metal− organic frameworks (MOFs) have attracted extensive attention for their adsorption applications. Here, we report that Zr(IV)-based MOFs (defective UiO-66, and MOF808) have extraordinary adsorption ability to remove nonsteroidal anti-inflammatory pharmaceuticals from water. Excellent adsorption performances are obtained for UiO66 and MOF-808, particularly for UiO-66, of which the adsorption capacities are the highest in a wide series of adsorptive materials previously reported. It is elucidated that the incomplete-coordinated cationic Zr in the cluster has high affinity for the anionic pharmaceutical (chemical adsorption) and that the adsorption interaction between the benzene ring of the pharmaceutical and MOF’s ligand is involved to enhance or as an alternative to the adsorption interactions (π−π interaction). In particular, adsorption of ibuprofen, ketoprofen, naproxen, indomethacin, and furosemide by UiO-66 and MOF-808 and the synergetic effect of chemical adsorption and π−π interaction are outstanding, leading to extremely higher binding energies (Ebind) and sorption abilities. KEYWORDS: pharmaceuticals removal, MOFs, UiO-66, MOF-808, chemical adsorption, π−π interaction degradation,19 ultrasonic irradiation,20 coagulation flocculation,21 and advanced oxidation processes.22 Unfortunately, these techniques have various drawbacks, including reduced removal efficiency, generation of poisonous effluent products, and additional capital and labor costs.23−26 Therefore, developing highly effective methods for the removal of such NSAIDs from water is of considerable importance. Metal−organic frameworks (MOFs) are envisioned to be an attractive new approach for the effective capture and removal of NSAIDs from water. MOFs represent a particular class of crystalline materials synthesized by the self-assembly of metallic ions/clusters and organic linkers/ligands, which results in the one-, two-, or three-dimensional (1-, 2-, or 3D, respectively) formation of well-defined networks.27,28 Owing to the diverse geometries of organic ligands and the diverse coordination mode of inorganic metal ions/clusters, MOFs with tunable crystal structures with ultrahigh porosities and extremely large internal surfaces can be designed.29 Consequently, MOF materials find applications in various fields, such as proton conduction,30,31 sensing,32,33 gas storage and separation,34−36 catalysis,37,38 and drug delivery.39,40 Furthermore, MOFs can facilitate the adsorptive removal of various

1. INTRODUCTION The large pharmaceutical class of nonsteroidal anti-inflammatory drugs (NSAIDs) as anti-inflammatory and analgesic for relieving pain and pain-related conditions are the most extensively utilized pharmaceutical compounds. The National Health Interview Survey carried out by the U.S. Department of Health and Human Services revealed that around 72 million adults (31.1% of U.S. population) were regular users of NSAIDs in 2010, indicating a 40% increase from 2005.1 The extensive and increasing production and consumption of NSAIDs enhances the threats of leakage of these pharmaceuticals to the environment. Over the past few decades, emergence of pharmaceuticals in groundwater, surface water, and sewage water have been frequently reported.2−6 Thereafter, the leaked NSAIDs as emerging organic pollutants in the environments could be poisonous to aquatic organisms or damage human health through contaminated potable water.7−10 However, the majority of NSAIDs are anionic aromatic compounds with stable structures, high polarity, and water solubility. Thus, it is not easy for the conventional water treatment plants to completely eliminate NSAIDs from water.11,12 Consequently, alternative approaches have been explored for trapping or degrading pharmaceuticals in water, including the use of modified activated carbon,13−15 solidphase extraction,16 electrochemical degradation,17,18 photo© XXXX American Chemical Society

Received: May 24, 2018 Accepted: July 27, 2018

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DOI: 10.1021/acsami.8b08596 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Scheme 1. Zr Cluster [Zr6O4(OH)4(−CO2)n] with Various Coordination Numbers (n-c) are Connected with Various Ligands (left two columns) to Form Unit Cell of UiO-66 (a), MOF-808 (b), and MOF-802 (c) (middle) and the Packing of Several Unit Cells (right) of Various Topologiesa

a Color code: Zr, cyan; C, gray; O, red; N, blue; H, white. Terminal −OH/−OH2 groups and some H atoms are omitted for clarity. The green and blue balls indicate the space in the framework.

adsorption data, the pore size distributions of the MOFs were calculated by a nonlocal density functional theory (NLDFT) model. The initial and final concentrations of pharmaceuticals were determined by high-performance liquid chromatography (HPLC, LC-20AD, Shimadzu, Japan). X-ray photoelectron spectroscopy (XPS, JEOL, Japan) was carried out using a JSM-6400 spectrometer for core-level spectra of Zr 3d, C 1s, and N 1s, and XPSPeak version 4.1 was used to resolve and optimize their peaks. 2.3. Synthesis of MOF Materials. 2.3.1. UiO-66 [Zr6O4(OH)4(BDC)5]. ZrCl4 (0.125 g, 0.54 mmol) and BDC (0.123 g, 0.75 mmol) were completely dissolved in a mixed solvent of DMF/ concentrated HCl (15:1 mL) in a 60 mL vial. The mixture was reacted at 120 °C for 24 h. The obtained white precipitate was washed for several times with DMF and anhydrous ethanol (EtOH).56 Materials were freeze-dried for further adsorption experiments and characterization. EA of material: calculated for [Zr6O4(OH)4(BDC)5(OH)0.5Cl1.5(H2O)2(EtOH)2.5]: Zr, 7.19; C, 53.89; O, 37.13; Cl, 1.80%. Calculated for [Zr6O4(OH)4(BDC)6Cl1.5(H2O)2(EtOH)2.5]: Zr, 6.32; C, 55.79; O, 36.32; Cl, 1.58%. Observed: Zr, 7.23; C, 52.37; O, 38.47; Cl, 1.92%; thus, the former formula was adopted. 2.3.2. MOF-808 [Zr6O4(OH)4(BTC)2]. ZrOCl2·8H2O (0.160 g, 0.50 mmol) and BTC (0.110 g, 0.50 mmol) were completely dissolved in a mixed solvent of DMF/formic acid (20:20 mL) in a 60 mL vial. The mixture was reacted at 150 °C for 7 days. The obtained white precipitate was washed several times with DMF and EtOH.57 Materials were freeze-dried for further adsorption experiments and characterization. EA of material: calculated for [Zr6O4(OH)4(BTC)2(OH)6]: Zr, 8.82; C, 44.12; O, 47.06%. Observed: Zr, 8.84; C, 43.66; O, 47.51%. 2.3.3. MOF-802 [Zr6O4(OH)4(PZDC)5]. ZrOCl2·8H2O (0.200 g, 0.65 mmol) and PZDC (0.135 g, 0.75 mmol) were completely dissolved in a mixed solvent of DMF/formic acid (25:15 mL) in a 60 mL vial. The mixture was reacted at 150 °C for 4 days. The obtained colorless crystals were washed several times with DMF and EtOH.57 Materials were freeze-dried for further adsorption experiments and characterization. EA of material: calculated for [Zr6O4(OH)4(PZDC)5(HCOO)2(OH)2(EtOH)2.5]: Zr, 7.10; C, 37.87; O, 43.20; N, 11.83%. Observed: Zr, 7.10; C, 38.17; O, 43.15; N, 11.58%. 2.4. Adsorption Experiments. The kinetic and isotherm adsorption experiments were carried out in a multishaking incubator (HB-201MS-2R, Hanbaek, Korea) at 25 °C and 120 rpm. The pharmaceutical solution (60 mL) and 6.0 mg of MOFs were mixed in a 120 mL vial. The solution with 1.0 mmol L−1 of pharmaceuticals

pollutants from water, due to their unique features of the adjustable internal surface and diverse and abundant ligands with functional sites for adsorption.41,42 Zr(IV)-based MOFs can be appropriate adsorbents for the capture and separation of NSAIDs from the aqueous medium because of their water stability and possible abundant binding sites.43−46 To date, Zr clusters with several connectivities [Zr6O4(OH)4(−CO2)n] have been reported, including 12 with full coordination47 and 10,48 8,49 and 6 with incomplete coordination.50 Incomplete-coordinated Zr clusters with cationic Zr sites were observed in Nu-1000, UiO-66, UiO67, and BUT-12, the remarkable adsorption performances of which for anions such as SeO32−, SeO42−, PdCl42−, PtCl62−, AuCl4−, and benzoic acid in aqueous phases have been reported.51−55 Herein, we synthesized Zr(IV)-based MOFs of UiO-66, MOF-808, and MOF-802 with incomplete-coordinated Zr clusters, distinct structures, and various porosities and studied their removal capacities and adsorption mechanisms for a series of nonsteroidal anti-inflammatory pharmaceuticals.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Zirconium chloride (ZrCl4), zirconium chloride octahydrate (ZrOCl2·8H2O), benzene-1,3,5tricarboxylic acid (BTC), and 3,5-pyrazoledicarboxylic acid monohydrate (PZDC) were purchased from Sigma-Aldrich (Yongin, Korea). Benzene-1,4-dicarboxylic acid (BDC) was obtained from Junsei Chemical Co., Ltd. (Tokyo, Japan). N,N-dimethylformamide (DMF) and formic acid were supplied by Daejung Chemicals and Metals Co., Ltd. Pharmaceuticals of ibuprofen, ketoprofen, naproxen, indomethacin, furosemide, salicylic acid, acetophenone, and amitriptyline were also supplied by Sigma-Aldrich. 2.2. Apparatus. Energy-dispersive X-ray detector (EDAX, AMETEK) was employed for elemental microanalysis (EA). Powder X-ray diffraction (PXRD, PANalytical B.V., the Netherlands) measurements were performed using a multipurpose high-performance X-ray diffractometer with a Cu-sealed tube (λ = 1.54178 Å) at 25 °C; measurements were made over a range of 4 < 2θ < 90° in a 0.05 step size at a scanning rate of 1°/min. The surface morphologies of the MOFs were examined by a field emission scanning electron microscope (JSM-6000, JEOL, Japan). N2 adsorption and desorption isotherm measurements were performed on a Belsorp-max (MicrotracBEL Corp., Japan) at 77 K; samples were activated under high vacuum by heating at 100 °C for 12 h. On the basis of the N2 B


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ACS Applied Materials & Interfaces was used in the kinetic experiment. The solution pH was maintained at 6.0 ± 0.1 for >30 h, and the samples were extracted at regular intervals for analysis. Various concentrations (0−2 mmol L−1) of pharmaceuticals were used for isotherm experiment at pH 6.0 ± 0.1 for 30 h. Diluted HCl and NaOH solutions were used for adjusting the solution pH. After the equilibrium, the adsorbent was separated by centrifugation at 12 000 rpm for 8 min. The concentration of pharmaceutical in the supernatant was determined by HPLC analysis after appropriate dilution. The following mass balance equation was used to calculate the uptake (q in mmol g−1) of the pharmaceutical on the MOFs q=

(Ci − Cf )V m

was observed, which is attributed to the lesser ligand interference in the 10-coordinated Zr clusters.56 PXRD data reveal that UiO-66 crystallizes in a cubic space group, with lattice parameter a = 20.92 (1) Å (Table 1). In UiO-66, each inorganic Zr cluster is connected to 10 organic ligands, with terminal −OH/−OH2 coordinating the other two coordination sites.52 The resultant 3D framework has a bct topology. Defective octahedral cages with inner pore size of 16.4 Å are formed, with the BDC ligands at the faces of the octahedron and the Zr clusters at the vertices. Among the contiguous octahedral cages is defective tetrahedral pore, which has an internal pore diameter of 11.5 Å. The porosities of UiO-66 were examined using N2 adsorption at 77 K. Saturated N2 uptakes of 346 cm3 (STP) g−1 were achieved, and the evaluated Brunauer−Emmett−Teller surface area (ABET) was 1507 m2 g−1, with a total pore volume (Vp) of 0.632 cm3 g−1 (Figure 2 and Table 1), which were much larger than those of reported conventional UiO-66 with 12-coordinated Zr clusters (ABET: 1290 m2 g−1 and Vp: 0.49 cm3 g−1).59 In addition, two types of pores of 10 and 15 Å were provided by the nonlocal density functional theory (NLDFT)-determined pore size distribution (Figure 2, inset), being consistent with the crystallographic structure of UiO-66. Furthermore, FE-SEM images showed that the UiO-66 crystal had a nanoparticle size of about 100 nm (Figure S5, Supporting Information). A tritopic ligand of BTC is used for the preparation of MOF808 (Scheme 1).57 The PXRD data reveal that MOF-808 crystallizes in the cubic space groups Fd3m with lattice parameter a = 35.23 (1) Å (Figure 1, Table 1; Figure S3, Supporting Information). In MOF-808, each Zr cluster is connected to six BTC ligands, with terminal −OH/−OH2 coordinating the other six coordination sites, and each ligand is coordinated to three Zr clusters. The resultant 3D framework has a spn topology. Tetrahedral cages with inner pore size of 4.8 Å are formed, with the BTC ligands at the faces and the Zr clusters at the vertices of the tetrahedron. Among the 10 contiguous tetrahedral cages, a large adamantane cage is formed with an internal pore diameter of 18.4 Å. The values of the saturated N2 uptakes for ABET and Vp were obtained at 302 cm3 (STP) g−1 as 1314 m2 g−1 and 0.621 cm3 g−1, respectively (Figure 2 and Table 1). Two types of pores, 6 and 18 Å, were provided according to pore size distribution, being consistent with the crystallographic structure of MOF-808. Furthermore, the FE-SEM images showed that the octahedral crystals of MOF-808 had a nanoparticle size of about 300 nm (Figure S6, Supporting Information). A bent ligand of PZDC is used for the preparation of MOF802 (Scheme 1).57 The PXRD data reveal that MOF-802 crystallizes in the Fdd2 space groups, with lattice parameters a = 39.49 (3) Å, b = 26.28 (2) Å, and c = 28.10 (2) Å (Figure 1, Table 1; Figure S4, Supporting Information). In MOF-802, each Zr cluster is connected to 10 PZDC ligands, with −OH/ −OH2 or formate coordinating the other two coordination sites. The resultant 3D framework has a bct topology, with a small pore of diameter 5.6 Å.57 The values of the saturated N2 uptakes for ABET and Vp were obtained at 1.1 cm3 (STP) g−1 as 4.8 m2 g−1 and 0.009 cm3 g−1, respectively (Figure 2, Table 1; Figure S8, Supporting Information,). Furthermore, the FESEM images showed that the crystal of MOF-802 had a nanoparticle size of about 500 nm (Figure S7, Supporting Information). The chemical stabilities of UiO-66, MOF-808, and MOF802 were tested separately in water, HCl (pH = 1), and NaOH

(1)

where Ci and Cf are pharmaceutical concentrations at initial and final times (mmol L−1), respectively; V is the adsorption work volume (L); and m is the mass of adsorbent used for the pharmaceutical adsorption (g). 2.5. Computation Method. The binding energies (Ebind) of pharmaceuticals in MOFs were investigated by the DFT calculations using the DMol3 tools in Material Studio 7.0 (Accelrys Software Inc.). The PWC functions and the DNP basis set in the local density approximation system were carried out. The value of Ebind is calculated as the energy difference between the products and reactants in the adsorption process, as defined by E bind = EMOF + pharmaceutical + Eextra − EMOF − Epharmaceutical

(2)

where EMOF+pharmaceutical is the total energy of the MOF/adsorbate adsorption system; Eextra is the energy of extra eluted ions or molecules after equilibrium; and EMOF and Epharmaceutical are the energies of MOFs and pharmaceuticals before adsorption, respectively.58

3. RESULTS AND DISCUSSION 3.1. Characterization of MOF Materials. Defective UiO66 with incomplete-coordinated Zr clusters is synthesized with unbent ditopic BDC as a ligand (Scheme 1) using hydrochloric acid for catalysis.56 PXRD data of the as-synthesized UiO-66 were consistent with the simulated pattern of UiO-66 with 10coordinated Zr clusters (Figure 1; Scheme S1 and Figure S1, Supporting Information). Compared to the simulated pattern of UiO-66 with 12-coordinated Zr clusters (Figure S2, Supporting Information), the intensification at the second reflection (200) in the pattern of the as-synthesized UiO-66

Figure 1. PXRD patterns of UiO-66, MOF-808, and MOF-802 upon treatment with water, HCl (pH = 1), and NaOH (pH = 10) solutions. C


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ACS Applied Materials & Interfaces Table 1. Lattice Parameters and Pore Structure Properties of UiO-66, MOF-808, and MOF-802 lattice parameters

pore structure properties

materials

a (Å)

b (Å)

c (Å)

α = β = γ (°)

N2 uptake ((cm3 STP) g−1)

ABET (m2 g−1)

Vp (cm3 g−1)

pore width (Å)

UiO-66 MOF-808 MOF-802

20.92 35.23 39.57

20.92 35.23 26.24

20.92 35.23 28.07

90 90 90

346 302 1.1

1507 1314 4.8

0.632 0.621 0.009

10, 15 6, 18 5.6a57

a

Due to the very low N2 adsorption capacity of MOF-802, the pore size distribution cannot be determined via NLDFT.

3.2. Pharmaceuticals Adsorption Studies. The adsorption kinetics of furosemide and salicylic acid was studied to gain insight into the reaction pathways of the adsorption reactions. As shown in Figure 3a, less than 420 min was required with UiO-66 and MOF-808 to achieve the adsorption equilibrium for a 1.0 mmol L−1 solution of salicylic acid, whereas 12 h or longer were required for furosemide. The faster uptake of salicylic acid by UiO-66 and MOF-808 compared to furosemide could be related to the molecular size of furosemide. As seen in Table S2 and Figure S9 (Supporting Information), furosemide has a much larger molecular size (min-width: 6.7 Å; max-length: 14.6 Å) than salicylic acid (min-width: 5.9 Å; max-length: 8.4 Å). As previously mentioned, UiO-66 and MOF-808 have larger pores (15 and 18 Å, respectively) than MOF-802 (5.6 Å). Thus, easy and convenient diffusions of furosemide and salicylic acid are possible through or in the pores of UiO-66 and MOF-808. In the case of MOF-802, however, the pores are relatively small, leading to less diffusion of large pharmaceuticals, i.e., furosemide and salicylic acid. These results suggest the importance of large MOF apertures for effective pharmaceuticals adsorption. To further confirm the adsorption capacities of UiO-66, MOF-808, and MOF-802 toward furosemide and salicylic acid, the adsorption isotherms for these MOFs were recorded at room temperature (Figure 3b), and the resultant isotherms were fitted by the Langmuir model (Table S3, Supporting Information). The maximum adsorption capacities of UiO-66 for furosemide and salicylic acid reached up to 1.17 and 1.13 mmol g−1, respectively, which were much higher than those of MOF-808 (0.39 and 1.05 mmol g−1, respectively). Besides,

Figure 2. Adsorption/desorption of N2 for UiO-66, MOF-808, and MOF-802 at 77 K, with inset showing pore size distribution as determined via NLDFT.

(pH = 10) solutions. After being immersed in each liquid for 1 week, the PXRD patterns of these three MOFs unchanged, implying retained crystallinity and structures (Figure 1; Table S1, Supporting Information). Meanwhile, Zr leaching from the three MOFs was carried out by determining the existence of zirconium in the centrifuged and supernatant solutions using inductively coupled plasma-optical emission spectroscopy. The result showed that zirconium was not detected, indicating no Zr leaching from the MOFs during immersion in water, HCl (pH = 1), and NaOH (pH = 10) solutions. These results revealed that the MOFs were stable in a large pH range of aqueous solution.

Figure 3. Kinetics (a) and isotherms (b) of furosemide and salicylic acid uptake in UiO-66, MOF-808, and MOF-802. In each case, 6.0 mg of the MOF was used at pH 6.0 ± 0.1. D


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Figure 4. Saturated adsorption capacities of pharmaceuticals in UiO-66, MOF-808, and MOF-802. In each case, 6.0 mg of the MOF was exposed to 1.0 mmol L−1 pharmaceutical at pH 6.0 ± 0.1.

Figure 5. XPS Zr 3d images of pristine MOFs (1) and furosemide (2)- and salicylic acid (3)-loaded UiO-66 (a), MOF-808 (b), and MOF-802 (c). (d) XPS C 1s images of pristine and pharmaceutical-loaded UiO-66, MOF-808, and MOF-802.

structure of Zr−OH2+,60 and the carboxyl groups (−COOH) in ibuprofen, ketoprofen, naproxen, indomethacin, furosemide, and salicylic acid were ionized (−COO−) as their pKa values were lower than 6.0 (Table S2, Supporting Information). Meanwhile, MOF-802 shows the least number of uptakes for these pharmaceuticals, as it has the narrowest pore. However, the neutral pharmaceutical of acetophenone, which has no anionic functional group (−COCH3) on the benzene ring and the smallest molecular size (min-width, 5.9 Å) among these pharmaceuticals, was still adsorbed by UiO-66, MOF-808, and MOF-802. This can be attributed likely to the π−π interaction between the benzene rings of acetophenone and ligands of UiO-66 (BDC), MOF-808 (BTC), and MOF-802 (PZDC) because the π−π interaction is generally present between aromatic molecules containing enriched π orbitals.61 Thus, the π−π interaction could be involved as an enhancing or an alternative adsorption interaction of MOFs for other pharmaceuticals. The larger uptakes of UiO-66 for ibuprofen, ketoprofen, naproxen, indomethacin, and furosemide than MOF-808 could be attributed to the larger quantity of functional groups (benzene rings) in UiO-66 for π−π interaction. As shown in Scheme 1 and calculated molecular formula, UiO-66 has more ligands with benzene rings [Zr6O4(OH)4(BDC)5] than MOF-808 [Zr6O4(OH)4(BTC)2]. Besides, the negligible uptakes of amitriptyline in UiO-66, MOF-808, and MOF-802 were attributed to the electrostatic

MOF-802 showed nonsignificant adsorption capacities for furosemide and salicylic acid due to the narrower pore size of MOF-802 versus pharmaceutical molecular sizes. Furthermore, the sharper isotherm curves at lower pharmaceutical concentrations as well as the large b values of furosemide adsorption in UiO-66 and MOF-808 (141.3 and 101.2 L mmol−1, respectively) than those of salicylic acid (6.8 and 4.4 L mmol−1, respectively) indicated much higher affinities of UiO-66 and MOF-808 for furosemide. To widely explore the adsorption capacities of UiO-66, MOF-808, and MOF-802 for the class of nonsteroidal antiinflammatory pharmaceuticals, the adsorption kinetics of ibuprofen, ketoprofen, naproxen, indomethacin, furosemide, salicylic acid, and other classes of pharmaceuticals (acetophenone and amitriptyline) were also evaluated besides furosemide and salicylic acid (Figures S11−S17 and Table S4, Supporting Information). The saturated adsorption capacities were collected and are illustrated as bar charts in Figure 4. The perfect adsorption abilities of UiO-66 and MOF808 for ibuprofen, ketoprofen, naproxen, indomethacin, furosemide, and salicylic acid were obtained, which could be attributed to the high affinities of the cationic Zr sites in MOF clusters (Scheme 1) for the negatively charged carboxyl groups and primary amine group in the pharmaceuticals. In an aqueous solution at pH 6.0, the terminal −OH groups in incomplete-coordinated Zr tend to form a positively charged E


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Figure 6. Optimized geometries for the interactions of ibuprofen, furosemide, salicylic acid, and acetophenone with UiO-66 (a−d), MOF-808 (e− h), and MOF-802 (i−l). Color code: Zr, cyan; C, gray; O, red; N, blue; H, white. Terminal −OH/−OH2 groups and some H atoms are omitted for clarity.

repulsion between the cationic sites (Zr−OH2+) and the protonated tertiary amine groups (NH+−) in amitriptyline in pH 6.0 solution (the pKa value of amitriptyline is 8.8, Table S2). Furthermore, as shown in Table S5, the adsorption capacities of UiO-66, especially for nonsteroidal antiinflammatory pharmaceuticals of ibuprofen, ketoprofen, naproxen, indomethacin, and furosemide, are the highest in a wide series of adsorptive materials, such as activated carbon, zeolites, graphenes, resins, carbon nanotubes, and other MOF materials. For UiO-66 and MOF-808, their adsorption kinetics in low-concentration (3.0 μmol L−1) pharmaceutical solutions was evaluated (Figures S18−S21 and Table S6, Supporting Information). The result revealed that the rapid and high pharmaceutical uptakes, achieving adsorption equilibrium in 30 min, were obtained for UiO-66, and the residual concentrations of ibuprofen, indomethacin, furosemide, and salicylic acid turned to be extremely low (0.61, 0.14, 0.06, and 0.80 μmol L−1, respectively), indicating the ability of UiO-66 to fully trap a trace quantity of these pharmaceuticals.

3.3. Investigation of the Interaction of Pharmaceuticals in MOFs. To gain insight into the mechanisms of pharmaceutical adsorption in UiO-66, MOF-808, and MOF802, XPS was performed before and after furosemide and salicylic acid loading (Figure 5). Compared to the Zr 3d spectra of the pristine UiO-66 and MOF-808 (Figure 5a,b, respectively), the pharmaceutical-adsorbed MOFs were significantly different. The deconvoluted Zr 3d spectra consisted of two main peaks (Zr 3d5/2) at 182.2 and 182.9 eV, which could be assigned to Zr in the Zr−O and terminal −OH occupied Zr (Zr−OH), and Zr in −COO−Zr or −NH2−Zr, respectively.62−64 The significant receding of the 182.2 eV peak and strengthening of the 182.9 eV peak were observed for the pharmaceuticals loaded with UiO-66 and MOF-808. These results verified that the cationic Zr sites (incompletecoordinated Zr sites with terminal −OH/−OH2+ eluted) were the binding sites toward the negatively charged carboxyl groups and primary amine group in the pharmaceuticals by electrostatic attraction or complexation. Additionally, two F


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Figure 7. Binding energy of pharmaceuticals with UiO-66, MOF-808, and MOF-802. (π) Binding energy of π−π interaction and (c) binding energy of chemical adsorption.

Figure 8. Schematic diagram of interactions of ibuprofen, furosemide, salicylic acid, and acetophenone with UiO-66 (a), MOF-808 (b), and MOF802 (c).

their TSs are 0) was attributed to the π−π interaction with the extensively delocalized structure, which effectively limits the delocalization.65,66 This confirmed the strong effect of the π−π interaction on the furosemide adsorption into UiO-66 and MOF-808, but no effect on salicylic acid adsorption. After reviewing the structures of the MOFs unit cell combined with the pharmaceutical molecules, we speculate that the complexation of the primary amine group with cationic Zr in the cluster as well as the π−π interaction between the benzene rings of the furosemide and MOF ligands (BDC and BTC) could occur at the same time for the adsorption of each furosemide molecule; however, after the carboxyl groups bond with cationic Zr, the salicylic acid molecule is too short and thus its benzene ring is not able to approach the MOF ligands; the π−π interaction could therefore not occur. Besides, for the pristine and pharmaceutical-adsorbed MOF-802, no significant difference among the C 1s spectra was observed. The C2 (285.4 eV) and C3 (286.5 eV) were assigned to C−N and −CH2−CO in PZDC, respectively, and the high intensity of the C4 peak in the spectrum of pristine MOF-802 could be attributed to the residual formate.62 To further understand pharmaceutical adsorption behaviors for UiO-66, MOF-808, and MOF-802, we carried out density functional theory (DFT) calculations to achieve optimized geometries for the interactions of ibuprofen, ketoprofen, naproxen, indomethacin, furosemide, salicylic acid, and acetophenone with three MOFs. As shown in Figures 6, S24, and S25 (Supporting Information), the carboxyl groups (phenyl−CRxCOOH) or the primary amine group of

emerging peaks at 399.1 and 399.9 eV in the N 1s spectra of furosemide-adsorbed UiO-66 and MOF-808 were observed (Figure S22, Supporting Information), which could be assigned to N in the phenylamine group (phenyl−NH2) and furosemide-loaded amine group (phenyl−NH2−Zr).62 Thus, the complexation between primary amine group in furosemide and Zr sites in MOFs was verified. Nevertheless, no significant difference of Zr 3d spectra of the pristine and pharmaceuticaladsorbed MOF-802 were observed (Figure 5c), which could be due to its low pharmaceutical uptake. To investigate the effect of the π−π interaction between the pharmaceuticals and MOFs on the adsorption performance, the XPS C 1s images were obtained from pristine, and furosemide- and salicylic acid-loaded UiO-66, MOF-808, and MOF-802 (Figure 5d; Figure S23, Supporting Information). Compared to the C 1s spectra of the pristine UiO-66 and MOF-808, the furosemide-loaded MOFs were significantly different. For the furosemide-loaded UiO-66 and MOF-808, the deconvoluted C 1s spectra consisted of three peaks at 284.5 eV (C1), 285.6 eV (C2), and 288.5 eV (C4) (Figure S23). The C1 was assigned to extensively delocalized sp2 bonds of benzene; C2 was assigned to localized sp2 bonds of benzene; and C4 was assigned to −COO−.65,66 As shown in Figure 5d, the TS asymmetry index of the Dexter spectral line shape, the resolved and optimized peaks of which were C1 and C2, was obtained as C2/C1 intensity ratio. The increasing TS asymmetry index value in the C 1s spectra of furosemideloaded UiO-66 (TS = 0.6) and MOF-808 (TS = 0.54) compared to the pristine and salicylic acid-loaded MOFs (all of G


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4. CONCLUSIONS The adsorption of a series of nonsteroidal anti-inflammatory pharmaceuticals from water by three porous, water-stable Zr(IV)-based MOFs of UiO-66, MOF-808, and MOF-802 was explored. Excellent adsorption performances for ibuprofen, ketoprofen, naproxen, indomethacin, furosemide, salicylic acid, and acetophenone were obtained on UiO-66 and MOF-808, especially for the former, for which the adsorption capacities for nonsteroidal anti-inflammatory pharmaceuticals of ibuprofen, ketoprofen, naproxen, indomethacin, and furosemide were the highest in a wide series of adsorptive materials. The incomplete-coordinated cationic Zr in the cluster showed high affinity for the anionic pharmaceuticals of ibuprofen, ketoprofen, naproxen, indomethacin, and salicylic acid (chemical adsorption); additionally, the π−π interactions between the benzene rings of pharmaceuticals and MOF ligands (UiO-66: BDC; MOF-808: BTC; and MOF-802: PZDC) were involved to enhance or as an alternative to the adsorption interactions (π−π interaction) (Figure 8). In the case of ibuprofen, ketoprofen, naproxen, indomethacin, and furosemide adsorption in UiO-66 and MOF-808, the synergetic effect of chemical adsorption and π−π interaction was demonstrated with significantly higher binding energies of UiO-66 and MOF-808 toward these pharmaceuticals compared to salicylic acid and acetophenone, the quality of which is also consistent with the adsorption results. However, MOF802 showed the lowest or negligible uptake for all pharmaceuticals owing to its low porosity and narrow inner pore size. These results suggest the importance of both high adsorption affinity and large MOF apertures for effective pharmaceuticals adsorption and removal.

ibuprofen, ketoprofen, naproxen, indomethacin, and furosemide locate near the cationic Zr in a cluster; meanwhile, the benzene rings are stacked on the ligands of BDC and BTC as parallel-displaced configurations or on the PZDC as T-shaped configurations. These two types of π−π stackings were formed by a strong electronic attraction between good electron donors (π-electron clouds) and electron acceptors (σ-framework), namely, π−π interaction.67−69 However, the salicylic acid locates only near the cationic Zr, and its benzene ring is on the same plane of the carboxyl group (phenyl−COOH) and far from the ligand. The far distance and the inappropriate geometrical positions of salicylic acid and MOF ligands caused a very weak π−π interaction. Besides, the acetophenone locates only by stacking on BDC, BTC, and PZDC as paralleldisplaced configurations, owing to its only one possibility of π−π interaction. These results demonstrated that ibuprofen, ketoprofen, naproxen, indomethacin, and furosemide could bond with MOFs by the synergetic effect of chemical adsorption and π−π interaction, while the length of the cationic Zr bond salicylic acid is insufficient to establish a π−π interaction with the MOF ligands. Herein, the binding energies (Ebind) of ibuprofen, ketoprofen, naproxen, and indomethacin in UiO-66 and MOF-808 are extremely high (above −300 kJ mol−1) (Figure 7), suggesting that the synergetic effect of chemical adsorption and π−π interactions between pharmaceuticals and these two MOFs are stronger than the single adsorption interaction for salicylic acid and acetophenone (Figure 8). And the quality of Ebind (Figure 7) is consistent with the adsorption experimental results (Figure 4), indicating that the highly effective adsorption could be owing to the high Ebind of MOFs for the pharmaceuticals. Furthermore, we retained the optimized geometries for the interactions of pharmaceuticals with MOFs but changed the functional branches to inert methyl groups; the DFT-calculated binding energies of these structures were obtained and considered as the binding energies of the π−π interaction (Eπ−π) (Figures S26 and S27, Supporting Information). Hereafter, the binding energies of chemical adsorption (Echem) were obtained from the difference between Ebind and Eπ−π. As shown in Figure 7, Echem is higher than Eπ−π for almost all anionic pharmaceuticals, especially for salicylic acid with extremely low Eπ−π (less than −10 kJ mol−1), suggesting the advantageous effect of chemical adsorption compared to π−π interaction (Figure 8, salicylic acid was bound to MOFs without π−π interaction). Finally, these DFT calculations do not consider the diffusion of pharmaceuticals in the pores of MOFs, and a higher Ebind for MOF-802 was thus obtained, although the pharmaceutical uptake into MOF-802 had reduced. 3.4. Regeneration of Pharmaceutical-Loaded MOFs. After adsorption of furosemide and salicylic acid onto UiO-66 and MOF-808 at pH 6.0 in 60 mL containing 1.0 mmol L−1 each, the pharmaceutical-loaded MOFs were regenerated by desorption using 8 mL of 0.1 M HCl/methanol mixture three times at 40 °C (i.e., 24 mL total) and then by washing with deionized water for several times. The regenerated MOFs were reused in the subsequent adsorption cycles. As shown in Figure S28, the stable adsorption performances of the MOFs were maintained up to three cycles of adsorption−regeneration, indicating the excellent regeneration performance of UiO-66 and MOF-808. Furthermore, Zr was not detected in the desorption solutions, implying the stabilities of UiO-66 and MOF-808 during the adsorption−regeneration cycles.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b08596. FE-SEM images; additional lattice parameters calculations and N2 adsorption isotherms data; information of physicochemical properties; additional kinetic curves; XPS images and DFT calculations; and adsorption− regeneration cycles (PDF) Crystallographic of UiO-66 with 12- and 10-coordinated

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Zr clusters, MOF-808, and MOF-802 (ZIP)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82-63-270-2308. Fax: +8263-270-2306. ORCID

Yeoung-Sang Yun: 0000-0002-2583-8278 Author Contributions †

S.L. and Y.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the Korean Government through NRF (2017R1A2A1A05001207) grant. H


Research Article


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Highly Effective Removal of Nonsteroidal Anti-inflammatory

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