Highly efficient adsorptive removal of sulfamethoxazole from aqueous solutions by porphyrinic MOF-525 and MOF-545

Journal Pre-proof Highly efficient adsorptive removal of sulfamethoxazole from aqueous solutions by porphyrinic MOF-525 and MOF-545 Kwangsun Yu, Imteaz Ahmed, Dong-Il Won, Wan In Lee, Wha-Seung Ahn PII:

S0045-6535(20)30326-X

DOI:

https://doi.org/10.1016/j.chemosphere.2020.126133

Reference:

CHEM 126133

To appear in:

ECSN

Received Date: 20 November 2019 Revised Date:

1 February 2020

Accepted Date: 4 February 2020

Please cite this article as: Yu, K., Ahmed, I., Won, D.-I., Lee, W.I., Ahn, W.-S., Highly efficient adsorptive removal of sulfamethoxazole from aqueous solutions by porphyrinic MOF-525 and MOF-545, Chemosphere (2020), doi: https://doi.org/10.1016/j.chemosphere.2020.126133. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Credit Author Statement

Highly efficient adsorptive removal of sulfamethoxazole from aqueous solutions by porphyrinic MOF-525 and MOF-545

Kwangsun Yua,†, Imteaz Ahmeda,†, Dong-Il Wonb, Wan In Leeb, Wha-Seung Ahna,* a.

Department of Chemical Engineering, Inha University, Incheon 22201, Republic of Korea.

E-mail: [email protected]; Fax: +82 328720959; Tel; +82 328607466 b.

Department of Chemistry, Inha University, Incheon 22201, Republic of Korea.

†. Equal contribution

Authors’ credits Kwangsun Yu : Data curation, Formal analysis, Writing- review and editing. Imteaz Ahmed: Methodology, Investigation, Visualization, Writing – original draft. Dong-Il Won: Formal analysis. Wan In Lee: Resources. Wha-Seung Ahn: Resources, Funding acquisition, Supervision.

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Highly efficient adsorptive removal of sulfamethoxazole from aqueous solutions by porphyrinic MOF-525 and MOF-545 Kwangsun Yua,†, Imteaz Ahmeda,†, Dong-Il Wonb, Wan In Leeb, Wha-Seung Ahna,* a.

Department of Chemical Engineering, Inha University, Incheon 22201, Republic of Korea. E-

mail: [email protected]; Fax: +82 328720959; Tel; +82 328607466 b.

Department of Chemistry, Inha University, Incheon 22201, Republic of Korea.

†. Equal contribution

Abstract The metal-organic frameworks MOF-525 and MOF-545 comprised of Zr-oxide clusters and porphyrin moieties in different geometries were synthesized solvothermally and applied for the adsorptive removal of the broadly used organic contaminant sulfamethoxazole (SMX) from water. Both MOFs were found highly efficient for the adsorption of SMX with the maximum adsorption capacities of 585 and 690 mg/g for MOF-525 and MOF-545, respectively. The latter value is the highest adsorption capacity reported so far for the adsorption of SMX molecules on any adsorbent. The adsorption equilibrium could be modeled successfully by the Langmuir model, which showed close to matching with the experimental data. Their adsorption equilibriums were attained within 120 and 30 minutes for MOF-525 and MOF-545, respectively. MOF-545 with mesopores demonstrated superior adsorption kinetics to MOF-525 with micropores, and the simulation by the pseudo-second-order kinetic model indicated ca. 20 times faster adsorption by MOF-545 than MOF-525. Both showed pH-dependent adsorption of SMX with a gradual reduction at high pH due to the repulsion between negatively charged adsorbent and SMX. The adsorption of SMX conducted over a group of representative MOFs with different physicochemical properties and detailed characterization confirmed that the high 1

adsorption capacity of the porphyrin MOFs is achieved by H-bonding between the SMX molecule and the N-sites of the porphyrin units in the MOFs, π-π interaction, and the high surface area. The adsorbents were easily regenerated by simple washing with acetone and reusable with > 95% efficiency during 4 repeated adsorption-desorption cycles.

KEYWORDS: Sulfamethoxazole; Aqueous phase adsorption; Porphyrinic MOFs; Hydrogen bonding; π-π interaction.

1. Introduction Sulfamethoxazole (SMX, Scheme 1(a)) is an antibiotic drug that is being used extensively to treat bacterial diseases, e.g. bronchitis, prostatitis, and urinary tract infections. After administration, however, only 10-50% of the drug is kept in the body and the rest leaves to the surroundings without any degradation. The widespread consumption of sulfa drugs, their unregulatory use and administration, high stability and low biodegradability made them a serious environmental concern (Hwang et al., 2016); these drugs introduce toxicity in aquatic ecosystems, and may lead to the evolution of drug-resistant pathogenic strains which would threaten human health. Therefore, necessary steps must be taken against the indiscriminate exposure of SMX to the aquatic environment, and adsorption is one of the most effective methods known to remove a wide range of organic contaminants from water (J. Liu et al., 2019; S. Liu et al., 2017; Yang et al., 2018). Adsorption can be carried out under mild operating conditions and efficient to capture the trace amount of antibiotics including SMX from water. Different adsorbents have been applied for the removal of SMX. These were mostly carbon-based materials: activated carbon (Akhtar et al., 2011), metal-organic framework (MOF)derived carbon (Ahmed et al., 2018), graphene (Rostamian and Behnejad, 2016), carbon 2

nanotubes (Zhang et al., 2010), and templated micro- and mesoporous carbons (Ji et al., 2010). Carbons have been most successful for the adsorptive removal of SMX owing to the strong π- π interaction between the π electrons of graphitic carbon and aromatic rings of SMX. However, other functional materials with similar specific interactive properties to the carbons, such as surface-modified MOFs (Xu et al., 2018), can also be considered as a potential adsorbent for organic pollutants such as SMX. MOFs are hybrid organic-inorganic materials with metal or metal clusters linked by organic ligands through coordination bonding (Furukawa et al., 2010) and have been extensively investigated for various potential industrial applications (Kuppler et al., 2009; Czaja et al., 2009). Porphyrinic MOFs are a special type of MOFs, where porphyrin derivatives are used as the linkers for their synthesis; porphyrins are frequently used in coordination chemistry owing to their easy complex formation at its N-binding site, and the coordinated metalloporphyrins are well known for their biomimetic catalysis (Lee et al., 2009; Lu and Kobayashi, 2016; Zucca et al., 2014). Porphyrin MOFs have been applied for catalysis (Jiang et al., 2016; Li et al., 2015), CO2 capture (Epp et al., 2018), liquid-phase adsorption (Meng et al., 2017), and sensors (Zhang et al., 2016). The porphyrinic MOFs could be a promising alternative to the carbon materials for SMX adsorption owing to their desirable textural properties and their in-built nitrogen functionality within the structure, which is expected to enhance the adsorption of antibiotic molecules via hydrogen bonding. In this study, we have prepared two porphyrinic MOFs comprised of Zr-oxide clusters linked by tetrakis(4-carboxyphenyl)porphyrins (H2TCPP, Scheme 1(b)) for the adsorption of SMX molecules from aqueous solution for the first time. As shown in Scheme 1, the structure of MOF-525 belongs to the Pm-3m̅ space group (c)) with ftw topology associated with Zr6O4(OH)4

3

clusters, while MOF-545 crystallizes in the hexagonal space group P6/mmm (d)) with a csq topology based on Zr6O8(CO2)8(H2O)8 clusters linked by square H2TCPP units. Both are highly porous and are known for their excellent thermal and chemical stabilities (Feng et al., 2012; Gao et al., 2014), and are suitable for use in aqueous systems.

Scheme 1. Structures of (a) SMX, (b) H2TCPP, (c) MOF-525, and (d) MOF-545. Green polyhedrons indicate Zr atoms, while red, blue and black spheres indicate O, N, and C atoms, respectively. H atoms are omitted for clarity.

2. Experimental 2.1. Materials Formic acid (CH2O2, Sigma-Aldrich, Germany, ≥ 95 %), SMX, (C10H11N3O3S, TCI, Japan, > 98 %), tetrakis(4-carboxyphenyl)porphyrin (H2TCPP, TCI, Japan, > 97 %), benzoic 4

acid (C7H6O2, TCI, Japan, > 99 %), zirconyl chloride octahydrate (ZrOCl2⸳8H2O, SigmaAldrich, USA, 98 %), N,N-dimethylformamide anhydrous (DMF) (C3H7NO, Sigma-Aldrich, USA, 99.8 %), acetone (C3H6O, Sigma-Aldrich, USA, 99.9 %), ethanol (C2H6O, Duksan, Korea, 99.9 %), methyl p-formylbenzoate (C9H8O3, Sigma-Aldrich, China, 97%), propionic acid (C3H6O2, TCI, Japan, > 99 %), and pyrrole (C4H5N, Sigma-Aldrich, China, 98%) were used as received without further purification.

2.2. Synthesis of the MOFs 2.2.1. Synthesis of MOF-525 1.35 g of benzoic acid and 0.105 g of zirconyl chloride octahydrate were added in a vial with 8 mL of DMF. The solution was sonicated for 20 min and heated in a conventional oven at 100 ⸳ for 1 h. After cooling it down, 0.047 g of H2TCPP was added to the solution and sonicated for 20 min. The solution was placed in a conventional oven and heated to 80 ⸳ for 24 h. After cooling it down to room temperature, the purple solid obtained was washed with DMF and acetone three times each and dried under vacuum condition (Su et al., 2016).

2.2.2. Synthesis of MOF-545 0.06 g of zirconyl chloride octahydrate and 0.012 g of H2TCPP were charged into a vial with 15 mL of DMF, and 9 mL of formic acid was added. The solution was sonicated for 20 min and heated at 135 ⸳ for 80 h in a conventional oven. After cooling it down, the dark purple needle-shaped solid obtained was washed with DMF and acetone three times each and dried under vacuum (Chen et al., 2014).

5

2.2.3. Synthesis of other MOFs Synthesis details of other MOFs were given in the Supplementary Information (SI).

2.3. Characterizations X-ray powder diffraction (XRD) patterns of the materials were obtained with a Rigaku X-ray diffractometer with a Cu Kα radiation (λ = 1.54). The N2 adsorption-desorption isotherms were obtained at 77K by a BELsorp Max (BEL Corporation, Japan) analyzer. Prior to the measurements, the samples were heated at 120 oC for 12 h under vacuum to remove moisture and impurities. Fourier transform infrared spectroscopy (FTIR) analysis was done by a VERTEX 80 V spectrophotometer (Bruker, Germany) using a pellet prepared by mixing the powder sample with potassium bromide (KBr). The morphology of the materials was examined by scanning electron microscopy (SEM) with a Hitachi-S-4300 microscope (Japan). The surface charge of the MOFs was measured with an ELSZ-2000 zeta potential analyzer (Otsuka Electronics, Japan). X-ray photoelectron spectroscopy (XPS) was conducted using a monochromatic Al Kα X-ray source and a hemispherical analyzer (Thermo Scientific, USA).

2.4. Adsorption experiment Details of adsorption experiments and the related calculations were given in the SI.

3. Results and Discussion 3.1. Characterization of the adsorbents Fig. 1(a) shows the X-ray diffraction (XRD) patterns of the MOF-525 and MOF-545 synthesized in this work, which were in good agreement with the literature data (Su et al., 2016;

6

Chen et al., 2014). The Brunauer–Emmett–Teller (BET) surface areas of the MOFs were measured from the N2 adsorption isotherms, and the pore size distributions of these materials (Fig. 1(b) and inset) were obtained using the non-localized density functional theory (NLDFT). MOF-525 and MOF-545 were both highly porous, with surface areas of 2626 and 2490 m2/g and pore volumes of 1.40 and 1.73 cm3/g, respectively. Both were made of micropores with an average pore diameter of 1.6 nm, and additional mesopores with an average pore diameter of 3.1 nm were found for MOF-545. Scanning electron microscopy (SEM) images (Fig. 2(c) and (d)) showed that the MOF-525 and MOF-545 had uniform cubic and needle-like particle morphologies, respectively (Hod et al., 2015; Chen et al., 2014).

Fig. 1. (a) XRD patterns, (b) N2 adsorption-desorption isotherms (inset shows the corresponding pore size distributions), and SEM images of the (c) MOF-525 and (d) MOF-545.

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3.2. SMX adsorption by MOF-525 and MOF-545 3.2.1. Preliminary MOFs screening for SMX adsorption Initially, SMX adsorption capacities by the representative MOFs with high structural stabilities and high surface areas were measured and compared. For this, UiO-66, UiO-66-NH2, Cu-BTC, ZIF-8, MIL-101, and NU-1000 were considered (XRD patterns and N2 adsorption isotherms of these MOFs are shown in Fig. S1) other than MOF-525 and MOF-545, and the adsorption by the metalized MOF-545(Mn) was also examined to explain the adsorption mechanism of MOF-545 (see later).

Fig. 2. The adsorption of SMX over different MOFs for 6 h. The numbers above each bar indicate the adsorption of SMX per unit surface area in µg/m2.

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The results (Fig. 2) clearly showed superior performances by MOF-525 and MOF-545 followed by NU-1000 and UiO-66-NH2. The BET surface areas of the UiO-66, UiO-66-NH2, Cu-BTC, ZIF-8, MIL-101, NU-1000, and MOF-545(Mn) were 1585, 1205, 1309, 1330, 3450, 2315, and 1909 m2/g, respectively, and MIL-101 had the highest surface area among these materials. However, the adsorption of SMX by the MIL-101 was far less than by the porphyrinic MOFs, indicating that surface area made only a minor contribution to the SMX adsorption process: the amounts of SMX adsorbed per unit surface area for each MOF are given above the corresponding bars in Fig. 2. Despite the lower surface area of the UiO-66-NH2 compared with the UiO-66, on the other hand, the SMX adsorption of the former was several times higher than that of the latter. Apparently, the surface functionality of the materials (NH2) was more important than the surface area for SMX adsorption. As is to be elaborated later, the N atoms in the porphyrinic MOF-525 and 545 could have made a similar contribution. After confirming the superior adsorption of SMX by the porphyrinic MOFs, detailed adsorption performances by MOF-525 and MOF-545 were examined next.

3.2.2. Adsorption kinetics Fig. 3(a) shows the SMX adsorption kinetics on MOF-525 and MOF-545. Adsorption took place immediately and the final equilibrium was attained within 30 and 120 min for the MOF-545 and MOF-525, respectively. MOF-545 exhibited significantly faster adsorption than MOF-525, and this difference can be explained by the dominant mesopores present in MOF-545, which were absent in the microporous MOF-525 (see Fig. 1(b) inset). Judging by the molecular size of SMX, micropores in MOF-525 are expected to accommodate only one SMX molecule at

9

a time, and the mesopores in the MOF-545 influence the adsorption kinetics as well as the adsorption capacity (Hsieh and Teng, 2000). The adsorption kinetics were modeled using both pseudo-first-order (PFO, Fig. S2(a)) and pseudo-second-order (PSO, Fig. S2(b)) models. The data were more satisfactorily represented by the PSO model with a correlation factor of approximately 1.0, and the kinetic parameters and correlation coefficients for both models are provided in Table 1. The values of the kinetic constant for the PSO model (k2) indicated that the adsorption rate by MOF-545 was ca. 20 times faster than that by MOF-525.

Fig. 3. (a) Effects of time on the adsorption of SMX and (b) the equilibrium adsorption isotherms on MOF-525 and MOF-545.

Table 1. Kinetic parameters for the adsorption of SMX on MOF-525 and MOF-545. Pseudo-first-order kinetic model Materials

Pseudo-second-order kinetic model

k1 (min-1)

R2

k2 (g/mg.min)

R2

MOF-525

5.24 × 10-3

0.904

3.90 × 10-4

0.9996

MOF-545

154 × 10-3

0.980

80.0 × 10-4

1.000

10

3.2.3. Adsorption isotherms Fig. 3(b) shows the equilibrium adsorption isotherms of SMX on MOF-525 and MOF545. Both Langmuir and Freundlich isotherm plots were produced from these data, as presented in Fig. S3(a) and (b), respectively. The Langmuir plots produced a better correlation with the experimental data (see Table 2 for the Langmuir parameters), and so the maximum adsorption capacities (Qm) for the samples were calculated using the Langmuir plots. These values were approximately 585 and 690 mg/g, respectively, and closely matched the experimental adsorption data obtained. In contrast, the commercial activated carbon only showed a Qm value of 110 mg/g (Ahmed et al., 2018). As mentioned earlier, the higher adsorption capacities of MOF-545 than those of MOF-525 can be explained by the difference in the pore structure of the two MOFs: mesopores in MOF-545 against micropores in MOL-525. To the best of our knowledge, the adsorption of SMX on MOF-545 is among the highest reported for SMX adsorption on any adsorbent to date as listed in Table 3.

Table 2. Langmuir parameters for the adsorption of SMX on MOF-525 and MOF-545. Adsorbents

Qm mg/g

b L/mg

R2

MOF-525

585

0.174

0.998

MOF-545

690

0.206

0.999

11

Table 3. Maximum adsorption capacities of adsorbents for the adsorption of SMX from water. SABET (m2/g)

Adsorbed amount (mg/g)

Exp. pH

968

22

3.0

MOF derived carbon

1855

435

3.0

Commercial AC

1016

110

3.0

MOF derived

1731

625

5.8

Adsorbents

ZIF-8

References

(Ahmed et al., 2018) H-bonding

(Ahmed et al., 2018) (Ahmed et al., 2018)

H-bonding, π–electron

(X. Li et al., 2018)

polarization

nanoporous carbon Fe2O3/CeO2 loaded

Adsorption mechanism

1536

60

-

1541

329

4.0

1469

126

4.0

848

118

-

(Akhtar et al., 2011)

activated carbon Wood-based activated

polar interactions

(Vidal et al., 2015)

carbon Sulfur-doped wood-

(Vidal et al., 2015)

based activated carbon Commercially available

electrostatic interactions, H-

(Calisto et al., 2015)

bonding and/or van der Waals

activated carbon

forces Non-activated carbon

209

1.69

~1000

282

652

159

-

(Calisto et al., 2015)

produced by pyrolysis Activated carbon with a

(J. Li et al., 2011)

membrane bioreactor Activated carbon with

7.0

Hydrophobic interaction, π–π

(Wan et al., 2014)

interaction, and H-bonding

magnetic manganese ferrite nanoparticle Microporous carbon

2311

24

5.9

π-π interaction

(Ji et al., 2010)

Mesoporous carbon

969

21

5.9

π-π interaction

(Ji et al., 2010)

Pine wood biochars

434

24.7

6.0

π-π interaction and

(Xie et al., 2014)

hydrophobicity 148

23.8

6.2

π-π interaction

(Ji et al., 2009)

Graphite

4.5

2.4

5.7

π-π interaction

(Ji et al., 2009)

MOF-525

2626

585

4.7

π–π interaction and H-

Multi-walled carbon nanotubes

This work

bonding MOF-545

2490

690

4.7

π–π interaction and Hbonding

12

This work

3.2.4. Effect of pH Fig. 4(a) presents the effect of the pH of the solution on the adsorption of SMX. Both porphyrinic MOFs showed a similar trend in adsorption within the pH range considered. The adsorption remained nearly constant from pH 2.0 to 6.0, indicating that both materials can be applied to a broad range of acidic solutions, although at pH 2.0 the adsorption was slightly lower. At pH 6.0 and above, the adsorption generally decreased with increases in pH.

Fig. 4. (a) Effects of pH on SMX adsorption and (b) Surface charge of MOF-525 and MOF-545 at different pH.

3.2.5. Thermodynamic studies To understand the effect of temperature on SMX adsorption, adsorption experiments were carried out at different temperatures ranging from 25 oC to 45 oC for both MOF-525 and MOF-545. The results are shown in Figure 5(a). Adsorption capacity gradually decreased with increases in temperature for both adsorbents. The thermodynamic parameters (∆Go, ∆Ho and ∆So) of adsorption were calculated using the van’t Hoff equations obtained from the adsorption results at different temperatures, and from the linear plots of lnKd vs 1/T (Figure 5(b)). Details of the

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procedure are given in the SI, and the thermodynamic properties are tabulated in Table 4. The negative values of ∆Ho confirmed exothermic physisorption of SMX on both adsorbents. The adsorption was favorable at lower temperatures and became unfavorable with increasing temperature. The entropy values (∆So) are negative for both cases since SMX molecules become more organized on the surface of the adsorbent after adsorption. The ∆Go values for both of the adsorbents were negative at lower temperatures and gradually approached a positive value with increasing temperatures indicating favorable adsorption only at lower temperatures (Rostamian and Behnejad, 2016).

Fig. 5. (a) Effect of temperature and (b) lnKd versus 1/T relationship for the adsorption of SMX on MOF-525 and MOF-545.

Table 4. Thermodynamic properties for the adsorption of SMX on MOF-525 and MOF-545. ∆Ho

∆So

(kJ/mol)

(J/mol·K)

MOF-525

-17.9

-53.6

MOF-545

-17.5

-48.5

R

∆Go (kJ/mol)

2

298 K

303 K

308 K

313 K

318 K

0.996

-1.92

-1.70

-1.35

-1.16

-0.89

0.997

-3.06

-2.85

-2.61

-2.37

-2.12

14

3.2.6. Adsorption mechanism Several adsorption mechanisms could potentially be involved in the adsorption of SMX on the porphyrinic MOFs. These include electrostatic interactions (Calisto et al., 2015), π-π interactions (Ji et al., 2010; Wan et al., 2014), and H bonding (Ahmed et al., 2018). We initially considered electrostatic interactions, which are affected by pH conditions. The SMX has two pKa values of 1.7 and 5.6 (Nam et al., 2015). The state of SMX at various pH values is shown in Scheme 2. Below pH 1.7, SMX is in a protonated form, while above pH 5.6, it is deprotonated. Between pH 1.7 and 5.6, the SMX is in a neutral form and therefore, should have no electrostatic interactions with the adsorbents.

Scheme 2. Configuration of SMX molecule at different solution pH.

However, the zeta potential data for the MOFs (Fig. 4(b)) show a gradual change in surface charge for both materials from positive to negative within this pH range. The slight zigzag pattern exhibited by the surface charge data and the adsorption amounts can be explained by the different pKa values of the porphyrin ligands (Jiang et al., 2013). Because SMX is negatively charged at pH values higher than 5.6, it would experience a negative-negative repulsion interaction with the adsorbent in this pH range. This repulsion would increase with increasing pH since the negative surface charge of the adsorbent also increases with pH. 15

However, some amount of SMX remained adsorbed under these conditions, indicating that electrostatic forces are not the major interaction determining SMX adsorption on the porphyrinic MOFs. The π-π interaction mechanism could explain the adsorption of SMX on the surface of an adsorbent based on interactions between the aromatic rings of SMX and the adsorbent. SMX has two aromatic rings that can undergo π-π stacking with the aromatic rings of the ligands in the MOFs (see Scheme. 1 (a) and (b)). The MOFs considered in this work comprising ligands having one aromatic ring or imidazole moieties, including the UiO-66, UiO-66-NH2, Cu-BTC, ZIF-8, and MIL-101, showed little SMX adsorption. Conversely, the NU-1000 (incorporating Zr-oxide clusters with the same csq topology as those in the MOF-545 but prepared using 4,4’,4’’,4’’’(pyrene-1,3,6,8-tetrayl) tetrabenzoic acid (H4TBAPy) with eight aromatic rings showed significant adsorption (Fig. 2). Thus, the contribution of π-π interactions based on aromatic rings in the MOF ligands to SMX adsorption is important. Lastly, the hydrogen bond formation in conjunction with N atoms in the organic struts is likely to be another factor governing the adsorption of SMX on the porphyrinic MOFs. Adsorption results can often be explained by the effect of hydrogen bonding interactions, especially, in the case of N-containing adsorbents (Ahmed and Jhung, 2017). Two hydrogen bond donor sites and six hydrogen bond acceptor sites are present in the SMX molecule and can interact with the N sites on the porphyrin ligands of the MOFs. In these interactions, the N-containing moieties can act both as hydrogen bond donors and acceptors (Adsmond and Grant, 2001; Akhtar et al., 2011). To investigate the contributions by the N sites on the porphyrinic MOFs to SMX adsorption, MOF-545(Mn) was synthesized using TCPP-Mn(III)Cl instead of H2TCPP (Hambright, 1971). The adsorption capacities before and after the metallization of MOF-545 was measured to be 145 and 100 µg/m2, respectively. The adsorption amount was reduced by approximately 31% after Mn introduction to the

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porphyrin units, which confirmed the dependence of SMX adsorption on the presence of free N sites on the porphyrinic MOFs. Fig. 6 provides Fourier transform infrared (FTIR) spectra of the MOF-525 and MOF-545 before and after SMX adsorption. Reductions in the intensities of the characteristic peaks were observed after adsorption of the SMX due to the presence of guest molecules strongly interacting with the MOF surfaces. There are also shifts and disappearances of several peaks, especially in the ranges of 1650-1580, 1342-1266, and 1252-1020 cm-1 supporting the interactions between SMX and N sites in the MOFs (“Infrared Spectroscopy Absorption Table,” 2014).

Fig. 6. FTIR spectra of the adsorbents before and after adsorption.

The importance of N-atoms in porphyrin linkers for the adsorption of SMX was further confirmed by the surface analysis of the adsorbents before and after SMX adsorption via an XPS analysis (Figure 7 and 8). The adsorption of SMX is evident from Figure 7(a) and 7(b), which 17

shows S 2p peaks only after the adsorption; these peaks must be originated from the S-atoms of the adsorbed SMX molecules. Figure 8 shows the states of N 1s orbitals after peak deconvolution. Two prominent peaks for both MOF-525 and MOF-545 were detected at ~ 397.9 and 400.2 eV, respectively, which belong to the characteristic N-peaks in the porphyrin molecules; attributed to the N-atoms of the porphyrin rings of the organic linkers in MOF-525 and MOF-545 (Macquet et al., 1978). After SMX adsorption, the intensity of the peaks having the binding energy of 397.9 eV decreased, indicating that the N-atoms having a lone pair of electrons strongly interacted with the SMX molecules during the adsorption process. The peak with the bonding energy of 400.2 eV is attributed to the N-atom bonded with H-atom in the porphyrin unit. The intensity of this peak increased after adsorption, due to overlapping with the energy band of N-H bond in the SMX molecule. The additional peak after the adsorption at 401.2 eV is due to the N-O bonds of the adsorbed SMX molecules (Li et al., 2018).

Zr3d

C1s

(b)

O1s

Counts/s

Counts/s

(a)

N1s

Zr3d

O1s

C1s

N1s

S2p

S2p

MOF-525 (Fresh) MOF-525 (After adsorption) 200

400

600

MOF-545 (Fresh) MOF-545 (after adsoprtion)

800

200

Bond energy (eV)

400

600

Bond energy (eV)

Fig. 7. XPS spectra of (a) MOF-525 and (b) MOF-545.

18

800

Fig. 8. Deconvoluted N1s XPS spectra of (a) MOF-525 and (b) MOF-545.

3.2.7. Reusability of the adsorbents Fig. 9 summarizes the SMX adsorption values after regeneration of the MOF-525 and MOF-545 using an acetone wash. The regenerated MOF-525 and MOF-545 retained up to 97% and 95% of their preliminary adsorption capacities, respectively. XRD patterns (Fig. 10), FTIR spectra (Fig. 11) and SEM images (Fig. S4) obtained from the MOFs showed negligible changes after use, and the slight reduction in adsorption capacity after use is likely due to the difficulty in completely removing the adsorbed SMX molecules from the MOF pores. Washing with ethanol (Fig. S5), on the other hand, recovered only 78% and 60% of the original adsorption capacities of the MOF-525 and MOF-545, respectively, after the fourth reuse. Evidently, acetone is a better solvent for SMX desorption than ethanol.

19

Fig. 9. Adsorption of SMX after regeneration of (a) MOF-525 and (b) MOF-545 by acetone washing.

Fig. 10. XRD patterns of (a) MOF-525 and (b) MOF-545: fresh and regenerated states after SMX adsorption.

20

Fig. 11. FTIR spectra of the fresh and regenerated MOF-545.

4. Conclusions Two porphyrinic MOFs, MOF-525 and MOF-545, were applied to the adsorption of SMX from water, and the maximum adsorption capacity of the MOF-545 (690 mg/g), in particular, was found to exceed those previously reported by other adsorbents. Both MOFs showed fast adsorption, and MOF-545 showed approximately four times faster adsorption kinetics compared with the MOF-525, due to its mesoporous structure. Both π-π interactions and H bond formation based on N atoms in the MOF structure are likely governing the adsorption of SMX on these porphyrinic MOFs. The adsorbents were easily regenerated by washing with acetone and could be reused for four repeated adsorption–desorption cycles without significant performance deterioration.

21

Acknowledgment This work was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF), with funding by the Ministry of Science and ICT (grant number: NRF-2015R1A4A1042434).

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Highlights

Highly efficient adsorptive removal of sulfamethoxazole from aqueous solutions by porphyrinic MOF-525 and MOF-545 Kwangsun Yua,†, Imteaz Ahmeda,†, Dong-Il Wonb, Wan In Leeb, Wha-Seung Ahna,* a.

Department of Chemical Engineering, Inha University, Incheon 22201, Republic of Korea. E-

mail: [email protected]; Fax: +82 328720959; Tel; +82 328607466 b.

Department of Chemistry, Inha University, Incheon 22201, Republic of Korea.

†. Equal contribution

Highlights •

Porphyrinic MOFs were applied to remove antibiotics from water for the first time.

•

Fast adsorption kinetics to reach equilibrium in 30 minutes by MOF-545.

•

Highest adsorption capacity of 650 mg/g was achieved by MOF-545.

•

MOF-545 was stable and recyclable with acetone wash.

1

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: