Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Effective Removal of Naphthalenesulfonic Acid from Water Using Functionalized Metal−Organic Frameworks Huifang Zhao,† Xudong Zhao,*,† Zhuqing Gao,† and Dahuan Liu*,‡ †
College of Chemical and Biological Engineering, Taiyuan University of Science and Technology, 030024 Taiyuan, China State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, 100029 Beijing, China
‡
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S Supporting Information *
ABSTRACT: Effective removal of toxic 1,5-naphthalenedisulfonic acid (NDS) and 2-naphthalenesulfonic acid (NSA) from contaminated water still faces a challenge to date. Herein, porous MIL-101, MIL-101-NH2, and MIL-101-SO3H were systematically studied for their adsorption performances toward these toxic molecules. Upon the introduction of −NH2 groups, H-bond interaction and electrostatic interaction were enhanced, and accordingly MIL-101-NH2 can exhibit more excellent adsorption capacity (NDS, 188.1 mg g−1; NSA, 285.0 mg g−1) compared to MIL-101SO3H, MIL-101, and some other common adsorbents. Further study indicates that MIL-101-NH2 can be easily regenerated even after three adsorption processes. This work demonstrates that MIL-101-NH2 may have large potential in naphthalenesulfonic acids removal and rational selection of organic functional groups may be critical in construction of adsorbents.
1. INTRODUCTION
In general, for the removal of acid molecules excellent acid stability is necessary in constructing MOF-based adsorbents. In this respect, some Cr3+-based or Zr4+-based MOFs may exhibit potentials such as UiO-66, MIL-100, and MIL-101. Meanwhile, the existence of −SO3H groups in NDS and NSA as well as the strong ionization (−SO3H → −SO3− + H+) suggests that introduction of H-bond interaction and electrostatic interaction may be an effective method. Therefore, in this work the Cr3+-based MOFs, MIL-101-R (R = H, NH2, SO3H) were studied as adsorbents for NDS and NSA due to their excellent water and acid stability, large specific surface areas and pore sizes, and various organic functional groups. Experimental data indicates that MIL-101NH2 has larger adsorption capacity compared to original MIL101 upon enhanced electrostatic interaction and H-bond interaction, whereas MIL-101-SO3H exhibits poor adsorption performance due to strong electrostatic repulsion. Furthermore, the effect of coexisting ions and solution pH, and regeneration ability of MIL-101-NH2 were also investigated.
Over the past decades, aromatic sulfonic compounds have been widely used in chemical industry. However, long-term use of these sulfonic compounds can result in a large threat to environmental safety and human health for their xenobiotic characters.1−3 Therefore, effective removal of these toxic compounds from aqueous solution is of great importance. Up to date, several methods have been explored and adsorptive removal methods attract much attention due to low byproducts, easy operation, and high removal efficiency.4−6 In the past years, several adsorbents have been synthesized to remove naphthalenesulfonic acids from water such as activated carbon, macroporous polymer, and basic resin.1,6−8 However, these materials may face the drawbacks in adsorption capacity or adsorption rate. For example, the naphthalenesulfonate, detected in the river Elbe water from Germany, cannot be totally removed by activated carbon.9 Therefore, introduction of a new class of porous adsorbents is necessary. Metal−organic frameworks (MOFs), a novel class of porous solids prepared by self-assembly of metal ions and organic ligands,10 have aroused considerable interest due to their potential applications in luminescence,11,12 adsorption,13−15 separation,16,17 catalysis,18,19 and so on. In particular, hydrostable MOFs have been widely studied in liquid phase adsorption and separation.20−27 To date, MOFs as adsorbents for pH-neutral pollutants have been largely reported as dyes and ions,28−31 whereas the studies on acid compound removal in MOFs are rarely reported especially for the toxic 1,5naphthalenedisulfonic acid (NDS) and 2-naphthalenesulfonic acid (NSA). © XXXX American Chemical Society
2. MATERIALS AND METHODS 2.1. Chemicals. Chromic nitrate hydrate (Cr(NO3)3· 9H 2 O), chromium oxide (CrO 3 ), terephthalic acid (H2BDC), monosodium 2-sulfotephthalic acid (H2BDCSO3Na), 2-aminoterephthalic acid (H2BDC-NH2), solvents, and the naphthalenesulfonic acids (NDS and NSA), were purchased from HWRK Chem. and used without further Received: April 20, 2018 Accepted: June 6, 2018
A
DOI: 10.1021/acs.jced.8b00318 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
stability of MOFs toward the naphthalenesulfonic acids was studied via PXRD measurement. 2.5. Adsorption Experiments. All the adsorption experiments in this work were performed at the form of batch adsorption. In a 20 mL vessel, 10 mg MOF sample and 10 mL NSA- or NDS-containing solution were added and stirred in a shaking table at 25 °C. After being stirred for some time, the mixture was filtrated using a microfiltration membrane (0.22 μm) and the collected clear filtration solution was used for concentration measurement through a UV−vis spectrometer (TU-1901, Persee). In detail, in the adsorption isotherms measurement, the adsorption time was set as 20 h; in the kinetics experiments, the initial concentrations of the naphthalenesulfonic acids were set as 1000 mg L−1; in the adsorption experiments with coexisting ions, the initial concentrations of NDS and NSA were set as 500 mg L−1 and the molar concentrations of coexisting ions were equal to those of the naphthalenesulfonic acids. 2.6. Regeneration Experiments. MIL-101-NH2 after adsorption was dispersed into 30 mL of mixed aqueous solution (hydrochloric acid, 0.5 M; Na2SO4, 0.1 M) for 12 h. Then, the collected solid via centrifugation was washed using deionized water and ethanol for two times. At last, the solid was dried at 100 °C overnight.
purification. The molecular structures of NDS and NSA are listed in Figure 1.
Figure 1. Molecular structures of (a) NDS and (b) NSA (color: C, gray; H, white; O, red; S, yellow; the molecule structures are optimized by geometry optimization in Material Studio 7.0).
2.2. MOFs Synthesis. MIL-101, MIL-101-NH2, and MIL101-SO3H were synthesized through the typical hydrothermal methods according to the literature (see the Supporting Information for detailed description).32−34 2.3. Characterization Methods. The powder X-ray diffraction (PXRD) patterns of the MILs were recorded on a D8 Advance X diffractometer equipped with Cu Kα radiation (λ = 1.54178 Å) at room temperature. Nitrogen adsorption− desorption measurements at 77 K were performed on an Autosorb-iQ-MP surface area analyzer. The FT-IR spectroscopy was recorded on a Nicolet iS50 FTIR spectrophotometer. In addition, the ζ-potentials of the samples were obtained through a Zetasizer Nano ZS Zeta potential analyzer. 2.4. Stability Experiments. In a 100 mL vessel, 40 mg of MOFs were added into 40 mL of naphthalenesulfonic acids solutions with the concentration of 1000 mg L−1. After stirring for 20 h, a filtration operation was performed. At last, the collected solid was dried at 100 °C in an oven for 12 h. The
3. RESULTS AND DISCUSSION 3.1. Characterization of MOFs and Stability Investigation. As shown in Figure 2a, the PXRD patterns of synthesized MIL-101 and MIL-101-SO3H are almost consistent to the simulated XRD pattern of MIL-101 crystal; meanwhile, the main characteristic peaks in PXRD pattern of MIL-101-NH2 can also fit with those of MIL-101 even with not a very good crystallinity. To further evaluate the permanent porosity of the MOFs, N2 adsorption−desorption
Figure 2. Characterization results of MIL-101s: (a) PXRD patterns of as-synthesized MIL-101s; (b) N2 adsorption−desorption isotherms at 77 K; (c) FT-IR spectra; (d−f) PXRD patterns of MIL-101s after contacting with NDS and NSA in aqueous solutions. B
DOI: 10.1021/acs.jced.8b00318 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Figure 3. Adsorption capacities for (a) NDS and (b) NSA versus contact time.
Table 1. Kinetics Models Parameters of NDS and NSA Adsorbed on the MIL-101s pseudo-first-order model pollutant
Qe,exp (mg g )
Qe,cal (mg g )
k1 (min )
R
Qe,cal (mg g )
k2 (g min−1 mg−1)
R2
MIL-101-NH2
NDS NSA NDS NSA NDS NSA
188.1 285.0 129.2 244.0 30.0 60.4
75.28 13.42 29.79 56.65 9.54 23.05
0.02044 0.01436 0.01581 0.01835 0.01214 0.01753
0.8850 0.3441 0.7959 0.8463 0.5955 0.8583
190.11 284.09 128.70 243.90 29.56 60.09
0.0012 0.0213 0.0037 0.0026 0.0086 0.0046
0.9987 1 0.9996 0.9999 0.9989 0.9991
MIL-101-SO3H
−1
−1
pseudo-second-order model
MOF
MIL-101
−1
2
−1
Figure 4. Adsorption isotherms of the MOFs for (a) NDS and (b) NSA.
other, demonstrating the excellent stability of these MIL-101s toward NDS and NSA aqueous solutions. 3.2. Adsorption of NDS and NSA. 3.2.1. Adsorption Kinetics. The effect of contact time on the adsorption was first investigated. As shown in Figure 3a, the adsorption process of NDS in MIL-101 and MIL-101-NH2 can reach equilibrium at ∼130 min; the adsorption in MIL-101-SO3H is faster while the adsorption capacity is much poorer. For NSA, from Figure 3b it can be seen that MIL-101-NH2 can exhibit faster adsorption equilibrium at ∼15 min compared to other two MOFs and other materials in reported literatures.6,7 Further, to study the adsorption kinetic characteristics of the MIL-101s, two classical models, pseudo-first-order model and pseudo-second-order model (see the Supporting Information for detailed description), were used to fit the adsorption data.35 From the fitting results in Figure S1 and S2 and Table 1, the adsorption of NDS and NSA on all the three MIL-101s can fit well with pseudo-second-order model, indicating the rate-
isotherms were measured (Figure 2b) and the BET specific surface areas of MIL-101, MIL-101-NH2, and MIL-101-SO3H were calculated to be 3016.5, 1480.3, and 1546.2 m2 g−1, respectively, consistent to the values in reported works.32−34 Furthermore, the FT-IR spectra of samples were also studied to confirm the existence of characteristic organic functional groups in the MOFs. From Figure 2c, in the spectrum of MIL101-NH2, the peak at 1250 cm−1 can be assigned to the stretching vibration of −C−N bond originated from the ligand BDC-NH2;32 the peaks at 1160 and 1230 cm−1 in MIL-101SO3H spectrum can be attributed to the stretching vibration of −SO3 groups.33 These results demonstrate the successful preparation of the MOFs. Furthermore, the stability of these MOFs frameworks was investigated via PXRD measurement. From Figure 2d−f, it can be observed that the PXRD patterns of the MOFs before and after contacting with NDS and NSA can be consistent to each C
DOI: 10.1021/acs.jced.8b00318 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 2. Langmuir and Freundlich Isotherms Models Parameters of NDS and NSA Adsorbed on the MOFs Langmuir isotherm
Freundlich isotherm
MOF
pollutant
Qe,exp (mg g−1)
Qm (mg g−1)
kL (L g−1)
R2
KF ((L mg−1)1/n mg g−1)
1/n (g min−1 mg−1)
R2
MIL-101-NH2
NDS NSA NDS NSA NDS NSA
197.0 296 113.0 268 36.0 77
185.53 307.69 110.86 315.46 22.59 37.22
0.0336 0.0099 0.0273 0.0043 −0.0123 −0.0074
0.9748 0.9472 0.9890 0.8547 0.5460 0.3935
30.3206 21.269 15.4152 9.419 0.0974 0.0027
0.2768 0.3980 0.2977 0.4935 0.7029 1.2771
0.8681 0.9871 0.8249 0.9880 0.6968 0.4997
MIL-101 MIL-101-SO3H
Figure 5. Adsorption capacities of other common materials for NDS (a) and NSA (b).
Figure 6. Effect of coexisting ions on NDS (a) and NSA (b) adsorption over the MIL-101s. (The molar concentrations of the salts are consistent with NDS or NSA.)
Figures S3 and S4 and Table 2, it can be seen that NDS adsorption on MIL-101 and MIL-101-NH2 follows Langmuir isotherm model and NSA adsorption on these two MOFs follows Freundlich isotherm model. The adsorption data of MIL-101-SO3H for NDS and NSA cannot fit well with both the two models for its very low adsorption capacities at the low concentration range. Furthermore, some common adsorbents in the field of water treatment were measured for their adsorption capacities (Figure 5) and comparison was performed. Experimental results indicate that MIL-101-NH2 can exhibit superior performances than these materials such as activated carbon,
limiting step in the adsorption process of the three MOFs may be a chemisorption process.35 3.2.2. Adsorption Isotherms. Adsorption isotherms were further measured to further evaluate the potential of the MOFs. The adsorption time was set as 20 h according to the kinetic experiments. As shown in Figure 4a,b, MIL-101-NH2 exhibits larger adsorption capacities for both NDS and NSA than MIL-101 and MIL-101-SO3H. In detail, the largest adsorption capacities of MIL-101-NH2 for NDS and NSA in the selected concentration range are 188.1 and 285.0 mg g−1, respectively. Further, Langmuir and Freundlich isotherms models (see the Supporting Information for detailed description) were used to fit the adsorption data.35 From D
DOI: 10.1021/acs.jced.8b00318 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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13 X zeolite, activated Al2O3, silica gel, fly ash, MCM-41, and SBA-15. 3.2.3. Effect of Coexisting Ions. Real water commonly contains some inorganic ions such as Na+, K+, Ca2+, Mg2+, Zn2+, Cl−, NO3−, and SO42−, which may have influences on the adsorption performances of adsorbents. Therefore, in order to evaluate the practical application of these MOFs, the adsorption capacities in the simulated real sample were further measured. As shown in Figure 6, the effect of the ions depends on the types of adsorbents and ions. Detailed analysis for MIL101 and MIL-101-NH2 is as follows and in the case of MIL101-SO3H is omitted for its very poor adsorption performance: (i) For MIL-101-NH2, monovalent ions (e.g., Na+, K+, Cl−, NO3−) have negligible effects on the adsorption of both NDS and NSA; divalent ions especially for SO42− have negative effects on NDS and NSA adsorption, which may be attributed to the strong competition of SO42− for the adsorption sites.36,37 (ii) For MIL-101, almost all the ions have negative effects on NDS adsorption but negligible or positive effects on NSA adsorption. 3.2.4. Adsorption Mechanism. The natural pH values of NDS and NSA aqueous solutions (conc. 1000 mg L−1) are ∼2.5 and ∼3.5, respectively, indicating that these naphthalenesulfonic molecules exist at the form of anions in water. On the other hand, according to Figure 7, the surfaces of MIL-101
negatively charged in the whole pH range due to the strong ionization of −SO3H groups in the ligand BDC−SO3H (−SO3H → −SO3− + H+). These results indicate electrostatic interaction may play significant effect on the adsorption behavior of the MIL-101s. Accordingly, as shown in Figure 8, solution pH exhibits large effects on the adsorption of NDS and NSA over the MILs. The possible adsorption mechanism was analyzed: (i ) MIL-101 surface is positively charged in most of the tested pH range (
