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Selectivity Adsorptive Mechanism of Different Nitrophenols on UIO66 and UIO-66-NH2 in Aqueous Solution Guangran Lv, Jianming Liu,* Zhenhu Xiong,* Zhanhang Zhang, and Ziyang Guan School of Environmental and Municipal Engineering, Tianjin Key Laboratory of Aquatic Science and Technology, Tianjin Chengjian University, Tianjin 300384, P. R. China S Supporting Information *

ABSTRACT: UIO-66 and UIO-66-NH2 were synthesized by a solvent thermal method, and their textures were characterized. The adsorption selectivity, kinetics, isotherms, and mechanism of the two MOFs in a single solute system for phenol, 4-nitrophenol, 2,4dinitrophenol, and 2,4,6-trinitrophenol were studied. At pH 4.0, the two MOFs exhibited a greater adsorption capacity for 2,4dinitrophenol than for other nitrophenols; however, at lower substrate concentrations (C0 < 100 mg/L), the adsorption capacity of UiO-66-NH2 for 2,4-dinitrophenol was greater than that of UiO66. The study of the adsorption mechanism showed that the electrostatic attraction is the main way in which UIO-66 and UiO-66NH2 adsorb nitrophenols. Moreover, there was π-electron accumulation at a certain angle between the benzene ring of the adsorbents and the benzene ring of nitrophenols, whereas UiO-66-NH2 could also combine the nitrophenols through the hydrogen bonds originating from the amino groups. The fitting results of adsorption data showed the pseudo-second-order model, and the Langmuir isotherm model could well represent the adsorption kinetics and isotherm adsorption processes, respectively. In the mixed solution consisting of 4-nitrophenol, 2,4-dinitrophenol, and 2,4,6-trinitrophenol, the competitive adsorption ability of UIO-66 and UIO-66-NH2 for 2,4-dinitrophenol were greater than for the other two nitrophenols, showing that the size of the substrate and the surface area of the adsorbent were also factors that affect the adsorption efficiency. functions.8 MOFs have a high specific surface area and a highly ordered pore structure, and their hole size and shape are easily adjusted.9 They are more diverse than conventional adsorbents and have been widely used in gas storage,10 gas/liquid separation,11 CO2 capture,12 catalytic reaction,13 and the removal of harmful substances. Studies have shown that there are a variety of complex interactions between the outer surface of MOFs and the substrate molecules.14 The electrostatic effect can be predicted by measuring the zeta potential of the solid surface;15,16 in this sense, the electrostatic effect can be achieved by human control.17,18 Recently, some of the amino derivatives of MOFs were used for the adsorption of chemicals in different media,19,20 and it has been found that −NH2 can promote the electrostatic interaction between MOFs and a substrate. For example, the zirconium(IV)-based MOF (UiO-66) that is based on a Zr6O4(OH)4 octahedron was synthesized21 and formed lattices by a 12-fold connection through a 1,4-benzenedicarboxylate (bdc) linker. Soon UiO-66-NH2 had also been synthesized by substituting 2-amino-1,4-benzenedicarboxylic acid for 1,4-benzenedicarboxylic acid in UiO-66.22 UiO-66 and UiO-66-NH2 were observed to be water-stable MOFs exhibiting high hydrothermal stability up to 773 K.23 Mean-

1. INTRODUCTION Nitrophenols are found in a variety of industrial drainage, including (but not limited to) the oil, pharmaceutical, plastics, coal, and steel industries. Nitrophenols are toxic and carcinogenic even at very low concentration for humans, animals, and wildlife. The compounds have been classified as priority pollutants by the U.S. Environmental Protection Agency (EPA), and their maximum permissible concentration in wastewater was set to be less than 1 ppm (mg/kg).1 Therefore, the removal of nitrophenols from polluted water has attracted a great amount of attention. However, the removal of nitrophenols is still a challenge because of the high stability and solubility of nitrophenols in water.2 Some measures have been used to remove nitrophenols from water, such as photocatalytic degradation3 and adsorption.4,5 In these methods, adsorption has became a competitive method for effectiveness at low concentration,6,7 and activated carbon was the most widely used adsorbent for removing nitrophenols because of the simple preparation method. However, activated carbon also had some disadvantages, such as high price and low selectivity. Therefore, research and development pertaining to materials with high capacity and selectivity for the adsorption of nitrophenols is essential. Metal−organic frameworks (MOFs) are a new type of organic inorganic hybrid porous material composed of metal ion clusters/chains and an organic ligand with multiple complex © XXXX American Chemical Society

Received: July 3, 2016 Accepted: October 5, 2016

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DOI: 10.1021/acs.jced.6b00581 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 1. Physical and chemical properties of UiO-66 and UiO-66-NH2: (a) FT-IR spectra; (b) XRD patterns and (i, ii) SEM images of UiO-66 and UiO-66-NH2; (c) thermogravimetric curves; and (d) nitrogen adsorption isotherms.

(Tianjin, China). All of the chemicals were used as received without further purification. 2.2. Preparation of the Adsorbents. Zr-based framework UiO-66 was synthesized according to the reported procedure with modification.22 In a typical synthesis, ZrCl4 ((2.92 g, 12.5 mmol), 1,4-benzenedicarboxylic acid (2.075 g, 12.5 mmol), and 12.5 mmol HCl (HCl, 37%, 1.0739 mL) were dissolved in DMF (60 mL), and then the mixture was ultrasonicated for 30 min. The as-obtained mixture was transferred to a stainless steel Teflon-lined autoclave of 120 mL capacity, and the temperature was maintained at 493 K for 16 h. After this time, the autoclave was cooled to room temperature and the resulting solid was filtered, washed with methanol, and dried overnight at 423 K. UiO-66-NH2 was synthesized by the same method using 2amino-benzenedicarboxylic acid instead of 1,4-benzenedicarboxylic acid. 2.3. Characterization. The morphologies of UIO-66 and UIO-66-NH2 were observed with a scanning electron microscope (SEM; model EPMA-8705QH2, Shimadzu Co., Japan). Before SEM measurement, the samples were dispersed on a monocrystalline wafer and then were treated with spray gold for 45 s. X-ray diffraction (XRD) patterns of the solid samples were determined by a D/max-IIIC X-ray diffractometer (Shimadzu, Japan) with Cu Kα X-rays as the target in a scanning range of 5−50° at a scanning speed of 6°/min. N2 adsorption− desorption (BET) was performed on a TristarII 3020 M surface area and porosity analyzer. Before the test, all of the samples were reduced in pressure and underwent degassing at 150 °C for 12 h. The specific surface areas of the adsorbents were calculated by a BET method in the range of 0.05−0.15 P/ P0. The total pore volume was obtained from the adsorbed

while, the binding of an amino-modified MOF with phenolic compounds was studied, and it has been thought that the interaction between the amino-modified MOF (MIL-101(Al)NH2)24 and 4-nitrophenol was controlled by hydrogen bonding rather than the electrostatic interaction.21 In fact, the aqueous solution is a complex system in which the adsorption of organic compounds on MOFs is dominated by many factors and rarely controlled by a single factor. To confirm the hypothesis, it is necessary to study the interaction between amino derivatives of MOFs and the substrate so that the controllable design of the MOFs can be realized, which is appropriate for removing specific contaminants in aqueous solution. In this work, two MOFs (UIO-66 and UIO-66-NH2) were prepared, and their adsorption selectivity for different nitrophenols (phenol, 4-nitrophenol, 2,4-dinitrophenol and 2,4,6trinitrophenol) in both single and mixed solutions was studied for the first time. The role of amino groups on UIO-66-NH2 in the adsorption was discussed, and the adsorption kinetics and adsorption isotherms of the nitrophenols on UIO-66 and UIO66-NH2 were evaluated. Finally, the adsorption mechanism of UIO-66 and UIO-66-NH2 on the nitrophenols was proposed.

2. EXPERIMENTAL SECTION 2.1. Materials. ZrCl4, 1,4-benzenedicarboxylic acid, and N,N-dimethylformamide (DMF) were purchased from J&K Scientific Ltd. (Shanghai, China). 2-Amino-benzenedicarboxylic acid (99%) was purchased from Sigma-Aldrich Ltd. (Beijing, China). Methanol (HPLC and AR grade), ethanol (AR grade), hydrochloric acid (37%), ammonium acetate, formate, phenol (99.5%), 2,4,6-trinitrophenol, 2,4-dinitrophenol, and 4-nitrophenol were provided by the Jiangtian Chemical Co., Ltd. B



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was similar to the ligand of UiO-66. The SEM images of UiO66 and UiO-66-NH2 are shown in illustrations i and ii in Figure 1b. It can be found that UiO-66 crystals were intergrown nanoparticles and the crystal size ranged from 200 to 500 nm. In illustration b, UiO-66-NH2 crystals were also intergrown with a size range comparable to that of UiO-66. The thermogravimetric (TG) curves of UiO-66 and UiO-66-NH2 samples are displayed in Figure 1c. The TG curves showed the chemical composition of the samples. From ambient temperature to 900 °C, both TG curves exhibited a series of weight loss. The weight loss below 100 °C can be attributed to the loss of the free carboxyl group. From 100 to 400 °C, small weight losses were observed, and the phenomenon was due to the removal of water molecules and the decomposition of the organic ligands. However, because the temperature exceeded 400 °C, significant weight loss was found, suggesting that most organic matter had been decomposed and even the the skeleton had collapsed. In Figure 1c, the residual weight of UiO-66-NH2 was higher than that of UiO-66, which indicated that UiO-66NH2 contained a large amount of organic matter residue (amino group). The nitrogen adsorption isotherms (Figure 1d) revealed that the amino functionalization caused a reduction in porosity for UiO-66-NH2. In other words, the porosity of UiO-66 was greater than that of UiO-66-NH2. Meanwhile, the surface area and pore volume of UiO-66-NH2 were lower than those of UiO-66 (Table 1). Therefore, it can be said that the additional volume of amino groups is not conducive to improving the physical and chemical properties of UiO-66-NH2.

amount at a relative pressure of 0.99. Fourier transform infrared spectroscopy (FT-IR) of UiO-66 and UiO-66-NH2 was conducted with a Germany Bruker Company EQUI-NOX55, using the KBr pellet pressing method in the scanning wavelength range of 4000−400 cm−1. Thermogravimetric analysis of the adsorbent samples were carried out using a Rigaku TG-8120 by scanning from 25 to 900 °C at a heating rate 4 °C/min under the protection of nitrogen. The zeta potential of UiO-66 and UiO-66-NH2 was measured with a Zetasizer Nano-ZS from Malvern Instruments. 2.4. Adsorption Experiments. Batch adsorption experiments were carried out by using a thermostatic shaker at different temperatures. Before adsorption, the aqueous concentrations of 4-nitrnol (4-NP), 2,4-dinitrophenol (DNP) and 2,4,6-trinitrophenol (TNP) (20−400 mg/L) were prepared with deionized water, respectively, and the adsorbents were dried overnight at 423 K in a vacuum oven to ensure full activation. In the adsorption experiments, the MOFs (0.020 g) and the nitrophenol solutions (50 mL, with a concentration of 20−400 mg/L) were placed in a series of conical flasks, and then the flasks were placed vertically on the table and shaken for a period of time at a speed of 200 rpm/min to make the solute mixture reach complete adsorption equilibrium. 2.5. Analysis of the Nitrophenols. Before analysis, the reaction mixtures were filtered with injection filters (PTFE, hydrophilic, 0.45 μm) and the concentration of the solutes were determined with an HPLC meter (Agilent 1110, USA) with an Eclipse plus C18 (2.1 mm × 150 mm, 3.5 μm) column. The mobile phases of HPLC were (A) 10 mmol ammonium acetate + 0.1% formic acid and (B) acetonitrile (75/25, v/v) at a flow rate of 0.60 mL/min. The temperature of the column was maintained at 298 K, and the detector wavelength was 220 nm. The amounts of different species in solution were calculated by the following mass balance equations

Table 1. Surface Area and Pore Volume of UiO-66 and UiO66-NH2 2

qe =

V (C0 − Ce) M

surface area (m /g) pore volume (cm3/g)

(1)

UIO-66

UIO-66-NH2

1188 0.48

1023 0.45

3.2. Adsorption Selectivity of UiO-66 and UiO-66-NH2 for Different Nitrophenols. In general, there is a large difference in the adsorption selection and capacity of MOFs with different structures for pollutants in aqueous solution, and the difference is related to the composition of the organic ligand and metal (or cluster) of MOFs. In a single chemical adsorption experiment, in order to explore the interaction between UiO-66 (and/or amino derivatives) and nitrophenols, four 80 mg/L nitrophenols (phenol, 4-nitrophenol, 2,4dinitrophenol, and 2,4,6-trinitrophenol) were used for the model compounds to compare the adsorption capacity of the nitrophenols on UiO-66 and UiO-66-NH2, and the adsorption experiment results are shown in Figure 2. As shown in Figure 2a, the equilibrium adsorption capacities (qe) of UiO-66 and UiO-66-NH2 for the nitrophenols apart from TNP increased with the increase in the number of nitro groups. The pKa values of different nitrophenols [phenol (9.89) < 4-nitrophenol (7.15) < 2,4-dinitrophenol (4.09) < 2,4,6trinitrophenol (0.25)] decreased with the increase in the number of nitro groups. According to the literature,25 if the pH of a solution was lower than the pKa of a compound that was mixed in the solution, the compound would be positively or neutrally charged while the pH would be greater than the pKa and the compound would be negatively charge. Meanwhile, if the solution pH was lower than the zero-charge point of a solid, then the solid surface would be positively charge; when pH >

V (C0 − Ct ) (2) M where C0 and Ct (mg/L) are the initial and remaining concentrations of nitrophenol in the solutions, respectively, Ce is the remaining concentration of the nitrophenol in the solutions, qt and qe (mmol/g) are the weights of nitrophenol adsorbed per unit of adsorbent at time t and adsorption equilibrium, V is the volume of the liquid phase (L), and M is the mass of the adsorbent (g). qt =

3. RESULTS AND DISCUSSION 3.1. UIO-66 and UIO-66-NH2 Characterization. The infrared spectrum data of the sample are given in Figure 1a. There were some absorption peaks in the infrared spectra of UiO-66 that come from the aromatic rings and carboxyl groups. For example, the peak at 1420 cm−1 was attributed to the C−C vibrational mode, and the peak at 1580 cm−1 was the stretching vibration of the C−O bond in the carboxyl group. For UiO-66NH2, the C−N stretching band and N−H rocking vibration were present at 1258 and 764 cm−1, respectively, and the positions of these absorption peaks were consistent with the literature report.19 The XRD spectra of UiO-66 and UiO-66NH2 are shown in Figure 1b. It can be seen that the pattern of UiO-66 was consistent with that of UiO-66-NH2, the reason being that the UiO-66-NH2 ligand was NH2-H2BDC, which C



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Figure 2. Comparison of the adsorption of (a) UiO-66 and (b) UiO-66-NH2 on phenol, 4-nitrophenol, 2,4-dinitrophenol, and 2,4,6-trinitrophenol. Experimental conditions: pH 4.0, msorbent = 20 mg, V solution = 50 mL, C0 = 80 mg/L, and T = 15 °C.

Figure 3. Effects of contact time and temperature on the 2,4-dinitrophenol adsorption to (a) UiO-66 and (b) UiO-66-NH2 in water. Experimental conditions: pH 4.0, msorbent = 20 mg, Vsolution = 50 mL, and C0 = 50 mg/L.

nitro groups in the phenol ring lead to a larger binding capacity, which partially results from 4-nitrophenol and 2,4-dinitrophenol getting into the pores of UiO-66 and UiO-66-NH2 by intraparticle diffusion, but 2,4,6-trinitrophenol cannot get into the pores because the molecular volume of 2,4,6-trinitrophenol is much larger than that of 4-nitrophenol and 2,4-dinitrophenol, though 2,4,6-trinitrophenol is subjected to a larger electrostatic attraction. Therefore, it should be believed that there are different forces affecting the combination of MOFs with nitrophenol, and the electrostatic force is just one of the effective ways. The qe values of 2,4-dinitrophenol and 2,4,6-trinitrophenol on UiO-66-NH2 (Figure 2) were similar to that of UiO-66, which means that the introduced amino groups effectively failed to improve the adsorption capacity of UiO-66-NH2. The reason may relate to the specific surface area of the adsorbents. In Table 1, the specific surface area of UiO-66 is larger than that of UiO-66-NH2, so the qe value of UiO-66 was greater than that of UiO-66-NH2. However, it should be noted that the -NH2 groups can form hydrogen bonds with the -NO2 groups, which will aid the adsorption of nitrophenol on UiO-66-NH2. These are two opposing interactions that lead to similar qe values of UiO-66 and UiO-66-NH2 for nitrophenols. 3.3. Adsorption Kinetics of 2,4-Dinitrophenol on UiO66 and UiO-66-NH2. The first-order equation was used to evaluate the adsorption processes of nitrophenols on UiO-66 and UiO-66-NH2, which can be expressed as nonlinear and linear forms (eqs 3 and 4, respectively)27

the zero charge point, the solid surface is dominated by negative charge.26 Because the zero-charge points of UiO-66 and UiO-66-NH2 were about pH 5.5 and 6.5 in our experiment, respectively, the adsorbents were positively charged in pH 4.0 solutions (Figure S1). The effect of pH on the adsorption capacity was also given in Figure S2, where the adsorption capacity of nitrophenols under a stronger acidic aqueous condition (pH < 4) is low because the charge property of nitrophenols is neutral, which is difficult to combine with the positively charged adsorbents. However, the qe value increased gradually and reached the maximum value as the solution pH increased and reached the zero-charge point (about pH 6). Subsequently, the qe value decreased gradually as the solution pH value continued to increase. The result may relate to the surface charge of UiO-66 and UiO-66-NH2, in which the surface charge property would change from positive to negative near the zero-charge point. Besides, the number of nitro groups on the phenol ring is an important factor affecting the charge properties. (That is, the more nitro groups, the stronger the negative charge property.) The electrostatic attraction between the substrates (from 4-nitrophenol to 2,4-dinitrophenol) and the adsorbents (UiO-66 and UiO-66-NH2) increases rapidly with the increase in the number of nitro groups so that the qe values for 4-nitrophenol and 2,4-dinitrophenol were about 50 and 140 mg/g, respectively. On the other hand, it can be found (Figure 2) that the qe of 2,4,6-trinitrophenol was about 120 mg/g, which is lower than the qe of 2,4-dinitrophenol. However, this result is not consistent with the fact that more D



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Table 2. Model Constants for 2,4-Dinitrophenol Adsorption on UiO-66 and UiO-66-NH2 under Different Conditions as Derived from the Pseudo-First-Order and Pseudo-Second-Order Equations conditions adsorbent UIO-66 (50 mg/L)

UIO-66-NH2 (50 mg/L)

pseudo-first-order rate model temp (°C)

qe(mg/g)

15 35 55 15 35 55

89.32 76.55 60.17 93.02 80.99 65.96

K1(min

−1

1.1584 1.2536 1.3334 0.7328 0.6820 0.5903

)

pseudo-second-order rate model R

2

0.9913 0.9868 0.9843 0.8947 0.8837 0.8924

qe(mg/g)

K2(g·mg−1min−1)

R2

90.22 77.38 60.74 95.75 83.50 68.06

0.0425 0.0524 0.0780 0.0129 0.0133 0.0140

0.9961 0.9907 0.9918 0.9355 0.9289 0.9394

Figure 4. Adsorption isotherms of 2,4-dinitrophenol on (a) UiO-66 and (b) UiO-66-NH2 in water at different temperatures. Experimental conditions: pH 4.0, t = 360 min, msorbent = 20 mg, and Vsolution = 50 mL.

qt = qe − qee(−k1t )

(3)

ln(qe − qt ) = ln qe − k1t

(4)

pseudo-second-order equation because the correlation coefficient of the pseudo-second-order equations (R22) listed in Table 2 is much larger than the R12 value of the pseudo-firstorder equation. Therefore, the pseudo-second-order equation was more suitable for fitting the adsorption kinetic data of UiO66 and UiO-66-NH2 to 2,4-dinitrophenol than was the pseudofirst-order equation. The k values of the fitting curves relating to UiO-66 and UiO-66-NH2 were also summarized in Table 2. It could be observed that the k2 value of UiO-66 is greater than that of UiO-66-NH2 at the test temperature and that the phenomenon reflecting the adsorption rate of UiO-66 was greater than that of UiO-66-NH2; the reason may be that the pore volume of UiO-66-NH2 was smaller than the pore volume of UiO-66 (Table 1). In addition, comparing the qe values of pseudo-first order to those of pseudo-second order (Table 2), it can be found that the qe values of UiO-66-NH2 are relatively large; the reason may be attributed to the hydrogen bonding between amino groups of UiO-66-NH2 and 2,4-dinitrophenol. 3.4. Adsorption Isotherms of UiO-66 and UiO-66-NH2 to 2,4- Dinitrophenol. The adsorption isotherms of 2,4dinitrophenol on UiO-66 and UiO-66-NH2 at different temperature are shown in Figure 4. Obviously, the equilibrium adsorption capacity (qe) is generally related to the concentration of the adsorbate. It can seen from Figure 4a,b that the qe of the two adsorbents increased to 250−350 mg/g as Ce increased from 0 to 300 mg/L. Moreover, qe decreased when the temperature increased, which indicated that the adsorption of 2,4-dinitrophenol on UiO-66 and UiO-66-NH2 was exothermic. The Langmuir isotherm is based on the assumption that the adsorption process takes place at specific homogeneous sites within the adsorbent surface and that once a substrate molecule occupies a site no further adsorption can take place at that site,

−1

where k1(min ) is the first-order rate constant. The dotted lines in Figure 3a,b represent the fitted kinetic curve related to the adsorption of UiO-66 and UiO-66-NH2 to 2,4-dinitrophenol, respectively. According to the fitted result, the first-order rate equation did not fit the adsorption kinetic data of 2,4dinitrophenol on UiO-66 and UiO-66-NH2 well. The reason is that the correlation coefficients (Table 2) of the first-order rate equation (R12) at different temperatures were somehow low; in particular, the R2 value of UiO-66-NH2 was lower than 0.90, which shows that the first-order rate equation is not a satisfactory adsorption kinetic model. The second-order equation that can be expressed as nonlinear and linear forms (eqs 5 and 6, respectively) was also used to evaluate the adsorption results of nitrophenols on UiO-66 and UiO-66-NH2 to study the adsorption kinetics thoroughly28,29 qt =

k 2qe 2t 1 + k 2qet

(5)

t 1 t = + 2 qt qe k 2qe −1

(6) −1

where k2 (g mg min ) is the second-order rate constants. The k2, R2, and qe values of the second-order kinetics were calculated from the plot of t/qt versus t and are listed in Table 2. The solid lines in Figure 3a,b represent the kinetic curves fitted by the second-order rate mode (eq 6) that is related to the adsorption of UiO-66 and UiO-66-NH2 to 2,4-dinitrophenol. It can be seen that the kinetic data are well fitted by the E



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Table 3. Adsorption Isotherm Model Constants Derived from Langmuir and Freundlich Isotherms Langmuir adsorbent UIO-66 (50 mg/L)


Freundlich

temp (°C)

qm(mg/g)

KL(L/mg)

R2

KF(mg/g)

n

R2

15 35 55 15 35 55

573.0 519.0 277.1 261.9 211.7 187.2

0.0515 0.0513 0.0348 0.1711 0.3365 0.1221

0.9853 0.9726 0.9771 0.9704 0.9805 0.9920

43.77 37.37 25.98 69.48 70.29 36.38

2.726 2.746 2.651 4.458 5.830 4.079

0.9765 0.9677 0.9506 0.9163 0.9354 0.9632

Figure 5. Adsorption of the three nitrophenols on UiO-66 and UiO-66-NH2. Fitting results of the pseudo-first-order kinetics (dotted line) and pseudo-second-order kinetics (solid line) of three nitrophenols on UiO-66 (a) and UiO-66-NH2 (b) and the isotherms of three nitrophenols on UiO-66 (c) and UiO-66-NH2 (d). Experimental conditions: pH = 4.0, t = 360 min, msorbent = 20 mg, Vsolution = 50 mL, C0 = 50 mg/L, and T = 15 °C.

mechanism and the nonlinear and linear forms of the Freundlich isotherm model can be expressed by eqs 9 and 10, respectively,31

which showed that the adsorption process is monolayer in nature.30 The nonlinear and linear forms of isotherm models can be expressed by eqs 7 and 8, respectively,31 qe =

qe = KFCe1/ n

KLqmCe 1 + KLCe

Ce C 1 = + e qe qmKL qm

(7)

ln qe = ln KF + (1/n)ln Ce

(9) (10)

where KF (μmol/g) and n are the adsorption constants that indicate the adsorption capacity and the value of 1/n ranging from 0.1 to 1.0 represents a favorable adsorption condition. The dashed lines in Figure 4 are the isothermal curves fitted by the Freundlich model relating to the adsorption of 2,4dinitrophenol on UiO-66 and UiO-66-NH2. The adsorption isotherm data analyzed by the Langmuir model (dashed line) and the Freundlich model (solid line) are given in Figure 4, and the correlation coefficient (RL2) is summarized in Table 3. The parameters (qm and KL) of the model at different temperature (Table 3) were calculated,

(8)

where Ce is the equilibrium concentration of the adsorbate (μmol/L), qe is the equilibrium adsorption capacity (μmol/g), qm is the maximum adsorption capacity of the adsorbate (μmol/g), and KL represents the affinity constant (L/μmol). The isothermal parameters were also expressed by the Freundlich isotherm model, which is an empirical equation assuming that the adsorption process takes place on a heterogeneous surface through a multilayer adsorption F



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Table 4. Adsorption Isotherm Model Constants in the Mixture Solution Derived from Langmuir Isotherms UIO-66 (50 mg/L)


adsorbent

temp (°C)

qm(mg/g)

KL(L/mg)

R2

qm(mg/g)

KL(L/mg)

R2

4-NP DNP TNP

15

290.7 130.2 63.3

0.2092 0.0130 0.0027

0.9488 0.9633 0.9857

264.8 113.6 58.4

0.2506 0.3026 0.1221

0.9829 0.9745 0.9762

Figure 6. Schematic diagram of the main mechanism for the adsorption of nitrophenols on UiO-66 and UiO-66-NH2.

mixture solution was approximately equal to that in the single solute system (Figure 3a,b). However, the qe values of 4nitrophenol and 2,4,6-trinitrophenol were decreased from 35 to 20 mg/g and from 100 to 60 mg/g, respectively. This showed that there was competitive binding of different substrates to the adsorption sites but that the competitive binding had little effect on the qe of 2,4-dinitrophenol. In other words, the competitive ability of 2,4-dinitrophenol was higher than that of the other two nitrophenols, indicating that the number of nitro groups is not the only factor that determines the qe of nitrophenols. Besides, upon increasing the solute concentration in the solution containing the three nitrophenols, the qe values of the nitrophenols all increased. However, compared to the qe of 2,4-dinitrophenol in the single solute system (Figure 4), the qe in the mixed solution was decreased (from 250 to 350 to 200 mg/g), and the reason might be the result that the other two nitrophenols competed with 2,4-dinitrophenol. The Langmuir model was still used to fit the adsorption data of the three nitrophenols on UiO-66 and UiO-66-NH2 in the solution containing the three solutes (Figure 5c,d). Similar to the results in a single solute system, the Langmuir model fit the adsorption isotherm data well in the mixture solution, showing that the competition of different nitrophenols did not change the adsorption properties of UiO-66 and UiO-66-NH2 for nitrophenols. The parameters (KF and n) of the isothermal model in the solution containing three kinds of substrates are given in Table 4. 3.6. Adsorption Mechanism. Several mechanisms such as electrostatic interactions, coordination, π−π stacking, and hydrogen bonding have been proposed as possible interpretations for the adsorption of organic compounds on MOFs.33,34 Among these mechanisms, the electrostatic effect is the most common, and the important parameters of electrostatic force are the zeta potential of the adsorbent and the pKa value of the adsorbate. In this study, the suggested mechanism is based on the adsorption of nitrophenols at various pH values and zeta potentials of UiO-66 or UiO-66-NH2. As discussed in Section 3.2, although the modification with -NH2 and the resulting UiO-66-NH2 has a more negative zeta potential compared to

respectively, by the intercept and slope of eq 8. In Table 3, it can be seen that RL2 was greater than 0.97, which indicated that the Langmuir isotherm was suitable for describing the isothermal adsorption data. It can be observed that the correlation coefficient (RF2) of the Freundlich model was smaller than that of the Langmuir model (Table 3), which means that the Freundlich isotherm model could not efficiently describe the isothermal adsorption data. The parameters (KF and n) of the isothermal model are also given in Table 3. It could be observed that the qm was decreased with the increase in temperature, which also meant that the adsorption of 2,4-dinitrophenol was exothermic. In Figure 4, under a low substrate concentration (C0 < 50 mg/L), the qmax value of UiO-66-NH2 was similar to that of UiO-66; however, at a high substrate concentration (C0 > 100 mg/L), the qmax value of UiO-66-NH2 was less than that of UiO-66, which means that the amino group of the adsorbent is the main binding site at low substrate concentration but the amino group did not promote the adsorption of 2,4-dinitrophenol at high substrate concentration. Furthermore, the surface area of the adsorbent was also the main factor (SBET (UiO-66-NH2) < SBET (UiO-66)) controlling the adsorption process. Besides, the experimental n value of the Freundlich model is greater than 1, which indicates that there is a chemical affinity between the substrate (nitrophenol) and the adsorbents (UiO-66 and UiO66-NH2).32 3.5. Competitive Adsorption of Different Nitrophenols in a Mixed System. The kinetic curves of UiO-66 and UiO-66-NH2 adsorbing the three nitrophenols in the solution containing three kinds of substrates with same initial concentration are given in Figure 5a,b. It could be observed that the adsorption data of the mixture system was better fit by a pseudo-second-order equation than by a pseudo-first-order equation. Nevertheless, it should be emphasized that the adsorption kinetics in the mixture system were consistent with that in the single substrate system. In Figure 5a,b, the qe values of UiO-66 and UiO-66-NH2 for the three nitrophenols increased with time. The qe (∼100 mg/ g) of 2,4-dinitrophenol on UiO-66 and UiO-66-NH2 in the G



Article

2,4-dinitrophenol, and 2,4,6-trinitrophenol), and the two kinds of MOFs showed high selective abilities for 2,4-dinitrophenol. UiO-66-NH2 and UiO-66 gave the following adsorption capacity hierarchy for nitrophenols: 2,4-dinitrophenol > 2,4,6trinitrophenol > 4-nitrnol > phenol. In this study, the effects of adsorption time and temperature on the adsorption process were investigated, and a pseudo-second-order model was found to fit the adsorption kinetics well. The Langmuir model was better than the Freundlich model for describing the isothermal adsorption data. Calculation results showed that the adsorption of nitrophenol on UiO-66 and UiO-66-NH2 was exothermic. The analysis of the adsorption mechanism showed that the main driving force for the adsorption of nitrophenols on UiO66 and UiO-66-NH2 was the electrostatic effect, whereas the hydrogen bond between UiO-66-NH2 and nitrophenols as well π−π stacking between benzene rings of the two adsorbents and the nitrophenols may also play a role. In addition, the larger molecular size of 2,4,6-trinitrophenol does not favor its diffusion into the pores of the adsorbents; in this instance, its competitive adsorption capacity is low. The repeated use of the adsorbent showed that UiO-66 and UiO-66-NH2 can be regenerated by washing with water, and it is expected to be the effective adsorptive material for the removal of nitrophenols in aqueous solution.

that of UiO-66, this did not improve the adsorption capacity of UiO-66-NH2. This means that there are other ways to dominate the adsorption of the three nitrophenols on UiO66 and UiO-66-NH2. Because the -NH2 groups can form hydrogen bonds with the -NO2 groups, the three -NO2 groups in 2,4,6-trinitrophenol will aid the binding of UiO-66-NH2 so that the qe value of 2,4,6-nitrophenol is greater than that of 4nitrophenol, which has only one nitrophenol in its molecular structure. In brief, the higher adsorption efficiency of the three nitrophenols is attributed to the electrostatic attraction interactions between the adsorbents and UiO-66, although the hydrogen bond between 2,4,6-nitrophenol and UiO-66NH2 also helps to improve their combination. In addition, the π−π stacking interaction between benzenes of nitrophenols and the ligands of the adsorbents might be the way to the interaction. Finally, Figure 6 shows a diagram of the main adsorption mechanism for the combination of UiO-66 and UiO-66-NH2 with nitrophenols. 3.7. Regeneration of the Adsorbents. The reusability of adsorbents is an important factor in measuring the potential of their applications. For that reason, regeneration tests for used adsorbent were carried out. First, the used adsorbent (UiO-66 or UiO-66-NH2, 0.020g) was placed into a vial containing 100 mL water, and then the vial was vibrated in a ultrasonic bath for 60 min. After filtration and separation, the adsorbents were washed with 5 mL of ethanol three times. Then, the adsorbents were dried at 60 °C in a vacuum oven for 4 h for the next adsorption process. Finally, the adsorption capacity of 2,4dinitrophenol was calculated for each adsorption cycle by HPLC detection. The results, obtained by five repeated measurements for UiO-66 and UiO-66-NH2, are shown in Figure 7. It can be observed that UiO-66 and UiO-66-NH2 used for five cycles still had good adsorption capacities for 2,4dinitrophenol.

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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/acs.jced.6b00581. Change in zeta potential for UiO-66 and UiO-66-NH2 with pH and the effect of solution pH on the amount of adsorbed 2,4-dinitrophenol with UiO-66 and UiO-66NH2 (PDF)

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4. CONCLUSIONS UiO-66 and amino-functionalized UiO-66-NH2 were prepared in the study. In aqueous solutions of both a single substrate and multiple substrates, UiO-66-NH2 and UiO-66 displayed high adsorption capacities for nitrophenols (phenol, 4-nitrophenol,

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Funding

This work was supported by the National Natural Science Foundation of China (no. 50878138). Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 7. Reusability of UiO-66 and UiO-66-NH for the adsorption of 2,4-dinitrophenol. Experimental conditions: pH 4.0, t = 360 min, msorbent = 20 mg, Vsolution = 50 mL, C0 = 80 mg/L, and T = 15 °C. H



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