Adsorption Behaviors of Organic Micropollutants on Zirconium Metal

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Adsorption Behaviors of Organic Micropollutants on Zirconium Metal-Organic Framework UiO-66: Analysis of Surface Interactions Caiqin Chen, Dezhi Chen, Shasha Xie, Hongying Quan, Xubiao Luo, and Lin Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13443 • Publication Date (Web): 27 Oct 2017 Downloaded from http://pubs.acs.org on October 30, 2017

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ACS Applied Materials & Interfaces

Adsorption Behaviors of Organic Micropollutants on Zirconium Metal-Organic Framework UiO-66: Analysis of Surface Interactions Caiqin Chena, Dezhi Chena*, Shasha Xiea, Hongying Quanb, Xubiao Luoa*, Lin Guoa,c a

Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle,

School of Environmental and Chemical Engineering, Nanchang Hangkong University, Nanchang 330063, China Email: [email protected] (Dr. D. Chen); [email protected] (Prof. X. Luo) b

School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang

330063, China c

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of

Education, School of Chemistry and Environment, Beihang University, Beijing 100191, China

KEYWORDS: adsorption; organic micropollutants; UiO-66; surface interactions; water treatment

ABSTRACT

Herein, we studied the adsorption behaviors of organic micropollutants, such as anticonvulsant carbamazepine (CBZ) and antibiotic tetracycline hydrochloride (TC), on zirconium metal-

ACS Paragon Plus Environment

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organic framework UiO-66 in water. The maximum adsorption capacities of CBZ and TC on the UiO-66 were 37.2 and 23.1 mg·g-1 at 25 oC, respectively. The adsorption isotherms and kinetics of CBZ and TC were well described by using the Langmuir model and pseudo-second-order model, respectively, and the adsorptions on UiO-66 are endothermic reactions. The adsorption capacities of CBZ and TC on UiO-66 were decreased with the increase of solution pH. The presence of humic acid could improve the adsorption of CBZ and TC on UiO-66, but K+ ion inhibited their adsorption obviously. In addition, Ca2+, and Al3+ ions also suppressed the adsorption of TC on UiO-66. The competitive adsorption suggested the adsorption sites for CBZ on UiO-66 were different from those for TC. The surface interactions between UiO-66 and the two micropollutants were demonstrated by powder X-ray diffraction, Fourier transform infrared (FT-IR) spectra, scanning electron microscopy, nitrogen adsorption/desorption isotherms and Xray photoelectron (XPS) spectra. The characterizations showed that the adsorption of CBZ on UiO-66 is mainly a physisorption, and the hydrophobic effect played a crucial role during the adsorption of CBZ, meanwhile weak π–π electron donor-acceptor interaction and electrostatic attraction also existed. However, the adsorption of TC on UiO-66 is mainly a chemisorption, in addition to the strong electrostatic attraction and π–π electron donor-acceptor interaction forces, the nitrogenous groups of TC played an important role, which can replace the carboxylic groups coordinated with Zr-O clusters. The obtained results will aid us to comprehend the surface interaction between organic micropollutants and UiO-66, and expand the application of UiO-66 as sorbent for removal of pollutants from water.

1. INTRODUCTION


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Metal-organic framework is the kind of three-dimensional porous crystal material linked by an inorganic metal with multi-tooth organic ligand of the aromatic acid or alkali contained nitrogen, oxygen via a coordination bond, which has aroused broad attention in recent decades.1 UiO-66, with many predominant advantages such as tailorable structure and high surface area and super stability, is the most representative Zr-metal organic framework. UiO-66 consist of octahedral [Zr6O4(OH)4] clusters with twelve 1,4-benzenedicarboxylic acid.2-9 In light of this threedimensional structure, UiO-66 is one of the most stable metal-organic framework materials, and can maintain its own three-dimensional structure even at 500 oC.10 In the recent years, because of high surface area and quiet stability, UiO-66 has been promising porous materials for adsorption, such as CO2 uptake and hydrogen storage and the other gas adsorption.4-6, 11-13 Water-stable UiO-66 membranes for gas permeation and ion rejection also exhibited superior performance of ion retention rate and remarkable chemical stability.7 With these wonderful performance, so it is a deep incentive for the scientific researcher to apply UiO66 in the field of water purification. Water stability is the basic factor for MOFs to be applied for water purification. UiO-66 had showed an outstanding adsorption capacity for organic contaminants such as organic dyes in aqueous.14 The maximum adsorption amount of UiO-66 for acid orange 7(AO7) even could reach to 358 mg·g-1, which was imputed to an open active metal site lie in UiO-66 that observably heightened the adsorption, tremendously increasing the interaction force with the AO7.15 Acid-promoted UiO-66 exhibited the selective adsorption to anionic dyes in aqueous solution because of a more regular space structure, higher specific surface area and more positive Zeta potential.16 In addition, the superior adsorption performance for organic dyes was partially attributed to hydrophobicity and π-π interactions as well as electrostatic interactions. Owing to the electrostatic attraction and the π-π interaction between the


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benzene ring of the organic ligand of UiO-66 and the benzene ring of phenolic compounds, UiO66 also showed high adsorption performance for removal of phenolic compounds.17 UiO-66 showed a well potential adsorbent for adsorption of acetic acid, which the adsorption capacity of acetic acid was higher than other MOFs.18 The behavior of adsorption of acetic acid by UiO-66 was recognized as chemical adsorption, however, the adsorption mechanism was not clear. UiO66 could easily and effectively get rid of methylchlorophenoxypropionic acid from water.19 UiO-66 is also popular with removal of inorganic pollutants such as heavy metal ions in water pollution treatment. The UiO-66 is an efficient adsorbent for the removal of arsenic ions and Sb(III,V), the critical factor is that hydroxyl group and terephthalic acid ligand of UiO-66 devote to two adsorption of binding sites.20-21 Simultaneously, the functionalized UiO-66 for water pollution has also aroused a wide range of researchers’ attention. The amine groups functionalized UiO-66 is a hopeful candidate for removal in contaminated water remediation because of many advantages of superb stability, great adsorption capacity and so on. And it can remove heavy metal Cd2+, Cr3+, Pb2+, Hg2+

20

, Sb (III, V)

22

, SeO42-

23

and so on. And Br-

functionalized UiO-66 24 also exhibited a prominent removal rate for mercury, and the adsorption capacity was several times than that of UiO-66. The additional bromine on UiO-66 increases the chemical adsorption sites of mercury. And it is a crucial role that the Zr-O bond and the amino group of UiO-66-NH2 for adsorption of Sb. Besides, UiO-66 also as an effective adsorbent to remove boron with high adsorption capacity, and wonderful performance of regeneration and recycling, which could be attributed to the interaction with the Zr site.25 To further expand the application of UiO-66 as a sorbent for removal of pollutants from water, it is necessary to study the adsorption behaviors of emerging pollutants, such as pharmaceutical and personal care products (PPCPs), on UiO-66. PPCPs include drugs such as antibiotics,


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developers, painkillers, and personal care products such as spices, skin care products, hair care products and so on.26-27 Studies have shown that PPCPs can interfere with the energy transfer system and affect the secretion of hormones in the body.28 Some drugs that chemical structure has been specially designed have a potential and cumulative adverse effects on non-target organisms.29 The human who drinks water containing a low concentration of PPCPs in long-term will pose a greater risk to health.26, 28, 30-31 However, compared with the extensive attention on the adsorption of PPCPs using the carbonaceous materials

32-35

, so far, there has been a lack to

investigate the adsorption of PPCPs on UiO-66 to the best of our knowledge. Therefore, two typical PPCPs, anticonvulsant carbamazepine (CBZ) and tetracycline hydrochloride (TC), were chosen as the target contaminant (Table 1). The effluent containing CBZ was discharged into the surface water directly causing a certain harm to people's health

36-37

, for example a decrease as

the number of platelets, granulocytes, and leukocytes and liver and kidney failure.38-39 In the process of biological metabolism, the vast majority of (TC) enters into the water environment through the feces and urine directly from the body, which poses a hazard to the aquatic ecosystem and induces the production of resistance genes in the environment and results in persistent contamination.40-41 Herein, the adsorption behaviors of CBZ and TC on UiO-66 were studied in detail, and the underlying mechanisms were clarified. The obtained results would provide valuable insights into the development of eﬃcient MOF-based adsorbents for environmental remediation. 2. EXPERIMENTAL SECTION 2.1 Materials and reagents


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Zirconium tetrachloride (ZrCl4) and1,4-benzenedicarboxylic acid were purchased from Aladdin (Shanghai, China), N, N-dimethylformamide (DMF) was purchased from Damao chemical reagent factory (Tianjin, China). Chloroform was purchased from Xinguang Fine Chemical Plant (Nanchang, China). Sodium chloride (NaCl), potassium chloride (KCl) and aluminum chloride hexahydrate (AlCl3•6H2O) were purchased from Xilong Scientific Co., Ltd. (Guangdong, China). Calcium chloride anhydrous was purchased from Shanghai Fengxian Fengcheng Reagent Factory (Shanghai, China). Humic acid was purchased from Zhiyuan Chemical Reagent Co. Ltd. (Tianjing, China). All chemicals were of A.R. grade, and used as received without any further purification. 2.2 Synthesis of UiO-66 The UiO-66 sorbent was synthesized by reported method.2 0.053 g of ZrCl4 (0.227 mmol) was dissolved in 24.9 g of N, N-dimethylformamide (DMF) (340 mmol) with ultrasound, then 0.034 g of 1,4-benzenedicarboxylic acid (H2BDC) (0.227 mmol) was added to the above solution evenly. The reaction solution was transferred to a 50-mL vessel placed in an oven at 120 ˚C for 24 h. White precipitation was obtained, and washed by DMF for three times and then soaked by trichloromethane for 24 h with three times to remove the residual DMF. Finally, UiO-66 sorbent was acquired after drying at 100 ˚C in an oven. 2.3 Sorption Experiments 2.3.1 Batch adsorption All sorption experiments were carried out using an orbital shaker at 180 rpm under pre-set temperature. 20 mg of the UiO-66 sorbent was added into the conical flasks with 40 mL target


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pollutants with the concentration (C0) from (10 mg·L-1 to 100 mg·L-1). To study the effect of pH, the initial solution pH for CBZ solution (pH=2 to 10) and TC solution (pH=2 to 8) was adjusted by adding 1 M NaOH or 1 M HCl. A duration of 48 h was employed for the experiments. After reached adsorption equilibration, the sorbent was removed using filter method, and then the residual CBZ and TC in aqueous solution were analyzed by UV spectrophotometer (Hitachi U3010/3310) at 285 nm and 365 nm, respectively. The quantity of organic contaminants absorbed on per unit mass of UiO-66 was calculated by the following equation:

‫ݍ‬௧ = ሺ‫ܥ‬଴ − ‫ܥ‬௧ ሻ

௏

(1)

௠

where qt (mg·g-1) is the adsorbing capacity when the time is t, C0 (mg·L-1) is the initial concentration of target contaminant, Ct (mg·L-1) is the concentration of target contaminant when the time is t, V is the volume of the solution, and m is the mass of adsorbent. 2.3.2 Adsorption kinetic The adsorption kinetics of CBZ and TC on UiO-66 were revealed as followed. 200 mg adsorbent of UiO-66 into 400 mL target contaminant (100 mg·L-1) solution with constant magnetic stirrers at 25 oC. Then, 1.5 mL solution sample was collected at predetermined time intervals and immediately were filtered to remove the adsorbent by a 0.22 µm filters. The residual CBZ and TC in aqueous solution were analyzed by UV spectrophotometer at 285 nm and 365 nm, respectively. 2.3.3 Competitive adsorption The competitive adsorption of CBZ and TC on UiO-66 sorbent was carried as followed three procedures: 1) 20 mg of the UiO-66 sorbent was added into the conical flasks with 40 mL of 100


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mg L-1 CBZ solution. After the adsorption equilibrium, then other 40 mL of 100 mg·L-1 TC was added into the conical flasks; 2) 20 mg of the UiO-66 sorbent was added into the conical flasks with 40 mL of 100mg·L-1 TC solution. After the adsorption equilibrium, then other 40 mL of 100 mg L-1 CBZ was added into the conical flasks; 3) 20 mg of the UiO-66 sorbent was added into the conical flasks with 40 mL of 100 mg L-1 CBZ solution and 40 mL of 100 mg·L-1 TC solution. After the adsorption equilibrium, 1.5 mL of solution sample was obtained and the adsorbent was removed by a 0.22 µm filters. Finally, the mass of CBZ and TC was tested by UV spectrophotometer at 285 nm and 365 nm, respectively. 2.3.4 Effect of humic acid and cations In this part, humic acid and four different cations (Na+, K+, Ca2+, and Al3+) were selected to evaluate the practical environmental remediation of UiO-66 for removal of CBZ and TC, respectively. 80 mL of 100 mg L-1 CBZ or TC solution containing 100 mmol L-1 of NaCl, KCl, CaCl2, AlCl3 and humic acid were prepared, respectively. 40 mg of the UiO-66 sorbent was added into the conical flasks with the prepared solution, which placed into the temperaturecontrolled shaker at 25 oC shaken for 48 h and shaken at 180 rpm. After the adsorption equilibrium, 1.5 mL of solution sample was obtained and the adsorbent was removed by a 0.22 µm filters. Finally, the mass of CBZ and TC was tested by UV spectrophotometer at 285 nm and 365 nm, respectively. 2.3.5 Reusability of UiO-66 100 mg adsorbent of UiO-66 was added into the conical flasks with 200 mL target contaminant solution (100 mg·L-1 CBZ, TC respectively), and then placed into the temperature-controlled shaker at 25 oC shaken for 48 h at 180 rpm. After the adsorption equilibrium, 1.5 mL of solution sample was obtained and the adsorbent was removed by a 0.22 µm filters. Finally, the mass of


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CBZ and TC was tested by UV spectrophotometer at 285 nm and 365 nm, respectively. Then, the recycled solids were soaked in chloroform for 24 h to desorb the CBZ and TC. After washed using methanol and dried at 80 ˚C in an oven for 24 h, the obtained solids were re-added into the target contaminant solution to study the regeneration and reusability of UiO-66. This whole process was repeated for three times. 2.4 Characterization Structure and morphology of the UiO-66 sorbent were characterized using Bruker X-ray diffraction (XRD, D8 Advanced) at a scan rate of 4° min-1, FEI scanning electron microscope (SEM, Quanta 450) and JEOL transmission electron microscope (TEM, 2100). The nitrogen adsorption-desorption isotherms were acquired on a TriStar II 3020 surface area & pore size analyzer (Micromeritics). The specific surface areas and pore size distributions were calculated from Brunauer–Emmett–Teller (BET) method and Density Functional Theory (DFT), respectively. Fourier transform infrared (FTIR) spectra were obtained on a Vertex70 FTIR spectrometer (Bruker). The surface zeta potential of UiO-66 was measured by Malvern NanoZS90 Zetaszier. The surface properties of UiO-66 sorbent were analyzed using an X-ray photoelectron (XPS) spectra on an Axis Ultra (Kratos) XPS spectrometer. The pH was measured by PHS-3E (Shanghai INESA Scientific Instrument Co., Ltd) 3. RESULTS AND DISCUSSION 3.1 Adsorption behaviors 3.1.1 Sorption isotherms


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Figure 1a and 1b show adsorption isotherms of CBZ and TC on UiO-66 at a different temperature. The adsorption capacities of both CBZ and TC on UiO-66 increase with increasing temperature, indicating the processes of adsorption are all endothermic reaction. It can be well match with the results of thermodynamic studies (∆H°CBZ=70.9 kJ·mol-1 and ∆H°TC=56.6 kJ·mol1

, see Section S4 and Table S1). Figure 1a shows that the adsorption capacity of the CBZ on

UiO-66 gradually increased with the increasing initial concentration, and then the increase of adsorption capacity slowed down until to adsorption equilibrium. The adsorption capacity of TC on UiO-66 (Figure 1b) increased rapidly with the initial concentration of TC increasing from 0 to 40 mg·L−1. From 40 to 100 mg·L−1, the increase in the rate of adsorption capacity gradually decreases. Both Langmuir and Freundlich models (see Section S2) were used to fit the experimental data, the results are presented in Table 2. The values of the correlation coefficients (R2) in Table 2 suggest that the Langmuir model is more suitable for adsorption of CBZ and TC than the Freundlich model, which implies that the adsorption of CBZ and TC on UiO-66 may take place in a monolayer adsorption manner. The maximum adsorption capacity of CBA and TC on UiO66 calculated by the Langmuir equation were 37.2, 42.2, 60.3 and 23.1, 31.0, 46.4 mg·g-1 at 25, 35, and 45 oC, respectively. 3.1.2 Adsorption kinetics Figure 2a shows the adsorption kinetic of CBZ on sorbent at 25 oC, it is obvious that the adsorption capacity of CBZ on UiO-66 sorbent increased rapidly in the first 40 min and then increased slowly until the adsorption equilibrium. Figure 2b presents the adsorption kinetic of TC on UiO-66, we can see that the adsorption capacity of TC on UiO-66 sorbent increased


10

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rapidly in the first 50 min. The adsorption kinetics of CBZ and TC on UiO-66 were fitted using pseudo-first-order and pseudo-second-order kinetics model (Section S3), respectively. The obtained adsorption kinetic parameters of CBZ and TC are shown in Table 3. It is obvious that adsorption kinetic of CBZ and TC on UiO-66 well fit the pseudo-second-order model. 3.1.3 Effects of pH Effects of pH on the adsorption capacity of CBZ and TC can see from Figure 3. Our experiment has proved that CBZ is very stable even in strong acid (pH=1.8) and alkali (pH=12.1) conditions. With the increase of pH (2 to 6 and 8 to 12), the adsorption capacity of CBZ decreased. The zeta potential of UiO-66 decreased with increasing pH value of the solution. As we can see from Figure S2a, the isoelectric point of UiO-66 suspension solution is 4.81. When the pH value is less than 4.81, the material is positively charged. In contrast, the material becomes negatively charged. Under an acidic environment of pH < 4.81, the UiO-66 is covered positive charge, Figure S2b shows the zeta potential of CBZ and TC, TC is electronegative from pH=3 to 8, and the isoelectric point of CBZ is 2.46, indicating the CBZ is electronegative at pH > 2.46. Therefore, when the pH value is less than 4.81, electrostatic interaction force is the main force of adsorption; and when pH value is greater than 4.81, the adsorbent and the target contaminant covered the same charge, but the adsorption capacity of CBZ is changed not obvious until pH=11, probably because it existed π-π interaction and hydrogen-bond interaction between the benzene ring of organic link of UiO-66 and CBZ. From Figure 3b we can see that the adsorption capacity of TC was decreased gradually with increasing pH. As the pH increases, the number of positively charges on the surface of UiO-66 is gradually reduced. Meanwhile, TC is negatively charged, so the intensity of the electrostatic interaction is gradually weakened.


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3.1.4 Competitive adsorption Figure 4 shows the competitive adsorption of CBZ and TC on the UiO-66 sorbent. When only CBZ in system, the equilibrium sorption capacity (the black bar) of CBZ on sorbent was around 26.1 mg·g-1. Following, 40 mL of TC (100 mg·L-1) was added into the above system, we found that the equilibrium adsorption capacity of CBZ on sorbent was increased to 34.7 mg·g-1 (the red bar). Even after the adsorption equilibrium of TC on UiO-66, there still 27.8 mg of CBZ could be adsorbed on the per gram of sorbent (the blue bar). For TC, the equilibrium sorption capacity (the black bar) of TC on sorbent was around 16.7 mg·g-1. After that, the further adding of CBZ (40 mL, 100 mg·L-1) could increase the adsorption capacity of TC up to 28.2 mg·g-1 (the blue bar). Even after the adsorption CBZ on sorbent reaches to equilibrium, the adsorption capacity of TC remained to 15.4 mg·g-1 (the red bar). It indicates that the presence of TC can significantly promote the adsorption capacity of CBZ even after adsorption of CBZ to equilibrium. While it has little effect on the adsorption capacity of TC itself. Similarly, the adsorption capacity of CBZ was not observably promoted when CBZ solution was added after TC to adsorption equilibrium, but it greatly increased the adsorption capacity of TC. Therefore, it can be explained that when adsorption of one of the substances to adsorption equilibrium, and then another one was added, the latter can enhance the adsorption capacity of the former and has little effect on the adsorption of itself. Furthermore, when the 20 mg of the UiO-66 sorbent was added into the two-component system of CBZ and TC, we can see that the adsorption capacity of CBZ was increased up to 40.8 mg·g-1, and that of TC were still 15.0 mg·g-1. The obtained results suggest that the adsorption sites for CBZ on UiO-66 are different from those for TC. 3.1.5 Effect of humic acid and cations


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Humic acid and four different cations (Na+, K+, Ca2+, and Al3+) were selected to evaluate the practical environmental interference of UiO-66 for removal of CBZ and TC, respectively. Figure 5a shows the adsorption capacities of TC and CBZ on UiO-66 under the presence of humic acid, Na+, K+, Ca2+, and Al3+, respectively. As we can see that humic acid could effectively facilitate the adsorption of both TC and CBZ, and Na+ showed almost no effect on their adsorption, but K+ ions inhibited their adsorption. Besides, Ca2+ and Al3+ had almost no effect on the adsorption of CBZ, while suppressed the adsorption of TC. These interesting phenomena may be attributed to the effect of solution pH or the surface charge of sorbent. 3.1.6 Reuse of sorbent The reuse of UiO-66 for removal of CBZ and TC was evaluated and shown in Figure 5b. In this study, we found that the adsorbed CBZ on UiO-66 could be desorbed by chloroform completely. After washed by methanol and dried at 80 oC, the recycled UiO-66 still showed good adsorption affinity for CBZ. At the 1st cycle, the adsorption capacity of CBZ was 30.0 mg g-1, and remained to 29.3 mg g-1 with 97.7% capacity retention at the 2nd cycle. After two cycles, 20.9 and 14.4 mg g-1 still could be adsorbed at the 3rd and 4th cycle, respectively. However, because of the strong adsorption force between TC and UiO-66, the adsorbed TC on UiO-66 could not be desorbed by chloroform. Therefore, the recycled UiO-66 after treatment with chloroform showed almost no adsorption capacity for the TC. 3.2 Characterization 3.2.1 SEM Scanning electron microscopy was used to characterize the morphology of the UiO-66 crystals. Figure 6a reveals well-defined cube shaped nanocrystals of gilded bare UiO-66. After adsorption


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of CBZ and TC, respectively, the corresponding Figure 6b and Figure 6c show that the overall framework of UiO-66 remains, but the edges of the nanocrystals are slightly smoother than before. 3.2.2 XRD Powder X-ray diffraction (PXRD) was conducted to investigate the crystalline structure of the obtained UiO-66 and the UiO-66 samples after CBZ (UiO-66-CBZ) and TC adsorption (UiO-66TC). Figure 7 illuminates the XRD patterns of the as-synthesized UiO-66 and the UiO-66 after adsorption. For the as-prepared UiO-66 sorbent, six mainly characteristic peaks at 2θ = 7.4°, 8.5°, 14.8° 17.1°, 25.8°, 30.8°, which are assigned to the (111), (200), (222), (400), (442), and (711) planes of UiO-66, respectively, and are in great accordance with simulated result and previously reported.2 In addition, a weak peak at 12.0° is probably attributed to the presence of the diagonal linkers combined with the strong interaction of linker-inorganic brick in the framework of UiO-66. After adsorption of CBZ and TC on the sorbent, the position of characteristic peaks for the UiO-66 samples show no shift, indicating no change for the phase of UiO-66 after adsorption of CBZ or TC. However, compared with the patterns of UiO-66 before adsorption, the intensity of part of main peaks vary after adsorption of target pollutant, especially TC. The results are listed in Table S2, and intensity ratios of I(200)/I(111), I(400)/I(111), I(442)/I(111) and I(711)/I(111) decrease but the I(222)/I(111) increase before and after the adsorption of CBZ or TC, implying that the absorption process may aﬃect the crystal structure of UiO-66. In addition, the increase of full width at half maximum of these main planes after the adsorption TC also indicates that the adsorbed TC may enter and damage the framework of UiO-66. 3.2.3 FT-IR


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Figure 8 shows the FT-IR spectra of the UiO-66 before and after adsorption of CBZ and TC, which are in agreement with the reported data in literature.42-44 From Figure 8a, the spectrum of bare UiO-66 is dominated by an intense and broad band centred at 3400 cm-1 due to intercrystalline water and physisorbed water condensed inside the cavities. After adsorption of CBZ and TC, the width of the broad band increased clearly, and the centre of the broad band shift toward lower wavenumbers, which can be attributed to the stretching vibrations of NH2 in CBZ 45

and the stretching vibrations of NH2 and amine halide salt bands in TC

46

, respectively. It

confirms that CBZ and TC were adsorbed by UiO-66 in water system. As shown in Figure 8b, the bands at 1581 and 1398 cm−1 can be assigned to ν(OCO) asymmetric and symmetric stretching, respectively.43 The small bands at 1506 cm−1 represented the typical vibration present in a C=C of a benzene ring. Moreover, no band at 1658 cm−1 clearly shows the absence of DMF in the framework of UiO-66. Because of the overlap of bands, it still difficult to find the strong bands of NH2 scissoring (1676 cm-1) and NH2 rocking (1389 cm-1) in the UiO-66 spectra after the adsorption of CBZ and TC. The band at around 1100 cm-1 exhibited the stretching vibration of Zr-O single bond of the framework. The peaks at 818, 746, and 668 cm−1 were due to OH and C-H vibration in the terephthalic acid ligand. At lower frequencies modes due to OH and CH bending are mixed with Zr-O modes (main bands at 746, 668, 549 and 487 cm-1). The vibration mode at 667 cm−1 can be ascribed to µ3-O stretching of the Zr6O4(OH)4(–CO2)12 in the UiO-66 framework. The band at around 600 cm-1 was taken on the asymmetric stretching vibration of ZrO2 of framework.2 3.2.4 Nitrogen adsorption/desorption isotherm Nitrogen adsorption/desorption isotherm of UiO-6 before and after adsorption of CBZ and TC, presented in Figure 9a, indicates a typical type I isotherm (according the IUPAC classification)


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with prominent adsorption at low relative pressure and followed by a long horizontal plateau extending up to high relatively pressure. It is characteristic features of the micropore (pore widths

Adsorption Behaviors of Organic Micropollutants on Zirconium Metal

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