Efficient Capture and Effective Sensing of Cr - ACS Publications

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Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

Efficient Capture and Effective Sensing of Cr2O72− from Water Using a Zirconium Metal−Organic Framework Zu-Jin Lin,*,†,‡ He-Qi Zheng,† Huan-Yu Zheng,† Li-Ping Lin,† Qin Xin,† and Rong Cao*,‡ †

Department of Applied Chemistry, College of Life Science, Fujian Agriculture and Forestry University, Fuzhou, Fujian 350002, P. R. China ‡ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China S Supporting Information *

ABSTRACT: Highly efficient decontamination of heavily toxic Cr2O72− from water remains a serious task for public health and ecosystem protection. An easily regenerative and reused sorbent with suitable porosity may address this task. Herein, a series of water-stable and ecofriendly metal−organic frameworks (MOFs) with large surface areas were assessed for their ability to adsorb and separate Cr2O72− from aqueous solutions. Among these tested MOFs, NU1000 shows an extraordinary capability to efficiently capture (within 3 min) Cr2O72− with a sorption capacity of up to 76.8 mg/g, which is the largest one for the neutral MOF-based Cr2O72− sorbents. NU-1000 also shows remarkable selectivity for Cr2O72− capture and can effectively reduce the Cr(VI) concentration from 24 ppm to 60 ppb, which is below the acceptable limit for the drinking water standard (100 ppb by the U.S. Environmental Protection Agency). Moreover, this adsorbent can be easily regenerated by Soxhlet extraction with an acidic methanol solution (2.5 M HCl) and can be reused at least three times without a significant loss of it adsorption ability. More intriguingly, NU-1000 can also serve as an efficient photoluminescent probe for the selective detection of Cr2O72− in aqueous media. The Cr2O72− detection limit is as low as 1.8 μM, and the linear range is from 1.8 to 340 μM. Our work shows that NU-1000 is a unique material combining both efficient sorption and exceptional fluorescent sensing of Cr2O72− in aqueous media.

1. INTRODUCTION Because water pollution has become a pressing global threat, the sequestration of toxic species from contaminated water has attracted considerable research attention. Among the most commonly found pollutants in industrial effluents are various heavy-metal contaminants. Many of them, presented as their oxo−hydroxo anionic forms, have been listed as priority pollutants by the U.S. Environment Protection Agency (EPA).1 One of these priorities is dichromate (Cr2O72−), a well-known carcinogen,2 which is rapidly diffusing as a result of its wide applications in chromium electroplating, metallurgy, pigment production, leather tanning, and so on.3 Because Cr2O72− is severely harmful to human health and the environment, its detection and segregation from water streams have become prominent problems. So far, various methods, such as ion exchange,4,5 adsorption,6 resins,7 membrane separation,8 and photocatalytic degradation,9,10 have been introduced to remove Cr2O72− from aqueous solution. Among these methods, adsorption is regarded as the most promising technique owing to its efficiency, sensitivity, and convenience. Therefore, intensive studies have been carried out to exploit various adsorbents for Cr2O72− removal, and some commercial adsorbents are available as well. In general, the sorbents for this purpose are classified into two types. One is inorganic © XXXX American Chemical Society

sorbents, such as ACs, zeolites, layered double hydroxides (LDHs), etc. These sorbents are cost-effective but show relatively slow sorption kinetics and poor selectivity.11 The other is organic sorbents, like organic polymer resins. The commercially available amine-functional organic resins are actually oxidized−decomposed by dichromate and therefore cannot be recycled.12 Besides, the thermal and chemical stabilities of organic polymer resins are much lower than those of inorganic sorbents. These limitations actuate the seeking of a new sorbent with excellent performance on the Cr2O72− detection and capture. Metal−organic frameworks (MOFs), constructed by organic linkers and metal ions/clusters, have been emerging as a vital class of porous materials.13−16 The large surface areas, versatile architectures, tunable pore sizes and shapes, and customizable chemical functionalities make these materials suitable for a wide variety of applications, including adsorption/storage, catalysis, sensing, drug delivery, etc.17−22 Recently, MOFs (especially cationic MOFs) have also been explored for the detection and removal of toxic species such as Cr2O72− from aqueous water.23−36 For example, Ghosh et al. developed a water-stable Received: September 9, 2017

A

DOI: 10.1021/acs.inorgchem.7b02327 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry cationic MOF (1·SO42−) pillared by a neutral ligand and with nickel(II) nodes as an adsorbent of Cr2O72− through ion exchange.37 Manos et al. synthesized an imino- and pyridinefunctionalized zirconium-based MOF (Zr-MOF) combining both exceptional sorption and excellent fluorescent sensing response to hexavalent chromium species in aqueous solutions.38 Qian et al. developed a Uio-66-type cationic ZrMOF for the efficient removal of Cr2O72− in water.39 Although a few cationic MOFs show rapid capture and fluorescent probing of toxic Cr2O72−, regeneration of the cationic MOFs generally leans upon a high concentration of anions (such as Cl−, NO3−, etc.), which inevitably generates large amounts of chromium(IV)-containing solid waste and needs secondary separation work. Thus, the development of MOFs as easily regenerable and reusable sorbents for capture and real sequestration of Cr2O72− is still an enormous challenge. As a material for Cr 2 O 7 2− detection and removal, ecofriendliness and water stability are some of the prerequisites that need to be considered. To avoid the ion-exchange process for regeneration, neutral MOFs with large surface areas may be one of the optimal choices. With these criteria in mind, six water-stable neutral MOFs, that is, HKUST-1, ZIF-8, MIL100(Fe), Uio-66, Uio-67, and NU-1000, have been screened for the removal of Cr2O72− in aqueous solution. Remarkably, NU1000 displays the best performance for Cr2O72− capture in water media. It even shows larger Cr2O72− uptake capacity and superior sorption efficiency than the commercially available sorbent-activated carbon. NU-1000 can also be easily regenerated by Soxhlet extraction with an acidic methanol solution (2.5 M) and can be reused at least three times without significant loss of its adsorption ability. Furthermore, NU-1000 can serve as an efficient photoluminescent probe with a low detection limit (1.8 μM) and a wide linear range (1.8−340 μM) for sensing of Cr2O72− in aqueous media.

and chemicals were purchased from Energy Chemical and used directly unless stated otherwise. Fourier transform infrared (FTIR) spectra were recorded in the range of 4000−400 cm−1 on a PerkinElmer Spectrum One using KBr pellets. Powder X-ray diffraction (PXRD) data were recorded on a Rigaku MiniFlex2 diffractometer working with Cu Kα radiation, and the recording speed was 1° min−1 over the 2θ range of 5−50° at room temperature. Scanning electron microscopy (SEM) images were carried out on a Phenom ProX scanning electron microscope equipped with energy-dispersive spectrometer. Fluorescence measurements were performed on a FS5 spectrofluorometer (Edinburgh Instruments Ltd.) at room temperature. Fluorescence lifetimes were recorded on an Edinburgh Instruments FLS920 spectrofluorometer equipped with pulsed xenon lamps (excited at 395 nm and monitored at 518 nm). UV−vis spectra were measured in a UV-2600 (Shimadzu) ultraviolet spectrophotometer. ζ potentials were measured by a NanoBrook Omni particle size and a ζ potential analyzer. The inductively coupled plasma (ICP) measurement was performed on an Ultima2 spectrometer. N2 sorption isotherms were measured at 77 K using a Micrometrics ASAP 2020 surface area and a pore-size analyzer. The Brunauer−Emmett−Teller (BET) method was utilized to calculate the specific surface areas. Pore-size distribution data were calculated from the N2 sorption isotherms based on the density functional theory model in the Micrometrics ASAP 2020 software package (assuming slit pore geometry). Prior to measurements, the samples were degassed at 150 °C for 10 h. 2.3. Adsorption Experiments. General Procedures. An aqueous stock solution of Cr2O72− (500 ppm) was prepared by dissolving K 2 Cr 2 O 7 in deionized water. Aqueous solutions of various concentrations were prepared through successive dilutions of the stock solution with deionized water. A calibration curve was obtained from the spectra of standard solutions of Cr2O72− (1−250 ppm). The Cr2O72− concentrations were determined by using the absorbance (257 nm) of the solutions after getting the UV spectra. The Cr2O72− uptake (%) was given according to the formula c − ce Cr2O7 2 − uptake (%) = 0 × 100% c0 where c0 and ce (ppm) are the initial and equilibrium concentrations of Cr2O72− in the solution, respectively. MOF Screening. In each adsorption experiment, the same amount of adsorbent (50 mg) was added to aqueous Cr2O72− solutions (50 mL) with a concentration of 50 ppm. The mixture was mixed well with magnetic stirring and then allowed to stand for 12 h at 25 °C. Then, the solution was filtered by a syringe filter [polytetrafluoroethylene (PTFE); hydrophobic, 0.24 μm], and the residual concentrations of Cr2O72− in the supernatant liquid were evaluated by UV absorbance. Sorption Kinetics. A total of 50 mL of aqueous of Cr2O72− (50 ppm) was added to a dram vial. Then 50.0 mg of sorbent (NU-1000 or AC) was separately added to form a slurry. The mixture was stirred at 25 °C for 30 min. During the stirring period, 2 mL of the mixture was taken out and filtered by a syringe filter, and the residual concentration of Cr2O72− in the supernatant liquid was evaluated by UV absorbance. Sorption Isotherm. A total of 50 mg of sorbent (NU-1000 or AC) was added to each dram vial containing a Cr2O72− solution (50 mL) with different concentrations. The mixtures were stirred at 25 °C for 30 min and then filtered separately by syringe filters (PTFE; hydrophobic, 0.24 μm), and the residual concentrations of Cr2O72− in the supernatant liquid were evaluated by UV absorbance. If necessary, the Cr2O72− solutions were diluted for UV measurement. Effects of the Sorbent Amount, pH Values, and Competing Ions. The procedures were similar to that of MOF screening tests with the following changes: to investigate the effects of the NU-1000 amount on Cr2O72− adsorption, the addition amounts of NU-1000 were changed from 0, 10, 25, 50 to 100 mg, respectively; to determine the effects of the pH value, the pH values of the Cr2O72− solutions were preadjusted using aqueous NaOH (0.1 M) or HCl (0.1 M) solutions; to evaluate the effects of competing ions, adsorption tests were performed in Cr2O72− aqueous solutions containing the corresponding concentration of competing anions.

2. EXPERIMENTAL SECTION 2.1. Synthetic Procedures. Synthesis of NU-1000B, NU-1000, and Cr2O72−@NU-1000. NU-1000 was made according to recently published literature with some modifications.40 Briefly, 70 mg of ZrCl4 (0.30 mmol), 2.7 g (22 mmol) of benzoic acid, and N,Ndimethylformamide (DMF; 8 mL) were mixed and ultrasonically dissolved. The clear solution was incubated in an oven at 80 °C for 1 h. After cooling to room temperature, 40 mg (0.06 mmol) of 1,3,6,8tetrakis(p-benzoic acid)pyrene was added to this solution, and the mixture was sonicated for 20 min. The yellow suspension was heated in an oven at 120 °C for 72 h. The yellow material was isolated by filtration, then extracted by a Soxhlet extractor in methanol overnight, and dried under vacuum at 80 °C overnight to get the as-synthesized samples (denoted as NU-1000B). NU-1000B (400 mg) was soaked in 120 mL of DMF, and then 3.3 mL of concentrated HCl was added. The mixture was heated in an oven at 100 °C for 24 h. After cooling to room temperature, the mixture was filtered, and the solid was extracted by a Soxhlet extractor in methanol overnight. Then the solid was dried in 80 °C under vacuum to obtain the active NU-1000 sample (denoted as NU-1000 for convenience). NU-1000 (50 mg) was added to a K2Cr2O7 solution (50 g) with a Cr2O72− concentration of 50 ppm, stirred at 25 °C for 30 min, then recovered through filtration, and dried under vacuum at 80 °C overnight to obtain a NU-1000 sample with Cr2O72− encapsulated in the pore (denoted as Cr2O72−@NU-1000). 2.2. Materials and Methods. HKUST-1,41 ZIF-8,42 MIL100(Fe),43,44 Uio-66,45 and Uio-6745 were prepared according to their literatures. Activated carbon (Norit ROW 0.8 mm pellets, steam activated) was purchased from Alfer Aesar. All other solvents, reagents, B

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Figure 1. Structures of (a) HKUST-1, (b) ZIF-8, (c) Uio-66 and Uio-67, (d) MIL-100(Fe), and (e) NU-1000 [magnified view of the 8-connected Zr6(μ3-OH)8(OH)8 cluster]. Recycle Experiment. Cr2O72−@NU-1000 was extracted by a Soxhlet extractor with acidic methanol (2.5 M HCl) overnight and then dried in a vacuum at 80 °C overnight before reuse. 2.4. Luminescence Sensing Experiments. NU-1000 (10.0 mg) was ground and immersed in deionized water (100 mL). The mixture was sonicated for 15 min and then stirred for 1 h to get a suspension, which was then used for luminescent measurements.

H2BDC. Thus, Uio-67 also has two types of cages similar to those of Uio-66 but with large equilateral triangular windows with a size length of ca. 12 Å. NU-1000 [Zr 6 (μ 3 OH)8(OH)8(TBAPy)2, where TBAPy = 1,3,6,8-tetrakis(pbenzoate)pyrene]40 contains 8-connected Zr6(μ3-OH)8(OH)8 clusters linked by tetratopic TBAPy ligands to give a 3D structure with hexagonal channels of ca. 30 Å and triangular channels of ca. 12 Å shown in Figure 1e. The Zr6(μ3OH)8(OH)8 clusters, each capped by eight μ3-OH and linked by another eight μ1-OH, are easily accessible through the aforementioned channels. PXRD and N2 adsorption isotherms of selected MOFs are shown in Figures S1 and S2 and Table S1. 3.2. Screening of the Cr2O72− Uptake of MOFs. To understand the relationship between the Cr2O72− adsorption performance and the pore character of MOFs, the Cr2O72− sorption abilities of the six selected water-stable and ecofriendly MOFs were screened. For the initial screening, samples of each material (50 mg) were separately added into aqueous solutions of Cr2O72− (50 ppm, 50 g), and then the mixture was stirred at 25 °C for 12 h. As shown in Figure 2a, ZIF-8 shows a negative Cr2O72− uptake (0.33%); HKUST-1 and MIL-100(Fe) give minor Cr2O72− uptake (6.8% for HKUST-1 and 13% for MIL100(Fe)), Uio-66 and Uio-67 show moderate Cr2O72− uptake (39.5% for Uio-67 and 59.4% for Uio-66), and NU-1000 shows the highest degree of Cr2O72− uptake (98.1%). These results corresponded well with the color change evident for the supernatant solution after the exchange process (Figure 2b). For comparison, the Cr2O72− adsorption ability of the commercially available active carbon (AC) with a large BET surface area (shown in Figure S1) was also investigated. As shown in Figure 2a, the Cr2O72− uptake (42.5%) of AC is nearly identical with that of Uio-67 but more modest than the values of Uio-66 and NU-1000. Because NU-1000 exhibits the

3. RESULTS AND DISCUSSION 3.1. Structure and Characterization of Selected MOFs. Neutral MOFs from HKUST-1 (Figure 1a), ZIF-8 (Figure 1b), Uio-66 and Uio-67 (Figure 1c), MIL-100(Fe) (Figure 1d), and NU-1000 (Figure 1e) were assessed for their Cr2O72− sorption ability. HKUST-1 [Cu3(BTC)2, where BTC = 1,3,5-benzenetricarboxylate]46 is a prototypical carboxylate MOF composed of 4-connected Cu2 nodes bridged by tridentate BTC linkers to give the 3D structure having square channels with side lengths of ca. 5.8 Å shown in Figure 1a. ZIF-847 is a 3D framework assembled by 2-methylimidazolate and zinc(II). Sod cages with free apertures of ca. 12.0 Å are found in the structure, which can be accessible via narrow six-ring windows with a size of ca. 3.4 Å (Figure 1b). MIL-100(Fe)48,49 is a 3D iron(III) carboxylate framework with two types of mesoporous cages (Figure 1d). The free apertures of these two types of cages are ca. 25 and 29 Å, which are separately accessible through microporous windows with sizes of ca. 5.8 × 5.8 Å and 8.6 × 8.6 Å. Uio6650 was constructed of 12-connected Zr6(μ3-O)4(μ3-OH)4 clusters and 1,4-benzenedicarboxylate (H2BDC) ligands. The structure contains two types of cages: an octahedral cage with a free aperture of ca. 14 Å and a tetrahedral cage with a free aperture of ca. 10 Å, both of which are accessible by equilateral triangle windows with side lengths of ca. 6.0 Å (Figure 1c). Uio6751 has a structure similar to that of Uio-66, except it utilizes biphenyl-4,4′-dicarboxylate (H2BPDC) as a liner instead of C

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1000. On the basis of these results, 30 min was selected as the contact time to ensure full equilibrium in the following experiments. The kinetic data can be fitted (Figure 3c) with the pseudo-second-order kinetic model using the following equation: t 1 t = + 2 qt qe k 2qe

where k2 (g mg−1 min−1) is the kinetic rate constant for the pseudo-second-order model, qt (mg g−1) is the amount of Cr2O72− adsorbed at time t (min), and qe (mg g−1) is the amount of Cr2O72− adsorbed at equilibrium. An extremely high correlation coefficient (i.e., 0.9999) was obtained, and the calculated qe value (49.14 mg g−1) from the pseudo-secondorder model is in good agreement with the experimental value, qe,exp (49.05 mg g−1). The results suggest that Cr2O72− adsorbs on NU-1000 following the pseudo-second-order model. The control experiment shows that NU-1000 has both qe and K2 values larger than AC, further indicating that NU-1000 is a more efficient adsorbent for Cr2O72− than AC. 3.4. Adsorptive Isotherms. To further confirm the Cr2O72− adsorption capacities, which are also an important aspect of the sorbent’s performance metrics, adsorption isotherms were carried out at 25 °C. The description of the data can be provided by the Langmuir model, which was shown as the following equation: ce c 1 = e + qe qm qmKL

where qe is the amount of Cr2O72− adsorbed on the adsorbent at equilibrium (mg g−1), Ce is the equilibrium Cr2O72− concentration in the solution (mg L−1), qm is the maximum adsorption capacity at monolayer coverage (mg g−1), and KL is the Langmuir constant, quantitatively reflecting the affinity of the binding sites (L mg−1) to the energy of adsorption. As shown in Figure 4, the fitting results show that the maximum Cr2O72− sorption capacity reaches 76.80 mg g−1, which is in good agreement with the experimental value (75 mg g−1) and is also consistent with inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis (74 mg g−1). This value is larger than that of the cationic MOF adsorbents of FIR53 and FIR-5425 but smaller than that of the other cationic MOFs such as ABT·2ClO4,24 ZJU-101,39 MOR-1,27 and MOR2.38 Remarkably, as far as we know, NU-1000 shows the largest adsorption capacity among any neutral MOF-based adsorbents so far (Table S4).39,52,53 3.5. Effects of the Sorbent Amount, pH Values, and Competing Ions. Effects of the Sorbent Amount. To confirm the final chromium concentration’s relationship to the NU1000 amount, different amounts of NU-1000 were separately added to Cr2O72− solutions (50 g, 50 ppm) with an initial chromium concentration of ca. 24 ppm. As shown in Figure 5, with the increasing NU-1000 concentration, the equilibrium concentration of chromium rapidly dropped. Remarkably, when the concentration of NU-1000 is 2 g L−1, the residue of chromium is 60 ppb, which is below the acceptable safety limit in drinking water defined by the U.S. EPA (100 ppb).54 Effects of the pH Values. The effects of the pH values on the Cr2O72− sorption ability of NU-1000 were investigated in the pH range of 1−7. Thanks to the excellent chemical stability of NU-1000 in water under the pH range 1−11,55 NU-1000 shows moderate-to-excellent performance as a Cr2O72− sorbent

Figure 2. (a) Bar graph illustrating Cr2O72− uptake (%) over various adsorbents and (b) color of supernatant solutions after corresponding sorbent adsorptions (50 mg of adsorbents and 50 g of Cr2O72− aqueous solution with an initial concentration of 50 ppm, 25 °C).

highest Cr2O72− sorption abilities, further research focused on the investigation the sorption performances of NU-1000 and AC as well for comparison. 3.3. Adsorptive Kinetics. It is well-known that the adsorption process of heavy-metal oxoanions in porous materials relies on an amalgam of multiple factors such as the pore size, shape, and surface of adsorbents, etc. In order to evaluate the adsorption efficiency of Cr2O72− over NU-1000, the adsorption of Cr2O72− from aqueous solution onto NU1000 as a function of the contact time was carried out and monitored by liquid UV−vis spectroscopy. As shown in Figure 3a, the intensity of the main characteristic adsorption peak of Cr2O72− in solution at 257 and 352 nm quickly decreased and was nearly invariable within 3 min of Cr2O72− solution and NU1000 contact. Because more than 98% of the initial Cr2O72− amount (initial concentration, 50 ppm) was adsorbed by NU1000 in less than 3 min indicates that the capture of Cr2O72− by NU-1000 was extremely fast compared to that of AC, which took more than 30 min to reach absorption equilibrium with a Cr2O72− uptake of ca. 42% (Figure 3b). Remarkably, within only 5 s and 1 min of Cr2O72− solution and NU-1000 contact, more than 82% and 92% Cr2O72− were removed, respectively. These results demonstrated that NU-1000 shows one of the most efficienct sorbents among porous materials reported for the capture of Cr2O72− in the aqueous solution (Table S4). The rapid removal rate of Cr2O72− could be ascribed to the fact that an abundance of vacant adsorption sites are easily available because of the large pore sizes and BET surface areas of NUD

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Figure 3. UV−vis absorption spectra of the supernatant solution remaining after Cr2O72− adsorption over (a) NU-1000 and (b) AC at various time intervals (the arrows indicate the change in the adsorption spectra with time). (c) Effect of the contact time on the adsorption of Cr2O72− over NU1000 and AC. (d) Color change of the supernatant solution before and after AC or NU-1000 adsorption (30 min), respectively (50 mg of adsorbents, 50 g of Cr2O72− aqueous solution with an initial concentration of 50 ppm, 25 °C).

Figure 4. Adsorption isotherms for Cr2O72− adsorption over NU-1000 and AC. The solid line represents the fitting of the data with the Langmuir model (Ce, equilibrium concentration of Cr2O72−; Qe, the amount of Cr2O72− adsorbed).

Figure 5. Effects of the concentration of NU-1000 on the final equilibrium concentration of chromium (initial chromium concentration, 24 ppm).

in acidic water solutions (Figure 6a). Specifically, NU-1000 shows 94.4−98.1% Cr2O72− uptake in the pH range of 3−6. The result indicates that NU-1000 may be practically applied to remove Cr2O72− in industrial waste because the pH value of industrial waste, for example, tannery and metal plating wastewater, is acidic (the pH value is ca. 3). Effects of Competing Ions. In a comparison with the sorption capacity, the selectivity is a more important parameter to evaluate the efficiency of a sorbent in pollutant removal. Some common anions, such as nitrate, sulfate, etc., show a high affinity to sorbents and usually a lower sorbent selectivity

toward toxic anionic pollutants. Conventional ion-exchange resins and LDHs, as well as some cationic MOFs and porous organic polymers, have suffered from low selectivity toward competing anions.56−59 Because various anions frequently coexist with Cr2O72− as in a contaminated water source, it is important to examine the effect of competing ions on the sorption ability of Cr2O72− onto NU-1000. Thus, the selectivity of NU-1000 for Cr2O72− was carried out for the three most common competing anions, i.e., nitrate (NO3−), sulfate (SO42−), and chloride (Cl−). Samples of NU-1000 (10 mg) were separately immersed in a 10 mL solution of Cr2O72− (50 E

DOI: 10.1021/acs.inorgchem.7b02327 Inorg. Chem. XXXX, XXX, XXX−XXX

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extraction with acidic methanol (2.5 M HCl) and the chromate element can be simply enriched in the acidic methanol solution. As shown in Figure 7, NU-1000 can be reused at least three times with no significant loss of sorption ability.

Figure 7. Reusability of NU-1000 for the adsorption of Cr2O72−.

3.7. Plausible Adsorption Mechanism for NU-1000. Many factors, such as the channel/window sizes, surface properties, pore volumes, and surface areas of MOFs, would affect the sorbates’ uptake abilities. Because of the narrow windows (ca. 3.4 Å), Cr2O72− with a size of ca. 6.0 × 3.94 Å2 hardly penetrates into the cages of ZIF-8. Therefore, it is reasonable that only surface adsorption happens for ZIF-8, and thus only negative uptake of Cr2O72− was found for ZIF-8. Considering that the windows or channels of other tested MOFs are sufficient for Cr2O72− to freely diffuse, the distinct Cr2O72− uptake for the other five MOFs cannot be ascribed to the channel/window sizes of the MOFs. To evaluate whether the specific surface area and/or pore volume affects the Cr2O72− uptake over the selected MOFs, the Cr2O72− uptake, BET surface areas, and pore volumes of these MOFs were summarized (Table S1 for the pore volume) and are shown in Figure 8. From this graph, we can find that HKUST-1, MIL-100(Fe), and Uio-67 have large surface areas, whereas their Cr2O72− uptake is smaller than that of Uio-66. That means no direct relationship between the BET surface area/pore volumes with the Cr2O72− uptake was observed for these selected MOFs. Presumably, the result may be ascribed to

Figure 6. (a) Effects of the pH values and (b) disturbing ions on the Cr2O72− sorption ability over NU-1000 (50 mg of NU-1000, 50 g of Cr2O72− aqueous solution with an initial concentration of 50 ppm, 25 °C).

ppm) containing an n-fold molar excess of disturbing ions (n is equal to 0, 1, 5, and 10, respectively). As shown in Figure 6b, no significant Cr2O72− uptake changes were observed even when a 5-fold molar excess of disturbing ions was present, and only small drops (11% for Cl−, 12% for SO42−, and 28% for NO3−, respectively) of Cr2O72− uptake were observed when a 10-fold molar excess of disturbing ions coexists. It should be noted that few cationic MOFs were examined for their selectivity for Cr2O 72− to other competing ions. The selectivities of NU-1000 for Cr2O72− to nitrate, sulfate, and chloride are superior to those of the cationic MOFs [Ag8(tz)6](NO3)2·6H2O,4 and the selectivity of NU-1000 for Cr2O72− to sulfate is also better than that of MOR-2.38 This result indicates that NU-1000 exhibits a high selectivity for Cr2O72− to nitrate (NO3−), sulfate (SO42−), and chloride (Cl−). 3.6. Cycle Experiment. Regeneration and reusability are important indicators for a sorbent because they are directly related to the cost effectiveness of the adsorption processes. For most cationic sorbents, such as MOF- or POF-based cationic sorbents, however, their regeneration largely depends on the high concentration of metal salt solutions (Table S4), which inevitably generates large amounts of chromium(VI)-containing metal salt solutions (a secondary treatment of the solution is then needed). Thanks to the high stability of NU-1000, regeneration of NU-1000 can be obtained simply by Soxhlet

Figure 8. Illustration of the relationship between the Cr2O72− uptake and BET surface areas of selected MOFs (50 mg of adsorbents, 50 g of Cr2O72− aqueous solution with an initial concentration of 50 ppm, 25 °C). F

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mechanism. However, we can exclude other interactions such as the hydrogen bonding and complexion between the Cr2O72− and Zr6 nodes. 3.8. Photoluminescent Sensing of Cr2O72−. Pyrene and its derivatives are well-known chromophores that have been applied as fluorescent sensors because of their high quantum yields and lifetimes. As shown in Figure S5, the solid-state fluorescent measurements showed that NU-1000 exhibits an intense emission peak at ca. 490 nm upon excitation at 400 nm, which can be largely attributed to a ligand-centered fluorescent process because similar emissions are detected at 530 nm for the free ligand. The intense fluorescence of NU-1000, in combination with its ability to rapidly and efficiently adsorb Cr2O72− from water, prompted us to examine NU-1000 as a fluorescent sensor for Cr2O72− in aqueous solution. To test the potential of NU-1000 for sensing Cr2O72−, the fluorescence of NU-1000 was monitored in aqueous solution. Initially, 10.0 mg of NU-1000 was dispersed in 100 mL of aqueous solution to form a stable suspension (0.1 mg mL−1) by ultrasound for 15 min and then stirred for 1 h, which exhibits an intense broad band with a maximum at ca. 518 nm upon excitation at 418 nm. For fluorescent measurement, 3 mL of the above suspension was added to a cuvette, and then 30 μL of a KX solution [100 mM; X = F−, Cl−, Br−, N3−, HCO3−, OAc− (i.e., acetate), SO42−, H2PO4−, NO3−, and Cr2O72−, respectively] was separately injected. The resulting emulsion was separately mixed evenly and then tested by a fluorescence spectrophotometer. Interestingly, as shown in Figure 10a, F−, Br−, N3−, HCO3−, SO42−, H2PO4−, and NO3− do not cause any significant changes of the fluorescent intensities, and Cl− and OAc− only show a small decrease of the values. However, the fluorescence of NU1000 reduces quickly in the case of Cr2O72−. The different degrees of quenching effects on the fluorescent intensities imply that NU-1000 could be considered as a potential candidate for the selective probing of Cr2O72−. Moreover, no significant changes of the quenching effect of Cr2O72− were observed after the introduction of equivalent molars of disturbing ions, such as Cl−, Br−, OAc−, SO42−, and NO3− (Figure 10b). To assess the sensitivity of NU-1000 toward Cr2O72− in detail, a series of Cr2O72− with varied concentrations was separately added to the aforementioned NU-1000 suspension and the responses were monitored. As shown in Figure 10c, the emission intensities clearly decrease with an increase of the Cr2O72− concentrations. Remarkably, the quenching effects can be observed by the naked eye under a typical usual UV lamp (excited at 365 nm), and the brightness of turquoise fluorescence slowly vanishes with increasing Cr2O72− concentrations (Figure 10d, inset). Quantitatively, the quenching effect can be analyzed by the Stern−Volmer equation, expressed as

the fact that there is sufficient surface area/volume for the Cr2O72− uptake for all selected MOFs. Recently, inspired by the fact that hydrous zirconium oxides exhibit ion-exchange behavior and high affinity with various oxo anions, Hupp, Farha et al. discovered that the terminal water and hydroxyl ligands on the Zr6 nodes of zirconium-based MOFs are important adsorptive sites for selenate, selenite, and sulfate.60,61 Given that the hydrous zirconium oxide also has a high affinity with Cr2O72−,62 the adsorption mechanism of Cr2O72− in zirconium-based MOFs should be similar to those of selenate, selenite, and sulfate; that is, the unsaturated Zr6 nodes with substitutable terminal water and hydroxyl ligand groups can work as efficient adsorptive sites for Cr2O72−. In the structures of Uio-66 and Uio-67, the Zr6 clusters [Zr6(μ3O)4(μ3-OH)4] were fully occupied by 12 carboxylate groups, and thus only four capped hydroxyl ligands serve as adsorption sites. In the structure of NU-1000, however, only 8 carboxylates occupied in the Zr6 clusters [Zr6(μ3-OH)8(OH)8], and thus 16 hydroxyl ligand groups per Zr6 cluster can serve as the Cr2O72− adsorption sites. Considering the isostructural nature of Uio-66 and Uio-67, the sequence of the Cr2O72− adsorptive site amount is NU-1000 > Uio-66 > Uio-67, which is well consistent with the sequence of the Cr2O72− uptake. In comparison, NU-1000B, where the eight Zr-OH groups in the equatorial plane for each Zr6 cluster were replaced by four benzoic groups, shows a more modest Cr2O72− uptake (67%) than that of NU-1000 (98%) in the same experimental conditions. This result further confirms that the Zr-OH groups should be some of the most important adsorptive sites. Thus, the major Cr2O72− adsorptive sites are the coordinative terminal water and hydroxyl ligands in the Zr6 nodes in zirconium-based MOFs. To further confirm the interaction between the Cr2O72− and Zr6 nodes, the surface charge (ζ potential) of NU-1000 at various pH values was tested. As shown in Figure 9, the

Figure 9. Relationship between the Cr2O72− uptake (%) and ζ potentials of NU-1000.

I0 = 1 + KSV[M] I

isoelectric point of NU-1000 corresponds to ca. pH 5.6, and the surface charge can be negative and positive at pH values below and above 5.6, respectively. If electrostatic interaction is the main mechanism for Cr2O72− adsorption, the Cr2O72− uptake should decrease when the pH is larger than 6.0 because both adsorbates and adsorbents are negatively charged under these conditions, and vice versa, the Cr2O72− uptake should increase with the sorbent surface carrying a more positive charge. As shown in Figure 9, the pH-dependent adsorption results coincide well with those expected for electrostatic interactions. Thus, electrostatic interaction might be considered as a possible

where I0 and I are the fluorescent intensities before and after the addition of analytes, respectively, KSV is the Stern−Volmer quenching constant, and [M] is the concentration of the analytes. As shown in Figure 10d, the fluorescent intensity follows the Stern−Volmer equation in the range of 0.01−10 ppm with a linear fit coefficient of 0.972 and a KSV value of 13370 M−1. As far as we know, the resulting KSV value is only smaller than that of [Zn(2-NH2bdc)(bibp)]n and represents the second largest one in the reported MOF-based fluorescent G

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Inorganic Chemistry

Figure 10. (a) Fluorescent intensities of NU-1000 at 518 nm in 0.5 mM different anions. (b) Comparison of the fluorescent intensity of NU-1000 in a solution of Cr2O72− (0.5 mM) containing a disturbing anion (0.5 mM) with an equivalent concentration (Cr2O72− was simplified by Cr for clarification in the graph). (c) Fluorescent intensities of NU-1000 in different concentrations of Cr2O72− (from 0, 0.01, 0.02, 0.04, 0.06, 0.08, 0.1, 0.2, 0.4, 0.6, 1.0, 2.0, 3.0, 4.0, 6.0, 8.0, 10, 25, 50, 75, 100 to 150 ppm) when excited at 418 nm. (d) Stern−Volmer plot of NU-1000 quenched by Cr2O72−. Inset: Photographs taken under UV light (365 nm), showing fluorescence quenching upon the addition of Cr2O72− to an aqueous emulsion of NU-1000). The concentration of NU-1000 is 0.1 mg mL−1 for all fluorescent tests.

probe for detecting Cr2O72−.30,63 The high KSV value indicates the relatively high quenching efficiency of Cr2O72− in the emission of NU-1000. The detection limit of Cr2O72− is 1.8 μM, calculated by the ratio of 3δ/KSV. The performances of the other MOF-based fluorescent probes for Cr2O72− were summarized and are shown in Table S5. In comparison with other MOF-based fluorescent sensors, NU-1000 exhibits a larger quenching constant (KSV), a lower detection limit (1.8 μM), a wider linear range (1.8−340 μM), and a shorter response time (a few seconds). Good thermal and water stabilities, a high quenching constant, a low detection limit, and a fast response time make NU-1000 a promising fluorescent sensor for the practical detection of Cr2O72−. Generally, three pathways, including the inner filter effect (IFE), electron transfer, and fluorescent materials converted into nonfluorescent ones, can result in fluorescence quenching.64 Considering the high chemical stability of NU-1000, the third mechanism was first precluded. As shown in Figure S6, the adsorption spectrum of Cr2O72− and the excitation spectrum of NU-1000 overlap in a wide range of 250−500 nm, which opens up the possibility of IFE. To further improve the exact mechanism, fluorescence lifetime spectroscopy was carried out (Figure S7) and the fluorescence intensity decays were fitted by three exponential decay functions (Table S2). The experimental results showed that the fluorescence lifetime became shorter with increasing Cr2O72− concentration, from 9.77 ns for NU-1000 alone to 8.00, 5.52, and 3.09 ns in the 1,

10, and 50 ppm of Cr2O72−, respectively. The lifetime reduction indicated the presence of electron transfer between NU-1000 and Cr2O72−. Given that the fluorescence lifetime of the fluorescent materials would not change in the IFE process, the predominant fluorescence quenching mechanism was attributed to electron transfer. That is, the quenching of NU-1000 in the presence of Cr2O72− is most likely due to a decrease in energy transfer between the π and π* orbitals of the ligand TBAPy4− because of electron-transfer transitions of Cr2O72−, which was in agreement with previous reports.27,65 3.9. Other Physicochemical Determinations. FTIR spectroscopy confirmed the inclusion of Cr2O72− ions in NU1000, with a new band corresponding to Cr−O stretching modes appearing at ca. 950 cm−1 of Cr2O72−@NU-1000 (Figure S8). The inclusion of Cr2O72− anions in NU-1000 was further confirmed by energy-dispersive X-ray spectroscopy (Figure S3b) and the ICP result where a chromium element was observed. X-ray diffraction and BET measurements were employed to gain knowledge of the structural and textural evolvement after Cr2O72− adsorption. As shown in Figure S2f, the PXRD patterns of NU-1000 before and after Cr2O72− uptake are in good agreement with each other, indicating that the structural integrity of NU-1000 was well-retained. The N2 isotherm for Cr2O72−@NU-1000 maintains a pristine isotherm type, which further demonstrates that Cr2O72− adsorption has a negligible influence on the structural integrity of NU-1000 (Figure S9). The small decreases in the BET surface area and H

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Inorganic Chemistry pore volume as well after Cr2O72− uptake imply trapping of dichromate in the channel of NU-1000. The regenerated NU-1000 sample shows FTIR spectra (Figure S8) and PXRD patterns (Figure S 2f) identical with those of a pristine NU-1000 sample, indicating that NU-1000 could be well regenerated. Besides, only a slight reduction in the surface areas and no obvious change of the pore sizes were observed for recycled NU-1000, even after three adsorptive runs (Figure S9 and Table S3). Moreover, SEM images showed that recycled NU-1000 could well retain its morphology (Figure 3c). These results demonstrate that NU-1000 could well retain its structural integrity after three adsorptive runs.

Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (Grant 20170028).

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4. CONCLUSIONS In conclusion, six water-stable and ecofriendly MOFs have been evaluated as novel adsorbents for the removal of toxic Cr2O72−. Among these MOFs, NU-1000 was found to exhibit both a fast adsorption rate (the equilibrium time is less than 3 min) and a high adsorption capacity (76.8 mg g−1). In addition, it exhibits high selectivity toward Cr2O72− even in the presence of a high concentration of competing ions, and it can work in a wide pH range from 1 to 7. The high adsorption efficiency of NU-1000 toward Cr2O72− may be ascribed to the abundant active sites (Zr-OH groups), high specific surface area, and large pore size for NU-1000. Moreover, it can be readily regenerated and recycled without significant loss of Cr2O72− adsorption ability. Such excellent adsorption properties and easily regenerated and reused characteristics of NU-1000 prefigure its great potential for Cr2O72− decontamination from wastewater. Meanwhile, NU-1000 can effectively, selectively, and quantitatively detect Cr2O72− with a low detection limit (1.8 μM) and a wide linear range (1.8−340 μM) through fluorescence quenching. Therefore, NU-1000 is a promising candidate for the detection and segregation of Cr2O72− from water.

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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.inorgchem.7b02327. Gas adsorption, PXRD, IR, SEM, energy-dispersive spectroscopy, and the performances of MOF-based dichromate sorbents (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.-J.L.). *E-mail: [email protected] (R.C.). ORCID

Zu-Jin Lin: 0000-0003-2515-3356 Rong Cao: 0000-0003-2384-791X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 2152106, 21520102001, and 213310061), Fujian Agriculture and Forestry University (Grant 118360020), Outstanding Youth Research Training Program of Fujian Agriculture and Forestry University (Grant XJQ201616), and the State Key Laboratory of Structural I

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DOI: 10.1021/acs.inorgchem.7b02327 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02327 Inorg. Chem. XXXX, XXX, XXX−XXX