Cationic Amorphous MetalâOrganic Cage-Based Materials for the

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Cationic Amorphous Metal-Organic Cage-Based Materials for the Removal of Oxo-Anions from Water Xia Jin, Guang-Qing Wang, Ding Ma, Shu-Qi Deng, SongLiang Cai, Jun Fan, Wei-Guang Zhang, and Sheng-Run Zheng ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b01294 • Publication Date (Web): 06 Aug 2019 Downloaded from pubs.acs.org on August 7, 2019

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ACS Applied Nano Materials

Cationic Amorphous Metal-Organic Cage-Based Materials for the Removal of Oxo-Anions from Water Xia Jin, Guang-Qing Wang, Ding Ma, Shu-Qi Deng, Song-Liang Cai, Jun Fan, Wei-Guang Zhang, Sheng-Run Zheng* School of Chemistry and Environment, South China Normal University, Guangzhou, 510006, P. R. China. KEYWORDS: cationic framework, oxo-anion adsorption, aMOC, rapid kinetics, high capacity

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ABSTRACT Oxo-anion pollutants pose serious threat to natural water systems. It is highly necessary to develop cationic porous materials capable of removing oxo-anion from sewage. However, they are relatively rarely reported compared with anionic and neutral materials. Herein, two cationic amorphous metal-organic cage-based solids containing cages of different sizes, namely, aMOC-1 and aMOC-2, were constructed and use for removing oxo-anions from water. Because the cages in these materials have large cavities (with about 3.5 and 4.5 nm in diameter for aMOC-1 and aMOC-2, respectively) and high positive charges (positive +24 for both), both aMOC-1 and aMOC-2 exhibit rapid kinetics in carcinogenic chromate (CrO42- and Cr2O72-) and ReO4- (a surrogate anion of radioactive isotope TcO4-) adsorption. The adsorption equilibrium can be reached within only a few minutes. The adsorption capacities of aMOC-1 for CrO42-, Cr2O72- and ReO4- are 157.4, 203.8, and 350 mg/g, respectively. Because aMOC-2 contains larger metalorganic cage, the corresponding adsorption capacities are 242.0, 407.0, and 583.9 mg/g, respectively. The capacity of aMOC-2 for Cr(VI) oxo-anion adsorption ranks the highest among all related materials ever reported. In addition, the oxo-anions can be released rapidly within several minutes from the oxo-anion-loaded aMOCs in 2 M NaNO3 solution, which allows these materials to exhibit good reusability. Finally, the aMOCs have a potential application in the removal of Cr(VI) from electroplating bath wastewater, in which the concentration changes from 10 ppm to 0.17 ppm after treatment with aMOCs.


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1. INTRODUCTION Oxo-anions are one of the major pollutants in industrial effluents and nuclear waste. Some oxoanions are regarded as priority pollutants by the U.S. Environmental Protection Agency (EPA).1 In particular, Cr(VI) (CrO42- and Cr2O72-) was classified as a Group “A” human carcinogen by the EPA because it can cause severe damage to human health.2-5 Pertechnetate anion (TcO4-) is a radionuclide result from nuclear fuel cycle. It is a problematic pollutant because its mobility is high in the environment.6 These toxic oxo-anions are difficult to degrade into eco-friendly compounds compared with organic pollutants, and their accumulation in organisms can cause strong toxicity and carcinogenicity.7 Obviously, the discharge of wastewater containing these pollutants causes great harm to the human health and ecological environment. Thus, the development of technologies to remove them from wastewater is of great importance. Many technologies,

such

as

electrocatalytic/photocatalytic

oxidation/reduction,8-9

biological

treatments,10-11 chemical oxidation-/reduction,12-14 and membrane filtration15-16 have been applied to the removal of toxic oxo-anions from wastewater. Among these methods, adsorption via anionexchange methods is one of the better options due to its high feasibility, high efficiency, few byproducts, low cost, easy operation, ease of recycling, and good performance even for wastewater that contains pollutants at extreme low concentration.7, 17-32 To remove oxo-anions, the development of cationic porous frameworks is highly desirable. Recently, a small number of reports have shown that cationic metal-organic frameworks can be used to effectively remove oxo-anions from water.27-32 The cation coordination cavities or channels in the metal-organic framework (MOF) not only provide a space for anion exchange but also form interactions with oxo-anions, thus giving rise to good oxo-anion removal performance. However,


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most MOFs lack sufficient physiochemical stability and are not easy to be synthesized in bulkscale, which limits their practical application.7 Considering that the coordination space is an advantage for the removal of oxo-anions, metal-organic cages (MOCs) have attracted substantial attention. MOCs are a class of discrete metal-organic polyhedron that have been applied in many fields such as molecular recognition,33 drug delivery,34 separation,35 and homogeneous catalysis,3637

resulting from their coordination space. To use such coordination space for liquid phase

adsorption, amorphous solids based on MOCs (denoted as amorphous MOCs or aMOCs) have been synthesized by us and applied to dye adsorption and separation.38 aMOCs are amorphous solids, but contain cavities having regular shape and size that result from the MOCs. aMOCs may benefit the shaping and processing of powders, may be easy to prepare on a bulk-scale, and may facilitate anion diffusion and exchange due to the various additional pores that result from the irregular accumulation of MOCs.38 Therefore, it is expected that these materials are suitable for the adsorption and removal of anionic pollutants. However, there has been no relevant research report until now. The synthesis of aMOC consists of two steps. Firstly, Pd(II) and angular dipyridyl ligands are assembled to stable nanoscale cationic MOCs in solution. Then, poor solvents are selected to precipitate the MOCs rapidly to obtain aMOCs. In the present work, two aMOCs, aMOC-1 and aMOC-2, based on large, cationic M12L24 cages were synthesized. They were applied to adsorb CrO4-, Cr2O72-, and ReO4-. Interestingly, they showed a very rapid adsorption and release rate for these oxo-anions and high capacities for CrO42- and Cr2O72- adsorption. The main idea of the current study is showed in Scheme 1.


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Scheme 1. The construction of cationic aMOCs and their application in the removal of oxo-anion.

2. EXPERINMENTAL SECTIONS 2.1 Chemicals Ligand L1 and L2 was purchased from Guangzhou Research & Creativity Biotechnology Co. Ltd and Shanghai Kaiyulin Pharmaceutical Technology Co., Ltd, respectively. Palladium nitrate dihydrate was purchased from Beijing HWRK Chem Co. Ltd. K2CrO4, K2Cr2O7 and KReO4 were purchased from Aladdin. Other regents were obtained from Guangzhou Chemical Reagent Factory. All chemicals were used without further purification. 2.2 Synthesis of aMOC-1 and aMOC-2 aMOC-1 was synthesized according to the reference.38 For aMOC-2, Pd(NO3)2‧2H2O (26.6 mg, 0.1 mmol), L2 (61 mg, 0.2 mmol), as well as DMSO (6 mL) was mixed and put into a high pressure glass reaction bottle (15 mL), which was then filled it with argon. The mixture was heated


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at 50 °C for 8 h. Then, dioxane (24 mL) was quickly added to the yellow clear solution, and stirred to form a pale-yellow precipitate. The resulting pale-yellow precipitate was obtained by centrifugation, washed several times by ethyl ether (24 mL) and then immersed in ethyl ether for another 72 hours. The amorphous pale-yellow power was collected by centrifugation and vacuum dried for about 6 hours at 50 °C. Yield: 79% (based on Pd). 2.3 Oxo-anion uptake experiments All the oxo-anion uptake and desorption experiments were performed at room temperature. The pH value for CrO42-, Cr2O72- and ReO4- adsorption are 6.8, 5.5 and 7.0, respectively. Oxo-anion uptake by aMOC-1 and aMOC-2 was tested by soaking 10 mg of products in 30 ml aqueous solutions of 10-4 M CrO42-, Cr2O72- and ReO4-, respectively. During the adsorption process, the solution remains stirred. At a given time, about 2 mL of the solution was taken out from the supernatant and filtered by syringe driven filter. The filtrate was used to determinate the oxo-anion concentrations at a given time. The concentrations of CrO42- and Cr2O72- were determined using a UV-vis spectrophotometer. The concentration of ReO4- is detected by ICP. The adsorption isotherms of CrO42-, Cr2O72- and ReO4- were obtained by immersing 5 mg of aMOC-1 or aMOC-2 into a serials of oxo-anion aqueous solutions (25 mL) with a concentration range of 10-400 ppm. After one day, the concentrations of the three oxo-anions were measured by UV-vis spectrophotometry (CrO42- and Cr2O72-) or ICP (ReO4-). The removal efficiency of the three oxo-anions in the presence of competing anions was tested by adding 5 mg of aMOC-1 or aMOC-2 to a 25 ml mixed solution containing equimolar (1 mM, 1:1) of targeted oxo-anions and the selected competing anion (Cl-, Br-, NO3-, SO42-, and CO32-). The suspension was stirred for 3 h. After that, 3 mL solution was taken out for measuring the


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concentrations of the three oxo-anions. Their concentrations were again determined by UV-vis spectrophotometry (CrO42- and Cr2O72-) or ICP (ReO4-). After the adsorption experiments, the adsorbents were recycled by centrifugation and use for desorption experiments. The desorption experiments were performed by immersing 30 mg of aMOC@oxoanion in 30 mL of 2 M NaNO3 solution for 12 hours. The adsorption capacity of oxo-anion on the aMOCs at a given time (Qt) and equilibrium (Qe) are described by equations (1) and (2), respectively: Qt = V(C0-Ct)/m

(1)

Qe = V(C0-Ce)/m

(2)

where V is the volume of the oxo-anion solution, C0 is the initial concentration of the solution, Ct is solution concentration at a given time, Ce is the equilibrium concentration, and m is the mass of the aMOC sample. 2.4 Characterization methods 1H

nuclear magnetic resonance (1H-NMR) spectra were performed on a Bruker AVANCE NEO

(600MHz) taking TMS as an internal standard at 298K. X-ray powder diffraction was performed on an Ultima IV X-ray powder diffractometer (Kurary, Tokyo, Japan) at 40 kV, 40 mA equipped with Cu Kα radiation (k = 1.5406 Å). Thermogravimetric analyses were measured on a TG-209F3 with a heating rate of 10 C/min under N2 atmosphere. Raman spectroscopy was recorded on a Renishaw in Via confocal microscope Raman spectrometer. Infrared spectrum (IR) were measured on an IRPrestige-zl spectrophotometer (Shimazduo, Japan) in KBr pellets in the range from 4000 to 400 cm-1. SEM images were recorded on a ZEISS Gemini 500 scanning electron microscope. EDX were recorded on a Phenom Pro X Desktop Scanning Electron Microscope-Energy Spectrum


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Integrative Machine. Zeta potentials were recorded by a Zetasizer Nano ZS90 from Malvern Instruments with the liquid concentration of 0.75 mg/mL. BET surface area was measured on an Autosorb iQ Station 2 sorptometer (Quantachrome, US) using nitrogen adsorption at 77 K. ICP were recorded on an Inductively Coupled Plasma Emission Spectrometer ICAP-7400 (Thermo, USA). UV-vis were measured by an UV-2700 Ultraviolet-Visible Spectrophotometer (Shimadzu, Japan).

3. Results and discussion 3.1 Synthesis and characterization of aMOC-2 It has been reported that the reaction of L1 with Pd(NO3)2 gave rise to a M12L24 cage.38 This M12L24 cage exhibits a positive charge as high as +24, which means that twenty-four NO3- ions exist on the aMOC-1 per cage. Because NO3- anions are smaller than the selected oxo-anions, it is reasonable to think that the space occupied by NO3- may not be enough for the oxo-anions if all the NO3- anions are exchanged, thus reducing the exchange efficiency. Therefore, to enlarge the space to accommodating more oxo-anions, the ligand with longer arms, L2, was selected to synthesize larger MOCs (Figure 1). The reaction condition of MOC-2 is similar to that of MOC-1 with a slight modification. The suspension of Pd(NO3)2·2H2O with L2 in dimethyl sulfoxide (DMSO-d6) turned into a clear pale yellow solution after heating at 50 °C for 8 hours. The 1H NMR spectrum of the DMSO-d6 solution basically contains one set of resonances from L2, suggesting that a highly symmetric cage was formed (Figure S1). The 1H NMR resonances for the pyridyl groups were obviously shifted downfield compared with those of L2. Specifically, the chemical shifts of hydrogen atoms at positions a and b were shifted by 0.65 and 0.38 ppm, respectively, indicating the coordination of the Pd(II) ion with the pyridyl nitrogen atom. The hydrogen atoms at the position far from the


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Pd(II) ion (at position c, d, and e) showed significantly fewer shifts than those on the pyridyl groups. Such phenomena are often observed in the formation of the Pd12L24 cage reported by other groups.39-40 Due to the rigidly of such ligands, the size of the cage may similar to those based on the similar ligand (same skeleton, different substituents). From the single crystal structures of MOCs with similar ligands,39-40 the size of MOCs in aMOC-1 and aMOC-2 are estimated to be 3.2 and 4.5 nm, respectively.

Figure 1. The construction of two aMOCs based on two MOCs of different sizes. The aMOC-2 compound was formed by diffusing MOC-2 DMSO solution into dioxane. The resulting pale-yellow precipitate was washed and soaked with ethyl ether to remove the guest DMSO molecules. The PXRD patterns of the as-synthesized and activated samples indicated that the powder is basically amorphous with only a few broad peaks (Figure 2a). As shown in Figure 2b, the TG curve of the as-synthesized aMOC-2 shows a weight loss of approximately 7.0% before 150 °C, which is caused by the release of solvent molecules in aMOC-2. The activated aMOC-2


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showed less weight loss before 150 °C, indicating that some of the guest solvent molecules could be removed by the activated process. Although the cavity in MOC-2 is large, the weight loss is not high, which may imply that the major cavities were occupied by NO3- anions. Figure 2c demonstrates the FT-IR spectra of L2 and aMOC-2. The intense band at 1700 cm−1 for L2 is assigned to the asymmetric stretching vibrations of C=O, vas (C=O), which shows only a slight shift compared with that for aMOC-2 (the band at 1702 cm-1). The peaks at 1595 and 1583 cm−1 for L2 result from the asymmetric C=N and C=C stretching vibrations on the benzene or pyridyl groups. In aMOC-2, these peaks are shifted to 1611 and 1598 cm−1, respectively, indicating that the pyridyl groups are coordinated with the Pd(II) ions. In addition, the intense band at 1385 cm-1 attributed to the NO3- anions is observed in aMOC-2. The Raman spectra of L2 and aMOC-2 were also performed. However, a reasonable Raman spectrum of L2 cannot be obtained due to its strong luminescent emissions. Thus, only a Raman spectrum of aMOC-2 was obtained, as shown in Figure 2d. We found that this spectrum is similar to the reported aMOC-1; thus, similar bonds may also be formed in the aMOC-2. The Raman peaks at 1613 and 1589 cm-1 are result from the C=N and C=C bonds, respectively. The Raman peaks at 1038 and 991 cm-1 may be attributed to the pyridyl and benzene ring breathing, respectively.41 All of them are comparable with that in aMOC1. As shown in Figure 2e and Figure S2a, the N2 adsorption-desorption isotherms of aMOC-1 and aMOC-2 belong to type Ⅳ with H3 hysteresis loops in term of the classification recommended by IUPAC, suggesting the presence of mesopores.42-43 The BET surface areas of aMOC-1 and aMOC-2 are 24.6137 and 51.0044 m2/g, respectively. Furthermore, the BJH pore size distribution (Figure 2b and Figure 2c) indicated that the BJH average pore width (dV/dW) is approximately 30-40 nm. Considering that the cages sizes are approximately 3.2 nm and 4.2 nm for aMOC-1 and aMOC-2, respectively, the mesopores cannot be generated from the cavity of the cage. Thus, they


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may result from the random packing of the cages. The inner space of the cage may be occupied by the NO3- anions, which make little contribution to the BET surface. The SEM image (Figure 2f) showed that the aMOC-2 material exhibits an irregular shape that may be formed by the aggregation of many small particles.

Figure 2. (a) PXRD and (b) TG curve of aMOC-2. (c) FT-IR spectra of L2 and aMOC-2. (d) Raman spectra of aMOC-2. (e) N2 adsorption-desorption isotherms of aMOC-2. (f) SEM image of aMOC-2. 3.2 Oxo-anion removal studies Anion exchange with three selected oxo-anions was investigated by immersing 10 mg of aMOC-1 or aMOC-2 samples in 30 mL of solution containing 10-4 M oxo-anions. The concentrations of CrO42-, Cr2O72- and ReO4- as a function of adsorption time are shown in Figure 3 and Figure S3. For both aMOC-1 and aMOC-2, all the oxo-anions were removed rapidly within only a few minutes. After 5 mins, the removal rates of CrO42-, Cr2O72- and ReO4- were


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approximately 99.0%, 92.2%, and 94.3%, respectively, with aMOC-1 as the adsorbent and approximately 99.2%, 99.7%, and 97.9%, respectively, with aMOC-2 as the adsorbent. All removal rates were almost 100% when reaching adsorption equilibrium. The adsorption kinetics of the oxo-anion removal of aMOC-2 are slightly faster than that of aMOC-1, which may be due to the larger cavity in MOC-2. The adsorption kinetics of these aMOC materials are much faster than those of many other related materials.18-23, 46-51

Figure 3. UV-vis spectra of (a) CrO42- and (b) Cr2O72- in 30 mL of aqueous solution (10-4 M for each oxo-anion) at given times by using aMOC-1 (10 mg) as the adsorbent. (c) The removal efficiency of aMOC-1 over time for ReO4- (The amount of aMOC-2 is 10 mg and the concentration of ReO4- is 10-4 M). UV-vis spectra of (d) CrO42- and (e) Cr2O72- in 30 mL aqueous solutions (10-4 M each oxo-anion) at given times by using aMOC-2 (10 mg) as the adsorbent. (f) The removal efficiency of aMOC-2 over time for ReO4- (The amount of aMOC-2 is 10 mg and the concentration of ReO4- is 10-4 M).


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To further explore the adsorption ability of these aMOC materials, adsorption isotherms were also performed at room temperature. The results are depicted in Figure 4. The highest uptake capacities of CrO42-, Cr2O72- and ReO4- for aMOC-1 are 157.4, 203.8, and 350 mg/g, respectively. For aMOC-2, the highest adsorption capacities of CrO42-, Cr2O72- and ReO4- are 242.0, 407.0, and 583.9 mg/g, respectively. The adsorption capacities of them are relative high compared with those of other porous materials.17-32, 44-56 Notably, the adsorption capacity of aMOC-2 for Cr(IV) oxoanions ranks the highest among related anion-exchanged porous materials, as showed in Tables S1 and S2. The adsorption capacities of aMOC-1 and aMOC-2 for ReO4- are moderate, but the adsorption capacities at low concentration are high.

Figure 4. Adsorption isotherms of (a) CrO42-, (b) Cr2O72- and (c) ReO4- to aMOC-1 and aMOC-2 (t = 48 h, T = 30 °C). 3.3 Adsorption kinetics and isotherms. To examine the adsorption mechanism, the time-dependent adsorption capacity data were analyzed using pseudo-second-order equation (equation S1 in the supporting information). The results are shown in Figure S4 and Table S3, revealing that the experimental adsorption data are fit very well by the equation with R2 values higher than 0.999. Therefore, the uptake of oxo-anions


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may be controlled by a chemical process.48-53,

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The pseudo-second-order rate constants,

equilibrium capacities, and correlation coefficients (R2) are given in Table S3. The adsorption isotherm data were fit by the three common adsorption modes including the Langmuir model, the Freundlich isotherm model, and the Tempkin model (see equations S2-S4 in the supporting information). As shown in Tab. S4 and Figures S5-S10, for aMOC-1, the experimental data obviously show the best fit to the Langmuir model, as seen from the correlation coefficients (R2). Thus, the adsorption of the three oxo-anions on aMOC-1 was a typical monomolecular-layer adsorption.58 As shown in Table S4, the situation for aMOC-2 was different. For the removal of Cr(VI) oxoanions, both the Freundlich and Langmuir equations give a decent correlation. For the removal of ReO4-, the Langmuir and Thempkin equations showed similar correlations. Therefore, the removal of oxo-anions on aMOC-2 may not merely belong to a monomolecular-layer adsorption.59 The heterogeneous distribution of surface-active sites of aMOC-2 are also attributed to the removal of oxo-anions. All adsorption isotherm parameters are given in Table S4. 3.4 Release experiments, variable pH studies, selectivity and recycling testes. The advantage of removing oxo-anions via the anion-exchange process may be that it is good for the desorption of oxo-anions, as well as the recycling of materials, which is very important for the practical application of the adsorbents in industry. Therefore, release experiments were performed in 2 M NaNO3 aqueous solution for aMOC-1@ oxo-anions and aMOC-2@oxo-anions. As shown in Figure 5a-5f, all the loaded oxo-anions can be released rapidly into NaNO3 aqueous solution and reach desorption equilibrium within several mins. The pH of the NaNO3 aqueous solution is approximately 7.0; therefore, Cr2O72- changes into CrO42- when it is released from the aMOC-


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loaded sample. The oxo-anions are difficult to release in water without NaNO3 solution, indicating that the release of oxo-anions may also be due to the anion-exchange process and further confirming the cationic nature of the aMOC materials. The adsorption of Cr(VI) by aMOC-2 is selected to investigate the influence of pH on the adsorption performance. As shown in Figure S11, the adsorbent is able to absorb Cr(VI) oxoanions from aqueous solution of a wide pH range (1-10). Furthermore, the Cr(VI) removal rate have no significantly differences in pH range from 3 to 10. The anion-exchange selectivity for the tested CrO42-, Cr2O72- and ReO4- was explored by the removal of them with the coexistence of one equivalent of other common anions, including Cl-, Br-, SO42-, NO3- and CO32-. As shown in Figure 5g and Figure 5h, the removal percentage is still over 85% in most case, and only the mixtures of ReO4- & Cl-, ReO4- & Br- , ReO4- & CO32- on aMOC-1 and Cr2O72- & NO3-, ReO4- & Br- ReO4- & CO32- on aMOC-2 exhibit a relatively lower removal efficiency (range from 80% to 85%). These binary mixture studies indicated the good selectivity of both aMOC-1 and aMOC-2. The recycling adsorption experiments were performed after dye release. As shown in Figure 5i, the recovered aMOC-2 can be reused for more than seven cycles without obviously losing adsorption capacity.


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Figure 5. UV-vis spectra of (a) aMOC-1@CrO42- (30 mg) and (b) aMOC-1@Cr2O72- (30 mg) in 30 mL 2 M NaNO3 solutions at different times during the release experiment. (c) The concentration of ReO4- in 2 M NaNO3 solutions at different times in the presence of 30 mg aMOC-1@ ReO4-. UV-vis spectra of (d) aMOC-2@CrO42- (30 mg) and (e) aMOC-2@Cr2O72- (30 mg) in 30 mL 2 M NaNO3 solutions at different times during the release experiment. (f) The concentration of ReO4in 2 M NaNO3 solutions at different times in the presence of 30 mg aMOC-2@ ReO4-. The removal efficiency of (g) aMOC-1 and (h) aMOC-2 with the coexistence of competing anions. (i) The recycling test for aMOC-2 on the removal of CrO42- anion.


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3.5 Possible adsorption mechanism. The aMOC@oxo-anion samples were analyzed by IR and EDX. For aMOC-1, after the oxoanion adsorption, peaks at 938, 928 and 918 cm-1 that were assigned to Cr2O72-, CrO4- and ReO4anions,18, 51 respectively, were observed in the FT-IR spectra (Figure S12). In addition, the intensity of the peak at 1383 cm-1 that was assigned to NO3- decreased obviously. The situation for aMOC-2 is similar to that of aMOC-1. Thus, the results confirm the anion exchange between NO3- and oxoanions on these aMOCs. For aMOC-1, although the peak of NO3- in FT-IR is obviously reduced, it still has a certain strength, which means the exchange process is not very complete. For aMOC-2, the intensity of the NO3- peak of FT-IR seems to decrease more than that for aMOC-1, indicating that more NO3- in aMOC-2 is exchanged. This is one of the reasons for the higher adsorption capacity of aMOC-2. Furthermore, the presence of Cr and Re elements after uptake experiments were revealed by EDX, confirming the adsorption of the oxo-anions (Figures S13-S18). In addition, the UV spectra indicated that the released anions from the release experiment shown above were CrO4- and ReO4-, thus confirming that these oxo-anions are adsorbed without chemical reaction. The PXRD of aMOC after adsorption was also recorded, which indicated that the framework may remain unchanged as shown in Figures S19-S20. If all the NO3- anions in aMOC-1 can be exchanged by the oxo-anions, the adsorption capacities can be calculated to be approximately 154, 287 and 668 mg/g for CrO4-, Cr2O72- and ReO4-, respectively.51 The adsorption capacities are near the experimental data on the CrO42- and Cr2O72- adsorption (157.4 and 203 mg/g, respectively) and higher than the experimental data for ReO4- adsorption (350 mg/g), which imply that their removal may proceed mainly via anion-exchange. The calculated capacities for CrO4-, Cr2O72- and ReO4- are only approximately 137, 255, and 593 mg/g for aMOC-2, respectively. The calculated capacity for ReO4- is quite close to the experimental capacity (583.9


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mg/g). However, the calculated capacities for CrO42- and Cr2O72- are obviously lower than those observed in experiments (242 and 407 mg/g, respectively). In addition, the zeta potential of aMOCs before and after CrO42- and Cr2O72- adsorption are displayed in Figure S21. The results indicated after Cr(VI) adsorption, the zeta potential of aMOC-2 changed from high positive (53.5 eV) to negative with value of -8.01 and -12.2 mV for CrO42- and Cr2O72-, respectively. While it is changed from 42.4 eV to 0.269 and 5.28 eV, respectively, in the case of aMOC-2. It is reasonable to though that the zeta potential may be reduced if the NO3- are changed by Cr(VI)-anions because they bind to the cage host more tightly and more difficult to release to the surface. If more additional anions are adsorbed, the zeta potential may change from positive to negative. Consequently, the high adsorption capacities for aMOC-2 is not only attributed to the anionexchange process but also to interactions such as anion-π interactions between the cage wall and the oxo-anions or weak Pd‧‧‧O interactions between Pd(II) and oxo-anions. All of these factors may contribute to the improvement of the adsorption capacities of aMOC-2 on Cr(VI) oxo-anion adsorption. Because the oxo-anions are larger than the NO3- anion, the anion-exchange process of replacing NO3- anions with oxo-anions will reduce the leaving space in aMOC-1 and stop the oxoanion uptake. However, the larger cavity in aMOC-2 may be large enough to accommodate more oxo-anions even after complete anion-exchange; thus, it can remove more oxo-anions via other processes. The proposed mechanism of oxo-anion adsorption is depicted in Scheme 2.


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Scheme 2. Scheme of the possible mechanism of oxo-anion adsorption on aMOC-1 and aMOC2.

3.6 Cr(VI) removal from electroplating bath The use of Cr(VI) plating baths in the electroplating industry is one of the main sources of Cr(VI) wastewater. To investigate the industrial application prospects of these materials, the adsorption of aMOC-2 on Cr(VI) in plating solution was initially studied. To remove Cr(VI) from the plating solution, 10 mg of aMOC-2 was added to 30 mL of electroplating wastewater, and soaked for 10 minutes. The results are shown in Figure 6. From the UV-vis spectrum (Figure 6a), the absorption peak in the wavelength range of 310 nm-500 nm is significantly weakened, indicating that some Cr(VI) oxo-anions have been removed. The ICP test further showed that the Cr(VI) concentration was decreased from 10 ppm to 0.17 ppm, meeting the standards for pollutant discharge. The IR and PXRD results also showed that Cr(VI) is indeed adsorbed by the materials, and the structure of the materials does not change significantly (Figures. S22-S23). Therefore, aMOC-2 has potential application in the adsorption of toxic Cr(VI) oxo-anions from plating bath wastewater.


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Figure 6. (a) UV-vis spectra of the electroplating bath at different times during the adsorption of aMOC-2 on Cr. (b) The concentration of Cr in the electroplating bath at different times with the presence of aMOC-2 (the concentrations of Cr are measured by ICP).

CONCLUSION The amorphous MOC-based solids named aMOC-1 and aMOC-2 were applied to oxo-anion adsorption for the first time. Because of the high positive charge and large cage in the aMOCs structures, the adsorption of CrO42-, Cr2O72- and ReO4- on aMOCs exhibits rapid kinetics and high capacities. The high capacity is attributed to the anion-exchange process and the host-guest interaction between aMOC-2 and oxo-anions. In addition, the adsorbed oxo-anions can be released rapidly in NaNO3 solution and can be reused for more than seven cycles. The aMOCs also showed a potential application in treating electroplating bath wastewater. This work develops a new method for the construction of cationic porous solids for the treatment of wastewater that contains anionic pollutants.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. The additional figures and tables of characterization information. (PDF) AUTHOR INFORMATION Corresponding Author *[email protected] ORCID Sheng-Run Zheng: 0000-0002-9071-0748 Song-Liang Cai: 0000-0002-5399-9036 Jun Fan: 0000-0003-2986-8551

Notes The authors declare no competing financial interest.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (grant no. 21473062, 21603076 and 21571070).


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Table of Contents

Cationic Amorphous Metal-Organic Cage-Based Materials for the Removal of Oxo-Anions from Water Xia Jin, Guang-Qing Wang, Ding Ma, Shu-Qi Deng, Song-Liang Cai, Jun Fan, Wei-Guang Zhang, Sheng-Run Zheng*


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Scheme 1 301x174mm (300 x 300 DPI)



Figure 1 255x146mm (300 x 300 DPI)


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Figure 2 338x190mm (300 x 300 DPI)



Figure 3 338x190mm (300 x 300 DPI)


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Figure 4 338x94mm (300 x 300 DPI)



Figure 5 254x178mm (300 x 300 DPI)


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Scheme 2 250x125mm (300 x 300 DPI)



Figure 6 290x118mm (300 x 300 DPI)


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