Cation Exchange Superparamagnetic Al-Based Metal Organic

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Cation exchange superparamagnetic Al-based metal organic framework (Fe3O4/MIL-96(Al)) for high efficient removal of Pb(II) from aqueous solutions Ali Mehdinia, Davoud Jahedi Vaighan, and Ali Jabbari ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03301 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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ACS Sustainable Chemistry & Engineering

Cation exchange superparamagnetic Al-based metal organic framework (Fe3O4/MIL-96(Al)) for high efficient removal of Pb(II) from aqueous solutions

Ali Mehdinia a∗, Davoud Jahedi Vaighanb, Ali Jabbarib a

Department of Marine Living Science, Ocean Science Research Center, Iranian National

Institute for Oceanography and Atmospheric science, P.O.Box: 14155-4781, Tehran, Iran b

Department of Chemistry, Faculty of Science, K. N. Toosi University of Technology,

P.O.Box: 15875-4416, Tehran, Iran

ABSTRACT A cation-exchange metal-organic framework sorbent with high adsorption capacity, active

anionic surface, available adsorptive sites, good response in magnetic field and also available enclosed space between the particles for encapsulation of the analyte was in-situ prepared by magnetization of a metal organic framework structure of aluminum named as MIL-96(Al). The units of magnetic MIL-96(Al) was synthesized by embedding method under hydrothermal condition and well characterized. The mechanism of sorption can be the electrostatic interaction between the anionic structures of the adsorbent with hydrated Pb2+ ions. The high percent of OH groups on the surface of Fe3O4/MIL-96(Al) led to high adsorption capacity (> 301.5 mg g-1) of the sorbent for Pb2+ ions in aqueous media. The thermodynamic study of Pb2+ adsorption onto the sorbent surface demonstrated that the process was spontaneous, exothermic and physical. The face-centered central composite ∗

Corresponding author: Tel: +98 21 66944873; Fax: +98 66944869. E-mail address: [email protected] (A. Mehdinia)

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design was applied for optimizing the adsorption conditions, and the effective parameters were obtained as pH of 6.92, amount of sorbent of 32.4 mg and adsorption time of 25 min. KEYWORDS Fe3O4/MIL-96(Al); magnetic composite of MOF; removal of lead ions; magnetic metal organic trap (MMOT); cation-exchange MFCs; central composite design

INTRODUCTION Metal-organic frameworks (MOFs) or porous coordination polymers, consisting metal ions/clusters and organic struts, have been recognized as an excellent platform for host-guest chemistry applications1 such as gas storage2, ion-exchange1, separation3, catalysis4, drug delivery5, sensors6 and conduction7. The main advantage of MOFs in comparison to the alternative materials is the tunability of pore size, shape, topology, and functionality.8 Selective sensing and exchange of ions is one of the main application of porous materials9. Recently, ion-exchange MOFs are new category of MOFs with large surface area, high porosity and charged framework which led to their high efficiency for ion-exchange chromatography and ion-exchange solid-phase extraction. The strong columbic interaction between the charged stationary phase and charged analytes leads to high efficiency of separation process1. In addition, ion-exchange MOFs, due to their large surface area and porosity, have well capacity for solid-phase extraction of the charged analytes. The composition of MOFs and the materials with a variety of functional groups is performed to combine the merits and reduce the shortcomings of the both components10. MOFs of highly porous nanocomposites were made with active nanoparticles such as metal nanoparticles/nanorods11,

12

,

oxides13,

14

,

quantum

polyoxometalates19, graphene20, carbon nanotubes21,

dots

15,

16

,

polymers17,

18

,

22

, biomolecules23 and so on. In this

regard, magnetic framework composites (MFCs), fabricated by combination of magnetic 2 ACS Paragon Plus Environment

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nano- or micro-particles with MOFs crystals24, are good candidates for adsorption process. Several methods introduced for fabrication of MFCs such as embedding25, encapsulation26, mixing27 and layer-by-layer28 strategies. MFCs combine the favorable virtues of both magnetic Fe3O4 nanoparticles and metal organic framework. It makes them excellent candidates for adsorption process27, and also the phase separation can be rapidly and easily accomplished by applying an external magnetic field29. The 3D metal-organic framework of MIL-96 (Al) (MIL stands for Material of Institute Lavoisier), with unit cell of Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6.24H2O, involves the aluminum octahedral units interacting with the btc ligands. It was hydrothermally synthesized for the first time by Loiseau and co-workers and applied for separation of CH4, CO2 and H230. In the other works, MIL-96 (Al) was used for separation of C5-hydrocarbons31, vapor-phase adsorption of alkylaromatics32, adsorption of nitrogenous volatile organic compounds (VOCs) 33 and defluorination of drinking water34. The MIL-96 (Al) is a practical material due to using of inexpensive metal of aluminium35, high hydrothermal stability36,

37

, flexible

structure32, excellent stability in neutral-to-acidic aqueous solutions38 and high porosity30. Furthermore, existence of large number of hydroxyl groups in its structure makes it useful for extraction purpose. Also, the weak Lewis acid sites in the Al-MIL-96 structure could form a coordinated bond with water molecules, hydroxyl groups or other hard base groups in the aqueous solution33. It can also create negative charges in the structure of MIL-96 (Al). Therefore, MIL-96 (Al), due to its interaction by water molecules, has high potential for release of protons and well capacity for selective cation-exchange application. In the neutral pH, O- groups in the framework of MOF lead to form anionic structure for it. Hence, MIL96(Al) possesses several effective factors to enhance efficiency of the extraction process. First, combining the specific pore size and strong columbic interaction between the anionic framework of this MOF and cationic compounds can cause selective extraction of the 3 ACS Paragon Plus Environment


cationic analytes1. Second, according to the Pearson’s hard/soft acid/base concept39, hard base oxygen atoms in the framework of MOF can form stable complexes with the analytes with hard acids groups. Third, the replacement of water molecules in the hydrated structure of cationic metals by oxygen atoms in the framework of MOFs 40 can affect on the selectivity of MOF. Fourth, spontaneous aggregation of MIL-96(Al) nanoparticles, during extraction process, can create a trap with the available enclosed space between the particles for capsulation of the bulky compounds. The aggregation can occur due to the interaction of hard Lewis acid sites of Al in MIL-96 particles33 with surficial oxygen atoms in the adjacent MIL96(Al) particles. It also leads to further increasing in sorption capacity of the sorbent and its ability for separating of the cloudy precipitated metals in aqueous solution. Also, it can cause rapid and easily isolation of the sorbent from aqueous solution after sorption procedure by an external magnetic field. In present work, the MIL-96 (Al) was considered for design of a magnetic cationexchange metal-organic trap with the advantages of ion-exchange MOFs and MFCs. Also synthesis of a magnetic nanocomposite of MIL-96(Al) was illustrated and its potential ability for in-situ preparation of cation-exchange metal-organic trap was investigated. It was used for adsorption of lead (II) ions in aqueous solution by trapping and cation-exchange mechanism. Parameters affecting the efficiency of sorption were evaluated in detail by central composite design under response surface methodology (RSM).

EXPERIMENTAL SECTION Materials and solutions. The reagents were obtained from a commercial seller and used without further purification. Benzene-1,3,5-tricarboxylic acid (H3BTC) (Sigma-Aldrich, 99%), aluminum nitrate nonahydrate (Al(NO3)3·9H2O) (Merck, 98.5%), dimethylformamide (DMF) (Sigma-Aldrich, 99.8%), methanol (Sigma-Aldrich, 99.8%), iron (III) chloride 4 ACS Paragon Plus Environment

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hexahydrate (FeCl3·6H2O) (Merck, 99%), iron (II) chloride tetrahydrate (FeCl2·4H2O) (Merck, 99 %) and ammonium hydroxide solution (NH4OH) (Merck 25%) were used for synthesis

of

Fe3O4/MIL-96(Al)

nanocomposite.

Sodium

phosphate

monohydrate

(NaH2PO4.H2O) was obtained from Merck and used for functionalization of Fe3O4/MIL96(Al). Sodium hydroxide pellets (NaOH) (Merck, 99 %) and nitric acid supra pure (HNO3) (Sigma-Aldrich, 65 %) were required for the adsorption and adjusting pH of the solutions. Stock standard solution (1000 mg L−1) of Pb2+ was prepared by dissolving of 0.159 g of analytical grade Pb(NO3)2 salt (Merck) in 2 mL of 6N nitric acid and dilute to 100 mL by the deionized water. Working standard solutions were prepared daily by diluting the stock standard solution to the appropriate concentrations with the deionized water. Apparatus. The crystalline structure and composition of the Fe3O4/MIL-96(Al) were identified by Powder X-ray diffraction (PXRD, Philips X’pert diffractometer, model pw1730), field emission scanning electron microscopy (FE-SEM, MIRA3TESCAN-XMU) with EDX analysis, scanning electron microscopy (SEM, KYKY- EM3200 -26 kV), transmission electron microscopy (TEM, Philips), Fourier-transform infrared spectroscopy (FT-IR, Bruker, VERTEX 70), vibrating sample magnetometry (VSM, Daghigh Kavir Co., Kashan, Iran) and dynamic Light Scattering (DLS, Malvern, Zetasizer Nano ZS). The surface of the of the synthesis Fe3O4/MIL-96(Al) was analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi).The nitrogen adsorption/desorption isotherms were obtained at -196 °C with an adsorption unit (BET, Belsorp mini II, Japan Co.) after evacuation at 200 °C for 3 h. The surface area was calculated from nitrogen adsorption isotherms using the Langmuir and BET equations. Digital pH meter, Metrohm 827 (Titrino, Metrohm, Switzerland) was used for pH measurements. The GBC 932 plus (Australia) flame atomic absorption spectrophotometer (FAAS) equipped with a lead hallow cathode lamp (GBC, Australia) at the wavelength of 217 nm was applied for determination of lead ions in 5 ACS Paragon Plus Environment


the sample solutions and thermal energy created via air/acetylene flame, applied for generation of Pb gas phase atoms in all measurements. Synthesis of Fe3O4 nanoparticles. Fe3O4 nanoparticles were synthesized via the coprecipitation method41; 8.1 g iron (III) chloride (FeCl3 · 6H2O) and 3 g iron (II) chloride (FeCl2 · 4H2O) were dissolved in 150 mL deoxygenated deionized water during stirring and under nitrogen gas atmosphere. 25 mL NH4OH (25%, v/v) was added to this solution at 80 oC under strong stirring condition. Stirring of the reacted mixture was continued for 30 min, and then cooled to room temperature. The black Fe3O4 MNPs was isolated by a magnetic field and washed three times with the deionized water. Synthesis of MIL-96(Al). Particles of Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6·24H2O, MIL96(Al) were prepared via a hydrothermal method that has been reported previously38, 42. First, 750 mg Al(NO3)3·9H2O were dissolved in 10 mL of water-DMF mixture (4:1), then 420 mg 1,3,5-benzenetricarboxylic acid (btc) was added to the reaction mixture and dissolved with 2 h sonication. Afterwards, the resulting mixture was refluxed at 140 °C for 24 h. Finally a white color product was separated by centrifugation and washed three times with deionized water and methanol. Then the product was dried at 100 °C in an oven for 12 h. Synthesis of Fe3O4/MIL-96(Al). Hydrothermal method was used for embedding24 of Fe3O4 nanoparticles in MIL-96(Al) crystals. Initially, 2.25 g Al(NO3)3.9H2O was dissolved in 30 mL mixture of water:DMF (4:1). Then, 0.9 g Fe3O4 nanoparticles, synthesized in the previous step, were dispersed in the reaction mixture under sonication (15 min). Next, 1.26 g H3BTC was added to the reaction flask and sonicated in an ultrasonic bath for 2h under mechanical stirring. Resulting suspension was refluxed at 140 °C for 36 h. Produced nonmagnetic composite was separated by an external magnet and washed three times with


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deionized water. At the end, the product was washed three times by methanol and dried at 100 °C in an oven for 12 h. Phosphate functionalization of Fe3O4/MIL-96(Al). 100 mg NaH2PO4.H2O was dissolved in 20 mL deionized water, and then 100 mg of the prepared Fe3O4/MIL-96(Al) was added to this solution, the pH was set at 6.5 and it stirred for 12 h at 25 °C. Subsequently, the obtained product was isolated from the reaction mixture by external magnet, washed three times by deionized water and methanol. Finally the product was dried at 100 °C in an oven. Batch cation exchange experiments. The procedure of magnetic cation exchange adsorption of lead ions was performed in batch experiments. According to a preliminary experimental design, 32.4 mg Fe3O4/MIL-96(Al) was activated (Figure S1) via dispersing in 3 mL 1M NaOH solution by 20s sonication in an ultrasonic bath, and after 10 min the activated sorbent was collected by a permanent magnet (2 cm×4 cm×6 cm, 1.3 T) and washed by deionized water. Then, it was added in a 100 mL aqueous sample containing the lead ions, and pH was adjusted to 6.92 by the diluted solution of NaOH or HNO3. The solution was stirred softly for 25 min at room temperature (25 ±1°C) using a magnetic stirrer at a steady rate of 750 rpm to suspend MFCs thoroughly and facilitate adsorption lead ions onto the sorbent. At the end of the extraction time, the solution was transferred to the conical tubes and after collecting the sorbent by a magnet on one side of the tube, the supernatant was decanted. The amounts of Pb2+ ions were specified using FAAS. Experimental design methodology. RSM is an alternate statistical approach to obtain the effect of the experimental variables that can mainly affect on the extraction process43. The exclusive factors must be considered along with nonlinear effects and interaction terms44. Central composite design (CCD), appropriate for fitting a quadratic surface was used for optimization of the effective parameters with a minimum number of experiments and also for



analyzing of the interaction between the parameters45. In this case, face-centered central composite design (FC-CCD) factors were assessed at three levels and the axial points were set on the centers of faces of the cubic experimental area. As a result, center point replications in FC-CCDs are generally less than the amount required for the comparable rotatable CCD. So, it can reduce the excessive changing of factor levels, makes comfortable administration of the experiment, and also can decrease the required time and expenses46. For this reason, the FC-CCD was used for optimization of the adsorption process of Pb2+ ions and elution step. Then, Design-Expert 7.1.3 software was applied for analysis of the experimental design data and estimation of the predicted responses.

RESULTS AND DISCUSSION Characterization of the Fe3O4/MIL-96(Al). In a typical synthesis, embedding method was used for synthesis of Fe3O4/MIL-96(Al) in the hydrothermal condition. The magnetic Fe3O4 nanoparticles were added into the mixture solution of ligands and inorganic precursors of MIL-96(Al). Two mechanisms can be offered for embedding of Fe3O4 nanoparticles within the MIL-96(Al) crystals (Figure 1a). In a possible mechanism, according to this fact that Al(III) is a hard Lewis acid39 with empty d orbitals, and aluminum octahedral units is a label complex35, they can form a stable coordination bound with OH hard base groups on the surface of Fe3O4 nanoparticles. Then, nucleation can carry out around of the Fe3O4 nanoparticles. In another possible mechanism, H3BTC ligand can be coordinated with Fe(III) sites on the surface of Fe3O4 nanoparticles. Subsequently, MIL-96(Al) crystals, after the heatdriven process, can grow in the surrounding of Fe3O4 nanoparticles. To determine the surface chemical composition of the synthesized Fe3O4/MIL-96(Al) and evaluate the probable connection mechanism between Fe3O4 and MIL-96(Al) X-ray photoelectron spectrometry results were shown in Figure 2. The wide scan spectrum of Fe3O4/MIL-96(Al), as shown in


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Figure 2a, indicated the presence of aluminum, carbon, oxygen, and iron in the composite. The photo electron lines at binding energies of approximately 75, 285, 530 and 710 eV are attributed to Al2p, C1s, O1s and Fe2p, respectively. The high-resolution XPS spectrum for O1s is shown in Figure 2b. In the O1s spectrum of Fe3O4/MIL-96(Al), there were different oxygen containing groups including metal oxides (530.4 eV), Fe-O-C (532.1 eV), Al-OH and Aluminum carbonate (532.3 eV) and the peak at 533.7 eV should correspond to C-O groups 47, 48

. It is worth mentioning that the Fe-O-C peak at 532.1 eV was ascribed to some covalent

bonds between Fe3O4 and BTC ligand (Figure 2b). H3BTC ligand can be coordinated with Fe(III) sites on the surface of Fe3O4 nanoparticles. Subsequently, MIL-96(Al) crystals, after the heat-driven process, can grow in the surrounding of Fe3O4 nanoparticles. That is corresponded with proposed mechanism for synthesis of Fe3O4/MIL-96(Al), and confirms the connection between Fe3O4 and MIL-96(Al) through oxygen atoms in carboxylates groups of BTC ligands and Fe3+ in Fe3O4. Figure 1b shows the SEM image of pure Fe3O4 with granular structure. The spindle shaped particles, obtained from original MIL-96(Al) was shown in Figure 1c. It is in accordance with the proposed morphology for MIL-96(Al) in the literature.38 Typically, the polycrystalline Fe3O4/MIL-96(Al) particles tend to form crystal morphology similar to the original MIL96(Al). FE-SEM and TEM images (Figure 1d, f, g and q) of Fe3O4/MIL-96(Al) almost indicated these similarities in crystal morphology. Also, TEM images (Figure 1f, g and q) show that the Fe3O4 nanoparticles were dispersed in the MIL-96(Al) crystals. In addition, the composition of the sorbent was determined using EDS analysis as shown in Figure S2. The results confirmed the presence of elements of the Fe3O4 nanoparticles distributed in the structure of Fe3O4/MIL-96(Al) composite.



Figure 1. Schematic illustration of two possible mechanisms for embedding Fe3O4 nano-particles into the MIL96(Al) crystals (a). The SEM images of Fe3O4 nanoparticles (b) and MIL-96(Al) nanoparticles (c), FE-SEM image Fe3O4/MIL-96(Al) composite (d) TEM images of Fe3O4/MIL-96(Al) composite (f, g and q).

Figure 2. (a): XPS survey spectra of Fe3O4/MIL-96(Al). (b): Fitted results of XPS O1s spectra.


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XRD analysis of the sorbent is indicated in Figure 3. The peaks in the XRD analysis of Fe3O4/MIL-96(Al) can be related to crystalline Fe3O4 and MIL-96(Al) (Figure 3e). The diffraction peaks observed at ~30.0°, 35.4°, 43.0°, 53.4°, 56.9° and 62.5° (Figure 3e) can be attributed to the typical cubic spinel structure of Fe3O4 which embedded in MIL-96(Al). Also, they have a great conformity with the Fe3O4 XRD pattern simulated by O'Neill et al.49 (Figure 3b) and experimental XRD pattern of Fe3O4 (Figure 3c). The patterns (Figure 3d,e) illustrate that the diffraction peaks at ~5.651°, 7.72°, 9.14°, 13.65°, 14.63°, 15.49°, 16.7°, in the magnetic Fe3O4/MIL-96(Al) and MIL-96(Al) can be matched with the diffraction peaks of simulated MIL-96(Al), that presented by Loiseau and co-workers30 (Figure 3a). It indicates the successful synthesis of Fe3O4/MIL-96(Al) composite. Also, XRD pattern of Fe3O4/MIL96(Al) after activation by NaOH solution was prepared (Figure S3). Comparison of the XRD pattern of Fe3O4/MIL-96(Al) before and after activation indicates the appropriate stability of Fe3O4/MIL-96(Al) during the activation process.

Figure 3. (a) and (b): Simulated XRD patterns of MIL-96(Al) and Fe3O4, extracted from a cif-file presented by Loiseau and co-workers30 and cif-file presented by O'Neill and coworkers49, respectively. (c): experimental XRD pattern of Fe3O4 (d): XRD pattern of MIL-96(Al) and (e): XRD pattern of Fe3O4/MIL-96(Al).



The N2 adsorption–desorption isotherm plot was exhibited for Fe3O4/MIL-96(Al) in Figure S4. The Langmuir and BET surface areas, calculated from the adsorption isotherm, were around 98.3 and 73.8 m2g-1, respectively. BET surface areas for MIL-96(Al) was 216 m2 g-1 in literature36. It can prove that the Fe3O4 nanoparticles were embedded in the MIL-96(Al) crystals and reduced the effective surface area of MIL-96(Al). The FT-IR spectra of Fe3O4, MIL-96(Al) and Fe3O4/MIL-96(Al) are shown in Figure S5a, b and c, respectively. The two bands observed at 632 and 585 cm-1 in the Figure S5a, are as a result of splitting of the Fe-O absorption band of bulk Fe3O4 at 570 cm-1 that shifted to the higher wavenumber50. The characteristic absorption bands of Fe3O4 at 632 and 585 cm-1, observed at Figure S5c, indicated successful embedding of magnetic nanoparticles in the MOFs crystals. The FT-IR spectra in Figure S5b indicated conformity of the characteristic absorption bands of the synthesized MIL-96(Al). It confirmed with the FT-IR spectra obtained by Loiseau et al.30. The band at 625 cm-1 is assigned to (OH)–Al=O angle bending of aluminum cluster 51 (Figure S5b). The vibrational bands in the range of 1400-1600 cm-1 at the FT-IR spectra in Figure S5b and c, are related to aromatic rings in the ligand groups of MIL-96(Al). Also, there are asymmetric stretching vibrations of bridging carboxylate groups in the range of 1500-1700 cm-152 and the symmetric vibrations between 1459 and 1399 cm-1, which is assigned to the C-O bond30. Due to the fully coordination of aluminum units with the btc ligands, strong absorption bond of carboxylates C=O groups in the area of 1710-1740 cm-1 was disappeared. Figure 4 shows the magnetization curves (VSM curve) of Fe3O4 and Fe3O4/MIL-96(Al). As can be observed from the figure, both samples exhibited a superparamagnetic property. The saturation magnetization of Fe3O4 and Fe3O4/MIL-96(Al) were obtained as 54.5 and 24.6 emu g-1, respectively. Due to the embedding of Fe3O4 nanoparticles in the crystalin structure of MIL-96(Al), the obtained superparamagnetic Fe3O4/MIL-96(Al) particles showed enough 12 ACS Paragon Plus Environment

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magnetism for facile separation from sample matrix by applying a magnetic field. As can be seen from Figure 4, the saturation magnetization was gradually decreased with increasing MOFs contents on the superparamagnetic nanoparticles53,

54

. The obtained values of

saturation magnetization can be used to estimate the weight percentage (w/w %) of magnetic Fe3O4 nanoparticles ( %) and MOF contents (MIL96 Al %) in the particles of Fe3O4/MIL-96(Al). Equations 1 and 2 were proposed for estimation of the composition of each nanoparticle: %

/

!

" 100 1

%& 96 % 100 ' % 2 Which and /

!

are saturation magnetizations of pure Fe3O4

nanoparticles and Fe3O4/MIL-96(Al) particles, respectively. For synthesized magnetic composite, calculated % and MIL96 Al % were 45.14 % and 54.86 %, respectively. These data are an estimate of MOF growth around of the Fe3O4 nanoparticles for each synthesis.

Figure 4. VSM magnetization curve of Fe3O4 (a), Fe3O4/MIL-96(Al) (b)



Evaluation of chemical characteristics of Fe3O4/MIL-96 (Al). Study and comparison of the adsorption amount of different analytes on the surface of Fe3O4/MIL-96(Al) demonstrated the appropriative properties of the sorbent. Lewis acid behavior of the sorbent was studied by titration. 50 mL distilled water suspension containing 50 mg Fe3O4/MIL-96(Al) was titrated by 0.1 M NaOH solution. The titration plot (Figure 5a) indicated that the sorbent has high potential for release of proton in aqueous solution due to the presence of Lewis acid sites of Al on the sorbent. The coordination of water molecules (as hard base) with Al atoms (as hard Lewis acid) leads to release of proton of the coordinated water molecules, and creates anionic frameworks with high coordinated hydroxyl groups on the surface of Fe3O4/MIL-96(Al) particles in aqueous solution. Also, Zeta potential measurement, presented in Figure 5b, shows the surface charge of the Fe3O4/MIL-96(Al) particles at different pH values. The isoelectric point (IEP) of Fe3O4/MIL-96(Al) particles was found to be 5.13, and it confirms anionic nature of Fe3O4/MIL-96(Al) in neutral aqueous solution.

Figure 5. Titration curves of 50 mg Fe3O4/MIL-96(Al) by NaOH 0.1 M (a), Zeta-potential as a function of the pH for Fe3O4/MIL-96(Al) (b). Evaluation of adsorption of different metal ions on the surface of Fe3O4/MIL-96(Al). Different hydrated metals ions; hard, intermediate and soft metal ions, with different labilities, were investigated to study the adsorption mechanism. As presented in Table 1,


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there are several effective factors in the ion exchange adsorption process: content of negative charges on the surface of adsorbent, quantity of positive charges on the metals ions, amount of basic groups with different hardness on the surface of sorbent, hardness extent of metal ions and kinetic of water exchange in metal ions40, 55-57. For cation exchange sorption of metal ions by Fe3O4/MIL-96(Al), amounts of negative and positive charges on the surface pores of the sorbent and metal ions, respectively, produce the required force for immigration of metal ions from the bulk of solution onto the cavity surfaces of the adsorbent. This electrostatic force increases immigration speed of the metal ions, but this electrostatic force is not enough to provide required selectivity for the extraction of each metal ion. In this regard, two other factors can affect to obtain the stable and selective extraction. In the second sorption stage, the metal ions require to be retained by the sorbent after immigration of cationic metals to the surface of sorbent. It is due to the stable interaction of the hydroxyl groups on the surface of sorbent and cationic metal ions. But, these stable interactions may be time-consuming for some ions, because of the need to different activation energies for substitution of the coordinated water molecules with the different hydrated metal ions. Thus, each ion that can quickly enter in the substitution reaction was retained by the sorbent and other ions were exchanged by those ions. The primary experiments (Table 1) showed that the Fe3O4/MIL96(Al) represents good response for extraction of Cu2+ and Pb2+ ions. Because of the higher environmentally importance of Pb2+ ion, it was used as analyte for subsequent experiments. Extraction recoveries of different cations were also investigated for pure MIL-96(Al). The similar results were obtained for MIL-96(Al). It indicated that Fe3O4 nanoparticles embedded into the MIL-96(Al) can compensate the reduction in the BET surface area of Fe3O4/MIL96(Al) than MIL-96(Al). According to the results, the adsorption mechanism of Pb2+ onto the cavity of Fe3O4/MIL96(Al) can be proposed according to Figure S6. In the natural and alkaline pH values, the



sorbent has negative surface charges. The negative charges of the activated sorbent are neutralized by proton or Na+ cations. After entering the sorbent to the sample solution, the hydrated Pb2+ cations rapidly immigrated58 from the bulk of solution to the surface and cavities of

Fe3O4/MIL-96(Al). Charge equilibrium on the surface of the activated

Fe3O4/MIL-96(Al) was tuned by ejecting Na+ and other metal cations from the surface of the sorbent. Then, due to the suitable labiality of the hydrated Pb2+ ions at room temperature, substitution reaction and sorption step can be rapidly and completely performed. In addition, the aggregation of Fe3O4/MIL-96(Al) was observed slowly during the Pb+2 sorption. Figure S6 also indicated the schematic mechanism of aggregation of the particles. Actually in natural pH, Fe3O4/MIL-96(Al) particles have dual properties. The hydroxyl groups on a particle can interact by Lewis acid Al sites on the adjacent particle. Al3+ is located in the main groups of metal ions with high positive charge density and empty d orbitals, hence, it can interact successfully by hard bases56. But due to the high positive charge density and low ionic radius, Al3+ has stable56 interaction by the hydroxyl or water molecule in aqueous solution. Therefore, Al3+ has low lability compared to Pb2+ ions56, and aggregation of the particles occurs slower than adsorption of Pb2+ ions in the studied temperature. It helps the extraction process, because after completing the extraction of Pb2+, the aggregation of sorbent particles is completed. Thus, the aggregation has low effect on the extraction efficiency due to the decreasing in dispersity of the sorbent in the batch sorption process. In addition, the data obtained from the capacity measurement (Figure 6) indicated that the capacity of the sorbent highly enhanced by trapping the precipitated analyte during aggregation of Fe3O4/MIL96(Al) particles. Other benefits of magnetic sorbent particles aggregation was easy magnetic isolation of these particles after batch sorption stage. By aggregation of the nanoparticles, the lumped particles will have lower surface area than the dispersed sorbent particles, and a reduction can be occurred in friction force between solvent molecule and particles surfaces.


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Thus, aggregation after sorption stage can reduce the required magnetic force for isolation of the sorbent. The EDS analysis of Fe3O4/MIL-96(Al) after adsorption of Pb2+ was also performed to prove Pb2+ adsorption. The obtained results (Figure S7) indicated the presence of Lead ions in the surface of Fe3O4/MIL-96(Al) after adsorption process. Dual behavior of Fe3O4/MIL-96(Al). Due to the Lewis acid sites, MIL-96(Al) is recently used for adsorption of nitrogenous volatile organic compounds33. For further study on activity of Lewis acid sites of Al on the surface of Fe3O4/MIL-96(Al), phosphate ions were used as model compounds. Phosphate ions, having hard base groups could creative effective interaction with Lewis acid sites of Al on the surface of Fe3O4/MIL-96(Al). The FT-IR spectra (Figure S8) of the phosphate functionalized Fe3O4/MIL-96(Al) indicated the capability of Lewis acid sites of Fe3O4/MIL-96(Al) for establishing stable interaction with the phosphate ions. The band obtained at 1000–1200 cm-1 for coordinated phosphate confirmed the stable interaction of phosphate with the surface sites of the sorbent. The active Lewis acid sites of Fe3O4/MIL-96(Al) can be useful for using of this sorbent for extraction of organic compounds, and facile modification of Fe3O4/MIL-96(Al) for extraction of other metal cations.



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Table 1. Hard/soft and labile/inert effects on the extraction efficiency of different metals ions on the surface of Fe3O4/MIL-96(Al) and MIL-96(Al) Metals

)*+, (sec-1)55, 56

Hard/Soft39

Extraction % Fe3O4/MIL-96(Al)

MIL-96(Al)

Cd2+

Soft

108 < -./ < 109

0

0

Hg2+

Soft

109 < -./ < 1010

66 ± 3

69 ±2

Cu2+

Borderline

109 < -./ < 1010

98±2

97±3

Pb2+

Borderline

Labile

92±1

90±3

Ni2+

Borderline

104 < -./ < 105

0

1±1

Zn2+

Borderline

107 < -./ < 108

5±1

3±1

Cr3+

Hard

Inert

40±3

44±2

Na+

Hard

108 < -./ < 109

---

---

Al3+

Hard

-./ ~ 100

Aggregation of sorbent

---

particles

0 1+ 2 ) is water exchange rate constants55, 56. Optimization of Extraction Parameters. Face-centered central composite design (FCCCD), by three times replicate center point, was applied for optimization of the sorption step and estimation of the interaction of effective parameters including pH of solution (A), sorption time (B) and amount of magnetic sorbent (C). The extraction percentage was considered for the responses of FC-CCD. The levels of factors and design matrix with responses were shown in Table S1 and S2. Experiments were distributed randomly to minimize the effects of uncontrolled variables. Distance center point from axial point was introduced by α value

45

, and adjusted (α=1) for FC-CCD. After analysis of variance

(ANOVA) (Table S3), response was modeled by the following quadratic polynomial equation: Log10(Extraction % + 5.00) = 1.96 + 0.46A + 0.028B + 0.029C - 0.019AB – 0.018AC + 0.012BC – 0.42A2 – 0.020B2 18 ACS Paragon Plus Environment

(3)

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Where A, B and C are coded values of independent variables. The coefficients with one factor demonstrate effect of the specific factor, while the coefficients with two factors and those with quadratic phrases display the interaction between the two factors and second-order effect, respectively. The positive marks in front of the terms show synergistic effect, while negative marks display opposite effect45. It was found the quadratic model is the best, due to the low standard deviation and high R2 statistics. The values of standard deviation and R2 for the extraction percent of Pb2+ were 0.015 and 0.99, respectively. Coefficient of equation (eq. 3) clearly indicated that the pH of solution has the highest significantly positive effect in the adsorption step. The decrease in extraction percent at low pH values (Figure S9a,b and Figure S9c,d) can be attributed to the protonation of active sites of the magnetic sorbent and reducing the negative charges in the structure of sorbent. On the other hand, the low extraction percentages at high pH values can be due to the increasing in aggregation rate of Fe3O4/MIL-96(Al) particles versus sorption rate of Pb2+ at these pH values. Also, interaction plots (Figure S9b,d) indicated the highest antagonism interaction of pH by other variables in very low and high pH values. It can be attributed to decreasing in accessibility of the active sites of Fe3O4/MIL-96(Al) at very low and high pH values. Figure S9g and h represent the positive effects of increasing in active sites and their availability on the extraction efficiency by increasing the amount of sorbent and sorption time. Also, Figure S9h presents the synergic effect of the sorption time and amount on the extraction efficiency, due to enhancement in availability of active sites of sorbent for interaction with Pb2+ ions. Low slope of the curve, in plot of extraction % vs. sorption times, indicates rapid immigration of Pb2+ ions toward the sorbent through the electrostatic force between Pb2+ ions and the Fe3O4/MIL-96(Al) surface. In addition, the low difference of extraction percentages in low and high levels of sorbent amount at the constant extraction time and pH, could be the reason for the high porosity and capacity of anionic structure of Fe3O4/MIL-96(Al) for the extraction 19 ACS Paragon Plus Environment


of Pb2+ ions (Figure S9h). Conforming to the acquired results from the optimization study, the following experimental conditions were chosen: pH, of 6.92; amount of magnetic sorbent of 32.4 mg and extraction time of 25 min. Sorption capacity study. One of the important characteristics of the sorbents in SPE is the sorption capacity. The sorption capacity parameter indicated potential ability of the adsorbent for sorption of the target analyte(s). In this regard, 32.4 mg active adsorbent was added to the solutions (100 mL) with various concentrations of Pb2+ (0 –100 mg L-1). After adjusting the pH at 6.92 and stirring the solution for 25 min, the sorbent was separated by a magnet and the concentration of Pb2+ in the supernatant was determined by FAAS. The equilibrium concentration of Pb2+ ions, Qe (mg g-1) adsorbed per gram of sorbent, was calculated according to the following equation:

3

45 ' 4 6 4

Which C0 and Ce (mg L-1) are the primary and equilibrium concentrations of Pb2+ in solution, M (g) is the mass of the adsorbent and V (L) is the volume of solution. The results, presented in Figure 6, indicated that the amount of Qe increased by increasing the initial concentration of Pb2+ up to 100 mg L-1. The equilibrium sorption capacity (Qe = 301.5 mg g-1) indicated that Fe3O4/MIL-96(Al) has a very good capacity for sorption of Pb2+ ions. Also, the maximum amount of adsorption capacity was not achieved in the study range. This high equilibrium sorption capacity can be attributed to high porosity of the magnetic sorbent. Two strategies can be concluded by comparison of the plots of Qe and Qe/Ce vs. Ce in Figure S10. It is indicated that when Ce < 0.43 µg mL-1, the ratio of Qe/Ce was rapidly increased that shows the ion exchange sorption mechanism. But, when Ce > 0.43 µg mL-1, the ratio of Qe/Ce was gradually constant, due to the precipitation of Pb2+. Thus, the isotherm models were


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studied at low concentrations of Pb2+. Experimental data were analyzed using Freundlich, Temkin, and Dubinin–Radushkevich isotherm models. Linearized equations are expressed as below: 9

Freundlich model

&83 : &84 + &8

Cation Exchange Superparamagnetic Al-Based Metal Organic

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