Synthesis of magnetic metal-organic framework (MOF) for efficient removal of organic dyes from water

A novel, simple and efficient strategy for fabricating a magnetic metal-organic framework (MOF) as sorbent to remove organic compounds from simulated water samples is presented and tested for removal of methylene blue (MB) as an example. The novel adsorbents combine advantages of MOFs and magnetic nanoparticles and possess large capacity, low cost, rapid removal and easy separation of the solid phase, which makes it an excellent sorbent for treatment of wastewaters. The resulting magnetic MOFs composites (also known as MFCs) have large surface areas (79.52 m2 g−1), excellent magnetic response (14.89 emu g−1), and large mesopore volume (0.09 cm3 g−1), as well as good chemical inertness and mechanical stability. Adsorption was not drastically affected by pH, suggesting π–π stacking interaction and/or hydrophobic interactions between MB and MFCs. Kinetic parameters followed pseudo-second-order kinetics and adsorption was described by the Freundlich isotherm. Adsorption capacity was 84 mg MB g−1 at an initial MB concentration of 30 mg L−1, which increased to 245 mg g−1 when the initial MB concentration was 300 mg L−1. This capacity was much greater than most other adsorbents reported in the literature. In addition, MFC adsorbents possess excellent reusability, being effective after at least five consecutive cycles.

Recently, due to their new physicochemical properties compared with their single component analogues, hetero-structured nanocomposites have received attention from engineers 8 . Among various nanomaterials, metal-organic frameworks (MOFs), which are porous crystalline materials made of metal ions coordinated to organic ligands, have attracted significant attention due to their larger specific surface areas, easy separation, higher porosity, diversity of structures and functions [9][10][11] . MOFs built from various organic binding ligands and metal ions have allowed more systematic engineering of chemical and physical properties that make the useful for various applications in different fields including domains of gas adsorption and separation, sensors, drug delivery, catalysis or others 12 . MIL-101(Cr) exhibited greater sorption of benzene than activated carbon due to the larger pore diameter 13 . Hybrid zirconium-based MOF modified with cerium exhibited greater adsorption of NO 2 under both moist and dry conditions 14 . When crystalline Cu 3 (BTC) 2 was used as an electro-responsive, electro-rheological material dispersed in insulating oil it formed chain-like structures with excellent, controllable rheological properties.
Magnetic Fe 3 O 4 can be easily separated from reaction liquids by use of an external magnetic field. Combinations of MOFs and magnetic nanoparticles of build magnetic MOF composites (also known as MFCs) have obvious advantages in adsorption and separation. Generally, preparation and application of MOFs requires centrifugation, which is laborious and inconvenient, their applications have been limited. Especially, controlling growth of MOF crystals on magnetic nanoparticles remains a challenge. There is limited literature that has reported syntheses of MFCs [15][16][17] . However, the procedure was time consuming because the reaction required two separate synthesis solutions for at least 25 repetitions each lasting 45 min 16,18 . Therefore, it was determined to be necessary to develop a more efficient method to prepare magnetic MOF composites.
Herein, we present a novel, simple and efficient scheme to fabricate magnetic MOF by simply controlling the rate of dropping of the organic ligand (Fig. 1). The magnetic MOF composite, Fe 3 O 4 /Cu 3 (BTC) 2 , was obtained by incorporation of Fe 3 O 4 and Cu 3 (BTC) 2 . BTC is the acronym for benzene-1,3,5-tricarboxylate and the complex, Cu 3 (BTC) 2 , has been referred to in the literature as "HKUST-1". MB was chosen as a typical adsorbate to be removed from water by use of and used Fe 3 O 4 /Cu 3 (BTC) 2 as asorbent. Effects of pH, contact time, temperature and dosage on adsorption capacity were also investigated in detail. The Fe 3 O 4 /Cu 3 (BTC) 2 exhibited both magnetic characteristics and high porosity, making it an excellent sorbent for treatment of wastewaters.

Results and Discussion
The structure and morphology of the Fe 3 O 4 /Cu 3 (BTC) 2 was characterized and is described here. TEM images showed that Fe 3 O 4 NPs were mono-dispersed, spherical with an approximate diameter of 200 nm and Fe 3 O 4 (Fig. 2a) was encapsulated by a shell of Cu 3 (BTC) 2 (Fig. 2b) and these two components of Fe 3 O 4 /Cu 3 (BTC) 2 could be clearly identified. The network structure of Cu 3 (BTC) 2 is an octahedron network constructed from dimer Cu paddle wheels linked by BTC 19 . The Cu 2+ ions are connected through a weak bond and the residual axial coordination site is filled by a weakly bound water molecule. The BTC ligand combined these primary building blocks into a 3D octahedron network with an open pore system 20 .In this study, the functional MAA-Fe 3 O 4 combined with free state Cu 2+ ions first and the rate of nucleation by Cu 3 (BTC) 2 could be controlled by the speed at which the organic ligand was added.
Peaks in XRD diffraction analysis of Fe 3 O 4 /Cu 3 (BTC) 2 could be related to crystalline Fe 3 O 4 (JCPDS file 19-0629) and Cu 3 (BTC) 2 21,22 , respectively, and no peaks of impurities were detected, which indicates successful synthesis of Fe 3 O 4 /Cu 3 (BTC) 2 (Fig. 2c). It was also confirmed that the composite is a real Fe 3 O 4 /Cu 3 (BTC) 2 composite rather than a physical mixture of two separate phases of Fe 3 O 4 and Cu 3 (BTC) 2 .
In FT-IR spectra of Fe 3 O 4 , peaks observed at 585 cm −1 are due to the Fe-O vibration (Fig. 2d) 23 . The adsorption peak at 1400 cm −1 is related to the vibration of COO-groups from citrate on the surface  at 3434 cm −1 was due to surface-sorbed water and hydroxyl groups 22 . The band at 1570 cm −1 was due to the C = C stretching vibrations of the aromatic ring of trimesic acid 26 . Intensities of peaks at 3375 and 3251 cm −1 are attributed to symmetric and asymmetric stretching of primary amines, respectively 19 .
All the materials exhibited super-magnetic characteristics ( Fig. 2e). At 300 K, the magnetization saturation value of Fe 3 O 4 was 59 emu g −1 . Saturation magnetization of the microspheres decreased with the addition of Cu 3 (BTC) 2 MOF, which can be explained by the increasing thickness of the nonmagnetic component. The Fe 3 O 4 /Cu 3 (BTC) 2 exhibited super-paramagnetic properties at room temperature, indicating that, in the absence of an external magnetic field, they can distribute evenly in aqueous solution. The maximal saturation magnetization of Fe 3 O 4 /Cu 3 (BTC) 2 was 14.89 emu g −1 , which is sufficient for them to be isolated rapidly from large volumes of water samples by a strong Nd-Fe-B magnet.
The thermogram curve shows three different regions ( Fig. 2f): (1) The first mass loss (14.5%) region between 30 and 90 °C indicating the loss of moisture; (2)The region between 90 and 304 °C with a slow loss of mass of 10% is related to loss of water from the MOF together with the oxidation of Cu 2+ and FeO; (3) The third region starts at 304 °C, at which temperature the structure of Cu 3 (BTC) 2 collapses and the organic linker is buried. This loss of mass is completed by 313 °C and then stable up to 800 °C. The stable residue was 43.3% of the original mass. The percent remaining after 310 °C could be regard as absolutely CuO and Fe 2 O 3 .
XPS was employed to investigate the elemental composition of the surface of prepared composites (Fig. 3). In the 2p core level photoelectron spectra for Cu in Fe 3 O 4 /Cu 3 (BTC) 2 ( Fig. 3A), the peaks observed at 934.14 and 954.27 eV were related to Cu2p 3/2 and Cu2p 1/2 electrons, respectively. Similar satellite peaks were also observed in CuO, which indicates that Cu in the MOF is in the divalent form 27 , which is consistent with results of the FT-IR and XRD analyses. The binding energy of O1s core level was observed at 531.1 eV and corresponds to the characteristics of O 2+ ions in the crystalline network (Fig. 3B). These results are in good agreement with results reported previously 28 .
The porous properties and pore structure of particles of Fe 3 O 4 /Cu 3 (BTC) 2 were investigated by measuring nitrogen adsorption isotherms (Fig. 4). The Brunauer-Emmett-Teller (BET) surface area and total pore volume of Fe 3 O 4 /Cu 3 (BTC) 2 were determined to be 79.52 m 2 g −1 and 0.09 cm 3 g −1 , respectively. The average pore size calculated from desorption in the N 2 isotherm by Barrett-Joyner-Halenda (BJH) method was 4.4 nm. The properties of large specific surface area and high porosity could provide multiple accessible channels for MB immigrating.

Mechanism of adsorption of MB on Fe 3 O 4 /Cu 3 (BTC) 2 . Efficiencies of removal of MB were directly
proportional to pH in the range of 2-11 ( Fig. 5a). At greater pH, the surface of the adsorbent is negatively charged, which favors electrostatic interaction of cationic species of dye with the negatively charged surface. The electrostatic attraction force of MB with Fe 3 O 4 /Cu 3 (BTC) 2 is likely to be greater at greater values of pH. Also, adsorption was not drastically affected by pH, suggesting the π -π stacking interaction and/or hydrophobic interactions between MB and Fe 3 O 4 /Cu 3 (BTC) 2 . These results are consistent with reports in the literature 7,29,30 .
Kinetics of adsorption of MB onto Fe 3 O 4 /Cu 3 (BTC) 2 was analyzed using both the pseudo-first-order and pseudo-second order kinetic models. The pseudo-first-order kinetic model (Eq. 1) Where: q e (mg g −1 ) and q t (mg g −1 ) are the amounts of MB adsorbed at equilibrium and any time t (h), respectively, and k 1 (h −1 ) is the adsorption rate constant.
Where: k 2 (g mg −1 •h −1 ) is the pseudo-second-order rate constant. Adsorption of MB was fast in the first 3 h, after which it increased slowly and reached a constant value after20 h (Fig. 5b). A similar trend was observed for adsorption of MB onto porous carbon nanospheres 31 and tungstate oxide nanourchins 32 . The coefficient of determination (R) value of pseudo-second-order kinetic model was approximately 0.99 and the calculated q e,cal (88 mg g −1 ) is very close to the experimental q e,exp (84 mg g −1 ), demonstrating that the kinetics data fits well with the pseudo-second-order kinetic model. A similar phenomenon has been reported for adsorption of MB on wheat shells and activated carbon 33,34 .
The effect of temperature on adsorption of MB was studied by measuring adsorption at 303, 313 and 323K, respectively. Capacity for adsorption of MB to Fe 3 O 4 /Cu 3 (BTC) 2 increased slightly with increasing temperature (Fig. 5c), which indicated that adsorption is controlled by an endothermic process.
Langmuir and Freundlich isotherm models were used to describe the relationship between adsorption of MB onto Fe 3 O 4 /Cu 3 (BTC) 2 and its equilibrium concentration in water. The Langmuir model assumes that adsorption is monomolecular and occurs on a homogeneous surface with all the adsorption sites possessing identical affinities for the adsorbate, while the Freundlich isotherm model is often applicable to a heterogeneous adsorption surfaces with multilayer adsorption (Eq. 3 and 4 and Table 1).
Where: q e (mg g −1 ) is the amount of adsorbate adsorbed at equilibrium, c e (mg L −1 ) is the equilibrium MB concentration, Q 0 (mg g −1 ) and b (L mg −1 ) is the Langmuir constant related to the maximum adsorption capacity and the free energy of adsorption, respectively.

. Nitrogen adsorption-desorption isotherm of Fe 3 O 4 /Cu 3 (BTC) 2 (inset is the pore size distribution of Fe 3 O 4 /Cu 3 (BTC) 2 ).
which is consistent with previously reported results 31,32 . Because the experimental data does not fit the Langmuir model well, the adsorption of MB by Fe 3 O 4 /Cu 3 (BTC) 2 was compared to that of other adsorbents that have been reported in the literature. Capacities for adsorption, were calculated at certain initial concentrations instead of the monolayer adsorption value calculated by the Langmuir equation. When the initial concentration of MB was 30 or 300 mg L −1 , capacity for adsorption of MB by Fe 3 O 4 /Cu 3 (BTC) 2 was 25 and 244 mg g −1 , respectively. These values were much greater than most of those obtained on  other porous adsorbent materials that have been reported in the literature ( Table 2). For activated carbon, the largest surface area is made up of micro pores with a radius of less than 1 nm, which are easily clogged in aqueous solution, which makes adsorptive sites unavailable to adsorbants. The mean size of pores on Fe 3 O 4 /Cu 3 (BTC) 2 is 4.4 nm (meso pores). Therefore, MB can migrate from the solution through the pore channels to reach most of the potential adsorptive sites. This is one possible reason for the much greater capacity for adsorption of MB observed for Fe 3   show that adsorption is endothermic and random at the solid-solution interface. If the value of Δ H 0 is greater than 40 kJ moL −1 , the process of adsorption is thought to proceed via chemisorption; while if the Δ H 0 value is less than 40 kJ moL −1 , adsorption is physic-sorption innature 35 . The value of Δ H 0 observed in this study was 17 kJ moL −1 , which indicates that MB adsorption on Fe 3 O 4 /Cu 3 (BTC) 2 is likely due to physic-sorption.

Reusability of adsorbent.
To assess reusability of the absorbant, which contributes to reduce the cost of practical application process, methanol, ethanol and acetonitrile were used in desorption and regeneration experiment and acetonitrile shows the best desorption efficiency. Capacity for adsorption of Materials Q e (mg g −1 ) Q e (mg g −1 )   (Fig. 5d).

Conclusions
Nanoparticles of Fe 3 O 4 /Cu 3 (BTC) 2 were prepared by a novel convenient method and used to remove MB from aqueous solution. This material exhibits excellent adsorption performance for MB attributing to the large, specific surface area and meso-porous channels. In addition, the Fe 3 O 4 /Cu 3 (BTC) 2 adsorbent possessed excellent reusability, being effective after at least five consecutive cycles, indicating its potential for the purification of organic dyes water. Characterization. Several methods were used to characterize the particles. Morphology and particle size of the samples were determined by use of a transmission electron microscope (TEM; Hitachi H-7500, Japan) at 80 kV accelerated voltage. The crystal phase was investigated by use of a PAN analytical X'pert Pro diffractometer (XRD, Almelo, Netherlands) by using Cu Kα radiation ranging from 10° to 80° with a scan step of 0.02°. Thermo-gravimetric analysis (TGA) was carried out using a Q5000 TGA analyzer (TA Instruments, Japan) under a flow of air with a temperature ramp of 10 °C min −1 . FTIR spectra were performed on a NEXUS 670 Infrared Fourier Transform Spectrometer (Nicolet Thermo, Waltham, MA). The specific surface area, pore size and volume were measured by BET methods (ASAP2000 V3.01A; Micromeritics, Norcross, GA). X-Ray photoelectron spectroscopy (XPS) was acquired with an ESCA-Lab-200i-XL spectrometer (Thermo Scientific, Waltham, MA) with monochromatic Al Kα radiation (1486.6 eV).The magnetic properties were analyzed on a LDJ9600 vibrating sample magnetometer (VSM, LDJ9600, Troy, MI).

Batch Adsorption Experiments. Adsorption of MB on Fe 3 O 4 /Cu 3 (BTC) 2 was performed in batch
experiments. The optimal pH for adsorption of MB was determined by a series of experiments where the initial concentration of MB was maintained constant (100 mg L −1 ) at different pH values (pH 2-11).
To evaluate the thermodynamic properties, adsorption isotherms were obtained by varying concentrations of MB from 10 to 300 mg L −1 at 303, 313 and 323 K, respectively. Solution pH was adjusted with 0.5 M NaOH or HCl. Kinetic studies were performed with an initial concentration of MB of 100 mg L −1 . All batch tests were executed in 15 mL of polyethylene bottles by taking 10 mg, dm (dry mass) of where c 0 and c e are the initial and equilibrium concentrations of MB (mgL −1 ), respectively, mis the mass of dry adsorbent (g)and V is volume of the solution (L).
Regeneration of the exhausted Fe 3 O 4 /Cu 3 (BTC) 2 saturated with MB was examined by using solvent desorption techniques. Methanol, ethanol and acetonitrile were as the eluent to regenerate Fe 3 O 4 /Cu 3 (BTC) 2 . The eluent solution was added to the used Fe 3 O 4 /Cu 3 (BTC) 2 and the mixture was ultrasonicated for 30 min.
Concentrations of MB in the supernatant solution were measured before and after adsorption was determined using a UV-vis spectrophotometer at 660 nm. The supernatant from the Fe 3 O 4 /Cu 3 (BTC) 2 did not exhibit any absorbance at this wavelength and the calibration curve was reproducible and linear over the concentration range used in this work.