Synthesis, Characterization, and Application of Magnetized Lanthanum (III)-Based Metal-Organic Framework for the Organic Dye Removal from Water

A hybrid composite based on metal-organic framework (MOF) was chemically fabricated by embedding the magnetic Fe 3 O 4 nanoparticles within amino-functionalized porous La-MOF (MOF/NH 2 ) to produce a highly e ﬃ cient and reusable composite of MOF/NH 2 /Fe 3 O 4 . Di ﬀ erent proper techniques were used for the characterization of surface morphology and chemical arrangement of the prepared MOF/NH 2 /Fe 3 O 4 composite. The characterization results using various techniques including Fourier transform infrared spectroscopy (FT-IR), X-ray di ﬀ raction (XRD), scanning electron microscope (SEM), Brunauer, Emmett, and Teller analysis (BET), and vibrating sample magnetometer (VSM) approved the successful fabrication of MOF with amino arms on its surface besides the well magnetization using magnetic nanoparticles. The MOF/NH 2 /Fe 3 O 4 composite showed enhanced adsorption capacity (618mg/g) toward methyl orange (MO) anionic dye which is higher than many commercial reported adsorbents due to the presence of many types of adsorption sites (NH 2 groups and lanthanum sites), large surface area of MOF, and the synergetic e ﬀ ect of magnetic nanoparticles. Moreover, the MOF/NH 2 /Fe 3 O 4 composite showed selective adsorption of MO dye from dye mixtures owing to the electrostatic attraction. Also, the MOF/NH 2 /Fe 3 O 4 composite retained over 90% of its e ﬃ ciency for the dye removal even after six successive cycles. So, the present study provided a practical strategy for the design of functional MOF hybrid composites. Furthermore, due to the adaptability of its architectural form, it is a potential adsorbent material for industrial wastewater treatment uses.


Introduction
Water pollution due to the existence of organic dyes and toxic heavy metals is a serious problem that faces the world [1]. This problem resulted from the release of industrial wastes directly to the running water and sewage [2]. The real danger arises from the toxic effect of these dyes and metals on the human health and living organisms [3]. Among all pollutants, organic dyes represent the most dangerous class on the environment [4]. These dyes appear in water due to the discharge of industrial wastewater including paper industries, leather industries, and textile industries. Dyes' danger arises from its stable structure and the difficulty to biodegrade their aromatic complex structure [5]. This makes the organic dyes to be carcinogenic, mutagenic, and very toxic [6]. Subsequently, their removal from wastewater before discharge is mandatory to protect human health and other microorganisms. Great efforts have been done by scientists to achieve proper methods for dye removal from wastewater. Different methods have been accepted for the removal of organic dyes from water including ion exchange [7], membranes [8,9], adsorption [10][11][12][13][14][15][16][17][18][19], precipitation [20], reverse osmosis [21], coagulation-flocculation [22], ozonation [23,24], electrochemical oxidation [25], and biological treatment [26][27][28]. Among all methods, adsorption is the widely used method for the removal of dyes due to many factors including low cost of processing, high efficiency, fast and easy separation, no secondary pollution, and high recyclability [29,30]. The commonly used adsorbents such as natural fibers, zeolites, and activated carbon have limited applications for the removal of organic dyes due to the poor selectivity and low adsorption capacity. So, the fabrication of new adsorbents for dye removal has become an urgent necessity. In this context, new fabricated materials such as graphene [31], carbon nanotubes [32,33], quantum dots [34], nanocelluose [35], and metal-organic frameworks (MOFs) [36][37][38] have been investigated as adsorbents for water treatment. Among all adsorbents, MOFs have attracted the attention recently for the water treatment applications. MOFs are classes of fabricated inorganicorganic hybrid porous crystalline materials with magnificent properties such as high thermal stability, large surface area, large amounts of unsaturated metal sites, and tunable pore structure allowing their wide use in different fields of applications such as water treatment [39], gas separation [40], gas storage [41], sensors [42], luminescent materials [43], and drug delivery and storage [44]. In the recent decade, engineered MOFs are widely investigated for the removal of water pollutants. Because metal ion is frequently used as an active site for a variety of applications, choosing the right metal ion for the framework is always critical. Lanthanidebased metals are excellent choice for the fabrication of MOFs due to their high valence state (normally +3) giving high complexation mode and flexibility giving architectural diversity into the MOFs [45]. All these advantages of MOFs make them appropriate applicant for adsorption of different pollutants from water.
However, MOFs as adsorbents have limitations due to the difficulty of separation from adsorption environment as their separation requires filtration and high-speed centrifugation. This makes the magnetization of the MOFs essential for their application as adsorbents for water treatment to ease their separation using an external magnet. Magnetic nanoparticles are favored because they have various advantages, including a wide surface area, low toxicity, low cost, environmental friendliness, and reusability, in addition to being collected by an external magnet and not requiring centrifugation. The dispersibility and magnetism loss are the essential disadvantage of magnetic nanoparticles that result from its tendency to oxidation. Magnetic nanoparticles are usually coated and treated with different materials to pro-mote stability and prevent aggregation. The hybrid magnetic adsorbents have advantages over magnetic nanoparticles, like providing active sites for effective capturing of pollutants and enhance the selectivity and adsorption ability. Subsequently, the association of MOF blocks with magnetic nanoparticles will enhance the capacity of the hybrid during treatment process with solving the issues of adsorbent separation from the medium after adsorption process. This magnetic separation provides the ability for using the adsorbent several times for water treatment. This makes the magnetic MOFs as adsorbent cost-effective from the economical view. In the present study, all these features are taken into account during the fabrication of the adsorbent to enhance its largescale (industrial) application.
Herein, a nanosized, highly efficient, and recyclable adsorbent based on La-MOFs was synthesized. The La-MOFs were amino-functionalized to produce La-MOFs-NH 2 to allow the surface modification of the La-MOFs using the magnetic Fe 3 O 4 nanoparticles and fabricate a composite, namely, La-MOFs/NH 2 /Fe 3 O 4 . The synthesized nanomaterials were characterized using proper techniques including scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and Brunauer−Emmett −Teller (BET). The synthesized composite was examined for the removal of methyl orange (MO) as a model dye. Moreover, the adsorption kinetics and adsorption isotherm for MO uptake on the surface of MOFs/NH 2 /Fe 3 O 4 composite were studied. Also, the reusability of the MOFs/NH 2 / Fe 3 O 4 composite as adsorbent was determined for MO removal for up to five cycles. Finally, the efficiency of MOFs/NH 2 /Fe 3 O 4 composite as adsorbent for dye removal was evaluated. The originality and novelty of the present study are based on the following criteria: (1) the magnetization of La-based MOF to enhance its adsorption capacity and reusability toward the adsorption of organic dyes; (2) to the best of our knowledge, there is no information about the capacity of the synthesized composite toward MO dye removal; (3) information on the removal of MO dye will be added to the current literature as a result of this study.

Experimental
2.1. Chemicals. Methyl orange (MO) was purchased from Aladdin Co. N,N-Dimethylformamide (DMF), hydrochloric acid, and sodium acetate were purchased from El-Gomhouria Co., Egypt. Ethylene glycol, ferric chloride hexahydrate, lanthanum nitrate hexahydrate, and 2-aminoterephthalic acid were purchased from Sigma-Aldrich. All chemicals were used as received without any further purification. Deionized water was used for the preparation of all solutions.
2.2. Synthesis of Magnetic Nanoparticles. The magnetic Fe 3 O 4 nanoparticles were synthesized using solvothermal method as described in the literature [46]. Typically, a colloidal solution was prepared at room temperature under strong stirring by dissolving 8.2 g of sodium acetate and 2.7 g of ferric chloride hexahydrate in 60.0 mL of ethylene glycol. The stirring process was continued for half-hour then poured In each experiment, a certain amount of adsorbent is shaken with 80 mL of dye solution of a certain concentration at 200 rpm. After that, the adsorbent was collected using an external magnet and the solution was examined for the presence of MO dye using UV-Visible spectrophotometer at λ =465 nm. During the study of pH effect, the pH values were adjusted using 0.1 M of KOH and HCl solutions. The adsorption isotherm study was performed using initial concentration ranging from 50 ppm to 700 ppm. The adsorption kinetics was performed using initial concentration of 400 ppm at different contact times. The adsorption capacities were calculated according to Eq. (1).
where q e , C o , C e , m, and V denote the adsorption capacity at equilibrium, the initial dye concentration, the dye concentration at equilibrium, the mass of adsorbent, and the volume of solution, respectively. The removal efficiency (%) was calculated according to Eq.(2).

Removal efficiency
Moreover, the reusability of MOFs/NH 2 /Fe 3 O 4 composite as adsorbent was studied for up to five successive cycles. During the adsorption-desorption, the adsorbent was used to adsorb the dye followed by desorption of the dye from  3 Adsorption Science & Technology the surface of the adsorbent using methanol. Then, the adsorbent was used for another cycle. After each adsorption experiment, the adsorbent was collected using an external magnet. Finally, depending on the removal efficiencies, the reusability of the adsorbent was evaluated.

Results and Discussion
3.1. The Characterization of Materials. The characterization of synthesized materials was achieved using familiar techniques. For the determination of the present functional groups, FT-IR spectra for the synthesized MOF/NH 2 / Fe 3 O 4 , MOF/NH 2 , and magnetic Fe 3 O 4 nanoparticles were performed in the range of 4000 to 400 cm −1 as shown in Figure 1(a). According to Figure 1(a), the FT-IR spectrum of magnetic nanoparticles showed the appearance of a broad strong band of Fe-O at 584 cm -1 [48]. This characteristic band of magnetic nanoparticles was also observed in the spectrum of MOF/NH 2 /Fe 3 O 4 which indicated the successful embedding of magnetic nanoparticles within the synthesized composite. Moreover, the MOF/NH 2 /Fe 3 O 4 and MOF/ NH 2 spectra showed the appearance of absorption bands at 3475 cm -1 , 3354 cm -1 , and 1546 cm -1 which are corresponding to the bending and stretching vibration of the NH 2 group [49].
For the determination of the phase structure of synthesized materials, XRD patterns of MOF/NH 2 /Fe 3 O 4 , MOF/ NH 2 , and magnetic Fe 3 O 4 nanoparticles were performed in the range of 2θ =5 to 80°as shown in Figure 1(b). According to Figure 1(b), the XRD of magnetic nanoparticles indicated the cubic phase of magnetic nanoparticles due to the appearance of planes (440), (511), (422), (400), (311), and (220) at 62.6°, 57.0°, 39.7°, 43.0°, 35.3°, and 30.0°, respectively [50]. The size of magnetic nanoparticles was determined using the Scherrer equation and their size was found to be equal to 47 nm. Additionally, the XRD of MOF/NH 2 showed the appearance of characteristic diffraction peaks at 19.8°, 17.0°, and 10.0° [47]. We can understand from the XRD of MOF/NH 2 /Fe 3 O 4 composite that the appearance of mag-netic nanoparticles peaks beside the peaks of MOF/NH 2 , the successful combination between the magnetic nanoparticles and MOF/NH 2 . Also, the reduction of peak intensity indicated the interaction between the Fe 3 O 4 nanoparticles and MOF/NH 2 . The XRD results confirmed the FT-IR results for the successful fabrication of the target composite.
For the characterization of size and morphology, the SEM analysis was performed for the synthesized Fe 3 O 4 nanoparticles, MOF/NH 2 , and MOF/NH 2 /Fe 3 O 4 composite as shown in Figure 2. To study the thermal stability of the synthesized materials, TGA curves are shown in Figure 3(a). According to Figure 3(a), Fe 3 O 4 nanoparticles showed the highest thermal stability, while MOF/NH 2 showed a similar trend with lower thermal stability than MOF/NH 2 /Fe 3 O 4 which was attributed to the addition of magnetic Fe 3 O 4 nanoparticles. MOF/NH 2 /Fe 3 O 4 composite showed gradual weight loss within two stages. The first stage occurred in the temperature range of 30°C to 400°C with a weight loss of 19% and attributed to the evaporation of adsorbed solvent molecules. The second stage occurred in the temperature range of 400°C to 700°C with a weight loss of 21% and attributed to the decay of organic linkers and decomposition of MOF. The thermal stability results according to

Adsorption Science & Technology
One of the advantages of magnetic adsorbents is the magnetic separation that allows the ease and simple separation from the adsorption medium. Subsequently, the magnetic behavior of the synthesized Fe 3 O 4 nanoparticles and MOF/NH 2 /Fe 3 O 4 composite was studied using VSM as shown in Figure 3(b). According to Figure 3 . This is attributed to the addition of nonmagnetic MOF/ NH 2 to the magnetic Fe 3 O 4 nanoparticles that caused the drop in the saturation magnetization. So, the paramagnetic behavior of the synthesized MOF/NH 2 /Fe 3 O 4 composite allows their simple separation using an external magnetic field and enables their reusability several times for water treatment that in turn reduces the overall cost of the treatment.
Furthermore, N 2 adsorption-desorption isotherm results of the synthesized materials were used for the characterization of geometrical properties as shown in Figure 3(c). These properties include pore volume, average pore diameter, and specific surface area (S BET ). According to Figure 3(c), the MOF/NH 2 /Fe 3 O 4 composite (36.2 m 2 g -1 ) showed a higher S BET than MOF/NH 2 (32.11 m 2 g -1 ) due to the addition of magnetic Fe 3 O 4 nanoparticles. Additionally, the MOF/ NH 2 /Fe 3 O 4 composite and MOF/NH 2 showed average pore diameters of 16.35 nm and 16.21 nm (BJH method), respec-tively, with mesoporous structure. This average pore diameter is appropriate enough for entrapping the dye molecules within their building. So, the addition of magnetic Fe 3 O 4 nanoparticles to MOF/NH 2 provides more spaces and active sites to dye uptake and enhances the adsorption capacity.

Adsorption Science & Technology
Fe 3 O 4 composite (618 mg/g) indicating the enhanced adsorption capacity resulted from the magnetization of MOF crystals. This enhanced capacity is attributed to the highly porous construction of MOF/NH 2 /Fe 3 O 4 besides the large surface area of MOF that speeds the mass transfer for quick dye uptake [51]. Another important factor affecting the adsorption process is the contact time. Subsequently, the adsorption capacity of Fe 3 O 4 nanoparticles and MOF/ NH 2 /Fe 3 O 4 composite for MO adsorption was studied in the range of 0 to 250 min as shown in Figure 4(b). According to Figure 4 Moreover, pH as a significant factor greatly affecting the adsorption process was studied in the range of 4 to 10 as shown in Figure 4 [54,55].
ln q e = 1 n ln C e + ln K F , ð4Þ where K L , q m , K F , and n refer to the Langmuir constant, maximum monolayer adsorption capacity, the Freundlich constant of adsorption capacity, and the Freundlich constant of adsorption intensity, respectively. Langmuir and Freundlich's parameters are presented in Table 1.
The Langmuir model suggests that the adsorption active sites on the surface of adsorbent are energetically identical and therefore capturing the adsorbate molecules as a monolayer over the homogenous surface of adsorbent [56,57], while the Freundlich model suggests that the adsorption active sites on the surface of adsorbent are not the same energetically and therefore capturing the adsorbate molecules as multilayers over the heterogeneous surface of adsorbent [58,59].  resulted from the strong electrostatic attraction between the negative charge on the dye molecule and the positive charge provided by La(III) in the MOF that has major effect in the uptake mechanism.
For more understanding of the adsorption mechanism and speed, the adsorption data were analyzed using kinetic models. So, the experimental adsorption data for the removal of MO dye on the surface of MOF/NH 2 /Fe 3 O 4 composite and Fe 3 O 4 nanoparticles were fitted using the well-known kinetic models, pseudo 1 st order model and pseudo 2 nd order model as shown in Figures 5(c) and 5(d). The pseudo 1 st order and pseudo 2 nd order models can be expressed according to Eq.(5) and Eq.(6), respectively [60,61].
where k 1 and k 2 refer to the pseudo 1 st order and pseudo 2 nd order rate constants with the units (min -1 ) and (g mg -1 min -1 ), respectively. The pseudo 1 st order and pseudo 2 nd order constants are presented in Table 1.
The pseudo 1 st order model suggested that the adsorption of adsorbate ions was achieved on the adsorbent surface via physisorption mechanism and subsequently depending on the number of the vacant active sites [62]. But, the pseudo 2 nd order model suggested that the adsorption of adsorbate ions was achieved on the adsorbent surface via chemisorption mechanism through exchange or sharing the electrons [63]. Like the isotherm study, the regression coefficient (R 2 ) value indicates the most suitable model to describe the experimental data. According to Table 1, the R 2 value is higher for the pseudo 2 nd order than the pseudo 1 st order for the adsorption of MO dye on the surface of MOF/ NH 2 /Fe 3 O 4 composite and Fe 3 O 4 nanoparticles. This indicates that the removals of MO dye on the surface of MOF/ NH 2 /Fe 3 O 4 composite and Fe 3 O 4 nanoparticles were achieved via a chemisorption mechanism. Additionally, the practical q e value was much closer to the calculated q e value of the pseudo 2 nd order. The electrostatic interaction between MOF/NH 2 / Fe 3 O 4 composite and MO dye could arise from the amino group and La(III) of the MOF with the functional groups of MO dye such as sulfonate, azo group, and aromatic ring. So, the functionalization of MOF/NH 2 /Fe 3 O 4 composite with active adsorption sites was achieved powerfully.

Thermodynamic Parameters of Adsorption.
For a better understanding of the behavior of MO dye adsorption on the surface of MOF/NH 2 /Fe 3 O 4 composite, adsorption thermodynamic parameters were calculated. Moreover, the thermodynamic parameters help the determination of whether the adsorption process is spontaneous. The thermodynamic parameters include entropy change (ΔS 0 ), enthalpy change (ΔH 0 ), and Gibs free-energy change (ΔG 0 ) and can be calculated using Eq.(7) and Eq. (8).
where T, R, and K 0 denote the temperature (K), gas constant (8.314 J mol -1 ), and distribution coefficient, respectively. The thermodynamic parameters of MO adsorption on the surface of MOF/NH 2 /Fe 3 O 4 composite were calculated and tabulated in Table 2. The entropy change (ΔS 0 ) and enthalpy change (ΔH 0 ) of adsorption were calculated using Van't Hoff plot and their values were determined from the slope and intercept. According to Table 2, the MO adsorption on MOF/NH 2 / Fe 3 O 4 surface is a spontaneous process as the ΔG 0 values are negative [64,65]. Moreover, the ΔG 0 values are in the range of 0 to -20.0 kJ mol -1 that indicate the chemical adsorption of MO on the adsorbent surface. The positive value of entropy change (ΔS 0 ) is an indication of random motion resulting from the disorder during the adsorption process at the solid-liquid interface, while the positive value of enthalpy    Figure 6. The mixtures of RB/MO and MB/MO were used to perform the selective adsorption study with dye initial concentrations of 10 mg/L for RB and MB dyes and 100 mg/L for MO dye, mixture volume of 50 mL, and an adsorbent dose of 20 mg. The solution was mixed for 30 minutes for adsorption followed by the collection of the adsorbent using an external magnetic field and the solution was examined for the presence of dyes. Then, the selective adsorption efficiency was calculated according to Eq. (9).
where C 1 and C 2 are representing the concentration of the mixture of two dyes after reaching the adsorption time. According to Figure 6, MO dye was adsorbed selectively in the presence of RB and MB dyes on the surface of the MOF/NH 2 /Fe 3 O 4 composite.
Uv-vis spectra of dye mixtures after adsorption showed the reduction of the MO dye band indicating selective adsorption. After adsorption, the selective adsorption efficiency of MO dye was 93% and 90% in the presence of MB and RB dyes, respectively. These results indicate the efficiency of the synthesized composite for the removal of MO from an aqueous solution. The results can be interpreted based on electrostatic attractions where the positively charged composite can adsorb the anionic dyes stronger than the cationic dyes.

Desorption and Reusability Study.
A reusable adsorbent is cost-effective and in agreement with green chemistry

10
Adsorption Science & Technology [77][78][79]. In this context, the reusability of MOF/NH 2 /Fe 3 O 4 composite was examined using methyl alcohol as an eluent for up to six successive cycles. The adsorption-desorption cycles were investigated six times as shown in Figure 7(a). During each cycle, the adsorbent was mixed with dye solution for adsorption. After that, the adsorbent was collected using an external magnet for desorption. The dye was desorbed from the surface of the adsorbent using methyl alcohol as eluent and the adsorbent was dried for usage in the next cycle. After each cycle, the solution was examined for the presence of dye, and the removal efficiency was calculated. According to Figure 7(a), the 1 st cycle has the highest efficiency due to the presence of a large number of fresh adsorption sites. The next cycles showed a little drop in the removal efficiency due to the damage to nonrenewable adsorption sites. However, the last cycle showed that MOF/NH 2 /Fe 3 O 4 composite retained more than 90% of its efficiency. As can be shown in Figure 7(b), the composite's XRD after the sixth cycle of MO dye adsorptiondesorption is identical to that obtained before dye adsorption, emphasizing the composite's stability during the reusability study. According to the reusability results, MOF/NH 2 /Fe 3 O 4 composite is considered a good regenerable adsorbent. The MOF/NH 2 /Fe 3 O 4 composite had advantages such as reusability at several stages without any change in the magnetic property, easy magnetic separation from aqueous solutions by the magnet, and fast pollutant adsorption.

Conclusion
A reusable, highly efficient, and low-cost dye adsorbent, amino-functionalized MOF embedded with magnetic nanoparticles (MOF/NH 2 /Fe 3 O 4 ) was well fabricated and the proper techniques were used for characterization. The synthetic approach involved the functionalization of porous MOF with amino groups to act as arms for dye capturing followed by the embedding of magnetic nanoparticles within MOF pores to increase its adsorption efficiency besides the ease of collection with an external magnet. The lanthanum sites and the large surface area of the synthesized adsorbent could be responsible for the effective adsorption of MO dye from an aqueous solution with q m of 618 mg/g which is higher than many reported adsorbents. The kinetics and isotherm studies indicated that the MO uptake on MOF/NH 2 / Fe 3 O 4 composite surface was achieved as a monolayer via a chemisorption mechanism in which the lanthanum ion played a key role besides the amino group arms. Additionally, MOF/NH 2 /Fe 3 O 4 composite showed selective adsorption toward MO dye from cationic/anionic dye mixtures which is related to the electrostatic attraction between anionic MO dye and positively charged surface of the composite. Also, the reusability study of MOF/NH 2 /Fe 3 O 4 composite toward MO adsorption was examined up to six successive cycles with excellent efficiency. As a result, MOF/NH 2 /Fe 3 O 4 composite showed excellent ability for effective applications in dye removal from aqueous solutions.

Data Availability
The research data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare that they have no conflicts of interest.