# Magnetic Nanoparticles@Metal-Organic Framework Composites as Sustainable Environment Adsorbents

Authors: Gege Zhao; Nianqiao Qin; An Pan; Xiaoyan Wu; Chuanyi Peng; Fei Ke; Mudassar Iqbal; Karna Ramachandraiah; Jing Zhu
Journal: Journal of Nanomaterials
 (2019)

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## Abstract

Metal-organic frameworks (MOFs) are an intriguing class of porous inorganic-organic hybrid networks synthesized from metal ions with multidentate organic ligands. MOFs have uniform and tunable cavities and tailorable chemistry, making them promising materials for hazardous component removal from the environment. Controllable integration of magnetic nanoparticles (NPs) and MOFs is leading to the creation of many novel multifunctional MOF-based composites, which exhibit advanced performance that is superior to both of the individual units. This review summarizes the recent significant advances in the development of MOF-based magnetic heterostructure materials for the removal of hazardous contaminants from the environment. The successful methods reported till date for the magnetic MOF synthesis are also provided. In the final section, we provide our views on the future development of the magnetic MOF heterostructure materials for the pollution management.


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## Body

## 1. Introduction

In recent years, environmental pollution is increasing and posing serious threat to the ecosystem and human health [1]. Inorganic pollutants such as heavy metal ions in water have drawn much attention due to their long half-life and nonbiodegradability. For these reasons, numerous technologies have been developed for water purification, such as ion exchange [2], biological treatment [3], chemical precipitation [4], and reverse osmosis [5]. Although these technologies are effective, their practical applications are usually hampered by the high cost and poor selectivity. On the other hand, dyes are considered as serious organic pollutants, which are produced by various industrial wastewater such as textile, leather, printing, plastics, cosmetics, pharmaceutical, and food wastewater [6, 7]. The presence of dyes not only gives rise to high visibility but also can reduce the solubility of gas in water and even more has a strong toxicity and carcinogenicity to the human body. Therefore, the removal of inorganic and organic pollutants from wastewater is very necessary for water safety and human health protection. Compared with these methods, adsorption is considered as an ideal pollution treatment method due to its low cost, strong universality, and simple operation [8]. Traditional adsorbents, such as zeolite, metal oxide, and activated carbon, cannot show satisfactory adsorption capacity or require long contact time [9, 10]. Hence, the development of a novel high efficiency adsorbent with large capacity will be an important challenge.Metal-organic frameworks (MOFs) are constructed by ditopic or polytopic organic ligands and transition metal ions or clusters [11]. Owning to their diverse structures, adjustable aperture, large surface area, and coordinated unsaturated metal sites, MOFs have been widely used in social and industrial fields, such as gas storage [12], separation [13], catalysis [14], sensing [15], and drug delivery [16]. In particular, adsorption is one of the most potential applications for MOFs during the past 20 years. Compared with traditional adsorbents, MOFs have huge porosity and tunable pore sizes, endowing highly selective adsorption of hazardous contaminants from the environment [17]. However, these novel MOF-based adsorbents are difficult to be recycled from the mixture solution. To overcome this problem, combining MOFs with magnetic nanoparticles (NPs) has been made due to their high adsorption capacity, easy functionality, and easy isolation with an external magnetic field. The methods of preparing magnetic MOF composites include the hydrothermal method [18] and layer-by-layer assembly method [19]. These composite materials have the magnetic response on the basis of magnetic particles, which facilitates product recovery and lower operational cost in MOF separation. Moreover, the components of MOF-based magnetic composites can be easily engineered [20–22]. Hence, compared to conventional adsorbents used in the environmental pollution treatment, the MOF-based magnetic adsorbents are more suitable for industrial applications. Although magnetic composites at the industrial scale are still facing great challenges, numerous magnetic particles are commercially available, making the magnetic MOFs compatible with commercial applications in the near future [23].In this review, we summarize the recent significant progress in the development of MOF-based magnetic nanocomposites for hazardous contaminant removal from the environment (Figure1). The structures, properties, and the methods for the synthesis of the magnetic nanocomposites are discussed briefly. Particular challenges of MOF-based magnetic NPs for further development toward adsorption applications are critically discussed.Figure 1
Schematic illustration of magnetic NP@MOF heterostructure for hazardous contaminant removal from the environment.


## 2. Design and Synthesis of Magnetic NP@MOF Structure

Magnetic metal-organic framework nanocomposites are new functional materials that have emerged in recent years [24]. They are composed of porous MOFs and magnetic NPs. It not only retains the structure and performance of the MOF material but also increases the magnetic properties of the granular material. It can be separated and recovered from the mix aqueous solution or soil by using a magnet and has the advantages of high selectivity, good dispersion, and multiple reuse [25]. It can be recycled again and is in line with the green concept. The synthesis of magnetic NP@MOFs generally can be seen as the following four methods.


### 2.1. Presynthesized Magnetic NP Template Method

In this method, the presynthesized magnetic NPs are used as seeds to grow MOFs. Firstly, the presynthesized magnetic NPs should be functionalized by capping agents or surfactants. For instance, Li et al. reported MOF-5@SiO2@Fe3O4 core-shell magnetic catalysts, which were prepared through coating the typical MOF-5 on the surface of SiO2@Fe3O4 NPs [26]. The results displayed that the as-synthesized magnetic core-shell nanocomposites can be easily separated from the mixture reaction system by a magnet. Zhao et al. also reported the synthesis of magnetic Fe3O4@MOF and demonstrated as an immobilization vector for enzymes [27]. Fe3O4 NPs were functionalized by the polydopamine (PDA) layer, and then Cu3(btc)2 was synthesized on the surface of Fe3O4@PDA by introducing Fe3O4@PDA into an ethanol solution containing copper acetate and 1,3,5-benzenetricarboxylic acid (Figure 2). The MOF shell has a large surface area to ensure high load carrying capacity. Due to the strong affinity for the enzyme, the Fe3O4@PDA@[Cu3(btc)2]-enzyme composites achieved excellent digestion efficiency, good reusability, durability, and reproducibility.Figure 2
Schematic illustration of Fe3O4@PDA@[Cu3(btc)2] microspheres and the procedure for enzyme immobilization. Reprinted from Ref. [27] with permission of the Royal Society of Chemistry.


### 2.2. Step-by-Step Method

Our group fabricated a series of Fe3O4@MOFs (e.g., Fe3O4@Cu3(btc)2, Fe3O4@MIL-100(Fe), and Au-Fe3O4@MIL-100(Fe)) core-shell nanocomposites with a controllable MOF shell thickness by the versatile step-by-step strategy (Figure 3) [28–30]. Functionalization of magnetic Fe3O4 core with the mercaptoacetic acid (MAA) before the coating process was vitally important during the initial stage of the step-by-step assembly, because no core-shell structures could be obtained using the unfunctionalized magnetic Fe3O4 core [28]. The growth of MOF shell on the MAA-functionalized Fe3O4 core can be initiated by first the metal ions binding to the MAA on the Fe3O4 surface, to which then the organic ligands from the solution bind. The thickness of the MOF shell can be facile controlled by tuning the number of step-by-step assembly cycles. Zhang et al. also prepared the novel porous Fe3O4@MIL-100(Fe) core-shell nanospheres by this method to achieve large enrichment capacity and high size exclusion selectivity for phosphopeptides [31].Figure 3
Schematic representation of the step-by-step assembly strategy (a) and TEM images of Fe3O4@[Cu3(btc)2] core-shell magnetic microspheres after (b) 10, (c) 20, (d) 25, (e) 30, (f) 40, and (g) 50 assembly cycles at 25°C. Reprinted from Ref. [28] with permission of the Royal Society of Chemistry.


### 2.3. Self-Template Method

In the self-template method, the magnetic metal or metal oxide composites will provide metal ions by sacrificing themselves and then coordinate to organic ligand. For example, Fe3O4@SiO2@HKUST-1 magnetic core-shell composite has been obtained by the self-template strategy in which magnetic Fe3O4@SiO2 were first coated with Cu(OH)2 shell as the sacrificial template and then HKUST-1 grew around the core [32]. Here, Cu(OH)2 shell not only was the sacrificial template but also provides copper ion sources for the formation of HKUST-1 [32]. Compared to other template strategies, this approach shows decisive economy advantage and does not require additional surface modification. Moreover, the Bi-I-functionalized Fe3O4@SiO2@HKUST-1 magnetic composite exhibited excellent adsorption for Hg2+ from water (Figure 4). By using this method, Cai and coworkers also reported the novel magnetic Prussian blue (PB) composite using the self-template method [33]. PB cube was used both as the sacrificial template and as the iron source of Fe3O4 for the formation of PB-Fe3O4 composite.Figure 4
Scheme of the preparation of Bi-I functionalized Fe3O4@SiO2@HKUST-1 composites. Reprinted from Ref. [32] with permission of the Royal Society of Chemistry.


### 2.4. Dry Gel Conversion Method

Tan et al. demonstrated a dry gel conversion (DGC) method to fabricate HKUST-1/Fe3O4 composites for desulfurization and denitrogenation applications [34]. In this method, the solvent is first separated from the mixed Fe3O4 and MOF precursors, and then solvent vapor is generated into the mixture to induce MOF formation around magnetic NPs. With this simple method, HKUST-1/Fe3O4 composites have been successfully constructed without blockage of the MOF pores (Figure 5). Significantly, the obtained magnetic porous adsorbents not only can undergo efficient adsorption of various aromatic sulfur and nitrogen compounds from model fuels but also can be easily separated from mixture by an external magnetic field.Figure 5
SEM and TEM images of (a, b) Fe3O4, (c, d) HKUST-1, and (e, f) the HKUST-1/Fe3O4 composites. Reprinted from Ref. [34] with permission of Elsevier.
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## 2.1. Presynthesized Magnetic NP Template Method

In this method, the presynthesized magnetic NPs are used as seeds to grow MOFs. Firstly, the presynthesized magnetic NPs should be functionalized by capping agents or surfactants. For instance, Li et al. reported MOF-5@SiO2@Fe3O4 core-shell magnetic catalysts, which were prepared through coating the typical MOF-5 on the surface of SiO2@Fe3O4 NPs [26]. The results displayed that the as-synthesized magnetic core-shell nanocomposites can be easily separated from the mixture reaction system by a magnet. Zhao et al. also reported the synthesis of magnetic Fe3O4@MOF and demonstrated as an immobilization vector for enzymes [27]. Fe3O4 NPs were functionalized by the polydopamine (PDA) layer, and then Cu3(btc)2 was synthesized on the surface of Fe3O4@PDA by introducing Fe3O4@PDA into an ethanol solution containing copper acetate and 1,3,5-benzenetricarboxylic acid (Figure 2). The MOF shell has a large surface area to ensure high load carrying capacity. Due to the strong affinity for the enzyme, the Fe3O4@PDA@[Cu3(btc)2]-enzyme composites achieved excellent digestion efficiency, good reusability, durability, and reproducibility.Figure 2
Schematic illustration of Fe3O4@PDA@[Cu3(btc)2] microspheres and the procedure for enzyme immobilization. Reprinted from Ref. [27] with permission of the Royal Society of Chemistry.


## 2.2. Step-by-Step Method

Our group fabricated a series of Fe3O4@MOFs (e.g., Fe3O4@Cu3(btc)2, Fe3O4@MIL-100(Fe), and Au-Fe3O4@MIL-100(Fe)) core-shell nanocomposites with a controllable MOF shell thickness by the versatile step-by-step strategy (Figure 3) [28–30]. Functionalization of magnetic Fe3O4 core with the mercaptoacetic acid (MAA) before the coating process was vitally important during the initial stage of the step-by-step assembly, because no core-shell structures could be obtained using the unfunctionalized magnetic Fe3O4 core [28]. The growth of MOF shell on the MAA-functionalized Fe3O4 core can be initiated by first the metal ions binding to the MAA on the Fe3O4 surface, to which then the organic ligands from the solution bind. The thickness of the MOF shell can be facile controlled by tuning the number of step-by-step assembly cycles. Zhang et al. also prepared the novel porous Fe3O4@MIL-100(Fe) core-shell nanospheres by this method to achieve large enrichment capacity and high size exclusion selectivity for phosphopeptides [31].Figure 3
Schematic representation of the step-by-step assembly strategy (a) and TEM images of Fe3O4@[Cu3(btc)2] core-shell magnetic microspheres after (b) 10, (c) 20, (d) 25, (e) 30, (f) 40, and (g) 50 assembly cycles at 25°C. Reprinted from Ref. [28] with permission of the Royal Society of Chemistry.


## 2.3. Self-Template Method

In the self-template method, the magnetic metal or metal oxide composites will provide metal ions by sacrificing themselves and then coordinate to organic ligand. For example, Fe3O4@SiO2@HKUST-1 magnetic core-shell composite has been obtained by the self-template strategy in which magnetic Fe3O4@SiO2 were first coated with Cu(OH)2 shell as the sacrificial template and then HKUST-1 grew around the core [32]. Here, Cu(OH)2 shell not only was the sacrificial template but also provides copper ion sources for the formation of HKUST-1 [32]. Compared to other template strategies, this approach shows decisive economy advantage and does not require additional surface modification. Moreover, the Bi-I-functionalized Fe3O4@SiO2@HKUST-1 magnetic composite exhibited excellent adsorption for Hg2+ from water (Figure 4). By using this method, Cai and coworkers also reported the novel magnetic Prussian blue (PB) composite using the self-template method [33]. PB cube was used both as the sacrificial template and as the iron source of Fe3O4 for the formation of PB-Fe3O4 composite.Figure 4
Scheme of the preparation of Bi-I functionalized Fe3O4@SiO2@HKUST-1 composites. Reprinted from Ref. [32] with permission of the Royal Society of Chemistry.


## 2.4. Dry Gel Conversion Method

Tan et al. demonstrated a dry gel conversion (DGC) method to fabricate HKUST-1/Fe3O4 composites for desulfurization and denitrogenation applications [34]. In this method, the solvent is first separated from the mixed Fe3O4 and MOF precursors, and then solvent vapor is generated into the mixture to induce MOF formation around magnetic NPs. With this simple method, HKUST-1/Fe3O4 composites have been successfully constructed without blockage of the MOF pores (Figure 5). Significantly, the obtained magnetic porous adsorbents not only can undergo efficient adsorption of various aromatic sulfur and nitrogen compounds from model fuels but also can be easily separated from mixture by an external magnetic field.Figure 5
SEM and TEM images of (a, b) Fe3O4, (c, d) HKUST-1, and (e, f) the HKUST-1/Fe3O4 composites. Reprinted from Ref. [34] with permission of Elsevier.
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## 3. Applications of Magnetic NP@MOF Composites in the Environment

MOF-based magnetic nanostructures have been widely used for many applications due to their outstanding physicochemical performance [24]. In this review, we are particularly interested in applying these magnetic composites as sustainable environment adsorbents. Environmental pollution has become one of the major problems worldwide at present. Due to the decline of water quality, water bodies continue to deteriorate, leading to the suspension of relevant factories and agricultural production. The adverse social impact and economic loss caused threaten the sustainable development of society and the healthy development of mankind. There are a large number of organic, inorganic, and biological pollutants in water and soil, such as herbicides [35] and dyes [36]. The advantages of magnetic NP@MOF composites for the adsorption of hazardous materials from the environment will be discussed in later sections. Further, we will also point out the state-of-the-art progress in magnetic NP@MOF composite adsorption applications categorized by the hazardous compound type.


### 3.1. Adsorption of Organic Contaminants

Magnetic NP@MOF composites are promising porous adsorbents for the adsorption of organic contaminants from the environment due to their magnetic core for easy magnetic separation along with porous MOF shell for highly selective removal of contaminants. For example, Zhou et al. developed a novel magnetic Fe3O4@MIL-100(Fe) NPs for mechanochemical magnetic solid phase extraction (MCMSPE) of organochlorine pesticides from tea leaves [37]. Fe3O4@MIL-100(Fe) magnetic NPs were synthesized by the step-by-step method and further successfully used for the separation of organochlorine pesticides from tea. Moreover, Fe3O4@MIL-100(Fe) magnetic NPs can be reused with no significant changes in the organochlorine pesticide recovery after five adsorption cycles. The results indicated that such magnetic NPs are ideal recyclable adsorbents for removal of organochlorine pesticides from plant samples. In the same time, Fe3O4@SiO2@Zr-MOF magnetic composites were also reported for the effective removal of pharmaceutical compounds from water [38]. The obtained Fe3O4@SiO2@UiO-66-NH2 displayed a high adsorption capacity and rapid separation rate for the adsorption of salicylic acid (SA) and acetylsalicylic acid (ASA) due to the magnetic NPs in combination with porous Zr-MOF (Figure 6). The saturated magnetization value of Fe3O4@SiO2@UiO-66-NH2 was measured to be 25.4 emu g-1. The easy separation, high capacities, and reusability of the magnetic Zr-based MOF make it as superior adsorbents for removal of pharmaceutical contaminants.Figure 6
Scheme of (a) the preparation of Fe3O4@SiO2@Zr-MOF composites and (b) the adsorption of SA and ASA. Reprinted from Ref. [38] with permission of the American Chemical Society.
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(b)Based on a similar process, four other magnetic composites, magG@SiO2@ZIF-8 [39], Fe3O4@SiO2@MOF/TiO2 [40], Fe@SiO2@MOF-5 [41], and Fe3O4@m-SiO2/PSA@Zr-MOF [42], were successfully synthesized based on the magnetic NPs coated with a layer of SiO2. The magnetic NPs@SiO2 cores help adsorption of metal ions and organic linkers for the growth of outer shell of MOF layers. The magG@SiO2@ZIF-8 composites displayed high extraction efficiency and reusability for adsorption of phthalate esters with the linear range of 50-8000 ng mL−1 and up to 92% recoveries [39]. Fe3O4@SiO2@MOF/TiO2 [40] and Fe@SiO2@MOF-5 [41] core-shell nanocomposites were used as efficient adsorbents for MSPE of five triazole fungicides as well as N- and S-containing polycyclic aromatic hydrocarbons from contaminated water. Here, TiO2 immobilized on the surface of Fe3O4@SiO2@MOF could enhance the adsorption properties of magnetic MOF with the detection and quantification limits of 0.19-1.20 ng L−1 and 0.61-3.62 ng L−1, respectively [40]. The Fe@SiO2@MOF-5 exhibited a good adsorption for N- and S-containing polycyclic aromatic hydrocarbons with LODs in the range of 0.025-0.033 μg L−1 [41]. In addition to these, Xu and coworkers developed a novel Fe3O4@m-SiO2/PSA@Zr-MOF magnetic nanocomposite for bifenthrin determination from water [42]. Considering the outstanding performance and limitless of MOFs, it is expected that such MOF-based magnetic core-shell nanocomposites will open a new doorway in the field of adsorption of organic contaminants from the environment.


### 3.2. Adsorption of Dyes from Wastewater

Dye has become an important industrial hazardous contaminant in water. In recent years, with the development of the dye industry, it has a great adverse impact on the environment and human health. Therefore, it is crucial to devise a strategy for the treatment and removal of these dyes from polluted water. Wang et al. reported the use of magnetic Fe3O4/MIL-101(Cr) composite for effective adsorption of two dyes, acid red 1 (AR1) and orange G (OG) [6]. They fabricated Fe3O4/MIL-101(Cr) magnetic composites by a reduction-precipitation method with large surface areas, strong magnetism, and excellent dispersion effect. The adsorption capacities of AR1 and OG with Fe3O4/MIL-101(Cr) were 142.9 and 200.0 mg g-1, respectively [6]. The authors suggested that the adsorption of AR1 and OG was spontaneous, exothermic, and randomness decreased with monolayer adsorption during this process. Very recently, perfect MgFe2O4@MOF [43] and Fe3O4@SiO2@Zn-TDPAT [44] core-shell magnetic materials were reported by the presynthesized magnetic NP template method. The MgFe2O4@MOF magnetic composites were fabricated by a mercaptoacetic acid- (MAA-) functionalized MgFe2O4 NPs as the template method [43]. The obtained MgFe2O4@MOF hybrid nanomaterials displayed excellent removal of Rhodamine B (RB, 219.78 mg g−1) and Rhodamine 6G (Rh6G, 306.75 mg g−1) from wastewater. Moreover, these magnetic hybrid nanomaterials showed good reusability even after 10 times reused. The as-synthesized Fe3O4@SiO2@Zn-TDPAT core-shell magnetic material also displayed a high performance activity in adsorption of polluted dyes [44]. The adsorption efficiencies can reach 100 and 99% for Congo red (CR) and methylene blue (MB) by this magnetic material, respectively. Significantly, such Fe3O4@SiO2@Zn-TDPAT core-shell magnetic material can be stable under different acid-alkaline conditions. The results suggested that MOF-based magnetic core-shell materials are promising adsorbents for dye removal from wastewater.These MOF-based magnetic composites have high capacity toward a certain dye; the application of selective removal of specific dye from a mixture of multiple dye-polluted water needs more development. In 2018, Yang et al. reported a novel Fe3O4-PSS@ZIF-67 magnetic core-shell composite for selective adsorption of methyl orange (MO) from MO and methylene blue (MB) mixed solution (Figure 7) [45]. The results demonstrated that the adsorption capacity of the magnetic composites for MO was measured to be 738 mg g-1 with the separation rate of up to 92%. The selective adsorption mechanism can be attributed to charge-selectivity between the dye molecule and the MOF. The sizes of the MO and MB molecules are 1.54×0.48×0.28nm3 and 1.38×0.64×0.21nm3, while ZIF-67 has pore cage of 1 nm. Therefore, MO and MB molecules can be adsorbed in the pore cage of Fe3O4-PSS@ZIF-67. Furthermore, the negatively charged MO can be adsorbed with the Lewis base of Co2+ centrals because of the electrostatic attraction while the positively charged MB is hard to be adsorbed because of the electrostatic repulsion.Figure 7
(a) Scheme of selective adsorption of MO from the mixed MO/MB solution by Fe3O4-PSS@ZIF-67 magnetic composites; photographs (b) and UV-vis spectra (c) of the mixed MO/MB solution before and after magnetic separation. Reprinted from Ref. [45] with permission of Elsevier.
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### 3.3. Adsorption of Heavy Metal Ions

Recently, MOF-based magnetic composites have been also used as porous adsorbents for the removal of heavy metal ions from the environment. MOF-based magnetic composites are promising adsorbents for the removal of heavy metal ions because of their easy modification and isolation. Karimi et al. reported a chemical bond between the NHSO3H-functionalized Fe3O4 and the HKUST-1 method for the synthesis of magnetic Fe3O4-NHSO3H@HKUST-1 nanocomposites for the adsorption of lead ions (Pb2+) from wastewater [46]. According to this work, the maximum adsorption capacity of Pb2+ with Fe3O4-NHSO3H@HKUST-1 was 384.6 mg g-1, which corresponds to 46.3% of the magnetic adsorbent occupied sites. After the adsorbent is separated by the magnet and washed with 0.1 M of HCl and distilled water, it can be used for another adsorption experiment. As a result, Fe3O4-NHSO3H@HKUST-1 could be reused four times without significant loss of adsorption activity (>90%). Huang et al. reported two amino-modified Zr-based magnetic MOF composites (Fe3O4@SiO2@UiO-66-NH2 and Fe3O4@SiO2@UiO-66-Urea) for the extraction of heavy metal ions [47]. Fe3O4@SiO2@UiO-66-NH2 and Fe3O4@SiO2@UiO-66-Urea were prepared by a simple one-pot strategy with different precursors. The obtained amine-decorated magnetic composites exhibited high adsorption for heavy metal ions compared to pure magnetic composites. In particular, Fe3O4@SiO2@UiO-66-NH2 showed the highest adsorption capacity for Pb2+ (102 mg g-1). The authors concluded that the improvement in the removal of Pb2+ by the amine-decorated magnetic composites compared with Fe3O4@SiO2@UiO-66 is due to the fact that the –NH2 groups on the magnetic composites provide more binding sites for the adsorption of Pb2+ by chelating. Further, hierarchically HPU-13@Fe3O4 (HPU-13 = {[Cu3(L)2]·OH·2CH3CH2OH·10H2O}n; HL = 2-(5-pyridin-4-yl-2H-[1,2,4]triazol-3-yl)pyrimidine) magnetic hybrid composites were synthesized for high removal and excellent reuse of Cr(VI) ions from water [48]. HPU-13@Fe3O4 showed high adsorption capacities for Cr2O72− (398.41 mg g-1) and CrO42− (471.69 mg g-1). The results proved that oxidation of Cu(I) to Cu(II) on the magnetic adsorbents occurred during the adsorption process and partial reduction of Cr(VI) to Cr(III) in the solution at the same time. Finally, the authors revealed that the high adsorption of Cr(VI) under these conditions is due to the synergistic reaction of Cr(VI) reduction and adsorption [48].The radioactive elements are also a major issue to the environment such as U(VI) and Th(IV). These radioactive metal ions can create numerous diseases including liver and lung cancers. In this case, the adsorption of these metal ions from the environment is a challenge and critical issue for the environmental remediation. Alqadami et al. prepared an Al-based magnetic MOF nanocomposite (Fe3O4@AMCA-MIL53(Al)) for the adsorption of U(VI) and Th(IV) from wastewater [49]. The adsorption capacities for U(VI) and Th(IV) were measured to be 227.3 and 285.7 mg g-1, respectively. The adsorption equilibrium time of Fe3O4@AMCA-MIL53(Al) for both radioactive metal ions was demonstrated within 90 min. The results suggested the adsorption mechanism for U(VI) and Th(IV) over Fe3O4@AMCA-MIL53(Al) through the electrostatic interactions between the organic part of the magnetic nanocomposite and the radioactive metal ions and coordinate interactions between the metal ions and nitrogen in the framework. Min et al. reported a novel Fe3O4@ZIF-8 magnetic nanocomposite for selective removal of UO22+ from water, which showed a high adsorption capacity of 523.5 mg U g-1 [50]. Furthermore, the selective separation of the UO22+ from lanthanide results indicated that the obtained Fe3O4@ZIF-8 displayed remarkable selectivity performance for UO22+ in the presence of lanthanides at pH 3.Selectivity is one of the most primary issues of a good adsorbent in the practical application. In order to solve this problem, magnetic adsorbents can be modified by various functional groups for the removal of target metal ions. The functional groups can be easily introduced into the MOF-based magnetic composites by a facile postsynthetic modification (PSM) method. Very recently, we prepared thiol-functionalized Fe3O4@Cu3(btc)2 core-shell magnetic microspheres and investigated their application in selective adsorption of heavy metal ions in the presence of other background ions from water [51]. Fe3O4@Cu3(btc)2 core-shell magnetic microspheres were synthesized by a step-by-step assembly fashion. Further, Cu3(btc)2 shell of the magnetic microspheres was modified by thiol groups of dithioglycol using such PSM method (Figure 8). Significantly, the thiol-functionalized Fe3O4@Cu3(btc)2 showed high selective adsorption for Pb2+ (Kd=1.23×104mLg−1) and Hg2+ (Kd=5.98×104mLg−1) in the presence of background ions of Ni2+, Na+, Mg2+, Ca2+, Zn2+, and Cd2+ [51].Figure 8
Scheme of the preparation of thiol-functionalized Fe3O4@Cu3(btc)2 and its use for selective removal of Pb2+ and Hg2+ from the mixed heavy metal ion solution. Reprinted from Ref. [51] with permission of Elsevier.


## 3.1. Adsorption of Organic Contaminants

Magnetic NP@MOF composites are promising porous adsorbents for the adsorption of organic contaminants from the environment due to their magnetic core for easy magnetic separation along with porous MOF shell for highly selective removal of contaminants. For example, Zhou et al. developed a novel magnetic Fe3O4@MIL-100(Fe) NPs for mechanochemical magnetic solid phase extraction (MCMSPE) of organochlorine pesticides from tea leaves [37]. Fe3O4@MIL-100(Fe) magnetic NPs were synthesized by the step-by-step method and further successfully used for the separation of organochlorine pesticides from tea. Moreover, Fe3O4@MIL-100(Fe) magnetic NPs can be reused with no significant changes in the organochlorine pesticide recovery after five adsorption cycles. The results indicated that such magnetic NPs are ideal recyclable adsorbents for removal of organochlorine pesticides from plant samples. In the same time, Fe3O4@SiO2@Zr-MOF magnetic composites were also reported for the effective removal of pharmaceutical compounds from water [38]. The obtained Fe3O4@SiO2@UiO-66-NH2 displayed a high adsorption capacity and rapid separation rate for the adsorption of salicylic acid (SA) and acetylsalicylic acid (ASA) due to the magnetic NPs in combination with porous Zr-MOF (Figure 6). The saturated magnetization value of Fe3O4@SiO2@UiO-66-NH2 was measured to be 25.4 emu g-1. The easy separation, high capacities, and reusability of the magnetic Zr-based MOF make it as superior adsorbents for removal of pharmaceutical contaminants.Figure 6
Scheme of (a) the preparation of Fe3O4@SiO2@Zr-MOF composites and (b) the adsorption of SA and ASA. Reprinted from Ref. [38] with permission of the American Chemical Society.
(a)
(b)Based on a similar process, four other magnetic composites, magG@SiO2@ZIF-8 [39], Fe3O4@SiO2@MOF/TiO2 [40], Fe@SiO2@MOF-5 [41], and Fe3O4@m-SiO2/PSA@Zr-MOF [42], were successfully synthesized based on the magnetic NPs coated with a layer of SiO2. The magnetic NPs@SiO2 cores help adsorption of metal ions and organic linkers for the growth of outer shell of MOF layers. The magG@SiO2@ZIF-8 composites displayed high extraction efficiency and reusability for adsorption of phthalate esters with the linear range of 50-8000 ng mL−1 and up to 92% recoveries [39]. Fe3O4@SiO2@MOF/TiO2 [40] and Fe@SiO2@MOF-5 [41] core-shell nanocomposites were used as efficient adsorbents for MSPE of five triazole fungicides as well as N- and S-containing polycyclic aromatic hydrocarbons from contaminated water. Here, TiO2 immobilized on the surface of Fe3O4@SiO2@MOF could enhance the adsorption properties of magnetic MOF with the detection and quantification limits of 0.19-1.20 ng L−1 and 0.61-3.62 ng L−1, respectively [40]. The Fe@SiO2@MOF-5 exhibited a good adsorption for N- and S-containing polycyclic aromatic hydrocarbons with LODs in the range of 0.025-0.033 μg L−1 [41]. In addition to these, Xu and coworkers developed a novel Fe3O4@m-SiO2/PSA@Zr-MOF magnetic nanocomposite for bifenthrin determination from water [42]. Considering the outstanding performance and limitless of MOFs, it is expected that such MOF-based magnetic core-shell nanocomposites will open a new doorway in the field of adsorption of organic contaminants from the environment.


## 3.2. Adsorption of Dyes from Wastewater

Dye has become an important industrial hazardous contaminant in water. In recent years, with the development of the dye industry, it has a great adverse impact on the environment and human health. Therefore, it is crucial to devise a strategy for the treatment and removal of these dyes from polluted water. Wang et al. reported the use of magnetic Fe3O4/MIL-101(Cr) composite for effective adsorption of two dyes, acid red 1 (AR1) and orange G (OG) [6]. They fabricated Fe3O4/MIL-101(Cr) magnetic composites by a reduction-precipitation method with large surface areas, strong magnetism, and excellent dispersion effect. The adsorption capacities of AR1 and OG with Fe3O4/MIL-101(Cr) were 142.9 and 200.0 mg g-1, respectively [6]. The authors suggested that the adsorption of AR1 and OG was spontaneous, exothermic, and randomness decreased with monolayer adsorption during this process. Very recently, perfect MgFe2O4@MOF [43] and Fe3O4@SiO2@Zn-TDPAT [44] core-shell magnetic materials were reported by the presynthesized magnetic NP template method. The MgFe2O4@MOF magnetic composites were fabricated by a mercaptoacetic acid- (MAA-) functionalized MgFe2O4 NPs as the template method [43]. The obtained MgFe2O4@MOF hybrid nanomaterials displayed excellent removal of Rhodamine B (RB, 219.78 mg g−1) and Rhodamine 6G (Rh6G, 306.75 mg g−1) from wastewater. Moreover, these magnetic hybrid nanomaterials showed good reusability even after 10 times reused. The as-synthesized Fe3O4@SiO2@Zn-TDPAT core-shell magnetic material also displayed a high performance activity in adsorption of polluted dyes [44]. The adsorption efficiencies can reach 100 and 99% for Congo red (CR) and methylene blue (MB) by this magnetic material, respectively. Significantly, such Fe3O4@SiO2@Zn-TDPAT core-shell magnetic material can be stable under different acid-alkaline conditions. The results suggested that MOF-based magnetic core-shell materials are promising adsorbents for dye removal from wastewater.These MOF-based magnetic composites have high capacity toward a certain dye; the application of selective removal of specific dye from a mixture of multiple dye-polluted water needs more development. In 2018, Yang et al. reported a novel Fe3O4-PSS@ZIF-67 magnetic core-shell composite for selective adsorption of methyl orange (MO) from MO and methylene blue (MB) mixed solution (Figure 7) [45]. The results demonstrated that the adsorption capacity of the magnetic composites for MO was measured to be 738 mg g-1 with the separation rate of up to 92%. The selective adsorption mechanism can be attributed to charge-selectivity between the dye molecule and the MOF. The sizes of the MO and MB molecules are 1.54×0.48×0.28nm3 and 1.38×0.64×0.21nm3, while ZIF-67 has pore cage of 1 nm. Therefore, MO and MB molecules can be adsorbed in the pore cage of Fe3O4-PSS@ZIF-67. Furthermore, the negatively charged MO can be adsorbed with the Lewis base of Co2+ centrals because of the electrostatic attraction while the positively charged MB is hard to be adsorbed because of the electrostatic repulsion.Figure 7
(a) Scheme of selective adsorption of MO from the mixed MO/MB solution by Fe3O4-PSS@ZIF-67 magnetic composites; photographs (b) and UV-vis spectra (c) of the mixed MO/MB solution before and after magnetic separation. Reprinted from Ref. [45] with permission of Elsevier.
(a)
(b)
(c)


## 3.3. Adsorption of Heavy Metal Ions

Recently, MOF-based magnetic composites have been also used as porous adsorbents for the removal of heavy metal ions from the environment. MOF-based magnetic composites are promising adsorbents for the removal of heavy metal ions because of their easy modification and isolation. Karimi et al. reported a chemical bond between the NHSO3H-functionalized Fe3O4 and the HKUST-1 method for the synthesis of magnetic Fe3O4-NHSO3H@HKUST-1 nanocomposites for the adsorption of lead ions (Pb2+) from wastewater [46]. According to this work, the maximum adsorption capacity of Pb2+ with Fe3O4-NHSO3H@HKUST-1 was 384.6 mg g-1, which corresponds to 46.3% of the magnetic adsorbent occupied sites. After the adsorbent is separated by the magnet and washed with 0.1 M of HCl and distilled water, it can be used for another adsorption experiment. As a result, Fe3O4-NHSO3H@HKUST-1 could be reused four times without significant loss of adsorption activity (>90%). Huang et al. reported two amino-modified Zr-based magnetic MOF composites (Fe3O4@SiO2@UiO-66-NH2 and Fe3O4@SiO2@UiO-66-Urea) for the extraction of heavy metal ions [47]. Fe3O4@SiO2@UiO-66-NH2 and Fe3O4@SiO2@UiO-66-Urea were prepared by a simple one-pot strategy with different precursors. The obtained amine-decorated magnetic composites exhibited high adsorption for heavy metal ions compared to pure magnetic composites. In particular, Fe3O4@SiO2@UiO-66-NH2 showed the highest adsorption capacity for Pb2+ (102 mg g-1). The authors concluded that the improvement in the removal of Pb2+ by the amine-decorated magnetic composites compared with Fe3O4@SiO2@UiO-66 is due to the fact that the –NH2 groups on the magnetic composites provide more binding sites for the adsorption of Pb2+ by chelating. Further, hierarchically HPU-13@Fe3O4 (HPU-13 = {[Cu3(L)2]·OH·2CH3CH2OH·10H2O}n; HL = 2-(5-pyridin-4-yl-2H-[1,2,4]triazol-3-yl)pyrimidine) magnetic hybrid composites were synthesized for high removal and excellent reuse of Cr(VI) ions from water [48]. HPU-13@Fe3O4 showed high adsorption capacities for Cr2O72− (398.41 mg g-1) and CrO42− (471.69 mg g-1). The results proved that oxidation of Cu(I) to Cu(II) on the magnetic adsorbents occurred during the adsorption process and partial reduction of Cr(VI) to Cr(III) in the solution at the same time. Finally, the authors revealed that the high adsorption of Cr(VI) under these conditions is due to the synergistic reaction of Cr(VI) reduction and adsorption [48].The radioactive elements are also a major issue to the environment such as U(VI) and Th(IV). These radioactive metal ions can create numerous diseases including liver and lung cancers. In this case, the adsorption of these metal ions from the environment is a challenge and critical issue for the environmental remediation. Alqadami et al. prepared an Al-based magnetic MOF nanocomposite (Fe3O4@AMCA-MIL53(Al)) for the adsorption of U(VI) and Th(IV) from wastewater [49]. The adsorption capacities for U(VI) and Th(IV) were measured to be 227.3 and 285.7 mg g-1, respectively. The adsorption equilibrium time of Fe3O4@AMCA-MIL53(Al) for both radioactive metal ions was demonstrated within 90 min. The results suggested the adsorption mechanism for U(VI) and Th(IV) over Fe3O4@AMCA-MIL53(Al) through the electrostatic interactions between the organic part of the magnetic nanocomposite and the radioactive metal ions and coordinate interactions between the metal ions and nitrogen in the framework. Min et al. reported a novel Fe3O4@ZIF-8 magnetic nanocomposite for selective removal of UO22+ from water, which showed a high adsorption capacity of 523.5 mg U g-1 [50]. Furthermore, the selective separation of the UO22+ from lanthanide results indicated that the obtained Fe3O4@ZIF-8 displayed remarkable selectivity performance for UO22+ in the presence of lanthanides at pH 3.Selectivity is one of the most primary issues of a good adsorbent in the practical application. In order to solve this problem, magnetic adsorbents can be modified by various functional groups for the removal of target metal ions. The functional groups can be easily introduced into the MOF-based magnetic composites by a facile postsynthetic modification (PSM) method. Very recently, we prepared thiol-functionalized Fe3O4@Cu3(btc)2 core-shell magnetic microspheres and investigated their application in selective adsorption of heavy metal ions in the presence of other background ions from water [51]. Fe3O4@Cu3(btc)2 core-shell magnetic microspheres were synthesized by a step-by-step assembly fashion. Further, Cu3(btc)2 shell of the magnetic microspheres was modified by thiol groups of dithioglycol using such PSM method (Figure 8). Significantly, the thiol-functionalized Fe3O4@Cu3(btc)2 showed high selective adsorption for Pb2+ (Kd=1.23×104mLg−1) and Hg2+ (Kd=5.98×104mLg−1) in the presence of background ions of Ni2+, Na+, Mg2+, Ca2+, Zn2+, and Cd2+ [51].Figure 8
Scheme of the preparation of thiol-functionalized Fe3O4@Cu3(btc)2 and its use for selective removal of Pb2+ and Hg2+ from the mixed heavy metal ion solution. Reprinted from Ref. [51] with permission of Elsevier.


## 4. Adsorption Mechanism

The adsorption method is widely accepted for the removal of hazardous pollutants from the environment [52]. Magnetic MOF composites are superior to the traditional porous materials, due to their rational design, tunable porosity, controllable dimensions, large internal surface area, and easy isolation. Particularly, the pore shape and size of the magnetic MOFs can be controlled to selective adsorption of targeted hazardous molecules [53]. The mechanism for the removal of hazardous pollutants can be summarized as the four major types [17, 53]: (I) The hazardous molecules can be bound to the coordinatively unsaturated metal centers of magnetic MOFs. (II) The hazardous molecules can be adsorbed by π‐π stacking interactions between the organic part of the MOFs and the hazardous molecules. (III) There are electrostatic interactions between the hazardous molecules and the magnetic MOFs. (IV) There is molecule bonding between the decorate functional groups on ligands of magnetic MOFs and the hazardous molecules. With the rapid new multifunctional MOF development, the mechanisms for the adsorption of pollutants over the magnetic MOFs become increasingly clear.


## 5. Conclusion and Outlook

Here, we summarize a review of recent developments in the MOF-based magnetic nanocomposites for the removal of hazardous contaminants from the environment. The design and synthesis of the magnetic core-shell MOF composites are promising methods to achieve synergies of magnetic particle core and porous MOF shell. Compared with traditional adsorption materials, magnetic MOF adsorbents have a larger specific surface area and more surface active sites and can be quickly and easily recovered by an external magnetic field, which is in line with the modern green concept. Because of these excellent properties, magnetic MOF adsorbents have great application prospects in the removal of hazardous materials from the environment. The magnetic MOF composites with highly selective adsorption of target toxic compounds can be constructed by chemical modification, which will further improve the removal efficiency. This review has demonstrated several synthetic methods for the MOF-based magnetic core-shell composites. However, the current challenge is that most of the reported magnetic MOFs are still in the small-scale application stage and the far distance from large-scale industrial production and application to safeguard the environment. In addition, the incorporated magnetic particle shape and size are highly desirable, which are key issues for the application in adsorption with high capacity and selectivity. Meanwhile, the understanding of the electronic structure and interactions present between the magnetic core and MOF shell remains a challenge. Furthermore, the exact mechanism of enhanced adsorption activity by the magnetic MOF composites is still unclear. Although challenges still exist, it is expected that with the in-depth development and discussion of the magnetic MOF composites at home and abroad, it will show a broad prospect in the future practical application.



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