Application of magnetic adsorbents based on iron oxide nanoparticles for oil spill remediation: A review

Journal of the Taiwan Institute of Chemical Engineers 97 (2019) 227–236

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Application of magnetic adsorbents based on iron oxide nanoparticles for oil spill remediation: A review Kaili Qiao a, Weijun Tian a,b,∗, Jie Bai a,b, Liang Wang a, Jing Zhao a, Zhaoyang Du a, Xiaoxi Gong a a b

College of Environmental Science and Engineering, Ocean University of China, Qingdao 266100, China Key Laboratory of Marine Environment and Ecology, Ministry of Education, Qingdao 266100, China

a r t i c l e

i n f o

Article history: Received 15 October 2018 Revised 8 January 2019 Accepted 23 January 2019 Available online 8 February 2019 Keywords: Magnetic adsorbents Functionalization Iron oxidation nanoparticle Oil removal

a b s t r a c t Oil pollution has posed a great threat to marine ecosystems and human health. In recent years, many studies have focused on the functionalization of magnetic iron oxide nanoparticles to obtain efficient adsorbents for oil removal from water. This review focuses on recent studies on magnetic materials based on iron oxide nanoparticles and their applications as oil sorbents. The main techniques used to obtain iron oxide nanoparticles are also discussed. The aim of this literature review is to obtain a better understanding of the materials that could be used to obtain efficient oil adsorbents with magnetic iron oxide nanoparticles and suitable synthesis strategies by encapsulating iron oxides with organic or inorganic coatings or embedding them in a matrix/support. Another purpose is to identify the desirable characteristics of magnetic materials required for high oil removal efficiency. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Currently, oil pollution is considered a major environmental concern. It is estimated that the spilled oil volume reaches four billion tons per year globally [1]. Oil spill examples such as the Exxon Valdez oil spill in 1989 [2], Prestige in 2002 (Spain), the storage tank explosion in Dalian, China, in 2010 and the Deepwater Horizon oil spill in the Gulf of Mexico in 2010 [3] seriously damaged marine ecosystems [4]. Except for large-scale events, many smaller crude oil spills also threaten the aquatic environment as they occur more frequently. In the United States, approximately fifteen oil spill accidents occur daily in navigable waters, and hundreds of oil spills have occurred in Nigeria [5,6]. Oil spills could cause large-scale pollution in a very short time, and it has been reported that after spillage, the oil could spread quickly and form oil films on the water surface [7]. According to Aguilera [8], when an oil spill occurs, seaside flora and some fauna, such as birds and bivalve mollusks, are affected, and human health could be affected by oil-contaminated food via the food chain because many of the oils are persistent contaminants and could be carcinogenic [9]. Currently, common oil removal techniques can be classified into physical, chemical and biochemical methods. The main advantages and limitations of each technique are summarized in Table 1. It is still ∗ Corresponding author at: College of Environmental Science and Engineering, Ocean University of China, Qingdao 266100, China. E-mail address: [email protected] (W. Tian).

a challenge to develop effective remediation solutions to separate the spilled oils from the water without releasing residues into the environment [6]. Recently, the utilization of magnetic materials based on iron oxide nanoparticles as oil sorbents has attracted increasing attention after the first attempts made by Turbeville [18]. Iron oxide nanoparticles are widely studied due to their high adsorption capacity of oil, magnetic property and biocompatibility [19]. However, the unavoidable problems of naked iron oxide nanoparticles, such as agglomeration and oxidization by air due to their high chemical activity, would cause a loss of magnetism and limit their application [20]. It is thus necessary to develop strategies to protect naked ferromagnetic nanoparticles against degradation. Forming protecting shells on the surface of nanoparticles is considered a good way to stabilize and functionalize nanoparticles [21]. Over recent years, strategies including grafting organic species, coating with organic or inorganic layers [22] have been established to transform iron oxide nanoparticles into advanced materials that are able to possess high oil adsorption capacity, fast recovery and promising reusability. In the following section, we will review the functionalization methods of iron oxide nanoparticles and their applications as oil adsorbents. The main techniques performed to synthesize iron oxide nanoparticles are concluded. The key characteristics that lead to a high oil adsorption capacity are identified. Then, the functionalized iron oxide nanoparticles and related materials, as well as the removal results of oils obtained, are presented.

https://doi.org/10.1016/j.jtice.2019.01.029 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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Table 1 The summary of the techniques for removal of the spilled oils in water. Methodology

Techniques

Conditions of application

Advantages

Limitations

References

Physical method

Booms and skimmers

Large-scale and thick slick of freshly spilled oil.

Treat emulsified oils and oils of variable viscosities (6–20,0 0 0 mPa s) efficiently. No adverse environmental effects.

Ineffective in rough weather and sea conditions.

[10,11]

Chemical method

Biological method

Adsorption

Floating slicks of oil, the final stages of the clean-up in relatively calm water.

High removal rate.

In-situ burning

Freshly spilled oil in vast water.

Remove large quantities of oil rapidly. Effective in ice or cold water.

Dispersants

Fresh oil slicks with low-medium viscosity.

Biodegradation

Thin slick of oil.

Eliminate large volumes of oil rapidly and effectively. Not effective in calm water. Efficient, cheap and environment-friendly.

Time consuming and expensive in a large scale. Large amount of personnel and equipment. Difficult to be recovered.

Difficult to be degraded. Affected by sea weather conditions. Minimum oil thickness of 2–3 mm. Large quantities of smoke and toxic compounds. Effective for oils with viscosity < 20 0 0 cSt. Harmful to aquatic creatures. Limited to nutrient substance, pH and temperature. Ineffective in spill with large coherent mass.

[12]

[13,14]

[14,15]

[16,17]

2. Preparation of iron oxide nanoparticles

2.3. Hydrothermal synthesis

In the past few decades, considerable efforts have been made to develop synthesis methods for iron oxide nanoparticles. The commonly used methods include coprecipitation, thermal decomposition, hydrothermal synthesis and microemulsion. As follows, discussions about these synthesis techniques are presented.

Hydrothermal synthesis involves chemical reactions in aqueous solutions under hydrothermal conditions, namely, a high temperature aqueous solution (130–250 °C) and high vapor pressure (0.3–4 MPa). Bhavani et al. [26] successfully prepared iron oxide nanopowders through a hydrothermal process. The iron-based precursors could influence the size and shape of the magnetic products, and an appropriate hydrothermal temperature could cause an increase in the saturation magnetization. Attallah et al. [27] also investigated whether parameters such as the reaction temperature, reactant concentration and reaction time in the hydrothermal process could influence the structural and magnetic properties of iron oxide nanoparticles.

2.1. Coprecipitation Coprecipitation is a conventional method to prepare iron oxides that involves the chemical reaction between Fe3+ and Fe2+ salt in a highly basic solution in the absence of oxygen. To obtain functionalized iron oxide nanoparticles, the technique could be improved by adding functional materials or surface active agents in the reaction media to reduce the aggregation and oxidation of naked iron oxide nanoparticles. The size, shape, structure and magnetic properties of iron oxide nanoparticles could be affected by the conditions of preparation, such as the type of Fe3+ or Fe2+ salt, the Fe3+ /Fe2+ ratio, the pH value, the reaction temperature and the ionic strength of the media [22]. One disadvantage of the method is that during both the synthesis and purification process, the pH value has to remain high, which adversely affects the formation of uniform and monodispersed nanoparticles.

2.2. Thermal decomposition Thermal decomposition is also a common way to prepare highquality monodispersed magnetic iron oxide nanoparticles. The raw materials used frequently are organometallic compounds primarily including Fe(cup)3 (cup = N-nitrosophenylhydroxylamine) [23], Fe(acac)3 (acac = acetylacetonate) [24], or Fe(CO)5 [25] in an organic solution phase containing stabilizing surfactants under relatively high temperatures of 20 0–30 0 °C. The obtained iron oxide particles obtained via the thermal decomposition method have a narrow size distribution and are highly monodispersed while only dissolvable in nonpolar solvents.

2.4. Microemulsion A microemulsion is a transparent and thermodynamically stable isotropic synthesis medium of two immiscible liquids (water and oil) in the presence of surfactants. In water-in-oil microemulsions, the aqueous phase is dispersed as nanodroplets surrounded by surfactant molecules in a continuous oil phase [28]. These nanodroplets provide restricted reaction media to control the shape and size distribution of particles prepared by precipitating iron salts. Vidal-Vidal et al. [29] applied a one-pot microemulsion method to produce coated nanoparticles. The results showed that the nanoparticles were spherical and coated with an oleylamine monolayer. Moreover, they had a narrow size distribution of 3.5 ± 0.6 nm and had high saturation magnetization values. Chin and Yaacob [30] compared the characterizations of iron oxide nanoparticles prepared by a water/oil microemulsion and Massart’s procedure. The results showed that the nanoparticles synthesized via a microemulsion had a smaller size and higher saturation magnetization. 3. Desired properties of the magnetic sorbents for oil spill remediation Ideal oil sorbents should possess favorable properties, including high oil adsorption capacity, suitable reusability and absence

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Fig. 1. (a) Images of (a and b) a water and oil droplet placed on naked Fe3 O4 ; (c and e) a water droplet placed on polymer coated-Fe3 O4 nanocomposites; (d and f) an oil droplet placed on polymer coated-Fe3 O4 nanocomposites (reprinted with permission from Ref. [32]). 1(b) The removal process of oil from the water surface by magnetic materials under an external magnetic field (a and d) (reprinted with permission from Ref. [33]).

of secondary pollution to the environment [12,31]. Ideal oil sorbents should have the merits of easy preparation, low cost and environmental friendliness [6]. We present the desired properties that define magnetic sorbents based on iron oxide nanoparticles as suitable oil sorbents.

oil film. After addition of the polymer-modified Fe3 O4 nanocomposites, the black-modified Fe3 O4 could also float on water. This result might be related to the low density and highly hydrophobic coatings, which can overcome the weight of the magnetic material and repel water to enable the material to float on the water surface [7,34].

3.1. Hydrophobicity and oleophilicity 3.3. Ferromagnetism and reusability Hydrophobicity and oleophilicity represent the ability of the material to adsorb oil and repel water. Wettability refers to the ability or tendency of a liquid to spread on a solid surface and is related to the selectivity of iron oxide to water and oil. When present on solid surfaces, liquid drops can show different shapes because of wettability. The wettability of solids could be represented by the contact angle, which is defined as the angle at which a liquid interface meets the solid surface. A higher water contact angle is an indication of a more hydrophobic surface. The surface of the material is hydrophilic/oleophilic when the water/oil contact angle is less than 90°, and hydrophobic/oleophobic when the contact angle is greater than 90° [6]. Fig. 1a shows the water contact angle and the oil contact angle of naked iron oxide and polymer-coated iron oxide. By appropriately modifying the magnetic nanoparticles, it is possible to simultaneously obtain high hydrophobicity and oleophilicity with increasing water contact angle from 0° to greater than 150°. 3.2. Unsinkable property After oil is spilled, the oil would float and spread on the surface of the water for a period of time. According to the characteristics of oil, oil adsorbents should possess unsinkable properties to contact the oil fully and obtain the greatest removal efficiency. Additionally, oil sorbents with a low density are easier to collect after adsorption. Functionalized iron oxide nanoparticles could meet these demands and could float on the surface of water when they are added in oil and water mixtures. Fig. 1b shows the processes of adsorption and recovery of the floatable magnetic material. It was observed that the oil (red color) could float on water and form an

The recovery and reuse of absorbents have always been of concern to researchers. If the adsorbents could not be collected after adsorption, the cost might be higher, and the probability of secondary pollution might increase. An impressive advantage of magnetic materials is that they can be easily recycled with an external magnetic field (Fig. 1b). Due to the ferromagnetic properties of Fe3 O4 nanoparticles, the magnetic materials could not only be manipulated to move to the oil-polluted region but could also be collected easily and quickly after adsorption by an external magnetic field [34]. The saturation magnetization of magnetic materials based on iron oxide nanoparticles is 15–92 emu/g. Compared with naked Fe3 O4 nanoparticles, even though the saturation magnetization decreased after modification, the magnetic oil-absorbed materials could still be collected in 1-2 s with a magnetic bar [7,35]. In this way, the recovery operation becomes easier with simple recovery equipment, and the cost of recovery also decreases. Another desirable property is that the functionalization of magnetic iron oxide nanoparticles could be reused after easy treatment. The methods of regeneration of magnetic particles are simple, mainly including ultrasonic washing in ethanol or hexane for several minutes [7]. The magnetic materials could be applied successively without notable changes in contact angle and oil adsorption capacity. According to recent studies, magnetic oil adsorbents can be reused several to 10 0 0 times [36]. 4. Magnetic oil sorbents based on iron oxide nanoparticles As mentioned above, naked iron oxide nanoparticles are easily oxidized in air and tend to accumulate together. To apply iron

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Fig. 2. The fabrication of sodium oleate-modified Fe3 O4 nanoparticles (reprinted with permission from Ref. [33].

oxide nanoparticles more widely, the protection and functionalization procedures performed during or after the synthesis processes are important. Except for encapsulating the iron oxide nanoparticles with organic compounds and inorganic compound coatings, embedding the iron oxide nanoparticles in polymer composites or combining the iron oxide nanoparticles with activated carbon by physical and chemical methods are also effective techniques to prepare magnetic oil sorbents. In the following section, the synthesis of effective magnetic oil sorbents based on iron oxide nanoparticles will be discussed. 4.1. Combining iron oxides with organic compounds Magnetic sorbents combining organic compounds and iron oxide nanoparticles possess the basic magnetic characteristics of magnetic nanoparticles and suitable biocompatibility characteristics of organic materials. The functional groups of the organic materials could offer active sites for iron oxide nanoparticles, which makes the functionalization process possible and broadens their application as sorbents for pollutants [37–39]. In addition, the usage of organic materials could passivate the iron oxide nanoparticles during or after synthesis to avoid agglomeration. Surfactants and polymers are often chemically anchored or physically adsorbed on iron oxide nanoparticles to synthesize single or double organic compound layer-coated Fe3 O4 nanoparticles [40]. 4.1.1. Oleic acid One of the main methods for the functionalization of iron oxide nanoparticles is the coating of small surfactants on the surface. Oleic acid is an excellent surfactant and is widely used to modify ferrite nanoparticles by chemical precipitation. The -COOH groups of oleic acid have a high affinity with the Fe atoms of iron oxide, in addition to the outward facing hydrophobic tails of oleic acid and generation of nonpolar shells [41]. The obtained oil-solute magnetic iron oxide nanoparticles have a strong affinity for oil and can be used to adsorb oil on the surface of water. The modified nanoparticles can be obtained by a two-step method that consists of a synthesis of iron oxide consisting of adding an alkaline solution into a mixture of di- and/or tri-valent iron salts and a modification process of adding oleic acid [42]. Oleic moleculeFe3 O4 hybrid nanomaterials were prepared via the microemulsion method and used to treat crude oil. The removal percentage reached 95 wt.%. The saturation magnetization of the products was 16.57 emu/g. Zhu et al. [33] also used sodium oleate to modify iron oxide nanoparticles, and the obtained nanoparticles demonstrated suitable properties for oil removal (Fig. 2). After modification, the saturation magnetization of the modified iron oxide nanoparticles was 49.40 emu/g, and the water contact angle was 155°, which demonstrated that the magnetic nanoparticles could adsorb oil and repel water. 4.1.2. Macromolecules Macromolecules are natural materials that have several advantages, including nontoxicity, biocompatibility and biodegradability.

The common macromolecules used for stabilization of magnetic nanoparticles include starch [43], alginic acid [44] and chitosan [45–47], and the formed composites could be used for different adsorption purposes. Among them, chitosan had already been applied to modified iron oxide nanoparticles to synthesize oil adsorbents. Chitosan-coated iron oxide nanoparticles produced by grafting chitosan onto Fe3 O4 coated with silica and 3-aminopropyltriethoxysilane (APTES) could separate 90% of the oil from a diesel-in-water emulsion (Fig. 3) [48]. Additionally, the oil-water separation ability of chitosan-coated iron oxide nanoparticles was less affected by the pH. After treatment, the magnetic chitosan-coated adsorbent could be reused 8 times and could maintain excellent separation efficiency [49]. Li et al. [50] prepared magnetic composite resin by suspension copolymerization. Fe3 O4 nanoparticles were synthesized and modified by oleic acid and then cross-linked with composite resin. The obtained magnetic composite resin with a diameter from 50 to 20 0 0 μm had a high oil adsorption ability and could collect almost all of the spilled oil. After adsorption, the magnetic composite resin could be expediently recovered by magnetic separation techniques. 4.1.3. Biomolecules Protein wastes from the leather industry were recycled to prepare superparamagnetic collagen/iron oxide nanoparticles for oil removal [51]. The Fe3 O4 nanoparticles were first modified with citric acid organic hulls and thereafter interacted with collagen fibers. The interaction between the citric acid coated on the Fe3 O4 nanoparticles and side chain amino functional groups of collagen molecules is depicted in Fig. 4. This nanocomposite could selectively adsorb oil from water and could be manipulated by a magnetic field. The studies demonstrated that the maximum oil adsorption capacity for used motor oil could reach 2 g/g. An oil-absorbed magnetic collagen-SPION nanobiocomposite could be converted into a bifunctional graphitic nanocarbon material or bifunctional carbon material via heat treatment and could be used for other applications, providing a sustainable cycle for waste recycling and environmental cleanup. 4.1.4. Polymer Recently, polymers have gradually become important and attractive coating materials and have received increasing attention. Polymer-coated iron oxide nanoparticles remain sterically stable even in complex environments and pose little hazard [52]. Polymers have excellent properties, such as a large surface area, low skeleton density and high chemical stability [32]. Polymer modification involves the covalent connection between the rich functional groups of polymers (such as carboxylic acids, phosphates and sulfates) and the iron oxide nanoparticles via substitution or ligand exchange reactions. The first step is to synthesize iron oxide nanoparticles with oxygen bonds. Then, the synthesized nanoparticles and the polymers are connected by a chemical grafting method or atom transfer radical polymerization [48,53]. To obtain more hydroxyl groups and improve the stability of the particles during the synthesis stage, the SiO2 outer layer of iron oxide nanoparticles is usually coated functional modification. Following this approach, polymer-coated nanoparticles could be synthesized by a two-step procedure. First, magnetic Fe3 O4 was prepared, and then, the silanization of Fe3 O4 was performed by adding sodium dodecyl benzene sulfonate (SDBS) or sodium dodecyl sulfonate (SDS) followed by grafting of the polymer. The most impressive results following the aforementioned approach were obtained by Yu et al. [34] and Chen et al. [32]. They obtained Fe3 O4 @PS nanoparticles that could adsorb oils 3 times their weight. In addition, the saturation magnetization reached 34.62 and 61.25 emu/g, and the water contact angles were 141.2 and 153°, respectively. Another example involves the polymer-coated

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Fig. 3. Synthetic scheme of the CS-grafted Fe3 O4 (reprinted with permission from Ref. [48]).

Fig. 4. The synthesis of collagen-SPION nanobiocomposites (reprinted with permission from Ref. [51]).

MIONs with double layers prepared by Gu et al. [54]. After the synthesis of Fe3 O4 @P(St/DVB) nanoparticles, methyl methacrylate (MMA) was added, and the Fe3 O4 @P(St/DVB) nanoparticles were coated with a P(MMA/St/DVB) layer through secondary polymerization. The water contact angle of the nanoparticles was 141.2°, and the nanoparticles could maintain their highly hydrophobic property in aqueous solutions with different pH values (pH 1–13). After 10 cycles, the nanoparticles still had a high water contact angle of 140.4° and a high oil absorption capacity of 3.22 g/g. Another functionalization approach to obtain polymer-coated iron oxide nanoparticles is through a one-step method by heating a mixture of the capping molecule and the iron precursor under alkaline conditions. The simple and cost-effective technique provided a simple and powerful tool to remove oil from water. The approach was employed by Palchoudhury and Lead [52] through coprecipitation and by Mirshahghassemi and Lead [55] through a hydrothermal technique to obtain Fe3 O4 @PVP nanoparticles. Despite the difference in the synthesis methods, in both cases, the oil removal under optimum conditions could reach approximately 100%. An impressive example is that Abdullah et al. [56] obtained sulfonated asphaltene (SAS) by extracting asphaltenes from heavy crude oil via sulfonation and applying the SAS as a coating agent to create hydrophobic Fe3 O4 nanoparticles. SAS-Fe3 O4 was synthesized by the coprecipitation method in the presence of FeCl3 ·6H2 O, Na2 SO3 and SAS (Fig. 5). The obtained nanomaterials could adsorb crude oil up to 22.5 g/g with a removal rate of 90%. In general, the modified iron oxide nanoparticles with organic compounds were easily leached by acidic solutions [57], leading to a loss of their magnetization. The organic compound-coated iron oxide nanoparticles have another disadvantage: the intrinsic stability of the coating is relatively low at higher temperatures [20]. Another drawback is that the saturation magnetization value

Fig. 5. The synthesis processes of SAS magnetite nanomaterials (reprinted with permission from Ref. [56]).

of modified Fe3 O4 will decrease if the organic shell around Fe3 O4 is too thick. Therefore, it is important to develop other methods for modifying magnetic nanoparticles. 4.2. Combining iron oxide with inorganic materials It is clear that the process of inorganic compound coating can greatly increase the stability of naked iron oxide nanoparticles and extend their application scope. Although different types of inorganic materials have been employed to modify iron oxide nanoparticles, such as silica [58,59] and metal oxide [60–61], attempts to functionalize iron oxide nanoparticles as oil sorbents are still inadequate. Nonetheless, as will be shown in the next paragraphs, nanomaterials coated with inorganic materials are also appropriate materials for oil removal.

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4.2.1. Carbon Recently, carbon-coated magnetic nanoparticles have received considerable attention because magnetite-carbon nanoparticles have many advantages, such as high chemical and thermal stability and biocompatibility [20]. Banerjee et al. [62] synthesized a carbon-Fe3 O4 nanocomposite by pyrolysis of an iron-containing metal organic framework (MOF) and tested the oil adsorption capacity of the material. The water contact angle of the nanoparticles was 143°, indicating a near superhydrophobic character and the particles could adsorb an amount of oil of more than 40 times its own weight. Zhu et al. [7] obtained core-shell Fe2 O3 @C nanoparticles by thermal decomposition of the precursor material. The specific surface area of Fe2 O3 @C was 94.04 m2 /g, and the water contact angle was approximately 162.9°. These magnetic nanoparticles were unsinkable, hydrophobic and superoleophilic and could adsorb up to 3.8 times their own weight. After absorbance, the magnetic nanoparticles could be quickly and easily collected and regenerated. The magnetic nanoparticles still maintained a large water contact angle, which was greater than 150° after six cycles. 4.2.2. Silica Silica is one of the most common inorganic compounds for preparing functionalized iron oxide nanoparticles [22]. A silica shell could not only protect the iron oxide cores from oxidation by air and agglomeration generation but also provide stability to the iron oxide cores in solution. Yu et al. [63] fabricated Fe3 O4 @SiO2 submicrometer materials via a simple template approach at room temperature. Tetraethyl orthosilicate (TEOS) was used as the silicon source, and hexadecyl trimethylammonium bromide (CTAB) was used as the template. The water contact angle of the magnetic nanoparticles was 148.8°, and the specific surface area reached 306.45 m2 /g. The magnetic nanoparticles had a significant absorption capacity for several types of oils, including lubricating oil, salad oil, edible oil and diesel oil. The magnetic nanoparticles could adsorb oils up to 11.51 times their own weight. In addition, the magnetic nanoparticles could be reused for 20 cycles with high oil removal efficiencies and without significant changes in the water contact angle. The process of coating iron oxide with inorganic materials is much less developed; therefore, understanding the formation mechanisms and effective synthetic methods to obtain stable, dispersible magnetic nanoparticles is needed. 4.3. Inserting iron oxides into fibers or membranes Electrospinning is a method to fabricate micrometer or nanometer fibers through nonmechanical fiber drawing under a strong electric field. An electrostatic force was used to accelerate polymer droplets at the Taylor cone vertex of a capillary. When the electric field force was sufficiently large, polymer droplets could overcome the surface tension and form a jet stream. During the injection process, after the evaporation or curing of solvent, the stream finally fell on the receiving device to form a nonwoven fabric mat [64,65]. At present, a variety of magnetic nanofibers, such as Fe3 O4 /PVA nanofibers [66], C/Fe3 O4 composite nanofibers [67], magnetic PAN/Fe3 O4 nanocomposite fibers [68] and Fe3 O4 /PVP nanofiber films [69], have been prepared by electrospinning techniques. Song [35] designed a magnetic fibrous sorbent with a high oil adsorption capacity using the electrospinning method. The synthesized PS/Fe3 O4 fibrous sorbent was homogeneous, and the Fe3 O4 nanoparticles were well dispersed in the fibers. The pore diameter of the fibrous sorbent (7.63 μm) was smaller than that of pristine PS (8.34 μm), which might be attributed to the Fe3 O4 induced magnetic attraction. The magnetic PS/Fe3 O4 had a high lipophilicity and could selectively adsorb oil from water. The magnetic PS/Fe3 O4 exhibited a rapid oil adsorption process that lasted

Fig. 6. (a) The preparation of magnetic sponges (AIBN: azodiisobutyronitrile) (reprinted with permission from Ref. [77]); 6(b) The synthesis of magnetic P(StDVB) foams (reprinted with permission from Ref. [78]).

only 1–2 s. The magnetic fibrous sorbent could be collected quickly by an external magnetic field as the saturation magnetization was 14 emu/g. 4.4. Magnetic porous materials A widely employed approach to synthesize efficient magnetic oil sorbents is to attach iron nanoparticles to the pore walls and struts of certain porous material by different techniques. Threedimensional porous materials in the form of aerogels, sponges and foams are considered oil sorbents due to their low density, high porosity and specific surface area [70]. Fe3 O4 nanoparticles should be modified with a SiO2 shell in advance to make them compatible with the organosilanes and to obtain a uniform magnetic porous material, otherwise phase separation is inevitable [71]. The magnetic particles increased the roughness, hydrophobicity and oleophilicity of the porous material. A simple method to synthesize magnetic porous material is to transfer iron oxide nanoparticles to porous materials by dipcoating and adsorption [13,72]. This method could improve the hydrophobicity and oil adsorption efficiency of the porous material. However, there might be a loss of iron oxide particles during the processes of oil adsorption and magnetic collection because the particles are weakly bound, which impairs the reusability of the oil adsorbent and threatens environmental safety [73,74]. An example of this approach is the work of Calcagnile et al. [75], where the PTFE-PU foams obtained by triboelectric charging were placed onto a glass substrate coated with toluene-based Fe3 O4 . The Fe3 O4 solution was transferred throughout the entire thickness of the reticulated network of foam pores by capillary action. The magnetic PU foam with superhydrophobic and superoleophilic

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Table 2 The summary of the properties of different magnetic sorbents for oil removal. Magnetic sorbents

Functional material

Synthesis Method

Saturation Magnetization (emu/g)

Water Contact Angle

Application

Sorption Capacity/Removal

Fe3 O4 @OA Fe3 O4 @SO QC-coated Fe3 O4

Oleic acid Sodium oleate Quaternized chitosan

16.57 49.40 30.5

N/A 155° N/A

Crude oil Engine oil Emulsified oil

95 wt.% N/A 90%

[42] [33] [48]

Magnetic composite resin Magnetic collagen-SPION Fe3 O4 @PS

Gelatin glue

Microemulsion Co-precipitation Modified stober method and “grafting to” reaction Suspension co-polymerization Heat treatment

N/A

N/A

Kerosene

100%

[50]

N/A

N/A

Motor oil

2 g/g

[51]

Emulsion polymerization

34.62/61.25

141.2°/153°

Diesel oil/ Lubricating oil

3 times of its own weight

[32,34]

Hydrothermal method Modified polyol/ Hydrothermal method Co-precipitation Pyrolysis

N/A

141.2°

Diesel oil

[54]

N/A

N/A

Crude oil/ MC252 oil

3.63 times of their weight Near 100%

75–92 26–49

N/A 143°

Crude oil Oil/hydrocarbon

Thermal decomposition

N/A

162.9°

Lubricating oil

Fe3 O4 @P(St/DVB) @P(MMA/St/DVB) Fe3 O4 @PVP

Collagen Styrene (St) and sodium dodecyl sulfonate (SDS) Methyl methacrylate Polyvinyl pyrrolidone

Fe3 O4 @SAS Fe2 O3 @C composite system Polysiloxanecoated Fe2 O3 @C Fe3 O4 @SiO2

Sulfonated asphaltene Iron containing metal organic framework Terephthalic acid and polysiloxane Tetraethyl orthosilicate

Template approach

58.3

148.8°

PS/Fe3 O4 fibrous sorbent Magnetic foams

Polystyrene

Electrospinning

14

N/A

Lubricating oil, salad oil, edible oil, diesel oil Edible oil

PTFE-PU foams

Adsorption

N/A

160°

Mineral oil

Cellulose aerogel

In-situ incorporation

N/A

N/A

Paraffin oil

Sponge

Polymerization induced macrophase separation High internal phase emulsions Chemical vapor deposition Co-precipitation

N/A

140.1°

engine oil, crude oil

N/A

150°

Diesel oil

N/A

140°

15

N/A

Diesel oil and gasoline Premium oil and used oil/ Palm oil

Magnetic cellulose aerogel Magnetic sponge

Magnetic P(St-DVB) foams Fe3 O4 –CNT sponges Magnetic carbon

P(St-DVB) foams Carbon nanotube sponges Coconut/ Palm shell-based carbon

properties could float on the surface of water and adsorb oil up to more than 13 times its weight. However, the maximum amount of iron oxide nanoparticles that could be reclaimed was 80% of the initial quantity due to the loss of magnetic particles upon interaction with oil. A type of cellulose aerogel with hydrophobic, magnetic and highly porous properties was prepared and applied to selectively adsorb oil from the water surface [76]. The obtained magnetic aerogel could reach an oil adsorption capacity up to approximately 28 times its own weight within 10 min. An external magnetic field was sufficient to remove and recover the magnetic material from the water surface. The magnetic aerogel could be reused by washing with ethanol. Another method is to crosslink or graft iron oxide nanoparticles onto porous materials, which decreases the leaching of Fe3 O4 from the porous material. An impressive example is that of Wu et al. [77] involving the insertion of modified Fe3 O4 nanoparticles into a sponge by polymerization-induced macrophase separation (Fig. 6a). This approach provided magnetic sponges with a high hydrophobicity (water contact angles of 140.1°) and oil adsorption capacity from 9.9 to 20.3 g/g. In addition, the adsorption processes were fast and lasted only 10 min at which point adsorption saturation was reached. Zhang et al. [78] developed another one-step method (high internal phase emulsions)

Reference

[52,55]

90% 40 times of its own weight 3.8 times of their own weight

[56] [62]

11.51 times of their own weight

[63]

near 100%

[35]

13 times of its own weight 28 times of its own weight 9.9–20.3 g/g

[75]

[7]

[76] [77]

24.5–57.6 times of its own weight 99%

[78] [36]

9.33 g/g and 80%

[82,83]

to prepare magnetic P(St-DVB) foams (Fig. 6b). The magnetic P(St-DVB) foams showed a very high degree of hydrophobicity and oleophilicity (water contact angle >150° and oil contact angle approximately equal to 0), and they could adsorb oil up to 57.6 times their own weight. In addition, the magnetic P(St-DVB) foams exhibited superior reusability and could be reused 20 times without a notable decline in the adsorption capacity. A remarkable performance was obtained by Li et al. [79] in preparing a magnetic carbon fiber aerogel through one-step pyrolysis of Fe(NO3 )3 -coated cotton in an argon atmosphere. The magnetic carbon fiber aerogel could adsorb oil selectively and rapidly due to its superhydrophobicity and superoleophilicity. After adsorption, the magnetic carbon fiber aerogel could be controlled by a magnetic bar for retrieval. The aerogel exhibited a good reuse performance and still maintained a high oil absorption capacity after ten times of use. According to Gui et al. [36], Fe3 O4 –CNT sponges with a high affinity for diesel oil and gasoline could be fabricated using ferrocene and dichlorobenzene as precursors by chemical vapor deposition. The oil adsorption capacity reached 56 g/g with a removal efficiency of 99%. Remarkably, the magnetic sponges could be reused for 10 0 0 cycles without great changes in the structure, contact angle and sorption capacity.

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4.5. Combining iron oxide with activated carbon Activated carbon is an impressive adsorbent with which to address environmental problems because it has a high specific surface area and a rich pore structure. The synthesis and application of magnetic activated carbon have received considerable attention in recent years because the materials combine the magnetism of magnetite and the high adsorption capacity of activated carbon [80,81]. According to Raj and Joy [82], activated carbon was first prepared by pyrolyzing coconut shells. Then, activated carbon/Fe3 O4 composites were obtained by in situ coprecipitation. An ammonia solution was added to the solution containing Fe3+ , Fe2+ and activated carbon, and the mixture was heated to facilitate the binding of iron oxide nanoparticles to surface functionalities of the activated carbon. The activated carbon had a higher adsorption capacity of premium oil (9.33 g/g) than that of used oil (5.54 g/g). After modification by iron oxide nanoparticles, the adsorption capacity of premium oil and used oil increased to 12.93 g/g and 7.65 g/g, respectively. After adsorption, the magnetic activated carbon could be recovered by heat treatment and an external magnetic field. The premium and used oil adsorption capacity decreased to 4.26 g/g and 4.89 g/g, respectively. After the first cycle, the adsorption values remained almost constant from the second to the sixth cycle. Palm shell-based carbon was also used to prepare magnetic sorbents for oil removal [83]. First, palm shell-based carbon was prepared by carbonization (combined with activation). Then, carbon/iron oxide composites were synthesized via a coprecipitation technique by heating a mixture of carbon, Fe3+ and Fe2+ in an alkaline environment. The removal efficiency of oil could reach 80%. The magnetic activated carbon could be collected after adsorption by a magnetic process due to its magnetic properties. The magnetic iron oxide nanoparticles could roughen the surface of activated carbon and increase the specific area of carbon. Additionally, the functionalization of carbon by magnetic nanoparticles could increase the oil adsorption efficiency and reusability. However, the main synthesis method of magnetic activated carbon is coprecipitation, which may result in agglomeration and oxidation during the adsorption process, such as iron oxide nanoparticle binding on the surface of carbon. It is important to develop synthesis techniques to obtain stable magnetic activated carbon. Table 2 illustrates the properties of magnetic sorbents and their oil removal efficiencies. The water contact angles of these magnetic materials were usually greater than those of naked iron oxide nanoparticles. The synthesis method and functionalization material influence the saturation magnetization. Generally, a thick shell could lead to a decrease in the saturation magnetization. Moreover, magnetic porous materials and magnetic carbon may have a lower level of saturation magnetization because the synthesis processes require the adsorption or immobilization of magnetic nanoparticles on/into the support materials, which may result in a lower content of iron oxide nanoparticles. The functionalization materials also have a great effect on the oil removal efficiency. Hydrophobic/lipophilic materials are considered ideal functionalization materials because they have a strong affinity for oil. The magnetic nanoparticle adsorbents obtained by encapsulating iron oxide nanoparticles with organic compounds usually presented a high oil removal efficiency (near 100%). There are two main reasons, one was that the organic compound possessed hydrophobicity/lipophilicity, and the other was that the nanoparticles contained a network of nanoscale structures and a large surface area, which could lead to a high adsorption capacity for some contaminants [84]. The magnetic porous materials also had a high oil adsorption capacity of 23.3 g/g (or nearly 100%). The ideal characteristics of porous materials for oil sorption are high porosity, large surface area, and low density. Another benefit of

porous materials for oil spill treatment is the recovery of oil [85]. Several recovery methods could be applied, including mechanical squeezing, extraction, and distillation [86–88]. Magnetic porous materials could combine the advantages of porous materials and the magnetic properties of magnetic iron oxide nanoparticles.

5. Conclusions and perspectives The review shows that magnetic oil sorbents have several advantages. One impressive property is magnetism, which allows a quick and efficient retrieval; another key parameter for efficient oil adsorption is the degree of hydrophobicity, which permits the sorbent to adsorb oil and repel water; the third necessary trait of an efficient magnetic sorbent is a low density, which allows full contact with the oil film, resulting in a considerable removal efficiency of oil and allowing efficient retrieval by an external magnetic field. Magnetic nanomaterials could be obtained by encapsulating iron oxide nanoparticles with organic or inorganic compounds. Furthermore, magnetic porous sorbents and magnetic activated carbon are also desirable oil sorbents. Although there are various strategies to obtain functionalized iron oxide nanoparticles, it is still a challenge to synthesize highquality and stable magnetic oil sorbents. In the future, development of high performance adsorbents is required. Sorbents with a high adsorption capacity, high reusability, low cost, and toxinfree performance are attracting attention. Meanwhile, the increasing population rate has increased the rate of food consumption, resulting in massive amounts of biowaste globally. For this reason, an inventive way of producing magnetic sorbents using easily biodegradable biomass to produce low-coast sorbents with higher oil sorption capacity has been developed. In the near future, the preparation of magnetic oil sorbents with a high stability and availability and realization of largescale or industrial applications will be greatly investigated. To accomplish the application under real case conditions, researchers have to resolve several practical questions, such as the amount of magnetic materials to add and how wave action might complicate the recovery of swelled-up particles. Research on the strength of the external magnetic field used to retrieve the magnetic material after adsorption should also be conducted. To realize industrial commercialization, the synthesis method of magnetized nanoparticles should be simple and cost-effective. Furthermore, the synthesis method should not introduce harmful chemicals and should enable recovery and recycling use. Recyclability is an important parameter for industrial commercialization. The adhesion between magnetic nanoparticles and supporting materials is largely affected by the synthesis method of magnetic materials. Certain magnetic materials were produced by inserting Fe3 O4 nanoparticles into fibers and membranes or by crosslinking or grafting in sponges and foams. After the regeneration treatment, these magnetic adsorbents could be recycled several times without a clear decline in the adsorption capacity, and no great changes were observed in the structures or contact angles. However, certain magnetic materials were synthesized by procedures where magnetic nanoparticles were transferred to porous materials through simple dip-coating or triboelectric charging or being attached to the surface of biochar by coprecipitation. After several cycles of oil-water separation, the magnetic materials still maintained suitable oil adsorption capacities. However, the drawbacks of these materials are mainly connected to the partial release of weakly bound particles into the environment during the oil absorption process. The latter will compromise the reusability of the sorbent and result in diverse environmental safety issues. In the future, researchers should consider the regeneration efficiency and cyclic stability when synthesizing magnetic oil adsorbents.

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However, future work should also focus on research on the toxicity and degradability of naked or functionalized iron oxide nanoparticles. If any nanoparticles were left behind, they would float in oceans indefinitely, possibly agglomerating with existing pollutants such as plastic garbage; therefore, it may be better to produce particles made of degradable polymers. In the future, polymers made of natural products, such as sugar, that will dissolve into harmless components if left in the environment should be concentrated on. From environmental and economic points of view, the optimal solution of an oil spill might be a combination of several different methods. In the case of oil spills, the treatment method could include oil adsorption and oil biodegradation. Magnetic adsorbents with a high oil adsorption capacity could be used as supports to carry microorganisms such as bacteria and fungi to treat oil pollution.

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