Magnetic adsorbents based on micro- and nano-structured materials

Abstract

Micro- and nano-sized magnetic adsorbents based on elemental metals, iron oxides, and ferrites and supported by inorganic (carbon, graphene, silica, and zeolites) or organic (macromolecules, polysaccharides, polymers, and biomolecules) compounds are reviewed. The main focus is on magnetic hydrogels, xerogels, and aerogels due to their high number of environmental and biomedical applications. The roles of carbohydrates and polysaccharides (starch, alginic acid, chitosan, etc.) in the stabilisation of magnetic nanoparticles and the use of the formed composites for adsorption purposes are described. Magnetic adsorbents are mainly used for heavy metal removal, for the separation, destruction and adsorption of oil, dyes, toxic organic compounds, several biomolecules and drugs, as well as in a wide variety of catalytic processes. These last applications of magnetic adsorbents, particularly ferrites, are emphasised. The main advantage of magnetic absorbents for these or other possible applications is that the adsorbent or catalyst can be easily removed from the reaction medium after adsorption or reaction completion via a simple magnet. Magnetic behaviour studies of magnetic adsorbents are also discussed.

---

1. Introduction

Adsorbents formed with nano- to micro-sized particles are currently in the stage of fast development. Recently, a series of books^1–4 and reviews^5,6 have been published in this field; among them, we note an excellent comprehensive contribution dedicated to nanoadsorbents,⁷ in which adsorbent materials based on metal and metal oxide nanoparticles, carbon-, silica-, and polymer-based nanomaterials, nanofibers, nanoclays, xerogels, aerogels, and destructive adsorbents are described in detail. At the same time, within this huge area of research and technology, adsorbents possessing magnetic properties are very valuable since their application allows a simple magnetic recovery after the completion of the adsorption process using magnets of distinct force. Several partial aspects of this problem including magnetically-modified biological materials containing magnetic nanoparticles as labels,⁸ magnetic cellulose nanocomposites,⁹ and magnetic hydrogel nanoparticles for proteins¹⁰ are covered in recent reviews and book chapters.

In this review, we summarise the most recent achievements in the preparation and main applications of magnetic adsorbents, focusing mainly on nano- and micro-sized magnetic particles in inorganic and organic supports in the form of aerogels, hydrogels, and xerogels.

2. Useful definitions

2.1. Nanocomposites

A composite is a multiphase material where a significant proportion of its properties come from the constituent phases, resulting in improved properties in the final product. It is possible to combine various types of materials in a single composite in order to optimise its properties for the desired application. When one of the phases has nanosized dimensions, the system is called a nanocomposite. Interest in the preparation of magnetic nanocomposites has increased in recent years due to the properties presented by these materials, which depend on the particle size, concentration and distribution of the particles in the matrix. Nanosystems such as Fe/SiO₂, Ni/SiO₂, Fe₃O₄/SiO₂, CoFe₂O₄/SiO₂, and NiFe₂O₄/SiO₂ have been intensively studied in recent years, revealing different behaviours from those of bulk magnetic systems and serving as models for the study of small particles.^11,12

2.2. Xerogels and aerogels

Drying is one of the more important steps in the sol–gel process because it is possible to obtain different materials by changing the drying route. During drying, the solvent adsorbed inside the porous gel is removed. In this process, the gel network can collapse. There are several types of drying processes including controlled drying and supercritical drying.¹³ In the controlled process, the solvent is evaporated slowly at room temperature and pressure, which contracts the material, decreasing the pore size due to the surface tension. The dry gels obtained by this process are called xerogels and have high porosities and specific surface areas. In supercritical drying, the wet gels are put in a reactor at a high temperature and pressure above the critical point of the system, where there is no discontinuity between the liquid and gaseous phases, avoiding capillary forces. The dry gels obtained are called aerogels and have higher porosities than xerogels. As an example, carbon aerogels and xerogels were prepared by a sol–gel process involving the polycondensation of resorcinol and formaldehyde catalysed by Na₂CO₃ followed by drying, either in supercritical CO₂ conditions to form the aerogel or in normal conditions to obtain the xerogel, and pyrolysis in Ar atmosphere at 750 °C for 2 h.¹⁴ Silica xerogels consist of a silica network with micro-, meso- and macropores interconnected throughout the bulk.¹⁵ Micro-pores are pores smaller than 2 nm in diameter, meso-pores are the pores with diameters between 2 and 20 nm, and macro-pores are larger than 20 nm. These xerogels represent monolithic porous matrices (Fig. 1) without defects after drying and change in size after thermal treatments at high temperatures. An image of a silica–titania aerogel¹⁶ is shown in Fig. 2.

Fig. 1 Silica xerogel. Reproduced from ref. 15 with permission.

Fig. 2 Silica–titania aerogel. Reproduced from ref. 16 with permission.

2.3. Hydrogels

Hydrogels^17–19 can be defined as cross-linked polymer networks that can absorb large amounts of water or biological fluids. Hydrogels themselves do not dissolve in water at a physiological temperature and pH, but swell considerably in aqueous media. Hydrogels are currently being considered for various biological applications such as components of drug delivery devices, microfluidic devices, biosensors, tissue implants and contact lenses. On the basis of their synthetic route, hydrogels can be classified as:²⁰

- homopolymer hydrogels (made up of only one type of hydrophilic monomer);

- copolymer hydrogels or network gels (composed of two types of monomers); and

- mutipolymer hydrogels (made up of three types of monomers or interpenetrating polymeric network).

Another mode of classification is based on the type of ionic charge present on the polymer network:

- anionic hydrogels (anionic thermoassociative carboxymethylpullulan hydrogels);

- cationic hydrogels (thermosensitive, cationic hydrogels of N-isopropylacrylamide (NIPAM) and (3-acrylamidopropyl)trimethylammonium chloride (AAPTAC));

- neutral hydrogels (miscible blends from water-insoluble polymers like poly(2,4,4-trimethylhexamethylene terephthalamide)); and

- ampholytic hydrogels (acrylamide-based ampholytic hydrogels).

On the basis of physical structures, hydrogels can be classified as:

- amorphous hydrogels (chains are randomly arranged);

- semi-crystalline hydrogels (dense regions of ordered macromolecular arrangement); and

- hydrogen-bonded or complexation structures (the three dimensional networks formed due to hydrogen bonding).

2.4. Ferrogels

Composed of a soft polymer matrix and magnetic filler particles, ferrogels are smart materials that are responsive to magnetic fields. Due to the viscoelasticity of the matrix, the behaviours of ferrogels are usually rate-dependent. One of their synthetic methods is to introduce monodomain, magnetite particles of colloidal size into chemically cross-linked poly(vinyl alcohol) hydrogels.²¹ Their ability to absorb aqueous solution without losing their shape and mechanical strength depends on the properties of the polymer along with the nature and density of the network; accordingly, they can absorb and retain various amount of water. Chains are connected by the electrostatic forces of hydrogen bonds and hydrophobic interactions; therefore, such gels are not permanent. They can be converted into polymers by heating.²²

2.5. Ferrofluids

Ferrofluids or so-called magnetic liquids are suspensions of colloidal magnetic particles stabilised by surfactants in liquid media. The magnetic phase in ferrofluids can be magnetite, ferrites (a group of non-metallic, ceramic-like, usually ferromagnetic compounds of ferric oxide with other oxides) and Fe_xC_y particles resulting from the thermal decomposition of Fe(CO)₅.

2.6. Iron oxides and ferrites

The presence of magnetic nanoparticles (such as iron oxides or CoFe₂O₄) inside the inert porous matrices of xerogels and aerogels reinforces their structures, avoiding large changes in specific surface area, porosity and matrix microstructure after preparation at temperatures varying between 300 and 900 °C. These characteristics influence the properties of the nanocomposites, such as their chemical reactivity, catalytic activity and magnetisation. As will be seen below, iron oxides and ferrites are generally used as the magnetic phase in adsorbents. Iron oxide is a collective term for oxides, hydroxides and oxy-hydroxides composed of Fe(II) and/or Fe(III) cations and O²⁻ and/or OH⁻ anions. Sixteen pure phases of iron oxides, i.e., oxides, hydroxides or oxy-hydroxides, are known to date: Fe(OH)₃, Fe(OH)₂, Fe₅HO₈·4H₂O, Fe₃O₄, FeO, five polymorphs of FeOOH and four of Fe₂O₃. Maghemite (γ-Fe₂O₃) in itself is a material of great technological importance due to its use in magnetic recording systems and in catalysis; moreover, maghemite properties are particularly enhanced when the size of the particles reaches the nanometre range. Although nanosized γ-Fe₂O₃ transforms into α-Fe₂O₃ (hematite) at rather low temperatures (350 °C), it can be stabilised through the incorporation of the nanoparticles into polymeric, glassy or ceramic matrices. Another important component in magnetic adsorbents is magnetite, the cubic spinel Fe₃O₄, which is ferrimagnetic at temperatures below 858 K. This mineral exhibits strong magnetic properties, easily allowing the creation of devices and processes for both upstream (adsorption) and downstream/deposition processes such as fixed bed adsorption, magnetically-stabilised beds, magnetic separation, and the remote deposition of dangerous materials. The adsorption/desorption capability of both natural and synthetic magnetite with respect to hazardous species dissolved in aqueous solutions has been reviewed.²³ Ferrites are chemical compounds obtained as powder or ceramic bodies formed by iron oxides (Fe₂O₃ and FeO) as their main component; they have ferrimagnetic properties and can be partly changed by other transition metal oxides. Ferrites can be classified according their crystalline structures: hexagonal (MFe₁₂O₁₉), garnet (M₃Fe₅O₁₂) and spinel (MFe₂O₄), where M represents one or more bivalent transition metals (Mn, Fe, Co, Ni, Cu, and Zn).

3. Metal-based adsorbents

3.1. Nano zero-valent iron (nZVI), other metals and alloys

As will be seen below, iron oxides are generally used as a magnetic component in adsorbents. However, zero-valent iron has also been applied in a few magnetic nanocomposites as adsorbent, mainly on the basis of carbon-containing materials. Thus, magnetic cellulose nanomaterials containing nanoscale zero-valent iron (nZVI) and cellulose were prepared by a reduction method.²⁴ With a saturation magnetisation of 57.2 emu g⁻¹, the cellulose@nZVI composites could be easily separated from solutions in 30 s using an external magnetic field. Arsenite adsorption by this nanoadsorbent followed the pseudo-second-order kinetic model and Langmuir isotherm model, showing a maximum removal of 99.27%. It is necessary to mention that As(V) as well as Pb(II), Congo red and methylene blue can be also effectively eliminated, even after five cycles, using supported nano zero-valent iron on barium ferrite microfibers.²⁵ These composite microfibers are fabricated from nano zero-valent iron and nano BaFe₁₂O₁₉ grains. The enhanced adsorption characteristics can be attributed to the porous structure, strong surface activity and electronic hopping. Related zero-valent iron nanoparticle (10 nm)–graphene composite (G–nZVI) is also known;²⁶ G–nZVI showed great potential as an efficient adsorbent for lead immobilisation from water as it exhibited stability, reducing power, a large surface area, and magnetic separation.

Among other carbon-containing magnetic adsorbents, mesoporous Fe₇Co₃/carbon nanocomposite was prepared via a co-casting method.^27,28 The authors noted that the introduction of Fe₇Co₃ alloy nanoparticles into mesoporous carbon greatly improved the magnetic property of the nanocomposite compared to using nanoparticles of a single metal (Fe or Co). The nanocomposite was applied as a magnetically-separable adsorber and exhibited an excellent performance for the adsorption of bulk dyes due to its high surface area (≤1429 m² g⁻¹) and pore volume (≤1.93 cm³ g⁻¹). Related cobalt magnetic nanoparticles coated by carbon (Co/C) were prepared by catalytic chemical vapour deposition (CCVD) with ethanol.²⁹ It was shown that in the CCVD process, cobalt oxide (Co₃O₄) reacts with ethanol in the temperature range of 700–900 °C to produce magnetic metallic cobalt coated by different carbon materials such as graphite and filaments. The presence of multiwalled carbon nanotubes encapsulating a cobalt metal core was confirmed. The produced Co/C nanoparticles were used as an adsorbent for organic compounds using methylene blue dye as a model molecule. In the case of other metals, a green and low-cost adsorbent with both magnetic property and a high adsorption capacity was prepared based on a nickel magnetic core with a silica shell (Ni@SiO₂).³⁰ The synthesised adsorbent exhibited a higher adsorption capacity for dyes with negative charge/hydroxyl groups compared to dyes with positive charge/without hydroxyl groups due to the hydrogen bonding and electrostatic interactions between the adsorbent and dyes. The binding of these dyes with the surfaces of the magnetic adsorbent mainly involves physical adsorption according to the D-R model. After regeneration, the adsorbent still showed a high adsorption capacity, even for four cycles of desorption–adsorption.

4. Iron oxides as magnetic adsorbents

4.1. Adsorbents based on the carbon matrix

A series of ferrite–carbon aerogel (FCA) monoliths with different iron/carbon ratios was synthesised³¹ directly from metal–resin precursors accompanied by phase transformation (Fig. 3). The self-doped ferrite nanocrystals and carbon matrix were formed synchronously via moderate condensation and sol–gel processes, leading to a homogeneous texture. The optimal FCA contained 5% ferric content and was composed of coin-like carbon nano-plates with continuous porous structure, and the ferric particles with diameters of dozens of nanometres were uniformly embedded into the carbon framework. When FCA was used as an electro-Fenton cathode (Fig. 4), metalaxyl (N-methoxyacetyl-N-(2,6-dimethylphenyl)-DL-alaninate, 1) degradation results demonstrated that 98% TOC elimination was realised after 4 h. This value was 1.5 times higher than that of the iron oxide-supported electrode. This result was attributed to the self-doped Fe@Fe₂O₃, which ensured Fe(II) as the mediator and maintained high activity via reversible oxidation and reduction by electron transfer among iron species with different valences (reactions (1)–(7)). Another carbon-based magnetic aerogel photocatalyst with strong rhodamine B (RhB) dye adsorption ability, tri-functional mesoporous composite γ-Fe₂O₃/α-Fe₂O₃/carbon aerogel (CA; its synthesis is described in ref. 32), was prepared by a hydrothermal process from FeSO₄ as a precursor in hexamethylenetetramine (HMTA).³³ The as-prepared mesoporous composite γ-Fe₂O₃/α-Fe₂O₃/CA structures have ferrimagnetic properties due to their γ-Fe₂O₃ component, and they also have photocatalytic properties because of their antiferromagnetic α-Fe₂O₃ component (Fig. 5). The removal capacity of RhB dyes of the as-prepared mesoporous structures was increased from 83.5% to 91% under visible light irradiation. The mesoporous structures can also be separated by an external magnetic field to avoid further separation steps. Curiously, carbon-based aerogels can also be obtained from biomass; for example, a nanocomposite of carbon aerogel and iron oxide was prepared by a facile hydrothermal treatment of watermelon.³⁴ Graphene-based magnetic adsorbents are also known; thus, macroscopic multifunctional graphene-based hydrogels with robust interconnected networks were fabricated under the synergistic effects of the reduction of graphene oxide sheets by ferrous ions and the in situ simultaneous deposition of nanoparticles on graphene sheets.³⁵ α-FeOOH nanorods and magnetic Fe₃O₄ nanoparticles can be easily incorporated with graphene sheets to assemble macroscopic graphene monoliths just by controlling the pH value under mild conditions. These functional graphene-based hydrogels exhibit excellent capability for the removal of pollutants, and thus could be used as promising adsorbents for water purification. We consider graphene-based adsorbents as promising materials under the condition of reduced cost in their large-scale production. For example, the present price of graphene nanoplatelets is $385–525 per kg, and it is predicted that nanoplatelets could be produced at $11 per kilogram.

≡Fe^III–OH + e⁻ → ≡Fe^II–OH (1)

≡Fe^III–OH + H₂O₂ ↔ ≡Fe^III–OH(H₂O₂) (s) (2)

≡Fe^III–OH(H₂O₂) (s) → ≡Fe^II–OH(HO₂˙) (s) + H⁺ (3)

≡Fe^II–OH(HO₂˙) (s) → ≡Fe^II–OH + HO₂˙ + H⁺ (4)

≡Fe^II–OH + H₂O₂ → ≡Fe^III–OH + ˙OH + OH⁻ (5)

˙OH ↔ H⁺ + O₂˙⁻ (6)

˙OH + MET → CO₂ + H₂O (7)

Fig. 3 Illustration of the fabrication of an FCA electrode. Reproduced from ref. 31 with permission.

Fig. 4 MET degradation process in a heterogeneous E-Fenton system with an FCA cathode. Reproduced from ref. 31 with permission.

Fig. 5 Scheme of RhB dye removal using the as-prepared tri-functional γ-Fe₂O₃/α-Fe₂O₃/CA structures under visible light irradiation. The tri-functional materials can be separated with an external magnetic field. Reproduced from ref. 33 with permission.

4.2. Adsorbents based on silica and related supports

Silica is also a conventional substance for many adsorbents. Aerogel and xerogel iron oxide–silica nanocomposites have been prepared by sol–gel methods.³⁶ Starting from both xerogel and aerogel samples, pure maghemite nanoparticles with average particle sizes of around 5 nm, which tend to aggregate in the silica matrix, were obtained. Varying the synthetic conditions could change the resulting products. Thus, depending on the synthetic parameters, ferrihydrite or maghemite particles were obtained in similar conditions.³⁷ The materials were obtained via sol–gel hydrolysis/condensation reactions of silicon alkoxides {Si(OCH₂CH₃)₄ (TEOS) or Si(OCH₃)₄ (TMOS)} with water in an alcoholic solvent, further drying by supercritical evacuation of the solvent, and growth of the magnetic phase in the silica aerogel starting from Fe(NO₃)₃·9H₂O alone or with FeNa(EDTA)·2H₂O as precursors (Table 1). The nanoparticle phase assignments were maghemite (γ-Fe₂O₃), magnetite (Fe₃O₄) ferrihydrite (Fe₅HO₈·4H₂O). At room temperature, the system was found to behave as an ensemble of non-interacting superparamagnetic particles, indicating that particle aggregation can be avoided by using a sol–gel preparation method with supercritical drying. In the temperature range of 15 to 300 K, the magnetic ac susceptibility χ(T) displayed a broad peak that shifts to higher temperatures upon increasing the ac applied field frequency.

Table 1 Aerogel alkoxide precursor and solvents, iron salt precursor, density (ρ), mean particle diameter (d_m) and standard deviation (σ_d) from TEM determination, activation energy (E) for magnetic moment reversal, microscopic attempt time (τ₀) from the ac susceptibility, effective anisotropy constant (K_eff) and magnetic phase identification. Reproduced from ref. 37 with permission

Magnetic phase	Alkoxide and solvent	Iron salt precursor	ρ (g cm^−3)	dm (nm)	σd (nm)	E/kB (K)	τ0 (s)	Keff (J m^−3)
Ferrihydrite	TEOS ethanol	Fe(NO3)3·9H2O	0.52	3.4	1.6
Maghemite	TMOS methanol	Fe(NO3)3·9H2O	0.44	5.0	1.3	1940	10^−11	1.02 × 10^5
Maghemite	TMOS methanol	Fe(NO3)3·9H2O + FeNa(EDTA)·2H2O	0.44	4.0	1.4	1700	10^−11	1.4 × 10^5

---

The stabilities of these nanocomposites were distinct in comparison with pure iron oxide nanoparticles. Thus, Fe₃O₄/SiO₂ nanocomposite aerogel powders were synthesised by a two-step process including the formation of Fe₃O₄ nanoparticles, which were further embedded in the SiO₂ matrix by the hydrolysis and subsequent condensation of silicic acid.³⁸ The Fe₃O₄/SiO₂ nanocomposites were found to exhibit an enhanced thermal stability against oxidation compared with pure Fe₃O₄. In addition, functionalised silica-supported magnetic adsorbents were prepared and studied. Thus, the magnetic adsorbents with functional –NH₂ groups were synthesised by the immobilisation of amino-silane on the surface of the magnetic silica supports, which were prepared by a co-precipitation method (Fig. 6).³⁹ These adsorbents based on Fe₃O₄ were applied in the separation of flavonoids {flavonoids are polyphenolic compounds with basic structures consisting of two aromatic rings (noted A and B) linked through three carbons that usually form an oxygenated heterocycle (C ring), formula (2); they are frequently used for “greener” syntheses of metal nanoparticles using plant extracts⁴⁰} from licorice root and were shown to have selectivity for the flavonoids to some extent. The selectivity of the adsorbents is based on the formation of hydrogen bonding between the –NH₂ groups of the magnetic adsorbents and –OH and –CO groups of the flavonoids. Another representative example of functionalisation is a versatile and robust solid phase with both magnetic property and a very high adsorption capacity based on the modification of iron oxide–silica magnetic particles with a Schiff base L (formula (3); Fe₃O₄/SiO₂/L).⁴¹ This magnetic solid phase offered an efficient and cost-effective method for the preconcentration of trace amounts of Pb(II), Cd(II) and Cu(II) in environmental and biological samples (Fig. 7).

Fig. 6 Scheme of the coating reaction of APTS {3-aminopropyltriethoxysilane NH₂(CH₂)₃Si(OC₂H₅)₃} with magnetic silica particles.

Fig. 7 Procedure for synthesising Schiff base (L) modified silica-coated magnetic nanoparticles and for the proposed magnetic solid-phase extraction. Reproduced from ref. 41 with permission.

4.3. Adsorbents based on zeolites

Natural and artificial zeolites are also widespread inorganic supporting materials for micro- and nanoparticles with distinct applications.^42–47 Adsorbents comprised of their composites with iron oxides can be fabricated by several routes. For example, a zeolite was synthesised by the alkaline hydrothermal treatment of coal fly ash (a major solid waste from coal-firing power stations),^48,49 and the zeolite–iron oxide magnetic nanocomposite was prepared by mixing zeolite from coal fly ash with magnetite nanoparticles in suspension. The magnetic nanocomposite was used for the removal of U(VI) from aqueous solutions by a batch technique as well as for the adsorption of Reactive Orange 16 dye. Sometimes, the states of presence of nanoparticles in zeolites can be “non-conventional”. Thus, nanoparticles and nanoclusters of FeO(OH)/Fe₃O₄ were incorporated by co-precipitation in clinoptilolite channels.⁵⁰ The intriguing aspect of this report was the demonstration of the coexistence of paramagnetic and superparamagnetic Fe(III) in clinoptilolite. The binding of trivalent iron in clinoptilolite results in the formation of iron oxide nanoclusters in microvoids and the stabilisation of tetrahedral AlO₄ in the aluminosilicate matrix. The resulting non-framework and bound Fe(III) exhibited superparamagnetic and paramagnetic properties, respectively.

Among other zeolite-based magnetic nanocomposites, we note the adsorption features of classic zeolites (NaY, Beta, mordenite and ZSM-5) combined with the magnetic properties of iron oxides in a composite to produce a magnetic adsorbent.⁵¹ These magnetic composites can be used as adsorbents for contaminants in water and subsequently removed from the medium by a simple magnetic process. In addition, magnetic MCM-41 (from a family of silicate and aluminosilicate solids) with a large surface area (ca. 800 m² g⁻¹) and a high magnetisation (ca. 8.3 emu g⁻¹) was prepared at a reasonable iron oxide nanoparticle loading of 10 wt% by a two-step synthetic process.⁵² Eight-nanometre iron oxide nanoparticles (i.e., 30 m² g⁻¹) were embedded in MCM-41 with no observable effects on the particle morphology and pore symmetry, although slight changes (i.e., <20%) in the textural properties and surface chemistry were detected. After the absorption of As(V) and Cr(VI) oxyanions, this magnetic adsorbent was easily dispersed in aqueous solution and could be removed by a magnet (1550 G) at a rate of 1 cm min⁻¹, much faster than the removal rate of gravity (0.004 cm min⁻¹).

4.4. Adsorbents based on the macromolecules

Magnetic cucurbituril (MQ[n]), a functional material compound, was prepared (Fig. 8) via chemical co-precipitation as a high-capacity adsorbent for humic acid (HA).⁵³ Q[n], a family of macrocyclic host molecules,⁵⁴ possesses a unique structure of a centre hydrophobic cavity and hydrophilic portals that can form stable clathrate compounds with guest molecules through van der Waals interaction, hydrophobic interaction, electrostatic interaction, hydrogen bonding, and ion–dipole or dipole–dipole interactions. Q[n] was found to be grafted on the surface of Fe₃O₄. MQ[n] demonstrated good adsorption capacity at pH 7 in adsorption experiments; moreover, the capacity of MQ[n] was also above 80% after being used four times; thus, it may have potential industrial applications.

Fig. 8 Schematic diagrams for the formation of MQ[n].

A series of different carbohydrates and polysaccharides have been applied for the stabilisation of magnetic nanoparticles, and the formed composites were used for adsorption purposes. Thus, starch (4, Table 2)-stabilised magnetic nanoparticles (SSMNPs) were prepared by reacting FeCl₃·6H₂O with FeCl₂·4H₂O in 0, 0.025, 0.5, 0.1 and 0.5% w/v cassava waste water starch solution.⁵⁵ The SSMNPs were then applied in the removal of nickel ions from crude oil. The sample with the lowest starch concentration exhibited the strongest attraction and surface affinity for nickel complexes in crude oil and removed up to 93% of nickel complexes. Other important related compounds include derivatives of alginic acid 5; their magnetic calcium alginate hydrogel beads (m-CAHBs, 3.4 mm average diameter; Table 2) composed of γ-Fe₂O₃ nanoparticles and calcium alginate were prepared (Fig. 9).⁵⁶ These beads were used to develop a methodology to remove Cu(II) from aqueous solution by adsorption using m-CAHBs. The optimal conditions for the maximum percent removal were a pH of 2.0 and an adsorbent dosage of 2.0 g L⁻¹ for an initial Cu(II) ion concentration of 250 mg L⁻¹. The percent removal of Cu(II) on m-CAHBs could still be maintained at 73% at the fifth cycle. Compared to electrostatic interaction, chelation was found to be the more favourable adsorption mechanism of Cu(II). In addition, a spray gelation-based method was offered to synthesise a series of magnetically responsive hydrogel nanoparticles based on Fe₂O₃ and alginate–oligoguluronate for biomedical and drug delivery applications.⁵⁷ This method is based on the production of hydrogel nanoparticles from sprayed polymeric microdroplets obtained by an air-jet nebulisation process immediately followed by gelation in a crosslinking fluid (Fig. 10).

Table 2 Polymers and polysaccharides used as supports in magnetic sorbents

Starch
Alginic acid, also called algin or alginate
Chitosan {poly-β-(1,4)-2-amino-2-deoxy-D-glucose}
Poly(N-isopropylacrylamide)
Polyacrylic acid (PAA)
Poly(ethylene glycol)(700) diacrylate, (PEGDA)
Polyvinyl acetate (PVAC)
Cetyl trimethylammonium bromide, CTAB
Carboxymethylcellulose
Cellulose
Polyvinyl alcohol (PVA)

---

Fig. 9 Schematic presentation of the preparation process of m-CAHBs. Reproduced from ref. 56 with permission.

Fig. 10 (a) An illustration of the spray gelation-based method used in the development of the hydrogel nanoparticles. (b) Schematic illustration of the developed magnetically responsive hydrogel nanoparticles. Reproduced from ref. 57 with permission.

Chitosan (6, Table 2), one of the most common polysaccharides, is widely applied in nanotechnology despite its low solubility. A modified chitosan derivative that is cross-linked with glutaraldehyde and functionalised with magnetic nanoparticles (Fe₃O₄) was prepared for Hg(II) removal from aqueous solutions (Fig. 11).⁵⁸ The same Fe₃O₄ nanoparticles with different sizes (2 μm and 10 nm) were used for incorporation in chitosan beads (chitosan/Fe₃O₄ = from 4 : 1 to 1 : 1).⁵⁹ The obtained magnetic beads were modified by physical cross-linking with CuSO₄, chemical cross-linking with epichlorohydrin, or coating with a polyelectrolyte complex, chitosan/poly(2-acryloylamido-2-methylpropanesulfonic acid). The magnetic beads completely sorbed a model dye (reactive red) from its aqueous solution, implying that such materials might be used for wastewater treatment in the textile industry. In addition, with the aid of oil–chitosan composite spheres synthesised by the encapsulation of sunflower seed oil in chitosan droplets (Fig. 12), hydrophilic materials (iron oxide nanoparticles) and lipophilic materials (i.e., rhodamine B or epirubicin) were encapsulated.⁶⁰ The diameters of the prepared spheres were found to be approximately 2.5 mm (pure chitosan spheres), 2.3 mm (oil–chitosan composites), 1.5 mm (iron-oxide embedded oil–chitosan composites), and 1.7 mm (epirubicin and iron oxide encapsulated oil–chitosan composites). Epirubicin, a lipophilic drug, could be loaded in the spheres with an encapsulation rate of 72.25%. The lipophilic fluorescent dye rhodamine B was also loadable in the spheres, and red fluorescence was observed under a fluorescence microscope. In addition, a flat magnetic separator was used long ago to separate magnetic adsorbents with bioaffinity from litre volumes of suspensions.⁶¹ Both magnetic cross-linked erythrocytes and magnetic chitosan were efficiently separated; at least 95% adsorbent recovery was achieved at the maximum flow rate (1680 mL min⁻¹).

Fig. 11 Preparation of CS and CSm and possible interaction with Hg(II). Reproduced from ref. 58 with permission.

Fig. 12 Schematic of the preparation of oil–chitosan spheres. Reproduced from ref. 60 with permission.

4.5. Adsorbents based on polymers

Polymers can be considered as the most common supports for magnetic nanoparticles among organic compounds and are widely used for the creation of magnetic adsorbents. Thus, hydrogel nanocomposites were synthesised by the incorporation of superparamagnetic Fe₃O₄ particles in negative temperature-sensitive poly(N-isopropylacrylamide) (7, Table 2) hydrogels.⁶² Applying pulses of alternating magnetic field (AMF) resulted in uniform heating within the nanocomposites, leading to accelerated collapse and the squeezing out of large amounts of imbibed drug (faster release rate). These smart biomaterials promise numerous potential applications in externally actuated drug delivery systems for the release of drug molecules. As an example, the nanocomposite was incorporated as a valve in one of the channels of the device.⁶³ An AMF with a frequency of 293 kHz was then applied to the device, and ON-OFF control of flow was achieved (Fig. 13). A pressure transducer was placed at the inlet of the channel, and pressure measurements were carried out for multiple AMF ON-OFF cycles to evaluate the reproducibility of the valve. A closely related application of the same hydrogel was also reported for Fe₃O₄ nanoparticles embedded in microrobots, which can be used for propulsion and tracking in the human vascular network using an MRI platform.⁶⁴ The same magnetic nanoparticles can also be exploited to act as hyperthermic actuators. When embedded in an N-isopropylacrylamide thermo responsive hydrogel, vascular microrobots capable of changing their size to adapt to various blood vessel diameters could be synthesised. This type of hydrogel is not only able to reduce its size in response to temperature elevations, but, as shown above, can also be used to release possible therapeutic agents previously trapped within the hydrogel.

Fig. 13 Schematic of the concept of remote-controlled hydrogel nanocomposite valves with an alternating magnetic field (AMF). Application of the AMF results in collapse of the hydrogel, leading to opening of the valve. Reproduced from ref. 64 with permission.

Magnetic field-induced heating, as described above, was also explored for the purpose of developing a more effective way to recover water from swollen hydrogel draw agents.⁶⁵ The composite hydrogel particles were prepared by the copolymerisation of sodium acrylate and N-isopropylacrylamide in the presence of magnetic nanoparticles (γ-Fe₂O₃, size <50 nm). It was indicated that magnetic heating is an effective and rapid method for hydrogel dewatering because the heat was more uniformly generated throughout the draw agent particles (Fig. 14); thus, the dense skin layer commonly formed via conventional heating from the outside of the particle is minimised. Significantly enhanced liquid water recovery (53%) is achieved under magnetic heating, as opposed to only around 7% liquid water recovery obtained via convection heating. A magnetic nano-adsorbent was also developed long ago using superparamagnetic Fe₃O₄ nanoparticles (13.2 nm, spinel structure) as cores and polyacrylic acid (PAA, 8, Table 2) as ionic exchange groups.⁶⁶ The Fe₃O₄ magnetic nanoparticles were prepared by co-precipitating Fe²⁺ and Fe³⁺ ions in an ammonia solution and treating under hydrothermal conditions. PAA was covalently bound onto the magnetic nanoparticles via carbodiimide activation. The maximum weight ratio of PAA bound to the magnetic nanoparticles was revealed to be 0.12. The ionic exchange capacity of the resultant magnetic nano-adsorbents was estimated to be much higher than those of commercial ionic exchange resins. When the magnetic nano-adsorbents were used for the recovery of lysozyme, the adsorption/desorption of lysozyme was completed within 1 min due to the absence of pore-diffusion resistance.

Fig. 14 Schematic diagram of the effect of magnetic and conventional heating on the dewatering of nanocomposite polymer hydrogels being used as draw agents in the forward osmosis process. Reproduced from ref. 65 with permission.

Superparamagnetic hydrogel microparticles in spherical and non-spherical forms (γ-Fe₂O₃)⁶⁷ were prepared in the T-junction microfluidic channel (Fig. 15 and 16). The precursor (magnetic solution) consisted of poly(ethylene glycol) (700) diacrylate, (PEGDA, 9, Table 2), deionised water, and the ferrofluid (4 : 4 : 2 in volume) with Darocur 1173 and Irgacure 819 photoinitiators. Light mineral oil with 3% ABIL EM 90 served as a continuous phase. In addition, well-defined, magnetic shell cross-linked nanoparticles (MSCKs) with hydrodynamic diameters of ca. 70 nm were constructed through the co-assembly of amphiphilic block copolymers of PAA₂₀-b-PS₂₈₀ and oleic acid-stabilised magnetic iron oxide nanoparticles using tetrahydrofuran, N,N-dimethylformamide, and water, ultimately transitioning to a fully aqueous system (Fig. 17).⁶⁸ These well-defined nanoparticles showed an efficient oil sorption capacity of 10-fold their initial dry weight when introduced into an aqueous environment polluted with a complex crude oil. Once loaded, the crude oil-sorbed nanoparticles were easily isolated via the introduction of an external magnetic field.

Fig. 15 Schematic diagram of the T-junction microfluidic channel with aluminium reflectors for spherical and non-spherical magnetic hydrogel synthesis: sphere, disk, and plug. P_m indicates the input pressure for the hydrogel precursors (dispersed phase) and P_o the input pressure for the mineral oil (continuous phase). Reproduced from ref. 67 with permission.

Fig. 16 Optical images of (a) deformed spherical magnetic microhydrogels in the absence of the aluminium UV reflector and (b)–(e) collections of magnetic microhydrogels in the presence of the aluminium UV reflector: (b) spheres, (c) disks, (d) plugs, and (e) high resolution of the disks. The scale bars are (a)–(d) 30 mm and (e) 10 mm. Reproduced from ref. 67 with permission.

Fig. 17 Schematic representation of the construction of magnetic shell cross-linked (MSCK) nanoparticles. Reproduced from ref. 68 with permission.

A microsized (average size 1 μm, range 500 nm–2 μm) magnetic polymer adsorbent (MPA) coupled with metal chelating ligands of iminodiacetic acid (IDA) for the removal of Cu(II) ion was synthesised based on magnetite.⁶⁹ Fe₃O₄ nanoparticles (10 nm in size) were prepared via chemical co-precipitation and then coated with polyvinyl acetate (PVAC, 10, Table 2) via suspension polymerisation with vinyl acetate (VAC), yielding magnetite–PVAC (denoted as M-PVAC). Several sequential procedures including alcoholysis, epoxide activation, and IDA coupling were subsequently employed to introduce functional groups on the surfaces of super-paramagnetic M-PVAC particles, yielding magnetite–polyvinyl alcohol (M-PVAL), magnetite–polyvinyl propene oxide (M-PVEP), and magnetite–polyvinyl acetate–IDA (M-PVAC-IDA), respectively. Copper ions can be adsorbed from solution with the aid of M-PVAC-IDA; the adsorption capacity is maximised at a pH of 4.5 and decreases with decreasing pH. After removing Cu(II) ion at a moderate pH value of 4.5, the exhausted M-PVAC-IDA can be regenerated at low pH of 1.

4.6. Adsorbents based on biomolecules

A stable magnetic nanocomposite of collagen and superparamagnetic iron oxide nanoparticles (SPIONs, 10 nm) was prepared by a simple process utilising protein wastes from the leather industry.⁷⁰ This nanocomposite exhibited selective oil absorption and magnetic tracking ability, allowing it to be used in oil removal applications.

5. Ferrites as magnetic adsorbents

In addition to magnetic iron oxides, several metal ferrites have also been used as components in magnetic adsorbents, although less commonly than mono-metal iron-containing composites. As far back as 1997, fine powders of zinc ferrite nanoparticles (ZFN) ZnFe₂O₄ with an average particle size of 10 nm and an inversion parameter of 0.21 were synthesised by the aerogel procedure.⁷¹ Portions of the powders were calcined in air at 500 and 800 °C, while other portions were ball-milled for 10 h. The magnetic state of the as-produced and calcined samples is best described as disordered and highly dependent on temperature. The magnetic properties of the ball-milled sample are similar to those of ferrimagnetic MgFe₂O₄ powders with comparable grain size and inversion parameters. In a related report,⁷² monolithic zinc ferrite aerogels were produced by the epoxide addition sol–gel method (addition of propylene oxide to a 2-propanol solution of either the hydrated metal nitrate salts or the hydrated metal chloride salts, resulting in the formation of stable red-brown gels). All the synthesised aerogels exhibited low densities and high surface areas (340 m² g⁻¹). Annealing the aerogel at relatively lower temperatures (below 450 °C) compared to the above method yielded a highly crystalline porous material composed of nanometre-sized particles (Fig. 18). In addition, zinc–ferrite nanoparticles can be non-stoichiometric; thus, their properties were studied as a function of particle size.⁷³

Fig. 18 Photograph of ZnFe₂O₄ aerogel fabricated using Zn(NO₃)₂·6H₂O and Fe(NO₃)₃·9H₂O in 2-propanol. Reproduced from ref. 72 with permission.

Zinc ferrite nanoparticles that were surface modified with cetyl trimethylammonium bromide {11, Table 2} were applied⁷⁴ as an adsorbent for Direct Green 6 (DG6), Direct Red 31 (DR31) and Direct Red 23 (DR23) dyes. It was found that the dye adsorption onto ZFN–CTAB followed the Langmuir isotherm. It was concluded that as a magnetic adsorbent, ZFN–CTAB has a high dye adsorption capacity and might be a suitable alternative to remove dyes from coloured aqueous solutions. Nanocrystalline zinc ferrite nanoparticles supported on a silica aerogel porous matrix differ in size (in the range 4–11 nm) and inversion degree (from 0.4 to 0.2) compared to bulk zinc ferrite, which has a normal spinel structure.⁷⁵ The nanocomposites are superparamagnetic at room temperature, and the temperature of the superparamagnetic transition in the samples decreases with particle size; therefore, the superparamagnetic transition in the samples is mainly determined by the inversion degree rather than by the particle size, which would have the opposite effect on the blocking temperature. In addition, mixed-metal zinc-containing ferrite–silica adsorbents are also known; a ferrite (Ni_0.5Zn_0.5Fe₂O₄)–silica aerogel nanocomposite was found to be an ultra-superlight and highly porous material, and the magnetic property of ferrite was maintained in the aerogel.⁷⁶

Cobalt ferrite is also a widespread component in magnetic adsorbents, mainly as aerogels and more rarely as hydrogels. Thus, porous cobalt ferrite aerogel catalysts (the crystallite sizes range between 6.3 and 28.1 nm) were obtained by a 1,2-epoxide sol–gel process (by reacting cobalt and iron salts with propylene oxide (similarly to ZnFe₂O₄ above) in methanol, drying by supercritical carbon dioxide, and calcining between 200 and 800 °C) and investigated in the hydrolysis of 4-nitrophenyl phosphate.⁷⁷ The as-prepared aerogel surface exhibited M–OH groups that disappear after annealing, which enhances the spinel structure. The catalysts revealed high porosities that decrease as the annealing temperature increases. The catalytic properties of these sol–gel materials were found to be due to the existence of residual surface OH groups that did not undergo condensation. A hybrid hydrogel with CoFe₂O₄ NPs as cross-linker agents of carboxymethylcellulose (CMC, 12, Table 2) polymer was obtained with the aim of testing it as a system for controlled drug release.⁷⁸ The NPs were functionalised with (3-aminopropyl)-trimethoxysilane (APTMS) in order to introduce NH₂ groups on the surface (Fig. 19–21). The hybrid hydrogel combines the magnetic proprieties of CoFe₂O₄ nanoparticles and the typical behaviours of the hydrogel to create a new system that overcomes some of the drawbacks of NP use in the field of drug release. This system can be loaded with a large amount of drug and positioned near the target site. In a related report,⁷⁹ magnetic hybrid cellulose (13, Table 2) aerogels were prepared in two steps. First, cellulose hydrogel films were prepared from LiOH/urea solvent, and CoFe₂O₄ nanoparticles were then synthesised in the porous-structured cellulose scaffolds, which, after freeze-drying, gave CoFe₂O₄/cellulose magnetic aerogels. The porosities of the composite aerogels ranged from 78 to 52% with a pore size distribution of a few tens of nanometres. These hybrid aerogels showed improved mechanical strengths and superparamagnetic properties. Unlike solvent-swollen gels and ferrogels, the magnetic composite aerogels are lightweight and flexible and have high porosities. Because cellulose is sustainable and readily available in large quantities from plants (wood), this route is suitable for industrial-scale production and may be used with many types of nanoparticles, which will expand the fields of application of cellulose-based functional materials. As an example, freeze-dried bacterial cellulose nanofibril aerogels based on cobalt ferrite can be used as templates for making lightweight porous magnetic aerogels that can be compacted into a stiff magnetic nanopaper.⁸⁰

Fig. 19 Schematic representation of the functionalisation reaction of (3-aminopropyl)-trimethoxysilane (APTMS) with the surface of a nanoparticle (NP).

Fig. 20 Bare (left suspension) and silanized (right suspension) images of CoFe₂O₄ nanoparticles (0.02 mg mL⁻¹, pH 4). The instability of bare nanoparticles is clearly evident. Reproduced from ref. 78 with permission.

Fig. 21 Reaction scheme for the formation of carboxymethylcellulose (CMC) hybrid hydrogels. The reaction involves the formation of an amide bond between the carboxylic groups of CMC and the amine groups of NP–NH₂ (the cross-linker agent) in the presence of N-(3-dimethylaminopropyl)-N-ethylcarbodiimidehydrochloride (EDC) and N-hydroxysuccinimide (NHS).

Unusual nanostructures based on ferrite can be obtained by varying the CoFe₂O₄ concentration in supports. In this way, magnetic nanocomposite materials consisting of 5 and 10 wt% ultrasmall stoichiometric CoFe₂O₄ nanoparticles in a silica aerogel matrix were synthesised by a sol–gel method.⁸¹ The CoFe₂O₄-10 wt% sample showed many “needle-like” nanostructures with typical lengths of ≈10 nm and widths of ≈1 nm that frequently appeared to be attached to nanoparticles. These needle-like nanostructures were observed to contain layers with interlayer spacings of 0.33 ± 0.1 nm; this is consistent with Co silicate hydroxide, a known precursor phase in these nanocomposite materials.

Other ferrite-based magnetic adsorbents are rare. Thus, CuFe₂O₄/activated carbon (mass ratios of 1 : 1, 1 : 1.5 and 1 : 2) magnetic adsorbents that combine the adsorption features of activated carbon with the magnetic property and excellent catalytic properties of powdered CuFe₂O₄ were developed using a co-precipitation procedure.⁸² The prepared magnetic composites can be used to adsorb acid orange II (AO7, 14) in water and can then be easily separated from the medium by a magnetic technique. The magnetic phase present is spinel copper ferrite, and the presence of CuFe₂O₄ did not significantly affect the surface area and pore structure of the activated carbon. The composite has a much higher catalytic activity than that of activated carbon, which is attributed to the presence of CuFe₂O₄. Manganese ferrite nanospheres constructed by nanoparticles were synthesised in high yield via a general, one-step, and template-free solvothermal method.⁸³ The resulting sphere-like manganese ferrite particles had a porous structure with a narrow pore-size distribution. Their saturation magnetisation is high (the maximum saturation magnetisation value of the product is 75 emu g⁻¹), and they exhibit an excellent ability for the magnetic removal of chromium in wastewater.

6. Magnetic behaviour studies

In a predominant number of the above reports, the statement “the adsorbent (or catalyst) can be easily removed from reaction media after adsorption or completion of reactions via a simple magnet” appears frequently to indicate an advantage for possible applications of synthesised magnetic adsorbents. However, according to our experience, it is not always possible to effectively remove magnetic nanoparticles from solution using classic small magnets, especially in the case of ultrasmall particles that are well stabilised in dispersion. A large amount of time is frequently required, or the magnet force is insufficient for large volumes of dispersions. Sometimes, stronger magnets must be applied for effective separation, when even strong centrifugation does not allow for the complete process of precipitate formation. The use of Fe₃O₄ magnetic nanoparticles as recoverable adsorbents for lignin removal from aqueous solutions was investigated.⁸⁴ Magnetic separation was performed using a strong super magnet with 1.4 Tesla magnetic fields (2.5 × 5 × 5 cm). Another interesting “magnetic” study was carried out for micron-sized magnetic particles (Fe₃O₄) dispersed in a polyvinyl alcohol (PVA, 15, Table 2) hydrogel.⁸⁵ This multiferroic ferrogel (Fig. 22 and 23) combines the elastic properties of PVA gel and the magnetic properties of Fe₃O₄ particles. The response of the ferrogel (with Fe₃O₄ concentration in the range of 1–10 wt%) to the application of a static magnetic field (up to a maximum of 40 mT) was studied. It was shown that the extent of deflection depended strongly on the Fe₃O₄ content and the magnetic field strength (Fig. 24). For each Fe₃O₄ concentration, there existed a threshold value of magnetic field strength before a large deflection occurred. This implied that the ferrogel system can be used as an ‘ON-OFF’ type transducer. The threshold value decreases with increasing Fe₃O₄ content.

Fig. 22 Picture of a ferrogel made of PVA + Fe₃O₄. Reproduced from ref. 85 with permission.

Fig. 23 Flexibility of (PVA + Fe₃O₄) ferrogel. Reproduced from ref. 85 with permission.

Fig. 24 Concentration graded ferrogel, the concentrations are 25, 15 and 3 wt% of iron oxide in the three regions; the highest concentration is closest to the magnet. Reproduced from ref. 85 with permission.

An interesting finding was observed in the case of iron oxide in PEG-containing systems. The high binding affinity of poly(ethylene glycol)–gallol (PEG–gallol) {PEG, 16, Table 2} allows the freeze drying and re-dispersion of 92 nm iron oxide cores individually stabilised with 9 nm thick stealth coatings, yielding particle stability for at least 20 months.⁸⁶ Fig. 25 shows the prevention of particle agglomeration, even in the presence of a small external magnet, when the surfactant is used. For all the other interactions between suspended MNPs and magnets observed in a series of reports, the MNPs were attracted by the magnets.

Fig. 25 (a) The biotin–PEG(3400)-gallol/mPEG(550)–gallol dispersant layer surrounding the iron oxide nanoparticle cores is stable and thick enough to prevent particle agglomeration in the presence of a small external magnet, even after particles have been dispersed in PBS for more than 1 year. (b) In the absence of the dispersant layer, iron oxide cores agglomerate and thus sediment instantaneously upon approaching a small external magnet. Reproduced from ref. 86 with permission.

7. Environmental applications

The magnetic adsorbents discussed above have a host of applications in distinct areas of science and technology. As has been observed, they are able to adsorb a series of contaminants in water, for instance, trace amounts of Pb(II), Cd(II), Hg(II) and Cu(II) ions in environmental and biological samples, along with several organic compounds (lignin and various dyes). They can separate flavonoids, serve as catalysts, and take part in drug delivery systems for the release of drug molecules, in other biomedical applications⁸⁷ and, as has long been known, in the food industry.⁸⁸ Symmetric supercapacitors and asymmetric supercapacitors that use carbon xerogels with different porous textures as the negative electrode and manganese oxide as the positive electrode are also known.⁸⁹

Due to their efficiency in oil sorption, magnetic adsorbents could be useful in a nascent research area: applications of nanomaterials (in particular xerogels and aerogels) in the processes of oil extraction when oil spills (mixtures of petroleum with water) frequently occur.^90–92 This field of technology could have a brilliant future and needs to be discussed in more detail. Thus, a technology for producing a magnetic adsorbent for the removal of oil from wastewater and surface water bodies using controlled magnetic fields was proposed⁹³ using magnetite and degradable steel slag as the absorbent material. The slag consisted of CaO, SiO₂, Al₂O₃, Fe₂O₃, MnO, MgO, and Cr₂O₃. The weight ratio of slag : magnetite was 1 : (1.5–2.0), and the optimal particle size of the components was 70–100 microns. Another representative example is 3D macroporous Fe/C nanocomposites {Fe–C-1 ÷ 4 (Fig. 26),⁹⁴ which {depending on the diameter of polystyrene (PS) microsphere (0.67 ÷ 4.2) and pore size (0.35 ÷ 3.0)} were investigated as highly selective absorption materials for removing oils from the surface of water. The macroporous nanocomposites were synthesised by sintering a mixture of closely packed polystyrene microspheres and a ferric nitrate precursor (Fig. 27). These nanocomposites exhibited superhydrophobic and superoleophilic properties without the modification of low-surface-energy chemicals. The macroporous nanocomposites readily, quickly, and selectively absorbed a wide range of oils and hydrophobic organic solvents under a magnetic field (Fig. 28). The oil absorption capacity of these nanocomposites was found to be much higher than that of reported Fe₂O₃@C nanoparticles. Moreover, the nanocomposites retained their highly hydrophobic and oleophilic characteristics after repeatedly removing oils from the water surface for many cycles.⁹⁴

Fig. 26 SEM images of (a) Fe/C-1, (b) Fe/C-2, (c) Fe/C-3, and (d) Fe/C-4 samples; inset images are the water contact angles of the corresponding macroporous nanocomposites. Reproduced from ref. 94 with permission.

Fig. 27 Illustration for the synthesis of three-dimensionally macroporous Fe/C nanocomposites. Reproduced from ref. 94 with permission.

Fig. 28 Removal of lubricating oil from water surface by Fe/C-2 sample under magnetic field. The oil was dyed with oil red 24 for clear observation. (a) Initial oil; (b) oil after adding magnetic samples; (c) magnetic removal of oil; (d) water after magnetic oil removal. Reproduced from ref. 94 with permission.

Another iron–carbon composite, a magnetic carbon nanotube sponge (M-CNT sponge) with a porous structure consisting of interconnected CNTs with rich Fe encapsulation, was fabricated by CVD using ferrocene and dichlorobenzene as the precursors. It showed⁹⁵ (Fig. 29) a high mass sorption capacity for diesel oil that reached 56 g g⁻¹, corresponding to a volume sorption capacity of 99%. The sponges are mechanically strong, and oil can be squeezed out by compression (Fig. 30). They can be recycled through reclamation by magnetic force and desorption by simple heat treatment. The M-CNT sponges maintain the original structure, high capacity, and selectivity after 1000 sorption and reclamation cycles.

Fig. 29 Removal of spilled oil from water surface by M-CNT sponges under magnetic field: (a) optical image of a box of sponges with a volume of approximately 5 L; (b) SEM image of the porous CNT; (c) TEM image of a CNT filled with magnetic Fe nanowires; (d) oil (dyed blue) spreading on water; four M-CNT sponge blocks have been placed onto the oil; (e) clean water surface after complete oil absorption by the sponges; and (f) collecting the sponges using a magnet. Reproduced from ref. 95 with permission.

Fig. 30 Schematic of the recycling of M-CNT sponges used for spilled oil sorption. (I) Sprinkled on the oil; (II) adsorbed spilled oil; (III) collected by magnet; (IV) regeneration; and (V) reuse. Reproduced from ref. 95 with permission.

A composite material based on commercially available polyurethane foams (PU) functionalised with colloidal superparamagnetic iron oxide nanoparticles (SPIONs) and sub-micrometre polytetrafluoroethylene (PTFE) particles can efficiently separate oil from water (Fig. 31).^96,97 It was found that the combined functionalisation of the PTFE-treated foam surfaces with colloidal iron oxide nanoparticles significantly increases the speed of oil absorption. In addition to the water-repellent and oil-absorbing capabilities, the functionalised foams also exhibit magnetic responsivity. Finally, due to their light weight, they float easily on water. This low-cost process can easily be scaled up to clean large-area oil spills in water. In a related report,⁹⁸ ultralight magnetic Fe₂O₃/C, Co/C, and Ni/C foams (with densities <5 mg cm³) were fabricated (Fig. 32) on the centimetre scale by pyrolysing commercial polyurethane sponges grafted with polyelectrolyte layers based on the corresponding metal acrylate at 400 °C. After modification with low-surface-energy polysiloxane, the ultralight foams showed superhydrophobicity and superoleophilicity and quickly and selectively absorbed a variety of oils from a polluted water surface under magnetic field. The oil absorption capacity reached 100-times the weight of the foams itself, exhibiting one of the highest values among existing absorptive counterparts. In addition, a stable magnetic nanocomposite of collagen and SPIONs was prepared⁹⁹ by a simple process utilising protein wastes from the leather industry. This nanocomposite exhibited selective oil absorption and magnetic tracking ability, allowing it to be used in oil removal applications. The environmental sustainability of the oil adsorbing nanobiocomposite was also demonstrated through its conversion into a bifunctional graphitic nanocarbon material via heat treatment.

Fig. 31 (a) Prolonged water squirt on the PU/NPs/PTFE sample. (b1–4) Mixed oil (coloured with blue dye) and water drop are phase separated, and the oil is immediately absorbed while water remains on the surface. The arrows in (b2) represent the absorption of the oil. Time interval between frames (b2) and (b3) is less than 1 s. (c) Experimental setup used to determine the foams' oil-absorption capacity. The sample is kept in horizontal position by two needles, and the excess of oil lost through the foam is collected. Reproduced from ref. 96 with permission.

Fig. 32 Illustration of the fabrication of ultralight magnetic foams from a polyacrylic acid (PAA)-grafted polyurethane sponge. The foams were constructed from 3D interconnected microtubes having nanoscale wall thickness. Reproduced from ref. 98 with permission.

Discussing the economic aspect of using magnetic aerogels for oil clean, we note that although the aerogel is one of the prime candidates for the mitigation of oil spills, its high cost is a major inhibiting factor for its widespread adoption. Currently, the typical cost of aerogels produced by supercritical drying is about $2870 per kg, while aerogels made from discarded rice husks are reported to cost only $276 per kg, thereby making the latter a commercially viable proposition. Xerogels are of a lower cost, but their efficiency in oil clean-up is also lower.

Selected applications of magnetic adsorbents are shown in Table 3.

Table 3 Selected applications of magnetic adsorbents

Adsorbent composition	Application	Reference
Environmental applications
Sodium alginate-coated Fe3O4 nanoparticles (Alg-Fe3O4)	Removal of malachite green from aqueous solutions using batch adsorption technique	100
Mn-doped maghemite nanoparticles	Removal of As(V) and As(III) from water	101
Shell–core structured Fe3O4/MnO2 magnetic adsorbent	Pb(II) removal from aqueous solutions	102
Carboxymethyl-β-cyclodextrin (CM-β-CD) polymer modified Fe3O4 nanoparticles (CDpoly-MNPs)	Selective removal of Pb^2+, Cd^2+, Ni^2+ from wastewater	103
Magnetic Fe–Al binary oxide adsorbent	Chromate removal from aqueous solution with permanent and superconducting magnets (superconducting magnetic separation)	104
Magnetite–porphyrin nanocomposite	Removal of heavy cations	105
Polystyrene–magnetite composite	Solid-phase extraction of uranium(VI) from aqueous nitrate solutions	106
Cobalt ferrite hollow spheres	Extraction of uranium(VI) ions from aqueous solutions	107
Fe3O4-embedded graphene oxide	Removal of methylene blue from aqueous solution	108
Magnetic-activated coke incorporated with Fe3O4 particles	Removal of organic materials from 2,4,6-trinitrotoluene red water	109
Fe3O4/Au composites	Extraction of benzo[a]pyrene from aqueous solution	110
Activated carbon (PAC)/iron oxide composite	Removal of surfactants from water	111
Magnetic mesoporous silica microspheres (Fe3O4@mSiO2)	Fast and convenient enrichment of toxic trace-level microcystin-LR in water	112
Graphene grafted silica-coated Fe3O4 (Fe3O4@SiO2-G)	Extraction of four neonicotinoid pesticides (thiamethoxam, imidacloprid, acetamiprid and thiacloprid) from pear and tomato samples	113
Analytical applications
Fe3O4/C/PANI (polyaniline) microbowls	Determination of five pyrethroids in tea drinks (cyhalothrin, beta-cypermethrin, esfenvalerate, permethrin and bifenthrin)	114
Hybrid materials based on magnetic Fe3O4 nanoparticles and synthetic macrocyclic receptor, carboxylato-pillar[5]arene	Extraction adsorbent for pesticide residue analysis in beverage samples	115
Magnetite/poly(styrene-divinylbenzene) nanoparticles	Adsorbent for enrichment-determination of fenitrothion	116
Adsorbent based on 2,2′-thiodiethanethiol grafted with tetra-Et orthosilicate-modified Fe3O4 nanoparticles	Separation and preconcentration of Hg, Pb, and Cd in environmental and food samples	117
Ionic liquid-coated Fe3O4@graphene nanocomposite	Determination of five nitrobenzene compounds	118
Biomedical applications
Magnetite/graphene oxide/chitosan (Fe3O4/GO/CS) composite	Protein adsorption	119
Adsorbent based on nickel nanoparticle decorated graphene	Separation/isolation of His6-tagged recombinant proteins from a complex sample matrix (cell lysate)	120
MWCNTs/Fe3O4 nanocomposite	Extraction of fluoxetine from human urine samples	121
Adsorbent based on multi-walled carbon nanotubes and iron oxide nanoparticles	Removal of amoxicillin	122
Cetyltrimethylammonium bromide adsorbed on the surface of Fe3O4 nanoparticles	Extraction and preconcentration of ultra-trace amounts of mefenamic acid in biologic fluids	123

---

8. Conclusions

Magnetic nano- and micro-adsorbents are based on magnetic nanoparticles (mainly iron oxides and ferrites) and supporting agents that consist of inorganic (carbon, graphene, silica, zeolites) or organic (macromolecules, polysaccharides, polymers, biomolecules) compounds. In the last decade, considerable attention has been paid to magnetic adsorbents in the form of hydrogels, aerogels,¹²⁴ and xerogels due to their numerous applications in the removal of heavy metals,^125–132 the degradation of dyes and other organic compounds,¹³³ the separation of oil and water mixtures, and in several biomedical fields.

In these composites, the same chemical compound, being in distinct morphological forms, could play different roles. For example, in the mesoporous composite γ-Fe₂O₃/α-Fe₂O₃/CA, structures have ferrimagnetic properties due to their γ-Fe₂O₃ component, and they also have photocatalytic properties because of their antiferromagnetic α-Fe₂O₃ component.

The main advantage for the possible applications of synthesised magnetic adsorbents is that the adsorbent (or catalyst, if it is used in organic reactions in this role) can be easily removed from the reaction medium after adsorption or reaction completion via a simple magnet. In combination with polymers, ferrogels combine the elastic properties of polymer gels and the magnetic properties of iron oxide particles. It is expected that applications of magnetic polymer–nanoparticle composites as well as others described above will be expanded in the near future, in particular for environmental clean-up purposes.

Acknowledgements

OVK and BIK are grateful to the CONACYT-Mexico for financial support.

References

1. A. K. Haghi, Foundations of Nanotechnology, Volume One: Pore Size in Carbon-Based Nano-Adsorbents: 1 (AAP Research Notes on Nanoscience and Nanotechnology), Apple Academic Press, 1 edn, 2014, p. 278.
2. S. Nasreen and U. Rafique, Novel Adsorbents for Removal of Aqueous Pollutants: Nano-Synthesis: Silica and Alumina: Meso and Nano-Adsorbents, LAP LAMBERT Academic Publishing, 2012, p. 96.
3. D. E. Reisner and T. Pradeep, Aquananotechnology: Global Prospects, CRC Press, 2014, p. 863.
4. A. Hu and A. Apblett, Nanotechnology for Water Treatment and Purification (Lecture Notes in Nanoscale Science and Technology), Springer, 2014, p. 373.
5. D. Aditya, P. Rohan and G. Suresh, Nano-adsorbents for Wastewater Treatment: A Review, Res. J. Chem. Environ., 2011, 15(2), 1033.
6. Y. C. Sharma, V. Srivastava, V. K. Singh, S. N. Kaul and C. H. Weng, Nano-adsorbents for the removal of metallic pollutants from water and wastewater, Environ. Technol., 2009, 30(6), 583–609.
7. M. Khajeh, S. Laurent and K. Dastafkan, Nanoadsorbents: Classification, Preparation, and Applications (with Emphasis on Aqueous Media), Chem. Rev., 2013, 113(10), 7728–7768,  DOI:10.1021/cr400086v.
8. E. Mosiniewicz-Szablewska, M. Safarikova and I. Safarik, Magnetically modified biological materials as perspective adsorbents for large-scale magnetic separation processes, Horiz. World Phys., 2010, 266(Applied Physics in the 21st Century), 301–318.
9. S. Liu, X. Luo and J. Zhou, Magnetic Responsive Cellulose Nanocomposites and Their Applications, in Cellulose - Medical, Pharmaceutical and Electronic Applications, ed. Theo van de Ven and Louis Godbout, InTech, 2013.
10. M. Franzreb, M. Siemann-Herzberg, T. J. Hobley and O. R. T. Thomas, Protein purification using magnetic adsorbent particles, Appl. Microbiol. Biotechnol., 2006, 70, 505–516.
11. R. Kasemann and H. K. Schmitd, A new type of a sol-gel derived inorganic-organic nanocomposite, Mater. Res. Soc. Symp. Proc., 1994, 346, 915–921.
12. Y. X. Gan, Structural assessment of nanocomposites., Micron, 2012, 43, 782–817.
13. N. Della Santina Mohallem, J. Batista Silva, G. L. Tacchi Nascimento and V. L. Guimarães, Study of Multifunctional Nanocomposites Formed by Cobalt Ferrite Dispersed in a Silica Matrix Prepared by Sol-Gel Process, in Nanocomposites - New Trends and Developments, ed. F. Ebrahimi, InTech, 2012.
14. L. C. Cotet, C. I. Fort, V. Danciu and A. Maicaneanu, Cu and Cd Adsorption on Carbon Aerogel and Xerogel, E3S Web Conf., 2013, 1, 25007.
15. N. Della Santina Mohallem, J. Batista Silva, G. L. Tacchi Nascimento and V. L. Guimarães, Study of Multifunctional Nanocomposites Formed by Cobalt Ferrite Dispersed in a Silica Matrix Prepared by Sol-Gel Process, in Nanocomposites - New Trends and Developments, ed. Farzad Ebrahimi, InTech, 2012.
16. J. L. Gurav, I.-K. Jung, H.-H. Park, E. S. Kang and D. Y. Nadargi, Silica Aerogel: Synthesis and Applications, Journal of Nanomaterials, 2010, 11.
17. Z. Liu, H. Wang, C. Liu, Y. Jiang, G. Yu, X. Mu and X. Wang, Magnetic cellulose-chitosan hydrogels prepared from ionic liquids as reusable adsorbent for removal of heavy metal ions, Chem. Commun., 2012, 48, 7350–7352.
18. S. Kyung Suh, S. C. Chapin, T. Alan Hatton and P. S. Doyle, Synthesis of magnetic hydrogel microparticles for bioassays and tweezer manipulation in microwells, Microfluid. Nanofluid., 2012, 13, 665–674.
19. F. Xu, C. Max Wu, V. Rengarajan, T. Dylan Finley, H. Onur Keles, Y. Sung, B. Li, U. A. Gurkan and U. Demirci, Three-Dimensional Magnetic Assembly of Microscale Hydrogels, Adv. Mater., 2011, 23, 4254–4260.
20. A. Vashist and S. Ahmad, Hydrogels: Smart Materials for Drug Delivery, Orient. J. Chem., 2013, 29(3), 861–870.
21. M. Zrínyi, L. Barsi and A. Büki, Ferrogel: a new magneto-controlled elastic medium, Polym. Gels Networks, 1997, 5(5), 415–427.
22. M. Venkatesan and P. Pazhanisamy, Synthesis and Dye Adsorption Studies of Poly (N-tert-butylacrylamide-co-acrylamide/Maleic acid), Ferrogel, Ind. J. Res., 2014, 3(5), 20–22.
23. T. M. Petrova, L. Fachikov and J. Hristov, The Magnetite as Adsorbent for Some Hazardous Species from Aqueous Solutions: a Review, Int. Rev. Chem. Eng., 2012, 3(2), 134–152.
24. S. Zhou, D. Wang, H. Sun, J. Chen, S. Wu and P. Na, Synthesis, Characterization, and Adsorptive Properties of Magnetic Cellulose Nanocomposites for Arsenic Removal, Water, Air, Soil Pollut., 2014, 225(5), 1–13.
25. X. Yang, X. Shen, M. Jing, R. Liu, Y. Lu and J. Xiang, Removal of heavy metals and dyes by supported nano zero-valent iron on barium ferrite microfibers, J. Nanosci. Nanotechnol., 2014, 14(7), 5251–5257.
26. H. Jabeen, K. C. Kemp and V. Chandra, Synthesis of nano zerovalent iron nanoparticles - Graphene composite for the treatment of lead contaminated water, J. Environ. Manage., 2013, 130, 429–435.
27. Z. Wang, X. Liu, M. Lv and J. Meng, A new kind of mesoporous Fe₇Co₃/carbon nanocomposite and its application as magnetically separable adsorber, Mater. Lett., 2010, 64(10), 1219–1221.
28. Z. Wang, R. Liu, F. Zhao, X. Liu, M. Lv and J. Meng, Facile Synthesis of Porous Fe₇Co₃/Carbon Nanocomposites and Their Applications as Magnetically Separable Adsorber and Catalyst Support, Langmuir, 2010, 26(12), 10135–10140.
29. D. A. S. Costa, R. V. Mambrini, L. E. Fernandez-Outon, W. A. A. Macedo and F. C. C. Moura, Magnetic adsorbent based on cobalt core nanoparticles coated with carbon filaments and nanotubes produced by chemical vapor deposition with ethanol, Chem. Eng. J., 2013, 229, 35–41.
30. Z. Jiang, J. Xie, D. Jiang, Z. Yan, J. Jing and D. Liu, Enhanced adsorption of hydroxyl contained/anionic dyes on non functionalized Ni@SiO2 core-shell nanoparticles: Kinetic and thermodynamic profile, Appl. Surf. Sci., 2014, 292, 301–310.
31. Y. Wang, G. Zhao, S. Chai, H. Zhao and Y. Wang, Three-Dimensional Homogeneous Ferrite-Carbon Aerogel: One Pot Fabrication and Enhanced Electro-Fenton Reactivity, ACS Appl. Mater. Interfaces, 2013, 5, 842–852.
32. S. H. Hsu, Y.-F. Lin, T. W. Chung, T.-Y. Wei, S.-Y. Lu, K.-L. Tunga and K.-T. Liu, Mesoporous carbon aerogel membrane for phospholipid removal from Jatropha curcas oil, Sep. Purif. Technol., 2013, 109, 129–134.
33. Y.-F. Lin and C. Y. Chang, Magnetic mesoporous iron oxide/carbon aerogel photocatalysts with adsorption ability for organic dye removal, RSC Adv., 2014, 4, 28628–28631.
34. X. Wu and W. Jia, Biomass-derived multifunctional magnetite carbon aerogel nanocomposites for recyclable sequestration of ionizable aromatic organic pollutants, Chem. Eng. J., 2014, 245, 210–216.
35. H.-P. Cong, X.-C. Ren, P. Wang and S.-H. Yu, Macroscopic Multifunctional Graphene-Based Hydrogels and Aerogels by a Metal Ion Induced Self-Assembly Process, ACS Nano, 2012, 6(3), 2693–2703.
36. C. Cannas, M. F. Casula, G. Concas, A. Corrias, D. Gatteschi, A. Falqui, A. Musinu, C. Sangregorioc and G. Spano, Magnetic properties of γ-Fe₂O₃–SiO₂ aerogel and xerogel nanocomposite materials, J. Mater. Chem., 2001, 11, 3180–3187.
37. M. B. Fernández van Raap, F. H. Sánchez, C. E. Rodríguez Torres, L. Casas, A. Roig and E. Molins, Detailed magnetic dynamic behaviour of nanocomposite iron oxide aerogels, J. Phys.: Condens. Matter, 2005, 17, 6519–6531.
38. J. S. Lee, E. J. Lee and H. J. Hwang, Synthesis of Fe₃O₄-coated silica aerogel nanocomposites, Trans. Nonferrous Met. Soc. China, 2012, 22, s702–s706.
39. B. Zhang, J. M. Xing, Y. Q. Lang and H. Z. Liu, Synthesis of amino-silane modified magnetic silica adsorbents and application for adsorption of flavonoids from Glycyrrhiza uralensis Fisch, Sci. China, Ser. B: Chem., 2008, 51(2), 145–151.
40. O. V. Kharissova, H. V. Rasika Dias, B. I. Kharisov, B. O. Pérez and V. M. Jiménez Pérez, The greener synthesis of nanoparticles, Trends Biotechnol., 2013, 31(4), 240–248.
41. H. Bagheri, A. Afkhami, M. Saber-Tehrani and H. Khoshsafar, Preparation and characterization of magnetic nanocomposite of Schiff base/silica/magnetite as a preconcentration phase for the trace determination of heavy metal ions in water, food and biological samples using atomic absorption spectrometry, Talanta, 2012, 97, 87–95.
42. S. Wang and Y. Peng, Natural zeolites as effective adsorbents in water and wastewater treatment, Chem. Eng. J., 2010, 156(1), 11–24.
43. J. C. White and P. K. Dutta, Assembly of Nanoparticles in Zeolite Y for the Photocatalytic Generation of Hydrogen from Water, J. Phys. Chem. C, 2011, 115, 2938–2947.
44. K. Shameli, M. Bin Ahmad, M. Zargar, W. M. Z. W. Yunus and N. Azowa Ibrahim, Fabrication of silver nanoparticles doped in the zeolite framework and antibacterial activity, Int. J. Nanomed., 2011, 6, 331–341.
45. S. K. Kulshreshtha, R. Vijayalakshmi, V. Sudarsan, H. G. Salunke and S. C. Bhargava, Iron oxide nanoparticles in NaA zeolite cages, Solid State Sci., 2013, 21, 44–50.
46. K. Ulucan, C. Noberi, T. Coskun, C. Bulent Ustundag, E. Debik and C. Kaya, Disinfection By-Products Removal by Nanoparticles Sintered in Zeolite, J. Clean Energy Technol., 2013, 1(2), 120–123.
47. T. M. Salem Attia, X. Lin Hu and D. Qiang Yin, Synthesized magnetic nanoparticles coated zeolite for the adsorption of pharmaceutical compounds from aqueous solution using batch and column studies, Chemosphere, 2013, 93(9), 2076–2085.
48. D. A. Fungaro, M. Yamaura and G. R. Craesmeyer, Uranium Removal from Aqueous Solution by Zeolite from Fly Ash-Iron Oxide Magnetic Nanocomposite, Int. Rev. Chem. Eng., 2012, 4(3), 353–358.
49. M. Yamaura and M. Alves Fungaro, Synthesis and characterization of magnetic adsorbent prepared by magnetite nanoparticles and zeolite from coal fly ash, J. Mater. Sci., 2013, 48(14), 5093–5101.
50. R. Bosinceanu and N. Sulitanu, Synthesis and characterization of FeO(OH)/Fe₃O₄ nanoparticles encapsulated in zeolite matrix, J. Optoelectron. Adv. Mater., 2008, 10(12), 3482–3486.
51. S. B. C. Pergher, L. C. Oliveira, A. Smaniotto and D. I. Petkowicz, Magnetic zeolites for removal of metals in water, Quim. Nova, 2005, 28(5), 751–755.
52. X. Chen, K. F. Lam, Q. Zhang, B. Pan, M. Arruebo and K. L. Yeung, Synthesis of Highly Selective Magnetic Mesoporous Adsorbent, J. Phys. Chem. C, 2009, 113, 9804–9813.
53. Q. Yang, Y. Jiang, X. Li, Y. Yang and L. Hu, Magnetic-supported cucurbituril: A recyclable adsorbent for the removal of humic acid from simulated water, Bull. Mater. Sci., 2014, 37(5), 1167–1174.
54. E. Masson, X. Ling, R. Joseph, L. Kyeremeh-Mensah and X. Lu, Cucurbituril chemistry: a tale of supramolecular success, RSC Adv., 2012, 2, 1213–1247.
55. J. L. Konne and K. Okpara, Remediation of Nickel from Crude Oil Obtained from Bomu Oil Field Using Cassava Waste Water Starch Stabilized Magnetic Nanoparticles, Energy Environ. Res., 2014, 4(1), 25–31.
56. H. Zhu, Y. Fu, R. Jiang, J. Yao, L. Xiao and G. Zeng, Optimization of Copper(II) Adsorption onto Novel Magnetic Calcium Alginate/Maghemite Hydrogel Beads Using Response Surface Methodology, Ind. Eng. Chem. Res., 2014, 53, 4059–4066.
57. I. M. El-Sherbiny and H. D. C. Smyth, Smart Magnetically Responsive Hydrogel Nanoparticles Prepared by a Novel Aerosol-Assisted Method for Biomedical and Drug Delivery Applications, Journal of Nanomaterials, 2011, 13.
58. G. Z. Kyzas and E. A. Deliyanni, Mercury(II) Removal with Modified Magnetic Chitosan Adsorbents, Molecules, 2013, 18, 6193–6214.
59. D. Paneva, O. Stoilova, N. Manolova and I. Rashkov, Magnetic hydrogel beads based on chitosan, e-Polymers, 2004, 4(1), 673–683.
60. K. S. Huang, C.-Y. Wang, C.-H. Yang, A. Mihai Grumezescu, Y.-S. Lin, C. P. Kung, I.-Y. Lin, Y.-C. Chang, W.-J. Weng and W.-T. Wang, Synthesis and Characterization of Oil-Chitosan Composite Spheres, Molecules, 2013, 18, 5749–5760.
61. I. Šafarık, L. Ptackov and M. Šafarıkova, Laboratory of Biochemistry and Microbiology, Institute of Landscape Large-scale separation of magnetic bioaffinity adsorbents, Biotechnol. Lett., 2001, 23, 1953–1956.
62. N. S. Satarkar and J. Zach Hilt, Magnetic hydrogel nanocomposites for remote controlled pulsatile drug release, J. Controlled Release, 2008, 130, 246–251.
63. N. S. Satarkar, W. Zhang, R. E. Eitel and J. Zach Hilt, Magnetic hydrogel nanocomposites as remote controlled microfluidic valves, Lab Chip, 2009, 9, 1773–1779.
64. S. N. Tabatabaei, J. Lapointe and S. Martel, Magnetic Nanoparticles Encapsulated in Hydrogel as Hyperthermic Actuators for Microrobots Designed to Operate in the Vascular Network, Intelligent Robots and Systems, (IROS 2009. IEEE/RSJ International Conference on), 2009, pp. 546–551,  DOI:10.1109/iros.2009.5354162.
65. A. Razmjou, M. Reza Barati, G. P. Simon, K. Suzuki and H. Wang, Fast Deswelling of Nanocomposite Polymer Hydrogels via Magnetic Field-Induced Heating for Emerging FO Desalination, Environ. Sci. Technol., 2013, 47, 6297–6630.
66. M.-H. Liao and D.-H. Chen, Preparation and characterization of a novel magnetic nano-adsorbent, J. Mater. Chem., 2002, 12, 3654–3659.
67. D. K. Hwang, D. Dendukuri and P. S. Doyle, Microfluidic-based synthesis of non-spherical magnetic hydrogel microparticles, Lab Chip, 2008, 8, 1640–1647.
68. A. Pavia-Sanders, S. Zhang, J. A. Flores, J. E. Sanders, J. E. Raymond and K. L. Wooley, Robust Magnetic/Polymer Hybrid Nanoparticles Designed for Crude Oil Entrapment and Recovery in Aqueous Environments, ACS Nano, 2013, 7(9), 7552–7561.
69. J.-Y. Tseng, C.-Y. Chang, Y. H. Chen, C. F. Chang and P. C. Chiang, Synthesis of micro-size magnetic polymer adsorbent and its application for the removal of Cu(II) ion, Colloids Surf., A, 2007, 295, 209–216.
70. P. Thanikaivelan, N. T. Narayanan, B. K. Pradhan and P. M. Ajayan, Collagen based magnetic nanocomposites for oil removal applications, Sci. Rep., 2012, 2(230), 7.
71. H. H. Hamdeh, J. C. Ho, S. A. Oliver, R. J. Willey, G. Oliveri and G. Busca, Magnetic properties of partially-inverted zinc ferrite aerogel powders, J. Appl. Phys., 1997, 81, 1851.
72. P. Brown and L. J. Hope-Weeks, The synthesis and characterization of zinc ferrite aerogels prepared by epoxide addition, J. Sol-Gel Sci. Technol., 2009, 51, 238–243.
73. D. Makovec and M. Drofenik, Non-stoichiometric zinc-ferrite spinel nanoparticles, J. Nanopart. Res., 2008, 10, 131–141.
74. D. M. Mahmoodi, J. Abdi and D. Bastani, Direct dyes removal using modified magnetic ferrite nanoparticle, J. Environ. Health Sci. Eng., 2014, 12(96), 10.
75. S. Bullita, A. Casu, M. F. Casula, G. Concas, F. Congiu, A. Corrias, A. Falqui, D. Lochea and C. Marrasa, ZnFe₂O₄ nanoparticles dispersed in a highly porous silica aerogel matrix: a magnetic study, Phys. Chem. Chem. Phys., 2014, 16, 4843–4852.
76. N. Katagiri, N. Adachi and T. Ota, Preparation of a magnetic aerogel from ferrite-silica nanocomposite, ChemXpress, 2014, 4(2), 221–227.
77. J. Akl, T. Ghaddar, A. Ghanem and H. El-Rassy, Cobalt ferrite aerogels by epoxide sol–gel addition: Efficient catalysts for the hydrolysis of 4-nitrophenyl phosphate, J. Mol. Catal. A: Chem., 2009, 312, 18–22.
78. G. Giani, S. Fedi and R. Barbucci, Hybrid Magnetic Hydrogel: A Potential System for Controlled Drug Delivery by Means of Alternating Magnetic Fields, Polymers, 2012, 4, 1157–1169.
79. S. Liu, Q. Yan, D. Tao, T. Yu and X. Liu, Highly flexible magnetic composite aerogels prepared by using cellulose nanofibril networks as templates, Carbohydr. Polym., 2012, 89, 551–557.
80. R. T. Olsson, M. A. S. Azizi Samir, G. Salazar-Alvarez, L. Belova, V. Strom, L. A. Berglund, O. Ikkala, J. Nogues and U. W. Gedde, Making flexible magnetic aerogels and stiff magnetic nanopaper using cellulose nanofibrils as templates, Nat. Nanotechnol., 2010, 5, 584–588.
81. A. Falqui, A. Corrias, P. Wang, E. Snoeck and G. Mountjoy, A Transmission Electron Microscopy Study of CoFe₂O₄ Ferrite Nanoparticles in Silica Aerogel Matrix Using HREM and STEM Imaging and EDX Spectroscopy and EELS, Microsc. Microanal., 2010, 16, 200–209.
82. G. Zhang, J. Qu, J. Liu, A. T. Cooper and R. Wu, CuFe₂O₄/activated carbon composite: A novel magnetic adsorbent for the removal of acid orange II and catalytic regeneration, Chemosphere, 2007, 68, 1058–1066.
83. L.-X. Yang, F. Wang, Y.-F. Meng, Q.-H. Tang and Z.-Q. Liu, Fabrication and Characterization of Manganese Ferrite Nanospheres as a Magnetic Adsorbent of Chromium, Journal of Nanomaterials, 2013, 5.
84. S. M. Mostashari, S. Shariati and M. Manoochehri, Lignin removal from aqueous solutions using Fe₃O₄ magnetic nanoparticles as recoverable adsorbent, Cellul. Chem. Technol., 2013, 47(9–10), 727–734.
85. R. V. Ramanujan and L. L. Lao, The mechanical behavior of smart magnet–hydrogel composites, Smart Mater. Struct., 2006, 15, 952–956.
86. E. Amstad, S. Zurcher, A. Mashaghi, J. Y. Wong, M. Textor and E. Reimhult, Surface Functionalization of Single Superparamagnetic Iron Oxide Nanoparticles for Targeted Magnetic Resonance Imaging, Small, 2009, 5(11), 1334–1342.
87. Y. Li, G. Huang, X. Zhang, B. Li, Y. Chen, T. Lu, T. J. Lu and F. Xu, Magnetic Hydrogels and Their Potential Biomedical Applications, Adv. Funct. Mater., 2013, 23, 660–672.
88. D. R. Dixon, Magnetic adsorbents: Properties and applications, J. Chem. Technol. Biotechnol., 1980, 30(1), 572–578.
89. F. Lufrano, P. Staiti, E. G. Calvo, E. J. Juárez-Pérez, J. A. Menéndez and A. Arenillas, Carbon Xerogel and Manganese Oxide Capacitive Materials for Advanced Supercapacitors, Int. J. Electrochem. Sci., 2011, 6, 596–612.
90. Mahajan, Y. R., Nanotechnology-based solutions for oil spills. Nanotechnology-based Solutions for Oil Spills, Nanotech Insights, 2011, 2 (1), http://www.nanowerk.com/spotlight/spotid=20215.php.
91. T. Viswanathan, Use of magnetic carbon composites from renewable resource materials for oil spill clean up and recovery, WO 2011008315 A1, 2011.
92. Apblett, A.W.; Al-Fadul, S.M.; Trad, T., Removal of petrochemicals from water using magnetic filtration. The 8th International Petroleum Environmental Conference, November 6-9, 2001, Houston, TX. http://ipec.utulsa.edu/Conf2001/apblett_101.pdf.
93. Y. K. Rubanov, Y. F. Tokach, O. E. Kustitckaya and A. I. Vezentcev, Intensification of Adsorption Processes of Hydrocarbons Extraction from Aqueous Media by the Controlled Magnetic Field, Middle East J. Sci. Res., 2013, 18(10), 1473–1478.
94. Y. Chu and Q. Pan, Three-Dimensionally Macroporous Fe/C Nanocomposites As Highly Selective Oil-Absorption Materials, ACS Appl. Mater. Interfaces, 2012, 4(5), 2420–2425.
95. X. Gui, Z. Zeng, Z. Lin, Q. Gan, R. Xiang, Y. Zhu, A. Cao and Z. Tang, Magnetic and Highly Recyclable Macroporous Carbon Nanotubes for Spilled Oil Sorption and Separation, ACS Appl. Mater. Interfaces, 2013, 5, 5845–5850.
96. P. Calcagnile, D. Fragouli, I. S. Bayer, G. C. Anyfantis, L. Martiradonna, P. D. Cozzoli, R. Cingolani and A. Athanassiou, Magnetically Driven Floating Foams for the Removal of Oil Contaminants from Water, ACS Nano, 2012, 6(6), 5413–5419.
97. http://www.nanoparticles-microspheres.com/water-treatment-nanoparticles.html.
98. N. Chen and Q. Pan, Versatile Fabrication of Ultralight Magnetic Foams and Application for Oil-Water Separation, ACS Nano, 2013, 7(8), 6875–6883.
99. P. Thanikaivelan, N. T. Narayanan, B. K. Pradhan and P. M. Ajayan, Collagen based magnetic nanocomposites for oil removal applications, Sci. Rep., 2012, 2, 230.
100. A. Mohammadi, H. Daemi and M. Barikani, Fast removal of malachite green dye using novel superparamagnetic sodium alginate-coated Fe₃O₄ nanoparticles, Int. J. Biol. Macromol., 2014, 69, 447–455.
101. L. Yu, X. Peng, J. Li, F. Ni and Z. Luan, Removal of As (V) and As(III) from water using Mn-doped maghemite nanoparticles, Fresenius Environ. Bull., 2014, 23(2a), 508–515.
102. X. Zhang, J. Chen, J. Han and G. Zhang, Preparation and evaluation of shell-core structured Fe₃O₄/MnO₂ magnetic adsorbent for Pb(II) removal from aqueous solutions, Huanjing Kexue Xuebao, 2013, 33(10), 2730–2736.
103. A. Z. M. Badruddoza, Z. B. Z. Shawon, W. J. D. Tay, K. Hidajat and M. S. Uddin, Fe₃O₄/cyclodextrin polymer nanocomposites for selective heavy metals removal from industrial wastewater, Carbohydr. Polym., 2013, 91(1), 322–332.
104. D.-w. Ha, Y.-J. Lee, J.-M. Kwon, H.-J. Hong, R.-K. Ko, M.-H. Sohn, S.-K. Baik and Y.-H. Kim, Preparation of magnetic Fe-Al binary oxide and its application in superconducting magnetic separation, IEEE Trans. Appl. Supercond., 2013, 23(3, Pt. 2), 3700304-1–3700304-4.
105. S. Bakhshayesh and H. Dehghani, Synthesis of magnetite-porphyrin nanocomposite and its application as a novel magnetic adsorbent for removing heavy cations, Mater. Res. Bull., 2013, 48(7), 2614–2624.
106. M. G. Mahfouz, H. M. Killa, M. E. Sheta, A. H. Moustafa and A. A. Tolba, Synthesis, characterization, and application of polystyrene adsorbents containing tri-n-butylphosphate for solid-phase extraction of uranium(VI) from aqueous nitrate solutions, J. Radioanal. Nucl. Chem., 2014, 301(3), 739–749.
107. J. Wei, X. Zhang, Q. Liu, Z. Li, L. Liu and J. Wang, Magnetic separation of uranium by CoFe₂O₄ hollow spheres, Chem. Eng. J., 2014, 241, 228–234.
108. C. Zhou, Z. Wenjie, W. Huixian, H. Li, J. Zhou, S. Wang, J. Liu, J. Luo, B. Zou and J. Zhou, Preparation of Fe₃O₄-Embedded Graphene Oxide for Removal of Methylene Blue, Arabian J. Sci. Eng., 2014, 39(9), 6679–6685.
109. P. Hu, Y. Zhang, K. Tong, F. Wei, Q. An, X. Wang, P. K. Chu and F. Lv, Removal of organic pollutants from red water by magnetic-activated coke, Desalin. Water Treat., 2014 , http://www.tandfonline.com/doi/abs/10.1080/19443994.2014.903204?journalCode=tdwt20, ahead of print.
110. X. Deng, K. Lin, X. Chen, Q. Guo and P. Yao, Preparation of magnetic Fe₃O₄/Au composites for extraction of benzo[a]pyrene from aqueous solution, Chem. Eng. J., 2013, 225, 656–663.
111. M. Zahoor, Magnetic adsorbent used in combination with ultrafiltration membrane for the removal of surfactants from water, Desalin. Water Treat., 2014, 52(16–18), 3104–3114.
112. H. Liu, X. Lu, C. Deng and X. Yan, Highly sensitive MC-LR detection by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry with magnetic mesoporous silica for fast extraction, Rapid Commun. Mass Spectrom., 2013, 27(21), 2515–2518.
113. X. Ma, J. Wang, M. Sun, W. Wang, Q. Wu, C. Wang and Z. Wang, Magnetic solid-phase extraction of neonicotinoid pesticides from pear and tomato samples using graphene grafted silica-coated Fe₃O₄ as the magnetic adsorbent, Anal. Methods, 2013, 5(11), 2809–2815.
114. Y. Wang, Y. Sun, Y. Gao, B. Xu, Q. Wu, H. Zhang and D. Song, Determination of five pyrethroids in tea drinks by dispersive solid phase extraction with polyaniline-coated magnetic particles, Talanta, 2014, 119, 268–275.
115. M.-m. Tian, D.-X. Chen, Y.-L. Sun, Y.-W. Yang and Q. Jia, Pillararene-functionalized Fe₃O₄ nanoparticles as magnetic solid-phase extraction adsorbent for pesticide residue analysis in beverage samples, RSC Adv., 2013, 3(44), 22111–22119.
116. H. Eskandari and A. Naderi-Darehshori, Preparation of magnetite/poly(styrene-divinylbenzene) nanoparticles for selective enrichment-determination of fenitrothion in environmental and biological samples, Anal. Chim. Acta, 2012, 743, 137–144.
117. A. Beiraghi, K. Pourghazi and M. Amoli-Diva, Thiodiethanethiol Modified Silica Coated Magnetic Nanoparticles for Preconcentration and Determination of Ultratrace Amounts of Mercury, Lead, and Cadmium in Environmental and Food Samples, Anal. Lett., 2014, 47(7), 1210–1223.
118. X. Cao, L. Shen, X. Ye, F. Zhang, J. Chen and W. Mo, Ultrasound-assisted magnetic solid-phase extraction based ionic liquid-coated Fe₃O₄@graphene for the determination of nitrobenzenes in environmental water samples, Analyst, 2014, 139(8), 1938–1944.
119. N. Ye, Y. Xie, P. Shi, T. Gao and J. Ma, Synthesis of magnetite/graphene oxide/chitosan composite and its application for protein adsorption, Mater. Sci. Eng., C, 2014, 45, 8–14.
120. J.-W. Liu, T. Yang, L.-Y. Ma, X.-W. Chen and J.-H. Wang, Nickel nanoparticle decorated graphene for highly selective isolation of polyhistidine-tagged proteins, Nanotechnology, 2013, 24(50), 505704-1–505704-8.
121. D. Bigdelifam, M. Mirzaei, M. Hashemi, M. Amoli-Diva, O. Rahmani, P. Zohrabi, Z. Taherimaslak and M. Turkjolar, Sensitive spectrophotometric determination of fluoxetine from urine samples using charge transfer complex formation after solid phase extraction by magnetic multi-walled carbon nanotubes, Anal. Methods, 2014, 6, 8633–8639.
122. H. Fazelirad, M. Ranjbar, M. A. Taher and G. Sargazi, Preparation of magnetic multi-walled carbon nanotubes for an efficient adsorption and spectrophotometric determination of amoxicillin, J. Ind. Eng. Chem. http://www.sciencedirect.com/science/article/pii/S1226086X14002263 2014, ahead of print.
123. A. Beiraghi, K. Pourghazi, M. Amoli-Diva and A. Razmara, Magnetic solid phase extraction of mefenamic acid from biological samples based on the formation of mixed hemimicelle aggregates on Fe₃O₄ nanoparticles prior to its HPLC-UV detection, J. Chromatogr. B: Anal. Technol. Biomed. Life Sci., 2014, 945–946, 46–52.
124. F. J. Heiligtag, M. J. I. Airaghi Leccardi, D. Erdem, M. J. Süess and M. Niederberger, Anisotropically structured magnetic aerogel monoliths, Nanoscale, 2014, 6, 13213–13221.
125. R. Hua and Z. Li, Sulfhydryl functionalized hydrogel with magnetism: Synthesis, characterization, and adsorption behavior study for heavy metal removal, Chem. Eng. J., 2014, 249, 189–200.
126. L. Jiang and P. Liu, Design of Magnetic Attapulgite/Fly Ash/Poly(acrylic acid) Ternary Nanocomposite Hydrogels and Performance Evaluation as Selective Adsorbent for Pb²⁺ Ion, ACS Sustainable Chem. Eng., 2014, 2(7), 1785–1794.
127. Y. Zhou, S. Fu, L. Zhang, H. Zhan and M. V. Levit, Use of carboxylated cellulose nanofibrils-filled magnetic chitosan hydrogel beads as adsorbents for Pb(II), Carbohydr. Polym., 2014, 101, 75–82.
128. Z. Yu, X. Zhang and Y. Huang, Magnetic Chitosan-Iron(III) Hydrogel as a Fast and Reusable Adsorbent for Chromium(VI) Removal, Ind. Eng. Chem. Res., 2013, 52(34), 11956–11966.
129. Z. Liu, H. Wang, C. Liu, Y. Jiang, G. g. Yu, X. Mu and X. Wang, Magnetic cellulose-chitosan hydrogels prepared from ionic liquids as reusable adsorbent for removal of heavy metal ions, Chem. Commun., 2012, 48(59), 7350–7352.
130. Y.-F. Lin and J.-L. Chen, Magnetic mesoporous Fe/carbon aerogel structures with enhanced arsenic removal efficiency, J. Colloid Interface Sci., 2014, 420, 74–79.
131. S. C. N. Tang, D. Y. S. Yan and M. C. Lo Irene, Sustainable Wastewater Treatment Using Micro-sized Magnetic Hydrogel with Magnetic Separation Technology, Ind. Eng. Chem. Res., 2014, 53, 15718–15724.
132. C. Rosales-Landeros, C. E. Barrera-Díaz, B. Bilyeu, V. Varela Guerrero and F. Ureña Núñez, A Review on Cr(VI) Adsorption Using Inorganic Materials, Am. J. Anal. Chem., 2013, 4, 8–16.
133. J. G. Mahy, L. Tasseroul, A. Zubiaur, J. Geens, M. Brisbois, M. Herlitschke, R. Hermann, B. Heinrichs and S. D. Lambert, Highly dispersed iron xerogel catalysts for p-nitrophenol degradation by photo-Fenton effects, Microporous Mesoporous Mater., 2014, 197, 164–173.

---