Ferrites: Synthesis and Applications for Environmental Remediation

Chapter 4

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Ferrites: Synthesis and Applications for Environmental Remediation Manpreet Kaur,* Navneet Kaur, and Vibha Department of Chemistry, Punjab Agricultural University, Ludhiana-141 004, India *E-mail: [email protected]

Ferrite materials are being widely used in magnetic, electronic and microwave devices. They have high resistivity and low eddy current losses which makes them better choice over metals. Apart from their promising magnetic properties, ferrites have now been explored for their adsorptive and catalytic properties. This chapter presents applications of ferrite nanoparticles in the diverse fields from magnetic devices totheadsorbentsforremediation of heavy metals and organic contaminants. Their role in facile synthesis of organic compounds as catalysts where they can replace conventional processes is also discussed. Environmental remediation is the most recently explored area. Magnetic separation of nanoparticles facilitates their reuse age. The works reported on lab scale can be explored at large scale for environmental remediation.

Introduction In recent years ferrites have attracted considerable attention as magnetic nanoparticles (NPs) with diverse applications.The term ferrite is derived from latin word ‘Ferrum’ meaning iron. Theseare mixed metal oxides with iron oxide as their main component. The properties of ferrite NPs are determined by chemical composition, particle size and interaction of the particles with the surrounding matrix. Same type of NPs can have markedly different properties as they can be tailored by varying their size, composition and structural properties (1). Ferrite NPs have intensiveusage in the electronic devices (2), magnetic © 2016 American Chemical Society Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


drug delivery and catalysis (3). They are better than pure metals because of their low cost, ease of synthesis and stable nature. Presently, ferrite and ferrates are explored for for environmental remediation (4, 5). Ferrite NPs have large surface area and presence of unbalanced forces at the surface is ideal for adsorption.Coating the NPs with surfactants and their composites with activated carbon, silica and bentoniteetc makes them stable against aggregation and increases their physic-chemical stability to provide functionalized surface (6). Their usage for the removal of heavy metals and organic contaminants has been widelyexplored. Water pollution by these contaminants is a serious health hazard and their removal from water is a thrust area of research. Ferrite NPs help in synergistic adsorption anddegradation of organic contaminants .Ferrites can act as photocatalyst for oxidation of compounds, reduction of nitro compounds, hydrogenation of alkanes,degradation of dyes and organic contaminants (7–9). Excited electrons and holes favour redox reaction resulting in the formation of stable non toxic products. Ferrites offer an advantage of having a band gap capable of absorbing visible light as well as spinel structure which enhances efficiency due to the available extra catalytic sites by virtue of the lattice structure (10). They have also been used in combination with other photocatalysts in an effort to enhance their photocatalytical activity. This chapter gives an overview of commonly used low temperature chemical routes for the synthesis of ferrite NPs. This is followed bytheirusefulnessfor adsorption of heavy metals/ synergistic degradation of organic compounds, as catalyst for organic synthesis and enzyme mimics.

Classification of Ferrites Ferrites are ceramic materials having iron oxide as their principal component. The arrangement of oxygen anions around metal cations may vary, thus giving rise to different crystal structures. Accordingly ferrites have been classified as:Spinel,Garnets and Magnetplumbites. Spinel, is the most important class of ferrites, which is isomorph with the mineral spinel (MgAl2O4) and having the general composition AB2O4, composed of a rigid anion sub-lattice of oxygen anions and a cation sub-lattice formed by cations A and B (11). Oxygen anions have cubic close packing with a variety of A and B cations filling the interstitial sites (Figure 1). The crystallographic unit cell contains eight formula units of AB2O4, resulting in 64 tetrahedral sites (8 are occupied) and 32 octahedral sites (16 are occupied) (12). These sites have radii in the following ranges:-

A wide variety of transition metal cations could fit into these interstitial sites and the ‘d’ electronic configuration ranges from d0 to d10. Thus, it becomes possible to make a large number of spinel ferrites, each having specific magnetic interactions. The difference in the electron environment at the fourfold [Td] and eight fold [Oh] 114 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


sites and interaction of that environment with the ‘d’ electrons of various transition metal cations results in the magnetic properties of the spinel ferrites.

Figure 1. Structure of unit cell of spinel ferrite The spinel structure may be formed from various anions including sulphur (thiospinels), chlorine (halospinels) and oxygen. The cations in the structure must satisfy both size and charge neutrality consideration. Depending upon the arrangement of metal cation M2+, the spinels are classified as: • •

•

Normal spinels, A[B2O4] with A2+ occupying tetrahedral sites only and B3+ ions in octahedral sites e.g. ZnFe2O4 and CdFe2O4. Inverse spinels, B[ABO4] with A2+ ions occupying the octahedral sites and B3+ ions are distributed over both octahedral and tetrahedral sites, e.g. NiFe2O4, MgFe2O4 and CoFe2O4. Mixed spinels with A2+ and B3+ ions distributed over both octahedral and tetrahedral sites AxBy[A1-x B2-y O4] e.g. MnFe2O4.

Nanosized spinel ferrites find potential application in the field of high density magnetic recording media, magnetic ferrofluids, microwave absorber and magnetically guided drug carriers. The peculiar magnetic properties of the spinel ferrites are strongly dependent on the distribution of cations (13, 14). Garnet ferrites have the structure of the silicate mineral garnet, X3Y2(SiO4)3, where the X site is usually occupied by divalent cations [Ca2+, Mg2+, Fe2+] and the Y site by trivalent cations [Al3+, Fe3+, Cr3+]in an octahedral/tetrahedral framework with [SiO4]4- providing the tetrahedra. Garnet ferrites have the chemical formula, M3Fe5O12, where M is yttrium or a rare-earth cation. In addition to the octahedral and tetrahedral sites, such as those seen in spinels, garnets have dodecahedral (12coordinated) sites. The net ferrimagnetism is thus a complex result of antiparallel 115 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


spin alignment among the three types of sites. Garnets are also magnetically hard. Whereas spinels have superior magnetization properties, garnets have superior dielectric properties, as they are more stress sensitive than spinels. Considerable interest has been shown in the rare earth iron garnets, M3Fe5O12 [M = Y, Dy, Sm] because of their ferrimagnetic properties . Magnetplumbites have the general composition, MIIFe12O19 (M= Ba, Pb, Sr). Since the structure is hexagonal, so they are referred to as hexagonal ferrites. These ferrimagnetic oxides contain a principal component, Fe2O3 in combination with divalent oxides (BaO, PbO or SrO). These ferrites have high uniaxial anisotropy, high magnetization values and good chemical stability, thus fitting very well in the needs of recording technology (15). Hexaferrites films of BaFe12O19 are promising candidates for both high density recording media and microwave/millimeter wave devices (16). Ferrites can be further classified into two major groups i.e.: soft ferrites and hard ferrites on the basis of coercive force (Hc) which is one of the most important properties of magnetic substances. Materials with low Hc are termed as magnetically soft while with higher Hc are termed as magnetically hard materials. The soft materials have high permeability and are used in application at low and high frequencies while hard materials have large energy product of (BH)max and are used as permanent magnetic materials. Ferrites that are used in transformer or electromagnetic cores containing Ni, Zn or Mn compounds have a low coercivity and are called soft ferrites. Because of their comparatively low losses at high frequencies, they are extensively used in the cores of Switched-Mode Power Supply (SMPW) and RF transformers and inductors. In contrast, permanent ferrite magnets (or hard ferrites) have a high remanence after magnetization, and are prepared with iron oxide and barium/strontium oxides. In a magnetically saturated state they conduct magnetic flux very well and have a high magnetic permeability. This enables these so-called ceramic magnets to store stronger magnetic fields than pure iron.

Synthesis of Ferrite NPs Various methods employed for the synthesis of ferrite NPs, can be broadly classified into: physical and chemical methods. The widely used low temperature chemical methods which yield ferritesNPswithuniform size distribution are discussed in this chapter. Sol-Gel Method Sol-gel method is commocnly used for preparing inorganic oxides (Scheme 1). This is used for producing highly reactive homogeneous powders with various advantages such as good stoichiometric control and formation of active submicronsize particles in a relatively shorter processing time at low temperature (17). The method involves exothermic and self-sustaining redox reaction of xerogel, which is obtained from aqueous solution containing desired metal salts (oxidizer) and organic complexant (reductant).During the xerogel combustion, rapid evoluation 116 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


of a large volume of gases leads to the formation of NPs (Figure 2). MgFe2O4 and CaFe2O4NPs having average particle size 16 nm and 5-10 nm respectively have been synthesized by the sol-gel method (18, 19). Nickel ferrite nanoparticles having grain size in the range 24-31 nm are alsoreported. The low temperature synthesis results in the formation of awaruite (FeNi3) peaks in the XRD pattern along with ferrite peaks, which can be removed after microwave treatment of the ferrite (approx. 400° C), but with slight distortion in the cubic structure (20). The NPs exhibit saturation magnetization (Ms) value of 27 emu/g. ZnFe2O4NPs have been synthesized by xerogel combustion on the hot plate and results in the formation of monophasic ferrite with particle size 20 nm (21). Green sol-gel auto combustion method have been utilized to synthesize a composite of CuFe2O4 with Fe2O3 and chitosan. Onion was used as a green reductant. Chitosan helped in modifying the surface properties by functionalization.TheMsvalue of chitosan comes out to be 9.65 emu/g (22). Nanocomposite of MgFe2O4 NPs with activated charcoal and bentonite have also been synthesized by sol-gel method, results in increased surface area of ferrite (4, 23).

Figure 2. Flow chart of processing steps

Scheme 1. Flow chart for the synthesis of ferrite NPs by sol-gel method Co-Precipitation Method 2).

Co-precipitation is a simple method for the synthesis of powders (Scheme In this method, the required metal cations, are coprecipitated from a 117

Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


common medium, usually as hydroxides, carbonates, oxalates or citrates. If all metal ions do not form insoluble precipitates, it becomes difficult to control the stoichiometry. This is the main drawback of co precipitation method. Mn-ferrite NPs have been reported with diameter in the range 17-45 nm created by co-precipitation technique. The growth of particle size is controlled by concentration of metal nitrate solutions (24). Hexagonal ferrite, having composition 3BaO•2CoO•12Fe2O3 (Co2Z), 2BaO•2CoO•6Fe2O3 (Co2Y) and BaO•6Fe2O3 (BaM) was synthesized by chemical co-precipitation method (25). The average particle diameter of manganese ferrite nanoparticles synthesized by co-precipitation method ranges between 10.5 and 19.0 nm from TEM. The particle diameter increases in the aqueous medium, indicating that the particles form aggregates (26). In further study, MnFe2O4 NPs synthesized by co-precipitation method and calcined at 900˚ C with average crystallite size of 37.93 nm.This method was compared with hydrothermal method which yield particles with average crystallite size of 31.90 nm (27). The saturation magnetization of MgFe2O4 NPs synthesized by different chemical methods have been compared. Magnesium ferrite synthesized by PEG, ODH and sol-gel methods displayed Msvalues ranging from 13.22–13.55emu/g whereas NPs synthesized by urea method had lower Msvalue of 10.73emu/g due to lower crystallinity. Due to incomplete precipitation the ferrite synthesized by coprecipitation method haslower value of Ms (1.95emu/g) (18).

Scheme 2. Flow chart for the synthesis of ferrite NPs by co-precipitation method Combustion Method Combustion process is an exothermic redox reaction between an oxidizer and a fuel (28). Oxidizer (O) and fuel (F) are used to carry out combustion. Stoichiometry ɸe = (O/F) is maintained unity by balancing the oxidising (O) and reducing valency (F) of the reactants (29). It is of great importance because 118 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


of its potential advantages like fast production rate, low temperature/cost and relatively simple preparation process (Scheme 3). Several researchers have preparedNiCuZn (30), MgCuZn, MgCu and NiZn ferrite (31) by auto combustion method. NPs of composition CoxFe(3−x)O4 with x ranging from 0.05 to 1.6 by a combustion reaction using iron nitrate, cobalt nitrate and urea as fuel were synthesized (32). Citrate-nitrate auto combustion method was employed to prepare the nano-sized particles of Co1−x Cax Fe2O4 (x = 0.0, 0.01, 0.03, 0.05, 0.07 and 0.09) (33).

Scheme 3. Flow chart for the synthesis of ferrite NPs by combustion method

Ternary ferrite (NPs) having stoichiometery Co0.6-xMgxZn0.4 Fe2O4 (x=0.0, 0.2, 0.4, 0.6) and pure spinel ferrites of Mg and Co have been synthesized by self-propagating oxalyldihydrazide-metal nitrate combustion method (Scheme 4). The average particle diameter was observed to be 25-45 nm (34).

Scheme 4. Synthesis of pure ferrites by redox reaction

Microemulsion Method Microemulsionmethod yields NPswith a narrow size distribution andsize control is readily achievable by minor adjustments to the synthesis conditions. Microemulsion methods can be classified into normal micelle methods [oil-in-water (o/w)] and reverse micelle methods [water-in-oil (w/o)]. In both methods surfactants are used and their concentration is above the critical micelle concentration (CMC). The effect of solvents on the synthesis of MgFe2O4 NPs by reverse micelle method was carried out. The particle morphology, particle size and Msvaluewas strongly dependent on the phase used. Toluene and cyclohexane formed aggregated particles while average particle size was found to be 20.9 ± 4.3 nm using heptane as a solvent (35). Various advantages and disadvantages of discussed chemical methods are given in Table 1. 119 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Table 1. Advantages and Disadvantages of various chemical methods for the synthesis of ferrite nanoparticles


Method

Advantages

Disadvantages

Sol-gel

• Low processing temperature • Molecular-level homogeneity • Porous

• Low yield • Evolution of large amount of gases

Co-precipitation

• low temperature • cost effective

• Incomplete precipitation • non-homogeneous particle size

Combustion

• high purity, yield • low processing time

• evolution of large amount of gases

Microemulsion

• Simple • Controlled properties • Uniform, homogeneous and size controllable nanoparticles

• Low yield • Expensive • Surfactant get adsorbed on the surface of nanoparticles • liquids are used in large quantity.

Microwave

• Simple, easy to operate • Rapid volumetric heating • Short reaction period • Increased yield

• Size and shape of nanoparticles is not uniform • Costly microwave system

Applications Ferrites are widely used in our day to day life as an important component of electronic inductors, transformers and electromagnets owing to their high electric resistance that leads to very low eddy curent losses. Ferrite powders are also used in the coatings of magnetic recording tapes, radars, coatings used in stealth-aircraft, as they possess spontaneous magnetic moment below the Curie temperature (36). Due to their application in our daily life, more studies are going on to identify their properties and their uses in versable areas like magnetic drug discovery, photocatalysis and enzyme mimics. Novel applications of ferrites and ferrates are their role as adsorbents for removal of environment contaminants (4, 5). Thus various applications of ferrites are shown in Figure 3. Catalytic Degradation of Organic Contaminants Catalytic degradation of dyes by ferrite NPs has been extensively studied. Photocatalytic degradation activities under UV light irradiation by fluorine-doped γ-Fe2O3 samples for RhB dye has been studied and its activity was found to be 2–5 times higher than that of the undoped sample. It was also proposed that increase in photocatalytic activity was due to pores and hollow internal cavity in fluorine-doped γ-Fe2O3 hollow hierarchitecture, which caused the 120 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


penetration of light waves and RhB solution deep in this catalyst (37). Cadmium sulfide–ferrite (CdS–MFe2O4, M = Zn, Co) nanocomposites were synthesized through hydrothermal method and their potential as a magnetically recyclable photocatalyst for degradation of rhodamine B (RhB) and 4-chlorophenol (4-CP) was reported. Doped ferrites showed better catalytic activity than pure CdS due to increase in surface area of composite and synergic effect of CdSand ZnFe2O4 and CuFe2O4 which produces active species (hydroxyl radicals, holes and superoxide anion radicals) (Figure 4) (38).

Figure 3. Applications of ferrites in different areas

Figure 4. Photocatalytic degradation of rhodamine B (RhB) and 4-chlorophenol (4-CP) using doped and undopedCdS ferrites. (Reproduced with permission from reference (38). Copyright 2013 American Chemical Society).

121 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Cobalt-doped zinc ferrite (Zn1-xCoxFe2O4),nanostructured spinel ferrites and CoMnxFe2-xO4 (x = 0.2, 0.4, 0.6, 0.8 and 1.0) have been used as heterogeneous catalysts in H2O2 assisted degradation of methylene blue. Doping with cobalt reduced band gap of ferrites thus enhanced photocatalytic activity in the degradation of methylene blue by destroying large conjugated pi system under visible light irradiation (39). Cobalt ferrites showed significant results for decomposition of H2O2. However, for oxidation of methylene blue, copper ferrites showed better results by exhibiting the redox pairs Cu+/Cu2+ which can also produce radicals OH- and ·OH from hydrogen peroxide (40). Mn ions in Co ferrite enhanced degradation of dyes and degradation increases with increase in Mn content due to increase in concentration of Mn3+ ions in octahedral sites of ferrite sub-lattice (41). NiFe2–xNdxO4 photocatalyst with different neodymium contents were studied.Nd substitution enhanced absorption in the whole visible region and increased photocatalytic activity for the degradation of organic pollutants (Figure 5). At x=0.5 band gap is narrower as new energy due appeared close to conduction band. Moreover, substitution of Fe3+ with Nd3+ induced significant lattice strains thus enhanced photocatalytic activity (42).

Figure 5. Photocatalytic degradation of organic pollutants using NiFe2–xNdxO4. (Reproduced with permission from reference (42). Copyright 2013 American Chemical Society). A facile route to synthesize SrFeO3−δ is reported and their potential for photocatalytic degradation of aqueous nitrobenzene in the presence and absence of H2O2 is studied. Results revealed that the degradation of nitrobenzene over the SrFeO3−δ catalyst (UV/SFO) was better compared to SrFeO3−δ in the presence of H2O2(UV/SFO/H2O2) as formation of strontium carbonate in the presence of H2O2 block active sites of photocatalytic reaction (Figure 6) (43).



Figure 6. Photocatalytic degradation of nitrobenzene using SrFeO3−δ. (Reproduced with permission from reference (43). Copyright 2015 American Chemical Society).

Oxidation of malachite green (MG) and reactive red 120 (RR120) by CuFe2O4 NPs is reported. MW irradiation significantly increases CuFe2O4 activity for MG degradation. MW-induced holes activation process, while both ·O2- and ·OH also participated in catalytic oxidation of MG (7). The degradation of RR120 was most effective at pH 3 and 75 °C. The catalyst was also recovered and reused 6 times without loss of catalytic activity (8). Magnetic MnFe2O4 particles were evaluated to enhance the degradation efficiency of p-nitrophenol (PNP) by activating peroxymonosulfate (PMS) catalytic oxidation. The degradation efficiency of PNP was significantly enhanced using microwave radiation as 97.2% of PNP was degraded in the time period of 2 min. Increase in concentration of catalyst also increased photocatalytic activity due to increase in number of active sites for activation of PMS (44). Photocatalytic degradation of acetic acid byMFe2O4 (M = Mg, Zn, and Cd) was studied. It was observed that MgFe2O4 exhibited the highest degradation, i.e. 196 μmol/g/h because of its small size and hence large surface area (45). Photocatalytic activity of CoFe2O4coupled with Fe and N co-doped TiO2 nanocompositewasevaluated for the degradation of methyl orange. It was observed to behigherthan pristine CoFe2O4. This is due to decrease in the band gap of TiO2 by introducing CoFe2O4 and FeN dopant. Furthermore, it was magnetically separable from the solution, which made it useful for industrial applications (46). Pillared montmorillonitecontaining iron oxide showed potential catalytic activity for degradation of dichlorophenol. Further, the presence of peroxymonosulfate degraded 85% of DCP in 2.5 hrs, whereas the presence of hydrogen peroxide and peracetic acid degrade 50% and 70% DCP respectively, in 3.5 hrs. The synthesized compound was proved as eco-friendly and applicable for environmental decontamination (47).



As Enzyme Mimic Enzymes are extensively used in different industries and are also important from medical point of view as they possess therapeutic applications. Work is going on to isolate the molecules exhibiting similar properties like enzymes as natural enzymes have some disadvantages. Enzymes denatured quickly onchanging pH, heat or solvent and many other chemicals.Hence developing synthetic “artificial enzymes” is of great interest which can work in solvents of choice and can catalyze a particular reaction. Peroxidase like catalytic activity of Fe3O4 nanoparticles and high electrocatalysis when supported with conductive carbon nanotube (CNT) using graphene oxide nanosheets as surfactant was studied. GCNT–Fe3O4nanocomposite showed higher aqueous stability and stronger peroxidase-like activity and electrocatalysis to H2O2.These enzyme mimics are recyclable and overcome some disadvantages of natural enzymes and common inorganic catalysts and showed environmental stability (48). Iron oxide nanoparticles (IONPs) were synthesized and evaluated pH-dependent peroxidase-like and catalase-like activities It was concluded that IONPs had a concentration-dependent cytotoxicity on human glioma U251 cells and increased H2O2-induced cell damage. Both Fe3O4 and γ-Fe2O3 nanoparticles catalyzed H2O2 to produce hydroxyl radicals in acidic lysosome mimic conditions, which was consistent with their peroxidase-like activities (Figure 7) (49).

Figure 7. Dual enzyme-like activities of iron oxide nanoparticles. (Reproduced with permission from reference (49).Copyright 2012 American Chemical Society). CoxNi1-xFe2O4 magnetic nanoparticleswere evaluated for their potential as enzyme mimics for the eletroctrocatalytic oxidation of H2O2 with different (Co/Ni) molar ratio toward H2O2 oxidation. Co0.5Ni0.5Fe2O4 modified carbon paste electrode (Co0.5Ni0.5Fe2O4/CPE) owned best electrocatalytic activity for 124 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


H2O2 oxidation. The calibration curve for H2O2determination was linear with low detection limit. Synthesized nanoparticles were also used to the determine H2O2 in commercial toothpastes,thus, a promising hydrogen peroxidase mimics for the detection of H2O2 (50). CoxFe3−xO4nanocubes (CF)were found to have a large specific surface area thus supposed to have potential catalytic activityas colorimetric biosensor for H2O2 and glucose and as a peroxidase mimic. Low manufacturing cost and potential catalytic activity make it important in field of medicine and biotechnology research (51).

As Drug Carrier Magnetic drug carrier property exhibited by nanoparticles is excellent for cancer treatment which avoids the side effects of conventional chemotherapy. They also increase the specificity of drug targeting. Other applications including magnetic resonance imaging, separation of DNA, hyperthermia tumor treatment, site specific gene and cell separation have been reported. Copper ferrite NPs induced cell viability reduction and membrane damage in MCF-7 cells.Degree of induction was dose and time dependent. It also induced oxidative stress in MCF-7 cells. Cytotoxicity due to CuFe2O4NPs exposure was effectively abolished by N-acetyl-cysteine (ROS scavenger) suggesting that oxidative stress could be the plausible mechanism of copper ferrite NPs toxicity (52). Manganese ferrite/graphene oxide (MnFe2O4/GO) nanocompositeswere evaluated for their potential for controlled targeted drug delivery. They exhibited negligible cytotoxic activity even at concentration 150 μg/mL in vitro conditions. Doxorubicin hydrochloride (DOX) was used as standard drug. The drug loading capacity of this nanocarrier was 0.97 mg/g and the drug release behavior was sustained and pH dependent.Enhanced drug delivery was due the large surface area of GO and adsorption on manganese ferrite. Hydrogen bonding and pi-pi stacking interactions also increased the loading capacity (53). Nickel ferrites were embedded within polyacrylamide based hydrogel matrix by original polymerization-crosslinking method. The synthesized compounds were evaluated for microbiological activities against Staphylococcus aureus and their kinetic studies were also performed. The compounds showed good anti-microbial results in both the cases i.e. for ferrites -hydrogel-drug and ferrite-hydrogel with gelatin drug systems for longer period of time (54).

Remediation of Heavy Metal and Dyes Now a days, dyes are extensively used in paper, cosmetics industries, textiles, rubber and plastics. These are major waste from these industries and is found to be hazardous as they affect aquatic life and food chain. They are resistant to fading on exposure to light, water,chemicals and thus difficult todecolourised once released into water sources. Hence, it is necessary to develop cost effective and environmental friendly methods to remove them from aqueous solution. 125 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Most effective pathway is to use magnetic adsorbent to solve this environmental problem. Adsorption capacity of different MFe2O4 (M = Mn, Fe, Co, Ni) ferrite NPs were studied for Congo red (CR). High-efficient magnetic separation from wastewater was due to ferromagnetic behaviour of MFe2O4NPs. Desorption of MFe2O4 nanoparticles loaded by CR was done by acetone. Among them, CoFe2O4 NPs exhibited excellent adsorption capacity (55). In another study, CaFe2O4 nanoparticles were found to have high affinity to CR. Various factors like pH, ionic strengths and co-existing ions affected extent of adsorption. The process was found to be spontaneous and exothermic.Adsorbed dye could be recovered by ethanol/ Na3PO4 (2mM) (1/1, v/v) upto 96.6% and was reusable (9). Mn ferrite nanospheres and magnetic nanocomposite (NC) of Mg ferrite NPs with activated charcoal was found effective for removal of Cr (VI) from water. Magnetic property of these ferritesshowedpromising results for magnetic removal of Cr (VI) in wastewater,hence helps to reduce environmental pollution (56). Nanocomposite was found to be more effective for adsorption of Cr (VI) than MgFe2O4 (4). Further, surface-modified jacobsite (MnFe2O4) nanoparticles (Figure 8) were used for theremovaland recovery of Cr(VI) from synthetic wastewater. Modified MnFe2O4NPscan be efficiently reused without loss of adsorption capacity and the recycling of Cr(VI) without changing the valence (57).

Figure 8. Surface-modified jacobsite (MnFe2O4) nanoparticles. (Reproduced with permission from reference (57). Copyright 2005 American Chemical Society).

Monodisperse porous ferrites hollow nanospheres(Fe3O4,CoFe2O4, ZnFe2O4, MnFe2O4) effectively adsorb As(V) and Cr (VI) ions (Figure 9(a-d)). The maximum adsorption capacity was found to be 340 mg/g based on Langmuir model, which shows excellent As(V) and Cr(VI) ions uptake capacity in contrast to other solid nanosphere materials (58).



Figure 9. TEM images (a) Fe3O4 (b) CoFe2O4 (c) ZnFe2O4 (d) MnFe2O4. (Reproduced with permission from reference (58). Copyright 2013 American Chemical Society). Pb (II) can be efficiently adsorbed onto NiFe2O4 and CoFe2O4 NPs. Threedimensional (3D) porous NiFe2O4 (PNA) having significant magnetic properties were also used for Pb (II) in another study. PNA was found to be effective for removal of Pb(II) from contaminated wastewater (Figure 10) (59, 60).

Figure 10. Detoxification of Pb(II) contaminated aqueous solution using three-dimensional (3D) porous NiFe2O4 adsorbent (PNA). (Reproduced with permission from reference (60). Copyright 2013 American Chemical Society). A series of doped and undoped ferrite CuCexFe2-xO4 (x = 0.0–0.5) NPs were evaluated for fluoride adsorption. Doped ferrites showed better results as compared to the undoped ferrites for active sites, functional groups and fluoride adsorption. Thus fluoride adsorption was increased with doping and also easy separation with simple magnet (61). The presence of ferrite nanoparticles increased activity of magnetically separable N-doped TiO2 for the removal of cyanotoxinmicrocystine-LR from aqueous medium as compared to non-magnetic doped TiO2 (62).



Ferrites as Catalysts The advantage of using ferrites as catalyst over other catalysts theirmagneticbehavior.Theyget easily recovered after completion of reactionsandcanbeused several times without loss of catalytic activity. Tremendous work has been done on catalytic activity of ferrites in various organic reactions. Mostly Co, Zn, Ni, Cu and their mixed metal combinations with Cr, Cd and Mn have been used as catalyst. Core-shell nanostructures with silica and titania have also been reported. Catalytic application of iron oxide, metal ferrites and multicomponent nanoparticles in organic transformations has also been reported (63). Use of ferrites as catalysts is listed in Table 2. Magnetic carbon-supported Pd catalyst wasfound to be efficient for the hydrogenation of alkenes and reduction of aryl nitro compounds (Figure 11) (64). The efficiency of a photoactive catalyst, Fe@g-C3N4 for the hydrogenation of alkenes and alkynes was evaluated using hydrazine hydrate as a source of hydrogen. The advantage of magnetically separable Fe@g-C3N4 is that it excludes the use of high pressure hydrogenation. Moreover, reaction is cost-effective as visible light was used instead of external sources (65). Chain like MFe2O4 (M = Cu, Ni, Co, and Zn) nanoaggregates (NAs) were used as heterogeneous catalysts. CuFe2O4 NAs exhibited highest catalytic activity for the reduction of nitroaromatic compounds compared to others. Further, these spinel ferrites showed high selectivity for the oxidation of benzyl alcohol and significant difference was observed within the activities of these spinel ferrites. CoFe2O4 showed best catalytic activity, also 93% selectivity of benzaldehyde was achieved when 63% reaction was progressed (66, 67). Toluene combustion was studied by 3D macroporousperovskite type oxides. The molecule possessed best pore quality i.e. 72, 64 and 52 nm and highest surface area, proved as best catalyst with T50% of ca. 270 °C and T90% of ca. 310 °C at space velocity = 5000 mL/(gh) (68). The catalytic property of pure LaFeO3 was compared with substituted Cr and Cu nano-crystalline lanthanum ferrites (LaMxFe1−xO3) for the decomposition of H2O2 solution (0.17 M). Addition of Cu increased catalytic activity about 25 times than pure LaFeO3. However, no significant results were obtained in case of Cr substitution (69). Ni ferrites, synthesized using electrochemical method,were used as a catalyst for the oxidation of glucose, NADH and methanol using paste graphite electrode. The ferrites showed good catalytic potential for these compounds with oxidation potential around 0.75, 0.5 and 0.8 V for glucose, NADH and methanol respectively (70). Ni0.3Co0.5ScxFe2-xO4 (x=0. 0.05, 0.1 and 0.2) spinel ferrites were evaluated for catalytic potential in combustion of acetone, propane and benzene. Partial substitution of Fe(III) by Sc (III) ions on the octahedral sites of spinel structure of Ni0.5Co0.5Fe2O4 ferrites enhances catalytic activity. For propane and acetone Ni0.5Co0.5Sc0.2Fe1.8O4 composition was found to be most effective at moderate temperatures while for acetone 400 °C temperature was required for the conversion above 90% (71).


Table 2. Ferrrites used as a catalyst in chemical reactions


Reaction

Ferrites used as catalyst

Reference

Decomposition of H2O2 and oxidation of methylene blue

Spinel ferrites of Co, Cu and Ni

(40)

Oxidation of benzyl alcohol

Spinel ferrites of Co, Cu, Zn and Ni

(67)

Combustion of toluene

3D macroporousperovskite type oxides SrFeO3-δ SFO-xEG; x = 1, 3 and 6)

(68)

Decomposition of H2O2

Cr and Cu nano-crystalline lanthanum ferrites (LaMxFe1−xO3) and LaFeO3

(69)

Oxidation of glucose, NADH and methanol

Ni ferrites

(70)

Degradation of methylene blue and Remazol dyes

Mn substituted Co ferrites CoMnxFe2-xO4

(41)

Microwave-induced-catalytic oxidation for malachite green

CuFe2O4

(7)

Combustion of acetone, propane and benzene

Ni0.3Co0.5ScxFe2-xO4

(71)

Reduction of nitrophenols

CoMnxFe2−xO4

(72)

Electro-reduction of H2O2 and electro-oxidation of NADH

CoFe2O4/EGO

(75)

Oxidation of formaldehyde

Mn substituted spinel ferrites

(73)

Oxidation of formaldehyde

Mn substituted spinel ferrites

(74)

Selective catalytic reduction of NO with hydrogen

Pd doped ferrite spinels of divalent metals (Co, Cu or Zn)

(76)

Wet peroxide oxidation of C.I

Cu ferrite nanoparticles

(8)

Degradation of P-nitrophenol

MnFe2O4

(44)

Photocatalytic degradation of acetic acid

MFe2O4 (M = Mg, Zn, and Cd)

(45)

Photocatalytic degradation of methyl orange

CoFe2O4 coupled with Fe and N co-doped TiO2

(46)

As a photocatalyst for methyl orange

CoFe2O4–PANI

(77)

Binding with (YADH) enzyme complex

Ni-Co nanoferrites

(81)



Figure 11. Hydrogenation of alkenes and reduction of aryl nitro compounds using carbon-supported Pd catalyst. (Reproduced with permission from reference (64). Copyright 2014 American Chemical Society). Catalytic properties of Mn substituted cobalt ferrite nanoparticles with the composition CoMnxFe2−xO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were evaluated. Pure CoFe2O4 were inefficient for the reduction of nitrophenols.The addition of Mn ions significantly increases the catalytic property of ferrites. Order of the reduction rate of three isomers of nitrophenols was 2-nitrophenol > 4-nitrophenol > 3-nitrophenol (72). In another study, Mn substituted spinel ferrites were found to be effective for the oxidation of formaldehyde (HCHO). Catalytic potential was increased with the increase of calcination temperature of ferrites upto 400 °C, but then decreased. Reaction mechanism and variations in cationic microstructure of Mn-doped ferrites also the calcination which further affected the catalytic activity of Mn-doped spinel ferrites for HCHO oxidation. The catalyst also displayed high stability and superior activity in the presence of water vapours (73, 74). Co ferrite nanohybrid embedded with graphene oxide (CoFe2O4/EGO) were found to be effective in electro-reduction of H2O2 and electro-oxidation of NADH. Rotating disk chronoamperometry showed calibration curves in range of 0.50 to 100.0 mol/L NADH and 0.9 to 900.0 mol/L H2O2 with detection limit of 0.38 and 0.54 mol/L respectively (75). Synthesis ofPd doped ferrite spinels of divalent metals (Co, Cu or Zn) using the sol–gel auto-ignition method was reported and evaluated for selective catalytic reduction of NO with hydrogen (H2-SCR). The activity of the Co–FePd catalyst was significantly increased by adding a small amount of palladium and 96% NO conversion was observed in the 170–250 °C range. The catalyst followed the order for their catalytic activity: Co–FePd > Zn–FePd > Cu–FePd. Addition of H2O slightly decreased NO conversion by the Co–FePd oxide upto certain limit. The presence of 100 ppm SO2 in the gas phase decreased the SCR activity of Co–FePd by approximately 19%, also SO2 has both a reversible and an irreversible effect on the H2-SCR reaction (76). Hollow cobalt ferrite–polyanilinenanofibers (CoFe2O4–PANI) were used as a photocatalyst for degradation of methyl orange. The remarkable improvement of visible light photocatalysis was observed owing to the heterojunction built between CoFe2O4 and PANI. The catalyst was also recovered and found 130 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


to be reusable with the excellent characteristics formagnetic separation of CoFe2O4–PANI hollow nanofibers (77). Use of magnetically recoverable ferrite (Fe3O4) as a catalyst for functionalization of metals, organocatalysts, N-heterocyclic carbenes for catalytically active metals such as Pd, Pt, Cu, Ni etc. has been reported (78–80). The enzyme complex of magnetic crystalline Ni-Co nanoferrites covalently binded with yeast alcohol dehydrogenase (YADH) was thermally more stable as compared to the free enzyme over a wide range of temperature and pH.It was also found to be more durable after recovery by magnetic separation and can be used repeatedly (81).

Conclusion Science and technology of ferrite NPs is an emerging area of research. They are extensively studied for their magnetic, adsorptive and catalytic properties.Conventional ceramic method is the commonly used approach for the bulk synthesis of ferrites but it leads to ferrite particles in micrometer regime. Milling process used in this method introduces lattice defects and strains.Chemical methods discussed herein yield nanosized ferrite with desired stoichiometery although in most of these methods yield israther low. More research is needed on bulk production methods forferrite NPs by chemical methods as research from laboratory to the industry scale requires qualitative as well as quantitative products.

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Ferrites: Synthesis and Applications for Environmental Remediation

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