**2.2 Catalysts for alternative energies**

The use of magnetic iron oxide nanoparticles as catalyst supports dates back to the 70's when Robinson *et al.* reported the synthesis of enzymatic biocatalysts supported over magnetic iron oxides. The initial interest for the use of iron oxide was to facilitate the catalyst recovery and the immobilization of the catalyst in the reactor with magnetic fields [62]. Since then, the interest for IONPs as catalyst has emerged for an extensive list of chemical reactions including the ones that contribute to diminishing environmental harmful effects triggered by anthropogenic activities. Some of these reactions are based on developing alternatives to the highly pollutant use of fossil fuels, *e.g.* biodiesel production, Fischer-Tropsch synthesis and catalytic cracking of used engine oil, among others.

The design of IONPs for this application, should prevent mechanical breakdown of the catalyst and increase its lifetime by avoiding the possible particle growth or sintering during the process. One way is by introducing IONPs in mesoporous materials, assuring better catalytic process due the relatively large pores with high surface area that facilitate mass transfer and increase the concentration of active sites per mass of material. A recent study of Wei, *et al.* consisted in the Fischer-Tropsch

synthesis catalyzed by a mesoporous iron oxide nanoparticle-decorated graphene oxide [63]. Here, they showed that the designed hierarchically mesoporous material can hinder the contact of the syngas with the active sites, highlighting that the improved mesoporous structure of the IONPs is extremely beneficial for reactants access and products release. Zhang *et al.* postulated that maybe metals atoms with multiple valences, including Fe and their oxides, result partially reduced during pyrolysis processes and generate oxygen vacancies that might transform some volatile biomass compounds into bio-oil [64]. The high specificity in the catalytic cracking of spent engine oils, reducing the undesired aromatics and high molecular weight constituents in a produced diesel-fuel, has been demonstrated also for simple natural magnetite particles in the micrometer range [65].

More complex nanostructures, like CaO@Fe3O4 composites have been shown to increase yields in the transesterification reaction of vegetable oils with no need of additional base compounds. It is also possible to improve the biodiesel production by immobilizing enzymes like lipase over IONPs. In this way there will be no need of purification after the reaction as these catalysts decrease yields of toxic byproducts [66]. Furthermore, Teo *et al.* prepared a highly recyclable CaSO4/Fe2O3-SiO2 catalyst for biodiesel production, showing its efficiency in the reaction [67].

In spite of the promising results in the use of magnetic colloids in this area, the literature is quite limited, and it is needed to strengthen the efforts in the years ahead to evaluate the potential of these catalysts in the efficient production of alternative fuels. It should be mentioned that besides the benefits of high surface areas, high selectivity and specificity and the ability to be functionalized, IONPs can offer a selective heating at the nanoparticle surface under an alternating magnetic field that may enhance the reaction rate and yield as will be described in Section 4.

#### **3. Preparation of iron oxide magnetic colloids**

The past two decades have seen tremendous advances in the synthesis and application of IONPs that take advantage of their distinct properties and functionalities. As seen in **Figure 3**, the economical perspectives for the market of magnetite nanoparticles is in continuous growth mainly boosted by their use in biomedical applications but also in fields like energy and wastewater treatment. As the interest for IONPs in different applications rises, the demand of new ways and technologies to produce them also increases.

Iron is one of the most abundant elements in nature presenting multiple crystalline phases with different structural and magnetic properties. Specifically, magnetite

**173**

*Magnetic Iron Oxide Colloids for Environmental Applications*

*Gesellschaft. Reproduced by permission of IOP Publishing. CC BY-NC-SA.*

(Fe3O4) and maghemite (γ-Fe2O3, the oxidized form of magnetite) have an interesting magnetic crystalline structure that can be described as a cubic inverse spinel structure with O2 shaping an fcc structure and Fe cations in the tetrahedral and octahedral sites, as presented in **Figure 4**. These are the two most common phases used in environmental applications due to their high magnetic susceptibility and good chemical stability. Other iron oxides like hematite (α-Fe2O3) or goethite (α-FeOOH) frequently found in nature, present poor magnetic properties although they present

*Typical inverse spinel crystalline structure of magnetite [70]. © IOP Publishing and Deutsche Physikalische* 

Different methods of synthesis have been studied and optimized so far in order to improve the physicochemical features of the magnetic iron oxide colloids. By selecting the proper method and controlling their key parameters (solvent, temperature, reaction time…) it is possible to generate IONPs of specific morphologies, size distributions and to control their colloidal stability. Each synthesis method presents specific advantages and drawbacks, therefore none of them can be declared as the universal method for producing IONPs. Between top-down or bottom-up approaches, the last ones are the most commonly used for large scale production because they offer a better control on the production of uniform nanoparticles with less defects, more homogeneous in shape, and better short and long range ordering (better crystallinity). This bottom-up category can be subclassified as a function of the reaction media, as aqueous synthesis (coprecipitation, hydrothermal and electrochemical, among others) or organic synthesis (thermal decomposition, polyol process, etc.) [71, 72]. In this section we will focus our attention in three different methods for comparison purposes: coprecipitation, thermal decomposition and

This is probably the most common and simplest method of synthesis of IONPs. Magnetic nanoparticles of magnetite or maghemite can be produced by coprecipitation of a stoichiometric mixture of Fe (II) and Fe (III) salts in an alkaline medium such as sodium hydroxide or ammonium hydroxide, for example. It is possible to obtain particles with diameters between 5 and 15 nm by controlling the synthesis key parameters like pH, temperature, addition rate of iron precursors and concentration of precursors

2 3 28 4 34 2

++ − *Fe Fe OH Fe O H O* ++ → + (1)

[73]. For magnetite formation the overall reaction can be described as Eq. (1):

unique and different advantages for other specific applications [69].

polyol-based hydrothermal method.

**3.1 Synthesis techniques**

*3.1.1 Coprecipitation*

**Figure 4.**

*DOI: http://dx.doi.org/10.5772/intechopen.95351*

*Magnetic Iron Oxide Colloids for Environmental Applications DOI: http://dx.doi.org/10.5772/intechopen.95351*

**Figure 4.**

*Colloids - Types, Preparation and Applications*

synthesis catalyzed by a mesoporous iron oxide nanoparticle-decorated graphene oxide [63]. Here, they showed that the designed hierarchically mesoporous material can hinder the contact of the syngas with the active sites, highlighting that the improved mesoporous structure of the IONPs is extremely beneficial for reactants access and products release. Zhang *et al.* postulated that maybe metals atoms with multiple valences, including Fe and their oxides, result partially reduced during pyrolysis processes and generate oxygen vacancies that might transform some volatile biomass compounds into bio-oil [64]. The high specificity in the catalytic cracking of spent engine oils, reducing the undesired aromatics and high molecular weight constituents in a produced diesel-fuel, has been demonstrated also for

More complex nanostructures, like CaO@Fe3O4 composites have been shown to increase yields in the transesterification reaction of vegetable oils with no need of additional base compounds. It is also possible to improve the biodiesel production by immobilizing enzymes like lipase over IONPs. In this way there will be no need of purification after the reaction as these catalysts decrease yields of toxic byproducts [66]. Furthermore, Teo *et al.* prepared a highly recyclable CaSO4/Fe2O3-SiO2 catalyst for biodiesel production, showing its efficiency in the reaction [67].

In spite of the promising results in the use of magnetic colloids in this area, the literature is quite limited, and it is needed to strengthen the efforts in the years ahead to evaluate the potential of these catalysts in the efficient production of alternative fuels. It should be mentioned that besides the benefits of high surface areas, high selectivity and specificity and the ability to be functionalized, IONPs can offer a selective heating at the nanoparticle surface under an alternating magnetic field that may enhance the reaction rate and yield as will be described in Section 4.

The past two decades have seen tremendous advances in the synthesis and application of IONPs that take advantage of their distinct properties and functionalities. As seen in **Figure 3**, the economical perspectives for the market of magnetite nanoparticles is in continuous growth mainly boosted by their use in biomedical applications but also in fields like energy and wastewater treatment. As the interest for IONPs in different applications rises, the demand of new ways and technologies

Iron is one of the most abundant elements in nature presenting multiple crystalline phases with different structural and magnetic properties. Specifically, magnetite

simple natural magnetite particles in the micrometer range [65].

**3. Preparation of iron oxide magnetic colloids**

*U.S. market perspectives on the application of magnetite nanoparticles [68].*

to produce them also increases.

**172**

**Figure 3.**

*Typical inverse spinel crystalline structure of magnetite [70]. © IOP Publishing and Deutsche Physikalische Gesellschaft. Reproduced by permission of IOP Publishing. CC BY-NC-SA.*

(Fe3O4) and maghemite (γ-Fe2O3, the oxidized form of magnetite) have an interesting magnetic crystalline structure that can be described as a cubic inverse spinel structure with O2 shaping an fcc structure and Fe cations in the tetrahedral and octahedral sites, as presented in **Figure 4**. These are the two most common phases used in environmental applications due to their high magnetic susceptibility and good chemical stability. Other iron oxides like hematite (α-Fe2O3) or goethite (α-FeOOH) frequently found in nature, present poor magnetic properties although they present unique and different advantages for other specific applications [69].

Different methods of synthesis have been studied and optimized so far in order to improve the physicochemical features of the magnetic iron oxide colloids. By selecting the proper method and controlling their key parameters (solvent, temperature, reaction time…) it is possible to generate IONPs of specific morphologies, size distributions and to control their colloidal stability. Each synthesis method presents specific advantages and drawbacks, therefore none of them can be declared as the universal method for producing IONPs. Between top-down or bottom-up approaches, the last ones are the most commonly used for large scale production because they offer a better control on the production of uniform nanoparticles with less defects, more homogeneous in shape, and better short and long range ordering (better crystallinity). This bottom-up category can be subclassified as a function of the reaction media, as aqueous synthesis (coprecipitation, hydrothermal and electrochemical, among others) or organic synthesis (thermal decomposition, polyol process, etc.) [71, 72]. In this section we will focus our attention in three different methods for comparison purposes: coprecipitation, thermal decomposition and polyol-based hydrothermal method.

#### **3.1 Synthesis techniques**

#### *3.1.1 Coprecipitation*

This is probably the most common and simplest method of synthesis of IONPs. Magnetic nanoparticles of magnetite or maghemite can be produced by coprecipitation of a stoichiometric mixture of Fe (II) and Fe (III) salts in an alkaline medium such as sodium hydroxide or ammonium hydroxide, for example. It is possible to obtain particles with diameters between 5 and 15 nm by controlling the synthesis key parameters like pH, temperature, addition rate of iron precursors and concentration of precursors [73]. For magnetite formation the overall reaction can be described as Eq. (1):

$$\rm{Fe^{2+}} + 2Fe^{3+} + 8OH^- \rightarrow Fe\_3O\_4 + 4H\_2O \tag{1}$$

The complete precipitation of magnetite is obtained in pH ranging from 8 to 14 with Fe3+/Fe2+ ratios of 2:1, usually in an oxygen free environment to avoid premature Fe2+ oxidation. As magnetite usually is sensitive to oxidation by air, for environmental applications results much better to work with maghemite that will preserve its properties throughout the processes [72]. Magnetite is transformed to maghemite by heating up to 250°C or by acid treatment with nitrate/nitric acid. The main problem using this synthesis method is that the IONPs obtained, usually present a wide particle size distribution and poor crystallinity since they are prepared at temperatures below 100°C.

#### *3.1.2 Thermal decomposition*

This synthesis technique is based on the decomposition of organometallic precursors of Fe in high boiling point organic solvents in the absence of oxygen and the presence of massive amount of surfactants. It is possible to finely tune the size and shape of the IONPs just by controlling the boiling temperature of the solvent, the reactivity and concentration of the iron precursor, and the surfactants (typically fatty acids) chain lengths. The high reaction temperature used in thermal decomposition (>300°C), creates IONPs with narrow size distributions and excellent crystallinity in a range size between 5 and 100 nm [74, 75]. Due to the presence of organic surfactants (oleic acid, oleylamine…), the raw product has hydrophobic character and forms stable dispersions in many organic solvents like hexane, cyclohexane and toluene. However, these particles cannot be dispersed in water, being necessary a second step, like a ligand exchange reaction or the coating with an amphiphilic polymer, to transfer the synthesized IONPs to aqueous medium [76].

#### *3.1.3 Polyol method*

The polyol synthesis method was specifically designed for the development of nanostructured materials. Polyols are a family of solvents whose characteristics and properties (boiling temperature, viscosity, polarity) depends mainly on the length and alcohol substitution of methylene chains. They take advantage of the boiling temperatures of multivalent alcohols in their liquid phase to fix the temperature of reaction. It is interesting to highlight that the boiling temperature increases with the number of –OH moieties, the same with the molecular weight, viscosity and polarity [77, 78]. The main advantage of the use of polyols is that they provide reaction temperatures like the ones obtained in organic media, but the obtained IONPs are hydrophilic and can be dispersed in water like the ones produced in aqueous media.

Due to the diversity of polyols, it is possible to control the reaction temperature, just by selecting one with the interested boiling point. These temperatures can range between 200 and 320°C. Besides, glycol chains of the polyols can be used to control the particle sizes. **Figure 5** shows the TEM images of different IONPs with their size distribution obtained by Hachani *et al.* [79] where they proved that the length of the polyol chain is strictly related to the size of the obtained particles. They confirmed by thermal gravimetric analyses that each polyol was attached to the surface supporting the crucial role of the solvent on the growth of the IONPs. Moreover, thanks to the high polarity of the polyols, many common metallic salts are soluble and can be used as precursors for the synthesis of magnetite or other cobalt or zinc ferrites [80].

This well-known synthesis method was firstly described in 1989 by Fievet *et al.* when they synthesized metal powders of gold, copper, cobalt and lead in the micrometer range [81]. In that seminal work, they carried out reactions in a polyolbased media from the ionic form of each oxide, hydroxide or salt. In general, they

**175**

parameters [83].

**Figure 5.**

**3.2 Colloidal stabilization**

*Magnetic Iron Oxide Colloids for Environmental Applications*

demonstrated that polyols can act simultaneously as solvents, reducing agents and in certain cases as protective agents. Moreover, surfactant abilities of polyols are usually considered weak due to their relatively low molecular weighted molecules, thus they can be easily removed and exchanged by specific functional groups

*Iron oxide nanoparticles synthesized with different polyols: (A) Tetraethylene glycol, (B) triethylene glycol and* 

An interesting approach to synthesize single (5–15 nm) and multicore (20–300 nm) IONPs and other metal ferrites is to combine the polyol method with a microwave assisted heating or with high pressure autoclaves. The microwave assisted polyol method is a versatile technique with improved yields, shorter residence times and highly reproducibility. The polyol molecules are able to adsorb microwave radiation due to their high polarity, with dielectric constants ranging from 20 to 45 [80, 82]. On the other hand, the polyol synthesis performed in a high pressure autoclave can decrease reaction times in a well-sealed environment and control of the size and the aggregation of the particles by controlling the synthesis

Magnetic attraction between nanoparticles compromise the colloidal stability of the suspension inducing agglomeration or even large precipitates [72, 84]. Depending on the application, the lack of colloidal stability may reduce the efficiency of the material. Therefore, it is necessary to develop compatible coatings that increase either the electrostatic or the steric repulsion between IONPs. Usually, IONPs are coated with polymers, surfactant agents, ligands, or inorganic materials like noble

*DOI: http://dx.doi.org/10.5772/intechopen.95351*

depending on the IONPs application [81].

*(C) diethylene glycol [79] - Published by The Royal Society of Chemistry.*

*Magnetic Iron Oxide Colloids for Environmental Applications DOI: http://dx.doi.org/10.5772/intechopen.95351*

#### **Figure 5.**

*Colloids - Types, Preparation and Applications*

temperatures below 100°C.

*3.1.2 Thermal decomposition*

*3.1.3 Polyol method*

The complete precipitation of magnetite is obtained in pH ranging from 8 to 14 with Fe3+/Fe2+ ratios of 2:1, usually in an oxygen free environment to avoid premature Fe2+ oxidation. As magnetite usually is sensitive to oxidation by air, for environmental applications results much better to work with maghemite that will preserve its properties throughout the processes [72]. Magnetite is transformed to maghemite by heating up to 250°C or by acid treatment with nitrate/nitric acid. The main problem using this synthesis method is that the IONPs obtained, usually present a wide particle size distribution and poor crystallinity since they are prepared at

This synthesis technique is based on the decomposition of organometallic precursors of Fe in high boiling point organic solvents in the absence of oxygen and the presence of massive amount of surfactants. It is possible to finely tune the size and shape of the IONPs just by controlling the boiling temperature of the solvent, the reactivity and concentration of the iron precursor, and the surfactants (typically fatty acids) chain lengths. The high reaction temperature used in thermal decomposition (>300°C), creates IONPs with narrow size distributions and excellent crystallinity in a range size between 5 and 100 nm [74, 75]. Due to the presence of organic surfactants (oleic acid, oleylamine…), the raw product has hydrophobic character and forms stable dispersions in many organic solvents like hexane, cyclohexane and toluene. However, these particles cannot be dispersed in water, being necessary a second step, like a ligand exchange reaction or the coating with an amphiphilic

polymer, to transfer the synthesized IONPs to aqueous medium [76].

The polyol synthesis method was specifically designed for the development of nanostructured materials. Polyols are a family of solvents whose characteristics and properties (boiling temperature, viscosity, polarity) depends mainly on the length and alcohol substitution of methylene chains. They take advantage of the boiling temperatures of multivalent alcohols in their liquid phase to fix the temperature of reaction. It is interesting to highlight that the boiling temperature increases with the number of –OH moieties, the same with the molecular weight, viscosity and polarity [77, 78]. The main advantage of the use of polyols is that they provide reaction temperatures like the ones obtained in organic media, but the obtained IONPs are hydrophilic and can be dispersed in water like the ones produced in aqueous media. Due to the diversity of polyols, it is possible to control the reaction temperature,

just by selecting one with the interested boiling point. These temperatures can range between 200 and 320°C. Besides, glycol chains of the polyols can be used to control the particle sizes. **Figure 5** shows the TEM images of different IONPs with their size distribution obtained by Hachani *et al.* [79] where they proved that the length of the polyol chain is strictly related to the size of the obtained particles. They confirmed by thermal gravimetric analyses that each polyol was attached to the surface supporting the crucial role of the solvent on the growth of the IONPs. Moreover, thanks to the high polarity of the polyols, many common metallic salts are soluble and can be used as precursors for the synthesis of magnetite or other

This well-known synthesis method was firstly described in 1989 by Fievet *et al.* when they synthesized metal powders of gold, copper, cobalt and lead in the micrometer range [81]. In that seminal work, they carried out reactions in a polyolbased media from the ionic form of each oxide, hydroxide or salt. In general, they

**174**

cobalt or zinc ferrites [80].

*Iron oxide nanoparticles synthesized with different polyols: (A) Tetraethylene glycol, (B) triethylene glycol and (C) diethylene glycol [79] - Published by The Royal Society of Chemistry.*

demonstrated that polyols can act simultaneously as solvents, reducing agents and in certain cases as protective agents. Moreover, surfactant abilities of polyols are usually considered weak due to their relatively low molecular weighted molecules, thus they can be easily removed and exchanged by specific functional groups depending on the IONPs application [81].

An interesting approach to synthesize single (5–15 nm) and multicore (20–300 nm) IONPs and other metal ferrites is to combine the polyol method with a microwave assisted heating or with high pressure autoclaves. The microwave assisted polyol method is a versatile technique with improved yields, shorter residence times and highly reproducibility. The polyol molecules are able to adsorb microwave radiation due to their high polarity, with dielectric constants ranging from 20 to 45 [80, 82]. On the other hand, the polyol synthesis performed in a high pressure autoclave can decrease reaction times in a well-sealed environment and control of the size and the aggregation of the particles by controlling the synthesis parameters [83].

#### **3.2 Colloidal stabilization**

Magnetic attraction between nanoparticles compromise the colloidal stability of the suspension inducing agglomeration or even large precipitates [72, 84]. Depending on the application, the lack of colloidal stability may reduce the efficiency of the material. Therefore, it is necessary to develop compatible coatings that increase either the electrostatic or the steric repulsion between IONPs. Usually, IONPs are coated with polymers, surfactant agents, ligands, or inorganic materials like noble

metals, oxides or silica that can prevent the dissolution of the particle, stabilize them at the working pH or introduce functional groups for the attachment of specific molecules. For environmental applications, tuning the nanoparticle surface, either in aqueous media by using small molecules or inorganic coating, or in organic media using surfactants, has been shown to improve its adsorption and catalytic capacities.

#### *3.2.1 Small molecules*

The modification of the nanoparticle surface with small molecules having a carboxylic or phosphate group with high coordination capacity to the iron atoms assures long term colloidal stability. The variety of molecules used is immense, some of the most commonly used are phosphonates, dimercaptosuccinic acid (DMSA), 11-mercaptoundecanoic acid or citric acid, small molecules with charged functional groups that provide excellent electrostatic stability.

If the IONPs were synthesized in aqueous media, the coating molecules can be introduced directly in the reaction media or preferably in a second step after the synthesis of the nanoparticles. In the case of IONPs obtained by the thermal decomposition method, it is possible to introduce functional groups by ligand exchange during the water transference. For example, in the coating with DMSA, one of the carboxylic groups of this molecule would replace the one at the surface of the IONPs with oleic acid, and the other carboxylic groups will remain facing outwards providing high negative surface charge in a wide pH range and a carboxylic functional group for further functionalization depending on the application [85].

#### *3.2.2 Engineered silica coating*

Coatings with silica (SiO2) have now become a promising and important pathway for the development of coated magnetic colloids for different applications due to its biocompatibility, stability, easy conjugation with different functional groups that offer high selectivity and specificity [72]. Most of the SiO2 coating strategies for magnetic colloids result in core-shell structures with an ionic positive charge that activate the surface and avoids aggregation (isoelectric point = 2–3). This diamagnetic coating reduces the magnetization per gram of material, but also increase the colloidal stability avoiding aggregation issues that results inconvenient for many applications [86].

The coating routes to obtain IONPs@SiO2 can be divided into three categories: pre-synthesized silica matrices, in-situ fabrication of core-shell structures, and silica coating in already synthesized nanoparticles. This last one been the most common technique, where Stöber method is the easiest pathway to obtain homogeneous particles by hydrolysis and polycondensation of tetraethyl orthosilicate under alkaline conditions with temperatures above 60 °C [86]. SiO2 layer over the IONPs can also be growth by a sol–gel process, where the silica shell is limited by a water-in-oil reverse microemulsion [87]. With these processes it is possible to control the shell thickness and to design a matrix with enhanced properties for specific applications.

It is also possible to design a high surface area Fe3O4@SiO2 nanostructures where mesopores can be potentiated by porogenic agents that allow its in-situ formation through the SiO2 shell. In this approach, nanoparticles are first coated by reverse microemulsion to add a first protective silica layer and the porogenic-doped shell is added by the Stöber method in a secondary step [58]. A schematic pathway for these kind of approach is showed in **Figure 6** were it is also pointed out how the porosity of silica engineered structures can be incremented by creating a hollow structure with hydrothermal or etching methods [88]. This example shows how the multitude of designing parameters of this kind of grafting molecules convert the Fe3O4@SiO2 nanocomposites in a versatile material for environmental processes.

**177**

*Magnetic Iron Oxide Colloids for Environmental Applications*

solvents like hexane, toluene, cyclohexane, etc. [84].

*Grafting iron oxide nanoparticles with engineered silica structures.*

**4. Promising magnetic features**

Since many alternative energies processes are performed with oils or organic solvents, it is also important to develop IONPs soluble in organic media. Just as hydrophobic particles can undergo a ligand exchange to be redispersed in water, hydrophilic ones can be coated with molecules that allow their dispersion in organic media. For example, an interesting way to increase the stability in organic solvents is to add, in an additional step after the synthesis of the nanoparticles, a surfactant with a hydrophobic end. This step consists in the mixture of the surfactant on the IONPs aqueous suspension at ~80 °C and is frequently used for ferrofluids preparation. Oleic acid is one the most common compounds used for these purposes as it is a fatty acid formed by a terminal carboxylic acid group (–COOH) and a long hydrophobic alkyl chain. The resulting particles are quite stable in many organic

For biodiesel production some studies have shown that iron oxides catalysts modified with polymers presented higher efficiencies [89]. Calcining organic compounds over nanoparticles surfaces can lead to a high surface area material with

The unique magnetic features of the iron oxide colloids represent one of the most exploitable characteristics of these materials for developing novel applications in the environmental area. In this section, we will review the physical principles of the superparamagnetic behavior observed in magnetic particles at the nanoscale

At the macroscale, the magnetic materials minimize their magnetic energy breaking the alignment of their atomic moments into regions of coherent magnetization known as magnetic domains. The size of these domains depends on the anisotropy and saturation magnetization of the material. When the size of the solid is reduced to the size of a single magnetic domain, all the atomic magnetic moments of the material are oriented in the same direction and rotate coherently with the applied field (macrospin approximation). If we continue decreasing their size,

carbonaceous residues for enhanced adsorption and catalytic properties.

and how it can be used in water remediation and biofuel generation.

*DOI: http://dx.doi.org/10.5772/intechopen.95351*

*3.2.3 Organic coatings*

**Figure 6.**

*Magnetic Iron Oxide Colloids for Environmental Applications DOI: http://dx.doi.org/10.5772/intechopen.95351*

#### **Figure 6.**

*Colloids - Types, Preparation and Applications*

*3.2.1 Small molecules*

*3.2.2 Engineered silica coating*

metals, oxides or silica that can prevent the dissolution of the particle, stabilize them at the working pH or introduce functional groups for the attachment of specific molecules. For environmental applications, tuning the nanoparticle surface, either in aqueous media by using small molecules or inorganic coating, or in organic media using surfactants, has been shown to improve its adsorption and catalytic capacities.

The modification of the nanoparticle surface with small molecules having a carboxylic or phosphate group with high coordination capacity to the iron atoms assures long term colloidal stability. The variety of molecules used is immense, some of the most commonly used are phosphonates, dimercaptosuccinic acid (DMSA), 11-mercaptoundecanoic acid or citric acid, small molecules with charged

If the IONPs were synthesized in aqueous media, the coating molecules can be introduced directly in the reaction media or preferably in a second step after the synthesis of the nanoparticles. In the case of IONPs obtained by the thermal decomposition method, it is possible to introduce functional groups by ligand exchange during the water transference. For example, in the coating with DMSA, one of the carboxylic groups of this molecule would replace the one at the surface of the IONPs with oleic acid, and the other carboxylic groups will remain facing outwards providing high negative surface charge in a wide pH range and a carboxylic functional group for further functionalization depending on the application [85].

Coatings with silica (SiO2) have now become a promising and important pathway for the development of coated magnetic colloids for different applications due to its biocompatibility, stability, easy conjugation with different functional groups that offer high selectivity and specificity [72]. Most of the SiO2 coating strategies for magnetic colloids result in core-shell structures with an ionic positive charge that activate the surface and avoids aggregation (isoelectric point = 2–3). This diamagnetic coating reduces the magnetization per gram of material, but also increase the colloidal stability avoiding aggregation issues that results inconvenient for many applications [86]. The coating routes to obtain IONPs@SiO2 can be divided into three categories: pre-synthesized silica matrices, in-situ fabrication of core-shell structures, and silica coating in already synthesized nanoparticles. This last one been the most common technique, where Stöber method is the easiest pathway to obtain homogeneous particles by hydrolysis and polycondensation of tetraethyl orthosilicate under alkaline conditions with temperatures above 60 °C [86]. SiO2 layer over the IONPs can also be growth by a sol–gel process, where the silica shell is limited by a water-in-oil reverse microemulsion [87]. With these processes it is possible to control the shell thickness and to design a matrix with enhanced properties for specific applications. It is also possible to design a high surface area Fe3O4@SiO2 nanostructures where mesopores can be potentiated by porogenic agents that allow its in-situ formation through the SiO2 shell. In this approach, nanoparticles are first coated by reverse microemulsion to add a first protective silica layer and the porogenic-doped shell is added by the Stöber method in a secondary step [58]. A schematic pathway for these kind of approach is showed in **Figure 6** were it is also pointed out how the porosity of silica engineered structures can be incremented by creating a hollow structure with hydrothermal or etching methods [88]. This example shows how the multitude of designing parameters of this kind of grafting molecules convert the Fe3O4@SiO2

nanocomposites in a versatile material for environmental processes.

functional groups that provide excellent electrostatic stability.

**176**

*Grafting iron oxide nanoparticles with engineered silica structures.*

#### *3.2.3 Organic coatings*

Since many alternative energies processes are performed with oils or organic solvents, it is also important to develop IONPs soluble in organic media. Just as hydrophobic particles can undergo a ligand exchange to be redispersed in water, hydrophilic ones can be coated with molecules that allow their dispersion in organic media. For example, an interesting way to increase the stability in organic solvents is to add, in an additional step after the synthesis of the nanoparticles, a surfactant with a hydrophobic end. This step consists in the mixture of the surfactant on the IONPs aqueous suspension at ~80 °C and is frequently used for ferrofluids preparation. Oleic acid is one the most common compounds used for these purposes as it is a fatty acid formed by a terminal carboxylic acid group (–COOH) and a long hydrophobic alkyl chain. The resulting particles are quite stable in many organic solvents like hexane, toluene, cyclohexane, etc. [84].

For biodiesel production some studies have shown that iron oxides catalysts modified with polymers presented higher efficiencies [89]. Calcining organic compounds over nanoparticles surfaces can lead to a high surface area material with carbonaceous residues for enhanced adsorption and catalytic properties.

#### **4. Promising magnetic features**

The unique magnetic features of the iron oxide colloids represent one of the most exploitable characteristics of these materials for developing novel applications in the environmental area. In this section, we will review the physical principles of the superparamagnetic behavior observed in magnetic particles at the nanoscale and how it can be used in water remediation and biofuel generation.

At the macroscale, the magnetic materials minimize their magnetic energy breaking the alignment of their atomic moments into regions of coherent magnetization known as magnetic domains. The size of these domains depends on the anisotropy and saturation magnetization of the material. When the size of the solid is reduced to the size of a single magnetic domain, all the atomic magnetic moments of the material are oriented in the same direction and rotate coherently with the applied field (macrospin approximation). If we continue decreasing their size,

we observe the characteristic magnetic response of small magnetic nanoparticles known as superparamagnetism. For magnetite nanoparticles smaller than 50 nm, the thermal fluctuations observed at room temperature are able to disorder the moments, cancelling the global magnetization of the sample. Consequently, in the absence of a magnetic field, the superparamagnetic nanoparticles present no remanent magnetization, avoiding the instability problems related to magnetic aggregation. However, when a magnetic field is applied, for example by approaching a magnet, the nanoparticles recover their magnetism with a high susceptibility and will be dragged towards the magnet proximity [90, 91].

Interestingly, when the superparamagnetic nanoparticles are subjected to an alternating magnetic field, they are able to absorb the magnetic energy and dissipate it as heat. The applied AMF forces the inversion of the spins of the atoms in a hysteretic process. During this process of magnetization reversal, the AMF energy will be transformed into heat increasing the temperature of the close environment of the nanoparticles. The way in which IONPs dissipate energy depends on the relaxation process and it is a function of the particle size, magnetic anisotropy and the viscosity of the media. The two principal relaxation mechanisms reported are Brown and Néel [92].

In the first mechanism, Brownian relaxation, the magnetic moment rotates with the particle within the medium, thus it is only observed when the particles are dispersed in a liquid medium. In this case, the time required to reverse moments by this mechanism (τB) depends on the hydrodynamic volume (Vh), the viscosity of the solvent where the particles are located (η) and the absolute temperature (T), as shown by the following expression, Eq. (2) where kB is Boltzmann's constant [93]:

$$
\tau\_{\mathcal{B}} = \frac{\Im V\_h \,\eta}{k\_{\mathcal{B}} T} \tag{2}
$$

On the other hand, the Néel mechanism describes the relaxation of the magnetic moment within the particle crystal axis. This mechanism is always present, and it is the only one that intervenes in the relaxation of magnetic moments when the particles are in compacted powder or in a frozen liquid where they cannot physically rotate. The expression for the relaxation time (τN) of the magnetic moments by Néel mechanism is as follows in Eq. (3):

$$\mathfrak{r}\_N = \mathfrak{r}\_0 e^{\int\_{\mathcal{H}} V\_{\text{MAC}}} \Big\{ \!\!\!/ \!\!/ \!\!/ \!\!/ \!\!/ \!\!/ \!\!/ \!\!/ \!\!/ \!\!/ \!\!/ \!\!/ \!\!/ \!\!/ \!\!/ \!\!/ \!\!/ \!\!/} \mathfrak{r}\_N \tag{3}$$

**179**

**Figure 7.**

*Magnetic Iron Oxide Colloids for Environmental Applications*

pollutant on their surface. On the other hand, they can be selectively heated under an AMF using moderate field conditions, which can rise reaction yields and shorten

Apart of the fact that IONPs can be efficiently recovered with an external magnetic field, facilitating its regeneration and reuse, environmental processes like the presented in previous sections can also be enhanced by taking advantage of the magnetic heating power of IONPs in the presence of AMF. In the case of adsorption with IONPs, Rivera *et al.* presented the improvement of the adsorption capacity of chromium under an AMF [95]. Here, they showed much higher adsorption yields for IONP systems heated up with AMF than with common thermal heating even though both systems were set at the same global temperature. The heat generated by the IONPs is dissipated in their surface what generates local temperature much greater than those measured in the reaction media, giving rise to better adsorption yields. Furthermore, in a more recent work they used the same principle to improve the reaction yields of the methylene blue degradation in the presence of IONPs and

The advanced oxidation processes have also been benefited by the use of IONPs, although the influence of the AMF has not been deeply studied yet. **Figure 7** shows the reaction mechanism that these particles can undertake in combination with hydrogen peroxide using the potential of IONPs as in situ nanoheaters. There are just a few references on this matter, where typical studies only focus in the increasing degradation yields with the increasing temperatures in common thermal reactors but they do not take advantage of the IONPs selective heating. Among the limited references on the subject, Munoz *et al.* were also able to prove that the catalytic wet peroxide oxidation of antibiotic sulfamethoxazole presented rates

significantly faster with an AMF than in a typical CWPO system [97].

Recently, magnetic colloids have been used as catalysts to enhance biomass hydrodeoxygenation reaction with magnetic induction heating, proving that this heating can provide a better environment for the reaction to take place in [98]. Furthermore, the potential heating of these catalysts have only been analyzed in a few environmental reaction mechanisms. One of them is the CO2 methanation presented by Rivas-Murias *et al.* where they achieved conversions >95% using a cobalt ferrite catalyst under a 93 kHz and 53 mT AMF with a SAR value of 270 W/g [99].

*An environmental catalytic process: Advanced oxidation of organic pollutants using iron oxide nanoparticles* 

*under an alternating magnetic field (AMF). (R: degradation products).*

*DOI: http://dx.doi.org/10.5772/intechopen.95351*

**4.1 Boosting environmental processes**

residence times.

AMF [96].

where, T is the temperature, VMAG the magnetic volume of the particle, Keff the energy by unit of volume needed to reverse the magnetic moment orientation (effective anisotropy) and τ0 the inverse value of the Larmor frequency [94].

When the particles are small (<~20 nm), we can consider that τN < < τB and the relaxation of the magnetic moment takes place by Néel relaxation. On the other hand, for larger nanoparticles in which the magnetic moment is blocked in the direction of the easy axis of magnetization within the particle, it is satisfied that τB < < τN, and the main relaxation mechanism is the Brownian rotation. The superparamagnetic IONPs that are usually employed in environmental applications are in an intermediate range in which both relaxation mechanisms might be present [94].

This superparamagnetic behavior is beneficial for wastewater treatment and catalysis in two aspects. On one hand, IONPs can be dispersed in the absence of a magnetic field without problems of magnetic aggregation and later be separated with a magnet once they have achieved their purpose like adsorbed a specific

pollutant on their surface. On the other hand, they can be selectively heated under an AMF using moderate field conditions, which can rise reaction yields and shorten residence times.
