**4.1 Boosting environmental processes**

*Colloids - Types, Preparation and Applications*

Brown and Néel [92].

Néel mechanism is as follows in Eq. (3):

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

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

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]:

> <sup>3</sup> <sup>=</sup> *<sup>h</sup> B*

τ

*B V k T* η

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

<sup>0</sup> τ =τ *eff MAG*

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

*K V*

*B*

*k T*

(2)

*<sup>N</sup> e* (3)

and will be dragged towards the magnet proximity [90, 91].

**178**

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 AMF [96].

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].

#### **Figure 7.**

*An environmental catalytic process: Advanced oxidation of organic pollutants using iron oxide nanoparticles under an alternating magnetic field (AMF). (R: degradation products).*

Our research group has recently tested the potential of IONPs as AOPs catalyst and local heating source on the decontamination of landfill leachate and complex textiles wastewater. The mineralization efficiency of the AOP of industrial wastewater was increased by magnetic induction heating of the nanocatalyst resulting in much quicker degradations in the presence of AMF compared to conventional heating. In general, magnetic induction-driven processes are a promising alternative for the improvement of well-known chemical reactions and real wastewaters remediation techniques.

### **5. Conclusion**

This chapter shows the wide range of possible applications of magnetic iron oxide colloids in the field of environmental applications. It probes how iron oxide nanoparticles are excellent platforms to overcome many of the current technological challenges of this area. The chapter reviews the most recent references on water treatment and alternative fuels production in which iron oxide colloids has been used as treatment agents or catalysts. We have covered the areas of adsorption, advanced oxidation processes, hydrocarbon synthesis, biofuels and catalytic cracking of long chain hydrocarbons. Using iron oxide colloids has been proved to be a promising alternative for these processes and recent works in the field have shown an excellent performance of this material.

However, in order to obtain a competitive material, it is important to control parameters such as the particle size, coating and stability of the magnetic colloids. The analysis made here provides details of the most common synthesis and colloidal stabilization methods to create magnetic iron oxide colloids for this application.

The prospection of using iron oxide colloids tends to take advantage of the presence of Fe atoms at the surface working as catalysts for the degradation of contaminants through Fenton reactions, and their magnetic properties, not only for magnetic separation but also for the possibility of heating under an alternating magnetic field, enhancing catalytic and remediation processes.

#### **Acknowledgements**

This research was funded by the Spanish Ministry of Economy and Competitiveness under grant - MAT2017-88148R (AEI/FEDER, UE), PIE-201960E062 project and the EU project H2020-FETOPEN- RIA 829162, HOTZYMES. This study was also supported by USFQ with Collaboration Grant 2018 N° 11197 and PoliGrant 2018-2019 N° 12501. Finally, we acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).

**181**

**Author details**

and Jesús G. Ovejero1

28049 Madrid, Spain

Alvaro Gallo-Cordova1,2\*, Daniela Almeida Streitwieser2

1 Institute of Materials Science of Madrid, ICMM/CSIC, Sor Juana Inés de la Cruz 3,

2 Institute for the Development of Alternative Energies and Materials, IDEMA, Chemical Engineering Department, Universidad San Francisco de Quito USFQ,

\*Address all correspondence to: alvaro.gallo@csic.es and jesus.g.ovejero@csic.es

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Av. Diego de Robles and Vía Interoceánica, EC 170901 Quito, Ecuador

\*

provided the original work is properly cited.

, María del Puerto Morales1

*Magnetic Iron Oxide Colloids for Environmental Applications*

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

### **Conflict of interest**

"The authors declare no conflict of interest."

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

*Colloids - Types, Preparation and Applications*

an excellent performance of this material.

remediation techniques.

**Acknowledgements**

**Conflict of interest**

**5. Conclusion**

Our research group has recently tested the potential of IONPs as AOPs catalyst and local heating source on the decontamination of landfill leachate and complex textiles wastewater. The mineralization efficiency of the AOP of industrial wastewater was increased by magnetic induction heating of the nanocatalyst resulting in much quicker degradations in the presence of AMF compared to conventional heating. In general, magnetic induction-driven processes are a promising alternative for the improvement of well-known chemical reactions and real wastewaters

This chapter shows the wide range of possible applications of magnetic iron oxide colloids in the field of environmental applications. It probes how iron oxide nanoparticles are excellent platforms to overcome many of the current technological challenges of this area. The chapter reviews the most recent references on water treatment and alternative fuels production in which iron oxide colloids has been used as treatment agents or catalysts. We have covered the areas of adsorption, advanced oxidation processes, hydrocarbon synthesis, biofuels and catalytic cracking of long chain hydrocarbons. Using iron oxide colloids has been proved to be a promising alternative for these processes and recent works in the field have shown

However, in order to obtain a competitive material, it is important to control parameters such as the particle size, coating and stability of the magnetic colloids. The analysis made here provides details of the most common synthesis and colloidal stabilization methods to create magnetic iron oxide colloids for this application. The prospection of using iron oxide colloids tends to take advantage of the presence of Fe atoms at the surface working as catalysts for the degradation of contaminants through Fenton reactions, and their magnetic properties, not only for magnetic separation but also for the possibility of heating under an alternating

magnetic field, enhancing catalytic and remediation processes.

through its Unit of Information Resources for Research (URICI).

"The authors declare no conflict of interest."

This research was funded by the Spanish Ministry of Economy and Competitiveness under grant - MAT2017-88148R (AEI/FEDER, UE), PIE-201960E062 project and the EU project H2020-FETOPEN- RIA 829162,

HOTZYMES. This study was also supported by USFQ with Collaboration Grant 2018 N° 11197 and PoliGrant 2018-2019 N° 12501. Finally, we acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative

**180**
