**Abstract**

This chapter deals with magnetic colloids with catalytic properties for the treatment of polluted waters and the efficient production of fuel alternatives. This kind of materials presents great advantages such as high surface/volume ratio, reproducibility, selectivity, ability to be magnetic harvested, functionalizable surfaces (e.g. with tunable pores and selective chelators deposited on them), high efficiencies and reusability. In particular, this chapter will consider the case of magnetic iron oxide colloids, which can be easily synthesized at low cost, are biocompatible and presents a well-developed surface chemistry. The most common techniques for the synthesis and functionalization of these magnetic nanoparticles will be reviewed and summarized. The iron oxide nanoparticles present outstanding properties that can be exploited in different aspect of the wastewater treatment such as heavy metals and organic pollutants removal by ionic exchange or adsorption, and degradation of the contaminants by advanced oxidation processes, among others. In the field of alternative energies, they have also been used as catalysts for biofuels production from oil crops, in Fischer-Tropsch reactions for liquid hydrocarbons and many other processes with potential environmental impact.

**Keywords:** magnetic colloids, iron oxide nanoparticles, renewable energies, water remediation, biofuels, pollutant, degradation, adsorption

### **1. Introduction: Environmental challenges**

The incessant deterioration of the environment caused by anthropogenic activities, including industrial ones, has been an issue of great concern over the last few decades. The modern society demands the development of novel technological solutions able to create a more efficient and eco-friendly industry. Nanotechnology has the potential to improve traditional environmental remediation technologies through cleaner processes at a reduced cost. This global "nanorevolution" has located engineered nanomaterials and in particular, magnetic iron oxide colloids, under the spotlight for environmental applications such as water treatments and renewable energies solutions.

#### **1.1 Water management**

Water management has emerged as a global issue while the governmental agencies in many countries are stepping up to combat climate change. Pollution of oceans, water sources eutrophication, pollution of effluents by heavy metals and industrials wastes, the spread of desserts and the restricted access to drinking water are all challenges that demand the development of alternative treatment techniques and cleaner industrial processes [1].

Water and wastewater treatment generally includes up to four different stages that encompasses chemical, physical and biological processes [2]. Usually, wastewater needs to go through a preliminary treatment aimed to easily separate large residues, and in some cases a pre-aeration process. Once pretreated, the following stage known as primary treatment consists on processes of sedimentation and smaller sieving. The secondary treatment comprises more complex biological and physicochemical techniques. Finally, in a tertiary treatment, water is disinfected and processed to adequate the biologic oxygen demand and heavy metals concentrations by simple methods like adsorption or filtration [3]. Magnetic iron oxides colloids are a well-known alternative for adsorption processes mentioned in ternary treatments, since they present a high relative surface area and ease of functionalization that increase their adsorption capacity, selectivity and facilitate the separation by magnetic harvesting [4, 5].

Typically, the secondary water treatment stage requires more efforts and presents more challenging inconveniences [6]. Depending on the wastewater effluent composition, this stage may comprise different processes either physicochemical or biological. Biological treatments like aerobic and anaerobic processing are usually implemented to efficiently eliminate and remove organic matter by transforming it into harmless compounds. These treatments may fail when the effluent is not biodegradable with a relation between chemical oxygen demand and biological oxygen demand above 4 (COD/BOD>4) [7]. In those cases, a physicochemical alternative results more convenient, being advanced oxidation processes (AOPs) the most promising and exploitable ones [8]. These AOPs are based on the in-situ generation of highly oxidative hydroxyl radicals or other oxidative species able to purify water by mineralizing the harmful organic matter into CO2, water, salts or inorganic acids. The oxidative species can be generated with the help of oxidizing agents, irradiation or with a catalyst [9, 10]. In the last years, many efforts have been place to improve these AOPs by combining them with photochemical, electrochemical and catalytic techniques and looking for cheaper, eco-friendly and more efficient agents that enable the transference of these technologies to industrial processing of wastewater [11]. In this sense, iron oxide colloids have been proposed as a very interesting catalyst for the degradation of contaminants by AOPs, through the Fe-mediated Fenton and Fenton-like reactions [12].

#### **1.2 Fuels and alternatives**

Fossil fuels are a non-renewable energy resource produced by organic matter from different living beings accumulated hundreds of millions of years at the bottom of lakes or sees with very little oxygen and covered with several layers of sediment. Humanity has known about the existence of fossil fuels since ancient times, becoming the main energy source during the industrial revolution [13]. At present, fossil fuel along with natural gas, are still fundamental for modern's society economy and alternative sources have not yet been found to replace them. The current energy model, based on these fuels, presents serious concerns of sustainability, either due to the emissions of polluting greenhouse gases or the economic and political tensions and therefore, there is a need to search for new alternative energies [14, 15]. In the last decade, different resources have been used to overcome the problematic carried out by fossil fuels, among these are solar, wind, water, natural gas, coal and biomass. **Figure 1** shows how some alternative fuels can be processed

**167**

*Magnetic Iron Oxide Colloids for Environmental Applications*

from different feedstock. The idea of waste valorization comes as a strong alternative for fuel and bio-fuels production. It is possible to reuse, recycle and compost waste materials and convert them into fuels. This is a way to overcome the environmental impact of common fossil fuels while taking advantage of useless wastes like food and wood residues, agricultural and municipal waste and used engine oil,

Biodiesel is a synthetic fuel obtained from natural components such as vegetable oils or animal fats through a transesterification reaction. It is mainly used for the preparation of diesel substitutes and can be mixed with it for commercial purposes. The use of biodiesel offers many advantages against traditional fossil fuels. As this combustible is synthesized from vegetable sources, such as rape, soy or sunflower seeds, it can be considered as an environmentally friendly fuel. It is even possible to manufacture it from recycled oils produced by different food industries [18–20]. Moreover, biodiesel generates less emissions of polluting gases and harmful substances like soot or benzenes. During the last decade the efficient production of biodiesel has been of great importance, and novel catalytic pathways, using for example nanocatalysts, are currently explored to increase the production yields [21]. Another alternative for fuel production comes from the waste valorization for hydrocarbons synthesis. Fischer-Tropsch process obtains gasoline from synthesis gases (syngas). This process was developed in the early 20's to obtain liquid fuels from coal as raw material as an alternative for fossil fuels [22]. But the syngas can be obtained from more sustainable sources, being biomass the one with the less environmental impact. The Fischer-Tropsch alternative process consists in transforming dry biomass into hydrocarbons via gasification with oxygen and using the syngas generated in the process as organic source [23]. As any chemical reaction, an efficient catalyst could lead this process to better yields of the desired product.

Finally, a third strategy to obtain alternative fuels consist on taking advantage of highly pollutant wastes such as used engine oils to produce liquid hydrocarbons. By thermal cracking, it is possible to break down the long, branched and cyclic chains, to obtain less heavy hydrocarbons that are in the order of the 15 to 20 carbons,

Two important parameters play a fundamental role in the enhanced production of alternative fuels: the efficient heating and the catalyst separation. Both of them are usually expensive and time consuming and there is still a lack of efficient processes

which are usually the compounds of diesel [24].

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

among others [16, 17].

*Different feedstock for alternative fuels.*

**Figure 1.**

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

#### **Figure 1.**

*Colloids - Types, Preparation and Applications*

and cleaner industrial processes [1].

by magnetic harvesting [4, 5].

and Fenton-like reactions [12].

**1.2 Fuels and alternatives**

oceans, water sources eutrophication, pollution of effluents by heavy metals and industrials wastes, the spread of desserts and the restricted access to drinking water are all challenges that demand the development of alternative treatment techniques

Water and wastewater treatment generally includes up to four different stages that encompasses chemical, physical and biological processes [2]. Usually, wastewater needs to go through a preliminary treatment aimed to easily separate large residues, and in some cases a pre-aeration process. Once pretreated, the following stage known as primary treatment consists on processes of sedimentation and smaller sieving. The secondary treatment comprises more complex biological and physicochemical techniques. Finally, in a tertiary treatment, water is disinfected and processed to adequate the biologic oxygen demand and heavy metals concentrations by simple methods like adsorption or filtration [3]. Magnetic iron oxides colloids are a well-known alternative for adsorption processes mentioned in ternary treatments, since they present a high relative surface area and ease of functionalization that increase their adsorption capacity, selectivity and facilitate the separation

Typically, the secondary water treatment stage requires more efforts and presents more challenging inconveniences [6]. Depending on the wastewater effluent composition, this stage may comprise different processes either physicochemical or biological. Biological treatments like aerobic and anaerobic processing are usually implemented to efficiently eliminate and remove organic matter by transforming it into harmless compounds. These treatments may fail when the effluent is not biodegradable with a relation between chemical oxygen demand and biological oxygen demand above 4 (COD/BOD>4) [7]. In those cases, a physicochemical alternative results more convenient, being advanced oxidation processes (AOPs) the most promising and exploitable ones [8]. These AOPs are based on the in-situ generation of highly oxidative hydroxyl radicals or other oxidative species able to purify water by mineralizing the harmful organic matter into CO2, water, salts or inorganic acids. The oxidative species can be generated with the help of oxidizing agents, irradiation or with a catalyst [9, 10]. In the last years, many efforts have been place to improve these AOPs by combining them with photochemical, electrochemical and catalytic techniques and looking for cheaper, eco-friendly and more efficient agents that enable the transference of these technologies to industrial processing of wastewater [11]. In this sense, iron oxide colloids have been proposed as a very interesting catalyst for the degradation of contaminants by AOPs, through the Fe-mediated Fenton

Fossil fuels are a non-renewable energy resource produced by organic matter from different living beings accumulated hundreds of millions of years at the bottom of lakes or sees with very little oxygen and covered with several layers of sediment. Humanity has known about the existence of fossil fuels since ancient times, becoming the main energy source during the industrial revolution [13]. At present, fossil fuel along with natural gas, are still fundamental for modern's society economy and alternative sources have not yet been found to replace them. The current energy model, based on these fuels, presents serious concerns of sustainability, either due to the emissions of polluting greenhouse gases or the economic and political tensions and therefore, there is a need to search for new alternative energies [14, 15]. In the last decade, different resources have been used to overcome the problematic carried out by fossil fuels, among these are solar, wind, water, natural gas, coal and biomass. **Figure 1** shows how some alternative fuels can be processed

**166**

*Different feedstock for alternative fuels.*

from different feedstock. The idea of waste valorization comes as a strong alternative for fuel and bio-fuels production. It is possible to reuse, recycle and compost waste materials and convert them into fuels. This is a way to overcome the environmental impact of common fossil fuels while taking advantage of useless wastes like food and wood residues, agricultural and municipal waste and used engine oil, among others [16, 17].

Biodiesel is a synthetic fuel obtained from natural components such as vegetable oils or animal fats through a transesterification reaction. It is mainly used for the preparation of diesel substitutes and can be mixed with it for commercial purposes. The use of biodiesel offers many advantages against traditional fossil fuels. As this combustible is synthesized from vegetable sources, such as rape, soy or sunflower seeds, it can be considered as an environmentally friendly fuel. It is even possible to manufacture it from recycled oils produced by different food industries [18–20]. Moreover, biodiesel generates less emissions of polluting gases and harmful substances like soot or benzenes. During the last decade the efficient production of biodiesel has been of great importance, and novel catalytic pathways, using for example nanocatalysts, are currently explored to increase the production yields [21].

Another alternative for fuel production comes from the waste valorization for hydrocarbons synthesis. Fischer-Tropsch process obtains gasoline from synthesis gases (syngas). This process was developed in the early 20's to obtain liquid fuels from coal as raw material as an alternative for fossil fuels [22]. But the syngas can be obtained from more sustainable sources, being biomass the one with the less environmental impact. The Fischer-Tropsch alternative process consists in transforming dry biomass into hydrocarbons via gasification with oxygen and using the syngas generated in the process as organic source [23]. As any chemical reaction, an efficient catalyst could lead this process to better yields of the desired product.

Finally, a third strategy to obtain alternative fuels consist on taking advantage of highly pollutant wastes such as used engine oils to produce liquid hydrocarbons. By thermal cracking, it is possible to break down the long, branched and cyclic chains, to obtain less heavy hydrocarbons that are in the order of the 15 to 20 carbons, which are usually the compounds of diesel [24].

Two important parameters play a fundamental role in the enhanced production of alternative fuels: the efficient heating and the catalyst separation. Both of them are usually expensive and time consuming and there is still a lack of efficient processes

to provide a profitable alternative. The use of nanocatalysts in all these processes has been extensively studied since they make possible to work at moderated temperatures and increase mass transfer. Recent works in this area have shown that magnetic nanoparticles based on iron oxide could be a powerful tool to address many of the limitations exposed for the efficient waste valorization for fuels production [25, 26].

#### **1.3 Magnetic colloids as alternative materials**

In both applications, wastewater treatment processes and catalytic biodiesel production at industrial scale, it is crucial the use of efficient and inexpensive materials. This is the case of magnetic iron oxide nanoparticles (IONPs) that, as mentioned before, can be used in the former case as adsorbents for heavy metals or organic compounds and for the advanced oxidation of organic matter, and for the latter case, they have demonstrated a high-performance as reaction catalysts [5, 27].

It is essential to design and produce an efficient colloid for the mentioned environmental processes taking into account parameters such as the particle size and the colloidal stability. It should be emphasized that in the case of these colloids, the magnetic properties of the nanoparticles provide important advantages over other commonly used materials as it is the possibility of easy separation by using a magnetic gradient and heating them under an alternating magnetic field [28, 29]. However, magnetic properties are also responsible for the formation of aggregates and agglomerates due to the magnetic interactions inter-particles that reduce the specific surface area and the colloidal stability, limiting the efficiency and possible re-use of the particles [30]. Therefore, it results crucial to design colloids with nanoparticle sizes in the nanometer range (<100 nm) and coatings that provide them electrostatic and, if possible, steric repulsion, to keep magnetic interactions at the minimum and assuring in this way long term colloidal stability.
