**2. Environmental applications of magnetic iron oxide colloids**

**Figure 2** summarizes in a schematic way some of the applications of IONPs colloids that can be beneficial for environmental conservation. Compared to conventional macroscale materials such as activated carbons or zeolites, IONPs can achieve similar performance with minimum masses reduction on materials and energy costs. To understand how important IONPs are turning out to be, in this section the most recent works on this field will be reviewed.

#### **2.1 Wastewater treatment agents**

IONPs have been extensively studied for water purification and remediation of wastewaters. The three most important properties for these applications are the surface area/volume ratios, the possibility of magnetic harvesting and their surface reactivity. Among the possible uses of IONPs in this field, some of the most highlighted are the adsorption for the removal of contaminants and, less-known is the advanced oxidation processes (AOPs) for its degradation [31–33].

Adsorption is a surface phenomenon where the molecules of a sorbate are bound to the solid surface of the sorbent. In this phenomenon, mass transfer has a remarkable significance in the three steps involved: external diffusion, pore diffusion and surface reaction. In the first step, the adsorbate is transported from the bulk phase to the external surface of the sorbent. The pore diffusion refers to the transport through the sorbent pores so it can get to the final step were the sorbate molecules are attached to the internal surface, for the surface reaction [34]. Once

**169**

**Figure 2.**

mental remediation [41, 42].

*Magnetic Iron Oxide Colloids for Environmental Applications*

adsorbed, the pollutants can be removed by magnetic collection using permanent magnets or alternatively degraded by the characteristic surface chemistry of iron oxide. As mentioned before, AOPs are based on physicochemical processes capable of producing profound changes in the chemical structure of pollutants. The concept was initially established by Glaze and collaborators, who defined AOPs that involve the generation and use of powerful transient species, mainly the hydroxyl radical (HO•) [35]. This radical, and others like HO•, OOH• can be generated by photochemical means (including sunlight) or by other forms of energy and are highly effective for the oxidation of organic matter. Some toxic pollutants that are not very susceptible to oxidation, such as metal ions or halogenated compounds, requires the use of chemical catalyst such as iron oxide [36]. An additional advantage of AOPs, is that they do not only change the pollutant phase but transform it chemically by its

*Examples of applications of magnetic iron oxide colloids with positive impact on the environment.*

**Table 1** summarizes some recent works in the field of adsorption and AOPs using different iron oxide nanoparticles formulations. As it can be observed, IONPs are widely used in this matter due to their unique properties. Their applications for water remediation range from the elimination of inorganic and organic pollutants to the elimination of bacteria, proving that IONPs are materials with great versatility. Between the inorganic contaminants that can be removed by magnetic colloids, the most common are arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb) and zinc (Zn), which can cause, above their maximum levels in water, adverse health effects [38]. On the other hand, organic compounds like azo dyes, which present ecological hazardous effects due to their capacity of obstruct light within streams causing distress in aquatic environments, can also be removed using magnetic colloids [39]. Another class of organic pollutants that have been intensively studied for many researchers in the last decade are the emerging contaminants [40]. IONPs have demonstrated their ability to successfully remove many of these novel contaminants like for example pharmaceutical active chemicals, pesticides, personal cleaning compounds and others proving to be an efficient tool for environ-

complete mineralization with no sludge generation [37].

*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*

*Colloids - Types, Preparation and Applications*

**1.3 Magnetic colloids as alternative materials**

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].

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.

advanced oxidation processes (AOPs) for its degradation [31–33].

most recent works on this field will be reviewed.

**2.1 Wastewater treatment agents**

**2. Environmental applications of magnetic iron oxide colloids**

**Figure 2** summarizes in a schematic way some of the applications of IONPs colloids that can be beneficial for environmental conservation. Compared to conventional macroscale materials such as activated carbons or zeolites, IONPs can achieve similar performance with minimum masses reduction on materials and energy costs. To understand how important IONPs are turning out to be, in this section the

IONPs have been extensively studied for water purification and remediation of wastewaters. The three most important properties for these applications are the surface area/volume ratios, the possibility of magnetic harvesting and their surface reactivity. Among the possible uses of IONPs in this field, some of the most highlighted are the adsorption for the removal of contaminants and, less-known is the

Adsorption is a surface phenomenon where the molecules of a sorbate are bound to the solid surface of the sorbent. In this phenomenon, mass transfer has a remarkable significance in the three steps involved: external diffusion, pore diffusion and surface reaction. In the first step, the adsorbate is transported from the bulk phase to the external surface of the sorbent. The pore diffusion refers to the transport through the sorbent pores so it can get to the final step were the sorbate molecules are attached to the internal surface, for the surface reaction [34]. Once

**168**

**Figure 2.** *Examples of applications of magnetic iron oxide colloids with positive impact on the environment.*

adsorbed, the pollutants can be removed by magnetic collection using permanent magnets or alternatively degraded by the characteristic surface chemistry of iron oxide. As mentioned before, AOPs are based on physicochemical processes capable of producing profound changes in the chemical structure of pollutants. The concept was initially established by Glaze and collaborators, who defined AOPs that involve the generation and use of powerful transient species, mainly the hydroxyl radical (HO•) [35]. This radical, and others like HO•, OOH• can be generated by photochemical means (including sunlight) or by other forms of energy and are highly effective for the oxidation of organic matter. Some toxic pollutants that are not very susceptible to oxidation, such as metal ions or halogenated compounds, requires the use of chemical catalyst such as iron oxide [36]. An additional advantage of AOPs, is that they do not only change the pollutant phase but transform it chemically by its

complete mineralization with no sludge generation [37]. **Table 1** summarizes some recent works in the field of adsorption and AOPs using different iron oxide nanoparticles formulations. As it can be observed, IONPs are widely used in this matter due to their unique properties. Their applications for water remediation range from the elimination of inorganic and organic pollutants to the elimination of bacteria, proving that IONPs are materials with great versatility. Between the inorganic contaminants that can be removed by magnetic colloids, the most common are arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb) and zinc (Zn), which can cause, above their maximum levels in water, adverse health effects [38]. On the other hand, organic compounds like azo dyes, which present ecological hazardous effects due to their capacity of obstruct light within streams causing distress in aquatic environments, can also be removed using magnetic colloids [39]. Another class of organic pollutants that have been intensively studied for many researchers in the last decade are the emerging contaminants [40]. IONPs have demonstrated their ability to successfully remove many of these novel contaminants like for example pharmaceutical active chemicals, pesticides, personal cleaning compounds and others proving to be an efficient tool for environmental remediation [41, 42].


#### **Table 1.**

*Different iron oxide nanoparticles as adsorbents or catalysts for water remediation (references taken from recent works).*

To maximize the adsorption capacity of the magnetic colloids, several parameters can be adjusted to the compound to be removed, that include the surface chelated molecules, the surface charge and the surface area available for the adsorption, being the later one determined by the nanoparticle size and the porosity of the coating.

**171**

*Magnetic Iron Oxide Colloids for Environmental Applications*

chromium usually found in its anionic form (HCrO4−, Cr2O7

with its sorbent is an important parameter to be considered.

IONPs can be efficiently chelated with certain molecules like (3-Aminopropyl) triethoxysilane (APTES) to modify the surface charge and promote the heavy metals adsorption process. Gallo et al. showed how the efficiency of IONPs increased with the increasing positive surface charge [56]. Moreover, the way IONPs are chelated can influence their selectivity for certain compounds. Removal of uranium from nuclear power polluted wastes has become a major issue in water processing since it can cause severe health and ecological problems. Helal et al. developed and efficient IONPs nanosorbent coated with APTES and succinyl-β-cyclodextrin molecules to increase its selectivity for uranium [57]. For other heavy metal ions such as

Silica coating of magnetic nanoparticles is an innovative way to modify the porosity of the sorbents surface and has been widely tested for the adsorption and degradation of different pollutants. Gallo *et al.* designed an interesting mesoporous silica coated IONPs for the adsorption of heavy metals and organic compounds [58]. They observed an interesting remark in which mesopores, growth with porogenic agents (e.g. octadecyltrimethoxysilane), present more affinity for the adsorption of organic compounds than heavy metals, in spite of having larger molecules. In this sense, it is not only possible to optimize the IONPs surface area but also their selectivity for specific compounds. On the other hand, as proved by Wu, *et al.* it is possible to grow the IONPs by spontaneous infiltration over a mesoporous SiO2 template, which results in a much faster and easy way of producing these hybrid materials for the degradation of harmful azo dyes [59]. Even though the active sites available where occupied by IONPs and the adsorption decreased, they observed that the removal was enhanced by the Fenton-like degradation, proving that the hybrids are efficient agents for dyes remediation. Likewise, an interesting approach for wastewater treatment using IONPs@SiO2 is to chelate the shell to increase selectivity for certain compounds. Uranium selectivity of cobalt ferrite nanoparticles coated with SiO2 was studied by Huang *et al.* where they decorated the shell with 2-Phosphonobutane-1,2,4-tricarboxylic acid to increase the affinity sorbent/sorbate [60]. The SiO2 coating was performed by the Stöber method and by coexisting ions tests they proved that the silica matrix can be efficiently chelated to improve selectivity. These examples show the importance of the physico-chemical nature of the coating on the stability, adsorption capacity and catalytic activity of the IONPs [61].

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

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

2−), the ionic attraction

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

**2.2 Catalysts for alternative energies**

cracking of used engine oil, among others.

*Colloids - Types, Preparation and Applications*

Chromium adsorption

Organic and inorganic compounds

Pharmaceuticals adsorption

Fungicides adsorption

Methylene blue adsorption

Cadmium adsorption

Combined adsorption and Fenton oxidation of chlorophenol

Fenton-like degradation of emerging pollutants and arsenic

Photo-Fenton for bacteria inactivation

Sonocatalytic degradation of Rhodamine B and Bisphenol A

Fe3O4 immobilized

Resin-based hydrated

on sand

Fe/La oxide microspheres

iron oxide

Pristine γ-Fe2O3 nanoparticles

Fe3O4/Douglas fir biochar

Zr metal organic framework immobilized onto Fe3O4@SiO2

Chitosan-polyglycidol coated iron oxide

α-Fe2O3/lignosulfonate

Fe3O4 grafted with β-cyclodextrin

Fe3O4/activated carbon

Fe3O4 immobilized on graphene oxide

Fe3O4/SiO2 coated with Polyethylene and polyacrilyc acid

(Ag3PO4)-(Fe3O4)@ activated biochar

composite (no-magnetic)

**Material Process Highlights Ref.**

Arsenic adsorption SiO2 template used for the microsphere

Arsenite adsorption Material presented binding affinity to

times.

Bacteria adsorption Pathogenic bacteria in water can be

harvesting processes.

oxide

Supportive matrix for nanoparticles to avoid agglomeration and enhance adsorption.

synthesis and a double shell material composed of Iron oxide and Lanthanum

Simultaneous removal of ρ-arsanilic acid

arsenite when coexisting with arsenate.

Adsorbent with maximum adsorption capacities for triclosan and triclocarban (476 and 602 mg/g). Material recycled up to 11

Interactions between the coating molecules where analyzed. It was proved that this material can be used for dye removal

The incorporation of organics onto magnetic sorbent can improve the adsorption process of heavy metals

adsorbed and removed by magnetic

Iron oxide was impregnated over porous activated carbon. Adsorption followed the intraparticle diffusion model and 90% degradation was achieved. 5 times recycled.

Simultaneous degradation of ρ-arsanilic acid and adsorption of arsenic.

*E. coli* bacteria photo-Fenton inactivation

H2O2 production was achieved by pyrolysis of water molecules on catalyst surface. Degradation of synthetic dyes, endocrinedisrupting compounds/pharmaceutical active chemicals, and chlorinated compounds was tested.

was achieved at natural pH

Byproduct waste of syn-gas production was used for developing the adsorbent. Caffeine, ibuprofen and acetylsalicylic acid were removed in a fast equilibrium process.

and adsorption of arsenic.

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

[53]

[54]

[55]

**170**

**Table 1.**

*recent works).*

To maximize the adsorption capacity of the magnetic colloids, several parameters can be adjusted to the compound to be removed, that include the surface chelated molecules, the surface charge and the surface area available for the adsorption, being the later one determined by the nanoparticle size and the porosity of the coating.

*Different iron oxide nanoparticles as adsorbents or catalysts for water remediation (references taken from* 

IONPs can be efficiently chelated with certain molecules like (3-Aminopropyl) triethoxysilane (APTES) to modify the surface charge and promote the heavy metals adsorption process. Gallo et al. showed how the efficiency of IONPs increased with the increasing positive surface charge [56]. Moreover, the way IONPs are chelated can influence their selectivity for certain compounds. Removal of uranium from nuclear power polluted wastes has become a major issue in water processing since it can cause severe health and ecological problems. Helal et al. developed and efficient IONPs nanosorbent coated with APTES and succinyl-β-cyclodextrin molecules to increase its selectivity for uranium [57]. For other heavy metal ions such as chromium usually found in its anionic form (HCrO4−, Cr2O7 2−), the ionic attraction with its sorbent is an important parameter to be considered.

Silica coating of magnetic nanoparticles is an innovative way to modify the porosity of the sorbents surface and has been widely tested for the adsorption and degradation of different pollutants. Gallo *et al.* designed an interesting mesoporous silica coated IONPs for the adsorption of heavy metals and organic compounds [58]. They observed an interesting remark in which mesopores, growth with porogenic agents (e.g. octadecyltrimethoxysilane), present more affinity for the adsorption of organic compounds than heavy metals, in spite of having larger molecules. In this sense, it is not only possible to optimize the IONPs surface area but also their selectivity for specific compounds. On the other hand, as proved by Wu, *et al.* it is possible to grow the IONPs by spontaneous infiltration over a mesoporous SiO2 template, which results in a much faster and easy way of producing these hybrid materials for the degradation of harmful azo dyes [59]. Even though the active sites available where occupied by IONPs and the adsorption decreased, they observed that the removal was enhanced by the Fenton-like degradation, proving that the hybrids are efficient agents for dyes remediation. Likewise, an interesting approach for wastewater treatment using IONPs@SiO2 is to chelate the shell to increase selectivity for certain compounds. Uranium selectivity of cobalt ferrite nanoparticles coated with SiO2 was studied by Huang *et al.* where they decorated the shell with 2-Phosphonobutane-1,2,4-tricarboxylic acid to increase the affinity sorbent/sorbate [60]. The SiO2 coating was performed by the Stöber method and by coexisting ions tests they proved that the silica matrix can be efficiently chelated to improve selectivity. These examples show the importance of the physico-chemical nature of the coating on the stability, adsorption capacity and catalytic activity of the IONPs [61].
