**Abstract**

Photocatalysis and high adsorption coupling in a same nanoparticle have been emerged as a prominent class of cost-effective materials to degrade recalcitrant contaminants in wastewater. α-Hematite, metal-organic frameworks and TiO2 nanocomposites have been investigated due to their features that overcome the other conventional photocatalysts and adsorbents to remove contaminants in aqueous medium. Several methods are applied to synthesize these nanostructures with different properties and physicochemical features and a brief review is shown to these well-established techniques to provide an understanding for the construction and application of these advanced materials.

**Keywords:** photocatalytic adsorbents, α-hematite, metal-organic frameworks, TiO2, nanoparticles

#### **1. Introduction**

Most conventional technologies applied to the removal of contaminants dispersed in the aqueous effluent employ the adsorption techniques. However, a class of contaminants, not yet regulated in relation to disposal parameters and/or presence in the environment by the government agencies, may have potentially harmful effects on aquatic biota and humans. However, the adsorption techniques need to be investigated in the removing processes of an important class of contaminants that not yet was regulated in relation to discard in the environment by the government agencies, but that have potentially harmful effects on aquatic biota and humans. These emerging contaminants may be synthetic or natural and their most prominent compounds are the drugs, the personal care products, hormones, flame retardants, perfluoroalkylated substances and pesticides [1, 2]. These compounds are called emerging contaminants and their nomenclature originates from the low concentrations detected in surface waters (μg•l −1 or ng•l −1 ) [3, 4]. The incidences in very small quantities could only be identified in the last 20 years, due to the appearance of new analytical equipment [5].

Compounds of difficult degradation by conventional sewage processes can affect the lives of millions of people due to contamination of water resources [6]. The incidence of recalcitrant molecules introduced into water resources is a serious problem of toxicity and deserves the development and research of specific affluent and effluent treatment systems, with greater process efficiency, greater safety and lower costs [7, 8]. Physicochemical and biological methods currently employed are capable of degrading some of these compounds, but they have a high-cost application, which makes it difficult to implement in regions with limited financial resources [9]. Furthermore, these methods can generate complex residues for treatment or storage, such as sludge and toxic or recalcitrant compounds [10].

Thus, alternative methods have been researched in an attempt to eliminate these contaminants effectively, as the adsorption techniques. The adsorption methods have some important vantages: can be operated with low energy consumer and with low operational cost, and cannot generate secondary compounds [11, 12]. The adsorbents applied in the adsorption techniques must have a high surface area, an adequate pore density and surface charge capable of interacting with specific compounds to be removed from the aqueous solution, which makes assure a high removal capacity [13, 14]. These adsorbents are produced from a variety of sources, such as clays, zeolites, silica and carbonaceous materials, which can be applied directly or after chemically or physically activation [15].

However, these processes need be improved because additional costs are still necessary for the waste disposals or for the regeneration of the adsorbents [16]. But recent combinations with other technologies have been reported to improve the removal of the organic pollutants from the water by nanoparticles adsorbents. One of the most attractive methods with nanoparticles is the combination of the adsorption processes and the advanced oxidation processes (AOPs) to ensure high efficient for the capture and for the degradation of the contaminants [17]. Advanced oxidative processes are characterized mainly by the production of •OH radicals in the reaction media or on the catalyst surface of the nanoparticles, which are oxidative species with high redox potential (*E*<sup>0</sup> = 2.80 V). Due to this feature, these radicals have capacity to degrade a great variety of organic compounds, transforming them into smaller molecules or reducing them completely in carbon dioxide (CO2) and water (H2O), mineralizing them [18, 19].

Among the advanced oxidation process, heterogeneous photocatalysis is widely researched with the nanoparticles adsorbents. In this process, very reactive oxygen species (•OH, 1 O2 e •O2 <sup>−</sup>) are generated from the use of semiconductor catalysts nanostructures induced by ultraviolet or visible radiation [20]. The semiconductors capacity to absorb energy and degrade molecules is due to the energy bands created by atomic orbitals arranged when a new compound is formed on the surfaces of the nanoparticles. These regions are denominated conduction band (CB) and valence band (VB) [21]. In the semiconductors, there will be a region without available energy levels, which are described as band gap [22]. In these cases, only the nanoparticles will be a catalyst if an amount of energy equal or greater than the band gap is provided, which will allow electrons transfer from valence band to conduction band, as depicted in **Figure 1**. The electron (e<sup>−</sup>) in the CB can interact with oxygen molecules and form reactive groups, such as the hydroxyl (•OH) and superoxide (•O2 <sup>−</sup>) radicals and singlet oxygen (1 O2) [23–25]. On the other hand, the vacancies (h+ ) created due to the electrons transfer are oxidants with potential to convert water molecules in •OH [26].

Therefore, a nanoparticle that can combine the adsorption affinity with photocatalysis will give rise to novel materials, which would join the advantages of both techniques to degrade organic pollutants, instead of only change the phase of these contaminants [27, 28]. The major photocatalytic adsorbents researched are α-hematite (Fe2O3), meta-organic frameworks (MOFs) and TiO2 nanocomposites, which are described further.

**65**

*Photocatalytic Adsorbents Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.79954*

**2. α-Hematite (Fe2O3)**

*Semiconductors photocatalytic mechanism.*

**Figure 1.**

gap energy (*E*<sup>0</sup>

anionic molecules is stronger [30, 31].

contaminants from aqueous systems.

Hematite is the most thermodynamically stable phase of iron oxide and is formed by a hexagonal crystalline structure. It is used in catalysis processes and as pigments, in the lithium ion batteries and for the water treatment as an adsorbent [29, 30]. The strong adsorption capacity of organic and inorganic species by the hematite is related to its intrinsic chemical and electronic properties [31, 32]. The pH ZPC of the hematite is approximately 8.0, which implies that in aqueous media with pH below 8.0, the surface is positively charged and the interaction between

This semiconductor is also employed in photocatalysis due to its reduced band

Many methods have been researched and applied to produce hematite nanoparticles, but the most common techniques are the thermal decomposition, sol-gel, solvothermal and hydrothermal [34–36]. These different methodologies are used to control parameters like size, shape and crystal structure of the particles produced, with the aim to ensure interesting adsorption capacity and photocatalysis [37].

Thermal decomposition usually applies inorganic salts, such as nitrates, oxalates and chlorides, with low thermal stability to produce metal oxide nanoparticles in air atmosphere [38]. However, this process can be accomplished by calcination of magnetite (Fe3O4) or maghemite (γ-Fe2O3) at 500°C [39]. Al-Gaashani et al. [40] synthesized hematite by thermal decomposition of iron (III) nitrate 9-hydrate at different temperatures (300, 400, 500 and 600°C) under ambient conditions within a short time (20 min). The nanostructures showed crystallite sizes in a range to 10–30 nm with high surface area and optical band gap near 3.0 eV. The iron oxide α-Fe2O3 nanoparticles with different crystalline structure were prepared by [41], which calcinated the iron precursor ferric ammonium citrate at different temperatures (300, 400 and 550°C) under air atmosphere. The mean particle sizes of the samples were near 20 nm with excellent dye photocalytic degradation under visible light after 10 h. Yang et al. [42] applied direct thermal decomposition of goethite

energy level higher than water oxidation potential. Moreover, the α-Fe2O3 phase is a material with low cost and environment friendly and has good chemical and corrosion stability [33]. It is found that because of these characteristics, the hematite nanoparticles are potential candidates to good adsorbents for the removal of

= 2.2 eV), which allows the visible light absorption and with an

*Photocatalytic Adsorbents Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.79954*

*Advanced Sorption Process Applications*

Compounds of difficult degradation by conventional sewage processes can affect the lives of millions of people due to contamination of water resources [6]. The incidence of recalcitrant molecules introduced into water resources is a serious problem of toxicity and deserves the development and research of specific affluent and effluent treatment systems, with greater process efficiency, greater safety and lower costs [7, 8]. Physicochemical and biological methods currently employed are capable of degrading some of these compounds, but they have a high-cost application, which makes it difficult to implement in regions with limited financial resources [9]. Furthermore, these methods can generate complex residues for treat-

ment or storage, such as sludge and toxic or recalcitrant compounds [10].

directly or after chemically or physically activation [15].

Thus, alternative methods have been researched in an attempt to eliminate these contaminants effectively, as the adsorption techniques. The adsorption methods have some important vantages: can be operated with low energy consumer and with low operational cost, and cannot generate secondary compounds [11, 12]. The adsorbents applied in the adsorption techniques must have a high surface area, an adequate pore density and surface charge capable of interacting with specific compounds to be removed from the aqueous solution, which makes assure a high removal capacity [13, 14]. These adsorbents are produced from a variety of sources, such as clays, zeolites, silica and carbonaceous materials, which can be applied

However, these processes need be improved because additional costs are still necessary for the waste disposals or for the regeneration of the adsorbents [16]. But recent combinations with other technologies have been reported to improve the removal of the organic pollutants from the water by nanoparticles adsorbents. One of the most attractive methods with nanoparticles is the combination of the adsorption processes and the advanced oxidation processes (AOPs) to ensure high efficient for the capture and for the degradation of the contaminants [17]. Advanced oxidative processes are characterized mainly by the production of •OH radicals in the reaction media or on the catalyst surface of the nanoparticles, which are oxidative species with high redox poten-

 = 2.80 V). Due to this feature, these radicals have capacity to degrade a great variety of organic compounds, transforming them into smaller molecules or reducing them completely in carbon dioxide (CO2) and water (H2O), mineralizing them [18, 19]. Among the advanced oxidation process, heterogeneous photocatalysis is widely researched with the nanoparticles adsorbents. In this process, very reactive oxygen

nanostructures induced by ultraviolet or visible radiation [20]. The semiconductors capacity to absorb energy and degrade molecules is due to the energy bands created by atomic orbitals arranged when a new compound is formed on the surfaces of the nanoparticles. These regions are denominated conduction band (CB) and valence band (VB) [21]. In the semiconductors, there will be a region without available energy levels, which are described as band gap [22]. In these cases, only the nanoparticles will be a catalyst if an amount of energy equal or greater than the band gap is provided, which will allow electrons transfer from valence band to conduction band, as depicted in **Figure 1**. The electron (e<sup>−</sup>) in the CB can interact with oxygen molecules and form

O2) [23–25]. On the other hand, the vacancies (h+

electrons transfer are oxidants with potential to convert water molecules in •OH [26]. Therefore, a nanoparticle that can combine the adsorption affinity with photocatalysis will give rise to novel materials, which would join the advantages of both techniques to degrade organic pollutants, instead of only change the phase of these contaminants [27, 28]. The major photocatalytic adsorbents researched are α-hematite (Fe2O3), meta-organic frameworks (MOFs) and TiO2 nanocomposites,

reactive groups, such as the hydroxyl (•OH) and superoxide (•O2

<sup>−</sup>) are generated from the use of semiconductor catalysts

<sup>−</sup>) radicals and

) created due to the

**64**

tial (*E*<sup>0</sup>

species (•OH, 1

singlet oxygen (1

which are described further.

O2 e •O2

**Figure 1.** *Semiconductors photocatalytic mechanism.*
