1. Introduction

Currently, the use of pesticides has increased in order to eliminate the pests that limit and reduce agricultural production in all countries of the world. Consequently, this has caused these substances to run off into natural aquatic bodies contaminating this medium. Diuron and methyl parathion have manifested this problem; hence, it is possible to find concentrations of these pesticides in aquatic bodies close to where they are applied [1]. Although the solubility in water is low, they can dissolve due to the surrounding environment. The presence of diuron and methyl parathion in water is difficult due to the persistence and stability they present [2].

In recent years, the elimination of these compounds in water has been reported by methods, physical [3], biological [4], and chemical [5]. Of the latter, advanced oxidation processes, such as heterogeneous photocatalysis, have been shown to be very efficient in the chemical transformation of pollutants up to their mineralization to CO2 and other harmless compounds [6].

This process starts when a semiconductor is excited, with light that has a wavelength greater than or equal to the band energy of the semiconductor, to generate electron-hole pairs, which combine with the water and oxygen of the medium to form radicals that oxidize and mineralize the polluting organic matter [7]. TiO2 is the ideal semiconductor used for this process; unfortunately, its spectral response is carried out at wavelengths corresponding to the UV region, which limits its use with natural sunlight because its spectrum only has a 5% UV light. Therefore, the investigations related to this semiconductor are made to improve its spectral response in the visible region, which has been achieved by doping the titania with different elements such as non-metals [8], transition metals [9], noble metals [10], and rare earth [11]. In this chapter, we analyzed the photocatalytic behavior under the natural sunlight of TiO2 doped with La, Ce, Nd, Pr, Sm, Eu, and Gd at 0.1, 0.3, and 0.5% by weight thermally stabilized at 500 and 800°C for the degradation of diuron and methyl parathion.

#### 1.1 Rare earth elements in photocatalysis

Rare earth ions have been used for doping TiO2 aiming to modify the spectral response of the semiconductor to the visible light region to enhance its photocatalytic properties. Specifically, these ions can displace the phase transformation of anatase to rutile due to high temperature. Furthermore, have the capacity to form complexes with various base Lewis such as amines, aldehydes, carboxylic acids, alcohols, thiols etc., by the presence of electrons coming f-orbitals that interact with these functional groups, consequently allows in improving the absorptivity of organic pollutants in the aqueous medium and to elevate the photocatalytic activity [12]. These trivalent ions possess energy levels with a form of stair that as a dopant in a semiconductor can emit UV or visible light, through sequential absorptions from many near-infrared photons. The transformation of light from near-infrared and visible spectra toward UV range can be used to excite band gap of the titania [13].

On the other hand, luminescent properties of rare earth ions are originated by the electronic transition in the f-orbitals, which are partially full. These are sterically shielded from surrounding microenvironment by filled 5 s and 5p orbitals, generating narrow bands with specific emission energy for each rare earth ion. This process provides properties unique to rare earth ions in photocatalytic applications [14].

At the end of the 1990s, the first investigations involving rare earth ions as dopants in TiO2 for the photocatalytic oxidation were started [15]. Lin and Yu use a commercial photocatalyst (TiO2-P25) as a semiconductor for the acetone oxidation, doping this material with La. Then many reports appeared describing the doping of TiO2 with rare earth ions applying methods of preparation such as solvothermal, microemulsion, impregnation, electrospinning, magnetron sputtering, and sol-gel [16]. The latter has been the most used due to its easy process and its low cost.

In previous studies, it has been found that the insertion of rare earth ions such as lanthanum in titania, cannot replace the position of Ti, due to the large size of the lanthanum ion with respect to Ti [17]. Typically, the rare earth ions on the surface of the TiO2 are adsorbed in the form of oxides; only the titanium surface can be replaced by rare earth ions in the network of adsorbed lanthanide oxides, forming the Ti-O-L bond [18]. However, the substitution of a trivalent rare earth ion by a

#### Photocatalytic Treatment of Pesticides Using TiO2 Doped with Rare Earth DOI: http://dx.doi.org/10.5772/intechopen.84677

tetravalent titanium ion creates an imbalance, favoring centers with positive charges, which could adsorb anions such as OH ions, to compensate the charge balance [19]. Therefore, the photo-generated holes can be consumed immediately after the load carriers are transferred to the surface, which increases the efficiency in the separation of charges. The photocatalytic benefits of the anatase and rutile phase in titania are widely known; the addition of rare earth ions in materials with these crystalline phases shows a growth in the crystal size, due to the presence of the Ti-O bonds, in the interface between TiO2 and rare earth oxide formed [20]. In the interaction with anatase, the presence of these mentioned bonds inhibits the thermal transformation at the critical temperature of change, manifesting mixtures of crystalline phases at temperatures above 700°C in titania.

The rare earth oxides modified with titania show a growth in the intensity of light absorbed compared to pure TiO2. According to Yan et al., incident photons can be scattered and lost by reflection on a smooth surface, while on a rough surface, formed by the presence of rare earth oxides, allows a large number of scattered photons penetrate the interior of the particle to activate the separation of charges [21].

In inorganic semiconductors such as TiO2, light absorption is mainly attributed to the transition from the valence band to the conduction band, which is commonly referred to as band transitions. However, it is believed that, in the presence of lanthanide oxides, the increase in the intensity of light absorption is due to the transition of the electrons belonging to layer 4f of the lanthanides, known as the transition f ! f. The corresponding energy can be transferred to the titania to separate the charges [22].

#### 1.2 Pesticides treated by photocatalysis

#### 1.2.1 Diuron

Diuron (3-(3, 4-dichlorophenyl)-1, 1-dimethylurea) is a white, crystalline and odorless powder, which has low solubility in water (36.4 mg/L). Herbicide is employed for weed control in non-crop areas and to control weeds in a range of tree crops. Its mechanism of action mainly acts inhibiting photosynthesis by blocking electron transport at photosystem II [23]. When it is applied to soils, it is leached from 3 to 5 cm and strongly adsorbs persisting up to 330 days. In aqueous medium, it is partially absorbed due to its solubility, by the action of solar photolysis and OH radicals present in the environment, almost completely degraded, but this process is too slow and depends on environmental conditions [24]. For this reason, advanced oxidation processes such as heterogeneous photocatalysis with TiO2 have been used to eliminate this herbicide as a water pollutant.

When TiO2 is used as a colloidal particle in an aqueous solution of diuron, only one transformation is observed in the aliphatic chain, where the OH radicals attack the benzene ring causing its opening to aliphatic chains. In the presence of acetonitrile, the reaction mechanism indicates a reductive discoloration of the benzene ring, without it an oxidative demethylation of the aliphatic chain is observed [25].

The modification of TiO2 with noble metals has improved the activity of this semiconductor for photodegradation and mineralization of diuron in an aqueous medium. Katsumata et al. impregnated the P25 at different doses of Pt in an oxidized state, stabilizing thermally up to 700°C. They described that 0.2% of Pt in TiO2 showed the best performance in the photodegradation of diuron in a period of 20 min, and this material is four times more active than pure P25. Nevertheless, 97% of mineralization was reached after 8 h [26].

#### 1.2.2 Methyl parathion

Methyl parathion (O, O-dimethyl O-p-nitrophenyl phosphorothioate) is a white crystalline powder that has a pungent smell like garlic and has low solubility in water (55 mg/L). As insecticide helps to control the biting and sucking of insects in fruit and vegetable crops, it is also applied in the fight against mites, Coleoptera, and caterpillars [27]. Furthermore, methyl parathion is capable to inhibit the action of acetylcholinesterase of nerve tissue, following its metabolic conversion to its corresponding phosphates methyl paraoxon and paraoxon. Organophosphate pesticides are generally regarded as safe for use on crops and animals due to their relatively fast biodegradation, but depend on microbial composition, pH, temperature, and sunlight. This compound can be degraded rapidly by hydrolysis in the presence of sunlight and air [28]. Physical, chemical, and biological methods have been used to minimize the toxic effect generated by this pollutant in water. One of the methods most used for this purpose is heterogeneous photocatalysis with titania, due to its high effectiveness to mineralize organic pollutants in an aqueous medium.

Many reports have been cited in the literature describing the photodegradation of the methyl parathion using TiO2 with UV light under different conditions. Evgenidou et al. analyzed the photocatalytic behavior of TiO2 and ZnO in the degradation of methyl parathion in an aqueous medium. They determined that the titania is more effective as a photocatalyst, presenting a higher reaction rate; in addition, this material could complete the mineralization process, without introducing unwanted intermediaries in the reaction [29].

On the other hand, investigations have been carried out involving the modification of TiO2 to improve its photocatalytic behavior in the degradation of methyl parathion. Senthilnathan and Philip doped the titania with N using different precursors such as triethylamine, ethylamine, urea, and ammonium hydroxide. Their results show that the highest photoactivity is obtained using triethylamine, however, this catalyst when used under UV light did not show a higher performance than P25-TiO2, but when used with visible irradiation its effectiveness was the best [30].

#### 1.3 TiO2 doped with rare earth

Rare earth ions have been doped in TiO2 as a strategy to increase the response of the semiconductor to the visible light region and enhance photocatalytic activity. It was reported in the literature that the optimum level of rare earth doping is less than 2% to hinder the crystal growth of titania during calcination [31]. Also, it is known that the rare earth ions occupy substitutional sites in the titania according to the analyzes carried out by XRD, but in many publications, this statement is contrary, due to the effect of the large ionic radionics of the rare earth ions, which they can only occupy interstitial sites or form aggregates such as oxides or hydroxide at the boundaries of the titania grain by creating Ti-O-RE bonds. The presence of this link generates an imbalance of charges with a positive charge center, which allows adsorbing anions to reach equilibrium. Therefore, the photo-generated holes can be consumed immediately after being transferred to the surface of the titania, whereby the separation of charges is improved; the process of recombination of hollow electron pairs is avoided, and consequently, the photocatalytic activity is favored [16]. Another effect, which inhibits the recombination process and therefore increases the photocatalytic yield of titania, is the formation of the Ti3+ species and the oxygen vacancies, both act as photo-generated hole capturers (valence band),

Photocatalytic Treatment of Pesticides Using TiO2 Doped with Rare Earth DOI: http://dx.doi.org/10.5772/intechopen.84677

and together they are charged and at the same time, the oxygen of the medium traps the photo-generated electrons (conduction band); and this increases the separation of the photo-generated species. Ti3+ is oxidized in the presence of the oxygen present to generate the anion superoxide (O2 ˜), which reacts with the photo-generated holes in order to produce hydroxyl radicals (OH \*) in an aqueous medium, and thus be able to oxidize any organic compound present in this system [6].

Rare earth dopant ions such as Pr, Ce, Nd, Eu, Sm, Dy, Gd, and La show significant enhancement in dye photodegradation compared with TiO2 pure, due to the higher adsorption and the 4f electron transition of rare earth ions. Between 0.5 and 1% wt of doping ions, the best photocatalytic behavior of the doped samples are shown [31]. On the other hand, La, Nd, Sm, Eu, Gd, and Yb as dopants in TiO2, increase the titania yield and raise the stability of the anatase phase and prevent the segregation of titania. Likewise, these ions play a role in providing a means of concentrating the contaminants to be eliminated on the surface, and consequently, increasing the photocatalytic activity of semiconductor [32]. Recently, La, Nd, Eu, Sm, Gd, Er, Tb, Yb, Pr, and Ce when used as dopants improve the performance of titania, because it increases the absorption capacity of light, to surface and structural modifications, which has allowed the development of catalysts with environmental applications such as the degradation of pollutants in aqueous medium [33].
