**2. Advanced oxidation processes to remove pesticides from water**

lays down environmental quality standards in accordance with the EWFD, for the 33 priority substances. The EWFD also sets out general provisions for the protection and conservation of groundwater. The establishment of detailed quality criteria for the assessment of groundwater chemical status in Europe was laid down in the European Groundwater Directive [20]. For this reason, the EU established the following groundwater quality standards: 0.1 μg L−1 for individual pesticide and 0.5 μg L−1 for the sum of all individual pesticides to safeguard people

During the 1990s of the last century, atrazine (herbicide) and endosulfan (insecticide) were found most often in surface waters in the USA and Australia due to their widespread use. In addition, although in lower proportions, other pesticides such as pronofos, dimethoate, chlordane, diuron, prometryn and/or fluometuron were detected [21]. Studies that are more recent also reported the presence of several pesticides in environmental waters (surface water, groundwater and seawater) close to agricultural lands over the world [22–34]. In addition, different studies have corroborated the presence of some pesticides in drinking

Pesticides are being continuously released into the aquatic environment through anthropogenic activities. Their detection in storm and wastewater effluent has been reported to be a major obstacle as regards wide-ranging acceptance of water recycling [39]. In addition, their variety, toxicity and persistence present a threat to humans through pollution of drinking water resources (e.g. surface water and groundwater). The frequent occurrence of pesticides in surface water and groundwater has prompted the search for suitable methods to destroy them. Although conventional biological treatments of water offer some advantages such as their low cost and easy operation, most studies concerning the treatment of pesticides have concluded that they are not very effective due to their low biodegradability [40, 41]. Other technologies such as adsorption or coagulation merely concentrate pesticides by transferring them in other phases but still remain and not being completely eliminated. To solve this problem, apart from reducing emissions, two main water strategies are followed: (i) chemical treatment of drinking water, surface water and groundwater and (ii) chemical treatment of wastewaters containing biocides and bio-recalcitrant pollutants as pesticides. Chemical treatments of polluted surface water, waste water and groundwater are part of a long-term strategy to improve the quality of water by removing toxic compounds of anthropogenic origin before returning the water to its natural cycle. The Directive 2013/39/EU [19] promotes the preventive action and the polluter pays principle, the identification of pollution causes, dealing with emissions of pollutants at the source, and finally the development of innovative wastewater treatment technologies, avoiding expensive solutions. Therefore, effective, low-cost and robust methods to decontaminate waters are needed, as long as they do not further stress the environment or endanger human health, particularly prior to direct or indirect reuse of reclaimed water. In this context, the development of Solar Chemistry Applications is of special relevance, especially photochemical processes where solar photons are absorbed by reactants and/or a catalyst causing a chemical reaction. Consequently, in recent years there has been growing interest in the use of advanced oxidation processes (AOPs) to remove pesticide residues as alternative to methods that are more conventional because they allow the

from harmful effects.

150 Application of Titanium Dioxide

water [35–38].

abatement of them by mineralisation.

AOPs have been commonly defined as *near-ambient temperature treatment processes based on highly reactive radicals*, especially the hydroxyl radical (•OH). Other radicals and active oxygen species involved are superoxide radical anions (O2 •−), hydroperoxyl radicals (HO2 •−), triplet oxygen (3O2 ) and organic peroxyl radicals (ROO<sup>−</sup> ). In all probability, the •OH (*E*<sup>0</sup> = 2.8 V) is among the strongest oxidising species used in water treatment and confers the potential to greatly accelerate the rates of pesticide oxidation. Hydroxyl radicals can degrade indiscriminately micropollutants with reaction rate constants usually around 109 L mol−1 s−1 [42], yielding CO2 , H2 O and, eventually, inorganic ions as final products. After fluorine (*E*<sup>0</sup> = 3.1 V), •OH is the strongest oxidant [43], and its production can be achieved by many pathways, which allows one to choose the appropriate AOP according to the specific characteristics of the target water/ wastewater and treatment requirements. Regarding the methodology to generate hydroxyl radicals, AOPs can be divided into chemical, electrochemical, sono-chemical and photochemical processes. Typical AOPs can be also classified as homogeneous that occur in a single phase and heterogeneous processes because they make use of a heterogeneous catalyst like metalsupported catalysts, carbon materials or semiconductors such as TiO2 , WO3 , ZnO, CdS, SnO2 , ZnS and others [44]. **Figure 1** shows some homogeneous and heterogeneous processes [41, 45].

When the chemical process (mineralisation) destroys the contaminants and their reaction intermediate products (metabolites), critical secondary wastes are not generated and, thus, post-treatment or final disposal is not required [46]. However, if complete mineralisation is not achieved or the reaction period is too long, a final post-treatment may be necessary. A higher biodegradability and lower toxicity of the reaction by-products, in comparison with the parent compounds, are desirable benefits of applying AOPs to treat wastewaters. However, in some cases, these by-products are less biodegradable and/or more toxic than the parent compounds. For this reason, AOPs can be applied as post- or pretreatment of biological processes. The integration of different AOPs in a sequence of treatment processes is a common

**Figure 1.** Scheme for conventional AOPs.

approach to achieve a biodegradable effluent, which can be further treated by a conventional biological process, reducing the residence time and reagent consumption in comparison with AOPs alone. **Figure 2** shows possible integration of AOPs in wastewater and drinking water treatment plants.

One drawback of these processes is the presence of scavengers in wastewaters because these species consume •OH, competing with pesticides. They can be organic matters (e.g. humic and/or fulvic acids, amino acids, proteins and carbohydrates) or inorganic ions (CO3 = , HCO3 − , S= , Br<sup>−</sup> , NO3 − and others). Because most natural waters contain these scavengers, optimisation of AOPs must be performed bearing them in mind. The knowledge of their effect on the process efficiency is difficult because they have different reactivities, as well as due to the constant variations in the aqueous phase when the parent pollutants are continuously transformed into many different intermediates. Among these techniques, photocatalytic methods in the presence of artificial or solar light, like heterogeneous photocatalysis (HP), have been proven very effective for the degradation of a wide range of pesticides [48].

#### **2.1. Basis of heterogeneous photocatalysis**

Photocatalysis may be defined as the *acceleration of a photoreaction by the presence of a catalyst*. HP, *the use under irradiation of a stable solid semiconductor for stimulating a reaction at the solid/ solution interface*, is a technique of environmental interest for the treatment of pesticide-polluted water combining the low cost, the mild conditions and the possibility of using natural sunlight as the source of irradiation [39, 49]. Progress and challenges of HP can be reviewed in recent published papers [50, 51]. In brief, HP is based on the irradiation of semiconductor (SC) particles, usually suspended in aqueous solutions, with wavelength energy *hv* ≥ *Eg* (band-gap energy). Thus, an electron (e<sup>−</sup> ) is driven to the conduction band (cb), remaining a positive hole (h+ ) in the valence band (vb). Both the e<sup>−</sup> and h+ migrate to the particle surface (**Figure 2**). The e<sup>−</sup> cb and the h+ vb can recombine on the surface or in the bulk of the particle in

**Figure 2.** Possible implementation of AOPs in waste water and drinking water treatment plants (adapted from Petrovic et al. [47].

short time, and the energy dissipated as heat. In addition, they can be trapped on the surface reacting with donor (D) or acceptor (A) species adsorbed or close to the surface of the particle [52] as can be seen in **Figure 3**. The wavelength (λ) of radiation required to activate the catalyst must be equal or lower than the calculation by Planck's equation, λ = *hc*/*Eg* , where *h* is Planck's constant (6.626176 × 10−34 J s−1), *c* is the speed of light and *Eg* is the semiconductor band-gap energy.

As a rule, TiO2 is considered the best photocatalyst due to different qualities such as high photochemical stability, high efficiency, non-toxic nature and low cost, whose behaviour is very well documented in the recent literature [53, 54]. Excellent reviews have been published during the last years on the photoactivity of TiO2 to purify pesticide-polluted waters [39, 48, 55–60].

#### **2.2. Properties and characteristics of titanium dioxide photocatalysts**

approach to achieve a biodegradable effluent, which can be further treated by a conventional biological process, reducing the residence time and reagent consumption in comparison with AOPs alone. **Figure 2** shows possible integration of AOPs in wastewater and drinking water

One drawback of these processes is the presence of scavengers in wastewaters because these species consume •OH, competing with pesticides. They can be organic matters (e.g. humic and/or fulvic acids, amino acids, proteins and carbohydrates) or inorganic ions

engers, optimisation of AOPs must be performed bearing them in mind. The knowledge of their effect on the process efficiency is difficult because they have different reactivities, as well as due to the constant variations in the aqueous phase when the parent pollutants are continuously transformed into many different intermediates. Among these techniques, photocatalytic methods in the presence of artificial or solar light, like heterogeneous photocatalysis (HP), have been proven very effective for the degradation of a wide range of

Photocatalysis may be defined as the *acceleration of a photoreaction by the presence of a catalyst*. HP, *the use under irradiation of a stable solid semiconductor for stimulating a reaction at the solid/ solution interface*, is a technique of environmental interest for the treatment of pesticide-polluted water combining the low cost, the mild conditions and the possibility of using natural sunlight as the source of irradiation [39, 49]. Progress and challenges of HP can be reviewed in recent published papers [50, 51]. In brief, HP is based on the irradiation of semiconductor (SC) particles, usually suspended in aqueous solutions, with wavelength energy *hv* ≥ *Eg*

**Figure 2.** Possible implementation of AOPs in waste water and drinking water treatment plants (adapted from Petrovic

and others). Because most natural waters contain these scav-

) is driven to the conduction band (cb), remaining a

migrate to the particle surface

and h+

vb can recombine on the surface or in the bulk of the particle in

treatment plants.

152 Application of Titanium Dioxide

pesticides [48].

positive hole (h+

(**Figure 2**). The e<sup>−</sup>

et al. [47].

, NO3 −

**2.1. Basis of heterogeneous photocatalysis**

(band-gap energy). Thus, an electron (e<sup>−</sup>

cb and the h+

) in the valence band (vb). Both the e<sup>−</sup>

(CO3 = , HCO3 − , S= , Br<sup>−</sup>

> Titanium dioxide is by far, the most investigated photocatalyst to remove organic pollutants from water. The photocatalytic activity of TiO2 slurries depends on physical properties of the catalyst (crystal and pore structure, surface area, porosity, band gap, particle size and surface hydroxyl density) [61, 62]. On the other hand, operating conditions such as light intensity and wavelength, initial concentration and type of pollutants, catalyst loading, oxygen content, interfering substances, presence of oxidants/electron acceptor, pH value and configuration of photoreactor have a key role [39, 58, 63–66]. Finally, the mode of TiO2 application (suspended, immobilised or doped) is fundamental to rate the photocatalytic activity.

**Figure 3.** Scheme for the heterogeneous photocatalysis.

#### *2.2.1. Composition of TiO2 and its types*

Titanium dioxide is known to occur in nature as anatase (At), brookite (Bk) and rutile (Rl) (**Figure 4**). Rl is usually considered to be the high-temperature and high-pressure phase relative to At, whereas Bk is often considered to be of secondary origin. Rl is the most common, most stable and chemically inert and can be excited by both visible and ultraviolet (UV) light (wavelengths smaller than 390 nm) [67]. At is only excited by UV light and can be transformed into Rl at high temperatures. Both Rl and At have a tetragonal ditetragonal dipyramidal crystal system but have different space group lattices. UV light does not excite Bk, but its orthorhombic crystal system can be transformed into Rl with the application of heat.

Generally, At exhibits higher photocatalytic activities than Rl. However, the reasons for the differences in photocatalytic activity between At and Rl are still being debated. Although At has lower absorbance ability to solar light than Rl because its band gap is larger (3.2 eV) than that of Rl (3.0 eV), the photocatalytic activity of At is higher than that of Rl. This can be explained because At has a higher surface adsorption capacity to hydroxyl groups and a lower charge carrier recombination rate than Rl [68]. Also, the lower photocatalytic activity of Rl is due to its larger grain size [69], lower specific surface areas and less capacity for surface adsorption. In addition, the lifetime of photo-generated e<sup>−</sup> and h+ in Rl is about an order of magnitude smaller than that of At. Consequently, the chance of participation of photoexcited e- and h+ of At in surface chemical reactions is greatly enhanced. According to Zhang et al. [70], Rl and Bk

**Figure 4.** (1) The schematic conventional cells for anatase (a), rutile (b) and brookite (c) phases [73]; (2) SEM image of TiO2 P25 and (3) XRD pattern of TiO<sup>2</sup> P25.

belong to the direct band-gap SC category, while At appears to be an indirect band-gap SC. As a consequence, At exhibits a longer lifetime of photoexcited e<sup>−</sup> and h+ than Rl and Bk because the direct transition of photo-generated electrons from the conduction band to valence band of At is not possible. Moreover, At has the lightest average effective mass of photo-generated e− and h+ than Rl and Bk suggesting the fastest migration of photo-generated e<sup>−</sup> and h+ from the interior to the surface of At. This results in the lowest recombination rate of photo-generated charge carriers within At. Therefore, it is not surprising that Rt and Bk show a smaller photocatalytic activity than At.

TiO2 can use natural sunlight because it has an appropriate energetic separation between its valence and conduction bands which can be surpassed by the energy content of a solar photon (380 nm > λ >300 nm). The sunlight puts 0.2–0.3 mol photons of m−2 h−1 in the 300–400 nm range with a typical UV flux of 20–30 W m−2. In addition, photons can be generated by artificial irradiation although it is the most important source of costs during the treatment of wastewater [65]. Different available TiO<sup>2</sup> catalysts (with different surface areas, crystal sizes and compositions) such as Degussa P25, Hombikat UV100, PC500, PC10, PC50, Rhodia and others have been tested for the photolytic degradation of pesticides in aqueous environments [39]. From them, Degussa P25 has been the most used because it has good properties (i.e. typically a 70:30 At:Rl composition, non-porous, Brunauer, Emmett and Teller (BET) around 55 m<sup>2</sup> g−1 and average particle size 30 nm) and a substantially higher photocatalytic activity than other commercial TiO2 [65]. The higher photocatalytic activity of P25 has been attributed to its crystalline composition of Rl and At. It is known that the smaller band gap of rutile absorbs the photons and generates e<sup>−</sup> /h+ pairs. Then, the electron transfer takes place from the rutile to electron traps in the At phase. Thus, the recombination is inhibited and allows the hole to move to the surface of the particle to react [71].

#### *2.2.2. Operating conditions*

*2.2.1. Composition of TiO2*

154 Application of Titanium Dioxide

TiO2

P25 and (3) XRD pattern of TiO<sup>2</sup>

P25.

 *and its types*

In addition, the lifetime of photo-generated e<sup>−</sup>

Titanium dioxide is known to occur in nature as anatase (At), brookite (Bk) and rutile (Rl) (**Figure 4**). Rl is usually considered to be the high-temperature and high-pressure phase relative to At, whereas Bk is often considered to be of secondary origin. Rl is the most common, most stable and chemically inert and can be excited by both visible and ultraviolet (UV) light (wavelengths smaller than 390 nm) [67]. At is only excited by UV light and can be transformed into Rl at high temperatures. Both Rl and At have a tetragonal ditetragonal dipyramidal crystal system but have different space group lattices. UV light does not excite Bk, but its ortho-

Generally, At exhibits higher photocatalytic activities than Rl. However, the reasons for the differences in photocatalytic activity between At and Rl are still being debated. Although At has lower absorbance ability to solar light than Rl because its band gap is larger (3.2 eV) than that of Rl (3.0 eV), the photocatalytic activity of At is higher than that of Rl. This can be explained because At has a higher surface adsorption capacity to hydroxyl groups and a lower charge carrier recombination rate than Rl [68]. Also, the lower photocatalytic activity of Rl is due to its larger grain size [69], lower specific surface areas and less capacity for surface adsorption.

and h+

smaller than that of At. Consequently, the chance of participation of photoexcited e- and h+ of At in surface chemical reactions is greatly enhanced. According to Zhang et al. [70], Rl and Bk

**Figure 4.** (1) The schematic conventional cells for anatase (a), rutile (b) and brookite (c) phases [73]; (2) SEM image of

in Rl is about an order of magnitude

rhombic crystal system can be transformed into Rl with the application of heat.

The e<sup>−</sup> /h+ formation in the photochemical reaction is strongly dependent on the light intensity at a given wavelength [72]. Therefore, the dependency of pollutant degradation rate on the light intensity has been studied in numerous investigations of various organic pollutants. According to Herrmann [73], the reaction rate is proportional to the radiant flux (φ) < 25 mW cm−2, while above this value, the rate varies as φ<sup>½</sup>, which indicates a too high value of the flux increasing the e− /h+ recombination rate. When the intensity is high, the reaction rate does not depend on light intensity because at low intensity, reactions involving e<sup>−</sup> /h+ formation are predominant, while e− /h+ recombination is not significant [74].

Several authors have indicated that when the level of the target pesticide increases, a large number of molecules of the compound are adsorbed on the photocatalyst surface and, consequently, the reactive species (•OH and O2 •−) required for pesticide degradation also increase. However, the formation of •OH and O2 •− on the catalyst surface remains constant for a given catalyst amount, light intensity and irradiation time. Hence, at higher concentrations the available •OH is inadequate for pollutant degradation. Therefore, as the concentration increases, the pollutant degradation rate decreases [75]. In addition, an increase in pesticide concentration leads to the generation of intermediates (metabolites), which may be adsorbed on the surface of the catalyst.

Because TiO2 is often used as suspension, the photocatalytic degradation rate initially increases with catalyst loading and then decreases at high concentrations due to light scattering and screening effects. Although the number of active sites in solution will increase with catalyst loading, light penetration is compromised because of excessive particle concentration. The interaction between particles (agglomeration) increases at high concentration, and, consequently, the surface area available for light adsorption is reduced and photocatalytic activity decreases. The optimum catalyst loading has to be found in order to avoid excess catalyst and ensure the maximum absorption of photons. Although the results in the literature are very different, it may be deduced that the incident radiation and path length inside the photoreactor are of special interest in determining the optimum catalyst mass.

Waste water, surface water, groundwater and drinking water pHs vary significantly and play an important role in the photodegradation of pesticides since it determines the size of aggregates it forms and the surface charge of the photocatalyst. The surface charge of the photocatalyst and the ionisation or speciation (p*K*<sup>a</sup> ) of a pesticide can be seriously affected by the solution pH. Electrostatic interaction between the surface of the semiconductor, substrate, solvent molecules and radicals formed during photocatalytic oxidation strongly depends on the solution pH. At pH below its p*K*<sup>a</sup> value, an organic compound exists as neutral state. Above this p*K*<sup>a</sup> value, organic compounds attain a negative charge, which can significantly influence their photocatalytic degradation. The point of zero charge (PZC, the pH at which the surface has a neutral net electrical charge) of TiO2 is not very sensitive to the crystallographic structure (At *vs* Rl) and the experimental method used. The common value is pH 5.9 for both phases. Although the PZC of TiO2 depends on the production method, the most frequent value for TiO2 P25 is 6.3 [76]. Below or above this value, the charge of the catalyst surface is positive or negative, respectively, according to the following reactions:

$$\text{pH} \gets \text{pzc} \cdot \text{TiOH} + \text{H}^+ \rightarrow \text{TiOH}\_2^+ \quad \text{pH} \geqslant \text{pzc} \cdot \text{TiOH} + \text{OH}^- \rightarrow \text{TiO}^- + \text{H}\_2\text{O} \tag{1}$$

At low pH, the positive holes are considered as the major oxidation step, whereas at neutral or high pHs, •OH is the predominant species [77]. It is expected that the generation of •OH will be higher due to the presence of more available OH<sup>−</sup> on the TiO2 surface. Thus, the degradation efficiency of the process will be enhanced at high pH. A very important feature of the photocatalytic process is that in many cases, a great number of different metabolites are produced, which may behave in a different way depending on the pH of the solution. As regards temperature, photocatalytic systems do not require heating and operate at room temperature because of photonic activation [65].

The e<sup>−</sup> /h+ recombination is one of the main drawbacks in the application of semiconductor photocatalysis as it causes waste of energy. In the absence of suitable electron acceptor, recombination step is predominant, and thus, it limits the quantum yield [39]. In HP reactions, O2 is generally used as electron acceptor. Addition of exogenous oxidant/electron acceptors into a semiconductor suspension has been shown to improve the photocatalytic degradation of many pesticides because they can eliminate the e<sup>−</sup> /h+ recombination by accepting the conduction band electron, increase the •OH concentration and oxidation rate of intermediate compound and produce more radicals and oxidising species to accelerate the degradation efficiency of intermediate compounds. Because •OH plays an important role in photodegradation, several researchers have investigated the effect of addition of different electron acceptors (i.e. H2 O2 , KBrO3 or Na2 S2 O8 ) on the photocatalytic degradation of many pesticides [75, 78].

Because TiO2

156 Application of Titanium Dioxide

Above this p*K*<sup>a</sup>

frequent value for TiO2

is often used as suspension, the photocatalytic degradation rate initially increases

with catalyst loading and then decreases at high concentrations due to light scattering and screening effects. Although the number of active sites in solution will increase with catalyst loading, light penetration is compromised because of excessive particle concentration. The interaction between particles (agglomeration) increases at high concentration, and, consequently, the surface area available for light adsorption is reduced and photocatalytic activity decreases. The optimum catalyst loading has to be found in order to avoid excess catalyst and ensure the maximum absorption of photons. Although the results in the literature are very different, it may be deduced that the incident radiation and path length inside the photoreactor

Waste water, surface water, groundwater and drinking water pHs vary significantly and play an important role in the photodegradation of pesticides since it determines the size of aggregates it forms and the surface charge of the photocatalyst. The surface charge of the photo-

solution pH. Electrostatic interaction between the surface of the semiconductor, substrate, solvent molecules and radicals formed during photocatalytic oxidation strongly depends on

influence their photocatalytic degradation. The point of zero charge (PZC, the pH at which

lographic structure (At *vs* Rl) and the experimental method used. The common value is pH

At low pH, the positive holes are considered as the major oxidation step, whereas at neutral or high pHs, •OH is the predominant species [77]. It is expected that the generation of •OH

dation efficiency of the process will be enhanced at high pH. A very important feature of the photocatalytic process is that in many cases, a great number of different metabolites are produced, which may behave in a different way depending on the pH of the solution. As regards temperature, photocatalytic systems do not require heating and operate at room temperature

photocatalysis as it causes waste of energy. In the absence of suitable electron acceptor, recombination step is predominant, and thus, it limits the quantum yield [39]. In HP reactions, O2 is generally used as electron acceptor. Addition of exogenous oxidant/electron acceptors into a semiconductor suspension has been shown to improve the photocatalytic degradation of

duction band electron, increase the •OH concentration and oxidation rate of intermediate compound and produce more radicals and oxidising species to accelerate the degradation

recombination is one of the main drawbacks in the application of semiconductor

/h+

surface is positive or negative, respectively, according to the following reactions:

+

value, organic compounds attain a negative charge, which can significantly

P25 is 6.3 [76]. Below or above this value, the charge of the catalyst

, pH > pzc TiOH + O H<sup>−</sup> → Ti O<sup>−</sup>

on the TiO2

) of a pesticide can be seriously affected by the

depends on the production method, the most

is not very sensitive to the crystal-

+ H2 O (1)

surface. Thus, the degra-

recombination by accepting the con-

value, an organic compound exists as neutral state.

are of special interest in determining the optimum catalyst mass.

catalyst and the ionisation or speciation (p*K*<sup>a</sup>

5.9 for both phases. Although the PZC of TiO2

pH < pzc TiOH + H+ → TiO H2

because of photonic activation [65].

The e<sup>−</sup> /h+

the surface has a neutral net electrical charge) of TiO2

will be higher due to the presence of more available OH<sup>−</sup>

many pesticides because they can eliminate the e<sup>−</sup>

the solution pH. At pH below its p*K*<sup>a</sup>

The ability of peroxydisulfate is not only attributed to the promotion of charge separation but also to the production of sulphate radicals (SO4 •−), which are very strong oxidising agents (*E0* =2.6 V), and the appearance of more hydroxyl radicals (•OH) according to the following reactions:

$$\rm{S}\_{2}\rm{O}\_{8}^{\*} + e^{-} \rightarrow \rm{S}\rm{O}\_{4}^{\*} + \rm{S}\rm{O}\_{4}^{\*}\tag{2}$$

$$\rm{SO}\_4^{\bullet} + e^- \rightarrow \rm{SO}\_4^{\bullet} \tag{3}$$

$$\rm{SO}\_4^{\bullet} + \rm{H}\_2\rm{O} \rightarrow \rm{SO}\_4^{\bullet} + \rm{^{\bullet}OH} + \rm{H}^\* \tag{4}$$

$$\rm SO\_4^{\bullet+} \cdot RH \to \rm intermediates \to SO\_4^{\bullet+} \cdot CO\_2 \tag{5}$$

Besides, the addition of H2 O2 enhances the degradation due to the increase in the •OH concentration as follows:

$$\mathrm{H}\_{2}\mathrm{O}\_{2} \star \mathrm{e}^{-} \rightarrow \ \mathrm{"OH} \star \mathrm{OH} \mathrm{-}\tag{6}$$

$$\mathrm{H\_2O\_2\text{+O\_2^{\bullet-}}}\rightarrow\ \mathrm{^{\bullet}OH}\text{+OH^-}\mathrm{^{\bullet}O\_2}\tag{7}$$

$$\text{H}\_2\text{O}\_2 \text{+ hv } \rightarrow 2\text{"OH}\tag{8}$$

The quantum yield of S2 O8 2− (1.8 mol Einstein−1) is much larger than that of H2 O2 (1 mol Einstein−1), which can be related to the rate of recombination of •OH (5.3×109 M−1 s−1) and SO4 •− (8.1×108 M−1 s−1) [79].

As reviewed by Ahmed et al. [39], many studies have demonstrated that water components like Ca2+, Mg2+, Fe2+, Zn2+, Cu2+, HCO3 − , PO4 3−, NO3 − , SO4 2− and Cl<sup>−</sup> and dissolved organic matter (DOM) can affect the photodegradation rate of organic pollutants since they can be adsorbed onto the surface of TiO2 [80, 81]. These dissolved components can compete with the pesticide for the active sites depending on the solution pH, reducing the formation of •OH. Anions result in corresponding anion radicals scavenged by •OH. However, they have lower oxidation potential. In addition, DOM, ubiquitously present in storm and wastewater effluent, also plays an important role regarding pesticide degradation. The observed slowdowns are related to the inhibition (surface deactivation), competition and light attenuation effects. Moreover, the presence of humic acids in the reaction solution has been reported to significantly reduce light transmittance and consequently the photooxidation rate.

As usual, two types of photoreactors are used for photocatalytic wastewater treatment processes: (i) slurry photoreactors and (ii) fixed-bed photoreactors [82]. The slurry photoreactors utilise suspended photocatalyst particles, while the other type utilises immobilised photocatalyst particles on a surface. Currently, the catalysts are applied in the form of a slurry in most cases. However, the separation of catalyst after the reaction in the slurry systems is an expensive and tedious stage, which adds to the overall running costs of the plant. Therefore, immobilised catalyst systems are preferred in order to avoid an increase in cost and time. However, the immobilised catalyst systems have low interfacial surface areas and consequently a very low activity, being also difficult to scale [83]. It is noted that configuration of photoreactor has an important role in the efficiency of the photocatalytic wastewater treatment processes.

## *2.2.3. Bare, doped and immobilised application of TiO2*

Generally, the use of TiO2 slurries has been demonstrated to have higher photocatalytic activity as compared to the same immobilised catalyst. This is due to changes on the surface of the catalyst by blocking pores and the appearance of by-products causing the loss of active sites on its surface. Usually, TiO2 is prepared in the form of nanopowders, crystals, thin films, nanotubes and nanorods. As a rule, the immobilisation of TiO2 onto supporting material has been carried out via one of two major routes: (i) physical (thermal treatment) and (ii) chemical (sol-gel, electrodeposition, etc.). The evolution of different supports and the benefits and drawbacks of various immobilisation techniques to obtain a high-surface-area TiO2 can be seen in the reviews by Shan et al. [84] and Dahl et al. [85]. Nonetheless, the interest for the development of TiO2 supported on different materials is growing because the use of the bare TiO2 phases presents some drawbacks as (i) necessity of irradiation with UV light due to the small amount of photons absorbed in the Vis region, (ii) high recombination rate for the photoproduced e<sup>−</sup> /h+ pairs, (iii) difficulty to improve the performance by doping with some materials that often act as recombination centres, (iv) deactivation in the absence of H2 O vapour when aromatic molecules must be abated and (v) difficulty to support powdered TiO<sup>2</sup> on some materials [45]. Consequently, the research line in HP has been driven to modify some electronic and morphological properties of TiO2 to enhance its photoefficacy. In this context, nanosized particles and films on glasses or other supports and powdered samples with high specific surface areas have been obtained to increase the possibility for the involved species to avoid the separation step [86]. Doping, loading and sensitisation of TiO2 are methods aimed to shift the light absorption towards visible light and/or to increase the lifetime of the photoproduced e<sup>−</sup> /h+ pairs.

A number of approaches have been suggested in recent years to enhance photocatalytic activity of TiO2 in the visible light region for its use in water detoxification [87]. Metal ion doping and co-doping with non-metals can improve trapping of the photoexcited conduction band electrons at the surface, thereby minimising charge carrier recombination. Several dopants used (e.g. Sn, Ag, Pd, Re, Bi, V, Mo, Th or Pt among others), have been shown to enhance photocatalytic activity for the systems examined. However, the photoactivity of the metal-doped TiO2 photocatalyst significantly depends on the dopant ion nature and concentration, preparation method and operating conditions [88]. The deposition of metal ions on TiO2 can modify the photoconductive properties by increasing the charge separation efficiency between electrons and holes, which will enhance the formation of both free hydroxyl radicals and active oxygen species [89]. Recent research indicates that the desired narrowing on the band gap of TiO2 can be achieved using non-metal elements such as N, F, S and C. Thus, modified TiO<sup>2</sup> showed a significant improvement on the absorption in the Vis light region due to band-gap narrowing and enhancing the degradation of pesticides under Vis light irradiation, mainly under sunlight [90, 91]. Also, it is possible to produce coupled colloidal structures using TiO2 such as TiO2 -SnO2 , TiO2 -CdS, TiO2 -Bi2 S3 , TiO2 -WO3 or TiO2 -Fe2 O3 , in which illumination of one semiconductor produces a response in the other at the interface between them by increasing the charge separation and extending the energy range of photoexcitation. On the other hand, the coating of one semiconductor or metal nanomaterial on the surface of another semiconductor or metal nanoparticle core is called capping. Semiconductor nanoparticles of TiO2 can be coated with another semiconductor (i.e. SnO2 ) with a different band gap to enhance its emissive properties [92].

#### **2.3. Heterogeneous photocatalytic degradation of pesticide residues in water over titanium dioxide**

and tedious stage, which adds to the overall running costs of the plant. Therefore, immobilised catalyst systems are preferred in order to avoid an increase in cost and time. However, the immobilised catalyst systems have low interfacial surface areas and consequently a very low activity, being also difficult to scale [83]. It is noted that configuration of photoreactor has an

ity as compared to the same immobilised catalyst. This is due to changes on the surface of the catalyst by blocking pores and the appearance of by-products causing the loss of active

been carried out via one of two major routes: (i) physical (thermal treatment) and (ii) chemical (sol-gel, electrodeposition, etc.). The evolution of different supports and the benefits and

seen in the reviews by Shan et al. [84] and Dahl et al. [85]. Nonetheless, the interest for the

 phases presents some drawbacks as (i) necessity of irradiation with UV light due to the small amount of photons absorbed in the Vis region, (ii) high recombination rate for the pho-

drawbacks of various immobilisation techniques to obtain a high-surface-area TiO2

rials that often act as recombination centres, (iv) deactivation in the absence of H2

avoid the separation step [86]. Doping, loading and sensitisation of TiO2

when aromatic molecules must be abated and (v) difficulty to support powdered TiO<sup>2</sup>

some materials [45]. Consequently, the research line in HP has been driven to modify some

nanosized particles and films on glasses or other supports and powdered samples with high specific surface areas have been obtained to increase the possibility for the involved species to

to shift the light absorption towards visible light and/or to increase the lifetime of the photo-

A number of approaches have been suggested in recent years to enhance photocatalytic activ-

and co-doping with non-metals can improve trapping of the photoexcited conduction band electrons at the surface, thereby minimising charge carrier recombination. Several dopants used (e.g. Sn, Ag, Pd, Re, Bi, V, Mo, Th or Pt among others), have been shown to enhance photocatalytic activity for the systems examined. However, the photoactivity of the metal-doped

photocatalyst significantly depends on the dopant ion nature and concentration, prepa-

 can be achieved using non-metal elements such as N, F, S and C. Thus, modified TiO<sup>2</sup> showed a significant improvement on the absorption in the Vis light region due to band-gap narrowing and enhancing the degradation of pesticides under Vis light irradiation, mainly

the photoconductive properties by increasing the charge separation efficiency between electrons and holes, which will enhance the formation of both free hydroxyl radicals and active oxygen species [89]. Recent research indicates that the desired narrowing on the band gap of

ration method and operating conditions [88]. The deposition of metal ions on TiO2

in the visible light region for its use in water detoxification [87]. Metal ion doping

slurries has been demonstrated to have higher photocatalytic activ-

supported on different materials is growing because the use of the bare

pairs, (iii) difficulty to improve the performance by doping with some mate-

is prepared in the form of nanopowders, crystals, thin films,

onto supporting material has

to enhance its photoefficacy. In this context,

can be

O vapour

are methods aimed

can modify

on

important role in the efficiency of the photocatalytic wastewater treatment processes.

*2.2.3. Bare, doped and immobilised application of TiO2*

nanotubes and nanorods. As a rule, the immobilisation of TiO2

Generally, the use of TiO2

158 Application of Titanium Dioxide

development of TiO2

toproduced e<sup>−</sup>

produced e<sup>−</sup>

ity of TiO2

TiO2

TiO2

/h+ pairs.

TiO2

sites on its surface. Usually, TiO2

/h+

electronic and morphological properties of TiO2

A desirable feature in the photodegradation of pesticides in water is the transformation of the parent compounds in order to avoid their toxicity. However, the main objective is the mineralisation of the pesticides. As previously commented, •OH is the main species involved for organic substrate oxidation, but the free radical HO2 • and its conjugate O2 •− also play an important role although those radicals are much less reactive than •OH. All these free radicals react with pesticides by hydrogen abstraction or electrophilic addition to double bonds. Further, the radicals react with O2 to give organic peroxyl radicals (ROO<sup>−</sup> ) initiating different oxidative reactions that may lead to the complete mineralisation of the pesticides. Since •OH is non-selective, numerous and different transformation products (intermediates) can be formed at low concentrations being in certain cases more persistent and toxic than the parent compounds.

$$\begin{aligned} \text{Mineralization:} & \rightarrow \text{Organic peptide} \rightarrow \text{Intermediate} \\ & \rightarrow \text{CO}\_2 + \text{H}\_2\text{O} + \text{Cl}^- + \text{NO}\_3^- + \text{SO}\_4^{2-} + \text{PO}\_4^{3-} \text{etc.} \end{aligned} \tag{9}$$

The presence of pesticide residues in water is usually monitored using chromatographic techniques such as gas chromatography (GC) and liquid chromatography (LC) coupled to mass spectrometry (MSn) and time-of-flight (TOF) detection systems. However, since identification of all the transformation products generated during the photooxidation is not possible, the measure of total organic carbon (TOC) and more specifically dissolved organic carbon (DOC) is crucial in the process because determination of CO2 must be stoichiometric with the organic carbon in the parent pesticide. This determination can be carried out in a simple and rapid way to know the mass balance and the remaining amount of metabolites. As example, **Figure 5** shows the photodegradation pathways proposed for chlorantraniliprole [93] and tebuconazole [94] dissolved in water when illuminated in the presence of TiO2 .

In the case of chlorantraniliprole, a new class of anthranilic diamide insecticide, several transformation products are generated during irradiation as a result of different reactions such as hydroxylation, deamination, hydrolysis of the amide bridge and rearrangement followed by cyclisation, methyl amine transfer and fragmentation. For tebuconazole, a common triazole fungicide with numerous agricultural and urban uses, the transformation pathway was found to proceed through tert-butyl chain cleavage, hydroxylation, oxidation and dechlorination and showed that its degradation mechanism was mainly driven by •OH and h+ .

**Figure 5.** Photometabolic pathways proposed for chlorantraniliprole (insecticide) and tebuconazole (fungicide) in water slurries when illuminated in the presence of TiO2 .

The use of HP has showed a powerful growth in recent years. Today, pesticides constitute an important group concerning pollutant treatment. The large number of papers published in the last years proves this interest. A review to the literature extracted from the Web of Science™ (formerly ISI Web of Knowledge, www.isiknowledge.com) managed by Thomson Reuters (Philadelphia, USA) using the following keywords, *TiO2* , *pesticides* and *water*, shows 463 papers only in the period 2005−2016.

**Table 1** shows some of the most popular journal publishing on the topic "photocatalysis and pesticides" according to the following criteria: impact factor > 2 and Eigenfactor score > 0.01 (Journal Citation Reports (JCR) Science Edition 2015).

The *Journal Impact Factor* is defined as all citations to the journal in the current JCR year to items published in the previous 2 years, divided by the total number of scholarly items (these comprise articles, reviews and proceeding papers) published in the journal in the previous 2 years. The *Eigenfactor* score calculation is based on the number of times articles from the journal published in the past 5 years have been cited in the JCR year, but it also considers which journals have contributed these citations so that highly cited journals will influence the network more than lesser cited journals. References from one article in a journal to another article from the same journal are removed, so that *Eigenfactor* scores are not influenced by journal self-citation.

Following the criteria above, some of the most representative publications are presented in this summary (**Table 2**). Results show that sunlight photoalteration (photolysis) processes are well now to play an important role in the degradation of pesticides and other contaminants in the aquatic environment. These technologies allow the removal of pesticides by mineralisation. When the exciting energy used comes from the Sun, the process is called *solar photocatalysis* [95]. Photocatalytic oxidation by semiconductor oxides is an area of environmental interest for the treatment of polluted water, particularly relevant for Mediterranean agricultural areas, where solar irradiation is highly available making this process quite attractive. An ideal photocatalyst is characterised by photostability, biologically and chemically inert


The use of HP has showed a powerful growth in recent years. Today, pesticides constitute an important group concerning pollutant treatment. The large number of papers published in the last years proves this interest. A review to the literature extracted from the Web of Science™ (formerly ISI Web of Knowledge, www.isiknowledge.com) managed by Thomson

**Figure 5.** Photometabolic pathways proposed for chlorantraniliprole (insecticide) and tebuconazole (fungicide) in water

.

**Table 1** shows some of the most popular journal publishing on the topic "photocatalysis and pesticides" according to the following criteria: impact factor > 2 and Eigenfactor score > 0.01

The *Journal Impact Factor* is defined as all citations to the journal in the current JCR year to items published in the previous 2 years, divided by the total number of scholarly items (these comprise articles, reviews and proceeding papers) published in the journal in the previous 2 years. The *Eigenfactor* score calculation is based on the number of times articles from the journal published in the past 5 years have been cited in the JCR year, but it also considers which journals have contributed these citations so that highly cited journals will influence the network more than lesser cited journals. References from one article in a journal to another article from the same journal are removed, so that *Eigenfactor* scores are not influenced by

Following the criteria above, some of the most representative publications are presented in this summary (**Table 2**). Results show that sunlight photoalteration (photolysis) processes are well now to play an important role in the degradation of pesticides and other contaminants in the aquatic environment. These technologies allow the removal of pesticides by mineralisation. When the exciting energy used comes from the Sun, the process is called *solar photocatalysis* [95]. Photocatalytic oxidation by semiconductor oxides is an area of environmental interest for the treatment of polluted water, particularly relevant for Mediterranean agricultural areas, where solar irradiation is highly available making this process quite attractive. An ideal photocatalyst is characterised by photostability, biologically and chemically inert

, *pesticides* and *water*, shows

Reuters (Philadelphia, USA) using the following keywords, *TiO2*

(Journal Citation Reports (JCR) Science Edition 2015).

463 papers only in the period 2005−2016.

slurries when illuminated in the presence of TiO2

160 Application of Titanium Dioxide

journal self-citation.


**Table 1.** Some of the main and influential journals of interest to readers where the authors usually publish their works about 'photocatalytic degradation of pesticides in water' (source: JCR, 2015).


Recent Overview on the Abatement of Pesticide Residues in Water by Photocatalytic Treatment Using TiO2 http://dx.doi.org/10.5772/intechopen.68802 163


**Pesticides Photocatalysts Light source Main findings References**

**Table 1.** Some of the main and influential journals of interest to readers where the authors usually publish their works

**Journal title (ISO) Editorial/country ISSN a IF b 5Y-IF c ES d**

American Chemical Society (USA) 0021-8561 2.857 3.308 0.08776

Springer Heidelberg (Germany) 0944-1344 2.760 2.876 0.02617

Wiley-Blackwell (England) 0268-2575 2.738 2.744 0.01051

Elsevier Science SA (Switzerland) 1010-6030 2.477 2.573 0.01025

Science Press (Mainland China) 1001-0742 2.208 2.699 0.01282

Solar TiO2

UV ZnO > TiO2

UV ZnO and TiO2

Solar Similar photoefficiences in mineralisation

than sol-gel TiO2

7000 > Zn2

0.3–2 min)

respectively

Na2 S2 O8 and TiO2

UV Disappearance after 45–80 min [96]

P25 > TiO2

compared to artificial light for the removal of these insecticides (t½ =

> /Na2 S2 O8 systems,

TiO4

the degradation rate

UV/solar Solar irradiation was more efficient

UV Half-lives of 53 and 71 min for ZnO/

+ fly ash is 2–3 times less active

1474-905X 2.235 2.673 0.01040

> ZnTiO3

Kronos vlp

oxides strongly enhance

[81]

[97]

[98]

[99]

[100]

[93]

Pyrimethanil TiO2

Imidacloprid TiO2

Flubendiamide ZnO/Na2

Chlorantraniliprole ZnO, TiO2

Acephate, omethoate, methyl parathion

Spirotetramat, spirodiclofen, spiromesifen

Journal of Agricultural and Food Chemistry

Journal of Photochemistry and Photobiology A: Chemistry

Journal of Environmental Science

Impact factor.

5-Year impact factor. dEigenfactor score.

a

b

c

Environmental Science and Pollution Research

Journal of Chemical Technology and Biotechnology

162 Application of Titanium Dioxide

Photochemical and Photobiological Science

Thiamethoxam, imidacloprid, acetamiprid

P25 and home

about 'photocatalytic degradation of pesticides in water' (source: JCR, 2015).

Royal Society of Chemistry

(England)

International Standard Serial Number.

nanomicrospheres

and TiO2

Kronos vlp 7000,

S2 O8 and TiO2

Kronos vlp 7000,

@mTiO2

P25, TiO2

and ZnTiO3

P25, TiO2


made

hybrid

Zn2 TiO4

P25/Na2 S2 O8

ZnO/Na2 S2 O8 and TiO2

P25/Na2 S2 O8

Zn2 TiO4

ZnO, TiO2

Fe3 O4 @SiO2


**Table 2.** A brief summary of recent research studies in which TiO2 was used for treating pesticide-polluted waters in the period 2005–2016.

nature, low cost and availability and capability to adsorb reactants under efficient photonic activation. Due to these characteristics, titanium dioxide (TiO2 ) has been demonstrated to be an excellent catalyst, and its behaviour is very well documented for the photodegradation of pesticide residues in water.

#### **2.4. Environmental impact and treatment cost**

As previously stated, there is a very extensive literature (at laboratory and pilot plant scale) on the photocatalytic degradation of organic pollutants in water. However, there are not many works devoted to the study of the impact of the process from an environmental and economic point of view. In this context, the well-known life cycle impact assessment (LCIA) tool has been successfully used to assess the environmental impact of chemical processes. This tool finds the potential impacts associated with the entire life cycle of a product or a process. This methodology has been devised to study and compare processes at the industrial level but can be perfectly used at the laboratory level. On the other hand, there are several methods of estimating the costs of implementing each of the different AOPs. In general, the following items are proposed: (i) facility cost, (ii) project contingency, (iii) engineering project and (iv) replacement costs. The sum of these four concepts is the total installed cost, based on which the yearly economic impact can be evaluated. Then, operating costs have to be calculated. These costs are normally yearly and consist of the following items: (i) personnel, (ii) maintenance, (iii) electricity and (iv) materials and services. These costs added to the annual facility costs are the total annual costs [128].
