**5. Photocatalytic reactors**

crystal composition, surface area, particle size distribution, porosity and band gap energy [12]. TiO2 is also known as titania, titanic oxide, titanium white, titanic anhydride, or titanic acid anhydride. It is prepared using ilmenite and rutile in crystalline forms called anatase and rutile. The anatase form is achieved by processing of titanium sulphate, which is ach‐ ieved when ilmenite is treated with sulphuric acid. Rutile crystalline form is obtained when raw rutile is chlorinated and the resulting titanium tetrachloride is subjected to vapor phase oxidation [13]. When photon energy (hv) of higher than or equal to the bandgap energy of TiO2 is illuminated onto its surface, typically 3.2 eV (anatase) or 3.0 eV (rutile), the lone elec‐ tron is photoexcited to the empty conduction band in femtoseconds [7]. Degussa P25 which is the most widely used form of TiO2 is composed of 75% anatase and 25% rutile and has a

/hVB+

conductive band to the electron traps occurs in the anatase phase [14].

**Table 1.** Band gap energies of various semiconductors at relevant wavelengths [15]

**4. Radiation sources for photocatalysis**

degradation applications.

sorbs photons and generates electron-hole pairs and the electron transfer from the rutile

**Semiconductor Band gas energy (eV) Wavelength** TiO2 (rutile) 3.0 413

TiO2(anatase) 3.2 388

ZnO 3.2 388

ZnS 3.6 335

CdS 2.4 516

Fe2O3 2.3 539

Both artificial UV lamps and sunlight can be used as the radiation source for photocatalytic process. Artificial UV lamps containing mercury are the most commonly used source of UV irradiation. These can be divided into low pressure mercury lamp, medium pressure mercu‐ ry lamp and high pressure mercury lamp. Sunlight has also been used in the photocatalytic process as nearly 4-5% of the sunlight that reaches the earth's surface is in the 300-400 nm near UV light range. Furthermore solar energy has limitations due to the graphical varia‐ tions when compared with the artificial UV lamps. However ongoing interests and develop‐ ments in harnessing solar energy are expected to increase its use in photocatalytic

/g. The high effectiveness of D25 is related to the inhibition

) due to the smaller band gap of rutile that ab‐

specific BET surface area of 50 m2

198 Organic Pollutants - Monitoring, Risk and Treatment

of recombination process on TiO2 (eCB<sup>−</sup>

Photocatalytic reactors can be classified based on the deployed state of the photocatalyst, i.e., suspended or attached. Photocatalytic reactors can use either UV or solar radiation. So‐ lar photocatalytic reactors have been of great interest for the photoxodation of organic con‐ taminants in water. Such kind of reactors can be divided into concentrating or nonconcentrating reactors [16]. Both the reactor types extend certain advantages and disadvantages. For example, non-concentrating reactors have negligible optical losses and therefore can use direct and diffuse sun irradiation but are larger in size compared with the concentrating reactors and have high frictional pressure losses [16]. However, the use of so‐ lar radiated photoreactors is limited due to the intrinsic nature of the TiO2 particles. Follow‐ ing section provides details on the type of reactors used in various studies for the degradation of organic pollutants in water.

#### **5.1. Slurry reactors**

Until recently, TiO2 slurry reactors are most commonly type used in water treatment. These show largest photocatalytic activity compared with the immobilized photocatalyst and pro‐ vide a high total surface area of photocatalyst per unit volume which is one of the most impor‐ tant factor configuring a photocatalytic reactor [7]. However, these reactors require separation of the sub-micron TiO2 particles from the treated water which complicates the treatment proc‐ ess. Several techniques were proposed to achieve post-treatment separation such as the use of settling tanks (overnight particle settling) or external cross-flow filtration system [7]. However the use of filtrations systems increases the cost of the treatment process.

### **5.2. Immobilized TiO2 reactors**

Photocatalytic reactors with immobilized TiO2 are those in which catalyst is fixed to support via physical surface forces or chemical bonds. These reactors extend the benefit of not re‐ quiring catalyst recovery and permit the continuous use of the photocatalyst [16]. Hybrid photocatalytic membrane reactors have been developed to achieve the purpose of down‐ stream separation of photocatalyst. The photocatalytic membrane reactors can be general‐ ized in two categories (1) irradiation of the membrane module and (2) irradiation of feed tank containing photocatalyst in suspension [17]. Various membranes such as microfiltra‐ tion, ultrafiltration, and nanofiltration membranes may be used for this purpose depending on the requirements of the treated water quality [7]. Photocatalytic membrane reactors have been successfully used for the degradation of tricholoroethylene and 4-nitrophenol [18, 19]. However, these reactors possess drawbacks such as low surface area to volume ratios, cata‐ lyst fouling and significant pressure drop [16]. Another problem associated with the mem‐ brane photocatalytic reactors is the diffusion of organic compounds to the catalyst surface which is slow particularly when the organic compounds concentration is low [20]. One pos‐ sible solution to the slow diffusion is using pores of nano size to enable photocatalyst to per‐ form selective permeation and to produce an oxidized permeate stream [21].

It can be observed that the photocatalytic reactors can be either slurry or immobilized sys‐ tems and each possess certain advantages and disadvantages related to their design and effi‐ ciency. Further research on the design and energy efficiency of photocatalytic reactors could make photocatalytic degradation process more feasible for future applications in water treatment. Membrane photoreactors appear to be a promising alternative to conventional photoreactors and more research in this area can assist overcome some of the problems faced with the use of conventional reactors.

charged under alkaline conditions. The maximum oxidizing capacity of the titania is at lower

tion of pH is thus need to be appropriate in order to achieve maximum degradation efficiency.

**Target compound Photocatalyst Optimum dosage (g/L) References** Erioglaucine TiO2 0.3 [23] Tebuthioron TiO2 5 [33] Propham TiO2 5 [33] Triclopyr TiO2 2 [34] Phorate TiO2 0.5 [54] Turbophos TiO2 0.5 [55] Trichlorfon TiO2 8 [56] Methamodiphos Re- TiO2 1 [57] Methylene blue La-Y/TiO2 4 [58] Carbendazim TiO2 0.07 [59] Direct red 23 Ag-TiO2 3 [60] Phenol Pr- TiO2 1 [61] Carbofuran TiO2 0.1 [62] Beta-cypermethrin RuO2-TiO2 5 [63] Aniline Pt-TiO2 2.5 [64] Benzylamine Pt-TiO2 2.5 [64] Glyphosate TiO2 6 [65] Picloram TiO2 2 [66] Floumeturon TiO2 3 [67] Imazapyr TiO2 2.5 [68]

[29]. The selec‐

201

http://dx.doi.org/10.5772/53699

Photocatalytic Degradation of Organic Pollutants in Water

pH however the reaction rate is known to decrease at low pH due to excess H+

**Table 2.** Optimum dosage of photocatalyst for degradation of organic compounds

Surface morphology such as particle size and agglomerate size, is an important factor to be considered in photocatalytic degradation process because there is a direct relationship between organic compounds and surface coverage of the photocatalyst [30]. The number of photon striking the photocatalyst controls the rate of reaction which signifies that the reaction takes place only in the absorbed phase of the photocatalyst [2, 31]. A number of different forms of TiO2 have been synthesized to achieve the desired characteristics of the photocatalyst [32]. Some of the examples include UV100, PC500 and TTP. For the degra‐ dation of various organic compound such as pesticides and dyes, the efficacy of these

**6.3. Size and structure of the photocatalyst**

## **6. Factors affecting the degradation performance**

#### **6.1. Catalyst loading**

The amount of TiO2 being directly proportional to the overall photocatalytic reaction rate, the concentration of the TiO2 particles affects the overall photocatalysis reaction rate in a true heterogeneous catalytic regime [2]. However, when the amount of TiO2 is above certain level (saturation stage), the light photon adsorption co-efficient decreases radially and the excess photocatalyst can create a light screening effect that leads to the reduction in the sur‐ face area exposed to irradiation and thus reduces the photocatalytic efficiency of the process [7]. A number of studies have reported the effect of TiO2 loadings on the treatment efficiency of the photocatalytic reactor [2, 22-24]. Although a direct comparison between these studies is difficult to be made due to the differences in the working geometry, radiation fluxes and wavelengths used, it was evident that the optimum dosages of photocatalyst loading were dependent on the dimension of the reactor. The importance of the determination of the reac‐ tor diameter has been emphasized to achieve effective photon absorption [25]. The optimum dosage of TiO2 used by various authors either alone or in combination with other catalysts is given in Table 2.

#### **6.2. pH of the solution**

The effect of pH on the photocatalytic reaction has been extensively studied [26, 27] due to the fact that photocatalytic water treatment is highly dependent on the pH as it affects the charge on the catalyst particles, size of aggregates and the position of conductance and val‐ ance bands [7]. Furthermore the surface of the TiO2 can be protonated or deprotonated un‐ der acidic or alkaline conditions [2], respectively according to the reaction given below.

$$\text{TiOH} + \text{H}^+ \rightarrow \text{TiOH}\_2^+ \tag{6}$$

$$\text{TiOH} + \text{OH}^- \rightarrow \quad \text{TiO} + \text{H}\_2\text{O} \tag{7}$$

The point of zero discharge for P25 Degussa, the most commonly used form of TiO2 is 6.9 [28]. Therefore the surface of the TiO2 is positively charged under acidic conditions and negatively


charged under alkaline conditions. The maximum oxidizing capacity of the titania is at lower pH however the reaction rate is known to decrease at low pH due to excess H+ [29]. The selec‐ tion of pH is thus need to be appropriate in order to achieve maximum degradation efficiency.

**Table 2.** Optimum dosage of photocatalyst for degradation of organic compounds

#### **6.3. Size and structure of the photocatalyst**

It can be observed that the photocatalytic reactors can be either slurry or immobilized sys‐ tems and each possess certain advantages and disadvantages related to their design and effi‐ ciency. Further research on the design and energy efficiency of photocatalytic reactors could make photocatalytic degradation process more feasible for future applications in water treatment. Membrane photoreactors appear to be a promising alternative to conventional photoreactors and more research in this area can assist overcome some of the problems

The amount of TiO2 being directly proportional to the overall photocatalytic reaction rate, the concentration of the TiO2 particles affects the overall photocatalysis reaction rate in a true heterogeneous catalytic regime [2]. However, when the amount of TiO2 is above certain level (saturation stage), the light photon adsorption co-efficient decreases radially and the excess photocatalyst can create a light screening effect that leads to the reduction in the sur‐ face area exposed to irradiation and thus reduces the photocatalytic efficiency of the process [7]. A number of studies have reported the effect of TiO2 loadings on the treatment efficiency of the photocatalytic reactor [2, 22-24]. Although a direct comparison between these studies is difficult to be made due to the differences in the working geometry, radiation fluxes and wavelengths used, it was evident that the optimum dosages of photocatalyst loading were dependent on the dimension of the reactor. The importance of the determination of the reac‐ tor diameter has been emphasized to achieve effective photon absorption [25]. The optimum dosage of TiO2 used by various authors either alone or in combination with other catalysts is

The effect of pH on the photocatalytic reaction has been extensively studied [26, 27] due to the fact that photocatalytic water treatment is highly dependent on the pH as it affects the charge on the catalyst particles, size of aggregates and the position of conductance and val‐ ance bands [7]. Furthermore the surface of the TiO2 can be protonated or deprotonated un‐ der acidic or alkaline conditions [2], respectively according to the reaction given below.

+ +


The point of zero discharge for P25 Degussa, the most commonly used form of TiO2 is 6.9 [28]. Therefore the surface of the TiO2 is positively charged under acidic conditions and negatively

<sup>2</sup> TiOH + H TiOH ® (6)

<sup>2</sup> TiOH + OH TiO + H O ® (7)

faced with the use of conventional reactors.

200 Organic Pollutants - Monitoring, Risk and Treatment

**6.1. Catalyst loading**

given in Table 2.

**6.2. pH of the solution**

**6. Factors affecting the degradation performance**

Surface morphology such as particle size and agglomerate size, is an important factor to be considered in photocatalytic degradation process because there is a direct relationship between organic compounds and surface coverage of the photocatalyst [30]. The number of photon striking the photocatalyst controls the rate of reaction which signifies that the reaction takes place only in the absorbed phase of the photocatalyst [2, 31]. A number of different forms of TiO2 have been synthesized to achieve the desired characteristics of the photocatalyst [32]. Some of the examples include UV100, PC500 and TTP. For the degra‐ dation of various organic compound such as pesticides and dyes, the efficacy of these photocatalysts has generally been reported in the order of Degussa P25 > UV100 > PC500 >TTP [33-36].

The inorganic anions such as nitrate, chlorides, carbonates and sulphates are also known to inhibit the surface activity of the photocatalyst. The presence of salts diminishes the colloi‐ dal stability, increases mass transfer and reduces the surface contact between the pollutant and the photocatalyst [7]. Other than fouling of the TiO2 surface certain anions such as chlor‐ ides, carbonates, phosphate and sulphates also scavenge both the hole and the hydroxyl rad‐ icals [52]. The mechanism of hole and radical scavenging by chloride has been proposed by

The inhibitory effect of chloride ions occurs through preferential adsorption displacement mechanism which results in reducing the number of OH- available on the photocatalyst sur‐

The fouling of photocatalytic surface can be reduced by pre-treatment of water such as with ion exchange resins which have been reported to reduce the fouling and so the cost of treat‐ ment (Burns et al., 1999). Similarly the fouling induced by sulphates and phosphates has been reported to be displaced by NaOH, KOH and NaHCO3 [41]. However, most of studies conducted on the effect of inorganic ions are based on the model compounds and therefore do not necessarily represent their effect in real water matrix where several ions exist. More work concentrating on the effect of complex mixtures of inorganic ions is thus required.

Photocatalytic degradation of organic pollutants is promising technology due to its advant‐ age of degradation on pollutants instead of their transformation under ambient conditions. The process is capable of removing a wide range of organic pollutants such as pesticides, herbicides, and micropollutants such as endocrine disrupting compounds. Although signifi‐ cant amount of research has been conducted on TiO2 photocatalysis at laboratory scale, its application on industrial scale requires certain limitations to be addressed. However the ap‐ plication of this treatment is constrained by several factors such as wide band gap (3.2eV), lack and inability of efficient and cost-effective catalyst for high photon-efficiency to utilize wider solar spectra. The effect of variables is required to be further studied in real water ma‐ trix to achieve representative results. The results achieved can be used to optimize the proc‐ ess and design appropriate reactor for potential large scale applications. The use of solar radiation has to be improved by virtue of the design of the photoreactor in order to reduce the cost of treatment. Further research to investigate the degradation of the real water con‐

stituents is required to better comprehend the process applications.



Photocatalytic Degradation of Organic Pollutants in Water

http://dx.doi.org/10.5772/53699

203

Matthews and McEnvoy [53] as follows.

face [7].

**7. Conclusions**

#### **6.4. Reaction temperature**

An increase in reaction temperature generally results in increased photocatalytic activity however reaction temperature >80o C promotes the recombination of charge carriers and dis‐ favor the adsorption of organic compounds on the titania surface [2]. A reaction tempera‐ ture below 80o C favours the adsorption whereas further reduction of reaction temperature to 0o C results in an increase in the apparent activation energy [7]. Therefore temperature range between 20-80o C has been regard as the desired temperature for effective photominer‐ alization of organic content.

#### **6.5. Concentration and nature of pollutants**

The rate of photocatalytic degradation of certain pollutant depends on its nature, concentra‐ tion and other existing compounds in water matrix. A number of studies have reported the de‐ pendency of the TiO2 reaction rate on the concentration of contaminants in water [37]. High concentration of pollutants in water saturates the TiO2 surface and hence reduces the photonic efficiency and deactivation of the photocatalyst [38]. In addition to the concentration of pollu‐ tants, the chemical structure of the target compound also influences the degradation perform‐ ance of the photocatalytic reactor. For example, 4-chlorophenol requires prolonged irradiation time due to its transformation to intermediates compared with oxalic acid that transforms di‐ rectly to carbon dioxide and water, i.e., complete mineralization [39]. Furthermore if the nature of the target water contaminants is such that they adhere effectively to the photocatalyst sur‐ face the process would be more effective in removing such compounds from the solution. Therefore the photocatalytic degradation of aromatics is highly dependent on the substituent group [2]. The organic substrates with electron withdrawing nature (benzoic acid, nitroben‐ zene) strongly adhere to the photocatalyst and therefore are more susceptible to direct oxida‐ tion compared with the electron donating groups [40].

#### **6.6. Inorganic ions**

Various inorganic ions such as magnesium, iron, zinc, copper, bicarbonate, phosphate, ni‐ trate, sulfate and chloride present in wastewater can affect the photocatalytic degradation rate of the organic pollutants because they can be adsorbed onto the surface of TiO2 [41-43]. Photocatalytic deactivation has been reported whether photocatalyst is used in slurry or fixed-bed configuration which is related to the strong inhibition from the inorganic ions on the surface of the TiO2 [44]. A number of studies have been conducted on the effect of inor‐ ganic ions (anions and cations) on TiO2 photocatalytic degradation [30, 45-51]. Some of the cations such as copper, iorn and phosphate have been reported to decrease the photodegra‐ dation efficiency if they are present at certain concentrations whereas calcium, magnesium and zinc have little effect on the photodegradation of organic compounds which is associat‐ ed to the fact that these cations have are at their maximum oxidation states that results in their inability to have any inhibitory effect on the degradation process [7].

The inorganic anions such as nitrate, chlorides, carbonates and sulphates are also known to inhibit the surface activity of the photocatalyst. The presence of salts diminishes the colloi‐ dal stability, increases mass transfer and reduces the surface contact between the pollutant and the photocatalyst [7]. Other than fouling of the TiO2 surface certain anions such as chlor‐ ides, carbonates, phosphate and sulphates also scavenge both the hole and the hydroxyl rad‐ icals [52]. The mechanism of hole and radical scavenging by chloride has been proposed by Matthews and McEnvoy [53] as follows.

$$\text{Cl}^\bullet \text{+}^\bullet \text{OH} \rightarrow \text{Cl}^\bullet \text{+} \text{OH}^\bullet \tag{8}$$

$$\text{Cl}^{\cdot} + \text{h}^{\cdot} \rightarrow \text{Cl}^{\bullet} \tag{9}$$

The inhibitory effect of chloride ions occurs through preferential adsorption displacement mechanism which results in reducing the number of OH- available on the photocatalyst sur‐ face [7].

The fouling of photocatalytic surface can be reduced by pre-treatment of water such as with ion exchange resins which have been reported to reduce the fouling and so the cost of treat‐ ment (Burns et al., 1999). Similarly the fouling induced by sulphates and phosphates has been reported to be displaced by NaOH, KOH and NaHCO3 [41]. However, most of studies conducted on the effect of inorganic ions are based on the model compounds and therefore do not necessarily represent their effect in real water matrix where several ions exist. More work concentrating on the effect of complex mixtures of inorganic ions is thus required.

## **7. Conclusions**

photocatalysts has generally been reported in the order of Degussa P25 > UV100 > PC500

An increase in reaction temperature generally results in increased photocatalytic activity

favor the adsorption of organic compounds on the titania surface [2]. A reaction tempera‐

The rate of photocatalytic degradation of certain pollutant depends on its nature, concentra‐ tion and other existing compounds in water matrix. A number of studies have reported the de‐ pendency of the TiO2 reaction rate on the concentration of contaminants in water [37]. High concentration of pollutants in water saturates the TiO2 surface and hence reduces the photonic efficiency and deactivation of the photocatalyst [38]. In addition to the concentration of pollu‐ tants, the chemical structure of the target compound also influences the degradation perform‐ ance of the photocatalytic reactor. For example, 4-chlorophenol requires prolonged irradiation time due to its transformation to intermediates compared with oxalic acid that transforms di‐ rectly to carbon dioxide and water, i.e., complete mineralization [39]. Furthermore if the nature of the target water contaminants is such that they adhere effectively to the photocatalyst sur‐ face the process would be more effective in removing such compounds from the solution. Therefore the photocatalytic degradation of aromatics is highly dependent on the substituent group [2]. The organic substrates with electron withdrawing nature (benzoic acid, nitroben‐ zene) strongly adhere to the photocatalyst and therefore are more susceptible to direct oxida‐

Various inorganic ions such as magnesium, iron, zinc, copper, bicarbonate, phosphate, ni‐ trate, sulfate and chloride present in wastewater can affect the photocatalytic degradation rate of the organic pollutants because they can be adsorbed onto the surface of TiO2 [41-43]. Photocatalytic deactivation has been reported whether photocatalyst is used in slurry or fixed-bed configuration which is related to the strong inhibition from the inorganic ions on the surface of the TiO2 [44]. A number of studies have been conducted on the effect of inor‐ ganic ions (anions and cations) on TiO2 photocatalytic degradation [30, 45-51]. Some of the cations such as copper, iorn and phosphate have been reported to decrease the photodegra‐ dation efficiency if they are present at certain concentrations whereas calcium, magnesium and zinc have little effect on the photodegradation of organic compounds which is associat‐ ed to the fact that these cations have are at their maximum oxidation states that results in

their inability to have any inhibitory effect on the degradation process [7].

C results in an increase in the apparent activation energy [7]. Therefore temperature

C favours the adsorption whereas further reduction of reaction temperature

C has been regard as the desired temperature for effective photominer‐

C promotes the recombination of charge carriers and dis‐

>TTP [33-36].

ture below 80o

range between 20-80o

**6.6. Inorganic ions**

alization of organic content.

to 0o

**6.4. Reaction temperature**

however reaction temperature >80o

202 Organic Pollutants - Monitoring, Risk and Treatment

**6.5. Concentration and nature of pollutants**

tion compared with the electron donating groups [40].

Photocatalytic degradation of organic pollutants is promising technology due to its advant‐ age of degradation on pollutants instead of their transformation under ambient conditions. The process is capable of removing a wide range of organic pollutants such as pesticides, herbicides, and micropollutants such as endocrine disrupting compounds. Although signifi‐ cant amount of research has been conducted on TiO2 photocatalysis at laboratory scale, its application on industrial scale requires certain limitations to be addressed. However the ap‐ plication of this treatment is constrained by several factors such as wide band gap (3.2eV), lack and inability of efficient and cost-effective catalyst for high photon-efficiency to utilize wider solar spectra. The effect of variables is required to be further studied in real water ma‐ trix to achieve representative results. The results achieved can be used to optimize the proc‐ ess and design appropriate reactor for potential large scale applications. The use of solar radiation has to be improved by virtue of the design of the photoreactor in order to reduce the cost of treatment. Further research to investigate the degradation of the real water con‐ stituents is required to better comprehend the process applications.
