*3.2.6 Effect of inhibitors*

Large amount of sodium chloride is used in the dyeing process in the textile industries and hence it usually comes out in the dye effluent along with sectional water of textile mills. Sodium carbonate also plays an important role in fixing the dye on fabrics and in the fastness of colour and hence it is mainly used in the dye bath to adjust the pH of the bath. Alcohols are used for padding process during dyeing the fabrics. Alcohols such as ethanol are commonly used to quench hydroxyl radicals. Hence, it is important to study the influence of chloride ions, carbonate ions and ethanol on the photomineralisation efficiency of the catalyst. Irradiation experiments were carried out by taking 250 mL of different dyes of various concentration (TAZ =1 × 10<sup>−</sup><sup>4</sup> M, RY-17 and RB-5 = 1 × 10<sup>−</sup><sup>5</sup> M), 1.5 g of TiO2 (P-25 Degussa) under neutral pH and with different concentrations of (0–2 g) for both sodium chloride and sodium carbonate and 0 to 1 mL of ethanol. The irradiation was carried out by using 125 W low pressure mercury arc lamp and 85 W tungsten lamp as UV and visible light sources, respectively. The results obtained under both UV-visible irradiations are shown in **Figure 8**.

*The presence of chloride ion in the dye samples had a negative effect on decolourisation. The percentage decolourisation decreased drastically with increase in the concentration of sodium chloride. This decrease in the percentage decolourisation of dyes in the presence of chloride ion is due to the hole-scavenging properties of chloride ion (equations 7 and 8).*

$$\text{TiO}\_2 \rightarrow \text{TiO}\_2 \left(\text{h}^+\text{.}\_{\text{vb}}\text{.}\text{e}^-\_{\text{cb}}\right) \tag{8}$$

$$\text{Cl}^{-} \star \text{h}^{+} \star \text{Cl}^{-} \tag{9}$$

$$\text{Cl}^{\cdot} \star \text{Cl}^{\cdot} \Rightarrow \text{Cl}\_{2}^{\cdot \cdot} \tag{10}$$

#### **Figure 8.**

*Effect of (A) sodium chloride, (C) sodium carbonate and (E) ethanol on decolourisation of dyes under UV and (B, D, F) visible irradiations. (reaction conditions: Dye concentration: TAZ = 1 × 10<sup>−</sup><sup>4</sup> M, RY-17 = 1 × 10<sup>−</sup><sup>5</sup> M, RB-5 = 1 × 10<sup>−</sup><sup>5</sup> M, weight of catalyst (P-25 Degussa) = 1.5 g, volume of dye solution = 250 mL, pH = neutral and irradiation time = 6 or 6 ½ h).*

This is a distinctive competitive inhibition reaction where the reaction of dye molecules with the holes has to compete with the chloride ions, which are not readily oxidisable. This inhibitive effect was also observed by [38]. However, the photocatalytic activity of the catalyst can be fully restored by washing the catalyst with pure water.

The decrease in the degradation of dyes in the presence of carbonate and bicarbonate ions is due to the hydroxyl scavenging property of carbonate ions according to the following Eqs. (11) and (12).

$$\text{OH}^{\cdot} \star \text{CO}\_{3}^{2-} \xrightarrow{2} \text{OH}^{-} \star \text{CO}\_{3}^{\cdot-} \tag{11}$$

$$\text{OH}^{\cdot} \star \text{HCO}\_{3}^{-} \rightarrow \text{H}\_{2}\text{O} \star \text{CO}\_{3}^{\cdot-} \tag{12}$$

**89**

*Detoxification of Carcinogenic Dyes by Noble Metal (Ag, Au, Pt) Impregnated Titania…*

electron/hole recombination is to add other electron acceptors such as hydrogen

Hydrogen peroxide is extensively used in commercial bleaching of textiles, especially cellulose fibres and wool. In order to study the influence of hydrogen peroxide in the decolourisation of dyes, experiments were performed by adding different quantities of hydrogen peroxide(30% v/v) (0–10 mL) and 0 to 2 g of potassium persulphate to 250 mL of different dyes of various concentration (TAZ = 2 × 10<sup>−</sup><sup>4</sup>

irradiation was carried out by using 125 W low pressure mercury arc lamp and 85 W

Since hydrogen peroxide is better than molecular oxygen as electron acceptor it leads to 100% decolourisation in comparison to the results obtained in the absence of H2O2. But, in the absence of TiO2, and with only H2O2 the percentage decolourisation was found to be less under both the light sources as shown in **Figure 9**.

The percentage decolourisation increased on increasing the volume of H2O2 from 0 to 10 mL. However, percentage decolourisation decreased on increasing the volume of H2O2 beyond 10 mL. Since, H2O2 is a powerful hydroxyl scavenger, the percentage decolourisation decreased at the higher concentration as shown in Eqs. (14) and (15).

H2O2 + OH˙→ HO2˙ +H2O (14)

HO2˙+OH˙→ H2O + O2 (15)

The above decolourisation results carried out by both UV and visible sources show the necessity of choosing the proper dosage of hydrogen peroxide for maximum decolourisation. Thus, it is clear from the **Figure 9** that addition of H2O2 up to 10 mL concentration showed a beneficial effect on the photocatalytic decolourisa-

The rate of photo-assisted decolourisation of dyes is significantly improved by

The percentage decolourisation increased with increase in the amount of K2S2O8 and 100% degradation was attained at highest concentration of K2S2O8 for both UV and visible irradiations. The added persulphate ion acts as electron traps resulting in

cb → SO4˙<sup>−</sup> + SO4

The sulphate radical anion is a strong oxidant and removes electron from neutral

The persulphate ions oxidise the dyes as well as act as electron scavenger. Due to

<sup>2</sup><sup>−</sup> ions produce sulphate radical anion (SO4˙

produce highly oxidative hydroxyl radicals. These hydroxyl radicals degrade the

M), 1.5 g of TiO2 (P-25 Degussa) under neutral pH. The

M,

) pair recombination

+ OH˙ (13)

<sup>−</sup> as shown in Eq. (16).

2− (16)

2− + H<sup>+</sup> (17)

<sup>−</sup>) which in turn

pair recombination at the semiconduc-

*DOI: http://dx.doi.org/10.5772/intechopen.80467*

RY-17 and RB-5 = 2 × 10<sup>−</sup><sup>5</sup>

tion of the dyes [41].

tor surface [42]. S2O8

dyes at a faster rate.

the presence of persulphate ions.

S2O8

the formation of reactive radical intermediate SO4˙

molecules like water and generates hydroxyl radical.

their scavenging property they inhibit e<sup>−</sup>/h+

SO4˙<sup>−</sup> + H2O → OH˙+SO4

2− + e −

peroxide, persulphate ions to the reaction mixture.

tungsten lamp as UV and visible light sources respectively. The presence of hydrogen peroxide will prevent the (e<sup>−</sup>/h+

and enhances the formation of hydroxyl radicals (Eq. 13).

TiO2(e−) <sup>+</sup> H2O2 <sup>→</sup> TiO2 <sup>+</sup> OH<sup>−</sup>

The presence of carbonate and bicarbonate ions reduces the concentration of hydroxyl radicals significantly and hence the percentage decolourisation of the dyes decreases [39].

When the volume of ethanol was increased the photodecolourisation efficiency gradually decreased. This shows that the dye degradation occurs via positive holes (h+ vb) that are not involved in recombination with electrons during illumination process. Further it is evident that hydroxyl radicals are not the only species responsible for degradation and minor degradation also occurs through holes generated during illumination [40].

#### *3.2.7 Effect of electron acceptors*

The practical problem in using TiO2 as a photocatalyst is the undesired electron/ hole recombination. In the absence of proper electron acceptors or donors the electron/hole recombination is extremely efficient and represents the major energy wasting step which limits the achievable quantum yield. One strategy to inhibit

*Detoxification of Carcinogenic Dyes by Noble Metal (Ag, Au, Pt) Impregnated Titania… DOI: http://dx.doi.org/10.5772/intechopen.80467*

electron/hole recombination is to add other electron acceptors such as hydrogen peroxide, persulphate ions to the reaction mixture.

Hydrogen peroxide is extensively used in commercial bleaching of textiles, especially cellulose fibres and wool. In order to study the influence of hydrogen peroxide in the decolourisation of dyes, experiments were performed by adding different quantities of hydrogen peroxide(30% v/v) (0–10 mL) and 0 to 2 g of potassium persulphate to 250 mL of different dyes of various concentration (TAZ = 2 × 10<sup>−</sup><sup>4</sup> M, RY-17 and RB-5 = 2 × 10<sup>−</sup><sup>5</sup> M), 1.5 g of TiO2 (P-25 Degussa) under neutral pH. The irradiation was carried out by using 125 W low pressure mercury arc lamp and 85 W tungsten lamp as UV and visible light sources respectively.

The presence of hydrogen peroxide will prevent the (e<sup>−</sup>/h+ ) pair recombination and enhances the formation of hydroxyl radicals (Eq. 13).

$$\text{TiO}\_{2(e-)} \star \text{H}\_2\text{O}\_2 \rightarrow \text{TiO}\_2 \star \text{OH}^- \star \text{OH}^- \tag{13}$$

Since hydrogen peroxide is better than molecular oxygen as electron acceptor it leads to 100% decolourisation in comparison to the results obtained in the absence of H2O2. But, in the absence of TiO2, and with only H2O2 the percentage decolourisation was found to be less under both the light sources as shown in **Figure 9**.

The percentage decolourisation increased on increasing the volume of H2O2 from 0 to 10 mL. However, percentage decolourisation decreased on increasing the volume of H2O2 beyond 10 mL. Since, H2O2 is a powerful hydroxyl scavenger, the percentage decolourisation decreased at the higher concentration as shown in Eqs. (14) and (15).

$$\text{H}\_2\text{O}\_2 \star \text{OH}' \rightarrow \text{HO}\_2^\cdot \quad \text{+H}\_2\text{O} \tag{14}$$

$$\rm{HO}\_2^- \star \rm{OH}^\cdot \Rightarrow \rm{H}\_2\rm{O} + \rm{O}\_2 \tag{15}$$

The above decolourisation results carried out by both UV and visible sources show the necessity of choosing the proper dosage of hydrogen peroxide for maximum decolourisation. Thus, it is clear from the **Figure 9** that addition of H2O2 up to 10 mL concentration showed a beneficial effect on the photocatalytic decolourisation of the dyes [41].

The rate of photo-assisted decolourisation of dyes is significantly improved by the presence of persulphate ions.

The percentage decolourisation increased with increase in the amount of K2S2O8 and 100% degradation was attained at highest concentration of K2S2O8 for both UV and visible irradiations. The added persulphate ion acts as electron traps resulting in the formation of reactive radical intermediate SO4˙ <sup>−</sup> as shown in Eq. (16).

$$\text{S}\_2\text{O}\_8^{2-} + \text{e}^-\_{\text{cb}} \rightarrow \text{SO}\_4^{\cdot-} + \text{SO}\_4^{2-} \tag{16}$$

The sulphate radical anion is a strong oxidant and removes electron from neutral molecules like water and generates hydroxyl radical.

$$\text{SO}\_4\text{'}-\text{}\text{+}\text{H}\_2\text{O} \rightarrow \text{OH'}\text{'}\text{+}\text{SO}\_4\text{'}{}^2\text{+}\text{H}^+\tag{17}$$

The persulphate ions oxidise the dyes as well as act as electron scavenger. Due to their scavenging property they inhibit e<sup>−</sup>/h+ pair recombination at the semiconductor surface [42]. S2O8 <sup>2</sup><sup>−</sup> ions produce sulphate radical anion (SO4˙ <sup>−</sup>) which in turn produce highly oxidative hydroxyl radicals. These hydroxyl radicals degrade the dyes at a faster rate.

*Gold Nanoparticles - Reaching New Heights*

*Effect of (A) sodium chloride, (C) sodium carbonate and (E) ethanol on decolourisation of dyes under UV and* 

This is a distinctive competitive inhibition reaction where the reaction of dye molecules with the holes has to compete with the chloride ions, which are not readily oxidisable. This inhibitive effect was also observed by [38]. However, the photocatalytic activity of the catalyst can be fully restored by washing the catalyst

The decrease in the degradation of dyes in the presence of carbonate and bicarbonate ions is due to the hydroxyl scavenging property of carbonate ions according

> 2− → OH<sup>−</sup>

> > −

The presence of carbonate and bicarbonate ions reduces the concentration of hydroxyl radicals significantly and hence the percentage decolourisation of the dyes

When the volume of ethanol was increased the photodecolourisation efficiency gradually decreased. This shows that the dye degradation occurs via positive holes

The practical problem in using TiO2 as a photocatalyst is the undesired electron/

hole recombination. In the absence of proper electron acceptors or donors the electron/hole recombination is extremely efficient and represents the major energy wasting step which limits the achievable quantum yield. One strategy to inhibit

vb) that are not involved in recombination with electrons during illumination process. Further it is evident that hydroxyl radicals are not the only species responsible for degradation and minor degradation also occurs through holes generated

 *M, weight of catalyst (P-25 Degussa) = 1.5 g, volume of dye solution = 250 mL, pH = neutral* 

 *M, RY-17 = 1 × 10<sup>−</sup><sup>5</sup>*

+ CO3˙<sup>−</sup> (11)

→ H2O + CO3˙<sup>−</sup> (12)

 *M,* 

*(B, D, F) visible irradiations. (reaction conditions: Dye concentration: TAZ = 1 × 10<sup>−</sup><sup>4</sup>*

**88**

(h+

**Figure 8.**

*RB-5 = 1 × 10<sup>−</sup><sup>5</sup>*

with pure water.

decreases [39].

during illumination [40].

*3.2.7 Effect of electron acceptors*

*and irradiation time = 6 or 6 ½ h).*

to the following Eqs. (11) and (12).

OH˙+CO3

OH˙+HCO3

#### **Figure 9.**

*Effect of (A) H2O2 and (B) K2S2O8 on decolourisation of dyes under visible and (C and D) UV irradiations. (reaction conditions: Dye concentration: TAZ = 2 × 10<sup>−</sup><sup>4</sup> M, RY-17 = 2 × 10<sup>−</sup><sup>5</sup> M, RB-5 = 2 × 10<sup>−</sup><sup>5</sup> M, weight of catalyst (P-25 Degussa) = 1.5 g, volume of dye solution =250 mL, pH = neutral and irradiation time = 7 h).*
