*3.2.5 Recycling of TiO2*

In order to test the recyclability of TiO2 catalyst, the experiment was repeated with 250 mL of dye solutions (TAZ =1 × 10<sup>−</sup><sup>4</sup> M, RY-17 and RB-5 = 1 × 10<sup>−</sup><sup>5</sup> M) with the used catalyst. The photocatalytic experiments were carried out with 1.5 g of TiO2 (P-25 Degussa) for 5 h. At the end of reaction, the dye samples were filtered and the catalyst was filtered off. There was not much difference between the weight of the recovered catalyst with its initial weight (<1%). Repeated experiments were carried out for another 5 h by taking 250 mL of fresh dye solution. The pseudo first order rate constant values were calculated for each cycle as mentioned before and the values are given in **Table 3**.

#### **Figure 7.**

*Mineralisation of dyes using TiO2 (P-25 Degussa) under (A) UV and (B) 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, volume of dye solution = 250 mL, irradiation time = 9 h and pH = 7).*

**87**

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

The rate constant values were found to be almost the same for all the dyes under both UV and visible irradiations even after two cycles. Nevertheless at the end of II recycle, 91% decolourisation occurred in the illumination period of 5 h. This shows that the TiO2 (P-25 Degussa) is a very good catalyst which does not undergo significant deactivation even after 2 cycles. A very slight decrease in the rate constant values obtained in the photocatalysis experiments may be due to the adsorption of dye on the active sites of the catalysts which could not be removed during filtering.

Fresh 8.7 × 10<sup>−</sup><sup>3</sup> 7.0 × 10<sup>−</sup><sup>3</sup> 9.4 × 10<sup>−</sup><sup>3</sup> 8.3 × 10<sup>−</sup><sup>3</sup> 6.3 × 10<sup>−</sup><sup>3</sup> 5.4 × 10<sup>−</sup><sup>3</sup> I-recycle 8.15 × 10<sup>−</sup><sup>3</sup> 6.9 × 10<sup>−</sup><sup>3</sup> 9.1 × 10<sup>−</sup><sup>3</sup> 8.1 × 10<sup>−</sup><sup>3</sup> 5.9 × 10<sup>−</sup><sup>3</sup> 5.2 × 10<sup>−</sup><sup>3</sup> II-recycle 8.09 × 10<sup>−</sup><sup>3</sup> 6.2 × 10<sup>−</sup><sup>3</sup> 8.2 × 10<sup>−</sup><sup>3</sup> 7.9 × 10<sup>−</sup><sup>3</sup> 5.3 × 10<sup>−</sup><sup>3</sup> 4.9 × 10<sup>−</sup><sup>3</sup>

**Pseudo first order rate constant K (min<sup>−</sup><sup>1</sup>**

**Tartrazine Reactive Yellow-17 Reactive Black-5 UV Visible UV Visible UV Visible**

**)**

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 con-

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

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

*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* 

+h<sup>+</sup>

vb, e<sup>−</sup>

→ Cl2˙

M), 1.5 g of TiO2 (P-25

cb) (8)

<sup>−</sup> (10)

vb→Cl˙ (9)

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

*Effect of catalyst recyclability on the degradation of textile dyes.*

**No. of cycles**

**Table 3.**

*3.2.6 Effect of inhibitors*

centration (TAZ =1 × 10<sup>−</sup><sup>4</sup>

UV-visible irradiations are shown in **Figure 8**.

*chloride ion (equations 7 and 8).*

Cl<sup>−</sup>

Cl˙+Cl<sup>−</sup>

TiO2 →TiO2(h<sup>+</sup>

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


**Table 3.**

*Gold Nanoparticles - Reaching New Heights*

*3.2.4 Photocatalytic degradation studies*

*3.2.5 Recycling of TiO2*

the values are given in **Table 3**.

with 250 mL of dye solutions (TAZ =1 × 10<sup>−</sup><sup>4</sup>

which are considered as the predominant species at neutral or alkaline pH values are generated by oxidising more hydroxide ions [29]. At low pH values (pH < 5) the photodecolourisation of dyes is retarded under both UV and sunlight sources by the high concentration of the proton. Lack of availability of hydroxyl radicals in the pH range less than 5 leads to the decrease in decolourisation of dyes. In highly alkaline conditions (>pH 9) the % decolourisation of dyes decreased drastically due to the electrostatic columbic repulsion between the anionic dye surface (negatively charged) and the hydroxyl anions. Due to this repulsion, the dyes do not interact closely with the anions [30–33]. Thus it is deduced that the efficient condition for the maximum degradation of all the mentioned dyes is at neutral pH (pH = 7).

The extent of degradation of dyes was followed by total organic carbon analyser (TOC). The experimental results revealed that as the irradiation time increased, some dye molecules may degrade into components of lower molecular fragments and they mineralise. TOC analysis was carried out for all the dye samples collected

Since the decrease in the TOC content is the direct measure of degradation, TOC studies have been carried out to check whether the photocatalyst converts the harmful dye into harmless products. The studies revealed that, about 45% of TOC of dye samples was reduced under UV and 35% under visible irradiations. Similar type of observations was also made by [34, 35]. The degradation of dye involved the cleavage of ▬N〓N▬ and sequential evolution of N2 in the early stage of degradation [36]. The fate of nitrogen containing compounds in the photodegradation was also explained by [37].

In order to test the recyclability of TiO2 catalyst, the experiment was repeated

the used catalyst. The photocatalytic experiments were carried out with 1.5 g of TiO2 (P-25 Degussa) for 5 h. At the end of reaction, the dye samples were filtered and the catalyst was filtered off. There was not much difference between the weight of the recovered catalyst with its initial weight (<1%). Repeated experiments were carried out for another 5 h by taking 250 mL of fresh dye solution. The pseudo first order rate constant values were calculated for each cycle as mentioned before and

*Mineralisation of dyes using TiO2 (P-25 Degussa) under (A) UV and (B) irradiations (reaction conditions:* 

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

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

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

M) with

 *M, volume of dye solution = 250 mL,* 

at different intervals of time and the results are shown in the **Figure 7**.

**86**

**Figure 7.**

*Dye concentration: TAZ = 1 × 10<sup>−</sup><sup>4</sup>*

*irradiation time = 9 h and pH = 7).*

*Effect of catalyst recyclability on the degradation of textile dyes.*

The rate constant values were found to be almost the same for all the dyes under both UV and visible irradiations even after two cycles. Nevertheless at the end of II recycle, 91% decolourisation occurred in the illumination period of 5 h. This shows that the TiO2 (P-25 Degussa) is a very good catalyst which does not undergo significant deactivation even after 2 cycles. A very slight decrease in the rate constant values obtained in the photocatalysis experiments may be due to the adsorption of dye on the active sites of the catalysts which could not be removed during filtering.
