**3.2 Photocatalytic studies on TiO2 (P-25 DEGUSSA)**

#### *3.2.1 Effect of initial concentration of dye*

The effect of initial concentration of dye on the rate of decolourisation was studied by taking 250 mL of dye solutions and varying the concentrations between 8 × l0<sup>−</sup><sup>5</sup> and 1.3 × l0<sup>−</sup><sup>4</sup> M for Tartrazine (TAZ), 8 × l0<sup>−</sup><sup>6</sup> and 1.6 × l0<sup>−</sup><sup>5</sup> M for RY-17 and 6 × l0<sup>−</sup><sup>6</sup> and 1.6 × l0<sup>−</sup><sup>5</sup> for RB-5 with constant catalyst weight (1.5 g). The irradiation was carried out for 6 h by using 125 W low pressure mercury arc lamp (wave length 254 nm) and 85 W tungsten lamp (wave length 365 nm) as UV and visible light sources, respectively. **Figure 4** shows that percentage decolourisation decreases as the initial concentration of the dye increases under both UV and visible light illumination.

Similar results in the photocatalytic degradation of phenol were reported in the literature [18, 19]. The decolourisation rate relates to the probability of formation of hydroxyl radicals (OH˙) on the catalyst surface and the probability of hydroxyl radicals reacting with dye molecules. Hence the rate constant depends on the probabilities of the formation of these two. Hence k' can be expressed as

$$\mathbf{k'} = \mathbf{k}\_0 \, \, \mathbf{x} \, \, \mathbf{P}\_{\text{OH}} \, \mathbf{x} \, \, \mathbf{P}\_{\text{dry molecules}} \tag{6}$$

where k' is the overall rate constant and k0 is the reaction rate constant. POH˙ and Pdye molecules refer to the probabilities of generation of OH˙ radicals and OH˙ radicals reacting with dye molecules. We all know that the reaction rate constant k0 is independent of initial dye concentration but POH˙ and Pdye molecules will be affected by the dye concentration. Literature suggests that photocatalytic degradation of aromatic compounds mainly occurs by hydroxyl radicals [20]. The rate determining step of the reaction may be the formation of OH˙ radicals that are formed through the reaction of holes with adsorbed OH<sup>−</sup> and H2O [21, 22].

**Figure 4.**

*Effect of initial concentration of (A)TAZ, (B) RY-17 and (C) RB-5 dyes on the % decolourisation under UV and visible irradiations. (reaction conditions: Weight of catalyst (P-25 Degussa) = 1.5 g, volume of dye solution = 250 mL, irradiation time = 6 h (UV): 6 h (visible) and pH = neutral).*

As the dye concentration increases, the available hole sites may be occupied by dye ions which are generated from the sodium salt of the dye molecules,

$$\left[\text{Dye - Na}\right] \rightarrow \left[\text{Dye}\right]^{-} + \text{Na}^{\*}\tag{7}$$

Since there are only a few active sites available for the generation of OH˙ radicals the generation of OH˙ will be reduced.

It is concluded that as the initial dye concentration increases, the catalyst surface needs to generate more amount of OH˙ radicals and other oxidising species. But, illumination time and required amount of the catalyst are constant, so that OH˙ radicals and other oxidising species formed on the TiO2 surface are also constant. So the relative number of OH˙ attacking the dye molecules decreases with increasing dye concentration [23]. Moreover, as the initial concentration of the dye increases, the path length of photons entering the solution decreases and at low dye concentration the reverse effect is observed [24, 25]. Also, at higher concentration, degradation decreases at sufficiently long distances from the light source or reaction zone due to retardation in the penetration of light. Hence, under both UV and visible light sources the rate of degradation decreases considerably with increase in dye concentration above the optimal concentration.

#### *3.2.2 Effect of catalyst weight*

A series of experiments were carried out to assess the optimum weight of the catalyst by varying the amount of TiO2 (P-25 Degussa) from 0 to 3 g for the decolourisation of 250 mL of dye solution (TAZ = 1 × 10<sup>−</sup><sup>4</sup> M, RY-17 and RB-5 = 1 × 10<sup>−</sup><sup>5</sup> M) at the irradiation time of 6 h at neutral pH. The effects of catalyst weight on the percentage decolourisation of all the dyes are shown in **Figure 5**.

In the absence of the catalyst no decolourisation occurred. From the figure it is also evident that the rate of photodecolourisation increased linearly with the weight of catalyst up to 1.5 g irrespective of light sources. On increasing the catalyst weight further, the percentage decolourisation decreased. This decrease in decolourisation on increasing the catalyst weight may be due to the formation of turbidity (Shadowing effect). Similar observation was also reported [26]. Optimum catalyst

**85**

**Figure 6.**

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

*irradiation time = 6 h).*

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

loading is always essential and the higher amount of TiO2 may not be useful both in view of aggregation as well as reduced irradiation field due to shadowing effect [27]. As the weight of the catalyst increased, the quantity of photons adsorbed increased and consequently the decolourisation rate increased [28], Hence, optimisation of the catalyst loading for a given dye concentration is an important parameter to avoid excess catalyst and to ensure total absorption of light either UV

*Effect of weight of catalyst on the decolourisation of dyes under (A)UV and (B) visible irradiations (reaction* 

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

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

 *M, volume of dye* 

The effect of pH on decolourisation of dyes irradiated under UV and visible

The percentage decolourisation of all the dyes was found to be maximum at the pH of 7 for UV and visible irradiations. On increasing the pH from acidic to neutral under UV and visible irradiations, the percentage decolourisation increased significantly. However, when the pH was increased beyond these values to the basic ranges the percentage decolourisation decreased drastically. The main reaction is presented by the hydroxyl radical attack on the dye anion. Hydroxyl radicals

*Effect of pH on decolourisation of dyes under (A) UV and (B) visible irradiations (reaction conditions: Dye* 

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

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

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

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

or visible light for efficient photodecolourisation.

*3.2.3 Effect of pH*

**Figure 5.**

irradiation are given in **Figure 6**.

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

*solution = 250 mL, irradiation time = 6 h and pH = 7).*

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

**Figure 5.**

*Gold Nanoparticles - Reaching New Heights*

As the dye concentration increases, the available hole sites may be occupied by

*Effect of initial concentration of (A)TAZ, (B) RY-17 and (C) RB-5 dyes on the % decolourisation under UV and visible irradiations. (reaction conditions: Weight of catalyst (P-25 Degussa) = 1.5 g, volume of dye* 

Since there are only a few active sites available for the generation of OH˙ radicals

It is concluded that as the initial dye concentration increases, the catalyst surface needs to generate more amount of OH˙ radicals and other oxidising species. But, illumination time and required amount of the catalyst are constant, so that OH˙ radicals and other oxidising species formed on the TiO2 surface are also constant. So the relative number of OH˙ attacking the dye molecules decreases with increasing dye concentration [23]. Moreover, as the initial concentration of the dye increases, the path length of photons entering the solution decreases and at low dye concentration the reverse effect is observed [24, 25]. Also, at higher concentration, degradation decreases at sufficiently long distances from the light source or reaction zone due to retardation in the penetration of light. Hence, under both UV and visible light sources the rate of degradation decreases considerably with increase in dye

A series of experiments were carried out to assess the optimum weight of the catalyst by varying the amount of TiO2 (P-25 Degussa) from 0 to 3 g for

lyst weight on the percentage decolourisation of all the dyes are shown in **Figure 5**. In the absence of the catalyst no decolourisation occurred. From the figure it is also evident that the rate of photodecolourisation increased linearly with the weight of catalyst up to 1.5 g irrespective of light sources. On increasing the catalyst weight further, the percentage decolourisation decreased. This decrease in decolourisation on increasing the catalyst weight may be due to the formation of turbidity (Shadowing effect). Similar observation was also reported [26]. Optimum catalyst

M) at the irradiation time of 6 h at neutral pH. The effects of cata-

the decolourisation of 250 mL of dye solution (TAZ = 1 × 10<sup>−</sup><sup>4</sup>

−

+ Na<sup>+</sup> (7)

M, RY-17 and

dye ions which are generated from the sodium salt of the dye molecules,

*solution = 250 mL, irradiation time = 6 h (UV): 6 h (visible) and pH = neutral).*

[Dye − Na] → [Dye]

concentration above the optimal concentration.

*3.2.2 Effect of catalyst weight*

RB-5 = 1 × 10<sup>−</sup><sup>5</sup>

**Figure 4.**

the generation of OH˙ will be reduced.

**84**

*Effect of weight of catalyst on the decolourisation of dyes under (A)UV and (B) 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, volume of dye solution = 250 mL, irradiation time = 6 h and pH = 7).*

loading is always essential and the higher amount of TiO2 may not be useful both in view of aggregation as well as reduced irradiation field due to shadowing effect [27].

As the weight of the catalyst increased, the quantity of photons adsorbed increased and consequently the decolourisation rate increased [28], Hence, optimisation of the catalyst loading for a given dye concentration is an important parameter to avoid excess catalyst and to ensure total absorption of light either UV or visible light for efficient photodecolourisation.

#### *3.2.3 Effect of pH*

The effect of pH on decolourisation of dyes irradiated under UV and visible irradiation are given in **Figure 6**.

The percentage decolourisation of all the dyes was found to be maximum at the pH of 7 for UV and visible irradiations. On increasing the pH from acidic to neutral under UV and visible irradiations, the percentage decolourisation increased significantly. However, when the pH was increased beyond these values to the basic ranges the percentage decolourisation decreased drastically. The main reaction is presented by the hydroxyl radical attack on the dye anion. Hydroxyl radicals

#### **Figure 6.**

*Effect of pH on decolourisation of dyes under (A) UV and (B) 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, volume of dye solution =250 mL and irradiation time = 6 h).*

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).

### *3.2.4 Photocatalytic degradation studies*

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

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].
