*4.2.3.3. Effect of the initial dye concentration*

blue [74, 82]. It is evident that in the presence of Ag, the photocatalytic performance of the

light is higher than that of unmodified one. The role of the Ag in the improvement of the dye photodegradation under UV light can explain similarly to the effect of Ag under visible light.

UV light. In the case of rhodamine-B degradation, the dye can be adsorbed by Ag particle

reduces UV light excitation [66]. This unexcitability of the photocatalyst leads to the low dye

Ag in photocatalysts and irradiation light. Also, the effectiveness of the dye photodegradation is affected by operating conditions such as photocatalyst dose, initial concentration, contact

The dye photodegradation increases with the increase in the photocatalyst dose. The effectiveness of the photodegradation reduces when the photocatalyst dose is further increased [66–68, 81]. The maximum photodegradation is obtained by using 1 g photocatalyst/100 mL [68, 74, 81]. In other work, the use of 0.6 g photocatalyst/L is also reported [67]. Such data can be explained based on the number of active sites available for photocatalytic reactions. More active sites of the photocatalyst are available when the dose of the photocatalyst increases. However, the use of a large number of photocatalysts may cause agglomeration of the material to produce big particle size. The large particle size gives small surface area, which decreases the number of active sites on the surface [36, 38]. Another reason for the decrease in the degradation can be attributed to the increase in the turbidity of suspension due to more suspended photocatalyst solids. The light scattering by the catalyst particles leads to the blockage of

photon absorption. Moreover, less OH radicals can be created [1, 2].

In the acidic pH, the effectiveness of the dye photocatalytic degradation over TiO<sup>2</sup>

is found to be low. The photodegradation improves as pH increasing, but when the pH is increased further the photodegradation declines. For methyl orange photodegradation, the optimum pH is reached at 3 [68]. The dye degradation over heterogeneous photocatalyst of

 is initiated by adsorption on photocatalyst surface, leading to sequentially or simultaneously dye degradation. The effectiveness of the adsorption and degradation of dye depends on the surface charge of the catalyst and solution pH. The pH is an effective parameter to








The photodegradation performance of TiO2

342 Titanium Dioxide - Material for a Sustainable Environment

TiO2

in TiO2

catalytic activity of TiO2

*4.2.3. Effect of the process conditions*

The dye photodegradation of dye by TiO2

*4.2.3.1. The effect of the photocatalyst dose*

photodegradation.

time and solution pH.

*4.2.3.2. Effect of initial pH*

TiO2

It is apparent that the dye photodegradation reduces gradually when dye concentration improves [66, 75]. At low dye concentration, a few dye molecules in solution can move freely into the active surface of the photocatalyst. When the abundant active sites of the photocatalyst are available to absorb the dye, the dye photodegradation becomes efficient. High dye concentration gives more dye molecules that hinder their movement close to the photocatalyst. Therefore, the adsorption and the photodegradation decrease. For the photocatalyst, the surface has been occupied by much dye that diminishes the active sites at the surface. It leads to less dye adsorption and declines photodegradation [66, 74, 75].

#### *4.2.3.4. Effect of the irradiation time*

The UV light irradiation time can represent: (1) how long the photocatalyst contact with irradiating light, for further formation of OH radicals and (2) how long the contact between dye with OH radicals to proceed photodegradation. A general trend shows that the extension of the irradiation time enhances photodegradation, but the photodegradation stays constant or even decreases slightly for extended irradiation. Long UV light exposure produces more OH radicals, which helps the photodegradation take place more efficiently. However, further addition of irradiation time leads to surface saturation of the photocatalyst to release OH radicals [66, 75].

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