*3.4.2. Effect of light intensity*

64 2 2 2 2 3 *C H OHNO O CO H O HNO* +® + + 76 2 (6)

Overall reaction stoichiometry shows complete mineralization of 4-NP with the involvement of HO• (Eq. (6)). Devi and Rajashekhar [9] described the possible degradation mechanism for phenol under natural sunlight/UV light using nitrogen-doped TiO2. Phenol mineralization went through the formation of dihydroxybenzene (catechol or resorcinol), pent 2-enedioic acid, and oxalic acid. In a parallel reaction path, benzoquinone and maleic acid were formed during

**3.4. Effect of different experimental parameters on degradation of phenol and phenolic**

Different parameters such as solution pH, light intensity, initial concentration of target compounds, photocatalyst concentration, and electron acceptors play a significant role on photocatalytic degradation of phenol and phenolic compounds. The following section will provide a review of recent studies on the degradation of phenol and phenol derivatives.

Solution pH plays a vital role in the photocatalytic degradation of phenol and phenolic compounds since it influences two surface properties of the photocatalyst: (i) band edge position and (ii) surface charge. TiO2 P25 shows a point zero charge at pH 6.8. Thus at pH < 6.8, TiO2 surface attains positive charge and can easily adsorb anionic species at the photocatalyst surface [54]. Again, the protonation and deprotonation of phenols greatly depend on solution

the mineralization (**Figure 4**).

406 Phenolic Compounds - Natural Sources, Importance and Applications

**Figure 4.** Phenol degradation mechanism (adapted from Ref. [9]).

**compounds**

*3.4.1. Effect of solution pH*

Photodegradation rates of different organic compounds improve with increasing light intensity. At high light intensity when mass transfer limitation is low, the reaction rate is found to be proportional to the square root of light intensity. However, at the low-intensity level, the photodegradation rate is directly proportional to the light intensity [14, 54]. Al-Sayyed et al. [57] observed a similar rate shift from first order to half order in intensity while they studied photocatalytic degradation rate of 4-CP in the light intensity range of 2–50 mW cm−2. Chen and Ray [14] correlated 4-CP degradation rate constant (k) with light intensity (I): k ∝ I0.84 indicating that the degradation was independent of mass transfer limitation.
