**3.3. Photocatalysis reaction mechanism for the degradation of phenols**

Here we are discussing the photocatalysis reaction mechanism for TiO2 photocatalyst only. The first step in the photocatalytic degradation is the formation of electron-hole pairs within the TiO2 photocatalyst. Most of the electron-hole pairs are recombined producing heat energy. However, hydroxyl radicals (HO•) are formed in the presence of electron acceptor (dissolved O2) while hole (h+ ) oxidizes water or TiO2 surface active ─OH group. Dissolved O2 reacts with the electron (e− ) and generates superoxide ion (O2 −•). Finally, the HO• reacts with either phenol or phenolic compounds until complete mineralization. Photodegradation mechanism of 4 nitrophenol (4-NP) under UV light is presented as follows [14]:

$$TtO\_2 \xrightarrow{h\theta,\ \ \lambda \in 398\,\nm} TtO\_2(e\_{CB}^- + h\_{VB}^+) \tag{2}$$

$$\text{OH}\_{\text{VB}}^{+} + \text{H}\_{2}\text{O}\_{\text{ad}} \rightarrow \text{HO}\_{\text{ad}}^{\bullet} + \text{H}^{+} \tag{3}$$

$$\text{H}\_{\text{VB}}^{+} + \text{HO}\_{\text{ad}}^{-} \rightarrow \text{HO}\_{\text{ad}}^{\bullet} \tag{4}$$

$$
\epsilon\_{\text{CB}}^{-} + O\_{2} \rightarrow O\_{2}^{-\bullet} \tag{5}
$$

$$\rm C\_6H\_4OHNO\_2 + 7O\_2 \rightarrow 6CO\_2 + 2H\_2O + HNO\_3 \tag{6}$$

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 the mineralization (**Figure 4**).

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

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

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.

### *3.4.1. Effect of solution pH*

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 pH. Different phenolic compounds show different optimum pH during photodegradation. Venkatachalam et al. [55] studied the photocatalytic activity of Mg2+ and Ba2+-doped TiO2 nanoparticles for the degradation of 4-chlorophenol (4-CP). In the acidic pH (pH 5), 4-CP was well adsorbed on the photocatalyst surface and showed higher degradation rate than alkaline pH. Lathasree et al. [56] reported the photocatalytic degradation of phenol and chlorophenols with ZnO under UV light. Significant phenol degradation was achieved at neutral and mildly acidic pH. The zero point charge for ZnO was 8, and at alkaline pH, chlorophenols exist as negatively charged chlorophenolate anion. Thus the photodegradation rate was higher at acidic pH (pH < 8).
