**3.1. Basic principle of photocatalysis**

where *Rankcost*, *<sup>i</sup>*

least expensive AOP.

402 Phenolic Compounds - Natural Sources, Importance and Applications

economically more efficient.

**Figure 1.** Cost comparison on AOPs for the degradation of 4C-2-NP.

**Figure 2.** Cost comparison on AOPs for the degradation of phenol.

is the cost rank of AOP, *i*. *Cost*max is the most expensive AOP, and *Cost*min is the

The different process costs are compared by few authors. Saritha et al. [30] compare UV, UV/TiO2, UV/H2O2, UV/Fenton, Fenton, and H2O2-based AOPs for the degradation of 4 chloro-2 nitrophenol (4C-2-NP). Based on the overall costs, we find that AOP carried out using H2O2 and Fenton are least expensive having ranks of ~5, while UV is the most expensive, assigned a rank of 0. **Figure 1** shows the cost ranking of the different AOPs for the degradation of 4C-2-NP. Esplugas et al. [31] compare UV, O3/H2O2, O3/UV, O3/UV/H2O2, UV/H2O2, and O3 processes for phenol degradation. Again, based on the overall costs, the different O3-based AOPs are least costly, while UV is the most expensive, as evident from **Figure 2**. We can infer from the cost comparison that incorporating a photocatalyst such as TiO2 with UV lowers the overall cost by one-third [30]. In the future, using sunlight in place of UV could make AOPs

The precise definition of heterogeneous photocatalysis is a tricky one; particularly as in many cases, the complete mechanism of the reactions is uncertain [27]. In photocatalytic reactions, liquid or gas phase reactants and/or products come into contact with the light-absorbing semiconductor photocatalyst [44]. The semiconductor material can be activated by photons with sufficient energy equal to or greater than the band gap energy (Eg) between the conduction band and valence band of the material [45]. A photocatalytic reaction initiates with the formation of electron-hole pairs followed by oxidation and reduction reactions [46]. In the presence of hole scavenger, the reduction reactions become predominant, whereas in the presence of electron scavenger, the oxidation reactions are the key reactions. However, there are some unwanted reactions such as recombination of electron-hole pairs which reduces the photocatalysis efficiency [47]. **Figure 3** provides a detailed mechanism of photocatalytic reactions.

**Figure 3.** Schematic diagram of photocatalytic reactions for the degradation of organic contaminants.
