**1. Introduction**

Phenol and phenolic compounds (chlorophenols, nitrophenols, etc.) detected in water and wastewater are toxic in nature and treated as primary water pollutants as per different countries' regulations. Phenol is one of the first compounds included in the US EPA list of priority pollutants [1]. Chlorophenols and nitrophenols are even more toxic than phenol itself. The exposure, health effects, and regulatory limits of phenols are mentioned in **Table 1** [2]. High concentration of phenolic compounds is present in the effluents from different industries, such as textiles, plastics, paint, paper, petroleum refining, coal processing, wood products, pharmaceuticals, and steel manufacturing [3]. Phenols can be removed by conventional techniques such as (i) physicochemical processes and (ii) biological processes. A comparative study of different phenol degradation methods is presented in the following section. Because of several limitations of the conventional phenol degradation processes, researchers are now relying on advanced oxidation processes (AOPs) for the complete mineralization of phenols. AOPs provide much faster degradation rate with the participation of hydroxyl radical (HO•), and phenols are mineralized to CO2 and water instead of transferring the pollutants from one phase to another [4]. Heterogeneous photocatalysis process became very popular among the AOPs, and it requires mainly three components such as (i) semiconductor photocatalyst, (ii) light energy (UV or visible or solar), and (iii) electron donor or hole acceptor. When semiconductor photocatalyst is illuminated with light energy greater than the band gap of the photocatalyst, charge carriers (i.e., electron-hole pair) are produced which ultimately generate hydroxyl radicals (HO•) in the system. Recently, photocatalytic degradation of phenol and phenolic compounds in wastewater has been extensively studied by several research groups [4–13]. Titanium dioxide (TiO2) photocatalyst is frequently used in the degradation of phenols under ultraviolet light [14–16]. TiO2 is nontoxic, photostable, insoluble under most conditions, and inexpensive and has exceptional chemical and biological inertness [17]. There are few other photocatalysts such as ZnO, CuO, and *β*-Ga2O3 which are also used for phenol degradation under UV light. TiO2 shows highest efficiency among the photocatalysts [6]. However, the use of UV light is neither feasible nor economical for the degradation of a larger volume of industrial effluent containing phenols. Again, sunlight contains only a small fraction of UV light (4% of solar spectrum) in comparison to visible light (46% of solar spectrum) [18]. For this reason, visible-light-active photocatalyst development is necessary to utilize sunlight in photocatalytic degradation of phenols. There are two approaches to achieve the visible-light-active photocatalysts: (i) modifying existing photocatalysts (via techniques such as doping, composite semiconductor, and dye sensitization) and (ii) developing novel undoped single-phase mixed oxide photocatalysts [19]. Phenol degradation is achieved successfully under visible light with doped-TiO2 photocatalysts, where different dopants such as iodine [20], nitrogen [21], sulfur [12], praseodymium [22], and iron [23] are used to expand their photoresponses into the visible spectrum. In the case of a composite semiconductor, a large band gap semiconductor is coupled with a small band gap semiconductor with a more negative conduction band level. Therefore, the conduction band electrons can be injected from the small band gap semiconductor to the large band gap semiconductor providing a better charge carrier separation [24]. There are few composite photocatalysts such as Co3O4/ BiVO4 [19], TiO2/multiwalled carbon nanotubes [13], and coke-containing TiO2 [25] that are

reported for the degradation of phenols under visible light. In dye-sensitization process, electron injection occurs from the excited dye into the conduction band of the semiconductor photocatalyst, followed by interfacial electron transfer [7]. Dyes are naturally visible light active, and upon light illumination, they get excited. Vinu et al. [11] used eosin Y and fluorescein as sensitizers of TiO2 to degrade 4-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol under visible light. Chowdhury et al. [7] used eosin Y-sensitized Pt-loaded TiO2 for phenol degradation. Qin et al. [8] applied N719 dye-sensitized TiO2 for the degradation of 4-chlorophenol. Degradation of 4-nitrophenol is studied with Cu(II)-porphyrin and Cu(II)-phthalocyanine sensitized TiO2 under visible light [26].

**1. Introduction**

396 Phenolic Compounds - Natural Sources, Importance and Applications

Phenol and phenolic compounds (chlorophenols, nitrophenols, etc.) detected in water and wastewater are toxic in nature and treated as primary water pollutants as per different countries' regulations. Phenol is one of the first compounds included in the US EPA list of priority pollutants [1]. Chlorophenols and nitrophenols are even more toxic than phenol itself. The exposure, health effects, and regulatory limits of phenols are mentioned in **Table 1** [2]. High concentration of phenolic compounds is present in the effluents from different industries, such as textiles, plastics, paint, paper, petroleum refining, coal processing, wood products, pharmaceuticals, and steel manufacturing [3]. Phenols can be removed by conventional techniques such as (i) physicochemical processes and (ii) biological processes. A comparative study of different phenol degradation methods is presented in the following section. Because of several limitations of the conventional phenol degradation processes, researchers are now relying on advanced oxidation processes (AOPs) for the complete mineralization of phenols. AOPs provide much faster degradation rate with the participation of hydroxyl radical (HO•), and phenols are mineralized to CO2 and water instead of transferring the pollutants from one phase to another [4]. Heterogeneous photocatalysis process became very popular among the AOPs, and it requires mainly three components such as (i) semiconductor photocatalyst, (ii) light energy (UV or visible or solar), and (iii) electron donor or hole acceptor. When semiconductor photocatalyst is illuminated with light energy greater than the band gap of the photocatalyst, charge carriers (i.e., electron-hole pair) are produced which ultimately generate hydroxyl radicals (HO•) in the system. Recently, photocatalytic degradation of phenol and phenolic compounds in wastewater has been extensively studied by several research groups [4–13]. Titanium dioxide (TiO2) photocatalyst is frequently used in the degradation of phenols under ultraviolet light [14–16]. TiO2 is nontoxic, photostable, insoluble under most conditions, and inexpensive and has exceptional chemical and biological inertness [17]. There are few other photocatalysts such as ZnO, CuO, and *β*-Ga2O3 which are also used for phenol degradation under UV light. TiO2 shows highest efficiency among the photocatalysts [6]. However, the use of UV light is neither feasible nor economical for the degradation of a larger volume of industrial effluent containing phenols. Again, sunlight contains only a small fraction of UV light (4% of solar spectrum) in comparison to visible light (46% of solar spectrum) [18]. For this reason, visible-light-active photocatalyst development is necessary to utilize sunlight in photocatalytic degradation of phenols. There are two approaches to achieve the visible-light-active photocatalysts: (i) modifying existing photocatalysts (via techniques such as doping, composite semiconductor, and dye sensitization) and (ii) developing novel undoped single-phase mixed oxide photocatalysts [19]. Phenol degradation is achieved successfully under visible light with doped-TiO2 photocatalysts, where different dopants such as iodine [20], nitrogen [21], sulfur [12], praseodymium [22], and iron [23] are used to expand their photoresponses into the visible spectrum. In the case of a composite semiconductor, a large band gap semiconductor is coupled with a small band gap semiconductor with a more negative conduction band level. Therefore, the conduction band electrons can be injected from the small band gap semiconductor to the large band gap semiconductor providing a better charge carrier separation [24]. There are few composite photocatalysts such as Co3O4/ BiVO4 [19], TiO2/multiwalled carbon nanotubes [13], and coke-containing TiO2 [25] that are


**Table 1.** Exposure and regulatory limits of phenol and phenolic compounds [1, 2].

In the first part of this chapter, conventional treatment methods for the degradation of phenol and phenolic compounds are presented, followed by the application of AOPs for such treatment. The process economics and efficiencies of different AOPs are also discussed for the degradation of phenolic compounds.

In the second part of the chapter, we focus on the photocatalytic degradation processes concerning different areas such as (i) basic principle of photocatalysis, (ii) experimental details of photocatalytic degradation of phenols, (iii) photocatalysis reaction mechanism for the degradation of phenols, and (iv) effect of different experimental parameters on degradation of phenol and phenolic compounds.

Finally, we demonstrate a dye-sensitized method to improve the photocatalytic activity and visible-light response of TiO2-based photocatalyst to perform visible-light-driven phenol degradation.
