*2.3. Aromatic Water Contaminants*

Nowadays, the removal of low-concentration organics recalcitrant for minimizing environmental and human health risks is of growing concern [62,63]. Among different aromatic substrates, phenol is a representative pollutant due to its presence in petroleum refinery wastewater affecting the ecosystems. It is often used as a model chemical for investigating the degradation of more complex and harmful compounds. Therefore, we will consider this pollutant focusing on its photocatalytic mineralization. This section of the paper outlines the general physicochemical properties of the phenol and its usages, describing the mechanism of the photocatalytic reaction. It also discusses and evaluates various types of photocatalysts and their performance in the photomineralization of phenol.

## 2.3.1. Phenol: General Properties and Uses

Phenol is a well-known contaminant introduced continuously into the environment through its usage in industry and human activities (Figure 8) [64]. Phenol is an aromatic compound, colorless to light-pink, with a burning taste and a distinctive odor. It is moderately soluble in water, soluble in most organic solvents, and less soluble in aliphatic hydrocarbons.

**Figure 8.** Phenol concentration in the effluent of major industries. Reproduced with permission from ref. [64]. Copyright 2019 Elsevier.

Phenols are produced naturally during the decomposition of plants and microorganisms, being released slowly in soils compared to other soil organic matter [65]. The oxidation of cumene (Hock process) is by far the dominant technology for the synthesis of phenol, with a production rate of more than 6.7 × <sup>10</sup><sup>6</sup> tons per year [66]. Other industrial processes include the toluene oxidation with benzoic acid as an intermediate product and the recovery of phenol from coal tar. Phenol is commonly used in different branches of industry comprising plastics and related materials, production of polycarbonates, epoxies, Bakelite, detergents, nylon, pesticides, explosives, dyes, textiles production, and pharmaceuticals [67]. Because of its impact on human health, the United States Environmental Protection Agency (USEPA) enlisted the phenol as a pollutant of priority concern. It is highly irritating to the skin and genotoxic, while repeated exposure causes weight reduction and infertility; it has endocrine-disrupting effects and can damage the liver, kidneys, and nervous system [68]. The World Health Organization (WHO) recommended a maximum permissible concentration of phenol in drinking water of 2 mg L−<sup>1</sup> [69]. However, even at very low concentrations, contamination of drinking water supplies represents a human threat, directly affecting the health of ecosystems.
