**6.1. Photocatalytic degradation of phenolic compounds**

similar manner, the interaction between hydroxyl radical and phenol in water produces 2‐ nitrophenol [32]. Conversion of phenol to nitrophenol also occurs with the availability of nitric ions. Photolysis of phenol in the presence of charge transfer complexes results in the formation of hydroquinone, while the formation of chlorophenol occurs through chlorination of aromatic compounds in water [33]. Some phenolic compounds also coordinate with metal cations of water enhancing their ionisation with the subsequent increase in their solubility in water [34].

Most phenolic compounds can easily penetrate the skin through absorption and can read‐ ily be absorbed from the gastrointestinal tract of humans. Once in the system, they undergo metabolism and transform to various reactive intermediate forms particularly quinone moi‐ eties, which can easily form covalent bonds with proteins, resulting in their ability to exert

Chlorophenols, aminophenols, chlorocatechols, nitrophenols, methylphenols and other phe‐ nolic compounds have all been characterised as exerting toxic influence on humans [36]. Bisphenol A and some alkylphenols have been identified to exert endocrine disrupting effects on humans by altering the development of mammary glands in exposed animals [37]. Similar work also discloses the tendency of bisphenol A to delay the onset of puberty in girls [38]. Consumption of liquids, including drinking water, containing a high concentration of phenol results in problems with the gastrointestinal tract and muscle tremor with difficulty in walk‐ ing. Application of products containing a high concentration of phenol to the skin causes blisters and burns on the skin; heart, kidneys, and liver damage may occur with exposure to high levels of phenol [39]. Because of their tendency to readily oxidise to quinone radicals, which tend to be more reactive, catechols have the tendency to cause DNA damage or ary‐ lation, destroy some proteins in the body and disrupt transportation of electrons in energy transducing membranes [35]. Caffeic and dihydrocaffeic acids, in the presence of copper, also cause damage to DNA [40]. Chlorophenol poisoning causes mouth burning, throat burning and necrotic lesions in the mouth, stomach and oesophagus. It also induces abnormal tem‐ perature and pulse fluctuation, weak muscles and convulsions [41]. Other effects of chloro‐ phenol poisoning include damage to the liver, kidneys, lungs, skin and the digestive tract [42]. Hydroquinone also damages chromosomes. Para‐cresol and 2,4‐dimethyl phenol have been

classified as a chemical with the potential of inducing carcinogenic effects [43].

**6. Techniques for the removal of phenolic compounds from water**

Recovery of phenolic compounds from the aquatic environment is a mandatory requirement in order to safeguard the life of humans and aquatic organisms through possible contamina‐ tion of these toxic chemicals. Deployment of appropriate technologies for effective removal of these compounds will not only eliminate problems of possible harm associated with pol‐ lutants, as well as waste disposal problems, but also allow the attainment of value‐added

**5. Toxic effects of phenolic compounds on humans**

426 Phenolic Compounds - Natural Sources, Importance and Applications

toxic effects on humans [35].

Photocatalytic degradation is the use of metal oxide catalysts to degrade pollutants where the catalyst is usually activated by absorption of a photon of appropriate energy and is capable of speeding up the reaction without being used up [44]. Photocatalytic properties of metal oxide catalysts are due to the fact that excitation of electrons from the valence to the conduc‐ tion band of the catalyst occurs upon its irradiation with a light of appropriate wavelength. Promotion of the electrons (e‐ ) creates positive charges or holes (h<sup>+</sup> ) on the valence band, and accumulation of electrons on the conduction band of the catalyst. Generation of these charge carriers (e‐ and h+ ) initiates the photocatalytic degradation process. The valence band holes attack and the oxidised surface absorbs water molecules to form hydroxyl radicals (OH•). Conduction band electrons reduce oxygen molecules and produce oxygen radicals or super‐ oxide radicals (O<sup>2</sup> •). These highly reactive radicals then attack and convert the pollutants to harmless products such as carbon dioxide and water [45, 46].

Photocatalytic degradation is regarded as an efficient technique for the elimination of pollut‐ ants from polluted water as a result of its ability to completely degrade the pollutant instead of their transformation into other products. The degree of effectiveness of the degradation process is known to rely heavily on the catalyst dose, exposure time, solution pH and light intensity [47].

There have been a number of reports where photocatalytic degradation techniques have been utilised effectively to degrade phenol and its derivative from the water. Natural clinoptilolite zeolite and FeO‐based nanoparticles were used by Mirian and Nezamzadeh‐Ejhieh [48] in photocatalytic degradation of phenol in polluted water under simulated solar light irradiation. The results confirmed that using zeolite as a support for FeO enhanced its photocatalytic deg‐ radation efficiency. The improved photocatalytic activity of the FeO‐zeolite composite was attributed to the fact that the zeolite prevented agglomeration of the FeO nanoparticles and minimised the charge carrier recombination rate. In their study, Shahrezaei et al. [49] explored the photocatalytic degradation ability of TiO2 in the degradation of phenolic compounds present in wastewater from a refinery. Highest degradation efficiency of the phenol and its derivatives was identified at an optimum temperature of 318 K, pH 3 and 100 mg/l catalyst concentration. A 90% degradation efficiency of phenol was achieved within 2 hours at these optimum conditions. Photocatalytic degradation of phenolic compounds from wastewater has also been demonstrated by many researchers using various catalysts including TiO2 /reduced graphene [50], ZnO [51], Fe2 O3 decorated on carbon nanotubes [52] and CuO [53].

### **6.2. Ozonation**

Ozone (O<sup>3</sup> ) is formed naturally when ultraviolet (UV) rays from the sun enter the earth's atmosphere. It is also formed whenever lightning strikes during a thunderstorm. Under these conditions, oxygen molecules (O<sup>2</sup> ) split to form highly reactive oxygen radicals (O•), which in turn react with O2 to form ozone. Ozone has a very high oxidising potential (‐2.74 V) which is much higher than that of hypochlorite ion (‐1.49 V) and chlorine (‐1.36 V) [54] which are all employed as oxidants for pollutant removal from water. This high oxidation potential forms the basis of the use of ozone as an oxidant for removal of organic pollutants from water.

Ozonation process begins with the formation of ozone through corona discharge simulation of lightning, or the use of UV‐type ozone generator for simulation of ultraviolet radiation from the sun, by passing clean and dry air through high voltage ozone generators. The waste‐ water is then allowed to flow along a venture throat, which generates a vacuum and pulls the ozone into the wastewater, or the ozone is simply bubbled up through the wastewater. The ozone then oxidises and decomposes the pollutants leading to their elimination from the water. UV ozonation is mostly used for small‐volume wastewater treatment while the corona discharge method is employed in large‐scale wastewater treatment processes. Some advan‐ tages of ozonation include [55]:


Based on the above advantages, several research works have been performed on the use of ozo‐ nation technique for phenolic compounds removal from wastewater. Treatment of olive mill wastewaters containing garlic, p‐hydroxybenzoic and p‐coumaric acids based on ozonation was studied by Chedeville et al. [56]. They identified that the highest ozonation process was attained when the gas‐liquid contractor was adopted to HaN<sup>3</sup> regime. The gas/liquid contrac‐ tor used permitted a comprehensive removal of the phenolic compounds within a short time. A maximum of 80% of the pollutants was eliminated with up to 95% ozone mass transfer. Ozonation was also used to treat ethylene glycol containing wastewaters with emphasis on the impacts of pollutant dose, process time, and pH on the decontamination efficacy. After 180 min, ethylene glycol removal efficiencies were 93.31, 89.96 and 85.01% at 10, 20 and 50 mg/l pollutant concentrations, respectively. Removal efficiency was observed to be highest in alkaline medium [57].
