**7. Conclusion**

This solvent was identified to possess higher hydrogen bonding, basicity and low viscosity

Fenton reaction is a reaction between iron (II) and hydrogen peroxide resulting in the forma‐ tion of hydroxyl radicals (OH**•**), a non‐selective and strong oxidising agent [91]. As a result, this process has been used to oxidise organic pollutants in aqueous solution to carbon dioxide and water. However, Fenton process has the disadvantage of the high cost of procuring the reactants (hydrogen peroxide and iron (II)) and sludge generation. An improved technique, electro‐Fenton process, which is capable of overcoming the above‐mentioned hindrances and permits improved control of hydroxyl radical generation [92], has therefore been devised. Electro‐Fenton process involves cathodic reduction of iron (III) in solution to iron (II) with a

duces the iron (II) at a faster rate and thus promotes the production of hydroxyl radical for

Evaluation of the electro‐Fenton process as an appropriate substitute technique for elimina‐ tion of phenol from a phenol simulated wastewater was conducted by Abdelaziz et al. [93]. They used a sacrificial iron anode as the source of iron (II), added hydrogen peroxide to the system externally and used nitrogen gas sparging to stir the batch reactor. They also examined the influence of some factors on the effectiveness of the electro‐Fenton process. Their results revealed 97% overall COD reduction of 50 mg/l pollutant concentration at optimum conditions

peroxide concentration of 1500 mg/l. They observed that COD removal percentage increased with increasing current density, hydrogen peroxide concentration and sodium chloride concen‐ trations but started decreasing, in all cases, beyond their respective optimum values. In addi‐ tion, consumption of energy and iron decreased as the initial pollutant (phenol) and sodium hydroxide concentrations were increased, but increased with an increase in the current density. In their study, where they investigated the effectiveness of the electron‐Fenton process for removal of COD from paper mill wastewater, Un et al. [94] concluded that electro‐Fenton process is effective for removal of COD from tissue paper waste water. They achieved a COD

gen peroxide concentration at pH 2. They noticed that though increasing the current density caused a corresponding increase in the COD removal, it also resulted in increasing energy consumption. Similarly, Rahmani et al. [95] have effectively applied the electro‐Fenton tech‐ nique to degrade phenol from aqueous solution. In this study, they applied a disposable iron anode as the source of ferrous iron and added the hydrogen peroxide manually. Highest degradation efficiency of 100% was obtained within 30 minutes at pH 3, 100 mg/l hydrogen

optimum conditions.

In an ion exchange process, an interchange of ions between two phases, usually a solid and a liquid phase, occurs. The ion change resin forms the solid phase while the sample under

of pH 3, the superficial gas velocity of 1.18 cm/s, current density (1.7 mA/cm<sup>2</sup>

removal efficiency of 80% within 60 min at 20 mA/cm<sup>2</sup>

peroxide concentration and 5 mA/cm<sup>2</sup>

**6.8. Adsorption and ion exchange**

= 0.77 V/SHE. This process, often referred to as electrochemical catalysis, pro‐

) and hydrogen

current density with 0.1 M hydro‐

compared to the other two solvents used.

434 Phenolic Compounds - Natural Sources, Importance and Applications

enhanced pollutant oxidation process [92].

**6.7. Electro‐Fenton method**

potential of *E*<sup>o</sup>

The rapid increase in industrial and domestic activities as a result of the desire to meet the demands of the ever‐increasing human population creates the possibility of phenolic com‐ pounds introduction into water bodies. Extensive research has been performed on these com‐ pounds resulting in the elucidation of their structure or classification, their sources of entry into the aquatic environment and their reactivity or interaction with other components of the aquatic environment. Research has also unveiled the significant toxic effects that these compounds exert on humans and wildlife upon exposure. Significant efforts have been made for the total elimination of phenolic compounds from water before use. This resulted in the development of water treatment technologies including the conventional methods such as activated carbon adsorption, solvent extraction and advanced technologies such as electro‐ Fenton method, membrane‐based separation method, photocatalysis and so on, which have all been successfully used for removal of phenolic compounds from water.
