**3. Removal methods of phenolic compounds from water**

Pollution of environment is one of the main problems facing humans today. Recently, the prob‐ lem of environmental pollution has increased exponentially and reached worrying level in terms of its impact on the life of human beings. Among the contaminants that have harmful effects in animals and humans are considered the toxic organic compounds. As mentioned earlier, dis‐ solved phenolic compounds that are present in industrial wastewater cause pollution of ground‐ water and owing to its harmful effect these compounds generate a serious problem in this type of water resources. Exposure to this type of chemical reagents, once they enter into human body can cause damage to the nervous and respiratory systems, kidney and blood system. Phenolic compounds have been classified as the top 45th in the list of priority hazardous substances by the Agency for Toxic substances and Disease Registry, USA, which require immediate treatment before disposal in the environment [4]. Consequently, removing organic compounds or reduc‐ ing their concentrations to the permitted levels by environmental standards represents a big

The approach of this section is to present a summary of physicochemical properties of

Phenols are in some respects as alcohols, due to the presence of hydroxyl groups in their structures. They have the ability to form strong hydrogen bonds. Moreover, these compounds present higher boiling points than hydrocarbons of the same molecular weight. Phenols are also slightly soluble in water because of their ability to form strong hydrogen bonds with water molecules [5]. Phenols are stronger acids than alcohols. They react with bases like sodium hydroxide to form phenoxide ions. However, they are weaker acids than carboxylic

Phenolic compounds behave as nucleophiles in most of their reactions and also the reagents that interact on them behave as electrophiles. In phenolic compounds, the site of nucleophilic reactivity may occur at the hydroxyl group or the aromatic ring. The reactions are carried out

*Halogenation*: Bromination and chlorination of phenols occur easily even in the absence of a catalyst. Substitution occurs primarily at the para position to the hydroxyl group. When the

*Nitration*: Phenol reacts with dilute nitric acid in either water or acetic acid. It is not necessary to use mixtures of nitric and sulfuric acids, due to the high reactivity of phenolic compounds. The o‐nitrophenol is a phenolic compound ortho‐substituted and therefore, this compound has considerably lower boiling point than the meta and para isomers. This is due to the hydrogen bond that is produced between the hydroxyl group and the substituent partially

compensates for the energy required to go from the liquid to the vapor phase.

challenge.

phenolic compounds.

*2.2.2. Reactions of phenols*

**2.2. Chemistry of phenolic compounds**

346 Phenolic Compounds - Natural Sources, Importance and Applications

acids and do not react with sodium hydrogen carbonate [6].

para position is blocked, ortho‐substitution is carried out.

on the aromatic ring results in electrophilic aromatic substitution [7].

*2.2.1. Physical properties of phenols*

Phenolic compounds are priority contaminants with high toxicity even at low concentrations. These compounds are present in industrial effluents, where increase biochemical and chemical oxygen demands resulting in detrimental effects on the environment. Some of them are highly toxic as well as carcinogenic and can remain in the environment for a long time due to their stability and bioaccumulation. Owing to the high toxicity of phenolic compounds, treatment of the organic wastewater has an important effect on the lives of human beings [9].

Many phenolic compounds can be removed efficiently by conventional treatments such as extraction, distillation, chemical oxidation, electrochemical oxidation and adsorption among others. On the other hand, some advanced treatments use less chemical reagents compared to the conventional processes, but they have the disadvantage of having high energy costs. Within the advanced treatments are as follows: Fenton, ozonation, wet air oxidation and photochemical method. Biological treatments have certain advantages compared to physico‐ chemical treatments; among these advantages may be mentioned: environmentally friendly and energy saving. However, it has the disadvantage that cannot treat high concentration of contaminants. One of the best ways to treat phenolic compounds under mild conditions is the enzymatic treatment, which uses different enzymes such as peroxidases, laccases and tyrosinases [10]. Thus, there is a need to treat waste contaminated with phenolic compounds at low and high concentrations before discharge. Some methods used today are described below:

*Adsorption*: Adsorption method for removal of phenols from water is effective from low con‐ centrations to high concentrations, depending on the economics and recycling the required sec‐ ondary material, adsorbent. Activated carbon (AC) is the most used in industry as adsorbent. It is expensive but has been shown to be effective for removal of trace organic compounds. Therefore, new options are being developed including impregnation with nanoparticles, differ‐ ent sources of carbon, different activation methods, carbon nanotubes (CNTs), graphene‐based materials, as well as substitution with low cost biosorbents, such as chitin/chitosan which are promising alternatives to remove phenolic compounds [11–13].

*Membrane processes*: Membrane processes are applied in water and wastewater treatment to remove organic contaminants. At present, this technology has been investigated for the phe‐ nolic compounds removal. Low energy consumption, low operating cost and easy scale up by membrane modules are the main advantages of these technologies. Today, separation mem‐ branes have many uses with a growing potential for industrial applications in biotechnology, nanotechnology and purification processes.

*Reverse osmosis and nanofiltration*: Reverse osmosis (RO) is a membrane‐based demineraliza‐ tion technique that is used to separate dissolved solids, especially ions, mainly from aqueous solutions. On the other hand, nanofiltration (NF) is widely used for removing organic pollut‐ ants, inorganic salts, color and hardness from aqueous solutions. NF is useful to use prior to an RO unit in order to decrease the pressures associated with organic matter [14].

*Chemical oxidation*: Chemical oxidants provide destructive methods of phenolic compounds. The processes have low consumption of reagents and energy costs, operating under mild con‐ ditions (temperature and pH). Ozone, chlorine, chlorine dioxide, chloramines, ferrate [Fe (VI)] and permanganate [Mn (VII)] are the most common chemicals applied in oxidative treatment of contaminated water.

*Electrochemical oxidation*: This technique can effectively oxidize many organic contaminants at high chloride concentration, usually larger than 3 g/L. Electrochemical oxidation is an alter‐ native destructive of phenols which does not require addition of reagents. This technique is divided into direct and indirect oxidation. Direct or anodic treatment occurs through adsorp‐ tion of the contaminants on the anode surface. Various anode materials are used with Pt, PbO<sup>2</sup> , SnO2 , IrO2 and BDD (boron‐doped diamond) being the most investigated ones. Parameters such as current density, pH, anode material and electrolytes used have significant impact on process efficiency.

*Advanced oxidation processes*: Advanced oxidation processes (AOP) are techniques that pres‐ ent the common feature that they form hydroxyl radical (OH•) in situ and this free radical is capable of mineralizing most organics, including phenolics compounds. AOP are used mainly for the treatment of contaminated waters that contain recalcitrant organics (e.g., pesticides, surfactants, coloring matters, pharmaceuticals) [10].

*Fenton and fenton‐like treatment*: An AOP with the capability to oxidize aromatic compounds is the Fenton reagent, which consists of hydrogen peroxide (H2 O2 ) and ferrous ion at low pH. The iron (II) reacts with H2 O2 to produce iron (III) and hydroxyl radicals. Then, Iron (III) is regenerated to Fe (II) by H2 O2 in acid medium. Some of the variants of the Fenton process are as follows: Fenton‐like, photo‐Fenton and electro‐Fenton [15].

*Biological treatment*: Biological treatment is the most commonly applied treatment for aque‐ ous phenols. The treatment is an inexpensive method, simple design and maintenance, for transforming phenolic solutions into simple end products.

Wide research is carried out daily on phenolic compounds removal from water, from conven‐ tional methods to new technologies. Optimization and modification of conventional processes provide attractive alternatives on contaminants removal. Some other methods used in the removal of phenol are as follows: wet air oxidation (WAO), catalytic wet air oxidation (CWAO), solvent extraction, extractive membrane bioreactors (EMBR), photocatalytic membrane reactors (PMR), UV/H<sup>2</sup> O2 treatment with microwave, etc.
