**3.1. Adsorption**

*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,

*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

*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

*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>

such as current density, pH, anode material and electrolytes used have significant impact on

*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,

*Fenton and fenton‐like treatment*: An AOP with the capability to oxidize aromatic compounds

*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

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

O2

to produce iron (III) and hydroxyl radicals. Then, Iron (III) is

in acid medium. Some of the variants of the Fenton process are

) and ferrous ion at low pH.

and BDD (boron‐doped diamond) being the most investigated ones. Parameters

an RO unit in order to decrease the pressures associated with organic matter [14].

nanotechnology and purification processes.

348 Phenolic Compounds - Natural Sources, Importance and Applications

surfactants, coloring matters, pharmaceuticals) [10].

is the Fenton reagent, which consists of hydrogen peroxide (H2

as follows: Fenton‐like, photo‐Fenton and electro‐Fenton [15].

transforming phenolic solutions into simple end products.

O2

O2

of contaminated water.

SnO2

, IrO2

process efficiency.

The iron (II) reacts with H2

regenerated to Fe (II) by H2

Adsorption process is preferred over all methods because it is nondestructive and with this method is possible recover the organics pollutants through regeneration, relatively simple. Due to the high adsorption capacity of adsorbents, adsorption seems to be the best process, especially for the removal of moderate and low concentration phenolic compounds from an effluent [9].

The most common method for the removal of dissolved organic material is the adsorption with activated carbon, a product that is produced from a variety of carbonaceous materials, including wood, pulp mill char, peat, lignite, etc. Adsorption is the physical and/or chemical process in which a substance is accumulated at an interface between phases. The substance which is being removed from the liquid phase to the interface is called as adsorbate and the solid phase in the process is known as adsorbent. Physical adsorption (physisorption) is rela‐ tively nonspecific and is due to the operation of weak forces between molecules. Chemical adsorption (chemisorption) is also based on electrostatic forces, but much stronger forces act a major role on this process. In chemisorption, the attraction between adsorbent and adsorbate is due to a covalent bond or electrostatic forces among atoms [16].

### *3.1.1. Zeolites and clays*

,

Zeolites and clays are two adsorbent materials commonly used on the adsorption process. Different investigations have shown interesting results on the phenolic compound removal.

Khalid [17] has carried out a research on phenol removal using four kinds of zeolites as adsor‐ bents and adsorption properties were compared to those of an activated carbon. In this inves‐ tigation, phenol diluted in water was used as contaminant and adsorption was carried out in batch and continuous flow. Siliceous BEA zeolite was successfully used; the adsorption capacity was slightly higher at low phenol concentration (1.6 g/L) than the one of activated carbon. Siliceous BEA zeolite showed to be efficient as adsorbent able to be easily regenerated.

Investigation of removal of 3‐nitrophenol isomers (ortho, meta and para) was studied by Huong [18]. They used nano zeolite (NZ) as adsorbents. The adsorption of nitrophenols onto NZ reached equilibrium within 150 min at pH 6.0. The maximum adsorption capacities of NZ for meta‐, ortho‐ and para‐nitrophenols were 125.7, 143.8 and 156.7 mg/g, respectively. The removal percentages of nitrophenols were maintained at more than 70% of the initial values. The regeneration process showed that desorption efficiency of nitrophenols remained above 70% even after five adsorption‐desorption cycles.

Adsorption capacity of a modified zeolite was evaluated by Xie [19] for the removal of ion‐ izable phenolic compounds (phenol, p‐chlorophenol and bisphenol A) and nonionizable organic compounds (aniline, nitrobenzene and naphthalene). The isotherm data of ionizable compounds fitted well to the Langmuir model but those of non‐ionizable chemicals followed a linear equation. Adsorption capacity of ionizable compounds depended greatly on pH, increasing at alkaline pH conditions. On the other hand, adsorption of non‐ionizable com‐ pounds was practically the same at all pH levels studied.

Djebbar et al. [20] employed as adsorbent, a natural clay, for the removal of phenol from aqueous solutions. This clay was easily activated. Some parameters such as pH solution, tem‐ perature, contact time and initial phenol concentration were studied. The adsorption experi‐ ments were carried out employing 100 mg of adsorbent and 100 mL of phenol solution at different initial concentrations of phenol at 23°C. The results indicated that up to 60 and 70% of phenol was removed by activated and natural clay after of 5 h of contact time. The acti‐ vated process improved the adsorption of phenol onto natural clay. The adsorption capacity of phenol decreased when the temperature was increasing. The best results were obtained at pH 5. Adsorption equilibrium data were well fitted to both Freudlich and Langmuir isotherm indicating that the adsorption was favorable. The adsorption of phenol onto activated natural clay was exothermic.

Other investigation of phenol removal from water with clay of low cost was investigated by Nayak and Singh [21]. The influence of pH phenol solution, temperature and particles size was studied. Results indicated that the higher adsorption capacity of phenol was achieved when the particle size decreased from 140 to 50 μm, the pH decreased from 10 to 2 and the temperature increased from 30 to 50°C. The adsorption process was found to be spontaneous.

The removal of p‐chlorophenol (PCP) and p‐nitrophenol (PNP) from water with two types of organoclays prepared from different surfactants such as dodecyltrimethylammonium bromide (DDTMA) and didodecyldimethylammonium bromide (DDDMA) was investigated by Park [22]. In the experiments, 200 mg of adsorbents were dispersed into 30 mL of PNP and PCP solutions at initial concentration of 100 mg/L and pH 5–6. In the isotherm studies, the initial concentration of PNP and PCP was studied in the range of 5–250 g/L. The best adsorption results were found on the organoclays where DDDMA surfactant was used. The adsorption of PNP and PCP onto organoclays was more efficient than in unmodified clay which was attributed to hydrophobic behavior. The adsorption equilibrium data were well fitted to Freundlich isotherm, indicating the presence of multilayer sorption.

### *3.1.2. Membranes*

Membranes are considered a process to separate two streams, a barrier to facilitate the selec‐ tive mass transport between fluids; feed and permeate [23]. Before to select the optimal membrane to remove or recover a specific compound, it is important to know the macro and molecular separation level. Munirasu et al. [24], divided into two categories, inorganic; anion and cation and organic compounds; these later compounds are quite complex and due to its nature, they can be classified such as oil, grease, dissolved, disperse and emulsi‐ fied organic forms, solids and/or particles, such as clays, waxes, bacteria, sand or any solids based on chemical productions. Membranes separation efficiency depends on diverse factors including physic‐chemical composition; as type, weight, polarity and solute charge, operat‐

ing parameters; as feed flow rate, transmembrane pressure, temperature, permeate flux; also it is important to contemplate the membrane characteristics, for example, membrane mate‐ rial, porous size and configuration of membranes (modules). On the other hand, these aspects play an important key role on specific phenomena related to the concentration polarization and membrane fouling, contributing directly to the solute retention and hydrophobic interac‐ tions between the solute and membrane surface [25].

a linear equation. Adsorption capacity of ionizable compounds depended greatly on pH, increasing at alkaline pH conditions. On the other hand, adsorption of non‐ionizable com‐

Djebbar et al. [20] employed as adsorbent, a natural clay, for the removal of phenol from aqueous solutions. This clay was easily activated. Some parameters such as pH solution, tem‐ perature, contact time and initial phenol concentration were studied. The adsorption experi‐ ments were carried out employing 100 mg of adsorbent and 100 mL of phenol solution at different initial concentrations of phenol at 23°C. The results indicated that up to 60 and 70% of phenol was removed by activated and natural clay after of 5 h of contact time. The acti‐ vated process improved the adsorption of phenol onto natural clay. The adsorption capacity of phenol decreased when the temperature was increasing. The best results were obtained at pH 5. Adsorption equilibrium data were well fitted to both Freudlich and Langmuir isotherm indicating that the adsorption was favorable. The adsorption of phenol onto activated natural

Other investigation of phenol removal from water with clay of low cost was investigated by Nayak and Singh [21]. The influence of pH phenol solution, temperature and particles size was studied. Results indicated that the higher adsorption capacity of phenol was achieved when the particle size decreased from 140 to 50 μm, the pH decreased from 10 to 2 and the temperature increased from 30 to 50°C. The adsorption process was found to be

The removal of p‐chlorophenol (PCP) and p‐nitrophenol (PNP) from water with two types of organoclays prepared from different surfactants such as dodecyltrimethylammonium bromide (DDTMA) and didodecyldimethylammonium bromide (DDDMA) was investigated by Park [22]. In the experiments, 200 mg of adsorbents were dispersed into 30 mL of PNP and PCP solutions at initial concentration of 100 mg/L and pH 5–6. In the isotherm studies, the initial concentration of PNP and PCP was studied in the range of 5–250 g/L. The best adsorption results were found on the organoclays where DDDMA surfactant was used. The adsorption of PNP and PCP onto organoclays was more efficient than in unmodified clay which was attributed to hydrophobic behavior. The adsorption equilibrium data were well fitted to Freundlich isotherm, indicating

Membranes are considered a process to separate two streams, a barrier to facilitate the selec‐ tive mass transport between fluids; feed and permeate [23]. Before to select the optimal membrane to remove or recover a specific compound, it is important to know the macro and molecular separation level. Munirasu et al. [24], divided into two categories, inorganic; anion and cation and organic compounds; these later compounds are quite complex and due to its nature, they can be classified such as oil, grease, dissolved, disperse and emulsi‐ fied organic forms, solids and/or particles, such as clays, waxes, bacteria, sand or any solids based on chemical productions. Membranes separation efficiency depends on diverse factors including physic‐chemical composition; as type, weight, polarity and solute charge, operat‐

pounds was practically the same at all pH levels studied.

350 Phenolic Compounds - Natural Sources, Importance and Applications

clay was exothermic.

spontaneous.

*3.1.2. Membranes*

the presence of multilayer sorption.

The application of a driven force, measure as transmembrane pressure (TPM), divided the conventional membrane separation in microfiltration (MF), ultrafiltration (UF) and nanofil‐ tration (NF) processes (**Figure 2**), the membranes separate the feed solution into permeate and retentate. The permeate stream contains the solvent passing through the membrane, this stream is rich in solutes with a nominal weight cut‐off (NMWCO) below the porous size of membranes and the retentate stream are particles and dissolved compounds which are kept inside membrane. When a pressure force is applied, the membrane operation and the hydrodynamic resistance fluctuate, depending on the pore size; thus, the operating pressure increases while the pore size of the membrane decreases [24, 25]. Pressure driven force is a strategy to improve low weight molecules removal, for example, salt or organic compounds; however, despite the excellent rejection of salts, the process frequently present low rejection levels to organic molecules, which include aliphatic or aromatic chemical structures; with polar or nonpolar properties, as well as different kinds of alcoholic, amino, carboxyl, phe‐ nol, or hydroxyl functional groups, just to mention some examples [24, 26]. Bellona et al. [27] reported the factors affecting the permeation of solutes in high‐pressure membrane and

**Figure 2.** Scheme for microfiltration (MF), ultrafitltration (UF) and nanofiltration (NF) technologies. Adapted from Castro‐Muñoz et al. [25].

some important solute parameters were identified to those that affect mainly solute rejec‐ tion: molecular weight, molecular size (length and width), acid disassociation constant (pKa), hydrophobicity/hydrophilicity character and diffusion coefficient. Besides, membrane properties play an important role on the separation process: molecular weight cut‐off, pore size, surface charge (zeta potential), hydrophobicity/hydrophilicity (contact angle) and sur‐ face morphology (roughness). Furthermore, water composition, such as pH, ionic strength, hardness and the presence of organic matter, was also associated to influence on the solute rejection.

Membrane process has been used to remove organic pollutants, among these technologies the liquid, anion exchange, nanofiltration, reverse osmosis and pervaporation membranes are precedents of this technology [28]. Hybrid processes is a recent tendency to get better removal of these compounds. These procedures are based on combination of diverse techniques, such as adsorption pretreatment process accomplish with reverse osmosis [29] and pervaporation with reverse osmosis [30], forward osmosis (FO) and reverse osmosis (RO) [31], NF/RO membranes [32], polymerization of phenolic compounds and UF/MF removal membranes [33].

Therefore, there is still a necessity to find advanced techniques to remove nonbiodegradable, high concentration organic substances from wastewater, not only those come from refineries but all complex wastewater from industry. In this sense, researchers are trying to design a combination of treatment methods for a complete and successful removal of such pollutants, due to variability of wastewater composition, the traditional methods become inadequate and could not be used individually in full scale [34]. Phenol and phenol stability usually offers difficulties to remove them, some of the main advantages of applying advanced techniques are the interfacial area; lower solvent losses, downstream phase separation and easy scale up. In the case of membrane process, the higher interfacial mass transfer, overcome the lower mass transfer rate in this kind of systems [25, 35–37].

In recent years, also composite membranes have been investigated to remove phenol com‐ pounds. In these materials, surface properties are controlled during membrane synthesis. Membrane aromatic recovery system (MARS) is a promising technology to recover phenol and aromatic amines. Composite membranes including poly(dimethylsiloxane) (PDMS) are com‐ monly investigated to control porosity and the operational stability by synthetizing nonporous selective layer coated on a microporous support layer cast; reinforcement polymers could be poly(vinylidine fluoride), polyethersulfones, polyetherimides, polyacrylonitrile, polyester, polyphenylenesulphones. Xiao et al. [38] developed a pertraction membrane (pervaporation and extraction combine process), through plate composite polydimethylsiloxane/polyvinyli‐ dene fluoride polymers to recover phenol compound. Results show that mass transfer coeffi‐ cient is five times higher compared to silicon rubber membranes nonreinforced (from 15 × 10‐7 to 3.5 × 10‐7 m/s), another property improved was the mass flux (2.38 × 10‐2 kg/m<sup>2</sup>  h), however, diminish the activation energy of permeation (9.7 kJ/mol), permeability (5.9 × 10‐12 m<sup>2</sup> /s) and dif‐ fusion coefficient (2.4 × 10‐11 m<sup>2</sup> /s). Lee et al. [39] also found a higher permeate flux using a wet phase inversion process polydimethylsioloxane/polysulfone (PDMS/PS) composite. Permeate flux is influenced by controlling the skin layer thickness of the asymmetric membrane during formation reaction, phenol concentration and recirculation rate. Additionally, the relatively hydrophilic nature of phenol and specificity of membranes are related to chemical moieties, both point of views allow making highly flexible polymer, with hydrophobicity and organophilicity properties and controlled free volume; this is the case of the poly(dimethylsiloxane) (PDMS) and poly(vinylmethylsiloxane) (PVMS) to synthesize extractive membrane bioreactor (EMBR). Main characteristic to the EMBR system is to permeate organic compounds, meaning it should have a high organic flux while being effectively impermeable to inorganic and water, such as silicon‐based rubbers [40].

some important solute parameters were identified to those that affect mainly solute rejec‐ tion: molecular weight, molecular size (length and width), acid disassociation constant (pKa), hydrophobicity/hydrophilicity character and diffusion coefficient. Besides, membrane properties play an important role on the separation process: molecular weight cut‐off, pore size, surface charge (zeta potential), hydrophobicity/hydrophilicity (contact angle) and sur‐ face morphology (roughness). Furthermore, water composition, such as pH, ionic strength, hardness and the presence of organic matter, was also associated to influence on the solute

Membrane process has been used to remove organic pollutants, among these technologies the liquid, anion exchange, nanofiltration, reverse osmosis and pervaporation membranes are precedents of this technology [28]. Hybrid processes is a recent tendency to get better removal of these compounds. These procedures are based on combination of diverse techniques, such as adsorption pretreatment process accomplish with reverse osmosis [29] and pervaporation with reverse osmosis [30], forward osmosis (FO) and reverse osmosis (RO) [31], NF/RO membranes

Therefore, there is still a necessity to find advanced techniques to remove nonbiodegradable, high concentration organic substances from wastewater, not only those come from refineries but all complex wastewater from industry. In this sense, researchers are trying to design a combination of treatment methods for a complete and successful removal of such pollutants, due to variability of wastewater composition, the traditional methods become inadequate and could not be used individually in full scale [34]. Phenol and phenol stability usually offers difficulties to remove them, some of the main advantages of applying advanced techniques are the interfacial area; lower solvent losses, downstream phase separation and easy scale up. In the case of membrane process, the higher interfacial mass transfer, overcome the lower mass

In recent years, also composite membranes have been investigated to remove phenol com‐ pounds. In these materials, surface properties are controlled during membrane synthesis. Membrane aromatic recovery system (MARS) is a promising technology to recover phenol and aromatic amines. Composite membranes including poly(dimethylsiloxane) (PDMS) are com‐ monly investigated to control porosity and the operational stability by synthetizing nonporous selective layer coated on a microporous support layer cast; reinforcement polymers could be poly(vinylidine fluoride), polyethersulfones, polyetherimides, polyacrylonitrile, polyester, polyphenylenesulphones. Xiao et al. [38] developed a pertraction membrane (pervaporation and extraction combine process), through plate composite polydimethylsiloxane/polyvinyli‐ dene fluoride polymers to recover phenol compound. Results show that mass transfer coeffi‐ cient is five times higher compared to silicon rubber membranes nonreinforced (from 15 × 10‐7

to 3.5 × 10‐7 m/s), another property improved was the mass flux (2.38 × 10‐2 kg/m<sup>2</sup>

diminish the activation energy of permeation (9.7 kJ/mol), permeability (5.9 × 10‐12 m<sup>2</sup>

phase inversion process polydimethylsioloxane/polysulfone (PDMS/PS) composite. Permeate flux is influenced by controlling the skin layer thickness of the asymmetric membrane during formation reaction, phenol concentration and recirculation rate. Additionally, the relatively

/s). Lee et al. [39] also found a higher permeate flux using a wet

 h), however,

/s) and dif‐

[32], polymerization of phenolic compounds and UF/MF removal membranes [33].

transfer rate in this kind of systems [25, 35–37].

352 Phenolic Compounds - Natural Sources, Importance and Applications

fusion coefficient (2.4 × 10‐11 m<sup>2</sup>

rejection.

Due to complexity of phenol and phenolic compounds, researchers are focused on to develop new technologies to improve efficiency removal, mainly on the size/steric exclusion, elec‐ trostatic repulsion, fouling and energy consumption. Hybrid or combine process involving two or more steps, previously mention, could be the answer to the problem. Heo et al. [41] suggest that forward osmosis/reverse osmosis (FO/RO) provide the advantage to enhance the driven pressure force, diminishing the fouling property and making it less energy costly process. Due to, FO depends on the molar concentration of the solution instead of the nature of the solutes and RO offers higher selectivity characteristics, internal concentration polariza‐ tion and low flux; the efficiency of both membranes (FO/RO) was attributed to porous, mesh fabric, hydrophobicity and steric hindrance. Surface fouling is one of the issues to overcome on the wastewater treatment; thus, forward osmosis alleviated the reverse osmosis membrane fouling as demonstrated by Choi et al. [31], after repeated cleaning membrane process where the permeate flux was recovered. Biopolymer‐like substances were persistently accumulated on the membrane surface as seen in **Figure 3**.

Some authors have paid attention on the porosity as main property to develop integrated systems on phenol removal and/or recovery, taking into account the molecular weight (MW) of the species, such as the case of the combined nanofiltration and reverse osmosis [32, 42]. However, in case that a specific compound is desired to isolate, the complexity of the system could include more than two combine steps, for example, an innovative integrated process to recover an important food polyphenol, such as the Gallic acid. The proposal consisting of purification steps based on the MW of the specific molecule with UF‐NF‐RO and their fur‐ ther separation with an adsorption/desorption resins, where the final product had a phenol concentration of 378 g/L in Gallic acid equivalents and the initial quantity was 2.64 g/L [43].

The membranes contactor is an alternative technology, based on solvent extraction using hol‐ low fiber membranes (membrane‐based extraction method), to recover or remove low concen‐ tration of aromatic compounds. Two fluid phases flow in adjacent channels with the interface maintained in the intermediate membranes pores; in other words, the process involves extrac‐ tion of the compound to a second phase stabilizing the aqueous and organic phases within the pores of the polymeric membrane. If compared to the conventional solvent extraction, mem‐ brane contactors offer a large interfacial area, a reduction in solvent by‐products and lower sol‐ vent losses. Nevertheless, the operational pressure range limits the applications of the process. Research challenger is the interface stabilization of fluids in the membrane pores, which affect the operational conditions and the properties of both, membrane materials and fluids [44, 45]. Diverse factors to the design of this kind of membranes include modified surface properties. It can improve the gradual erosion caused by the shear forces generated by the aqueous phase

**Figure 3.** Concentration of DOMs (byopolymers, humic, buildings blocks, low‐MW neutrals and low‐MW acids) on the fouled membranes surface. Adapted from Choi et al. [31].

flowing over the liquid membrane. Alternatives, such as hollow fiber, supported liquid mem‐ brane (HFSLM), with solids‐extractants immobilized on the pores of the hydrophobic surface, serve to dissolve a nonvolatile carrier solvent to maintain a high distribution coefficient for the solute. Trioctylphosphine oxide (TOPO) act as extractants in the liquid membranes; due to in solid crystalline powder can be impregnated on a polypropylene membrane, changing from a conventional liquid‐liquid extraction into a solvent extraction liquid membrane of aro‐ matic compounds; to study model molecules [46] or applied as osmotic membranes bioreac‐ tor (TPPOMBR) [47]. A variant of liquid‐liquid phenol extraction and hollow fiber contactor membranes are the liquid membrane processes. Mainly, the process combines liquid‐liquid extraction and stripping operations in a single unit operation; pertraction. In a pertraction system, the liquid membrane phase (organic solution phase) separates into two additional liq‐ uid phases (feed and stripping phase) which are immiscible (aqueous solution phase). There exist different types of liquid membranes, bulk liquid membranes (BLM), supported liquid membranes (SLM) and emulsion liquid membranes (ELM). The bulk liquid membrane is con‐ sidering one of the simplest arrangements of liquid membrane systems. Mass transfer, solvent extractants, pH, temperature and rate migration are the foremost drawbacks to study [48–50].

### *3.1.3. Carbon‐based materials (activated carbon and nanomaterials)*

Due to structural and surface properties, carbon materials as activated carbon obtained of dif‐ ferent sources have been used as effective adsorbents of pollutants from water. However, in addition to the structural and surface properties, the size of the adsorbents is an important factor that can improve the efficiency of the process, therefore, carbon nanomaterials as carbon nanotubes and graphene have been used as adsorbents of pollutants from water obtaining good results. Thus, investigations related to carbon activated, carbon nanotubes and graphene‐based materials used as adsorbents of phenolic compounds from water are reviewed in this section.

Activate carbon (AC) is a carbon material very effective in the removal of pollutants from water. Its effectivity was probed on the removal of bisphenol A (BA). Liu et al. [51] inves‐ tigated the effect of modification treatments of two AC (W20 and F20) onto removal of BA. The ACs treated with nitric acid were labeled as W20A and F20A and the AC modified with thermal treatment under a flow of N<sup>2</sup> was labeled as W20N and F20N. The highest adsorp‐ tion capacities of BA were found on W20 (382.12 mg/g) and W20N (432.34 mg/g) samples. The thermal treatment favored more the BA adsorption on AC than the acid treatment. The surface charge density of the different ACs and their content of oxygenated groups are factors very important that affect the BA adsorption. Similarly than other experiments with phenolic compounds, the adsorption of BA onto ACs decreased when the temperature was increasing. The removal by adsorption of Bisphenol A also has been investigated with AC derivates of coconut and coal with good results [52]. The best results were found in the pH range from 3 to 9. The adsorption capacity of both ACs decreased in pH range (>10), it was attributed to the electrostatic repulsion between the negative charge surface of the ACs and the bispheno‐ late anion from the ionization of bisphenol A. Factors as surface area and pore volume were determinants for the good adsorption of BA onto ACs; however, surface polarity also played an important role in this adsorption process.

Fasfous et al. [53] studied the adsorption of tetrabromobisphenol (TBBPA) on multiwalled carbon nanotubes (MWCNTs). The results showed that MWCNTs have a high potential for removal of TBBPA from water. The removal of TBBPA after of 60 min was 90%. The adsorp‐ tion capacity was increasing when the initial TBBPA concentration and contact time were increased. Oppositely, the adsorption capacity of TBBPA decreased when the temperature and pH (pH > 7) were increased. The experimental kinetic data were well adjusted to the pseudo‐ second‐order model and the Freundlich and Langmuir models described well the experimen‐ tal equilibrium data. The adsorption of TBBPA on MWCNTs was spontaneous with exothermic nature. In other investigation for TBBPA removal, Zhang et al. [54] employed graphene oxide (GO) as adsorbent. The experiments were conducted modifying the pH of TBBPA solution in the range from 2 to 12 and the temperature from 288 to 318 K. The results indicated that at 0.3 and 1 mg/L TBBPA concentrations, the maximum adsorption capacities were of 70–90% after of 120 min, indicating an influence of the initial concentration of TBBPA. The adsorption capacity of TBBPA on GO decreased when the temperature of the solutions was increased. The adsorption process of TBBPA on GO was exothermic. These effect of the temperature and type of process were similar when were used MWCNTs. The main interaction mechanisms between TBBPA and GO were π‐ π interaction and hydrogen bonding. Ji et al. [55] also investigated the removal of TBBPA but using Fe<sup>3</sup> O4 nanoparticles loaded on MWCNTs (MWCNTs‐Fe<sup>3</sup> O4 ) com‐ posite and MWCNTs‐Fe<sup>3</sup> O4 modified with 3‐aminopropyltriethoxysilane (APTS) (MWCNTs/ Fe3 O4 ‐NH<sup>2</sup> ). The solution concentration of the TBBPA was 10 mg/L and the adsorbent dos‐ age was 0.5 g/L for all experiments. The solution pH was adjusted from 1.4 to 9 in order to

flowing over the liquid membrane. Alternatives, such as hollow fiber, supported liquid mem‐ brane (HFSLM), with solids‐extractants immobilized on the pores of the hydrophobic surface, serve to dissolve a nonvolatile carrier solvent to maintain a high distribution coefficient for the solute. Trioctylphosphine oxide (TOPO) act as extractants in the liquid membranes; due to in solid crystalline powder can be impregnated on a polypropylene membrane, changing from a conventional liquid‐liquid extraction into a solvent extraction liquid membrane of aro‐ matic compounds; to study model molecules [46] or applied as osmotic membranes bioreac‐ tor (TPPOMBR) [47]. A variant of liquid‐liquid phenol extraction and hollow fiber contactor membranes are the liquid membrane processes. Mainly, the process combines liquid‐liquid extraction and stripping operations in a single unit operation; pertraction. In a pertraction system, the liquid membrane phase (organic solution phase) separates into two additional liq‐ uid phases (feed and stripping phase) which are immiscible (aqueous solution phase). There exist different types of liquid membranes, bulk liquid membranes (BLM), supported liquid membranes (SLM) and emulsion liquid membranes (ELM). The bulk liquid membrane is con‐ sidering one of the simplest arrangements of liquid membrane systems. Mass transfer, solvent extractants, pH, temperature and rate migration are the foremost drawbacks to study [48–50].

**Figure 3.** Concentration of DOMs (byopolymers, humic, buildings blocks, low‐MW neutrals and low‐MW acids) on the

Due to structural and surface properties, carbon materials as activated carbon obtained of dif‐ ferent sources have been used as effective adsorbents of pollutants from water. However, in

*3.1.3. Carbon‐based materials (activated carbon and nanomaterials)*

fouled membranes surface. Adapted from Choi et al. [31].

354 Phenolic Compounds - Natural Sources, Importance and Applications

optimize the pH for the maximum adsorption capacity of TBBPA on the composites. The results showed that the adsorption capacity of TBBPA on the two composites was increasing when the pH increasing from 1.5 to about 5.5, where the TBBPA is not dissociated. The donor‐ acceptor interactions between TBBPA and the magnetic nanocomposites through the graphene sheets of MWCNTs, the aromatic structure of TBBPA and π‐π interactions between the ben‐ zene‐ring structure on both of TBBPA and MWCNTs and Hydrogen bonding are the main possible adsorption mechanism of TBBPA on the two nanocomposites. The functionalization of the MWCNTs/Fe<sup>3</sup> O4 with amine groups improved the adsorption of TBBPA. The maximum adsorption capacity of TBBPA was found on MWCNTs/Fe<sup>3</sup> O4 nanocomposite (33.72 mg/g). The experimental kinetic data were well adjusted to the pseudo‐second‐order model.

The removal of phenol, 2‐chlorophenol and 4‐chlorophenol from aqueous solutions using as adsorbents activated carbon, multiwalled carbon nanotubes and carbon‐encapsulated iron nanoparticles (CEINs) was investigated by Strachowski and Bystrzejewski [56]. The surface area of the different materials was found to be 1187, 156 and 36 m<sup>2</sup> /g for AC, MWCNTs and CEINs, respectively. All adsorption kinetic experiments were carried out with a 150 mg/L initial concentration of the phenolic compounds and the relation of adsorbent mass and solution volume was 0.5 g/L. The results showed that the maximum adsorption capacity of the phenolic compounds was obtained for AC followed of activated carbon nanotubes (act‐ MWCNTs), MWCNTs and CEINs in this order. However, the maximum adsorption kinetic rate of the studied adsorbates was found in MWCNTs followed of act‐MWCNTs = CEINs and AC, in this order. The highest adsorption capacity was found for 2‐chlorophenol (549.5 mg/g). MWCNTs showed a rapid adsorption kinetic and the equilibrium concen‐ tration was achieved around of 5 min. The adsorption kinetic data were well fitted to the pseudo‐second‐order model.

Different studies have investigated the removal of phenol from water employing different carbon materials. de la Luz‐Asunción et al. [13] realized the removal of phenol from aqueous solutions with carbon nanomaterials of 1D and 2D. The adsorbents used were MWCNTs and oxidized MWCNTs (O‐MWCNTs), pristine single‐walled carbon nanotubes (SWCNTs) and oxidized SWCNTs (O‐SWCNTs), GO and reduced graphene oxide (rGO). The oxidation of the carbon nanotubes was carried out in microwave using H2 O2 as oxidant agent. This method reduced the time of oxidation. GO and rGO were obtained by Hummer's method. The results showed that the pristine 1D nanomaterials (MWCNTs and SWCNTs) have a better adsorption capacity than rGO, however, GO presents a higher adsorption capacity than O‐MWCNTs and O‐SWCNTs. The best adsorption capacities of phenol were obtained by GO and O‐SWCNTs. Oxygenated functional groups play an important role in the removal of phenol with carbon nanomaterials of 1D and 2D. The kinetic adsorption data and the adsorption equilibrium data were well fitted to the pseudo‐second‐order model and the Freundlich model, respectively. **Figure 4** shows the fit of Freundlich isotherms on the adsorption of phenol onto carbon nanomaterials. The main mechanism of adsorption of phenol onto the different carbon structure was due to π‐ π interac‐ tions between the aromatic structure of the graphitic layers and aromatic rings of the phenol structure. Other mechanism proposed in the adsorption of phenol onto carbon nanomaterials was hydrogen bonding. Li et al. [57] also studied the removal of phenol from aqueous solutions onto rGO. The effect of phenol solutions pH on the adsorption capacity of rGO was analyzed

optimize the pH for the maximum adsorption capacity of TBBPA on the composites. The results showed that the adsorption capacity of TBBPA on the two composites was increasing when the pH increasing from 1.5 to about 5.5, where the TBBPA is not dissociated. The donor‐ acceptor interactions between TBBPA and the magnetic nanocomposites through the graphene sheets of MWCNTs, the aromatic structure of TBBPA and π‐π interactions between the ben‐ zene‐ring structure on both of TBBPA and MWCNTs and Hydrogen bonding are the main possible adsorption mechanism of TBBPA on the two nanocomposites. The functionalization

The removal of phenol, 2‐chlorophenol and 4‐chlorophenol from aqueous solutions using as adsorbents activated carbon, multiwalled carbon nanotubes and carbon‐encapsulated iron nanoparticles (CEINs) was investigated by Strachowski and Bystrzejewski [56]. The surface

CEINs, respectively. All adsorption kinetic experiments were carried out with a 150 mg/L initial concentration of the phenolic compounds and the relation of adsorbent mass and solution volume was 0.5 g/L. The results showed that the maximum adsorption capacity of the phenolic compounds was obtained for AC followed of activated carbon nanotubes (act‐ MWCNTs), MWCNTs and CEINs in this order. However, the maximum adsorption kinetic rate of the studied adsorbates was found in MWCNTs followed of act‐MWCNTs = CEINs and AC, in this order. The highest adsorption capacity was found for 2‐chlorophenol (549.5 mg/g). MWCNTs showed a rapid adsorption kinetic and the equilibrium concen‐ tration was achieved around of 5 min. The adsorption kinetic data were well fitted to the

Different studies have investigated the removal of phenol from water employing different carbon materials. de la Luz‐Asunción et al. [13] realized the removal of phenol from aqueous solutions with carbon nanomaterials of 1D and 2D. The adsorbents used were MWCNTs and oxidized MWCNTs (O‐MWCNTs), pristine single‐walled carbon nanotubes (SWCNTs) and oxidized SWCNTs (O‐SWCNTs), GO and reduced graphene oxide (rGO). The oxidation of the carbon

O2

time of oxidation. GO and rGO were obtained by Hummer's method. The results showed that the pristine 1D nanomaterials (MWCNTs and SWCNTs) have a better adsorption capacity than rGO, however, GO presents a higher adsorption capacity than O‐MWCNTs and O‐SWCNTs. The best adsorption capacities of phenol were obtained by GO and O‐SWCNTs. Oxygenated functional groups play an important role in the removal of phenol with carbon nanomaterials of 1D and 2D. The kinetic adsorption data and the adsorption equilibrium data were well fitted to the pseudo‐second‐order model and the Freundlich model, respectively. **Figure 4** shows the fit of Freundlich isotherms on the adsorption of phenol onto carbon nanomaterials. The main mechanism of adsorption of phenol onto the different carbon structure was due to π‐ π interac‐ tions between the aromatic structure of the graphitic layers and aromatic rings of the phenol structure. Other mechanism proposed in the adsorption of phenol onto carbon nanomaterials was hydrogen bonding. Li et al. [57] also studied the removal of phenol from aqueous solutions onto rGO. The effect of phenol solutions pH on the adsorption capacity of rGO was analyzed

with amine groups improved the adsorption of TBBPA. The maximum

O4

nanocomposite (33.72 mg/g). The

as oxidant agent. This method reduced the

/g for AC, MWCNTs and

of the MWCNTs/Fe<sup>3</sup>

pseudo‐second‐order model.

nanotubes was carried out in microwave using H2

O4

356 Phenolic Compounds - Natural Sources, Importance and Applications

adsorption capacity of TBBPA was found on MWCNTs/Fe<sup>3</sup>

area of the different materials was found to be 1187, 156 and 36 m<sup>2</sup>

experimental kinetic data were well adjusted to the pseudo‐second‐order model.

**Figure 4.** Freundlich isotherms of phenol adsorption onto carbon nanomaterials. Reprinted and adapted with permission from de la Luz‐Asunción et al. [13]. Copyright © 2015, Hindawi Publishing Corporation.

when the pH increased from 2.3 to 11.5. The highest rGO adsorption capacities were found when the pH of phenol solutions was adjusted between 4 and 6.6. In this pH range, the complexation capability of the oxygenated groups present on the surface of rGO increased. Besides, the gra‐ phitic structure of rGO and the aromatic ring of phenol interact through of π–π interactions. The phenol removal increased when the rGO concentration was increasing gradually in the range from 0.5 to 1.7 g/L. Thermodynamic study revealed that the adsorption process of phenol onto rGO was endothermic and spontaneous.

Others phenolic compounds have been studied through of the adsorption process with carbon materials. Mehrizad and Gharbani [58] used rGO as adsorbent for the removal of 4‐Chloro‐2‐ nitrophenol (4C2NP) from aqueous solutions. The adsorption capacity of rGO onto 4C2NP removal decreased with increasing dosage of adsorbent from 0.2 to 0.8 g/L but in all cases the adsorption velocity was rapid (10 min) reaching the equilibrium at about 60 min. The best results of adsorption were found in the pH range from 3 to 7, 298 K and initial concentration of 10 mg/L. The pseudo‐second‐order model described well the adsorption kinetic data and the equilibrium adsorption data were well fitted to the Freundlich model. The adsorption of 4C2NP was found to be spontaneous and exothermic process in the temperature range from 298 to 328 K.

The adsorption of other phenol chloride compound onto carbon materials was studied by Pei et al. [59]. For the adsorption of 2,4,6‐trichlorophenol (246TCP) from aqueous solutions were used rGO and GO as adsorbents. The adsorption of 246TCP onto rGO and GO was favored to pH range from 2.0 to 6.0. After of pH 6.0, the fraction of negatively charged 246TCP species increase in the solution causing electrostatic repulsion with negatively charged surfaces of rGO and GO. The adsorption capacity of rGO was higher than GO. The adsorption experimental data of 246TCP onto rGO and GO were well fitted to Freundlich equation.

Definitely, the application of carbon materials on the removal of phenolic compounds by adsorption process is a good alternative for the remediation of the contamination problem of water by this kind of compounds.
