Removal of phenol from aqueous medium using micellar solubilization followed by ionic ﬂ occulation

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Introduction
The contamination of water bodies caused by organic compounds, such as the phenolic ones, is of great concern to researchers and government agencies involved in pollution control.According to the US Environmental Protection Agency, phenol is one of the most aggressive pollutants, which can affect micro biota even at low concentrations (5 μg/L).These compounds are present in the wastewater of various industrial segments, such as resin, gas, and coke manufactures; textile, plastic, rubber, pharmaceutical, and petroleum industries; and also from domestic effluents [1,2].
There is a consensus in the industry that phenol compounds should be removed from wastewaters prior to its disposal into water streams.Numerous researchers have addressed the problem by proposing methods such as dialysis, microbial degradation, oxidation, solvent extraction, impregnated resin extraction, flocculation/coagulation process, and adsorption [1][2][3][4][5] to treat phenol-bearing wastewaters.Some of these processes involve the application of oxidizing agents, such as ozone, Fenton reagents, and photochemical sonolysis.Although efficient in promoting a rapid mineralization of numerous chemical species, these processes offer several disadvantages, including the need for using artificial radiation sources, challenges in the distribution of radiation in the reactor, operational complexity for implementing largescale continuous systems, high cost, and the formation of by-products [6].
The coagulation/flocculation (C/F) process has traditionally been used in water and wastewater treatment facilities to remove turbidity, color, metals, organic matter, and pathogens.The overall principle in C/F process involves the formation of large particles that can be removed by settling or filtration processes [7].In general, coagulation is a process that involves reducing the repulsive potential of electrical double layers of colloids in such a way that micro particles can be produced.Flocs in the flocculation process are formed when these micro particles collide with each other forming larger structures [8].Several mechanisms are involved in the removal of pollutants: charge neutralization, entrapment, adsorption, and complexation with coagulant ions into the insoluble aggregates [9].
In this context, the presence of natural organic matter (NOM) in water becomes a major concern.NOMs should be removed from drinking water for causing undesirable effects to the water treatment processes and to the environment.NOMs affect organoleptic properties, reduce the disinfection power of most disinfectants used in water treatment, produce disinfection by-products (DBPs), affect the removal of inorganic particles, and influence coagulant demand [10].
Enhanced coagulation is a technique normally used to remove NOM (measured as total organic carbon -TOC) in conventional water treatment plants.The coagulant dose must be in excess, when compared to the one used to remove turbidity, to obtain the required efficiency in TOC removal [11][12][13].Edzwald [13] published a list of coagulants used in drinking water treatment, which includes aluminum-based, ferricbased, and cationic organic polyelectrolytes.
In recent decades, the use of surfactants has gained prominence as a new way for removing phenolic compounds from effluents.These agents are used in processes such as the cloud point extraction, which uses non-ionic surfactants in a process involving two-phase separation [14].Industrial effluents containing other contaminants, such as dyes, have also been investigated using ionic and non-ionic surfactants, microemulsion, and ultra-micellar filtration [15,16].
The use of surfactants to treat phenol-bearing wastewater is the object of this work.An alternative methodology, using micellar solubilization followed by ionic flocculation, was used to remove these organic compounds from aqueous solutions.Unlike conventional methods based on micelle separation, this novel process applied the surfactant as a floc.An anionic surfactant in aqueous media was used to promote the micellar solubilization of phenol.Following, calcium ions were added to promote surfactant flocculation.Due to the floc hydrophobic characteristic, phenol molecules migrated from the micellar core toward floc surface [17].Finally, the solute adhered to the floc surface was removed from the effluent through a filtration and a desorption step.

Materials
Surfactant (base soap -BS) was synthesized using sodium hydroxide and a mixture of 95 wt% animal fat and 5 wt% coconut oil.The experiment used a stoichiometric ratio (1:3 ratio) between the fatty acids, present in the animal fat and the coconut oil, and the sodium hydroxide.The surfactant obtained had an average molecular mass of 289 g/ mol.All reagents used in this work were of analytical grade.Aqueous solutions were prepared using distilled water.
The synthetic wastewater was prepared using phenol (Scientific Exodus; molar mass = 94.11g/mol; λ max = 272 nm).The required amount to obtain a 1000 ppm solution was weighed and dissolved in distilled water using a 100 mL volumetric flask.

Experiments for phenol removal
Initially, in 100 mL test tubes, the phenol solution was added at a fixed concentration (100 ppm).Following, the surfactant was weighed and dissolved according to the concentration required for each experiment (400-2500 ppm).Subsequently, calcium was added to the solution to promote surfactant flocculation.The concentration of calcium, corresponding to half of the surfactant one, was determined preliminarily in jar tests to assure total flocculation [17].To evaluate the calcium/surfactant ratio in the phenol removal process, this parameter was varied considering values below and above the established one as optimum value (0.25; 0.35; 0.50; 0.65; 0.75; 1).
The residual phenol concentration in the filtrate was analyzed using High Performance Liquid Chromatography (HPLC).The equipment used was a HPLC-DAD (Shimadzu series Prominence) equipped with a degasser (DGU-20As), a ternary pump (LC-20AT), an automatic sampler (SIL-20A HT), an oven (CTO-(SPD-M20A)), and an interface (CBM-20A).HPLC solutions software (version 1.25) was used in data acquisition and processing.The column used was a C18 (4.6 mm × 150 mm, 0.46 μm).It was injected 20 μL of the filtrate, at 1 mL/min.The mobile phase consisted of 70% acetonitrile and 30% miliQ water.The samples analyzed on HPLC were previously treated with sodium carbonate (2000 ppm) to precipitate the calcium of the sample, thus, preventing its entry into the HPLC system.The phenol removal efficiency, R(%), was calculated using Eq. ( 1).
Where C initial is the initial concentration of phenol and C diluted is its concentration in the diluted phase after floc separation.

Influence of phenol concentration in the removal process
The influence of phenol concentration in the synthetic wastewater (25-300 ppm) was evaluated by keeping the surfactant concentrations fixed at three different concentrations: 700; 1000; and 1300 ppm, at 35 °C.This step followed the procedure described in Section 2.2.1.

Evaluation of the calcium/surfactant ratio
The calcium/surfactant ratio was evaluated by using the surfactant concentrations fixed at three different values: 700; 1000; and 1300 ppm, at 35 °C.The following ratios were evaluated: 0.25; 0.35; 0.50; 0.65; 0.75, and 1.The main objective of this study was to evaluate phenol removal (R(%)) and visually verify changes occurred in the structure of the flocs.

Effect of pH on phenol removal efficiency
The experiments to study the effect of pH (3-12.7)were performed using different concentrations of surfactant (700; 1000; and 1300 ppm), under constant temperature (35 °C) and phenol concentration (100 ppm).The pH was adjusted after surfactant addition and before the addition of the calcium solution by using hydrochloric acid solution or sodium hydroxide solution.The experiment used a potentiometer (Digimed, DM-22) to measure the pH.

Effect of electrolytes in the phenol removal process
This study used the following concentrations of sodium chloride (Anidrol, analytical grade): 0.01; 0.05; and 0.1 M.These concentrations were chosen to guarantee that the precipitation of the surfactant would occur only due to the presence of calcium in solution considering that surfactant precipitation was observed at higher NaCl concentrations [18].Phenol concentration and temperature were maintained constant at 100 ppm and 35 °C, respectively.Surfactant concentration ranged from 400 to 1450 ppm.

Effect of equilibrium time and stirring speed
The concentration of surfactant was set at 1000 ppm to ensure the formation of flocs [17], and the phenol concentration was set at 100 ppm to determine the equilibrium time.The experiments were performed in a thermostatic batch, at 35 °C.Samples were stirred at 100 rpm (3 min) and then at 50 rpm (2 min).The contact times evaluated were: 5, 30, 60, 90, 120, and 150 min.The system was also evaluated without stirring to verify the influence of this parameter.
The stirring speed of the system was also studied.Samples were prepared under the same conditions used for the equilibrium time study.The fast stirring speed varied between 100, 200, 300, 400, and 500 rpm.The slow stirring speed was always equivalent of half of the fast one; for example, for 100 rpm fast stirring speed, 50 rpm was used as the slow one [17].

Study of phenol desorption
The kinetic desorption study was carried out to determine the desorption rate of the phenol from the surfactant floc.The desorption efficiency (DE(%)) was calculated using Eq. ( 2): Where q 0 (g phenol /g surfactant ) is the initial amount of phenol in the surfactant floc and q (g phenol /g surfactant ) is the amount of phenol remaining bounded to the floc after desorption.Desorption kinetics modeling was performed using Lagergren's zeroorder, pseudo-first order, and pseudo-second order models [18].The modified zero order model (Eq.( 3)), in a linearized form, assumes that the desorption rate is constant and independent of the amount of phenol at the filled sites.
Where k 0,des is the desorption velocity constant for the model (g/g.min) and t is the time (min).
The pseudo-first order model assumes that the desorption rate is proportional to the number of phenol molecules in the filled sites, and it can be expressed in a linearized form as: Where k 1,des is the rate of desorption for the pseudo first order model (min −1 ).
The modified pseudo second order model assumes that the rate of desorption is proportional to the square of the amount of phenol molecules at the filled sites and can be expressed in a linearized form as: Where k 2,des is the velocity constant of the pseudo-second order of desorption (g/g.min).

Effect of surfactant concentration and temperature
The ionic flocculation, using BS as surfactant, was applied to the treatment of phenol synthetic wastewater.Fig. 1 shows the phenol removal efficiency for different surfactant concentrations and temperatures.
One can notice that the removal of phenol increases with surfactant concentration (C t ) until it reaches 1300 ppm, with a maximum value of 60%, falling sharply from this point.Fig. 2 shows the suggested mechanism for micellar solubilization followed by the ionic flocculation process.Fig. 3 illustrates phenol removal steps.The whole procedure takes place in three steps: (1) surfactant addition to the phenol aqueous solution and solubilization of the pollutant in the micellar core (micellar solubilization); (2) addition of Ca 2+ ions to react with the surfactant, forming a precipitate (Na + RCOO − ) (aq) → Ca(RCOO − ) 2(s) + 2 Na + (aq) ) that, with continuous stirring, aggregates to form flocs; (3) after floc formation, the phenol molecules move from the micellar core to the surface of the floc.At last, the floc/phenol aggregate can be separated from the aqueous phase.The entire procedure occurs in 5 min.
Based on the suggested reaction mechanisms and Fig. 1, there is an optimum value for surfactant concentration.If dosages in excess of surfactant are used, changes in the micellar structure may occur, also altering the location in the micelle where phenol solubilization occurs.According to Suratkar and Mahapatra [19], the solubilization in micellar structures can occur at a number of different sites, including the micelle-water interface, between the hydrophilic head groups, in the core of the micelle, and between the hydrophilic groups and the first few carbon atoms of the hydrophobic groups that comprise the outer core of the micellar interior (palisade layer).
By increasing surfactant concentration, a modification in the micellar structure formed may occur (spherical to lamellar form), thus, changing the location where phenol molecules are solubilized.According to Talens-Alesson et al. [20], when addressing the use of anionic micelles and phenol molecules, phenol molecules solubilize near the polar barrier around the micelle or as phenoxide within the hydrophobic core but close to the core-water interface.With an increase in surfactant concentration, it is also important to consider the partition of Ca 2+ ions between micelles and solution, changing the hydrophobicity of the flocs and their affinity for phenol molecules (Talens-Alesson et al. [20]).
Concerning to temperature, at the same concentration of surfactant, the maximum removal was reached at 35-40 °C (Fig. 1).This phenomenon is associated with the stability of the floc and phenol interaction with its surface, which decreases with an increase in temperature, changing its affinity by water.Thus, for the subsequent studies, the authors of the present study suggest that the temperature is set at 35 °C, where the best results were obtained.

Influence of initial phenol concentration
Phenol concentration ranged from 25 to 300 ppm, and the surfactant one from 700 to 1300 ppm, at 35 °C, as one can observe in Fig. 4.
Fig. 4 shows that the removal of phenol increases with phenol concentration, reaching a maximum at 100 ppm, using 1300 ppm of surfactant.From this value, there is a decrease in phenol removal efficiency, probably due to the saturation of the surfaces of flocs with phenol, and also because phenol forms hydrogen bonds with water (strong interactions), tending to return to the solution.This observation shows that there is a synergistic effect in the process, which corroborates with the results for the effects of temperature and surfactant concentration presented previously.

Effect of calcium/surfactant ratio in the removal process
To evaluate the influence of calcium/surfactant ratio on the phenol removal efficiency this ratio was set at 0.25; 0.35; 0.50; 0.65; 0.75; and 1. Fig. 5 shows phenol removal (%) as a function of calcium/surfactant ratio for three different concentrations of surfactant.
One can notice that phenol removal efficiency increases with calcium concentration, reaching values close to 65% for 1300 ppm of surfactant.The maximum values were reached at a calcium/surfactant ratio equal to 1.0.The formation of more defined and larger flocs in solution was also observed.Nevertheless, in low calcium proportions, in the range of 0.25-0.35,low flocculation and a decrease in the size of the flocs were observed, explaining lower phenol efficiency.The 0.5 calcium/surfactant ratio reached 60% phenol removal.The present study chose to use this ration to avoid high calcium consumption, since only a 5% increase in the efficiency of the process was observed.
The stoichiometry of the reaction corroborates with the results obtained.Theoretically, the Ca 2+ ion causes precipitation of the anionic surfactant in the solution.The precipitation reaction can be represented by Eq. ( 6) [21]:

Effect of pH
In this study the pH varied from 3 to 12.7, as one can see in Fig. 6.In the acid pH range (3-7) the medium becomes cloudy, indicating the formation of an oil-in-water emulsion.This occurred due to surfactant degradation, which returns to its respective fatty acid.The formation of flocs is then impaired due to the absence of the carboxylate anion, decreasing the efficiency of the process (Fig. 6).At pH 9.7, phenol removal efficiency reached around 60%.This condition corresponds to an experimental run where only the surfactant was added, presenting a basic character.One can observe that even when making the medium more basic (pH equal to 11, 12, and 12.7), phenol removal remains practically constant, showing that it is not necessary to adjust the pH to have an increment on phenol removal.

Evaluation of the presence of electrolytes
Anionic surfactants undergo physicochemical changes when electrolytes are added to the environment.Generally, electrolytes are present at high concentrations in industrial effluents [22].In this sense, the evaluation of the effects of the addition of electrolytes is very important.Fig. 7 shows phenol removal efficiency as a function of the concentration of surfactant and sodium chloride.
According to Fig. 7, the presence of electrolyte leads to a decrease in process efficiency, with a reduction from 58%, without NaCl addition,  to 10%, with 0.1 M NaCl.The increase in NaCl concentration promotes an increase in Na + ions in the medium (see Eq. ( 6)).These Na + ions decrease the dissociation of surfactant (Na + + RCOO − ), making the interaction between the surfactant anion with Ca 2+ ions more difficult, hindering the floc formation process [21,23].

Effect of contact time and stirring speed
Fig. 8 presents the data for phenol removal as a function of contact time, with and without stirring.The main objective of this study was to evaluate the retention capacity of phenol by the flocs after its formation in the aqueous medium.
As one can observe in Fig. 8, phenol removal decreases with increasing contact time, either in a stirred or non-stirred process, but more pronounced in the former one.The micellar solubilization followed by ionic flocculation process occurs in a few minutes.As explained previously, phenol is solubilized in the micellar structures and migrates to the floc surface after its formation.After 150 min contact time, the removal efficiency tends to stabilize to values between 5 and 10%, showing that a desorption process occurred due to a decreased affinity of phenol molecules to the floc surface, as compared with the aqueous bulk solution.Phenol (C 6 H 6 O) has a hydrophilic characteristic and, in solution, it ionizes forming H + and C 6 H 5 O − .This anion may interact with water molecules by means of hydrogen bonding (strong bonds), hindering its presence in the floc surface for long periods of time.
This is an interesting observation, since phenol desorbs from the floc surface without the addition of a third agent, which in conventional processes ends up leading to increased costs and, in some cases, to the production of environmentally incompatible wastes.
One can also observe that the desorption rate of phenol molecules from the floc surface increases with the presence of stirring, reaching 12.5% efficiency after 25 min of contact time.In the process without stirring, this occurred after 100 min.This is related to the breaking of the floc structure by the stirring, which facilitates the desorption process [24].
The effect of the stirring speed on the removal efficiency is shown in Fig. 9.The efficiency of the process decreases with the increase in stirring speed.Rapid mixing is important to the process to disperse rapidly the surfactant and Ca 2+ into the phenol aqueous solution, but only for a few minutes to avoid interference in the mechanisms involved in the floc-formation process.According to Ghernaout and    Boucherit [25], concerning to the removal of natural organic matter (NOM) using the coagulation/flocculation process, initial mixing intensity and duration are critical in a coagulation process, since rapid and uniform dispersion of metal coagulant has advantages for the formation of charge-neutral products for NOM removal.As the stirring speed increased, a reduction in the size of the flocs was observed (Fig. 10).The increase in turbulence caused by the high stirring speed affects the diffusion of phenol molecules to floc surface and its adsorption.

Desorption model
The study of desorption efficiency over time (see Fig. 11) was made using a fixed concentration of surfactant (1000 ppm).
By examining the desorption kinetics of the process, one can observe that for 150 min the desorption reaches values close to 90%, showing that practically all phenol returns to the aqueous phase.
The desorption kinetics was studied by using the order zero, pseudofirst order, and pseudo-second order modified models of desorption [21], see Fig. 11.Table 1 presents the model constants and correlation coefficient for all models.
In this figure one can see that the model that best describes the experimental data is the pseudo-first order one, demonstrating that the desorption rate is proportional to the amount of phenol molecules in the filled sites [21].As observed in Table 1, the experimental results were better fitted using the Lagergren's pseudo-first order model.Eq. ( 7) describes the obtained model.

Conclusion
The ionic flocculation of the surfactant used in this work showed to be efficient for phenol removal from aqueous medium.The process occurs in two steps: phenol solubilization in micellar cores followed by surfactant flocculation.Based on the experimental results, the following conclusions can be drawn: -The optimal operation conditions were obtained using 100 ppm phenol, 1300 ppm surfactant, at 35 °C, in 5 min of contact time, 100 rpm rapid stirring, 50 rpm slow stirring, and pH of 9.7, reaching 60% phenol removal; -The desorption process is spontaneous and occurs without the addition of a desorbing agent, which makes the process more versatile; -A study using a real wastewater is necessary to evaluate the feasibility of the application of this method in a real wastewater treatment plant; This paper sought to investigate a novel separation process for pollutant removal by using surfactants.Many studies developed in the field of surfactant science for this purpose involve the formation of two phases, such as cloud-point extraction and the use of microemulsions (Winsor systems), which are based on liquid-liquid extraction principles.Differently, the ionic flocculation used in this research is based on a surface phenomena and it involves the interaction between the surface of surfactant flocs and pollutants dispersed in aqueous media.

Fig. 3 .
Fig. 3. Picture showing the stages involved in the removal of phenol.Test tube 1solution of phenol and surfactant, Test tube 2calcium is added and the flocs are formed, Test tube 3treated effluent after removal of the flocs by filtration.

Fig. 5 .
Fig. 5. Removal of phenol as a function of calcium/surfactant ratio.