Physicochemical Characterization of Regional Clay: Application to Phenol Adsorption

Phenol is one of the most toxic pollutants found in industrial waste. &is work focuses on the removal of phenol using clay from the Sale region. Adsorbent was characterized by X-ray fluorescence spectroscopy, X-ray diffraction, infrared spectroscopy, and scanning electron microscopy.&e chemical analysis of this clay shows that the percentage of silicon and aluminium is quite high, and the percentage of calcium and iron is relatively high, so this material is rich inmuscovite, quartz, and calcite. In addition to the presence of titanium dioxide (TiO2), which can give it a property of degradation of organic compounds under ultraviolet light, the pHPZC zero point of our material is 7.4. &e results showed that the adsorption of phenol was well fitted by the pseudo-secondorder kinetic model and the Langmuir and Freundlich isotherms and that the best retention is obtained at a pH between 3 and 8.


Introduction
e pollution of water by certain organic and inorganic chemicals is a serious source of degradation of our environment. Among these products is phenol, which is the product used in some industrial sectors in the region, such as the industry of plastics, synthetic petroleum fibres, detergents, and the industry of dyes and oil additives [1]. e effluents of the above sectors are loaded by phenolic compounds, which are spilled into wastewater networks and are in turn spilled into marine areas or rivers, producing adverse effects for human health as well as disruption of ecosystem life cycles. Concentration of 1 μg·L −1 is the maximum permissible amount of phenol in drinking water according to the World Health Organization [2]; also phenols are classified among the priority pollutants by the U.S. Environmental Protection Agency [3]. Faced with this problem, two techniques are applied: destructive techniques [4], such as thermal oxidation and catalytic oxidation [5], and recovery techniques, such as adsorption [6], precipitation [7], separation by membrane, and ion exchange [8]. Adsorption is among the most used techniques for the recovery of some organic and inorganic chemicals, thanks to its ease and simplicity unit operation [9,10]. Nowadays activated carbon is the strongest adsorbent used for water purification and the removal of organic matter [11]. But the high price of activated carbon has prompted many researchers to look for other alternatives. e abundance of many cheaper materials in nature attracted the attention of many researchers including Chaari et al. [12,13], Bentahar et al. [14], Sakin Omer et al. [15], Khosla [16], and Kyzioł-Komosinska et al. [17]. Clay materials have a good adsorption capacity of organic pollutants, heavy metals, and dyes to aqueous media [18]. Several studies have been targeted by the use of Moroccan clay adsorbent for the recovery of methylene blue [19], basic yellow cationic dye [20], methyl violet [19], adsorption of amino acids [21], adsorption of gallic acid on montmorillonite [22], adsorption of acid dyes red 1 and acid green 25 [23], removal of phenol and its derivatives by adsorption on petroleum asphalts [3], and adsorption of phenol by modified bentonite [24]. e surface charge of a clay material and its structure and chemical composition are parameters sufficiently capable of determining their adsorption capacity [25]. Note that 2 : 1 clay materials have higher adsorption capacities than 1 : 1 materials that adsorb species on the external surface [26].
In the present work, we aimed at the physicochemical characterization of a regional raw clay and its application to the adsorption of phenol in a liquid medium.

Material.
In this study, we used a clay from the city of Sale located in the region of Rabat-Sale-Kenitra. It is located on the Atlantic Ocean, on the right bank (north) of the mouth of the Bouregreg River, opposite Rabat, the national capital of Morocco ( Figure 1).

Characterization of the Adsorbent.
e adsorbent used in this study was characterized, before and after its contact with phenol, by X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and scanning electron microscopy (SEM). e FTIR spectrum of the studied sample was performed using Fourier Transform Infrared Spectrometer (Shimadzu FTIR, JASCO 4100) in the range from 400 to 4000 cm −1 .

Adsorption Tests.
A mass m � 0.05 g of the adsorbent was mixed with V � 20 mL of a synthetic phenol solution with an initial concentration of 10-3(mol/L). e mixture was then stirred for a well-determined contact time and after separation of the solid by centrifugation. e phenol remaining in the solution was analyzed with the UV/Visible Spectrophotometer (Shimadzu UV-1240) at a wavelength equal to 270 nm, and the amount of adsorbed phenol was calculated by the following equation [27]: where Q e denotes amount of pollutant adsorbed at equilibrium (mg g −1 ); C 0 denotes initial concentration (mg L −1 ); C e denotes concentration of the solute in the solution at equilibrium (mg L −1 ); V denotes volume of the solution (L); and m denotes mass of the adsorbent (g).
Kinetic adsorption experiments were performed to establish the effect of time on adsorption, and equilibrium experiments were performed to establish the adsorption isotherms. e thermodynamic study was done at different temperatures (20°C, 30°C, and 60°C). Initially, experiments were performed at 20°C to study the effects of contact time and pH. e conditions were pH 6.0 and contact time of 90 min. e removal efficiency of phenol on the adsorbent, R%, was calculated by the following equation [28]: where C 0 and C e are, respectively, the initial and equilibrium concentrations of the adsorbate (mg/L), respectively. Table 1 presents the equations of the models used in this work [29]. q t (mg·g −1 ) and q e (mg·g −1 ) are the quantities of adsorbed phenol at time t and at equilibrium; k 1 (min −1 ) and k 2 (g·mg −1 ·min −1 ) are the constants of pseudo-first-order and pseudo-second-order models, respectively. C e (mg/L) is the equilibrium concentration; q m (mg/g) is the maximum adsorption capacity; K L (L/mg) is the Langmuir constant; K F (mg·g −1 ) (L·mg −1 ) 1/n and n are the Freundlich constants.

Chemical Composition of the Adsorbent.
Our material has undergone analytical techniques that highlight its physical and chemical properties and is studied at the molecular level using X-ray spectroscopy, X-ray diffraction, and infrared spectroscopy (IR). e chemical analysis of the raw clay sample ( Table 2) shows that the percentage of silicon and aluminium is quite high, and the percentage of calcium and iron is relatively high, so this material is rich in muscovite, quartz, and calcite. In addition, the presence of titanium dioxide (TiO 2 ) can give it a property of degradation of organic compounds under ultraviolet light.

Analysis by X-Ray Diffraction.
e X-ray diffraction spectrum is shown in Figure 2. e X-ray diffraction spectrum ( Figure 2) shows that this clay is a mixture of

Scanning Electron Microscopy.
Scanning electron microscopy analysis of materials used in this study was performed to obtain additional information about their morphologies. Figure 3 shows the images obtained by scanning electron microscopy; it is apparent that the shape of the sample is dispersed and characterized by grains whose size is between 2 and 4 μm. ese grains are characterized by the presence of various microcavities and large pores that are favorable for the diffusion of phenol molecules [2]. e local analysis performed by EDXS ( Figure 4 and Table 3) is in fairly good agreement with those of X-ray diffraction ( Figure 2) [2]. Salt clay has a porous rock surface form with significant structural shapes that can adsorb phenol.

Infrared Analysis.
e infrared spectrum of our adsorbent is displayed below. Figure 5 represents the infrared spectra of the Sale clay before and after the adsorption of phenol. According to this analysis, one observes on the two infrared spectra of material two adsorption bands characterizing the O-H bonds located, respectively, between 3200 and 3800 cm −1 . Another average band between 1600 and 1700 cm −1 is attributed to the

Determination of the Zero Charge Point (PZC).
e point of zero charge pH pzc is the parameter corresponding to the pH for which the surface of the solid presents a zero charge.
If the hydrogen potential (pH) is lower than PZC, then the net charge is positive, and if the pH is higher than PZC, then the net charge is negative.
To determine the point of zero charge (PZC) of our material, we use the Drift method [30], which consists of preparing five samples of 50 ml of 0.01 M NaCl solution; for the stabilization of the pH, the samples must be bubbled with nitrogen, using a few drops of HCl and/or NaOH to adjust the pH of four samples, two of which are in the acidic zone and the other two are basic. e fifth sample, which has an intermediate pH, is left as a reference; then 0.15 g of the product is added to each sample, and the final pH is measured after 24 hours of agitation.
To determine the point of zero charge (PZC), we draw the curve pH f � f (pH i ), the point pH pzc is the intersection of    this curve with the right pH i � pH f , and we find that pH pzc of the material is 7.4 ( Figure 6).

Adsorption Kinetics of the Adsorbate on the Material
e results of the experiments related to the effect of time on the adsorption of phenol on clay are shown in Figure 7. According to Figure 7, the adsorption capacity increases very rapidly with time until equilibrium is reached after 20 minutes. Indeed, from this moment on, almost all the available sites on the surface of the adsorbent are occupied by phenol.

Adsorption Kinetics of the Different Granulometries of
Material. Figure 8 shows that the adsorbed amount of phenol on different particle sizes of this clay is almost the same. is explains that the particle size for this adsorbent has no effect.

Determination of the Order of the Adsorption Reaction.
Modeling of the phenol adsorption kinetics was done using the first-order (Lagergren model) and second-order (Ho et al.'s model) kinetic equations.

Pseudo-First-Order Model.
e graphical representation of the pseudo-first-order equation (Lagergren) is given in Figure 9. e plot of ln (Q abs -Q t ) versus time (t) ( Figure 9) shows a straight line with a regression coefficient equal to 0.99; Table 4 groups the parameters of the graphical representation of the pseudo-first-order equation (Lagergren). e correlation coefficient is R 2 � 0.99, which confirms the straight line pattern obtained; however, the calculation of the adsorbed quantity (Q e ) shows that the theoretical adsorbed quantity is rather low compared to the experimental quantity. e relative differences between the values of the quantity of adsorption (Q e ) were determined experimentally and were theoretically strong (>97%); these observations lead us to say that the adsorption of phenol on this clay does not express a process of controlled diffusion since it does not follow the equation of the pseudo-first-order, given by Lagergren.

Pseudo-Second-Order Model.
e graphical representation of the pseudo-second-order equation (Ho et al.) is given in Figure 10. Figure 10 shows the application of the pseudo-secondorder kinetics model to the result obtained for the adsorption of phenol. e straight line obtained is perfectly linear with a correlation coefficient closer to 1. e theoretical and experimental adsorbed quantity (Q e ) values, the pseudo-second-order constant K 2 , and the regression coefficient R 2 for this adsorbent used are displayed in Table 5.
From these results, it appears that the calculated adsorbed quantity (Q e ) of the order of 17.89 mg/g is closer to that of the experimental adsorbed quantity (Q e ) whose value is of the order of 17.55 mg/g, which explains the small relative difference between these two quantities; therefore, the pseudo-second-order model is the most suitable to describe the kinetics of adsorption of phenol on this adsorbent support.

Adsorption Isotherm Study.
e study of the phenol adsorption isotherm was carried out under the same experimental conditions as the kinetic study, and the modeling   From the results obtained for the two models studied, we can say that the adsorption of phenol on our clay support in the range of initial concentrations studied can be described by the Langmuir and Freundlich models, indicating that the phenol molecules are adsorbed on the material surface without interactions on energetically homogeneous sites and probably form a monolayer. e parameters deduced from the Langmuir and Freundlich lines are shown in Table 6.

Effect of Hydrogen Potential (pH).
To evaluate the influence of the pH treatment on the adsorption of phenol by our adsorbent, we carried out adsorption tests according to the following operating conditions (Table 7). e pH is adjusted successively to 3.35, 6.17, 8.59, and 12.51 and is kept constant during one hour of stirring, using NaOH (0.1 N) and HCl (0.1 N) solutions. e results (Figure 13) show the influence of pH on the adsorption of phenol    on the adsorbent, and we can see that the best retention yield is obtained in the acidic medium, removing the phenol with a yield of 50% for a mass concentration equal to 0.9 g/L. e removal of phenol was maximum and unperturbed when the initial pH of the phenol solution was between 3 and 8; a similar ability of pH effect was observed for the adsorption of phenol on activated carbon prepared from biomass material [4], as well as a similar trend for the adsorption of phenol red from aqueous solution on modified bentonite with maximum uptake at pH 3.0 and a remarkable decrease upon increasing the pH from 3.0 to 9.0 [24].
Phenol behaves as an anion at high pH values [28], which produces at high pH a competition between OHions and phenol molecules on the sorption sites resulting in a weak adsorption due to repulsion. However, at acidic pH, a cooperative retention of phenol occurs. First, with the protonation of the -OH function which gives rise to nucleophilic sites on the function, these nucleophilic sites will be more attracted by the negative surface of the clay. Second, the presence of a very acidic proton favors the hydrogen bonds.

ermodynamic Study of Adsorption.
In the course of adsorption, the surface energy of the adsorbent can be increased or decreased, and the standard enthalpy (∆H°), standard entropy (∆S°), and standard free energy are the parameters that can describe the thermodynamics of adsorption to calculate the values of ∆H°and ∆S°using the following equation [31]: where R denotes perfect gas constant (R � 8.314 J·mol −1 ·K −1 ); T denotes absolute temperature of solution (K); K d denotes distribution coefficient equal to Q e /C e. Equation (4) is used to calculate the values of ΔG° [27,29]: e graphical presentation of the van't Hoff equation for the adsorption of phenol on our adsorbent is displayed in Figure 14.
According to Table 8, the positive value of ΔS resulting from the adsorption of phenol corresponds to an increase in the degree of freedom in the system and indicates an affinity    of the adsorbent for the adsorbate molecules [31]. e negative value of ΔH, 9.8 kJ/mol, indicates an exothermic adsorption process. From this table, we noted that the values of ΔG are between −23855.01 and −26998.01 kJ/mol, indicating that the adsorption of phenol on this material is physical in nature.

Conclusion
is work aims to study the removal of phenol on a Moroccan clay support. e kinetics of adsorption of phenol on this clay show that this material adsorbs phenol rapidly in aqueous solution. e kinetics follow the pseudo-secondorder equation. e adsorption of phenol on this material is exothermic and physical in nature, and the adsorption capacity decreased slightly with increasing temperature. Study of the effect of particle size on the adsorption capacity showed that the particle size of our material has no effect on the diffusion of phenol. e Langmuir-Freundlich isotherm model describes well the adsorption equilibrium data for phenol. ese isotherms also show significant adsorption capacity with percent removal exceeding 50%. e parameter optimization study showed that the adsorption yield increases as the pH decreases. is increase is explained by the protonation of the phenol, which leads to the increase of nucleophilic sites on the aromatic ring. ese sites will be more attracted by the negative surface of the clay.

Data Availability
All the basic data that support the results of this study are contained in the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.