Kinetic Study of the Adsorption of Polyphenols from Olive Mill Wastewater onto Natural Clay: Ghassoul

Laboratory of Chemistry-Biology Applied to the Environment, Research Team “Applied Materials and Catalysis”, Chemistry Department, Faculty of Sciences, Moulay-Ismaı̈l University, BP. 11201-Zitoune, Meknes 50000, Morocco Research Team “Membrane Materials and Separation Processes,” Chemistry Department, Faculty of Science, Moulay-Ismaı̈l University, BP. 11201-Zitoune, Meknes 50000, Morocco Research Team “Biomolecular and Macromolecular Chemistry,” Chemistry Department, Faculty of Science, Moulay-Ismaı́l University, BP. 11201-Zitoune, Meknes 50000, Morocco


Introduction
e olive mill wastewaters (OMWs) from two-phase extraction systems are deemed to be one of the main environmental problems in region Fes-Meknes, Morocco (Figure 1), due to presence of toxic elements such as polyphenols. In 2016, Morocco generated 4000 to 5000 tonnes of OMW diverse in the rivers [1] because we used greater quantities of water which generate large volumes of the latter [2].
OMW is an environmental threat, and it became a problem that needs to be solved by the olive industry [3]. e composition of OMW may vary significantly depending on several factors: climate conditions, olive storage period, extraction process, and period of production [4].
Many physicochemical and biological techniques have been developed to treat OMW. ese methods include coagulation/flocculation [5,6], oxidation, ozonation [7], and membrane filtration [3,7,8]. Despite the availability of the processes above, the adsorption method is most extensively employed for treatments of the OMW.
For instance, Curi and Velioglu [9] and Azzam [10] utilized activated charcoal and natural clay for adsorption of hydroxytyrosol and other phenolic mixes from OMW. e goal of this research is the elimination of OMW polyphenols onto a low-cost clay called ghassoul and the characterization of this material. For this purpose, the quantity of polyphenols retained has been determined at the equilibrium. e isotherms to Langmuir and Freundlich models have been described. Moreover, the kinetic of adsorption has been analyzed uses pseudo-first-order (PFO), pseudo-second-order (PSO), and Weber-Morris intraparticle diffusion (IPD) models.

Material.
e material used is the commercial clay labelled "Ghassoul Chorafa Al Akhdar" without any further treatment, native from a site called "Ksabi" in the Province of Missour, East of Middle Atlas (Fez-Morocco). e particles of size <63 nm are crushed and dried during 24 hours at 80°C in the steam room. e prepared product was called Gh-B, referring to unprocessed ghassoul.

Olive Mill Wastewater and Pretreatment.
e origin olive from Taza (Morocco) and the OMW was obtained from a two-phase discontinuous extraction factory in the Fes-Meknes region (Morocco) on 20 November 2018. e gathered OMW was kept in separate plastic containers until use and then treated under nitrogen stream to remove dissolved oxygen to protect polyphenols. e sample obtained was then filtered and conserved to prepare a stock solution for kinetic study.
For X-ray diffraction analysis, the Philips PW 1800 instrument has been utilized. e quickening voltage was 40 kV, the current was 20 mA, and the copper Kα radiation was λ � 1.5418Å. e spectra of the different samples were registered in an interval of 2θ (5°-70°) with an accurate addition of 0.04°. FTIR investigation was directed by using Fourier Transform Infrared Spectrometer (JASCO 4000), out fitted in with a detector (TGS) and a ceramic source isolated by an optical framework utilizing an interferometer of Michelson. FTIR spectra are extended somewhere in the range of 4.000 and 400 cm −1 .
e Micromeritics ASAP 2010 Gas Sorption System was used to measure the surface area, and both methods of BET and BJH were utilized for determination of the specific surface and the pore size. ermogravimetric (TGA-DTA) investigation was completed by using Shimadzu TA-60 type contraption, working under air with a direct warming rate of 10°C·min −1 from surrounding temperature to 600°C. e technique of SEM-EDX was utilized to determine the morphology and elemental composition of the Gh-B.
e Gh-B dried at 105°C was analyzed by using X-ray fluorescence Philips PW 1666 type to determine the chemical composition, such as P 2 O 5 , Al 2 O 3 , MgO, Fe 2 O 3 , BaO, and SiO 2 .

Kinetic of Adsorption of Polyphenols from OMW onto Gh-B.
Adsorption tests were done in black bottles to avoid the degradation of polyphenols. 50 mg of Gh-B with 50 mL of OMW was diluted in water (starting focus C 0 � 30 mg·L −1 ). e blends were waved at temperatures of 25°C, 35°C, and 45°C during different times (20 min to 180 min). After each time, the blend is segregated by centrifugation at 3400 rpm for 8 min, and the supernatant was examined for determination of total polyphenols utilizing the Folin-Ciocalteu [11] technique and analyzed by UV-Vis spectroscopy. e absorbance at the wavelength of 760 nm was determined to calculate the leftover concentration of polyphenols (C e , g·L −1 ), and amount of polyphenols adsorbed at equilibrium time (q e , in mg·g −1 ) was calculated utilizing the following equation [12]: where C 0 is the initial concentration of polyphenols, C e is the leftover concentration of polyphenols which are expressed by g·L −1 , m (mg) is the lump of Gh-B, and V (mL) is the volume of OMW diluted. e adsorption isotherms were done under identical conditions from those of the adsorption kinetic utilizing a larger concentration from 0 to 58 mg·L −1 of polyphenols. e solutions were mixed for 3 hours until the equilibrium time was attained and then centrifuged. e determination of residual concentrations and the adsorbed amounts was done using (1).

eoretical Background.
We present in this part the expressions utilized to represent the kinetic and isotherms of the examined models. e kinetic model Lagergren [13] of pseudo-first-order (PFO) is represented by the following equation: where q t is the capacity adsorbed at time t; q e is the capacity adsorbed at balanced, which are expressed by mg·g −1 ; and K 1 (min −1 ) is the speed constant of PFO. K 1 and q e can be determined by plotting ln(q e − q t ) versus the time t.

Kinetic of PSO.
e expression of the pseudo-secondorder (PSO) model [13,14] is represented by the following equation: where K 2 (g·mg −1 ·min −1 ) is the speed constant for the PSO and q e is the quantity of polyphenols adsorbed at the balanced (mg·g −1 ). e slope and the y-intercept are utilized to calculate K 2 of PSO and q e .
is model is used to determine the limiting step in the adsorption mechanism: where K d is the IPD constant in mg·g −1 ·min −1/2 and C represents the value of the thickness of the boundary layer. ey can both be determined from slope and the y-intercept (equation (4)).

Adsorption of Isotherm Studies.
In the literature, various models have been published to compare experimental and theoretical data of adsorption isotherms. Freundlich and Langmuir models were utilized to describe isotherm adsorption.

Langmuir Model.
e nonlinear shape of the Langmuir model [14] is expressed by the following equation: where K L is the Langmuir constant (L·mg −1 ), C e is the equilibrium polyphenol concentration (mg·L −1 ), q e is the adsorption capacity of polyphenols at equilibrium (mg·g −1 ), and q m is the maximum adsorption amount for a monolayer (mg·g −1 ). Another parameter labelled separation factor (R L ) [16,17] is expressed in the following equation: where C 0 is the initial concentration of the adsorbate (mg·L −1 ). R L is the factor of separation which allows to check whether the isotherm is favorable or not provided that if the value of R L is between 0 and 1, it confirms the validity of the Langmuir model; when R L is close to 1 or 0, it signifies that the isotherms are linear and irreversible, respectively, and if R L is upper to 1, it indicate that isotherm is unfavourable [16,17].

Freundlich
Model. e nonlinear type of the Freundlich model [15,18] can be calculated by the following equation: where q e is the equilibrium polyphenol concentration on the ghassoul, C e is the equilibrium polyphenol concentration of solution, K F is the Freundlich constant, and n is the adsorption intensity characterizing the affinity of the pollutant for the adsorbent; when n is close to 1, it signifies a chemical adsorption process, and when n is greater than 1, it indicates a physical adsorption mechanism.

ermodynamic Parameters of Adsorption.
e enthalpy (ΔH 0 ), free energy (ΔG 0 ), and entropy (ΔS 0 ) thermodynamic parameters are calculated by the following relations [19]: where K C is the equilibrium constant defined as follows: in which C ads is the adsorbed concentration (g/L) and C 0 is the initial concentration of polyphenols in OMW (g/L). ne also notices the presence of free silica in the shape of quartz and dolomite in very small amount. On the contrary, the stevensite and magnesia poles of the smectites series are dominant in the Gh-B. ese outcomes are congruent with those obtained in the literature [19][20][21].

XRF Analysis.
XRF was carried out to identify the chemical composition of the minerals present in the Gh-B. e information given in Table 1 Figure 4) shows that the morphology of the Gh-B is close to hectorite, and the particles from different sizes have the appearance of sheets which oriented parallel to each other, as indicated by Caillere and Henin [22]. e chemical elements contained in natural clay (Gh-B) were detected by EDX analysis, and the results show that Gh-B has a higher percentage of silica (Table 2) mainly due to the presence of majority of quartz followed by magnesia. ese results are in agreement with XRD and XRF analysis. Table 2 gives the chemical elements and their mass percentages determined by the EDX analysis. e nitrogen adsorption/desorption isotherms of Gh-B show that, according the IUPAC classification, isotherm obtained is type IV, characteristic of solid mesoporous with onset the hysteresis of H3 type. After calculating using the BET method, the specific surface is 296 m 2 /g. e pore size distribution is determined from desorption isotherm by the BJH method, shown in Figure 5. is latter shows that the pore diameter is in the order of 73Å and thus confirms the mesoporosity of the structure of the Gh-B.

3.1.5.
ermal Analysis (DTA/TGA). TGA/DTA thermogram ( Figure 6) shows that the breakdown of Gh-B is done on three exothermic steps and one endothermic step:  (ATD), which is manifested by wide and asymmetric peak corresponding to decomposition of the early mixed carbon of magnesia and the calcium.   Figure 7 demonstrates that the quantity of polyphenols adsorbed at different temperatures is in the order 161 mg/g and considered important because of the high specific surface area (296 m 2 ·g −1 ) of Gh-B. e curves demonstrate that the adsorption kinetics is very quick at the start, due to the presence of the active sites at the start of adsorption, and the equilibrium was established after 2 h (about 60 min at 25°C). e quantities of polyphenols adsorbed onto Gh-B diminished from 161 to 123 mg·g −1 when the temperature increases from 25°C to 45°C, indicating that the temperature higher than 25°C destabilizes the force of adsorption and also decreases the interaction between Gh-B and polyphenols, and therefore, the adsorption process is exothermic. e same observation was found by De Chimie et al., works of adsorption of polyphenols from OMW by pomace olive which is utilized as an active carbon [23]. e plots of ln (q e −q t ) and t/q t according to time (equations (2) and (3), respectively) are obtained in Figures 8(a) and 8(b), respectively. We can wind up that polyphenols are adsorbed onto Gh-B and excellently followup the PSO model (Figure 8(a)). is is endorsed by these adsorbed quantities determined theoretically (q th ) and are very near to those obtained experimentally (q exp ), R 2 � 0.99 (Table 3).

Kinetics Adsorption.
In this study, we notice that when the temperature of the solution raises, the apparent constant of the PSO speed K 2 increased probably due to chemisorption phenomena. e same observation has been obtained in the adsorption of phenolic compounds from OMW on orange peel [24], on active carbon [25][26][27][28], onto resin [29], on onion [30], in removal of basic yellow cationic dye, [21] and in methyl violet by the same adsorbent (ghassoul) [20].

IPD.
e plot of the adsorbed quantity q t versus t 1/2 shows that polyphenols are adsorbed in two steps (Figure 9). e first one is quick; this is due to the transfer of polyphenols from OMW to the outside of the adsorbent. e second step is typified by a slight evolution to equilibrium, and it represents interaction between ghassoul and polyphenols. ese results validate an adsorption according to a kinetic of the PSO. However, the values of constant C are different to 0 ( Table 4) that shows the rate of polyphenol adsorption onto Gh-B is not controlled only by IPD step.
is results is an agreement with Valderrama et al. and-Lavinia et al. [31,32].

Study of Activation Energy E a .
e tracing of ln K 2 as function to 1/T allows to determine activation energy E a from the slope of the equation line Arrhenius. Figure 10 shows that the experimental points give a line when R 2 is very near to 1. e value of activation energy (90.622 kJ/mol) given by the slope of the Arrhenius plot demonstrates that the adsorption of polyphenols from OMW can be controlled by a chemisorption phenomenon. is phenomenon is confirmed by the obtained kinetic results.
is is in accordance with the works of M. kessoum [33] in the investigation of adsorbed polyphenols on a commercial active carbon (Picachem 150), but it is in disagreement with the results of Senol et al. [28] in the kinetic studies of biophenol adsorption onto commercial activated carbon with different particle sizes and at varied temperature. ey have found the physisorption phenomenon because the values of E a are included between 27.22 and 33.76 kJ/mol.

Adsorption Isotherms.
e curves of nonlinear transforms obtained by Langmuir and Freundlich models are shown in Figure 11, and the different parameters deduced from the two models are grouped in Table 5.
In Figure 11, it is clearly demonstrated that the adsorbed quantity of the polyphenols q max increases when the initial polyphenol concentration C 0 grows until the saturation. Table 5 shows a good linear correlation coefficient R 2 close to 1 for both isotherms Langmuir and Freundlich.
However, the values n from the Freundlich model for various temperatures (Table 5)  Journal of Chemistry Table 4: Parameters of IPD.
Step 1 Step 2    near to 1 in the concentration domain from 0 to 58 mg/L, but the amount of adsorbed q max calculated from this model is much different from those found experimentally. erefore, the two models are favorable to describe the adsorption phenomena of polyphenols onto ghassoul clay, but Freundlich model is more suitable because the theoretical amount of polyphenols (q max ) calculated from the Langmuir model is very far to the experimental value for all temperatures. ese results are similar to those obtained by Mounia et al. and Jedi et al. who worked on removing phenolic compound by adsorption onto wheat bran and bentonite, respectively [33,34].

3.3.1.
ermodynamic Parameters. e negative value of ΔH 0 (−0.14 kJ/mol) ( Table 6) shows that the adsorption of polyphenols onto Gh-B is an exothermic process in accordance with the kinetic studies (q max decreases with the increase in temperature). e order of the process is indicated by the negative value of ΔS 0 (−46.25 J/K·mol). e adsorption process is spontaneous because of the negative  1/T (K -1 )

Analysis of the Adsorption by FTIR.
e spectrum of Gh-B after adsorption in Figure 12 shows new bands of vibration. A shouldering to 3472 cm −1 and an intense band at 3437 cm −1 corresponding to hydroxyl stretching vibration of free and bonded −OH groups of the polyphenols, respectively. We also noted the presence of new bands at 2922 cm −1 and 2855 cm −1 corresponding to aromatic C-H and other bands at 1732 and 1385 cm −1 attributed to the C-O group of polyphenols ( Figure 12). ese results indicate the presence of polyphenols and confirmed that these compounds are adsorbed onto ghassoul.

Conclusion
In this study, we are interested in testing the effectiveness of natural clay "Gh-B" in the elimination of polyphenols from olive mill wastewater (OMW). e obtained results are as follows: (i) e natural clay ghassoul is majority constituted of silica and magnesia. is result is in agreement with XRD, XRF, and SEM/EDX. (ii) e quantity of the polyphenols adsorbed at different temperature has been of order 161 mg/g, and it is considerably important because of high specific surface area (296 m 2 ·g −1 ) of Gh-B. (iii) e examination of the adsorption kinetic of polyphenols onto Gh-B demonstrates that adsorption is done in two steps. e initial step is fast, and the balance comes at 2 h of contact. e next step is typified by slow evolution to equilibrium and adsorption kinetic realized at pseudo-second-order (PSO) model. (iv) e experimental isotherms are preferentially described by the Freundlich model, and the thermodynamic study indicates that the adsorption of polyphenols was exothermic in nature ΔH 0 < 0, ordered ΔS 0 < 0, and spontaneous ΔG 0 < 0. (v) All outcomes demonstrated that ghassoul was an effective and feeble cost adsorbent for the elimination of polyphenols from "OMW".

Data Availability
e authors affirm that all information fundamental to the discoveries of this examination are completely accessible without limitation.

Conflicts of Interest
e authors declare that there are no conflicts of interest.  Figure 12: FTIR spectra of Gh-B before and after adsorption.