# Adsorption, Modeling, Thermodynamic, and Kinetic Studies of Methyl Red Removal from Textile-Polluted Water Using Natural and Purified Organic Matter Rich Clays as Low-Cost Adsorbent

Authors: Dalila Fkih Romdhane; Yosra Satlaoui; Rawya Nasraoui; Abdelkrim Charef; Rim Azouzi
Journal: Journal of Chemistry
 (2020)

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## Abstract

Clay minerals have large surface areas that contribute to their high adsorption capacity. Pure clays were often used. However, their prices remain expensive. However, the natural clay minerals that are locally available can have economic and environmental benefits for textile wastewater treatment. The tested natural clays had given low removal yields. Therefore, we wanted to test particular rich organic matter clay for adsorbing azo dye, which is a very toxic molecule. In order to make the use of this clay type have a better efficiency for removal of this dye from the polluted waters, the optimal conditions had been specified. The results indicated that advised conditions were as follows: 5 min was the contact time of dye-clay; the better adsorbent masses were 0.25 g and 0.5 g per 100 ml solution for raw (ANb) and purified clays (ANp), respectively; the initial dye concentrations were 1 gL−1 for raw clay and 50 mgL−1 for purified clay; pH solution had any effect on the yield of dye removal only when raw clay was used; however, acid environment was advised when purified clay was the adsorbent and for the two tested clays about 20–30°C was the better solution temperature. X-Ray diffraction, Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM) analysis confirmed that functional groups of clay adsorbed the dye. Langmuir maximum adsorption capacities of ANb and ANp were found to be 397 mgg−1 and 132.3 mgg−1 at pH 7 and 5, respectively. Raw and chemically activated samples gave similar results. Adsorption of ANb and ANp data showed better agreement with the pseudo-second-order kinetic model. Thermodynamic parameters of the two adsorbents confirmed that the adsorption was endothermic (ΔH > 0) and spontaneous (ΔG0 < 0). Energy level was high when purified clay was used; however, it was significantly lower when the adsorbent was raw clay. Therefore, it was likely that adsorption by carbonates and organic matter involved small energy amounts. Comparing between these and other previous results, Jebel Louka natural clay type is better recommended for MR removal from textile wastewater, since the removal yield was about 98%. Hence, this tested clay type could provide an alternative low-cost material that could be used in treatment of the textile wastewater rich in MR and the obtained adsorption model and desorption tests provided a background for pilot and industrial scale applications.


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## Body

## 1. Introduction

Annual productions of the quantities and varieties of textile dyes are more and more important [1, 2] and have extensive industrial applications. These coloring molecules are often very toxic. Hence, these toxic chemical agents discharged into textile used water can often endanger the equilibrium of natural ecosystem [3] and have negative impacts on living organisms and environment [4–6]. We add that industrials prefer actually stable and hardly degradable colors. Hence, they generate molecules that are more complex and more difficult to eliminate from textile used water. Actually, there are more than 100,000 commercially existing dyes (azo dyes represent about 70% on weight of used dyes) and over one million tons of dyes are manufactured per year, of which 50% are textile dyes [7, 8]. Therefore, during the dying process, significant amounts of toxic stable molecules were lost in used water [9]. Because of the molecules complexity and the affinity degrees of colorants to the textile fibers, the discharge of this water kind in the environment is very worrying today [10, 11]. Hence, these pollutants of used water need to be well treated to obtain permissible concentrations before discharge.Nowadays, many technologies are used to purify the contaminated water (e.g., [12, 13]). These technologies permit the elimination of total impurities or decrease of their concentrations for having permissible limits. However, the cost remains high. Therefore, finding low-cost technologies and materials for textile wastewater treatment is still an objective for industrials.Azo dye methyl red (MR) is among undesirable substances and is often used for coloring textile fibers. Various methods, processes, and materials were used for MR-dye elimination. Always, the principal objective has been finding the best conditions for extracting this dye from polluted wastewater (e.g., [14, 15]). Treatment using biodecolorization with many simple sort and consortium of bacteria (mixture of bacteria) was tested [16–19]. Different chemical adsorbents such as SiO2-coated, Fe3O4 magnetic nanoparticles and modified iron oxide [20, 21] were tested. Natural minerals such as activated clay and carbon (e.g., [22–24]) were also used for MR elimination. But we are still looking for even cheaper adsorbents that can eliminate this pollutant type with more important yields.Clay minerals have large surface areas and ionic characters that contribute to their high adsorption capacity [25]. Pure clays were often used. However, their prices remain expensive. Hence, the natural clay minerals that are locally available could have economic and environmental benefits for textile wastewater treatment. These raw materials have highly variable mineralogical compositions and proportions. This variability always imposes their investigation before their use as a potential alternative adsorbent for removal of dyes from textile wastewater (e.g., [26, 27]).In Tunisia, clay minerals are abundant and have heterogenic composition. Many attempts were realized for utilizing clays as occurring adsorbents. These tested natural clays were not very efficient to eliminate important dye quantities (e.g., [28]). In order to search other qualities of materials, we are still looking for even cheaper adsorbents that can eliminate this pollutant type with more important yields. We proposed Jebel Louka clay type particularly rich in organic matter. Similar to the majority of natural materials, Jebel Louka clay type contains many minerals and essentially an important organic matter content that could adsorb important dye quantities. To be able to apply this natural clay in textile industry, it was always asked to have a strong and fast performance of dye elimination per volume unit. Therefore, the effects of some controllable parameters on the MR removal were studied.The saturated clays with methyl red were often rejected in controlled and uncontrolled landfills. Therefore, it was important to perform desorption tests to quantify the binding energies of the MR-clay to assess the risk degree of rejection of polluted clays.Hence, the objectives of this work were (1) to evaluate the adsorption behaviors of MR onto raw and purified Jebel Louka clay type, (2) to test the effects of contact time between the adsorbent and the aqueous solution of MR, of adsorbent dose, of concentration of initial MR-ion, and of pH and temperature of aqueous solution, (3) to perform adsorption isotherms, adsorption kinetics, and thermodynamic parameters, (4) to identify and characterize the adsorption mechanism by X-ray diffraction (XRD), Fourier transform infrared (FTIR), and scanning electron microscopy (SEM), and (5) to evaluate the efficiency of using Jebel Louka clay type for MR removal.


## 2. Materials and Methods

### 2.1. Clay Sampling and Analysis

The used deposit is from Jebel Louka (Tunisia). The site is located in the extreme northwestern part of Tunisia. Its Lambert coordinates are E: 529590 and N: 412268. It is a sedimentary series of Oligocene to Miocene age (Aquitanian, Burdigalian) [29, 30]. From this facies, about 5 composite samples were collected from five plots (3 × 3m) and from each point 10 × 10 × 20 cm volume of sediments was taken. Sampled sediments were homogenized and passed through 2 mm stainless steel sieves. All samples were lyophilized, milled with a mechanical grinder, and maintained at temperature about 4°C for programmed analyses.For mineralogical characterization of total sediments (powder sample), raw and purified clays before and after MR adsorption were identified by X-ray diffraction (XRD) SIEMENS D-5000 type diffractometer with a scanning speed of 1°2θ min and Cu-Kα radiation (40 kV, 20 mA) [31]. Semiquantitative estimation of detected minerals was based on peaks’ area [32]. Granulometric study of sediment fraction <63 μm was performed using MAVERN X-ray grain size analyzer [33]. Major and trace element concentrations were determined with atomic adsorption Perkin Elmer spectrophotometer type AAS 200 associated with a graphite furnace and inductively coupled plasma-mass spectroscopy (ICP-MS). Clay pH was measured in clay: distilled water volume ratios of [1 : 2.5]. Samples were stirred every 5 minutes, until the saturation period was reached (0.5 hours) [34]; pH was determined with pH-meter LPH 230 T-type. Electric conductivity (EC) was measured in extracts soil: water ratio [1 : 5] [35] with a conduct meter model ORION 150. Total organic carbon (TOC) was analyzed by ANNE method [36]. Based on methylene blue spot and Bergaya and Vayer’s methods, respectively (e.g., [37]), specific surface area (Ss) and the cation exchange capacity (CEC) values were determined. Casagrande method (e.g., [38]) was applied for the determination of samples Atterberg limits. Experimental error was about ±3%.In order to purify natural representative sample (ANb) of Jebel Louka, distilled water and H2O2 were used to remove organic matter (OM). Then, hydrochloric acid (10%) was added while controlling pH to dissolve the carbonate fractions. Sample was washed with distilled water in order to rid the excess acid and to allow the deflocculating of clay. Finally, the suspension was centrifuged at 2500 rpm for 10 minutes to obtain the pure Jebel Louka clay fraction (ANp).UV-Visible spectrophotometry analysis using a Perkin Elmer Lambda 25 Spectrophotometer, at wavelengths between 200 and 800 nm, was used to measure the remaining MR in separated suspension phase and FTIR spectra of the methyl red (MR) dye and raw and purified clay before and after adsorption using wavelengths ofλ = 455 nm.Appearance and morphology of the adsorbent surface before and after adsorption of MR-dye were carried out with a SEM (scanning electron microscope JEOL JSM 5400).


### 2.2. Adsorbate

Methyl red (dimethyl amino-4 phenylazo-2 benzoic acid or diamino-4 phenylazo-2 carboxybenzene) is a monoazo anionic dye (Table1).Table 1
Characteristics of methyl red (MR).
Chemical structureChemical formulaC15H15N3O2Change range of pH4.4 < pH < 6.0λmax525 nmMethyl red (MR) is soluble in water, ethanol, glacial acetic acid, and ether [39]. The group of azo dyes is characterized by the presence of –N = N- bond and synthesized by primary amine [40, 41].


### 2.3. Adsorption Experiments

Adsorption experiments were carried out by adding three adsorbent dosages ranging from 0.25 g to 1 g with a constant dye concentration (750 mgL−1) in aqueous solution (100 ml) at temperature of 25°C (±2°C) and pH 9. The colored solutions at specified concentrations were put in glass bottles. After shaking at a fixed speed of 450 rpm to attain equilibrium state, the liquid-solid phases were separated by centrifuging at a rate of 3000 trs min−1 for 20 min.Effects of adsorbent doses (0.25, 0.5, 1 gL−1), contact times (0 to 120 minutes), MR concentrations (30–1000 mgL−1), pH (3 to 11), and temperatures (20°C, 30°C, and 40°C) on dye removal were investigated. In Table 2 all chemical reagents and their concentrations used in the experimental study were detailed. We note that the purity degree of all reagents is about 99.7.Table 2
Chemical reagents used in experimental essays of adsorption of MR dye.
Chemical reagentsConcentrationsMR30, 50, 250, 500, 750, and 1000 gL−1HCl1.1 N, 0.5 N, and 3 NH2SO42NNaOH1 MH2O230%In order to ensure the reproducibility of obtained results, all analyzed parameters collected from batch experiments tested onto raw, purified, and activated clays of Jebel Louka were carried out three times under identical conditions. The reproducible results were only considered. Statistical analysis of the data was implemented using SPSS Statistics 21 software.The adsorbed MR quantity (qe, mgg−1) at equilibrium state was calculated by the following equation:(1)qe=C0−Cr∗Vm,where C0 (mgL−1) is initial dye concentration, Cr (mgL−1) is residual concentration at equilibrium (mgL−1), V is volume of solution (l), and m is mass of adsorbent (g).In order to investigate adsorption process of methyl red (MR) dye, infrared spectroscopy was carried out before and after adsorption. ANb and ANp were mixed with spectroscopically pure KBr (2 mg of clay was mixed with 200 mg of KBr). Infrared spectra were obtained by Fourier infrared spectroscopy [42] using Bruker FTIR-2000 spectrometer with reflection mode at a 4 cm−1 resolution in the 400–4000 cm−1 range.


### 2.4. Modeling

Adsorption isotherms described the equilibrium processes [43, 44]. They were also used to determine clay-fixing capacities of methyl red (MR) and adsorption type. Langmuir, Freundlich, and Dubinin-Radushkevich adsorption models based on experimental data were used to identify isotherms of various adsorption systems. Thermodynamic parameters such as free energy (ΔG°), standard enthalpy (ΔH°), and standard entropy (ΔS°) were calculated. Pseudo-first-order and pseudo-second-order models were implicated to precise adsorption kinetic model of MR into tested Jebel Louka clay.


## 2.1. Clay Sampling and Analysis

The used deposit is from Jebel Louka (Tunisia). The site is located in the extreme northwestern part of Tunisia. Its Lambert coordinates are E: 529590 and N: 412268. It is a sedimentary series of Oligocene to Miocene age (Aquitanian, Burdigalian) [29, 30]. From this facies, about 5 composite samples were collected from five plots (3 × 3m) and from each point 10 × 10 × 20 cm volume of sediments was taken. Sampled sediments were homogenized and passed through 2 mm stainless steel sieves. All samples were lyophilized, milled with a mechanical grinder, and maintained at temperature about 4°C for programmed analyses.For mineralogical characterization of total sediments (powder sample), raw and purified clays before and after MR adsorption were identified by X-ray diffraction (XRD) SIEMENS D-5000 type diffractometer with a scanning speed of 1°2θ min and Cu-Kα radiation (40 kV, 20 mA) [31]. Semiquantitative estimation of detected minerals was based on peaks’ area [32]. Granulometric study of sediment fraction <63 μm was performed using MAVERN X-ray grain size analyzer [33]. Major and trace element concentrations were determined with atomic adsorption Perkin Elmer spectrophotometer type AAS 200 associated with a graphite furnace and inductively coupled plasma-mass spectroscopy (ICP-MS). Clay pH was measured in clay: distilled water volume ratios of [1 : 2.5]. Samples were stirred every 5 minutes, until the saturation period was reached (0.5 hours) [34]; pH was determined with pH-meter LPH 230 T-type. Electric conductivity (EC) was measured in extracts soil: water ratio [1 : 5] [35] with a conduct meter model ORION 150. Total organic carbon (TOC) was analyzed by ANNE method [36]. Based on methylene blue spot and Bergaya and Vayer’s methods, respectively (e.g., [37]), specific surface area (Ss) and the cation exchange capacity (CEC) values were determined. Casagrande method (e.g., [38]) was applied for the determination of samples Atterberg limits. Experimental error was about ±3%.In order to purify natural representative sample (ANb) of Jebel Louka, distilled water and H2O2 were used to remove organic matter (OM). Then, hydrochloric acid (10%) was added while controlling pH to dissolve the carbonate fractions. Sample was washed with distilled water in order to rid the excess acid and to allow the deflocculating of clay. Finally, the suspension was centrifuged at 2500 rpm for 10 minutes to obtain the pure Jebel Louka clay fraction (ANp).UV-Visible spectrophotometry analysis using a Perkin Elmer Lambda 25 Spectrophotometer, at wavelengths between 200 and 800 nm, was used to measure the remaining MR in separated suspension phase and FTIR spectra of the methyl red (MR) dye and raw and purified clay before and after adsorption using wavelengths ofλ = 455 nm.Appearance and morphology of the adsorbent surface before and after adsorption of MR-dye were carried out with a SEM (scanning electron microscope JEOL JSM 5400).


## 2.2. Adsorbate

Methyl red (dimethyl amino-4 phenylazo-2 benzoic acid or diamino-4 phenylazo-2 carboxybenzene) is a monoazo anionic dye (Table1).Table 1
Characteristics of methyl red (MR).
Chemical structureChemical formulaC15H15N3O2Change range of pH4.4 < pH < 6.0λmax525 nmMethyl red (MR) is soluble in water, ethanol, glacial acetic acid, and ether [39]. The group of azo dyes is characterized by the presence of –N = N- bond and synthesized by primary amine [40, 41].


## 2.3. Adsorption Experiments

Adsorption experiments were carried out by adding three adsorbent dosages ranging from 0.25 g to 1 g with a constant dye concentration (750 mgL−1) in aqueous solution (100 ml) at temperature of 25°C (±2°C) and pH 9. The colored solutions at specified concentrations were put in glass bottles. After shaking at a fixed speed of 450 rpm to attain equilibrium state, the liquid-solid phases were separated by centrifuging at a rate of 3000 trs min−1 for 20 min.Effects of adsorbent doses (0.25, 0.5, 1 gL−1), contact times (0 to 120 minutes), MR concentrations (30–1000 mgL−1), pH (3 to 11), and temperatures (20°C, 30°C, and 40°C) on dye removal were investigated. In Table 2 all chemical reagents and their concentrations used in the experimental study were detailed. We note that the purity degree of all reagents is about 99.7.Table 2
Chemical reagents used in experimental essays of adsorption of MR dye.
Chemical reagentsConcentrationsMR30, 50, 250, 500, 750, and 1000 gL−1HCl1.1 N, 0.5 N, and 3 NH2SO42NNaOH1 MH2O230%In order to ensure the reproducibility of obtained results, all analyzed parameters collected from batch experiments tested onto raw, purified, and activated clays of Jebel Louka were carried out three times under identical conditions. The reproducible results were only considered. Statistical analysis of the data was implemented using SPSS Statistics 21 software.The adsorbed MR quantity (qe, mgg−1) at equilibrium state was calculated by the following equation:(1)qe=C0−Cr∗Vm,where C0 (mgL−1) is initial dye concentration, Cr (mgL−1) is residual concentration at equilibrium (mgL−1), V is volume of solution (l), and m is mass of adsorbent (g).In order to investigate adsorption process of methyl red (MR) dye, infrared spectroscopy was carried out before and after adsorption. ANb and ANp were mixed with spectroscopically pure KBr (2 mg of clay was mixed with 200 mg of KBr). Infrared spectra were obtained by Fourier infrared spectroscopy [42] using Bruker FTIR-2000 spectrometer with reflection mode at a 4 cm−1 resolution in the 400–4000 cm−1 range.


## 2.4. Modeling

Adsorption isotherms described the equilibrium processes [43, 44]. They were also used to determine clay-fixing capacities of methyl red (MR) and adsorption type. Langmuir, Freundlich, and Dubinin-Radushkevich adsorption models based on experimental data were used to identify isotherms of various adsorption systems. Thermodynamic parameters such as free energy (ΔG°), standard enthalpy (ΔH°), and standard entropy (ΔS°) were calculated. Pseudo-first-order and pseudo-second-order models were implicated to precise adsorption kinetic model of MR into tested Jebel Louka clay.


## 3. Results and Discussion

### 3.1. Adsorbent Characterization

The analyzed parameters of natural clay were granulometry, pH, electric conductivity (EC), cation exchange capacity (CEC), specific surface area (Ss), and carbonates (CaCO3) and organic matter (TOC) contents. Only arithmetic mean, maximum, minimum, and standard deviation of statistical analysis were presented in Table 3.Table 3
Statistic values of the maximum physiochemical parameters of raw and purified clays of Jebel Louka.
ANbANppH6.3 (5.8–6.6)±0.3∗∗∗4.8 (4.7–4.8)±0.06∗∗EC (mScm−1)0.4 (0.4–0.5)±0.01∗0.3 (0.1–0.2)±0.44∗TOC %8.3 (9.1–7.6)±0.13∗∗0CaCO3 %2.5 (2.9–2.0)±0.03∗∗0CEC (méq100 g−1)26.7 (26.7–26.9)±0.10∗21.5 (21.4–21.5)±0.04∗Ss (m2g−1)37.7 (37.6–37.9)±0.11∗∗∗39.2 (39.1–39.3)±0.07∗∗∗Mean (minimum-maximum); ±SD : standard deviation;∗p<0.05; ∗∗p<0.01; ∗∗∗p<0.001.Purified clay sample has more acid pH (4.7) compared to raw material. EC shows that purified clay ANp is more salty (0.1) than ANb clay (0.4). Natural clay is relatively rich in organic matter (about 7.5%). Clay treatments have improved particles specific surface areas (Ss) (from 39.7 to 39.2 m2g−1) and decreased cation exchange capacities (from 26.7 to 21.4 meq100 g−1) and electric conductivities (from 0.4 to 0.1 mScm−1) (Table 2). This clay type is considered anomaly rich in organic matter (8.2%).Principal minerals of natural clay (ANb) are kaolinite, illite, quartz, and CaCO3. The percentages of dolomite and sulfate are lower than 2%, that is, the XRD (X-ray diffraction) detection limit. Thus, they are observed only by microscopic study. Mean values of granulometric and physicochemical analysis indicated that Jebel Louka natural clay is dominated by fine fraction (65.8%) that is composed of 50.5% silt and 15.3% of clay. Proportion of sandy fraction is 32.43%.Chemical composition of raw and purified clays, as shown in Table4, is dominated by silica (54.8% and 57.1%), alumina (18.2% and 22.7%), and high iron quantity (7.2 % and 11. 8 %). This clay type is distinguished by the high yield of waste fire that confirms its high percentage of organic matter content.Table 4
Chemical composition of raw and purified clays of Jebel Louka.
Component weight (%)SiO2Al2O3FeO3MgOK2OCaOSO3Na2OFire wasteANb54.818.27.20.51.722.71.30.1112.75ANp57.122.711.780.062.690.30.320.763.68


### 3.2. Adsorption Characteristics

In order to characterize adsorption process of MR onto Jebel Louka clay, we discussed effects of adsorbent dose, contact time, initial MR concentration, pH, and temperature on clay capacity of dye removal from tested solution.


#### 3.2.1. Effects of Adsorbent Dosage and Contact Time

0.25 g of clay was separately added to 100 mL solution with variable initial methyl red concentrations ranging from 30 mgL−1 to 1 gL−1, at a fixed pH 9. After shaking of colored solutions in glass bottles at a fixed speed (450 rpm) to attain equilibrium state, the liquid and solid phases were separated by centrifugation.Adsorbed amount of dye increased for natural (ANb) and purified clays (ANp). It was more important for ANb (262 mgg−1) than for ANp (81 mgg−1). Amount of adsorbed dye was inversely proportional to clay mass. For the two cases, adsorption process was rapid during the first 5 minutes. Continued agitation for 115 minutes did not allow an adsorption of additional quantities of MR on natural clay. However, after 5 min, sorption became less efficient for ANp. The highest decrease was for the smaller clay amount. Thus, contact time needed to reach equilibrium conditions was only 5 min for three tested quantities (0.25 g, 0.5 g, and 1 g) of natural clay. This quickly obtained equilibrated state was probably due to high availability of active sites on clay surface (Figure 1).Figure 1
Effect of adsorbent dose and contact time on raw (a) and purified (b) clays.
(a)(b)It was expected that the increase of clay amount promoted an increase in number of active sites [45] and consequently adsorbed quantity of MR. However, the increase of clay mass (0.5 and 1 g) induced a decrease in adsorbed quantity of MR per mass unit of clay minerals. Therefore, small amount of adsorbent favored ease access of MR to adsorption sites. With increasing adsorbent quantity, MR molecule found difficulty to reach active sites of clay (e.g., [46]). Therefore, we suggested that the overcrowding phenomenon and agglomerations of clay particles created with large amounts of adsorbent had reduced total adsorption areas. Consequently, the decrease of the ratio “clay mass/solution volume” avoided creation of physical and chemical phenomena, discussed in the above section, that promoted MR adsorption [47]. For purified clay, probably the long shaking provoked desorption of some MR quantity that did not have a hard relation with adsorbent. However, for raw clay, this desorbed quantity was readsorbed onto other minerals and particularly onto carbonate and organic matter.Saxena and Sharma [48] showed similar results while they were studying the removal of MR by Guar Gum (GG) Powder. They confirmed that when adsorbent quantities increased, MR molecule found difficulty to reach active sites of clay and the increase in the adsorbate dose led also to a decrease in the adsorbed amount of dye.


#### 3.2.2. Effect of Initial Dye Concentration

Effect of initial azo dye concentration on adsorption capacity of natural (ANb) and purified (ANp) clays was investigated in the range from 30 mgL−1 to 1 gL−1. It was studied at a fixed shaking speed of 450 rpm, dye solution of 100 mL, clay mass of 0.25 g, and ambient temperature about 25°C.Adsorption capacities of ANb and ANp increased with increase of initial concentration of dye. The highest adsorbed quantity (qe) of dye was obtained with 1 gL−1 of MR initial concentration (Figure 2). The equilibrium state that marked the saturation of active sites of adsorbents was reached at the end of the first 5 minutes of the experience period [47]. After 10 min of shaking, due to the resistance of dye uptake, we had an adsorption decrease of MR as long as mass transfer driving forces increased [49].Figure 2
Effect of initial dye concentration on adsorbed quantities onto raw (a) and purified (b) clays.
(a)(b)Laabd et al. [50] showed also similar results while studying the removal of monoazoic dye by polyaniline. Sure enough, the increase of initial dye concentration led to an increase in the adsorbed amount of dye.


#### 3.2.3. Effect of pH

pH of aqueous solution is an important factor in any atom or molecule adsorption. In this study, 0.25 g of sorbent was mixed in glass bottles with a solution of 1 gL−1 of MR and shacken at a fixed speed (450 rpm) to attain equilibrium state. The pH solution values ranging from 3 to 11 were adjusted by either 1 M HCl or 1 M NaOH solutions. With continuous shaking, many aliquots of 5 ml were taken after different contact times (every 5 minutes at first and then every 20 minutes).Removal batch experiments of MR with natural clay indicated that all tested pH offered favorable conditions for uptake of this dye, since slight qe variations were observed (between 364 and 387 mgg−1) (Figure 3). Therefore, we suggested that different minerals of natural clay had probably different surface charge characteristics. It was probable that dominant mineral that fixed MR varied with the change of solution pH. Hence, overall, the adsorbed quantity of MR remained almost stable. The low fluctuation of the isotherms observed supports this explanation (Figure 3).Figure 3
Effect of variation of pH of aqueous solution on raw (ANb) and purified (ANp) clays.Adsorption capacity of purified clay showed a gradual drop of retained amount of MR per mass unity of clay with pH increase. At pH 3 (376 mgg−1) and 5 (396 mgg−1), the uptake of MR was appreciable. However, at basic pH (pH = 11), the removed quantity of MR was only about 66 mgg−1 (Figure 3). Adsorption efficiency at a very acid pH was also signaled by other previous studies (e.g., [51]). Acid pH could change several characteristics of adsorbent such as surface charges and ionization and dissociation degree of functional groups from its active sites. It could also affect the dissociation of functional groups of adsorbents and the affinity to adsorbents [52] and an electrostatic attraction could exist between surface of the adsorbent and adsorbate molecules [46]. However, adsorption capacity decrease at basic pH was mostly due to the dominance of the negative charges at the surface of the adsorbent or due to the low competition between OH− and dye anions in basic pH [53–55].


#### 3.2.4. Temperature Effect

Effect of temperature on adsorption processes is always mentioned in several researches [56, 57]. Batch adsorption experiments were carried out with 100 mL solutions of 1 gL−1 MR with 0.25 g of adsorbent and at 20, 30, and 40°C (commonly registered in arid and semiarid countries).Obtained experimental results showed that there was no great difference of MR removal at the three tested temperatures for natural clay (Figure4). Hence, temperature had a small influence on adsorption capacity of natural clay. It was possible that quartz, small fractions of carbonates, sulfate, and the relatively high amount of organic matter had equilibrated the energetic balance and had favored adsorption of dye amounts not fixed by pure clay fractions.Figure 4
Effect of temperature on raw (ANb) and purified (ANp) clays.However, an important reduction of adsorption capacities of purified clay with temperature increases was observed (Figure4). In many cases, with increasing temperatures, the adsorption capacities of adsorbents increased by an increase of diffusion rate of MR across external boundary layer and in internal pores of clay minerals. It was also possible that the increase of temperature increased solubility of sorbent that might acquire sufficient energy to undergo an interaction with active sites [58]. Thus, ambient temperature changes could change the adsorbent capacity [44]. However, for Jebel Louka sorbent, temperature increases led to decreases in its adsorption capacity. The same phenomenon was also observed in other previous studies [59–61]. It was considered that the increasing temperature probably produced a swelling effect within the internal structure of absorbent. This structure variation probably had further effects on the penetration of the large MR molecules [62]. This decrease could be also explained by the exothermicity and spontaneity of adsorption process and by the weakening of bonds between dye molecules and active sites of adsorbents for high temperatures (e.g., [63]). In addition, we think that with acidic pH of pure clay fraction temperature had an important effect on the adsorption process.


### 3.3. Adsorption Characterization

To characterize MR adsorption, clay adsorption tests were done under optimal batch conditions which were clay mass, initial dye concentration, contact time, pH, and temperature.


#### 3.3.1. DRX Characterization

To highlight the intercalation of MR molecules on clay minerals and to estimate their influence on structure and mineralogy of adsorbate clay, qualitative mineralogical study dye by diffraction X-rays was also done after MR adsorption. Comparison of ANb (AB) clay diffractograms before and after adsorption (AB-MR) showed an increase in reticular distance of phyllosilicate (located at 4.47) and a slight increase in quartz peak characteristic (located at 3.35) (Figure5). These variations were related to an increase in secondary peaks of clay minerals and minerals.Figure 5
XRD spectra of raw clay before (AB) and after (AB + MR) MR adsorption.Diffractograms of purified clay ANp before (AP) and after (AP-MR) adsorption showed some variations such as an increase in the reticular distance of Kaolinite (Figure6). These findings confirmed the intercalation of MR molecules on the surface of clay materials and in the interfoliar space. Therefore, adsorption of MR on clay surface of Jebel Louka was not a physical process, but it was a chemisorption that was produced by exchange between ions of dye and clay of interfoliar layer. Electrostatic forces probably produced this clay-color interaction [64].Figure 6
XRD spectra of purified clay before (AP) and after (AP + MR) adsorption.


#### 3.3.2. FTIR Spectra Characterization

The identification of Fourier transform infrared (FTIR) spectra was obtained through John Coates proposition [65]. Group frequencies and functional groups of MR dye were resumed in Table 5. The bands obtained at 3570 cm−1 and 3200 cm−1 indicated the hydroxyl group (O-H stretch) and normal “polymeric” O-H stretch, respectively. The primary, secondary, and tertiary amine stretch were obtained at 3400.22 cm−1, 1606 cm−1, 1374.43 cm−1, and 1210.06 cm−1 respectively. The methyl-amino stretch (N-CH3) was obtained at 2820.13 cm−1 and the open-chain azo (-N = N-) at 1630 cm−1. In addition, the simple C-H stretching vibration and aromatic and alkyne C-H bend were obtained at 2820.34 cm−1, 672 cm−1, and 683 cm−1. The conjugated and C = C stretch vibration were obtained at 1680 cm−1 and 1606 cm−1, respectively. Also, the C-O-C stretch was indicated at 1151 cm−1. The –COOH bending vibrations for carboxylic acid occurred between 1780 and 1714 cm−1.Table 5
Group frequencies (cm−1) and functional groups of the methyl red.
Group frequency (cm−1)Functional group3570–3200Hydroxy group, H-bonded O-H stretch3400–3200Normal “polymeric” O-H stretch3400–3380Primary amino: aliphatic primary amine N-H stretch3000–2800Simple C-H stretching vibrations for saturated aliphatic species2820–2780Methyl-amino, N-CH3, C-H stretch1780–1714Carboxylic acid (–COOH)1630–1575Open-chain azo (-N = N-)1650–1550Secondary amine, NH bend (>N-H)1680–1601Conjugated C = C, C = C stretch vibration1360–1280Aromatic secondary and tertiary amine CN stretch1210–1150Tertiary amino compound1151–1070C-O-C stretch, alkyl-substituted ether900–670Aromatic C-H out of plane bend680–610Alkyne C-H bendAdsorption of azo dye (MR) was controlled also by using the FTIR spectroscopy. The comparison between FTIR spectra of natural clay after adsorption (ANb + MR) indicated a clear difference, which confirmed the adsorption of dye (Figure7). The spectrum of ANb + MR indicated the disappearance of the stretching vibrations between 3500 and 3200 cm−1, between 1800 and 1250 cm−1, and between 692 and 683 cm−1. The removal of those stretching bends confirmed the adsorption process that happened for the MR dye. The bending vibrations between 1110 and 1020 cm−1 displayed the organic siloxane and silicone (Si-O-Si).Figure 7
FTIR spectra of methyl red (MR) dye and after its adsorption by raw clay (MR + ANb).The FTIR spectrum of purified clays after adsorption (ANp + MR) was mentioned in Figure8. The appearance of bending vibrations at 1603, 1375, and 1080 cm−1 indicated the presence of carboxylic acid, aromatic tertiary amine, and Si-O, respectively. These results confirmed the partial adsorption of azo dye.Figure 8
FTIR spectra of methyl red (MR) dye and after its adsorption by purified clay (MR + ANp).The FTIR spectrum results confirmed that minerals and organic matter of natural clay had a more adsorption capacity compared to pure clay fraction.


#### 3.3.3. SEM Characterization

SEM micrograph ANb before adsorption showed the clay grains and leaflets lamellar structures and aggregates were heterogeneous (different sizes and shapes and rough appearance of particles) (Figure9(a)). However, after adsorption, the micrograph showed a homogeneous surface indicating that clay surface was covered with the MR-dye molecules. Therefore, we had a disappearance of pores and spaces between clay aggregates (Figure 9(b)).Figure 9
SEM image of raw clay ANb before (a) and after (b) MR-dye uptake.Surface morphology of ANp was determined by SEM (Figures10(a) and 10(b)) and showed the SEM micrographs of ANp sample before and after adsorption of MR dye.Figure 10
SEM image of purified clay ANp before (a) and after (b) MR-dye uptake.The purified material (ANp) had pores and spaces between clay aggregates that were well apparent. Aggregates were very heterogeneous and presented in platelet forms of sticks with irregular contours. Lamellar structure was also observed (Figure10(a)).The scanning view of ANp after adsorption showed homogeneous surface morphology that was covered with MR-dye molecules (Figure10(b)).These results confirmed physisorption model and this dye could be easily liberated from saturated clays during adsorption process.Similar results were observed by Saxena and Sharma [48]. Their SEM figures showed that GG (Guar Gum) was covered with dye molecules and presented a rough surface morphology.


### 3.4. Adsorption Isotherms

Adsorption isotherms that describe adsorbed MR (qe) onto tested clay at equilibrium condition (Ce: concentration of solute in mgL−1at equilibrium), constant temperature, and pH [66] were proposed. Several equilibrium adsorption isotherm equations were available. Langmuir, Freundlich, and Dubinin-Radushkevich models (D-R) were applied to understand the mechanism of MR adsorption by tested clays.Generally, adsorption of a solute is considered as monolayer adsorption; that is, the adsorption layer is one molecule in thickness with no migration of the adsorbate in the surface of adsorbent [65]. Langmuir isotherms model assumes the existence of a monolayer adsorption with homogeneity of the surface [43, 66]. The linear form is expressed in the following equation:(2)Ceqe=1qmKL+Ceqm,where qe (mgg−1) is the adsorbed amount of the solute at equilibrium, Ce (mgL−1) is the concentration of the solute at equilibrium, qm (mgg−1) is the maximum adsorption capacity, and KL (Lmg−1) is the ratio of adsorption and desorption rate constant or Langmuir parameter. Then, by plotting fCe=Ce/qe, the value of the maximum capacity of the monolayer is determined.It is also proposed that the solute-solid is strong enough to allow solvent-solid interactions in monolayer and there is no similarity in the following layers [67].Freundlich isotherm model assumes that adsorption occurs on a heterogeneous surface with interaction between adsorbed molecules [68, 69]. At equilibrium, this model is an empirical equation expressed as follows:(3)qe=Ce1/nKf,where Kf (Lg−1) is the ratio of adsorption capacity of the adsorbent and n is the heterogeneity factor (Lg−1).Meanwhile the linear form is given by the following equation:(4)Lnqe=LnKf+1nLnCe.This model suggests that adsorption energy is not constant but exponentially decreases upon the completion of adsorption process.Adsorption equilibrium data were fitted to Freundlich (b, e) and Langmuir (a, d) isotherm models for ANb and ANp (Figure11). Two model parameters that were obtained for the adsorption of the methyl red onto natural and purified clays were shown in Table 6.Figure 11
Isotherm models for MR-dye adsorption onto raw (ANb) and purified (ANp) clays. (a) Langmuir, (b) Freundlich, and (c) D-R for ANb; (d) Langmuir, (e) Freundlich, and (f) D-R for ANp.
(a)(b)(c)(d)(e)(f)Table 6
Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) parameters.
LangmuirFreundlichD-RANbQ0 (mgg−1)59.180KF (Lg−1)3.480V’m (molg−1)6.529KL (Lmg−1)0.0071/n2.125K’ (mol2kJ−2)−0.321R20.788N0.471E (KJmol−1)1.248RL0.999R20.962ANpQ0 (mgg−1)−86.207KF (Lg−1)1.809V’m (molg−1)1.954KL (Lmg−1)−0.0031/n1.973K’ (mol2kJ−2)−0.111R20.344n0.507E (KJmol−1)2.119RL1.000R20.754At first, separation factorRL indicated a linear adsorption (RL = 1) for natural (ANb) and purified (ANp) clays. The intensity parameter 1/n > 1 showed that adsorption was defavorable because adsorption bonds became weak. Adsorption capacity decreases confirmed the linear adsorption process. Therefore, the best fitting results were obtained with Langmuir isotherm model. Consequently, monolayer adsorption capacity was in accordance with the experimental result for the two adsorbents (ANb and ANp). Similar results were obtained in obvious studies (e.g., [54]).Dubinin–Radushkevich (D–R) isotherm determines the energy balance of the adsorption process [70, 71]. Based on this model, E is the energy quantity necessary to remove one molecule from the adsorbent to the infinity and the adsorption processes are classified as a physical adsorption if E < 8 kJmol−1 and chemical adsorption if 8 < E < 168 kJmol−1. The model has often successfully fitted high solute activities and the intermediate range of concentrations data. D-R isotherm is expressed by the following equation:(5)Lnqe=LnVm′−K′ε2,where qe is the amount of dye which is adsorbed at equilibrium time (mgg−1) by solid matrix, V′m is the maximum adsorption capacity (molg−1), K’ is the activity coefficient related to the adsorption energy, and ɛ is the Polanyi potential, which is equal to(6)ε=RTLn1+1Ce.R is the constant of the gases (JK−1mol−1) and T is the temperature (K).Adsorption energy E (kJmol−1) is expressed by the following equation:(7)E=−2K′−0.5,E=12β,it gives information on physical and chemical characteristics of adsorption.Adsorption equilibrium data were fitted to the Dubinin–Radushkevich (c, f) isotherm models for ANb and ANp. The linear curve for the D-R isotherm obtained by tracingε2 as a function of Ln qe was presented with the experimental data in Figure 11. Average adsorption energy calculated from D-R isotherm confirmed that the physisorption was the dominate mechanism (E < 8 kJmol−1) for two adsorbents (Table 5).


### 3.5. Kinetic Models

The kinetics of adsorption define the efficiency of an adsorbent. Adsorption kinetics of MR onto natural and purified were investigated using different MR concentrations (30–1000 mgL−1). When the solution had 25°C (±2°C) and acid pH 3, the optimal quantity of adsorbed dye was obtained. Several models were used to fit experiment data for adsorption kinetics identification. MR adsorption dynamics processes were studied by Pseudo-first-order and pseudo-second-order models. These two models describe the solute uptake rate at solid-solution interface [48] and control the resident time of adsorbate and possibilities of desorption [41, 54]. The linear form of pseudo-first-order model was explained by Lagergren equation (8):(8)logqe-qt=logqe-k1t2.303,where qe and qt are the amounts of dye adsorbed (mgg−1) on the adsorbents at the equilibrium and at time t.k1 is the rate constant of adsorption (min−1). The values of k1 were calculated by plotting log (qe − qt) versus time t (min).Linear form of pseudo-second-order model is expressed by the following equation:(9)tqt=1k2qe2+1qet,where k2 is the rate constant of pseudo-second-order model (gmg−1min−1). Values of k2 and qe were calculated from intercepting a linear plot of t/qt versus t.Linear form of the two models and their calculated parameters for both adsorbents were listed in Figures12 and 13. Initially, removal rate of MR was rapid and then it was gradually slowed down until equilibrium state beyond which there was no significant increase. The maximum MR adsorption onto ANb and ANp (equilibrium state) was observed at 10 min. The kinetic experimental parameters of pseudo-first-order and pseudo-second-order models (qe and qt) and the rate constant of pseudo-first-order model (k1 and R2) and pseudo-second-order model (k2 and R2) were determined. The slopes and intercepts of plots of log (qe – qt) versus t (0–80 min) were used to determine the first-order rate constant k1 and equilibrium adsorption density qe for ANb and ANp. k1 ranges were from 0 to 0.001 for ANb and from −0.001 to 0.001 for ANp and their correlation coefficients (R2) ranges were 0.04–0.27 for ANb and 0.09–0.88 for ANp (Figure 12).Figure 12
Pseudo-first-order adsorption kinetics of MR onto ANb (a) and ANp (b).
(a)(b)Figure 13
Pseudo-second-order adsorption kinetics of MR onto ANb (a) and ANp (b).
(a)(b)However, for the linear plots of t/qt versus t for the pseudo-second-order model, the k2 ranges were from about 0.4 to 105 for ANb and from about −103 to 34.104 for ANp, and R2 values were from 0.95 to 1 for ANb and 1 for ANp (Figure 13). These experimental and calculated values showed that the adsorption of MR onto Jebel Louka clay followed the pseudo-second-order model.


### 3.6. Thermodynamic Study

Adsorption is a phenomenon that can be endothermic or exothermic. It depends on the adsorbent material and the nature of the adsorbed molecules [72, 73]. Thermodynamic parameters of equilibrium adsorption reaction are standard entropy ΔS°, standard enthalpy ΔH°, and standard free enthalpy ΔG° [69]. These parameters were determined using the following equations:(10)kd=QeCel g−1,(11)Lnkd=ΔS°R−ΔH°RT,(12)ΔG°=−RTLnkd.Calculated values of thermodynamic parameters of dye adsorption onto natural and purified clays (ANb and ANp) were given in Table7. The obtained values of ΔH° at 20, 30, and 40°C for ANb and ANp indicated that adsorption of MR onto Jebel Louka clay was an endothermic process. The positive ΔS° value was for all cases about 0.07 kJmol−1K−1. Hence, the low positive values of entropy variations showed that there was a random interference at solid-liquid interface during the fixation of MR on the active sites of adsorbent. The negative or nil values of Gibbs free energy (ΔG°) at the three tested temperatures indicated the feasibility and the spontaneity of adsorption process without an induction period.Table 7
Thermodynamic parameters of MR adsorption on ANb and ANp.
AdsorbentsTemperature T(°K)Thermodynamic parametersΔG° (KJmol−1)ΔH° (KJmol−1)ΔS° (KJmol−1K−1)ANb293.15020.200.0690303.15−6326.2720.860.0688313.15021.550.0688ANp293.15−40.4020.200.0689303.15−41.7300.0688313.15000.0688


### 3.7. Efficiency of Jebel Louka Clay

Several methods, processes, and components were used to remove MR (Table8). The principal objective was to define an adsorbent or a process which had the highest capacity of methyl red (MR) elimination. Chemical components, fibers of bananas, pretreated sugarcane bagasse, activated carbon, modified zeolite, and isolate or consortium bacteria were used as adsorbent or degrading tool. The best results were given by iron oxide modified MIL-100 (Fe) (62.5 %), but a long time was necessary for finishing the process and biodegradation by one consortium of three bacteria (about 99%).Table 8
Yields of methyl red removed using different methods, processes, and materials.
MethodsMaterials (adsorbents)Removal yields (%)ReferencesChemical componentsSterchamol0.022[20]SiO2-coated Fe3O4 magnetic nanoparticles4.95[21]Iron oxide modified MIL-100 (Fe)62.5[74]Biodegradation (or biodecolorization)Staphylococcus arlettae PF4 isolated from garden soil2.5[18]Saccharomyces cerevisiae ATCC 976399.1[19]Consortium of bacteria (mixture of bacteria): Three bacteria (Sphingomonas paucimobilis, Bacillus sp., and Staphylococcus epidermidis)98.0[11]Hazardous materials (biocomponents)Banana pseudostem fibers8.85[75]Sugarcane bagasse, pretreated with phosphoric acid (SBC)1.10[76]Untreated sugarcane bagasse (SB)0.57[76]NBP: neem tree bark powder/MBP: mango tree bark powder/LBP : locust bean tree bark powderMBP: 12%NBP: 8.5%LBP: 8%[77]Nano-sized calcium hydroxide catalyst prepared from clam shells0.2[78]Guar Gum powder5.76[47]Activated carbonCommercially available powdered activated carbon (PAC)4.84[76]Annona squamosa seeds activated carbon4.05[79]Oxidized multiwalled carbon nanotubes10.87[22]Mesoporous-activated carbon from durian seed38.47[80]Zeolite commercialModified zeolite commercial activated charcoal3.0[81]Modified zeolite commercial activated charcoal0.7[81]The yields of the azo dye MR removal with tested natural (ANb) and purified clays (ANp) were 89.5% and 92.2% when the initial concentrations of MR were 30 and 50 mgL−1, respectively. In these cases, biodegradation of MR was more effective (Figure 14). When initial MR concentrations in solutions were equal to or higher than 750 mgL−1, removal yields were at least equal to bacterial biodegradation yields. Because of low cost and facility of Jebel Louka natural clay use compared with bacterial consortium biodegradation, natural clay is better recommended for MR removal from textile wastewater.Figure 14
Cross between color removal (%) of MR onto raw (ANb) and purified (ANp) clays and the bacterial treatment.We note that adsorption yields of chemically activated clays with HCl and H2SO4 were between 98.1% for AN4 and 99.4% for ANp (Figure 15). Hence, chemical activation and raw material (99.7%) gave similar results. However, thermal activation decreased adsorption capacity of treated clay (78.2%). This decline was probably due to destruction of some adsorption active sites. However, for Lu et al. [82, 83], thermal and chemical activations improved the adsorption capacity of the material.Figure 15
Influence of initial concentration dye and adsorbent dose onto different samples. AN2: activated clay with 0.5 N HCl; AN3: activated clay with 3N HCl; AN4: activated clay with 2N H2SO4; AN5: thermally activated clay (500°C).


### 3.8. Desorption Studies

Desorption analyses had allowed seeing the possibility of the reuse of adsorbent or making the process more economic [84]. MR adsorbed onto natural Jebel Louka clay could be desorbed by water when the dye adsorption was by weak bonds (ion exchange or electrostatic attraction). In this case, if the clay is rejected in controlled or noncontrolled landfill the released toxic molecules could infiltrate into natural resources and have dangerous environmental impact. Therefore, it was considered that it is very important to estimate the possible desorbed amounts of MR.Desorption tests were done using adsorbents at different pH (3, 5, 7, 9, and 11) and ambient temperature (±25°C) in 100 mL solution of double-distilled water. At pH 3, 5, and 11, the dye amount desorbed from natural clay was more important compared to the purified clay. At pH 7 and 9, the amount of MR released by purified clay was the most important (Figure16). These obtained experimental results showed that the MR was easily desorbed and this pollutant could be transferred and potentially contaminated the natural resources (soil and water). Thus, rejection of saturated clays with MR can cause environmental problems. To limit the danger, the pH of rejected clay must be about 7 for natural clays and 11 for purified clay. In any case, we must use this saturated clay for brick making or take some measures for the isolation of this pollution source.Figure 16
Desorbed yields of MR dye from raw (ANb) and purified (ANp) clays.


### 3.9. Comment

The used kaolinite rich clay (called Tabarka clay (TBK)) and standard kaolinite (KGa-2: the Source Clay Repository of the Clay Minerals Society) gave similar results [85]. Abidi et al. [85] advanced that an increase in the amount of adsorbed dye (qe) occurred when the concentration of RR120 dye increased from 10 to 120 mg L−1. The higher adsorption amount was at pH 3. Therefore, pH always played an important role in dye adsorption processes. This phenomenon was also observed when RR120 dye was adsorbed onto raw clay [86], Brilliant Green dye onto red clay [87], and acid dye onto various modified clays [88]. It was also remarked that adsorption capacity decreased with temperature increase. This effect could be explained by weakening of the adsorptive forces between the active sites of the clays and the dye molecules [85].In order to investigate the adsorption mechanism and the nature of interaction forces between adsorbate molecules and adsorbent surfaces, Langmuir and Freundlich isotherms models were considered by all authors. They obtained also similar results. For example, Abidi et al. [85] indicated that Langmuir isotherm fitted well with their experimental data and the most suitable model to describe the experimental data was also the second-order model.It was also observed that the desorbed dye amount from the clays was high. For example, Abidi et al. [28] showed that the desorbed dye amount from KGa was 16 % and that from Tabarka clays was 18 %. These results were comparable to the data of this work.


## 3.1. Adsorbent Characterization

The analyzed parameters of natural clay were granulometry, pH, electric conductivity (EC), cation exchange capacity (CEC), specific surface area (Ss), and carbonates (CaCO3) and organic matter (TOC) contents. Only arithmetic mean, maximum, minimum, and standard deviation of statistical analysis were presented in Table 3.Table 3
Statistic values of the maximum physiochemical parameters of raw and purified clays of Jebel Louka.
ANbANppH6.3 (5.8–6.6)±0.3∗∗∗4.8 (4.7–4.8)±0.06∗∗EC (mScm−1)0.4 (0.4–0.5)±0.01∗0.3 (0.1–0.2)±0.44∗TOC %8.3 (9.1–7.6)±0.13∗∗0CaCO3 %2.5 (2.9–2.0)±0.03∗∗0CEC (méq100 g−1)26.7 (26.7–26.9)±0.10∗21.5 (21.4–21.5)±0.04∗Ss (m2g−1)37.7 (37.6–37.9)±0.11∗∗∗39.2 (39.1–39.3)±0.07∗∗∗Mean (minimum-maximum); ±SD : standard deviation;∗p<0.05; ∗∗p<0.01; ∗∗∗p<0.001.Purified clay sample has more acid pH (4.7) compared to raw material. EC shows that purified clay ANp is more salty (0.1) than ANb clay (0.4). Natural clay is relatively rich in organic matter (about 7.5%). Clay treatments have improved particles specific surface areas (Ss) (from 39.7 to 39.2 m2g−1) and decreased cation exchange capacities (from 26.7 to 21.4 meq100 g−1) and electric conductivities (from 0.4 to 0.1 mScm−1) (Table 2). This clay type is considered anomaly rich in organic matter (8.2%).Principal minerals of natural clay (ANb) are kaolinite, illite, quartz, and CaCO3. The percentages of dolomite and sulfate are lower than 2%, that is, the XRD (X-ray diffraction) detection limit. Thus, they are observed only by microscopic study. Mean values of granulometric and physicochemical analysis indicated that Jebel Louka natural clay is dominated by fine fraction (65.8%) that is composed of 50.5% silt and 15.3% of clay. Proportion of sandy fraction is 32.43%.Chemical composition of raw and purified clays, as shown in Table4, is dominated by silica (54.8% and 57.1%), alumina (18.2% and 22.7%), and high iron quantity (7.2 % and 11. 8 %). This clay type is distinguished by the high yield of waste fire that confirms its high percentage of organic matter content.Table 4
Chemical composition of raw and purified clays of Jebel Louka.
Component weight (%)SiO2Al2O3FeO3MgOK2OCaOSO3Na2OFire wasteANb54.818.27.20.51.722.71.30.1112.75ANp57.122.711.780.062.690.30.320.763.68


## 3.2. Adsorption Characteristics

In order to characterize adsorption process of MR onto Jebel Louka clay, we discussed effects of adsorbent dose, contact time, initial MR concentration, pH, and temperature on clay capacity of dye removal from tested solution.


### 3.2.1. Effects of Adsorbent Dosage and Contact Time

0.25 g of clay was separately added to 100 mL solution with variable initial methyl red concentrations ranging from 30 mgL−1 to 1 gL−1, at a fixed pH 9. After shaking of colored solutions in glass bottles at a fixed speed (450 rpm) to attain equilibrium state, the liquid and solid phases were separated by centrifugation.Adsorbed amount of dye increased for natural (ANb) and purified clays (ANp). It was more important for ANb (262 mgg−1) than for ANp (81 mgg−1). Amount of adsorbed dye was inversely proportional to clay mass. For the two cases, adsorption process was rapid during the first 5 minutes. Continued agitation for 115 minutes did not allow an adsorption of additional quantities of MR on natural clay. However, after 5 min, sorption became less efficient for ANp. The highest decrease was for the smaller clay amount. Thus, contact time needed to reach equilibrium conditions was only 5 min for three tested quantities (0.25 g, 0.5 g, and 1 g) of natural clay. This quickly obtained equilibrated state was probably due to high availability of active sites on clay surface (Figure 1).Figure 1
Effect of adsorbent dose and contact time on raw (a) and purified (b) clays.
(a)(b)It was expected that the increase of clay amount promoted an increase in number of active sites [45] and consequently adsorbed quantity of MR. However, the increase of clay mass (0.5 and 1 g) induced a decrease in adsorbed quantity of MR per mass unit of clay minerals. Therefore, small amount of adsorbent favored ease access of MR to adsorption sites. With increasing adsorbent quantity, MR molecule found difficulty to reach active sites of clay (e.g., [46]). Therefore, we suggested that the overcrowding phenomenon and agglomerations of clay particles created with large amounts of adsorbent had reduced total adsorption areas. Consequently, the decrease of the ratio “clay mass/solution volume” avoided creation of physical and chemical phenomena, discussed in the above section, that promoted MR adsorption [47]. For purified clay, probably the long shaking provoked desorption of some MR quantity that did not have a hard relation with adsorbent. However, for raw clay, this desorbed quantity was readsorbed onto other minerals and particularly onto carbonate and organic matter.Saxena and Sharma [48] showed similar results while they were studying the removal of MR by Guar Gum (GG) Powder. They confirmed that when adsorbent quantities increased, MR molecule found difficulty to reach active sites of clay and the increase in the adsorbate dose led also to a decrease in the adsorbed amount of dye.


### 3.2.2. Effect of Initial Dye Concentration

Effect of initial azo dye concentration on adsorption capacity of natural (ANb) and purified (ANp) clays was investigated in the range from 30 mgL−1 to 1 gL−1. It was studied at a fixed shaking speed of 450 rpm, dye solution of 100 mL, clay mass of 0.25 g, and ambient temperature about 25°C.Adsorption capacities of ANb and ANp increased with increase of initial concentration of dye. The highest adsorbed quantity (qe) of dye was obtained with 1 gL−1 of MR initial concentration (Figure 2). The equilibrium state that marked the saturation of active sites of adsorbents was reached at the end of the first 5 minutes of the experience period [47]. After 10 min of shaking, due to the resistance of dye uptake, we had an adsorption decrease of MR as long as mass transfer driving forces increased [49].Figure 2
Effect of initial dye concentration on adsorbed quantities onto raw (a) and purified (b) clays.
(a)(b)Laabd et al. [50] showed also similar results while studying the removal of monoazoic dye by polyaniline. Sure enough, the increase of initial dye concentration led to an increase in the adsorbed amount of dye.


### 3.2.3. Effect of pH

pH of aqueous solution is an important factor in any atom or molecule adsorption. In this study, 0.25 g of sorbent was mixed in glass bottles with a solution of 1 gL−1 of MR and shacken at a fixed speed (450 rpm) to attain equilibrium state. The pH solution values ranging from 3 to 11 were adjusted by either 1 M HCl or 1 M NaOH solutions. With continuous shaking, many aliquots of 5 ml were taken after different contact times (every 5 minutes at first and then every 20 minutes).Removal batch experiments of MR with natural clay indicated that all tested pH offered favorable conditions for uptake of this dye, since slight qe variations were observed (between 364 and 387 mgg−1) (Figure 3). Therefore, we suggested that different minerals of natural clay had probably different surface charge characteristics. It was probable that dominant mineral that fixed MR varied with the change of solution pH. Hence, overall, the adsorbed quantity of MR remained almost stable. The low fluctuation of the isotherms observed supports this explanation (Figure 3).Figure 3
Effect of variation of pH of aqueous solution on raw (ANb) and purified (ANp) clays.Adsorption capacity of purified clay showed a gradual drop of retained amount of MR per mass unity of clay with pH increase. At pH 3 (376 mgg−1) and 5 (396 mgg−1), the uptake of MR was appreciable. However, at basic pH (pH = 11), the removed quantity of MR was only about 66 mgg−1 (Figure 3). Adsorption efficiency at a very acid pH was also signaled by other previous studies (e.g., [51]). Acid pH could change several characteristics of adsorbent such as surface charges and ionization and dissociation degree of functional groups from its active sites. It could also affect the dissociation of functional groups of adsorbents and the affinity to adsorbents [52] and an electrostatic attraction could exist between surface of the adsorbent and adsorbate molecules [46]. However, adsorption capacity decrease at basic pH was mostly due to the dominance of the negative charges at the surface of the adsorbent or due to the low competition between OH− and dye anions in basic pH [53–55].


### 3.2.4. Temperature Effect

Effect of temperature on adsorption processes is always mentioned in several researches [56, 57]. Batch adsorption experiments were carried out with 100 mL solutions of 1 gL−1 MR with 0.25 g of adsorbent and at 20, 30, and 40°C (commonly registered in arid and semiarid countries).Obtained experimental results showed that there was no great difference of MR removal at the three tested temperatures for natural clay (Figure4). Hence, temperature had a small influence on adsorption capacity of natural clay. It was possible that quartz, small fractions of carbonates, sulfate, and the relatively high amount of organic matter had equilibrated the energetic balance and had favored adsorption of dye amounts not fixed by pure clay fractions.Figure 4
Effect of temperature on raw (ANb) and purified (ANp) clays.However, an important reduction of adsorption capacities of purified clay with temperature increases was observed (Figure4). In many cases, with increasing temperatures, the adsorption capacities of adsorbents increased by an increase of diffusion rate of MR across external boundary layer and in internal pores of clay minerals. It was also possible that the increase of temperature increased solubility of sorbent that might acquire sufficient energy to undergo an interaction with active sites [58]. Thus, ambient temperature changes could change the adsorbent capacity [44]. However, for Jebel Louka sorbent, temperature increases led to decreases in its adsorption capacity. The same phenomenon was also observed in other previous studies [59–61]. It was considered that the increasing temperature probably produced a swelling effect within the internal structure of absorbent. This structure variation probably had further effects on the penetration of the large MR molecules [62]. This decrease could be also explained by the exothermicity and spontaneity of adsorption process and by the weakening of bonds between dye molecules and active sites of adsorbents for high temperatures (e.g., [63]). In addition, we think that with acidic pH of pure clay fraction temperature had an important effect on the adsorption process.


## 3.2.1. Effects of Adsorbent Dosage and Contact Time

0.25 g of clay was separately added to 100 mL solution with variable initial methyl red concentrations ranging from 30 mgL−1 to 1 gL−1, at a fixed pH 9. After shaking of colored solutions in glass bottles at a fixed speed (450 rpm) to attain equilibrium state, the liquid and solid phases were separated by centrifugation.Adsorbed amount of dye increased for natural (ANb) and purified clays (ANp). It was more important for ANb (262 mgg−1) than for ANp (81 mgg−1). Amount of adsorbed dye was inversely proportional to clay mass. For the two cases, adsorption process was rapid during the first 5 minutes. Continued agitation for 115 minutes did not allow an adsorption of additional quantities of MR on natural clay. However, after 5 min, sorption became less efficient for ANp. The highest decrease was for the smaller clay amount. Thus, contact time needed to reach equilibrium conditions was only 5 min for three tested quantities (0.25 g, 0.5 g, and 1 g) of natural clay. This quickly obtained equilibrated state was probably due to high availability of active sites on clay surface (Figure 1).Figure 1
Effect of adsorbent dose and contact time on raw (a) and purified (b) clays.
(a)(b)It was expected that the increase of clay amount promoted an increase in number of active sites [45] and consequently adsorbed quantity of MR. However, the increase of clay mass (0.5 and 1 g) induced a decrease in adsorbed quantity of MR per mass unit of clay minerals. Therefore, small amount of adsorbent favored ease access of MR to adsorption sites. With increasing adsorbent quantity, MR molecule found difficulty to reach active sites of clay (e.g., [46]). Therefore, we suggested that the overcrowding phenomenon and agglomerations of clay particles created with large amounts of adsorbent had reduced total adsorption areas. Consequently, the decrease of the ratio “clay mass/solution volume” avoided creation of physical and chemical phenomena, discussed in the above section, that promoted MR adsorption [47]. For purified clay, probably the long shaking provoked desorption of some MR quantity that did not have a hard relation with adsorbent. However, for raw clay, this desorbed quantity was readsorbed onto other minerals and particularly onto carbonate and organic matter.Saxena and Sharma [48] showed similar results while they were studying the removal of MR by Guar Gum (GG) Powder. They confirmed that when adsorbent quantities increased, MR molecule found difficulty to reach active sites of clay and the increase in the adsorbate dose led also to a decrease in the adsorbed amount of dye.


## 3.2.2. Effect of Initial Dye Concentration

Effect of initial azo dye concentration on adsorption capacity of natural (ANb) and purified (ANp) clays was investigated in the range from 30 mgL−1 to 1 gL−1. It was studied at a fixed shaking speed of 450 rpm, dye solution of 100 mL, clay mass of 0.25 g, and ambient temperature about 25°C.Adsorption capacities of ANb and ANp increased with increase of initial concentration of dye. The highest adsorbed quantity (qe) of dye was obtained with 1 gL−1 of MR initial concentration (Figure 2). The equilibrium state that marked the saturation of active sites of adsorbents was reached at the end of the first 5 minutes of the experience period [47]. After 10 min of shaking, due to the resistance of dye uptake, we had an adsorption decrease of MR as long as mass transfer driving forces increased [49].Figure 2
Effect of initial dye concentration on adsorbed quantities onto raw (a) and purified (b) clays.
(a)(b)Laabd et al. [50] showed also similar results while studying the removal of monoazoic dye by polyaniline. Sure enough, the increase of initial dye concentration led to an increase in the adsorbed amount of dye.


## 3.2.3. Effect of pH

pH of aqueous solution is an important factor in any atom or molecule adsorption. In this study, 0.25 g of sorbent was mixed in glass bottles with a solution of 1 gL−1 of MR and shacken at a fixed speed (450 rpm) to attain equilibrium state. The pH solution values ranging from 3 to 11 were adjusted by either 1 M HCl or 1 M NaOH solutions. With continuous shaking, many aliquots of 5 ml were taken after different contact times (every 5 minutes at first and then every 20 minutes).Removal batch experiments of MR with natural clay indicated that all tested pH offered favorable conditions for uptake of this dye, since slight qe variations were observed (between 364 and 387 mgg−1) (Figure 3). Therefore, we suggested that different minerals of natural clay had probably different surface charge characteristics. It was probable that dominant mineral that fixed MR varied with the change of solution pH. Hence, overall, the adsorbed quantity of MR remained almost stable. The low fluctuation of the isotherms observed supports this explanation (Figure 3).Figure 3
Effect of variation of pH of aqueous solution on raw (ANb) and purified (ANp) clays.Adsorption capacity of purified clay showed a gradual drop of retained amount of MR per mass unity of clay with pH increase. At pH 3 (376 mgg−1) and 5 (396 mgg−1), the uptake of MR was appreciable. However, at basic pH (pH = 11), the removed quantity of MR was only about 66 mgg−1 (Figure 3). Adsorption efficiency at a very acid pH was also signaled by other previous studies (e.g., [51]). Acid pH could change several characteristics of adsorbent such as surface charges and ionization and dissociation degree of functional groups from its active sites. It could also affect the dissociation of functional groups of adsorbents and the affinity to adsorbents [52] and an electrostatic attraction could exist between surface of the adsorbent and adsorbate molecules [46]. However, adsorption capacity decrease at basic pH was mostly due to the dominance of the negative charges at the surface of the adsorbent or due to the low competition between OH− and dye anions in basic pH [53–55].


## 3.2.4. Temperature Effect

Effect of temperature on adsorption processes is always mentioned in several researches [56, 57]. Batch adsorption experiments were carried out with 100 mL solutions of 1 gL−1 MR with 0.25 g of adsorbent and at 20, 30, and 40°C (commonly registered in arid and semiarid countries).Obtained experimental results showed that there was no great difference of MR removal at the three tested temperatures for natural clay (Figure4). Hence, temperature had a small influence on adsorption capacity of natural clay. It was possible that quartz, small fractions of carbonates, sulfate, and the relatively high amount of organic matter had equilibrated the energetic balance and had favored adsorption of dye amounts not fixed by pure clay fractions.Figure 4
Effect of temperature on raw (ANb) and purified (ANp) clays.However, an important reduction of adsorption capacities of purified clay with temperature increases was observed (Figure4). In many cases, with increasing temperatures, the adsorption capacities of adsorbents increased by an increase of diffusion rate of MR across external boundary layer and in internal pores of clay minerals. It was also possible that the increase of temperature increased solubility of sorbent that might acquire sufficient energy to undergo an interaction with active sites [58]. Thus, ambient temperature changes could change the adsorbent capacity [44]. However, for Jebel Louka sorbent, temperature increases led to decreases in its adsorption capacity. The same phenomenon was also observed in other previous studies [59–61]. It was considered that the increasing temperature probably produced a swelling effect within the internal structure of absorbent. This structure variation probably had further effects on the penetration of the large MR molecules [62]. This decrease could be also explained by the exothermicity and spontaneity of adsorption process and by the weakening of bonds between dye molecules and active sites of adsorbents for high temperatures (e.g., [63]). In addition, we think that with acidic pH of pure clay fraction temperature had an important effect on the adsorption process.


## 3.3. Adsorption Characterization

To characterize MR adsorption, clay adsorption tests were done under optimal batch conditions which were clay mass, initial dye concentration, contact time, pH, and temperature.


### 3.3.1. DRX Characterization

To highlight the intercalation of MR molecules on clay minerals and to estimate their influence on structure and mineralogy of adsorbate clay, qualitative mineralogical study dye by diffraction X-rays was also done after MR adsorption. Comparison of ANb (AB) clay diffractograms before and after adsorption (AB-MR) showed an increase in reticular distance of phyllosilicate (located at 4.47) and a slight increase in quartz peak characteristic (located at 3.35) (Figure5). These variations were related to an increase in secondary peaks of clay minerals and minerals.Figure 5
XRD spectra of raw clay before (AB) and after (AB + MR) MR adsorption.Diffractograms of purified clay ANp before (AP) and after (AP-MR) adsorption showed some variations such as an increase in the reticular distance of Kaolinite (Figure6). These findings confirmed the intercalation of MR molecules on the surface of clay materials and in the interfoliar space. Therefore, adsorption of MR on clay surface of Jebel Louka was not a physical process, but it was a chemisorption that was produced by exchange between ions of dye and clay of interfoliar layer. Electrostatic forces probably produced this clay-color interaction [64].Figure 6
XRD spectra of purified clay before (AP) and after (AP + MR) adsorption.


### 3.3.2. FTIR Spectra Characterization

The identification of Fourier transform infrared (FTIR) spectra was obtained through John Coates proposition [65]. Group frequencies and functional groups of MR dye were resumed in Table 5. The bands obtained at 3570 cm−1 and 3200 cm−1 indicated the hydroxyl group (O-H stretch) and normal “polymeric” O-H stretch, respectively. The primary, secondary, and tertiary amine stretch were obtained at 3400.22 cm−1, 1606 cm−1, 1374.43 cm−1, and 1210.06 cm−1 respectively. The methyl-amino stretch (N-CH3) was obtained at 2820.13 cm−1 and the open-chain azo (-N = N-) at 1630 cm−1. In addition, the simple C-H stretching vibration and aromatic and alkyne C-H bend were obtained at 2820.34 cm−1, 672 cm−1, and 683 cm−1. The conjugated and C = C stretch vibration were obtained at 1680 cm−1 and 1606 cm−1, respectively. Also, the C-O-C stretch was indicated at 1151 cm−1. The –COOH bending vibrations for carboxylic acid occurred between 1780 and 1714 cm−1.Table 5
Group frequencies (cm−1) and functional groups of the methyl red.
Group frequency (cm−1)Functional group3570–3200Hydroxy group, H-bonded O-H stretch3400–3200Normal “polymeric” O-H stretch3400–3380Primary amino: aliphatic primary amine N-H stretch3000–2800Simple C-H stretching vibrations for saturated aliphatic species2820–2780Methyl-amino, N-CH3, C-H stretch1780–1714Carboxylic acid (–COOH)1630–1575Open-chain azo (-N = N-)1650–1550Secondary amine, NH bend (>N-H)1680–1601Conjugated C = C, C = C stretch vibration1360–1280Aromatic secondary and tertiary amine CN stretch1210–1150Tertiary amino compound1151–1070C-O-C stretch, alkyl-substituted ether900–670Aromatic C-H out of plane bend680–610Alkyne C-H bendAdsorption of azo dye (MR) was controlled also by using the FTIR spectroscopy. The comparison between FTIR spectra of natural clay after adsorption (ANb + MR) indicated a clear difference, which confirmed the adsorption of dye (Figure7). The spectrum of ANb + MR indicated the disappearance of the stretching vibrations between 3500 and 3200 cm−1, between 1800 and 1250 cm−1, and between 692 and 683 cm−1. The removal of those stretching bends confirmed the adsorption process that happened for the MR dye. The bending vibrations between 1110 and 1020 cm−1 displayed the organic siloxane and silicone (Si-O-Si).Figure 7
FTIR spectra of methyl red (MR) dye and after its adsorption by raw clay (MR + ANb).The FTIR spectrum of purified clays after adsorption (ANp + MR) was mentioned in Figure8. The appearance of bending vibrations at 1603, 1375, and 1080 cm−1 indicated the presence of carboxylic acid, aromatic tertiary amine, and Si-O, respectively. These results confirmed the partial adsorption of azo dye.Figure 8
FTIR spectra of methyl red (MR) dye and after its adsorption by purified clay (MR + ANp).The FTIR spectrum results confirmed that minerals and organic matter of natural clay had a more adsorption capacity compared to pure clay fraction.


### 3.3.3. SEM Characterization

SEM micrograph ANb before adsorption showed the clay grains and leaflets lamellar structures and aggregates were heterogeneous (different sizes and shapes and rough appearance of particles) (Figure9(a)). However, after adsorption, the micrograph showed a homogeneous surface indicating that clay surface was covered with the MR-dye molecules. Therefore, we had a disappearance of pores and spaces between clay aggregates (Figure 9(b)).Figure 9
SEM image of raw clay ANb before (a) and after (b) MR-dye uptake.Surface morphology of ANp was determined by SEM (Figures10(a) and 10(b)) and showed the SEM micrographs of ANp sample before and after adsorption of MR dye.Figure 10
SEM image of purified clay ANp before (a) and after (b) MR-dye uptake.The purified material (ANp) had pores and spaces between clay aggregates that were well apparent. Aggregates were very heterogeneous and presented in platelet forms of sticks with irregular contours. Lamellar structure was also observed (Figure10(a)).The scanning view of ANp after adsorption showed homogeneous surface morphology that was covered with MR-dye molecules (Figure10(b)).These results confirmed physisorption model and this dye could be easily liberated from saturated clays during adsorption process.Similar results were observed by Saxena and Sharma [48]. Their SEM figures showed that GG (Guar Gum) was covered with dye molecules and presented a rough surface morphology.


## 3.3.1. DRX Characterization

To highlight the intercalation of MR molecules on clay minerals and to estimate their influence on structure and mineralogy of adsorbate clay, qualitative mineralogical study dye by diffraction X-rays was also done after MR adsorption. Comparison of ANb (AB) clay diffractograms before and after adsorption (AB-MR) showed an increase in reticular distance of phyllosilicate (located at 4.47) and a slight increase in quartz peak characteristic (located at 3.35) (Figure5). These variations were related to an increase in secondary peaks of clay minerals and minerals.Figure 5
XRD spectra of raw clay before (AB) and after (AB + MR) MR adsorption.Diffractograms of purified clay ANp before (AP) and after (AP-MR) adsorption showed some variations such as an increase in the reticular distance of Kaolinite (Figure6). These findings confirmed the intercalation of MR molecules on the surface of clay materials and in the interfoliar space. Therefore, adsorption of MR on clay surface of Jebel Louka was not a physical process, but it was a chemisorption that was produced by exchange between ions of dye and clay of interfoliar layer. Electrostatic forces probably produced this clay-color interaction [64].Figure 6
XRD spectra of purified clay before (AP) and after (AP + MR) adsorption.


## 3.3.2. FTIR Spectra Characterization

The identification of Fourier transform infrared (FTIR) spectra was obtained through John Coates proposition [65]. Group frequencies and functional groups of MR dye were resumed in Table 5. The bands obtained at 3570 cm−1 and 3200 cm−1 indicated the hydroxyl group (O-H stretch) and normal “polymeric” O-H stretch, respectively. The primary, secondary, and tertiary amine stretch were obtained at 3400.22 cm−1, 1606 cm−1, 1374.43 cm−1, and 1210.06 cm−1 respectively. The methyl-amino stretch (N-CH3) was obtained at 2820.13 cm−1 and the open-chain azo (-N = N-) at 1630 cm−1. In addition, the simple C-H stretching vibration and aromatic and alkyne C-H bend were obtained at 2820.34 cm−1, 672 cm−1, and 683 cm−1. The conjugated and C = C stretch vibration were obtained at 1680 cm−1 and 1606 cm−1, respectively. Also, the C-O-C stretch was indicated at 1151 cm−1. The –COOH bending vibrations for carboxylic acid occurred between 1780 and 1714 cm−1.Table 5
Group frequencies (cm−1) and functional groups of the methyl red.
Group frequency (cm−1)Functional group3570–3200Hydroxy group, H-bonded O-H stretch3400–3200Normal “polymeric” O-H stretch3400–3380Primary amino: aliphatic primary amine N-H stretch3000–2800Simple C-H stretching vibrations for saturated aliphatic species2820–2780Methyl-amino, N-CH3, C-H stretch1780–1714Carboxylic acid (–COOH)1630–1575Open-chain azo (-N = N-)1650–1550Secondary amine, NH bend (>N-H)1680–1601Conjugated C = C, C = C stretch vibration1360–1280Aromatic secondary and tertiary amine CN stretch1210–1150Tertiary amino compound1151–1070C-O-C stretch, alkyl-substituted ether900–670Aromatic C-H out of plane bend680–610Alkyne C-H bendAdsorption of azo dye (MR) was controlled also by using the FTIR spectroscopy. The comparison between FTIR spectra of natural clay after adsorption (ANb + MR) indicated a clear difference, which confirmed the adsorption of dye (Figure7). The spectrum of ANb + MR indicated the disappearance of the stretching vibrations between 3500 and 3200 cm−1, between 1800 and 1250 cm−1, and between 692 and 683 cm−1. The removal of those stretching bends confirmed the adsorption process that happened for the MR dye. The bending vibrations between 1110 and 1020 cm−1 displayed the organic siloxane and silicone (Si-O-Si).Figure 7
FTIR spectra of methyl red (MR) dye and after its adsorption by raw clay (MR + ANb).The FTIR spectrum of purified clays after adsorption (ANp + MR) was mentioned in Figure8. The appearance of bending vibrations at 1603, 1375, and 1080 cm−1 indicated the presence of carboxylic acid, aromatic tertiary amine, and Si-O, respectively. These results confirmed the partial adsorption of azo dye.Figure 8
FTIR spectra of methyl red (MR) dye and after its adsorption by purified clay (MR + ANp).The FTIR spectrum results confirmed that minerals and organic matter of natural clay had a more adsorption capacity compared to pure clay fraction.


## 3.3.3. SEM Characterization

SEM micrograph ANb before adsorption showed the clay grains and leaflets lamellar structures and aggregates were heterogeneous (different sizes and shapes and rough appearance of particles) (Figure9(a)). However, after adsorption, the micrograph showed a homogeneous surface indicating that clay surface was covered with the MR-dye molecules. Therefore, we had a disappearance of pores and spaces between clay aggregates (Figure 9(b)).Figure 9
SEM image of raw clay ANb before (a) and after (b) MR-dye uptake.Surface morphology of ANp was determined by SEM (Figures10(a) and 10(b)) and showed the SEM micrographs of ANp sample before and after adsorption of MR dye.Figure 10
SEM image of purified clay ANp before (a) and after (b) MR-dye uptake.The purified material (ANp) had pores and spaces between clay aggregates that were well apparent. Aggregates were very heterogeneous and presented in platelet forms of sticks with irregular contours. Lamellar structure was also observed (Figure10(a)).The scanning view of ANp after adsorption showed homogeneous surface morphology that was covered with MR-dye molecules (Figure10(b)).These results confirmed physisorption model and this dye could be easily liberated from saturated clays during adsorption process.Similar results were observed by Saxena and Sharma [48]. Their SEM figures showed that GG (Guar Gum) was covered with dye molecules and presented a rough surface morphology.


## 3.4. Adsorption Isotherms

Adsorption isotherms that describe adsorbed MR (qe) onto tested clay at equilibrium condition (Ce: concentration of solute in mgL−1at equilibrium), constant temperature, and pH [66] were proposed. Several equilibrium adsorption isotherm equations were available. Langmuir, Freundlich, and Dubinin-Radushkevich models (D-R) were applied to understand the mechanism of MR adsorption by tested clays.Generally, adsorption of a solute is considered as monolayer adsorption; that is, the adsorption layer is one molecule in thickness with no migration of the adsorbate in the surface of adsorbent [65]. Langmuir isotherms model assumes the existence of a monolayer adsorption with homogeneity of the surface [43, 66]. The linear form is expressed in the following equation:(2)Ceqe=1qmKL+Ceqm,where qe (mgg−1) is the adsorbed amount of the solute at equilibrium, Ce (mgL−1) is the concentration of the solute at equilibrium, qm (mgg−1) is the maximum adsorption capacity, and KL (Lmg−1) is the ratio of adsorption and desorption rate constant or Langmuir parameter. Then, by plotting fCe=Ce/qe, the value of the maximum capacity of the monolayer is determined.It is also proposed that the solute-solid is strong enough to allow solvent-solid interactions in monolayer and there is no similarity in the following layers [67].Freundlich isotherm model assumes that adsorption occurs on a heterogeneous surface with interaction between adsorbed molecules [68, 69]. At equilibrium, this model is an empirical equation expressed as follows:(3)qe=Ce1/nKf,where Kf (Lg−1) is the ratio of adsorption capacity of the adsorbent and n is the heterogeneity factor (Lg−1).Meanwhile the linear form is given by the following equation:(4)Lnqe=LnKf+1nLnCe.This model suggests that adsorption energy is not constant but exponentially decreases upon the completion of adsorption process.Adsorption equilibrium data were fitted to Freundlich (b, e) and Langmuir (a, d) isotherm models for ANb and ANp (Figure11). Two model parameters that were obtained for the adsorption of the methyl red onto natural and purified clays were shown in Table 6.Figure 11
Isotherm models for MR-dye adsorption onto raw (ANb) and purified (ANp) clays. (a) Langmuir, (b) Freundlich, and (c) D-R for ANb; (d) Langmuir, (e) Freundlich, and (f) D-R for ANp.
(a)(b)(c)(d)(e)(f)Table 6
Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) parameters.
LangmuirFreundlichD-RANbQ0 (mgg−1)59.180KF (Lg−1)3.480V’m (molg−1)6.529KL (Lmg−1)0.0071/n2.125K’ (mol2kJ−2)−0.321R20.788N0.471E (KJmol−1)1.248RL0.999R20.962ANpQ0 (mgg−1)−86.207KF (Lg−1)1.809V’m (molg−1)1.954KL (Lmg−1)−0.0031/n1.973K’ (mol2kJ−2)−0.111R20.344n0.507E (KJmol−1)2.119RL1.000R20.754At first, separation factorRL indicated a linear adsorption (RL = 1) for natural (ANb) and purified (ANp) clays. The intensity parameter 1/n > 1 showed that adsorption was defavorable because adsorption bonds became weak. Adsorption capacity decreases confirmed the linear adsorption process. Therefore, the best fitting results were obtained with Langmuir isotherm model. Consequently, monolayer adsorption capacity was in accordance with the experimental result for the two adsorbents (ANb and ANp). Similar results were obtained in obvious studies (e.g., [54]).Dubinin–Radushkevich (D–R) isotherm determines the energy balance of the adsorption process [70, 71]. Based on this model, E is the energy quantity necessary to remove one molecule from the adsorbent to the infinity and the adsorption processes are classified as a physical adsorption if E < 8 kJmol−1 and chemical adsorption if 8 < E < 168 kJmol−1. The model has often successfully fitted high solute activities and the intermediate range of concentrations data. D-R isotherm is expressed by the following equation:(5)Lnqe=LnVm′−K′ε2,where qe is the amount of dye which is adsorbed at equilibrium time (mgg−1) by solid matrix, V′m is the maximum adsorption capacity (molg−1), K’ is the activity coefficient related to the adsorption energy, and ɛ is the Polanyi potential, which is equal to(6)ε=RTLn1+1Ce.R is the constant of the gases (JK−1mol−1) and T is the temperature (K).Adsorption energy E (kJmol−1) is expressed by the following equation:(7)E=−2K′−0.5,E=12β,it gives information on physical and chemical characteristics of adsorption.Adsorption equilibrium data were fitted to the Dubinin–Radushkevich (c, f) isotherm models for ANb and ANp. The linear curve for the D-R isotherm obtained by tracingε2 as a function of Ln qe was presented with the experimental data in Figure 11. Average adsorption energy calculated from D-R isotherm confirmed that the physisorption was the dominate mechanism (E < 8 kJmol−1) for two adsorbents (Table 5).


## 3.5. Kinetic Models

The kinetics of adsorption define the efficiency of an adsorbent. Adsorption kinetics of MR onto natural and purified were investigated using different MR concentrations (30–1000 mgL−1). When the solution had 25°C (±2°C) and acid pH 3, the optimal quantity of adsorbed dye was obtained. Several models were used to fit experiment data for adsorption kinetics identification. MR adsorption dynamics processes were studied by Pseudo-first-order and pseudo-second-order models. These two models describe the solute uptake rate at solid-solution interface [48] and control the resident time of adsorbate and possibilities of desorption [41, 54]. The linear form of pseudo-first-order model was explained by Lagergren equation (8):(8)logqe-qt=logqe-k1t2.303,where qe and qt are the amounts of dye adsorbed (mgg−1) on the adsorbents at the equilibrium and at time t.k1 is the rate constant of adsorption (min−1). The values of k1 were calculated by plotting log (qe − qt) versus time t (min).Linear form of pseudo-second-order model is expressed by the following equation:(9)tqt=1k2qe2+1qet,where k2 is the rate constant of pseudo-second-order model (gmg−1min−1). Values of k2 and qe were calculated from intercepting a linear plot of t/qt versus t.Linear form of the two models and their calculated parameters for both adsorbents were listed in Figures12 and 13. Initially, removal rate of MR was rapid and then it was gradually slowed down until equilibrium state beyond which there was no significant increase. The maximum MR adsorption onto ANb and ANp (equilibrium state) was observed at 10 min. The kinetic experimental parameters of pseudo-first-order and pseudo-second-order models (qe and qt) and the rate constant of pseudo-first-order model (k1 and R2) and pseudo-second-order model (k2 and R2) were determined. The slopes and intercepts of plots of log (qe – qt) versus t (0–80 min) were used to determine the first-order rate constant k1 and equilibrium adsorption density qe for ANb and ANp. k1 ranges were from 0 to 0.001 for ANb and from −0.001 to 0.001 for ANp and their correlation coefficients (R2) ranges were 0.04–0.27 for ANb and 0.09–0.88 for ANp (Figure 12).Figure 12
Pseudo-first-order adsorption kinetics of MR onto ANb (a) and ANp (b).
(a)(b)Figure 13
Pseudo-second-order adsorption kinetics of MR onto ANb (a) and ANp (b).
(a)(b)However, for the linear plots of t/qt versus t for the pseudo-second-order model, the k2 ranges were from about 0.4 to 105 for ANb and from about −103 to 34.104 for ANp, and R2 values were from 0.95 to 1 for ANb and 1 for ANp (Figure 13). These experimental and calculated values showed that the adsorption of MR onto Jebel Louka clay followed the pseudo-second-order model.


## 3.6. Thermodynamic Study

Adsorption is a phenomenon that can be endothermic or exothermic. It depends on the adsorbent material and the nature of the adsorbed molecules [72, 73]. Thermodynamic parameters of equilibrium adsorption reaction are standard entropy ΔS°, standard enthalpy ΔH°, and standard free enthalpy ΔG° [69]. These parameters were determined using the following equations:(10)kd=QeCel g−1,(11)Lnkd=ΔS°R−ΔH°RT,(12)ΔG°=−RTLnkd.Calculated values of thermodynamic parameters of dye adsorption onto natural and purified clays (ANb and ANp) were given in Table7. The obtained values of ΔH° at 20, 30, and 40°C for ANb and ANp indicated that adsorption of MR onto Jebel Louka clay was an endothermic process. The positive ΔS° value was for all cases about 0.07 kJmol−1K−1. Hence, the low positive values of entropy variations showed that there was a random interference at solid-liquid interface during the fixation of MR on the active sites of adsorbent. The negative or nil values of Gibbs free energy (ΔG°) at the three tested temperatures indicated the feasibility and the spontaneity of adsorption process without an induction period.Table 7
Thermodynamic parameters of MR adsorption on ANb and ANp.
AdsorbentsTemperature T(°K)Thermodynamic parametersΔG° (KJmol−1)ΔH° (KJmol−1)ΔS° (KJmol−1K−1)ANb293.15020.200.0690303.15−6326.2720.860.0688313.15021.550.0688ANp293.15−40.4020.200.0689303.15−41.7300.0688313.15000.0688


## 3.7. Efficiency of Jebel Louka Clay

Several methods, processes, and components were used to remove MR (Table8). The principal objective was to define an adsorbent or a process which had the highest capacity of methyl red (MR) elimination. Chemical components, fibers of bananas, pretreated sugarcane bagasse, activated carbon, modified zeolite, and isolate or consortium bacteria were used as adsorbent or degrading tool. The best results were given by iron oxide modified MIL-100 (Fe) (62.5 %), but a long time was necessary for finishing the process and biodegradation by one consortium of three bacteria (about 99%).Table 8
Yields of methyl red removed using different methods, processes, and materials.
MethodsMaterials (adsorbents)Removal yields (%)ReferencesChemical componentsSterchamol0.022[20]SiO2-coated Fe3O4 magnetic nanoparticles4.95[21]Iron oxide modified MIL-100 (Fe)62.5[74]Biodegradation (or biodecolorization)Staphylococcus arlettae PF4 isolated from garden soil2.5[18]Saccharomyces cerevisiae ATCC 976399.1[19]Consortium of bacteria (mixture of bacteria): Three bacteria (Sphingomonas paucimobilis, Bacillus sp., and Staphylococcus epidermidis)98.0[11]Hazardous materials (biocomponents)Banana pseudostem fibers8.85[75]Sugarcane bagasse, pretreated with phosphoric acid (SBC)1.10[76]Untreated sugarcane bagasse (SB)0.57[76]NBP: neem tree bark powder/MBP: mango tree bark powder/LBP : locust bean tree bark powderMBP: 12%NBP: 8.5%LBP: 8%[77]Nano-sized calcium hydroxide catalyst prepared from clam shells0.2[78]Guar Gum powder5.76[47]Activated carbonCommercially available powdered activated carbon (PAC)4.84[76]Annona squamosa seeds activated carbon4.05[79]Oxidized multiwalled carbon nanotubes10.87[22]Mesoporous-activated carbon from durian seed38.47[80]Zeolite commercialModified zeolite commercial activated charcoal3.0[81]Modified zeolite commercial activated charcoal0.7[81]The yields of the azo dye MR removal with tested natural (ANb) and purified clays (ANp) were 89.5% and 92.2% when the initial concentrations of MR were 30 and 50 mgL−1, respectively. In these cases, biodegradation of MR was more effective (Figure 14). When initial MR concentrations in solutions were equal to or higher than 750 mgL−1, removal yields were at least equal to bacterial biodegradation yields. Because of low cost and facility of Jebel Louka natural clay use compared with bacterial consortium biodegradation, natural clay is better recommended for MR removal from textile wastewater.Figure 14
Cross between color removal (%) of MR onto raw (ANb) and purified (ANp) clays and the bacterial treatment.We note that adsorption yields of chemically activated clays with HCl and H2SO4 were between 98.1% for AN4 and 99.4% for ANp (Figure 15). Hence, chemical activation and raw material (99.7%) gave similar results. However, thermal activation decreased adsorption capacity of treated clay (78.2%). This decline was probably due to destruction of some adsorption active sites. However, for Lu et al. [82, 83], thermal and chemical activations improved the adsorption capacity of the material.Figure 15
Influence of initial concentration dye and adsorbent dose onto different samples. AN2: activated clay with 0.5 N HCl; AN3: activated clay with 3N HCl; AN4: activated clay with 2N H2SO4; AN5: thermally activated clay (500°C).


## 3.8. Desorption Studies

Desorption analyses had allowed seeing the possibility of the reuse of adsorbent or making the process more economic [84]. MR adsorbed onto natural Jebel Louka clay could be desorbed by water when the dye adsorption was by weak bonds (ion exchange or electrostatic attraction). In this case, if the clay is rejected in controlled or noncontrolled landfill the released toxic molecules could infiltrate into natural resources and have dangerous environmental impact. Therefore, it was considered that it is very important to estimate the possible desorbed amounts of MR.Desorption tests were done using adsorbents at different pH (3, 5, 7, 9, and 11) and ambient temperature (±25°C) in 100 mL solution of double-distilled water. At pH 3, 5, and 11, the dye amount desorbed from natural clay was more important compared to the purified clay. At pH 7 and 9, the amount of MR released by purified clay was the most important (Figure16). These obtained experimental results showed that the MR was easily desorbed and this pollutant could be transferred and potentially contaminated the natural resources (soil and water). Thus, rejection of saturated clays with MR can cause environmental problems. To limit the danger, the pH of rejected clay must be about 7 for natural clays and 11 for purified clay. In any case, we must use this saturated clay for brick making or take some measures for the isolation of this pollution source.Figure 16
Desorbed yields of MR dye from raw (ANb) and purified (ANp) clays.


## 3.9. Comment

The used kaolinite rich clay (called Tabarka clay (TBK)) and standard kaolinite (KGa-2: the Source Clay Repository of the Clay Minerals Society) gave similar results [85]. Abidi et al. [85] advanced that an increase in the amount of adsorbed dye (qe) occurred when the concentration of RR120 dye increased from 10 to 120 mg L−1. The higher adsorption amount was at pH 3. Therefore, pH always played an important role in dye adsorption processes. This phenomenon was also observed when RR120 dye was adsorbed onto raw clay [86], Brilliant Green dye onto red clay [87], and acid dye onto various modified clays [88]. It was also remarked that adsorption capacity decreased with temperature increase. This effect could be explained by weakening of the adsorptive forces between the active sites of the clays and the dye molecules [85].In order to investigate the adsorption mechanism and the nature of interaction forces between adsorbate molecules and adsorbent surfaces, Langmuir and Freundlich isotherms models were considered by all authors. They obtained also similar results. For example, Abidi et al. [85] indicated that Langmuir isotherm fitted well with their experimental data and the most suitable model to describe the experimental data was also the second-order model.It was also observed that the desorbed dye amount from the clays was high. For example, Abidi et al. [28] showed that the desorbed dye amount from KGa was 16 % and that from Tabarka clays was 18 %. These results were comparable to the data of this work.


## 4. Conclusion

The main conclusions were as follows: (1) the optimal adsorbed quantity of the methyl red (MR) was obtained by the use of 0.25 g and o.5 g of raw clays and purified clays, respectively; (2) the pollutant removal increased with a decrease in pH for purified clay and approximately independent of temperature for raw clay; (3) the sorption isotherm showed that physisorption type and Langmuir model (good correlation coefficientR2 > 0.96) were appropriate to the tested process; (4) the adsorption process was relatively rapid (about 5 min) and well described by pseudo-second-order model; (5) the adsorption was endothermic and spontaneous; (6) FTIR identified that the sites of surface of adsorbent were active sites; (7) the organic matter and carbonate fractions played an important role in MR elimination; (8) raw material had better performance than the purified fraction; (9) the dye desorption confirmed the physisorption was dominant and the chemisorption was secondary produced by exchange between ions of dye and clay; (10) SEM of saturated ANb and ANp clays confirmed the disappearance of pores and spaces between clay aggregates and MR dye covered the clay surfaces.Compared with other technologies, processes, and materials, efficiency of natural Jebel Louka clays in the methyl red removal could provide an alternative low-cost material for removal of methyl red from the textile wastewater. The presented adsorption and desorption model provided a background for pilot and industrial scale applications.



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