# Characterization and Analysis ofArgania spinosa Shells from Souss-Massa Area: Application in the Adsorption of Methylene Blue in Aqueous Solution

Authors: Ousama Ifguis; Rachid Bouhdadi; Younes Ziat; Béatrice George; Mohamed Mbarki
Journal: Journal of Nanomaterials
 (2022)

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## Abstract

This study focused on the adsorption in aqueous solution of methylene blue (MB), a cationic dye used in the textile industry, by crushed Argan shells (used as raw materials) from the southern region of Morocco. These crushed Argan shells were first characterized by different technical analyses: elemental analysis, biochemical analysis, IR spectroscopy, thermogravimetric analysis, scanning electron microscopy, mercury porosimetry, and zetametry. The adsorption kinetics was then studied by varying the experimental parameters such as the dye concentration in solution. We note that atpH=8, there is an elimination up to 232.6 mg/g, during an equilibrium time of 180 min. The correlation coefficient showed that the pseudosecond-order kinetic equation best describes the adsorption kinetics for the tested material. The adsorption isotherms of MB by Argan shells are suitably described by the Langmuir model which provide the best theoretical correlation of the experimental data. The thermodynamics of MB adsorption on Argan shells indicates a spontaneous and endothermic process. It was concluded that an increase in temperature leads to a greater adsorption of MB by the studied biosorbent.


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## Body

## 1. Introduction

Nowadays, water treatment is a major concern in the context of environmental protection [1]. Industrial wastewater is characterized by the presence of significant amounts of organic and inorganic pollutants, including dyes such as methylene blue [1, 2]. When they are present in river water, the consequences are multiple and harmful: reduction of light penetration due to the coloration of the environment and negative effect on photosynthesis, and these discharges are very difficult to eliminate because of their complex molecular structure [2, 3] and can have dramatic effects on fauna and flora.The treatment of polluted effluents is of main environmental concern. Whatever the field of application, more constraining laws about the rejected effluents and particularly those contaminated by dyes require the implementation of efficient treatments enabling the recycling of some products and the reuse of purified water mainly in arid countries and desert regions [4].Among these treatments, we can find those based on flocculation-decantation, ionic exchange, electrolysis, separation on membranes, biological treatments, degradation and adsorption, etc. [5–10].Nevertheless, the adsorption onto low-cost adsorbents including waste from agriculture or industry and natural materials appears as one of the most interesting process because of its low cost and ease of implementation. In this context, a lot of studies have been carried out, dealing with, for example, biosorption of heavy metals by olive pomace [11], biosorption of cadmium by bagasse of sugar cane [12], use of cellulosic waste as wheat shells for eliminating methylene blue [13, 14], and acylation of cellulose for lead II adsorption [4].This present work was aimed at studying the adsorption capacity of Argan shells for removal of methylene blue from aqueous solutions. This dye—in which toxicity increases as the duration of exposure increases—is frequently chosen as a model compound in order to evaluate the removal capacity of the substrates. After the characterization of adsorbent by different techniques, we investigated adsorption kinetics by varying contact time and MB concentrations. The thermodynamic parameters of these adsorption phenomena are also determined to consider further developments.


## 2. Materials and Methods

### 2.1. Materials

#### 2.1.1. Adsorbent (Argan Shells)

Argan shells used in this study as biosorbents are cheap and highly available lignocellulosic wastes in North Africa. They come more particularly from Souss-Massa-Draâ in the south of Morocco. These wastes are obtained after fruit pulping and crushing to extract pits. Generally, they are not recycled but just degraded by combustion [15].The dried shells were crushed with a ring grinder (10 s, 1100 rpm) and manually sieved to keep only the size range below 6 mm. The powdered materials were thoroughly washed with distilled water to remove adhering dirt and then dried at85±5°C in a stove for 24 hours.Figures1 illustrate the different stages of Argan shell preparation.Figure 1
The different stages of preparation of Argan shells.


#### 2.1.2. Cationic Dye (Methylene Blue)

The methylene blue (or methylthioninium chloride) whose chemical structure is given in Figure2 is used. The dye is soluble in water and more slightly in alcohol. The methylene blue used in this work was purchased from Janssen Chimica and used without further purification. It was chosen for this study because of its known strong adsorption to solids [16, 17].Figure 2
Chemical structure of methylene blue (MB).


### 2.2. Characterization of the Supports

#### 2.2.1. Elemental Analysis

Final analyses were performed in duplicate on a Thermo Finnigan Flash EA 1112 elemental analyzer equipped with a sample changer and a Porapak chromatographic column. The procedure of this analysis was carried out according to the treatment developed in our previous work [9].


#### 2.2.2. Biochemical Analysis

The biochemical analysis of Argan shells was aimed at determining the quantities: lignin, holocellulose, and extractives found in Argan shells. The determination of the quantities of lignin and holocellulose Klason was made according to the protocols described in the literature [18] and already carried out in our previous work [9]. The determination of the extractives content was done by refluxing a mass of about 2-3 g of ground Argan shells (size less than 6 mm) in a Soxhlet extractor with 350 mL of a 95% (2/1 v/v) toluene/ethanol solution for 24 hours. Once the solvents were evaporated under vacuum, the percentage of total extractables was deduced from the initial mass mi and final mass mf of the dried sample, after drying the samples at 105°C for 48 hours according to
(1)Extractives%=mi−mfmi×100,wheremi is the initial mass of the dried sample and mf is the final mass of the dried sample.


#### 2.2.3. Infrared Spectroscopy

For the identification of the different chemical functions of the molecules present in the crushed Argan shells, the latter was analyzed in ATR mode using a spectrophotometer PERKIN ELMER Spectrum 2000 from 400 to 4000 cm-1 (16 scans).


#### 2.2.4. Thermogravimetric Analysis

In this study, the apparatus used is the SETARAM Setsys TG 12 thermobalance where a sample of Argan shells (about 80 mg) was introduced. The sample is subjected to a program of temperature increase (5°C min-1) from room temperature to 700°C under a flow of nitrogen (80 mL.min-1) [9–19].


#### 2.2.5. Scanning Electron Microscopy

The Hitachi FEG S 4800 high-resolution electron microscope (resolution of 1 nm and the acceleration voltage was 15 kV) was used in order to obtain SEM images of the ground Argan shells. Before analysis, the dry samples of the ground Argan shells are covered with a layer of carbon to increase their conductivity.


#### 2.2.6. Mercury Porosimetry

Mercury porosimetry was performed on Pascal 140 (Hgpressures<400kPa) and Pascal 240 (100kPa<Hgpressures<400MPa) mercury porosimeters from CE Instruments (England). A mass of crushed Argan shells of about (300-350 mg) was introduced into the measuring cell and degassed under vacuum to reach a residual pressure of 0.01 kPa. Then, the measuring cell was filled with mercury. Information on the particle size, bulk density, and specific surface area of the sorbents assuming a cylindrical pore shape is given by the curve giving the intrusion volume of mercury as a function of the applied pressure.


#### 2.2.7. Determination of pH at Zero Charge Point and Measurement of Zeta Potential

To measurepHPZC (pH at the point of zero charge), the method described by Altenor et al. [20] was followed. It consists in placing crushed Argan shells (0.1 g) in 100 mL of a KCl solution at 10-2 mol.L-1 at various pH values in a pH range from 2 to 12 (solution acidified or basified by hydrochloric acid solution or sodium hydroxide at 0.1 mol.L-1). The final pH (at equilibrium) is measured after 2 days using a pH meter PHM 210 (MeterLab).The zeta potential is a valuable characteristic to understand hydrophilic or hydrophobic properties of materials. The measurements were done using a zetameter Surpass from Anton PAAR. In a first experiment, the zeta potential was measured for Argan shells (0.5 g) in contact of different KCl solutions, acidic, basic, or neutral, at 10-3 mol.L-1. In a second one, the measurement was carried out in a neutral solution as a function of time.


#### 2.2.8. Adsorption Procedure

The study of the adsorption of MB on Argan shells was performed by mixing a mass of 100 mg of adsorbent in a volume of 100 mL of MB. The working solutions were prepared by dilution from a stock solution of MB (1 g.L-1). The concentrations of the solutions are between 50 and 500 mg.L-1. This mixture was stirred (250 rpm) at room temperature (25±1°C) for the time necessary to reach equilibrium. After separation of the liquid phase and the adsorbent, the residual concentration of the filtrates was monitored by spectrophotometry in the visible range using a UV-visible spectrophotometer (UV-2550 Shimadzu). The maximum adsorption wavelength of MB is 664 nm.The equation of the calibration curve is as follows:Absorbance664nm=0.2102MBwithMBinmg.L−1.The amount adsorbed is calculated using [9]
(2)Qt=V×C0−Ct1000×m,where Qt is the MB uptake quantity (mg.g-1), V is the sample volume (mL), C0 is the initial MB concentration (mg.L-1), Ct is the residual MB concentration (mg.L-1), and m is the dry weight of the biosorbent (g).


## 2.1. Materials

### 2.1.1. Adsorbent (Argan Shells)

Argan shells used in this study as biosorbents are cheap and highly available lignocellulosic wastes in North Africa. They come more particularly from Souss-Massa-Draâ in the south of Morocco. These wastes are obtained after fruit pulping and crushing to extract pits. Generally, they are not recycled but just degraded by combustion [15].The dried shells were crushed with a ring grinder (10 s, 1100 rpm) and manually sieved to keep only the size range below 6 mm. The powdered materials were thoroughly washed with distilled water to remove adhering dirt and then dried at85±5°C in a stove for 24 hours.Figures1 illustrate the different stages of Argan shell preparation.Figure 1
The different stages of preparation of Argan shells.


### 2.1.2. Cationic Dye (Methylene Blue)

The methylene blue (or methylthioninium chloride) whose chemical structure is given in Figure2 is used. The dye is soluble in water and more slightly in alcohol. The methylene blue used in this work was purchased from Janssen Chimica and used without further purification. It was chosen for this study because of its known strong adsorption to solids [16, 17].Figure 2
Chemical structure of methylene blue (MB).


## 2.1.1. Adsorbent (Argan Shells)

Argan shells used in this study as biosorbents are cheap and highly available lignocellulosic wastes in North Africa. They come more particularly from Souss-Massa-Draâ in the south of Morocco. These wastes are obtained after fruit pulping and crushing to extract pits. Generally, they are not recycled but just degraded by combustion [15].The dried shells were crushed with a ring grinder (10 s, 1100 rpm) and manually sieved to keep only the size range below 6 mm. The powdered materials were thoroughly washed with distilled water to remove adhering dirt and then dried at85±5°C in a stove for 24 hours.Figures1 illustrate the different stages of Argan shell preparation.Figure 1
The different stages of preparation of Argan shells.


## 2.1.2. Cationic Dye (Methylene Blue)

The methylene blue (or methylthioninium chloride) whose chemical structure is given in Figure2 is used. The dye is soluble in water and more slightly in alcohol. The methylene blue used in this work was purchased from Janssen Chimica and used without further purification. It was chosen for this study because of its known strong adsorption to solids [16, 17].Figure 2
Chemical structure of methylene blue (MB).


## 2.2. Characterization of the Supports

### 2.2.1. Elemental Analysis

Final analyses were performed in duplicate on a Thermo Finnigan Flash EA 1112 elemental analyzer equipped with a sample changer and a Porapak chromatographic column. The procedure of this analysis was carried out according to the treatment developed in our previous work [9].


### 2.2.2. Biochemical Analysis

The biochemical analysis of Argan shells was aimed at determining the quantities: lignin, holocellulose, and extractives found in Argan shells. The determination of the quantities of lignin and holocellulose Klason was made according to the protocols described in the literature [18] and already carried out in our previous work [9]. The determination of the extractives content was done by refluxing a mass of about 2-3 g of ground Argan shells (size less than 6 mm) in a Soxhlet extractor with 350 mL of a 95% (2/1 v/v) toluene/ethanol solution for 24 hours. Once the solvents were evaporated under vacuum, the percentage of total extractables was deduced from the initial mass mi and final mass mf of the dried sample, after drying the samples at 105°C for 48 hours according to
(1)Extractives%=mi−mfmi×100,wheremi is the initial mass of the dried sample and mf is the final mass of the dried sample.


### 2.2.3. Infrared Spectroscopy

For the identification of the different chemical functions of the molecules present in the crushed Argan shells, the latter was analyzed in ATR mode using a spectrophotometer PERKIN ELMER Spectrum 2000 from 400 to 4000 cm-1 (16 scans).


### 2.2.4. Thermogravimetric Analysis

In this study, the apparatus used is the SETARAM Setsys TG 12 thermobalance where a sample of Argan shells (about 80 mg) was introduced. The sample is subjected to a program of temperature increase (5°C min-1) from room temperature to 700°C under a flow of nitrogen (80 mL.min-1) [9–19].


### 2.2.5. Scanning Electron Microscopy

The Hitachi FEG S 4800 high-resolution electron microscope (resolution of 1 nm and the acceleration voltage was 15 kV) was used in order to obtain SEM images of the ground Argan shells. Before analysis, the dry samples of the ground Argan shells are covered with a layer of carbon to increase their conductivity.


### 2.2.6. Mercury Porosimetry

Mercury porosimetry was performed on Pascal 140 (Hgpressures<400kPa) and Pascal 240 (100kPa<Hgpressures<400MPa) mercury porosimeters from CE Instruments (England). A mass of crushed Argan shells of about (300-350 mg) was introduced into the measuring cell and degassed under vacuum to reach a residual pressure of 0.01 kPa. Then, the measuring cell was filled with mercury. Information on the particle size, bulk density, and specific surface area of the sorbents assuming a cylindrical pore shape is given by the curve giving the intrusion volume of mercury as a function of the applied pressure.


### 2.2.7. Determination of pH at Zero Charge Point and Measurement of Zeta Potential

To measurepHPZC (pH at the point of zero charge), the method described by Altenor et al. [20] was followed. It consists in placing crushed Argan shells (0.1 g) in 100 mL of a KCl solution at 10-2 mol.L-1 at various pH values in a pH range from 2 to 12 (solution acidified or basified by hydrochloric acid solution or sodium hydroxide at 0.1 mol.L-1). The final pH (at equilibrium) is measured after 2 days using a pH meter PHM 210 (MeterLab).The zeta potential is a valuable characteristic to understand hydrophilic or hydrophobic properties of materials. The measurements were done using a zetameter Surpass from Anton PAAR. In a first experiment, the zeta potential was measured for Argan shells (0.5 g) in contact of different KCl solutions, acidic, basic, or neutral, at 10-3 mol.L-1. In a second one, the measurement was carried out in a neutral solution as a function of time.


### 2.2.8. Adsorption Procedure

The study of the adsorption of MB on Argan shells was performed by mixing a mass of 100 mg of adsorbent in a volume of 100 mL of MB. The working solutions were prepared by dilution from a stock solution of MB (1 g.L-1). The concentrations of the solutions are between 50 and 500 mg.L-1. This mixture was stirred (250 rpm) at room temperature (25±1°C) for the time necessary to reach equilibrium. After separation of the liquid phase and the adsorbent, the residual concentration of the filtrates was monitored by spectrophotometry in the visible range using a UV-visible spectrophotometer (UV-2550 Shimadzu). The maximum adsorption wavelength of MB is 664 nm.The equation of the calibration curve is as follows:Absorbance664nm=0.2102MBwithMBinmg.L−1.The amount adsorbed is calculated using [9]
(2)Qt=V×C0−Ct1000×m,where Qt is the MB uptake quantity (mg.g-1), V is the sample volume (mL), C0 is the initial MB concentration (mg.L-1), Ct is the residual MB concentration (mg.L-1), and m is the dry weight of the biosorbent (g).


## 2.2.1. Elemental Analysis

Final analyses were performed in duplicate on a Thermo Finnigan Flash EA 1112 elemental analyzer equipped with a sample changer and a Porapak chromatographic column. The procedure of this analysis was carried out according to the treatment developed in our previous work [9].


## 2.2.2. Biochemical Analysis

The biochemical analysis of Argan shells was aimed at determining the quantities: lignin, holocellulose, and extractives found in Argan shells. The determination of the quantities of lignin and holocellulose Klason was made according to the protocols described in the literature [18] and already carried out in our previous work [9]. The determination of the extractives content was done by refluxing a mass of about 2-3 g of ground Argan shells (size less than 6 mm) in a Soxhlet extractor with 350 mL of a 95% (2/1 v/v) toluene/ethanol solution for 24 hours. Once the solvents were evaporated under vacuum, the percentage of total extractables was deduced from the initial mass mi and final mass mf of the dried sample, after drying the samples at 105°C for 48 hours according to
(1)Extractives%=mi−mfmi×100,wheremi is the initial mass of the dried sample and mf is the final mass of the dried sample.


## 2.2.3. Infrared Spectroscopy

For the identification of the different chemical functions of the molecules present in the crushed Argan shells, the latter was analyzed in ATR mode using a spectrophotometer PERKIN ELMER Spectrum 2000 from 400 to 4000 cm-1 (16 scans).


## 2.2.4. Thermogravimetric Analysis

In this study, the apparatus used is the SETARAM Setsys TG 12 thermobalance where a sample of Argan shells (about 80 mg) was introduced. The sample is subjected to a program of temperature increase (5°C min-1) from room temperature to 700°C under a flow of nitrogen (80 mL.min-1) [9–19].


## 2.2.5. Scanning Electron Microscopy

The Hitachi FEG S 4800 high-resolution electron microscope (resolution of 1 nm and the acceleration voltage was 15 kV) was used in order to obtain SEM images of the ground Argan shells. Before analysis, the dry samples of the ground Argan shells are covered with a layer of carbon to increase their conductivity.


## 2.2.6. Mercury Porosimetry

Mercury porosimetry was performed on Pascal 140 (Hgpressures<400kPa) and Pascal 240 (100kPa<Hgpressures<400MPa) mercury porosimeters from CE Instruments (England). A mass of crushed Argan shells of about (300-350 mg) was introduced into the measuring cell and degassed under vacuum to reach a residual pressure of 0.01 kPa. Then, the measuring cell was filled with mercury. Information on the particle size, bulk density, and specific surface area of the sorbents assuming a cylindrical pore shape is given by the curve giving the intrusion volume of mercury as a function of the applied pressure.


## 2.2.7. Determination of pH at Zero Charge Point and Measurement of Zeta Potential

To measurepHPZC (pH at the point of zero charge), the method described by Altenor et al. [20] was followed. It consists in placing crushed Argan shells (0.1 g) in 100 mL of a KCl solution at 10-2 mol.L-1 at various pH values in a pH range from 2 to 12 (solution acidified or basified by hydrochloric acid solution or sodium hydroxide at 0.1 mol.L-1). The final pH (at equilibrium) is measured after 2 days using a pH meter PHM 210 (MeterLab).The zeta potential is a valuable characteristic to understand hydrophilic or hydrophobic properties of materials. The measurements were done using a zetameter Surpass from Anton PAAR. In a first experiment, the zeta potential was measured for Argan shells (0.5 g) in contact of different KCl solutions, acidic, basic, or neutral, at 10-3 mol.L-1. In a second one, the measurement was carried out in a neutral solution as a function of time.


## 2.2.8. Adsorption Procedure

The study of the adsorption of MB on Argan shells was performed by mixing a mass of 100 mg of adsorbent in a volume of 100 mL of MB. The working solutions were prepared by dilution from a stock solution of MB (1 g.L-1). The concentrations of the solutions are between 50 and 500 mg.L-1. This mixture was stirred (250 rpm) at room temperature (25±1°C) for the time necessary to reach equilibrium. After separation of the liquid phase and the adsorbent, the residual concentration of the filtrates was monitored by spectrophotometry in the visible range using a UV-visible spectrophotometer (UV-2550 Shimadzu). The maximum adsorption wavelength of MB is 664 nm.The equation of the calibration curve is as follows:Absorbance664nm=0.2102MBwithMBinmg.L−1.The amount adsorbed is calculated using [9]
(2)Qt=V×C0−Ct1000×m,where Qt is the MB uptake quantity (mg.g-1), V is the sample volume (mL), C0 is the initial MB concentration (mg.L-1), Ct is the residual MB concentration (mg.L-1), and m is the dry weight of the biosorbent (g).


## 3. Results and Discussion

### 3.1. Characterization of Adsorbents

Table1 illustrates the results of the elemental analysis. The device is equipped to measure carbon, hydrogen, nitrogen, sulfur, and oxygen. The combustion of the sample takes place at high temperature (940°C) in the presence of tungstic anhydride (WO3) under a current of oxygen for a very short time (15 s). This decomposition gives CO2, H2O, SO2, and NOx; nitrogen oxides are reduced to N2 (nitrogen) by copper. The whole system is then swept by a current of helium. The (gaseous) combustion products are then measured by gas chromatography (column filled with Chromosorb, katharometer detector) under the conditions determined by the manufacturer and which do not vary. The results are recorded and analyzed by the Eager 300 software. To determine the oxygen content, the samples are pyrolyzed under a stream of helium and the CO formed is quantified by gas chromatography in the stationary phase. Due to the reaction tube filling system, the dosage oxygen cannot be achieved with samples containing fluorine.Table 1
Elemental analysis of raw Argan shells.
Element1st measurement2nd measurementCarbon48.13%47.86%Hydrogen5.39%5.60%Nitrogen0.60%0.53%Oxygen40.62%41.11%Total97.74%95.10%Both measurements give similar results: large percentages of carbon and oxygen can be observed because of large amounts of cellulose and lignin precursors in the shells. The differences between 100% and the total correspond to inorganic content (Ca, K, Mg, etc.). The proportions of C, N, and O obtained are in quite good agreement with the values reported in literature for similar lignocellulosic substrates [21, 22].The biochemical analysis illustrated in Table2 shows that Argan shells are rich in holocellulose (cellulose+hemicelluloses). The percentage of lignin corresponds to the theoretical percentage reported in literature for this kind of lignocellulosic materials [23]. The extractive content is quite low compared to other biomasses such as orange peels but is quite of the same order as that found for almond peels, for example [9].Table 2
Biochemical analysis of raw Argan shells.
CompoundPercentage by weight (%)Lignin16.11Holocellulose80.63Extractives3.26Extractives are generally phenolic substances able to interact with cationic dyes, and we could expect acceptable MB adsorption capacity for Argan shells.FTIR spectra of ground Argan shells are shown in Figure3 which summarizes the infrared bands in the 4000-400 cm-1 spectral region. The IR spectroscopy was aimed at identifying the characteristic functional groups of the adsorbent surface. Generally, these groups are responsible of linkages between adsorbent and adsorbed substance. The spectrum presents the characteristic bands that can be found on IR spectra of cellulose and lignin [24] and are in good agreement with literature [19, 25]. The most intense band in the higher energy region between 3400 and 3200 cm-1 is due to a large amount of OH groups in the carbohydrates (carboxyles, phenols, or alcohols) and in adsorbed water [9]. The intense band around 1034 cm-1 corresponds to the linkage C-O-H or C-O-R (alcohols or esters), while the band around 2922 cm-1 can be assigned to C-H stretching vibration together with bending vibrations around 1424 cm-1 of aliphatic chains (-CH2 and –CH3) or also methoxyl groups O-CH3 on which lignocellulosic materials are based. The band around 1733 cm-1 corresponds to stretching of C=O groups from ketones, aldehydes, lactones, and carboxylic groups. The band at 1606 cm-1 can be attributed to aliphatic and/or unsaturated aromatic compounds and can be correlated to the band at 1236 cm-1. The band at 1606 cm-1 could also be attributed to O-H bending in adsorbed water.Figure 3
Infrared spectrum of Argan shells in the 4000-400 cm-1 spectral region.Lignocellulosic substrates are complex mix of organic materials, and consequently, thermal treatment produces a variety of chemical and physical changes. Their thermal stability can be studied using thermogravimetric analysis (TGA). Figure4 shows the thermogravimetric analysis (TGA) and derivative thermogravimetric (DTG) curves of Argan shell sample heated in the conditions described in Materials and Methods. We identify three peaks in the DTG curve and can assign them according to the previous works [26]. We can attribute the first one around 105°C (378 K) to moisture traces release. We assign the second one centered at 287°C (560 K) to the degradation of hemicelluloses and organic and/or aqueous extractives. The third peak is observed at 355°C (628 K) and corresponds to thermal degradation of cellulose. Above this last temperature, lignin degradation occurs. Our results are in good agreement with those reported in literature on Chinese forest peat or other lignocellulosic materials [27, 28].Figure 4
TG/DTA curve analysis of Argan shells at 5°C/min in nitrogen flow (80 mL.min-1).The morphology of the samples was characterized before adsorption of MB using SEM, and we present the microphotographs in Figure5. It can be seen that pores within Argan shells are quite homogeneous and they are well distributed on the sample.Figure 5
SEM micrograph of Argan shells before adsorption (2 magnifications).We give the results of porosity measurements in Table3. Regarding size distribution of particles and porosity of the substrate used, it appears that Argan shells present pores of nearly the same size as almond peels [9], hence a quite interesting specific surface area. By considering that the higher the area, the higher the available adsorption sites, we can assume that the adsorption onto Argan shells should occur in good conditions.Table 3
Porosity characterization of Argan shells.
SampleArgan shellsTotal cumulative volume (mm3/g)288.603Total specific surface area (m2/g)12.112Average pore diameter (μm)4.073Considering the determination ofpHPZC (pH at the point of zero charge) value, the main results obtained are shown in Table 4 and Figure 6.Table 4
Evolution of pH for different KCl solutions (100 mL) in contact with Argan shells (mass 0.1 g).
Time (h)Solution 1Solution 2Solution 3Solution 4Solution 501.784.177.969.8111.4311.503.456.107.929.3321.462.985.346.147.9331.412.104.975.276.3441.361.994.575.016.1551.351.884.394.816.08481.251.774.124.715.97Figure 6
Correlation between initial and final pH values.◆ Symbols: experimental points; ▲ symbols: theoretical points.It appears that pH decreases in a great extent in the first hours of test, particularly in basic solutions and acidic solutions with pH above 4, to finally reach a constant value considered as final pH value. By reporting initial pH values versus final ones, thepHPZC can be estimated to 1.5-2.0 from the extrapolation of the experimental curve to the theoretical one (initial pH = final pH). This means that the surface of the adsorbent is positively charged at pH below 1.5-2.0 and negatively charged for higher pH.Two experiments were performed with the zetameter. The first was to measure the zeta potential of crushed Argan shells in contact with a solution of potassium chloride to 10-3 mol.L-1 in acidic, basic, or neutral and the second to measure the zeta potential over time on the same sample of shells at neutral pH.In acidic medium (Figure7), it is observed that for a pH<2.6, the zeta potential becomes positive while beyond, it is negative. The observation of an isoelectric point can reveal the presence of proteins and uronic acids in the lignocellulosic material with pKa between 1.8 and 3 [29].Figure 7
Zeta potential in acid medium.In a basic medium (Figure8), the zeta potential is negative but tends to increase as soon as we reach very high pH values (note: measurement problems can occur at such pH values because we reach the limits of use of the Anton PAAR zetameter).Figure 8
Zeta potential in basic medium.When the potential is measured by passing a neutral KCl solution, it is negative, and we observe that over time, it varies very little, remaining around -20 mV (Table5). This shows that time is not a factor that influences the zeta potential for our measurements.Table 5
Zeta potential over time (raw Argan shells and 1 mM KCl solution).
Time in hoursZeta potential0-24.610.5-20.521-20.681.5-21.162-21.3518-22.75On the other hand, a measured negative potential means that the tested material has a hydrophilic character and is subject to swelling [30].


### 3.2. Adsorption Studies and Modelling

#### 3.2.1. Influence of the Contact Time

Figures9(a)–9(c) show the evolution of the adsorbed amount of MB as a function of the contact time for different concentrations of MB solutions (50, 100, and 260 mg.L-1) at three different temperatures 25°C, 35°C, and 45°C (298, 308, and 318 K).Figure 9
Adsorption of MB onto Argan shells (1 g.L-1) as a function of contact time at different temperatures: (a) 25°C (298 K), (b) 35°C (308 K), and (c) 45°C (318 K).
(a)(b)(c)We obtained the same kind of curves whatever the concentration used, and we can identify three zones. The first zone is characterized by a very fast adsorption, particularly at low concentrations, as already described [31, 32]. This important increase is followed by a slower adsorption until a stationary state is reached corresponding to approximately 16 mg/g for C=50 mg.L-1 and 206 mg/g for C=260 mg.L-1 after 15 and 180 minutes, respectively (experiment carried out at 25°C).Increasing temperature generally leads to higher values of adsorbed quantitiesQt at shorter times. For example, Qt increases from 16 mg/g (25°C) to 36 mg/g (45°C) for C=50 mg.L-1. An increase of temperature can improve the interactions between adsorbent and dye as already observed and reported in literature [33, 34].


#### 3.2.2. Determination of Rate Constants

In order to approach the real reaction mechanism, we used the modelling of the results obtained. The rate constants for adsorption were determined from simple mathematical models (first order and pseudosecond order) generally used to describe adsorption phenomena for organic compounds onto different substrates [35, 36].For first-order reaction, the model of Lagergren (1898) [32] gives
(3)logQe−QtQe=−K1×t2.3.While carrying logQe−Qt versus t, we should obtain a straight line whose slope gives K1, the rate constant of adsorption (min-1).For a pseudosecond-order reaction, we deduce the rate constantK2 from
(4)tQt=12K2Qe2+tQe.where, in all the cases, Qe is the quantity adsorbed at equilibrium (mg/g), Qt is the quantity adsorbed at time t (mg/g), and t is the time of contact (min).By plottingt/Qt versus t, we can determine K2 (g.min/mg).Figures10(a) and 10(b) show the first-order and pseudosecond-order modelling of data corresponding to experimental values obtained from experiments carried out at 25°C (298 K).Figure 10
Determination of the rate constant for (a) first-order model and (b) pseudosecond-order model (Argan shells 1 g.L-1 at 298 K).
(a)(b)From this figure, it appears that the values are best fitted with pseudosecond-order model, and this result was also observed at higher temperatures whatever the concentration. The coefficients of determination are always close to unity (R2>0,99) as shown in Table 6 which summarizes all the results deduced from both models (i.e., constants, Qe, and R2).Table 6
Determination of kinetics constants for first and pseudosecond order.
First orderLogQe−Qt=ftPseudosecond ordert/Qt=ftK1 (min-1)Qe calc. (mg/g)R2K2 (g.min/mg)Qe calc. (mg/g)R250 mg.L-10.0188617.820.7570.0021427.770.9919298 K100 mg.L-10.0236926.040.7620.0017286.210.9999260 mg.L-10.0110460.970.70780.00045217.390.999450 mg. L-10.0308220.130.93740.0015431.750.9944308 K100 mg.L-10.0250721.660.72870.0023189.290.9998260 mg.L-10.0305985.450.910.00052212.770.999750 mg.L-10.036113.900.70680.021337.591318 K100 mg.L-10.025313.960.62410.0042290.090.9999260 mg.L-10.01656100.950.83250.000274222.220.9956


#### 3.2.3. Adsorption Equilibrium

An adsorption isotherm is characterized by constants which give some indications about surface properties and affinity between adsorbate and adsorbent. In our case, the equilibrium data were fitted to the Langmuir and Freundlich isotherm models.According to Langmuir’s model, adsorption occurs through the formation of a monolayer of adsorbate and leads to [37]
(5)Qe=Qm×KL×Ce1+KL×Ce,whereQm is the maximum adsorption capacity (mg/g), KL is the equilibrium constant, characteristic of the adsorbent (L/mg), depending on temperature and operating conditions, and Ce is the concentration of the adsorbate at equilibrium (mg.L-1).The Freundlich’s model which application is limited to dilute media does not provide an upper limit to the adsorption and admits the existence of interactions between adsorbed molecules [36–38]. It is based on the empirical
(6)Qe=KF×Ce1/n,where Ce is the concentration of adsorbate at equilibrium (mg.L-1), KF is the constant related to the adsorption capacity, and n is the heterogeneity factor. In most cases, the adsorption of dye does not follow a simple law.Figures11(a) and 11(b) show the modelling of adsorption isotherms of MB using Argan shells according to Langmuir and Freundlich. The Qm, KL, and R2 (coefficient of determination for Langmuir isotherm) and KF, n, and R2 (coefficient of determination for Freundlich isotherm) are given in Table 7.Figure 11
MB adsorption according to (a) Langmuir and (b) Freundlich models on Argan shells (Argan shells 1 g.L-1, temperature 318 K).
(a)(b)Table 7
Maximum amounts adsorbed and constants determined according to Langmuir and Freundlich adsorption models (Argan shells 1 g.L-1, temperature 318 K).
LangmuirFreundlichQm (mg.g-1)KL (L.mg-1)RL2KFnRF2Argan shells232.60.2390.985134.862.910.7003It can be noted that the Freundlich model is not adapted to model the adsorption of MB on the studied crushed Argan shells. On the other hand, the Langmuir model is well applicable to the experimental results obtained. For the boyated Argan shells, it seems that the Langmuir model is the most representative of the adsorption mechanism with correlation coefficients worth 0.9851.We assume that the surface of the biosorbent is covered by a monolayer of dye and the determination of the dimensionless equilibrium parameterKR=1/1+KLC0 with KL the Langmuir constant (L/mg) and C0 the initial dye concentration (mg.L-1) which leads to values between 0 and 1 indicating favorable adsorption of MB onto Argan shells. These results are in good agreement with those in literature as several researches carried out on adsorption of cationic dyes onto different cellulosic substrates lead to the same observations [39]. Figure 12 gives the variation of KR as a function of initial concentration.Figure 12
Equilibrium parameter determined by considering a Langmuir model for adsorption on Argan shells (experiments carried out atT=298 K).


#### 3.2.4. Thermodynamic Parameters of Biosorption

To estimate the effect of temperature on the adsorption of MB on Argan shells, thermodynamic parameters were determined from the apparent equilibrium constantKc of biosorption which is defined as
(7)Kc=C0−CeCe,where C0is the initial concentration (mg.L-1), Ce is the equilibrium concentration (mg.L-1), and Kc is linked to ΔG° hence to ΔH° and ΔS° as follows:
(8)ΔG°=ΔH°−TΔS°=−R.T.lnKc,and consequently leading to(9)lnKc=−∆H0RT+∆S0R,where ΔG° is the standard Gibbs free energy change (J/mol), ΔH° is the standard enthalpy change (J/mol), ΔS° is the entropy change (J/mol.K), R is the universal gas constant (8.314 J/mol.K), and T is the absolute temperature (K).The plot of ln (Kc) versus 1/T leads to values of ΔH° and ΔS°, and the results are given in Table 8.Table 8
Thermodynamics associated with adsorption onto Argan shells.
SubstrateΔH° (kJ/mol)ΔS° (J/mol/K)ΔG° (kJ/mol) at298 K308 K318 K328 KArgan shells8.19242.26-4.329-4.506-5.375-5.725Whatever the temperature is, the negative values of Gibbs free energy change indicate that the adsorption process is spontaneous. Besides, the positive value ofΔH° shows that it is also endothermic, i.e., the adsorption increases as the temperature increases. The positive value of ΔS° suggests the increased randomness at the solid/solution interface during the adsorption of dye on the sorbent.The adsorption capacity of Argan shells with respect to methylene blue dye was compared with other adsorbents reported in the literature. The results are grouped together in Table9.Table 9
Comparison of the maximum adsorption capacity of Argan shells with different adsorbents previously reported.
AdsorbentMaximum adsorption capacity,Qm (mg/g)ReferenceTypha latifolia54.73[40]Native biochar106.38[41]Strong acid-treated biochar153.84[41]Weak acid-treated biochar113.63[41]Aragn shells232.6This work


#### 3.2.5. Methylene Blue Adsorption Mechanism

The possible MB adsorption mechanism onto lignocellulosic biomass (Argan shells) is schematically illustrated in Figure13.Figure 13
Possible MB adsorption mechanism onto lignocellulosic biomass (Argan shells).The mechanism of adsorption techniques involves four steps: migration of dye molecules from the bulk solution to the sorbent surface, diffusion through the boundary layer to the sorbent surface, adsorption at a single site, and intraparticle diffusion onto the sorbent [42]. Figure 13 represents the possible mechanism of MB adsorption on lignocellulosic biomass (Argan shells). The adsorption capacity depends on the structural and functional behavior of the sorbent. MB can be readily adsorbed to lignocellulosic bioadsorbents by forces such as van der Waals force and hydrogen bonding interactions [43, 44].According to biochemical analysis, Argan shells are rich in cellulose which consists of -OH groups, arranged in a sequence based onβ-D-glucopyranose-1,4-β-D-glucopyranose. The binding between the cellulose-rich adsorbent (Argan shells) and methylene blue can be achieved by hydrogen bonds and van der Waals bonds.


## 3.1. Characterization of Adsorbents

Table1 illustrates the results of the elemental analysis. The device is equipped to measure carbon, hydrogen, nitrogen, sulfur, and oxygen. The combustion of the sample takes place at high temperature (940°C) in the presence of tungstic anhydride (WO3) under a current of oxygen for a very short time (15 s). This decomposition gives CO2, H2O, SO2, and NOx; nitrogen oxides are reduced to N2 (nitrogen) by copper. The whole system is then swept by a current of helium. The (gaseous) combustion products are then measured by gas chromatography (column filled with Chromosorb, katharometer detector) under the conditions determined by the manufacturer and which do not vary. The results are recorded and analyzed by the Eager 300 software. To determine the oxygen content, the samples are pyrolyzed under a stream of helium and the CO formed is quantified by gas chromatography in the stationary phase. Due to the reaction tube filling system, the dosage oxygen cannot be achieved with samples containing fluorine.Table 1
Elemental analysis of raw Argan shells.
Element1st measurement2nd measurementCarbon48.13%47.86%Hydrogen5.39%5.60%Nitrogen0.60%0.53%Oxygen40.62%41.11%Total97.74%95.10%Both measurements give similar results: large percentages of carbon and oxygen can be observed because of large amounts of cellulose and lignin precursors in the shells. The differences between 100% and the total correspond to inorganic content (Ca, K, Mg, etc.). The proportions of C, N, and O obtained are in quite good agreement with the values reported in literature for similar lignocellulosic substrates [21, 22].The biochemical analysis illustrated in Table2 shows that Argan shells are rich in holocellulose (cellulose+hemicelluloses). The percentage of lignin corresponds to the theoretical percentage reported in literature for this kind of lignocellulosic materials [23]. The extractive content is quite low compared to other biomasses such as orange peels but is quite of the same order as that found for almond peels, for example [9].Table 2
Biochemical analysis of raw Argan shells.
CompoundPercentage by weight (%)Lignin16.11Holocellulose80.63Extractives3.26Extractives are generally phenolic substances able to interact with cationic dyes, and we could expect acceptable MB adsorption capacity for Argan shells.FTIR spectra of ground Argan shells are shown in Figure3 which summarizes the infrared bands in the 4000-400 cm-1 spectral region. The IR spectroscopy was aimed at identifying the characteristic functional groups of the adsorbent surface. Generally, these groups are responsible of linkages between adsorbent and adsorbed substance. The spectrum presents the characteristic bands that can be found on IR spectra of cellulose and lignin [24] and are in good agreement with literature [19, 25]. The most intense band in the higher energy region between 3400 and 3200 cm-1 is due to a large amount of OH groups in the carbohydrates (carboxyles, phenols, or alcohols) and in adsorbed water [9]. The intense band around 1034 cm-1 corresponds to the linkage C-O-H or C-O-R (alcohols or esters), while the band around 2922 cm-1 can be assigned to C-H stretching vibration together with bending vibrations around 1424 cm-1 of aliphatic chains (-CH2 and –CH3) or also methoxyl groups O-CH3 on which lignocellulosic materials are based. The band around 1733 cm-1 corresponds to stretching of C=O groups from ketones, aldehydes, lactones, and carboxylic groups. The band at 1606 cm-1 can be attributed to aliphatic and/or unsaturated aromatic compounds and can be correlated to the band at 1236 cm-1. The band at 1606 cm-1 could also be attributed to O-H bending in adsorbed water.Figure 3
Infrared spectrum of Argan shells in the 4000-400 cm-1 spectral region.Lignocellulosic substrates are complex mix of organic materials, and consequently, thermal treatment produces a variety of chemical and physical changes. Their thermal stability can be studied using thermogravimetric analysis (TGA). Figure4 shows the thermogravimetric analysis (TGA) and derivative thermogravimetric (DTG) curves of Argan shell sample heated in the conditions described in Materials and Methods. We identify three peaks in the DTG curve and can assign them according to the previous works [26]. We can attribute the first one around 105°C (378 K) to moisture traces release. We assign the second one centered at 287°C (560 K) to the degradation of hemicelluloses and organic and/or aqueous extractives. The third peak is observed at 355°C (628 K) and corresponds to thermal degradation of cellulose. Above this last temperature, lignin degradation occurs. Our results are in good agreement with those reported in literature on Chinese forest peat or other lignocellulosic materials [27, 28].Figure 4
TG/DTA curve analysis of Argan shells at 5°C/min in nitrogen flow (80 mL.min-1).The morphology of the samples was characterized before adsorption of MB using SEM, and we present the microphotographs in Figure5. It can be seen that pores within Argan shells are quite homogeneous and they are well distributed on the sample.Figure 5
SEM micrograph of Argan shells before adsorption (2 magnifications).We give the results of porosity measurements in Table3. Regarding size distribution of particles and porosity of the substrate used, it appears that Argan shells present pores of nearly the same size as almond peels [9], hence a quite interesting specific surface area. By considering that the higher the area, the higher the available adsorption sites, we can assume that the adsorption onto Argan shells should occur in good conditions.Table 3
Porosity characterization of Argan shells.
SampleArgan shellsTotal cumulative volume (mm3/g)288.603Total specific surface area (m2/g)12.112Average pore diameter (μm)4.073Considering the determination ofpHPZC (pH at the point of zero charge) value, the main results obtained are shown in Table 4 and Figure 6.Table 4
Evolution of pH for different KCl solutions (100 mL) in contact with Argan shells (mass 0.1 g).
Time (h)Solution 1Solution 2Solution 3Solution 4Solution 501.784.177.969.8111.4311.503.456.107.929.3321.462.985.346.147.9331.412.104.975.276.3441.361.994.575.016.1551.351.884.394.816.08481.251.774.124.715.97Figure 6
Correlation between initial and final pH values.◆ Symbols: experimental points; ▲ symbols: theoretical points.It appears that pH decreases in a great extent in the first hours of test, particularly in basic solutions and acidic solutions with pH above 4, to finally reach a constant value considered as final pH value. By reporting initial pH values versus final ones, thepHPZC can be estimated to 1.5-2.0 from the extrapolation of the experimental curve to the theoretical one (initial pH = final pH). This means that the surface of the adsorbent is positively charged at pH below 1.5-2.0 and negatively charged for higher pH.Two experiments were performed with the zetameter. The first was to measure the zeta potential of crushed Argan shells in contact with a solution of potassium chloride to 10-3 mol.L-1 in acidic, basic, or neutral and the second to measure the zeta potential over time on the same sample of shells at neutral pH.In acidic medium (Figure7), it is observed that for a pH<2.6, the zeta potential becomes positive while beyond, it is negative. The observation of an isoelectric point can reveal the presence of proteins and uronic acids in the lignocellulosic material with pKa between 1.8 and 3 [29].Figure 7
Zeta potential in acid medium.In a basic medium (Figure8), the zeta potential is negative but tends to increase as soon as we reach very high pH values (note: measurement problems can occur at such pH values because we reach the limits of use of the Anton PAAR zetameter).Figure 8
Zeta potential in basic medium.When the potential is measured by passing a neutral KCl solution, it is negative, and we observe that over time, it varies very little, remaining around -20 mV (Table5). This shows that time is not a factor that influences the zeta potential for our measurements.Table 5
Zeta potential over time (raw Argan shells and 1 mM KCl solution).
Time in hoursZeta potential0-24.610.5-20.521-20.681.5-21.162-21.3518-22.75On the other hand, a measured negative potential means that the tested material has a hydrophilic character and is subject to swelling [30].


## 3.2. Adsorption Studies and Modelling

### 3.2.1. Influence of the Contact Time

Figures9(a)–9(c) show the evolution of the adsorbed amount of MB as a function of the contact time for different concentrations of MB solutions (50, 100, and 260 mg.L-1) at three different temperatures 25°C, 35°C, and 45°C (298, 308, and 318 K).Figure 9
Adsorption of MB onto Argan shells (1 g.L-1) as a function of contact time at different temperatures: (a) 25°C (298 K), (b) 35°C (308 K), and (c) 45°C (318 K).
(a)(b)(c)We obtained the same kind of curves whatever the concentration used, and we can identify three zones. The first zone is characterized by a very fast adsorption, particularly at low concentrations, as already described [31, 32]. This important increase is followed by a slower adsorption until a stationary state is reached corresponding to approximately 16 mg/g for C=50 mg.L-1 and 206 mg/g for C=260 mg.L-1 after 15 and 180 minutes, respectively (experiment carried out at 25°C).Increasing temperature generally leads to higher values of adsorbed quantitiesQt at shorter times. For example, Qt increases from 16 mg/g (25°C) to 36 mg/g (45°C) for C=50 mg.L-1. An increase of temperature can improve the interactions between adsorbent and dye as already observed and reported in literature [33, 34].


### 3.2.2. Determination of Rate Constants

In order to approach the real reaction mechanism, we used the modelling of the results obtained. The rate constants for adsorption were determined from simple mathematical models (first order and pseudosecond order) generally used to describe adsorption phenomena for organic compounds onto different substrates [35, 36].For first-order reaction, the model of Lagergren (1898) [32] gives
(3)logQe−QtQe=−K1×t2.3.While carrying logQe−Qt versus t, we should obtain a straight line whose slope gives K1, the rate constant of adsorption (min-1).For a pseudosecond-order reaction, we deduce the rate constantK2 from
(4)tQt=12K2Qe2+tQe.where, in all the cases, Qe is the quantity adsorbed at equilibrium (mg/g), Qt is the quantity adsorbed at time t (mg/g), and t is the time of contact (min).By plottingt/Qt versus t, we can determine K2 (g.min/mg).Figures10(a) and 10(b) show the first-order and pseudosecond-order modelling of data corresponding to experimental values obtained from experiments carried out at 25°C (298 K).Figure 10
Determination of the rate constant for (a) first-order model and (b) pseudosecond-order model (Argan shells 1 g.L-1 at 298 K).
(a)(b)From this figure, it appears that the values are best fitted with pseudosecond-order model, and this result was also observed at higher temperatures whatever the concentration. The coefficients of determination are always close to unity (R2>0,99) as shown in Table 6 which summarizes all the results deduced from both models (i.e., constants, Qe, and R2).Table 6
Determination of kinetics constants for first and pseudosecond order.
First orderLogQe−Qt=ftPseudosecond ordert/Qt=ftK1 (min-1)Qe calc. (mg/g)R2K2 (g.min/mg)Qe calc. (mg/g)R250 mg.L-10.0188617.820.7570.0021427.770.9919298 K100 mg.L-10.0236926.040.7620.0017286.210.9999260 mg.L-10.0110460.970.70780.00045217.390.999450 mg. L-10.0308220.130.93740.0015431.750.9944308 K100 mg.L-10.0250721.660.72870.0023189.290.9998260 mg.L-10.0305985.450.910.00052212.770.999750 mg.L-10.036113.900.70680.021337.591318 K100 mg.L-10.025313.960.62410.0042290.090.9999260 mg.L-10.01656100.950.83250.000274222.220.9956


### 3.2.3. Adsorption Equilibrium

An adsorption isotherm is characterized by constants which give some indications about surface properties and affinity between adsorbate and adsorbent. In our case, the equilibrium data were fitted to the Langmuir and Freundlich isotherm models.According to Langmuir’s model, adsorption occurs through the formation of a monolayer of adsorbate and leads to [37]
(5)Qe=Qm×KL×Ce1+KL×Ce,whereQm is the maximum adsorption capacity (mg/g), KL is the equilibrium constant, characteristic of the adsorbent (L/mg), depending on temperature and operating conditions, and Ce is the concentration of the adsorbate at equilibrium (mg.L-1).The Freundlich’s model which application is limited to dilute media does not provide an upper limit to the adsorption and admits the existence of interactions between adsorbed molecules [36–38]. It is based on the empirical
(6)Qe=KF×Ce1/n,where Ce is the concentration of adsorbate at equilibrium (mg.L-1), KF is the constant related to the adsorption capacity, and n is the heterogeneity factor. In most cases, the adsorption of dye does not follow a simple law.Figures11(a) and 11(b) show the modelling of adsorption isotherms of MB using Argan shells according to Langmuir and Freundlich. The Qm, KL, and R2 (coefficient of determination for Langmuir isotherm) and KF, n, and R2 (coefficient of determination for Freundlich isotherm) are given in Table 7.Figure 11
MB adsorption according to (a) Langmuir and (b) Freundlich models on Argan shells (Argan shells 1 g.L-1, temperature 318 K).
(a)(b)Table 7
Maximum amounts adsorbed and constants determined according to Langmuir and Freundlich adsorption models (Argan shells 1 g.L-1, temperature 318 K).
LangmuirFreundlichQm (mg.g-1)KL (L.mg-1)RL2KFnRF2Argan shells232.60.2390.985134.862.910.7003It can be noted that the Freundlich model is not adapted to model the adsorption of MB on the studied crushed Argan shells. On the other hand, the Langmuir model is well applicable to the experimental results obtained. For the boyated Argan shells, it seems that the Langmuir model is the most representative of the adsorption mechanism with correlation coefficients worth 0.9851.We assume that the surface of the biosorbent is covered by a monolayer of dye and the determination of the dimensionless equilibrium parameterKR=1/1+KLC0 with KL the Langmuir constant (L/mg) and C0 the initial dye concentration (mg.L-1) which leads to values between 0 and 1 indicating favorable adsorption of MB onto Argan shells. These results are in good agreement with those in literature as several researches carried out on adsorption of cationic dyes onto different cellulosic substrates lead to the same observations [39]. Figure 12 gives the variation of KR as a function of initial concentration.Figure 12
Equilibrium parameter determined by considering a Langmuir model for adsorption on Argan shells (experiments carried out atT=298 K).


### 3.2.4. Thermodynamic Parameters of Biosorption

To estimate the effect of temperature on the adsorption of MB on Argan shells, thermodynamic parameters were determined from the apparent equilibrium constantKc of biosorption which is defined as
(7)Kc=C0−CeCe,where C0is the initial concentration (mg.L-1), Ce is the equilibrium concentration (mg.L-1), and Kc is linked to ΔG° hence to ΔH° and ΔS° as follows:
(8)ΔG°=ΔH°−TΔS°=−R.T.lnKc,and consequently leading to(9)lnKc=−∆H0RT+∆S0R,where ΔG° is the standard Gibbs free energy change (J/mol), ΔH° is the standard enthalpy change (J/mol), ΔS° is the entropy change (J/mol.K), R is the universal gas constant (8.314 J/mol.K), and T is the absolute temperature (K).The plot of ln (Kc) versus 1/T leads to values of ΔH° and ΔS°, and the results are given in Table 8.Table 8
Thermodynamics associated with adsorption onto Argan shells.
SubstrateΔH° (kJ/mol)ΔS° (J/mol/K)ΔG° (kJ/mol) at298 K308 K318 K328 KArgan shells8.19242.26-4.329-4.506-5.375-5.725Whatever the temperature is, the negative values of Gibbs free energy change indicate that the adsorption process is spontaneous. Besides, the positive value ofΔH° shows that it is also endothermic, i.e., the adsorption increases as the temperature increases. The positive value of ΔS° suggests the increased randomness at the solid/solution interface during the adsorption of dye on the sorbent.The adsorption capacity of Argan shells with respect to methylene blue dye was compared with other adsorbents reported in the literature. The results are grouped together in Table9.Table 9
Comparison of the maximum adsorption capacity of Argan shells with different adsorbents previously reported.
AdsorbentMaximum adsorption capacity,Qm (mg/g)ReferenceTypha latifolia54.73[40]Native biochar106.38[41]Strong acid-treated biochar153.84[41]Weak acid-treated biochar113.63[41]Aragn shells232.6This work


### 3.2.5. Methylene Blue Adsorption Mechanism

The possible MB adsorption mechanism onto lignocellulosic biomass (Argan shells) is schematically illustrated in Figure13.Figure 13
Possible MB adsorption mechanism onto lignocellulosic biomass (Argan shells).The mechanism of adsorption techniques involves four steps: migration of dye molecules from the bulk solution to the sorbent surface, diffusion through the boundary layer to the sorbent surface, adsorption at a single site, and intraparticle diffusion onto the sorbent [42]. Figure 13 represents the possible mechanism of MB adsorption on lignocellulosic biomass (Argan shells). The adsorption capacity depends on the structural and functional behavior of the sorbent. MB can be readily adsorbed to lignocellulosic bioadsorbents by forces such as van der Waals force and hydrogen bonding interactions [43, 44].According to biochemical analysis, Argan shells are rich in cellulose which consists of -OH groups, arranged in a sequence based onβ-D-glucopyranose-1,4-β-D-glucopyranose. The binding between the cellulose-rich adsorbent (Argan shells) and methylene blue can be achieved by hydrogen bonds and van der Waals bonds.


## 3.2.1. Influence of the Contact Time

Figures9(a)–9(c) show the evolution of the adsorbed amount of MB as a function of the contact time for different concentrations of MB solutions (50, 100, and 260 mg.L-1) at three different temperatures 25°C, 35°C, and 45°C (298, 308, and 318 K).Figure 9
Adsorption of MB onto Argan shells (1 g.L-1) as a function of contact time at different temperatures: (a) 25°C (298 K), (b) 35°C (308 K), and (c) 45°C (318 K).
(a)(b)(c)We obtained the same kind of curves whatever the concentration used, and we can identify three zones. The first zone is characterized by a very fast adsorption, particularly at low concentrations, as already described [31, 32]. This important increase is followed by a slower adsorption until a stationary state is reached corresponding to approximately 16 mg/g for C=50 mg.L-1 and 206 mg/g for C=260 mg.L-1 after 15 and 180 minutes, respectively (experiment carried out at 25°C).Increasing temperature generally leads to higher values of adsorbed quantitiesQt at shorter times. For example, Qt increases from 16 mg/g (25°C) to 36 mg/g (45°C) for C=50 mg.L-1. An increase of temperature can improve the interactions between adsorbent and dye as already observed and reported in literature [33, 34].


## 3.2.2. Determination of Rate Constants

In order to approach the real reaction mechanism, we used the modelling of the results obtained. The rate constants for adsorption were determined from simple mathematical models (first order and pseudosecond order) generally used to describe adsorption phenomena for organic compounds onto different substrates [35, 36].For first-order reaction, the model of Lagergren (1898) [32] gives
(3)logQe−QtQe=−K1×t2.3.While carrying logQe−Qt versus t, we should obtain a straight line whose slope gives K1, the rate constant of adsorption (min-1).For a pseudosecond-order reaction, we deduce the rate constantK2 from
(4)tQt=12K2Qe2+tQe.where, in all the cases, Qe is the quantity adsorbed at equilibrium (mg/g), Qt is the quantity adsorbed at time t (mg/g), and t is the time of contact (min).By plottingt/Qt versus t, we can determine K2 (g.min/mg).Figures10(a) and 10(b) show the first-order and pseudosecond-order modelling of data corresponding to experimental values obtained from experiments carried out at 25°C (298 K).Figure 10
Determination of the rate constant for (a) first-order model and (b) pseudosecond-order model (Argan shells 1 g.L-1 at 298 K).
(a)(b)From this figure, it appears that the values are best fitted with pseudosecond-order model, and this result was also observed at higher temperatures whatever the concentration. The coefficients of determination are always close to unity (R2>0,99) as shown in Table 6 which summarizes all the results deduced from both models (i.e., constants, Qe, and R2).Table 6
Determination of kinetics constants for first and pseudosecond order.
First orderLogQe−Qt=ftPseudosecond ordert/Qt=ftK1 (min-1)Qe calc. (mg/g)R2K2 (g.min/mg)Qe calc. (mg/g)R250 mg.L-10.0188617.820.7570.0021427.770.9919298 K100 mg.L-10.0236926.040.7620.0017286.210.9999260 mg.L-10.0110460.970.70780.00045217.390.999450 mg. L-10.0308220.130.93740.0015431.750.9944308 K100 mg.L-10.0250721.660.72870.0023189.290.9998260 mg.L-10.0305985.450.910.00052212.770.999750 mg.L-10.036113.900.70680.021337.591318 K100 mg.L-10.025313.960.62410.0042290.090.9999260 mg.L-10.01656100.950.83250.000274222.220.9956


## 3.2.3. Adsorption Equilibrium

An adsorption isotherm is characterized by constants which give some indications about surface properties and affinity between adsorbate and adsorbent. In our case, the equilibrium data were fitted to the Langmuir and Freundlich isotherm models.According to Langmuir’s model, adsorption occurs through the formation of a monolayer of adsorbate and leads to [37]
(5)Qe=Qm×KL×Ce1+KL×Ce,whereQm is the maximum adsorption capacity (mg/g), KL is the equilibrium constant, characteristic of the adsorbent (L/mg), depending on temperature and operating conditions, and Ce is the concentration of the adsorbate at equilibrium (mg.L-1).The Freundlich’s model which application is limited to dilute media does not provide an upper limit to the adsorption and admits the existence of interactions between adsorbed molecules [36–38]. It is based on the empirical
(6)Qe=KF×Ce1/n,where Ce is the concentration of adsorbate at equilibrium (mg.L-1), KF is the constant related to the adsorption capacity, and n is the heterogeneity factor. In most cases, the adsorption of dye does not follow a simple law.Figures11(a) and 11(b) show the modelling of adsorption isotherms of MB using Argan shells according to Langmuir and Freundlich. The Qm, KL, and R2 (coefficient of determination for Langmuir isotherm) and KF, n, and R2 (coefficient of determination for Freundlich isotherm) are given in Table 7.Figure 11
MB adsorption according to (a) Langmuir and (b) Freundlich models on Argan shells (Argan shells 1 g.L-1, temperature 318 K).
(a)(b)Table 7
Maximum amounts adsorbed and constants determined according to Langmuir and Freundlich adsorption models (Argan shells 1 g.L-1, temperature 318 K).
LangmuirFreundlichQm (mg.g-1)KL (L.mg-1)RL2KFnRF2Argan shells232.60.2390.985134.862.910.7003It can be noted that the Freundlich model is not adapted to model the adsorption of MB on the studied crushed Argan shells. On the other hand, the Langmuir model is well applicable to the experimental results obtained. For the boyated Argan shells, it seems that the Langmuir model is the most representative of the adsorption mechanism with correlation coefficients worth 0.9851.We assume that the surface of the biosorbent is covered by a monolayer of dye and the determination of the dimensionless equilibrium parameterKR=1/1+KLC0 with KL the Langmuir constant (L/mg) and C0 the initial dye concentration (mg.L-1) which leads to values between 0 and 1 indicating favorable adsorption of MB onto Argan shells. These results are in good agreement with those in literature as several researches carried out on adsorption of cationic dyes onto different cellulosic substrates lead to the same observations [39]. Figure 12 gives the variation of KR as a function of initial concentration.Figure 12
Equilibrium parameter determined by considering a Langmuir model for adsorption on Argan shells (experiments carried out atT=298 K).


## 3.2.4. Thermodynamic Parameters of Biosorption

To estimate the effect of temperature on the adsorption of MB on Argan shells, thermodynamic parameters were determined from the apparent equilibrium constantKc of biosorption which is defined as
(7)Kc=C0−CeCe,where C0is the initial concentration (mg.L-1), Ce is the equilibrium concentration (mg.L-1), and Kc is linked to ΔG° hence to ΔH° and ΔS° as follows:
(8)ΔG°=ΔH°−TΔS°=−R.T.lnKc,and consequently leading to(9)lnKc=−∆H0RT+∆S0R,where ΔG° is the standard Gibbs free energy change (J/mol), ΔH° is the standard enthalpy change (J/mol), ΔS° is the entropy change (J/mol.K), R is the universal gas constant (8.314 J/mol.K), and T is the absolute temperature (K).The plot of ln (Kc) versus 1/T leads to values of ΔH° and ΔS°, and the results are given in Table 8.Table 8
Thermodynamics associated with adsorption onto Argan shells.
SubstrateΔH° (kJ/mol)ΔS° (J/mol/K)ΔG° (kJ/mol) at298 K308 K318 K328 KArgan shells8.19242.26-4.329-4.506-5.375-5.725Whatever the temperature is, the negative values of Gibbs free energy change indicate that the adsorption process is spontaneous. Besides, the positive value ofΔH° shows that it is also endothermic, i.e., the adsorption increases as the temperature increases. The positive value of ΔS° suggests the increased randomness at the solid/solution interface during the adsorption of dye on the sorbent.The adsorption capacity of Argan shells with respect to methylene blue dye was compared with other adsorbents reported in the literature. The results are grouped together in Table9.Table 9
Comparison of the maximum adsorption capacity of Argan shells with different adsorbents previously reported.
AdsorbentMaximum adsorption capacity,Qm (mg/g)ReferenceTypha latifolia54.73[40]Native biochar106.38[41]Strong acid-treated biochar153.84[41]Weak acid-treated biochar113.63[41]Aragn shells232.6This work


## 3.2.5. Methylene Blue Adsorption Mechanism

The possible MB adsorption mechanism onto lignocellulosic biomass (Argan shells) is schematically illustrated in Figure13.Figure 13
Possible MB adsorption mechanism onto lignocellulosic biomass (Argan shells).The mechanism of adsorption techniques involves four steps: migration of dye molecules from the bulk solution to the sorbent surface, diffusion through the boundary layer to the sorbent surface, adsorption at a single site, and intraparticle diffusion onto the sorbent [42]. Figure 13 represents the possible mechanism of MB adsorption on lignocellulosic biomass (Argan shells). The adsorption capacity depends on the structural and functional behavior of the sorbent. MB can be readily adsorbed to lignocellulosic bioadsorbents by forces such as van der Waals force and hydrogen bonding interactions [43, 44].According to biochemical analysis, Argan shells are rich in cellulose which consists of -OH groups, arranged in a sequence based onβ-D-glucopyranose-1,4-β-D-glucopyranose. The binding between the cellulose-rich adsorbent (Argan shells) and methylene blue can be achieved by hydrogen bonds and van der Waals bonds.


## 4. Conclusion

In this study, we first analyzed Argan shells by different techniques (elemental analysis, biochemical analysis, FTIR, thermogravimetric analysis, scanning electron microscopy, mercury porosimetry, and zetametry) and then used them as potential biosorbent. It appears that this substrate is efficient for eliminating MB from aqueous solutions. The monolayer adsorption capacity was estimated to 232.6 mg/g. The adsorption process depends on contact duration, and equilibrium is reached after 180 minutes. In this study, it appears that the adsorption kinetics are well described by a pseudosecond-order model and that the process is conveniently modelled by a Langmuir model. The determination of thermodynamic parameters shows that the adsorption spontaneously occurs onto the substrate and is all the more favored than temperature increases since the process is endothermic.



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