Exploring the potential of olive pomace derived activated charcoal as an efficient adsorbent for methylene blue dye: a comprehensive investigation of isotherms, kinetics, and thermodynamics

Methylene blue (MB) is a heterocyclic aromatic chemical compound used as a dye in various dyeing processes. The accumulation of such an organic compound poses a significant threat to both the environment and human health. Therefore, numerous biological, physical, and chemical processes have been established to remove MB dye, with adsorption being the most predominant dye-based treatment technology. In this context, the aim of this work was to evaluate the adsorption properties of activated carbon derived from olive pomace against methylene blue. To this end, scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), and Fourier transform infrared spectroscopy (FT-IR) analyses were carried out to confirm the adsorption of MB on carbon structures. In addition, the effect of contact time, pH, initial dye concentration, adsorbent dose, and temperature on the adsorption efficiency of MB was investigated. On the other hand, kinetic and isothermal models were used to further understand the adsorption mechanism, which showed a good correlation with the pseudo-second-order kinetic model and the Langmuir isotherm. Finally, thermodynamic analysis showed favorable conditions for physisorption, with the process being both endothermic and spontaneous.


Introduction
Wastewater from the textile industry contains various types of pollutants, such as the methylene blue (MB) dye [1].MB dye is used in various dyeing processes.These include chemical indicators, and biological dyes.Nevertheless, the persistent accumulation of this organic compound has had a substantial and detrimental impact on the environment and human health [2].Indeed, wastewater loaded with MB is characterized by high chromaticity and organic matter content, which reduces its biodegradability and causes negative effects on both human health and the environment [3][4][5].To tackle these environmental issues, many chemical, physical and biological removal processes have been developed [6].However, these processes suffer from the drawbacks of being more costly and energy-intensive.They are also less suitable for large-scale production facilities, where large quantities of sludge or hazardous waste are produced [7].Consequently, in the absence of practical, straightforward and secure techniques for treating industrial wastewater, the ecosystem could be significantly impacted [3,8].
In this context, the adsorption technique is considered as one of the leading dye-based wastewater treatment technologies.It offers numerous advantages, including high efficiency, low cost, ease of use, and insensitivity to toxic substances.However, the technique's effectiveness is dependent on various parameters, specifically the physicochemical properties of the adsorbent and adsorbate.
Activated carbon (AC) is a widely used adsorbent for the removal of a variety of contaminants from wastewater [9].This substance exhibits high adsorption ability, a well-developed microporous structure, a substantial specific surface area, and effective reactive surface chemistry [2,10].As a result, the demand for activated carbon has substantially grown with its increased application in various industries.A significant focus has been placed on the preparation of activated carbon (AC) using agro-industrial by-products, which present a renewable and potentially cost-effective source for production [11].In this regard, multiple studies have explored the production of AC from various sources such as palm kernels, coconut shells [5], olive stones [12], date cores, eucalyptus residues [13], fruit pits, and walnut shells [14].
In this line, several studies have focused on the production of activated carbon from olive by-products [15][16][17][18][19].However, most of the published processes have used olive stones as the main raw material for AC production [12,[15][16][17][18][19].As a result, an additional step was added to the whole process the need to separate the olive stones from the olive pulp before starting the production process.
To address this issue, we have recently improved the production process of the activated carbon derived from olive by-products by the use of olive pomace as solid waste [20].The used olive pomace combines shredded stone shell, skin, ground olive pulp, water, and a certain amount of oil.We do not require a preceding separation step [20].
Therefore, the aim of the present study was to investigate the effectiveness of activated charcoal derived from olive pomace in terms of its adsorptive properties for the removal of MB dye.The adsorption of MB on the activated charcoal structure was analyzed by SEM, EDS, FTIR, and XRD spectroscopy.In addition, a systematic investigation was carried out to evaluate the effect of adsorption parameters, including contact time, pH, dye concentration, adsorbent dose, and temperature, on the removal efficiency of methylene blue.Finally, the adsorption kinetics, equilibrium isotherms, and thermodynamics associated with the process have also been comprehensively established and discussed in our study.

Adsorbent: activated charcoal
Methylene blue was adsorbed using activated charcoal derived from olive pomace (ACp).The ACp was obtained through a chemical activation procedure that involved phosphoric acid (H 3 PO 4 ) as the activating agent.To achieve this, 2.5 g of olive pomace was impregnated in a 22% H 3 PO 4 solution under magnetic stirring at 50 °C for 2 h.This step was followed by drying in an oven at 50 °C for 24 h.After drying, the activated olive pomace was heated in a programmable muffle furnace under air at temperatures ranging from room temperature to 500 °C.Following pyrolysis, the resultant powder was subjected to isothermal treatment for 60 min, as described by Alouiz et al [20].Table 1 shows the primary physical and chemical characteristics of the activated charcoal.

Adsorbate: methylene blue
Methylene blue, classified as a cationic dye, has been selected as a representative molecule for medium-sized organic pollutants.This dye is commonly used as the adsorbate to assess the absorbency of materials.The physical and chemical characteristics of MB are summarized in table 2.

Adsorption procedure
The adsorption process followed these steps: firstly, a solution of MB dye was prepared by dissolving a specified amount of dye in deionized water, resulting in an initial concentration of 1000 mg/l.This solution was then diluted to reach the desired initial concentrations.
For each experiment, 20 ml of the dye solution was added to a flask in the presence of activated charcoal derived from olive pomace.The mixture was stirred for a predetermined time to allow interaction between the dye and the activated charcoal.
After the agitation period, the solution was separated from the adsorbent by centrifugation at 6000 rpm for 10 min.The absorbance of the supernatant was measured using a UV/visible spectrometer (VWR Spectrophotometers, UV/Vis, UV-3100PC) at the wavelength corresponding to the maximum absorbance of the MB solution (λ = 660 nm).The concentration of residual dye was assessed using a calibration curve based on various concentrations of methylene blue (MB).This produced a correlation coefficient (R 2 ) of 0.997, indicating a robust linear fit and thus highlighting the reliability of the calibration curve.
The removal efficiency (R) and adsorption capacity of methylene blue ((q e ) in mg/g) were determined by the following equations (1)-(2), respectively [21]: Where Ci and Ce represent the initial and equilibrium concentrations, respectively, expressed in mg/L, V is the volume of the solution in liters and m is the mass of ACp introduced into the solution in grams.
To evaluate the influence of adsorption parameters such as contact time, pH, initial MB concentration, adsorbent dosage and temperature, similar experiments were carried out by varying these variables while keeping others constant.The optimal adjustment of these parameters determined the effectiveness of adsorption.

Effect of contact time
To evaluate the adsorption equilibrium time, 20 ml of MB dye solutions with a concentration of 5 mg L −1 ) were added to flasks containing 0.1 g of activated charcoal.The solutions were stirred at a constant speed of 300 rpm, and samples were analyzed after contact times ranging from 5 min to 5 h.

Effect of pH
The methylene blue solutions pH was adjusted in the range of 2 to 12 using HCl (0.1 M) or NaOH (0.1 M) and a pH meter (Jenco Microcomputer 6171).Approximately 0.1 g of adsorbent was then added to the solution and stirred for 2 h.

Effect of initial concentration of MB
Dye solutions were prepared with initial concentrations ranging from 0.1 mg l −1 to 20 mg L −1 .Each solution was placed in a flask containing a fixed amount of activated carbon.The flasks were then stirred at a constant speed of 300 rpm for 2 h.

Effect of adsorbent dosage
Different quantities of adsorbent, from 0.05 g to 0.4 g, were added to dye solutions with an initial concentration of 5 mg L −1 .The mixtures were stirred until equilibrium was achieved.The thermodynamic parameters associated with the adsorption phenomenon were assessed by varying the solution temperature within the range of 16 to 50 °C.

Effect of temperature
In order to gain insights into the energetics and spontaneity of the MB adsorption on ACp, a series of experiments were conducted at different temperatures ranging from 16 to 50 °C.The initial concentration of MB and other adsorption parameters were kept constant.The thermodynamic parameters related to the adsorption phenomenon were evaluated to gain insights into the energetics and spontaneity of the MB adsorption on ACp.

Adsorption isotherm and kinetic models
The adsorption isotherm indicates how adsorption molecules are partitioned between the liquid and solid phases when the adsorption process reaches a state of equilibrium [14].Therefore, parameters obtained from different models provide important useful information about adsorption mechanisms and adsorbent surface properties [22].In this line, data from the methylene blue bindings on activated charcoal were treated according to the linear equations of Langmuir, Freundlich, and Temkin (table 3).
On the other hand, to formulate an effective adsorption model, it is essential to understand the kinetics and dynamics of adsorption.The process's nature is dependent on the physical and/or chemical properties of the adsorbent system and the system conditions [23].Hence, studying experimental kinetic data modeling may help to elucidate the binding mechanism.In this context, the experimental kinetics of MB were modeled using kinetic models such as pseudo-first and pseudo-second order, and intra-particle diffusion to investigate the rate and the adsorption mechanism [12,[22][23][24].Table 3 summarizes the expression of the linear equations used in the kinetic models.
The parameters of the isotherm models were: q e (mg/g) is the adsorbed amount at equilibrium; -C e is the equilibrium dye concentration (mg/L); -Langmuir model: K L is Langmuir equilibrium constant and q m the maximum adsorption capacity (mg/g); -Freundlich model: K F (L/mg) is the Freundlich constant and n is the heterogeneity factor.The K F value is related to the adsorption capacity; while 1/n value is related to the adsorption intensity.
-Temkin isotherm: B 1 is Temkin isotherm equilibrium binding constant and A is constant related to heat of adsorption (L/g).
The parameters of the kinetic models where: q t (mg/g) refers to adsorption capacity of adsorbent at time t, k 1 (min −1 ) is the adsorption rate constant of quasi first-order kinetic, t (min) is adsorption time.where k 2 (min −1 ) is the adsorption rate constant of quasi second-order kinetic.Where, K i denotes the intra particle rate constant and the Xi value establishes the boundary layer effect's thickness.

Samples characterization
The adsorption capacity of activated carbon depends on its porosity and the chemical reactivity of the functional groups on its surface [25][26][27].
The crystallographic structure of the fabricated ACp was characterized by x-ray diffraction -XRD (Bruker D8 Advance, USA) with Cu Kα radiation (40 kV, λ = 0.15418 nm) and scanning speed of 3°min −1 between 5°and 80°.Then, the ACp morphology before and after methylene blue adsorption was investigated by SEM (JEOL, JEOL-IT500HR, Japan) equipped with an EDX analyzer, an accelerating voltage of 5 kV, in high vacuum mode.The surface organic structures of ACp before and after MB adsorption were studied by FTIR spectrum recorded at 1 cm −1 resolution and 10 scans between 400 and 4000 cm −1 using an (FTR-Vertex 70-Bruker, USA) spectrometer.

X-ray diffraction (XRD) analysis
The structural composition of ACp was characterized crystallographically using x-ray diffraction (figure 1).XRD is generally used to obtain information about the spacing between graphene layers, which is determined from Intra-particle diffusion ID q K t X t i the position of diffraction peaks (002) and (100) [4].The broad peak of (002) at 22-26°indicates small, irregular domains of coherent, parallel stacking of graphene sheets, while the weak peak of (100) at around 43°suggests the presence of nest structures and random layers formed by sp2 hybrid carbons.The diagram illustrates that the obtained product exhibits the typical characteristics of amorphous carbon (figure 1).

Textural characterization by SEM and EDX
SEM analyses were carried out in order to confirm the adsorption of MB on the ACp surface.Before MB adsorption, SEM images of ACp show a very porous morphology with the presence of different pores sizes (figure 2(a)).These images also reveal the presence of a high number of cavities in the external surfaces suggesting a large specific surface area developed by the supports.These results were confirmed, by the iodine index (higher > 923 mg g −1 ) and the specific surface (higher > 1400 m 2 /g) of the tested supports.Regarding the morphological aspect of charcoal after methylene blue adsorption, SEM images reveal a modification of its morphology and the pores filling by methylene blue (figure 2(b)).These findings are in accordance with the EDX analysis showing the presence of methylene blue sulfur compound in the ACp pores (figure 2(b)).Altogether, these results confirm the adsorption of MB on the ACp surface.

FTIR spectroscopy
FTIR analysis was performed to analyze the molecular structure of activated charcoal before and after contact with methylene blue dye.Analyses of IR spectra showed the positional shift of some peaks, the disappearance of others, and the appearance of new ones.These findings indicate the possible involvement of these functional groups on the ACp surface in the adsorption process (figure 3).In the activated charcoal spectrum (in black), three bands were detected between 1580-1700 cm −1 (figure 3).These bands were attributed to the stretching vibration of the lactone C=O and carbonyl groups by the C=C groups conjugated to the aromatic rings (figure 3).The bands between 1250 and 1300 cm −1 may be related to the angular deformation in the plane of the C-H bonds of aromatic rings.Finally, the 1100-1180 cm −1 band is generally found with oxidized carbons and has been assigned to C-O stretching in acids, alcohols, phenols, ethers, and/or ester groups [22,44].The red spectrum of ACp after methylene blue adsorption revealed the presence of new peaks.The observed band in the 1700-1850 cm −1 regions was identified as the stretching vibration of C=O to C=C appearing with a more intense band.The bands between 1500-1570 cm −1 correspond to the axial deformation of the C-N bond of aromatic amines [44].Furthermore, a small band has appeared in the 3650 cm −1 regions, possibly associated with the stretching of the O-H bond in free phenols or vibrations of the O-H bond in carboxylic acid.Therefore, both spectra reveal significant changes in the molecular structure of the activated carbon after contact with methylene blue dye, highlighting the crucial role of specific functional groups in the adsorption process.

Influence of adsorption parameters 3.2.1. Effect of contact time
The effect of contact time on MB adsorption was evaluated using an aqueous solution of methylene blue ([MB] = 5 mg L −1 ) in the presence of 0.1 g of the activated carbon.As shown in figure 4(a), the MB removal is clearly dependent on the contact time with ACp.The MB removal rate increased proportionally with the contact time.In addition, the adsorption kinetics showed two distinct phases: a first phase, during which a fast adsorption rate was observed, followed by a second slower phase until reaching a plateau.The equilibrium was reached around 120 min (MB removal efficiency = 92.24%).These findings could be attributed to the strong attractive forces between the MB and ACp, as well as the adsorption process occurring from the bulk solution onto the surface adsorbent [10,27].

Effect of pH
The effect of pH on the adsorption process was studied, at room temperature, in the range of 2 to 12.As shown in figure 4(b), the removal efficiency of methylene blue strongly depends on the pH value.Indeed, higher is the pH, higher is the adsorption capacity of ACp (MB removal efficiency equal to 68.21% at pH = 2, 91.67 at pH = 7 and 94.81% at pH = 12).Similar results were reported for the adsorption of methylene blue dye on clay [9], activated carbon prepared from treated olive stone [12], nanographene oxide [28], and activated carbon produced from steam-activated bitumen [28].
According to these observations, two possible adsorption mechanisms could be considered.Firstly, electrostatic interactions between the ACp surface groups and the MB functional groups.Accordingly, the lower the pH, the higher the number of positively charged sites, and the lower is the number of negatively charged sites on the ACp.Therefore, an electrostatic repulsion could be observed between the adsorbent and the adsorbate.Secondly, hydrogen, hydrophobic, and π-π interactions may also occur between ACp and the MB dye as described for other absorbents [10,28,29].
On the other hand, MB is a cationic dye and its adsorption is promoted by the ACp negative charges, especially since the zero-charge point of our adsorbent (pH PZC = 8.8) promotes the adsorption of positively charged sites in a basic medium at pH >pH PZC .

Effect of adsorbent dosage
The MB adsorption on ACp was examined by varying the adsorbent quantity from 0.05 to 0.4 g.The concentration of MB solution was maintained at 5 mg L −1 , and the contact time was fixed at 2 h.The maximum adsorption of 93.97% was achieved with 0.1 g of ACp (figure 4(c)).The proportional variation of the adsorption and adsorbent quantity could be attributed to an increased surface area and the availability of more adsorption sites [30,31].

Effect of temperature
In order to investigate the effect of temperature on the adsorption process, this parameter was varied from 16 to 50 °C while keeping the charcoal in contact with the solution for 2 h. Figure 4(d) illustrated a slight increase in MB removal efficiency (from 90.17% to 95.77%) as the temperature increased from 16 °C to 50 °C.In agreement with the Arrhenius law, this phenomenon suggests that the adsorption reaction is endothermic, and the increase in temperature may promote the adsorption process.This phenomenon could be related to an increased diffusion rate of the adsorbate molecules through the outer boundary layer and into the inner pores of the adsorbent particle [32][33][34].

Isotherm, kinetic, and thermodynamic data analysis 3.3.1. Analysis of adsorption isotherm data
In general, the adsorption isotherm is used to give useful information about the mechanism, and properties, and also indicates how the adsorption molecules partition between the liquid and solid phases when they reach equilibrium [14,24].As presented in figure 5, the adsorption capacity of MB on activated charcoal increased with the increasing MB concentration.To analyze the equilibrium characteristics of the adsorption process, the collected data was used to fit the equations of three specific isotherms: Langmuir, Freundlich, and Temkin (as shown in table 3 and figure 5).
The Langmuir isotherm assumes that the adsorption process occurs on a uniform surface where sites possess equal energy and availability for adsorption.This model holds true for complete adsorption monolayers, where adsorbate molecules remain within the surface plane, and adsorption sites on the adsorbent are consistent in their capacity and homogeneity [35].For this isotherm, the obtained q m (2.3078 mg g −1 ) was close to the experimental value.In addition, the correlation coefficient showed a good fit to the experimental data (R 2 = 0.989, figure 6(a)).On the other hand, the separation factor (R L ) was used to evaluate the adsorption process in the studied system: Unfavorable (R L > 1), linear (R L = 1), favorable (0 < R L < 1), or irreversible (R L = 0) [12,30,32] this parameter is defined by: In the studied concentrations, our results indicate favorable adsorption.The decrease in R L associated with an increase in initial BM concentration indicates that adsorption is more favorable at higher concentrations (figure 6(b)).
The Freundlich isothermal model admits surface heterogeneity and assumes that adsorption occurs at sites with different adsorption energy [9,14].The heterogeneity factor n F is used to indicate if the adsorption is linear (n F = 1), a chemical process (n F < 1), or a physical process is favorable (n F > 1).On the other hand, 1/n F < 1 and 1/n F > 1 indicate a normal Langmuir isotherm and cooperative adsorption, respectively [37].In the current study, n F = 1.4841 and 1/n F = 0.6738 indicating that the physical process and normal Langmuir isotherm are favorable.The fit of the Freundlich isotherm to the experimental data (R 2 = 0.9452) is shown in figure 6(c).
The Temkin isotherm is commonly applied in several adsorption processes.According to this model, the interactions between adsorbates considered by this model assume that the heat of adsorption ( Q ∆ ) of all molecules decreases linearly as the layer is covered, and that the adsorption has a uniform distribution of the maximum binding energy [14,22].The constants B 1 and A are determined from the linear plot of q e versus Ln (C e ).The linear fit to the experimental data (R 2 = 0.9083) in figure 6(d) shows that the Temkin isotherm failed to explain the MB adsorption on ACp compared to the Langmuir and Freundlich isotherms.In order to determine the best-fitting isothermal model for the methylene blue adsorption on ACp, the correlation factors of the used isothermal models were compared.The parameters of Langmuir, Freundlich, and Temkin isotherms were calculated and listed in table 4. Based on the correlation coefficients, the Langmuir isotherm shows the best fit for the equilibrium data for MB adsorption on activated charcoal ACp.

Analysis of the adsorption kinetic data
The adsorption kinetics provides information on the different adsorption stages between the adsorbate and adsorbent particles.In the current study, the adsorption equilibrium was reached after 2 h of contact time (figure 7(a), dye removal percentage = 92.24%).In order to determine the adsorption kinetics of MB, we studied the kinetic equations of the most commonly used models such as the pseudo-first-order model, pseudo-secondorder model, and the intra-particle diffusion model.All the kinetic parameters were calculated by fitting three models (figures 7(b)-(c), table 5).The correlation coefficients for the three kinetic models were greater than  0.91 0.97, indicating that these models provide a good correlation for the MB adsorption on ACp.However, a higher correlation coefficient was obtained with the pseudo-second-order model (R 2 = 0.999) allowing a better description of the methylene blue adsorption.On the other hand, in order to study the diffusion mechanism, the kinetic results were analyzed using the intra-particle diffusion model.The obtained qt = f(t 1/2 ) line does not cross the origin, indicating the involvement of multi-linear steps in the adsorption process (figure 7(d), table 5).
The first stage corresponds to the diffusion of MB molecules through the liquid film followed by rapid attraction to the outer surface of the activated carbon and subsequent movement into the inner pores [38,39].The second step represents the adsorption equilibrium of MB molecules.Moreover, the fact that the curves do not cross the origin indicates that intra-particle diffusion is not the only step controlling the rate of adsorption [12,[40][41][42].
Based on the results, the adsorption capacities of methylene blue dye on activated charcoal derived from different raw materials and the comparison of previous studies as listed in table 6. Comparatively, the charcoal carbon using olive pomace as raw material showed better adsorption capacity compared to the carbonaceous Table 5. Pseudo-first-order, pseudo-second-order kinetics, and intra-particle diffusion parameters for the adsorption of methylene blue onto activated charcoal.

Kinetic models Parameters
Pseudo-first-order q e ,cal (mg/g) 0.35 K 1 (min −1 ) 0.016 R 2 0.97 Pseudo-second-orde q e ,cal (mg/g) 0.95 K 2 (min −1 ) 0.13 R 2 0.99 Intra-particle diffusion First stage K 0.85 R 2 0.99 absorbents obtained from other waste materials.And we note that the removal rate of MB on ACp is significantly higher under the same experimental conditions for this study.

Adsorption thermodynamics
Thermodynamic parameters such as free enthalpy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) are useful to describe the adsorption reaction (endothermic or exothermic), and the spontaneity of the adsorption process [35,36].These parameters are calculated from the following equations: Where, T is the temperature in Kelvin (K), the universal gas constant is represented by R (8.314 J /m o l .K), and K C is the thermodynamic constant.The parameters of Van't Hoff graph for ln (K C ) versus 1/T are shown using figure 8 and are used to calculate the values of ΔH°and ΔS°, which are presented in table 7.
The obtained results (table 7) indicate that the adsorption process occurs with a spontaneous and favorable reaction (ΔG°< 0).The positive enthalpy (ΔH°) is in favor of an endothermic process.The heat of adsorption (ΔH°< 40 kJ mol −1 ) indicates that MB adsorption is a physical process [44].These results are similar to Hameed et al (2007) finding which shows that the adsorption of methylene blue on bamboo-based activated charcoal is spontaneous and physical in nature.The positive entropy value (ΔS°) for MB dye adsorption on ACp, shows increased randomness at the solid/liquid interface during dye adsorption and reflects the affinity of adsorbents for MB [46][47][48][49].

Conclusions
These investigations indicate that olive pomace presents a suitable raw material for the production of an effective methylene blue adsorbent.The activated charcoal produced from this source has demonstrated its ability to eliminate methylene blue, with an adsorption efficiency exceeding 90%.The adsorption process was influenced by various parameters, including the initial concentration of the compound, the duration of contact, pH, quantity of adsorbent and temperature.Furthermore, the investigation of adsorption mechanisms by kinetic and isothermal models showed good correlation with the pseudo-second-order kinetic model and the Langmuir isotherm.Subsequently, thermodynamic analysis indicated favorable conditions for physisorption, with the process being both endothermic and spontaneous.

Figure 1 .
Figure 1.X-ray diffraction of the activated charcoal ACp.

Figure 2 .
Figure 2. SEM images and EDX analysis of methylene blue dye onto activated charcoal before (a) and (b) after the adsorption.

Figure 3 .
Figure 3. FTIR spectrum of activated charcoal before (black-ACp) and after (red-ACp-MB) the removal process of methylene blue.

Figure 4 .
Figure 4. Variation in MB removal rate as a function of contact time (a), pH (b), ACp quantity (c) and temperature (d).

Figure 7 .
Figure 7. Kinetics curves for the methylene blue adsorption by activated charcoal (a) effect of the contact time, (b-c) fitting of pseudofirst-order and pseudo-second-order kinetics, respectively, and (d) intra-particle diffusion model.

Table 2 .
Main physical and chemical characteristics of methylene blue.

Table 3 .
Expression of linear equations for kinetic and isotherm models.

Table 4 .
Values of parameters of the Langmuir, Freundlich and Temkin isotherm adsorption constants.

Table 6 .
Comparison of developed activated charcoal with other previously developed adsorbents reported in the literature.