Valorization of Oued Sebou Natural Sediments (Fez-Morocco Area) as Adsorbent of Methylene Blue Dye: Kinetic and Thermodynamic Study

The objective of this study is to evaluate the efficiency of methylene blue removal using Oued Sebou sediments as an adsorbent. The presence of carboxyl functional group demonstrated by infrared (IR) analysis of the sediment favorized the methylene blue (MB) adsorption. Sediment collected from Oued Sebou could remove the most MB molecules at pH 8. The Freundlich model described suitably the adsorption process. The experimental measured enthalpy (ΔH) and entropy (ΔS°) are 118.1 kJ mol−1 and 395.2 J mol−1 K−1, respectively, indicating that the reaction was endothermic with an increase of randomness at the solid/liquid interface during the adsorption. The kinetics of MB adsorption by sediment were adequately fitted to the pseudo-second-order model. Experimental results showed that the adsorption capacity of the methylene blue dye depends on the solution pH, the initial dye concentration, the adsorbent mass, the sediment particle diameter, and the temperature of the reaction medium. The removal efficiency of the MB molecules reaches 100% after 60 minutes under the optimum conditions.


Introduction
Many industries such as textiles and printing are continuously releasing various pollutants into the hydrosphere, and these pollutants have adverse impacts on the environment and human health [1]. ey can cause eye burns, which result in permanent damage to human and animal's eyes, its inhalation may give rise to breathing difficulties, and its ingestion through the mouth causes a burning sensation, nausea, vomiting, sweating, and an abundance of cold sweats [2]. Many research works have studied the removal of dyes from aqueous solution [3,4], and the adsorption property of several natural adsorbents was widely studied [4][5][6][7]. Nevertheless, according to the bibliography, little effort has been made to study the adsorption of MB by using sediments as an adsorbent. e color produced by minute amounts of organic dyes in water is of a great concern because the color in water is aesthetically unpleasant. As such, few dyes and their degraded forms are carcinogenic and toxic [8]. Methylene blue is one of the most commonly used dyes for dyeing cotton, wool, silk, and leather. e dye is not toxic in nature, and it causes several harmful effects to the health of living beings, though. Once consumed, methylene blue can cause increased heartbeat, nausea, and vomiting. Today, several physical or chemical processes are used to treat dye laden wastewaters such as photodegradation [9]. e sediments are often called "vases", and we define the constitution as a more or less large set of particles or precipitates which have undergone transport in their individual parts [10]. e current work aims at investigating the adsorption capacity of Oued Sebou sediments on the elimination of methylene blue. e effect of operational parameters such as adsorbent loading, initial dye concentration, particles size, and pH was studied. Besides, the modeling of the adsorption kinetics and thermodynamics was studied as well. e adsorption isotherm models were investigated to explain the adsorption mechanism at the equilibrium during the elimination of MB from aqueous solutions.

Materials.
e sediment adsorbent is obtained from Moroccan Oued Sebou. e sediment samples were taken with a shovel at the top of the fine sediment deposit in a layer between 5 and 15 cm of the depth. Next, it was dried, grinded, and sieved before adsorption applications. Selected concentrations of MB solutions (10-50 mg L −1 ) were prepared in distilled water at room temperature. During the adsorption experiment, HCl (0.5 M) and NaOH (0.5 M) were used to regulate the solution pH.

Characterization Techniques.
e X-ray diffraction (X′ PERT PRO), infrared spectroscopy (VERTEX 70), and SEM-EDAX (QUANTA 200) were used to identify the composition and the morphology of the sediment material. During adsorption tests, the remaining concentration of MB was determined by measuring the absorbance at 66 2 nm using a spectrophotometry UV-visible (VR-2000).

Adsorption Experiment.
e adsorption experiments of MB by the sediment were carried out by using a mass of adsorbent mixed with a solution of MB at room temperature under continuous stirring. In order to investigate the kinetic adsorption, a 5 mL of the sample was collected every 5 min to measure its concentration by spectrophotometric UV-vis, but, before that, the suspension was filtered with a filter paper with 0.45 µm pore size to separate the sediment particles from the liquid. e removal of MB molecules was calculated using following formula [11,12]: where C 0 and C t are the concentrations of MB at t � 0 and at t ≠ 0, respectively. e adsorption capacity of the sediment for MB removal was obtained applying the following equation [13]: where q e (mg. g −1 ) is the adsorption capacity at equilibrium; C 0 (mg. L −1 ) is the initial concentration of MB; C e (mg. L −1 ) is the equilibrium concentration of MB; V (L) is the MB solution volume; and m (g) is the sediment mass. e influence of operating conditions on the adsorption of MB onto the sediment was studied in a batch system by varying the sediment mass from 1 to 2 g L −1 , the initial MB concentration from 10 to 50 mg·L -1, and the pH solution in the range of 4-10.

X-Ray Diffraction (XRD).
e mineralogical characterization of sediments is presented in Figure 1

Scanning Electron Microscopy (SEM-EDX).
Scanning electron microscopy (SEM) was used to observe the morphology and the particles size of the sediment. Figure 3 illustrates the corresponding images. It was demonstrated that there are grains of different morphologic materials. e quantitative chemical analysis is performed by EDX. Figure 4 shows the existence of a proportion of iron, aluminum, calcium, and magnesium in the sediments, which is related to the presence of hematite, calcium carbonate, and dolomite phases [17].

Effect of the Particle Size.
In order to study the effect of sediment particles sizes, a series of experiments were performed with a constant adsorbent mass of 1g·L −1 , initial dye concentration of 40 mg·L -1 , initial pH, and different sizes of sediment particles thanks to grinding and sieving with sieves of different diameter (40, 63, 125, and 200 µm). Figure 5 illustrates that decreasing particle size enhanced the adsorption capacity: the 40 µm particle size has the highest methylene blue removal rate (95%). Other treatments showed lower removal rate between 72% and 85.6%, although the 200 µm processing showed slow adsorption about 51.64% at 60 min. is evolution could be explained by the link between the effective surface area of sediment particles and adsorption efficiency [18].

Effect of pH.
e study of the effect of pH on the adsorption of MB using the sediment shows the affectation of its surface properties, particularly its functional groups, which influences the overall adsorption process. e effect of the MB solution pH on the adsorption experiment using 1g·L −1 of Oued Sebou sediments and 40 mg·L -1 as MB concentration at room temperature was studied. e pH 2 e Scientific World Journal range of the solution has been adjusted from 4 to 10 using HCl (0.5 M) and NaOH (0.5 M). It has been observed that MB removal increases by rising the dye solution pH [19] where it reaches its maximum at pH 8 and 10 ( Figure 6). is behavior can be explained by the case of basic dyes (MB) at acidic pH, and H + ions will compete with blue methylene cations, which will reduce adsorption efficiency. On the other hand, when the pH is considered, the basic of this competition decreases when the active sites become more negatively charged on the sediment surface, which enhances the MB adsorption by electrostatic attraction forces [20]. As a result, we can conclude that the surface sediment is negatively charged. For further MB adsorption tests, pH 8 was fixed as the optimum solution pH .

Effect of Initial Dye Concentration.
For evaluating the adsorption capacity of sediments according to initial dye concentration, this parameter was varied in a range of 10-50 mg·L -1 . Figure 7 illustrates the evolution MB adsorption by sediments for different initial MB concentrations. e results revealed that the adsorption removal efficiency of MB decreases from 98% to 78% with the decrease of the initial MB concentration from 50 to 10 mg·L -1 .
is rate evolution could be explained by the low dispersion of MB molecules into sediments due to the high MB concentration. Furthermore, the high MB concentration saturates the sediments, which decreases the adsorption capacity [21].

Effect of Adsorbent Mass.
e results of sediment mass effect on the MB adsorption are shown in Figure 8.
e results indicate that the removal efficiency of MB adsorption onto the Oued Sebou sediment increased from 79% to 94% when the sediment mass increased from 1 to 2.5 g L −1 . ese observations are in accordance with the increase of active sites on the sediment surface [22]. Nevertheless, the dye removal remains constant while the sediment dose is up to 2 g L −1 . erefore, the optimal sediment dose for maximal MB elimination was fixed at 2 g L −1 .

Effects of Temperature and Activation Energy.
Although the effect of temperature on adsorption has been carefully studied, no universal law has been found. Indeed, these studies have shown that an increase in temperature can lead to either an increase or a decrease in the amount of molecules adsorbed [23,24]. To study the effect of temperature on the kinetics of MB adsorption by the sediment, the experiments were carried out in a temperature ranging from 283 K to 323 K. e results obtained are shown in Figure 9, which indicate that there is a higher temperature, and the sediment produces higher removal efficiency of MB. According to this result, enough energy ameliorates the adsorption fixation of MB [21].

Kinetic Models.
In order to investigate the kinetic model of methylene blue adsorption, the following equations were used [25]: where q e is the adsorption capacity at equilibrium, q t is the adsorption capacity at different time, k is the kinetic constant, and t is the time. It is concluded from Table 1 that the values of the pseudo-first-order (PFO) R 2 were 0.967, 0.991, 0.998, 0.75, and 0.947; the values of the pseudo-second-order (PSO) R 2 were 0.997, 0.994, 0.986, 0.996, and 0.991; thus, R 2 of the PSO was bigger than that of the PFO, implying that the pseudosecond-order kinetic was the main kinetic process as long as it gives the best prediction for the kinetic data [26]. Similar results were observed by Lamia Dali et al. using an Algerian  e Scientific World Journal palygorskite [21], erefore, adsorption is an irreversible reaction, and it implies both the rapid fixation of the dye molecules and long fixation on the weak energy sites [27].

Isotherm Models.
Isotherm models (Langmuir, Freundlich, and Dubinin-Radushkevich) were studied to explain the adsorption system at equilibrium [28]. All isotherms are executed with a variation of MB concentration from 10 to 50 mg·L -1 . e results of adsorption isotherms show that the adsorption process of MB onto sediment adsorbate fitted more strictly the Freundlich isotherm equation (equation (5)) than the Langmuir (equation (4)) and Dubinin-Radushkevich (equation (6)) isotherms [29]. is result is based on the higher value of R 2 (0.998), which is considered as a measure of the excellent fit of experimental data on the isotherms models. us, this process is a nonideal sorption on heterogeneous surfaces and multilayer sorption [30].  e Langmuir isotherm admits that adsorptions happen at specific homogeneous active sites on the adsorbent surface. Its corresponding equation is explained as follows: where K L is the Langmuir constant (L·mg −1 ). e Freundlich model describes nonuniform and multilayer adsorption on heterogeneous surfaces. Its equation model is detailed as follows: where K F the adsorption capacity and 1/n the intensity of adsorption.
e Dubinin-Radushkevich (D-R) model is used to determine the nature of adsorption [25,27,28]. e equation form of the D-R isotherm is as follows: where q mDR is the sediment adsorption capacity at equilibrium (mg·g −1 ), K DR is the Dubinin-Radushkevich constant (mol 2 ·kJ −2 ), and ε is the Polanyi potential (J·mol −1 ). e corresponding data of the three models are presented in Table 2.

Adsorption
ermodynamic Studies. e Van't Hoff equations were used to determine the thermodynamic parameters, mainly Gibbs free energy change (ΔG°), enthalpy where ΔG°is the standard free energy, kJ mol −1 ; T is the absolute solution temperature, K; ΔH°is the standard enthalpy, kJ mol −1 ; R is the universal gas constant, 8.314 J mol −1 K −1 ; and ΔS°is the standard entropy, J K −1 . e linear plots of ln K L versus 1/T afford ΔH and ΔS values for the adsorption of methylene blue at different temperatures. e enthalpy values imply that the adsorption is endothermic [31,32]. e value of ΔS o is positive, which means the increase of randomness at the solid/liquid interface during the adsorption process, as reported by Aroguz et al. in their work on MB removal by the pyrolyzed petrified sediment [25]. Contrariwise, Gülen et al. found that the enthalpy, entropy, and Gibbs values are negative during the removal of methylene blue by using an porous carbon adsorbent [33], which confirms that the adsorbent type controls the thermodynamic parameters over the adsorption process. Table 3Values of studied thermodynamic parameters.

Comparison of the Adsorption Efficiency with Literature
Studies. e efficiency of the adsorption removal and the contact time on methylene blue, according to the literature, is presented in Table 4 compared to our work. As it can be noticed in Table 4, the different materials used for the adsorption of the MB dye compared with the adsorption of MB by the sediment shows an important effective removal yield with a fast contact time.

Conclusion
During this work, batch studies on adsorption of methylene blue molecules using a Moroccan sediment of Oued Sebou without any further treatment obviously suggest that the maximum adsorption yield was found as 100% at pH equal to 8, 323 K as the solution temperature, 1g·L −1 of mass sediment, and 10 mg·L -1 as MB concentration, which proves that those operational parameters greatly affected the adsorption process. roughout limited conditions, the adsorption behaves well at low concentration and becomes more efficient at increasing sediment masses. e highest adsorption removal is obtained at basic pH at higher temperatures.
e pseudo-second-order and Freundlich isotherm models are well fitted with the adsorption process.
is experimental result showed that the adsorption removal

Data Availability
No data were used to support this study.

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