Adsorption of cationic dye onto Moroccan natural rock

This paper evaluates the ability of a natural rock found in Morocco to remove the pollutant properties of methylene blue (MB) dye. In this regard, the experiments were conceived to test the hypothesis that the rock native of the country’s region of Khenifra can be used as a new abundant adsorbent to remove cationic dyes from water through the adsorption technique in order to avoid extra cost and contribute to the valorization of this material. For this, several factors such as pH of the solution, granulometry, mass of adsorbent, initial dye concentration and stirring rate were tested. The adsorption kinetics process was tested through three models, namely, the pseudo-first-order model, pseudo-second-order model and the intraparticle diffusion model, to predict which type of adsorption is best suited in the removal of dye pollution. The batch study proves that only the granulometry (G ≤ 63μm = 91%, G ≤ 80μm = 88.4% and G ≤ 100μm=70.7%) and the adsorbent mass (the more the mass of the adsorbent increases the more the elimination is achieved) can influence the elimination rate of our rock. Meanwhile, the data of the adsorption kinetics test show that the experimental adsorption could be described by the mechanism of the pseudo-second-order model (correlation coefficients near the unit ‘R = 0.99’ ), confirming chemical sorption as a rate-limiting step of the intraparticle diffusion mechanism. According to the results of this study, the rock collected from the area of Khenifra can be used as a new and efficient adsorbent in the field of wastewater treatment.


Introduction
Industries in Morocco consume vast quantities of water in their manufacturing processes, which generates tons of polluted water 1 . Once released, the polluted water causes alterations in the aquatic environment because of its stabilities, toxicities and low biodegradability [1][2][3][4][5] . These pollutants pose a severe threat to the environment, which requires prior treatment to reduce the polluting load before spilling it into the environment [6][7] .
A wide variety of physical, chemical and biological techniques have been developed and tested in the treatment of dye-loaded effluents [8][9][10] . The adsorption technology is one of the most effective methods for dye removal because of its low cost, high efficiency, simplicity, and insensitivity to toxic substances [11][12][13][14][15] .
The adsorbent that is widely used in the removal of dyes from aqueous solutions is activated carbon [16][17][18][19][20] . However, it manifests several disadvantages such as high cost, flammability and difficulty to regenerate, which urges us to look for another efficient and less expensive adsorbent [21][22][23] .
Since Morocco is abundant with different varieties of rocks, especially in the Atlas Mountains, we sought to find an adsorption support (adsorbent) alternative to the activated carbon in rocks. In this context, the use of natural resources found in rocks as an adsorbent is particularly advantageous because of its abundance and accessibility in the environment 24 .
This work aims to evaluate the retention capacity of Methylene Blue by a Moroccan natural rock native of Khenifra region in northern central Morocco, in order to contribute in the understanding of adsorption processes and valorize rocks in the field of wastewater treatment. For this purpose, we carried out two studies: a parametric study of the adsorption phenomenon by examining the effect of several factors on the bleaching power of the rock, and another study of the adsorption kinetics using several models.

Adsorption support
The adsorbent used in this work is a "natural rock" collected from the mountain range of the Middle Atlas, in Khenifra region (Morocco). The chosen rock was crushed and sieved to obtain three types of particle sizes in the range of 63 and 100µm. The samples were stored in closed bottles to be used in the adsorption studies without any chemical or physical modification in order to avoid extra costs.

Adsorbate
The dye chosen as a representative of the significant textile pollutants in this study is "Methylene Blue" (MB), belonging to the family of the cationic dyes; in particular, the powerful dye with many chemical properties 25,26 . Its main features are represented in Table 1, and its structural formula is shown in Fig. 1.

Experimental protocol
Batch adsorption experiments were conducted in 250 ml beakers with different masses of the natural rock (0.1-1.0 g) and 100 mL of dye solution indifferent initial concentrations (10−25 mg/L of MB).
The different concentrations used in this study were prepared from a dilution of the methylene blue stock solution; this latter was prepared by dissolving 100 mg of the methylene blue powder in one liter of distilled water.
The dye concentration was determined calorimetrically by measuring at maximum absorbance of the dye (λ max= 664 nm) using a Shimadzu UV/Visible mini-1240 UV spectrophotometer.
A calibration curve was plotted between absorbance and concentration of the dye to obtain the absorbanceconcentration profile of the dye-based on Beer-Lambert's law.
For high concentration, dye samples were diluted before absorbance measurements.
The adsorbed amount of Methylene Blue was calculated using the following formula 27 : Qads= (Ci − Ct) )× M Where: Qads: quantity adsorbed at time t (mg/g). V: volume of the solution (ml). Ci and Ct: are respectively the initial concentration and the concentration at time t of the dye (mg/l). M: mass of the adsorbent (g).
The rate of decrease R% of the MB was calculated using the following equation: Knowing that: Ci and Ce are the initial and equilibrium dye concentrations in solution (mg/L), respectively.

Desorption studies
The main objective of this test is an attempt to answer one of the questions asked by the kinetic studies: is the mechanism of the adsorption chemical or physical sorption?
The desorption potential of the natural rock was evaluated using 1 g of adsorbent in contact with 100 mL, 10 mg/L dye solution. The quantity of adsorbed dye was noted, and the adsorbent was filtered and dried in the oven for 12 hours at 105 °C. Afterwards, the adsorbent was soaked into 100 mL of distilled water, agitated for 2 hours, and left in rest for 3 months.
In this research, we will initially study the effect of the main parameters influencing the adsorption capacity such as the pH of the solution, the granulometry, the mass of the adsorbent, the initial concentration of the adsorbate, and the stirring rate. Second, we will determine which kinetic model represents the mechanism of MB adsorption on natural rock. Finally, and as a complementary component, we will answer the kinetic questions related to the mechanism of adsorption.

Effect of the pH
The initial pH of the solution is an important parameter that must be taken into consideration during adsorption studies. It can influence the structure of the adsorbent, the adsorbate and also the adsorption mechanism 28 . The effect of this factor on the adsorption capacity was analyzed over a range of pH 2-12, including the normal pH of the colored solution.
The experiments were carried out by adding 0.5g of the raw rock to 100 ml of the dye solution (Ci = 10 mg/L) at room temperature T=25±1°.
The pH adjustments to the desired values were made by adding HCL (0.1N) or NaOH (0.1N).
The results shown in Figure 2 indicate that the removal rate of MB dye is generally very little influenced by pH variations, which makes the effect of this parameter negligible. These results can be explained by the resistance of the adsorbent's surface charge to pH changes 29 . This is due to the basic nature of the rock, which can balance the pH and make it partially neutral.
The results obtained in this experiment allowed us to work with the normal pH of the solution in the following tests.

Effect of particle size
In order to study the effect of granulometry on dye adsorption, we used three types of granulates (G ≤ 63µm, G ≤ 80µm, and G ≤ 100µm), while the mass of the adsorbent was fixed at 0.5g and the initial concentration of the MB at 10 mg/L. Figure 3 shows the influence of the particle size of the adsorbent on the adsorption of methylene blue. The results illustrated in this figure show that the adsorption equilibrium of methylene blue is quickly achieved in the three types of granulates, but the finest granulometry (≤63µm) has the higher yield (91.6%) compared to other granulates (≤ 80µm=88.4% and ≤ 100µm=70.7%). Consequently, the external surface of the rock determines the retention rate of the dye's molecule. In other words, the tests carried out show that particle size plays a vital role in adsorption kinetics, which allows us to conclude that the more exceptional particle size of the rock, the higher the adsorption capacity becomes. Therefore, we will carry on our research using the finest size of granulometry.

Effect of the mass of adsorbent
To estimate the optimal quantity of rock added to the colored solution, we will investigate the effect of mass on the adsorption of MB. The experiments were carried out by adding masses between 0.1 and 1g to a volume of 100 ml of BM at initial concentration Ci = 10 mg/L. The supernatant produced was analyzed so we can calculate the residual concentration of the dye over time, and Figure 4 illustrates the results obtained.

Effect of the initial dye concentration:
In order to study the initial concentration effect on the rock's retention capacity, 1g of the rock was added to 100 ml of MB solutions with an initial concentration range of 10-25 mg/l while fixing the previous parameters. In Figure 5, it is clearly shown that the adsorption of MB dye was fast at all concentrations, due to the available sites, and as the concentration increases, the amount of the adsorbed dye increases as well. In fact, by increasing concentrations, the ratio between the first number of dye molecules and the available surface area becomes high, which explain the total occupancy of the adsorption sites and the persistence of few free dye molecules in the solution. Based on these results, it appears that the equilibrium time is independent of initial concentration and the amount adsorbed at equilibrium increases with increasing concentration.

Effect of stirring speed
Agitation is an essential parameter in the adsorption phenomenon; it influences the distribution of the solute in the solution and the formation rate of the external boundary film 30 . The effect of agitation speed on the removal of MB dye by the rock was studied at different agitation speeds (400, 800, 1200 rpm) while fixing the previous parameters (pH = normal pH of the solution, granulometry = ≤63µm, mass of the adsorbent = 1g, initial concentration of the dye = 15 mg/l.) The data in Figure 6 show a significant removal percentage. However, this removal is independent of the stirring rate used, which means that the stirring rate does not affect the adsorption capacity of the natural rock.

The study of adsorption kinetics
The kinetics study of the adsorption is a crucial step to define the sorption link between the adsorbent and the adsorbate 31 . Several kinetic models can be used to investigate the retention mechanism and test the experimental results of adsorption.
In this work, the adsorption reaction is carried out between the surface of the adsorbent (the rock) and the ions of the basic dye with a positive charge (methylene blue) in aqueous solution. The kinetics tests were carried out by mixing 100 ml of colored solutions at concentrations between 10 and 25 mg/l, with 0.5g of the natural rock, at room temperature and normal pH of the dye solution.
The solid-liquid homogenization is ensured by a magnetic stirring rod with a constant stirring speed of 800 rpm. Samples were taken at different time intervals, then the adsorbent and adsorbate were separated, and the residual concentration of the dye was determined by spectrophotometry.
In this study, the models chosen to evaluate the adsorption kinetics and define the adsorption mechanism of the natural rock are pseudo-first-order, pseudo-second-order, and intraparticle diffusion.

Pseudo-First order model
The first-order model is described by the following equation 32 : Where: qe1: amount adsorbed at equilibrium (mg/g); qt: amount adsorbed at time t (mg/g); k1: the first order kinetic constant (min-1) t: adsorption time (min). The linearity expression of the equation with a plot of ln(qe-qt) versus t gives 33 : The first-order model suggests that adsorption is reversible and fast when the system is far from equilibrium time.

Pseudo-Second order model
The second-order model is represented by the following equation 34 : Where: qe2: amount adsorbed at equilibrium (mg/g); qt: amount adsorbed at time t (mg/g); k2: the second order kinetic constant (min-1) The linearity expression of the equation with a plot of t/qt versus t gives 34 : The second-order model suggests the existence of chemisorption 35 , which describes adsorption by an exchange of electrons between adsorbate and adsorbent molecules 36 .

Intra-particle diffusion model
The intraparticle diffusion model proposed by Weber and Morris (1963) is used to explain the mechanism of diffusion in the adsorbent pores 37 .
The model is represented by the following equation 37 : = . ⁄ + With :  KD: the rate constant of the intra-particle diffusion model (mg/g.min1/2).  CD: constant associated with the thickness of the boundary layer (mg/g).
In case the lines do not pass through the origin, it indicates that the diffusion into the pores is not the only mechanism limiting sorption kinetics 37 .
The parameters of the pseudo-first-order model, the pseudo-second-order model and intraparticle diffusion are given in the following tables The data shown in Table 2 represent the parameters of the pseudo-first-order model, which give as an idea about the application of this model on our adsorbent. The plot of the PSO model shows low values of the coefficient correlation (R 2 ), and the Qeex is different of Qeth, which leads as to conclude that the kinetic adsorption does not follow this model.
Based on the results obtained from the kinetic study (Table 3), we can observe that the experimental equilibrium could follow the pseudo-second-order model. This is due to the high values of correlation coefficients (R 2 ) close to the unit. Meanwhile, the theoretical values of Qe are equal to experimental; which confirms that the adsorption kinetics is pseudosecond-order and suggests chemisorption.
The study of the adsorption mechanism carried out by the intraparticle diffusion model indicates that the diffusion rate constant KD increases in the same direction as the initial dye concentration, which makes the diffusion stronger when the concentration is high 38 .
The curves plotted in this model (q vs. t 0.5 ) do not pass through the origin, and according to Weber and Morris, this mechanism will not be the only one limiting the adsorption, and there are most undoubtedly multistep mechanisms involved, like chemisorption (valence forces, sharing or exchange ions) confirmed by the best fit shown in the PSO model.

Desorption studies
The desorption test was used to confirm the chemisorption hypothesis of the kinetic study. After leaving the mixture in rest for 3 months, we can observe that the water is clear and there is no desorption (Cres= 0mg/L), which confirms the mechanism of the pseudo-second-order, chemisorption.

Conclusion
The main objective of this study is to find a new adsorbent to enable us to avoid the extra charge and evaluate the adsorption of dyes existing in wastewater using a natural rock in Morocco's central region of Khenifra. This is undertaken in an attempt to reduce the pollutants contained in this wastewater, which pose a severe threat to the environment spilt into. Based on the results obtained, the rock used in this work can remove MB dye with a high yield that depends on various determining factors such as granulometry, and rock's mass while the other effect (pH solution and agitation rate) showed a low action on adsorption yield which demonstrates their futility in future experiments.
The execution of the kinetic study on adsorption shows that the process is speedy, which explains the 95% disappearance of the MB during the first ten minutes, then the rate slows down to 20 minutes, which corresponds to the experimental equilibrium time. The mechanism determining the adsorption reaction can describe by the pseudo-second-order model and the intraparticle diffusion model, explained by their high R 2 and the equality between the values of Qeexp and Qeth.
The results obtained in this study show that our natural rock could be used as an excellent natural adsorbent that allows the removal of methylene blue and other cationic dyes at low cost and high efficiency.