Adsorption of methylene blue dye onto bentonite clay: Characterization, adsorption isotherms, and thermodynamics s tudy by using UV-Vis technique

This s tudy uses the UV-Vis technique to describe the elimination of methylene blue dye from an aqueous solution by adsorption on an Iraqi bentonite clay surface. The batch approach was used to conduct adsorption s tudies carried out to evaluate the in ﬂ uence of factors of experimental like contact time (0–90 min), clay dose (0.1–0.35 g), and initial dye concentration (10–125 mg L -1 ) at the range of temperatures (25-40 o C). The Langmuir and the Freundlich isotherms were used to analyze the data; the Langmuir isotherm (R2 = 0.998) proved more appropriate for the equilibrium data. The thermodynamic properties of the adsorption process, including Gibbs free energy ( Δ G O ), entropy( Δ S O ), and enthalpy ( Δ H O ), were also s tudied. Since the ( Δ G O ) and ( Δ H O ) values were negative, it was clear that the adsorption process cons tituted an exothermic, spontaneous reaction. This inves tigation revealed that Iraqi bentonite clay e ﬀ ectively removed the dye methylene blue because of its high surface area. Methylene blue may be removed with an adsorption e ﬃ ciency of up to 99.39 % at 25 o C. By employing bentonite clay as an adsorbent surface, this research o ﬀ ers practical adsorption technology that is a ﬀ ordable and e ﬀ ective for treating was tewater.


Introduction
The production of textiles, leather tanning, cosmetics, paper, food processing, and pharmaceuticals are jus t a few indus tries whose effluents frequently contain dyes [1].They have long been a source of ecological worry because of their toxic effects, carcinogenicity, mutagenicity, and teratogenicity [2].Many physicochemical techniques, including adsorption, coagulation, chemical oxidation, advanced oxidation, and flocculation, are presently available to treat was tewater containing dyes [3][4][5].Due to its incredible effectiveness, cheap operating cos t, and s traightforward operation technique, adsorption has drawn particular attention from researchers across the globe [6].The main challenge in adsorption has continuously been developing new adsorbent materials to increase removal effectiveness.The greates t significant commercial adsorbents are alumina, activated carbon, polymers, clays, and silica [7,8].Particularly, inexpensive natural clay is regarded as a suitable adsorbent since it is readily available, eco-friendly, non-toxic, has a high surface area, and has a variety of active sites on its surface [9,10].This abundant clay may be used in many water purification processes [11].Several researchers have documented using natural clays like halloysite, ------------------------bentonite, montmorillonite, kaolinite, sepiolite, and smectite to purify several organic contaminants [12][13][14][15][16][17].Also, modern and more advanced techniques have been used in the adsorption of organic pollutants from aqueous solutions in the recent period.These s tudies showed the possibility of using the adsorption process as a significant process for purifying was tewater and the possibility of using it in water purification sys tems [18,19].The uses of Methylene Blue in the other domains mus t be carefully considered and supported by early safety research.This necessitates developing and optimizing a s traightforward approach for detecting Methylene Blue in chemical samples.Numerous analytical techniques can be used to determine the amount of Methylene Blue because of its physicochemical characteris tics.Several techniques can be lis ted, including capillary electrophoresis (CE), liquid chromatography (LC) combined with mass spectrometry (MS), diode array detection (DAD), and ultraviolet-visible spectroscopy (UV-VIS) [16].This research s tudies the Methylene Blue adsorption as a sample of organic dye on natural Iraqi clay without treated (Bentonite) at several temperatures.Key operational parameters such as contact time, clay dose, and initial dye concentration have been considered for their effects.The adsorption process was computed using the isotherm models: Langmuir and Freundlich's models were used to assess the connection between experimental data, inves tigate the adsorbent's ability to maximize adsorption efficiency and compute thermodynamics parameters for the adsorption process.

Ins truments
Water Bath Oscillator (WB-80 -Bioevopeak Co., China), UV-Vis spectrophotometer (Edinburgh Ins truments Ltd., UK), The amount of discrete wavelengths of UV or visible light that are absorbed by or transmitted through a sample in comparison with a reference or blank sample is measured by the analytical technique known as UV-Vis spectroscopy.Digital ultrasonic cleaner (VEVOR-USA), hotplate magnetic stirrer (Bioevopeak Co., Ltd., China), electronic balance (PR2202/E-Thermo Fisher Scientific Inc., USA), mesh sieves (20 μm-Retsch GmbH, Germany), and digital drying oven (Alterlab Co., Indonesia) were used.

Chemicals
All the chemical subs tances utilized in this inves tigation were sourced from reliable sources and were of the highes t purity, including Methylene blue (C 16 H 18 ClN 3 S, CAS number: 61-73-4) from Sigma-Aldrich.Iraqi Bentonite clay (Fig. 1) was obtained from the wes tern desert of Al-Anbar Governorate in Iraq.It is a fine particle of yellow colour and can absorb water.Table 1 shows the chemical components of Bentonite clay.

Preparation of clay powder
The Iraqi clay (Bentonite) used in this s tudy was washed with Deionized water many times to eliminate contaminants and remove water-soluble subs tances such as salts and others.These clays were dried for five hours in an oven whose temperature was (200 o C).The clays were ground in the physical chemis try laboratory at Alayen University, and the ground clay powders were subjected to a size sorting process using sieves designated for this purpose.The sorted particles used in this s tudy were 20 μm (Particle size).After completing the sieving process, the clays were dried and kept in containers with tight lids (Fig. 1).

Preparation of s tandard solution
To prepare different concentrations of Methylene blue (MB) dye, a s tandard concentration of dye (1000 mg L -1 ) was diluted in double-dis tilled water (DDW).The s tock solution was diluted appropriately to provide solutions for adsorption experiments.Using a UV-Vis spectrophotometer with a range of 200 -800 nm, the maximum wavelength (λmax) of 664 nm was determined, corresponding to the highes t absorption of the dye solution, as shown in Figure 2. Six concentrations were used to create the calibration curve, illus trating the relationship between absorbance and concentration: 5, 10, 15, 20, 40 and 50 mg L -1 .The s tandard curve between absorbance and concentration was created in Figure 3 after measuring the absorbance of these concentrations at the dye's λmax.

Adsorption process
The batch approach was used to conduct s tudies on MB adsorption onto Bentonite clay since it is s traightforward and dependable at several temperatures.A set quantity of adsorbent was combined with 40 mL of an aqueous MB solution to conduct all batch adsorption s tudies in 100 ml Erlenmeyer flasks.Filtration was used to remove the adsorbent from the dye solution after a predetermined time.At the maximum adsorption wavelength of MB, λmax =664nm, the UV-Vis spectrophotometer was used to measure the dye concentration in the filtrate.The variation between the initial and final concentrations of the MB solution acquired before and after a connection between the adsorbent and the cationic MB solution was used to compute the concentration of dye absorbed by the adsorbent.Equations ( 1) are used to calculate the amount of MB dye adsorbed per unit mass of the adsorbent (mg g -1 ) at time (t) [22].
V As : Total volume of the adsorbent solution (L).
Co: Initial concentration of the dye (mg L -1 ).
Ce: Concentration of the adsorbent solution at equilibrium (mg L -1 ).m: weight of the adsorbent (g).Qe: amount of adsorbent at equilibrium (mg g -1 ).

Determination of equilibrium time for adsorption sys tem
The equilibrium time between the adsorbed clay surface and the adsorbed amount of dye mus t be evaluated.So, a concentration of 50 mg L -1 of MB dye solution was chosen and made in contact with (0.2 g) of bentonite clay powder at a temperature of (25 o C).Then, the samples were taken from the solution at successive times and analyzed to find out the change in concentration with time.Determine the equilibrium time for the concentration of the dye adsorbed on the surface was equal to 45 minutes, as shown in Figure 4.

Determination of the clay dose
The influence of the adsorbent surface dose on the adsorption process of bentonite clay was inves tigated using a cons tant concentration of a solution of methylene blue dye (50 mg L -1 ) and by taking different doses of bentonite clay at a temperature of (25 o C) and 45 min of contact time.
From the observation of Figure 5, we find that the shape of the curve begins to rise with the increase in the adsorbent surface dose.This height value indicates that the amount of adsorbent surface has reached saturation, which depends on the bentonite clay's physical properties [23].The results show that the bes t dose of bentonite clay for adsorption of the methylene blue dye solution is 0.2 gm.

Adsorption percentage
The adsorption percentage was calculated by Equation 2 [24]. (Eq.2) Since Co and Ce are the initial concentration (mgL - 1 ) of the dye solution and the concentration of dye (mgL -1 ) adsorbed at equilibrium time, respectively.Table 2 shows the adsorption capacity percentage of MB dye on the surface of bentonite clay at the equilibrium time for s tudied temperatures.The amount of adsorbent (Qe) corresponding to each value of the equilibrium concentrations (Ce) shown in Table 3 was calculated using Equation 1.The amount of adsorbent (Qe) was plotted agains t the equilibrium concentration (Ce) to give the general shape of the adsorption isotherms, as shown in Figure 6; it was possible to conclude that the adsorption follows the Langmuir equation (L 2 -Type according to Giles classification).
The major shape of the adsorption isotherms of methylene blue dye on the surface of bentonite is identical to the type (L 2 -Type according to Giles classification), which is characterized by a decrease in slope when increasing the concentration as the active sites decrease gradually until reaching the full coverage of the surface [25].This behavior is attributed to the high affinity for the surface of the adsorbent at low concentrations of the adsorbent, which decreases with increasing concentrations, and this was consis tent with previous s tudies [26,27].Here, there is a need to give a clear idea of the nature and behavior of the clay inside the  Table 3. Adsorption values of MB dye on the bentonite surface clay at s tudied temperatures.solution, as the surface of the clay material contains an abundance of active centers for adsorption due to the possession of wet clay particles by an Electros tatic double-layer [26].We note from the adsorption isotherms on the surfaces of bentonite clay that the quantity of adsorption rises with rising concentration.This increase s tops at a certain concentration, and after this concentration, the value of adsorption capacity s tabilizes and remains cons tant no matter how the concentration of the dye in the solution increases, i.e. it reaches the s tate of saturation of the surface (Plateau Shape).That is, the adsorption is with a monomolecular layer, and the surface coverage of the material is homogeneous [28].It can be explained that the rise in the amount of adsorption of methylene blue dye on the surface of bentonite may be because this clay surface contains effective sites that differ in physical and chemical properties, as well as the s teric form [29], in addition to having the property of ion exchange (Ion -exchange) with other ionic species [30], as bentonite is one of the highly colloidal clays, in addition to its ability to retain the solution inside the layer due to the high plas ticity and colloidal property that gives the characteris tic of spreading the materials on the surface of the bentonite [31].The adsorption data of methylene blue dye on the surface of bentonite clay were processed in the same s tudied range of temperatures according to Langmuir in Equation 3 [32].

Langmuir isotherm model
(Eq.3) Qe is the amount of dye adsorbed at equilibrium, and Ce is the concentration of the dye absorbed at equilibrium (mg L -1 ).Both Qu max and kl are Langmuir experimental cons tants.From drawing the values of (Ce/Qe) vs. the concentration at equilibrium (Ce), a linear relationship was obtained (Fig. 7).From it, we find the values of the Langmuir cons tants shown in Table 4 as the slope is equal to (1/Qe).The intersection is equal to (1/Qe max kl).
In general, it is noted from Table 4 that there is a gradual decrease in the value of kl and the maximum quantity of adsorption (Qe max ) with rising temperature and for all s tudied temperatures.This behavior indicates a decrease in dye adsorption with increasing temperature on the adsorbent surface.On the other hand, when taking the numerical average of the values of the correlation coefficient (R 2 ) for all the s tudied temperatures, it is noticed that its compatibility is on the surface of bentonite clay, where the numerical average of the values of the correlation coefficient (R 2 ) reached (0.998), and this means that the adsorption of methylene blue dye on the surface of bentonite made the Langmuir model more compatible.) Y = 0.04588*X + 0.01464

Frendlish isotherm model
The adsorption data were treated according to the linear of the Frendlish Equation 4 [33].
Ln Qe=ln kf + 1/n ln Ce (Eq.4) Since: Qe: the quantity of dye adsorbed at equilibrium (mg g -1 ).Ce: concentration of dye adsorbed at equilibrium (mg g -1 ).Both (n) and (K f ) are experimental Frendlish cons tants.From drawing lnQe agains t lnCe, the values of the cons tant K f were obtained from the s traight-line segment, which expresses the surface adsorption capacity, while the values of the cons tant n were obtained from the slope of the s traight line, which tells the intensity of adsorption.Table 5 shows the values of the Frendlish model's experimental cons tants and the correlation coefficient (R 2 ) values for the adsorption of methylene blue dye on the surface of bentonite clay.Figure 8 also shows the Frendlish isotherms for dye adsorption on the clay surface at the same  It is noted from Table 5 that the values of the K f cons tant generally decrease with rising temperature for the adsorption of methylene blue dye on the surface of bentonite clay, and this means that the adsorption process decreases with increasing temperature in general.As for the cons tant n, a relative decrease in its value is observed with an increase in temperature, and this means that an increase in temperature leads to a reduction in the affinity of the dye to s tay in the solvent on the one hand and an increase in the adsorption of the MB on the surface of the bentonite on the other hand [34].In comparison, the value of the numerical average of the correlation coefficient (R 2 ) for the bentonite surface according to the Frendlish model was (0.890).As opposed to that, when comparing the values numerical average of the correlation coefficient (R 2 ) of the Langmuir equation (0.998) and the Frendlish equation, we find that it is more applicable to the Langmuir equation.Thus, this equation is more suitable for describing the adsorption isotherm.

Adsorption Thermodynamics
The thermodynamics of the adsorption of MB dye on the surface of bentonite clay were calculated.Table 6 shows the computed values for those functions.The adsorption enthalpy (ΔH O ) value was determined from the slope of the linear Vant Hoff (Equation 5) from the Ln k plot agains t the reciprocal of temperature 1/T [35].The plot of 1/T vis ln Qe for the adsorption of MB dye on the bentonite clay is shown in Figure 9.
ΔG o = ̶ RT LnK (Eq. 6) ∆G° = ∆H° -T∆S (Eq.7) Anal.Methods Environ.Chem.J. 6 (3) (2023) 5-18  The results in Table 6 show that the values (∆H°) for the adsorption of methylene blue dye on the surface of bentonite are negative, which means that the adsorption process is an exothermic reaction.The number of adsorbent particles decreases with the increase in temperature because the thickness of the adsorption layer will decrease with the increase in the temperature of the solution; this is because the rise in the temperature of the solution leads to the increase in the kinetic energy for the dye molecules adsorbed on the adsorbent surface, which leads to their separation from the adsorbent surface and their return to the solution [37].This was confirmed by the kinetic s tudy, where the adsorption capacity decreases with increasing temperature and for all s tudied concentrations.The experimental outcomes of the negative values of ΔG° for dye adsorption on the bentonite surface and for all temperatures indicate that the adsorption reaction is spontaneous in all its s teps.The calculated negative entropy values (ΔS°) also indicate that the dye molecules are more uniform on the surface than in the solution.

Conclusion
The findings of this s tudy show how effective Iraqi bentonite clay is at removing Methylene blue (MB) dye from aqueous solutions when used as an adsorbent surface.The optimal conditions for removing MB were 298 K of temperature, 0.2 gm of clay dose, and 45 min of contact time.
The Langmuir isotherm (R2 = 0.998), according to the experimental results of adsorption isotherms, was more suitable for the equilibrium data.By analyzing the thermodynamic characteris tics, it is possible to determine that the MB adsorption on Iraqi bentonite clay is an exothermic and spontaneous reaction.Also, this inves tigation revealed that Iraqi bentonite clay was effective at removing the dye MB because of its high surface area.MB dye was removed with an adsorption efficiency of up to 99.39 % at 25 o C.This research offers practical adsorption technology that is affordable and effective for treating was tewater using bentonite clay as an adsorbent surface.
The adsorption s tudy of MB dye from aqueous solutions on the surface of bentonite clay was conducted at different temperatures(25,30,35, and 40 o C) at initial concentrations of 10, 20, 30, 40, 50, 75,100, and 125 (mg L -1 ) at a s tirring speed of 150 rpm and at an equilibrium time of 45 min.

1 )Fig. 6 .
Fig. 6.Plot of Ce vis Qe for adsorption of MB dye on the Bentonite clay.

18 Fig. 7 .
Fig. 7. Plot of Ce vis Ce/Qe for adsorption of MB dye on the Bentonite clay.

Fig. 8 .
Fig. 8. Plot of ln Ce vis ln Qe for adsorption of MB dye on the Bentonite clay.

Fig. 9 .
Fig. 9. Plot of 1/T vis ln Qe for adsorption of MB dye on the Bentonite clay.

Table 2 .
Adsorption percentage of dye on bentonite at s tudied temperatures.

Table 4 .
Experimental Langmuir cons tants for adsorption of MB dye on the Bentonite clay

Table 5 .
Experimental Frendlish cons tants for adsorption of MB dye on the Bentonite clay