Adsorption thermodynamics of cationic dyes (methylene blue and crystal violet) to a natural clay mineral from aqueous solution between 293.15 and 323.15 K

Omer Sakin Omer; Mohammed Ali Hussein; Belal H.M. Hussein; Arbi Mgaidi

doi:10.1016/j.arabjc.2017.10.007

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Original article

11 (

); 615-623

doi:

10.1016/j.arabjc.2017.10.007

Adsorption thermodynamics of cationic dyes (methylene blue and crystal violet) to a natural clay mineral from aqueous solution between 293.15 and 323.15 K

Omer Sakin Omer^{^a,^b}, Mohammed Ali Hussein^{^b}, Belal H.M. Hussein^{^a,^c}, Arbi Mgaidi^{^a,^d,⁎}

a Chemistry Department, Faculty of Sciences and Arts, Al Ula Branch, Taibah University, Saudi Arabia

b Department of Industrial and Applied Chemistry, Faculty of Pure and Applied Science, International University of Africa, Sudan

c Department of Chemistry, Faculty of Science, Suez Canal University, Ismailia, Egypt

d University Tunis El Manar, Faculty of Sciences, Chemistry Department, Tunisia

⁎Corresponding author at: Chemistry Department, Faculty of Sciences and Arts, Al Ula Branch, Taibah University, Saudi Arabia. amgaidi@taibahu.edu.sa (Arbi Mgaidi)

Received: 2017-8-23, Accepted: 2017-10-26, Published: 2017-07-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Wastewater from dyestuff production is one of the main water pollutants. Several methods have been applied for the remediation of contaminated water. Currently, adsorption using a cheap, abundant, and environmental-friendly adsorbent such as natural clay is the simplest and most useful method. This study aimed to determine the enthalpies of adsorption of the organic cationic dyes, methylene blue (MB) and crystal violet (CV), from polluted water onto a natural clay mineral. Early on, we performed mineralogical and textural analyses of a clay sample using various techniques, namely X-ray diffraction, scanning electron microscopy/energy dispersive X-ray spectroscopy, Brunauer-Emmett-Teller analysis, Fourier-transform infrared spectroscopy, and differential scanning calorimetry, before and after adsorption. The experimental results showed that this adsorbent is a mesoporous and non-swelling clay with illite and kaolinite as the major components. The effects of various parameters such as contact time, pH, and temperature were examined. The experimental data were analyzed using the linear forms of the Langmuir and Freundlich isotherm models and showed a good fit with the Langmuir equation for MB adsorption. Thermodynamic parameters such as the changes in Gibbs free energy, enthalpy, and entropy were determined from batch experiments. Results revealed that the adsorption of MB onto illitic clay was endothermic, while that of CV was an exothermic and spontaneous process.

Keywords

Adsorption

Cationic dyes

Illitic clay

Temperature effect

Enthalpies of adsorption

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1 Introduction

More than 1000 types of dyes are commercially available and widely employed in the pharmaceutical, food, cosmetics, plastics, photography, paper, and textile industries. The textile dyeing industry consumes large volumes of clean water, which places it in the forefront of the polluters (Abidi et al., 2015). Currently, more than 7 × 10⁵ tons of dyestuffs are produced annually and a significant fraction is discharged directly in aqueous effluents. The presence of colored effluents in an aquatic ecosystem reduces the penetration of sunlight to benthic organisms, thus hindering the process of photosynthesis. Dyes can be classified into three types according to their nuclear structures: anionic dyes (acidic and reactive), nonionic dyes (disperse), and cationic dyes (basic) (Robinson et al., 2001). Organic cationic dyes like methylene blue (MB) and crystal violet (CV) are considered more toxic than anionic ones (Gao et al., 2016). In general, those dyes adsorb on clay in amounts higher than the cation exchange capacity (CEC maximum number of cations adsorbed on the clay). Previous studies (Rytwo and Ruiz-Hitzky, 2003) have concluded that clay-dye interactions at the adsorbent surface are endothermic while dye-dye interactions are exothermic. The enthalpy of adsorption may be derived from adsorption experiments at different temperatures or measured directly by using the microcalorimetric technique. For MB-mineral clay-aqueous solution and CV-mineral clay-aqueous solution systems, thermodynamic data, i.e. ΔG, ΔH, and ΔS, evaluated using these two methods, are divergent and sometimes contradictory with respect to the nature of the adsorption phenomena of the dyes onto the clay mineral surface (see Table 1). Accurate determination of these parameters is useful for the design of adsorption unit operations and for successful implementation of adsorption technology. In general, adsorption is a less energy-intensive unit operation (Gopinathan et al., 2017), and it is well known that |ΔG| range between 0 and 20 kJ/mol for physical processes and that |ΔG| is ≥20 kJ/mol for chemical reactions. Numerous factors can affect the enthalpy of adsorption of a cationic dye onto natural clay such as the structure of the adsorbent, the extent of hydration, and their negatively charged layers. Clay minerals that have been tested, adsorbed organic cationic dyes with maximum adsorption capacities between 10 and 370 mg/g, but increasing the temperature produced a swelling effect. This effect facilitates the diffusion of dyes into the internal structure of the clay. On the other hand, the adsorption of MB onto natural clay has been reported to be an athermal process (Hajjaji and Alami, 2009) and less influenced by increases in temperature (Elmoubarki et al., 2015). The main objective of this study was to investigate the effect of temperature on the adsorptions of cationic dyes MB and CV onto a natural clay mineral. For this purpose, mineralogical and textural analyses of a clay sample were performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) analysis, Fourier transform infrared (FTIR) spectroscopy, and differential scanning calorimetry (DSC), and some operating experimental factors such as contact time, pH, and adsorbent dose were studied using a batch-contact-time method.

Table 1 Reported ΔG°, ΔH°, and ΔS° values from literature.

Dye	Clay mineral used	Nature	ΔG° kJ/mol	ΔH° kJ/mol	ΔS° J/(mol·K)	T range °C	Ref.
Methylene blue(MB)	Montmorillonite (Na,Ca)_0.3(Al,Mg)₂Si₄O₁₀(OH)₂·nH₂O	Endo		+3.9		Cal.^a	Rytwo and Ruiz-Hitzky (2003)
	Kaolinite Al₂Si₂O₅(OH)₄	Endo	−0.27 to −1.72	+14.11	+48.13	25–55	Gopinathan et al. (2017)
	Montmorillonite	Endo	−6.70 to −19.0	+145.0	+492	35–60	Almeida et al. (2009)
	Bentonite (Al,Mg)₈(Si₄O₁₀)₄(OH)₈·12H₂O	Endo	−17.0 to −19.4	+9.21	+92.2	10–35	Hong et al. (2009)
	Smectite (Na,Ca)_0.3(Al,Mg)₂Si₄O₁₀(OH)₂·nH₂O	No effect				15–35	Hajjaji and Alami (2009)
	Sepiolite Mg₄Si₆O₁₅(OH)₂·6H₂O	Endo	–	+56.28	–	30–60	Özdemir et al. (2006)
	Illite associated with chlorite (K,H₃O)(Al,Mg,Fe)₂(Si,Al)₄O₁₀[(OH)₂,(H₂O)]/ (Fe,Mg,Al)₆(Si,Al)₄O₁₀(OH)₈	Exo	+0.318 to + 1.42	−7.5	−27.4	10–50	Elmoubarki et al. (2015)
	Illite/kaolinite (K,H₃O)(Al,Mg,Fe)₂(Si,Al)₄O₁₀[(OH)₂,(H₂O)]	Exo	−2.3 to −0.75	−13.6	−40.3	10–50	Elmoubarki et al. (2015)
	Montmorillonite associated with nontronite K_y(Fe³⁺,2 − xAl_x)(Si_4−yAl_y)O₁₀(OH)₂·nH₂O	Exo 20–40		−7.99 (isosteric enthalpy)		20–60	Gürses et al. (2006)
		Endo 40–60		−7.99 (isosteric enthalpy)		20–60	Gürses et al. (2006)

Crystal violet	Montmorillonite	Exo		−13.5		Cal.^a	Rytwo and Ruiz-Hitzky (2003)
	Kaolinite	Exo	−4.11 to −4.48	−0.235	+13.14	22–50	Nandi et al. (2008)
	Pyrophyllite Al₂(Si₄O₁₀)(OH)₂	Endo	−10.1 to −12.2	+9.28	+66.37	20–50	Miyah et al. (2016)
	Bentonite	Exo	−4.41 to −0.64	−83.81	−260.0	22–36	Eren and Afsin (2008)
	Montmorillonite	Exo	−3.63 to −2.06	−18.97	−52.38	20–50	Kumar et al. (2016)
	Sepiolite	Exo	−1.3 to −0.46	−18.79	−60.0	22–36	Eren and Afsin (2007)
	Mixed clay (kaolinite + pyrophyllite)	Exo	−19.5 to −18.0	−25.59	−9.77	30–50	Monash and Pugazhenthi (2009)

a Cal.: Determined directly using calorimetric measurements.

2 Experimental

2.1

2.1 Materials

Methylene blue (C.I.52015, chemical formula C₁₆H₁₈ClN₃S, mol. wt. = 319.86, λ_max = 664 nm) and crystal violet (C.I. 42555, chemical formula C₂₅H₃₀N₃Cl, mol. wt. = 407.99, solubility in water = 16 g/L at 25 °C, λ_max = 590 nm) were purchased from Sigma Aldrich. All chemicals were used as received. The structures of the dyes are given in Fig. 1. Raw clay sample was purchased from AlMarwani for Spices Company, Saudi Arabia. A certain amount of the clay was suspended in a 1 M NaCl solution at a 5% clay-to-solution ratio, and the suspension was agitated for 12 h using a mechanical stirrer. The suspended solid was separated from the solution by centrifugation for 10 min at 4000 rpm using a Hettich EBA 20 centrifuge, dried in an oven for 72 h at 60 °C, ground, and sieved to obtain a fine texture.

Structures of (a) methylene blue and (b) crystal violet.

2.2

2.2 Characterization of adsorbent

The clay was characterized using a powder X-ray diffractometer (Phillips Xpert-pro) between 2θ values of 5° and 60° using Cu K_α radiation. The microstructure of the adsorbent was examined using a JEOL JED2200 series apparatus coupled with an energy dispersive X-ray detector. The specific surface areas and pore size distribution were estimated through N₂ physisorption at 77 K by applying the BET equation on a Micromeritics ASAP2000 apparatus. The sample was degassed at 373 K for 1 h prior to BET measurements. FTIR spectra in the 4000–400 cm⁻¹ region were obtained for the samples before and after adsorption using a Thermo Scientific spectrometer with a deuterated triglycine sulfate (DTGS) detector and a KBr splitter. The KBr pressed disc technique (1 mg of sample and 200 mg of KBr) was used. Spectra were manipulated using the OMNIC software package (Thermo Instruments Corp.). Finally, raw sample and two samples after dye adsorption were also examined using DSC under an air atmosphere using a Shimadzu DSC-60 detector. The heating rate was 2 °C/min.

2.3

2.3 Batch equilibrium studies

Batch equilibrium investigations were carried out using a set of Erlenmeyer flasks (50 mL) with 25 mL of dye solutions at different temperatures (20 °C, 30 °C, 40 °C, and 50 °C). Initially, batch experiments were performed at 30 °C to study the effects of contact time, pH, and adsorbent dose. The optimal conditions were a pH of 7.0, contact time of 180 min, and adsorbent dose of 0.5 g per liter of dye solution for MB, while for CV, the optimal pH and contact time were 7.5 and 180 min, respectively. The flasks were placed in a shaker and the solutions were shaken until equilibrium time, at which the flasks were removed from the shaker and the solutions were centrifuged at 4000 rpm for 20 min. The supernatant was analyzed using an ultraviolet–visible (UV–Vis) spectrophotometer at λ = 664 nm for MB and λ = 590 nm for CV.

2.4

2.4 Calculation

The amount of dye adsorbed by the clay in each time interval, t, was calculated according to the following mass balance equation:

(1)

q_{e} = (C_{0} - C_{e}) \times \frac{V}{m}

where q_e is the amount of dye adsorbed per unit weight of clay (mg/g), C₀ and C_e are the initial and equilibrium dye concentrations (mg/L), respectively, V is the volume of the solution (L), and m is the dry weight of the clay (g).

3 Results and discussion

3.1

3.1 Characterization of the adsorbent

XRD patterns of clay samples (washed and unwashed samples) are shown in Fig. 2. The two samples show the characteristic (0 0 1), (0 0 2), and (0 0 3) peaks of illite and kaolinite, which indicates that illite and kaolinite are the dominant clay minerals with a small amount of quartz.

Results of XRD analyses of the clay mineral samples used.

Morphological characterization of the adsorbent using a field emission scanning electron microscope revealed an irregular texture and porous cavities on the surface (see Fig. 3).

The specific surface area (S_BET), average pore size by BET, and total pore volume of the clay obtained from N₂ adsorption-desorption isotherms at 77 K were 128 m²/g, 75.6 Å, and 0.320 cm³/g, respectively. Investigation of the pore size distribution using Barrett-Joyner-Halenda (BJH) analysis showed that the pore size distribution was in the mesopore range (5–50 nm) with a significant contribution from pores ∼40 nm in diameter. Our results are in agreement with those published for an illitic clay sample (Ghorbel-Abid et al., 2010).

FTIR analysis was conducted to evaluate the roles of functional groups on the clay surface in the adsorption process. Fig. 4 shows the FTIR spectra of the adsorbent samples before and after dye adsorption. A broad band at 1024 cm⁻¹ is characteristic of Si—O stretching vibrations. The peaks for the OH stretching of hydroxide groups coordinated to Al³⁺ ions are located at 3697 and 3614 cm⁻¹while the peak for the OH deformation of water is located at 1630 cm⁻¹. New bands were observed at 1340, 1370, and 1595 cm⁻¹ after the dyes were adsorbed. These bands could be assigned to —C—N—, —C⚌N—, and —C⚌C— stretching in polyheterocycles. These bands could be used as evidence for the adsorption of the dyes on clay.

FTIR spectra: raw clay, clay after CV adsorption, and clay after MB adsorption.

In the DSC curves (see Fig. 5), there is an endothermic peak at around 90 °C for the clay before adsorption, which can be assigned to the loss of adsorbed water. After adsorption, we observed two exothermic peaks at around 260 °C for MB and 370 °C for CV. For CV, our observation is in agreement with previous work (Eren and Afsin, 2008). The exothermic oxidation of the dye under an air atmosphere is the principal thermal reaction in this temperature range.

DSC curves: raw clay, clay after CV adsorption, and clay after MB adsorption.

3.2

3.2 Effects of contact time and pH

The effect of contact time on the adsorptions of MB and CV at 30 °C is illustrated in Fig. 6. It appears that a rapid initial uptake occurs and equilibrium is reached in less than 50 min.

Effect of contact time on the adsorptions of MB and CV by a clay mineral (adsorbent dose = 1 g/L, T = 30 °C, pH = 7.5 for CV and pH = 7 for MB).

For both MB and CV, the adsorption is very rapid in the first 5 min and then becomes slower until equilibrium is reached. The rapid initial increase in the amount of adsorbed dye may be due to an increase in the number of vacant sites available at the beginning. The adsorption behaviors of MB and CV on the clay were studied over a wide pH range of 3–12. Fig. 7 shows the effect of initial pH on the removal of MB and CV by clay. A reduction in the amount adsorbed was observed for both dyes as the pH decreased. The lower adsorptions of MB and CV at acidic pH values are probably due to the presence of excess H⁺ ions competing with the dye cations for the adsorption sites. At a higher pH (pH > 7), the clay surface will be more negatively charged, facilitating association between the cationic dyes and the adsorbent surface.

Effect of solution pH on the adsorptions of MB and CV by a clay mineral (adsorbent dose = 1 g/L, time = 120 min, T = 30 °C).

3.3

3.3 Effects of initial concentration and temperature on adsorption

The effect of the initial concentration of the dye solution on the adsorptions of MB and CV onto clay is illustrated in Fig. 8. It was observed that the adsorption of MB decreased as the concentration of the dye solution increased from 90 to 300 mg/L. It was also noted that the adsorption of MB was not favored at high temperatures (see Fig. 8a). In contrast, the adsorption of CV increased with increasing dye concentration and is favored at high temperatures (see Fig. 8b).

Effects of initial concentration and temperature on the adsorption of the dyes onto illitic clay.

The difference between these isotherms is probably due to the different adsorption mechanisms of the two monovalent organic cationic dyes (Rytwo et al., 1995). Those studies concluded that the interactions between CV molecules and natural clay differ in certain aspects from those between MB and clay due to the amount of charged complexes formed between two dye molecules (monomer, dimmer or trimer). The aggregation of MB molecules in solution also reduces its adsorption on the clay. But, at low temperature, there is competition between MB molecules and water molecules in the inter-lamellar spaces of clay and it is well known that the adsorption of water molecules on a solid is higher at lower adsorption temperature.

The experimental data were analyzed using the Langmuir and Freundlich isotherm models. The linear forms of these equations are as follows:

(2)

\frac{C_{e}}{q_{e}} = \frac{1}{q_{\max}} \times C_{e} + \frac{1}{k_{L \times} q_{\max}}

(3)

Ln (q_{e}) = \frac{1}{n} \times Ln (C_{e}) + Ln (k_{F})

where q_max, k_L,n, and k_F are the constants for the Langmuir and Freundlich models.

Fig. 9 shows the Langmuir and Freundlich plots for the adsorptions of MB and CV onto illitic clay at 20 °C, 30 °C, 40 °C, and 50 °C. This figure shows that the Langmuir approach accommodated the equilibrium data well, with a high correlation coefficient (R²), as shown in Table 2. A similar result was obtained in a previous investigation (Al-Futaisi et al., 2007). The authors found that the Langmuir model described the equilibrium adsorption data well for MB adsorption and the Freundlich model was effective for CV adsorption onto palygorskite.

Langmuir and Freundlich plots of the adsorptions of MB and CV onto illitic clay at 20 °C, 30 °C, 40 °C, and 50 °C: (a) Langmuir plot for MB adsorption; (b) Langmuir plot for CV adsorption; (c) Freundlich plot for MB adsorption; and (d) Freundlich plot for CV adsorption.

Table 2 Fitted isotherm parameters and their goodness of fit to the adsorption data.

Dye	Parameter	q_max (mg/g)	K_L (L/mg)	k_F (mg^1−1/ng⁻¹L^1/n)	n	R² Langmuir	R² Freundlich
Methylene blue	T = 293.15 K	100.0	−0.0649	494.2	−5.7	0.99	0.88
	T = 303.15 K	76.9	−0.0425	1075.9	−3.1	0.98	0.93
	T = 313.15 K	71.4	−0.0375	1314.1	−2.7	0.98	0.95
	T = 323.15 K	62.5	−0.0320	2024.5	−2.1	0.97	0.97

Crystal violet	T = 293.15 K	330.0	0.0492	150.3	3.95	0.98	0.98
	T = 303.15 K	330.0	0.0612	214.2	5.12	0.99	0.96
	T = 313.15 K	330.0	0.0857	1314.1	5.88	0.99	0.96
	T = 323.15 K	330.0	0.0789	2024.5	6.06	0.98	0.96

The results in Table 2 also show that an increase in temperature resulted in a decrease in the maximum adsorption of MB. At 50 °C, the Langmuir monolayer capacity, q_max, is 62.5 mg/g. This result is in agreement with previous results (Özdemir et al., 2006; Elmoubarki et al., 2015). For CV adsorption, q_max appears to be temperature independent. The parameters k_L, n, and k_F are also on the same orders of magnitude as those reported in literature (Gao et al., 2016; Kumar et al., 2016; Auta and Hameed, 2012 ). The calculated Langmuir constant (k_L) and Freundlich constant (n) can be interpreted as the free energy of adsorption and the adsorption intensity, respectively. High k_F values reflect a high adsorption capacity of the adsorbent.

Thermodynamic studies are an essential component of proposing adsorption mechanisms (e.g., chemical and physical). To calculate the thermodynamic parameters, we used the following equation:

(4)

Δ G ° = - RT \ln K °

where R is the constant of perfect gas (R = 8.314 J/mol. K), T is the absolute temperature of solution (K), K° is the standard equilibrium constant, which is a dimensionless parameter (without units).

The relationship between ΔG°, ΔH° and ΔS° is given by:

(5)

Δ G ° = Δ H ° - T Δ S °

Substituting Eq. (4) into Eq. (5), we obtained the well-known vant'Hoff equation:

(6)

\ln K ° = \frac{- Δ H °}{R} \times \frac{1}{T} + \frac{Δ S °}{R}

K° is related to the Langmuir constant k_d(q_e/C_e) which is dimensional with common units (L/mg) and can be calculated as follows (Sakin et al., 2017; Tran et al., 2017):

(7)

K ° = \frac{q_{e}}{c_{e}} \times M_{Adsorbate} \times C^{0}

where q_e is the amount of dye adsorbed per unit mass of the clay, C_e is the equilibrium dye concentration (mg/L), M is the molecular weight of the adsorbed species, and C° is the concentration of the solution in the chosen standard state (C° = 1 mol/L).

The values of the thermodynamic parameters determined in this study are summarized in Table 3. The enthalpy of adsorption of MB, +6.5 kJ/mol, indicates that the process is endothermic and is on the same order of magnitude as those reported in the past (Rytwo and Ruiz-Hitzky, 2003; Gopinathan et al., 2017; Hong et al., 2009 ). The negative values of ΔS° for MB adsorption suggest a decrease in randomness at the solid/solution interface, whereas the positive value of ΔS° as a result of CV adsorption corresponds to an increased degree of freedom in the system and indicates high affinity of the adsorbent for CV molecules. The negative value of ΔH°, −9.8 kJ/mol, indicates an exothermic adsorption process for CV. From this table, we noted that the values of ΔG° are between −9.25 and +7.44 kJ/mol, which indicate that the adsorptions of MB and CV are physical in nature.

Table 3 Thermodynamic constants for the adsorption of cationic dyes on illite + kaolinite clay at various temperatures (time = 3 h, pH = 7–7.5, and adsorbent dose = 0.5 g/L).

Dye	T (K)	$Δ G °$ (kJ/mol)	$Δ H °$ (kJ/mol)	$Δ S °$ (J/(mol·K))
Methylene blue	293.15	+6.75	+6.5 (+3.9)^a	−23.0
	303.15	+6.98
	313.15	+7.2
	323.15	+7.44

Crystal violet	293.15	−8.31	−9.8 (−13.5)^a	+28.5
	303.15	−8.63
	313.15	−8.90
	323.15	−9.25

a Values are from Rytwo and Ruiz-Hitzky (2003).

No comparative study has been reported scarcely up to now. In this table we reported for comparison the enthalpies of adsorption of methylene blue and crystal violet to montmortillonite determined using calorimetric measurements (values in parentheses from Rytwo and Ruiz-Hitzky (2003). We note that our values are in the order of magnitude of those published data.

4 Conclusion

In the present study, the effect of temperature on the removal of methylene blue and crystal violet from synthetic wastewater is studied using a natural clay mineral as an adsorbent. The adsorption of MB to illitic-kaolinite natural clay is better modeled by Langmuir model. On the other hand, the adsorption equilibrium of CV is predicted by Freundlich approach. The results also show that the enthalpy of adsorption of MB is slightly positive (ΔH° = +6.5 kJ/mol) signifying that the process is endothermic. However, the enthalpy of adsorption of CV is negative (ΔH° = −9.8 kJ/mol) which is in accordance with an exothermic process. These results are in line with the reported values on the direct determination of enthalpies of adsorption using calorimetry. Physisorption is the prevailing mechanism for both monovalent cationic dyes adsorption onto natural clay.

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Al-Futaisi A., Jamrah A., Al-Hanai R., . Aspects of cationic dye molecule adsorption to palygorskite. Desalination. 2007;214:327-342.
[CrossRef] [Google Scholar]
Auta M., Hameed B.H., . Modified mesoporous clay adsorbent for adsorption isotherm and kinetics of methylene blue. Chem. Eng. J.. 2012;198–199:219-227.
[CrossRef] [Google Scholar]
Sakin O., Hussein B.H.M., Hussein M.A., Mgaidi A., . Mixture of illite-kaolinite for efficient water purification: removal of As(III) from aqueous solutions. Desalin. Water Treat.. 2017;79:273-281.
[CrossRef] [Google Scholar]
Tran H.N., Jie You S., Hosseini-Bandegharei A., Chao H.P., . Mistakes and inconsistences regarding adsorption of contaminants from aqueous solutions: a critical review. Water Res.. 2017;120:88-116.
[CrossRef] [Google Scholar]

Introduction

Experimental

Materials
Characterization of adsorbent
Batch equilibrium studies
Calculation

Results and discussion

Characterization of the adsorbent
Effects of contact time and pH
Effects of initial concentration and temperature on adsorption

Conclusion

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[Google Scholar]

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[CrossRef] [Google Scholar]

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[CrossRef] [Google Scholar]

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[CrossRef] [Google Scholar]

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[CrossRef] [Google Scholar]

Adsorption thermodynamics of cationic dyes (methylene blue and crystal violet) to a natural clay mineral from aqueous solution between 293.15 and 323.15 K