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Abstract

In this study, three adsorbents were prepared with the raw kaolin issued from the Tamazert region in Algeria (Tamazert kaolin (KT)), for the removal of basic dye methylene blue (MB) from aqueous solutions. The first absorbent, KT-1, was obtained by thermal treatment; the second, KT-2, was obtained by thermal followed by acid treatment; and the third, KT-3, was obtained by thermal followed by acid and alkaline treatments. These adsorbents were characterized by powder X ray diffraction, Fourier transform infrared spectroscopy, Brunauer–Emmett–Teller surface area, and scanning electron microscopy. Adsorption equilibrium isotherms of MB were correlated with common isotherm equations such as Langmuir and Freundlich models. The modified kaolins (KT-1, KT-2, and KT-3) showed different capacities of the equilibrium adsorption in comparison with raw kaolin (KT). The maximal adsorption capacity (111 mg g⁻¹) was observed with modified kaolin (KT-3). The effect of operating parameters such as the initial dye concentration, contact time, adsorbent dose, pH, and temperature were studied onto KT-3. The removal of MB by adsorbent increased with an increase in adsorbent dose and initial concentration. Kinetics models, the pseudo-first-order, and pseudo-second-order rate equations were applied. The obtained results show that the adsorption of MB by KT-3 was well described by the pseudo-second-order kinetic model. The thermodynamic study revealed that the adsorption process was spontaneous, endothermic, and the positive value of $Δ S^{\circ}$ indicated the affinity of MB molecules to the adsorbent surface. This study showed that this new adsorbent could be a good candidate for some activated carbons.

Keywords

Adsorption modified kaolin methylene blue isotherms kinetics

Introduction

The day-to-day human and industrial activities have influenced the quality of fresh water. Many industries like textile refineries, chemical, plastic, and food-processing plants produce wastewaters, characterized by a perceptible content of organics with strong color (Mohan et al., 2007). Due to the low levels of dye-textile retention, textile industry generates a great amount of polluted water with high contents of different kinds of dyes (Salleh et al., 2011). Methylene blue (MB) was one of the very famous cationic dyes. It has wide applications that include coloring papers, dyeing cottons, wools, silk, leather, and coating for paper stock. Although MB is not strongly hazardous, it can cause some harmful effects, such as heartbeat increase, vomiting, shock, cyanosis, jaundice, quadriplegia, and tissue necrosis in humans (Kushwaha et al., 2011).

Many techniques are used for the removal of dye compounds from wastewater such as sedimentation, filtration technology, chemical treatment with coagulating flocculating agent, oxidation by using oxidizing agents, electrochemical methods, advanced oxidation processes (AOPs), biological treatments, and adsorption and ion exchange (Gupta et al., 2009). In addition to the previously mentioned methods, the adsorption process has been widely used for color removal. Adsorption is one of the processes which are widely used for dye removal and also have wide applicability in wastewater treatment (Gupta et al., 2009). Due to its effectiveness and versatility, activated carbon is widely employed in water and wastewater treatment. However, the operating cost of activated carbon adsorption is high. Problems of regeneration and difficulty in separation from the wastewater after use are the two major concerns of using this material (Trevino-Corderoa et al., 2013).

In recent years, there is an increasing interest in utilizing clay minerals such as bentonite, kaolinite, diatomite, and Fuller’s earth as adsorbents to remove not only inorganic but also organic molecules. The adsorption efficiency of clays generally results from abundance, high potential for ion-exchange, net negative charge on the structure of minerals, mechanical and chemical stability, and high surface area. The negative charge gives clay the capability to adsorb positively charged species (Gupta et al., 2009). However, kaolinite is a clay mineral that can be used as an adsorbent for MB dye, but its adsorption capacity remains low in comparison with others adsorbents. This is due to its low surface area and cationic exchange capacity (Jiang et al., 2009). Recently, clay minerals can be modified in different ways to obtain specific chemical, surface, and structural properties for different applications by pre-treatment with chemicals or combined treatment such as heating and chemical attacks (Adeyemo et al., 2015; Auta and Hameed, 2012; Gao et al., 2015; Ijagbemi et al., 2010; San Cristobal et al., 2009; Yavuz and Saka, 2013).

The present work investigates the feasibility of using raw kaolin of Tamazert and their modified kaolin forms, for removal of MB from aqueous solution by adsorption under various conditions. Characterization of the raw and modified kaolin was studied by different techniques (X-ray diffraction (XRD), differential thermal analysis thermogravimetric analysis (DTA-TG), Fourier transform infrared (FT-IR), Brunauer–Emmett–Teller (BET) surface area, and scanning electron microsocopy (SEM)). The equilibrium data were tested with the Langmuir and Freundlich isotherms models. Effects of adsorbent dosage, initial dye concentration, pH and temperature were studied. Adsorption kinetics models were employed to analyze the kinetics and mechanisms of MB adsorption on the modified kaolin KT-3. Thermodynamics parameters of the adsorption were also calculated to understand the adsorption process.

Materials and methods

Materials

The clay mineral used in the study, Tamazert kaolin (KT), was obtained from the National Society of Sanitary Ceramics located in the region of El Milia province of Jijel, Algeria. It was used as received without any other treatment apart from drying at 100℃ for 24 h to remove excess moisture, and then kept in a desiccator. The percentage of the main minerals in starting material is shown in Table 1.

Table 1.

Chemical composition of Tamazert kaolin.

Chemical composition (% wt)
SiO₂	Al₂O₃	Fe₂O₃	TiO₂	CaO	MgO	K₂O	Na₂O	LOI
48	37	0.85	0.05	0.07	0.3	1.75	0.1	12.1

LOI: lost on ignition.

The cationic dye, methylene blue (MB), was purchased from Sigma Aldrich. The chemical structure of MB is shown in the Figure 1.

Figure 1.

Structure of MB dye.

Preparation of clay adsorbents

The natural kaolin (KT) was calcined at 800℃ for 5 h in air flow. Where its structure was destroyed and any undesired volatile matter was removed, the obtained material was metakaolin (KT-1). The obtained sample was treated with 2.5 M solution of chlorhydric acid at 80℃ for 7 h, washed with distilled water, and dried in an oven at 110℃ for 3 h (designated as KT-2).The obtained material KT-2 was treated with 0.5 M solution of sodium hydroxide, in the same conditions as in the case of the acid treatment. The final product was designated as KT-3.

Characterization

Thermogravimetric analysis (TG) and differential thermal analysis (DTA) of the natural kaolin (KT) were performed on Perkin–Elmer TGA7 and Perkin–Elmer DTA7 devices, respectively, from 25 to 960℃ at heating rate of 10℃ min⁻¹and under an air ﬂow of about 60 mL min⁻¹.

The XRD patterns for the samples were recorded on a Philips X’Pert instrument using Cu Ka radiation (λ = 1.54184 nm). The excitation conditions were 40 kV and 50 mA. A secondary graphite monochromator and a mini-proportional counter were also used, and patterns recorded at a scan rate of 0.02° 2θ/s from 5° to 90°. All measurements were made on disoriented powder samples.

FT-IR spectra were recorded on a Shimadzu IRAffinity-1 spectrometer using an amount of 1 mg of powdered sample that was mixed with 200 mg of KBr to obtain a pellet for analysis. The wave number range was 400–4000 cm⁻¹.

The morphological features and surface characteristics of samples were obtained from SEM using a Philips XL20 Scanning Electron Microscope at an accelerating voltage of 10 kV.

The specific surface area was determined by nitrogen adsorption at −196℃ on a Micromeritics ASAP 2000 instrument. Specific surfaces areas (S_BET) were calculated by applying the BET method. Before measurements all samples were outgassed at 200℃ to ensure that they were clean, dry, and completely free of loosely adsorbed particles (Konan et al., 2009; San Cristobal et al., 2009).

To determine the point of zero charge (pH_ZPC) of KT, 0.2 g of adsorbent was added to 40 mL of NaCl solution (0.1 M) at different initial pH values (pH_i) (2–12) and agitated for 24 h at room temperature. The final pH (pH_f) values of solutions were then measured (Deng et al., 2010; Kumar et al., 2008).

The cationic exchange capacity was calculated using MBT test. The procedure is as follows. We added 1 g of sample to 15 mL of fresh water under vigorous stirring until the dispersion of the sample. Then, we added 15 mL of hydrogen peroxide (3%) with 12 drops of sulfuric acid (5 N); this mixture was boiled gently for 10 min and diluted with 20 mL of fresh water. We added MB solution (3.2 g L⁻¹) in 1.0 mL increments. After each addition, we stirred vigorously for at least 20 s. Then, we removed a drop of the sample at the end of stirring rod. The approximate end point is reached when a blue ring spreads out from the blue spot on the filter paper. The cation exchange capacity (CEC) was calculated using the following relation (Khar and Madsen, 1995; Yukselen and Kaya, 2008):

CEC = \frac{V_{mb}}{W}

(1) where V_mb is the MB dye add volume (mL) and W is the weight of sample (g).

Total acidity and basicity of adsorbents were determined using standard Boehm titrations (Issa et al., 2014; Priscila et al., 2015). About 0.25 g of each sample was mixed with 50 mL of 0.01 mol/L aqueous reactant solution (HCl or NaOH). The mixtures were stirred for 24 h at constant speed at room temperature. Then, the suspensions were filtrated and centrifuged to get a clean supernatant and titrated in the presence of phenolphthalein as indicator.

Adsorption experiments

Batch experiments were carried out using 1 -L capacity glass beaker at ambient temperature (18.5℃), the stock solution of MB dye was prepared, by dissolving 0.32 g of solid dye in 1 L of deionized water. Different concentrations were obtained by dilution of the stock solution. The initial and the final dye concentrations were determined at characteristic wavelength of MB (λ_max = 665 nm), by double beam UV–visible spectrophotometer (Shimadzu UV-1601). Initial dye concentrations were varied between 30 and 100 mg L⁻¹. To observe the effect of others parameters, the concentration of 150 mg L⁻¹ was chosen. To observe the effect of pH, experiments were carried out at different pH values varied from 2 to 12.5. To observe the effect of adsorbent dose on dye adsorption, different amounts of adsorbent were used (varying from 1 to 6 gL⁻¹). The effect of temperature has been studied at 18℃, 30℃, and 50℃. A common adsorbent dose of 1 gL⁻¹ with a stirring speed of 450 r/min were used for all the above experiments.

The amount of dye adsorbed per unit weight of adsorbent; q_t (mg g⁻¹) was calculated using the mass balance equation given as follows:

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

(2)

The dye removal percentage can be calculated as follows:

% DyeRemoval = \frac{(C_{0} - C_{t})}{C_{0}} \times 100

(3) where C₀ is the initial dye concentration (mg L⁻¹), C_t is the concentration of dye at any time t, V is the volume of solution (L), and m is the mass of adsorbent (g).

In order to study the adsorption isotherm, 0.05 g of kaolin was kept in contact with 50 mL dye solution of different concentrations for 2 h (to conﬁrm that the equilibrium has been reached). After 2 h, the solution attained equilibrium and the amount of dye adsorbed (mg g⁻¹) on the surface of the adsorbent was calculated using the following relation:

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

(4) Where C_e (mg L⁻¹) is the concentration of dye at equilibrium time.

Results and discussion

Characterization

Thermal analysis

Thermal analyses of KT are given in Figure 2. DTA curve presents two endothermic transformations which begin at 120℃ and 520℃, respectively. The first peak of low intensity is associated with a small weight loss (about 1.5 wt.%) due to the departure of hydration water without any structural modification of the material. The second peak, which starts at 450℃, exhibits an important intensity and the weight loss is significant (about 12.5 wt. %). As known, during the dehydroxylation of kaolinite, the weight loss is equal to 13.76%. The reaction is given by

{Al}_{2} {Si}_{2} O_{5} (OH)_{4} \to {Al}_{2} {Si}_{2} O_{7} + {2 H}_{2} O

Figure 2.

Thermal analysis of KT kaolin. DTA: differential thermal analysis; TGA: thermogravimetric analysis.

The difference between the two values is due to the presence of impurities (San Cristobal et al., 2009).

XRD characterization

The XRD patterns of KT and obtained samples at different treatment steps are shown in Figure 3. Sample KT is constituted by the predominant simple phase as kaolinite, a small amount of quartz, muscovite, and anatase. The thermal treatment of KT at 800℃ (KT-1) transforms the kaolinite into a typical amorphous metakaolinite phase. This transformation explains the decrease in the crystallinity of the sample. There is no evidence of any kaolinite peaks due to the loss of water confirmed by the thermal analysis (Figure 2) and deterioration of its crystal structure (Al-Harahsheh et al., 2009). The XRD pattern of the KT-2 sample is the same as KT-1, where during the acid attack, the interlayer cations are replaced by protons and the octahedral cations (Al³⁺) are dissolved. The product is X-ray amorphous, of high SiO₂ content, and shows a high specific surface area and micro- and mesopores (Hussin et al., 2011; Ijagbemi et al., 2010). The XRD pattern of KT-3 is similar to KT-2 sample; we can say that NaOH treatment after acid attack has no influence on the structural proprieties of the acid-activated metakaolin.

Figure 3.

XRD patterns of the obtained samples. M: muscovite; K: kaolinite; Q: quartz; A: anatase.

FT-IR spectroscopy

The FT-IR spectra in the range 400–4000 cm⁻¹ of the different samples are shown in Figure 4. The main characteristic bands of KT are listed as follows.

Figure 4.

Infrared spectroscopy of KT kaolin and treated samples.

The bands in the 3000–4000 cm⁻¹ correspond to the hydroxyl groups sitting at the edges of the clay platelets, hydroxyl groups at the surface of the octahedral layers that interact with the oxygen atoms of the adjacent tetrahedral layers, and the internal hydroxyl groups. The band at 1674 cm⁻¹ is attributed to the vibration of water molecules. The bands at 937 and 912 cm⁻¹ are due to stretching Al–OH groups. The bands at 795, 754, 696, and 536 cm⁻¹ correspond to the vibrations of Si–O–Al groups. The bands at 1115, 1032,1006, and 740 cm⁻¹ correspond to the elongation of Si–O–Si, and the bands at 671 and 471 cm⁻¹ are assigned to the bending and symmetric deformation vibrations of Si-O-Si groups. All these bands are typical of kaolinite clay mineral (Daud et al., 2010; Volzone et al., 2006).

For the IR spectrum of the sample KT-1, there is a disappearance of the peaks at 3693, 3670, and 3650 cm⁻¹. In fact, after the thermal treatment, most of the hydroxyl groups have been removed by dehydroxylation (Hassan et al., 2011). All peaks that correspond to Si–O–Si groups are grouped in a single large band at 1060 cm⁻¹which are attributed to amorphous silica. Therefore, thermal treatment leads not only to a dehydroxylation of kaolinite but also to a structural modification. The disappearance of the bands at 912 and 937 cm⁻¹ indicates the loss of Al–OH units, while the changes in Si–O stretching bands and the disappearance of the Si–O–Al bands at 754 cm⁻¹ are consistent with distortion of the tetrahedral and octahedral layers. The peak at 795 cm⁻¹, which is narrow for kaolinite, becomes in the case of metakaolinite a broad band. This broad band is a characteristic of the degree of disorder in metakaolinite.

KT-2 exhibits the same IR spectra with KT-1; only the peaks at 1060 and 795 cm⁻¹ become narrow (San Cristobal et al., 2009). For KT-3, we observe the appearance of new peaks at 775, 536, and 471 cm⁻¹.

Scanning electron microscopy

The surface morphology of these four adsorbents was observed under SEM analysis (Figure 5). KT sample reveals the presence of well-crystallized kaolinite consisting of hexagonal particles of different sizes forming books of variable thickness. KT-1 saves the hexagonal form but with high distortion in the SiO₄ tetrahedral layers. After acid treatment, the kaolinite layers are shown to have randomly arranged pores, leading to further increase in surface area (Al-Harahsheh et al., 2009; San Cristobal et al., 2009; Volzone et al., 2011). KT-3 seems to save the random structure of KT-2.

Figure 5.

SEM images of KT kaolin and treated samples.

Specific surface area

The BET specific surface areas for the samples are summarized in Table 2. After thermal treatment, there is a very small increase in surface area, due to the effect of calcination on the structure of kaolinite. According to the literature, the books of metakaolinite are more open than the ones of kaolinite. This gives a greater pore diameter of slit-shaped pores of the structure. The specific surface area of the KT-1 increases by acid attack from 16 to 191 m²g⁻¹.The increase in porosity is due to the partial dissolution of the exchangeable cations (Al³⁺, Fe³⁺), which causes an opening in the edges of the platelets (Bhattacharyya and Gupta, 2006; Lenarda et al., 2007; Noyan et al., 2007). A large decrease in the BET surface area is observed after NaOH treatment, between KT-2 and KT-3 samples. This difference is due essentially to the insertion of Na⁺.

Table 2.

Physical and chemical properties of adsorbents.

Adsorbents	S_B_ET (m²g⁻¹)	CEC (meq/100 g)	pH_zpc	Acidity (mmolg⁻¹)	Basicity (mmolg⁻¹)
KT	14	12	3.1	1.70	0.02
KT-1	16	7	6	0.38	0.02
KT-2	191	16	3.5	1.14	0.025
KT-3	45	23	7.4	1.42	1.025

Point zero charge

The point of zero charge (pH_pzc) is an important factor that determines the linear range of pH sensitivity and then indicates the type of surface active centers and the adsorption ability of the surface. The values of pH_zpc are shown in Table 2 as a function of the various processing carried out on kaolin. The pH_pzc of KT is 3.1 and after thermal treatment, the value increases to achieve 6 due to the elimination of hydroxyl groups. The acid attack decreases pH_zpc to 3.5. This can be explained by the insertion of positive charges as H⁺ proton to the structure of KT-1. However, the alkaline treatment increases this value to 7.4 due to a modification in the global charge of adsorbent which caused by the addition of more sodium oxide into the structure of KT-2 (Al-Harahsheh et al., 2009).

Cation exchange capacity (CEC)

According to Table 2, the CEC of KT is 12 meq/100 g. This value is in good agreement with literature results (3–15 meq/100 g), but it is lower to that of montmorillonite (Ghosh and Bhattacharyya, 2002; Karaoglu et al, 2009; Kooli, 2013; Koswojo et al., 2010, Ma et al., 2012). After thermal treatment, the decrease in CEC to 7 meq/100 g is due to the dehydroxylation reaction during the thermal treatment, in which most hydroxyl groups and interlayer cations, which are H⁺ in the case of kaolinite, are eliminated as water molecules. This leads to a significant decrease in the amount of H⁺ in the interlayer space. The acid treatment from KT-1 sample shows an increase above the double CEC (16 meq/100 g). This improvement can be explained by the important insertion of H⁺ ions in the structure of the KT-2 sample. The maximum value of the CEC is obtained with NaOH treatment of the sample KT-2. This increase up to 23 meq/100 g can be explained by the cation exchange process between H⁺ and Na⁺.

Acidity and basicity

Table 2 presents the results of the acidity and the basicity of the adsorbents studied. The high acidity of the KT sample is due to the availability of more –OH groups. In the case of the KT-1 sample, we observe very low value of acidity due to the elimination of –OH groups after the thermal treatment. This confirms the obtained result from the cation exchange capacity. After both acid and base treatments, both samples KT-2 and KT-3 show an increase in their values of acidity1.14 and 1.42, respectively, due to the insertion of more H⁺ ions for KT-2 and the enrichment of KT-3 with –OH groups.

The basic character (0.02 mmol/g) is the same for the three samples KT, KT-1, and KT-2. However, after alkaline treatment with NaOH, the surface of the sample KT-3 shows a normal enrichment of –OH groups and the value of the basicity become1.025 mmol/g.

We can say that KT-3 exhibits a high acidity and basicity due to the presence of –OH groups, which play the act of base and acid in the same time (Karaoglu et al., 2009) as illustrated:

K - OH + H^{+} \to K - {OH}^{+ 2}

K - OH + {OH}^{-} \to K - O^{-} + H_{2} O

Adsorption equilibrium

Adsorption properties and equilibrium parameters, commonly known as adsorption isotherms, describe how the adsorbate interacts with adsorbents, and comprehensive understanding of the nature of interaction. Isotherms help to provide information about the optimum use of adsorbents (Foo and Hameed, 2010).Figure 6 shows a significant increase in adsorption capacity of MB dye after alkaline treatment of the acid-activated calcined kaolin. The improvement achieves more than 100% with KT-3 in comparison with KT. So, in order to design an adsorption system to remove dye from solutions, several isotherm equations are available, and two important isotherms were selected for this study: the Langmuir and Freundlich models.

Figure 6.

Adsorption isotherms.

The linear form of Langmuir isotherm model is represented by the following equation:

\frac{1}{q_{e}} = \frac{1}{q_{m}} + \frac{1}{K_{L} . q_{m}} \times \frac{1}{C_{e}}

(5) where q_e is the amount of dye adsorbed per unit of adsorbent (mg g⁻¹), C_e is the concentration of dye solution at adsorption equilibrium (mg L⁻¹), q_m is the maximum amount of adsorption which complete monolayer coverage on the adsorbent surface (mg g⁻¹), and K_L is the Langmuir constant (L mg⁻¹).

The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor R_L that is given by the following equation (Bera et al., 2013):

R_{L} = \frac{1}{1 + K_{L} . C_{0}}

(6) where C₀ (mg L⁻¹) is the initial adsorbate concentration in aqueous solution, K_L (L mg⁻¹) is the Langmuir constant, and R_L indicates the type of isotherm to be irreversible (R_L = 0), favorable (0 < R_L < 1), linear (R_L = 1), or unfavorable (R_L > 1).

A linear form of the Freundlich isotherm model can be expressed as:

{lnq}_{e} = {lnK}_{F} + \frac{1}{n_{F}} . {lnC}_{e}

(7) where K_F and n_F are Freundlich constants, which represent adsorption capacity (mg^1−(1/ ⁿ ⁾l^(1/ ⁿ ⁾g⁻¹) and adsorption strength, respectively. The magnitude of the exponent n_F gives an indication on the favorability of adsorption. We distinguish three cases: values of n_F in the range 2–10 represent good adsorption; 1–2, moderately difficult, and less unity poor adsorption characteristics (Hamdaoui and Naffrechoux, 2007).

Langmuir and Freundlich constant values and the correlation coefficients R² obtained from the linear regression are shown in Table 3.

Table 3.

Adsorption isotherm parameters for the adsorption of MB on KT-3.

Adsorbents		Langmuir model			Freundlich model
Adsorbents	q _m	K_l	R_L	R ²	n _f	K_f	R²
KT	46.94	1.098	0.0091	0.92	4.17	22.66	0.84
KT-1	0.86	1.69	0.0059	0.77	0.95	2.69	0.95
KT-2	47.39	0.58	0.0168	0.94	4.88	24.96	0.95
KT-3	104.16	1.31	0.0076	0.94	5.88	64.68	0.90

The value of the coefficient of correlation (R²= 0.92) which is obtained from Langmuir expression indicates that Langmuir expression provides a better fit to the experimental data of MB on KT, KT-2, and KT-3 samples. The R_L value for the adsorption of MB onto these samples is, respectively, 0.0091, 0.0168, and 0.0076. This indicates that the adsorption is a favorable process. In this case, the adsorption of MB on KT takes place as monolayer adsorption on a surface that is homogenous in adsorption affinity. The Langmuir isotherm is unable to describe adsorption process on KT-1 (R²= 0.77). The maximum adsorption capacity calculated is very low in comparison with the experimental value.

The equilibrium data analyzed using the linearized form of Freundlich isotherm show a good linearity in the case of KT-1, KT-2, and KT-3samples. The value of n_F is greater than unity, n_F = 4.88 and 5.88, for KT-2 and KT-3, respectively, which indicates that the dye is favorably adsorbed on these samples, in contrast with KT-1 which exhibits an inferior value to the unity (n_F = 0.95), this confirms the poor adsorption characteristics of KT-1.

Adsorption kinetics

The kinetic study will be on the adsorbent KT-3 because it shows the best adsorption capacity.

Effect of contact time and initial dye concentration

The effects of the initial dye concentration and the contact time on the removal rate of MB dye on KT-3 are shown in Figure 7. We observe for all initial concentrations that the extent dye uptake increases with contact time, and the maximum amount of dye adsorption takes place in the first 5 min, which indicates that the rate of adsorption is very fast. We can also observe that with the increase in initial dye concentration equilibrium, time also increases as 5 min for 30 mg/L initial dye concentration and 70 min for 80 and100mg/L initial dye concentrations. This is explained by the increase in competition for the active adsorption sites and the adsorption process will increasingly be slowing down.

Figure 7.

Effect of contact time and initial concentration of MB dye on the adsorbed quantity. (Adsorbent dose: 1gL⁻¹, temperature: 18.5℃, and stirring speed: 400 r/min).

Effect of adsorbent dose

Usually, the percentage of dye removal increases with increasing adsorbent dosage, where the number of adsorption sites at the adsorbent surface will increase by increasing the dose of the adsorbent (Vimonses et al., 2009), and as a result an increase in the percentage of dye removal from the solution. The effect of adsorbent dose on removal of MB was studied in range of 1.0–6.0 gL⁻¹ and initial dye concentration of 150 mgL⁻¹. Figure 8 shows that the percentage removal of dye increases from 66% to 100%, as adsorbent dose increases from 1.0 to 1.8 gL⁻¹.

Figure 8.

Effect of adsorbent dose. (C₀ = 150 mgL⁻¹, temperature: 18.5℃, stirring speed: 400 r/min, and time: 2 h).

Effect of pH

The initial pH of dye solution is an important parameter which controls the adsorption capacity. This pH can be influenced by the surface charge of adsorbent, the degree of ionization of adsorbate molecule, and extent of dissociation of functional groups on the active sites of the adsorbent (Nandi et al., 2009). Generally for a cationic dye, at a low pH solution, the percentage of dye removal will decrease, and at a high pH solution, the percentage of dye removal will increase. According to Figure 9, we can say that the maximum dye uptake (111 mgg⁻¹) is observed in basic environment, pH = 11.2. This value of pH is located in the region in which the adsorbent has the highest negative charge. The minimum adsorption capacity of 96 mgg⁻¹ is observed between 2.6 and 4.8 pH range, which corresponds to the highest positive charge. In a general manner, at a high pH solution (pH > pH_zpc = 7.4), the particles acquire a negative surface charge leading to a bigger dye uptake. Since pH is smaller than pH_zpc, the surface possesses a positive charge which causes an electrostatic repulsion between dye molecule and adsorbent, leading to a decrease in dye uptake.

Figure 9.

Effect of pH solution on the adsorbed quantity. (C₀ = 150 mgL⁻¹, temperature: 18.5℃, stirring speed: 400 r/min, and time: 2 h).

Kinetic studies

Adsorption kinetics

In order to investigate the adsorption of MB dye on the surface of KT-3, different kinetics models were used to examine controlling mechanism of adsorption process. Pseudo-ﬁrst-order kinetic model, equation (7), pseudo-second-order kinetic model, equation (8), and intra-particle-diffusion model, equation (9), were investigated to ﬁnd the best ﬁtted model for the experimental data.

\ln (q_{e} - q_{t}) = {lnq}_{e} - k_{1} t

(8)

\frac{t}{q_{t}} = (\frac{1}{k_{2} q_{e}^{2}}) + (\frac{1}{q_{e}}) t

(9)

q_{t} = k_{int} t^{1 / 2} + I

(10) where q_e and q_t (mgg⁻¹) are the adsorption capacities at equilibrium and time t(min), respectively, k₁ (min⁻¹) is the pseudo-first-order rate constant, k₂ (gmg⁻¹min⁻¹) is the pseudo-second-order rate constant,

k_{int}

(mg g⁻¹min^0.5) is the intraparticle-diffusion constant rate, and I(mg g⁻¹) is the intercept. The calculated kinetic parameters are given in Table 4.

Table 4.

Adsorption kinetic parameters for the adsorption of MB on KT-3.

C₀ (mg L⁻¹)	q_e.exp (mgg⁻¹)	Pseudo first order			Pseudo second order			Intraparticle diffusion
C₀ (mg L⁻¹)	q_e.exp (mgg⁻¹)	k₁ (min^-1)	q_e	R ²	k₂ (g mg⁻¹ min⁻¹)	q_e	R ²	k_i.D (mg g⁻¹ min^0.5)	I (mgg⁻¹)	R ²
30	28.5	7.62.10^-5	0.90	0.12	0.0013	27.3	0.999	0.26	26	0.94
80	76.5	0.0052	16.20	0.35	0.005	77.9	0.999	5.33	42.9	0.99
100	93.24	0.0346	23.05	0.89	0.004	94.42	0.999	2.737	70.13	0.98

The first-order kinetic curves do not fit well with the experimental data. The plots of $t / q_{t}$ versus t give a straight line for all the initial dye concentrations as shown in Figure 10. At all the initial concentrations studied, the values of R² for the pseudo-second-order model are close to the unity (R² > 0.99), and the theoretical $q_{e . cal}$ values obtained from this model are also closer to the experimental $q_{e, \exp}$ (Table 4). These results indicate that the adsorption of MB onto KT-3 could be better explained based on the pseudo-second-order kinetics model. A number of authors have reported pseudo-second-order kinetics, for adsorption of MB on different adsorbents such as kaolin and NaA zeolite (Rida et al., 2013), activated carbon issued from a biomass (Theydan and Muthanna, 2012), and natural zeolite (Han et al., 2009).

Figure 10.

Pseudo-second-order kinetic model for the adsorption of MB dye on KT-3. (Adsorbent dose: 1 gL⁻¹, stirring speed: 400 r/min, and tempertaure: 18.5℃).

The plots of the intraparticle-diffusion model of Weber–Morris at different initial concentrations are shown in Figure 11. The first sharper portion is the instantaneous adsorption or external surface adsorption; the second portion is the gradual adsorption stage where the intraparticle diffusion is the rate limiting; and the third portion is the final equilibrium. In order to say that the intraparticle diffusion is the rate controlling step, the plot of q_t versus $t^{1 / 2}$ should be linear and pass through the origin. As can be noticed from Figure 11, the plot does not pass through the origin, and it can be concluded that intraparticle diffusion is not the dominating mechanism for the adsorption of MB from aqueous solution by KT-3. A similar behavior was reported for MB adsorption onto papaya seeds (Hameed, 2009), mixture of monmorillonite and nontronite (Gurses et al., 2006), and bamboo-based activated carbon (Hameed et al., 2007).

Figure 11.

Intraparticle diffusion model for the adsorption of MB dye on KT-3. (Adsorbent dose: 1gL⁻¹, stirring speed: 400 r/min, and temperature: 18.5℃).

Activation parameters

The pseudo-second-order rate k₂ in function of temperature has been used to determine the energy activation of the adsorption process using the Arrhenius equation:

k_{2} = k_{0} e^{- \frac{E_{a}}{RT}}

(11) where k₀ is the independent temperature Arrhenius factor (g mg⁻¹min⁻¹),E_a is the apparent activation energy of the adsorption reaction (J mol⁻¹), R is the gas constant (8.314 J K⁻¹mol⁻¹), and T is the solution temperature (K). A plot of ln(k₂) versus

1 / T

(Figure 12) gives a straight line, and the corresponding activation energy is determined from the slope of the linear plot. The obtained result is E_a = 986 J mol⁻¹, this indicates that the adsorption possesses a low potential barrier and corresponds therefore to physisorption process (Al-Ghouti et al., 2005; Zhu et al.,2009).

Figure 12.

Plot of ln(k₂) and ln(K_d) versus 1/T.

Thermodynamic parameters

The temperature is another important factor which inﬂuences the process of adsorption. The effect of temperature on adsorption of MB dye onto KT-3 has been investigated at 291, 303, and 323 K. Thermodynamic parameters for the adsorption are calculated using the equation:

\ln (K_{d}) = - \frac{Δ H^{\circ}}{RT} + \frac{Δ S^{\circ}}{R}

(12) where K_d is the distribution coefficient of adsorbent and equal to

\frac{q_{e}}{C_{e}}

;

Δ H^{0}

(kJ mol⁻¹) and

Δ S^{0}

(J mol⁻¹) are, respectively, the standard enthalpy and the standard entropy; T is the absolute temperature; and R is the gas constant. The Gibbs free energy change

Δ G^{0}

, indicating the spontaneity of the adsorption process, is calculated as:

Δ G^{0} = Δ H^{0} - T Δ S^{0}

(13)

The plot of

\ln K_{d}

versus 1/T (Figure 12) is linear with the slope and the intercept giving values of

Δ H^{0}

and

Δ S^{0}

. The thermodynamic parameters obtained are shown in Table 5. The negative value of

Δ G^{0}

indicates that the adsorption process is spontaneous and the increase in positive value of with increasing temperature shows an increase in feasibility of adsorption at higher temperatures. The standard enthalpy of adsorption

Δ H^{0}

= 50.59 kJ mol⁻¹ indicates an endothermic process, and the positive value of

Δ S^{0}

suggests an increase in disorder of solid–liquid interface of KT-3 during adsorption of MB (Zhang et al., 2013).

Table 5.

Thermodynamic parameters for the adsorption of MB dye on KT-3 at different temperatures.

T (K)	ΔG°(kJ mol⁻¹)	ΔH° (kJmol⁻¹)	ΔS°(JK⁻¹mol⁻¹)
291	−75	50.59	257.77
303	−78.08
323	−83.24

Evaluation of modified kaolin as an adsorbent

Adsorption capacity based on the Langmuir isotherm, q_max is commonly used as an important parameter to evaluate the performance of adsorbents. The q_max values of some adsorbents from literature are summarized in Table 6. By comparison of the obtained results in this study, with those in the previously reported works, modified kaolin presents a good adsorption capacity for MB dye. Because of the availability, the low cost, and the interesting adsorption capacity of modified kaolin, its utilization for removal of MB dye is very promising.

Table 6.

Comparison of adsorption capacities of various adsorbents for MB dye.

Adsorbents	pH	Temperature (℃)	Adsorption capacity (mg g⁻¹)	Reference
Spent activated clay	5.5	25	56	Weng and Pan (2007)
Thermal treated palygorskite	7	30	78	Chen et al. (2011)
Montmorillonite and nontronite	5.65	20	58	Gurses et al. (2006)
Acid-activated milled pyrophylite	7	20	4.2	Sheng et al. (2009)
Natural zeolite	7	25	5.5	Han et al. (2009)
Zeolite type X	-	20	360	Yan et al. (2009)
Zeolite type A	7	30	64.8	Sapawe et al. (2013)
Modified ZSM-5 zeolite	4	25	13	Jin et al. (2008)
Monmorillonite clay	11	35	292	Almeida et al. (2009)
Cold plasma modified kaolin	8	30	23	Yavuz and Saka (2013)
Natural montmorillonite	5.95	30	224	Fil et al. (2012)
Playgorskite	7	30	111	Liu et al. (2012)
Modified clay	12	30	100	Auta and Hameed (2012)
Modified playgorskite	6.5	20	33	Zhang et al. (2013)
Clay	7	20	255	Issa et al. (2014)
Perlite	11	30	0.7	Acemioglu (2005)
Tamazert kaolin	6.51	25	45	Rida et al. (2013)
Modified kaolin	11.19	18.5	111	Present study

Conclusion

The results of this investigation show that the thermal treatment of KT is unable to improve its adsorption capacity due to the loss of the most hydroxyl groups during the reaction of dehydroxylation. After acid activation of KT-1, we obtained KT-2 which is characterized by a high surface area. This improves the adsorption capacity in comparison with KT-1, but it remains equal to that of KT.

The modified kaolin KT-3 which was obtained after three treatments (thermal treatment at 800° C, acid, and alkaline activation), has a suitable adsorption capacity for the removal of MB from aqueous solutions. This higher adsorption capacity is attributed to higher specific surface area and the availability of more hydroxyl groups, which gives higher cationic exchange capacity and best basic-acid properties. The adsorption of MB onto KT-3 is highly dependent on various operating parameters like adsorbent dose, contact time, pH, initial dye concentration, and temperature. The kinetic modeling of the MB onto KT-3 adsorbent indicates that adsorption process is pseudo-second order with the correlation coefficients higher than 0.999. Thermodynamic constants were also evaluated using equilibrium constants at different temperatures. Thermodynamic study reveals the endothermic, the spontaneity, and the physical nature of adsorption. The positive value of entropy change indicates that adsorption is accompanied with an increase in randomness in the solid/liquid interface. This modified kaolin is an effective and economical adsorbent for the removal of MB from aqueous solution.

Footnotes

Acknowledgements

Thanks are due to the Instituto de Catálisis y Petroleoquímica (CSIC, Spain) Unidad de Apoyo staff for the performance of characterization experiments. Thanks are also due to Drs I. Pacheco and E. Sastre of the ICP-CSIC for performing TGA/DTA experiments.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors thank the Ministère Algérien de l'Enseignement supérieur et de la Recherche for their financial support.

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Improvement adsorption capacity of methylene blue onto modified Tamazert kaolin

Abstract

Keywords

Introduction

Materials and methods

Materials

Preparation of clay adsorbents

Characterization

Adsorption experiments

Results and discussion

Characterization

Thermal analysis

XRD characterization

FT-IR spectroscopy

Scanning electron microscopy

Specific surface area

Point zero charge

Cation exchange capacity (CEC)

Acidity and basicity

Adsorption equilibrium

Adsorption kinetics

Effect of contact time and initial dye concentration

Effect of adsorbent dose

Effect of pH

Kinetic studies

Adsorption kinetics

Activation parameters

Thermodynamic parameters

Evaluation of modified kaolin as an adsorbent

Conclusion

Footnotes

Acknowledgements

Declaration of Conflicting Interests

Funding

References