This study investigated the adsorption of methylene blue with natural and artificial zeolite. The effect of pH, contact time, initial concentration and adsorbent dose on adsorption was also investigated. An artificial dye was prepared. Adsorption removal efficiency was low at pH = 2, 3 and 4 but it was quite high at pH = 7. It was determined that the contact time reaches equilibrium within 60 minutes in the adsorption of methylene blue with natural and artificial zeolite. The initial dyestuff concentration for both adsorbents was 5 mg/L. For the removal of methylene blue, a 0.5 g natural and artificial zeolite dosage was sufficient. In order to express the adsorption of natural and artificial zeolite on methylene blue, Langmuir, Freundlich, Temkin and Dubinin-Radushkevich (D-R) isotherm models were examined. In the isotherm study, both natural and artificial zeolite adapted to the Langmuir isotherm model. Langmuir correlation coefficient was 0.998 for artificial zeolite and 0.993 for natural zeolite. Both adsorbent materials best fit into the pseudo-second kinetic model with similar correlation coefficient values of 0.999.

  • In methylene blue adsorption, physical and chemical analyses of artificial and natural zeolites were made.

  • Langmuir isotherm is the most suitable adsorption model for the artificial and natural zeolites.

  • Methylene blue adsorption followed the pseudo-second-order kinetics.

  • Physicochemical properties of zeolite forms found to be effective for dye removal.

In the industrial sector, the chemicals used to colour materials such as fabric, fibre and leather are called dyestuffs. These are either natural or artificial dyes. Most artificial dyes are synthetic. Chemical reactions are effective in the formation of synthetic dyes. They penetrate the surface of the material by chemical or physico-chemical processes. It is known that approximately 15–20% of dyes from industry are discharged to various environments. Dyestuffs are widely used in textile, paper, leather and rubber industries. When these dyes are discharged into the environment, they are very harmful. They have transferrable capacities affecting lakes, seas and even underground waters (Önal & Tantekin 2018). Light transmittance of water is reduced due to colour changes caused by mixing dyes into water. Depending on the nature of the dyestuff in the water, the occurrence of photosynthetic events may be prevented and there may be pH changes, a decrease in oxygen demand, mutagenic, teratogenic and toxic effects. In addition, direct or indirect contact to colorants can cause health problems such as cancerous tumours, nausea, vomiting, increased pulse, and following direct contact, skin and eye irritation in humans (García et al. 2014; Brião et al. 2018; Önal & Tantekin 2018). Dyestuffs are very difficult to remove because they have large molecules, do not break down in aerobic or anaerobic microbial degradation, and do not oxidize with chemicals. Therefore, the adsorption process is one of the commonly used applications. It is a physical method that is cheap, easy to operate, and less waste is generated by using natural adsorbents and regenerating some adsorbents (Göçenoğlu Sarıkaya 2019). Many different sources of adsorbents have been used in this process, such as olive seeds, bamboo powder, coconut husk, peanut husk, straw, rice hull, peach seed husk, almond husk, walnut husk, hazelnut shell, pumice, perlite, bentonite and zeolite (Acar & Kılıç 2019). Mineral adsorbent, such as zeolite, resources are one of the most promising substances due to their very nature to multiple reuse after suitable recharging and increasing their adsorption capacity with modification. Zeolite is a natural mineral, with a structure that is thought to have been formed many years ago by a change in volcanic clays as a result of contact with water. Zeolites are formed by bonding oxygen atoms to each other. The most important species of zeolites are clinoptilolites (Mgbemere et al. 2017). The low pressure and temperature caused by water particles in the zeolites give it a hollow structure, with the appearance of cages and honeycomb due to their cavities. In addition, they have less density than other silica groups. Since zeolites are bonded to each other by weak bonds, they can be very good ion exchangers (Demir & Polat 2003; Jiang et al. 2019). Jawada et al. examined the removal of methylene blue dye using mesoporous Iraqi red clay, and also examined the isotherm and kinetics. The effects of parameters such as adsorbent dose (0.02–0.20 g), contact time (0–300 min) and initial methylene blue concentration (10–120 mg/L) on adsorption were investigated. Using adsorption data, the Langmuir, Freundlich and cautious isotherms were investigated. It was observed that the Langmuir (R2 = 0.99) and Freundlich (R2 = 0.98) isotherms provided the best fit. The best fit was found to be a pseudo-first order kinetic model. As a result of the study, it was reported that mesoporous Iraqi red clay is a promising adsorbent in the removal of methylene blue (Jawada & Abdulhameed 2020). The removal of Basic Blue 41 textile dyestuff on natural zeolite (Nerejo-Romania) was investigated by using adsorption isotherm and kinetic.

Table 1

Langmuir, Freundlich, Temkin, Dubinin-Radushkevich (D-R) isotherm equations

Isotherm modelEquationsCGraph
Langmuir Equation (1)  Ce/qe vs. Ce 
Freundlich Equation (2) . lnqe vs. lnCe 
Temkin Equation (3)  qe vs. lnCe 
Dubinin-Radushkevich (D-R) Equation (4) 
Ɛ: RTln(1 + 1/Ce)
Ɛ: Adsorption potential
E = (2 Kı) − 1/2 = Energy of adsorption 
Ɛ2 vs. lnqe 
Isotherm modelEquationsCGraph
Langmuir Equation (1)  Ce/qe vs. Ce 
Freundlich Equation (2) . lnqe vs. lnCe 
Temkin Equation (3)  qe vs. lnCe 
Dubinin-Radushkevich (D-R) Equation (4) 
Ɛ: RTln(1 + 1/Ce)
Ɛ: Adsorption potential
E = (2 Kı) − 1/2 = Energy of adsorption 
Ɛ2 vs. lnqe 
Table 2

Pseudo-first-degree and pseudo-second-degree kinetic equations

Kinetic modelEquationsLinear equationGraph
Pseudo-first degree Equation (5)  log(qe − qt) vs. t 
Pseudo-second degree Equation (6)  t/q vs. t 
Kinetic modelEquationsLinear equationGraph
Pseudo-first degree Equation (5)  log(qe − qt) vs. t 
Pseudo-second degree Equation (6)  t/q vs. t 

In this study, the characterisation properties of samples were examined with SEM, EDX and instruments. After the appropriate conditions were met, the adsorption data were checked for compatibility with Langmuir, Freundlich, Temkin, Dubinin-Radushkevich isotherms. The Langmuir isotherm was reported to be the best fit. When adsorption kinetics were examined, it was observed that pseudo-second order kinetic adsorption was more suitable. As a result of these studies, zeolite was found to be a natural adsorbent substance in the removal of dyes (Humelnicu et al. 2017). Conventional methods have been proposed for the removal of methylene blue. In a study of production and characterization of Gum arabic-cl-poly(acrylamide) nanohydrogel used for effective adsorption of crystal violet dye. Microwave synthesis of Gum arabic-cl-poly(acrylamide) nanohydrogel was performed. The crystal violet dye was adsorbed onto the nanohydrogel. They achieved good adsorption efficiency of 90.90 mg/g for crystal violet dye. In this adsorption, Langmuir isotherm adapted (Sharma et al. 2018).

The aim of this study was to investigate the use of natural zeolite and artificial zeolite to remove a cationic dye of methylene blue from aqueous media. The effect of parameters such as initial concentration of methylene blue, pH, amount of adsorbent material and contact time were examined. The suitability of the different kinetic and isotherm models was also investigated by using the absorption data. In addition, the adsorption isotherms and kinetics of methylene blue on the zeolite surface were discussed.

Materials

UV-spectrophotometer (WTW 7600 UV-VIS) was used for adsorption measurements. All shaking operations were done by Magnetic Stirrer (2 mag magnetic motion-mix 15 eco) model shaker. Methylene blue (Merck, Darmstadt, Germany) solution and adsorbent weighing were measured with Ohaus Adventurer Pro brand precision scales. It was made with deionized water (ELGA Purelab Option DV-25) in dilution of stock solution and initial dyestuff concentrations.

Zeolite

Zeolites are minerals with a colourless structure which are found as crystal salts in nature. They are used in many different sectors due to their homogeneous, compact and robust structure with low weight and high porosity. The densities of zeolites are generally known to be 1.9, 2, 3 g/cm3 because they vary according to the type of cations they contain. However, when the pore sizes were examined, the analysis found that the pore size of the artificial zeolite was higher than that of the natural zeolite. Zeolites have four-sided AlO4 and SiO4 network structures (Jiang et al. 2020).

Methylene blue

The chemical structure of methylene blue (MB) used as adsorbate in the study is shown in Figure 1. Its molecular weight is known as 319,85 g λmax and it has a wavelength of 663 nm. It was used without any treatment for the preparation of aqueous solutions (Deng et al. 2009; Türkyılmaz 2018). Since they act as protons due to their structure, they are bound by anionic group-containing fibres. The methylene blue dye was obtained from Merck-Darmstadt.

Figure 1

Chemical structure of methylene blue (Deng et al. 2009).

Figure 1

Chemical structure of methylene blue (Deng et al. 2009).

Close modal

Method

Initially, stock solution was prepared by adding deionized water to a 1,000 ml flask as artificial dye wastewater and 1 g of methylene blue ingredient. Then, 0.5, 1.0, 2.0, 3.0, 4.0 and 5.0 mg/L methylene blue initial concentrations were prepared from these stock solutions in 1,000 ml flasks. The amount of adsorbent material was also weighed on a precision scale at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 grams. In order to determine the equilibrium, contact time in the adsorption process, the initial concentration was determined as 5 mg/L and 0.5 grams in the amount of adsorbent material. The prepared solutions were mixed in the magnetic stirrer at 200 rpm for 5, 10, 30, 60 and 120 minutes and then were left in the centrifuge for five minutes at 3,000 rpm for separation of the solids from the aqueous solution. The aqueous samples were then measured on a spectrophotometer at a wavelength of 663 nm of methylene blue. The equilibrium contact time after the treatments was determined to be 60 minutes. To determine the amount of equilibrium adsorbent, the contact time was determined to be 60 minutes and 50 ml was taken from the 5 mg/L initial concentration. Next, the solution was measured by a spectrophotometer after the zeolites were weighed at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 grams, respectively. The variation of adsorption depending on the initial concentration was determined as 60 minutes of contact time and 0.5 grams of adsorbent material. Then, the adsorbed amount in all solutions, 0.5, 1, 2, 3, 4 and 5 mg/L, respectively, was measured by spectrophotometer.

Adsorption isotherm studies

The adsorption data of the methylene blue on zeolites was obtained, and the suitability of the Langmuir (Langmuir 1918; Ceyhan & Baytar 2018; Cheruiyot et al. 2019), Freundlich (Freundlich 1906; Mahmoud et al. 2016), Temkin (Temkin & Pyzhev 1940; Ngulube et al. 2018) and Dubinin-Radushkevich (D-R) (Ngulube et al. 2018) isotherms was examined with the help of Table 1.

Adsorption kinetic study

Adsorption kinetics were examined in discrete systems in order to show the speed limiting stage of the adsorption process. Two different kinetic models were applied to use the zeolite as an adsorbent material in methylene blue removal (Ho & Mckay 1998; Mar Areco et al. 2012; Buldağ 2018).

The SEM and BET analysis was carried out in the Ataturk University Research Laboratory in Erzurum and the Düzce University Research Laboratory, respectively, and SEM images are shown in Figure 2. Table 3 shows the analysis of the BET surface area of natural and artificial zeolite. Natural zeolite was found to have more surface area than artificial zeolite. In BET analysis, the surface area of natural and artificial zeolite measurements was 22.0218 m2/g and 16.3460 m2/g, respectively. Also, the average particle size was measured as 272.4576 nm and 367.0623 nm for natural and artificial zeolite, respectively.

Table 3

BET surface area of natural and artificial zeolite

Surface areaNatural zeolite (m2/g)Artificial zeolite (m2/g)
Single point surface area 21.3408 16.0781 
BET surface area 22.0218 16.3460 
Langmuir surface area 199.7677 128.9351 
t-Plot micropore area 0.8837 2.3446 
t-Plot external surface area 21.1381 14.0014 
BJH adsorption cumulative surface area of pores
between 17,000 nm and 3,000,000 nm width 
20.7580 14.4329 
BJH desorption cumulative surface area of pores
between 17,000 nm and 3,000,000 nm width 
26.3501 15.8170 
D-H adsorption cumulative surface area of pores
between 17,000 nm and 3,000,000 nm width 
20.2659 14.0290 
D-H desorption cumulative surface area of pores
between 17,000 nm and 3,000,000 nm width: 
25.3957 15.3634 
Pore volume Natural zeolite (cm3/g) Artificial Zeolite(cm3/g) 
Single point adsorption total pore volume of pores
less than 403,122 nm width 
0.052791 0.046938 
Single point desorption total pore volume of pores
less than 403,122 nm width 
0.066421 0.059114 
t-Plot micropore volume 0.000155 0.001032 
BJH adsorption cumulative volume of pores
between 17,000 nm and 3,000,000 nm width 
0.096560 0.076822 
BJH desorption cumulative volume of pores
between 17,000 nm and 3,000,000 nm width 
0.098004 0.077401 
D-H adsorption cumulative volume of pores
between 17,000 nm and 3,000,000 nm width 
0.095516 0.075837 
D-H desorption cumulative volume of pores
between 17,000 nm and 3,000,000 nm width 
0.097450 0.076544 
Pore size Natural zeolite (nm) Artificial Zeolite (nm) 
Adsorption average pore diameter 9.5889 11.4861 
Desorption average pore diameter 12.0646 14.4657 
BJH adsorption average pore width 18.6068 21.2910 
BJH desorption average pore width 14.8772 19.5733 
D-H adsorption average pore width 18.8526 21.6228 
D-H desorption average pore width 15.3491 19.9290 
Average particle size 272.4576 367.0623 
Surface areaNatural zeolite (m2/g)Artificial zeolite (m2/g)
Single point surface area 21.3408 16.0781 
BET surface area 22.0218 16.3460 
Langmuir surface area 199.7677 128.9351 
t-Plot micropore area 0.8837 2.3446 
t-Plot external surface area 21.1381 14.0014 
BJH adsorption cumulative surface area of pores
between 17,000 nm and 3,000,000 nm width 
20.7580 14.4329 
BJH desorption cumulative surface area of pores
between 17,000 nm and 3,000,000 nm width 
26.3501 15.8170 
D-H adsorption cumulative surface area of pores
between 17,000 nm and 3,000,000 nm width 
20.2659 14.0290 
D-H desorption cumulative surface area of pores
between 17,000 nm and 3,000,000 nm width: 
25.3957 15.3634 
Pore volume Natural zeolite (cm3/g) Artificial Zeolite(cm3/g) 
Single point adsorption total pore volume of pores
less than 403,122 nm width 
0.052791 0.046938 
Single point desorption total pore volume of pores
less than 403,122 nm width 
0.066421 0.059114 
t-Plot micropore volume 0.000155 0.001032 
BJH adsorption cumulative volume of pores
between 17,000 nm and 3,000,000 nm width 
0.096560 0.076822 
BJH desorption cumulative volume of pores
between 17,000 nm and 3,000,000 nm width 
0.098004 0.077401 
D-H adsorption cumulative volume of pores
between 17,000 nm and 3,000,000 nm width 
0.095516 0.075837 
D-H desorption cumulative volume of pores
between 17,000 nm and 3,000,000 nm width 
0.097450 0.076544 
Pore size Natural zeolite (nm) Artificial Zeolite (nm) 
Adsorption average pore diameter 9.5889 11.4861 
Desorption average pore diameter 12.0646 14.4657 
BJH adsorption average pore width 18.6068 21.2910 
BJH desorption average pore width 14.8772 19.5733 
D-H adsorption average pore width 18.8526 21.6228 
D-H desorption average pore width 15.3491 19.9290 
Average particle size 272.4576 367.0623 
Figure 2

SEM images of natural and artificial zeolite, (a) SEM images of artificial zeolite at 2.00KX, (b) SEM images of natural zeolite at 2.00KX, (c) SEM images of artificial zeolite at 20.00KX, (d) SEM images of natural zeolite at 20.00KX.

Figure 2

SEM images of natural and artificial zeolite, (a) SEM images of artificial zeolite at 2.00KX, (b) SEM images of natural zeolite at 2.00KX, (c) SEM images of artificial zeolite at 20.00KX, (d) SEM images of natural zeolite at 20.00KX.

Close modal

The EDS analysis of zeolites is shown in Figures 3 and 4. The EDS analysis of natural and artificial zeolites were carried out in Düzce University Research Laboratory, as seen in Figure 3; in the EDS analysis of natural zeolite, the oxidation rate was 45%. However, as seen in the spectrums in Figure 3, high rates of oxide level change were observed in the EDS analysis of artificial zeolite. The variation was from minimum 40% to maximum 46%. At the same time, the Si ratio varied between 9 and 26% in the spectrum analysis of artificial zeolite in Figure 4.

Figure 3

EDS analysis of natural zeolite.

Figure 3

EDS analysis of natural zeolite.

Close modal
Figure 4

EDS analysis of artificial zeolite.

Figure 4

EDS analysis of artificial zeolite.

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Effect of pH and contact time on methylene blue adsorption

The pH of the solution affects both the structure of the adsorbent and the ionization of the adsorbate. In high pH solution, the adsorbent surface becomes more negative. Therefore, adsorption of a positively charged adsorbate occurs more on this surface. On the other hand, in low pH solution, the adsorbent surface becomes more positive and adsorption of a negatively charged adsorbate is more efficient. From the methylene blue stock solution at a concentration of 1,000 mg/L, seven different methylene blue sub-solutions with pHs ranging from 2 to 11 were prepared with 1 M NaOH and 1 M HCl with appropriate dilutions. It was agitated for 60 minutes at 500 rpm by adding natural and artificial zeolite. As seen in Figure 5(a), pH had a significant effect on adsorption. Adsorption removal efficiency was low at pH = 2, 3 and 4 but it was quite high at pH = 7. The low adsorption at low pH was because the surface of the adsorbent became positive due to the excess hydrogen ions entering the environment. Then, it caused a decrease in the amount of dye adsorbed due to the intermolecular repulsion of positively charged methylene blue.

Figure 5

(a) The effect of pH on the adsorption of methylene blue with natural and artificial zeolite was investigated at an initial concentration of 5 mg/L and at 25 °C, (b) effect of time on the removal of natural and artificial zeolite on methylene blue (5 mg/L methylene blue concentration, pH: 7–8, 0.5 g adsorbent amount, 50 ml volume, temperature 25 °C).

Figure 5

(a) The effect of pH on the adsorption of methylene blue with natural and artificial zeolite was investigated at an initial concentration of 5 mg/L and at 25 °C, (b) effect of time on the removal of natural and artificial zeolite on methylene blue (5 mg/L methylene blue concentration, pH: 7–8, 0.5 g adsorbent amount, 50 ml volume, temperature 25 °C).

Close modal

Contact time is an important parameter to understand the effectiveness of the adsorption process. The contact time is the time period required for the adsorption conditions to reach equilibrium, and it covers the time period when the ions or molecules in the solution are in contact with the adsorbent. Considering the technological and economic conditions in scientific studies, it was shown that this time can be limited between 10 minutes and 3 hours. For this reason, the adsorption equilibrium time was determined as 60 minutes in the contact time study. In this study, dyestuff removal percentages were calculated by keeping the initial concentration of 5 mg/L, pH: 7–8, adsorbent dose 0.5 g and a range of contact times. It is apparent from Figure 5(b) that the amount of colorant, whose contact time is removed between 0 and 5 minutes, increases with time. However, the amount of adsorption does not increase much in the following minutes. Percentages of removal were 87 and 88%, respectively, for artificial zeolite and natural zeolite. As the adsorption centres of the zeolite had a large surface area, the dyestuff easily settled, and therefore adsorption occurred quickly in the first actions. As the adsorption centres filled, the dyestuffs moved from the outer surface of the zeolite to the inner surfaces, and the adsorption reached balance (Altun & Parlayıcı 2018).

The effect of adsorbent content and initial concentration on methylene blue adsorption

In the adsorption experiments, it was important to determine the most appropriate value of the adsorbent in terms of the efficiency of the process. A series of experiments were performed at different concentrations of zeolite at a concentration of 5 mg/L methylene blue. The effect of varying the adsorbent amount on adsorption was calculated as a % yield (Figure 6).

Figure 6

(a) Effect of the amount of adsorbent in the removal of natural and artificial zeolite and methylene blue (adsorption conditions: 5 mg/L initial concentration, pH: 7–8, 50 mL adsorption volume, 0.1–10 g adsorbent amount, temperature 25 °C, contact time: 60 min.), (b) effect of initial concentration on methylene blue removal with natural and artificial zeolite (adsorption conditions: 0.5 grams of natural zeolite and artificial zeolite, pH: 7–8, 50 mL adsorption medium, 60 min. contact time, temperature 25 °C).

Figure 6

(a) Effect of the amount of adsorbent in the removal of natural and artificial zeolite and methylene blue (adsorption conditions: 5 mg/L initial concentration, pH: 7–8, 50 mL adsorption volume, 0.1–10 g adsorbent amount, temperature 25 °C, contact time: 60 min.), (b) effect of initial concentration on methylene blue removal with natural and artificial zeolite (adsorption conditions: 0.5 grams of natural zeolite and artificial zeolite, pH: 7–8, 50 mL adsorption medium, 60 min. contact time, temperature 25 °C).

Close modal

As shown in the Figure 6(a), the best removal efficiency for artificial zeolite was 87% at 0.2 grams, while it was 84% at 2 grams in natural zeolite. In the adsorption process that reached a certain value, there was not much change in the removal efficiency with the increase of the amount of adsorbent material. Since the adsorption takes place in the surface area, the yield obtained is directly proportional to the surface area. The relationship between the amount of adsorbed substance per unit of adsorbent and the equilibrium obtained was examined in the experimental series conducted at different initial concentrations. As the initial concentration increases, there is an increase in the amount of methylene blue removed per unit of adsorbent. Due to the increase in initial concentrations, achieving a low percentage of adsorption can be explained by the saturation of the appropriate areas on the adsorbent surface. The methylene blue removal efficiencies at 60 minutes’ contact time using 0.5-gram adsorbent amount are shown in Figure 6(b). The amount of adsorbent substance decreases and the efficiency decreases with increasing concentrations. As seen in Figure 6(b), when artificial zeolite and natural zeolite in methylene blue removal were compared, the best efficiency was 95 and 88%, respectively.

Adsorption isotherm studies

Adsorption isotherms of natural and artificial zeolite on methylene blue were investigated according to the Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm models (Figures 7 and 8).

Figure 7

Langmuir and Freundlich isotherms of adsorption of methylene blue on natural and artificial zeolite: (a) Langmuir isotherm of adsorption of methylene blue on natural and artificial zeolite, (b) Freundlich isotherm of adsorption of methylene blue on natural and artificial zeolite.

Figure 7

Langmuir and Freundlich isotherms of adsorption of methylene blue on natural and artificial zeolite: (a) Langmuir isotherm of adsorption of methylene blue on natural and artificial zeolite, (b) Freundlich isotherm of adsorption of methylene blue on natural and artificial zeolite.

Close modal
Figure 8

Temkin and D-R Isotherms of adsorption of methylene blue on natural and artificial zeolite: (a) Temkin isotherm of adsorption of methylene blue on natural and artificial zeolite, (b) D-R isotherm of adsorption of methylene blue on natural and artificial zeolite.

Figure 8

Temkin and D-R Isotherms of adsorption of methylene blue on natural and artificial zeolite: (a) Temkin isotherm of adsorption of methylene blue on natural and artificial zeolite, (b) D-R isotherm of adsorption of methylene blue on natural and artificial zeolite.

Close modal

As shown in Figure 7(a), a correlation value of 0.993 for natural zeolite and 0.998 for artificial zeolite was obtained in the Langmuir isotherm. These high correlation values indicate that the adsorbent surface is covered with a single layer. The Q0 value that indicates the adsorption capacity in Langmuir constants and the b value that indicates the adsorption energy are included in Tables 4 and 5 for both zeolite types.

Table 4

Isotherm constants for artificial zeolite

LangmuirFreundlichTemkinDubinin-Radushkevich
Q0 (mg/g−1) = 0.3326 k (mg g−1) = 0.3970 B (J mol−1) = −0.0873 Xm (mg g−1) = 0.3524 
b (L mg−1) = −6.375 n (g L−1) = −4.683 At (L g−1) = 0.0104 K (mol2 J−2) = −3*10−8 
R2 = 0.9981 R2 = 0.985 R2 = 0.990 R2 = 0.947 
LangmuirFreundlichTemkinDubinin-Radushkevich
Q0 (mg/g−1) = 0.3326 k (mg g−1) = 0.3970 B (J mol−1) = −0.0873 Xm (mg g−1) = 0.3524 
b (L mg−1) = −6.375 n (g L−1) = −4.683 At (L g−1) = 0.0104 K (mol2 J−2) = −3*10−8 
R2 = 0.9981 R2 = 0.985 R2 = 0.990 R2 = 0.947 
Table 5

Isotherm constants for natural zeolite

LangmuirFreundlichTemkinDubinin-Radushkevich
Q0 (mg/g−1) = 0.204 k (mg g−1) = 0.388 B (J mol−1) = −0.151 Xm (mg g−1) = 0.237 
b (L mg−1) = −2.180 n (g L−1) = −2.138 At (L g−1) = 0.075 K(mol2 J−2) = −1.10*10−7 
R2 = 0.993 R2 = 0.973 R2 = 0.985 R2 = 0.880 
LangmuirFreundlichTemkinDubinin-Radushkevich
Q0 (mg/g−1) = 0.204 k (mg g−1) = 0.388 B (J mol−1) = −0.151 Xm (mg g−1) = 0.237 
b (L mg−1) = −2.180 n (g L−1) = −2.138 At (L g−1) = 0.075 K(mol2 J−2) = −1.10*10−7 
R2 = 0.993 R2 = 0.973 R2 = 0.985 R2 = 0.880 

In Figure 7(b), the adsorption of methylene blue with artificial and natural zeolite was obtained for the Freundlich isotherm with a correlation value of 0.985 and 0.973, respectively. These correlation values show that the adsorption also matches the Freundlich isotherm. The k value expressing the adsorption capacity from Freundlich constants and the n value expressing the adsorption intensity are shown in Tables 4 and 5. Compliance with the Freundlich isotherm also indicates that the surface has heterogeneous properties and adsorption occurs physically.

The correlation values obtained in the adsorption isotherm in Figure 8(a) were 0.990 and 0.985 for artificial and natural zeolite, respectively. The b values for the heat of adsorption and equilibrium binding constants expressing At value are reported in Tables 4 and 5. This means that the adsorption adapts to the Temkin isotherm and the artificial and natural zeolite surface is covered with methylene blue, which also means that a decrease in the temperature of the adsorption has occurred. In Figure 8(b), correlation values of 0.947 and 0.88 for artificial and natural zeolite were obtained in the D-R isotherm, respectively. The Xm values expressing the adsorption capacity and the values of the K1 constants expressing the adsorption energy are given in Tables 4 and 5. In the case of adaptation to the D-R isotherm, it can be said that artificial and natural zeolite do not have homogeneous properties. However, natural and artificial zeolite had the highest correlation value for the Langmuir isotherm, in which adsorption occurs in a single layer and the adsorbent surface is homogeneous. The homogeneous surface fills up to the moment of equilibrium and reaches the maximum adsorption capacity.

Adsorption kinetic studies

The adsorption kinetics of natural and artificial zeolite of methylene blue were investigated according to the pseudo-first order and pseudo-second order models in Table 2.

The kinetic models of the experimental data were calculated linearly. As seen in Figure 9(a), the correlation values for the pseudo-first order kinetic model were small. For this reason, it does not comply with the pseudo-first degree kinetic model applied to methylene blue adsorption of natural and artificial zeolites. In Figure 9(b), it is apparent that the correlation value is closer to the pseudo-first kinetic model and, accordingly, it fits better to the pseudo-second degree kinetic model (Table 6).

Table 6

Kinetic constants for artificial and natural zeolites

SubstancePseudo-first degree kinetic model
Pseudo-second degree kinetic model
qden experimental (mg/g)k1(1/dk)qden calculated (mg/g)R2k2 (g/mg.dk)qden calculated (mg/g)R2
Artificial zeolite 0.416 −6.909*10−4 0.048 0.0003 6.711 0.428 0.999 
Natural zeolite 0.405 0.053 6.267*10−3 0.684 −1.949 0.390 0.999 
SubstancePseudo-first degree kinetic model
Pseudo-second degree kinetic model
qden experimental (mg/g)k1(1/dk)qden calculated (mg/g)R2k2 (g/mg.dk)qden calculated (mg/g)R2
Artificial zeolite 0.416 −6.909*10−4 0.048 0.0003 6.711 0.428 0.999 
Natural zeolite 0.405 0.053 6.267*10−3 0.684 −1.949 0.390 0.999 
Figure 9

The adsorption kinetics of natural and artificial zeolite of methylene blue: (a) pseudo-first order kinetics of adsorption of methylene blue on natural and artificial zeolite, (b) pseudo-second order kinetics of adsorption of methylene blue on natural and artificial zeolite.

Figure 9

The adsorption kinetics of natural and artificial zeolite of methylene blue: (a) pseudo-first order kinetics of adsorption of methylene blue on natural and artificial zeolite, (b) pseudo-second order kinetics of adsorption of methylene blue on natural and artificial zeolite.

Close modal

The dyestuff methylene blue has been successfully removed from aqueous solution by natural adsorbents; natural and artificial zeolites within the scope of this study. The investigated parameters; namely, contact time, initial dyestuff concentration, and the adsorbent dosage, were significant factors for affecting the adsorption yield.

Based on the tested experimental conditions, the optimum adsorbent amount was 0.5 grams for the removal of 5 mg/L dyestuff from aqueous solution, and the contact time required to ensure sufficient adsorbent yield was 60 minutes. The results obtained from adsorption experiments were subjected to the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) adsorption isotherms. The correlation coefficients in Langmuir isotherms were 0.998 and 0.993 for artificial zeolite and natural zeolite, respectively. The correlation coefficients for artificial and natural zeolite estimated in Freundlich isotherms were 0.985 and 0.973, respectively. In the Temkin isotherm, the correlation coefficient for artificial zeolite and natural zeolite was 0.990 and 0.985, respectively.

In Dubinin-Radushkevich isotherms, the correlation coefficients for artificial zeolite and natural zeolite was 0.947 and 0.880, respectively. According to the applied adsorption isotherms and kinetic models, methylene blue could be expressed by the pseudo-second-order kinetic model for both artificial and natural zeolites. The correlation coefficient in the pseudo-second degree kinetic model of both adsorbent substances was 0.999. Based on applied experimental protocols, adsorption isotherms and kinetic models, it was concluded that artificial and natural zeolite could successfully be used as an adsorbent for the adsorption of methylene blue from aqueous solution.

All relevant data are included in the paper or its Supplementary Information.

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