Methylene Blue Removal by Chitosan Cross-Linked Zeolite from Aqueous Solution and Other Ion Effects: Isotherm, Kinetic, and Desorption Studies
Developing innovative technology for removing methylene blue (MB) from water is essential since the widespread discharge of MB from industrial effluents causes problems for humans and the environment. In this study, we conducted the adsorption method, a simple technique that utilizes an adsorbent. Chitosan is cross-linked with zeolite as a promising adsorbent material and environmentally friendly. For the characterization, FTIR, SEM-EDS, DLS, and pHzpc were analyzed. It was discovered that the removal percentage reached 97% with an adsorption capacity of 242.51 mg/g for 60 minutes at pH 10. The adsorption isotherm and kinetic model were investigated. As a result, the Freundlich model and pseudo-second-order model were fitted to the adsorption process. Moreover, the effect of other ions was investigated for 5 minutes of mixing time. The results showed that the removal percentage increased in the presence of H2O2 ion. Contrary to sodium chloride, glucose, and citric acid ions, the effectiveness of H2SO4 as a desorbing agent was 99.65% for 30 minutes at 45°C.
The use of dyes in industry generates a considerable amount of printing and dyeing wastewater, which is harmful to the environment and human health [1, 2]. Methylene blue (MB) is a cationic dye that is often used as a coloring agent, medicine, and chemical indicator . MB was detected in either low or high amounts in the effluents . However, dye is a hazardous chemical whose breakdown may cause water contamination. Therefore, scientists have developed various physical, chemical, and biological ways to remove the dye from the water. Adsorption is a common technique for dye removal from an aqueous solution due to its simplicity and low cost.
Zeolites attract significant attention due to their high surface area and porous structure with exchangeable cations [5, 6]. Zeolite is a crystalline aluminosilicate with a 3D structure linked to oxygen atoms . Additionally, zeolites may be recycled and reused several times . However, zeolites cannot be used for dye adsorption since their adsorption ability decreases with time. As a result, modifying zeolite is a method for increasing dye removal. To overcome these constraints, chitosan was utilized as a cross-linking agent. Chitosan is a unique polymer that is nontoxic to the environment, has an abundance of functional groups (amino, methyl, and hydroxyl groups), and is simple to modify or react with other compounds .
Several researchers have reportedly used chitosan modified for dye removal [10–12]. However, there are no reports of using chitosan cross-linked with zeolite as an adsorbent to remove MB from water. The current study investigated the influence of pH, initial MB concentration, adsorbent dosage, contact time, and other ions such as NaCl, glucose, citric acid, and H2O2 on MB adsorption.
2. Materials and Methods
Zeolite synthetic (ZL) was from previous research . Methylene blue (MB), chitosan (CH), sodium hydroxide (NaOH), hydrochloric acid (HCl), sulfuric acid (H2SO4), citric acid (C6H8O7), D+ glucose (C6H12O6), sodium chloride (NaCl), and hydrogen peroxide (H2O2) were purchased from Kanto Chemical Co. Inc. (Tokyo, Japan). EDTA-Na solution was purchased from Dojindo Moleculer Technologies, Inc. (Tokyo, Japan). The MB characteristics are shown in Table 1.
2.2. Preparation of Adsorbent
The adsorbent preparation followed previous research with modifications . The process was conducted at room temperature. Chitosan was mixed with different ratios of acetic acid 0.5 : 1 and 1 : 1, referred to as ZLCH-a and ZLCH-b, respectively. 1 g of zeolite was mixed with 25 mL of chitosan solution for two hours (Rotator RT-50). Afterward, 25 mL of 1 M NaOH was added and mixed for thirty minutes. Then filtered, and dried at 60°C for 48 hours. Lastly, the adsorbent was sieved at <100 μm.
2.3. Adsorption Experiments
All experiments were conducted for three replicates on a magnetic stirrer at room temperature (25°C). The pH effect, adsorbent dose, initial MB concentration, and contact time on the adsorption of MB were investigated. The adsorption capacity and percent removal were calculated using the following equations, respectively. where is the adsorption capacity (mg/g), is the initial MB concentration (mg/L), is the equilibrium of MB (mg/L), is the adsorbent mass (g), and is the volume of MB (L).
2.4. Adsorption Kinetics
ZLCH-b was mixed with 100 mL (25 mg/L of MB) in intervals time from 1 to 60 minutes. The adsorption capacity at the time was calculated using the following equation. where is the MB concentration at time (min) (mg/L), is the initial concentration of MB (mg/L), is adsorbent mass (g), and is the volume of MB solution (L).
2.5. The pH of Zero Point Charge
The salt addition was employed to determine the pH of the zero point charge of the ZLCH-b. A constant amount of solute was added to a series of solutions containing different pH strengths . In a 100 mL beaker, 0.1 g of ZLCH-b was distributed in a 50 mL solution containing 0.01 M NaCl. 0.01 M HCL and 0.01 M NaOH were used for pH adjustment in acidic and alkaline conditions. The samples were agitated for 24 hours using a rotary agitator (Rotator RT-50). The point zero charges were calculated by plotting () versus pH initial.
2.6. Desorption Studies
Under optimal conditions of MB adsorption, desorption studies were employed. 25 mg/L of MB-loaded to ZLCH-b at pH 10. It was soaked in 0.01 M (NaOH, EDTA-Na, HCl, and H2SO4). The desorption percentage is calculated using the following equation. where is the desorption percentage (%), is the initial MB concentration before adsorption (mg/L), is the equilibrium MB concentration after adsorption (mg/L), and is the equilibrium MB concentration after desorption (mg/L).
MB concentration was analyzed using a UV-Vis spectrophotometer (Jasco V-530, Jasco Corporation, Tokyo, Japan). The functional group of the adsorbent was investigated by ATR-FTIR in the region of 400–4000 cm−1 before and after adsorption at 1 cm−1 (Thermo Scientific Nicolet iS10, Thermo Fisher Scientific Inc., Waltham, MA, USA). The photograph and EDS data were analyzed using scanning electron microscopy (SEM) (Hitachi TM3000, Tokyo, Japan) and SEM-EDS (JIED-2300, Shimadzu, Kyoto, Japan), respectively. The original unit results of specific surface area (SSA) are cm2/cm3 and converted to m2/g with assumed . It was analyzed using dynamic light scattering (DLS) (Horiba LB-550, Kyoto, Japan).
3. Results and Discussion
3.1. Adsorbent Characteristics
The SEM images and EDS spectra of the adsorbent (ZLCH-a and ZLCH-b) are shown in Figure 1. As can be seen, both surface adsorbents had interlayers, containing porous and adhesive structures. The EDS data revealed that the value of carbon and sodium in ZLCH-a is higher than that in ZLCH-b. By comparing SSA data (Table 2), it can be concluded that ZLCH-b shows the lowest result than ZLCH-a from 96.85 mg/g to 88.51 mg/g, respectively (data not shown), indicating that ZLCH-b had much of the mass unit number. The mass unit number has correlated with the adsorbent sites to catch the number of the ions. It could confirm the adsorption process’s effectiveness (Figure 2). Hence, ZLCH-b was used for the next treatment.
3.2. Optimum pH
The pH of the dye solution is critical to the adsorption capacity mechanisms . The influence of pH on the MB adsorption onto ZLCH-b with an initial MB concentration of 10 mg/L is shown in Figure 2(a). The result shows that the adsorption capacity and removal percentage increased from 37.68 to 97.35 mg/g and 37.7 to 97.3%, respectively, when the initial pH was changed from 2 to 10. It could be confirmed with the pH zero point charge of the adsorbent (Figure 2(b)). When the pH is below , the surface charge of the adsorbent may become positively charged, forcing H+ ions to compete with MB cations, resulting in a decrease in the amount of dye adsorbed (Azza [15, 16]). However, when the pH is above , the surface charge becomes negatively charged favouring MB removal. This result is similar to Maryam et al.  for MB removal by corn husk carbon activation ZnCl2.
3.3. Effects of Initial MB Concentration
The effect of initial MB concentration (10-25 mg/L) on the MB adsorption onto ZLCH-b is shown in Figure 3. The adsorption capacity increased with increasing initial MB concentration from 97.35 to 207.16 mg/g, and the removal percentage decreased with the increase in initial MB concentration from 97.3 to 82.9%. It may cause the ratio of the initial number of MB molecules to the surface area to be low at lower concentrations . However, at high concentrations, the number of accessible adsorption sites decreases. Hence, the removal percentage depends on the initial concentration .
3.4. Effects of Adsorbent Doses
Getting a higher amount of adsorption capacity with the lowest adsorbent dose is essential for eliminating contaminants . Figure 4 shows the effect of adsorbent dose from 0.1 to 0.3 g/L on ZLCH-b adsorption. As a result, the impact on adsorption capacity decreased with the increasing dose from 207.16 to 82.86 mg/g, and the removal percentage increased from 82.9 to 99.4%. Similar results have been reported for dye removal [18, 19]. The decrement of MB adsorption capacity at a higher dose of adsorbent is because active adsorbent sites are less accessible due to adsorbent particles collected and overlapped , while the increment of the removal percentage due to the increased number of binding sites would lead to a decrease in the total surface area of the adsorbent [21, 22].
3.5. Effects of Contact Time
As shown in Figure 5, the effect of contact time on the adsorption capacity of ZLCH-b for MB removal was studied between 1 and 60 minutes. The result indicates that the removal percentage increases rapidly from 0 to 1 minute (71.3%) and then gradually up to 60 minutes (97%), indicating that the electrostatic interaction between MB and adsorbent is responsible, and MB molecules were caught and filled up in the adsorbent sites.
3.6. Adsorption Kinetic Studies
Adsorption kinetic studies are important to provide information on the MB adsorption mechanism onto ZLCH-b. The most commonly used are pseudo-first- and pseudo-second-order kinetic models [6, 23, 24]. The experimental data of kinetic studies for first and second order are illustrated in Figures 6(a) and 6(b), respectively. Pseudo-first-order and pseudo-second-order kinetic models are shown in the following equations (5) and (6), respectively
Table 3 shows the correlation linear of the pseudo-first- and pseudo-second-order kinetic models ( and ), respectively, indicating that the pseudo-second-order kinetic model fits onto the ZLCH-b adsorption process.
3.7. Adsorption Isotherm Studies
In the current study, we used the Langmuir and Freundlich adsorption isotherm models, which are common by many authors [6, 25, 26]. Langmuir occurs on the single layer of the adsorbent surface, while Freundlich on the multilayer of the adsorbent surface. The equation for Langmuir (7), Langmuir separation factor characteristics (8), and Freundlich (9) are shown in below: where is the adsorbent amount (mg/g), is the equilibrium time (mg/g), is the adsorption equilibrium constant (L/mg), is the single layer of maximal adsorption capacity, and is the equilibrium concentration (mg/L). is the equilibrium parameter of Langmuir characteristics. is the multilayer of absorption capacity (mg/g), and is the adsorption intensity.
The experimental adsorption isotherm data for Langmuir, Freundlich, and the correlation coefficient of MB are shown in Figures 7(a) and 7(b) and Table 4, respectively. As a result, the value of Freundlich was higher than the Langmuir isotherm model and the value of in these studies (0.00002), indicating that the adsorption isotherm of MB onto ZLCH-b is a Freundlich isotherm model and favorable .
3.8. Influence of Ion Strength
The influence of other ions on the MB adsorption is essential because the water solution in the environment has diverse ions as shown in Figure 8. 0.1 g/L of adsorbent, 25 mg/L MB, and 50 mg/L (sodium chloride, glucose, citric acid, and H2O2) were investigated. The adsorption process was conducted on a magnetic stirrer for 5 minutes. We found that the removal percentage decreased by adding citric acid, sodium chloride, and glucose. This is due to the surface of the adsorbent competing with other ions which inhibited MB molecules trapped on the adsorbent surface sites. In contrast, the removal percentage increased by adding H2O2 compared with no contaminant from 83.4 to 85.3%, respectively, which indicates that an oxidation reaction simultaneously occurred with the adsorption process. The proposed mechanism’s interaction is shown in Figure 9.
3.9. FTIR Data
The FTIR spectra of the adsorbent before and after MB adsorption in the absence of other ions are shown in Figure 10. After the MB adsorption process, the range and intensity of peaks in the adsorbent structure were changed, which could be due to the interaction and placement of the MB on the adsorbent surface. For example, after the adsorption process, the range of –OH vibrations in the ZLCH-b increased to 3363 cm-1, indicating that hydrogen bonds have been formed in the adsorption process. A decreased band peak was found from 1645 to 1604 cm-1, which corresponded to the N-H group, resulting in increased electrostatic interaction between MB and the ZLCH-b adsorbent . Moreover, the other peaks in the band also decreased after the adsorption process, from 1423 to 1393 cm-1. It may cause N-O stretching to correspond to C-H in OH- or NH2 groups. The band from 1050 to 950 cm-1 was assigned to the internal asymmetric stretching of Si-O-Si or Si-O-Al . New peaks after adsorption were found at 2924 and 1731 cm-1, which corresponded to C-H and C=O, respectively .
3.10. Desorption Studies
MB recovery is important in economic terms. Desorption of percentage with various solvents is shown in Figure 11(a). As can be seen, using an alkaline agent (NaOH), the desorption percentage reached 1.64%. While using an acidic agent (HCl and H2SO4), the desorption percentage reached 49.75% and 57.05%, respectively. This is due to hydrochloric acid having one proton, whereas sulfuric acid contains two protons, making sulfuric acid more effective in separating (-OH) from the NH2- to H+ of ZLCH-b. After that, sulfuric acid is employed as the subsequent treatment (Figure 11(b)). Due to the low percentage results, we increased the temperature. The results obtained that the desorption percentage increased to 99.65% at 45°C. This indicates that oxidizing plays an important role in releasing hydrogen groups from adsorbent surface. This agrees with Momina et al.  for the desorption of MB from bentonite adsorbent coating.
3.11. Comparison with Other Results
Table 5 shows the comparison of the adsorption capacity of MB using several adsorbents. We can see that the highest adsorption capacity derived from zeolite-activated carbon from oil palm ash was 285.71 mg/g with an initial MB concentration of 400 mg/L. However, our study reaches 252.51 mg/g with an initial MB concentration of 25 mg/L. These results concluded that ZLCH could be practical to remove MB from water.
Zeolite modified was evaluated and confirmed to be a promising adsorbent for MB removal from water. MB removal has been studied under various experimental conditions. The result shows that increased pH and initial MB concentration would increase the adsorption capacity, while increased adsorbent dose would decrease the adsorption capacity. Experimental data showed the Freundlich isotherm and pseudo-second-order kinetic models. The effect of sodium chloride, glucose, and citric acid could decrease the removal percentage. However, it increased after adding the H2O2 ion. These results indicate that adsorption and oxidation processes simultaneously occurred for MB removal. MB desorption was efficient used sulfuric acid.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
The authors (E.H.) express gratitude to the MEXT scholarship for sponsoring the studies at the Prefectural University of Hiroshima, Japan.
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