Methylene Blue Removal from the Aqueous Phase Using a Magnetic-Calcined Bamboo Composite Adsorbent

Abstract

In the present study, magnetic-calcined bamboo composite adsorbents (MCBC200, MCBC400, MCBC600, MCBC800, and MCBC1000) were prepared, and their physicochemical characteristics (scanning electron microscope images, differential thermogravimetric analysis, Fourier transform-IR, specific surface area, surface functional groups, and point of zero charge [pH_pzc]) were evaluated. Furthermore, the adsorption capacity of methylene blue (MB, cationic dye) using the prepared adsorbents was assessed. The value of pH_pzc and the specific surface area of MCBC400 were 7.8 and 50.6 m²/g, respectively. The amounts of acidic or basic functional groups of MCBC400 were relatively greater than those of the other adsorbents. The amount of MB adsorbed onto MCBC400 (31.9 mg/g) was higher than that onto other adsorbents. The adsorption of MB using MCBC400 was evaluated in relation to various parameters, including coexistence, solution pH, adsorption temperature, and contact time. The results followed the Langmuir isotherm model and a pseudo-second-order model with correlation coefficients of 0.980–1.000 and 0.996, respectively. MB was selectively adsorbed by MCBC400 in a binary solution system containing anionic dyes. Finally, one of the adsorption mechanisms was determined by analyzing the elemental distribution and the binding energy before and after the adsorption of MB. The current findings provide important information for removing MB with MCBC400 from the aqueous phase.

Introduction

Millions of tons of wastewater with intense color are released from a variety of sources, including the printing, dye, plastic, textile, leather, food, and cosmetic industries.¹⁾ A previous study reported that there were more than 100000 types of dyes that, in the process of producing more than 700000 tons of dyestuff per year, are divided into anionic and cationic groups based on their structural makeup.²⁾ Dye exposure can lead to harmful damage to organisms, such as impairment of the kidneys, reproductive system, liver, brain, and central nervous system.^3,4) Furthermore, compared to anionic dyes, cationic dyes are more hazardous.⁵⁾ In particular, methylene blue (MB), a cationic soluble dye or stain, has been used in medicine for centuries. It is commonly used as an antidotal treatment for acquired methemoglobinemia. However, MB is known to be poisonous, carcinogenic, and suspected to be a mutagenic agent. In addition, previous studies reported that MB causes harmful effects on the water environment.^6,7) In water environment fields, MB is often selected as a model organic cation in research devoted to assessing the interaction between cationic dyes and various biomaterials in the search for new adsorbent-based materials.⁸⁾ In addition, the United Nations announced 169 integrated and indivisible targets that are linked to the 17 sustainable development goals (SDGs).⁹⁾ In particular, the most recent iteration of the United Nations’ goal to address water-related concerns is SDG 6, which focuses on water and sanitation. Further, oceans, seas, and marine resources need to be conserved and used responsibly for sustainable development, according to SDG 14. Therefore, the development of an efficient and/or useful treatment for the removal of MB from wastewater is considered important to maintain a safe and/or healthy environment.

In addition, sustainable patterns of production and consumption are the focus of SDG 12. Hence, researchers have recently become interested in the reuse of residues and/or by-products to act as eco-friendly products. Various biomass, agricultural wastes, and industrial residues have been used to prepare useful adsorbents, such as carbonaceous materials, due to their multiple advantages, including renewability, low-cost precursors, and environmental friendliness.^10,11)

In recent years, researchers have exploited various renewable and/or biomass products, such as coconut,¹¹⁾ sugarcane bagasse,¹²⁾ bamboo,¹³⁾ and Bacillus subtilis,¹⁴⁾ amongst others, to produce carbonaceous materials. Prepared carbonaceous materials are useful adsorbents for the removal of dyes from the aqueous phase because of their unique physicochemical characteristics, which can include porous structure, specific surface area, surface functional groups, and cation exchange capacity. Therefore, various physical and/or chemical treatments have been used to improve the adsorption capability of prepared carbonaceous materials.^15,16) In addition, in the practical application of prepared carbonaceous materials, it is quite difficult to control (recover and/or regenerate) them after use, especially with a small particle size. Focused magnetic adsorbents can now be utilized to separate different chemicals from both suspensions and solutions.^14,17–19) Magnetic field-assisted separation treatment can relatively simply and selectively separate compounds from solutions due to their magnetic properties. This useful treatment with magnetic carbonaceous material is efficient for not only small but also large-scale operations.¹⁴⁾

To the best of our knowledge, among waste biomass products, bamboo is now abundant in developing countries, such as South Asia; however, suitable recycling technology for it has not been established. In addition, the preparation of magnetic-calcined bamboo composite adsorbent and the assessment of its physicochemical characteristics and/or adsorption capability have not been investigated. Thus, the present study focused on the removal of MB using a magnetic-calcined bamboo composite adsorbent from the aqueous phase. In order to clarify the MCBC in the adsorption mechanism, some isothermic and kinetic models were evaluated in batch experiments. Additionally, investigations of the binding energy and elemental distribution were done both before and after the adsorption of MB.

Experimental

Materials

Raw bamboo (RB, Bambusa nutans Wall. ex Munro, GRAMINEAE) was obtained from Chiang Mai, Thailand (Production area: Mae Chaem District, Chiang Mai Province, Thailand). MB (C₁₆H₁₈N₃SCl・3H₂O, CAS RN^®: 7220-79-3) was purchased from FUJIFILM Wako Pure Chemical Corporation (Japan), as was crystal violet (CV, C₂₅H₃₀ClN₃・9H₂O, CAS RN^®: 60662-33-1). Acid orange 7 (AO, C₁₆H₁₁N₂NaO₄S, CAS RN^®: 633-96-5) and Kayanol Milling Red BW (KMRB, C₃₀H₃₇N₃Na₂O₈S₂, CAS RN^®: 15792-43-5) were obtained from Tokyo Chemical Industry Co., Ltd. (Japan) and Shinko Co. (Japan), respectively.

Preparation of Magnetic-Calcined Bamboo Composite Adsorbents and Their Characterization

Magnetic-calcined bamboo composite adsorbents (MCBCs) were prepared by the previously reported method.¹²⁾ In particular, the mass ratio of 1:1.5 between RB and iron(III) nitrate nonahydrate was mixed in a crucible. Then, the calcination process was done at 200, 400, 600, 800, and 1000°C, which are denoted as MCBC200, MCBC400, MCBC600, MCBC800, and MCBC1000, respectively. The scanning electron microscope images, differential thermogravimetric analysis (TG-DTA), and Fourier-transform IR (FT-IR) spectroscopy were used for the characterization of the prepared adsorbents and had been previously reported.¹³⁾ In addition, the specific surface area was measured by a NOVA4200e instrument (Yuasa Ionics, Japan). Finally, the point of zero charge (pH_pzc) and surface functional groups were determined using previously published techniques.^20,21)

Adsorption Capacity of MB

Initially, a screening experiment was conducted. MB solution of 50 mg/L (50 mL) and 0.05 g of tested adsorbent were mixed at 100 rpm for 24 h at 25°C. Then, the filtrate was collected from a 0.45-µm membrane filter. The UV-visible spectrophotometer (UV-1280, Shimadzu, Japan) was used to measure the concentration of MB before and after adsorption at 664 nm, the maximum absorption wavelength of MB. The difference between the concentrations before and after adsorption was used to compute the quantity of MB adsorbed.

Additionally, the effects of the parameter on the MB adsorption utilizing MCBC400 were demonstrated. Adsorption isotherms were evaluated using the following procedures: MB solutions of 1, 5, 10, 20, 30, 40, or 50 mg/L and MCBC of 0.05 g were mixed at 100 rpm for 24 h at 5, 25, and 45°C. Second, the effect of pH or contact time on the adsorption of MB using MCBC400 was determined. The experimental conditions were similar to those described above. The pH value ranged from 3 to 11, and the contact time ranged from 1 to 24 h in this study. In these experiments, the suspension was shaken at 100 rpm for 24 h at 25°C. Then, the filtrate was collected from a 0.45-µm membrane filter. The absorbance of MB was measured at 664 nm by a UV-visible spectrophotometer. Additionally, the F-73 digital pH meter (HORIBA, Japan) was used to measure the pH.

Furthermore, the adsorption capacity of MB in binary solution systems, including CV, AO, or KMRB, was measured to clarify the effect of coexistence on the adsorption of MB. Binary solutions (MB + CV, MB + AO, or MB + KMRB) of 10 or 50 mg/L were prepared, and then, each solution and MCBC400 of 0.05 g were mixed at 100 rpm for 24 h at 25°C. Then, the filtrate was collected from a 0.45-µm membrane filter. The absorbance of MB was measured at 664 nm by a UV-visible spectrophotometer. The maximum absorption wavelengths of CV, AO, and KMRB were 591, 486, and 515 nm, respectively. The difference between the concentrations before and after adsorption was used to compute the quantity of each dye adsorbed. All sample solutions were prepared using distilled water in this study.

Finally, to elucidate the adsorption mechanism of MB, the elemental distributions of carbon (C), nitrogen (N), and sulfur (S) and the binding energies of these elements were evaluated by an electron probe microanalyzer JXA-8530F (JEOL, Japan) and AXIS-NOVA (Shimadzu), respectively.

Adsorption/Desorption Capacity of MB Using MCBC400

Desorption of MB from MCBC400 was evaluated with 0.1 mol/L sodium hydroxide solution, 0.1 mol/L hydrochloric acid solution, and 99.5% ethanol. Before desorption, MCBC400 was loaded with MB using the following procedures: MB solution of 50 mg/L (300 mL) and MCBC400 of 0.3 g were mixed at 100 rpm for 24 h at 25°C. Then, the filtrate was collected from a 0.45-µm membrane filter. The absorbance of MB was measured at 664 nm by a UV-visible spectrophotometer. After adsorption of MB, MCBC400 was collected and dried. Subsequently, the MB-loaded MCBC400 of 0.05 g was added to a 50 mL solution of 0.1 mol/L sodium hydroxide solution, 0.1 mol/L hydrochloric acid solution, and 99.5% ethanol. After shaking the suspension for 24 h at 100 rpm and 25°C, a 0.45-µm membrane filter was used to filter it. The amount of MB desorbed from MCBC400 was calculated by the difference between concentrations before and after desorption. In this study, all data are expressed as mean ± standard error (n = 2–3) values.

Results and Discussion

Characteristics of the Magnetic-Calcined Bamboo Composites

As mentioned earlier, the characteristics of MCBC adsorbents were reported in a previous study.¹²⁾ Briefly, a dense and constricted morphology of RB was observed. Figure 1 shows the TG-DTA of the MCBC adsorbent. RB was clearly thermally decomposed at approximately 250–300°C in this study. The number of MCBC adsorbent surface porosities increased with increasing temperature during the calcination process. FT-IR spectra demonstrated that some MCBC adsorbents showed C-O carbonyl stretching and hydrogen-bonded OH stretching vibrations. The values of pH_pzc and the specific surface area of MCBC tended to increase with increasing calcination temperatures (in particular, for MCBC400, pH_pzc: 7.8 and specific surface area: 50.6 m²/g). Moreover, the amounts of acidic or basic functional groups of MCBC400 were relatively greater than those of other adsorbents (amount of acidic functional groups: 0.10 mmol/g; amount of basic functional groups: 0.83 mmol/g). The number of acidic functional groups and surface hydroxyl groups decreased with increasing calcination temperature. For instance, phenolic and carboxyl groups were generated upon carbonization at 400–500°C; the number of these functional groups decreased upon carbonization at 600–700°C. Additionally, the hydroxyl and carboxyl groups disappeared at temperatures above 800°C. Similar trends were observed in waste biomass.^22,23) The obtained results showed that various characteristics of composite adsorbents could be produced by calcination treatment with RB and iron(III) nitrate nonahydrate under the present experimental conditions.

Fig. 1. Thermal Analysis of Magnetic-Calcined Bamboo Composite under an Air Atmosphere

Adsorption Capacity of MB Using MCBC Adsorbents

The amount of MB adsorbed using MCBC adsorbents is shown in Fig. 2. The amount of MB adsorbed was in the following order: MCBC1000 < MCBC < MCBC600 < MCBC200 < MCBC800 < MCBC400. Subsequently, the yield percentages of MCBC200, MCBC400, MCBC600, MCBC800, and MCBC1000 were 19.2, 18.0, 15.4, 4.8, and 1.9%, respectively, under the present experimental conditions. Consequently, it was found that 400°C was the optimal calcination temperature for adsorption.

Fig. 2. Amount of MB Adsorbed Using Magnetic-Calcined Bamboo Composite Adsorbents

Initial concentration: 50 mg/L; sample volume: 50 mL; adsorbent: 0.05 g; temperature: 25°C; contact time: 24 h; agitation speed: 100 rpm.

The relationships between physicochemical characteristics and the amount adsorbed were evaluated, and it was found that the specific surface area and/or surface acidic functional groups were crucial in enhancing the adsorption capacity of MB in the aqueous phase. In a separation study after adsorption, some studies reported an easy/simple separation treatment toward the use of a magnetically-assisted water purification agent to prevent clogging in the water purification system.^12,24) Therefore, one of the main themes in this study was wastewater purification, including dyes, to prevent clogging in adsorption treatment with adsorbents. It can be seen from Fig. 3 that MCBC400 could be separated from the liquid phase under the action of the magnetic field. MCBC400 was an ideal adsorbent for easy/simple magnetic separation from treated water by the external magnetic field.

Fig. 3. Photograph of Sample Solution before and after Adsorption of MB Using MCBC400

Adsorption Isotherms of MB Using MCBC400

Figure 4 displays the MB adsorption isotherms on MCBC400. The amount of MB adsorbed onto MCBC400 was in the following order: 5 < 25 < 45°C. Adsorption capacity increased significantly with increasing adsorption temperatures. The interaction behavior between the adsorbent and the adsorbate can be explained by the adsorption isotherm. Two models were employed in this study to evaluate the adsorption isotherm, the Langmuir (Eq. (1)), and Freundlich models (Eq. (2)).^25,26)

Fig. 4. Adsorption Isotherms of MB at Different Temperatures

Initial concentration: 1–50 mg/L; sample volume: 50 mL; adsorbent: 0.05 g; temperature: 5, 25, and 45°C; contact time: 24 h; agitation speed: 100 rpm.

The Langmuir model is a monolayer adsorption with localized physical adsorption on a homogeneous surface. Three hypotheses explain these phenomena: uniformly energetic adsorption sites, monolayer coverage, and no lateral interaction between adsorbed molecules.^27,28) In addition, the Freundlich model is an empirical equation describing the adsorption intensity of the adsorbent toward the adsorbates.²⁹⁾ These equations are provided as follows:

  

$$1/q\text{\hspace{0.17em}=\hspace{0.17em}}1/\left(q_{max}{K}_L{C}_e\right)+1/q_{max},$$(1)

  

$$\text{log}q=\frac{1}{n}\text{log}{C}_e+\text{log}{K}_F,$$(2)

where q represents the amount of MB adsorbed (mg/g), q_max represents the maximum adsorption capacity of MB (mg/g), and C_e represents the equilibrium concentration of MB (mg/L). 1/n and K_F represent MB’s adsorption strength and capacity, respectively. K_L represents the Langmuir isotherm constant (binding energy) (L/mg).

Table 1 lists the Freundlich and Langmuir constants for MB adsorption utilizing MCBC400. The r-values of the Langmuir model and the Freundlich model were 0.980–1.000 and 0.963–0.981, respectively. The adsorption of MB using MCBC400 followed the Langmuir’s model theory. This indicates that there are no molecule interactions with adjacent sites as the MB molecules form a homogeneous monolayer. It was found that MB’s maximum adsorption capacity was significantly increased with increasing adsorption temperatures. Moreover, it may be inferred that there is good MB adsorption onto MCBC400 based on the values of the Langmuir constant K_L. In contrast, the value of the Freundlich constant 1/n is an important factor for elucidating the adsorption mechanism. In the present study, the value of 1/n (0.38–0.56) showed that MB was easily adsorbed on MCBC400.³⁰⁾

Table 1. Langmuir and Freundlich Constants for the Adsorption of MB

Sample	Temperature (°C)	Freundlich constants			Langmuir constants
		LogKF	1/n	r	qmax (mg/g)	KL (L/mg)	r
	5	0.74	0.56	0.976	22.61	0.57	1.000
MCBC400	25	1.10	0.38	0.981	25.31	2.01	0.980
	45	1.19	0.50	0.963	45.36	0.94	0.997

Next, the elemental distribution and binding energy analyses were used to evaluate the adsorption mechanism. The qualitative analysis of the MCBC400 surface both before and after MB adsorption is displayed in Fig. 5, which demonstrates that high and low concentrations of MB are shown by the warm and cool colors, respectively. In this study, the components of MB such as carbon (C), nitrogen (N), and sulfur (S) were measured, and the intensities of carbon increased slightly after adsorption. These results indicate that MB was adsorbed onto the MCBC400 surface. Furthermore, the binding energies of each element onto the MCBC400 surface were measured. After adsorption of MB, the peak of carbon (C, 1s) was clearly detected and increased under the experimental conditions (Fig. 6). These findings suggest that the adsorption capacity of MB from aqueous media is significantly influenced by the physicochemical characteristics of the MCBC400 surface. However, the intensities of MB components such as nitrogen (N) and sulfur (S) did not change between before and after adsorption under our experimental conditions (Figs. 5 and 6). Therefore, further studies are needed to elucidate the adsorption mechanism of MB using MCBC400 in detail.

Fig. 5. Qualitative Analysis of MCBC400 Surface before and after Adsorption of MB

Color mappings of each element, such as carbon, nitrogen, and sulfur, onto the MCBC400 surface are shown. Warm colors and cold colors show high concentration and low concentration, respectively. The average value of each element’s intensity in the picture is shown.

Fig. 6. Binding Energies of Carbon (C), Nitrogen (N), and Sulfur (S) onto MCBC400 Surface

Adsorption Capacity of MB Using MCBC400 at Different Contact Times

The adsorption kinetics of MB onto MCBC400 is demonstrated in Fig. 7. As shown in the figure, MCBC400 showed rapid adsorption, with 22.6 mg/g MB adsorption occurring within the initial 3 h at an initial concentration of 50 mg/L. Thereafter, the adsorption was slower and reached equilibrium within 24 h. Both the increased surface area and increased functional groups facilitated the interaction between MCBC400 and MB, which is responsible for the initial adsorption.^15,31)

Fig. 7. Effect of Contact Time on the Adsorption of MB

Initial concentration: 50 mg/L; sample volume: 50 mL; adsorbent: 0.05 g; temperature: 25°C; contact time: 1–24 h; agitation speed: 100 rpm.

The mass transport process and the physicochemical characteristics of the adsorbent affect the adsorption mechanism.³²⁾ In this study, the regulating adsorption mechanism of MB was explained using 2 commonly utilized adsorption kinetic models. The pseudo-first-order model (PFOM) of Lagergren [Eq. (3)] is based on the assumption that the rate of change of adsorbate with time is proportional to the difference in equilibrium adsorption capacity and the amount adsorbed. On the other hand, the pseudo-second-order model [PSOM, Eq. (4)] is based on the assumption that the rate-limiting step involves chemisorption.^33,34)

The equations are as follows:

  

$$\text{ln}(q_e-q_t)=\text{\hspace{0.17em}ln}{q}_e-k_{1}t,$$(3)

  

$$\frac{t}{q_t}=\frac{t}{q_e}+\frac{1}{K_2\times {q_e}^2},$$(4)

where q_e (mg/g) is the adsorption capacity of MB at equilibrium, q_t (mg/g) is the adsorption capacity of MB at time t, k₁ (1/h) is the overall constant in the PFOM, and k₂ (g/mg/h) is the pseudo-second-order adsorption constant.

Table 2 lists the fitting results of kinetic data using PFOM and/or PSOM. The correlation coefficient (r) obtained from PSOM (0.996) was higher than that from PFOM (0.944). This result showed that the PSOM was the most effective for explaining the kinetics of MB adsorption onto MCBC400. In addition, the calculated value of (q_{e, cal}) obtained from the PSOM agreed perfectly with the experimental value of (q_{e, cal}) in this study. The result further indicated that the adsorption of MB using MCBC400 followed the PSOM well. According to the PSOM, the adsorption mechanism cannot be sufficiently described by the external resistance model, as boundary layer resistance is not the rate-limiting factor.^4,35) Moreover, chemical adsorption involving valency forces through electron sharing or exchange between MB and MCBC400 was the rate-limiting step.^4,36)

Table 2. Fitting Results of Kinetic Data Using PFOM and PSOM

Sample	qe, exp (mg/g)	PFOM			PSOM
		k1	qe, cal	r	k2	qe, cal	r
		(1/h)	(mg/g)		(g/mg/h)	(mg/g)
MCBC400	30.0	0.10	13.2	0.944	0.02	30.5	0.996

Adsorption Capacity of MB Using MCBC400 at Different pHs

Figure 8 shows the amount of MB adsorbed at different pH conditions. By controlling the adsorbent surface charge and the degree of ionization of the adsorbent in the aqueous solution, the pH of the solution has a significant impact on the adsorption capacity. It is clear from Fig. 8 that the adsorption capacity of MB gradually increased with increasing solution pH values from 3 to 11. In the acidic condition, the competition between the protonation of MB and H₃O⁺ ions for adsorption sites resulted in lower adsorption. In contrast, the number of negatively charged sites increased gradually, and the formation of an electric double layer changed the polarity of MCBC400 under basic conditions. These phenomena could be explained by the pH_pzc.

Fig. 8. Amount of MB Adsorbed at Different pH Conditions

Initial concentration: 50 mg/L; sample volume: 50 mL; adsorbent: 0.05 g; temperature: 25°C; pH: 3, 5, 7, 9, and 11; agitation speed: 100 rpm.

Characteristics of the Magnetic-Calcined Bamboo Composites

Collectively, our obtained results mean that the adsorption mechanism of MB using MCBC400 is promoted by physisorption (related to specific surface area), chemical adsorption (related to valency forces through electron sharing or exchange), electrostatic interaction (related to adsorbent surface charge), and interaction between MB and surface functional groups (Fig. 9).

Fig. 9. The Schematic Mechanism of MB Adsorption Using MCBC400

Adsorption Capacity of MB in a Binary Solution System

CV, AO, and KMRB in a binary solution system were selected due to widespread use in textiles such as cotton and silk, and in paints and printing ink. In addition, these dyes are responsible for causing moderate eye irritation, painful sensitization to light, aquatic toxicity, or allergenic effects.^37–39)

The amount of MB adsorbed using MCBC400 in a binary solution system is listed in Table 3. When using cationic dyes (containing CV), the amounts of MB and CV adsorbed onto MCBC400, and the competition between MB and CV, were observed under the present experimental conditions. These results indicate that cationic dyes were adsorbed more effectively using MCBC400. In contrast, when using anionic dyes (containing AO or KMRB), MB was selectively adsorbed in a binary solution system, indicating that anionic dyes were not adsorbed onto MCBC400. Therefore, MCBC400 was a useful adsorbent for the removal of cationic dyes from the aqueous phase.

Table 3. Amount of MB Adsorbed Using MCBC400 in Binary Solution System

Initial concentration (mg/L)	MB + CV		MB + AO		MB + KMRB
	Amount of MB adsorbed (mg/g)	Amount of CV adsorbed (mg/g)	Amount of MB adsorbed (mg/g)	Amount of AO adsorbed (mg/g)	Amount of MB adsorbed (mg/g)	Amount of KMRB adsorbed (mg/g)
10	9.2	9.2	7.1	0	6.9	0
50	20.2	29.9	19.4	0	25.1	1.6

Adsorption/Desorption Capacity of MB Using MCBC-400

The desorption percentages of MB from MCBC400 using 0.1 mol/L sodium hydroxide solution, 0.1 mol/L hydrochloric acid solution, and 99.5% ethanol were 18.4, 38.2, and 1.4%, respectively. Of these, the 0.1 mol/L hydrochloric acid solution was a useful desorption solution under the present experimental conditions. As shown in Fig. 8, the optimal pH condition was determined under basic conditions, so the acidic condition (using 0.1 mol/L hydrochloric acid solution) was unsuitable for removing MB from the aqueous phase. Further studies are needed to elucidate the adsorption/desorption efficiency of MB using MCBC400 in the field.

Finally, the findings of the current study were compared and confirmed with previous studies that employed various adsorbent types to remove MB from the aqueous phase (Table 4). The adsorption capacity of MB using MCBC400 was obtained by Langmuir constant q_max in Table 1. The comparison showed that MCBC400 is a useful adsorbent for removing MB, with the exception of sugarcane bagasse waste and karanj (Pongamia pinnata) fruit hulls.^40–46)

Table 4. Comparison of the MB Adsorption Capacity with Those of Other Reported Adsorbents

Samples	Adsorption capability (mg/g)	pH	Temperature (°C)	Initial concentration (mg/L)	Contact time (h)	Adsorbent (g/L)	References
Biomass fly ash geopolymer monoliths	15.4	—	r.t.	50	30	—	Novais et al.40)
Raw Posidonia oceanica fibers	5.56	6.0	30 ± 2	50	3	10	Ncibi et al.42)
Caulerpa racemosa var. cylindracea	5.23	7.0	18	100	1.5	16.7	Cengiz and Cavas41)
Sugarcane bagasse waste	136.5	8.0	30	250	12	0.8	Jawad et al.43)
Karanj (Pongamia pinnata) fruit hulls	239.4	3–13	50	400	28	1.0	Islam et al.44)
Oil palm shell	20.0	—	r.t.	50	—	1.0	Kong et al.45)
Biochar prepared from the pyrolysis of mixed municipal discarded material	7.3	5.0	30	100	6	5.0	Hoslett et al.46)
MCBC400	25.31	—	25	50	24	1.0	This study

Conclusion

Magnetic-calcined bamboo composite adsorbent (MCBC-400) was prepared, and its capacity for adsorption of MB was also demonstrated. The adsorption capacity of MB using MCBC400 was greater than that of other adsorbents. In addition, MCBC400 showed effective solid–liquid separation in the magnetic field. The present study indicated that the amount of MB adsorbed onto MCBC400 increased with increasing adsorption temperatures, contact time, and solution pH. These findings were fitted to the pseudo-second-order kinetic model and the Langmuir isotherm model with correlation coefficients of 0.996 and 0.980–1.000, respectively. Based on the elemental distributions and binding energy analyses before and after the adsorption of MB, it was determined that MB could be adsorbed onto MCBC400. Moreover, MB was selectively adsorbed onto MCBC400 in a binary solution system (containing anionic dyes). Finally, these results indicated the potential utility of the prepared MCBC400 for solid–liquid separation in wastewater purification containing dyes.

Acknowledgments

This research was partially supported in part by JSPS KAKENHI (Grant Number: JP22K06674).

Conflict of Interest

The authors declare no conflict of interest.

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