One-pot synthesis of purple benzene-derived MnO2-carbon hybrids and synergistic enhancement for the removal of cationic dyes

MnO2-carbon hybrid (MnO2-C-PBz) was simultaneously synthesized by a one-step solution plasma process (SPP) using a single precursor referred to as “purple benzene”, which was derived from the K+(dicyclohexano-18-crown-6 ether) complex. To clarify the synergistic effects on the cationic dye removal, MnO2-free carbon and carbon-free MnO2 samples were concurrently investigated. The results of adsorption for cationic dyes (methylene blue (MB) and rhodamine B (Rh B)) and anionic dye (methyl orange (MO)) revealed remarkably high affinity for cationic dyes. In particular, MnO2-C-PBz exhibited the highest adsorption capacity for MB, i.e., ~3 times greater than that of the others. In addition, MnO2-C-PBz exhibited a rapid, high decolorization ability at C0 = 10 mg L−1 (within a few seconds, ~99%) and at C0 = 100 mg L−1 (within 30 min, ~81%), and the theoretical maximum monolayer adsorption capacity was 357.14 mg g−1 as calculated from the Langmuir adsorption isotherm equation. Furthermore, compared with carbon-free MnO2, MnO2-C-PBz exhibited quite a good cyclic stability. We expect that our findings give rise to the understanding of the synergistic effects of MnO2-carbon hybrid, as well as role of each components for the cationic dye adsorption, and may open an innovative synthesis approach to inorganic-organic hybrid materials.


Results and Discussion
The MnO 2 -carbon hybrid (MnO 2 -C-PBz) was successfully synthesized from purple benzene by SPP. To verify the effects of crown ethers or KMnO 4 , reference samples, i.e., C-Bz and C-CE-Bz, were also prepared as described in the experimental section. From the XRD data ( Fig. 1(a)), broad peaks at 2θ = ~23.7° and ~43.7°, corresponding to the 002 and 100/101 planes of graphitic carbon, were observed for all samples 39 . C-Bz and C-CE-Bz exhibited almost the same XRD patterns, but the (002) peak of C-CE-Bz was slightly shifted toward a lower angle at 2θ = ~23.4°. Crown ethers and/or decomposed oxygen-containing by-products are considered to be embedded in the carbon matrix. In contrast, compared with C-Bz and C-CE-Bz, MnO 2 -C-PBz exhibited considerably weaker and broader peaks. In addition, new peaks at around 2θ = 12.9°, 25.8°, 35.4°, and 66.1° were observed for MnO 2 -C-PBz (inset: magnified XRD pattern). These peaks corresponded to potassium birnessite-type MnO 2 (JCPDS# 80-1098) 40 . These results indicate the possibility that MnO 2 is formed and distributed on the surface of the carbon matrix. This can be deduced from the XRD results of MnO 2 /C-Bz prepared by dispersing C-Bz in aqueous KMnO 4 solution (see supplementary information; Fig. S1  † ). The formation of the MnO 2 -carbon hybrid was further confirmed by Raman spectroscopy. Two strong peaks corresponding to the D-and G-bands were observed at 1338 and 1588 cm −1 , respectively, for all samples, indicating the presence of carbon material ( Fig. 1(b)) 41 . Clear peaks were observed at 576 and 645 cm −1 , corresponding to the Mn-O stretching vibration in the MnO 6 octahedral layer and the symmetric stretching vibration in the MnO 6 octahedral framework, respectively 42 . These results confirmed the successful synthesis of the MnO 2 -carbon hybrid by a single-step SPP.
The surface area and pore structure of the adsorbents were characterized by N 2 adsorption-desorption isotherm analysis. All samples exhibited type IV isotherms and a typical hysteresis loop of mesoporous materials (inset in Fig. 1(c)) 43 . The specific surface area (SSA BET ) values for C-Bz, C-CE-Bz, and MnO 2 -C-PBz were 148.67, 169.56, and 77.3 m 2 g −1 , respectively. With the addition of crown ether (i.e., C-CE-Bz), SSA BET slightly increased, whereas with the addition of KMnO 4 (i.e., MnO 2 -C-PBz), it considerably decreased. Figure 1(d) shows the pore size distributions. All samples exhibited a hierarchical porous structure with mesopores or macropores ranging between ~5 and 100 nm. The total pore volume and average pore size of C-Bz and C-CE-Bz were similar, whereas considerably lower values were observed for MnO 2 -C-PBz (Table 1). The effects of physical properties on adsorption capacity will be discussed in the section on adsorption studies.
XPS analysis was carried out to investigate the surface elemental composition of the products and the oxidation state of manganese in MnO 2 -C-PBz. In comparison with MnO 2 -free samples (i.e., C-Bz and C-CE-Bz), Mn, O, and K were observed for MnO 2 -C-PBz ( Fig. 2(a,b)), which clearly indicates that the MnO 2 -carbon hybrid is successfully synthesized by SPP. Figure 2(c) shows the high-resolution XPS spectra of Mn 2p and Mn 3 s. Two major peaks were observed at 653.74 and 642.07 eV, corresponding to the Mn 2p 1/2 and Mn 2p 3/2 , respectively, and a peak separation (ΔE) value of 11.67 eV was in good agreement with literature values 44,45 . As shown in the bottom side of Fig. 2(c), the ΔE for Mn 3 s was 5.09 eV, indicative of a mixture of trivalent (Mn 3+ ) and tetravalent (Mn 4+ ) states 46,47 . This result is in agreement with the as described deconvolution peaks observed in the Mn 2p 3/2 spectrum, which showed a mixture of Mn 4+ , Mn 3+ , and Mn 2+ . The O 1 s spectra ( Fig. 2(d)) provided clear evidence for the formation of MnO 2 because binding energy of metal oxide is different from those of the other oxygen species 48 . Compared with MnO 2 -free C-Bz and C-CE-Bz samples, MnO 2 -C-PBz exhibited new peaks at around 531-529 eV. Two strong peaks were observed at binding energy values of 529.8 and 531.3 eV, corresponding to oxygen species in the MnO 2 lattice (O latt. , Mn-O-Mn) and adsorbed oxygen species (O ads. , Mn-OH), respectively 49 .
FE-SEM and TEM were used to examine C-Bz, C-CE-Bz, and MnO 2 -C-PBz. C-Bz exhibited relatively uniform spherical particles with a size of ~20 nm ( Fig. 3(a,d)), and C-CE-Bz exhibited a slightly larger particle size of ~50 nm ( Fig. 3(b,e)). On the contrary, MnO 2 -C-PBz exhibited non-uniform flat particles in a wide particle size range from tens to hundreds of nanometers ( Fig. 3(c,f)). The selected-area electron diffraction (SAED) patterns of samples ( Fig. 3(d-f) inset images) exhibited three broad diffuse rings, corresponding to the 002, 100/101, and 110 planes of carbon materials from the inside diffraction ring, respectively 50 . C-Bz and C-CE-Bz exhibited almost identical diffraction patterns, whereas MnO 2 -C-PBz showed a weaker, broader diffraction pattern without any distinct diffraction patterns of MnO 2 . This result indicated that amorphous MnO 2 is formed and loaded on the carbon surface, as evidenced from the weaker, broader SAED pattern compared with those of C-Bz and C-CE-Bz as well as the XRD results. In addition, the EDS spectrum ( Fig. 3(g)) and elemental mapping results ( Fig. 3(h)) clearly indicated that C, O, and Mn are present and that Mn and O are uniformly dispersed in MnO 2 -C-PBz.
To observe the adsorption of MO and MB (pH = 6.5 ± 0.5) on the as-prepared samples, adsorption tests were carried out in MO and MB stock solutions. The adsorption behavior of MO and MB was clearly different ( Fig. 4(a)). All samples exhibited a higher adsorption capacity for MB than MO, indicating that the as-synthesized samples are suitable for the adsorption of cationic dye molecules. Notably, (i) C-Bz and (ii) C-CE-Bz exhibited higher adsorption capacities than (iii) MnO 2 -C-PBz in an MO solution, whereas the opposite tendency was observed in an MB solution. In particular, despite the fact that the SSA BET values of C-Bz and C-CE-Bz were greater than that of MnO 2 -C-PBz, MnO 2 -C-PBz exhibited around three times higher adsorption capacity than the other samples, i.e., (i) C-Bz, 49.31 mg g −1 , (ii) C-CE-Bz, 54.04 mg g −1 , and (iii) MnO 2 -C-PBz, 156.43 mg g −1 , under the same conditions for the MB solution. In addition, to investigate the effect of MnO 2 in the hybrid products, MB adsorption experiments were carried out using (iv) MnO 2 -SP (synthesized by the same method, i.e., SPP 28 . see supplementary information; Table S1 † ) and (v) commercial MnO 2 . Compared with (iv) MnO 2 -SP and (v) commercial MnO 2 , the MnO 2 -carbon hybrid exhibited higher adsorption capacity ( Fig. 4(a)). The excellent adsorption ability of the MnO 2 -carbon hybrid was clearly demonstrated by UV-visible spectra and MB dye
solution color (inset image in Fig. 4(b)) after equilibrium adsorption ( Fig. 4(b)). Additionally, Rh B adsorption behavior of MnO 2 -carbon hybrid also exhibited good adsorption capacity and removal efficiency compared with anionic dye MO (see supplementary information; Fig. S2 † ). Therefore, these results suggest that the as-synthesized MnO 2 -carbon hybrid is suitable for the removal of cationic dyes; particularly, the adsorption ability is enhanced using a hybrid with carbon materials. The adsorption behavior and effects of MnO 2 and carbon are investigated in detail in the following section on adsorption and isotherm studies. Based on the above results, the adsorption ability of MnO 2 -C-PBz with the best adsorption characteristics was investigated in detail. The dependence of MnO 2 -C-PBz adsorption capacity on initial MB concentrations was evaluated to confirm the efficacy of the adsorbent. With the increase in initial MB concentrations from 10 to 300 mg L −1 , the adsorption capacity of MnO 2 -C-PBz increased from 32.58 to 230.86 mg g −1 , whereas the dye removal efficiency decreased ( Fig. 5(a)). The removal efficiency was observed to be greater than ~93% up to C 0 = 100 mg L −1 , and it dramatically decreased to ~33.5% at C 0 = 200 mg L −1 and to ~22.3% at C 0 = 300 mg L −1 . In addition, with the immediate addition of a significantly low initial concentration (C 0 = 10 mg L −1 ) of MnO 2 -C-PBz, MB was rapidly removed within 10 s (~99%); hence, adsorption experiments are carried out at high concentrations of greater than 25 mg L −1 to confirm the dependence of time on adsorption behavior. Removal efficiencies of greater than ~97% and ~92% within 10 min were observed for C 0 values of 25 and 50 mg L −1 , respectively, and a removal efficiency of greater than ~81% at a C 0 of 100 mg L −1 was observed within 30 min ( Fig. 5(b)). Moreover, MB was effectively decolorized. As can be observed in the inset images of Fig. 5(b), the blue solution color clearly disappeared at a low initial MB concentration (25 mg L −1 ), indicative of the effective decolorization ability of MnO 2 -C-PBz. Although the blue color did not completely disappear at higher initial concentrations (i.e., 50 and 100 mg L −1 ), considerable decolorization was observed by comparison of the initial MB solution color. Thus far, the results indicate that MnO 2 -C-PBz possesses the highly fast and effective MB removal abilities.
The effects of pH were investigated because solution pH is an important factor affecting the adsorption of dye molecules in an aqueous system. With the increase in pH from 2 to 12, the adsorption capacity of MnO 2 -C-PBz for MB increased from around 111.6 to 151.8 mg g −1 (Fig. 6). Murray 8 and Fendorf et al. 9 independently investigated the pH of the zero point of charge of MnO 2 (especially δ-phase MnO 2 ), which was verified to be around ~2.4 and 2.7. These values indicated that MnO 2 is negatively charged over a wide range of pH (pH > ~3). The MnO 2 -C-PBz surface apparently became more negatively charged with increasing solution pH.   The adsorption mechanism in hybrid nanocomposite system, various interactions between adsorbent and adsorbate might have possibility to contribute adsorption capability. The possible interactions between MnO 2 -carbon hybrid and MB molecules can be summarized as follows; (i) MnO 2 : the strong electrostatic forces of attractions between negatively charged MnO 2 and positively charged MB + , (ii) π-π stacking interactions between bulk π-system of sp 2 -carbon and C=C or aromatic ring of MB molecules, and (iii) hydrogen bonding between oxygen-containing functional groups in carbon moieties derived from crown ether-containing precursor and MB molecules. Among these mechanism, we regard that electrostatic forces of attraction are main adsorption mechanism in our hybrid system based on the results of adsorption capability (Fig. 4) and pH-dependent of MB adsorption behavior (Fig. 6).
Three initial concentrations of dye solution, 25, 50, and 100 mg L −1 , respectively, were selected to investigate the dependence of adsorption capacity on contact time. The equilibrium adsorption capacity increased with increase in the initial concentration ( Fig. 7(a)). Rapid adsorption occurred at the initial stage (~10 min) and then attained equilibrium gradually. This result is related to the presence of a large number of active sites at the start of adsorption, followed by the decrease in the number of available active sites. To better understand the adsorption of MB on MnO 2 -C-PBz, adsorption kinetics was further examined. The obtained experimental kinetic data were fitted by the linear forms of two kinetic models: the pseudo-first-order (PFO, eq. (1)) and pseudo-second-order (PSO, eq. (2)) models, respectively 51,52 . These two linear forms can be described by eqs (1) and (2), respectively: where q e and q t represent the adsorption capacities (mg g −1 ) at equilibrium and contact time (t, min), respectively, and k 1 (min −1 ) and k 2 (g mg −1 min −1 ) are the equilibrium rate constants for the PFO and PSO models, respectively. k 1 was determined from the slope of log(q e − q t ) vs. t (Fig. 7(b)) obtained from the linear plots fitted using eq. (1), and k 2 and q e were determined from the slope and intercept of t/q t vs. t (see Fig. 7(c)) obtained from the linear plots fitted using eq. (2). In addition, the initial (t → 0) adsorption rate h (mg g −1 min −1 ) was calculated by eq. (3) 51 : As summarized in Table 2   capacity of MnO 2 -C-PBz is considered to be greater than those of the other reference samples (i.e., C-Bz and C-CE-Bz) despite its lower surface area (C-Bz: 148.67 m 2 g −1 ; C-CE-Bz: 169.56 m 2 g −1 ; MnO 2 -C-PBz: 77.3 m 2 g −1 ). In addition, the k 2 and h values decreased with the increase in initial MB concentrations, indicating that adsorption reaches a plateau more rapidly at a low initial concentration 53 . However, it is difficult to determine the diffusion mechanism using only two previous kinetic models; hence, the linear form of the intraparticle diffusion kinetic model (proposed by Weber and Morris) 54 was also applied at different initial MB concentrations to further investigate sorption using the following equation: The data obtained from eq. (4) exhibited two linear portions ( Fig. 7(d)), indicating that adsorption can be described by a two-stage process: (i) the external surface adsorption stage, i.e., mass transfer through boundary-layer diffusion, and (ii) the gradual sorption stage, i.e., intraparticle diffusion 55 . k ip represents the adsorption rate (mg g −1 min −1/2 ), and C (mg g −1 ) is the value related to boundary thickness. Steep slopes were observed at the initial portion, and plateau slopes were observed at the second stage ( Fig. 7(d)). Initially, rapid adsorption was observed, which slowly attained equilibrium. This result suggests that the adsorption of MB on MnO 2 -C-PBz is controlled by external surface adsorption, followed by intraparticle diffusion 56 . In addition, the rate constants (k ip ) increased with the increase in initial MB concentrations (k ip : 9.05 (at 25 mg L −1 ) < 12.60 (at 50 mg L −1 ) < 15.45 (at 100 mg L −1 )) because a high initial MB concentration provides a high driving force to overcome the mass-transfer resistance of MB between the aqueous and solid phases, leading to a high rate for the adsorption of MB molecules on the MnO 2 -C-PBz adsorbent 57 .
The study of adsorption isotherms can provide insights into adsorption behavior, and efficient adsorbents can be designed on the basis of the obtained parameters. Hence, equilibrium adsorption data are analyzed using the linear form of the Langmuir adsorption isotherm model, which is based on the hypothesis of monolayer adsorption using a homogeneous adsorbent surface, and the Freundlich adsorption isotherm model, which is based on the hypothesis of multilayer adsorption on a heterogeneous adsorbent surface 58 . The linear form of the Langmuir adsorption isotherm model is described as follows: where C e , q e , and q max represent the concentrations of the dye solution at equilibrium (mg L −1 ), adsorption capacity at equilibrium (mg g −1 ), and the theoretical maximum monolayer adsorption capacity of the adsorbent (mg g −1 ), respectively, and b is the Langmuir constant (L mg −1 ). The q max and b values were determined from the slope and intercept of the linear fitted plot of C e /q e vs. C e (Fig. S3(a)  † ). Furthermore, the separation factor R L (also known as the equilibrium parameter), providing information about the favorability of adsorption in the adsorbate/adsorbent system, was calculated using eq. (6) 59 : R L represents the shape of the isotherm: (i) R L > 1 (unfavorable), (ii) R L = 1 (linear), (iii) 0 < R L < 1 (favorable), and (iv) R L = 0 (irreversible) 59,60 .
The linear form of the Freundlich isotherm model is described as follows 61 : where K F is the Freundlich constant (mg g −1 ), related to the adsorption capacity, and 1/n is the heterogeneity parameter: (i) 1/n < 1 (favorable, heterogeneous adsorption) and (ii) 1/n > 1 (unfavorable) 62 . These factors can be used to describe the multilayer adsorption on a heterogeneous adsorbent surface 58,62 . Table 3 shows the calculated isotherm parameters from the linear forms of the Langmuir and Freundlich adsorption models and fitted lines presents in Fig. S3(b) † . The high R 2 values indicate that the two models successfully explain the favorable adsorption of MB on MnO 2 -C-PBz. The theoretical maximum monolayer adsorption capacity (q max ) was 357.14 mg g −1 as calculated from the Langmuir adsorption isotherm model. Compared with various adsorbents, including carbon, metal oxide, and metal-oxide-carbon hybrid adsorbents reported in literatures as shown in  hybrid adsorbents, suggesting that it could be a promising adsorbent for application of cationic pollutants removal. The R L values (0.00811-0.19693) in the range of 0-1 (Table 3)  To confirm the reusability of the MnO 2 -carbon hybrid adsorbent, multi-recycle tests were performed and compared to the carbon-free MnO 2 -SP synthesized by SPP. As can be seen in Fig. 8, in the first cycle, MnO 2 -C-PBz and MnO 2 -SP exhibited similar adsorption capacities (~149.66 and ~149.83 mg g −1 ) and removal efficiencies (~89.78% and ~89.81%). In the second cycle, however, the removal efficiency of MnO 2 -SP dramatically decreased from 89.8% to 49.2%, whereas that of MnO 2 -C-PBz slightly decreased to around 0.49%. As the cycle repeats, the removal efficiency of MnO 2 -SP gradually decreased, but that of MnO 2 -C-PBz remained stable. Consequently, after seven cycles, the MnO 2 -C-PBz still maintained considerable dye removal performance and stability as compared with those of MnO 2 -SP. Although large SSA of MnO 2 -SP provides high adsorption capacity and removal efficiency in the first cycle, MB molecules could be blocked in the micropores or small mesopores; therefore, the efficient desorption of MB molecules from the adsorbent does not occur. Meanwhile, MnO 2 -C-PBz shows no obvious degrading after cycle tests (see Fig. S4  † ), which demonstrates immense potential for reusable adsorbent. From the results obtained thus far, the synergistic effects related to the presence of MnO 2 and the high stability of carbon for the removal of cationic dyes have been clearly confirmed.

Conclusion
In this study, we first report a one-pot synthesis of MnO 2 -carbon hybrid (MnO 2 -C-PBz) by applying plasma in single-precursor "purple benzene" derived from K + (DCH18C6) complex formation, and its performance for the adsorption of cationic dye (i.e., MB) was investigated. The MnO 2 -C-PBz showed around 3 times greater adsorption capacity than those of MnO 2 -free carbon (C-Bz and CE-Bz), and exhibited quite a good cyclic stability than carbon-free MnO 2 (MnO 2 -SP). In addition, the adsorption kinetics and isotherm studies demonstrated that MnO 2 -C-PBz is a heterogeneous adsorbent, which is accompanied by complex adsorption, with both chemisorption and physisorption. From the results obtained thus far, the MnO 2 -carbon hybrid is an efficient and reusable adsorbent for the removal of cationic dyes, which attributed to adsorption properties of both MnO 2 and carbon. This finding will be useful for understanding the synergistic effects of the MnO 2 -carbon hybrid for the adsorption of cationic dye and contributing an innovative single-step synthetic approach for inorganic oxide-organic carbon hybrid materials.
Synthesis of the MnO 2 -carbon hybrid by SPP. The MnO 2 -carbon hybrid was successfully synthesized by a one-pot SPP in purple benzene derived from the K + (DCH18C6) complex. Purple benzene was prepared as  (Fig. 9). DCH18C6 (50 mM) was added to benzene (100 mL). After vigorous stirring for a few minutes, KMnO 4 (5 mM) was added and sufficiently mixed until a homogeneous purple solution was obtained without the remaining solid KMnO 4 . As-prepared purple benzene was transferred into a glass reactor with a volume of 100 mL, consisting of a pair of tungsten electrodes (Ø 1 mm, 99.5% purity, Nilaco, Japan) insulated with ceramic tubes and connected to a bipolar pulse power supply (Kurita, Japan) for generating plasma in the liquid-phase precursor. The plasma discharging conditions for frequency, pulse width, voltage, and gap distance of the metal electrode were maintained constant at 25 kHz, 0.8 μs, 1.4-1.6 kV, and 1 mm, respectively. Figure S5 † shows the photograph of the purple benzene precursor and the schematic of the SP apparatus in detail. SP was allowed to proceed at ambient temperature and pressure for 10 min under constant stirring during operation to ensure a homogeneous chemical reaction. The resulting MnO 2 -carbon hybrid obtained from SPP was collected by vacuum filtration and rinsed several times with ethanol, followed by air drying in an oven at 85 °C for 12 h. The production rate of MnO 2 -carbon hybrid was approx. 20 mg min -1 in the purple benzene during discharging. In addition, to demonstrate the effect of the MnO 2 -carbon hybrid, reference samples were prepared from pure benzene and benzene with DCH18C6 ethers without KMnO 4 under the same SP conditions as stated above. The resulting products obtained using pure benzene, benzene with crown ether (DCH18C6), and purple benzene precursors were designated as C-Bz (carbon synthesized from benzene), C-CE-Bz (carbon synthesized from crown ether-benzene), and MnO 2 -C-PBz (MnO 2 -carbon hybrid synthesized from purple benzene), respectively. Material characterization. The fabricated samples were characterized by X-ray diffraction (XRD, SmartLab, Rigaku Co., Ltd., Japan) with Cu Kα (λ = 1.5418 Å) radiation operating at 45 kV and 200 mA and by Raman spectroscopy (inVia Raman Microscope, Renishaw Co. Ltd., UK) with a solid-state laser operating at 532 nm to examine the structural features. N 2 adsorption/desorption isotherms were recorded on a BELSORP mini II analyzer (MicrotracBEL Corp.) at 77 K in liquid nitrogen to characterize the surface area and pore structure of the adsorbents. The specific surface area (SSA) was determined by the Brunauer-Emmett-Teller (BET) method, and the pore size distribution was calculated from the Barrett-Joyner-Halenda analysis. Morphologies were observed by field-emission scanning electron microscopy (FE-SEM, S-4800, HITACHI High Technologies Co., Ltd., Japan) at an accelerating voltage of 10 kV and by transmission electron microscopy (TEM, JEM-2500SE, JEOL, Japan) at an accelerating voltage of 200 kV. Energy-dispersive X-ray spectroscopy (EDS) and elemental mapping images were obtained on an FE-SEM system equipped with an EDS system (EMAX Energy, Horiba Ltd., Japan). X-ray photoelectron spectroscopy (XPS) was carried out on a PHI 5000 VersaProbe II (ULVAC-PHI, Inc., Japan) with Mg Kα radiation to examine the surface chemistry of materials.  (0-240 min). The pH was adjusted using a 0.1 M HCl or 0.1 M NaOH solution, and adsorption experiments were carried out at room temperature (297 K) and under natural pH. The adsorbent dose was maintained constant at 0.6 g L −1 , and adsorption experiments were carried out in a 150 mL conical flask with 60 mg of the adsorbent and 100 mL of the dye stock solution. During the tests, the conical flasks were wrapped in aluminum foil to intercept surrounding light so as to minimize other effects. UV-visible spectroscopy (Shimadzu UV-3600, Japan) was utilized to determine dye removal efficiency and adsorption capacity. The solution was filtered using a 0.45 μm membrane filter (PTFE, ADVANTEC, Japan) before characterization. The dye removal efficiency (R) and adsorption capacity at equilibrium (q e ) were evaluated from the UV-visible spectra using the following equations: e e 0 where C 0 , C t , and C e represent initial dye concentration, dye concentration at contact time (t), and dye concentration at equilibrium (mg L −1 ), respectively. W and V represent adsorbent dosage (mg) and dye solution volume (L), respectively.

Reusability tests.
Reusability tests were carried out to evaluate the stability and reusability of adsorbents.
A total of 60 mg of adsorbents was added in 100 mL of an MB solution with a C 0 of 100 mg L −1 at natural pH (6.5 ± 0.5) for 180 min. After the adsorption of MB on the adsorbent, the solution phase was obtained by filtration using a 0.45 μm membrane filter, and the solid phase (adsorbent) was collected by centrifugation, followed by washing two times using ethanol with 0.1 M NaOH (1 wt%). The washed adsorbent was dried at 65 °C for 2 hrs, and recyclability tests were carried out seven times under the same conditions and procedures as stated above.