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Article http://pubs.acs.org/journal/acsodf
Porous BiOBr/Bi2MoO6 Heterostructures for Highly Selective Adsorption of Methylene Blue Danjun Wang,*,† Huidong Shen,† Li Guo, Chan Wang, and Feng Fu* Shaanxi Key Laboratory of Chemical Reaction Engineering, College of Chemistry & Chemical Engineering, Yan’an University, Holy Land Road No. 580, Baota District, Shaanxi Province, Yan’an 716000, P. R. China S Supporting Information *
ABSTRACT: Porous BiOBr/Bi2MoO6 (Br/Mo) heterostructures were designed and successfully fabricated, in which BiOBr nanoparticles were deposited on the surface of the secondary nanoplate of three-dimensional porous Bi2MoO6 architectures through a deposition−precipitation process. The as-prepared Br/ Mo heterostructures were used as an adsorbent to remove methylene blue (MB) from aqueous solution. The batch adsorption results indicated that 50.0 wt % Br/ Mo heterostructures show an enhanced adsorption capacity compared with pure Bi2MoO6 and BiOBr. The effects of initial solution, initial concentration, and contact time were systematically investigated. The optimum adsorbent amount and the pH value were determined to be 0.8 g L−1 and 2, respectively. Meanwhile, the experiments also revealed that porous Br/Mo heterostructures possess higher preferential adsorptivity for MB than that for methyl orange (MO−) and rhodamine B (RhB+). The dynamic experimental result indicated that the adsorption process conforms to the pseudo-second-order kinetic model. Weber’s intraparticle diffusion model indicated that two steps took place during the adsorption process. Thermodynamic analysis results showed that the adsorption is a physisorption process, which conforms to the Langmuir isotherm model. Additionally, the possible adsorption mechanism was also investigated. The present study implied that Br/Mo heterostructures are promising candidates as adsorbents for MB removal. Therefore, fabrication of semiconductor-based heterostructures could be a strategy to design new efficient adsorbents for the removal of environmental pollutants. eco-friendly nature.25−27 In addition, the special layered structure endows BiOBr with good performance for separation of photogenerated electron/hole pairs.28,29 To date, many groups have devoted efforts toward studying the use of BiOBr as a photocatalyst and an adsorbent for the adsorption of organic dyes and heavy metal ions.30−34 According to previous investigations, the adsorption ability of BiOBr is vital to treat wastewater.35 Therefore, it is highly desirable to explore new BiOBr-based materials with enhanced adsorption abilities. Bi2MoO6 is a layered Aurivillius-related n-type semiconductor with a small band gap (2.5−2.8 eV); therefore, it is a promising photocatalyst.36−40 However, to our knowledge, studies on the adsorption properties of Bi2MoO6 for organic pollutant removal have not been reported. It is believed that BiOBr particles decorated on the surface of a Bi2MoO6 heterostructure to form a novel heterostructure will be an excellent adsorbent by utilizing the merits of BiOBr and Bi2MoO6. In this work, porous BiOBr/Bi2MoO6 (Br/Mo) heterostructures have been successfully fabricated via a facile deposition−precipitation process. The Br/Mo heterostructure
1. INTRODUCTION Organic dyes have been extensively used in various industries, including the dyestuff, leather, cosmetic, and textile industries.1,2 Until now, treatment of dye wastewater has been difficult due to its high stability and complicated compositions.3 Most of these dyes may have potential carcinogenic and mutagenic effects to humans and aquatic organisms.4,5 Therefore, the removal of organic dyes from wastewater is of great significance to the environmental safety.6−9 To date, a lot of techniques have been developed for removing dyes from aqueous media, such as photocatalytic degradation, chemical oxidation, chemical precipitation, biodegradation, coagulation, and adsorption.10−13 Among the numerous methods, adsorption is a simple, low-cost and effective method.6,14,15 In this regard, the well-known adsorbents, such as activated carbon, natural materials, agricultural waste materials, and so forth, have been widely studied.16,17 However, most of these adsorbents are not widely used due to their high cost, difficult disposal, and low adsorption efficiency. Recently, much attention has been paid to nanomaterial adsorbents, because it is possible to construct them with controllable sizes, morphologies, and functional surface groups.18,19 Recently, BiOBr has been widely used to degrade toxic organic pollutants owing to its special layered structure,20,21 high photocatalytic activity,22−24 good chemical stability, and © 2016 American Chemical Society
Received: July 31, 2016 Accepted: September 26, 2016 Published: October 12, 2016 566
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presents a hierarchical structure with a relatively high specific surface area to adsorb organic dyes and exhibits improved adsorption efficiency than pure BiOBr and Bi 2 MoO 6 . Furthermore, the kinetics and thermodynamics of the adsorption process were investigated. Moreover, on the basis of the experimental results, the enhanced adsorption mechanism of Br/Mo heterostructures was also discussed. The experimental result reveals that the as-fabricated Br/Mo heterostructures could be a high-efficiency adsorbent for the removal of MB from aqueous solution.
2. RESULTS AND DISCUSSION 2.1. Characterization of BiOBr/Bi2MoO6 Heterostructures. Synthetic details of the porous BiOBr/Bi2MoO6 heterostructures are provided in the Supporting Information. The Fourier transform infrared (FT-IR) spectra of pure BiOBr, Bi2MoO6, and Br/Mo heterostructures are presented in Figure 1. The band at 1630 cm−1 in the spectrum of BiOBr
Figure 2. XRD patterns of Bi2MoO6 (a), 50 wt % Br/Mo heterostructure (b), and pure BiOBr (c).
The XRD analysis indicates the coexistence of BiOBr and Bi2MoO6 phases in the as-prepared Br/Mo heterostructures, and no other new phases are observed. The characteristic peaks of BiOBr at 25.156° can be found at Br/Mo heterostructures. The XRD patterns of other Br/Mo heterostructures with the different mass ratios are listed in Figure S1. It can be seen that the diffraction peak intensities of BiOBr become stronger with an increase in BiOBr content, whereas the characteristic peaks of Bi2MoO6 decrease in intensity. The XRD result is consistent with the FT-IR analysis. To further investigate the chemical composition of the composites, X-ray photoelectron spectroscopy (XPS) analysis of 50 wt % Br/Mo heterostructures was carried out. The XPS survey spectrum (Figure 3a) clearly indicates that 50 wt % Br/ Mo is composed of Bi, O, Br, Mo, and C elements. The C 1s peak in the spectrum of Bi2MoO6 originates from adventitious carbon.46 Figure 3b−e shows the high-resolution XPS spectrum of Bi 4f, Mo 4f, Br 3d, O 1s, and C 1s regions, respectively. In Figure 3b, two peaks with binding energies of 159.0 and 164.4 eV are assigned to Bi 4f 5/2 and Bi 4f 7/2, respectively, which correspond to Bi3+.47 In Figure 3c, the peak with binding energy at 68.89 eV is assigned to Br in the Br/Mo heterostructure.48 Peaks at 235.44 and 232.29 eV, as shown in Figure 3d, corresponding to Mo 4f 5/2 and Mo 4f 7/2, respectively, can be assigned to the Mo6+ species in the MoO6 octahedron.49 Figure 3e shows that two different oxygen species are present on the surface of the Br/Mo heterostructures. The binding energy of O 1s located at 529.8 eV is attributed to the lattice oxygen, whereas the O 1s peak situated at 531.8 eV should be attributed to the hydroxyl groups adhered onto the surface of Br/Mo heterostructures.48 These results are consistent with the result of FT-IR analysis. Hence, the XPS result further confirmed that the Br/Mo heterostructures have been fabricated. Therefore, using a combination of FT-IR, XPS, and XRD investigations, we can deduce that both Bi2MoO6 and BiOBr species are present in the Br/Mo sample.
Figure 1. FT-IR spectra of the as-synthesized samples of BiOBr, Bi2MoO6, and 50 wt % Br/Mo heterostructure.
corresponds to the bending vibrations of O−H,41 which is ascribed to the water adsorbed. The absorption band at 510 cm−1 is ascribed to the Bi−O stretching mode.42 For the Bi2MoO6 sample, peaks at 3443 and 1636 cm−1 are ascribed to O−H vibrations.43 The absorption bands at 910−650 and 630−430 cm−1 are assigned to Mo−O stretching vibrations and Bi−O stretching and deformation vibrations,44 respectively. The peak located at 734 cm−1 is attributed to the asymmetric stretching vibrations of the equatorial oxygen atoms of MoO6, whereas the peak at 567 cm−1 corresponds to the bending vibration of the MoO6 octahedron.45 The FT-IR results indicate that the heterostructures contain two fundamental components, BiOBr and Bi2MoO6, and that no appreciable chemical reaction occurred between BiOBr and Bi2MoO6. Figure 2 shows the X-ray diffraction (XRD) patterns of pure BiOBr, Bi2MoO6, and 50 wt % Br/Mo heterostructures. As shown in Figure 2, all of the characteristic diffraction peaks of the Bi2MoO6 and BiOBr samples are assigned to the orthorhombic phase of Bi2MoO6 (JCPDS card no. 76-2388) and the tetragonal phase of BiOBr (JCPDS card no. 09-0393), respectively. For the 50 wt % Br/Mo heterostructures, all of the BiOBr diffraction peaks correspond to Br/Mo heterostructures. 567
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Figure 3. XPS spectra of the as-prepared 50 wt % Br/Mo heterostructure. (a) Survey of the sample; (b) Bi 4f; (c) Br 3d; (d) Mo 3d; (e) O 1s; and (f) C 1s.
The morphology of the Br/Mo heterostructures was investigated by field emission scanning electron microscopy (FE-SEM), as shown in Figure 4. It can be seen from Figure 4 that pure Bi2MoO6 exhibits three-dimensional (3D) sphere-like Bi2MoO6 architectures with a size of 1−2 μm (Figure 4a). Interestingly, the 3D Bi2MoO6 microsphere consists of numerous secondary nanoplates with a thickness of 20−50 nm (Figure 4b). Figure 4c−d shows the morphology of 50 wt % Br/Mo, and it can be seen that when BiOBr is deposited on the surface of the 3D Bi2MoO6 architectures, the resultant Br/ Mo composite sample exhibits similar morphology and size to those of pure Bi2MoO6. With increasing BiOBr content, the morphology of the as-fabricated Br/Mo heterostructures is similar to that of the 50 wt % Br/Mo heterostructure (Figure S2). Further information of Br/Mo heterostructures was obtained from transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images (Figure 5). It can be
seen that pure Bi2MoO6 architectures consist of a massive Bi2MoO6 nanoplate (Figure 5a,b). From Figure 5c, it is clear that BiOBr nanoparticles are loaded on the surface of the Bi2MoO6 nanoplate. By carefully measuring the lattice parameters, the (111) plane of Bi2MoO6 and the (101) crystallographic plane of BiOBr can be found clearly. In addition, a selected-area electron diffraction (SAED) pattern clearly shows that the Br/Mo heterostructure has polycrystalline subunits, which indicates that Bi2MoO6 and BiOBr have coupled together to form the heterostructure (Figure 5d). Furthermore, the energy-dispersive spectrometry (EDS) analysis demonstrated that Bi, Mo, O, and Br elements existed in the Br/Mo heterostructures (Figure S3). The molar weight ratio of BiOBr to Bi2MoO6 obtained from the EDS analysis was also close to the theoretically calculated value of Br/Mo heterostructures (Table S1). Therefore, the XRD, XPS, SEM, 568
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Figure 4. SEM images of pure Bi2MoO6 (a, b) and 50 wt % Br/Mo heterostructure (c, d).
Figure 5. (a) Low magnification and (b, c) high-resolution TEM images of Br/Mo heterostructures; (d) SAED of Br/Mo heterostructures.
shown in Figure 6, the pore size distribution curves for the assynthesized 50 wt % Br/Mo were obtained by the BJH method (the inset picture of Figure 6). The calculated pore size distribution using the BJH method indicates that the size of mesopores in Br/Mo heterostructures is not uniform, ranging from 2 to 100 nm. Furthermore, the size of mesopores in Br/ Mo heterostructures is much higher than that of pure Bi2MoO6 and BiOBr (Table S2). The abundantly porous structure can facilitate more efficient contact of Br/Mo heterostructures with organic contaminants and thus improve their adsorptive
and TEM investigation results confirmed that both Bi2MoO6 and BiOBr species are there in the Br/Mo heterostructure. To investigate the porous structure of the Br/Mo heterostructure, N2 adsorption−desorption isotherms and the corresponding Barrett−Joyner−Halenda (BJH) pore size distribution plots of the as-fabricated samples were obtained. The BET specific surface areas of the prepared BiOBr, Bi2MoO6, 12.5 wt % Br/Mo, 25 wt % Br/Mo, 50 wt % Br/ Mo, and 100 wt % Br/Mo are 31.04, 50.78, 64.94, 55.43, 55.57, and 49.62 m2 g−1, respectively (Figure S4 and Table S2). As 569
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The pH seriously affects the active sites of adsorbents as well as the dye speciation during the adsorption process.55−58 In addition, pH has a significant effect on the electrostatic charges that are imparted by the ionized dye molecules between the adsorbent and adsorbate. The experiments were conducted with methylene blue (MB+), methyl orange (MO−), and rhodamine B (RhB+) with initial concentrations of 30 mg L−1 and 0.8 g L−1 of the Br/Mo heterostructure. As shown in Figure 8, the removal amount of MB+, MO−, and RhB+ was highly dependent on the pH of the solution. The pH of the solution affected the surface charge of the Br/Mo heterostructures and the degree of ionization of the dyes. For cationic dyes MB+ and RhB+, with an increase in the initial pH, the removal capacities decreased drastically. When the initial pH of the MB+ solution was 2, the removal efficiencies of MB+ and RhB+ reached up to 98.94 and 98.42%, respectively. The maximum adsorption of MB+ and RhB+ at the initial pH of 2 was due to the electrostatic attraction between the positive charge of the protonated MB+ and RhB+ and the negative surface of the Br/Mo heterostructure. However, a different situation was observed for the anionic dye MO−. The maximum removal efficiency of MO− was 77.95%, observed at pH 4. The results could also be well explained by electrostatic interactions between the dye molecule and adsorbent. The maximum adsorption of MO− at pH 4 was due to the electrostatic attraction between the negatively charged deprotonated MO− dye and the positively charged surface of the Br/Mo heterostructures, whereas the electrostatic repulsion between the negatively charged surface and MO− anionic dye molecules increased with an increase in solution pH, thus decreasing the adsorption capacity. To investigate the surface charge of the adsorbent, the zero charge (pHPZC) of the 50 wt % Br/Mo heterostructures was tested and added as Figure S5. Generally speaking, the influence of the solution pH on dye adsorption can be explained on the basis of the pH at point zero charge (pHPZC) of the adsorbent. The pHPZC value of 50 wt % Br/Mo heterostructures determined by the solid addition method was about ca. 2.23 and ca. 5.37 (Figure S5). Hence, the net charge of the 50 wt % Br/Mo heterostructures is positive at pH 2 and 4. However, Bi2MoO6 is a layered oxide, consisting of [Bi2O2]2+ layers sandwiched between [MoO4]2− slabs. Hence, it exhibits both negative and positive surfaces, which results in its ability to adsorb MB+/RhB+ and MO−. Although the net charge of the 50 wt % Br/Mo heterostructures is positive at pH 2 and 4, the merit structure endows Br/Mo heterostructures with superior
Figure 6. Nitrogen adsorption/adsorption isotherms and BJH pore size distribution plot (inset picture) of the as-prepared 50 wt % Br/Mo heterostructures.
performances. Because of the hysteresis loop and the small mesopores predominant in the pore size distribution of Br/Mo, it is very reasonably expected that Br/Mo has the potential for high adsorptive removal of organic dyes. 2.2. Adsorption Properties of Br/Mo Heterostructures. 2.2.1. Effect of Initial pH on the Adsorption Efficiency. Figure 7 shows adsorption capacities for MB adsorption onto the surface of Br/Mo heterostructures. It can be seen that a negligible amount of MB is adsorbed on the surface of pure BiOBr. In Br/Mo heterostructures, the adsorption capacity of MB improves with increasing content of BiOBr and reaches a maximum value for the 50 wt % Br/Mo heterostructures (Figure 7b). The dye removal efficiencies of the 50 wt % Br/ Mo heterostructures, Bi2MoO6 nanostructures, and BiOBr are 72.21, 30.04, and 18.39%, respectively, which may be attributed to their novel heterostructures, abundant porous structure, and relatively large BET specific surface area (Figure S2). Therefore, a synergistic effect may exist between Bi2MoO6 and BiOBr during MB adsorption. Similar effects have been reported for BiPO4/BiOBr,50 Cu2O/BiOBr,51 g-C3N4/BiOBr, and BiOBr/BiOI heterostructures.52 MB+ adsorption on the Br/Mo heterostructures reaches equilibrium after about 60 min, which is significantly higher than that for some other adsorbents.53,54
Figure 7. (a) Effect of Br/Mo heterostructures with different weight ratios on MB adsorption onto Br/Mo microspheres (1: BiOBr, 2: Bi2MoO6, 3: 12.5 wt % Br/Mo, 4: 25 wt % Br/Mo, 5: 50 wt % Br/Mo, 6: 100 wt % Br/Mo); (b) UV−vis absorption spectra of MB+ after being adsorbed for 120 min with the initial concentration of MB: 30 mg L−1; mass of adsorbent: 1.0 g L−1; temperature: 298 K, respectively. 570
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Figure 8. Effects of initial pH on the adsorption of different dyes onto the 50 wt % Br/Mo heterostructures. Removal and temporal evolution of UV−vis absorption spectra of MB+ (a, b), MO− (c, d), and RhB+ (e, f).
adsorption capacity for MB+/RhB+ and MO− in acidic solutions. From the above results, it is suggested that the Br/ Mo heterostructure can be a high-efficiency adsorbent for the adsorption of MB+, MO−, and RhB+, and acidic solutions are beneficial for dye adsorption. On the other hand, the charge and size of the dye affect the adsorption efficiency.59 So, we have chosen three kinds of organic dyes for this study: MB+, MO−, and RhB+. The geometrical optimization of the dyes was carried out by using the B3LYP functional and the 6-31G(d,p) basis sets. The measured dimensions of the organic dye molecules from the calculation are shown in Table S3. Firstly, MB+ and MO− were chosen to investigate the charge effect due to their similar size but different charges (Table S3). As shown in Figure 9a, the absorption peak decreased gradually and the color faded to become transparent (the inset picture in the upper part of Figure 9a), suggesting that nearly all of the MB+ were adsorbed by the Br/Mo heterostructures. Then, a MB+/MO− mixture solution was chosen to investigate the charge effect on the adsorption efficiency. The results indicated that more than 90% of MB+ were adsorbed in 120 min, whereas less than 40% of
MO− were adsorbed (Figure 9b,c,f). This revealed that the Br/ Mo heterostructures adsorbed MB+ more efficiently than MO− (Figure 9b,c,f). Secondly, MB+ and RhB+ with the same charges but different molecular sizes were chosen to further investigate the size effect on the adsorption efficiency (Table S3). For the smaller sized MB+, the characteristic absorption peak at 664 nm decreased sharply in the MB+/RhB+ mixture solution. However, for the larger sized RhB+, the characteristic absorption peak at 553 nm decreased only slightly (Figure 9d). The molecular dimensions of the MB+ dye are 4.00 and 7.93 Å along the x and y directions, respectively, which are much smaller than the diameter of the channel of Br/Mo heterostructures. Thus, MB+ can enter the channel of Br/Mo heterostructures. For RhB+ molecules, the dimensions along the x and y directions are 10.89 and 15.72 Å, respectively, and they are much larger than the diameter of the channel of Br/Mo heterostructures (Table S3). Hence, the RhB+ molecule is too large to diffuse into the channel of the Br/Mo heterostructures, and the adsorption efficiency is low. Therefore, both size and charge of the dye molecule are responsible for the selective adsorption efficiency of Br/Mo heterostructures. 571
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Figure 9. Temporal evolution of UV−vis absorption spectra of (a) MB+, (b, c) MB+/MO− mixture solution at different pH values, (d, e) MB+/RhB+ mixture solution at different pH values, and the removal ratio of MB+/MO− at different pH values (f).
equilibrium.62 When the dosage of the adsorbent was above 0.8 g L−1, there was no significant increase in the removal rate of MB, but qe decreased rapidly. Therefore, considering qe and R %, an adsorbent dose of 0.8 g L−1 was determined as the optimum adsorbent dosage, which was used in other studies. 2.2.3. Effect of the Initial Concentration of MB. It can be seen from Figure 11 that the adsorption rate of MB+ onto the Br/Mo heterostructures is enhanced rapidly during the initial stage and is then reduced and reaches equilibrium after 80 min. During the adsorption process, the dye molecules rapidly reach the boundary layer by mass transfer, and then they slowly diffuse from the boundary layer onto the adsorbent surface and finally diffuse into the porous structure of the adsorbent.63 Figure 11 also showed that the equilibrium adsorption (qe) capacity increased from 37.23 to 58.85 mg g−1 with the initial concentration of MB increasing from 30 to 70 mg L−1. When the initial concentration of MB increased, the driving force for mass transfer became larger and the interaction between MB
2.2.2. Effect of Adsorbent Dosage. The values of qe and the removal percentage of dye (R%) at different dosages of Br/Mo heterostructures are presented in Figure 10. As the Br/Mo concentration was increased from 0.3 to 0.8 g L−1, the percentage of adsorbed MB increased from 39.28 to 99.50%. The increase in the removal rate of dye was due to the increased adsorbent surface area and the availability of more adsorption sites.60 However, the adsorption capacity (qe) presented the opposite trend with a further increase in adsorbent concentration. The decrease in qe from 53.52 to 18.72 mg g−1 with increasing adsorbent concentration from 0.3 to 1.6 g L−1 was attributed to the adsorption competition among adsorbents and the split in the concentration gradient.61 When the dosage of Br/Mo heterostructures was 0.8 g L−1, qe and R% were 37.33 mg g−1 and 99.50%, respectively. When the concentration of the Br/Mo heterostructures was higher than 0.8 g L−1, the equilibrium adsorption (qe) inversely turned to be very low, which resulted as the adsorption of MB reached 572
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second-order model is based on the assumption that the ratelimiting step involves chemisorption.67 Figure S6a,b shows the kinetics for the adsorption of MB+ onto Br/Mo heterostructures. As can be seen from Table 1, the correlation coefficients (R2) of the pseudo-second-order model were higher than 0.9999 for all concentrations and the qe (qcal) values perfectly agreed with the experimental values of qe,exp. However, the correlation coefficients (R2) obtained from the pseudo-firstorder model were low for all concentrations and the qe,cal values were much lower than the qe,exp values. The result further indicated that the adsorption of MB onto Br/Mo heterostructures followed the pseudo-second-order model well. According to the pseudo-second-order model, it is important to investigate the rate-controlling step of the adsorption process.68 Hence, the intraparticle diffusion model is employed to investigate the diffusion mechanism between MB+ molecules and Br/Mo heterostructures, and the kinetic model is given by eq 3 (Table S4),69,70 where ki (mg g−1 min−1/2) is the intraparticle diffusion rate constant and c (mg g−1) is the intercept. The value of c is related to the thickness of the boundary layer. Figure S6c reveals that two steps have taken place during the adsorption process. As shown in Figure S6c, the ki value of the film diffusion stage is larger than that of the intraparticle diffusion stage, indicating that the intraparticle diffusion stage is a gradual process, which is consistent with Weber’s intraparticle diffusion model (Table 2). Moreover, as seen from Figure S6c, the plot does not pass through the origin, indicating that the intraparticle diffusion is not the ratecontrolling step.71,72 The high c values indicate that external mass transfer is significant in the adsorption process. Furthermore, R12 and R22 of the two stages imply the good applicability of Weber’s intraparticle diffusion model in studying the MB adsorption process. 2.4. Adsorption Isotherm. For solid−liquid adsorption, the adsorption isotherm is significant for understanding the adsorption behavior. In the present study, both Langmuir and Freundlich isotherm models were employed to simulate the experimental data.71,73 The models can be described in the linear form, as listed in Table S5, and the isotherms are shown in Figure S7. KL, q0, KF, and n can be determined from these two isotherms, and the results are displayed in Table 3. The results indicated that the adsorption capacity calculated from the Langmuir isotherm is 54.82 mg g−1, which is similar to the experimental result (55.36 mg g−1). Besides, R2 of the Langmuir model is much larger than that of the Freundlich model. Hence, it is suggested that the adsorption of MB onto Br/Mo heterostructures follows the Langmuir model. In addition, the Temkin, Khan, Redlich-Peterson, and Sips isotherm models were also employed to simulate the experimental data. Parameters obtained using the four isotherm models are listed
Figure 10. Effect of adsorbent dosage on the removal of MB (C0 = 30 mg L−1; contact time: 120 min; pH = 2; temperature: 298 K).
Figure 11. Effects of contact time on the adsorption capacity of MB onto the 50 wt % Br/Mo heterostructures at different initial concentrations (C0 = 30, 50, and 70 mg L−1; adsorbent concentration = 0.8 g L−1; initial pH = 2; temperature: 298 K; contact: 160 min).
and the adsorbent was enhanced, which resulted in a higher adsorption capacity.64 2.3. Adsorption Kinetics. Generally speaking, the adsorption mechanism depends on the characteristics of the adsorbent and the mass transport process.65 To investigate the adsorption mechanism of MB+ adsorbed onto Br/Mo heterostructures, several adsorption kinetic models were employed (Table S4). The pseudo-first-order model of Lagergren is proportional to the difference in the equilibrium adsorption capacity and the adsorbed amount.66 The pseudo-
Table 1. Pseudo-First-Order and Pseudo-Second-Order Kinetic Parameters for the Adsorption of MB onto Br/Mo Heterostructure Powdera pseudo-first-order model −1
−1
−1
−1
pseudo-second-order model 2
C0 (mg L ) Br/Mo
qe,exp (mg g )
qe,cal (mg g )
k1 (min )
R
30 50 70
37.2337 49.5743 58.8532
1.8679 8.2352 9.1685
0.03242 0.03112 0.03052
0.6774 0.9723 0.8838
−1
qe,cal (mg g )
k2 (g mg−1 min−1)
R2
37.3972 50.3271 59.7015
0.03927 0.00864 0.00788
1.0000 0.9999 0.9999
Note: qe,exp (mg g−1) means the experimental result of equilibrium adsorption; qe,cal (mg g−1): calculated equilibrium adsorption according to the pseudo-first-order model and pseudo-second-order model, respectively. k1, k2: rate constant according to the pseudo-first-order model and pseudosecond-order model, respectively. a
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Table 2. Intraparticle Diffusion Model Parameters for the Adsorption of MB onto the Br/Mo Heterostructure Adsorbenta intraparticle diffusion model
a
C0 (mg L−1)
ki,1 (mg g−1 min−1/2)
c1 (mg g−1)
R12
ki,2 (mg g−1 min−1/2)
c2 (mg g−1)
R22
30 50 70
0.9535 0.7822 1.4144
31.15303 41.78396 46.97702
0.9967 0.9872 0.9989
0.0407 0.2561 0.1510
36.72057 46.53892 56.80093
0.7611 0.7381 0.6751
Note: ki,1: rate constant of the film diffusion stage; ki,2: rate constant of the intraparticle diffusion stage; R12, R22: linear correlation coefficients.
Table 3. Isotherm Parameters for MB Adsorption on Br/Mo Heterostructures at Different Temperaturesa Langmuir model −1
−1
Freundlich model −1
2
T (°C)
q0,exp (mg g )
q0,cal (mg g )
KL (L mg )
R
25 30 35 45
55.3656 58.0130 52.4807 55.4922
54.8246 57.5694 53.7634 53.8793
0.3802 0.5234 0.6276 0.7548
0.9991 0.9987 0.9970 0.9979
−1
KF (L mg )
n
R2
45.6873 45.9604 47.1880 49.0652
11.9005 15.4560 11.8245 26.7738
0.8688 0.8535 0.9152 0.9067
a
Note: q0,exp represents the experimental result of the maximum adsorption capacity value; q0,cal: calculated value of maximum adsorption capacity according to the Langmuir model; R2: linear correlation coefficients.
positive value of ΔS° (115.05 J mol−1 K−1) illustrates the structural changes in the adsorbate and the increasing randomness at the solid−solution interface during fixation of MB onto the active sites of the Br/Mo heterostructures.79 2.6. Adsorption Mechanism. To further verify the adsorption mechanism of the dye on the surface of Br/Mo heterostructures, the FT-IR spectra of Br/Mo, MB+, and MBadsorbed Br/Mo heterostructures (Br/Mo-MB) were also investigated (Figure S9). It can be seen that the FT-IR spectrum of Br/Mo slightly changed after the adsorption of MB in comparison to Br/Mo heterostructures. The absorption bands of Br/Mo heterostructures at 567 and 835 cm−1, corresponding to Bi−O and Mo−O, shift slightly to 549 and 843 cm−1, respectively, indicating that Bi−O and Mo−O of Br/ Mo heterostructures play an important role in the adsorption process, whereas after adsorption on the surface of Br/Mo heterostructures, the absorption bands of MB at 1858, 1389, and 1319 cm−1 shift slightly to 1608, 1391, and 1322 cm−1, respectively. These changes can be attributed to the following facts. First, the negatively charged surfaces of the Br/Mo heterostructures can provide adsorption sites for electrostatic interaction with MB+. Second, the porous Br/Mo heterostructures possess a regular hierarchical structure, a relatively high specific surface area, and an abundant pore structure, which can facilitate efficient contact of MB molecules with Br/ Mo heterostructures.80,81 These above-mentioned aspects synergistically contribute to the high adsorption capacity and removal efficiency of Br/Mo heterostructures for MB. The possible adsorption mechanism is schematically illustrated in Figure S10. In addition, the stability of the 50 wt % Br/Mo heterostructure was also investigated to demonstrate its potential performance. After adsorption of MB+, the 50 wt % Br/Mo adsorbent was collected by centrifugation, washed with 0.4 mol L−1 NaOH to remove the adsorbed MB+, and finally dried at 70 °C in air for another cycle. As shown in Figure S11, the 50 wt % Br/Mo heterostructure did not exhibit slight loss of adsorption activity even after five cycles. The equilibrium adsorption capacity of Br/Mo heterostructures at 25 °C was 55.36 mg g−1, which was larger than that of the reported adsorbents, such as Cu 2 Cl(OH) 3 microspheres,82 chitosan/kaolin/Fe2O3,83 Y-Fe2O3/SiO2/chitosan composites,84 and other adsorbents (Table S7). The
in Table S6. As seen in Tables 3 and S6, the Langmuir isotherm model fits the experimental data better than the Freundlich model and the other isotherm models, as evidenced by the correlation coefficients (R2). When the three-parameter isotherm is used to simulate the experimental data, such as the Khan, Redlich-Peterson, and Sips isotherm models, the correlation coefficient (R2) is close to that of the Langmuir isotherm model, which further demonstrates that the MB adsorption onto Br/Mo heterostructures follows the Langmuir model well.74,75 2.5. Thermodynamic Studies of Adsorption. Thermodynamic parameters were estimated to describe the effect of temperature on MB adsorption onto Br/Mo heterostructures and to provide insight into the adsorption mechanism. They were determined by van’t Hoff equations76 ΔG° = −RT ln KL
(1) (2)
ΔG° = ΔH ° − T ΔS° −1
where KL is the Langmuir equilibrium constant (L mg ) and R (8.314 J K−1 mol−1) represents the gas constant. The entropy change (ΔS°) and enthalpy change (ΔH°) are determined from the intercept and slope of the van’t Hoff plots of In (KL) versus 1/T (Figure S8). The thermodynamic parameters are shown in Table 4. The negative values of Gibbs free energy Table 4. Thermodynamic Parameters for the Adsorption of MB onto Br/Mo Heterostructures T (K)
ΔG° (kJ mol−1)
ΔH° (kJ mol−1)
ΔS° (J mol−1 K−1)
298 303 308 318
−11.99 −13.25 −13.96 −14.92
21.58
115.05
change (ΔG°) (−11.99, −13.25, −13.96, and −14.92 kJ mol−1 at 298, 303, 308, and 318 K, respectively) confirm the spontaneous nature of the adsorption process. Besides, the decrease in ΔG° with an increase in temperature suggests that the adsorption is more favorable at higher temperatures. The value of ΔH° (21.58 kJ mol−1) indicates that MB adsorption onto Br/Mo heterostructures is a physisorption process and that the adsorption process is endothermic.77,78 Meanwhile, the 574
DOI: 10.1021/acsomega.6b00160 ACS Omega 2016, 1, 566−577
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present study implied that the Br/Mo heterostructure is a promising candidate as a low-cost adsorbent for MB dye removal.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.6b00160. Experimental section; XRD patterns; FE-SEM images; EDS patterns; nitrogen adsorption/adsorption isotherms and BJH pore size distribution plot; pHpzc plot; the pseudo-first- and -second-order models, and intraparticle diffusion model; Langmuir and Freundlich isotherms; linear regressions of the van’t Hoff plot; FT-IR spectra; schematic illustration; and cycling runs (Figures S1− S11); BiOBr content in Br/Mo; structural parameters; molecular weight and dimensions; the adsorption kinetic model; adsorption isotherm models, various adsorption isotherm models, and equations; other adsorption isotherm models and correlation coefficient; and comparison of adsorption capacity (Tables S1−S7) (PDF)
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3. CONCLUSIONS Br/Mo heterostructures have been successfully fabricated via a facile in situ deposition−precipitation process. The experimental results indicated that the as-fabricated Br/Mo heterostructures exhibit enhanced adsorption performance for MB with respect to pure Bi2MoO6 and BiOBr, which we ascribed to the electrostatic interactions and structural properties of Br/Mo heterostructures. Adsorption of MB onto Br/Mo heterostructures followed a pseudo-second-order model, and the intraparticle diffusion was not the rate-limiting step. The selective adsorption of MB onto Br/Mo heterostructures followed the Langmuir isotherm. MB adsorption onto Br/Mo heterostructures was found to be spontaneous and endothermic. Br/Mo heterostructures could potentially become a new class of adsorbents for removal of organic dyes from wastewater.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Phone: +86-911-2586217 (D.W.). *E-mail:
[email protected]. Phone: +86-911-2332037 (D.F.). Author Contributions †
D.W. and H.S. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21666039 and 21663030). This work was also financially supported by the Project of Science & Technology Office of Shaanxi Province (2015GY174, 2013K11-08, and 2013SZS20-P01), Education Department of Shaanxi Province (15JS119), and Project of Yan’an University (YDZ2013-07). 575
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DOI: 10.1021/acsomega.6b00160 ACS Omega 2016, 1, 566−577
