pH-Dependent Degradation of Methylene Blue via Rational-Designed

Apr 10, 2014 - College of Materials Science and Engineering, Chongqing University, Chongqing 400044, People's Republic of China. ‡ National Key ...
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pH-Dependent Degradation of Methylene Blue via RationalDesigned MnO2 Nanosheet-Decorated Diatomites Yu Xin Zhang,*,†,‡ Xiao Dong Hao,† Fei Li,† Zeng Peng Diao,† Zao Yang Guo,§ and Jing Li*,∥ †

College of Materials Science and Engineering, Chongqing University, Chongqing 400044, People’s Republic of China National Key Laboratory of Fundamental Science of Micro/Nano-Devices and System Technology, Chongqing University, Chongqing 400044, People’s Republic of China § Department of Engineering Mechanics, Chongqing University, Chongqing 400044, People’s Republic of China ∥ Department of Chemistry, Tongji University, Shanghai 200092, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Hierarchical MnO2-modified diatomite (MnO2-D) nanostructures are synthesized through a one-pot hydrothermal method. By adjusting the preparative parameters, the morphology and surface pore size distribution of MnO2D composites can be well-controlled. Their nanostructures are examined by focused-ion-beam scanning electron microscopy (FIB/SEM), transmission electron microscopy (TEM), X-ray diffraction spectroscopy (XRD), and Fourier transform infrared (FT-IR) spectroscopy. The results show that birnessite-type MnO2 nanosheets are observed to grow vertically on the internal and external surfaces of purified diatomite, thus building a hierarchical architecture. Furthermore, the degradation ability of MnO2-D composites toward MB solution is fully investigated. It was found that MnO2-D composite degraded 97.7% of MB solution (0.02 g L−1, 100 mL) by oxidation within only 5 min in acid solution (pH 2); moreover, in neutral solution (pH 6−7), this composite catalyzed the oxidation decomposition reaction of MB (0.02 g L−1, 100 mL) by H2O2 with a high removal percentage of 92.4% in 2 h, and it also represented a high adsorption capacity of 714.3 mg g−1 for the removal of MB in alkaline solution (pH 11), respectively. In principle, these MnO2-D composites provided an appropriate way for the degradation of complex and various dye wastewater in practical applications.

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INTRODUCTION

Diatomite is one of the promising templates for the preparation of porous composites, because of its high porosity, low bulk density, stable chemical properties, and large specific area.22 During the last two centuries, the importance of diatomite frustules in the field of microtechnology and nanotechnology has become increasingly evident.23−26 In particular, considerable research efforts have been devoted to forming three-dimensional (3D) hierarchical porous structures, which aid the diffusion of guest species through ordered networks of pores and channels. As inspired by its novel complex structures and properties, the potentials for diatomite in device application such as high sensitivity solar cells, batteries, electroluminescent devices, drug delivery devices, and water treatment materials have been explored.27−30 For instance, Khraisheh et al. have synthesized MnO2-modified diatomite through multisteps of alkali-pretreatment/immerse/ precipitation/oxidation process for the adsorption of Methylene Blue (MB), which is considered to be the earliest known synthetic cationic dye and is widely used as a stain by many industries.20 Although the as-prepared MnO2-modified diatomite composites exhibited a high adsorption capacity for the removal of MB (376 mg g−1), MnO2 sheets that have large size and have aggregated on the surface of diatomite limit complete

Manganese dioxides (MnO2) have attracted intensive interest because of low cost, high activity/stability in alkaline/neutral media, environmental compatibility, and abundant availability.1−3 Because of the existing unique layers or tunnels in crystal lattices and high specific surface areas, MnO2 has been widely investigated and extensively used in adsorption,4−6 catalysis,7−11 oxidation,12−14 and electrochemical capacitor material.15−17 These reports have proven that MnO2 is one of the outstanding candidates for the practical application in the degradation of dye wastewater in different surroundings. Birnessite-type MnO2, which is also denoted as δ-MnO2, has a two-dimensional lamellar structure with an interlayer distance of 0.71 nm and hydrated alkaline cations (Na+, K+, ...) in the interlayers that compensate the small overall negative charge.18 Associated with their very open structure, they undergo cationexchange reactions and exhibit a larger adsorption and catalytic capacity than other manganese compounds.19 That is why these compounds continue to attract considerable scientific interest as inexpensive and nontoxic materials. Therefore, lamellar structures of birnessite-type MnO2 nanosheets have been prepared by the electrochemical and chemical routes and their performance in wastewater treatment application have been investigated.5,12,20,21 However, these methods are either complicated or require strict conditions. This stimulates an extensive interest to develop MnO2 nanosheet-based composites with high efficiency and flexible operations. © 2014 American Chemical Society

Received: Revised: Accepted: Published: 6966

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spectrophotometer (Model UV-2450, Shimadzu, Japan) was used to record the adsorption of UV-vis spectrum for the centrifuged solution. The MB concentration in the supernatant solution was determined through the measurement of the absorbance intensity at a wavelength of 663 nm. The efficiency of oxidant for the degradation of MB was estimated by eq 1:

fulfillment of the physical and chemical properties of this 3D porous structure. The mechanism to form uniform MnO2diatomites composites, to the best of our knowledge, has not been well-clarified. Very recently, uniform MnO2 nanosheets were successfully grown on the internal and external surfaces of purified diatomite for promising pseudo-supercapacitor electrodes.31 In this work, such a synthesis process is modified to scale up to a larger yield (∼0.5 g) of MnO2-modified diatomite (MnO2-D) composites in a 0.5-L reaction system. Controllable preparation of the MnO2 nanosheets with different crystal forms and thicknesses on the surface of diatomite was achieved by finetuning the preparative parameters. Moreover, we further studied their degradation ability toward MB solution in different pH values, such as the degradation of MB by oxidative decomposition (pH 2), catalytic oxidative decoloration in the presence of hydrogen peroxide (H2O2; pH 6−7), and adsorption uptake (pH 11). Thus, the as-prepared MnO2-D composite has proven to be an excellent candidate for the degradation of complex and various dye wastewater in practical applications.

⎛ C − Ct ⎞ MB degradation (%) = ⎜ 0 ⎟ × 100 ⎝ C0 ⎠

(1)

−1

where C0 and Ct (mg L ) are the liquid-phase concentrations of dye initially and at any time t, respectively. Effects of oxidant dosage (20−80 mg) and pH (2−5) on the oxidative properties of MnO2-D were investigated in order to optimize the experimental condition for the application. Catalytic Decomposition of MB. The obtained MnO2-D composites were used as catalysts for the oxidation and decomposition of the MB dye solution in the presence of H2O2. Typically, the catalytic reaction was carried out in a 250-mL glass flask, which contained 20 mL of MB dye solution (100 mg/L), 65 mL of distilled water, and 30 mg of MnO2-D composites. The pH values of the initial solutions in the degradation system were adjusted to be in the range of 5−6 with dilute HCl or NaOH solution. After adding 15 mL of 30 wt % H2O2 solution, the mixture was allowed to react at 35 °C under continuous stirring. For a given time, 4 mL of the suspension were sampled and centrifuged before the characterization, using UV-vis spectrophotometer (Model UV-2450, Shimadzu, Japan). The efficiency of catalysis for the degradation of MB was also estimated by eq 1. Adsorption Equilibrium and Kinetic Studies. Adsorption isotherm experiments were performed by adding 30 mg of MnO2-D (160 °C, 24 h) to a series of 250-mL conical flasks with MB solutions (100 mL, 30−300 mg L−1). The pH values of the initial solutions in the degradation system were adjusted to be 11 with dilute NaOH solution. Then, the conical flasks were sealed and stirred under room temperature for 24 h. The conical flasks were then taken out and the supernatant solution was separated from the adsorbent by centrifugation. The concentrations of MB in the supernatant were determined using a UV-vis spectrophotometer (Model UV-2450 Shimadzu, Japan) at 663 nm. Each experiment was conducted in duplicate under identical conditions. The amount of adsorption at equilibrium, qe (mg g−1), was calculated using eq 2:

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EXPERIMENTAL METHODS Materials. All the chemical reagents were purchased from Alfa Aesar, which were of analytical purity and used without any further purification. The natural diatomites employed in this study were provided by Tianjin Damao Chemical Reagent Company. Preparation of MnO2-Modified Diatomite. Before preparing the MnO2-modified diatomite composites, the natural diatomite was chemically treated via a modified onepot method without any surfactant.31 In a typical synthesis, purified diatomite (100−500 mg) was dispersed into the KMnO4 solution (360 mL, 0.05 M) to form a homogeneous precursor. Then, the mixture was placed into a Teflon-lined stainless steel autoclave (500 mL), which was subsequently maintained at 160 °C for 24 h, marked by MnO2-D. Finally, the suspension was then washed with distilled water and ethanol and dried at 60 °C to obtain the MnO2-D composites. Besides, some dried powders were calcined to investigate the effect of heat treatment on the crystal and morphology of the composites. The detailed experimental conditions for all the samples prepared in this work are listed in Table S1 in the Supporting Information. Characterization. The crystallographic information and chemical composition of as-prepared products were established by powder X-ray diffraction (XRD, D/max 1200, Cu Kα) and Fourier transform infrared (FTIR) spectroscopy (Nicolet, Model 5DXC). Nitrogen adsorption−desorption isotherms were measured at 77 K with a Micromeritics Model ASAP 2020 sorptometer. The structural and morphological investigations of the samples were investigated by focused-ion-beam scanning electron microscopy (FIB/SEM, Zeiss Model Auriga) and transmission electron microscopy (TEM, Zeiss Model Libra 200). Oxidative Degradation of MB. In the oxidative degradation process of MB, quantitative MnO2-D dry powder was added to the 100 mL of MB solution (0.02 g L−1) at room temperature under vigorous magnetic stirring. The pH values of the initial solutions in the degradation system were adjusted with dilute HCl solution. After different time intervals, aqueous samples (4 mL) were taken out and centrifuged to remove the oxidant particles. Then, ultraviolet−visible light (UV-vis)

qe =

(C0 − Ce)V M

(2)

where C0 and Ce (mg L−1) are the liquid-phase concentrations of dye initially and at equilibrium, respectively. V is the volume of the solution (L) and M is the mass of dry adsorbent used (g). The adsorption kinetic experiments were basically identical to those involving adsorption isotherm measurements. Aqueous samples (4 mL) were taken out at preset time intervals. The MB uptake at a certain time, qt (mg g−1), was calculated by eq 3: qt =

(C0 − Ct )V M

(3) −1

where C0 and Ct (mg L ) are the liquid-phase concentrations of dye initially and at any time t (h), respectively. V is the volume of the solution (L), and M is the mass of dry adsorbent used (g). 6967

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RESULTS AND DISCUSSION Structure and Morphology. Figure 1 shows the XRD spectra of MnO2-D and its corresponding calcinations products.

with the cristobalite structure (JCPDS File Card No. 39-1425). The other diffraction peaks of 12.4° and 24.8° are in great accordance with the (001) and (002) planes of birnessite-type manganese oxide crystal (JCPDS File Card No. 80-1098). In addition, Figures 1b and 1c display the effect of calcinations conditions on the crystallite of MnO2-D. After the heat treatment at 300 °C, the three main diffraction peaks of δMnO2 mentioned above are faded and become broad, while they disappeared completely by calcining at 500 °C. However, new weak diffraction peaks of 12.7°, 18.1°, and 37.4° emerge and could be indexed to α-MnO2 (JCPDS File Card No. 811947). Moreover, it is observed that the diffraction peaks of diatomite become sharp as the calcining temperature elevates. The FT-IR spectra (see Figure S1 in the Supporting Information) also confirm the existence of manganese oxide in the MnO2-D composites. The morphologies of the MnO2-D composites are presented in Figures 2a and 2b. The entire diatomite is decorated by MnO2 nanosheets with smooth and uniform coating. More importantly, the pores on the surface of MnO2-D composites are open with the uniform size diameter (∼175 nm). In order to obtain more information on the MnO2-modified diatomite, we check the inner part of the MnO2-D composites with a dualbeam focused ion beam (FIB) and SEM (Zeiss Auriga). Remarkably, it reveals that MnO2 nanosheets are also welldistributed on the inner surface of diatomite. Furthermore, the MnO2 nanostructures after etching diatomite in NaOH solution could partially retain the replicated diatomite

Figure 1. (a) XRD patterns of MnO2-D composites (160 °C, 24 h) and those under different calcinations conditions: (b) at 300 °C for 2 h and (c) 500 °C for 2 h.

It is found that diffraction peaks of 22.0°, 28.4°, 31.5°, and 36.1° are observed in Figure 1a, and these could be assigned to the (101), (111), (102), and (200) planes of crystalline SiO2

Figure 2. SEM images of (a,b) MnO2-D composites (160 °C, 24 h) and their calcinations product after the heat treatment at different temperature for 2 h ((c) 300 °C and (d) 500 °C). 6968

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Figure 3. (a,b) SEM images of purified diatomite and MnO2-D composites prepared through different hydrothermal synthesis conditions ((c) at 160 °C for 12 h and (d) at 160 °C for 30 h).

3D composites and their application in wastewater treatments.31 Meanwhile, the processing time under hydrothermal reaction is tuned (see Figures 3c and 3d). When the reaction time is shortened to 12 h or prolonged to 30 h, the pores of purified diatomite are also fully decorated by MnO2 nanosheets. However, the extent of the coverage of MnO2 nanosheets on the surface of diatomite MnO2 is influenced by the hydrothermal reaction time. We observe that the pore size is decreased as the reaction time increased from 12 h to 24 h, to 30 h. It is assumed that a long-time hydrothermal reaction results in much more MnO2 nanosheets growing on the surface of diatomite. But it is difficult that the SEM images are pictured in situ during the growth process of MnO2 nanosheets. Thus, it makes it uneasy to quantify the pore size distribution on the surface of diatomite. Therefore, we collect more than 100 pores on the surface of different diatomite monomers in order to make a clear trend for the pore size distribution (see details in Figure S3 in the Supporting Information). TEM images of purified diatomite and MnO2-D are shown in Figure 4. A thin shell of diatomite is peeled off by ultrasonic concussion during the sample preparation process, and its porous structure of diatomite is clearly observed in the magnified TEM image in Figure 4a. After the hydrothermal reaction with KMnO4 solution at 160 °C for 24 h, it is revealed that a two-dimensional (2D) structure of MnO2 nanosheets grows well and firmly on the surface of diatomite, with a layer thickness of ∼7.1 nm (Figure 4b). Moreover, the energydispersive spectra (EDS) line mapping of MnO2-D confirms the existence of elemental Mn, Si, and O and a small amount of

morphology. (See details in Figure S2 in the Supporting Information.) In addition, the effect of calcinations process on morphology of MnO2-D composites is studied in Figures 2c and 2d. After the heat treatment at 300 °C for 2 h, the decorated MnO2 nanosheets curl into particle clusters. In addition, these particle clusters become larger and aggregate under the calcinations procedure (500 °C for 2 h). Thus, a much greater amount of surface of diatomite is uncovered, because of the aggregation and growth of MnO2 nanoparticles, which results in the sharpness of diffraction peaks of diatomite mentioned above. Combined with the XRD diffraction (Figure 1), it is suggested that, under the calcination process at 300 °C, the morphology of MnO2 changed but the crystal form of the birnessite-type MnO2 remained; while at 500 °C, a crystal transformation of manganese oxide occurred, from the δ-MnO2 phase to the αMnO2 phase, with the growth of MnO2 nanoparticles on the surface of diatomite. This is reasonable, since δ-MnO2 is used as a precursor for other types of MnO2 materials prepared via thermal decomposition or hydrothermal treatment.32 Figures 3a and 3b depict the morphology of the purified diatomite, which reveal the porous structure after purification, indicating almost no impurities in the pores. Therefore, it is inferred that the acid treatment could remove the impurities and increase the amount of the open pores. In the magnified SEM images (Figure 3b), it can be seen that the fine pore distribution and the mean aperture is ∼500 nm. Besides, the diatomite has a large void volume in addition to its highly porous structure, suggesting that diatomite with high specific area and porosity is beneficial for the controllable synthesis of 6969

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Figure 4. TEM images of (a) purified diatomite and (b,c) MnO2-D composites prepared at 160 °C for 24 h. (d) SAED pattern of MnO2-D composites prepared at 160 °C for 24 h. Insets in panels a and b are magnified TEM images of the purified diatomite and MnO2-D composites, respectively.

trations of MnO2-D were examined. Figure 5 shows the UV-vis spectra of MB solution (0.02 g L−1) after changing different concentrations (0.2−0.8 g L−1) at pH 2. Clearly, in the presence of MnO2-D, the characteristic peak of MB dye (at the wavelength of 663 nm) fades, and even disappears after 5 min of the reaction time, comparing to that of the pristine MB solution (0.02 g L−1) (see details in Figure S6 in the Supporting Information). In addition, evidence of a blue shift in the λmax of the solution is observed, which implies the possible formation of some new composites in the degradation processes.12,21 Comparing the UV-vis spectra of different adding amount of MnO2-D in Figure 5, it is suggested that the initial concentration plays an important role on the degradation of MB dye. The decolorization efficiency of MB solution is increased greatly by adding MnO2-D at concentrations of 0.2− 0.5 g L−1. When the concentration is greater than 0.5 g L−1, there is no obvious change in the color of MB solution (inset in Figure 5). This can be explained based on the fact that the optimum oxidant loading is dependent on initial solute concentration. If the concentration is increased, the total active surface is increased correspondingly and the enhanced oxidative performance is obtained. However, the increased concentration of MnO2-D would have no extraordinary effect on promoting the decolorization efficiency after a maximum dosage is imposed. This may be ascribed to the increased aggregation of MnO2-D with a high concentration. Therefore, the MnO2-D concentration of 0.5 g L−1 is fixed for MB degradation for further studies.

K in the MnO2 nanosheets (see details in Figure S4 in the Supporting Information). The lattice fringes in the HRTEM image are indexed to a d001-spacing of 0.71 nm and (110) planes (0.24 nm).12 Moreover, there are only two weak reflection halos in the SAED pattern, which can be indexed to the (110) and (1̅14) crystal planes of the δ-MnO2 phase. The Brunauer−Emmett−Teller (BET) and nitrogen adsorption−desorption measurement are performed to examine surface properties of the purified diatomite and as-prepared MnO2-modified diatomite (see details in Figure S5 in the Supporting Information). For all four of these spectra, the isotherms are typical for a mesoporous material with a hysteresis loop at high partial pressures. The sharp increase in the N2 adsorbed quantity near the relative pressure of 1 indicates the existence of macropores in the MnO2-modified diatomite, which is due to the macropores on the surface of diatomite. According to the Brunauer−Emmett−Teller (BET) analysis, the as-synthesized MnO2-D (160 °C, 24 h) exhibits a large specific surface area of 17.04 m2 g−1, which is ∼6 times as much as that of purified diatomite (2.73 m2 g−1). Remarkably, the pore volume of purified diatomite is enlarged ∼40 times after the decoration of MnO2 nanosheets, which implies its excellent potential in the application of wastewater treatment (see details in Figure S5 in the Supporting Information). Oxidation Degradation of MB by MnO2-D Composites. Effect of MnO2-D Concentration on the Oxidative Degradation of MB. In order to obtain the optimal concentration for the degradation of MB, various concen6970

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Figure 5. UV-vis absorbance spectra of MB dye after different time intervals at pH 2 for different MnO2-D (160 °C, 24 h) concentrations: (a) 0.2 g L−1, (b) 0.35 g L−1, (c) 0.50 g L−1, and (d) 0.80 g L−1.

Effect of pH on the Oxidative Degradation of MB. The pH dependence on dye degradation plays a vital role, since dye effluents are often discharged at different pH values. Figure 6 shows the degradation of MB as a function of reaction time under different pH values. It is found that, at pH 3 and 4 (Figure 6a and b), the decolorization proceeds with high efficiency and an evident blue shift of λmax is observed. In detail, at pH 3, a short time (5 min) of reaction is enough for the characteristic peak of MB dye absorbance at 663 nm to vanish, while a longer time (∼45 min) is needed at pH 4. The degradation of MB dye solution by the MnO2-D is much faster, compared to other oxidative degradation systems, by using different MnO2-based composites, such as montmorillonite, SnO2 substrate, and SiO2 nanowires.21,33 At pH 5.0, the discoloration of MB dye solution is not so significant, and no shifted characteristic peaks are found with increasing interaction time (Figure 6c), which suggests no generation of intermediates during the interaction of MB dye molecules with MnO2-D composites. The decrease in peak intensity may be attributed to the adsorption removal of MB molecules. Figure 6d displays the oxidation decomposition percentage of MB by MnO2-D as the pH value increased from 2 to 5. It is observed that 97.7% of MB was degraded by oxidative decomposition with MnO2-D nanocomposites, which is much more efficient than those of the previous reports about the oxidation degradation of MB by MnO2-based composites.12,14 The zero point charge (pHzpc) of MnO2-D composites is determined to be 2.2, using the traditional potentiometric acid−base titration method.20 Moreover, it has been reported that catalysis exhibits its highest activity near the pHzpc value.12

It can be deduced that, at pH < pHzpc (2.2), the surface of the MnO2-D composites is positively charged, because of the protonation reaction, which would go against the adsorption of the cationic MB dye molecules onto the surface of MnO2-D composites, because of electrostatic repulsion. In this regard, it is easy to understand that the as-prepared MnO2-D composites exhibited much greater oxidation ability at the pH value range of 2−3. The extremely high discoloration efficiency may be due to the special layered MnO2 nanosheets, which brings in the high tendency for Mn(IV) to be reduced to Mn(II) under the sway of MB dye molecule as reductants. Besides, after the main absorbance peak of MB at 663 nm is rapidly diminished, the aromatic ring will be slowly decomposed into small molecules with one benzene ring and some inorganic ions, such as NO3− and SO42− in the later period. At pH > pHzpc (2.2), the surface is negatively charged, because of the deprotonation reaction, which favors precursor formation between the MB dye and the MnO2 nanosheets. If the decoloration is due to pure adsorption of MB on the MnO2-D composites, the degradation would be enhanced with increased pH of solution, which is inconsistent with the experimental data. Therefore, it can be inferred undoubtedly that the principal mechanism for the decoloration of MB dye solution is not attributed to pure adsorption, but to the oxidation degradation of MB. Catalytic Decomposition of MB by MnO2-D in the Presence of H2O2. The catalytic properties of as-prepared MnO2-D composites on the oxidation of MB dye have been studied in the presence of H2O2 at 35 °C for 2 h. All of the catalytic reactions are conducted under the same conditions. UV-vis absorption spectra are applied to demonstrate the 6971

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Figure 6. UV-vis absorbance spectra of MB dye solution after different time intervals with the fixed MnO2-D (160 °C, 24 h) dosage of 50 mg at different pH values: (a) pH 3, (b) pH 4, and (c) pH 5. (d) Comparison of the oxidation efficiency of MB at different pH values.

Figure 7. (a) UV-vis absorbance spectra of MB dye solution in the presence of MnO2-D (160 °C, 24 h) and H2O2 treated for different time intervals. (b) Time profiles of MB degradation: (1) H2O2 + MB; (2) MnO2-D (160 °C, 24 h) + MB; (3) MnO2-D (160 °C, 12 h) + MB + H2O2; (4) MnO2D (160 °C, 30 h) + MB + H2O2; (5) MnO2-D (160 °C, 24 h) + MB + H2O2. The pH values of the initial solution for all the tests were adjusted to be within the range of 6−7 and the reaction temperature was maintained at 35 °C.

reports,8,34 which makes it convenient to utilize in practical applications, since a short time is needed to decolor the textile wastewater. Upon further increasing the reaction time, the decrease in the intensity of the MB peaks continues and the peak of 664 nm is blue-shifted, which indicates that the catalytic degradation of MB is similar to the earlier reports.7,35 To clarify the catalyst performance, a systematic investigation is carried out and presented in Figure 7b. It is found that, for MB and H2O2 only (1), no obvious dye discoloration is observed after 2 h; for MnO2-D and MB only (2), the degree of

catalytic degradation activity of MB. A typical UV-vis curve and the performance of the degradation of MB by as-prepared MnO2-D composites are shown in Figure 7a. The spectrum at t = 0 is recorded from the starting solution of MB with a concentration of 20 mg L−1 (without H2O2). Once H2O2 is added, the intensities of the MB absorption peaks are decreased immediately. Within only 3 min, the intensities of the peaks at 614 and 664 nm are reduced by 86.6%. The color of the mixture turns from blue to gray quickly. It indicates that the catalytic decomposition reaction occurs faster than the previous 6972

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Figure 8. (a) Effect of contact time on MB adsorption on MnO2-D composites at various initial concentrations (30−300 mg L−1) at room temperature. (b) Pseudo-second-order kinetics for MB adsorption on MnO2-D (160 °C, 24 h) composites at room temperature. (c) Comparison of experimental and predicted amount of MB adsorbed onto MnO2-D composites for the isotherms models studied. (d) Plot of ln k2 vs 1/T for MB adsorption onto MnO2-D composites. The pH value for all of the adsorption tests was adjusted to 11.

during the catalytic oxidation reaction, leading to the degradation of the organic compounds.8,40−43 Once the highly oxidizing •OH are generated, it would subsequently attack the adsorbed pollutant molecules, leading to their decomposition. In the present work, MB molecules are first adsorbed onto the porous MnO2-D composites. Afterward, the addition of H2O2 causes its catalytic decomposition on the surface of MnO2-D composites immediately. Then, the adsorbed MB molecules are attacked and decomposed by the newly generated •OH, resulting in the destructive oxidation of MB. After the small molecules from the dye degradation are desorbed away from the surface of MnO2-D, the catalyst is recovered. Adsorption of MB on MnO2-D Composites. Adsorption Kinetic Studies. Adsorption kinetics is often used for adsorption studies, because they can describe the adsorption uptake rate at which a pollutant is removed from aqueous solutions and provide valuable data for understanding the mechanism of sorption reactions.4,44 The adsorption kinetics of MB on MnO2-D composites obtained by batch tests for the various initial MB concentrations, from 30 mg L−1 to 300 mg L−1 at room temperature, are shown in Figure 8a. It can be seen that the MB adsorption is fast at the initial contact time, and thereafter it becomes slower near the equilibrium. It indicates that the contact time needed for MB solutions with initial concentrations of 30 and 60 mg L−1 to reach equilibrium ranges between 4 and 12 h, while an equilibrium time of 24 h is required for the higher initial concentrations of 120−300 mg L−1 MB solutions. The pseudo-first-order kinetic model (see

discoloration is less and only 33.4% is detected even after 2 h. This might be due to the adsorption of the dye molecules on the MnO2-D surface; when both the MnO2-D and H2O2 are added to the MB solution, obvious discoloration occurs. Significantly, by using MnO2-D composites, the degradation of MB is up to 92.2% after only 8 min and 93.4% after 1 h. This performance is much higher than some other MnOx-based composites under similar conditions.7,8,35,36 For comparison, samples of purified diatomite and MnO2-D composites with different hydrothermal processing time are used for the degradation of MB under the same conditions. With a reaction time of 2 h, the catalytic degradation of MB is 83.1% by the MnO2-D (160 °C, 12 h)/H2O2 system and 92.4% by the MnO2-D (160 °C, 30 h)/H2O2 system, respectively. The difference of the surface area and pore volume of MnO2-D composites should be responsible for these different performances. However, for purified diatomite, only 17.3% of MB is removed after a reaction time of 2 h. Hydrogen peroxide (H2O2) has been commonly applied for removing the organic dye via the oxidative degradation.37 However, it often needs a catalyst to decompose to some active species, which are often used for the degradation of dye wastewater. Metal oxides, which possess the ability to form reduction and oxidation pairs, have been used as catalysts to the degradation of organic compounds with the assistance of H 2 O 2 . 8,38,39 It have been reported that the catalytic decomposition of H2O2 would generate the free radical species, •OH, which has been deemed to be the major active species 6973

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Figure 9. Outline of the preparation and application of MnO2-D (160 °C, 24 h) composites in this study.

where KF and n are Freundlich constants, with n giving an indication of how favorable the adsorption process is and KF (mg1−1/ n L1/n g−1) is the adsorption capacity of the adsorbent. KF is defined as the adsorption or distribution coefficient. This result implies that heterogeneity on the surface or pores of MnO2-D plays a key role in MB adsorption. Accordingly, comparing the correlation coefficient R2 of the Langmuir and Freundlich models (see details in Figure S9 in the Supporting Information), it finds that the Freundlich isotherm can well fit the experimental data for MB adsorption on the MnO2-D composites (see Figure 8c). The plot of ln qe versus ln Ce shows a straight line with slope of 1/n having a value of 0.6694 (see details in Figure S9 in the Supporting Information), indicating advantageous adsorption conditions.47 Comparatively, MnO2-D composites prepared in this work have a relatively larger adsorption capacity of 714.2 mg g−1 than other MnO2- or diatomite-based absorbents, such as diatomite (198 mg g−1), hierarchical MnO2 nanostructures (273.9 mg g−1), diatomitetemplated carbons (505.1 mg g−1), and MnO2-modified diatomite (376 mg g−1).29,48−50 Adsorption Thermodynamics. In an isolated system where energy cannot be gained or lost, the concept of thermodynamics assumes that the entropy change is the driving force.47,51 The Arrhenius equation has been applied to measure the activation energy of adsorption, which represents the minimum energy that reactants must have for the reaction to proceed, as shown by eq 6:4

details in Figure S7 in the Supporting Information) and pseudo-second-order kinetic models are applied to study the kinetics of the adsorption process. The linear form of the pseudo-second-order equilibrium adsorption model equation is given as eq 4:4 t 1 t = + 2 qt qe k 2qe

(4)

where k2 (g mg−1 h) is the rate constant of pseudo-secondorder adsorption. The linear plot of t/qt versus t gives 1/qe as the slope and 1/k2qe2 as the intercept. The linear plot of t/qt vs t (Figure 8b) shows good agreement between the calculated qe values and the experimental data. The calculated qe value is 485.4 mg g−1 for the initial MB concentration of 300 mg L−1, indicative of a high adsorption capacity. Besides, it suggests that the second-order kinetic model well fit the adsorption process of MB on the prepared MnO2-D composites, comparing the correlation coefficient R2 values of these two kinetics model (see details in Figure S7 in the Supporting Information). Adsorption Isotherms. Adsorption capacity of MB on the MnO2-D composites at room temperature can be obtained by the adsorption isotherms (see Figure 8c). The adsorption process is normally described by the Langmuir and Freundlich isotherm. The Langmuir isotherm assumes that there is no interaction between the adsorbate molecules and the adsorption is localized in a monolayer. In addition, once adsorbate molecules occupy a site, no further adsorption can take place at that site (see details in Figure S8 in the Supporting Information). The Freundlich isotherm, on the other hand, assumes heterogeneous surface energies, in which the energy term in the Langmuir equation varies as a function of the surface coverage. The well-known logarithmic form of the Freundlich isotherm is given by eq 5:45,46 ln qe = ln KF +

1 ln Ce n

ln k 2 = ln A −

Ea RT

(6)

−1

where k2 (g mg h) is the rate constant of pseudo-secondorder adsorption, Ea is the Arrhenius activation energy (kJ mol−1), A is the Arrhenius factor, R is the gas constant (R = 8.314 J mol−1 K), and T is the absolute temperature. When lnk2 is plotted against 1/T, a straight line with the slope −Ea/R is obtained in Figure 8d. The magnitude of activation energy gives an idea about the type of adsorption, such as chemical or physical adsorption. The physisorption processes often have

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activation energies in the range of 0−40 kJ mol−1, yet higher activation energies (40−800 kJ mol−1) imply chemisorption. The value of the activation energy is ca. 3.239 kJ mol−1, which is indicative of the physical adsorption process for MB adsorption onto the MnO2-D composites. Meanwhile, the values of these thermodynamic parameters are calculated (see details in Figure S10 in the Supporting Information). As can be seen, with increasing temperature, the decrease in the ΔG° values indicates an increase in the feasibility and spontaneity of the adsorption at higher temperatures. Moreover, a positive ΔH° value indicates the endothermic nature of the adsorption, whereas a positive value of ΔS° suggests that there is increased randomness at the solid/ solution interface during the adsorption of MB in aqueous solution on MnO2-D composites. Some structural changes may have taken place as a result of interactions between the MB molecules and the functional groups on the surface of the MnO2-D composites.52 Regeneration of MnO2-D Composites. Thermal regeneration tests are carried out by calcining the exhausted adsorbents, aiming at exploring the potential reutilization of MnO2-D composites as adsorbents for MB removal from aqueous solution. Regeneration process of MB adsorbed on MnO2-D composites for three times of thermal recycling is conducted (see details in Figure S11 in the Supporting Information). The percentages of MB adsorption for the three regenerations (R1−R3) are 77.76%, 73.45%, and 60.90%, respectively, compared to the original MnO2-D (R0, 93.4%). This phenomenon can be attributed to the structure and morphology transformation of MnO2 nanosheets during the regeneration process at 450 °C for 30 min. More importantly, adsorption kinetics is carried out in order to illustrate the loss of the adsorption capacities of the calcined MnO2-D products (see details in Figure S12 in the Supporting Information). These results reveal that the thermal regeneration of the MnO2D composites for reuse is feasible at least twice, after which the regenerated materials will suffer from a large loss in their adsorption capacities.

environmentally friendly materials for the degradation of complex and various dye wastewater in practical applications.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental conditions for all the samples; FT-IR, FIB-milled cross-section images, EDS spectra and BET data of MnO2-diatomite (160 °C for 24 h); the constants and coefficients of kinetic models (pseudo-first-order and pseudosecond-order) and isotherm models (Langmuir and Freundlich isotherm model); thermodynamics parameters and regeneration test experiment. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 23 65104131. Fax: +86 23 65104131. E-mail: [email protected] (Y.X. Zhang). *Tel.: +86 23 65104131. Fax: +86 23 65104131. E-mail: [email protected] (J. Li). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial supports provided by National Natural Science Foundation of China (Grant Nos. 51104194 and 21103127), Doctoral Fund of Ministry of Education of China (No. 20110191120014), No. 43 Scientific Research Foundation for the Returned Overseas Chinese Scholars, National Key Laboratory of Fundamental Science of Micro/Nanodevice and System Technology (No. 2013MS06, Chongqing University), State Education Ministry and Fundamental Research Funds for the Central Universities (Project Nos. CDJZR12248801, CDJZR12135501, and CDJZR13130035, Chongqing University, PR China).

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REFERENCES

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CONCLUSIONS In summary, a hierarchical architecture consisting of MnO2 nanosheets layer decorated on purified diatomite has been prepared via a facile hydrothermal treatment without any surfactant. Moreover, the controllable adjustment of crystal form of MnO2 nanosheets layer and the pore size on the surface of diatomite can be achieved. Based on the batch tests toward MB solutions in different pH values, the mechanism for the degradation of MB were proposed in Figure 9. As can be seen, MnO2-D composites exhibit excellent performance for the degradation of MB in a full range of pH values (2−11) in aqueous solution. Typically, the MnO2-D composite has shown a high oxidation degradation performance of MB solution (97.7% removed) in acidic solution (pH 2); and it can also be an excellent catalyst for the oxidation decomposition of MB solution (92.4% removed) by H2O2; and it represents a high adsorption capacity of 714.3 mg g−1 for the removal of MB in alkaline solution (pH 11), respectively. Moreover, MnO2-D composites also exhibit promising potential in the application of catalytic oxidation of phenol in the presence of H2O2 (see details in Figure S13 in the Supporting Information). Thus, these results indicate that these hierarchical and porous MnO2D composites could be used as large-scale, cost-effective and 6975

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