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Research Article pubs.acs.org/journal/ascecg

Antimony Removal from Aqueous Solution Using Novel α‑MnO2 Nanofibers: Equilibrium, Kinetic, and Density Functional Theory Studies Jinming Luo,†,‡,§ Chengzhi Hu,† Xiaoyang Meng,§ John Crittenden,*,§ Jiuhui Qu,*,† and Ping Peng#

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†

Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § School of Civil and Environmental Engineering and Brook Byers Institute for Sustainable Systems, Georgia Institute of Technology, 828 West Peachtree Street, Atlanta, Georgia 30332, United States # School of Materials Science and Engineering, Hunan University, Changsha, Hunan 410082, China S Supporting Information *

ABSTRACT: Herein, we report the synthesis and characterization of a novel α-MnO2 nanofibers (MO-2) prepared via morphological and phase transitions from δ-MnO2 nanoparticles under hydrothermal reaction in the presence of graphene oxide (GO) for the first time. The MO-2 shows long, compact, and uniform nanofiber morphology. The adsorption properties of antimonite (Sb(III)) and antimonate (Sb(V)) on MO-2 were investigated using batch experiments of adsorption isotherms and kinetics. Experimental results show that the adsorption behavior of Sb on MO-2 is spontaneous, exothermic, and pH-dependent and follows the monolayer Langmiur isotherm model, pseudo-second-order kinetic model and external mass transfer model. MO-2 has maximum Sb(III) and Sb(V) adsorption capacities of 111.70 and 89.99 mg/g, respectively. Density functional theory (DFT) calculations indicate that both Sb(III) and Sb(V) have monodentate and bidentate complexes on the (110) facet. The adsorption energies (Ead) analysis demonstrates that the formed monodentate and bidentate complexes of Sb(III) (−2.31 and −2.70 eV, respectively) and Sb(V) (−2.17 and −2.85 eV, respectively) on the (110) facet are stable. And it can be confirmed that Sb(III) and Sb(V) are chemisorbed on the surface of MO-2 according to the analyses of partial density of state (PDOS) and Dubinin−Radushkevich (DR) isotherm model. KEYWORDS: Antimony, Adsorption, α-MnO2 nanofibers, DFT study

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INTRODUCTION

and high efficiency. Currently, many adsorbents are available for Sb removal, including diatomaceous earth, activated alumina, iron−zirconium bimetal oxides, and nanoscale zerovalent iron.8−11 However, these adsorbents have some deficiencies, such as low adsorption capacities and long equilibrium contact time. The adsorption mechanisms of Sb on different adsorbents are not clear as well. Therefore, the developments of novel adsorbents, which simultaneously owing excellent adsorption capacities, fast equilibrium contact time, and elucidate the adsorption mechanism are urgently needed. Manganese (Mn), the twelfth most abundant element on the planet and the third most abundant transition metal in the Earth’s crust, is ubiquitous, inexpensive and indispensable in biogeochemical cycles.12,13 Mn-related oxides are particularly

Antimony (Sb) contamination in water has attracted worldwide attention in recent decades because of the large quantities consumed (over 100 000 tons annually worldwide) for the fabrication of a variety of industrial products, such as flame retardants, ceramics, and alloys.1,2 Because of their high toxicity and suspected carcinogenic properties to humans, Sb and its compounds are classified as priority pollutants by the United States Environmental Protection Agency (USEPA) and the European Union (EU, Council of the European Communities).3 The maximum contaminant levels specified by the USEPA and EU for Sb in drinking water are 6 and 5 μg/L, respectively. Many techniques for controlling Sb contamination, including ion exchange,4 precipitation/coagulation,5 reverse osmosis,6 and adsorption,7 have been reported. Compared with other methods, adsorption is a promising technology for Sb removal from water because of its advantages of safety, easy operation, © 2017 American Chemical Society

Received: October 26, 2016 Revised: January 9, 2017 Published: January 25, 2017 2255

DOI: 10.1021/acssuschemeng.6b02583 ACS Sustainable Chem. Eng. 2017, 5, 2255−2264

Research Article

ACS Sustainable Chemistry & Engineering

resulting mixture was stirred for 1 h and aged at room temperature overnight. Finally, the products were decanted and washed several times using DI water and dried at 80 °C overnight. Then, 300 mg of the as-prepared OL-1 was dissolved in 80 mL of DI water and mixed with 10 mL of GO solution that was prepared from natural graphite flakes by using a modified Hummers method.33 This solution was transferred to a Teflon-lined autoclave and then placed in an oven at 200 °C for 48 h. After allowing the sample to cool to room temperature naturally, the MO-2 was filtrated, washed several times with DI water, and freeze-dried overnight. Characterization. Fourier transform infrared (FTIR) spectra were recorded using KBr pellets with a VERTEX-70 spectrometer (Bruker, German). Raman spectra were collected using a Renishaw-5277 Raman spectrometer with a 514 nm laser at a power of 4.7 mW (Renishaw, England). The crystalline phases of the samples were investigated via X-ray diffraction (XRD, Rigaku UltimaIV) using graphite monochromatized Cu Kα (λ = 1.5406 Å) radiation. The XRD data for indexing and cell-parameter calculations were obtained in scanning mode with a scanning speed of 2°/min in the 2θ range of 10−80°. The sample morphologies were examined by using a scanning electron microscope (SEM, FEI Nova NanoSEM 450) and highresolution transmission electron microscope (HRTEM, JEOL JEM 2010). Energy dispersive X-ray spectroscopy (EDS) was performed by using an Oxford Aztec Energy microanalysis system with an X-Max 80 silicon drift detector. X-ray photoelectron spectroscopy (XPS) measurements were taken using a VG Escalab 250 spectrometer equipped with an Al anode (Al Kα = 1486.7 eV). The Brunauer− Emmett−Teller (BET) specific surface areas were determined by using nitrogen-sorption isotherms via a Micrometritics ASAP 2010 analyzer. Adsorption Experiments. First, 50 mL of Sb(III) and Sb(V) solutions, with initial concentrations ranging from 10 to 500 mg/L, were prepared by diluting the appropriate stock solution with DI water. The batch experiments were used to determine the adsorption isotherms of Sb(III) and Sb(V) at 20, 30, and 40 °C. The adsorbent dosage for all the experiments was 0.5 g/L. The adsorption capacity (qe) for Sb was calculated as follows:

interesting because of their high surface area, microporous structures, environmental compatibility, and excellent physicochemical properties. Additionally, Mn-related oxides adsorbents are important scavengers for heavy metals in natural water, and have shown good performance for Sb removal.14−16 Furthermore, manganese dioxide (MnO2) is one of the most stable manganese oxides under ambient conditions, and has many kinds of polymorphs, such as α-, β-, γ-, and δ-type, offering attractive physiochemical properties. Among the MnO2 group, α-MnO2 has some advantages: high phenol-degradation efficiency, high carbon monoxide (CO)-oxidation activity, and high charge storage capacity.17−19 Nowadays, amorphous MnO2 are usually applied on heavy metals removal.20−22 However, to the best of our knowledge, the specific crystal form MnO2, such as α-MnO2, has not been thoroughly studied on heavy metals removal, especially Sb. On the basis of the fact that α-MnO2 owns many superior characters in many other aspects, in this study, we aim to systematically investigate Sb removal by using α-MnO2 to improve our understanding of its adsorption performance and realize the material’s full applications. Graphene has shown great properties in many areas,23−26 and herein, we present a new approach to synthesize α-MnO2 (MO-2) with unique compact nanofiber morphology via phase transition from δ-MnO2 nanoparticles under a hydrothermal reaction in the presence of graphene oxide (GO). The morphology of MO-2 is distinct from other reported αMnO2.27−32 Due to the compact nanofiber morphology of MO2, the BET surface area is evidently increased, which exposes more active adsorption sites and lower the mass transfer resistance. Meanwhile, the intraparticle diffusion is largely inhibited; leading to surface diffusion as the dominant adsorption process. These facts can largely promote the adsorption capacity and shorten the equilibrium contact time simultaneously. Meanwhile, DFT calculations are also performed to simulate the adsorption of Sb(III) and Sb(V) on (110) facet of MO-2 to gain insight into the adsorption mechanism. Both batch experimental results and the intrinsic fundamental mechanism analysis of Sb adsorption on (110) facet demonstrate that MO-2 is a promising and efficient adsorbent for Sb removal from aqueous solutions. The novelty of this study including: (1) the new method was proposed to synthesize α-MnO2 with unique morphology and excellent adsorption performance for the removal of Sb; (2) the adsorption behavior of Sb on the α-MnO 2 was well investigated; and (3) the adsorption mechanism of Sb on the (110) facet was further identified by using density functional theory (DFT) calculations.

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qe =

V (C0 − Ce) m

(1)

Where qe (mg/g) is the equilibrium adsorption capacity; C0 and Ce (mg/L) are the initial and equilibrium concentrations in the presence of adsorbents in solution, respectively; V is the volume (mL) of the Sb aqueous solution; and m is the mass (mg) of adsorbent used in the experiment. The adsorption kinetics of Sb(III) and Sb(V) were examined with an initial Sb(III) and Sb(V) concentration of 20 mg/L under constant stirring. Samples were removed at a particular adsorption time. To analyze the effect of pH on Sb removal, the isotherms were determined for pH values from 1.0 to 12.0. The desired pH was obtained by adding 0.1 mol/L HCl or 0.1 mol/L NaOH to the solution. All the suspensions with three replicates were sealed and stirred at 240 rpm for 24 h. After the adsorption, all the suspensions were filtered through 0.45 μm cellulose acetate membranes to collect the clear permeate solution for Sb concentration analysis. The residual concentrations of Sb in the permeate were determined by using an 8220 atomic fluorescence spectrophotometer according to a previously reported method.34 Computational Methods. All the calculations were performed by using the Castep package of Material Studio 7.0. In the planewave calculations, a cutoff energy of 450 eV was applied. The exchangecorrelation interaction was treated within the generalized gradient approximation (GGA) with the parametrized function provided by Perdew−Burke−Ernzerhof (PBE). To further improve the model accuracy in the band gap estimation of (110) surface slab, an on-site Coulomb potential (DFT+U) correction was applied to Mn atoms.35−37 The optimized Hubbard U = 1.6 eV was chosen in this study, as in previous studies.38,39 All calculations were performed in a ferromagnetic spin polarized configuration. The SCF convergence criterion and energy tolerance were set as 5.0 × 10−5 and 1.0 × 10−5

MATERIALS AND METHODS

Chemicals and Reagents. All chemicals were analytical grade and used without further purification. The 1000 mg/L stock solutions of Sb(III) and Sb(V) were prepared in deionized (DI) water by adding potassium antimony tartrate (K2Sb2(C4H4O6)2) and potassium pyroantimonate (KSb(OH)6), respectively. Graphite powder, ethanol, potassium hydroxide (KOH), and potassium permanganate (KMnO4) were used to synthesize the α-MnO2 nanofibers (MO-2). Synthesis of α-MnO2 Nanofibers. MO-2 was synthesized via a hydrothermal method. First, δ-MnO2 (layered birnessite, denoted as OL-1) was synthesized according to the following procedure: 100 mL of solution (hereafter labeled A) was prepared by mixing 46 mL of ethanol and 16.8 g of KOH. Then, another 50 mL of aqueous solution (hereafter labeled B) containing 4.74 g of KMnO4 was prepared. Solution A was slowly added to solution B under vigorous stirring. The 2256


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Figure 1. (a, b) SEM images of MO-2 at different magnifications. (c) XRD pattern of MO-2. (d) EDS spectrum of MO-2. (e) HRTEM image of MO-2 showing the (310) interplanar distance. (f) HRTEM images of MO-2 with ① a specific amplification area in which the (110) and (211) interplanar distances are shown and ② SAED pattern analysis. eV/atom, respectively. A Monhorst-Pack scheme with a 2 × 1 × 1 kpoint grid was employed. The unit cell of MO-2 was first geometry optimized, resulting in lattice parameters a = b = 9.898 Å and c = 2.924 Å, which were comparable with the experimental data (a = b = 9.750 Å and c = 2.861 Å). These accurate results validated the applicability of the Castep package for calculating our α-MnO2 slab structures. The (110) surface slab was cleaved from the optimized structure of α‑MnO2, 14 layers of atoms were extracted, and then, a 2 × 3 supercell was built. Periodic boundary conditions were applied in all three dimensions, with a vacuum region of approximately 15 Å along the z‑axis. The atoms in the top seven layers of the slab were relaxed, whereas those in the bottom seven layers were constrained during the optimization to keep the computational time relatively low. The optimized Sb(OH)3 and Sb(OH)6 were added to the optimized (110) to build the initial interfacial complex. To further simulate the adsorption of Sb on (110) in water, four water molecules were added to surround Sb(OH)3 and Sb(OH)6. The similar calculation method has been reported by Yan et al., which added four water molecules to simulate arsenate adsorption on rutile (110) in aqueous solutions.40 The adsorption energies (Ead) required for Sb molecule to adsorb on (110) surface slab complex was calculated using eq 2:41,42

Ead = ESb + surf − (Esurf + ESb)

previously reported α-MnO2 are nanoparticles, nanorods, or nanotubes. This is the first report of the very long, uniform, and compact nanofiber morphology of α-MnO2. As the evident morphology changed from nanoparticles to nanofibers, the BET surface area of MO-2 reached 144 m2/g, far exceeding that of δ-MnO2 (56 m2/g). This is attributable to the new synthesis process and the distinctive structure of the as-prepared MO-2. The surface morphologies, crystalline structures, and compositions of MO-2 were characterized by using various methods, including SEM, HRTEM, XRD, and EDS. Figures 1a and b show the morphology of MO-2 nanofibers. The cross-sectional view of MO-2 (∼70-μm thick) shows that the nanofibers are uniform, compact, and well preserved (Figure 1a and inset and Figure S1d). As shown in Figure 1c, the sharp diffraction peaks in the XRD patterns of MO-2 indicate the high quality of MO2’s crystallinity. The diffraction patterns of MO-2 are in accordance with those of α-MnO2 (JCPDS 44-0141), and no other impurity peaks are found in the XRD patterns. The EDS spectrum demonstrates Mn, O and K are present in the MO-2 nanofibers (Figure 1d). The SEM mapping result (Figure S2) further confirms the EDS results. Figure 1e clearly shows the lattice fringe with a lattice spacing of 0.32 nm between adjacent planes, which corresponds to (310) facet. Combined with the Figure 1b, it shows the length and diameter of the MO-2 nanofiber is about 10 μm and 10 nm, respectively. Another HRTEM image is presented in Figure 1f, showing lattice fringes along the nanofiber. The lattice spacings are 0.69 and 0.24 nm between adjacent planes, in accordance with the distances separating the (110) and (211) facets of MO-2, respectively (Figure 1f①). This observation is further confirmed by the selected-area electron diffraction (SAED) pattern (Figure 1f②). Overall, the HRTEM investigations shown in Figure 1e and f demonstrate that MO-2 exhibits a uniform nanofiber morphology and high-quality single-crystalline features. To verify the absence of graphene or GO in the assynthesized MO-2, we analyzed the FTIR and Raman spectra of MO-2. As shown in Figure S3a, the four peaks in the Raman spectra are ascribed to α-MnO2.44−47 No other characteristic peaks of graphene or GO are present in the Raman spectra and

(2)

where ESb+surf is the total energy of the surface complexes, and Esurf and ESb represent the energy of (110) surface slab and Sb, respectively. The net charges of atoms during Sb adsorption were calculated as eq 3:

net charge = Mulliken chargeafter − Mulliken charge before

(3)

where Mulliken chargeafter is the charge of the atoms after Sb adsorption, and Mulliken chargebefore is the charge of the atoms before adsorption. Note that a positive value for the net charge suggests a loss of electrons.43

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RESULTS AND DISCUSSION Characterization. α-MnO2 is normally prepared via a hydrothermal, reflux, or other chemical method, using permanganates as reactants. For the first time, we synthesized the novel α-MnO2 nanofibers (MO-2) via phase transition from δ-MnO2 nanoparticles (Figures S1a and S1b) under a hydrothermal reaction in the presence of GO (Figure S1c). To the best of our knowledge, the morphologies of the 2257


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Figure 2. Adsorption isotherms collected at different temperatures for (a) Sb(III) and (b) Sb(V) on MO-2. The initial Sb(III) and Sb(V) concentrations were 10−500 mg/L, the adsorbent dose was 0.5 g/L, the solution volume was 50 mL, and the pH was 6.0.

Table 1. Langmuir and Freundlich Parameters for Sb(III) and Sb(V) Adsorption on the MO-2 Langmuir model temperature (°C) 20 30 40

Sb(III) Sb(V) Sb(III) Sb(V) Sb(III) Sb(V)

kL (L/mg)

R2

n

kF (mg1−(1/n) L1/n g−1)

R2

111.70 89.99 93.31 85.25 88.80 84.93

0.0239 0.0056 0.0194 0.0041 0.0169 0.0031

0.997 0.995 0.995 0.999 0.996 0.997

2.579 1.839 2.524 1.662 2.525 1.589

11.175 2.480 8.466 1.490 7.866 1.105

0.950 0.981 0.940 0.991 0.927 0.995

According to the Langmuir model, the monolayer coverage or maximum adsorption capacities (MTACs) of MO-2 are 111.70 and 89.99 mg/g for Sb(III) and Sb(V), respectively, at 20 °C. For comparison, we measured the performance of α-MnO2 nanowires synthesized according to the reported method.49 As shown in Figure S4 and Table S1, we found that the Langmuir model is more suitable for describing the adsorption isotherm than the Freundlich model. The MTACs of the reported αMnO2 nanowires are 71.82 and 53.24 mg/g for Sb(III) and Sb(V), respectively, at 20 °C. Clearly, MO-2 has a much higher capacity than the traditional α-MnO2 nanowires. Meanwhile, MO-2 also owns many advantages than many literature reports, including high adsorption capacity, stable performance within the wide pH range (1.0−12.0), and short adsorption kinetic equilibrium contact time (Table S2). We found that the adsorption capacity decreases as the temperature increases. Thus, Sb adsorption on MO-2 is an exothermic process, as confirmed by a thermodynamic analysis. We calculated the changes in the Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) using eqs 6 and 7:50

FTIR of MO-2 (Figures S3a and S3b). Combined with the above XRD and HRTEM results, we can conclude that the assynthesized MO-2 is pure α-MnO2 nanofibers. Moreover, GO could act as a structure-directing and morphology-controlling agent during the synthesis of MO-2, as described in the literature.48 Adsorption Isotherms. Figure 2a and b show the adsorption isotherms of Sb(III) and Sb(V) on MO-2 at 20, 30, and 40 °C and pH 6.0. Clearly, the equilibrium adsorption capacities of MO-2 for Sb(III) and Sb(V) rapidly increase in the low-concentration range and then gradually increase in the middle-concentration range, eventually reaching their maximum. Meanwhile, the adsorption capacities of Sb(III) and Sb(V) on MO-2 decrease as the increase of temperature increases. The equilibrium adsorption isotherm data were analyzed using the Langmuir (eq 4) and Freundlich (eq 5) adsorption models:

qe = qe =

qmkLCe 1 + kLCe kFCe1/ n

Freundlich model

qe (mg/g)

(4)

ΔG° = −RT ln kL

(6) −3

where R is the universal gas law constant (8.314 × 10 kJ/mol· K), T is the absolute temperature (K), and kL is the Langmuir adsorption equilibrium constant (L/mg). ΔH° and ΔS° were determined according to eq 7:

(5)

where qe is the amount (mg/g) of Sb adsorbed at equilibrium, Ce is the equilibrium Sb concentration (mg/L) in the water samples, qm (mg/g) and kL (L/mg) are the Langmuir parameters, qm represents the maximum adsorption capacity, kL is the adsorption equilibrium constant, and kF (mg1−(1/n) L1/n g−1) and n are the Freundlich parameters. The values of kF and n are in the range of 1−10, indicating favorable adsorption. The parameters are listed in Table 1. The Langmuir model fits the data better than the Freundlich model, and the correlation coefficient (R2) of the Langmuir model is higher than that of the Freundlich model. Accordingly, Sb(III) and Sb(V) adsorption onto MO-2 results in a homogeneous monolayer, and the adsorption site energies are uniform.

ln kL =

ΔS ° ΔH ° − R RT

(7)

On the basis of eq 7, ΔH° and ΔS° can be calculated as the slope and intercept of the plot of ln kL versus 1/T yields, respectively (Figure S5 and Table S3). For Sb(III), the adsorption of Sb(III) on MO-2 is an exothermic reaction between 20 and 40 °C because the value of ΔH° is negative (−13.36 kJ/mol). The negative value of ΔS° (−0.019 kJ/mol· K) is attributable to the decrease in the degree of randomness 2258


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Figure 3. Adsorption kinetics for Sb(III) and Sb(V) adsorption on MO-2, (a) fitted with pseudo-first and pseudo-second order kinetic models, and (b) fitted using the mass transfer model. The initial Sb(III) and Sb(V) concentration was 20 mg/L, the adsorbent dose was 0.5 g/L, the solution volume was 200 mL, the pH was 6.0, and the temperature was 20 °C.

of the adsorbed species. We determined ΔG° values of −7.73, −7.47, and −7.35 kJ/mol at 20, 30, and 40 °C, respectively. A similar trend was found for Sb(V). ΔH° is −22.57 kJ/mol, whereas ΔS° is −0.063 kJ/mol·K. We calculated ΔG° values of −4.20, −3.55, and −2.94 kJ/mol at 20, 30, and 40 °C, respectively. The nature of the adsorption process (i.e., physical or chemical) can be determined using the Dubinin−Radushkevich (DR) isotherm model. The linear equation of the DR isotherm is expressed as eq 8: ln qe′ = ln qm′ − βε

2

qt =

1 2β

(11)

Table 2. Kinetics Constants for Sb(III) and Sb(V) Adsorption on the MO-2

(8)

pseudo-first-order model

Sb(III) Sb(V)

pseudo-second-order model

K1 (min−1)

qe (mg/g)

R2

K2 (g mg/min)

qe (mg/g)

R2

0.552 0.039

30.22 18.42

0.975 0.983

0.0336 0.0023

31.35 21.06

0.998 0.993

The results show that the pseudo-second-order kinetic model provide a better correlation of the experimental kinetic data than the pseudo-first-order kinetic model, which suggest the rate-limiting step of the adsorption mechanism for MO-2 is chemical adsorption.52 In Figure 3b, adsorption kinetic data were analyzed using the mass-transfer model. The mathematical derivation of the mass flow rate equation on MO-2 is shown in Section 1 (Supporting Information). The mass balance equations are

(9)

A value of E between 8 and 16 kJ/mol indicates that the adsorption process is chemisorption.51 We determined E values of 8.73 and 10.04 kJ/mol for Sb(III) and Sb(V) adsorption, respectively. Thus, both Sb(III) and Sb(V) chemisorb on the surface of MO-2. Adsorption Kinetics. In practical applications, the adsorption rate and kinetic parameters are the key factor for designing adsorption reactors and optimizing the operation conditions. Both the adsorption amounts of Sb(III) and Sb(V) increase rapidly with the time during the first 20 min, then gradually reached the equilibrium after about 30 min time extension. As shown in Figure 3a, the adsorption kinetic data were simulated by using the pseudo-first-order and pseudosecond-order kinetic models. The pseudo-first-order and pseudo-second-order kinetic models are expressed as eqs 10 and 11, respectively. qt = qe(1 − e−K1t )

1 + qeK 2t

where qe is the amount of adsorbate at equilibrium (mg/g); qt is the amount of adsorbate (mg/g) at time t (min); and K1 (min−1) and K2 (g mg·min−1) are the rate constants for the pseudo first- and second-order sorption, respectively. All the parameters are listed in Table 2.

where qe′ is the amount of metal ions adsorbed per unit weight of the adsorbent (mol/L), qm′ is the maximum adsorption capacity (mol/g), β is the activity coefficient related to the mean free energy of adsorption (mol2/kJ2), and ε is the Polanyi potential (ε = RTln(1 + 1/Ce)). The DR isotherm model fits the equilibrium data well (Figure S6 and Table S4). The qm′ values were determined to be 5.57 × 10−3 and 1.23 × 10−3 mol/g for Sb(III) and Sb(V), respectively, based on the intercept of the plot. The mean free energy of adsorption (E; kJ/mol) is expressed as eq 9: E=

qe 2K 2t

−

dC = k f a(C − Cs) dt

C = bexp[−ht ] + C0 − b

(12) (13)

where C is the concentration of Sb (mg/L) in the bulk solution at any time t, Cs is the concentration of Sb at the interface (mg/ L), h and b are the fitting parameters, and kf is the mass-transfer coefficient (cm/s). The R2 values are 0.989 and 0.990 for Sb(III) and Sb(V), respectively, using the mass transfer model. The calculated kf value is 4.38 × 10−4 and 1.42 × 10−4 cm/s for Sb(III) and Sb(V), respectively. The kf of Sb(III) is higher than that of Sb(V). As a result, the adsorption kinetics is controlled by mass transfer and the surface reaction rate is very fast. Furthermore, the adsorption kinetic experiment was conducted in real waste lake water, which spiked with 325 μg/L Sb(III) and Sb(V). The real lake water was collected from Tianyi lake, Jiangxi Province, China (located at 28° 38′ 52″ N,

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Figure 4. Effect of pH on MO-2 adsorption capacities for (a) Sb(III) and Sb(V) and (b) is zeta potential of MO-2. The initial Sb(III) and Sb(V) concentrations were 150 and 250 mg/L, respectively; the adsorbent dose was 0.5 g/L; the solution volume was 50 mL; the pH was 1.0−12.0; and the temperature was 20 °C.

was recovered by shaking in 0.5 mol/L sodium hydroxide (NaOH) solution at 300 rpm for 2 h, and centrifugation. The regenerated adsorption capacity of MO-2 is shown in Figure 5.

115° 49′ 52″ E; details of water contents were mentioned in the caption of Figure S7). As shown in Figure S7, the MO-2 can effectively remove Sb(III) and Sb(V) under 5 μg/L (3.786 and 4.778 μg/L, for Sb(III) and Sb(V), respectively) within 90 min in real wastewater, which further identifies the MO-2 has great potential on Sb removal in real wastewater treatment. Effect of pH. pH can markedly affect an adsorbent’s adsorption capacity. The influence of pH on the adsorption capacity of the MO-2 was investigated based on the ionic form of Sb species and the surface charge of MO-2. As shown in Figure 4a, MO-2’s adsorption capacities for Sb(III) and Sb(V) are pH dependent, and the best adsorption performance of MO-2 can be achieved at pH 4.0. Meanwhile, compared with the previously reported absorbents, the adsorption capacity of MO-2 to Sb(III) and Sb(V) is relatively high and stable over a wide pH range (from 2.0 to 10.0).53−55 The zero charge point (pHpzc) of MO-2 is 4.8 (Figure 4b), indicating that the MO-2 surface is negatively charged when pH values exceeding 4.8. As for Sb(III), the adsorption capacity largely increases from pH 1.0 to 2.0 and decreases slightly with the pH increase from 4.0 to 10.0. And the evidence drop is identified from pH 10.0 to 12.0. As a result, the Sb(III)-adsorption capacity decreases by 20.77%. This phenomenon is mainly attributed to the Sb(III) species varying with different pH values. Sb(III) exists as Sb(OH)+ when pH below 2.0 and exists as Sb(OH)3 with the pH values ranging from 2.0 to 10.4, while it transforms to H2SbO3− or Sb(OH)4− when the pH exceeds 10.4.6 The low adsorption capacity at pH 1.0 and 12.0 are due to the fact that Sb(III) is positively and negatively charged at that pH condition, which is same to the charge of the adsorbent surface. Under this circumstance, the electrostatic adsorption force is greatly weakened. As for Sb(V), the adsorption capacity increases in the pH range 1.0−3.0, decreases as the increase of pH value (from 4.0 to 12.0). This behavior is rational because Sb(V) exists as SbO2+ when pH below 2.0, whereas Sb(V) exists as H3SbO4 in the range of pH 2.0−2.7. And when the pH exceeds 2.7 it exists as Sb(OH)6−,6,56 and the surface of MO-2 is positively charged when the pH is below 4.8. When the pH is 4.0, the adsorption capacity is maximized because the surface of MO-2 is positively charged and the electrostatic adsorption force is enhanced. Subsequently, the adsorption capacity gradually decreases because the surface becomes negatively charged when the pH exceeds 4.8, and the electrostatic adsorption force is weakened under this circumstance Regeneration and Reuse. The regeneration and recycling performance of the MO-2 was investigated. The spent MO-2

Figure 5. Regeneration and reuse of MO-2 cycles. The initial Sb(III) and Sb(V) concentration was 180 mg/L; the adsorbent dose was 0.5 g/L; the solution volume was 50 mL; the pH was 6.0 ± 0.2; and the temperature was 20 °C.

The capacities of reused MO-2 remained nearly unchanged for the first two cycles and later slightly dropped after the five cycles, which illustrating that the MO-2 can be used repeatedly. XPS Analyses. The elemental compositions, adsorbed species on surface solid material and metal oxidation states were determined with XPS. As shown in Figure S8, the Mn 2p, O 1s and Sb 3d peaks were observed, indicating the existence of Mn, O and Sb in MO-2 after adsorption. The Mn 2p2/3, O 1s, and Sb 3d XPS spectra of three samples with different treatment are shown in Figure 6, and the details of XPS results are shown in Table S5. In Figure 6a, it can be seen that the asymmetrical Mn 2p3/2 XPS spectrum of each sample can be deconvoluted to four components at binding energy (BE) = 640.7, 641.6, 643.0, and 644.5 eV, which assign to the surface Mn2+, Mn3+, and Mn4+ species and the satellite of the Mn3+ species, respectively.57,58 Besides, the surface Mn4+/Mn3+ molar ratio of the MO-2 is decreased from 1.08 (MO-2) to 0.84 (MO-2 + Sb(III)). This result could be caused by the redox reaction occurred during the Sb(III) adsorption process. Whereas, after Sb(V) adsorption, the Mn4+/Mn3+ molar ratio of MO-2 is 1.05 that nearly equals to that of Sb-free MO-2, indicating there is no redox reaction in the Sb(V) adsorption process. As shown in Figure 6b, the characteristic BEs of 530.5 and 539.7 eV are ascribed to Sb 3d5/2 and Sb 3d3/2, respectively, 2260


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Sb(OH)6− were used in DFT calculations to identify the adsorption site preference. Meanwhile, the (110) is a stable facet with a low energy (0.77 J/m2), which commonly exists in α-MnO2.35,61 In (110) surface slab, the Mn atom has two types of coordinate sites: pentacoordinate and hexacoordinate sites. Based on the DFT calculations, the adsorption complexes of Sb(III) and Sb(V) on the hexacoordinate site of (110) surface slab do not exist (data not shown). Thus, we present the adsorption complexes of Sb(III) and Sb(V) on the pentacoordinate site of (110) surface slab in this study. The optimized (110) surface slab was shown in Figure S9. Sb(III) forms two different adsorption complexes on (110), including the monodentate (M-1) and bidentate (M-2) complexes. As shown in Figure 7a, Sb(III) is monodentate on (110) and forms one Mn−O bond with the bond distance of 1.994 Å. The corresponding adsorption energy (Ead) is −2.31 eV. In Figure 7b, Sb(III) is bidentate on (110) and forms two Mn−O bonds. The bond distances of the two Mn−O bonds are 2.018 and 1.968 Å, respectively, with Ead of −2.70 eV. The Mulliken net charge of Sb(III) in M-1 and M-2 is +0.10 e and +0.13 e, respectively (details in Table S5), which clearly confirms partial Sb(III) was oxidized during the Sb(III) adsorption on (110), consistent with the XPS result. Meanwhile, Sb(V) also form monodentate (N-1) and bidentate (N-2) complexes on (110). In Figure 7c, Sb(V) is monodentate on (110) and forms one Mn−O bond with the bond distance of 1.948 Å. The Ead of this complex is determined to be −2.17 eV. In addition, the bidentate complex of Sb(V) is shown in Figure 7d. The bond distances of the two Mn−O bonds are 2.018 and 1.975 Å, respectively. The Ead of this bidentate complex is calculated to be −2.85 eV. And the Mulliken net charge remains identical before and after Sb(V) adsorption, indicating that no redox reaction occurred during Sb(V) adsorption (details in Table S6). The details of DFT calculated adsorption energies are summarized in Table 3. The Ead of bidentate complex of Sb(III) and Sb(V) is larger than their monodentate complex, which is reasonable and further confirms the existence of bidentate complex. Meanwhile, it is evidently that the Ead of both bidentate complexes is lower than monodentate complexes,

Figure 6. (a) Mn 2p3/2 and (b) O 1s XPS spectra of samples with three different treatments. Note: Olatt is lattice oxygen, Oads is surface adsorbed oxygen, and Oc is chemisorbed oxygen species.

confirming the existence of Sb on the MO-2.59 The Sb 3d3/2 region of the XPS data after Sb(III) adsorption indicate the existence of Sb(V) and the partial oxidization of Sb(III) to Sb(V).60 Furthermore, the O 1s spectrum can be decomposed into three individual components at BE = 529.7, 531.2, and 532.9 eV, which correspond to the surface lattice oxygen (Olatt), surface adsorbed oxygen (Oads), and chemisorbed oxygen species (Oc), respectively. The Oads/Olatt ratio of MO-2 is 0.16, whereas the Oads/Olatt ratio of MO-2 + Sb(III) and MO-2 + Sb(V) is 0.19 and 0.24, respectively, larger than that of the MO2. The results confirm that both Sb(III) and Sb(V) adsorb on the surface of MO-2. The difference in the Oads/Olatt ratio of the MO-2 + Sb(V) and MO-2 + Sb(III) is mainly due to Sb(III) coordinated with three −OH groups, while Sb(V) coordinated with six −OH groups. DFT Calculations. Because Sb(OH)3 and Sb(OH)6− are the dominant species of antimonite (Sb(III)) and antimonite (Sb(V)), respectively, under pH 6.0,56 hence, Sb(OH)3 and

Figure 7. Adsorption of Sb on optimized (110): (a) Sb(III) monodentate complex (M-1), (b) Sb(III) bidentate complex (M-2), (c) Sb(V) monodentate complex (N-1), and (d) Sb(V) bidentate complex (N-2). Antimony atoms are shown in purple, oxygen atoms are shown in red, manganese atoms are shown in green, and hydrogen atoms are shown in white. 2261


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Table 3. Adsorption Energy (eV) of Sb(III) and Sb(V) on MO-2(110) in the Water Model for Different Adsorption Structures structure

ESb+surf

Esurf

ESb(III)

M-1 M-2 N-1 N-2

−63551.305 −63551.695 −64909.396 −64910.087

−61595.577 −61595.577 −61595.577 −61595.577

−1953.419 −1953.419

which means the bidentate complexes is more stable than monodentate complexes. And the Ead value varies between the Sb(III) and Sb(V) in the same surface complexes (i.e., monodentate or bidentate complex). This difference is mainly attributed to the following reasons: (1) the different of molecular volume, (2) the slightly different bond distances to the (110), and (3) the difference of electron transfer in the different complexes. To reveal the nature of the interfacial bonding, the partial densities of states (PDOS) of Sb(III) and Sb(V) adsorption complexes on (110) were calculated and analyzed. After Sb(III) adsorbed on (110), the bonding peaks at −17.44 eV (labeled as I) and −4.94 eV (labeled as II) shift to lower energy levels, indicating M-1 and M-2 are the stable adsorption complexes. Further, the antibonding peaks (labeled as III) evidently decrease and shift to lower energy level, confirming Sb(III) is chemisorbed on the surface of (110) facet (Figure 8a−c).

ESb(V)

Ead

−3311.653 −3311.653

−2.31 −2.70 −2.17 −2.85

Figure 9. Partial density of states (PDOS) of Sb(V) and Mn for different surface complexes. (a) Sb(V), (b) Sb(V) (N-1) (monodentate complex), (c) Sb(V) (N-2) (bidentate complex), (d) Mn, (e) Mn (N-1) (monodentate complex), and (f) Mn (N-2) (bidentate complex). Note: The Fermi level (EF) is set at zero energy.

clearly identifies that the activity of Mn decreases but its stability increases and demonstrates the existence of chemireaction between Mn and Sb(V).

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CONCLUSIONS In summary, a novel α-MnO2 with very long and uniform nanofibers was first synthesized via phase transition from δMnO2 nanoparticles in present of GO. And the α-MnO2 exhibits a high adsorption capacity on Sb with the maximum adsorption capacity of 111.70 and 89.99 mg/g for Sb(III) and Sb(V), respectively. The adsorption can be conducted in a wide pH range of 1.0−12.0, and adsorption kinetic equilibrium can be reached within 30 min due to the novel morphology and excellent adsorption properties. Meanwhile, MO-2 shows good regenerated and reused properties. DFT calculations and analyses confirm that the existence of monodentate and bidentate complexes of Sb(III) and Sb(V) on (110), respectively. The PDOS analysis and DR model confirm that Sb(III) and Sb(V) are chemisorbed on the (110). The XPS analysis and Mulliken net charge further revealed that partial Sb(III) was oxidized to Sb(V) after Sb(III) adsorb on MO-2. The present work not only opens a new avenue to synthesize αMnO2 with novel nanofibers morphology and extends the application of α-MnO2 but also provides a new candidate for efficient decontamination of water containing Sb.

Figure 8. Partial density of states (PDOS) of Sb(III) and Mn for different surface complexes. (a) Sb(III), (b) Sb(III) (M-1) (monodentate complex), (c) Sb(III) (M-2) (bidentate complex), (d) Mn, (e) Mn (M-1) (monodentate complex), and (f) Mn (M-2) (bidentate complex). Note: The Fermi level (EF) is set at zero energy.

Moreover, in Figure 8d−f, the PDOS of Mn at Fermi level (labeled as IV) decreases from 0.31 eV to 0.12 (M-1) and 0.13 eV (M-2), respectively, after Sb(III) adsorbed on (110). The above result reveals the activity of Mn decreases, but its stability increase. And the results further verify that the chemi-reaction exists between Mn and Sb(III). Meanwhile, the PDOS result of Sb(V) adsorption on (110) is similar to that of Sb(III) adsorption. As shown in Figure 9a−c, the bonding peaks at −18.07 eV (labeled as I) and −5.88 eV (labeled as II) have slightly shift to lower energy levels after Sb(V) adsorbed on (110), confirming N-1 and N-2 are the stable adsorption complexes. The antibonding peaks (labeled as III) evidently decrease and slightly shift to lower energy level. These results confirm that Sb(V) adsorption on (110) is ascribed to chemisorption. In Figure 9d−f, the PDOS of Mn at Fermi level (labeled as IV) decreased from 0.39 eV to 0.11 (N-1) and 0.12 eV (N-2), respectively, after Sb(V) adsorbed on (110). It

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02583. Section 1, Figures S1−S9, and Tables S1−S6 (PDF) 2262


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.C.) *E-mail: [email protected] (J.Q.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate support from the Brook Byers Institute for Sustainable Systems, Hightower Chair and Georgia Research Alliance at the Georgia Institute of Technology. This work is also financially supported by the Major Program of the National Natural Science Foundation of China (No. 51290282) and the National Natural Science Foundation of China (Grant No. 51422813 and No. 51378490). The authors also appreciate the English editing by ChemWorx.

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