Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 447−456
pubs.acs.org/journal/ascecg
Recyclable Nanocomposite of Flowerlike MoS2@Hybrid Acid-Doped PANI Immobilized on Porous PAN Nanofibers for the Efficient Removal of Cr(VI) Jinli Qiu,† Fuqiang Liu,*,†,‡ Song Cheng,† Lidan Zong,† Changqing Zhu,† Chen Ling,† and Aimin Li†,‡ †
State Key Laboratory of Pollution Control and Resources Reuse, School of the Environment, Nanjing University, Nanjing 210023, P. R. China ‡ State Environmental Protection Engineering Center for Organic Chemical Industrial Waste Water Disposal Resource Reuse, Nanjing 210023, P. R. China S Supporting Information *
ABSTRACT: For the preparation of an ideal material for the emergency disposal of Cr(VI)-contaminated water, a recyclable nanocomposite of molybdenum disulfide@hybrid acid-doped polyaniline immobilized on porous polyacrylonitrile nanofibers (MoS2@PANI/PAN nanocomposite) was synthesized successively through electrospinning, a hydrothermal process, and in situ polymerization. The advantages of the flowerlike MoS2@ PANI/PAN nanocomposite include a fast equilibrium rate (30 min), a high removal capacity (6.57 mmol/g), and good recyclability. Thus, the material’s performance is much better than those of many earlier materials developed for similar applications. MoS2@PANI/PAN nanocomposite still showed high adsorption capacity toward Cr(VI) in the presence of coexisting anions and natural organic matter. The remarkable interaction of this material with Cr(VI) was shown to mainly occur through ion exchange, electrostatic adsorption, and reduction. Furthermore, the MoS2@PANI/PAN nanocomposite could be thoroughly regenerated and successfully reused for more than six cycles without a significant decrease in its removal capacity. Additional favorable properties of MoS2@PANI/PAN nanocomposite are its strong hydrophilic performance and high mechanical strength. Hence, MoS2@PANI/PAN provides a green, economic, and fast method to efficiently remove toxic Cr(VI) from aqueous environments. KEYWORDS: Nanofibers, Cr(VI), Polyaniline, Molybdenum disulfide, Ion exchange, Reduction
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INTRODUCTION
Among the technologies extensively employed to eliminate or rapidly minimize the potential effects of water pollution accidents associated with Cr(VI) are reverse osmosis,7,8 chemical oxidation/degradation,9−12 and adsorption,13,14 which is the most suitable in terms of removal efficiency, economic feasibility, and the avoidance of secondary pollution. In acidic media, Cr(VI) species exist primarily as anions and can thus be rapidly removed through electrostatic interactions and ion exchange using modified adsorbents, preferably those that can be recycled readily. Materials made up of small compounds offer a relatively large number of adsorption sites that promote the removal process. Nevertheless, materials made up of microspheres and nanoparticles are limited practically by their time-consuming and complex separation procedures. The synthesis of microspheres such as resins and carbon spheres is a lengthy process that includes the
The high frequency of water pollution accidents all over the world not only has harmful effects on the environment and human health but also causes enormous economic losses.1−3 Most contaminants contain a complex range of heavy metals and organic compounds, and their removal is therefore often challenging. At hazardous waste sites, chromium is the second most abundant inorganic groundwater contaminant.4 Once chromium ions enter water resources, such as drinking water and rivers, they diffuse rapidly to cause serious and widespread damage. In aqueous media, chromium primarily exists as Cr(VI) and Cr(III). While at trace levels Cr(III) is a nutrient, Cr(VI) is toxic, soluble, and carcinogenic. The toxicity of Cr(VI) is 10−100 times higher than that of Cr(III).5 In 2016, the Environmental Working Group evaluated the water systems of the United States and found that the levels of carcinogenic Cr(VI) in tap water exceeded 0.02 ppb.6 Accordingly, there is great interest in the development of effective methods to control Cr(VI). © 2017 American Chemical Society
Received: August 9, 2017 Revised: October 31, 2017 Published: November 23, 2017 447
DOI: 10.1021/acssuschemeng.7b02738 ACS Sustainable Chem. Eng. 2018, 6, 447−456
Research Article
ACS Sustainable Chemistry & Engineering Scheme 1. Synthetic Route of the MoS2@PANI/PAN Nanocomposite
composite material can be employed in the cleanup of water pollution involving Cr(VI) contamination. Flowerlike MoS2@ hybrid acid-doped polyaniline immobilized on porous polyacrylonitrile nanofibers (MoS2@PANI/PAN nanocomposite) was synthesized using PAN nanofibers as the substrate to facilitate recycling, and the loading of MoS2 and PANI nanoparticles onto the fibers to greatly enhance Cr(VI) removal. The mechanism of removal was then investigated using X-ray photoelectron spectroscopy (XPS), ζ potential measurement, and Discrete Fourier Transform (DFT) calculation.
modification of functional groups and the attainment of a useful removal equilibrium.15−18 Nanoparticles can remove heavy metals quickly, but they are not suitable for circulating use and are prone to aggregation.19,20 Despite simplified separation procedures, magnetic nanoparticles continue to exhibit difficulties in their modification and unsatisfactory removal efficiencies due to the shortcomings of aggregation.21 In the past few years, nanofibers have become increasingly appreciated materials owing to their high removal capacity, fast rate of equilibrium, and ease of recycling.22−26 Moreover, composite materials that include nanofibers can be easily modified with functional groups and combined with other types of materials.27 Well-defined fibers can be produced by electrospinning, in which a high voltage is applied to a viscous solution of a polymer. The polymer is then stretched to yield sticky filaments that at a certain density stack naturally to form a membrane. The unique morphology of this membrane facilitates recycling of its component nanofibers. In addition, the small size of the nanofibers results in a large surface area and therefore a large number of active sites or sites that can be modified with functional groups. The high removal capacity, fast removal rate, and good recycling performance of nanofibers or nanofiber-based materials are byproducts of these properties. One of the polymers most commonly used in conjunction with nanofibers is polyacrylonitrile (PAN), because of its high tensile strength, thermal and mechanical stabilities, and resistance to solvents and abrasion. The conductive polymer polyaniline (PANI) is suitable for the removal of heavy metals because of its excellent environmental stability and ease of synthesis.28 Its large numbers of amine and imine functional groups result in a strong affinity for many heavy metals. Molybdenum disulfide (MoS2), a graphite-like structure consisting of three stacked layers of atoms, is often employed as a photocatalyst because of its optical and electrical properties.29 However, its large surface area, low cost, and positive charge make MoS2 a feasible adsorbent for Cr(VI) ions, although the adsorption performance of MoS2 has not been well-studied. However, when used in aqueous media PANI and MoS2 nanoparticles may cause toxicity, due to their poor recyclability; these materials thus require a suitable substrate to avoid their release into the surrounding medium. Here we describe the use of MoS2 and PANI nanoparticles in combination with PAN nanofibers to achieve the fast and efficient removal of Cr(VI). This newly developed, recyclable,
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EXPERIMENTAL SECTION
Materials. Polyacrylonitrile (PAN, Mw = 150 000) and polyethylene glycol (PEG, Mw = 20 000) were purchased from SigmaAldrich and sulfuric acid (H2SO4, 98%), N,N-dimethylformamide (DMF), potassium dichromate (K2Cr2O7), and ethanol from Shanghai Chemical Industrial Co., Ltd. Aniline (C6H7N, An), ammonium persulfate [(NH4)2S2O8, APS], thioacetamide (C2H5NS, TAA), sodium molybdate dihydrate (H4MoNa2O6, AMT), silicotungstic acid hydrate (H6O41SiW12, STA,), and p-toluene sulfonic acid (C7H8O3S, TSA) were purchased from Macklin (Shanghai, China) Biochemical Technology Co., Ltd. Milli-Q water was used in all experiments. Synthesis of the MoS2@PANI/PAN Nanocomposite. A 15 wt % homogeneous polymer blend solution was obtained for electrospinning by dissolving a mixture of polymer powders of PAN and PEG (weight ratio of 1:2.5) in DMF under magnetic stirring for 24 h at room temperature. The solution was then loaded into a glass syringe and connected to a high-voltage power supply. PAN/PEG nanofibers were prepared by providing 15 kV between the cathode and anode at a distance of 15 cm; the feeding rate of electrospinning was 1.5 mL/h. The PAN/PEG nanofibers were then immersed in water for 24 h to remove the PEG and dried in a vacuum oven overnight at 333 K to obtain porous PAN nanofibers. MoS2 nanoparticles were synthesized through a simple hydrothermal treatment process.30 Briefly, TAA, AMT, and STA at a weight ratio of 1:1:1 were mixed with 50 mL of a 50% (v/v) aqueous ethanol solution under vigorous stirring for 1 h, followed by hydrothermal treatment at 473 K in a Teflon-lined stainless-steel autoclave for 24 h. The product was then washed with ethanol and with water and the pure product finally dried overnight in a vacuum oven at 333 K. The core−shell structure of MoS2@PANI/PAN nanocomposite was synthesized through one-step, in situ polymerization as follows: H2SO4 (7.0 mmol) and TSA (40 mmol) were added into 100 mL of an ethanol/water (50:50, v/v) solution. A 50 mg portion of porous PAN nanofibers was then immersed in this solution, followed by the 448
DOI: 10.1021/acssuschemeng.7b02738 ACS Sustainable Chem. Eng. 2018, 6, 447−456
Research Article
ACS Sustainable Chemistry & Engineering addition of MoS2 (1.2 mmol) and aniline monomer (20 mmol). The mixture was stirred for 30 min, after which 13.5 mmol of APS was added dropwise. After 8 h of incubation under continuous magnetic stirring at 273 K, the resulting dark-green MoS2@PANI/PAN nanocomposite was filtered and the filtrate washed successively with ethanol and water until it became colorless. The product was then dried in a vacuum oven overnight at 333 K. The synthesis of MoS2@ PANI/PAN nanocomposite is schematically illustrated in Scheme 1 and the final product in Figure S1. Characterizations of the MoS2@PANI/PAN Nanocomposite. Cr(VI) solutions of varying concentrations were prepared and then confirmed by inductively coupled plasma atomic emission spectroscopy (iCAP 8000). The morphology of the nanocomposite and of the other materials analyzed in this study was observed by field emission scanning electron microscopy (FESEM, SU8000, Hitachi) and transmission electron microscopy (TEM, JEOL). Fourier transform infrared (FT-IR) spectroscopy (NEXUS870) was used to analyze the functional groups of the materials. X-ray diffraction (XRD) patterns were recorded on a Philips X′ Pert Prosuper X-ray diffractometer using Cu radiation. Raman spectra were recorded at room temperature using a Horiba Jobin Yvon Xplo RA Raman spectrometer at an excitation wavelength of 532 nm and a 12.5 mW He−Ne laser source. N2 adsorption was measured on an ASAP 2020 accelerated surface area instrument (Micromeritics) at 90 K using Brunauer−Emmett−Teller (BET) calculations for surface area and pore structure analysis. XPS was performed on a VG Scientific ESCALAB Mark II spectrometer. The ζ potential of MoS2 was measured using a ZETASIZER Nano ZS instrument. The surface roughness of the materials was assessed by atomic force microscopy (AFM, Dimension Icon, Bruker) under the tapping mode. The contact angles of the materials were calculated using dynamic contact angle software (Drop Meter A100P) and the mechanical properties measured using the Electronic Pull and Push Strength Calculator (SH-20). Cr(VI) Removal Experiments. Solutions with different concentrations of Cr(VI) were prepared by dissolving K2Cr2O7 in Milli-Q water. A removal kinetic experiment was performed by adding 50 mg of as-prepared materials to 50 mL of a Cr(VI) solution (2.0 mmol/L). The removal isotherms and thermodynamics were determined on the basis of initial concentrations ranging from 1.0 to 24 mmol/L at three different temperatures (298, 308, and 318 K). The effect of pH 1.0− 6.0 on removal was studied at an initial Cr(VI) concentration of 2.0 mmol/L. The desired pH value of each sample was adjusted by adding negligible volumes of HNO3 or NaOH. The effects of coexisting substances on the Cr(VI) removal ability and reusability of the MoS2@ PANI/PAN nanocomposite were monitored for 24 h at 25 °C. The effects of co-occurring anions were determined using a series of solutions containing different molar ratios (1:0, 1:0.5, 1:1, and 1:2.5) of Cr(VI) (2.0 mmol/L) and NO3−, Cl−, SO42−, and PO43− (as the sodium salts) as the competing anions. The experiment was carried out at pH 3.0. For an investigation into the effect of the presence of humic acid (HA), solutions containing 2.0 mmol of Cr(VI)/L and different amounts of HA (5.0−250 mg/L) were prepared. The pH value of each mixture was adjusted to 3.0. All batch removal experiments were performed using a centrifugation speed of 180 rpm.
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the MoS2@PANI/PAN nanocomposite. The relationship between the amount of additive and the removal behavior of the resulting material was then investigated, optimizing the conditions by balancing the parameter values to obtain the maximum adsorption capacity. As shown in Figure S7, the optimal added amounts of MoS2, An, TSA, and APS were 1.2, 20, 40, and 13.5 mmol, respectively. Characterization of the MoS2@PANI/PAN Nanocomposite. The morphologies of the bare porous PAN nanofibers, MoS2, and PANI nanoparticles and of the MoS2@PANI/PAN nanocomposite are shown in Figure 1. As seen in the FESEM
Figure 1. SEM images of (a) bare PAN fibers, (c) MoS2, (e) PANI, and (g) MoS2@PANI/PAN nanocomposite. TEM images of (b) bare PAN fibers, (d) MoS2, and (f) PANI. (h, i) EDS spectrum and element mapping images of MoS2@PANI/PAN nanocomposite.
image (Figure 1a), the bare PAN nanofibers randomly overlapped and had a mean diameter of 244.37 ± 1.91 nm (Figure S8). The TEM image (Figure 1b) shows that the porous nanofibers were prepared successfully. The MoS2 nanoparticles (Figure 1c,d) adopted a 3D flowerlike architecture and had a mean size of 70.55 ± 0.63 nm (Figure S9). SEM (Figure 1e) and TEM (Figure 1f) revealed the synthesized PANI as rough nanorods with an average size of 63.84 nm (Figure S10) that form a cluster. The SEM images of the MoS2@PANI/PAN nanocomposite before and after Cr(VI) removal are shown in Figure 1g,h. Energy-dispersive X-ray spectroscopy element mapping (Figure 1i) confirmed that all elements, including Cr ions, adsorbed onto the MoS2@PANI/ PAN nanocomposite. The PAN nanofibers before (Figure 2a1,b1) and after (Figure 2a2,b2) modification were imaged using AFM to investigate the changes in surface roughness. Pristine PAN nanofibers had two separate phases whereas the MoS2@PANI/ PAN nanocomposite showed an obvious gradient-type transition between the fiber and matrix regions. The MoS2@ PANI/PAN nanocomposite had a higher surface roughness (Ra = 0.841 μm, Rq = 1.06 μm) than the bare PAN nanofibers (Ra = 0.411 μm, Rq = 0.598 μm) due to the formation of MoS2 and PANI nanoparticles on the surface. To some extent, the increase in the surface roughness from 0.411 to 0.841 μm was consistent with the change in the surface area and was confirmed by BET measurements (Table S1). According to the BET results, after modification of the pores by PEG, the surface area of the bare PAN nanofibers decreased
RESULTS AND DISCUSSION
Optimization of the MoS2@PANI/PAN Nanocomposite. The MoS2@PANI/PAN nanocomposite was prepared by electrospinning, a hydrothermal process, and in situ polymerization using low-cost and readily available raw materials. The conditions of the polymerization reaction were optimized using the response surface methodology.31,32 After the parameters to be optimized were selected, single-factor experiments were performed, varying only one parameter while keeping the others unchanged. The results of these experiments are shown in Figures S2−S6. According to the results, the range of parameter values was set at levels of low, middle, and high, after which different amounts of raw materials were added to prepare 449
DOI: 10.1021/acssuschemeng.7b02738 ACS Sustainable Chem. Eng. 2018, 6, 447−456
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ACS Sustainable Chemistry & Engineering
Figure 4. X-ray diffraction patterns of the MoS2@PANI/PAN nanocomposite and its components.
reaction occurred in the weak imine structure unit of the N atoms, followed by an inner molecular redox reaction that generated phenylenediamine dication free radicals. Separation of the charge interaction resulted in the formation of alternating structures of aniline and aniline cation radicals, with the charge on one of the N atoms delocalizing to the adjacent benzene ring and its para N atom through the conjugate effect, thereby reducing the CC bond in the benzene ring. The chemical environment of the N atoms in the molecular chain was homogenized, yielding an ordered structure after doping of the reaction with hybrid acid. For bare PAN nanofibers, the peak at 2θ = 18° corresponded to a linear structure and the two peaks at 23° and 26° to a disordered structure. The peaks at 2θ = 14.7°, 32.6°, and 35.7° were for MoS2 nanoparticles. The characteristic peaks of PANI and MoS2 were also seen in the XRD pattern of the MoS2@PANI/PAN nanocomposite. As shown in Figure 5, the dynamic contact angles of water and ethylene glycol for the MoS2@PANI/PAN nanocomposite and bare PAN nanofibers decreased rapidly (in seconds). The contact angles of both materials before and after modification were close to zero, reflecting the strong hydrophilic performance of the materials. The strong hydrophilic property of the MoS2@PANI/PAN nanocomposite favored Cr(VI) removal in aqueous media. The surface energies of the PAN fibers before and after modification were calculated according to eqs 1 and 2:27
Figure 2. AFM images of (a1, b1) bare PAN nanofibers and (a2, b2) MoS2@PANI/PAN nanocomposite (a1 and b1, morphology image; a2 and b2, phase image).
from 21.52 to 13.99 m2/g, but the pore diameter increased from 4.36 to 7.39 nm. This increase favors mass transfer and further accelerates Cr(VI) removal. Notably, as shown in Table S1, after modification with MoS2 and PANI nanoparticles, the surface area of the MoS 2 @PANI/PAN nanocomposite increased, providing a larger number of adsorption sites for Cr(VI) ions. In the FT-IR spectrum (Figure 3) of pure PANI, the strong adsorption bands at 1573, 1488, 1297, and 1240 cm−1 were
γl(1 + cos θ ) = 2(γlpγfp)1/2 + 2(γldγfd)1/2
(1)
γf = γfp + γfd
(2)
where γ is the surface energy, the subscripts l and f are the solvent and fiber, the superscripts d and p are the dispersive and polar components, respectively, and θ is the corresponding contact angle. There was little change in the surface energy of the PAN nanofibers before and after modification (101.7 and 100.5 mN/ m, respectively), due to the much smaller contact angles of both water and ethylene glycol. After modification with MoS2 and PANI nanoparticles, the breaking strength of the nanofibers decreased slightly, from 39.8 to 25.6 N/mm2, and the elongation at the break declined to 16%. The increase in the elasticity modulus from 150.5 to 164.2 MPa indicated the good flexibility of the MoS2@PANI/PAN nanocomposite (Table S2). Removal Behaviors. The species distribution of Cr(VI) at different pH values was calculated using the software Visual MINTEQ 3.0. The results are shown in Figure S11.33 The primary form of Cr(VI) at pH 3.0 is HCrO4−. The effects of pH
Figure 3. Fourier transform infrared spectra of the MoS2@PANI/PAN nanocomposite and its components.
assigned to the stretching vibration of CN bonds in NQ N, NBN, BNHB, and BNH+Q (Q, quinonoid ring; B, benzenoid ring), respectively, and the peak at 1001 cm−1 to the sulfo group (SO). The spectrum of bare PAN fibers exhibited a characteristic stretching vibration at 2243 cm−1, corresponding to the nitrile group (CN). The adsorption peak at 1731 cm−1 was assigned to the carbonyl group (CO) and the peak at 509.7 cm−1 to the MoS bond of the MoS2 nanoparticles. The presence of these peaks also in the FT-IR spectrum of MoS2@PANI/PAN confirmed the successful preparation of the nanocomposite. The XRD patterns (Figure 4) exhibited several sharp peaks characteristic of the analyzed materials. The sharp and remarkable peaks at 2θ = 15° and 25° represented the characteristic crystal structures of doped polyaniline. A proton 450
DOI: 10.1021/acssuschemeng.7b02738 ACS Sustainable Chem. Eng. 2018, 6, 447−456
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ACS Sustainable Chemistry & Engineering
Figure 5. Contact angles of (a, a′) the bare PAN nanofibers and (b, b′) the MoS2@PANI/PAN nanocomposite in water and ethylene glycol, respectively.
Figure 6. (a) Effect of pH on the removal of Cr(VI) ions by the MoS2@PANI/PAN nanocomposite. (b) Kinetics and (c) equilibrium curve for the removal of Cr(VI) ions by the MoS2@PANI/PAN nanocomposite.
Kinetic studies of Cr (VI) were performed at pH 3.0; the rapid dynamic removal process is shown in Figure 6b. The Cr(VI) removal capacity of the MoS2@PANI/PAN nanocomposite increased rapidly within a contact time of 30 min and then slowly until a removal equilibrium was reached. Pseudo-first- and pseudo-second-order kinetic models were used to explore the removal mechanisms.34 As shown in Table S3, the removal process was better fitted using the pseudo-firstorder kinetic model. Generally, materials with a nanoscale have a larger contact area than those of other sizes; accordingly, the nanosize of both the fibers and the particles would explain the fast removal process. Adsorption isotherm models were applied to describe the interaction behaviors of the liquid and solid phases.35 The
1.0−6.0 on the removal of Cr(VI) ions by MoS2@PANI/PAN nanocomposite was evaluated at the initial metal concentration of 2.0 mmol/L and an absorbent dosage of 1 g/L (Figure 6a). At low pH, the protonation of amino, sulfuric acid, and sulfonic acid groups caused the removal of negatively charged HCrO4− through ion exchange and electrostatic attraction. However, at pH 1.0, the removal performance decreased dramatically, because of the decomposition of the MoS2@PANI/PAN nanocomposite in the strongly acidic solution. At high pH, the surface charge of MoS2@PANI/PAN becomes negative, greatly weakening the electrostatic interaction between the nanocomposite and negatively charged Cr(VI) anions, thus decreasing the removal efficiency. 451
DOI: 10.1021/acssuschemeng.7b02738 ACS Sustainable Chem. Eng. 2018, 6, 447−456
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ACS Sustainable Chemistry & Engineering
Figure 7. Effect of (a) co-occurring anions and (b) humic acid (HA) on the removal of Cr(VI).
result also showed that at pH 3.0 the HA adsorbed on the nanocomposite can improve the removal Cr(VI) ions. Regeneration and Reusability. The results of the experiments on the effects of co-occurring substances showed that the prepared MoS2@PANI/PAN nanocomposite is highly selective for Cr(VI) ions and recommended its use in practical applications. However, given current environmental protection requirements and economic considerations, there is a need for materials that are chemically stable and reusable over several cycles. We therefore tested the MoS2@PANI/PAN nanocomposite for six cycles to investigate its reusability, keeping the initial concentration of Cr(VI) ions at 2.0 mmol/L in all removal tests. After each cycle, Cr(VI) and Cr(III) were desorbed using 0.5 M NaOH followed by a mixed acid solution (H2SO4 + TSA) with the amount added equal to that of the in situ polymerization reaction. Figure 8 shows the regeneration
Langmuir isotherm model is based on the hypothesis of monomolecular layer adsorption, with no interactions among adsorbed molecules. According to this model, the materials have equally active sites for the molecules. The Freundlich isotherm model suggests heterogeneous binding sites on the surface of materials. Both models were used to describe the equilibrium data of Cr(VI) ions adsorbed onto the nanocomposites. As seen in Figure 6c and Table S4, the experiment data were better fitted with the Langmuir isotherm model than with the Freundlich model. The maximum removal capacity was 6.57 mmol/g. Thermodynamic parameters, including the Gibbs energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) changes, were determined to reveal the energetic changes associated with the process of Cr(VI) removal. As shown in Table S5, the negative values of ΔG° indicated that removal was feasible and spontaneous; the decrease in ΔG° from −18.33 kJ/mol (T = 298 K) to −19.56 kJ/mol (T = 318 K) indicated that higher temperature facilitated the removal of Cr(VI), and the positive value of ΔH° indicated that the interaction process was endothermic. The positive value of ΔS° indicated the increasing randomness of the removal process. Effect of Co-Occurring Substances. In surface or underground water, Cr(VI) ions generally co-occur with other anions, such as NO3−, Cl−, SO42−, and PO43−. Since the Cr(VI) ions were adsorbed by the MoS2@PANI/PAN nanocomposite primarily through ion exchange and electrostatic interaction, other anions would potentially be strong competitors. We therefore investigated the selectivity of the MoS2@PANI/PAN nanocomposite. As shown in Figure 7a, interference by other anions was, in descending order, as follows: SO42− > PO43− > Cl− > NO3−, which can be attributed to their different hydrated radii and negative charges. Although the hydrated radii of SO42−, PO43−, and Cr(VI) ions are similar, the greater interference by SO42− than by PO43− reflected the occurrence of the latter at pH 3.0 as H2PO4−. In industrial wastewater or contaminated rivers, aside from inorganic anions, natural organic matter such as humic acid (HA) and fulvic acid are frequently present together with Cr(VI) ions. As shown in Figure 7b, the Cr(VI) removal capacity of MoS2@PANI/PAN increased slightly following the addition of HA. At the optimal pH for Cr(VI) removal of 3, HA (pKa = 4.5) remained undissociated and became hydrophobic, while the nanocomposite surface became positively charged. Thus, the nanocomposite attracted little undissociated HA by weak induction at pH 3.0,36 in contrast to the Cr(VI) removal mechanism of the nanocomposite (Figures S12 and S13). This
Figure 8. Regeneration performance and reusability of the MoS2@ PANI/PAN nanocomposite.
performance and reusability of the nanocomposite after six cycles. The slight decrease in removal capacity can be ascribed to incomplete desorption. Taken together, the results demonstrate the promise of the MoS2@PANI/PAN nanocomposite as a material for the fast removal of Cr(VI) from aqueous solutions, for example, in the emergency removal of heavy metal pollution. Moreover, the nanocomposite avoids the toxicity of MoS2 and PANI nanoparticles caused by exfoliation. Mechanisms Behind the Efficient Removal of Cr(VI). The XPS spectra of the nanocomposite before and after Cr(VI) removal are shown in Figure 9. Peaks corresponding to S 2p, Mo 3d, C 1s, O 1s, N 1s, and Cr 2p are seen in the survey scan spectrum. These were consistent with the results of FT-IR and 452
DOI: 10.1021/acssuschemeng.7b02738 ACS Sustainable Chem. Eng. 2018, 6, 447−456
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Figure 9. (a) XPS survey spectrum. (b) High-resolution XPS spectrum of Cr 2p. (c, d) N 1s before and after interaction of the nanocomposite with Cr(VI).
Figure 10. (a) Raman spectra and (b) schematic illustration of the coordination mode for Cr(III) on PANI (N−Cr−N).
Raman spectroscopy was also applied to explore the skeletal changes in the PANI nanoparticles on the nanocomposite. In Figure 10a, the peaks at 1596, 1563, and 1490 cm−1 represent CC and CC stretching vibrations, and the CN stretching vibration of the quinonoid ring, respectively. The strong peak at 1343 cm−1 corresponded to the C···N+ vibration of PANI.37 After the Cr(VI) ions were removed, the peak position of the PANI skeleton changed from 1300 to 1600 cm−1. The intensities of the peaks at 1596 and 1563 cm−1 increased, whereas that at 1343 cm−1 decreased because of the interaction between the Cr(VI) ions and PANI, in accordance with the XPS results. After the redox reaction, Cr(III) was presented on the solid phase probably as Cr2O3 or Cr(III) ions. Cr2O3 could retain in solid phase by deposition; however, it was possible for Cr(III) ions to release into the solution. To further explore whether Cr(III) ions on the nanocomposite were released into the aqueous solution by electrostatic repulsion, we
again confirmed the successful synthesis of the nanocomposite. From the spectrum of Cr 2p, the double peaks at the binding energies of 579.4 and 577.6 eV could be assigned to Cr(VI) and Cr(III), with molar ratios of 38.1% and 61.9%, respectively. The peak of Cr 2p3/2 made the presence of Cr(III) evident, indicating the reduction of a portion of the Cr(VI) loaded onto the solid phase by PANI in the nanocomposite. The highresolution spectra of N 1s before Cr(VI) removal could be separated into three peaks with binding energies of 398.5, 399.8, and >400 eV, corresponding to imine (N), amine (NH), and doped imine (NH*+) groups, respectively. After Cr(VI) removal, the NH*+ group disappeared, whereas the amounts of N and NH groups increased to 36.1% and 63.9%, respectively. This result was consistent with an increase in the proportion of the quinoid structures reflecting the oxidated state of the PANI molecular chains. 453
DOI: 10.1021/acssuschemeng.7b02738 ACS Sustainable Chem. Eng. 2018, 6, 447−456
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Figure 11. Schematic illustration of the Cr(VI) removal mechanisms.
Figure 12. Cr(VI) removal performance of (a) the MoS2@PANI/PAN nanocomposite and previously reported materials (indicated by the reference number) and (b) the individual components of the nanocomposite.
indicated that positive charges were present on the surface of MoS2 nanoparticles at pH 3.0, that is, when Cr(VI) ions occurred as HCrO4−, resulting in electrostatically mediated Cr removal. PANI and HCrO4− also underwent ion exchange, electrostatic interaction, and a redox reaction. First, Cr(VI) ions on the nanocomposite were enriched through ion exchange and electrostatic interaction, and then, the portion loaded by doped PANI participated in a redox reaction. The acidic condition also induced the redox reaction between HCrO4− and the electrondonor groups. After the redox reaction, the Cr(III) ions were able to combine with two of the nitrogen atoms in PANI chains by chelation, as verified by the DFT calculation.
analyzed the content of total chromium and Cr(VI) ions in the liquid phase by ICP-MS and UV−vis spectroscopy, respectively, and verified the absence of Cr(III) ions, which may instead have been immobilized on the nanocomposite through coordination with the imine (N) groups of PANI, because of the vacant d orbital of Cr(III). The coordination interaction of Cr(III) with PANI was verified using DFT. The total energies of a single Cr atom, PANI0 (oxidation state), and their complexation, as calculated using the Guass09 software, were −84.13, −1375.12, and −1460.41 (arbitrary units), respectively. The binding energy, calculated according to eq 3, was −794.43 kJ/mol. The details are given in Table S6. Chelation occurred between Cr(III) and PANI. The coordination mode for Cr(III) on PANI0 (N−Cr− N) is shown in Figure 10b. 0
3+
ΔE = E(complex) − [E(PANI ) + E(Cr )]
(3)
MoS2( +) + HCrO−4 → MoS2( +)/HCrO−4
(4)
Cr2O27 − + H+ → 2HCrO−4 + H 2O
(5)
PANI/H 2SO4 + HCrO−4 → PANI/HCrO−4 + SO24 −
During the synthesis of the MoS2@PANI/PAN nanocomposite, the PAN nanofibers provide a large space for PANI and the MoS2 nanoparticles a large surface area of the final material. These features further promoted the interaction between the active sites on the nanocomposite and Cr(VI) in aqueous solution. As noted above, Cr(VI) ions were loaded on the nanocomposite primarily through ion exchange, electrostatic adsorption, and redox interaction, as described by eqs 4−8 and in Figure 11. The ζ potentials shown in Figure S14
PANI/SO3H +
HCrO−4
→
PANI/HCrO−4
+
SO−3
(6) (7)
PANI (Reduction)/HCrO−4 → PANI (Oxidation)/Cr 3 + (8)
Comparison with Other Materials. Comparisons between the synthesized nanocomposite and previously reported materials are presented in Table S7 and Figure 12a. None of these other tested materials simultaneously satisfied the 454
DOI: 10.1021/acssuschemeng.7b02738 ACS Sustainable Chem. Eng. 2018, 6, 447−456
Research Article
ACS Sustainable Chemistry & Engineering
potential of MoS2@PANI/PAN in the emergency disposal of Cr(VI) in aqueous solutions.
requirements of a fast rate of equilibrium and high removal capacity, which are essential features for materials used in the emergency disposal of contaminants, and easy recycling. In terms of the removal rate, one to several hours were needed for most of the materials to reach removal equilibrium, which was far slower than the rate of the nanocomposite (0.5 h). Although some of the tested nanoparticles achieved complete removal within 1 h, the removal capacities were far lower than that of the MoS2@PANI/PAN nanocomposite (6.57 mmol/g), whose performance was within the optical area (Figure 12a) and attributable to the large surface area of the porous PAN nanofibers, which provide sufficient space for PANI and the MoS2 nanoparticles. Moreover, combining the fibers with nanoparticles improved the recyclability of MoS2 and PANI nanoparticles. The removal capacity of the MoS2@PANI/PAN nanocomposite was greater than those of any of the single components, and the growth proportion of the PAN fibers, MoS2, and PANI was 94.2%, 36.4%, and 54.1%, respectively (Figure 12b). However, the equilibrium times for single components and the nanocomposite are all in the range 0−1 h, which have no obvious difference due to the their small size. We also tested the efficacy of MoS2@PANI/PAN nanocomposite in groundwater and drinking water (Figure 13)
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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.7b02738. Photographic image; relationships between Cr(VI) removal capacity, MoS2@PANI/PAN polymerization, and several different variables; diameter distributions; surface area and pore data; kinetics data; species distribution; isothermal, thermodynamic, and geometrical parameters; ζ potentials; and comparisons with other materials (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. (F.L.) ORCID
Fuqiang Liu: 0000-0001-8753-1901 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the National Natural Science Foundation of P. R. China (Grant 51378253, 51708281 and 51522805), the Discipline Crossing Foundation of Nanjing and the Doctoral Candidate Innovation and Creativity Research Project of Nanjing University.
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REFERENCES
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Figure 13. Removal performance of MoS2@PANI/PAN nanocomposite in water samples spiked with 0.25 mg Cr(VI)/L, which is five times the maximum allowable concentration.
spiked with 0.25 mg Cr(VI)/L, which was five times the maximum allowable concentration. After the removal process, the concentrations in both spiked water samples were
