Heavy Metal Removal from Wastewater by a Polypyrrole-derived N-doped Carbon Nanotube Decorated with Fish Scale-like Molybdenum Disulfide Nanosheets

Haitao Zhang,1,# Xin Ding,1,# Shitao Wang,1,* Yan Huang,1 Xiao-Fei Zeng,1,* Srihari Maganti,2 Qinglong Jiang,3 Mina Huang,2, 4 Zhanhu Guo4 and Dapeng Cao1,*

1 State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China.

2 Advanced Materials Division, Engineered Multifunctional Composites (EMC) Nanotech LLC, Knoxville, TN 37934, USA.

3 Department of Chemistry and Physics, University of Arkansas, Pine Bluff, Arkansas 71601, USA.

4 Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, TN 37996, USA.

#Equally contributed to this work.

*Email: stwang@buct.edu.cn (S. Wang), zengxf@mail.buct.edn.cn (Zeng), caodp@mail.buct.edu.cn (D. Cao)

 

Abstract

Lead (Pb), duo to its toxicity, non-biodegradability and carcinogenicity, has huge harm to the environment and living beings. Therefore, developing an environmentally friendly material to remove lead from wastewater is of extraordinary significance for the health of human and other creatures. Here, we synthesized a novel N-doped carbon nanotube (derived from polypyrrole precursor) decorated with fish scale-like molybdenum disulfide nanosheet (C-Ppy@MoS2) by high temperature hydrothermal reaction and further explored the practical application for removal of Pb(II) from wastewater. Owing to the fish scale-like structure, the C-Ppy@MoS2 showed a maximum Pb(II) removal capacity of 381.87 mg/g, which is 50 times higher than that of the C-Ppy nanotube without MoS2 decoration. By one-time filtration removal experiment, it is found that the Pb(II) in the solution after filtration can be almost removed completely, which suggests that the C-Ppy@MoS2 is an extremely promising adsorbate for the efficient removal of Pb(II) from wastewater.

Table of Contents

Keywords: N-doped carbon nanotube; Fish scale-like structure; Removal of Pb((II); MoS2 nanosheet; Wastewater treatment.

 



1. Introduction

Currently, the heavy metal pollution in water causes serious environmental issues,[1-3] and severely affects the human health and living beings.[4-7] For example, Pb(II) is a notorious cumulative neurotoxin. The lead pollution in water is mainly from steel plant, battery factory and many other industries, and even low concentration level lead can cause serious health problems due to its non-degradable property and bioaccumulation.[8-11] Therefore, it is extremely significant for the removal of lead ions from contaminated water. Various methods have been adopted to remove heavy metal ions from wastewater, including membrane separation,[12,13] ion exchange,[14] chemical precipitation,[15] electrochemical deposition[16] and adsorption technology.[17-24] However, the operating complexity and expensive raw materials greatly limited the application of some techniques in wastewater treatment. Adsorption method shows some advantages for metal ion removal from dilute solutions owing to the high efficiency, design flexibility and operation convenience,[25,26] and it is therefore regarded as one of the most useful technologies.

Finding a kind of highly efficient adsorbent is urgently required, but still a huge challenge. Nano-adsorbents with many unique physical and chemical properties, like the strong operability, high stability and large specific surface area (SSA), have been developed.[27-34] Further modification of nanomaterials by introducing the sulfur-containing groups can enhance the adsorption and removal ability of heavy metal ions. Moreover, this structure of porous nanosheets can improve the SSA and provide more adsorption sites for the metal ions to increase adsorption performance.[35] Recently, the layered transition metal sulfides attract more and more attention because of the sulfur-functional group.[36-39] Moreover, their low cost, the control of valence states and high thermal stability make them a promising candidate for metal ion removal.[28,40-44] The investigation from Kong et al. indicated that the Pb(II) adsorption amount of iron sulfide (Fe3S4)/reduced graphene oxide composites reached 285.71 mg/g.[2] Jawad et al. prepared layered double hydroxide (LDH)-based iron-tetrathiomolybdate (Fe-MoS4) and its Pb(II) adsorption amount was 346 mg/g.[40] Manos et al. reported that the Pb(II) adsorption amount of the layered sulfide material K2xMnxSn3xS6 reached 329 mg/g.[45] Youngtak et al. used porous Zn2SnxS2x+2 (x = 1, 2, 4) as an adsorbent and found that its Pb(II) adsorption amount was 331 mg/g.[46] Among transition metal sulfides, the molybdenum sulfide (MoS2) is an emblematic two- dimensional (2D) material,[47] where each layer consists of one molybdenum sheet sandwiched by two sulfur sheets, forming a S-Mo-S layered structure with six S atoms coordinating with a Mo atom in a trigonal prismatic geometry.[48,49] The MoS2 is an excellent candidate, due to its special layered structure and forceful combination with heavy metal ions by metal chalcogenides,[50] because S exhibits a strong affinity for soft Lewis acidic metal ions.[51-53] Tong et al. synthesized MoS2/CeO2 nanohybrids as adsorbents to obtain the capacity of 333 mg/g for Pb(II).[54] Aghagoli et al. reported MoS2-rGO composite and its adsorption capacity reached 322 mg/g for Pb(II).[55] Zhu et al. also found that the Pb(II) capacity of MoS2/ biochar reached 189 mg/g.[56] Actually, all these adsorbents with sulfur-functional groups or sulfur edge sites show excellent selectivity for Pb(II) owing to the strong sulfur-lead interaction. Therefore, if the sulfur edge sites can be exposed more efficiently by increasing the SSA of adsorbents, the adsorption amount of adsorbents for Pb(II) would be improved significantly. Based on the fractal concept from mathematics, if the ultra-flaky MoS2 nanosheets can be used to decorate porous carbon nanotube (CNT), which can significantly increase the SSA of the MoS2-modified CNT adsorbent and therefore improve its adsorption amount of Pb(II) efficiently.

Here, we use a template method to synthesize the CNTs decorated with fish scale-like MoS2 nanosheets (marked as C-Ppy@MoS2), and investigate the performance of the synthesized C-Ppy@MoS2 sample for Pb(II) removal from wastewater. Then, the effects of different parameters (like adsorption time, adsorbent dose, metal ion concentration) on the adsorption performance of C-Ppy@MoS2 are explored systematically.

 

2. Experimental

2.1 Sample preparation

Fig. 1A shows the whole synthesis process by using a template method, which is divided into five steps.[57-59] First, we adopted a hydrothermal method to synthesize MoO3 nanorods (Fig. 1A-1) and further used one-pot oxidative polymerization to synthesize MoO3@Ppy (Fig. 1A-2) with a core-shell structure. Then, the MoO3 in the core-shell structure was removed by NH3·H2O (28%) solution to obtain Ppy nanotubes (Fig. 1A-3)[60] and the Ppy nanotubes were subsequently carbonized under an inert atmosphere to obtain N-doped C-Ppy nanotubes (Fig. 1A-4). Finally, the N-doped C-Ppy and (NH4)2MoS4 were dispersed in dimethylformamide (DMF), the filtrated powder was carbonized at high temperature to synthesize C-Ppy@MoS2 (Fig. 1A-5). Detailed synthesis steps are also presented in Supporting Information.

 

2.2 Quantitative Pb(II) uptake studies

Pb(NO3)2 was selected as a representative pollutant for the adsorption study due to its high solubility. The adsorption process was carried out in a fixed shaker. First, the solution with a maximum lead ion concentration of 1000 ppm was prepared using ultra-pure water, and the lower concentration solution could be obtained by subsequent dilution. Batch experiments were performed in 250 mL volumetric flasks under stirring (150 rpm). The effects of different variables, for example, adsorbent dose (5-30 mg), initial concentration (0-110 ppm), contact time (0-1440 min) on adsorption performance of Pb(II) were explored. Generally, a certain amount of sample was added to a 250 mL volumetric flask containing 100 mL of heavy metal solution. After the adsorption process was performed for a given period of time, the mixture was filtered through a nylon membrane filter containing 0.22 μm pores with a medical syringe, and the resulting solution was collected and its Pb(II) concentration was further analyzed by using the inductively coupled plasma-mass spectrometry (ICP-MS) (Agilent Technologies, 7700x (G3281A).

For adsorption kinetics, 10 mg of the as-prepared C-Ppy@MoS2 was dispersed in 100 mL solution containing 22.90 ppm Pb(II). The adsorption isotherm was obtained by adding 10 mg of C-Ppy@MoS2 in 100 mL solutions containing different initial Pb(II) concentrations (0-110 ppm). The amount of lead ion adsorption and the removal efficiency were calculated by the following two equations:

                    (1)                             

             (2)             

where  (mg/g) is the adsorption capacity of heavy metal,  (mL) is the volume of solution,  and  (mg/L) is the initial and equilibrium concentrations of Pb(II), respectively, and  (mg) is the mass of adsorbents.

 

3. Results and discussion

3.1 Characterization

The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the products in each step of the synthesis process are shown in Fig. 1B. MoO3 nanorods exhibit smooth surface and uniform diameter (Figs. 1B(1-1) and 1B(1-2)). After polymerization, the surface of MoO3 is coated by Ppy and the TEM image shows the formation of the translucent substances around the MoO3 nanorods, i.e. the formation of the core-shell structure of MoO3@Ppy (Fig. 1B(2-2)). The hollow tubes can be observed after the ammonia treatment of MoO3@Ppy (Figs. 1B(3-1) and (3-2)), and the tubes become more transparent and the tube wall becomes thinner after carbonization (Figs. 1B(4-1) and (4-2)). After C-Ppy@MoS2 nanotube was synthesized, the fish scale-like MoS2 nanosheets are evenly distributed on the surface of C-Ppy nanotubes (Figs. 1B(5-1) and 1B(5-2)).

Fig. 1 (A) The synthetic illustration of the C-Ppy@MoS2, (B) SEM and TEM images of MoO3 (1-1, 1-2), MoO3@PPy (2-1, 2-2), PPy (3-1, 3-2), C-Ppy (4-1, 4-2), C-Ppy@MoS2 (5-1, 5-2).

 

The X-ray diffraction (XRD) characterization was used to explore the crystal form of materials. As shown in Fig. 2a, all the identifiable sharp diffraction peaks prove the formation of the MoO3 nanorod crystals. After the pyrrole was polymerized on the surface of MoO3, the peak position and sharpness are almost the same, indicating that the pyrrole polymerization does not destroy the crystal structure of MoO3 nanorods. After the MoO3 nanorods are removed, the sharp peak disappears and only the characteristic peak of Ppy exists (Fig. 2b). After Ppy was carbonized, the degree of graphitization of C-Ppy was characterized by Raman spectrum (Fig. 2c), where the peak ratio of G-band (1560.6 cm-1) associated with graphite crystallinity and D-band (1361.2 cm-1) associated with carbon defects is equal to 1.103, indicating that the material is highly graphitized.

Fig. 2d shows that the C-Ppy@MoS2 sample possesses obvious diffraction peaks of MoS2 (JCPDS # 75-1539). The peak in the small-angle region, with a d spacing of 0.49 nm indexed to (002) reflections,[53,54] confirms the formation of the MoS2 nanosheet. In addition, the peaks attributed to (100) and (110) planes of 2H-MoS2 in the high-angle region can be found obviously, and the (103) plane could be readily indexed as hexagonal MoS2. By comparing Raman spectra of C-Ppy (Fig. 2c) and C-Ppy@MoS2 (Fig. 2e), it is found that an apparent characteristic peak of MoS2 appears in Fig. 2e.

Raman characterization is also used to investigate the number of layers of each piece of C-Ppy@MoS2.[61,62] A more detailed structure of the MoS2 peak in the high resolution is shown in Fig. 2f. Based on the Raman mode () difference between the in-plane  and the out-plane , where the  mode is from the opposite vibration of Mo and the two S atoms on either side, and the opposite vibration of the S atom in the opposite direction leads to the occurrence of the  mode. Therefore, the layer number of the material can be determined basically. With the increase of the layer number, the  mode shows a red-shift while the  mode has a blue-shift. The peak frequency difference for bulk MoS2 is 26 cm-1 approximately while it is 24 cm-1 for C-Ppy@MoS2, indicating that the C-Ppy@MoS2 comprises 3-5 layers of MoS2.[63] The fish scale-like MoS2 nanosheets exhibit a layered structure, which significantly increases the SSA of C-Ppy nanotube and provides more exposed sites for lead adsorption.

Fig. 2g clearly shows two peaks of Mo and S elements, suggesting its tubular distribution. The span of elemental distribution is about 200 nm, which is consistent with the tubular size from TEM images. The results further confirm that the sample is C-Ppy nanotube decorated with fish scale-like MoS2 nanosheets. The N2 isotherms of C-Ppy@MoS2 at 77 K were measured. The BET SSA C-Ppy@MoS2 reaches 181.95 m2/g (Fig. 2h), indicating that it contains rich micropores for lead adsorption (Fig. 2i). The detailed parameters are shown in Table S1.

Fig. 2 Structure characterization of as-synthesized samples. (a) (b) XRD pattern of MoO3, MoO3@PPy and Ppy, (c) Raman spectra of C-Ppy, (d) XRD pattern of C-Ppy@MoS2, (e) Raman spectra of C-Ppy@MoS2, (f) Magnified Raman spectra of bulk MoS2 and C-Ppy@MoS2, (g) Normalized line scan of C-Ppy@MoS2, (h) N2 adsorption/desorption isotherms of C-Ppy@MoS2, (i) Pore size distribution of C-Ppy@MoS2.

 

3.2 Batch Pb(II) absorption performance

3.2.1 Effects of adsorbent dose, initial concentration and adsorption time

Fig. 3a shows the effect of adsorbent dose on the Pb(II) adsorption in C-Ppy@MoS2. The as-synthesized samples with different doses (5-30 mg) were putted into lead nitrate solutions with the same volume (100 mL) of 54.26 ppm. All the adsorption experiments were conducted at T=25 ℃ for 24 h. As shown in Fig. 3a, the removal efficiency can reach 82.6% with increasing the adsorbent dose from 5 to 15 mg. When the adsorbent dose is increased to 20 mg, the removal rate reaches almost 100% and does not change anymore. The above results indicate that as long as the adsorbent C-Ppy@MoS2 is excessive, Pb(II) in wastewater can be removed almost completely.

Adsorption experiments of different concentrations of Pb(II) solutions (0-110 ppm) were performed, where the volume of Pb(II) solutions and the adsorbent doses were fixed at 100 mL and 10 mg, respectively. With the increase of initial Pb(II) concentration (i.e. the increase of adsorbate Pb(II) content), the adsorption capacity of Pb(II) in C-Ppy@MoS2 increases, as shown in Fig. 3b. After the initial Pb(II) concentration increases to 61.04 ppm, the adsorption capacity of C-Ppy@MoS2 for Pb(II) keeps unchanged, which means that the adsorption capacity of C-Ppy@MoS2 for Pb(II) reaches its maximum (here, it is 381.87 mg·g-1). When the initial Pb(II) concentration is increased sequentially, the adsorption capacity of Pb(II) in C-Ppy@MoS2 would be unchanged, i.e. the adsorbates are greatly excessive in this case. Interestingly, as a comparison, the maximum adsorption capacity of Pb(II) in bulk MoS2 (Fig. S2) and C-Ppy (Fig. 3c) is only about 5~10 mg·g-1. Apparently, the excellent performance of C-Ppy@MoS2 for Pb(II) adsorption and removal from wastewater are ascribed to the special structure of the fish scale-like MoS2 nanosheets on the C-Ppy nanotubes.

Fig. 3c shows the capacity of Pb(II) at different time intervals, which can reflect the adsorption kinetics of as-synthesized samples. In Fig. 3c, the rapid adsorption efficiency is shown within the first 200 minutes for C-Ppy@MoS2, and then the adsorption slowly reaches equilibrium, and finally the removal rate also can reach 100%. The initial adsorption rate is very fast because the adsorbents are greatly excessive and more adsorption sites are available. With the increase of time, the adsorptive sites were occupied gradually. When all the adsorptive sites were occupied entirely, the adsorption capacity of the absorbents would reach the maximum. Figs. 3d-3f show the removal rate and Pb(II) concentration changes with times at three different doses, where the amount of adsorbents obviously affects the adsorption kinetics. When the amount of adsorbent is 10 mg, it needs about 400 min to reach adsorption equilibrium (Fig. 3d), while when it increases to 30 mg, the adsorption process of Pb(II) will be finished basically in 60 minutes (Fig. 3e). When the adsorbent is increased to 50 mg, the adsorption process only costs about 10 minutes (Fig. 3f). However, the adsorption capacity of C-Ppy for Pb(II) has no significant change with time and concentration, and the adsorption efficiency also has no change (Figs. S3 and 3c). The comparison of C-Ppy and C-Ppy@MoS2 for Pb(II) adsorption indicates that the presence of MoS2 indeed affects the adsorption process, where the C-Ppy nanotubes only serve as a support, as expected.

Fig. 3 Adsorption properties of as-synthesized samples. (a) Removal efficiency of Pb(II) by C-Ppy@MoS2 of different doses, (b) Adsorption capacity of C-Ppy@MoS2 for Pb(II), (c) Adsorption capacity of Pb(II) in C-Ppy@MoS2 and C-Ppy changing with time, (d), (e), (f) Removal rate and concentration changes of Pb(II) by different doses of C-Ppy@MoS2 (10, 30, 50 mg).

 

3.2.2 One-time filtration removal experiment

In order to explore the applicability of C-Ppy@MoS2,[64] we filtered 50 mL Pb(II) solution with 20 ppm concentration by using C-Ppy@MoS2-made cake-like material. The operation diagram was shown in Fig. S4. The Pb(II) in the solution after passing the filtration cake (50 mg) can be almost removed completely. The filtration process is about several minutes. This observation indicates that the as-synthesized C-Ppy@MoS2 sample is an extremely promising adsorbent for the removal of Pb(II) from wastewater.

In addition, we also used Langmuir, Freundlich[65] and Temkin models[66] to investigate the thermodynamics and adsorption mechanism of Pb(II) in the as-synthesized sample(Fig. S5). Results indicates that the Langmuir model can satisfactorily depict the adsorption behavior of Pb(II), which suggests that it is mainly monolayer adsorption. The kinetic equations were also used to explore its adsorption kinetics process.[67] The detailed information is presented in Section S4 in supporting information.

3. 3 Reaction of C-Ppy@MoS2 with Pb(II)

X-ray diffraction analysis of the sample after adsorption reveals that its XRD is similar to the standard XRD pattern of PbMoO4, as shown in Fig. 4a, where the extra peaks may originate from the presence of other phases. The peak at the position of 28.7° (marked with ) indicates that the PbS may be formed due to the strong binding of S to Pb. A single tube for the surface sweep can further prove the substance type of the white block (Fig. S6). The multiple lead energy peaks appear in the corresponding X-ray photoelectron spectroscopy (XPS) spectrum (Fig. S7), and the intensity of S in the XPS may correspond to the inherent selectivity of chalcogenides for Pb(II) ions, where the S atoms replace a part of the O atoms in the PbMoO4. In short, the new type of compound PbMoO4−xSx may be formed after adsorption.

Figs. 4b and 4c show lots of white irregular multifaceted crystal, which can explain the emergence of new substances. At the edge, irregular white crystals were produced, and there were also smaller white blocks in the wall of the tube. The edges of the sheet material are wrinkled, which is similar to the C-Ppy@MoS2. Therefore, we believe that in the adsorption process, the fish scale-like MoS2 provided the adsorption sites and most MoS2 was transferred to PbMoO4−xSx. As a result, the uniform fish scale-like morphology on the surface of nanotubes becomes gully-like bark structure.

Fig. 4 Characterization of C-Ppy@MoS2 after adsorption. (a) XRD data and (b, c) SEM images.

The X-ray photoelectron spectroscopy (XPS) was also used to investigate the composition and the states of the elements in the Pb(II)-loaded adsorbents. Fig. S7 shows the full-range scale spectrum of C-Ppy@MoS2 after adsorption with loading Pb(II). Compared to the binding energies of virgin C-Ppy@MoS2, an obvious peak about Pb 4f appears. A more detailed structure of the Pb 4f species was obtained in the high-resolution spectrum (Fig. 5a), where the doublet peaks at 143.84 and 138.95 eV indicate that an energy separation of 4.89 eV is attributed to 4f7/2 and 4f5/2 of Pb(II) in PbMoO4−xSx. As shown in Fig. 5b, the XPS peak at 531.47 eV is in agreement with the binding energies of O 1S. In Fig. 5c, the S 2p XPS peaks at 161.71 eV and 162.97 eV are consistent with the binding energies of S 2p1/2 and S 2p3/2 related to PbMoO4−xSx. Furthermore, the scan in the Mo 3d region specificity shows two peaks at 235.30 and 232.19 eV, which correspond to the types of oxidation states for MoO42− (Fig. 5d). Moreover, it is found that hydrogen gas was generated (from 0 ppmv to 37.2 ppmv) and was detected by a high purity gas analyzer after adsorption test. So, we believe that the protons may have a driving effect in the adsorption process. Based on the above analysis on characterization data, we proposed a possible mechanism[68,69] to present Pb(II) adsorption on the C-Ppy@MoS2, given by:

Mo(IV) + 2H+ + 4e- + 4H2O (aq) → 5H2 (g) + MoO42-(VI)      (3)

   MoO42-(VI) + Pb2++S→ PbMoO4-xSx (s)            (4)                            

 Fig. 5 XPS data to support the formation of PbMoO4−xSx. (a) Pb 4f, (b) O 1S, (c) S 2p, and (d) Mo 3d.

4. Conclusion

In summary, the fish scale-like C-Ppy@MoS2 was successfully synthesized by the combination of template strategy and hydrothermal method. Owing to the fish scale-like special structure to provide abundant active sites, the C-Ppy@MoS2 shows an excellent performance for Pb(II) removal from wastewater. Results indicate that the C-Ppy@MoS2 with fish scale-like structure shows a maximum Pb(II) adsorption capacity of 381.87 mg/g. Actually, the material morphology changed significantly after the adsorption of Pb(II), because new crystals were formed due to the innate affinity of chalcogen for Pb(II) ions, and the possible mechanism was also proposed. By one-time filtration removal experiment, it is found that the Pb(II) in the solution after filtration can be almost removed completely. The excellent Pb(II) adsorption performance of as-synthesized materials is attributed to its special structure of the fish scale-like MoS2 nanosheets on the C-Ppy nanotubes. In short, this work suggests that the fish scale-like C-C-Ppy@MoS2 is an excellent absorbent for efficient removal of metal ions from wastewater.

 

Acknowledgements

This work is supported by Outstanding Talents Plan from BUCT and Open Fund of State Key Laboratory of Inorganic-Organic Composites, and the Fundamental Research Funds for the Central Universities (buctrc201918).

 

Conflict of Interest

There is no conflict of interest.

 

Supporting Information

Applicable.

 

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Author Information

Shitao Wang obtained B.Sc. degree from Zhengzhou University in 2010 and his M.Sc. degree in 2013 from Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences. He received his Ph.D. degree from The University of Tokyo in 2016 under the supervision of Prof. Makoto Fujita. After a short period of postdoctoral research in the same group, he joined Prof. Virgil Percec’s group at the University of Pennsylvania as a postdoctoral researcher (2017-2018), supported by the JSPS research fellowship. Since 2018, he is an associated Professor at Beijing University of Chemical Technology. His current research interests focus on the porous materials and their environment and energy related applications.

Xiaofei Zeng is a Professor of the State Key Laboratory of Organic Inorganic Composites at Beijing University of Chemical Technology (BUCT) (2001- ). She got her PhD from BUCT in 2010 under the supervision of Professor Jianfeng Chen, and was a visiting scholar at University of New South Wales in Australia (2013-2013). Her research interests are focused on the synthesis of polymer-based energy composites, photoelectric materials and their applications in engineering.

Dapeng Cao is a Fellow of the Royal Society of Chemistry (FRSC), and a Professor of State Key Laboratory of Organic Inorganic Composites at Beijing University of Chemical Technology (BUCT) (2005- ). He received his Ph D degree from BUCT in 2002, and was a research scientist at NanoMaterials Technology Pte. Ltd. in Singapore (2002-2003) and a postdoctoral researcher at the University of California at Riverside (2003-2005). His research interests are focused on the designed synthesis and applications of the porous carbon/organic materials and the energy/catalysis materials.

 

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