Role of structural characteristics of MoS2 nanosheets on Pb2+ removal in aqueous solution
Graphical abstract
Introduction
Rapid developments in the field of nanotechnology have triggered an increased tremendous interest in using unique structures, and excellent properties of two-dimensional (2D) nanomaterials to tackle environmental pollution issues. Molybdenum sulfide (MoS2), as a newly emerging 2D nanomaterial, is being researched to remove various environmental pollutants from aqueous solutions including: heavy metal ions (Li et al., 2019, Wang et al., 2018, Yadav et al., 2020, Zhan et al., 2019), organic dyes (Massey et al., 2016), antibiotics (Chao et al., 2017, Chao et al., 2014, Zeng et al., 2019), bacteria (Liu et al., 2016) and so forth. Each MoS2 monolayer with an interlayer spacing of 0.62 nm includes one Mo layer and two S layers (Wang and Mi, 2017). MoS2 has strong covalent bonds within the plane and weak van der Waals forces between adjacent layers (Xie et al., 2013). Generally, there exists two main crystalline phases including octahedral 1T phase and hexagonal 2H phase for the application of MoS2 (Yu et al., 2018). The 2H phase is stable and semiconducting, which is commonly found in natural MoS2 bulk, whilst in the 1T phase displays metallic and metastable properties (Chen et al., 2019a, Wang and Mi, 2017, Zhang et al., 2020). Over the last few decades, many efforts have been made to synthesize single-layer and few-layer MoS2 nanosheets for various applications such as catalysis (Bai et al., 2014), electronics and optoelectronic (Feng et al., 2016) and energy-related fields (Xie et al., 2013). Synthetic strategies to MoS2 nanosheets can be classified into top-down and bottom-up methods (Liu et al., 2019a, Wang and Mi, 2017). Top-down methods seeks to achieve peeling by overcoming the weak van der Waals force between layers, mainly including mechanical, ultrasonic, liquid, and chemical peeling methods (Hirunpinyopas et al., 2017, Li et al., 2015). In the bottom-up methods, MoS2 nanosheets are synthesized using precursors containing Mo and S atoms such as hydrothermal reaction and chemical vapor deposition (Najmaei et al., 2013, Xie et al., 2013).
MoS2 has proven performance for the adsorption of heavy metal ions in aqueous solution, owning to its intrinsic sulfur-rich characteristics, high specific surface area and negatively charged surface and active sites at the edges. Theoretical (DFT calculation) and experimental studies have demonstrated that MoS2 nanosheets and MoS2-based nanomaterials exhibited excellent adsorption abilities for heavy metal ions such as Au3+, Hg2+, Pb2+, Ag+, Cu2+, Cr6+, Cd2+, Co2+ and some radionuclides (Aghagoli et al., 2017a, Ai et al., 2016, Du et al., 2017, Feng et al., 2018, Liu et al., 2019a, Wang et al., 2017, Wang et al., 2018, Wang et al., 2020b, Yang et al., 2018, Yi et al., 2019, Zhan et al., 2019, Zhang et al., 2020). In their search for a promising adsorbent, most studies focus on improving adsorption performance, such as adsorption capacity, selectivity, liquid–solid separation by decoration of MoS2 nanosheets and/or optimization of adsorption conditions. Few studies, however, systematically describe the preparation methods involved, the structure of obtained MoS2 nanosheets and removal performance. Different preparation methodologies may lead to MoS2 samples with markedly different structural characteristics and physicochemical properties, which greatly affects the adsorption behavior of heavy metal ions. For example, the Hg adsorption capacities of 254 mg/g (Jia et al., 2017) , 1996 mg/g (Zhan et al., 2019), 2506 mg/g (Ai et al., 2016) were reported using MoS2 nanosheets as adsorbents prepared by different methods (i.e. the exfoliation and hydrothermal methods). The adsorption capacity of Pb2+ could reach 1479 mg/g using an ultrasound assisted electrochemical exfoliation method (Liu et al., 2017), while several studies reported that Pb2+ adsorption capacities (e.g. 46.5 mg/g (Aghagoli et al., 2017b), 279.93 mg/g (Du et al., 2017)) of the obtained MoS2 nanosheets by hydrothermal synthesis were significantly lower than this value. Besides, for current studies of heavy metal removal by MoS2 nanosheets, adsorption capacity is mostly used as a key evaluation indicator to assess the feasibility of MoS2 nanosheets as adsorbents. It is noting that the removal efficiency should also be considered from the perspective of potential application on water treatment and purification. More importantly, the structural defects, interlayer spacing and crystal phase of MoS2 nanosheets induced by preparation methods can play key roles in physicochemical characteristics such as surface area, exposed S atom, and surface charge (Chen et al., 2019a, Chen et al., 2019b, Liu et al., 2019b, Zhan et al., 2019, Zhang et al., 2020). To some extent, the structure of adsorbent materials determines the adsorption affinity, selectivity and mechanism. Therefore, it is a requirement to compare the effects of different preparation methods on adsorption behavior to provide profound insights into the role of MoS2 nanosheets structure on the adsorption of heavy metal ions, especially considering both adsorption capacity and removal efficiency.
This paper aims to highlight on the significance on the structure of MoS2 nanosheets in affecting the adsorption of heavy metal ions. MoS2 samples were respectively prepared by two methods including ultrasonic-assisted liquid-phase stripping (a typical top-down method) and hydrothermal synthesis (a typical bottom-up method). Pb2+ was selected as a representative heavy metal ion considering its moderate affinity between S atom and heavy metal ions according to Lewis acid–base interactions. More importantly, Pb2+ is one of the most used metal elements existing in industrial wastewater, and surface water (Zhang et al., 2020). The limited value of Pb is 10 g/L according to World Health Organization (WHO) guidelines for drinking-water quality (WHO, World Health Organization, Wang et al., 2020a) with Pb being listed as one of the 14 priority pollutants suggested by U.S. Environmental Protection Agency (Islam et al., 2014). In this study, the structure of MoS2 samples such as chemical composition, surface defects, interlayer spacing, the phase (i.e. crystal structures) were characterized employing various techniques. The relationship between the structure and preparation method was identified in detail. During the adsorption process, we focused on investigating the effects on the structure of MoS2 nanosheets obtained using two typical preparation methods on removal performance and mechanism.
Section snippets
Chemical materials
Crystalline Bulk MoS2 powder (particle size <, AR, 99%) was purchased from Sigma-Aldrich Co. (USA). Sodium cholate (from bovine and/or ovine bile, AR, 99%) was used to be a surfactant in ultrasonic-assisted liquid-phase stripping, which was obtained from Sigma-Aldrich Co. (USA). (NH4)6Mo7O24 4H2O (AR, 99%) and Thiourea (AR, 99%) were provided by Shanghai Macklin Biochemical Co. Ltd. (Shanghai, China). Pb(NO3)2 was bought from Guangfu Technology Development Co. Ltd. (Tianjin, China).
Synthesis of u-MoS2 and h-MoS2 nanosheets
The
The structure of u-MoS2 and h-MoS2
As shown in Figs. 1a and 1b, the u-MoS2 sample exhibited a thin sheet structure with small lateral sizes and straight edges. The h-MoS2 sample with multilayer and overlapped structure displayed the large lateral sizes, the curved edges, obvious wrinkles, which could provide more active sites for capturing Pb2+. Furthermore, the hexagonal symmetric structure of the u-MoS2 was obviously observed in Figs. 1c and 1e, which corresponds to the 2H phase (Wang and Mi, 2017), suggesting that
Conclusion
The two typical preparation methods (i.e. ultrasonic-assisted liquid-phase stripping and hydrothermal synthesis) were used to respectively obtain u-MoS2 and h-MoS2 nanosheets and then their adsorption performances were evaluated and compared. The h-MoS2 nanosheets prepared by hydrothermal synthesis was suitable for the adsorption of Pb2+ in this study. It should be emphasized that the structural properties of MoS2 nanosheets play essential roles in the adsorption performance rather than
CRediT authorship contribution statement
Yang Liu: Investigation, Writing - original draft, Methodology, Formal analysis, Data curation. Chanjuan Ma: Investigation, Writing - original draft, Formal analysis. Xinbo Zhang: Investigation, Conceptualization, Review & editing. Huu Hao Ngo: Investigation, Conceptualization, Review & editing. Wenshan Guo: Supervision, Investigation, Review & editing. Mingdong Zhang: Investigation, Review & editing. Dan Zhang: Methodology, Resources, Review.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by Tianjin Municipal Science and Technology Bureau of China (No. 18PTZWHZ00140, No. 20JCYBJC00560) and Research Project of Tianjin Education Commission, China (No. 2019KJ103).
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