Thiol-grafted magnetic polymer for preconcentration of Cd, Hg, Pb from environmental water followed by inductively coupled plasma mass spectrometry detection
Graphical abstract
Introduction
Metal mining and smelting, municipal solid waste, automobile exhaust, and even natural processes like precipitation would increase the heavy metal contamination. Heavy metals discharged into the environmental water would be gradually enriched by biological chains and finally affecting human health [1]. To monitor the environmental pollution and protect human health, crucial guide lines of heavy metals in drinking water were established by World Health Organization (2011) (0.003, 0.006, 0.010 mg L−1 for Cd, Hg, Pb, respectively). Therefore, establishing effective methods for monitoring pollution levels of heavy metals in the environment water is of great significance.
Inductively coupled plasma mass spectrometry (ICP-MS) is a kind of commonly used trace elements analysis technique, with advantages of low detection limit, simultaneous multi-element detection capability, high sensitivity, fast analysis and wide linear range. However, environmental samples with complex matrices usually cause matrix interferences during ICP-MS analysis. At the same time, instrumental detection limit (LOD) may be higher than the concentration of target metal ions in real samples. Therefore, effective sample pretreatment techniques are often needed to enrich target metal ions and separate complex matrix. Magnetic solid phase extraction (MSPE) is an efficient sample pretreatment technique with high enrichment factor, easy operation and convenient phase separation by an external magnetic field. It has been widely applied for trace elements analysis in various samples [2,3].
Magnetic materials are the key point of MSPE, which determine the extraction selectivity and dynamics. For example, specific functional groups (-COOH, -SH, -NH2, -CNS, etc.) are often grafted onto Fe3O4 NPs to improve the adsorption performance of magnetic materials for heavy metals [4,5]. Among them, sulfur-containing groups are often used for modification due to their good binding ability to sulphophile heavy metals. In situ synthesis and post-modification are two commonly used techniques to graft sulfur-containing groups onto NPs. The former one is often applied in the mercapto functionalization of carbon materials [6] or some specific monomers based polymers [7]. However, introducing thiol group into NPs during polymerization is not easy, since thiol group has quenching effect on free radical polymerization. In comparison, post-modification strategy is more widely used and a variety of reactions are available to functionalize the material with sulfur-containing group. In this context, bifunctional group modification strategy is very convenient and efficient, in which one functional group would bind with magnetic core and the other sulfur-containing functional group bind with interest metals.
In our previous work, Huang and Hu [2] grafted -SH onto the magnetic silicon nanoparticles by using 3-mercaptopropyltrimethoxysilane (γ-MPTS), a kind of bifunctional group modified reagent, leading to an adsorption capacity of 45.2, 56.8, 83.8 and 70.4 mg g−1 for Cd2+, Pb2+, Hg2+ and Cu2+ respectively. Although the magnetic sorbents had excellent dispersibility (enrichment factor of 500), the amount of active site on the sorbents and the adsorption capacity are expected for further improvement. Increasing specific surface area of the sorbents by forming porous structure or introducing polymers is a good way to load more active sites. Li et al. [8] used cetyltrimethylammonium as a template to prepare a γ-MPTS modified magnetic nanoparticles. This material has a larger specific surface area of 321 m2 g−1, and therefore a high adsorption capacity for Hg2+ and Pb2+ (262.8 and 91.2 mg g−1). To avoid tedious template-cleaning process, polymer-containing structure is often used to get large specific surface area and graft more functional groups for magnetic sorbents. Organic porous polymer (POPs) was a new type of polymer which is composed of light elements such as C, O, B, N and S. They had advantages of high chemical stability, large specific area and adjustable pore structure [9]. Li et al. [10] grafted sulfhydryl group onto a porous aromatic framework (PAF)-1 with a high specific surface area of 3274 m2 g−1. The synthesized PAF-1-SH exhibited excellent adsorption capacity for Hg (up to 1014 mg g−1), and the S content reached 17.6 wt%. However, it took 6 days to complete chloromethylation and thiolation. Cross-linked polymer is easy to synthesis; the active group introduced by the polymer surface can be artificially controlled (such as epoxy group, -C=C) [11,12], and combined with more effective groups (-SH, -NH2). Pan et al. [13] modified -C=C- onto Fe3O4 NPs with oil acid, and obtained cross-linked network by polymerizing methyl methacrylate (MMA), glycidyl methacrylate (GMA) and divinylbenzene (DVB) on the surface. The epoxy-containing polymer reacted with thiourea and NaHS, resulting in a mercapto engineered material with an adsorption capacity of up to 523 mg g−1 for Hg2+. Introducing more active sites into the polymer contributes to a large adsorption capacity. However, the thiolation step of this material is complicated.
Thiol click chemistry can get high yield under mild conditions with a rapid reaction rate [14], and it is a good functionalization method. Mercaptans are highly reactive with olefins, alkynes, epoxy groups, halogenated hydrocarbons. He et al. [3] used a double bond-modified magnetic NPs and GMA as a polymer monomer to form an epoxy-modified polymer magnetic sorbent. The high-performance of thiol click chemistry allows the small molecule of ethanedithiol to be attached to the polymer chain. A thiol-modified magnetic polymer particle was obtained for preconcentration of different Hg species, with adsorption capacity of 141 mg g−1 for Hg2+. While increasing the dosage of monomers cannot improve the adsorption capacity, probably because the chain structure on the magnetic ball cannot be extended infinitely.
Higher sulfur group content and better adsorption performance are expected if the material has a cross-linked network structure instead of a chain structure, in which the short chains with thiol groups can be extended. Using suspension polymerization instead of solvent polymerization can wrap single magnetic particles into a large polymer network, which can create more active sites. Based on this consideration, a thiol-grafted magnetic polymer (MPPs-SH) was synthesized with double-bonded magnetic NPs and GMA as the monomers and ethylene glycol dimethacrylate (EDGMA) as the crosslinking agent in this work. The scheme of the synthesis procedure is presented in Fig. 1. Multiple small silicon particles were wrapped in a large polymer network with epoxy on its surface by suspension polymerization. Then, a click reaction of ethanedithiol with epoxy group was carried out on its surface to obtain the thiol-modified polymer magnetic particles. The material exhibited high sulfur content, large specific surface area, strong magnetism, and was investigated as a magnetic sorbent for the analysis of trace heavy metals in environmental water samples.
Section snippets
Reagents and solutions
Stock standards solutions of Hg2+, Cd2+and Pb2+ (1000 mg L−1) were prepared by dissolving HgCl2 (Shanghai Chemical Reagent No. 4, Shanghai, China), Cd(NO3)2 (Shanghai Jinshanting New Chemical Reagent Factory, Shanghai, China), Pb(NO3)2 (Tianjin Dengfeng Chemical Reagent Factory, Tianjin, China) in 0.5 mol L−1 HNO3 (GR). The standard solution was obtained by gradually diluting the above stock solution daily with deionized water. Ferric chloride (FeCl3·6H2O), polyvinyl alcohol (PVA-124),
TEM and SEM characterization
The morphology and particle size of Fe3O4@SiO2@OA, MPPs and MPPs-SH were observed by TEM. The experimental results in Fig. S1 (a-c) (Appendix) revealed that the diameter of Fe3O4@SiO2@OA was about 20 nm, and the particle size of MPPs and MPPs-SH were both about 400 nm, which indicated that small Fe3O4@SiO2@OA NPs were wrapped successfully in large polymer network by suspension polymerization, and ethane-dithiol functionalization step has no obvious effect on particle size of MPPs. Besides, the
Conclusion
In this work, a cross-linked thiol grafted magnetic polymer nanoparticles was prepared by suspension polymerization and click chemistry. The prepared MPPs-SH is easy to synthesize, and has a high adsorption capacity (64.2, 254, 56.5 mg g−1) for Cd2+, Hg2+ and Pb2+, owing to its large specific surface area and high sulfur content. Using this material as sorbent, a method of MSPE-ICP-MS was proposed for the analysis of trace Cd2+, Hg2+ and Pb2+ in environmental water. The method has a fast
Credit author statement
Yujie Chen: Methodology, Investigation, Data curation, Writing- Original draft preparation. Man He: Visualization, Writing- Reviewing and Editing. Beibei Chen: Writing- Reviewing and Editing. Bin Hu: Conceptualization, Methodology, Supervision, Project administration, Funding acquisition, Writing- Reviewing and Editing.
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
Financial supports from the National Natural Science Foundation of China (Nos. 21775113, 21974100) and the Science Fund for Creative Research Groups of NSFC (Nos. 20921062) are gratefully acknowledged.
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