Magnetic supramolecular polymer: Ultrahigh and highly selective Pb(II) capture from aqueous solution and battery wastewater

Journal Pre-proof Magnetic supramolecular polymer: Ultrahigh and highly selective Pb(II) capture from aqueous solution and battery wastewater Zongwu Wang, Jing Zhang, Qing Wu, Xuexue Han, Mengna Zhang, Wei Liu, Xinding Yao, Jinglan Feng, Shuying Dong, Jianhui Sun PII:

S0045-6535(20)30235-6

DOI:

https://doi.org/10.1016/j.chemosphere.2020.126042

Reference:

CHEM 126042

To appear in:

ECSN

Received Date: 28 November 2019 Revised Date:

13 January 2020

Accepted Date: 26 January 2020

Please cite this article as: Wang, Z., Zhang, J., Wu, Q., Han, X., Zhang, M., Liu, W., Yao, X., Feng, J., Dong, S., Sun, J., Magnetic supramolecular polymer: Ultrahigh and highly selective Pb(II) capture from aqueous solution and battery wastewater, Chemosphere (2020), doi: https://doi.org/10.1016/ j.chemosphere.2020.126042. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

CRediT authorship contribution statement Zongwu Wang: Conceptualization, Investigation, Writing-Original Draft, Writing-Review & Editing. Jing Zhang: Data Curation, Formal analysis. Qing Wu: Validation, Supervision. Xuexue Hana: Investigation. Mengna Zhang: Investigation. Wei Liu: Resources, Visualization. Xinding Yao: Resources, Visualization. Jinglan Feng: Visualization, Writing - Review & Editing. Shuying Dong: Resources, Visualization, Writing-Review & Editing. Jianhui Suna: Project administration, Supervision, Writing-Review & Editing.

Magnetic supramolecular polymer: Ultrahigh and highly selective Pb(II)

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capture from aqueous solution and battery wastewater

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Zongwu Wanga,b, Jing Zhanga,c, Qing Wua, Xuexue Hana, Mengna Zhanga, Wei Liub, Xinding Yaob,

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Jinglan Fenga, Shuying Donga, *, Jianhui Suna, *

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a

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Henan Key Laboratory for Environmental Pollution Control, School of Environment, Henan

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Normal University, Xinxiang, Henan, 453007, P. R. China

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b

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Engineering Technology Research Center of Green Coating Materials, Kaifeng, Henan, 475004, P.

MOE Key Laboratory of Yellow River and Huai River Water Environmental and Pollution Control,

School of Environment Engineering, Yellow River Conservancy Technical Institute, Henan

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R. China

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c

Sanmenxia Polytechnic, Sanmenxia, Henan, 472000, P. R. China

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*Corresponding authors. E-mail: [email protected] (S. Dong), [email protected] (J. Sun).

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Tel.: +86-373-3325971

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Abstract

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For the practical capture of heavy metal ions from wastewater, fabricating environmental

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friendly adsorbents with high stability and super adsorption capacity are pursuing issue. In this

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work, we develop magnetic supramolecular polymer composites (M-SMP) by using a simple

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two-step hydrothermal method. Systematical characterizations of morphological, chemical and

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magnetic properties were conducted to confirm the formation of M-SMP composites. The resulting

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M-SMP composites were applied to remove Pb(II) from aqueous solution and from real battery

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wastewater, and easy separation was achieved using a permanent magnet. By investigating the

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effects of various parameters, we optimized their operating condition for Pb(II) adsorption by the

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M-SMP. The uptake of Pb(II) onto M-SMP fitted well the pseudo-second-order and Langmuir

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isotherm models, and favourable thermodynamics showed a spontaneous endothermic process.

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The SMP endowed M-SMP with ultrahigh adsorption capacity for Pb(II) (946.9 mg g–1 at pH = 4.0,

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T = 298 K), remarkable selectivity, satisfactory stability and desirable recyclability. In

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Pb-contaminated lead-acid battery industrial wastewater, the concentration of Pb(II) declined from

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18.070 mg L–1 to 0.091 mg L–1, which meets the current emission standard for the battery industry.

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These merits, combined with simple synthesis and convenient separation, make M-SMP an

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outstanding scavenger for the elimination of industrial Pb(II) wastewater.

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Keywords: Magnetic supramolecular polymer; Pb(II); Adsorption; Lead-acid battery wastewater

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1. Introduction

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On account of the growing industrial activities and urbanization, the discharging of heavy

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metals to environment poses continuous and increasing threats to the quality of the environment,

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the food chain and human health seriously due to its high toxicity, bioaccumulation and

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non-degradability (Ji et al., 2019; Li et al., 2019). Therefore, it is strategically and ecologically

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imperative to explore reliable technology to tackle this global dilemma. Among the diverse

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technologies, the adsorption method stands out for its simple operation, cost-effective and free of

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toxic sludge (Fu and Wang, 2011; Wu et al., 2015; Xu et al., 2018). However, the lack of

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adsorbents with excellent comprehensive performance has become a hindrance to the application

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of adsorption technology. To date, much efforts have been devoted to the fabrication of various

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engineered materials, including carbon, functionalized-silica, clay mineral, polymer-based and

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metal-organic frameworks etc., to removal heavy metal ions from wastewater (Da'na, 2017; Dai et

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al., 2018; Songwut Lapwanit et al., 2016; Wen et al., 2018; Yadav et al., 2019; Yang et al., 2019).

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Because of their low toxic and environment-friendly qualities, polymeric materials are one of

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the most promising candidates. Then also have excellent regulatable chemical functionality,

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dimensional stability and extraordinary adsorption property. Furthermore, considering the ease of

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separating a magnetic sample after treatment, magnetic polymeric adsorbents are becoming a focus

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of research (Huang et al., 2018; Venkateswarlu and Yoon, 2015). Therefore, it is important to

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develop a creative synthesis method and screen suitable raw material for producing high-efficiency

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magnetic polymeric materials.

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Among many supramolecular precursors, trithiocyanuric acid (TTCA) is the main ingredient

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of commercial highly potent chelating agent TMT15, which is widely adopted in the industrial

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wastewater management owing to its strong complex ability of sulfur ligands with many soft

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Lewis acidic metal ions (Pb2+, Cd2+, Hg2+, and others) (Fu et al., 2019; Huang et al., 2018).

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However, the aggregation, pH limitations and difficulty of recovery in practice seriously restrict its 3

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application value. Organodisulfide polymer has been investigated as an excellent heavy metal

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scavenger, but suffers from a cumbersome process and limited final yield, and requires the usage

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of toxic reagents, resulting in a typical impediment to its large-scale application for heavy metal

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recovery (Ko et al., 2017; Ko et al., 2018; Lin et al., 2019). Recent research indicates that the

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melamine (MA)-based dendrimer also presents high metal capture performance owing to the

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strong complexation between -NH2 groups and soft metal ions (Al et al., 2017; Sharahi and

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Shahbazi, 2017). However, it also has a complicated synthetic process, and its potential ecological

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toxicity does not meet the requirements of commercial application.

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For these reasons, it is still imperative to develop a smart strategy to overcome these

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problems while retaining the merits of TTCA and MA. There have been a few attempts to

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sequentially add TTCA and MA to aqueous solution for precious metal recovery (Nagai et al.,

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2015; Nagai et al., 2016). Nevertheless, to the best of our knowledge, the related studies for the

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synthesis of adsorbent material and investigation of adsorption behaviour have not been reported.

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Therefore, the development of a straightforward strategy to simultaneously address these issues

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and further improve adsorption ability is attractive but remains a great challenge.

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Hydrogen bonds abandoning the conventional covalent bonds is a popular synthetic strategy

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used widely in organic chemical synthesis (Shalom et al., 2013; Whitesides et al., 1991).

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Interestingly, the supramolecular polymer (SMP) derived from MA and TTCA comprises a

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hexagonal network stacked in three dimensions with channels. This could be constructed by a

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simple procedure via the highly hydrogen-bonding action of N–H···S and N–H···N, forming

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precipitates in water with satisfactory chemical stability. The SMP in non-covalent combination is

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the reorganization of MA and TTCA at molecular level (Fan et al., 2017; Ranganathan et al., 1999;

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Whitesides et al., 1995). Considering the high complexation capacity and recovery property for

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industrial and practical applications, we constructed magnetic supramolecular polymer (M-SMP)

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by incorporating SMP and magnetic Fe3O4 nanoparticles (NPs) to solve the aforementioned 4

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problems.

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The objectives of this current study were (1) to synthesize M-SMP composites by a simple

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hydrothermal method for effective capture of Pb(II) ions in acidic wastewater samples, (2) to

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characterize the morphology and composition systematically by various microscopic and

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spectroscopic techniques, (3) to investigate the adsorption performance of Pb(II) on M-SMP

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composites through routine experiments, (4) to reveal the plausible interaction mechanism by the

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results of batch experiments coupled with Fourier transform infrared (FT-IR) and x-ray

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photoelectron spectroscopy (XPS) characterization, and (5) to explore the application of M-SMP

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composites for attenuating Pb(II) concentrations in lead-acid battery wastewater. Given all that,

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the as-prepared M-SMP materials possess considerable practical applications for heavy metal

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contaminated industrial wastewater clean-up.

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2. Experimental procedures

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2.1 Materials and instrumentation

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All chemicals in this experiment were analytical grade and were received without further

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purification. Detailed information on the adsorbents and apparatus are listed in Supplementary

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Information (SI) (Text S1).

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2.2 Synthesis of magnetic supramolecular polymer (M-SMP) composites

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Magnetic supramolecular polymer (M-SMP) composites were prepared successfully by a

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simple two-step method. In the first step, the Fe3O4 nanoparticles (NPs) were obtained according

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to previous work with some modification (Text S2). In the second step, melamine (315 mg) and

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Fe3O4 (200 mg) were dispersed in 30 mL water and heated at 70 oC by sonication for 1 h to form a

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homogeneous solution. Afterwards, trithiocyanuric acid (443 mg) was mixed thoroughly with 40

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mL water, accompanied sonication for 0.5 h at 70 oC. Then the above two solutions were mixed

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together, transferred into Teflon-lined autoclave, and maintained at 100 oC for another 2 h. Finally,

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the resulting grey solid was collected by magnetic separation, cleaned with deionized water, and 5

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dried at 60 oC for 12 h, which is denoted as M-SMP. The light yellow SMP was also prepared

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without Fe3O4 NPs using the same method. The process is shown in Fig. 1a. Digital photographs

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of raw materials and as-prepared composites are shown in Fig. S1, and the proposed route of the

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synthesis and the chemical equation of M-SMP is presented in Fig. S2.

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Fig. 1. (a) Schematic illustration of the fabrication procedure for M-SMP. (b) SEM images of SMP. (c) SEM

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images of M-SMP. (d) TEM images of SMP. (e) TEM images of M-SMP. The top-left inset is a high-resolution

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TEM image and the bottom-right is the corresponding SAED pattern.

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2.3 Batch adsorption experiments

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For batch adsorption procedures, 25 mg M-SMP solid was dispersed into 25 mL desired Pb(II)

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solution in a 50 mL flask iodine, and the suspension was shaken under 120 rpm at the desired

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temperature in a thermostatic shaker bath. Finally, the solid was separated from the mixing

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solution via a permanent magnet, and the remaining metal ion concentrations were detected by

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inductively coupled plasma-mass spectrometry (ICP-MS). The removal efficiency R (%) and

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adsorption capacity qe (mg g–1) were calculated according to the relevant equations (Text S3).

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2.4 Industrial battery wastewater samples

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The industrial wastewater samples were obtained from the lead-acid battery factory located in

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Xinxiang city (35.20oN, 113.51oE), Henan province, China. The mass concentration of Pb(II)

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determined by ICP-MS was 18.070 mg L-1, Other quality indicators were listed in Table S1.

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3. Results and discussion

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3.1 Material characterizations

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The morphologies of SMP and M-SMP were observed by scanning electron microscope

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(SEM), as shown in Fig 1b-c. Compared with the SMP with the smooth surface (Fan et al., 2017),

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the M-SMP composites illustrate a rod-like structure with abundant Fe3O4 nanoparticles wrapped

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inside supramolecular polymer. This was further confirmed by transmission electron microscopy

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(TEM) of SMP (Fig. 1d) and M-SMP (Fig. 1e). Furthermore, the surface of the M-SMP was

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slightly rough compared with that of the SMP. Notably, the high-resolution TEM (HR-TEM)

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image in the top-left inset of Fig. 1e shows that M-SMP composites have interlayer spacing of

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about 0.253 nm, matching well with the (311) plane of Fe3O4 (Wang et al., 2017). The

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corresponding selected area electron diffraction (SAED) (bottom-right inset of Fig. 1e) further

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verified the crystalline structure of Fe3O4. 7

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Fig. 2. FT-IR spectra (a), XRD patterns (b), XPS spectra (c) and Raman shift spectra (d) of the intermediate and

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final as-prepared samples.

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The FT-IR spectra of raw materials and composites are shown in Fig. 2a. For TTCA, the

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characteristic peaks located at 1100, 1520, 2655 and 2900-3150 cm–1 represented the C=S, C-N,

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S-H, and N-H groups, respectively. For MA, the peaks of 3426 and 810 cm–1 are ascribed to the

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N-H stretching vibration and the triazine ring vibration. These results were in good agreement with

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the literature reported (Jun et al., 2013; Ko et al., 2017; Li et al., 2016; Shalom et al., 2013).

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Although the characteristic peaks of supramolecular polymers derived from MA and TTCA in the

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SMP and the M-SMP both showed almost the sum of those of virgin MA and TTCA, some new

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peaks appeared owing to the hydrogen bonding between TTCA and MA in composites. Compared

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with the signal of TTCA and MA, the hydrogen bonding of N–H···S and N–H···N resulted in the

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N-H stretching vibration shifting from 3426 cm–1 to 3418 cm–1 accompanied with the enhanced

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intensity, the blue shift of the C=S stretching vibration from 1100 cm–1 to 1135 cm–1, and the

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triazine ring vibration shifting from 810 cm–1 to 780 cm–1 (Jun et al., 2013; Li et al., 2016; Yang et

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al., 2016). Surprisingly, a new peak at 1633 cm–1 was the signal of thione (Fu and Huang, 2018).

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The peak at 554 cm–1 reflected the stretching vibration of the Fe-O bond (Song et al., 2019),

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indicating the successful incorporation of Fe3O4 component. The formation of supramolecular

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polymer composites was also demonstrated by the high crystallinity nature of SMP in Fig. 2b

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(Shalom et al., 2013). In the x-ray diffraction (XRD) pattern of SMP in contrast to those of MA

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and TTCA (Fig. S3), three significant peaks at 12.25o, 13.06o, and 18.41o can be indexed to the

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in-planar packing. Also, the well-resolved peak at 24.58o with a 0.362 nm d-spacing corresponded

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to the graphite-like structure of 2-D SMP sheets (Fan et al., 2017; Jun et al., 2013; Shalom et al.,

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2013), showing the formation of a new supramolecular polymer arrangement (Feng et al., 2014).

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The characteristic peaks of M-SMP sample offsetting the partial signals of Fe3O4 was in line with

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those of SMP, indicating the chemical stability of SMP in the incorporation of Fe3O4 (Fig. 2b).

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Furthermore, there was no difference in the XRD patterns of M-SMP before and after immersing

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the M-SMP in water for 12 h, confirming the stability of as-prepared M-SMP in aqueous solution.

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The surface composition and the chemical state of the element were investigated by XPS. For

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the SMP, the dominant peaks centered at 162.1, 227, 288.2 and 399.7 eV confirmed the

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coexistence of S, C and N elements (Fig. 2c), corresponding to S2p, S2s, C1s and N1s signals,

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respectively (Fu and Huang, 2018; Huang et al., 2019). In the high-resolution XPS spectra (Fig.

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S4a), there were three forms of C1s, including C bonds of the aromatic trithiol (25.0% of the total

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carbon) at 288.7 eV, the triazine ring (48.6%) at 287.8 eV and the thione (26.4%) at 284.8 eV in

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M-SMP. The high-resolution N1s XPS spectra of the M-SMP (Fig. 3a) were also deconvoluted by

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the aromatic trithiol (61.6.0% of the total nitrogen) at 399.3 eV, the -NH- (31.3%) at 400.2 eV and

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the thione (7.3%) at 400.8 eV (Dinda and Kumar, 2015; Fu and Huang, 2018; Fu et al., 2019; 9

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Kucharski and Szukiewicz, 2000). Fig. 3b showed the fitted S2p spectra of S2p3/2, S2p1/2 in aromatic

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trithiol, and S2p3/2 in thione, which were situated at 161.9, 162.6 and 163.2 eV, respectively (Kim et

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al., 2015; Ko et al., 2018; Wang et al., 2017). Compared with the spectrum of SMP, the existence

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of the Fe2p signal and the enhancement of O1s intensity accompanying the high-resolution spectra

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of Fe2p and O1s (Fig. S4b-c) demonstrated that Fe3O4 was successfully incorporated with SMP.

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Furthermore, the M-O bond at 531.2 eV confirmed the as-prepared M-SMP was not a simple solid

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mixing process of SMP and Fe3O4 (Tian et al., 2019).

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Fig. 2d and Fig. S5 depicted the Raman shift spectra of these samples. The dominant band at

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687 cm–1 was the characteristic Raman signal of MA (Wang et al., 2016; Zhang et al., 2019),

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which was attributed to the deformation modes of the in-plane triazine ring. Raman modes

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centered at 448, 1125 and 1275 cm–1 were assigned to the N-C-S, C=S and C-N groups,

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respectively, which originated from the direct evidence of TTCA structure (Kim et al., 2015; Li et

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al., 2016). After the reaction of TTCA with MA, the characteristic peak of MA shifted to 691 cm–1

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(Fig. 2d and Fig. S5a), and the representative peaks of TTCA shifted to 440, 1162 and 1264 cm–1

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(Fig. 2d and Fig. S5b) in the SMP and the M-SMP. Furthermore, the fact that there were no

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marked differences between the SMP and the M-SMP except for signal intensity confirmed that

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the introduction of Fe3O4 did not affect the formation of hydrogen bonds.

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The magnetization measurement was investigated by vibrating sample magnetometer (VSM)

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(Fig. 3c). The M-SMP showed superparamagnetic nature with saturation magnetization (Ms) of

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19.86 emu g–1, which was lower than that of naked Fe3O4 owing to a certain amount of

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non-magnetic SMP. In Fig. 3c, inset (L) and Inset (R) are digital photographs of the heavy metal

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aqueous solution with dispersed M-SMP before and after magnetic separation. Notably, the

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M-SMP showed rapid response to the magnetic field, meeting the requirement of magnetic

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separation. The synergy of the SMP and Fe3O4 nurtured the composite into a promising adsorbent

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with easy separation. The thermal stability of as-prepared M-SMP and SMP was determined using 10

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thermal gravimetric analysis (TGA) under N2 atmosphere (Fig. 3d). It can be seen that the quality

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of M-SMP and SMP remains constant without quality loss up to 300 oC, demonstrating the higher

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thermal stability of M-SMP. Compared with the 100% weight loss of SMP, approximate 25% of

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the stable quality for M-SMP was due to the existence of thermally stable Fe3O4 when the

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temperature was above 750 oC, which was consistent with the formulation of the material in this

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study. The specific surface area and pore size of M-SMP were determined by the N2

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adsorption-desorption isotherms measurement shown in Text S4, Fig S6 and Table S2. The pore

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diameter between 2 and 50 nm will help to provide faster accessibility between heavy metal ions

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and functional groups containing N and S atoms in the M-SMP (Ko et al., 2017).

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Fig. 3. High-resolution XPS spectra of N1s (a) and S2p (b). Hysteresis loop of M-SMP and Fe3O4 (c), TGA curves

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of M-SMP and SMP (d). At the bottom right of (c), the L and R insets are digital photographs of the heavy metal

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aqueous solution with dispersed M-SMP before and after magnetic separation.

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3.2 Adsorption experiments 11

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3.2.1 Effect of pH

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To avoid the formation of Pb(OH)2 precipitation and the dissociation of hydrogen bonds in

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M-SMP, adsorption experiments with M-SMP were performed over a wide pH range from 1.0 to

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6.0 (Nagai et al., 2015; Nassar, 2010). The result of zeta potential measurement showed that the

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surface charge of M-SMP was negative over the operating pH range, and the effect of different pH

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on the adsorption performance was also investigated (Fig. 4a). When the pH solution was below

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2.0, Pb2+ ions were difficult to be immobilized on the M-SMP because of the strong competition

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effects between H+ and Pb(II). Notably, the adsorption capacity of Pb(II) on M-SMP increased

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dramatically with the pH ranging from 2.0 to 4.0, which was mainly attributed to the rising

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electrostatic attraction between the sharply increasing negatively charged M-SMP surface sites and

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positively charged Pb(II) species. When pH was tuned from 4.0 to 6.0, the adsorption capacity of

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M-SMP toward Pb(II) decreased slightly. Furthermore, because most industrial wastewater

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containing heavy metals is considered acidic, the higher adsorption capacity of the as-prepared

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adsorbent under acidic conditions indicated that the M-SMP has better practical application

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prospect (Li et al., 2016). Given these conditions, we tested the performance of M-SMP toward

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Pb(II) at pH 4.0 in the following experiments.

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Fig. 4. (a) Effect of pH and Zeta potential (T = 298 K, C0 = 200 mg L–1). (b) Effect of M-SMP dosage on Pb(II)

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adsorption (pH = 4.0, T = 298 K, C0 = 200 mg L–1). (c) Effect of contact time on Pb(II) uptake onto M-SMP. (d)

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Adsorption capacities (qe) at different temperatures with initial Pb(II) concentrations from 55 to 4500 mg L–1

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(pH = 4.0), and (e) adsorption isotherm and removal efficiencies of Pb(II) onto M-SMP (pH = 4.0, T = 298 K). (f)

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Co-existing heavy metal ions removal performance from a mixed solution of heavy metal ions and the relevant

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heavy metal ions removal performance individually (pH = 4.0, T = 298 K, C0 = 200 mg g–1). 13

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3.2.2 Effect of dosage

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The adsorption capacity of Pb(II) on different M-SMP dosage was displayed in Fig. 4b.

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Interestingly, the highest adsorption capacity was achieved, and the removal process presented the

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lowest efficiency when the dosage was 0.2 g L–1. This could be caused by insufficient adsorption

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sites concerning the abundant target ions. When the dosage varied from 0.2 to 1.2 g L–1, there was

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an apparent effect on the inhibition of adsorption capacity. Meanwhile, the promotion of removal

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efficiency was discovered, which could be attributed to the constant amount of target ions and the

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increasing number of binding sites (Tang et al., 2018). Compared with the fixed amount of metal

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ions, the available adsorption sites on M-SMP became excessive with continuous increasing

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dosage (above 1.0 g L–1). Consequently, the removal efficiency increased slightly, and the

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adsorption capacity decreased sharply. Taking the two factors into account simultaneously, 1.0 g

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L–1 was the appropriate dosage for the preconcentration of 200 ppm Pb(II).

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3.2.3 Adsorption kinetics

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Fig. 4c shows the effect of contact time on Pb(II) adsorption onto M-SMP at 298 K. The

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adsorption capacity increased significantly from 0 to 189.4 mg g–1 within the first 100 min, and the

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removal efficiency was up to 95% accordingly. Later, as the time progressed after 100 min, the

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adsorption amount and the adsorption efficiency maintained high level. The removal of Pb(II) on

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M-SMP took place in two stages: in the first stage within 100 min, the rapid removal might be

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attributed to the ample adsorption active sites provided by M-SMP. In the second stage after 100

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min, the adsorption sites became exhausted, and the adsorption rate was dominated by the

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transport of Pb(II) onto the M-SMP from the exterior to the interior, resulting in a relatively slow

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increase stage. Based on the kinetic result, a shaking time of 100 min was considered appropriate

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in the following experiments.

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For better insight into the adsorption process, three classical kinetic models, including the

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pseudo-first-order, pseudo-second-order and intra-particle kinetic models, were used to simulate 14

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the adsorption data (Text S5). The fitting of the model-estimated result and experimental data was

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assessed by the linear correlation coefficients (R2). As shown in Fig. S7, the relevant parameters

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were fitted by the plots of ln(qe-qt) vs t, t/qt vs t and qt vs t1/2, as listed in Table S3. From the

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intra-particle kinetic model (Fig. S7c), the entire adsorption process of mass transfer had

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experienced three stages: the instantaneous diffusion from solution to M-SMP surface, the interior

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surface diffusion stage, and the final equilibrium stage. The R2 value of pseudo-second-order

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equation for Pb(II) removal was 0.9998, which was higher than that of the other two equations,

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indicating the pseudo-second-order equation can perfectly describe the kinetics for Pb(II) removal

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from the M-SMP. Furthermore, the calculated data of qe,exp from the pseudo-second-order model at

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298 K was 201.5 mg g–1, which was very close to the experimental one, qe,cal (Table S4). Given

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these findings, the strong surface complexation between Pb(II) and the M-SMP was mainly

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controlled by chemisorption (Huang et al., 2019; Tang et al., 2018).

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3.2.4 Effect of initial Pb(II) concentrations and adsorption isotherms

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The adsorption performance of Pb(II) onto M-SMP was further evaluated by varying initial

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Pb(II)concentrations from 55 to 4500 mg L–1. As shown in Fig. 4d-e, the adsorption capacities

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clearly increased with the increasing concentrations, originating from the fact that the driving force

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provided by the high concentration of metal ions overcame the mass transfer resistance from the

289

solution to solid phases. The isotherm model is an effective tool to better investigate the relevant

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adsorption equilibrium at various temperatures. Therefore, three representative isotherms

291

(including the Langmuir, Freundlich and Temkin isotherm models) were engaged to depict the

292

adsorption equilibrium Pb(II) onto the M-SMP (shown in Text S6). All the related parameters

293

tabulated in Table S5 can be calculated from the linear plots of ce/qe vs ce, lnqe vs lnce and qe vs

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lnqe, as shown in Fig. S8-10. According to the R2 values, Langmuir isotherm model matched the

295

adsorption data better than those on other isotherm models. Therefore, this phenomenon implied

296

that the adsorption of Pb(II) on the M-SMP was monolayer adsorption behavior owing to the 15

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identical adsorption activity on the M-SMP. The calculated maximum adsorption capacity (qm) is

298

946.9 mg g–1 for Pb(II), superior to that of recently reported magnetic materials (Table S6).

299

Furthermore, even at low concentration, there was also a higher equilibrium adsorption capacity,

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indicating that the M-SMP has excellent adsorption performance in low concentration solution

301

(Fig. 4e) and demonstrating that the as-prepared M-SMP exhibited great potential application

302

prospect in low-concentration wastewater purification.

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3.2.5 Effect of temperature and thermodynamic analysis

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To better understand the effect of temperature on the adsorption performance, the

305

thermodynamic studies were carried out, and detailed information is presented in Fig. 4d and Text

306

S7. It is found that no distinct influence was observed below 1400 mg L–1, whereas the adsorption

307

capacities were promoted slightly at a higher temperature when the initial Pb(II) concentrations

308

remained in the range of 1400-4500 mg L–1, indicating that higher temperatures were more

309

favourable for the increasing Pb(II) uptake. In other words, this phenomenon demonstrated the

310

endothermic nature of the adsorption process. The relevant parameters are listed in Table S7.

311

Clearly, all negative standard free energy changes (∆G) values and the decreasing trend along with

312

rising temperature indicated the spontaneity of the adsorption system. The positive enthalpy

313

change (∆H) value demonstrated that the adsorption process of Pb(II) onto M-SMP is an

314

endothermic reaction. Furthermore, the positive value of entropy change (∆S) suggested that the

315

randomness on the interface between Pb(II) ions and M-SMP was increased during the adsorption

316

process.

317

3.2.6 Effect of coexisting ions

318

The effect of various coexisting cations on the adsorption performance was conducted at pH

319

4.0. Five typical kinds of metal ions [Ni(II), Zn(II), Cd(II), Mg(II), and Ca(II)] associated with

320

Pb(II) competition were used to test the specificity of the M-SMP. After adsorption equilibrium,

321

the residual efficiency of Pb(II) individually was 0.60%, and that in the presence of co-existence 16

322

ions is 5.25%, which were both higher than that of other divalent ions (Fig. 4f). The result

323

indicated that the M-SMP exhibited high removal efficiency for Pb(II) individually or with the

324

presence of the competing divalent cations, confirming the high selectivity of the M-SMP toward

325

Pb(II). The phenomenon is attributed to the strong coordination bonds of Pb···N and Pb···S for

326

sharing lone pair electronic (Wang et al., 2019; Wang et al., 2020). In other words, heavy metal

327

ions (the Lewis soft-acid) forming strong interaction with S or N (the Lewis soft-base) plays an

328

important role in the adsorption process.

329

3.3 Regeneration and comparison with commercial adsorbents

330

Easy reusability of the M-SMP is another crucial factor for the wide application of adsorbent

331

to minimize the wastewater treatment cost and reduce secondary pollution to the environment. For

332

the desorption experiment, 0.05 M EDTA-2Na and 0.5 M HCl solutions were used to elute the

333

absorbed Pb(II) ions from the M-SMP, as listed in Table S8. The desorption efficiencies of 0.05 M

334

EDTA-2Na were significantly better than those of 0.5 M HCl solution due to the high chelating

335

ability of EDTA-2Na toward Pb(II). Therefore, 0.05 M EDTA-2Na was selected as the desorbent

336

in the subsequent regeneration experiment. The grey solids were collected by the magnet

337

separation. There was only slightly decrease of Pb(II) adsorption capacities after five cycles of

338

regeneration experiments (Fig. 5a), maintaining a high removal efficiency of 92%. What' more, the

339

saturation magnetization value of M-SMP (Fig. S11) maintained a constant value without obvious

340

decreasing, indicating that the magnetic property of the M-SMP is quite stable. Therefore, the

341

resulting composites had excellent stability, good reproducibility and easy regeneration.

342

17

343

344 345

Fig. 5. (a) Recycles performance of Pb(II) onto M-SMP (pH = 4.0, T = 298 K, C0 = 200 mg L–1), (b) FT-IR

346

spectra of M-SMP before and after Pb(II) adsorption. The specific high-resolution XPS spectra of Pb4f (c)，N1s (d)

347

and S2p (e) after Pb(II) adsorption.

348

18

349

Aiming at comparatively evaluating the adsorption performance, the adsorption experiments

350

were performed by using commercial adsorbent including such as silica gel, activated carbon,

351

aluminium oxide, D401 and D402 resins under the same conditions, instead of the synthesized

352

adsorbent in this study (Table S9). In both low and high initial Pb(II) concentrations, the uptake of

353

Pb(II) onto the M-SMP was higher than those of most commercial adsorbents. Similarity, the

354

removal percentage was clearly superior to above commercial adsorbents, especially in the case of

355

high Pb(II) concentration. These results demonstrated that M-SMP composites are a promising

356

candidate for capturing Pb(II) from wastewater.

357

3.4 Plausible adsorption mechanism

358

To reveal the plausible interaction mechanism of Pb(II) with the adsorbent, FT-IR and XPS

359

surveys of the M-SMP before and after adsorption were performed. In FT-IR spectra of Pb(II)

360

loaded M-SMP compared with pristine M-SMP, the represented peaks shifted to lower

361

wavenumbers (Fig. 5b), including C=S (from 1135 cm–1 to 1128 cm–1), C-N (from 1520 cm–1 to

362

1480 cm–1), N-H (from 3418 cm–1 to 3410 cm–1), and thione (from 1633 cm–1 to 1626 cm–1). The

363

complete disappearance of signal (780 cm–1) and the weakness of strong peaks (1128 cm–1, 1626

364

cm–1 and 3410 cm–1) in the M-SMP suggested the coordination bonds of Pb···N and Pb···S (Fu

365

and Huang, 2018; Fu et al., 2019). Furthermore, the good reusability confirmed that the C-S bond

366

was not broken. Fig. 2c showed the full XPS spectrum after Pb(II) adsorption. Compared with the

367

spectrum before adsorption, a new signal of Pb4f after adsorption demonstrated the presence of

368

Pb(II) cation in the adsorbed M-SMP sample. Compared with purified Pb(II) with the binding

369

energies Pb4f 5/2 at 139.6 eV and Pb4f 7/2 at 144.5 eV (Peng et al., 2014), the remarkable binding

370

energy shifts for Pb4f 5/2 (from 139.6 eV to 139.3 eV) and for Pb4f 7/2 (from 144.5 to 142.9 eV) after

371

adsorption demonstrated the strong affinity between functional groups and Pb(II) (Fig. 5c) (Wang

372

et al., 2016). In contrast with the intensity of N1s signal in Fig 3a, the weakness of that after

373

adsorption (Fig. S12a) and the becoming smaller of binding energy value for three peaks suggested 19

374

that the formation of Pb···N (Fig. 5d) (Fu and Huang, 2018). In high-resolution XPS scan of S2p in

375

the aromatic trithiol form after adsorption, the area of S2p 1/2 was significantly increased from

376

19.1% to 21.6%, whereas that of S2p 3/2 was reduced from 16.7% to 6.7% (Fig. 5e, Fig. S12b and

377

Table S10). These phenomena confirmed the strong Pb2+···S2- bonding interactions, which was

378

consistent with previous literatures (Wang et al., 2017; Zhuang et al., 2018). Due to the uniform

379

array of MA and TTCA in the M-SMP, S and N atoms derived from MA and TTCA were

380

homogeneously distributed over the entire M-SMP, providing abundant adsorption active sites for

381

the target heavy metals in all solid-liquid contact interfaces. Therefore, the as-prepared M-SMP

382

possess superior heavy metal capture ability (Fu and Huang, 2018; Xu et al., 2018). In addition to

383

the main role of complexation interaction, electrostatic attraction and ion exchange are also

384

possible causes, according to the effect of pH on adsorption in this study.

385

3.5 Application to real battery industry wastewater

386

To verify the applicability of the M-SMP to Pb(II) extraction from lead-acid battery industry

387

wastewater, we performed the same adsorption experiment as in our previous work (Wang et al.,

388

2019; Wang et al., 2020). Keeping in mind the complexity of real wastewater, it is necessary to

389

explore the effects of various solution pH and adsorbent dosage on the performance of Pb(II)

390

removal. The adsorption results were diverse in acidic conditions (Fig. 6a). Although there is

391

abundant free Pb(II) in acidic condition due to its weak binding ability to bind with the other

392

wastewater components (Table S1), the equilibrium concentration after adsorption was still high,

393

indicating the poor adsorption ability of M-SMP toward Pb(II) at pH = 1.2, which is consistent

394

with the results mentioned previously (Wang et al., 2019; Wang et al., 2020). Considering both the

395

chemical stability of adsorbent and the efficiency of adsorption, the effect of the M-SMP dosage

396

on Pb(II) capture was subsequently performed at pH= 4.0. Fig. 6b shows that the ideal dosage was

397

1.6 g L–1, higher than that on Pb(II) removal in simulated aqueous solution. This phenomenon may

398

be caused by the competition for adsorption active sites between Pb(II) and the other battery 20

399

industry wastewater components (Table S1). Furthermore, a substantially constant mass before and

400

after adsorption was maintained (Table S11).

401 402

Fig. 6. Effect of pH (a) and dosage (b) for Pb(II) removal in lead-acid battery industry wastewater.

403 404

Pleasant results were obtained under desired operating conditions, and the concentration of

405

Pb(II) in wastewater was reduced from 18.070 mg L–1 to 0.091 mg L–1, which was well below the

406

less than 0.70 mg L–1 concentration limit of China’s Emission Standard of Pollutants for Battery

407

Industry (GB 30484-2013). Furthermore, the removal efficiency reached 99.50%. These results

408

show that the as-prepared composites possessed excellent Pb(II) capture ability in real battery

409

industry wastewater and confirm that the M-SMP has potentially low-cost and ultrahigh adsorption

410

ability for Pb(II) cleanup from wastewater.

411

4. Conclusions

412

In conclusion, we successfully fabricated a magnetic supramolecular polymer composite via a

413

two-step method. Based on the systematic characterizations and batch adsorption experimental

414

results, superior adsorption uptake (946.9 mg L–1 to Pb(II) at pH = 4.0, T = 298 K), and favourable

415

reusability make the M-SMP an efficient adsorbent in the treatment of heavy metal pollution. In

416

addition, the accumulation of Pb(II) on the M-SMP was attributed mainly to the formation of

417

Pb···N and Pb···S bonding, which was jointly demonstrated by investigation of FT-IR and

418

analyzing of XPS. Furthermore, the exploration of capturing Pb(II) in lead-acid battery wastewater 21

419

was performed, and desired results were achieved. In consideration of its easy preparation and

420

separation, rapid adsorption rate and ultrahigh Pb(II) adsorption capacity, the as-prepared M-SMP

421

could be a competitive purifier adsorbent in the remediation of industrial wastewater containing

422

heavy metal.

423

Conflicts of interest

424

There are no conflicts to declare.

425

Acknowledgements

426

This work was supported by the NSFC (Grants No. 21677047, U1604137 and 51808200), Science

427

and Technology Key Program of Henan Province (No.172102310698), Natural Science foundation

428

of Henan Province (No.182300410154), the China Postdoctoral Science Foundation (Grant No.

429

2018M630825) and Plan for University Scientific Innovation Talent of Henan Province

430

(19HASTIT046).

431

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28

Highlights:


Magnetic supramolecular polymer was fabricated by facile hydrothermal strategy.




As-synthesized composites demonstrated ultrahigh adsorption capacity toward Pb(II).




High-performance was mainly account of coordination bonds of Pb···N and Pb···S.




Application in real Pb(II)-contaminated battery wastewater were performed.

Declaration of interests ☑ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: