Thiol-functionalized magnetic covalent organic frameworks by a cutting strategy for efficient removal of Hg2+ from water

Journal Pre-proof Thiol-functionalized magnetic covalent organic frameworks by a cutting strategy for efficient removal of Hg2+ from water Lijin Huang (Investigation) (Writing - original draft) (Funding acquisition), Rujia Shen (Software)Data analysis), Ruiqi Liu (Writing - review and editing), Qin Shuai (Supervision) (Resources) (Funding acquisition)

PII:

S0304-3894(20)30308-3

DOI:

https://doi.org/10.1016/j.jhazmat.2020.122320

Reference:

HAZMAT 122320

To appear in:

Journal of Hazardous Materials

Received Date:

8 December 2019

Revised Date:

9 February 2020

Accepted Date:

14 February 2020

Please cite this article as: Huang L, Shen R, Liu R, Shuai Q, Thiol-functionalized magnetic covalent organic frameworks by a cutting strategy for efficient removal of Hg2+ from water, Journal of Hazardous Materials (2020), doi: https://doi.org/10.1016/j.jhazmat.2020.122320

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Thiol-functionalized magnetic covalent organic frameworks by a cutting strategy for efficient removal of Hg2+ from water

Lijin Huang*, Rujia Shen, Ruiqi Liu, Qin Shuai* Engineering Research Center of Nano-Geomaterials of Ministry of Education, Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan), No.

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388, Lumo Road, Hongshan District, Wuhan 430074, P. R. China.

*

Corresponding Author.

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Tel: 86-27-67884283; E-mail address: [email protected]; [email protected]

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Lijin Huang: 0000-0002-0555-7823

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ORCID

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Graphical Abstract

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Highlights

Thiol functionalized Magnetic COFs have been synthesized through a facile

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

strategy.

The resulstant composite featuring high specific areas contains many accessible

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

chelating sites.

The material exhibited superior capture ability toward Hg2+ with high selectivity.

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The composites can be easily recycled for reuse without loss its adsorption ability.

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

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Abstract: Covalent organic frameworks (COFs) have attracted tremendous attention due to their excellent performance in wastewater remediation, but their practical application still suffers from various challenges. The development of highly-efficient magnetic COFs along with fast adsorption kinetic and high adsorption capacity is very promising. To achieve the purpose, thiol-functionalized magnetic covalent organic frameworks (M-COF-SH) with abundant accessible chelating sites were designed and

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synthesized by utilizing disulfide derivative as building blocks and subsequently cutting off the disulfide linkage. After the cutting process, the crystallinity, porosity,

superparamagnetism of pristine M-COF are well maintained, and the resultant M-COF-

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SH turned out to be an effective and selective platform for Hg2+ capture from water.

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Impressively, the resulting composite exhibited a maximum adsorption capacity of Hg2+ as high as 383 mg g-1. In addition, it also displays a rapid kinetic, where the adsorption

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equilibrium can be achieved within 10 min. More importantly, there is no significant

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loss of its adsorption performance even after recycling 5 times. This work not only offers a reliable platform for wastewater remediation but also provides a conceptual

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guide to prepare functionalized M-COF composites which cannot be obtained through

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conventional approaches.

Key-words: Covalent organic frameworks; cutting strategy; magnetic composites; heavy metal ion removal; wastewater remediation; adsorption

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1. Introduction Water pollution caused by organic/inorganic contaminants has become a serious worldwide problem and attracted tremendous attention. Among various contaminants, mercury has drawn increasing interest due to its potential adverse effects on the ecosystems and human health [1]. Therefore, scientists have been getting rather animated about this issue. It is imperative to develop powerful technologies to remove

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mercury pollutant from aqueous solution [2]. Various methods, including ion exchange [3], membrane separation [4] and adsorption [5], have been developed for the removal

of mercury from water/wastewater. Compared to other methods, adsorption stands out

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because of its low cost, high efficiency and simplicity [6,7]. In addition, secondary

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pollution such as generation of toxic by-product or resistance gene/bacteria to the environment can be dramatically reduced.

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Covalent organic frameworks (COFs) with orderly porous structure have emerged as

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one of the most promising porous materials [8]. They are constructed by organic building blocks via thermodynamically controlled reversible polymerization through

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strong covalent bonds [9-13]. Taking into account of their high specific surface area, pre-designed structure, excellent chemical stability, as well as regular pore size and

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facile functionality, COFs have been highlighted as versatile and functional platforms in the field of environmental remediation [14]. Recent studies have demonstrated that COFs with specific functionalization can be robust adsorbents for hazardous pollutant capture, such as heavy metal ions [15-17], perfluorinated alkyl substances [18] and pharmaceutical contaminants [19-21]. Furthermore, to improve their performance, a 4

variety of tailored functional groups (-SCH3 [22], -SH [23, 24], -COOH [25], -NH2 [18], -SO3H [21], etc.) have been integrated into the skeleton of COFs via delicate design. Among them, sulphide functionalized (thiol or thioether) materials are efficient for Hg capture due to the strong soft–soft interaction between sulphide and mercury [26]. Generally, two main strategies, i.e. direct synthesis (also known as de novo strategy) [22] and post-synthesis [27], are utilized to achieve the goal. For the former, the

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molecular building monomers with specific functional groups are employed to prepare target materials directly, such as thioether functionalized COFs [22, 28]. Despite great

success has been achieved by this strategy, the complex or incompatible monomers

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impede its further application greatly. As an alternative, post-synthesis strategy [27],

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being regarded as a more reliable approach towards functional COFs, has attracted tremendous worldwide attentions. Post-synthesis strategy has been employed to

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construct numerous functional COFs that are difficult to obtain directly. It is

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encouraging that various reactions, such as esterification reaction [29], “click” reaction [23], and hydrolysis reaction [15], can be utilized for post-modification. For instance,

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a 2D mesoporous COF decorated with flexible thiol and thioether chelating groups (COF-S-SH) could be prepared via modifying vinyl-functionalized COFs through

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thiol−ene “click” reaction [23]. Taken as adsorbent, COF-S-SH can remove mercury from aqueous solution or air with a high efficiency. Thiol grafted imine-based COFs (TPB-DMTP-COF-SH) was also prepared through post-synthetic modification of an ethynyl-functionalized COF, which has been testified as a robust and selective platform for mercury sorption. However, these methods are still encountered multiple challenges 5

due to the lack of reactive groups in most reported COFs and multistep post-synthetic modifications [30]. Therefore, developing a facile, cost-effective and convenient synthesis strategy to prepare sulphur-functionalized COFs that cannot be obtained directly is highly desirable. Magnetic COF composites (M-COFs) inheriting the advantages of COFs and magnetic materials simultaneously has been regarded as a promising and efficient

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platform for adsorptive removal of toxic contaminants [31-33]. However, the reported studies regarding M-COF-based adsorbents for metal ions removal is still limited because of the very early stage of fundamental research. Herein, for the first time, a

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facile salt-mediated crystallization approach and subsequent cutting strategy (Scheme

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1) are utilized to construct thiol functionalized M-COF by utilizing commercially available reagents, namely M-DPAS50-COF-SH, which can’t be obtained through direct

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synthesis. Taking advantage of salt-mediated crystallization, M-COF with high

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crystalline and ultra-porous structure can be obtained in a high yield within seconds. Subsequently, cutting off the disulfide linkage endows the resulting material with an

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exceptionally high density of -SH functional groups, and the topology of parent crystal can be well maintained. The resultant composite was well charaterized and

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subsequently utilized as adsorbent for the capture of Hg2+ from water. The adsorption kinetic and isotherm were investigated. The effects of solution pH and co-existing ions were assessed. What’s more, recycling experiment were carried out to evaluate the regeneration ability of M-DPAS50-COF-SH. Finally, the interaction mechanism between resultant material and Hg2+ was adequately discussed. 6

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Scheme 1 Schematic diagram for the synthesis of M-COF-SH.

2.1 Chemicals and materials

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2. Experimental

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4,4'-diaminodiphenyldisulfide (DAPS), p-toluenesulfonic acid (monohydrate) (PTSA), p-Azoaniline (Azo), dimethylacetamide (DMAc), acetone, tetrahydrofuran (THF) and

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tributylphosphineacid (TBP) were purchased from Sinopharm Chemistry Reagent Co. Ltd. All chemicals utilized in this study were analytical grade and used without further

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purification. 1, 3, 5-triformylphloroglucinol (Tp) were prepared using the existing

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method reported in the literature [34]. Amine functionalized magnetic nanoparticles Fe3O4@SiO2-NH2, termed as MNP-NH2, was prepared according to our previous method (see Supplementary Materials for detail) [35]. A stock solution of 1000 mg L-1 of Hg2+ was prepared from Hg(NO3)2, and the working solutions with specific concentrations were diluted step-wise accordingly. 2.2 Preparation of M-DAPSx-COFs 7

Three different M-DAPSx-COFs composites with different molar ratios of DAPS/Azo were synthesized, where X%=nDAPS/(nDAPS+nAzo)=0%, 50% and 100%. Remarkably, 5.0 mmol (951.0 mg) of PTSA, 0.45 mmol (95.5 mg) of Azo, 0.45 mmol (111.8 mg) of DAPS and 100.0 mg of MNP-NH2 were well mixed in a mortar for 5 min. Subsequently, 0.60 mmol (126.0 mg) of Tp was added and the mixture was thoroughly ground for another 10 min before 40 μL water was added. After the additional 5 minutes grounding,

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the well-mixed sample was transferred into a glass vial and placed in a preheated oven for 1 min at 170 °C. After cooling to room temperature, the resultant product was

thoroughly washed with water, DMAc and acetone several times, and then the resultant

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was dried under vacuum at 60 oC overnight. M-DAPS50-COF (405 mg) was obtained

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with a high yield of ~95%. 2.3 Preparation of M-DAPS50-COF-SH

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200 mg of M-DAPS50-COF was suspended in 12 mL of THF/water (Volume/Volume

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=5/1) mixture in an argon atmosphere. Then 0.5 mL TBP was added and the mixture was stirred at room temperature for 24 h. After that, the product was collected by using

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an external magnetic field and washed with THF and ethanol for several times. At last, the resultant M-DAPS50-COF-SH (in a yield of 95%) was dried at 25 °C for 24 h under

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vacuum.

2.4 Apparatus The crystallinity of the resultant materials was tested by Powder X-ray diffraction (PXRD) pattern, which was recorded on a D8-Discover X-ray diffractometer (Bruker, German) using Cu Kα radiation (40 kV, 40 mA) with a scan rate of 2° min-1. Thermal 8

stability was identified by means of thermal gravimetric analysis (TGA), which was carried out on PE diamond TG/DTA 6300 (USA) with a heating rate of 10 oC min-1 under N2 atmosphere. N2 adsorption-desorption curves were measured at 77 K on an ASAP 2020 apparatus (Micromeritics, USA). The specific surface areas and pore size distribution were estimated via Brunauer-Emmet-Teller (BET) and Barrett-JoynerHalenda (BJH) method, respectively. The determination of target metal ions was

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achieved on an Intrepid XSP Radial inductively coupled plasma optical emission

spectrometry (ICP-OES) (Thermo, Waltham, MA, USA) with a concentric model nebulizer and a cinnabar model spray chamber. Infrared spectra were collected on

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NEXUS 870 Fourier transform infrared spectrometer (FT-IR) (Thermo, Madison, USA).

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Transmission electron micrograph (TEM) images were obtained by means of JEM2010 electron microscope (Tokyo, Japan). The magnetic properties were investigated

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by a vibrating sample magnetometer (VSM, PPMS-9, QUANTOM, USA). Surface

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morphology and chemical composition of the as-prepared samples were recorded on X650 scanning electron microscopy (SEM, Hitachi, Tokyo, Japan) fitted with an energy

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dispersive X-ray (EDX, SUTW-SAPPHIRE) system. X-ray photoelectron spectroscopy (XPS) characterization was carried out using an ESCALAB 250 XPS (Thermo Fisher

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Scientific Inc., Waltham, UK) with Al Kα X-ray as the excitation source. 2.5 Adsorption investigation Hg2+ uptake experiment was carried out by mixing the adsorbent with the solution and then shaken for a certain time. Subsequently, the magnetic sorbent was isolated from the mixed solution by means of a permanent magnet. ICP-OES was utilized to 9

determine the concentration of Hg2+ in initial solution and supernatant after adsorption. To estimate the influence of solution pH, the pH of initial solution was adjusted by NaOH (0.1 M) or HNO3 (0.1 M), and measured by a pH meter. The adsorption capacity (Qe) was calculated according to the equation Qe=(C0-Ce)×V/m, where C0 and Ce are the concentrations of Hg2+ at initial and equilibrium; V represents the volume of solution and m is the mass of adsorbent.

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For adsorption isotherms, 5 mg as-prepared material was added into 10 mL Hg2+ solution with various initial concentrations (C0=50, 100, 150, 200, 250 and 300 mg L1

). After incubated at 25 oC for 12 h, the concentration of Hg2+ in the solution before

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and after treatment was determined by ICP-OES. And the Freundlich [36] (Eq. (1)),

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Langmuir [37] (Eq. (2)), Temkin [37] (Eq. (3)) and BET [36] (Eq. (4)) models were utilized to analyse the adsorption isotherms.

Eq. (1)

Eq. (2)

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Ce 1 Ce   Qe QmKL Qm

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1 log Qe  log KF  log Ce n

Eq. (3)

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Qe = BlnKt+BlnCe

Eq. (4)

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Ce 1 Kb  1 Ce   Qe(Cs- C) e KbQm KbQm Cs

Where Qe is the amount of Hg2+ ion adsorbed on the adsorbent at equilibrium (mg g−1), Ce is the equilibrium concentration of Hg2+ (mg L−1), KF is Freundlich constant (min-1); Qm is the mono-layer adsorption capacity (mg g−1), and KL is the Langmuir constant (L mg−1), Kt and B are the Temkin constant (L mg−1) and heat of sorption (J mol-1), Kb is the constant that represents the energy of interaction with the surface and Cs is 10

saturation concentration of solute (mg L-1). For adsorption kinetics, 50 mg M-COF-SH was added into 50 mL Hg2+ (20 mg L-1) solution at room temperature. 2.0 mL mixture were taken out at predetermined time intervals (2, 5, 10, 20, 30, 40, 50, 60 minutes), and then the corresponding supernatant was subjected to ICP-OES for analysis. In general, the pseudo first-order model (Eq. (5)), pseudo second-order model (Eq. (6)) and Weber and Morris model (Eq. (7)) are

ln(Qe-Qt)=lnQe-K1t

Qt  Kit1/2  C

Eq. (5) Eq. (6)

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t t 1   Qt Qe K 2Qe2

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used to simulate the adsorption kinetic.

Eq. (7)

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Where K1 (min-1), K2 (g mg-1min-1) and Ki (g mg-1min-0.5) are the pseudo-first-order,the

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pseudo-second-order and diffusion rate constants; Qt and Qe represent the amount of target adsorbed at time t (min) and equilibrium (mg g-1); C (mg g-1) is intercept of the

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intraparticle diffusion model.

Selectivity test was carried out by spiking 5 mg M-COF-SH into the mixed solution

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containing Hg2+, Cu2+, Co2+, Cd2+, Cr3+, Mn2+, Ni2+, Pb2+, K+, Na+ and Ca2+. The

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concentration of each ion was 10 mg L-1. To evaluate the reusability, Hg2+ loaded M-DAPS50-COF-SH sample was treated

with the mixed solution of 0.01M HCl+ 0.1% thiourea. After being isolated from the aqueous solution by applying an external magnet field, the sample was rinsed with deionized water until pH reached 6–7 and then used for next adsorption test. 3. Results and discussions 11

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3.1 Materials preparation and characterization

Fig. 1 XRD patterns (a), FT-IR spectra (b), N2 adsorption-desorption isotherms (c),

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and TG curves (d) of MNP-NH2, M-DAPSX-COFs and M-DAPS50-COF-SH.

condensation

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A three-component reaction system was employed to synthesis M-DAPSx-COFs by the of

Tp

with

DAPS

and

Azo

at

different

molar

ratios

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(X%=nDAPS/(nDAPS+nAzo)=0%, 50% and 100%). The density of disulfide functional groups in the frameworks was adjusted by varying the content of monomers.

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Subsequently, to achieve effective materials with enhanced adsorption performance, a cutting strategy was employed to prepare thiol functionalized M-COF. According to previous reports, disulfide can be easily converted into thiols by using NaBH4 or TBP as reductant [38, 39]. The formation of imine covalent bonds and integration of MNPs into COFs 12

framework was confirmed by FT-IR and XRD characterization (Fig. 1(a) and (b)). The peak at ~1090 cm-1 corresponding to Si-O stretching vibration provides solid evidence for the presence of MNPs [35]. After polymerization, the peaks assigned to C-N and C=C stretching were clearly observed at 1258 and 1568 cm-1, indicating the successful condensation between amine and aldehyde [40]. And the peak at ~512 cm-1 attributed to the stretching vibration of disulfide [41] was observed in the spectra of DAPS (Fig.

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S1(a)), M-DAPS50-COF and M-DAPS100-COF, which undeniably verified the

introduction of disulfide into the porous frameworks. But, after the reduction, its

intensity is reduced due to the rupture of disulfide. Nevertheless, M-DAPS50-COF-SH

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held a similar spectra with that of M-DAPS50-COF and the new peak assigned to -SH

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(around 2500 cm-1) was not well observed owing to its poor sensitivity in IR characterization [42].

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To further evaluate the disconnection of disulfide, XPS spectra of pristine M-

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DAPS50-COF and resultant M-DAPS50-COF-SH before and after reducing process were studied. Compared to that of M-DAPS50-COF, as illustrated in Fig. S1(b), the

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binding energy of S 2p for M-DAPS50-COF-SH shifted toward a lower level (0.3 eV), which is in agreement with the binding energy change previously reported [43]. These

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results indicated that the cleavage of disulfide has been achieved successfully. The content of sulfur was found to be 8.35 wt% through elemental analysis, corresponding to 2.6 mmol g−1 sulfur species. The corresponding EDX characterization (Fig. S2) also provided ample evidence for the presence of abundant sulphur (~2.4 mmol g-1) in the resultant composites. Such considerable high density of acceptable binding sites are 13

favorable for subsequent application. XRD diffraction patterns of different samples are present in Fig. 1 (a). For all magnetic samples, the characteristic patterns at 31o and 35o were observed, which are consistent with the published data of crystalline Fe3O4 (JCPDS file, No. 19-0629), demonstrating the successful incorporation of MNPs. And the same XRD patterns corresponding to TpAzo-COF [40] were both observed in MDAPS0-COF and M-DAPS50-COF, revealing that the intrinsic crystalline structure of

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COFs was well retained after the introduction of MNPs. However, the patterns

corresponding to COFs disappeared for M-DAPS100-COF, which exhibited in an

effect on the crystalline structure of MCOFs.

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amorphous form. Thus result manifested that the ratios of DAPS have a considerable

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The surface areas of M-DAPSX-COFs (Fig. 1(c) and Table 1) exhibited a decrease tendency with increasing the ratios of DAPS in the precursors, indicating that the

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introduction of DAPS profoundly affected the formation of crystalline structure, which

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was consistent with the result of XRD characterization. It is noteworthy that the BET surface area of M-DAPS50-COF-SH was reduced mainly due to the disconnection of

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disulfide. Nevertheless, it is still as high as 181.5 m2 g-1, implying that pores are still available after reduction reaction.

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TGA measurements were conducted to investigate the thermostability of resultant

materials. As shown in Fig. 1(d), the weight loss of MNP-NH2 was less than 10% even the temperature was up to 800 oC, demonstrating its high thermal stability. For others composites, a slight weight loss (<5%) was observed around 60 oC, which can be attributed to the escape of adsorbed water/solvent in the pores. No significant weight 14

loss was observed under 350 oC for M-DAPS0-COF and M-DAPS50-COF-SH, indicating that the introduction of disulfide into the frameworks of COF has a negligible effect on the thermal stability. In addition, after the cutting process, the resultant MDAPS50-COF-SH is still of high thermal stability. The corresponding morphologies of the resultant samples were characterized. SEM images clearly revealed the porous structure of resultant materials, resulting from the

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stacking interactions of sheets or platelet-like polymer crystallites (Fig. S3). TEM measurement was further utilized to explore the morphology of resultant samples. The

result clearly confirms that MNP-NH2 nanoparticles were incorporated into the porous

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framework of COFs successfully, which was verified by elemental mapping (Fig. S4).

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And no remarkable morphology difference was found before (Fig. S3 (a) and (c)) and after (Fig. S3 (b) and (d)) the reduction process.

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Magnetic hysteresis loops of MNP-NH2, M-DAPS50-COF and M-DAPS50-COF-SH

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were showed in Fig. S5. The saturation magnetization of MNP-NH2 was 46.0 emu g-1, which was much higher than that of M-DAPS50-COF and M-DAPS50-COF-SH. The

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decrease was attributed to the introduction of non-magnetic COFs. And there is no surprise to find that M-DAPS50-COF (20.2 emu g-1) and M-DAPS50-COF-SH (19.6

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emu g-1) displayed similar saturation magnetizations due to their similar composition. The saturation magnetization of M-DAPS50-COF-SH was strong enough to be easily manipulated and isolated rapidly from solution with a magnet. 3.2 Adsorption investigations

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Removal efficiency (%)

100

(b) 80

60

40

20

0

Cd Co Cr Cu Mn Ni Pb K Ca Na Mg Al Hg

Fig. 2 Comparison studies of adsorption amount of Hg2+ under different pH with M-

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DASP50-COF and M-DASP50-COF-SH (5 mg adsorbent in 10 mL 300 mg L-1 Hg2+ solution) (a); Hg2+ removal efficiency in the presence of other metal ions using M-

3) (b).

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DASP50-COF-SH as adsorbent (the concentration of each ion was 10 mg L-1 and pH

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During the ions removal process, solution pH plays an essential role. The adsorption behaviour of resultant materials toward Hg2+ was studied under pH2-7. As shown in

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Fig. 2(a), the adsorption amount of Hg2+ over M-DAPS50-COF was strongly affected

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by solution pH due to the competition of hydrogen ions in water [24]. Nevertheless, compared with that of M-DAPS50-COF, the removal ability of M-DAPS50-COF-SH for

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Hg2+ enhanced greatly, especially under acidic condition. These results provided further evidence that the disulfide functional groups have been cut off and turned into free -SH

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successfully [38]. Taking into account the pH values of industrial sewage fall in the range of 3-4 [22], M-DAPS50-COF-SH is an appropriate candidate for Hg2+ segregation from wastewater. Generally, there are numerous ions coexisting in practical Hg2+ contaminated water, so it is a very challenging task to remove Hg2+ effectively. To clarify the effect of 16

coexisting ions, the adsorption performance of Hg2+ over M-DAPS50-COF-SH in the presence of others ions (K+, Na+, Mg2+, Cu2+, Ca2+, Co2+, Cd2+, Pb2+, Mn2+, Ni2+, Cr3+, and Al3+) were investigated. As illustrated in Fig. 2(b), the removal efficiency of Hg2+ still remained above 99%, indicating that the competitive effect of additional foreign ions was fairly small. The reason for the comparatively high selectivity of DAPS 50COF-SH toward Hg2+ was mainly caused by the strong soft-soft interaction between S

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atom and Hg2+, which has also been found on other sulphur functionalized materials

[24, 26]. In short, the result suggests that DAPS50-COF-SH is a suitable candidate for the capture of Hg2+ with high selectivity.

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To reveal the kinetic involved in adsorption process, the influence of contact time

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was investigated by exposing 25 mg DAPS50-COF-SH to 50 mL Hg2+ solution (C0=20 mg L-1). As shown in Fig. 3(a), ~95% of Hg2+ in the test solution can be captured onto

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DAPS50-COF-SH within 20 min, and more than 99% were removed at equilibrium. The

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rapid kinetic is benefited from its high specific surface area. In addition, the strong affinity between –SH and Hg2+ is also responsible for the highly efficient capture. To

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understand the kinetic mechanism, the experimental data were further analysed by using the pseudo-first-order model [44], the pseudo-second-order [35] and Weber and

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Morris model [45] (Fig. S6). The corresponding parameters obtained from all models were summarized in Table S1. The results showed that the value of R2 for the pseudosecond-order kinetic model (0.999) was higher than that of the pseudo-first-order (0.964), indicating the pseudo-second-order kinetic model fitted better with experimental data. Moreover, the value Qe (20.69 mg g-1) calculated from the pseudo17

second-order kinetic model is also agreement with the experimental data (20.0 mg g-1). As can be seen from Fig. S6(c), the plots of Qt versus t0.5 was not a straight line that pass through the origin, according to the Webber and Morris model, suggesting that the rate-controlling process was not only determined by intra particle diffusion, in other word, both intra-particle diffusion and film diffusion are involved. [45] Therefore, the rate controlling step was deemed to be controlled by chemisorption, intra-particle

0 0

10

20

30

40

50

(b)

320 240 160 80

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5

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10

400

-p

15

-1

(a)

20

Adsorbed amount (mg g )

Concentration (mg L

-1

)

diffusion and external diffusion for the adsorption of Hg2+ over DAPS50-COF-SH.

0

60

30

60

90

-1

120

Concentration (mg mL )

Time (min)

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Fig. 3 Effect of contact time (a) and initial concentration (b) on Hg2+ removal by M-

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DAPS50-COF-SH.

For a valuable adsorbent, its adsorption capacity is also one of the most decisive factors.

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The determination of adsorption capacity was done by investigating its adsorption ability under different initial Hg2+ concentration. Remarkably, as initial concentration

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of Hg2+ increased, the adsorbed amount of Hg2+ climbed rapidly and then reached a plateau (Fig. 3(b)). The maximum adsorption capacity of Hg2+ over M-DAPS50-COFSH was 383 mg g-1, which is comparable to many other reported adsorbents (Table 2). It was lower than some COF-based materials, but much higher than other existing magnetic COFs composites. Besides, theadditional advantages including fast 18

construction, easy operation and cost-efficient make this resultant material as a fantastic adsorbent. Two well-known and widely accepted isotherm models, namely the Freundlich (Eq. (1)), Langmuir (Eq. (2)), Temkin (Eq. (3)) and BET (Eq. (4)) models [36, 37], were applied to study the experimental data (Fig. S7). Their corresponding parameters (Table S2) were obtained through Eq. (1)-(4), respectively. It is worthy

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360

270

180

90

0 1

2

3

-p

-1

Adsorption amounts (mg g )

noted that the Langmuir model fitted better with the experimental data with a higher

4

5

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Number of cycles

Fig. 4 Recycle investigation for Hg2+ removal by using M-DAPS50-COF-SH.

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Regeneration ability of the adsorbent plays a significant role for their practical

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application as it can not only avoid the secondary pollution, but also reduce the cost greatly. To achieve this goal, the adsorbent was rinsed with 0.01 M HCl+ 0.1 % thiourea.

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Inspiringly, as shown in Fig. 4, the removal capacity of the adsorbent is substantially unchanged after 5 times recycling. In addition, many characterizations, including FT-

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IR, XRD, N2 adsorption-desorption isotherms and TEM, were carried out to investigate the stability of reused M-DAPS50-COF-SH (Fig. S8). The results demonstrated that MDAPS50-COF-SH was robust enough to be readily available for wastewater remediation. To gain insight into the interaction mechanism between Hg2+ and DAPS50-COF-SH, the FT-IR spectra of DAPS50-COF-SH before and after Hg2+ loading were measured 19

primarily. As shown in Fig S8(a), compared to pristine sample, a new peak at 1388 cm1

, attributed to the S-Hg stretching, was clearly observed, suggesting the bond formation

between S and Hg [35]. In addition, the elemental distribution images, displayed in Fig. 5, provide additional evidence for the successful capture of Hg2+ by DAPS50-COF-SH. Interaction between Hg and DAPS50-COF-SH was further evaluated by XPS characterization. The appearance of peaks at 101.4 and 105.4 eV assigned to Hg 4f7/2

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and Hg 4f5/2 clearly confirms the existence of Hg [26]. Compared to Hg(NO3)2, the binding energy for both peaks shift (1.5 V) toward a higher value, which was in agreement with the energy change reported by previous work [24]. Moreover, compared

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with that of Hg-loaded sample, the corresponding binding energy of S 2p in M-DAPS50-

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COF-SH showed a slight shift (~ 0.4 eV), demonstrating the involvement of S functional groups during adsorption of Hg [38]. The results revealed that the rich thiols

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groups are essential for the effective capture of Hg2+ from water.

Fig. 5 XPS spectra of S 2p (a) and Hg 4f (b) of M-DAPS50-COF-SH and Hg-loading sample.

4. Conclusions In conclusion, thiol-functionalized M-COF composite has been successfully prepared 20

by incorporating disulfide monomer into the framework and subsequent cutting off strategy. Compared to other conventional methods, the preparation process was greatly simplified. Notably, M-DAPS50-COF-SH exhibited exceedingly capacity for Hg2+ sequestration due to its abundant thiol functional groups and porous structure. In addition to its high adsorption capacity (383 mg g-1), the resultant composite also can be easily recovered from the system after adsorption and shows excellent stability,

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demonstrating its tremendous application potential for the elimination of heavy metal

ions from contaminated water. The cutting-off strategy provides a feasible methodology for the fabrication of versatile materials in the field of wastewater remediation and

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beyond.

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Credit authorship statement

Lijin Huang: Investigation, Writing - original draft, Funding acquisition. Rujia Shen:

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Resources acquisition.

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Software, data analysis. Ruiqi Liu: Review & editing. Qin Shuai: Supervision,

Notes

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The authors declare no competing financial interest.

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Credit authorship statement Lijin Huang: Investigation, Writing - original draft, Funding acquisition.. Rujia Shen: Software, data analysis. Ruiqi Liu: Review & editing. Qin Shuai: Supervision, Resources acquisition.

21

Acknowledgements This work was supported by the National Natural Science Foundation of China (21806148) and Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (Grant No. CUG170102; Grant No. CUG180610). References [1] J. Yang, Y. Zhao, S. Ma, B. Zhu, J. Zhang, C. Zheng, Mercury removal by magnetic

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Table 1 Surface areas and pore volumes of different samples. BET Surface area (m2 g-1)

Pore volume (cm3 g-1)

MNP-NH2

48.6

0.10

M-DAPS0-COF

511.4

0.38

M-DAPS50-COF

446.1

0.41

M-DAPS100-COF

80.1

0.14

M-DAPS50-COF-SH

181.5

0.24

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Material

Table 2 Comparison of adsorption capacities of different adsorbents for removal of Hg2+.

Capacity (mg g-1)

Reference

COF-LZU8

236

[28]

1350

[23]

734

[22]

MPTS-mesoporous Fe3O4/C@SiO2

118.6

[46]

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COF-S-SH

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Adsorbents

Fe3O4@SiO2-SH

148.8

[47]

γ-Fe2O3@CTF-1

165.8

[48]

M-COFs

97.7

[49]

M-DAPS50-COF-SH

383

This work

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TAPB-BMTTPA-COF

correlation coefficient (R2=0.999), demonstrating the adsorption of Hg2+ over MDAPS50-COF-SH was a monolayer uniform sorption.

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