Thiol-Functionalized Magnetic Porous Organic Polymers for Highly

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Thiol-functionalized magnetic porous organic polymers for highly-efficient removal of mercury Lijin Huang, Chuyu Peng, Qian Cheng, Man He, Beibei Chen, and Bin Hu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03093 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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Industrial & Engineering Chemistry Research

Thiol-functionalized magnetic porous organic polymers for highly-efficient removal of mercury Lijin Huang, Chuyu Peng, Qian Cheng, Man He*, Beibei Chen, Bin Hu* Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), Department of Chemistry, Wuhan University, Wuhan 430072, Hubei Province, P. R. China ABSTRACT: Magnetic porous organic polymers (MOPs) with abundant thiol groups were synthesized successfully in high yield through a template-free and catalyst-free diazo-coupling reaction. The reaction was conducted under mild conditions in aqueous solution, in which the introduction of magnetism and thiol-functionalization was realized simultaneously, avoiding the use of environment-unfriendly organic solvents. The magnetic nanoparticles (MNPs) were embedded into hierarchical porous network structures of porous organic polymers (POPs) physically and the magnetism of thiol-functionalized MOPs (MOP-SH) was easily controlled by varying the amount of spiked MNPs. The obtained MOP-SH exhibited high thermal stability and chemical stability within a wide pH range (2-13), and good adsorption performance for Hg(II) over a wide pH range due to the abundant thiols in its hierarchical structure. After the adsorption process by using MOP-SH, the concentration of Hg in the spiked domestic sewage reached 1.1 µg L-1, which is even lower than the acceptable limit of national standard for drinking water (2 µg L-1). Besides, the prepared MOP-SH exhibited high adsorption capacity, fast adsorption kinetics and easy-recycling behaviour, providing a new avenue for the preparation of green functionalized adsorbents with good performance for water decontamination. KEYWORDS: Porous organic polymers, magnetic separation, diazo-coupling reaction, pollutant removal, mercury, adsorption

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1. INTRODUCTION Over the past decade, the discharge of heavy metal ions not only caused economic crisis but also deteriorated environmental pollution status.1-3 As the heavy metal ions are hardly decomposed in the nature, they show high toxicity and would cause a lot of severe diseases. Presently, removal of the toxic mercury from aqueous solution is very attractive, and also a great challenge to scientists. For the last few years, a variety of strategies have been developed for removing mercury from water, including chemical precipitation,4 ion exchange,5,6 membrane separation,7 and adsorption8. Among them, adsorption is one of the most promising water treatment technology due to its simple operation process and high efficiency.9 Different sorbents have been employed for the removal of Hg from polluted water. Traditional adsorbents such as activated carbon (AC), zeolite, and clays suffer from low capacity and poor selectivity due to the scanty adsorption sites on the inert surface. Taking advantage of strong soft-soft interaction of sulphur with mercury, a number of sulphur-functionalized adsorbents have been explored for mercury elimination, such as thiol-functionalized mesoporous silica,10 mesoporous carbons,11,

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metal-organic

frameworks (MOFs)13, 14 and porous organic polymers (POPs).2, 15-18 Due to their high surface areas and flexible structure, both of MOFs and POPs offer an attractive platform for Hg removal. While, the relative poor chemical stability and the presence of metal ions in MOFs seriously hampered their practical application19. In contrast, POPs, constructed through high stable covalent bonds between different organic monomer, are featured with low framework density due to the use of light elements (C, H, N, O, and B), low toxicity (no heavy metal ions involved), excellent porosity, and good physicochemical stability.20-22 Very recently, a variety of POPs with different functional groups have been constructed and used for the removal of metal ions.2, 15, 17, 18, 23, 24 Among them, the thiol/thioether functionalized POPs are attractive and have been demonstrated to be superior sorbents for Hg removal based on the soft-soft interaction between Hg and S.25 Nevertheless, the preparation process generally involves noble-metal catalysts, long reaction time, high reaction temperature, the use of organic 2


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solvents (e.g., dimethylformamide or dimethyl sulfoxide) and difficult recovery of products, which is relatively harsh and limits their practical application. Hence, developing convenient and cost-effective strategy for the preparation of functionalized POPs remains in urgent demand. In this contribution, a thiol-functionalized magnetic POPs (denoted as MOP-SH) with mesoporous structure, adjustable magnetism as well as abundant thiol functional groups were successfully achieved through a convenient one-step synthesis strategy. The MOP-SH was obtained from the commercially available benzenedithiol and monomers-1,3,5-tris(4-aminophenyl)benzene via diazo-coupling reaction in aqueous solution without any template under mild conditions, and MNPs were embedded into polymer network during the polymerization process (Scheme 1). The resulting MOP-SH possessed abundant accessible thiols, high surface areas with hierarchical mesoporous structure, and exhibited good adsorption performance for Hg(II) over a broad range of pH (2-13). The simple synthesis process, high density of inherent thiol groups, plenty of accessible Hg binding sites and easily-recycling property make MOP-SH an ideal candidate for mercury removal from wastewater.

SH

SH N N N

N

N N SH N

SH

NN

NH2

MNPs

1. HCl, NaNO2, 0-5 oC

SH

N

NN SH

HS

HS

SH NN

2. NaOH, MNPs, 0-5 oC

NN

SH H2N

NH2 N N

SH

SH N N NN

NN

SH

SH

Scheme 1. Synthesis route of MOP-SH. 2. EXPERIMENTAL 2.1 Chemicals and materials Benzenedithiol

was

bought

from

Tokyo

Chemical

Industry

Co.,

Ltd.

1,3,5-Tris(4-aminophenyl)benzene was supplied by Aladdin (Shanghai, China). 3



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Hg(NO3)2 was used for preparing Hg(II) stocking solution (1000 mg L-1) and all the working solutions were prepared by stepwise dilution of the stock solution accordingly. 2.2 Material Characterization The concentration of target metals ions in solution was determined by Intrepid XSP Radial inductively coupled plasma optical emission spectrometry (ICP-OES) (Thermo, Waltham, MA, USA). PPMS-9 vibrating sample magnetometer (VSM) (QUANTOM, USA) was used to investigate the magnetic properties of the MOPs. Transmission electron micrograph (TEM) images were collected by JEM-2010 electron microscope (Tokyo, Japan). Thermodynamic analysis (TGA) was measured in nitrogen atmosphere on PE diamond TG/DTA 6300 (USA) and the heating speed was 5 oC min-1. X-ray photoelectron spectroscopy (XPS) characterization was performed via an ESCALAB 250 XPS with Al Kα X-ray as the excitation source. ASAP 2020 apparatus (Micromeritics, USA) was used to measure the Brunauer-Emmet-Teller (BET) surface area. The Fourier transform infrared spectrums (FT-IR) of the as-prepared composites were measured by a NEXUS 870 spectroscopy (Thermo, Madison, USA). 2.3 Synthesis of MOP-SH The Fe3O4@SiO2 magnetic nanoparticles (MNPs) (~15 nm) were obtained via the previous method.26 1,3,5-Tris(4-aminophenyl)benzene (1.0 mmol) was dissolved in water (100 mL) containing 0.7 mL of concentrated hydrochloric acid. The mixture was stirred for 15 min under ice bath. Then, 10 mL of NaNO2 (3.1 mmol) aqueous solution was added. The mixture was stirred for 30 min and then neutralized with diluted NaOH solution, resulting in solution A. On the other hand, benzenedithiol (1.5 mmol) and MNPs (200 mg) were dissolved in 30 mL of 0.1 mol L-1 NaOH solution, resulting in solution B. Solution B was ultrasounded for 20 min and then added into solution A under stirring at 0-5 oC. The resulting mixture was stirred overnight. The product was collected by using external magnetic field and washed with water and ethanol for several times. MOP-SH was obtained through freeze drying with a high yield (93%). Similarly, the azo linked POPs with thiol functionalization (AzoPOP-SH) (in yield of 4


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95%) was prepared using the same process without MNPs addition. 2.4 Adsorption/desorption experiments The adsorption of Hg(II) on magnetic adsorbents were studied using batch operation. 10 mg of as-prepared MOP-SH was suspended in 10 mL of Hg(II) aqueous solutions (pH 3.0) with different concentrations (50-1000 mg L-1). Then the mixture was shaken for 2 h continuously and the concentration of Hg(II) in supernatant was determined by ICP-OES. All adsorption tests were conducted in triplicate. The regeneration of the magnetic adsorbents was conducted as follows: after adsorption experiments, the Hg(II) loaded MOP-SH was separated using a magnetic, stirred in 10 mL 1.0 M HCl containing 1% thiourea for 30 min and washed with deionized water. The regenerated adsorbent was then separated from solution and used for next adsorption. 3. RESULTS AND DISCUSSION 3.1. Characterizations FT-IR measurements

Fig. 1 FT-IR spectra (a) and N2 adsorption-desorption isotherms (b) of different samples. To validate the formation of MOP-SH, FT-IR characterization was carried out and the data are presented in Fig. 1 (a). Notably, the peaks around 1405 cm-1 and 1448 cm-1 are observed for MOP-SH and AzoPOP-SH, attributing to -N=N- stretching.26, 27 The peaks at 1011 and 1100 cm-1 confirm the presence of C-S bonds. The characteristic band of S-H around 2,558 cm-1 is observed for AzoPOP-SH, confirming benzenedithiol 5



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was incorporated into the skeleton of POPs. At the same time, a number of distinct feature peaks of 1,3,5-tris(4-aminophenyl)benzene and benzenedithiol are observed in the

spectra

of

MOP-SH

and

AzoPOP-SH.

The

peaks

of

1,3,5-tris(4-aminophenyl)benzene at 3354 cm-1 and 3209 cm-1 belong to NH2 disappears in the spectrum of AzoPOP-SH and MOP-SH. All these results indicate a successful conversion of monomers into polymer through diazo-coupling reaction. The peak at 561 cm-1 is ascribed to Fe-O stretching,28 demonstrating a successful incorporation of MNPs into AzoPOP-SH frameworks. It is also validated by EDX and TEM analysis as shown below. N2 adsorption-desorption isotherms (Fig. 1 (b)) were used to verify the formation of porous structure. Both of MOP-SH and AzoPOP-SH exhibit isotherms of type IV, indicating the presence of mesopores, and the adsorption average pore width was calculated to be 11 nm. The BET surface area of the as-prepared MOP-SH is 270 m2 g-1, which is a little lower than that of AzoPOP-SH (281 m2 g-1). This phenomenon is mainly caused by the integration of heavier and nonporous MNPs (70.5 m2 g-1).26 These results confirm a successful preparation of MOP-SH and MNPs has negligible effect on the formation of AzoPOP-SH. EDX, elemental mapping and TEM/SEM measurements

Fig. 2 SEM images of AzoPOP-SH (a) and MOP-SH (b); TEM images of AzoPOP-SH 6


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(c) and MOP-SH (d). The combination of MNPs and AzoPOP-SH was further confirmed via EDX measurements (Table 1) and element mapping characterization (Fig. S1). For MOP-SH, EDX measurement reveals a S content of 11.2% corresponding to 3.2 mmol g-1 of -SH groups in MOP-SH. Meanwhile, the obvious signal of S and Fe obtained via element mapping characterization indicates that the MNPs were embedded into AzoPOP-SH frameworks during the process of polymerization. These results are further confirmed by SEM/TEM analysis (Fig. 2). As can be seen in TEM images, compared with bare AzoPOP-SH, MOP-SH is enwrapped by a shell of POPs. Its morphology is further valuated by SEM, demonstrating that the MNPs were embedded in POPs during the in-situ polymerization. Table 1 The relative contents (wt%) of AzoPOP-SH and MOP-SH Materials

C

N

S

Fe

Si

O

AzoPOP-SH

83.4

4.2

12.4

-

-

-

MOP-SH

58.5

3.6

11.2

12.0

1.8

13.2

XPS analysis XPS test was carried out to provide further insights into the elemental composition of MOP-SH (Fig. 3). The peak at 399.9 eV and 163.9 eV in the XPS spectrum of MOP-SH is assigned to N 1s and S 2p (Fig. S2), indicating the presence of -N=N- and -SH, respectively.27, 30, 31 In addition, the presence of strong signals corresponding to Fe and Si in the spectrum of MOP-SH further confirms that the MNPs were incorporated into the frameworks of POPs. All these results demonstrate the formation of MOP-SH by this one-pot synthesis strategy.

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Fig. 3 High resolution N 1s data (a) and high resolution S 2p data (b) for MOP-SH. Thermal/chemical stability investigation

Fig. 4 TG curves of different samples (a); FT-IR spectra of MOP-SH after immersing in water for 24 h under different pH values (b). To investigate the thermal stability of MOP-SH and AzoPOP-SH, thermo gravimetric analysis (TGA) was carried out (Fig. 4). For AzoPOP-SH and MOP-SH, they show similar weight loss trends. The weight loss (~5 %) around 110 oC for both AzoPOP-SH and MOP-SH is observed, corresponding to the loss of water present in the polymer network. Due to the decomposition of the polymer network, the weight loss around 500 oC is observed for both of MOP-SH and AzoPOP-SH. However, MOP-SH show less weight loss (~28 %) compared to the pristine AzoPOP-SH (38 %), which could be ascribed to the presence of MNPs embedded in the polymer network. As can be seen from Fig. 4(b), the FT-IR spectra of MOP-SH after immersing in water for 24 h under a wide pH range (1-13) are in agreement with that of as-prepared sample, indicating that MOP-SH is very stable in water. Magnetic properties In order to clarify the magnetic properties of MOP-SH, VSM characterization was carried out. As shown in Fig. 5, the saturation magnetization of MOP-SH is determined to be 16 emu g-1, which s lower than the pristine MNPs (71 emu g-1).26 In addition, the saturation magnetization of MOP-SH can be easily modulated by varying the amount of spiked MNPs during the polymerization (Fig. 5). These results demonstrate that the MOP-SH is superparamagnetic and can be easily manipulated through an external 8


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magnet, which makes the recovery of MOP-SH easily realized.

Fig. 5 The magnetic hysteresis curves of MOP-SH with different amount of MNPs (A: 100 mg; B: 200 mg; C: 300 mg; D: 400 mg); insets: photographs of the dispersion of MOP-SH in water and their separation by a magnet. 3.2. Adsorption of Hg(II) by MOP-SH 3.2.1 Effect of pH and coexisting ions on the adsorption

Fig. 6 Effect of pH on the removal efficiency of Hg(II) (10 mg L-1) with different magnetic composites (adsorbent concentration: 1 mg mL-1) (a); Selectivity of MOP-SH for Hg(II) conditions: each ion of 10 mg L-1 (pH 3) (b). The adsorption performance of Hg(II) from water by MOP-SH under different pH condition was investigated (Fig. 6). As can be seen, MOP-SH exhibits great adsorption behaviour for Hg(II) at a wide pH range (2-13), which is attributed to the strong soft-soft acid-base interaction between -SH and Hg(II). The high removal efficiency of Hg(II) under strong acidic/base conditions indicates that the as-prepared MOP-SH is suitable for trapping Hg(II) from industrial wastewater. There are a variety of metal ions in waste water. Therefore, the selectivity of 9



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MOP-SH for different metal ions is of great significance for practical application. We investigated the removal efficiency of Hg(II) by employing a mixed solution containing different ions (Na+, Cd2+, Al3+, Cu2+, K+, Mn2+, Co2+, Ni2+, Pb2+, Cr3+, Mg2+, Hg(II) and Ca2+). Notably, MOP-SH exhibits very high removal efficiency for Hg(II) (>99%), but relatively low removal efficiency for other ions (

Thiol-Functionalized Magnetic Porous Organic Polymers for Highly

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