Imidazolium Salt Incorporated Poly(N-vinylimidazole-co-ethylene

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Imidazolium Salt Incorporated Poly(N‑vinylimidazole-co-ethylene glycol dimethacrylate) for Efficient Adsorption of Congo Red and Hg2+ from Aqueous Solution Zhen Liu,† Gui Chen,‡ Fa Zhou,† and Jianhan Huang*,† †

College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China College of Chemistry and Materials, Huaihua University, Huaihua 418000, China

J. Chem. Eng. Data Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/16/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: Herein, imidazolium salt incorporated poly(Nvinylimidazole-co-ethylene glycol dimethacrylate) was developed for adsorption of Congo Red (CR) and Hg2+ from aqueous solution. By the introduction of N-vinylimidazole and p-dichloroxylene on the skeletons, this polymer contained not only ionized imidazole moieties but also plenty of mesopores, endowing the polymer with excellent adsorption of CR and Hg2+. The maximum capacity at 298 K reached 503.0 and 240.0 mg/g, respectively. The Langmuir model described the equilibrium data well, and a chemical interaction in forms of electrostatic interaction, hydrogen bonding, and chelating interaction was involved in the adsorption with adsorption enthalpies of 9.99 and 17.28 kJ/mol, respectively. The adsorption was a fast process, and less than 20 min was sufficient to attain the equilibrium with pseudo-second-order rate constants of 4.003 × 10−3 and 9.830 × 10−4 g/(mg·min), respectively. For example, Welton 25 reported that the quaternary ammonium and phosphonium modified ILs had removal efficiency up to 90% for Cu2+, Ni2+, and Zn2+. Gharehbaghi and Shemirani26 prepared imidazolium-functionalized ILs that can purify CR and Hg2+ by liquid−liquid extraction. Poursaberi and Hassanisadi16 synthesized IL@Fe3O4 nanoparticles and found that they are effective for adsorption of Reactive black 5 (qmax: 161.3 mg/g). Particularly interestingly, in recent years poly(ionic liquids) (PILs) are known as among the prospective polymers for adsorptive removal of azo dyes and heavy metals, and they show wide applications in catalysis, energy storage and conversion, and wastewater treatment.27−29 For instance, Gao et al.30 fabricated hydroxyl functionalized PILs. They can adsorb anionic azo dyes efficiently by electrostatic interaction and hydrogen bonding because the functional IL moieties play a significant role in the adsorption. Li et al.31 synthesized an imidazolinium-based polymer by the simple one-pot Friedel− Crafts alkylation reaction, resulting in PILs that are efficient for CO2 capture and conversion. Herein we developed a simple and low-cost synthetic procedure for the fabrication of imidazolium salt incorporated POPs. This polymer not only has ionized imidazole moieties but also has predominant mesopores, endowing it with excellent adsorption of CR and Hg2+ from aqueous solution.

1. INTRODUCTION Azo dyes are widely used in the textile, paper, leather, and cosmetics industries.1 In particular, they frequently coexist with heavy metals such as Cd2+, Cu2+, and Hg2+ in industrial effluents because these heavy metals are commonly used as the mordant in the dyeing process. Heavy metals can evidently change the physiochemical properties of the dyes, making them highly toxic, carcinogenic, and mutagenic.2−4 Thus, the treatment of wastewater containing azo dyes and heavy metals is important and pressing. Many technologies such as biological treatment,5 coagulation and flocculation,6 chemical oxidation,7 membrane filtration,8 ion exchange,9 chemical precipitation,10 and adsorption11 are extensively applied to treat combined pollutants of azo dyes and heavy metals. Among them, adsorption is the most common due to its simplicity, efficiency, economy, and no secondary pollution. Various materials such as chitosan,12 activated carbons,13 carbon nanotube,14 biomass,15 nanoparticles,16−18 organic− inorganic nanohybrid,19 and porous organic polymers (POPs)20,21 are employed for adsorptive removal of azo dyes and heavy metals from aqueous solution. The POPs are proven promising due to their high Brunauer−Emmett−Teller (BET) surface area, outstanding porosity, and diversified chemical tailorability.11,20−23 Ionic liquids (ILs) have been receiving increasing attention in recent years owing to their high thermal stability, multifunctional groups, and high ionic conductivity,24 and they are efficient for extraction of azo dyes and heavy metals. © XXXX American Chemical Society

Received: January 28, 2019 Accepted: April 4, 2019

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DOI: 10.1021/acs.jced.9b00092 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Scheme 1. Synthetic Procedure of IL-PVE

Specifically, N-vinylimidazole (VIM) was used as the IL, and ethylene glycol dimethylacrylate (EGDMA) was applied as the cross-linking agent. The imidazole-functionalized POP, namely, PVE, was first synthesized by copolymerization. pDichloroxylene (DCX) was then introduced as the cationization agent, the nucleophilic substitution was executed for PVE, the imidazole moieties of the POPs were ionized, and the imidazolium salt embedded POP labeled as IL-PVE was fabricated accordingly. The as-prepared polymers were subsequently applied for adsorption of CR and Hg2+ from aqueous solution, and the possible adsorption mechanism was clarified in detail.

area (SBET) was derived from the BET method using the adsorption data ranging at P/P0 = 0.05−0.30. Pore volume (Vtotal) was calculated from the N2 adsorption isotherm at P/P0 = 0.99. Micropore area (Smicro) and micropore volume (Vmicro) were calculated from the V−t plot method, and pore size distribution (PSD) was calculated by applying the nonlocal density functional theory (NLDFT) model. The chlorine content of the polymers was measured by using titration according to the method in ref 32. Elemental analysis (EA) was performed using an elementar CHNOS analyzer (Vario Micro Cube, Germany). Thermogravimetric analysis (TGA) of the samples was performed using a thermobalance (STA-499C, NETZSCH). The concentration of CR was determined by a UV-2450 spectrophotometer at a wavelength of 398.0 nm, while Hg2+ was measured by a TAS-990 atomic absorption spectrophotometer. 2.4. Batch Adsorption. About 0.05 g of the polymers was mixed with 50 mL of CR or Hg2+ aqueous solution. The initial concentrations of CR C0 (mg/L) were set to be about 200, 400, 600, 800, and 1000 mg/L, while those of Hg2+ were about 100, 200, 300, 400, 500, and 600 mg/L. The solution pH was adjusted using 0.1 mol/L NaOH or 0.1 mol/L HCl. The mixed solutions were shaken at a desired temperature (298, 308, or 318 K) in a thermostatic oscillator until the equilibrium was reached. The residual concentration of CR or Hg2+ Ce (mg/L) was measured, and the equilibrium capacity qe (mg/g) was calculated as

2. MATERIALS SECTION 2.1. Materials. VIM and EGDMA were purchased from Gray West Chengdu Chemical Co. Ltd. They were washed with 5 wt % NaOH aqueous solution three times to remove the inhibitors, and after that, they were dried with anhydrous MgSO4 before use. The initiator 2,2-azobisisobutyronitrile (AIBN) was purified by recrystallization before use. DCX was purchased from Xiya Chemical Co. Ltd. o-Xylene, 1,2dichloroethane (DCE), CR, poly(vinyl alcohol) (PVA), and HgCl2 were obtained from Aladdin Chemical Co. Ltd. They were analytical reagents and used without further purification. 2.2. Synthesis of the Imidazolium Salt Incorporated Polymers. Scheme 1 shows the synthetic procedure of the imidazolium salt incorporated POPs, and two reactions, namely, copolymerization and ionization, were carried out. In the copolymerization, the organic phase including VIM (4 g, 20 wt %), EGDMA (16 g), AIBN (0.2 g), and o-xylene (40 g) were added to the water phase containing 200 mL of 0.05 wt % PVA aqueous solution at 318 K. Copolymerization was performed at 353 K for 8 h, and the VIM-co-EGDMA polymer, namely, PVE, was synthesized. Ionization was performed for PVE using DCX as the cationization agent, the imidazole moieties of PVE were ionized at 353 K for 24 h according to the nucleophilic substitution using DCE as the solvent, and hence, the imidazolium salt incorporated POP denoted as IL-PVE was prepared. 2.3. Characterization of the Polymers. Fourier transform infrared spectroscopy (FTIR) of the polymers was performed on a Nicolet 510P Fourier transform infrared spectrometer. The pore structure was determined by N2 adsorption−desorption isotherms at 77 K via a Micromeritics ASAP 2020 surface area and porosity analyzer. The surface

qe = (C0 − Ce)V /W

(1)

where V (L) is the volume of solution and W (g) is the weight of the polymers. The kinetic adsorption was similar to the equilibrium adsorption, and the adsorption capacity at contact time t (qt, mg/g) was calculated with the concentration at contact time t (Ct, mg/L).

3. RESULTS AND DISCUSSION 3.1. Structural Characterization of the Polymers. As shown in Figure 1, the CN stretching of the imidazole ring at 1481 cm−1 33,34 and CO stretching of the benzene ring at 1721 cm−1 35 imply the successful copolymerization of VIM and EGDMA. Gao et al.30 and Li et al.31 reported similar results. In addition, the ionization reaction induces a new peak at 1560 cm−1, and this peak can be assigned to the CC stretching of the benzene ring.36 Meanwhile, the ionization induces a new peak at 1263 cm−1, which is related to the C−Cl B



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Table 1. Textual Parameters of PVE and IL-PVE material

SBET (m2/g)

Smicro (m2/g)

Vtotal (cm3/g)

Vmicro (cm3/g)

PVE IL-PVE

175 217

17.7 37.4

0.57 0.94

0.008 0.018

increases from 0.008 to 0.018 cm3/g, suggesting that DCX inserts in the polymer chains through nucleophilic substitution, and hence, the imidazolium salt incorporated POP depicted in Scheme 1 is fabricated accordingly. 3.2. Equilibrium Adsorption. CR and Hg2+ were employed as the adsorbates in this study, and the adsorption of IL-PVE was assessed using PVE as the reference. As displayed in Figure 3a, the adsorption of CR on IL-PVE is

Figure 1. FTIR spectra of PVE and IL-PVE.

stretching.21,37 The chlorine content of the produced polymers was measured to be 4.56 wt %, which means that the imidazole moieties were ionized by DCX and imidazolium salts were successfully embedded on the polymers. The EA of the polymers was performed, and the values of carbon (C), hydrogen (H), and nitrogen (N) are 84.1, 6.25, and 5.88 wt %, respectively. Thermogravimetric analysis (TGA) of IL-PVE (Figure S1) reveals its high thermal stability. Figure 2a displays the N2 adsorption−desorption isotherms of the polymers. Following the IUPAC classification,38 these

Figure 3. Equilibrium isotherms for the adsorption of (a) CR and (b) Hg2+ on PVE and IL-PVE.

greatly enhanced as compared with PVE. At Ce = 100 mg/L and 298 K, the qe’s are predicted to be 45.5 and 199.8 mg/g. The embedded imidazolium salts on the polymers should have a positive contribution due to the strong chemical interaction including electrostatic interaction, hydrogen bonding, and chelating interaction. Noticeably, IL-PVE has a higher SBET especially Smicro and Vtotal, which can support more active sites for the adsorption. In addition, as different VIM concentrations (10 and 30 wt %) were adopted in the copolymerization, it is clear from Figure S2 that the maximum capacity (qmax) on ILPVE is the largest (Table S1), verifying that the feeding amount of the imidazole moieties and SBET are both key factors for the adsorption. The equilibrium data were fitted by the Langmuir and Freundlich models,39,40 and the experimental results are confirmed by the fitted results according to the Langmuir model (Table S2). The qmax of CR on IL-PVE is 503.0 mg/g, and it is larger than the reported results in ref 41 (NH4Al(OH)2CO3@Ni(OH)2, qmax = 425.5 mg/g), ref 42 (ZrO2 fibers, qmax = 103 mg/g), ref 43 (NiO-SiO2, qmax = 204 mg/g), and ref 44 (pyrimidine-based POPs, qmax = 400 mg/g). Additionally, the equilibrium isotherms of Hg2+ on the

Figure 2. (a) N2 adsorption−desorption isotherms and (b) PSD of PVE and IL-PVE.

isotherms can be classified as type-IV profiles with hysteresis loops, indicative of a mesoporous character. A rapid N2 adsorption occurs at P/P0 > 0.9, meaning that considerable mesopores exist, and this analysis is in accordance with the PSD in Figure 2b. According to the N2 isotherms, Table 1 shows the porous parameters such as SBET, Vtotal, Smicro, and Vmicro. PVE has SBET and Vtotal of 175 m2/g and 0.57 cm3/g, respectively. The ionization induces a further increase in SBET (217 m2/g) and large increase in Vtotal (0.94 cm3/g). Moreover, Smicro increases from 17.7 to 37.4 m2/g, and Vmicro C



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Table S4, the ΔH is positive, which implies an endothermic process. Notably, the ΔH’s for the adsorption of CR (9.995 kJ/ mol) and Hg2+ (17.28 kJ/mol) are relatively high, revealing that the adsorption probably involves a process of chemical bonding formation. In fact, the adsorbed CR on IL-PVE cannot be completely desorbed by 95% ethanol with a desorption ratio of 86.7%, and adding NaOH aqueous solution can sharply increase the desorption ratio to 96.2%. In a similar way, 1.0 mol/L HCl containing 1% thiourea could desorb ILPVE easily after Hg2+ adsorption. These results further confirm that chemical bonding including electrostatic interaction, hydrogen bonding, and chelating interaction may be formed during the adsorption. 3.3. Kinetic Adsorption. As displayed in Figure 5a, 15 min is sufficient for the adsorption of CR attaining the equilibrium

polymers were collected (Figure 3b), and it shows that the Hg2+ adsorption on IL-PVE is much enhanced after the ionization and the Langmuir model fits the equilibrium data better (Table S2), reflecting that the Hg2+ adsorption on ILPVE is a monolayer process. The qmax evaluated by the Langmuir model is 240.0 mg/g, which is higher than some other polymers reported in the literature (Table 2).45−49 Table 2. Comparison of Maximum Adsorption Capacities of Hg2+ in This Study with the Results Reported in the Literature material

qmax (mg/g)

ref.

SWCNT-SH CBAP-1(AET) Fe3O4-melamine based POP thiol-functionalized CNT/Fe3O4 TAPB-BMTTPA-COF Fe3O4@SiO2-SH amine-modified AC thiol-functionalized mesoporous silica IL-PVE

131 232 96 65.5 734 148.8 119 401 240

14 21 23 45 46 47 48 49 this work

Afterward, the effect of the temperature on the adsorption was investigated. The results in Figure 4 exhibit that the qe

Figure 5. Kinetic curves for the adsorption of (a) CR and (b) Hg2+ on PVE and IL-PVE.

on IL-PVE, while much more time (approximately 80 min) is needed for PVE. The strong chemical interaction should be the main reasons. As compared with the other results in ref 41 (NH4Al(OH)2CO3@Ni(OH)2, t = 30 min), ref 42 (ZrO2 fibers, t = 30 min), and ref 43 (NiOeSiO2, t = 70 min), IL-PVE needed much less time to attain the equilibrium. The pseudofirst-order and pseudo-second-order rate equations were adopted to characterize the kinetic data,50,51 and these two equations are suitable because R2 is less than 0.98 (Table S5). Noticeably, the calculated capacity (qcal) based on the pseudosecond-order rate equation is close to the final experimental qe, and the k2 of IL-PVE is determined to be 1.071 × 10−3 g/(mg· min), much greater than PVE (1.597 × 10−4 g/(mg·min)). The kinetic curves for the Hg2+ adsorption in Figure 5b show that Hg2+ is adsorbed more rapidly on IL-PVE than PVE. In the beginning 10 min, higher than 90% relative to the final qe is achieved for IL-PVE and about 15 min is enough to reach the equilibrium. Meanwhile, the pseudo-first-order rate

Figure 4. Equilibrium isotherms for the adsorption of (a) CR and (b) Hg2+ on IL-PVE at 298, 308, and 318 K.

increases as the temperature increases; the fitted qmax gives the same information (Table S3). The thermodynamic parameters such as adsorption enthalpy (ΔH) and adsorption entropy (ΔS) were calculated as ln K = ΔS /R − ΔH /RT

(2)

where K is the equilibrium constant by the Langmuir model (K = 1/KL), R is the universal gas constant (8.314 J/(mol·K)), and T is the absolute temperature (K). As can be seen from D



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equation is more appropriate for fitting the kinetic data since R2 is less than 0.99, and the k2 of IL-PVE (9.830 × 10−4 g/ (mg·min)) is a little greater than PVE (8.738 × 10−4 g/(mg· min)). 3.4. Effect of the Solution pH on the Adsorption. Solution pH is one of the most vital factors affecting the adsorption of ionic adsorbents. The adsorption of CR on ILPVE was plotted as a function of solution pH, and the results in Figure S3a show that the qe increases dramatically as the pH increases from 2 to 5 and the qe is almost stabilized as the solution pH is changed from 5.5 to 8. The much lower qe at a lower pH may be from the decrease of strong electrostatic attraction induced by the adsorbent surface protonation. Figure S3b displays that the qe of Hg2+ is first increased as the solution pH increases from 2 to 6 and then slightly decreased as the pH is higher than 6. The main reason is that the imidazole moieties are highly protonated due to the external H+ at a lower pH, which causes the surface potential to rise, resulting in a decrease of the chemical interaction. Meanwhile, the strong electrostatic repulsion will prevent Hg2+ from chelating with the imidazole groups, resulting in a lower qe. Notably, as the solution pH is higher than 6, Hg2+ is precipitated, leading to the lower qe. 3.5. Adsorption Selectivity and Regeneration of the Polymers. The adsorption selectivity of Hg2+ employing a mixed solution containing different metals such as Cu2+, K+, Mn2+, Ni2+, Zn2+, Fe3+, Mg2+, and Ca2+ was investigated with the same initial concentration (C0 = 500 mg/L). Figure 6

Hg2+, and the qe decreases to 95.0% after five adsorption− desorption cycles (Figure S4b). 3.6. Adsorption Mechanism. To have a better understanding of the adsorption of CR and Hg2+, the FTIR spectra of IL-PVE, IL-PVE-CR, and CR were measured and the results are listed in Figure 7. A new peak related to the −SO3−

Figure 7. FTIR spectra of IL-PVE before and after adsorption of CR.

stretching at 1041 cm−1 is shown in the FTIR spectra after adsorption of CR; the peak is blue-shifted compared to CR, which indicates that there is a strong electrostatic interaction between IL-PVE and CR. The peak at 3423 cm−1 is shifted to 3452 cm−1 after adsorption of CR; hence, hydrogen bonding may be involved in the adsorption. In addition, the bands of CR in the range of 500−750 cm−1 are greatly reduced after the adsorption of CR occurs, which further reflects the strong electrostatic interaction of CR with IL-PVE.30 XPS was exploited to identify the interaction of Hg2+ with IL-PVE. Figure 8 exhibits that new peaks related to Hg 4d and Hg 4f

Figure 6. Coexisting adsorption of various metal ions on IL-PVE for the coexisting metal ions from aqueous solution.

displays that IL-PVE exhibits a much higher qe for Hg2+ while relatively lower qe for the other metals, revealing that IL-PVE is a great candidate for selective adsorption of Hg2+. These results can be attributed to the soft−soft interaction.52 Hg2+ is a soft acid; Cu2+, Zn2+, Ni2+, and Fe3+ are boundary acids, and some other ions such as K+ are hard acid, which means that the electrons of Hg2+ are more powerful than the other ions and Hg2+ is more easily combined with the functional ionic groups by chemical interaction like electrostatic interaction and chelating coordination. Even though the active sites are occupied by the other ions at the initial stage, Hg2+ can exchange more metal ions than any other transition metals. After the equilibrium adsorption, the mixed solvent including 80% ethanol and 0.1 mol/L NaOH was used for desorption of CR and the regenerated polymers were repeatedly used for five adsorption−desorption cycles (Figure S4a), and the qe decreases to approximately 90.3%, exhibiting excellent reusability. In a similar way, using 1.0 mol/L HCl containing 1% thiourea, IL-PVE has a great regeneration performance for

Figure 8. XPS surveys of IL-PVE and IL-PVE-Hg (i.e., after adsorption of Hg2+ on IL-PVE).

are shown after Hg2+ adsorption, which means that Hg2+ was successfully adsorbed on IL-PVE. The O 1s (Figure 9a) is deconvoluted into two distinct peaks. The peak around 531.7 eV is related to carbonyl (CO), and another peak around 533.1 eV is related to C−O. Significantly, the peak (C−O) is blue-shifted to 533.2 eV after Hg2+ adsorption, which is blueshifted by about 0.1 eV and confirms the strong interaction between O and Hg2+. Similarly, Figure 9b exhibits that the N 1s related to the quaternary amino N with the binding energy at 401.0 eV is mostly blue-shifted to 401.2 eV after Hg2+ adsorption; the N 1s attributed to the pyrrole N is changed from 398.4 to 398.5 eV. The electrons are further donated to Hg2+, which makes the N atoms electron-deficient.52 As a E



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ORCID

Jianhan Huang: 0000-0002-3838-0622 Funding

The National Natural Science Foundation of China (No. 51673216) and the Fundamental Research Funds for the Central Universities Central South University (No. 2018zzts379) are gratefully acknowledged. Notes

The authors declare no competing financial interest.

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Figure 9. XPS O 1s (a) and N 1s (b) spectra of IL-PVE and IL-PVEHg.

result, the N and O of IL-PVE form coordination complexes with Hg2+ and the chelating coordination leads to the much enhanced adsorption of Hg2+.

4. CONCLUSIONS The novel imidazolium salt embedded POP, namely, IL-PVE, was prepared, and its adsorption of CR and Hg2+ was assessed from aqueous solution. The polymers contained not only ionized imidazole salts but also plenty of mesopores after introduction of VIM and DCX on the skeletons, and hence, they possessed a much larger qe to CR and Hg2+. The qmax’s of CR and Hg2+ on IL-PVE were predicted to be 503.0 and 240.0 mg/g, respectively, by the Langmuir model. The equilibrium data could be well fitted by the Langmuir model, and a chemical adsorption was involved with ΔH’s of 9.995 and 17.28 kJ/mol, respectively. The adsorption was a very fast process, and k2’s were fitted to be 1.070 × 10−3 and 9.830 × 10−4 g/(mg·min) for CR and Hg2+, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.9b00092. Additional tables and figures contain detailed description of the thermogravimetric analysis (TGA); the equilibrium isotherms of CR effect of the solution pH on Hg2+ and CR adsorption, effect of recycle times on the recovery ratios of Hg2+ and CR and CR kinetic parameters of Hg2+ and CR; equilibrium parameters of Hg2+ and CR (PDF)

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