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New crosslinked poly(ionic liquids) networks as As(V) extractants
Journal of Environmental Chemical Engineering 7 (2019) 103154
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New crosslinked poly(ionic liquids) networks as As(V) extractants
T
Umesh K. Dautoo, Yashwant Shandil, Sunita Ranote, Shivani Jamwal, Rohini Dharela, ⁎ Ghanshyam S. Chauhan Department of Chemistry, Himachal Pradesh University, Shimla, 171005, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Arsenic extraction Poly(N-vinylimidazole) Poly(4-vinylpyridine) Crosslinker effect
Arsenic (As) contamination of drinking water is a serious problem. We present here a series of four new materials which are effective extractants of As(V) ions. Four new polymer networks were obtained by crosslinking of synthesised poly(N-vinylimidazole) and poly(4-vinylpyridine) with two quaternizing agents viz. 1,4 and 1,6dichlorobutane. The networks were well characterized for investigation of their structural aspects and evaluated as As(V) ions extractants as a function of time, temperature, concentration and pH. The synthesised materials show 100% removal of As(V) ions both from a low concentration of 20 ppb and high concentration of 3.5 ppm thus showing high affinity for the targeted ions.
1. Introduction Arsenic in various forms is a serious water contaminant and poses huge health threat to the aquatic organisms as well to the human beings. It can easily accumulate in body through water and various drinking water or food cycles [1]. Arsenic (As) enters into water streams via various natural and anthropogenic activities [2]. Arsenicosis is a serious health manifestation of the consumption of arseniccontaminated groundwater [3]. Serious diseases such as diabetes, skin lesions, liver, lung, bladder, cardiovascular diseases and kidney cancer are common manifestation of uptake of As in different forms [4,5]. Hence it is imperative to remove As from the drinking water prior to its consumption. WHO has set the critical limit of 10 ppb for total As in the drinking water [6]. Therefore, reduction of As concentration to such a miniscule amount requires effective water treatment techniques. Adsorption has been widely accepted as simple to operate, efficient and cost-effective method among all the available techniques practiced for purification of contaminated water [7,8]. Recent advances in polymer science has attracted the attention of the researchers’ worldwide to design new polymer-based adsorbents for the efficient removal of toxic metal ions from wastewater [9,10]. In literature the use of quaternary ammonium, dialkylimidazolium or N-alkylpyridinium ion derivatives has been extensively reported as antimicrobial agents [11], disinfectants [12], surfactants, [13] etc., after simple quaternization of tertiary N functionality. Quaternization of N containing polymers with alkylating agents enlarges the spectrum of their end-uses in many advanced applications. One such advancement is the synthesis of poly (ionic liquids) [PILs]. These are technologically very attractive ⁎
materials, and have proven efficacy as surfactants [14,15], antimicrobial agents [16–20], CO2 adsorbents [21–23] and anion exchangers [24]. In view of the above stated we report the synthesis of a series of four polymer networks with immobilized PILs as As(V) ions extractants from the simulated wastewater. The As(V) ions removal efficiency was studied quantitatively as a function of time, temperature, concentration and pH by using ion chromatogram [25]. Ion chromatography is powerful and sensitive analytical tool to analyze even the trace amounts of As(V) ions in fast and simultaneous fashion. It has been used to detect various anions viz. I−, Br−, BrO4-, NO3-, As(III) and As(V) [26–29]. PILs reported in the present work exhibit bi-functional characteristics as apart from possessing exchangeable counter anions, these materials have quaternized nitrogen linked to the alkyl chains which makes them effective antimicrobial agents. N-vinyl imidazole (N-VIm) and 4-vinyl pyridine (4-Py) were used as the monomers and 1,4-dichlorobutane and 1,6-dichlorohexane as the bi-functional crosslinkers. The resulting networks were linked at the tertiary N atom to generate quaternized structures. Hence, these materials have inherent exchangeable Cl− counter anions on the cationic nitrogen and hence possess intrinsic bi-functional characteristics. There are numerous reports in literature where similar materials consisting of exchangeable counter anions and possessing antimicrobial properties have been reported [30,31]. But no work has been reported so far related to the synthesis of similar crosslinked PILs. To the best of authors’ knowledge there are no similar materials reported in literature for the removal of As(V) ions. Moreover, from the technological viewpoint, it is significant
Corresponding author. E-mail addresses: [email protected], [email protected] (G.S. Chauhan).
https://doi.org/10.1016/j.jece.2019.103154 Received 9 January 2019; Received in revised form 10 April 2019; Accepted 11 May 2019 Available online 10 June 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 103154
U.K. Dautoo, et al.
Scheme 1. Synthesis of [PVIm-(CH2)4-PVIm][Cl−]2 (left) and [PVP-(CH2)4-PVP][Cl−]2 (right). Table 1 CHN analysis of different polymeric networks. Polymer/PILs
%N
%C
%H
Poly(N-VIm) [PVIm-(CH2)4-PVIm][Cl−]2 [PVIm-(CH2)6-PVIm][Cl−]2 Poly(4-Py) [PVP-(CH2)4-PVP][Cl−]2 [PVP-(CH2)6-PVP][Cl−]2
20.267 15.080 15.860 11.522 6.425 6.861
45.784 42.253 45.026 69.203 45.930 51.433
6.726 6.867 7.516 6.938 7.572 7.942
Laboratories Pvt. Ltd., Mumbai, India), 1,4-dichlorobutane and 1,6-dichlorohexane (Sigma Aldrich, St. Louis), sodium arsenate (Loba Chemie Pvt. Ltd. Mumbai, India) were used as received. 2.2. Synthesis and quaternization of poly(N-VIm) and poly(4-Py) Poly(N-VIm) was synthesized from the monomer N-VIm by γ-radiation treatment for 24 h (dose rate = 0.998 KGy/h). It was subsequently quaternized by treatment with 1,4-dichlorobutane in 1:3 M ratio to ensure maximum extent of the reaction. Two components were taken in a water bath and heated at 100 °C for 18 h. The product was extracted with distilled water to remove the un-reacted 1,4-dichlorobutane and dried under vacuum. The resulting quaternized network polymer was designated as [PVP-(CH2)4-PVP][Cl−]2. Poly(4-Py) was synthesized by thermal polymerization technique from the monomer, 4-Py and APS as initiator. The polymerization was achieved in water bath maintained at 70 °C at 8 h. Poly(4-Py) thus obtained was washed with distilled water and dried in a vacuum desiccator prior to further use. The quaternization of poly(4-Py) was achieved by treating with 1,4-dichlorobutane in 1:5 wt ratio to ensure maximum extent of reaction. The crosslinked quaternized network polymer was designated as [PVP-(CH2)4-PVP][Cl−]2. In a set of parallel experiments, the quaternization of poly(N-VIm) and poly(4-Py) with a new bi-functional crosslinker, 1,6-dichlorohexane, was also carried out under the similar reaction conditions as reported for quaternization with 1,4-dichlorohexane. The two new network polymers thus obtained were marked as [PVIm-(CH2)6PVIm][Cl−]2 and [PVP-(CH2)6-PVP][Cl−]2.
Fig. 1. (a) FTIR spectra of poly(N-VIm), [PVIm-(CH2)4-PVIm][Cl−]2 and [PVIm-(CH2)6-PVIm][Cl−]2. (b) FTIR spectra of poly(4-Py), [PVP-(CH2)4-PVP] [Cl−]2 and [PVP-(CH2)6-PVP][Cl−]2.
that 100% removal of As(V) ions could be realized even from a low concentration of 20 ppb and the results obtained are not much affected by contact time, temperature and pH. In view of the significance of the reported data we are submitting this work as short communication while full laboratory scale development of the choicest adsorbent and its direct use in the field study is being undertaken.
2.3. Characterization of polymer networks Synthesized network polymers were characterized through various techniques to get evidence of their structure and formation. Fourier transform infrared (FTIR) spectra were recorded with Nicolet 5700 in transmittance mode in KBr pellets. Scanning electron microscope (SEM) images of representative samples were recorded with HITACHI SEM 8010 and EDX of the As(V) ions loaded samples were recorded with Nova-Nano SEM-450. CHN analysis was performed with Thermo Scientific (FLASH 2000).
2. Materials and methods 2.1. Materials N-vinyl imidazole (N-VIm) and 4-vinyl pyridine (4-Py), (Hi Media laboratories Pvt. Ltd. Mumbai, India), ammonium persulphate (APS, SR 2
Journal of Environmental Chemical Engineering 7 (2019) 103154
U.K. Dautoo, et al.
Fig. 3. Conductivity detection of As(V) ions by [PVIm-(CH2)4-PVIm][Cl−]2 at different contact time of 10, 30 and 60 min [20 ppb As(V), 25 °C]. Table 2 As(V) ions uptake study at 25 °C and 20 ppb concentration. PILs
[PVP-(CH2)4-PVP][Cl−]2 [PVP-(CH2)6-PVP][Cl−]2 [PVIm-(CH2)4-PVIm][Cl−]2 [PVIm-(CH2)6-PVIm][Cl−]2
Remaining concentration after adsorption (time in min) 10
30
60
120
15.44 10.10 15.46 11.77
10.10 6.211 6.637 0.000
8.519 5.441 0.000 0.000
0.000 0.000 0.000 0.000
2.4. As(V) extraction study As(V) ions removal capacity (q) of as-synthesized network polymers was studied as a function of contact time (10–120 min), temperature (25–45 °C), concentration (0.02–3.5) mg L−1 and pH (1.5, 4.5, 9.2 and 12.8). As(V) ions solution of different concentration (0.02, 0.03, 0.04, 0.05, 1.0, 1.5, 2.0, 3.5) mg L‒1, were prepared in distilled water using sodium arsenate. As-synthesized network polymers (0.01 g) were added to the sample tubes containing 10 mL of As(V) ions solutions of different concentrations. Thereafter, the sample tubes were put into water bath for different time intervals i.e. 10, 30, 60 and 120 min at 25 °C, 37 °C and 45 °C. Afterwards, the sample tubes were centrifuged and filtrate containing non-adsorbed As(V) ions was analysed for residual concentration with Ion Chromatogram (IC) [Metrohm, Switzerland]. The latter has Metrosep A Supp 5-250/4.0 and the mobile phase eluent
Fig. 2. SEM images of [PVIm-(CH2)4-PVIm][Cl−]2 (a), [PVIm-(CH2)6-PVIm] [Cl−]2 (b), As(V) ions loaded [PVIm-(CH2)4-PVIm][Cl−]2 (c) [PVIm-(CH2)6PVIm][Cl−]2 (d) and their corresponding EDX spectra, respectively (e,f); and SEM images of [PVP-(CH2)4-PVP][Cl−]2 (g), [PVP-(CH2)6-PVP][Cl−]2 (h), As (V) ions loaded [PVP-(CH2)4-PVP][Cl−]2 (i), [PVP-(CH2)6-PVP][Cl−]2 (j) and, their corresponding EDX spectra (k,l), respectively. 3
Journal of Environmental Chemical Engineering 7 (2019) 103154
U.K. Dautoo, et al.
Fig. 4. Conductivity detection of As(V) ions left after adsorption by PILs (A) [PVIm-(CH2)4-PVIm][Cl−]2 (B) [PVIm-(CH2)6-PVIm][Cl−]2 (C) [PVP-(CH2)4-PVP] [Cl−]2 (D) [PVP-(CH2)6-PVP][Cl−]2 on ion chromatogram at 37 °C and 20 ppb.
obtain poly(N-VIm) and poly(4-Py). After that the quaternization of poly(N-VIm) and poly(4-Py) was performed with 1,4-dichlorobutane and 1,6-dichlorohexane to develop intensely crosslinked polymer networks as in situ quaternization takes place during the crosslinking process at the tertiary N of poly(N-VIm) and poly(4-Py) with Cl− as the counter anions. Synthesized network polymers were characterized with FTIR, SEM,EDX and CHN analysis to get evidence of polymerization and quaternization.
is 3.2 mM Na2CO3 + 1.0 mM NaHCO3 as eluent (Scheme 1). The q values were calculated using the formula:
q=
(Ci − Ct ) ×V w
(1) −1
Where q is the adsorption capacity (mg g ); Ci and Ct, respectively, are the initial and final concentrations of As(V) ions (mg L−1), V is the volume of the solution in litres and w is weight of the network polymer in g. Calibration curve of arsenate standard with relative standard deviation (RSD) and correlation coefficient values were plotted at ppb and ppm concentrations (Supplementary material Fig. 1 S).
3.1. Characterization of different crosslinked PILs FTIR spectrum of poly(N-VIm) has adsorption bands at 3110 cm−1 (CeH stretching due to VIm ring), 2945 cm−1 (CeH stretching due to chain) and, 1500 cm−1, 916 cm−1 and 822 cm−1 due to CeC and CeN stretching in the N-VIm ring. After quaternization process, the
3. Results and discussion Four new crosslinked PILs were synthesized from N-VIm and 4-Py, using γ-irradiation and APS thermal polymerization, respectively, to 4
Journal of Environmental Chemical Engineering 7 (2019) 103154
U.K. Dautoo, et al.
Fig. 5. Conductivity detection of arsenate ions left after adsorption by PILs (A) [PVIm-(CH2)4-PVIm][Cl−]2 (B) [PVIm-(CH2)6-PVIm][Cl−]2 (C) [PVP-(CH2)4-PVP] [Cl−]2 (D) [PVP-(CH2)6-PVP][Cl−]2on ion chromatogram at 25 °C and 3.5 ppm. Table 3 Comparison of different adsorbents for the removal of As(V) ions. Adsorbent
Initial As(V) ion concentration (ppb)
Time
Final As(V) ion concentration (ppb)
Ref.
Chitosan Coconut shell Rice polish Titanium dioxide [PVIm-(CH2)4-PVIm][Cl−]2 [PVIm-(CH2)6-PVIm][Cl−]2 [PVP-(CH2)4-PVP][Cl−]2 [PVP-(CH2)6-PVP][Cl−]2
1000 50000 1000 2000 20 20 20 20
– 10 h 40 min 22 h 60 min 60 min 120 min 120 min
900 30000 146.05 1800 0 0 0 0
[35] [36] [37] [38] Present Present Present Present
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study study study study
Journal of Environmental Chemical Engineering 7 (2019) 103154
U.K. Dautoo, et al.
adsorption band at 1500 cm−1 is shifted to 1498 cm−1 along with a new band at 1565 cm−1 for [PVIm-(CH2)4-PVIm][Cl−]2 and [PVIm(CH2)6-PVIm][Cl−]2 which confirmed successful quaternization of VIm [32], (Fig. 1a). The poly(4-Py) spectrum has characteristic bands at 3028 cm-1 (CeH stretching), 1596 cm-1 (C]N stretching), 1456 cm‒1 (C]C stretching vibrations of the pyridine ring) and 1218 cm-1 (CeN stretching). After quaternization, the band at 1599 cm-1 disappeared and the band at 1456 cm‒1 is displaced to 1470 cm‒1 and 1218 cm-1 to 1222 cm-1, respectively, alongwith an appearance of new band at 1646 cm-1 which is the characteristic peak of the quaternary pyridinium moiety. [33] The FTIR spectra thus confirm successful synthesis of the quaternized materials (Fig. 1b). CHN analysis was performed to get supplementary evidence of polymerization and quaternization reaction. Low %N in [PVIm-(CH2)4PVIm][Cl−]2 and [PVIm-(CH2)6-PVIm][Cl−]2 than poly(N-VIm), supports quaternization, and similar inferences can be drawn for poly(4Py) (Table 1). [34] SEM images of the quaternized polymeric materials, [PVIm-(CH2)4PVIm][Cl−]2 and [PVIm-(CH2)6-PVIm][Cl−]2) show their porous morphology (Fig. 2a,b). After adsorption of As(V) ions onto the surface of [PVIm-(CH2)4-PVIm][Cl−]2 and [PVIm-(CH2)6-PVIm][Cl−]2), surface morphology changes with decreased porosity that confirms the successful attachment of As(V) ions onto the synthesized PILs (Fig. 2c,d). Adsorption of As(V) ions onto the [PVIm-(CH2)4-PVIm] [Cl−]2 and [PVIm-(CH2)6-PVIm][Cl−]2 was further affirmed by their corresponding EDX spectra having peaks corresponding to As (Fig. 2e,f). Similarly, SEM images of the ([PVP-(CH2)4-PVP][Cl−]2, [PVP(CH2)6-PVP][Cl−]2, poly(N-Vim), also has porous morphology (Fig. 2g,h). After adsorption of As(V) ions onto the surface of quaternized polymeric materials, their particles became spherical and surface became smoother. Thus change in the morphology of the synthesized PILs before and after adsorption of As(V) ions onto their surface successfully confirmed the attachment of As(V) ions onto the synthesized PILs (Fig. 2i,j). Adsorption of As(V) ions onto the ([PVP(CH2)4-PVP][Cl−]2, [PVP-(CH2)6-PVP][Cl−]2, was further affirmed by their corresponding EDX spectra having peaks corresponding to As (Fig. 2k,l).
solution. With increase in pH ranging from 2.0 to 12.0, this nonionic form (H3AsO4) is replaced by H2AsO4− and HAsO42− ions and there is increase in forces of attraction between these ions and PILs. Hence these forces help in effective adsorption of As(V) ions with increase in basic medium of the solution. When pH exceed (pH > 9.0), the hydroxyl ions in the solution compete with arsenate ions for binding site on PILSs resulting in low As(V) adsorption. The PILs favour wide pH range (pH 4–9) because of their quaternized structure and does not change structure with the variation of pH. Adsorption of As(V) ions by PILs from the water solutions was found to be dependent on the nature of monomer, N-VIm or 4-Py, and length of the crosslinker. Though all the PILs showed rapid and high As(V) removal, yet with the longer crosslinker, 1,6 hexyl chain, initial rapid removal of the contaminant ions was observed for both the polymers as it allows larger access of the active sites to the As(V) ions. Comparison of the efficiency of the synthesised PILs with other reported adsorbents in the literature is shown in Table 3, which revealed that the synthesised polymeric materials are rapid and efficient candidates for the removal of As (V) ions. 4. Conclusions We report synthesis of four new crosslinked poly(ionic liquids) derived from N-VIm and 4-Py for use as As(V) extractants from aqueous solution. Results obtained reveal that the synthesized PILs are effective extractants of As(V) ions with high performance of 100% removal from very low concentration of 20 ppb or 3.5 ppm solution. As per WHO and EU guidelines arsenic concentrations in groundwater should be below 10 ppb. As(V) adsorption increased with increase in contact time and temperature. Structure of the PILs or synthetic protocol does not have sharp effect on the removal efficiency of As(V) as all PILs were effective extractants though those with the longer crosslinker have somewhat rapid extraction rate. It can be concluded that PILs has a high capacity for As(V) ions removal. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jece.2019.103154.
3.2. As(V) ion exchange behaviour of PILs as a function of different parameters
References
As exits as H2AsO4− and HAsO42− within a pH range of 2–12. Using 20 ppb solution of As(V) ions and [PVIm-(CH2)4-PVIm][Cl−]2 as extractant, concentration left in filtrate after 10 min was reduced by 25% to 15 ppb. When the contact time was increased from 10 to 30 min the residual concentration was reduced to 6 ppb and whole of the As(V) ions were removed when contact time was 60 min (Fig. 3). Similar trends in results were observed in the case of other three PILs (Table 2). PVIm derivatives show complete removal of As(V) ions at 25 °C and 60 min. In case of [PVP-(CH2)4-PVP][Cl−]2 and [PVP-(CH2)6-PVP] [Cl−]2 8 ppb and 5 ppb of As(V) ions was the residual concentration under the same conditions. Similarly, As(V) ions removal by the synthesized PILs increases effectively when temperature was progressively increased. The synthesized PILs show effective extraction of As(V) ions at higher concentration of up to 3.5 (mg L−1), (Fig. 4). Hence these results show that the synthesized PILs are effective adsorbent at very low as well as high concentration (20 ppb and 3.5 ppm) of As(V) ions (Fig. 5). Adsorption study of As(V) ions was also carried out under media of different pH i.e.1.5, 4.5, 9.2 and 12.8 at 37 °C. As(V) ions removal was somewhat low in acidic medium than basic medium because at very low pH (˜2.0), the As(V) ions remains mainly as non-ionic form (H3AsO4). Due to this there is no electrostatic force and van der Waals forces between the amines on synthesized PILs and the As(V) ions in
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New crosslinked poly(ionic liquids) networks as As(V) extractants
T
Umesh K. Dautoo, Yashwant Shandil, Sunita Ranote, Shivani Jamwal, Rohini Dharela, ⁎ Ghanshyam S. Chauhan Department of Chemistry, Himachal Pradesh University, Shimla, 171005, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Arsenic extraction Poly(N-vinylimidazole) Poly(4-vinylpyridine) Crosslinker effect
Arsenic (As) contamination of drinking water is a serious problem. We present here a series of four new materials which are effective extractants of As(V) ions. Four new polymer networks were obtained by crosslinking of synthesised poly(N-vinylimidazole) and poly(4-vinylpyridine) with two quaternizing agents viz. 1,4 and 1,6dichlorobutane. The networks were well characterized for investigation of their structural aspects and evaluated as As(V) ions extractants as a function of time, temperature, concentration and pH. The synthesised materials show 100% removal of As(V) ions both from a low concentration of 20 ppb and high concentration of 3.5 ppm thus showing high affinity for the targeted ions.
1. Introduction Arsenic in various forms is a serious water contaminant and poses huge health threat to the aquatic organisms as well to the human beings. It can easily accumulate in body through water and various drinking water or food cycles [1]. Arsenic (As) enters into water streams via various natural and anthropogenic activities [2]. Arsenicosis is a serious health manifestation of the consumption of arseniccontaminated groundwater [3]. Serious diseases such as diabetes, skin lesions, liver, lung, bladder, cardiovascular diseases and kidney cancer are common manifestation of uptake of As in different forms [4,5]. Hence it is imperative to remove As from the drinking water prior to its consumption. WHO has set the critical limit of 10 ppb for total As in the drinking water [6]. Therefore, reduction of As concentration to such a miniscule amount requires effective water treatment techniques. Adsorption has been widely accepted as simple to operate, efficient and cost-effective method among all the available techniques practiced for purification of contaminated water [7,8]. Recent advances in polymer science has attracted the attention of the researchers’ worldwide to design new polymer-based adsorbents for the efficient removal of toxic metal ions from wastewater [9,10]. In literature the use of quaternary ammonium, dialkylimidazolium or N-alkylpyridinium ion derivatives has been extensively reported as antimicrobial agents [11], disinfectants [12], surfactants, [13] etc., after simple quaternization of tertiary N functionality. Quaternization of N containing polymers with alkylating agents enlarges the spectrum of their end-uses in many advanced applications. One such advancement is the synthesis of poly (ionic liquids) [PILs]. These are technologically very attractive ⁎
materials, and have proven efficacy as surfactants [14,15], antimicrobial agents [16–20], CO2 adsorbents [21–23] and anion exchangers [24]. In view of the above stated we report the synthesis of a series of four polymer networks with immobilized PILs as As(V) ions extractants from the simulated wastewater. The As(V) ions removal efficiency was studied quantitatively as a function of time, temperature, concentration and pH by using ion chromatogram [25]. Ion chromatography is powerful and sensitive analytical tool to analyze even the trace amounts of As(V) ions in fast and simultaneous fashion. It has been used to detect various anions viz. I−, Br−, BrO4-, NO3-, As(III) and As(V) [26–29]. PILs reported in the present work exhibit bi-functional characteristics as apart from possessing exchangeable counter anions, these materials have quaternized nitrogen linked to the alkyl chains which makes them effective antimicrobial agents. N-vinyl imidazole (N-VIm) and 4-vinyl pyridine (4-Py) were used as the monomers and 1,4-dichlorobutane and 1,6-dichlorohexane as the bi-functional crosslinkers. The resulting networks were linked at the tertiary N atom to generate quaternized structures. Hence, these materials have inherent exchangeable Cl− counter anions on the cationic nitrogen and hence possess intrinsic bi-functional characteristics. There are numerous reports in literature where similar materials consisting of exchangeable counter anions and possessing antimicrobial properties have been reported [30,31]. But no work has been reported so far related to the synthesis of similar crosslinked PILs. To the best of authors’ knowledge there are no similar materials reported in literature for the removal of As(V) ions. Moreover, from the technological viewpoint, it is significant
Corresponding author. E-mail addresses: [email protected], [email protected] (G.S. Chauhan).
https://doi.org/10.1016/j.jece.2019.103154 Received 9 January 2019; Received in revised form 10 April 2019; Accepted 11 May 2019 Available online 10 June 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 103154
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Scheme 1. Synthesis of [PVIm-(CH2)4-PVIm][Cl−]2 (left) and [PVP-(CH2)4-PVP][Cl−]2 (right). Table 1 CHN analysis of different polymeric networks. Polymer/PILs
%N
%C
%H
Poly(N-VIm) [PVIm-(CH2)4-PVIm][Cl−]2 [PVIm-(CH2)6-PVIm][Cl−]2 Poly(4-Py) [PVP-(CH2)4-PVP][Cl−]2 [PVP-(CH2)6-PVP][Cl−]2
20.267 15.080 15.860 11.522 6.425 6.861
45.784 42.253 45.026 69.203 45.930 51.433
6.726 6.867 7.516 6.938 7.572 7.942
Laboratories Pvt. Ltd., Mumbai, India), 1,4-dichlorobutane and 1,6-dichlorohexane (Sigma Aldrich, St. Louis), sodium arsenate (Loba Chemie Pvt. Ltd. Mumbai, India) were used as received. 2.2. Synthesis and quaternization of poly(N-VIm) and poly(4-Py) Poly(N-VIm) was synthesized from the monomer N-VIm by γ-radiation treatment for 24 h (dose rate = 0.998 KGy/h). It was subsequently quaternized by treatment with 1,4-dichlorobutane in 1:3 M ratio to ensure maximum extent of the reaction. Two components were taken in a water bath and heated at 100 °C for 18 h. The product was extracted with distilled water to remove the un-reacted 1,4-dichlorobutane and dried under vacuum. The resulting quaternized network polymer was designated as [PVP-(CH2)4-PVP][Cl−]2. Poly(4-Py) was synthesized by thermal polymerization technique from the monomer, 4-Py and APS as initiator. The polymerization was achieved in water bath maintained at 70 °C at 8 h. Poly(4-Py) thus obtained was washed with distilled water and dried in a vacuum desiccator prior to further use. The quaternization of poly(4-Py) was achieved by treating with 1,4-dichlorobutane in 1:5 wt ratio to ensure maximum extent of reaction. The crosslinked quaternized network polymer was designated as [PVP-(CH2)4-PVP][Cl−]2. In a set of parallel experiments, the quaternization of poly(N-VIm) and poly(4-Py) with a new bi-functional crosslinker, 1,6-dichlorohexane, was also carried out under the similar reaction conditions as reported for quaternization with 1,4-dichlorohexane. The two new network polymers thus obtained were marked as [PVIm-(CH2)6PVIm][Cl−]2 and [PVP-(CH2)6-PVP][Cl−]2.
Fig. 1. (a) FTIR spectra of poly(N-VIm), [PVIm-(CH2)4-PVIm][Cl−]2 and [PVIm-(CH2)6-PVIm][Cl−]2. (b) FTIR spectra of poly(4-Py), [PVP-(CH2)4-PVP] [Cl−]2 and [PVP-(CH2)6-PVP][Cl−]2.
that 100% removal of As(V) ions could be realized even from a low concentration of 20 ppb and the results obtained are not much affected by contact time, temperature and pH. In view of the significance of the reported data we are submitting this work as short communication while full laboratory scale development of the choicest adsorbent and its direct use in the field study is being undertaken.
2.3. Characterization of polymer networks Synthesized network polymers were characterized through various techniques to get evidence of their structure and formation. Fourier transform infrared (FTIR) spectra were recorded with Nicolet 5700 in transmittance mode in KBr pellets. Scanning electron microscope (SEM) images of representative samples were recorded with HITACHI SEM 8010 and EDX of the As(V) ions loaded samples were recorded with Nova-Nano SEM-450. CHN analysis was performed with Thermo Scientific (FLASH 2000).
2. Materials and methods 2.1. Materials N-vinyl imidazole (N-VIm) and 4-vinyl pyridine (4-Py), (Hi Media laboratories Pvt. Ltd. Mumbai, India), ammonium persulphate (APS, SR 2
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Fig. 3. Conductivity detection of As(V) ions by [PVIm-(CH2)4-PVIm][Cl−]2 at different contact time of 10, 30 and 60 min [20 ppb As(V), 25 °C]. Table 2 As(V) ions uptake study at 25 °C and 20 ppb concentration. PILs
[PVP-(CH2)4-PVP][Cl−]2 [PVP-(CH2)6-PVP][Cl−]2 [PVIm-(CH2)4-PVIm][Cl−]2 [PVIm-(CH2)6-PVIm][Cl−]2
Remaining concentration after adsorption (time in min) 10
30
60
120
15.44 10.10 15.46 11.77
10.10 6.211 6.637 0.000
8.519 5.441 0.000 0.000
0.000 0.000 0.000 0.000
2.4. As(V) extraction study As(V) ions removal capacity (q) of as-synthesized network polymers was studied as a function of contact time (10–120 min), temperature (25–45 °C), concentration (0.02–3.5) mg L−1 and pH (1.5, 4.5, 9.2 and 12.8). As(V) ions solution of different concentration (0.02, 0.03, 0.04, 0.05, 1.0, 1.5, 2.0, 3.5) mg L‒1, were prepared in distilled water using sodium arsenate. As-synthesized network polymers (0.01 g) were added to the sample tubes containing 10 mL of As(V) ions solutions of different concentrations. Thereafter, the sample tubes were put into water bath for different time intervals i.e. 10, 30, 60 and 120 min at 25 °C, 37 °C and 45 °C. Afterwards, the sample tubes were centrifuged and filtrate containing non-adsorbed As(V) ions was analysed for residual concentration with Ion Chromatogram (IC) [Metrohm, Switzerland]. The latter has Metrosep A Supp 5-250/4.0 and the mobile phase eluent
Fig. 2. SEM images of [PVIm-(CH2)4-PVIm][Cl−]2 (a), [PVIm-(CH2)6-PVIm] [Cl−]2 (b), As(V) ions loaded [PVIm-(CH2)4-PVIm][Cl−]2 (c) [PVIm-(CH2)6PVIm][Cl−]2 (d) and their corresponding EDX spectra, respectively (e,f); and SEM images of [PVP-(CH2)4-PVP][Cl−]2 (g), [PVP-(CH2)6-PVP][Cl−]2 (h), As (V) ions loaded [PVP-(CH2)4-PVP][Cl−]2 (i), [PVP-(CH2)6-PVP][Cl−]2 (j) and, their corresponding EDX spectra (k,l), respectively. 3
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Fig. 4. Conductivity detection of As(V) ions left after adsorption by PILs (A) [PVIm-(CH2)4-PVIm][Cl−]2 (B) [PVIm-(CH2)6-PVIm][Cl−]2 (C) [PVP-(CH2)4-PVP] [Cl−]2 (D) [PVP-(CH2)6-PVP][Cl−]2 on ion chromatogram at 37 °C and 20 ppb.
obtain poly(N-VIm) and poly(4-Py). After that the quaternization of poly(N-VIm) and poly(4-Py) was performed with 1,4-dichlorobutane and 1,6-dichlorohexane to develop intensely crosslinked polymer networks as in situ quaternization takes place during the crosslinking process at the tertiary N of poly(N-VIm) and poly(4-Py) with Cl− as the counter anions. Synthesized network polymers were characterized with FTIR, SEM,EDX and CHN analysis to get evidence of polymerization and quaternization.
is 3.2 mM Na2CO3 + 1.0 mM NaHCO3 as eluent (Scheme 1). The q values were calculated using the formula:
q=
(Ci − Ct ) ×V w
(1) −1
Where q is the adsorption capacity (mg g ); Ci and Ct, respectively, are the initial and final concentrations of As(V) ions (mg L−1), V is the volume of the solution in litres and w is weight of the network polymer in g. Calibration curve of arsenate standard with relative standard deviation (RSD) and correlation coefficient values were plotted at ppb and ppm concentrations (Supplementary material Fig. 1 S).
3.1. Characterization of different crosslinked PILs FTIR spectrum of poly(N-VIm) has adsorption bands at 3110 cm−1 (CeH stretching due to VIm ring), 2945 cm−1 (CeH stretching due to chain) and, 1500 cm−1, 916 cm−1 and 822 cm−1 due to CeC and CeN stretching in the N-VIm ring. After quaternization process, the
3. Results and discussion Four new crosslinked PILs were synthesized from N-VIm and 4-Py, using γ-irradiation and APS thermal polymerization, respectively, to 4
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Fig. 5. Conductivity detection of arsenate ions left after adsorption by PILs (A) [PVIm-(CH2)4-PVIm][Cl−]2 (B) [PVIm-(CH2)6-PVIm][Cl−]2 (C) [PVP-(CH2)4-PVP] [Cl−]2 (D) [PVP-(CH2)6-PVP][Cl−]2on ion chromatogram at 25 °C and 3.5 ppm. Table 3 Comparison of different adsorbents for the removal of As(V) ions. Adsorbent
Initial As(V) ion concentration (ppb)
Time
Final As(V) ion concentration (ppb)
Ref.
Chitosan Coconut shell Rice polish Titanium dioxide [PVIm-(CH2)4-PVIm][Cl−]2 [PVIm-(CH2)6-PVIm][Cl−]2 [PVP-(CH2)4-PVP][Cl−]2 [PVP-(CH2)6-PVP][Cl−]2
1000 50000 1000 2000 20 20 20 20
– 10 h 40 min 22 h 60 min 60 min 120 min 120 min
900 30000 146.05 1800 0 0 0 0
[35] [36] [37] [38] Present Present Present Present
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study study study study
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adsorption band at 1500 cm−1 is shifted to 1498 cm−1 along with a new band at 1565 cm−1 for [PVIm-(CH2)4-PVIm][Cl−]2 and [PVIm(CH2)6-PVIm][Cl−]2 which confirmed successful quaternization of VIm [32], (Fig. 1a). The poly(4-Py) spectrum has characteristic bands at 3028 cm-1 (CeH stretching), 1596 cm-1 (C]N stretching), 1456 cm‒1 (C]C stretching vibrations of the pyridine ring) and 1218 cm-1 (CeN stretching). After quaternization, the band at 1599 cm-1 disappeared and the band at 1456 cm‒1 is displaced to 1470 cm‒1 and 1218 cm-1 to 1222 cm-1, respectively, alongwith an appearance of new band at 1646 cm-1 which is the characteristic peak of the quaternary pyridinium moiety. [33] The FTIR spectra thus confirm successful synthesis of the quaternized materials (Fig. 1b). CHN analysis was performed to get supplementary evidence of polymerization and quaternization reaction. Low %N in [PVIm-(CH2)4PVIm][Cl−]2 and [PVIm-(CH2)6-PVIm][Cl−]2 than poly(N-VIm), supports quaternization, and similar inferences can be drawn for poly(4Py) (Table 1). [34] SEM images of the quaternized polymeric materials, [PVIm-(CH2)4PVIm][Cl−]2 and [PVIm-(CH2)6-PVIm][Cl−]2) show their porous morphology (Fig. 2a,b). After adsorption of As(V) ions onto the surface of [PVIm-(CH2)4-PVIm][Cl−]2 and [PVIm-(CH2)6-PVIm][Cl−]2), surface morphology changes with decreased porosity that confirms the successful attachment of As(V) ions onto the synthesized PILs (Fig. 2c,d). Adsorption of As(V) ions onto the [PVIm-(CH2)4-PVIm] [Cl−]2 and [PVIm-(CH2)6-PVIm][Cl−]2 was further affirmed by their corresponding EDX spectra having peaks corresponding to As (Fig. 2e,f). Similarly, SEM images of the ([PVP-(CH2)4-PVP][Cl−]2, [PVP(CH2)6-PVP][Cl−]2, poly(N-Vim), also has porous morphology (Fig. 2g,h). After adsorption of As(V) ions onto the surface of quaternized polymeric materials, their particles became spherical and surface became smoother. Thus change in the morphology of the synthesized PILs before and after adsorption of As(V) ions onto their surface successfully confirmed the attachment of As(V) ions onto the synthesized PILs (Fig. 2i,j). Adsorption of As(V) ions onto the ([PVP(CH2)4-PVP][Cl−]2, [PVP-(CH2)6-PVP][Cl−]2, was further affirmed by their corresponding EDX spectra having peaks corresponding to As (Fig. 2k,l).
solution. With increase in pH ranging from 2.0 to 12.0, this nonionic form (H3AsO4) is replaced by H2AsO4− and HAsO42− ions and there is increase in forces of attraction between these ions and PILs. Hence these forces help in effective adsorption of As(V) ions with increase in basic medium of the solution. When pH exceed (pH > 9.0), the hydroxyl ions in the solution compete with arsenate ions for binding site on PILSs resulting in low As(V) adsorption. The PILs favour wide pH range (pH 4–9) because of their quaternized structure and does not change structure with the variation of pH. Adsorption of As(V) ions by PILs from the water solutions was found to be dependent on the nature of monomer, N-VIm or 4-Py, and length of the crosslinker. Though all the PILs showed rapid and high As(V) removal, yet with the longer crosslinker, 1,6 hexyl chain, initial rapid removal of the contaminant ions was observed for both the polymers as it allows larger access of the active sites to the As(V) ions. Comparison of the efficiency of the synthesised PILs with other reported adsorbents in the literature is shown in Table 3, which revealed that the synthesised polymeric materials are rapid and efficient candidates for the removal of As (V) ions. 4. Conclusions We report synthesis of four new crosslinked poly(ionic liquids) derived from N-VIm and 4-Py for use as As(V) extractants from aqueous solution. Results obtained reveal that the synthesized PILs are effective extractants of As(V) ions with high performance of 100% removal from very low concentration of 20 ppb or 3.5 ppm solution. As per WHO and EU guidelines arsenic concentrations in groundwater should be below 10 ppb. As(V) adsorption increased with increase in contact time and temperature. Structure of the PILs or synthetic protocol does not have sharp effect on the removal efficiency of As(V) as all PILs were effective extractants though those with the longer crosslinker have somewhat rapid extraction rate. It can be concluded that PILs has a high capacity for As(V) ions removal. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jece.2019.103154.
3.2. As(V) ion exchange behaviour of PILs as a function of different parameters
References
As exits as H2AsO4− and HAsO42− within a pH range of 2–12. Using 20 ppb solution of As(V) ions and [PVIm-(CH2)4-PVIm][Cl−]2 as extractant, concentration left in filtrate after 10 min was reduced by 25% to 15 ppb. When the contact time was increased from 10 to 30 min the residual concentration was reduced to 6 ppb and whole of the As(V) ions were removed when contact time was 60 min (Fig. 3). Similar trends in results were observed in the case of other three PILs (Table 2). PVIm derivatives show complete removal of As(V) ions at 25 °C and 60 min. In case of [PVP-(CH2)4-PVP][Cl−]2 and [PVP-(CH2)6-PVP] [Cl−]2 8 ppb and 5 ppb of As(V) ions was the residual concentration under the same conditions. Similarly, As(V) ions removal by the synthesized PILs increases effectively when temperature was progressively increased. The synthesized PILs show effective extraction of As(V) ions at higher concentration of up to 3.5 (mg L−1), (Fig. 4). Hence these results show that the synthesized PILs are effective adsorbent at very low as well as high concentration (20 ppb and 3.5 ppm) of As(V) ions (Fig. 5). Adsorption study of As(V) ions was also carried out under media of different pH i.e.1.5, 4.5, 9.2 and 12.8 at 37 °C. As(V) ions removal was somewhat low in acidic medium than basic medium because at very low pH (˜2.0), the As(V) ions remains mainly as non-ionic form (H3AsO4). Due to this there is no electrostatic force and van der Waals forces between the amines on synthesized PILs and the As(V) ions in
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