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Facile and efficient removal of Pb(II) from aqueous solution by chitosan-lead ion imprinted polymer network
Chemosphere 240 (2020) 124772
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Facile and efficient removal of Pb(II) from aqueous solution by chitosan-lead ion imprinted polymer network Javad Gatabi a, Yaghoub Sarrafi a, *, Moslem Mansour Lakouraj a, Mehdi Taghavi b a b
Department of Organic Chemistry, Faculty of Chemistry, University of Mazandaran, Babolsar, 47416-95447, Iran Department of Chemistry, Faculty of Science, Shahid Chamran University of Ahvaz, 61357-43337, Iran
h i g h l i g h t s Chitosan and properties of it suggest that it can be a promising adsorbent. Lead ions were removed from ion imprinted polymer (IIPs) particles by nitric acid. The competitive adsorption studies showed excellent selectivity for the lead ions. The regeneration studies showed no significant decrease in adsorption capacities.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 April 2019 Received in revised form 26 August 2019 Accepted 4 September 2019 Available online 5 September 2019
A new chitosan based imprinted polymer was prepared by copolymerization of 4-Vinylpyridine (VP) as a functional monomer and N,N0 -Methylenebisacrylamide (MBA) as a crosslinker in the presence of Potassium peroxodisulfate (KPS) as an initiator to eliminate Pb(II) from aqueous solutions. The template ions were removed from ion imprinted polymer (IIPs) particles by leaching with 0.1 M nitric acid (HNO3) that leaves cavities in the particles with the capability of selective extraction of the Pb(II) ions. Some properties of the bioadsorbent were further identified using Field Emission Scanning Electron Microscopy (FE-SEM), Fourier Transform Infrared Spectroscopy (FT-IR) and Thermal Gravimetric Analysis (TGA). In addition, the equilibrium adsorption data were examined through Langmuir and Freundlich isotherm models and it was found that adsorption data fit well with the Freundlich isotherm. The competitive adsorption studies clearly showed that the Pb(II)-IP has a much higher adsorption capacity for Pb(II) than the non-imprinted polymer (NIP) with the same chemical composition; furthermore, it has excellent selectivity for the targeted ion. In addition, the studies regarding the regeneration and reuse studies revealed that the Pb(II)-IP beads showed no significant decrease in their adsorption capacities. © 2019 Published by Elsevier Ltd.
Handling Editor: Martine Leermakers Keywords: Chitosan Selective extraction Ion imprinted polymer Pb(II)-IP beads
1. Introduction Water pollution by heavy metals (Caeiro et al., 2005; Armitage et al., 2007) which is considered a global environmental issue as a result of activities such as mine exploitation (Dragovi c et al., 2008), industrialization and urbanization has been on the rise all over the planet (Hu and Cheng, 2012; Li et al., 2018). Heavy metal ions in the environment have bio-accumulation potential and they also have a bio-magnification along food chain. Therefore, their toxicity increases in animals at higher food levels. Among these
* Corresponding author. E-mail address: ysarrafi@umz.ac.ir (Y. Sarrafi). https://doi.org/10.1016/j.chemosphere.2019.124772 0045-6535/© 2019 Published by Elsevier Ltd.
metal ions, lead has severe toxicity (Mom cilovi c et al., 2011; Carocci et al., 2016) and is generally assumed as one of the most dangerous metals due to its high harmful effect on brain (Lange and Condello, 2017) (i.e., nervous system), kidney and digestive systems especially in children. Considering all the above, the release of heavy metals into the environment is a serious threat to human health and ecosystems. Different methods for recycling heavy metals (Khan et al., 2013) (Arias et al., 2002; Nadeem et al., 2006) from wastewater have been developed including filtration (Inyang et al., 2012), reverse osmosis (Greenlee et al., 2009; Al-Obaidi et al., 2017), chemical oxidation or reduction (Kurniawan et al., 2006; Barakat, 2011), adsorption and surface deposition (Guminilovych et al., 2013) have been developed. In this regard, employing biopolymers (Zhou et al., 2012) with valuable properties such as
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availability, low cost and toxicity has a profound influence on the adsorption methods of heavy metals. Cellulose, starch, lignin, gelatin, chitin and chitosan are taken into account as the most useful biopolymers for this purpose. Among them, chitosan (CS), a biodegradable polysaccharide which is usually obtained from deacetylation of chitin (Tianwei et al., 2001; Flores-Hern andez et al., 2014), plays an important role in the removal of the pollutant particularly hazardous heavy ion metals from aquatic environments (Fu and Wang, 2011). In recent decades, the molecular imprinted polymers (MIP) have widely been used as a target adsorption method for selective separation of molecules (Wulff, 1995) and ions (Shawky, 2009). The molecular imprinted polymers represent a new category of artificially produced materials (Alexander et al., 2003) with proprietary identification sites. Since their discovery in 1972, MIPs have attracted many scientists'attention. (Cheong et al., 2013). As a matter of fact, their remarkable characteristics including the convenient and cost-effective preparation, besides, considerable selective recognition of target molecules have led to their increasing applications in separation techniques such as membranes (Piletsky et al., 1999), biosensors (Yano and Karube, 1999), solid phase extraction (Hennion, 1999), high performance liquid chromatography (Bitas and Samanidou, 2018), electrophoresis chromatography (Turiel and Martin-Esteban, 2004) and so on. In terms of application and structure, molecular imprinting polymers are similar to antibodies which have a specific function in identifying the target molecule. However, as a prominent difference, MIPs possess several hundred to several thousand detection sites; on the contrary, the antibodies have one or limited positions. Practically, there are three steps to fabricate an imprinted polymer: (I) prior to polymerization, functional monomer and template are bound to each other by covalent or noncovalent linkages. (II) Then, this covalent or noncovalent conjugation is polymerized while the linkage remains intact during the process. (III) After the polymerization, the linkage is cleaved and the template is removed from the polymer network. Upon the guest binding by the imprinted polymers, the same covalent or noncovalent linkage is formed. Considering chitosan as a powerful complexing agent, its introduction into the structure of various imprinted polymers (Liu et al., 2011a, 2011b, 2013; Xu et al., 2015) provides an efficient and expedient technique which unveils enhanced features such as higher thermal stability and adsorption capacity (Birlik et al., 2006; Ren et al., 2008). In recent years, chitosan based ion imprinted polymers have been developed which is resulted in different selectivity and adsorption abilities (Pakdel and Peighambardoust, 2018). Additionally, various studies have been conducted on the design of novel imprinted polymer on the basis of chitosan for solid-phase extraction of lead ions (Shakerian et al., 2016) have been done and moreover, there are a few reports investigating Pb(II) imprinted chitosan based polymer via cross-linking with ethylene glycol dimethacrylate (EGDMA) (Hande et al., 2016). According to what have been mentioned above, and considering the fact that the most of corresponding MIPs have been designed and utilized in the organic media; the current study reports a new chitosan based imprinted polymer for the elimination of Pb(II) from aqueous solutions. Based on our experimental results, this adsorbent represents appropriate adsorption capacity as well as noticeable selectivity in lead ions against other metallic ions.
2. Experimental 2.1. Apparatus A digital pH meter, WTW Metrohm 827 Ion analyzer (Herisau, Switzerland), equipped with a combined glass calomel electrode was utilized for the pH control at the temperature of 25 ± 1 C. Field Emission Scanning Electron Microscope (FESEM) analyses were taken using JEOL JSM6390, coupled with energy-dispersive X-ray (EDX) analyzer (acceleration voltage 10 kV). Fourier transform infrared (FT-IR) spectra (4000e400 cm1) were recorded on Bruker vector 22 spectrophotometer. Thermogravimetric analysis (TGA) was studied on a METTLER TOLEDO analyzer (London, UK). The concentration of heavy metal ions was evaluated by atomic absorption spectroscopy (AAS) using a Perkin- Elmer Analyst 200 model. 2.2. Materials N,N0 -Methylenebisacrylamide (MBA, 99%), ammonium peroxydisulfate (APS, reagent grade, 98%), chitosan (CS, deacetylation degree of 90%) and 4-vinyl pyridine (95%) were purchased from Sigma Aldrich. Sodium hydroxide (analytical grade), acetic acid (99%), lead (II) nitrate and other metal salts were purchased from Merck (Darmstadt, Germany). All solvents were analytical grade and were used as soon as they were received. 2.3. Preparing IIPs and NIPs Synthesis of lead-imprinted polymer was performed by precipitation polymerization technique. For this purpose, in a three necked round bottom flask chitosan solution 3% (w/w) (prepared by adding chitosan into 2% (v/v) acetic acid solution) and 4-vinyl pyridine (14 mmol) were dissolved with continued stirring at room temperature for 1 h. Subsequently, 0.3 mmol lead nitrate and then 10 mmol MBA and 0.07 g KPS were added as an initiator to the reaction mixture. The system was kept under inert atmosphere and refluxed at 100 C for 18 h. Afterwards; the obtained polymer was filtered and washed with ethanol and distilled water several times and dried in a vacuum oven. Finally, Polymer network was leached with 0.2 M HNO3 to remove lead and afford imprinted cavities (Fig. 1.). In the same procedure non-imprinted polymer (NIP) was synthesized without lead ion. 3. Results and discussions 3.1. Characterization studies 3.1.1. FT-IR analysis Fig. 2 represents the Ft-IR spectra of Chitosan, 4-vinyl pyridine, N,N0 -Methylene bisacrylamide and IIPs. According to the Fig. (2-a), the absorption peaks at 3380 cm1 can be assigned to the NeH stretching of chitosan, in addition, the broad band observed at 3481 cm1 is related to the OeH stretching vibration of hydroxyl groups, so is it at 2937 and 2912 cm1 to the CeH stretching of chitosan. In MIPs spectrum (Fig. 2-c), the -C-O stretching and NeH bending vibration appeared at 1381 and 1595 cm1, respectively. Moreover, the characteristic peaks attributed to C]C stretching vibration of pyridine rings and C]O stretching of amide group were emerged at 1639 and 1679 cm1, respectively. In Fig. 2bed, the absorption peak at 1415 cm1pertains to stretching vibration of eCH2 in polymer skeleton. 3.1.2. SEM analysis The morphology of chitosan (A), unleached (B), leached (C) and
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Fig. 1. Schematic illustration of imprinting process for the preparation of chitosan based lead-imprinted polymer.
non-imprinted polymer (D) was assessed through Field Emission Scanning Electron Microscopy (FE-SEM). As it is depicted in Fig. 3, FE-SEM micrograph of leached polymer (Fig. 3-C) obviously revealed the porous polymer network in comparison to nonporous structure of non-imprinted framework (Fig. 3-D). Moreover, elemental composition of leached and unleached imprinted polymer was studied by energy dispersive X-ray (EDX) (Fig. 4). EDX analysis clearly shows the presence of Pb, C, N, and O which can be seen in Fig. 4- E and 4- F. The higher percentage of O along with N enhances the complexing ability of CS-IIP towards Pb(II). Absence of the Pb(II) in EDX spectra confirms its complete leaching of polymer.
3.2. Adsorption experiments Adsorption of Pb(II) was evaluated by employing different amounts of sorbent (10, 25, 50, 75 and 100 mg of IIP and NIP) in 100 mL solution including lead ion (10e100 mg L1) at different pH values. Afterwards, the polymer was filtered and subsequently elution was carried out by using 30 mL HNO3 solution (0.2 M) for 40 min. The Pb(II(ions adsorbed on polymer were determined by Flame Atomic Absorption Spectrometry (FAAS). Utilizing the following equation (Eq. (1)), the equilibrium capacity of the corresponding polymer was calculated:
qe ¼ 3.1.3. TGA Thermogravimetric analysis (TGA) was employed to survey the thermal stability of chitosan, leached and unleached imprinted polymer. As it is observable in TGA plot (Fig. 5), the maximum weight loss for chitosan is about 80% and the most sustainability for unleached polymer is determined ca. 60% of weight loss. In comparison to unleached polymer, the leached polymer showed less thermal stability owing to the absence of lead ions. These observations evidently proved the formation of Pb2þ imprinted polymer and elution of Pb(II) ions from the polymer.
ðC0 CeÞV m
where qe (mg g1) represents the equilibrium capacity of polymer, C0 (mg L1) and Ce (mg L1) point to the initial and equilibrium concentration of metal ions, V (L) shows the amount of ion solution and finally m (g) represents the mass of polymer. In order to investigate the adsorption performance and efficiency of the adsorbent, Langmuir and Freundlich isotherm models were applied. According to Langmuir model, the adsorption process occurs in a monolayer in or constant adsorption sites on the surface (Fig. 6). All of these adsorption sites contain the same amount of energy which leads to a homogenous adsorbent structure (Chen et al., 2008). Langmuir equation (Eq. (2)) (Sreejalekshmi et al., 2009) and Freundlich equation (Fig. 7) (Eq. (3)) (Chu, 2002) were applied to determine the adsorption capacity values:
1 1 1 ¼ þ qe qm Kl Ce qm lnqe ¼ lnKf þ
Fig. 2. FTeIR spectra of chitosan (a), 4-vinyl pyridine (b), N,N0 -Methylene bisacrylamide (c) and unleached polymer (d).
(1)
1 lnCe n
(2)
(3)
In these equations, qm (mg g1) and Kl (L mg1) refer to Langmuir constants which correspond to adsorption capacity and energy of adsorption, respectively. Ce (mg L1) and qe (mg g1) demonstrate the equilibrium concentration and the amount of Pb(II) adsorbed at equilibrium. KF and n (Freundlich constants) indicate adsorption capacity and adsorption intensity, respectively. Furthermore, the degree of suitability of adsorbent towards metal ions was estimated from the values of separation factor constant (RL), which has always been used to indicate the
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Fig. 3. FE-SEM micrographs of A) Chitosan, B) Unleached polymer C) lead-imprinted polymer D) Non-imprinted polymer.
adsorption, whether it is favorable or not (Hao et al., 2010). The value of RL will indicate the type of isotherm whether it is, irreversible (RL ¼ 0), favorable (0 1) which can be calculated from the following equation (eq. (4)):
RL ¼
1 1 þ KL C0
(4)
where KL (L mmol1) is the Langmuir constant, and C0 (mmol L1) is the initial concentration of metal ions. The RL values of polymer shown in Table S1 are greater than zero and less than unity, which suggests that the adsorption processes between the adsorbents (CS-IIP) and metal ion (Pb(II)) are favorable (Hao et al., 2010). The adsorption kinetic of Pb(II) on IIP and NIP was studied by adding 0.2 g adsorbent (IIP or NIP) to 30 mL of 200 mmol L1 of Pb(II) at pH ¼ 7 in different contact times. Then, concentration of Pb(II) ions in the solution was measured by employing FAAS after being eluted with 30 mL of 0.2 M HNO3. Determination of adsorption rate was accomplished on the basis of Lagergren's pseudo-first order (Eq. (5)) (Crist et al., 1981) and pseudo-second order kinetic equations (Eq. (6)) (Kannamba et al., 2010):
k t logðqe qt Þ ¼ logqe e 1 2:303
(5)
t 1 t ¼ þ qt k2 q2e qt
(6)
Herein, qe (mg g1) and qt (mg g1) relate to the quantities of adsorbate adsorbed at equilibrium and at time t, respectively. Furthermore, K1 (min1) and K2 (g mg1 min) describe the pseudofirst order (Fig. S1) and pseudo-second order adsorption rate constants, respectively. The kinetic parameters calculated from the pseudo-first and the pseudo-second order models (Fig. S2) are listed in Table S2. The results indicate that the pseudo-first order model fits the experiment data much better than the pseudo-second order model because its correlation coefficient (R2) values are beyond 0.994, which are higher than that of the pseudo-second order model (below 0.986). In addition, the theoretical calculated qe,cal. values obtained from the pseudo-first order model are closer to the experimental ones (qe,exp.) than that of the pseudo-second order equation. 3.3. Sorption and elution times The effect of sorption and elution times was evaluated according to the following procedure: first the pH of the analyte solution was regulated on 7. Then, 20 mg of leached sorbent was added to the 30 mL solution of the sample comprising of 10 mg/L Pb(II) and in the next step, the solution was stirred for 10, 11, 12, 13, 14, 15 and
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Fig. 4. (E) EDX spectrum of unleached (F) Leached polymer.
Fig. 5. TGA curve of leached polymer, chitosan and unleached polymer.
Fig. 6. Langmuir isotherm curve for different adsorbent in Pb(II) ions aqueous.
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with Pb(II), mainly due to the electrostatic disposal of lead ions (Ngah and Fatinathan, 2010). However, increasing the pH led to an increase in the negative charge density on the surface of the ligands, and thus their electrostatic gravity with metal ions and subsequently higher absorption percentage. At the pH values greater than 7, owing to the aggregation of hydroxyl ions, along with precipitation with Pbþ2 ions, a remarkable decrease in adsorption amounts was occurred (Zhao et al., 2011).
Fig. 7. Freundlich isotherm curve for different adsorbent in Pb(II) ions aqueous.
30 min. Separation of ion imprinted polymer through filtration was the next step. The amount of analyte was analyzed by means of FAAS after eluting the adsorbent with 30.0 mL 0.2 mol.L1 HNO3 solutions. The experiment showed that the adsorption reached to an equilibrium state after 15 min. Additionally, the elution time was estimated by adding 20 mg of unleached sorbent into 30 mL solution 0.2 mol L1 HNO3 solution and then being stirred for 8, 10, 11, 12, 14, 16, 18, 20 and 30 min. Subsequently, after isolation of the lead imprinted polymer by filtration, the resulting analyte was studied using FAAS. Eventually, the optimum desorption time was determined 16 min (Fig. S3). Putting adsorbed Cs-IIP into HNO3 solution, the hydroxyl group of chitosan was protonated. In comparison between protonation and chelation, a great amount of hydrogen from HNO3 solution replaced the metal ions to occupy the groups and most of the metal ions adsorbed were released. Then the adsorption capacity could be regained after washing with NaOH solution to be deporotonated.
3.6. Selectivity study The selectivity of IIP and NIP for Pb(II) was studied by shaking 0.1 g of the polymers with 25 mL of 50 mg L1of each individual metal ion at pH 6.0 against other metal ions (Table S3). The parameters of selectivity including distribution coefficient (Kd), selectivity coefficient (k) and relative selectivity coefficient (K0) of Pb(II(ions towards IIP were analyzed by the following equations:
Kd ¼
K¼
0
K ¼
Qe Ce
(7)
Kd ðPbðIIÞÞ Kd ðmetal ionsÞ
(8)
K imprinted K nonimprinted
(9)
where Qe indicates the adsorption capacity (mg g1) and Ce refers to the equilibrium concentrations of Pb(II) (mg L1).
3.7. Reusability 3.4. Effect of temperature on Pb(II) adsorption The effect of temperature on PbeCs IIP adsorbing Pb(II) ions was explored at pH 7.0 and the results have been shown in Fig. S4. Accordingly, the adsorption capacity of PbeCS-IIP for Pb(II) ions increased from 89 to 136 mg/g during temperature rise from 20 to 40 C. This may arise from the stronger interactions between Pb(II) and active groups (unreacted hydroxyl, and vinyl pyridine) of PbeCS-IIP that happen at elevated temperature, which also confirms the endothermic identity (nature) of adsorption process. Generally, the adsorption process proceeds with two successive sequences called “fast diffusion” and “slow complication” (Fu and Wang, 2011). Enhancing the temperature not only promotes the rate of Pb(II) ions diffusion from the inside of solutions to the surface of adsorbents, but it also accelerates the complexation of Pb(II) ions with the functional groups of adsorbents. 3.5. Effect of sorption pH In order to investigate the influence of pH on the Pb(II) adsorption, 20 mg of imprinted polymer was immersed to 30 mL of 10.0 mg L1 solution of Pb(II) under different pH conditions from pH ¼ 2 to pH ¼ 8, for 15 min. Increase in pH value was accompanied by an increase in Pb(II) adsorption amount. Eventually, the highest adsorption amount was observed in the range of pH 6e7 (Fig. S5). A crucial parameter on the lead adsorption process involves the protonation or non-protonation of amine and hydroxyl groups in chitosan moiety (Luk et al., 2014). In fact, decreasing the pH values provoked the decline in the number of active sites in chitosan which was consequently followed by decrease in complexation
The recovery and reusability of the corresponding imprinted polymer was studied as an important factor in evaluation of the adsorbent effectiveness and the results were summarized in Fig. S6. As it can be seen in Fig. S6, no considerable loss of adsorption capacity was detected until the fifth cycle, representing that the elution has a negligible effect on cavity structure as well as chemical features of CS-IIP. Obviously, these results confirm favorable stability and reusability of CS-IIPs. Moreover the CS-IIP after adsorption of Pb(II) was analyzed via map imaging and results revealed that the adsorption onto the polymer occurred uniformly in all regions (Fig. S7).
4. Conclusion This study is the first report on imprinted chitosan polymer for the selective removal of Pb(II) ions which has exhibited excellent characteristics properties. It is noteworthy that the Pb(II)-IP chitosan bead can significantly enhance the selectivity and adsorption capacity of the Pb(II) heavy metal ions. In comparison to the literature on performance of lead adsorption with the chitosan based polymer has indicated that it has a high adsorption capacity. The experimental data was fitted to the Freundlich model to explain the adsorption mechanism. The reusability experiments have been demonstrated that the process is suitable, and has favorable stability in several cycles. As potential biosorbent for the Pb(II) removal from waste water, the Pb(II)-IP chitosan bead demonstrated a great adsorption capacity throughout the multi cycle adsorption-desorption process in aquatic media.
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Selective adsorption behavior of Pb(II) by mesoporous silica SBA-15-supported Pb(II)-imprinted polymer based on surface molecularly imprinting technique. J. Hazard Mater. 186, 197e205. http://doi.org/10.1016/j.jhazmat.2010.10.105. Liu, B., Lv, X., Meng, X., Yu, G., Wang, D., 2013. Removal of Pb(II) from aqueous solution using dithiocarbamate modified chitosan beads with Pb(II) as imprinted ions. Chem. Eng. J. 220, 412e419. http://doi.org/10.1016/j.cej.2013.01. 071. Luk, C.J., Yip, J., Yuen, C.M., Kan, C., Lam, K., 2014. A comprehensive study on adsorption behaviour of direct, reactive and acid dyes on crosslinked and noncrosslinked chitosan beads. J. Fiber Bioeng. Inf. 7, 35e52. http://doi.org/10.3993/ jfbi03201404. Mom cilovi c, M., Purenovi c, M., Boji c, A., Zarubica, A., RanCelovi c, M., 2011. Removal of lead (II) ions from aqueous solutions by adsorption onto pine cone activated carbon. Desalination 276, 53e59. http://doi.org/10.1016/j.desal.2011.03.013. Nadeem, M., Mahmood, A., Shahid, S.A., Shah, S.S., Khalid, A.M., McKay, G., 2006. Sorption of lead from aqueous solution by chemically modified carbon adsorbents. J. Hazard Mater. 138, 604e613. http://doi.org/10.1016/j.jhazmat.2006.05. 098. Ngah, W.W., Fatinathan, S., 2010. Adsorption characterization of Pb(II) and Cu(II) ions onto chitosan-tripolyphosphate beads: kinetic, equilibrium and thermodynamic studies. J. environ. Management. 91, 958e969. http://doi.org/10.1016/j. jenvman.2009.12.003. Pakdel, P.M., Peighambardoust, S.J., 2018. Review on recent progress in chitosanbased hydrogels for wastewater treatment application. Carbohyd. polym. http://doi.org/10.1016/j.carbpol.2018.08.070. 263-278 Piletsky, S.A., Panasyuk, T.L., Piletskaya, E.V., Nicholls, I.A., Ulbricht, M., 1999. Receptor and transport properties of imprinted polymer membranesea review. J. Membr. Sci. 157, 263e278. http://doi.org/10.1016/S0376-7388(99) 00007-1. Ren, Y., Zhang, M., Zhao, D., 2008. Synthesis and properties of magnetic Cu (II) ion imprinted composite adsorbent for selective removal of copper. Desalination 228, 135e149. http://doi.org/10.1016/j.desal.2007.08.013. Shakerian, F., Kim, K.H., Kwon, E., Szulejko, J.E., Kumar, P., Dadfarnia, S., Shabani, A.M.H., 2016. Advanced polymeric materials: synthesis and analytical application of ion imprinted polymers as selective sorbents for solid phase extraction of metal ions. TrAC Trends Anal. Chem. (Reference Ed.) 83, 55e69. http://doi.org/10.1016/j.trac.2016.08.001. Shawky, H.A., 2009. Synthesis of ion-imprinting chitosan/PVA crosslinked membrane for selective removal of Ag (I). J. Appl. Polym. Sci. 114, 2608e2615. http:// doi.org/10.1002/app.30816. Sreejalekshmi, K.G., Krishnan, K.A., Anirudhan, T.S., 2009. Adsorption of Pb(II) and Pb(II)-citric acid on sawdust activated carbon: kinetic and equilibrium isotherm studies. J. Hazard Mater. 161, 1506e1513. http://doi.org/10.1016/j.jhazmat.2008. 05.002. Tianwei, T., Xiaojing, H., Weixia, D., 2001. Adsorption behaviour of metal ions on imprinted chitosan resin. J. Appl. Chem. Biotechnol. 76, 191e195. http://doi.org/ 10.1002/jctb.358. Turiel, E., Martin-Esteban, A., 2004. Molecularly imprinted polymers: towards highly selective stationary phases in liquid chromatography and capillary electrophoresis. Anal. Bioanal. Chem. 378, 1876e1886. http://doi.org/10.1007/ s00216-003-2331-1. Wulff, G., 1995. Molecular imprinting in cross-linked materials with the aid of molecular templatesda way towards artificial antibodies. Angew Chem. Int. Ed. Engl. 34, 1812e1832. http://doi.org/10.1002/anie.199518121. Xu, L., Huang, Y.A., Zhu, Q.J., Ye, C., 2015. Chitosan in molecularly-imprinted polymers: current and future prospects. Int. J. Mol. Sci. 16, 18328e18347. http://doi. org/10.3390/ijms160818328.
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Yano, K., Karube, I., 1999. Molecularly imprinted polymers for biosensor applications. TrAC Trends Anal. Chem. (Reference Ed.) 18, 199e204. http://doi.org/10. 1016/S0165-9936(98)00119-8. Zhao, D., Sheng, G., Hu, J., Chen, C., Wang, X., 2011. The adsorption of Pb(II) on Mg2Al layered double hydroxide. Chem. Eng. J. 171, 167e174. http://doi.org/10.
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Facile and efficient removal of Pb(II) from aqueous solution by chitosan-lead ion imprinted polymer network Javad Gatabi a, Yaghoub Sarrafi a, *, Moslem Mansour Lakouraj a, Mehdi Taghavi b a b
Department of Organic Chemistry, Faculty of Chemistry, University of Mazandaran, Babolsar, 47416-95447, Iran Department of Chemistry, Faculty of Science, Shahid Chamran University of Ahvaz, 61357-43337, Iran
h i g h l i g h t s Chitosan and properties of it suggest that it can be a promising adsorbent. Lead ions were removed from ion imprinted polymer (IIPs) particles by nitric acid. The competitive adsorption studies showed excellent selectivity for the lead ions. The regeneration studies showed no significant decrease in adsorption capacities.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 April 2019 Received in revised form 26 August 2019 Accepted 4 September 2019 Available online 5 September 2019
A new chitosan based imprinted polymer was prepared by copolymerization of 4-Vinylpyridine (VP) as a functional monomer and N,N0 -Methylenebisacrylamide (MBA) as a crosslinker in the presence of Potassium peroxodisulfate (KPS) as an initiator to eliminate Pb(II) from aqueous solutions. The template ions were removed from ion imprinted polymer (IIPs) particles by leaching with 0.1 M nitric acid (HNO3) that leaves cavities in the particles with the capability of selective extraction of the Pb(II) ions. Some properties of the bioadsorbent were further identified using Field Emission Scanning Electron Microscopy (FE-SEM), Fourier Transform Infrared Spectroscopy (FT-IR) and Thermal Gravimetric Analysis (TGA). In addition, the equilibrium adsorption data were examined through Langmuir and Freundlich isotherm models and it was found that adsorption data fit well with the Freundlich isotherm. The competitive adsorption studies clearly showed that the Pb(II)-IP has a much higher adsorption capacity for Pb(II) than the non-imprinted polymer (NIP) with the same chemical composition; furthermore, it has excellent selectivity for the targeted ion. In addition, the studies regarding the regeneration and reuse studies revealed that the Pb(II)-IP beads showed no significant decrease in their adsorption capacities. © 2019 Published by Elsevier Ltd.
Handling Editor: Martine Leermakers Keywords: Chitosan Selective extraction Ion imprinted polymer Pb(II)-IP beads
1. Introduction Water pollution by heavy metals (Caeiro et al., 2005; Armitage et al., 2007) which is considered a global environmental issue as a result of activities such as mine exploitation (Dragovi c et al., 2008), industrialization and urbanization has been on the rise all over the planet (Hu and Cheng, 2012; Li et al., 2018). Heavy metal ions in the environment have bio-accumulation potential and they also have a bio-magnification along food chain. Therefore, their toxicity increases in animals at higher food levels. Among these
* Corresponding author. E-mail address: ysarrafi@umz.ac.ir (Y. Sarrafi). https://doi.org/10.1016/j.chemosphere.2019.124772 0045-6535/© 2019 Published by Elsevier Ltd.
metal ions, lead has severe toxicity (Mom cilovi c et al., 2011; Carocci et al., 2016) and is generally assumed as one of the most dangerous metals due to its high harmful effect on brain (Lange and Condello, 2017) (i.e., nervous system), kidney and digestive systems especially in children. Considering all the above, the release of heavy metals into the environment is a serious threat to human health and ecosystems. Different methods for recycling heavy metals (Khan et al., 2013) (Arias et al., 2002; Nadeem et al., 2006) from wastewater have been developed including filtration (Inyang et al., 2012), reverse osmosis (Greenlee et al., 2009; Al-Obaidi et al., 2017), chemical oxidation or reduction (Kurniawan et al., 2006; Barakat, 2011), adsorption and surface deposition (Guminilovych et al., 2013) have been developed. In this regard, employing biopolymers (Zhou et al., 2012) with valuable properties such as
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availability, low cost and toxicity has a profound influence on the adsorption methods of heavy metals. Cellulose, starch, lignin, gelatin, chitin and chitosan are taken into account as the most useful biopolymers for this purpose. Among them, chitosan (CS), a biodegradable polysaccharide which is usually obtained from deacetylation of chitin (Tianwei et al., 2001; Flores-Hern andez et al., 2014), plays an important role in the removal of the pollutant particularly hazardous heavy ion metals from aquatic environments (Fu and Wang, 2011). In recent decades, the molecular imprinted polymers (MIP) have widely been used as a target adsorption method for selective separation of molecules (Wulff, 1995) and ions (Shawky, 2009). The molecular imprinted polymers represent a new category of artificially produced materials (Alexander et al., 2003) with proprietary identification sites. Since their discovery in 1972, MIPs have attracted many scientists'attention. (Cheong et al., 2013). As a matter of fact, their remarkable characteristics including the convenient and cost-effective preparation, besides, considerable selective recognition of target molecules have led to their increasing applications in separation techniques such as membranes (Piletsky et al., 1999), biosensors (Yano and Karube, 1999), solid phase extraction (Hennion, 1999), high performance liquid chromatography (Bitas and Samanidou, 2018), electrophoresis chromatography (Turiel and Martin-Esteban, 2004) and so on. In terms of application and structure, molecular imprinting polymers are similar to antibodies which have a specific function in identifying the target molecule. However, as a prominent difference, MIPs possess several hundred to several thousand detection sites; on the contrary, the antibodies have one or limited positions. Practically, there are three steps to fabricate an imprinted polymer: (I) prior to polymerization, functional monomer and template are bound to each other by covalent or noncovalent linkages. (II) Then, this covalent or noncovalent conjugation is polymerized while the linkage remains intact during the process. (III) After the polymerization, the linkage is cleaved and the template is removed from the polymer network. Upon the guest binding by the imprinted polymers, the same covalent or noncovalent linkage is formed. Considering chitosan as a powerful complexing agent, its introduction into the structure of various imprinted polymers (Liu et al., 2011a, 2011b, 2013; Xu et al., 2015) provides an efficient and expedient technique which unveils enhanced features such as higher thermal stability and adsorption capacity (Birlik et al., 2006; Ren et al., 2008). In recent years, chitosan based ion imprinted polymers have been developed which is resulted in different selectivity and adsorption abilities (Pakdel and Peighambardoust, 2018). Additionally, various studies have been conducted on the design of novel imprinted polymer on the basis of chitosan for solid-phase extraction of lead ions (Shakerian et al., 2016) have been done and moreover, there are a few reports investigating Pb(II) imprinted chitosan based polymer via cross-linking with ethylene glycol dimethacrylate (EGDMA) (Hande et al., 2016). According to what have been mentioned above, and considering the fact that the most of corresponding MIPs have been designed and utilized in the organic media; the current study reports a new chitosan based imprinted polymer for the elimination of Pb(II) from aqueous solutions. Based on our experimental results, this adsorbent represents appropriate adsorption capacity as well as noticeable selectivity in lead ions against other metallic ions.
2. Experimental 2.1. Apparatus A digital pH meter, WTW Metrohm 827 Ion analyzer (Herisau, Switzerland), equipped with a combined glass calomel electrode was utilized for the pH control at the temperature of 25 ± 1 C. Field Emission Scanning Electron Microscope (FESEM) analyses were taken using JEOL JSM6390, coupled with energy-dispersive X-ray (EDX) analyzer (acceleration voltage 10 kV). Fourier transform infrared (FT-IR) spectra (4000e400 cm1) were recorded on Bruker vector 22 spectrophotometer. Thermogravimetric analysis (TGA) was studied on a METTLER TOLEDO analyzer (London, UK). The concentration of heavy metal ions was evaluated by atomic absorption spectroscopy (AAS) using a Perkin- Elmer Analyst 200 model. 2.2. Materials N,N0 -Methylenebisacrylamide (MBA, 99%), ammonium peroxydisulfate (APS, reagent grade, 98%), chitosan (CS, deacetylation degree of 90%) and 4-vinyl pyridine (95%) were purchased from Sigma Aldrich. Sodium hydroxide (analytical grade), acetic acid (99%), lead (II) nitrate and other metal salts were purchased from Merck (Darmstadt, Germany). All solvents were analytical grade and were used as soon as they were received. 2.3. Preparing IIPs and NIPs Synthesis of lead-imprinted polymer was performed by precipitation polymerization technique. For this purpose, in a three necked round bottom flask chitosan solution 3% (w/w) (prepared by adding chitosan into 2% (v/v) acetic acid solution) and 4-vinyl pyridine (14 mmol) were dissolved with continued stirring at room temperature for 1 h. Subsequently, 0.3 mmol lead nitrate and then 10 mmol MBA and 0.07 g KPS were added as an initiator to the reaction mixture. The system was kept under inert atmosphere and refluxed at 100 C for 18 h. Afterwards; the obtained polymer was filtered and washed with ethanol and distilled water several times and dried in a vacuum oven. Finally, Polymer network was leached with 0.2 M HNO3 to remove lead and afford imprinted cavities (Fig. 1.). In the same procedure non-imprinted polymer (NIP) was synthesized without lead ion. 3. Results and discussions 3.1. Characterization studies 3.1.1. FT-IR analysis Fig. 2 represents the Ft-IR spectra of Chitosan, 4-vinyl pyridine, N,N0 -Methylene bisacrylamide and IIPs. According to the Fig. (2-a), the absorption peaks at 3380 cm1 can be assigned to the NeH stretching of chitosan, in addition, the broad band observed at 3481 cm1 is related to the OeH stretching vibration of hydroxyl groups, so is it at 2937 and 2912 cm1 to the CeH stretching of chitosan. In MIPs spectrum (Fig. 2-c), the -C-O stretching and NeH bending vibration appeared at 1381 and 1595 cm1, respectively. Moreover, the characteristic peaks attributed to C]C stretching vibration of pyridine rings and C]O stretching of amide group were emerged at 1639 and 1679 cm1, respectively. In Fig. 2bed, the absorption peak at 1415 cm1pertains to stretching vibration of eCH2 in polymer skeleton. 3.1.2. SEM analysis The morphology of chitosan (A), unleached (B), leached (C) and
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Fig. 1. Schematic illustration of imprinting process for the preparation of chitosan based lead-imprinted polymer.
non-imprinted polymer (D) was assessed through Field Emission Scanning Electron Microscopy (FE-SEM). As it is depicted in Fig. 3, FE-SEM micrograph of leached polymer (Fig. 3-C) obviously revealed the porous polymer network in comparison to nonporous structure of non-imprinted framework (Fig. 3-D). Moreover, elemental composition of leached and unleached imprinted polymer was studied by energy dispersive X-ray (EDX) (Fig. 4). EDX analysis clearly shows the presence of Pb, C, N, and O which can be seen in Fig. 4- E and 4- F. The higher percentage of O along with N enhances the complexing ability of CS-IIP towards Pb(II). Absence of the Pb(II) in EDX spectra confirms its complete leaching of polymer.
3.2. Adsorption experiments Adsorption of Pb(II) was evaluated by employing different amounts of sorbent (10, 25, 50, 75 and 100 mg of IIP and NIP) in 100 mL solution including lead ion (10e100 mg L1) at different pH values. Afterwards, the polymer was filtered and subsequently elution was carried out by using 30 mL HNO3 solution (0.2 M) for 40 min. The Pb(II(ions adsorbed on polymer were determined by Flame Atomic Absorption Spectrometry (FAAS). Utilizing the following equation (Eq. (1)), the equilibrium capacity of the corresponding polymer was calculated:
qe ¼ 3.1.3. TGA Thermogravimetric analysis (TGA) was employed to survey the thermal stability of chitosan, leached and unleached imprinted polymer. As it is observable in TGA plot (Fig. 5), the maximum weight loss for chitosan is about 80% and the most sustainability for unleached polymer is determined ca. 60% of weight loss. In comparison to unleached polymer, the leached polymer showed less thermal stability owing to the absence of lead ions. These observations evidently proved the formation of Pb2þ imprinted polymer and elution of Pb(II) ions from the polymer.
ðC0 CeÞV m
where qe (mg g1) represents the equilibrium capacity of polymer, C0 (mg L1) and Ce (mg L1) point to the initial and equilibrium concentration of metal ions, V (L) shows the amount of ion solution and finally m (g) represents the mass of polymer. In order to investigate the adsorption performance and efficiency of the adsorbent, Langmuir and Freundlich isotherm models were applied. According to Langmuir model, the adsorption process occurs in a monolayer in or constant adsorption sites on the surface (Fig. 6). All of these adsorption sites contain the same amount of energy which leads to a homogenous adsorbent structure (Chen et al., 2008). Langmuir equation (Eq. (2)) (Sreejalekshmi et al., 2009) and Freundlich equation (Fig. 7) (Eq. (3)) (Chu, 2002) were applied to determine the adsorption capacity values:
1 1 1 ¼ þ qe qm Kl Ce qm lnqe ¼ lnKf þ
Fig. 2. FTeIR spectra of chitosan (a), 4-vinyl pyridine (b), N,N0 -Methylene bisacrylamide (c) and unleached polymer (d).
(1)
1 lnCe n
(2)
(3)
In these equations, qm (mg g1) and Kl (L mg1) refer to Langmuir constants which correspond to adsorption capacity and energy of adsorption, respectively. Ce (mg L1) and qe (mg g1) demonstrate the equilibrium concentration and the amount of Pb(II) adsorbed at equilibrium. KF and n (Freundlich constants) indicate adsorption capacity and adsorption intensity, respectively. Furthermore, the degree of suitability of adsorbent towards metal ions was estimated from the values of separation factor constant (RL), which has always been used to indicate the
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Fig. 3. FE-SEM micrographs of A) Chitosan, B) Unleached polymer C) lead-imprinted polymer D) Non-imprinted polymer.
adsorption, whether it is favorable or not (Hao et al., 2010). The value of RL will indicate the type of isotherm whether it is, irreversible (RL ¼ 0), favorable (0
RL ¼
1 1 þ KL C0
(4)
where KL (L mmol1) is the Langmuir constant, and C0 (mmol L1) is the initial concentration of metal ions. The RL values of polymer shown in Table S1 are greater than zero and less than unity, which suggests that the adsorption processes between the adsorbents (CS-IIP) and metal ion (Pb(II)) are favorable (Hao et al., 2010). The adsorption kinetic of Pb(II) on IIP and NIP was studied by adding 0.2 g adsorbent (IIP or NIP) to 30 mL of 200 mmol L1 of Pb(II) at pH ¼ 7 in different contact times. Then, concentration of Pb(II) ions in the solution was measured by employing FAAS after being eluted with 30 mL of 0.2 M HNO3. Determination of adsorption rate was accomplished on the basis of Lagergren's pseudo-first order (Eq. (5)) (Crist et al., 1981) and pseudo-second order kinetic equations (Eq. (6)) (Kannamba et al., 2010):
k t logðqe qt Þ ¼ logqe e 1 2:303
(5)
t 1 t ¼ þ qt k2 q2e qt
(6)
Herein, qe (mg g1) and qt (mg g1) relate to the quantities of adsorbate adsorbed at equilibrium and at time t, respectively. Furthermore, K1 (min1) and K2 (g mg1 min) describe the pseudofirst order (Fig. S1) and pseudo-second order adsorption rate constants, respectively. The kinetic parameters calculated from the pseudo-first and the pseudo-second order models (Fig. S2) are listed in Table S2. The results indicate that the pseudo-first order model fits the experiment data much better than the pseudo-second order model because its correlation coefficient (R2) values are beyond 0.994, which are higher than that of the pseudo-second order model (below 0.986). In addition, the theoretical calculated qe,cal. values obtained from the pseudo-first order model are closer to the experimental ones (qe,exp.) than that of the pseudo-second order equation. 3.3. Sorption and elution times The effect of sorption and elution times was evaluated according to the following procedure: first the pH of the analyte solution was regulated on 7. Then, 20 mg of leached sorbent was added to the 30 mL solution of the sample comprising of 10 mg/L Pb(II) and in the next step, the solution was stirred for 10, 11, 12, 13, 14, 15 and
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Fig. 4. (E) EDX spectrum of unleached (F) Leached polymer.
Fig. 5. TGA curve of leached polymer, chitosan and unleached polymer.
Fig. 6. Langmuir isotherm curve for different adsorbent in Pb(II) ions aqueous.
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with Pb(II), mainly due to the electrostatic disposal of lead ions (Ngah and Fatinathan, 2010). However, increasing the pH led to an increase in the negative charge density on the surface of the ligands, and thus their electrostatic gravity with metal ions and subsequently higher absorption percentage. At the pH values greater than 7, owing to the aggregation of hydroxyl ions, along with precipitation with Pbþ2 ions, a remarkable decrease in adsorption amounts was occurred (Zhao et al., 2011).
Fig. 7. Freundlich isotherm curve for different adsorbent in Pb(II) ions aqueous.
30 min. Separation of ion imprinted polymer through filtration was the next step. The amount of analyte was analyzed by means of FAAS after eluting the adsorbent with 30.0 mL 0.2 mol.L1 HNO3 solutions. The experiment showed that the adsorption reached to an equilibrium state after 15 min. Additionally, the elution time was estimated by adding 20 mg of unleached sorbent into 30 mL solution 0.2 mol L1 HNO3 solution and then being stirred for 8, 10, 11, 12, 14, 16, 18, 20 and 30 min. Subsequently, after isolation of the lead imprinted polymer by filtration, the resulting analyte was studied using FAAS. Eventually, the optimum desorption time was determined 16 min (Fig. S3). Putting adsorbed Cs-IIP into HNO3 solution, the hydroxyl group of chitosan was protonated. In comparison between protonation and chelation, a great amount of hydrogen from HNO3 solution replaced the metal ions to occupy the groups and most of the metal ions adsorbed were released. Then the adsorption capacity could be regained after washing with NaOH solution to be deporotonated.
3.6. Selectivity study The selectivity of IIP and NIP for Pb(II) was studied by shaking 0.1 g of the polymers with 25 mL of 50 mg L1of each individual metal ion at pH 6.0 against other metal ions (Table S3). The parameters of selectivity including distribution coefficient (Kd), selectivity coefficient (k) and relative selectivity coefficient (K0) of Pb(II(ions towards IIP were analyzed by the following equations:
Kd ¼
K¼
0
K ¼
Qe Ce
(7)
Kd ðPbðIIÞÞ Kd ðmetal ionsÞ
(8)
K imprinted K nonimprinted
(9)
where Qe indicates the adsorption capacity (mg g1) and Ce refers to the equilibrium concentrations of Pb(II) (mg L1).
3.7. Reusability 3.4. Effect of temperature on Pb(II) adsorption The effect of temperature on PbeCs IIP adsorbing Pb(II) ions was explored at pH 7.0 and the results have been shown in Fig. S4. Accordingly, the adsorption capacity of PbeCS-IIP for Pb(II) ions increased from 89 to 136 mg/g during temperature rise from 20 to 40 C. This may arise from the stronger interactions between Pb(II) and active groups (unreacted hydroxyl, and vinyl pyridine) of PbeCS-IIP that happen at elevated temperature, which also confirms the endothermic identity (nature) of adsorption process. Generally, the adsorption process proceeds with two successive sequences called “fast diffusion” and “slow complication” (Fu and Wang, 2011). Enhancing the temperature not only promotes the rate of Pb(II) ions diffusion from the inside of solutions to the surface of adsorbents, but it also accelerates the complexation of Pb(II) ions with the functional groups of adsorbents. 3.5. Effect of sorption pH In order to investigate the influence of pH on the Pb(II) adsorption, 20 mg of imprinted polymer was immersed to 30 mL of 10.0 mg L1 solution of Pb(II) under different pH conditions from pH ¼ 2 to pH ¼ 8, for 15 min. Increase in pH value was accompanied by an increase in Pb(II) adsorption amount. Eventually, the highest adsorption amount was observed in the range of pH 6e7 (Fig. S5). A crucial parameter on the lead adsorption process involves the protonation or non-protonation of amine and hydroxyl groups in chitosan moiety (Luk et al., 2014). In fact, decreasing the pH values provoked the decline in the number of active sites in chitosan which was consequently followed by decrease in complexation
The recovery and reusability of the corresponding imprinted polymer was studied as an important factor in evaluation of the adsorbent effectiveness and the results were summarized in Fig. S6. As it can be seen in Fig. S6, no considerable loss of adsorption capacity was detected until the fifth cycle, representing that the elution has a negligible effect on cavity structure as well as chemical features of CS-IIP. Obviously, these results confirm favorable stability and reusability of CS-IIPs. Moreover the CS-IIP after adsorption of Pb(II) was analyzed via map imaging and results revealed that the adsorption onto the polymer occurred uniformly in all regions (Fig. S7).
4. Conclusion This study is the first report on imprinted chitosan polymer for the selective removal of Pb(II) ions which has exhibited excellent characteristics properties. It is noteworthy that the Pb(II)-IP chitosan bead can significantly enhance the selectivity and adsorption capacity of the Pb(II) heavy metal ions. In comparison to the literature on performance of lead adsorption with the chitosan based polymer has indicated that it has a high adsorption capacity. The experimental data was fitted to the Freundlich model to explain the adsorption mechanism. The reusability experiments have been demonstrated that the process is suitable, and has favorable stability in several cycles. As potential biosorbent for the Pb(II) removal from waste water, the Pb(II)-IP chitosan bead demonstrated a great adsorption capacity throughout the multi cycle adsorption-desorption process in aquatic media.
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