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Chitosan based Lead Ion Imprinted Interpenetrating Polymer Network by Simultaneous Polymerization for Selective Extraction of Pb(II) Pankaj Hande, Sanjay P. Kamble, Asit Baran Samui, and Prashant Shripad Kulkarni Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04889 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 5, 2016

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Industrial & Engineering Chemistry Research

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Chitosan based Lead Ion Imprinted Interpenetrating Polymer Network by Simultaneous Polymerization for Selective Extraction of Pb(II)

Pankaj E. Handea, Sanjay Kambleb, Asit B. Samuia, Prashant S. Kulkarnia* a

Energy and Environment Laboratory, Department of Applied Chemistry, Defence Institute of Advanced Technology (Deemed University), Pune-411025, India b

Chemical Engineering and Process Development Laboratory, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road Pune, India

ACS Paragon Plus Environment


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ABSTRACT In this study, we report synthesis of Pb(II) ion imprinted interpenetrating polymer network (IIIPN) by simultaneous polymerization for the selective extraction of Pb(II) from printed circuit board (PCB) recycling unit wastewater. Initially, polymer network was synthesized by the polymerization of methacrylic acid (MAA, monomer) and ethylene glycol dimethacrylate (EGDMA, crosslinker) and second polymer network by chitosan (complexing monomer) and tetra ethyl orthosilicate (TEOS, crosslinker). Chemical structure and morphology of the II-IPN were analyzed by using FTIR, FE-SEM, AFM and TEM. The interaction of the functionality in II-IPN with Pb(II) through chelation was studied by XPS analysis. The maximum adsorption capacities for II-IPN and NI-IPN were 37.5 and 10.3 mg g-1, respectively. The largest selectivity coefficient for the Pb(II) in presence of the W(VI) was 161.58. Developed Pb(II)-II-IPN was successfully employed for the selective extraction of Pb(II) from PCB recycling unit wastewater.

1. INTRODUCTION Printed circuit boards (PCBs) are being used in many electronic types of equipment such as mobile phones, televisions, computers, etc. Excessive use of these electronic equipments due to its vast application in day to day life results in increased electronic waste in urban areas. Management of huge quantity of electronic waste is a major task today in most of the cities all over the world. There are different variety of metals being used in PCB including Cr, Pb, Cu, W, Na, Mn, Mg, Zn, etc.1 Most of the metals are highly hazardous to the ecosystem and human being. Therefore, recovery of these metals is important in terms of protection of the environment and cost. Pb(II) is one of the major constituent after W(VI) present in PCB known for its higher toxicity. Its poisoning


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causes serious hazards in human beings such as memory and learning deficits, high blood pressure, damage to neurologic, hematologic and renal system, fertility damage, etc.2 Thus, selective extraction and preconcentration of Pb(II) generated in wastewater of PCB recycling unit is necessary to avoid contamination to the ecosystem. There are various methods reported for the recovery of Pb(II) such as ion exchange separation,3 chemical precipitation4 and adsorption.5–7 But these techniques are non-selective in nature. Imprinted polymers are known for their artificial specific recognition sites with higher thermal and chemical stability as well as selectivity.8 Therefore, imprinted polymers are reported to be used in wide applications such as biomimetic sensors,9 solid phase extraction,10 drug delivery,11 detection of environmental pollutants,12 etc. Imprinted polymers are synthesized by using template (target), monomer, cross-linker and initiator under nitrogen atmosphere. The synthesized polymer is considered to be highly selective for particular template. In order to improve reliability, various modifications have been done towards development of the IIP over the years.13 Recently various Pb(II)-ion imprinted polymers (IIP) have been developed for different applications.14–16 The Pb(II)-ion imprinted mesoporous SBA-15 was reported to be used for the selective extraction of Pb(II) from the highly acidic conditions with very high adsorption capacity.17 For fast adsorption, the Pb(II)-magnetic IIP have been developed for various applications.18–21 The Pb(II)-IIP membrane was used for the extraction of Pb(II) from the environmental samples.22 IIPs are also reported to be used for the sensing of Pb(II) using voltammetry.23–26



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Chitosan is a biopolymer known for its high complexing ability towards metal ions because of its free hydroxyl and amino groups.6 It is one of the cheap and easily available biomaterial. Swelling of chitosan in water reduces its direct use for metal chelation. Therefore, various modifications have been done through its high functionality.27 Chitosan has been reported to be used as a complexing monomer in developing various ion imprinted polymers.28–31 Literature survey reveals that chitosan based IIP shows good adsorption capacity, thermal stability and comparably lower selectivity.32,33 Recently, chitosan based Pb(II) ion imprinted polymer have been reported with very high adsorption capacity (>500 mg g-1) but possesses low selectivity as other metal ions get adsorbed with Pb(II) in a large quantity.34,35 The methacrylic acid (MAA) and ethylene glycol dimethacrylate (EGDMA) based IIPs have been developed over the years but these IIPs exhibits poor adsorption capacity and site accessibility.36,37 These limitations can be overcome by ion imprinted interpenetrating polymer network (II-IPN) known to have very high adsorption capacity, mechanical strength and very high selectivity.38–41 Thus, we have synthesized novel, cheap and highly selective chitosan-MAA-EGDMA based Pb(II)-ion imprinted interpenetrating polymer network (II-IPN) by simultaneous polymerization. II-IPNs have unique interpenetration among both kinds of polymer chains. This is expected to maintain high to very high level of selectivity. Chitosan with hydroxyl and MAA with carboxyl functionality forms complexes with Pb(II). The chitosan cross-linking with TEOS, and MAA cross-linking with EGDMA result in a formation of polymer matrix. Removal of Pb(II) from the polymer matrix establishes a three dimensional cage for Pb(II). Specific recognition of Pb(II) in II-IPN not only depends on size and charge, but also on various thermodynamic and kinetic parameters that differentiate one ion from other. Developed II-IPN was successfully used first time for the treatment of PCB recycling unit wastewater containing Pb(II). We have effectively studied the adsorption behavior of II-IPN for


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its selective adsorption of Pb(II). The kinetics of adsorption of Pb(II) on II-IPN have also studied in details. 2. EXPERIMENTAL 2.1 Instruments FTIR (Bruker ALPHA) with Zn-Se ATR crystal and gold coated mirror, monitored at 4 cm-1 resolution (25 scans). To study the morphology, Field Emission Scanning Electron Microscopy (FE-SEM) (Carl Zeiss Sigma at 2.50 kv EHT and 3.6 mm of WD), High Resolution Transmission Electron Microscopy (HR-TEM) (FEI Tecnai G230, Hillsboro, U.S.A) and Atomic Force Microscopy (AFM, Asylum research MPF3D) were used. Elemental composition of the polymer particles were studied with Energy Dispersive X-ray (EDX) (Carl Zeiss Sigma at 2.50 kv EHT and 3.6 mm of WD). Polymer particles were dispersed in a toluene and deposited over copper plate, carbon coated copper grid and silicon vapor for FE-SEM, HR-TEM and AFM studies, respectively. Thermal stability of the polymer was analyzed by using Thermogravimetric Analysis (TGA) (Perkin Elmer 1061608). Average specific surface area and pore volume of the polymer particles were determined with Quantachrome Autosorb instrument at 77 K by nitrogen adsorption using Brunauer–Emmett–Teller (BET) method. In order to study the X-ray Photoelectron Spectroscopy (XPS) of II-IPN, VG Microtech Multilab ESCA 3000 spectrometer was used. All the studies related to determination of other metal ions were carried out by using Microwave plasma atomic emission spectroscopy (4200-MP-AES, Agilent Tecnologies). Solid phase extraction studies were done with the help of Flash column chromatography (Bonna-Agela India Pvt. Ltd.) with column of 8 mm in diameter and 7 cm in length. Other instruments used are table



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top centrifuge (KUBOTA 4200), orbital shaker (Lab companion) and pH meter (HANNA Instruments). 2.2 Reagents Lead

nitrate,

methacrylic

acid

(MAA),

ethylene

glycol

dimethacrylate

(EGDMA),

azobisisobutyronitrile (AIBN), toluene (LR grade), tetra ethyl orthosilicate (TEOS), chitosan, acetic acid, NaOH, nitric acid, hydrochloric acid, ZnCl2, Na2WO4, MgCl2, KCl, CuCl2, MnCl2.(H2O)4, buffer tablets (pH 4 and 7) and all the chemicals were purchased from Sigma Aldrich (St. Louis, MO, USA). Deionized water was used in preparation of all solutions. 2.3 Polymer network synthesis The Pb(II)-ion imprinted interpenetrating polymer network was synthesized by simultaneous polymerization method. 0.3 mmol of lead nitrate was mixed with 2 mmol of complexing monomer methacrylic acid and 0.5 g of the chitosan (complexing monomer) in 2% acetic acid (5 ml) for 4 hr. Then, 10 mmol of cross-linker EGDMA, 50 mg of AIBN and 1 mmol of TEOS were added in to it. Toluene was used as a porogenic solvent. The reaction was carried out at 80 °C for 2 hr under nitrogen atmosphere. Then, it was refluxed at 110 °C for 12 hr. After precipitation the polymer was filtered and washed initially with toluene and then with water. Polymer network was leached with 1 M HNO3 to remove lead to get imprinted cavities. Complete removal of Pb(II) from polymer network was confirmed by using MP-AES. At the end, amino functionality of the polymer matrix was activated using 1 M NaOH solution. Complete procedure for the development of Pb(II)-IIIPN is shown in Scheme 1. Same method was used for the synthesis of non-imprinted interpenetrating polymer network (NI-IPN) in absence of the template Pb(II).


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Scheme 1: Schematic for the development of the Pb(II)-II-IPN 2.4 Optimization of pH, adsorption and kinetics experiments Effect of pH on adsorption is an important parameter for improving efficiency of the sorbent. Thus, adsorption of Pb(II) on II-IPN was studied using 5 mg L-1 solutions of Pb(II) at varying pH (2 to 10). The solutions were prepared with the help of buffer, 0.1 M HCl and 0.1 M NaOH. To study



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extraction efficiency at different pH, 20 mg of II-IPN powder was added into 25 ml solution of each pH. Extraction efficiency was calculated by using following equation (1)

𝐸(%) =

(𝐶𝑖 − 𝐶𝑒 ) × 100 𝐶𝑖

(1)

where 𝐶𝑖 and 𝐶𝑒 are the initial and equilibrated concentrations of Pb(II), respectively In order to study the adsorption capacities of the II-IPN and NI-IPN, solutions of Pb(II) were prepared at different concentrations such as 1, 5, 10, 25, 50, 100, 250, and 500 mg L-1, respectively. Then, 20 mg of II-IPN powder was added into 25 ml solution of each concentration. Each solution was maintained at pH 6. Adsorption experiment was carried out by agitating orbital shaker for 2 hr at speed of rotation of shaker is 150 rpm. Adsorption capacity 𝑞𝑒 was calculated by using following equation

𝑞𝑒 =

(𝐶𝑖 − 𝐶𝑒 )𝑉 𝑀

(2)

where 𝑞𝑒 (adsorption capacity) is the amount of Pb(II) (mg) adsorbed on the II-IPN (g); 𝐶𝑖 and 𝐶𝑒 are the initial and equilibrated concentrations of Pb(II), respectively; 𝑉 is the volume of the solution (ml); 𝑀 is the mass of the II-IPN (mg). Kinetics experiment was carried out to study the effect of contact time on adsorption. Kinetics study was done by taking 25 ml, 250 mg L-1 solution of Pb(II) along with a 20 mg II-IPN. The samples were collected at regular time interval of 5, 10, 30, 60, 90, 120, 150, 180 min, respectively and analyzed by using MP-AES.


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2.5 Selectivity studies Selectivity studies of II-IPN towards Pb(II) were done by preparing synthetic wastewater of W(VI), K(I), Mg(II), Mn(II), Zn(II), and Cu(II). The total volume of solution containing above metals was 70 ml with 10 mg L-1 concentration of each metal ion. The pH of the solution was maintained at 6 using buffer solution and 0.1 M. HCl. Then, 40 mg of the II-IPN powder was added into the solution. Thereafter, it was kept for continuous stirring on the orbital shaker for 2 hr. Same procedure was followed for the selectivity studies of the NI-IPN. All the samples were analyzed by using MP-AES. Selectivity of the II-IPN and NI-IPN were calculated by using following equations (3, 4 and 5). 𝐷𝑖𝑠𝑡𝑟𝑖𝑏𝑢𝑡𝑖𝑜𝑛 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡, 𝐾𝑑 =

𝑆𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡, 𝛼 =

𝑞𝑒 𝐶𝑒

(3)

𝐾𝑑1 𝐾𝑑2

𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑠𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡, 𝛽 =

(4) 𝛼1 𝛼2

(5)

where 𝑞𝑒 is the adsorption capacity and 𝐶𝑒 represents equilibrium concentration; 𝐾𝑑1 and 𝐾𝑑2 represents distribution coefficients of Pb(II) and competitor metals; 𝛼 is the selectivity coefficient; 𝛽 is the relative selectivity coefficient; 𝛼1 and 𝛼2 are the relative selectivity coefficient of II-IPN and NI-IPN, respectively



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2.6 II-IPN adsorbent for the treatment of actual sample 2.6.1 Analysis of PCB recycling unit wastewater using MP-AES We have selected PCB recycling unit wastewater for treatment with II-IPN. The wastewater was collected from the PCB recycling unit Pune, India. The collected samples were filtered through 0.22 µm filter. Literature survey reveals that PCB recycling unit wastewater contains metals like Zn(II), Cu(II), W(VI), Pb(II), K(II), Mg(II), and Mn(II) in major proportion.42 In order to find out the unknown concentration of the metal ions present in the PCB recycling unit wastewater, above metal ions were calibrated by using MP-AES. All the solutions were prepared in deionized water. Calibration was done in the concentration range of 1 to 1000 mg L-1 for each metal ion. Calibration was carried out by taking mixture of metal ions. 2.6.2 Selective extraction of Pb(II) from PCB recycling unit wastewater For the selective extraction of the Pb(II) from PCB recycling unit wastewater, flash chromatography was used by filling the column with II-IPN particles. 2 g of the II-IPN powder was filled inside the flash column cartridges. Initially column was eluted by passing deionized water for 5 min. Then, 100 ml solution of the PCB recycling unit wastewater containing 65 mg L1

of Pb(II) was allowed to pass through the column containing II-IPN with the flow rate of the 2

ml min-1. 1 M HNO3 was used as a desorption agent for the Pb(II) with the flow rate of 2 ml min1

. After desorption column containing II-IPN adsorbent was washed with 1 M NaOH solution to

activate functionality of the II-IPN for the further extraction of Pb(II). The extraction with same material was carried out in five different cycles. The collected samples were quantitatively analyzed by using MP-AES. 3. RESULTS AND DISCUSSIONS


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Ion imprinted interpenetrating polymer network was synthesized for removal of Pb(II) from PCBs recycling unit wastewater. Cross-linked chitosan and cross-linked MAA were synthesized simultaneously in presence of Pb(II) template. This is expected to make a strong template cavity interaction enhancing the selectivity. The applicability was verified satisfactorily by using possible user oriented flash column chromatography. 3.1 Characterizations The polymerization and cross-linking of the II-IPN was confirmed by using FTIR spectroscopy. The –C=C- (1622 cm-1) stretching frequencies related to the cross-linker (EGDMA) and monomer (MAA) are absent in polymer matrix indicating that all monomer and cross-linker are completely polymerized as shown in Fig. 1. The stretching frequency at 1723 cm-1 in both II-IPN and NI-IPN is due to the presence of the –C=O in MAA and EGDMA. Chitosan cross-linking with TEOS can be confirmed by studying the intensity of the –O-H stretching.43 Hydroxyl (–O-H: 3500 cm-1) stretching frequency corresponding to the chitosan is observed with lower intensity in polymer matrix, which indicates that the –OH groups of the chitosan is partially reacted with silanol groups of TEOS, forming chitosan-O-Si-O-chitosan linkage. It is difficult to differentiate –C-O-C and – Si-O-C stretch because, both appear nearly at the same stretching frequency range of 1100-1150 cm-1.44 The characteristic peak at 1640 cm-1 corresponds to the –NH2 group which apparently increases after cross-linking with TEOS. These observations indicate weakening of the hydrogen bonding interaction between amino groups and hydroxyl group of the chitosan.45



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Figure 1: FTIR spectra of a) II-IPN, b) NI-IPN, c) Chitosan, d) EGDMA and e) MAA The FE-SEM study was performed to understand the surface morphology of the II-IPN as well as NI-IPN. From the FE-SEM images, porous polymer network has been observed as shown in Fig. 2a. It has been observed in literature that MAA-EGDMA based polymer synthesized by free radical polymerization showed particle size in the range of 30-100 nm.46 The FE-SEM images show particles in the range of 30-50 nm. There is always disagreement in literature regarding the effect of the template on the particle size of the polymer. Some studies report that template affects the particle size of the polymer47,48 while in some studies it hasn’t been observed.49 In the present case, II-IPN and NI-IPN were observed with the same particle size thus it could be concluded that there is no effect of the template Pb(II) on particle size of the II-IPN. Elemental composition of the II-IPN was studied with the help of the EDX. EDX analysis clearly shows presence of Si C, N,


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and O which can be seen in Fig. 2c. The higher percentage of the O along with N enhances the complexing ability II-IPN towards Pb(II). Absence of the Pb(II) in EDX spectra confirms its complete leaching II-IPN which supports MP-AES results.

Figure 2: FE-SEM micrographs of A) II-IPN, B) NI-IPN; EDX spectrum of C) II-IPN The particles size of the II-IPN was further confirmed by using HR-TEM images. HR-TEM micrographs show particles in the range of 20-30 nm which supports particle size obtained from the FE-SEM images (Fig. 3a and b). It has been observed in literature that AFM can be used in adsorption-desorption studies of metal ion.50,51 Therefore, morphology of the II-IPN particles before and after removal of the Pb(II) was examined with AFM (Fig. 3c and d). The distribution of cavities is found to be mostly uniform before and after Pb(II) removal. Before removal of the Pb(II) from polymer matrix, arithmetic average roughness (Ra), surface height (Rz) and root mean square roughness (Rq) were 0.28, 7.84 and 0.56 nm, respectively (Fig. 3c). On the other hand, Ra, Rz and Rq after removal of Pb(II) were



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0.24, 6.5 and 0.38 nm, respectively (Fig. 3d). After removal of the template from polymer matrix decrease in surface height is observed in literature.51 After removal of the template Pb(II), decrease in Ra, Rz and Rq were observed in this case which reflects template embedded polymer network.

Figure 3: a) TEM image of II-IPN, b) HR-TEM image of II-IPN, c) 3D AFM image of II-IPN (before Pb(II) removal) and d) 3D AFM image of II-IPN (after Pb(II) removal) In order to study the porosity of II-IPN and NI-IPN, nitrogen adsorption-desorption experiment was carried out. The specific surface area and pore specific volume of II-IPN obtained by Brunauer–Emmett–Teller (BET) method were 66.29 m2 g-1 and 0.056 cm3 g-1, respectively. In case of NI-IPN, the specific surface area and pore specific volume were 8.4 m2 g-1 and 0.015 cm3 g-1, respectively. Thus, it could be concluded that the increase in surface area and pore specific volume of II-IPN is the effect of Pb(II) imprinting.


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Chemical status of the functional groups of the II-IPN and adsorbed Pb(II) was evaluated with the help of XPS to measure binding energy values of the II-IPN before and after Pb(II) adsorption (initial concentration-100 mg L-1, pH-6). Fig. 4a gives an overview of the chemical composition of the II-IPN before and after adsorption of Pb(II). Two peaks observed at 140.8 and 146 eV belong to Pb 4f7/2 and Pb 4f5/2, respectively (Fig. 4b). It has been previously reported that the binding energy for Pb 4f7/2 and Pb 4f5/2 were 138.9 and 143.9 eV, respectively.17 Fig. 4c and d show high resolution O1s spectra of II-IPN before and after adsorption of Pb(II) at 535.1 and 535.7 eV, respectively. This binding energy can be attributed to presence of the -C-OH.52 The shifting of peak from 535.1 to 535.7 eV after adsorption takes place due to the chelation of Pb(II) with O through its lone pair and thus decreases electron density which results in increase in binding energy.53 The same shifting was observed in case of N1s spectra after adsorption (supplementary material, Fig. S1a and b). A high resolution C1s spectrum of the II-IPN was composed of three different peaks (Fig. 4e). These peaks were attributed to the C-C and C-H (284.1 eV), -C-N (287.4 eV) and O-C=O (290.9 eV).54,55 After adsorption of the Pb(II) on II-IPN (Fig. 4f) binding energies for C-C and C-H, –C-N and –C=O were shifted to 283.5, 286.8 and 290.2, respectively. In summary, change in binding energies of II-IPN before and after adsorption of Pb(II) indicates the involvement of the O and N in adsorption through chelation. It results in increased electron density at the adjacent carbon which causes decrease in binding energy of the carbon.56 From the above discussion, it can be concluded that there is involvement of carboxyl and hydroxyl functionality during the adsorption of the Pb(II) on II-IPN.



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Figure 4: XPS spectra of II-IPN a) Over all spectra of the II-IPN before and after adsorption; b) Pb 4f spectrum of II-IPN (after adsorption); O1s spectra of c) II-IPN (before) d) II-IPN (after); C1s spectra of e) II-IPN (before) e) II-IPN (after) TGA study was used to investigate thermal stability of the II-IPN before and after adsorption of the Pb(II) as shown in Fig. 5. It has been observed that II-IPN show nearly same thermal stability


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before and after adsorption of the Pb(II) with the decomposition temperature in the range of the 240-450 °C. Complete degradation of the material is observed after reaching the temperature around 500 °C.

Figure 5: TGA plots of II-IPN before and after adsorption of the Pb(II) 3.2 The pH, adsorption and kinetics studies Literature study reveals that chitosan based IIP shows maximum adsorption in the pH range of 56.28,30 In this case Pb(II) II-IPN showed maximum extraction efficiency at pH 6 as shown in Fig. 6. The extraction efficiency goes on decreasing slightly towards acidic pH 2. This may be due to the protonation of lone pair of electron which forms complexes with the metal ions. With increase in pH above 6, extraction efficiency decreases sharply because of formation of lead hydroxide. Thus, pH 6 was used for the further studies of the II-IPN.



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Figure 6: Effect of pH on adsorption of Pb(II) on II-IPN and NI-IPN (𝐶𝑖 - 5 mg L-1; volume- 25 ml; adsorbent loading- 20 mg; shaker speed- 150 rpm) Adsorption of Pb(II) on II-IPN and NI-IPN at different initial concentrations is shown in Fig. 7a. From the plot it may be observed that II-IPN shows more adsorption capacity as compared to the NI-IPN. The maximum adsorption capacity obtained for the II-IPN and NI-IPN is 37.50 mg g1

and 10.30 mg g-1, respectively. This explains formation of the specific binding sites for Pb(II) in

II-IPN. The saturation for the Pb(II) on II-IPN is observed at the concentration of 250 mg L-1.


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Figure 7: a) Adsorption of Pb(II) on II-IPN and NI-IPN at different initial concentrations (𝐶𝑖 - 1 to 500 mg L-1; volume- 25 ml; pH- 6, adsorbent loading- 20 mg, shaker speed- 150 rpm); b) Effect of contact time on adsorption of Pb(II) on II-IPN and NI-IPN (𝐶𝑖 - 250 mg L-1; volume- 25 ml; adsorbent loading- 20 mg; pH- 6), Langmuir model was employed to study the distribution of Pb(II) between the liquid and solid phase. Langmuir isotherm model assumes the adsorption of molecules in fixed number in a specific recognition sites. The linearized and non-linearized Langmuir isotherms are expressed in terms of the following equations 6 and 7.

𝑞𝑒 =

𝑄. 𝐾. 𝐶𝑒 1 + 𝐾. 𝐶𝑒

1 1 1 1 = + . 𝑞𝑒 𝑄 𝐾𝑄 𝐶𝑒


(6)

(7)


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where 𝑞𝑒 is the amount of adsorbed Pb(II) on the adsorbents (mg g-1); 𝐶𝑒 is the equilibrium Pb(II) concentration in solution (mg L-1); 𝐾 the Langmuir constant (L mg-1) and 𝑄 is the maximum adsorption capacity (mg g-1). Langmuir adsorption isotherm parameters are estimated by plotting 1/ 𝐶𝑒 versus 1/𝑞𝑒 . The obtained experimental results are also fitted with Freundlich model as shown in following equations (8 and 9). It describes multilayer adsorption on the heterogeneous surface of sorbent. 1

𝑞𝑒 = 𝐾𝑓 𝐶𝑒𝑛

(8)

1 𝑙𝑜𝑔𝑞𝑒 = 𝑙𝑜𝑔𝐾𝑓 + 𝑙𝑜𝑔𝐶𝑒 𝑛

(9)

where 𝐾𝑓 and 𝑛 are the Freundlich constants, which affects the adsorption capacity and the intensity of the adsorption, respectively. Freundlich isotherm parameters are estimated by plotting 𝑙𝑜𝑔𝑞𝑒 versus 𝑙𝑜𝑔𝐶𝑒 . Table 1. Langmuir and Freundlich isotherm constants for adsorption of Pb(II) using II-IPN and NI-IPN particles Langmuir isotherm constants Polymer

𝑞𝑒 exp(mg g-1)

𝑄 max(mg g-1)

K (L mg-1)

R2

II-IPN

37.50

39.46

39.46×10-3

0.9999

NI-IPN 10.30 Freundlich isotherm constants

10.53

5.98×10-3

0.9995

Polymer

Kf

N

R2

II-IPN

1.98

1.741

0.8966

NI-IPN

0.120

1.321

0.9830


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Langmuir and Freundlich constants are summarized in the Table 1. By comparing the data obtained from the isotherm models, it has been observed that the Langmuir isotherm model shows higher correlation coefficient as compared with the Freundlich model. Thus, it can be concluded from the above results that, the adsorption of Pb(II) takes place at the fixed number of specific recognition sites in II-IPN. Adsorption of Pb(II) on II-IPN and NI-IPN was studied as function of time in order to evaluate the adsorption kinetics as shown in Fig. 7b. At the initial stage fast adsorption of Pb(II) is observed because of availability of the binding sites. Equilibrium for the adsorption of Pb(II) on II-IPN and NI-IPN is observed within 120 min. Fast adsorption kinetics suggested that II-IPN is suitable for preconcentration of the Pb(II) from the large volume of the solutions. In order to investigate rate determining step, kinetic data are fitted with the pseudo first and pseudo second order kinetics models. It is expressed in the form the following equations. 1 𝐾1 1 = + 𝑞𝑡 𝑞𝑒 𝑡 𝑞𝑒

(10)

where 𝑞𝑒 and 𝑞𝑡 are the amounts of Pb(II) adsorbed, in mg g-1,at equilibrium and at time t (min), respectively, and 𝐾1 is the pseudo first order rate constant (min-1). 𝑡 1 𝑡 = + 2 𝑞𝑡 𝐾2 𝑞𝑒 𝑞𝑒

(11)

where 𝐾2 is the pseudo second order rate constant (g. mg-1 min) The adsorption kinetics constant and correlation coefficients are summarized in the Table 2. Pseudo first order kinetics model shows equilibrium adsorption capacity of 41.23 mg g-1 with correlation coefficient (R2=0.9970). Equilibrium adsorption capacity (38.46 mg g-1) obtained from



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the pseudo second order model is far closer to experimental adsorption capacity (37.50 mg g-1) as compared to pseudo first order model. Correlation coefficient obtained from the pseudo second order kinetic model (R2=0.9990) is higher than the pseudo first order kinetic model (R2=0.9970). Therefore, Pb(II) adsorption on II-IPN fits very well with pseudo second order model which shows chemisorption through electron exchange between adsorbent and adsorbate. Table 2. Kinetic parameters of pseudo-first and -second orders for initial Pb(II) concentration of 250 mg L-1 Experimental 𝑞𝑒 (mg g-1)

K1 (min-1)

𝑞𝑒 (mg g-1)

R2

K2 (min-1)

𝑞𝑒 (mg g-1)

R2

II-IPN

37.50

17.34

41.23

0.9970

1.47 x 10-3

38.46

0.9990

NI-IPN

10.30

40.51

12.62

0.9931

1.53 x 10-3

13.62

0.9903

polymer

Pseudo first-order

Pseudo second-order

3.3 Selectivity studies Adsorption selectivity is one of the most important property in application of the adsorbent. The selectivity study was done by using synthetic wastewater. The Pb(II) separation studies have been done in presence of the W(VI), K(I), Mg(II), Mn(II), Zn(II), and Cu(II) at pH 6. These metal ions were selected because of their presence in PCB wastewater along with Pb(II). Selectivity coefficient and other parameters are listed in Table 3. The selectivity coefficient of the adsorbent for the Pb(II) ions in presence of the W(VI) is found to be very high 161.58. It may be due to the formation of the four co-ordination recognition sites in II-IIP susceptible for Pb(II) while, W(VI) forms six co-ordination complexes. As the concentration of W(VI) in PCB wastewater is higher than the Pb(II), we have studied selectivity in presence of the W(VI) with three times higher concentration.42 It shows decrease in selectivity coefficient of the adsorbent to 39.16. Improved selectivity of the adsorbent may be attributed to the size and the charge on tungsten which varies


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from other metals. It has also been observed that II-IPN shows very high selectivity towards Pb(II) as compared to the other metals. Thus, complimentary recognition sites have been formed in adsorbent. At pH 6 (slightly acidic condition), selectivity of the II-IPN towards Pb(II) may be improved due to the protonation of amino, thereby making lone pair of electron unavailable for complexation with concomitant metals. Here, we report II-IPN adsorbent with higher selectivity for Pb(II) in presence of W(VI) which has not been reported earlier. It could be concluded that the effect may due to the result of higher degree of cross-linking of the two polymer network. Table 3. Selective adsorption of Pb(II) from the mixtures of W(VI), Zn(II), Cu(II), Mn(II), K(I) and Mg(II) by II-IPN and NI-IPN (Ci- 10 mg L-1; adsorbent- 40 mg; shaking rate- 150 rpm; solution pH- 6.0) Pb(II)

Distribution ratio

Selectivity coefficient (α)

Relative

competitor

(ml g-1)

kPb(II)/ kCompetitor

coefficient, β

II-IPN

NI-IPN

II-IPN

NI-IPN

Pb(II)

2855

131.72

-

-

-

W(VI)

17.67

19.46

161.58

6.69

24.15

3[W(VI)]*

72.80

92.10

39.16

1.43

27.38

Zn(II)

92.10

111.70

31.00

1.17

26.92

Cu(II)

72.91

101.85

39.12

1.29

30.32

Mn(II)

54.12

44.87

52.75

2.93

18.00

K(I)

36.08

32.00

77.58

4.11

18.87

Mg(II)

63.47

72.91

44.98

1.80

24.98

selectivity

* Three times higher concentration as compared to Pb(II) 4. APPLICATION OF DEVELOPED II-IPN ADSOEBENT FOR TREATMENT OF ACTUAL SAMPLE 4.1 Analysis of PCB recycling unit wastewater



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Prior to the analysis of the metals present in PCBs wastewater, the calibration of metals Pb(II), W(VI), Zn(II), Cu(II), Mg(II), and Mn(II) using MP-AES was performed (supplementary material, Fig. S2 and S3). The wastewater collected from PCB recycling unit in Pune, India was used for its qualitative and quantitative analysis of metals present using MP-AES. The wastewater having pH 5 is directly used for the analysis. In previous study we used ion chromatograph technique for analysis of the metals present in PCB recycling unit wastewater and showed presence of the Pb(II) along with W(VI), Cu(II), Mg(II), Mn(II), K(I) and Zn(II).42 Analysis of wastewater samples by MP-AES showed presence of W(VI), Pb(II), K(I), Mg(II), Mn(II), Zn(II) and Cu(II) with different concentrations of 200, 65, 7, 2.5, 0.15, 0.3, and 3 mg L-1, respectively. 4.2 Treatment of PCB recycling unit wastewater with II-IPN In order to study the effectiveness of the extraction of Pb(II) from actual sample, II-IPN with flash column technique is employed. The extraction efficiency of the II-IPN towards Pb(II) from the wastewater at pH 6 is shown in Fig. 8a. The II-IPN showed very high selectivity towards Pb(II). Other metals being present in very low concentrations in PCB, gets extracted in small quantity with Pb(II). This extraction is very much pH sensitive because at lower pH protonation of the amino and hydroxyl groups takes place which attracts W(VI) through electrostatic force of attraction. The adsorption study showed 91.5, 5, 27.1, 40, 66.66 and 56.66 %, extraction of the Pb(II), W(VI), K(I), Mg(II), Zn(II) and Cu(II), respectively. The stability and reusability of the IIIPN adsorbent was investigated by carrying out five adsorption-desorption cycles, and the results are illustrated in Fig. 8b. Extraction efficiency of the II-IPN gradually decreased from first to fifth cycle. After five cycles, 11.5% loss of extraction of Pb(II) was observed indicating that the II-IPN is much stable for the repeated use. Therefore, it can be concluded that the synthesized Pb(II)-IIIPN could be used repeatedly with significant extraction efficiency.


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Figure 8: a) Extraction of Pb(II) in presence of competitive metals from the PCB recycling unit wastewater using II-IPN; b) % Extraction of Pb(II) and W(VI) in five different cycles with same adsorbant (adsorbent loading-2 g; pH-6; wastewater-100 ml (each cycle); column-8 mm in diameter and 7 cm in length; desorption agent- 1 M HNO3, Flow rate- 2 ml min-1) 5. CONCLUSIONS In this work, we have synthesized novel cheap and highly selective Pb(II) II-IPN based on biomaterial chitosan with better metal coordination ability. The II-IPN was characterized by using various techniques. AFM was used to study the morphology of II-IPN before and after Pb(II) removal. Decrease in surface height in 3D AFM image of the II-IPN particles after removal of Pb(II) is observed. These findings support few articles reporting on desorption study of IIP by AFM. Involvement of II-IPN functionality with Pb(II) was studied with the help of XPS analysis and showed interaction of carboxyl and hydroxyl groups. It means both chitosan and MAA of IIIPN are involved in complexation of Pb(II) during adsorption. Synthesized II-IPN showed four times higher adsorption capacity as compared to the non-imprinted polymer network. It indicates formation of the highly selective recognition sites in polymer network. The II-IPN could solve the



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low selectivity issue of IIP for Pb(II). Selectivity study of II-IPN showed very high selectivity for Pb(II) in presence of the W(VI). It may be due to the greater charge of the W(VI) as it forms six co-ordination complexes. The cavities formed are only available for a metal which forms four coordination complexes along with prefixed size. From the kinetic study, it is observed that the adsorption of Pb(II) on II-IPN is much faster. This technique was successfully employed for the remediation of Pb(II) from the PCB recycling unit wastewater. SUPPORTING INFORMATION XPS analysis of II-IPN before and after adsorption of Pb(II) and calibration of metals using MPAES. AUTHOR INFORMATION Corresponding author: [email protected], [email protected], Ph: 91-20-24304161. ACKNOWLEDGEMENTS The financial support from DRDO (ERIP/ER/1003883/M/01/908/2012/D, R&D/1416, dated, 283-2012) New Delhi, India, is gratefully acknowledged. P.E. Hande gratefully acknowledges DIAT-DRDO for supporting with the Ph.D. research scholarship.

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