A novel Cd(II) ion imprinted interpenetrating polymer network (Cd(II)IIP) was prepared by free radical polymerization using alginic acid and NNMBA-crosslinked polyacrylamide in presence of initiator potassium persulphate. Cd(II)IIP showed higher capacity and selectivity than the nonimprinted polymer (NIP). The sorption capacities of Cd(II)IIP and NIP for Cd(II) ions were 0.886 and 0.663
Water pollution is one of the most serious environmental problems of the present day. Water obtained from different sources is associated with a large number of impurities. Among them heavy metal toxicity is very crucial. Occurrence of toxic metals in lakes, ponds, ditch, and river water affect the lives of local people that depend on these water sources for their daily requirements [
In the present study, we aim to fabricate a novel type of Cd(II) ion imprinted interpenetrating polymer network using a natural biosorbent, alginic acid which is crosslinked by the hydrophilic crosslinker N,N′ methylene-bis-acrylamide and template Cd(II) ion. The hydrophilic nature of crosslinker increased the swelling ability of polymer network which favours effective cadmium ion binding. The newly prepared interpenetrating polymer network is advantageous because it showed high sorption capacity and remarkable selectivity in Cd(II) ion separation from mixture of metal ions. Compared to the previous reports, the present IIP showed high sorption capacity and good reusability for Cd(II) ion and the synthesis of IIP involves clean technology and can be used for waste water treatment. The biopolymers are capable of removing metal ions. The method of preparation of the IPN can be considered as “green” and also simple, rapid, low cost, and environment friendly due to the use of aqueous medium.
Reagents of analytical and spectral grade were used for all experiments. Cadmium acetate used in the present study is of analytical grade (99% pure) and solutions were prepared by dissolving the required quantity of CdCH3(COO)2 in millipore water. The monomers used in this study, namely, acrylamide (99.9% pure) and crosslinking agent NNMBA, were obtained from SRL (Mumbai). Alginic acid (98.5% pure) was obtained from Merk (India). Fourier transform infrared (FT-IR) spectra of the imprinted, nonimprinted, and the Cd(II) ion bound polymer networks were recorded between 4000 and 400 cm−1, using a Perkin Elmer 400 FT-IR spectrophotometer and far IR spectra of Cd(II) ion bound polymer networks were recorded between 400 and 100 cm−1 using the same instrument. UV-vis spectrophotometric measurements were carried out using Shimadzu 2400 UV-vis spectrophotometer. SEM-EDAX were taken on JEOL-JSM-840A scanning electron microscope in nitrogen atmosphere. TG curves were recorded on a Shimadzu D-740 thermal analyser at a heating rate of 10°C min from 20 to 900°C in nitrogen atmosphere. The amount of metal ion sorbed per gram of imprinted and nonimprinted polymers from metal ion solutions was determined, using Perkin Elmer atomic absorption analyzer 300.
Alginic acid (7.5 g) was mixed with (0.64 g) of cadmium acetate in aqueous medium. This mixture was added to an aqueous solution of acrylamide (10.66 g) and N,N-methylenebisacrylamide (NNMBA) (7.71 g) and kept at 70°C in a water bath with stirring, using potassium persulphate (100 mg) as initiator. The polymer obtained was washed with water to remove unreacted monomer and with 2 N HCl to remove Cd(II) ion. The bulk polymer obtained was dried, sieved, and weighed. Nonimprinted polymer networks were also prepared using the same procedure without metal ions.
In order to investigate specific rebinding capacity, Cd(II) ion imprinted and nonimprinted polymers (500 mg) were equilibrated with Cd(II), Co(II), Cu(II), and Ni(II) (1–5 ppm, 10 mL) ion solution. The concentration of template metal ion solution before and after binding was determined by atomic absorption spectrophotometry (AAS).
100 mg of Cd(II) ion imprinted, nonimprinted, and corresponding Cd(II) bound polymers was allowed to swell in 10 mL water for 24 h. After 24 h, the polymers were filtered and surface water was carefully wiped off, and the final swollen weight was determined. From the swollen and the dry weights of the sample, the EWC (%) was calculated, using the following equation:
In order to optimize the conditions of Cd(II) ion rebinding by imprinted and nonimprinted polymers, factors affecting rebinding such as effect of concentration, time, and pH of the Cd(II) ion solution on binding were investigated.
The batch-wise metal ion binding experiments were carried out using (500 mg) of imprinted and nonimprinted polymer to evaluate the effect of concentration of template solution on rebinding.
From the difference in concentration of template solution before and after incubation, the amount of Cd(II) ion bound was determined. Similar rebinding studies at various concentrations of metal ion (1–5 ppm, 10 mL) were carried out and analyzed by AAS.
The time required for maximum binding was determined by batch equilibration method. 500 mg of imprinted and nonimprinted polymers was equilibrated with (5 ppm, 10 mL) of Cd(II) ion solution and metal ion bound was determined at regular intervals of time and analyzed by AAS.
About 100 mg of imprinted and nonimprinted polymers was equilibrated with Cd(II) ion (5 ppm, 10 mL) at different pH. After the removal of polymer particles, the amounts of Cd(II) ion bound at each pH were determined by AAS.
Aqueous solution of Cd(II) ion (5 ppm, 10 mL) was added to (100 mg) of imprinted and nonimprinted polymers. The solutions were shaken in stoppered bottles. At regular time intervals, the concentration of Cd(II) ion was found out by atomic absorption spectrophotometry. Sorption capacity was investigated using Langmuir’s and Freundlich’s isotherms.
Different sets of imprinted polymer (100 mg) were equilibrated with Cd(II) ion solution (5 ppm, 10 mL) at room temperature using a thermostat. After removing the polymer particles, the remaining concentration of Cd(II) ion was determined at equal intervals of time and analyzed by AAS. To describe the adsorption kinetic behavior of Cd(II) ion imprinted polymer network, two types of kinetic models were tested, namely, the pseudofirst-order model and pseudosecond-order model. The binding agrees with pseudosecond-order equation.
The effect of temperature on the sorption of Cd(II) ion on imprinted polymer network was investigated. Imprinted polymer and nonimprinted polymer (100 mg) were equilibrated with cadmium ion solution (10 mL, 5 ppm), at temperature varying from 25 to 40°C. The Cd(II) ion bound at each temperature was determined by AAS. The thermodynamic parameters such as
Selectivity studies were carried out by column experiment. Cd(II) ion imprinted polymer (1 g) was slurred with demineralized water (DMW) and then poured into a Pyrex glass column (id.40 mm) plucked with small portion of glass wool at the bottom. The column was preconditioned by passing DMW followed by the mixture of metal ion solution (5 ppm, 10 mL) that was passed through the column at a flow rate of ~0.5 mL min−1. The eluted solution was collected and the amount of metal ion bound was determined by atomic absorption spectrophotometric method.
The Cd(II) ion imprinted polymer networks were synthesized by free radical polymerization of acrylamide and N,N-methylenebisacrylamide (NNMBA) in presence of alginic acid. Potassium persulphate (100 mg) was used as initiator and Cd(II) ion was used as template, and the polymerization was carried out at 70°C (Scheme
Synthesis of Cd(II) ion imprinted interpenetrating polymer networks.
FT-IR spectrum of the IPN is different from those of the pure polymers because in interpenetrating polymer networks there will be intermolecular interactions. The carboxylate group plays an important role in metal sorption property of alginic acid. FT-IR spectra of Cd(II) ion imprinted polymer networks showed absorption bands at 1643 cm−1 which is assigned to carboxylate group of alginic acid. This band is shifted to 1634 cm−1 after Cd(II) ion binding. This result showed that Cd(II) ion binding takes place at carboxylate group of alginate. Imprinted polymer showed bands at 2923 cm−1 and nonimprinted polymer showed bands at 2953 cm−1 due to C–H stretching vibrations.
UV-vis spectra of Cd(II) ion imprinted interpenetrating polymer networks and the corresponding Cd(II) ion bound polymer networks are shown in Figure
UV-vis spectra of (a) Cd(II) ion desorbed and (b) Cd(II) ion bound imprinted polymers.
X-ray diffraction patterns of Cd(II) ion imprinted and bound polymers are shown in Figure
XRD pattern of imprinted and Cd(II) bound polymers.
The chemical composition of the imprinted and nonimprinted polymer networks were confirmed by SEM-EDAX. The presence and complete removal of Cd(II) ion in imprinted and Cd(II) bound polymers, respectively, were confirmed by SEM-EDAX. As shown in Figure
SEM-EDAX of Cd(II) ion (a) desorbed and (b) Cd(II) bound imprinted polymers.
Thermogravimetric analysis of imprinted, nonimprinted, and the corresponding Cd(II) ion bound interpenetrating polymer networks reveals the variation of thermal stability with Cd(II) ion binding (Figure
TGA curves of Cd(II) ion imprinted, nonimprinted, and Cd(II) ion bound IPNs.
The efficiency of a functional polymer is governed by the accessibility of the reactive functional groups anchored on it, which in turn depends on the extent of swelling and solvation [
EWC (%) values of imprinted and nonimprinted polymers and their Cd(II) ion bound polymers.
Polymers used | EWC (%) |
---|---|
IIP | 88 |
NIP | 86 |
Cd(II) bound IIP | 85 |
Cd(II) bound NIP | 84 |
The effect of initial concentration of metal ion solution on its sorption was investigated by varying the concentration of the metal ions, such as Cd(II), Co(II), Cu(II), and Ni(II) ions. The imprinted and nonimprinted polymer networks (500 mg) were equilibrated with metal ion solution (1–5 ppm, 10 mL); the concentration of template before and after binding was determined by atomic absorption spectrophotometry. It was noted that, as the concentration increases, binding of metal ion increases (Figure
Effect of concentration of metal ion solution on its binding by imprinted and nonimprinted polymers.
To optimize the time taken for maximum binding of Cd(II) ion by imprinted and nonimprinted polymer networks, 100 mg of polymer networks was equilibrated with Cd(II) ion solution (10 mL, 5 ppm) and the binding was followed by AAS at definite intervals of time. The time dependence of adsorption capacities of Cd(II) ions on imprinted and nonimprinted polymers was given in Figure
Effect of time on Cd(II) ion binding by imprinted and nonimprinted polymers.
The pH of the medium is one of the most important factors controlling the sorption of metal ions by adsorbent. The sorption of Cd(II) ion uptake at different pH was examined at equilibration by equilibrating imprinted and nonimprinted polymer (100 mg) with (10 mL, 5 ppm) metal ion solution at varying pH and the metal ion bound was determined by AAS. Sorption of metal ion increases with increase in pH of the medium and then decreases (Figure
Effect of pH on Cd(II) ion binding.
The effect of concentration of Cd(II) ion solution on sorption rate and capacity was studied. Definite amounts of imprinted and nonimprinted polymers were added to fixed amount of Cd(II) ion solution (5 ppm). The solutions were shaken in closed flasks. At regular intervals of time, Cd(II) ion bound was determined by AAS. The sorption characteristics were assessed by plotting both Langmuir’s and Freundlich’s isotherms. The Langmuir equation can be written as
For Langmuir’s isotherm,
Langmuir isotherm of Cd(II) ion imprinted polymer network.
Freundlich’s equation can be written as
For Freundlich’s isotherm,
Freundlich’s isotherm.
See Figures
The effect of temperature on the adsorption process was investigated for the adsorption of cadmium ion on the imprinted polymer by batch equilibration method and the amount of template bound at each temperature was determined by AAS. The temperature was varied from 25 to 40°C. The obtained results showed that increase in temperature favours the sorption process. Thermodynamic parameters were calculated using the following equation:
When
The linear van’t Hoff equation.
The pseudosecond-order Lagergren equation was used to describe the sorption kinetics of cadmium ion imprinted IPN as described earlier. The sorption kinetics of cadmium ion imprinted IPN confirmed to the pseudosecond-order Lagergren equation with
Kinetic model of pseudosecond order.
The pseudosecond-order Lagergren equation was used to describe the adsorption kinetics of cadmium ion imprinted IPN. Consider
The selectivity of the imprinted polymer for Cd(II) ion was investigated by rebinding Cd(II) ion in presence of various competitor metal ions (Figure
Selectivity parameters of Cd(II) ion imprinted polymers.
Metal ions |
|
|
|
---|---|---|---|
Cd(II), Zn(II) | 1.90 | 0.30 | 6.30 |
Cd(II), Ni(II) | 1.64 | 1.01 | 1.62 |
Cd(II), Mn(II) | 1.06 | 0.71 | 1.51 |
Cd(II), Cu(II) | 1.01 | 0.92 | 1.11 |
Cd(II), Co(II) | 1.01 | 0.85 | 1.18 |
Summary of the selectivity study of Cd(II) ion imprinted interpenetrating polymer networks.
To investigate the Cd(II) ion selectivity of the imprinted polymer networks, competitive sorption of Mn(II), Co(II), Ni(II), Cu(II), and Zn(II) ions was carried out by column experiment, in which 1 g of imprinted polymer was treated with (10 mL, 5 ppm) solution of these metal ions. After sorption equilibrium was reached, the concentration of metal ions in the remaining solution was measured by AAS. The functional host molecules on the imprinted polymer networks are immobilized with the strict configuration suitable for cadmium ions, and the ionic recognition is influenced by the nature of metal ion and ionic radius and charge. The results revealed that Cd(II) ion imprinted polymer networks showed high selectivity towards cadmium ion from cadmium-zinc mixture, and maximum separation is obtained depending on the selectivity coefficient. In cadmium-copper mixture, low selectivity coefficient value is obtained. But from the cadmium-nickel mixture, the ion imprinted polymer showed considerable selectivity (Figure
Reusability of cadmium ion imprinted polymers was investigated by elution operations and the results are given in Figure
Reusability studies of Cd(II) ion binding by IIP.
Under the selected conditions, eight portions of standard solutions were enriched and analyzed simultaneously following the general procedure. The relative standard deviations (R. S. D.) of the method was lower than 2.2%, which indicated that the method had good precision for the analysis of trace Cd(II) ion in solution samples. In accordance with the definition of IUPAC, the detectionlimit of the method wascalculated based on three times of the standard deviation of 11 runs of the blank solution. The detection limit (3
In the literature, various ion imprinted polymers have been studied with a wide range of sorption capacities for Cd(II) ions. The results of this study are compared with them in Table
Comparative study of the Cd(II)IIP.
Monomer | Polymerization technique | Adsorption capacity (meq/g) | Reference |
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Epoxy resin triethylene tetra amine | Copolymerization | 0.0934 | 7 |
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2,2-{ethane-1,2-diylbis[nitrilo(E)metylylidene]}diphenolate 4-vinylpyridine | Suspension polymerization | 0.0042 | 18 |
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Alginic acid acrylamide netwok | Free-radical polymerization | 0.217 | This study |
The synthesized IPN was applied to the analysis of Cd(II) ion in environmental water samples collected from Vembanad lake and nearby canals. The samples were treated by the same procedure. The samples were introduced into the cadmium ion imprinted polymer by column method and analysed by AAS. The results obtained indicate the suitability of the present Cd(II) ion imprinted IPN for the removal of hazardous Cd(II) ion from environmental water samples. The results were listed in Table
Analysis of environmental water samples.
Cd(II) ion | Lake water (mg/L) | Canal water |
---|---|---|
Found | 0.047 | 0.035 |
Removed | 0.043 | 0.034 |
Recovered (%) | 98 | 99 |
The present paper demonstrates the preparation of Cd(II) ion imprinted and nonimprinted interpenetrating polymer networks using functional polymer alginic acid and NNMBA-crosslinked polyacrylamide. The synthesised imprinted and nonimprinted interpenetrating polymer networks were characterized by FT-IR, UV-vis, SEM-EDAX, XRD, and TGA. The Cd(II) ion sorption was relatively fast. The maximum sorption capacity for Cd(II) ions was 0.8861 meqmole−1 of imprinted polymer. The fast sorption equilibrium is most probably due to high complexation and geometric affinity between Cd(II) ions and the cavities in the network structure. The sorption values increased with increasing concentration of Cd(II) ions. Langmuir model was found to be applicable in interpreting Cd(II) ion sorption on the Cd(II) ion imprinted polymer and the adsorption kinetics was described by the pseudosecond-order kinetic model. Maximum swelling was obtained for Cd(II) ion imprinted and nonimprinted polymers rather than imprinted complexes. The sorption values increased with increase in pH and a saturation value was obtained at pH 6.9. Thermodynamic parameters were calculated using the van’t Hoff equation and the sorption of Cd(II) ion on imprinted polymer networks was spontaneous and endothermic in nature and entropy of sorption increases during the reaction. Cd(II) ion imprinted interpenetrating polymer networks exhibited much high selectivity. The analytical results obtained in these investigations suggested that the sorbent may be used as an inexpensive and effective polymer for the removal of cadmium ion from aqueous solution and was successfully applied for the separation of Cd(II) ion from environmental water samples.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The author Girija Parameswaran thanks UGC Bangalore for providing a Teacher Fellowship.