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Synthesis, adsorption and analytical applicability of Ni-imprinted polymer for selective adsorption of Ni2+ ions from the aqueous environment
Polymer Testing 77 (2019) 105871
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Synthesis, adsorption and analytical applicability of Ni-imprinted polymer for selective adsorption of Ni2+ ions from the aqueous environment
T
Ameet Kumara, Aamna Baloucha,*, Abdullaha, Ashfaque Ahmed Pathanb a b
National Centre of Excellence in Analytical Chemistry, University of Sindh, Jamshoro, Pakistan Department of Civil Engineering, Mehran University of Engineering and Technology, Jamshoro, Pakistan
ARTICLE INFO
ABSTRACT
Keywords: Precipitation polymerization Nickel imprinted polymer Selectivity Water treatment
A novel nickel imprinted polymer (Ni-IP) was synthesized by using precipitation polymerization method with 4vinyl pyridine as ligand and methacrylic acid as functional monomer and it was employed to eliminate the nickel (II) ions from the aqueous media. In this work, various effects of preparation conditions on adsorption performance were also investigated. Several ligands, functional monomers, and cross-linkers were employed in the polymerization process to obtained maximum adsorption capacity. The synthesized Ni-IP was characterized by scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX) and Fourier transform infrared spectroscopy (FT-IR). Several parameters i.e. pH, adsorbent dose, agitation time and shaking speed were optimized to obtained maximum adsorption capacity. The maximum adsorption capacity of Ni-IP was 125 mg/g and obtained in 6min of agitation time with the concentration of 5 mg/L at pH 6.0. The kinetic data were well described by pseudo-second-order while Langmuir model is best fitted to isotherm data. The relative standard deviation, the limit of detection and quantification of this method was found to be 4.2%, 0.85μg/L and 2.83μg/L. The relative selective coefficient of Ni2+ ions and selective ions are greater than one due to the imprinting effect. This Ni-IP polymer also possesses good reproducibility and stability.
1. Introduction Nickel is one of the toxic and carcinogenic elements found in water, air, and soil. The major sources of its discharges into air i.e. waste incineration, volcanic eruptions, fossil fuel combustion, and rock weathering while metal alloys, kitchen utensils, agricultural and industrial activities are the main sources of water and soil contamination [1–3]. The metal exposure to humans occurs due to the inhalation of gases and ingestion of food and drinks. Although nickel (II) is an essential and crucial element in the organism and strengthens the secretion of insulin, endorse the regeneration of red corpuscles and treat against anemia and cirrhosis but nickel at higher concentration causes the skin disorder i.e. dermatitis, damage kidneys, nickel eczema mostly in women and allergic reactions. The chronic exposure of nickel may cause cardiovascular disease, pulmonary fibrosis and carcinogenic activity [4,5]. In an aqueous environment, nickel is present in oxides, hydroxide and in sulfide compounds, therefore its availability depends on pH, concentration, and salinity. Due to these reasons nickel is known as toxic and carcinogenic to environment and it is necessary to remove the nickel from industrial wastewater by using common treatment *
techniques i.e. chemical precipitation [6], membrane filtration [7,8], ion exchange [9–11], electrochemical treatment [12] and adsorption [13] to save our environment. Adsorption is one of the simplest, inexpensive and most effective method for the remediation of heavymetal ions from the aqueous system [14]. Wide ranges of adsorbents are used for the remediation of heavy metal ions i.e. activated carbon, bio-sorbents, agricultural waste sorbents, zeolites and polymers [15]. Today ion-imprinted polymers have received great attention for the selective remediation of toxic metals from industrial wastewater. These polymers are prepared by using metal ion as a template and form complexation by using functional monomer. Then cross-linking agents and initiators are added in the polymerization reaction. After the formation of the polymer, the metal ion is leached out from the polymer by using strong acids and form selective recognition cavities [16,17]. Ion-imprinted polymers have many advantages ascompared to other adsorbents. They are stored for a long period without loss of any affinity, cost-effective, easily prepared and they are highly reusable for many time [18–21]. In this work, we have prepared novel Ni-imprinted polymer (Ni-IP) by using 4-vinyl pyridine (4-VP) and methacrylic acid (MAA) as ligand and a functional monomer for the faster removal of Ni2+ ions from
Corresponding author. E-mail address: [email protected] (A. Balouch).
https://doi.org/10.1016/j.polymertesting.2019.04.018 Received 14 March 2019; Received in revised form 17 April 2019; Accepted 18 April 2019 Available online 24 April 2019 0142-9418/ © 2019 Elsevier Ltd. All rights reserved.
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aqueous solution. The prepared polymer was characterized by using scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX) and Fourier transform infrared spectroscopy (FT-IR). The effect of various parameters has also been studied i.e. adsorption time, pH, temperature, adsorbent dose and initial concentration of Ni2+ ion. The selectivity study of Ni-IP was also investigated by choosing Zn2+, Pb2+, Cu2+, Co2+, Cr3+as interfering ions and applied on real water samples.
2.4. Instrumentation The characterization of Ni-IP and NIP were carried out by using SEM, EDX, and FT-IR. SEM was carried out to check the surface morphology of non-imprinted (NIP), Ni-imprinted (Ni-IP), and leached imprinted polymer (L-IP) by using (JSM 6380 of Joel, Japan) operating at 20 kV. The elemental composition of Ni-imprinted, non-imprinted and leached imprinted polymers were analyzed by Energy-dispersive Xray (EDX) analyzer which was equipped Scanning electron microscope. Fourier transform infrared microscopy of IIP and NIP were carried out on the Thermo Nicolet 5700 in transmission mode wave number range (4000-500 cm−1) with deuterated triglycine sulfate detector was employed.
2. Material and methods 2.1. Reagents Nickel chloride hexahydrate (NiCl2·6H2O),2,2-azobisisobutyronitrile (AIBN) and Hydrochloric acid were obtained from Daejung chemicals (South Korea), dimethylglyoxime (DMG) was obtained from Ever chemicals (China), 2-(Hydroxyethyl)methacrylate (2-HEMA), Methacrylic acid (MAA), Acrylamide (AM), Ethylene glycol dimethacrylate (EGDMA), Pentaerythrityl triacrylate (PETA), Trimethylolpropane triacrylate (TMPTA) were obtained from Alfa Aesar (England), lead nitrate (Pb(NO3)2) and Copper sulfate pentahydrate (CuSO4·5H2O) Merck AG (Darmstadt, Germany), 2-vinyl pyridine (2-VP),4-Vinylpyridine (4-VP), 1-vinyl imidazole (1-VI), Divinylbenzene (DVB) and methanol were purchased from Sigma Aldrich (USA),CobaltChloride hexahydrate (CoCl2·6H2O), Zinc Chloride tetrahydrate (ZnCl2·4H2O), Chromium Chloride hexahydrate (CrCl3·6H2O) (BDH AnalaR) (England) and ELGA ultrapure Milli-Q water (USA) used throughout the experiment. All the above chemicals belong to analytical grade and used without any purification.
2.5. Adsorption procedure Batch experiments were used to check the adsorption performance of Ni2+ ion from the aqueous system. The Ni-IP was added into 10 mL of an aqueous solution which contains the concentration of Ni2+ ion ranging from 5 to 50 mg/L. A 0.1 M solution of HCl and NaOH were prepared for adjusting the pH from 2.0 to 8.0. Adsorption method can take place by shaking the solution for 15 min at 50 °C. After shaking the solution was analyzed on UV–visible spectrophotometer. The adsorption capacity of the Ni-IP was calculated by the following formula. Adsorption capacity:
Q=
Ci
Cf m
*V
where Q represents the adsorption capacity (mg/g) of Ni2+ ion on imprinted polymer surface, Ci and Cf are the initial and final concentration (mg/L) of Ni2+ metal ion in aqueous solution, V is the volume (L) of aqueous solution and m is the mass (g) of imprinted polymer added into the aqueous solution. The selectivity studies of Ni-IP were also investigated by using batch adsorption method. 20 mL of the aqueous solution was prepared which contains 10 mg/L of Ni2+ ion and another competitive ion. The competitive ions were done on the basis of atomic radii. The 20 mg of Ni-IP was added into the aqueous solution and shacked for 15 min at pH 6.0 and temperature 50 °C. The distribution ratio (Kd), selectivity coefficient (K) and relative selective coefficient (K’) values were calculated by using these formulas
2.2. Real water sample To conduct the applicability of this imprinted polymer, real water samples were collected from Channel canal near Hyderabad, Sindh, Pakistan. Samples were collected in cleaned plastic containers which washed by detergent then rinsed with distilled water and soaked in 8% HNO3 for 15 h finally rinsed D/D water prior to usage. The water samples were filtered by 0.45 μm pore size membrane to eliminate the suspended particles. Then all samples were transferred into plastic bottles, labeled and stored in the refrigerator at about 4 °C prior to analysis.
Kd = 2.3. Preparation of Ni-IP
K=
In order to synthesize the Ni-IP, a precipitation polymerization technique was carried out as shown in Fig. 1. In the first step, 1-mmol of Ni2+ ion as template dissolved in 50 mL of methanol as a solvent for 30min. In the second step, 2-mmol of 4-VP and 4-mmol of MAA was added as ligand and functional monomer to form complexation with the template. In the third step, 20-mmol of EGDMA was added in the solution as cross-linker and 100 mg of AIBN was added as initiator and continuously stirring for 10 min at room temperature. Finally, the solution was purged with nitrogen gas for 20min to eliminate dissolved oxygen from the reaction mixture. The molar ratio of Ni2+, 4-VP, MAA and EGDMA were 1:2:4:20. The prepared solution was sealed and starts the polymerization reaction for 12 h at 60 °C. After polymerization, the polymer was washed with 1:4 v/v of water and methanol to remove the unreacted material and in last the imprinted polymer was washed with 0.1 M hydrochloric acid to leach the Ni2+ ion from the polymer matrix and form the selective recognition cavities and then rinsed the polymer with Milli-Q water up to the neutral pH. The resulting polymer was dried at 55 °C and used for the adsorption studies. The same procedure used for the synthesis of non-imprinted polymers (NIPs) without the addition of Ni2+ ion as a template.
K =
Ci
Cf Ci
*
V m
K d (targeted ion) K d (Mint.)
Kimprinted K (non
.
imprinted )
where Kd, K and K'represents the distribution ratio, selectivity coefficient, and relative selectivity coefficient and Mint. Shows the other selected competitive metal ion respectively. 2.6. Sample analysis A digital pH meter (Metrohm 781) was employed to measure the pH of the solution. The concentration of Ni2+ ion was measured by UV–Visible spectrophotometer (Biochrom Libra S22) with dimethylglyoxime (DMG) as a complexing agent at 465 nm. The concentration of other competitive metal ions was determined by using atomic absorption spectrophotometer (AAS) was used for metals quantification (Perkin Elmer Model Analyst 700, Norwalk, CT) and Ion Chromatography (IC) was used for anions quantifications (Metrohm, 861 Advanced Compact IC). 2
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Fig. 1. Graphical presentation for the preparation of Ni-IP.
3. Results and discussions
3.1.2. EDX EDX analysis was used to determine the composition of NIP, Ni-IP, and L-IP. EDX spectrum of NIP Fig. 3a shows two major peaks of carbon and oxygen. It shows that the polymer has no impurity. However, Fig. 3b shows the peak of Ni2+ ions in the spectrum which confirms that nickel is successfully imprinted into the polymer matrix. However, the nickel ion was leached from the polymer to obtain the selective cavities into the polymer matrix of similar size and shape. In Fig. 3c, the absence of Ni2+ peak confirmed that Ni2+is completely removed from the polymer matrix.
3.1. Characterization 3.1.1. SEM The morphological analysis of non-imprinted polymer (NIP), Niimprinted polymer(Ni-IP) and leached imprinted polymer (L-IP) was done by using SEM as shown in Fig. 2. Fig. 2a and b displayed some noticeable changes in the surface of Ni-IP and NIP. In Fig. 2a, particles are closely packed to each other and show less porous surface while the SEM image of Ni-IPpossesses a fractured, irregular and rough surface due to the imprinting of Ni2+ ions in the polymer matrix as shown in Fig. 2b. It could be seen in Fig. 2c that after the extraction of Ni2+ ions from polymer matrix the roughness is increased. The increase in roughness is may be due to the elimination of Ni2+ ions which generates selective recognition sites in the polymer matrix.
3.1.3. FT-IR FT-IR analysis was carried out to determine the interaction between the ligand and Ni2+ ion. The FT-IR spectra of Ni-imprinted polymer and non-imprinted polymer are shown in (Fig. 4). Both spectra are almost the same which shows they have a similar backbone. The peak at 3452.73 cm−1 shows the stretching vibration of the O-H group present
Fig. 2. SEM images of Non-Imprinted Polymer (NIP) (a), Nickel imprinted polymer Ni-IP (b) and leached imprinted polymer (c). 3
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Fig. 3. EDX images of Non-Imprinted Polymer (NIP) (a), Nickel imprinted polymer Ni-IP (b) and leached imprinted polymer (c).
in EGDMA. The peak at 2973.82 cm−1 and 1728.27 cm−1show the stretching vibration of C-H and C=O which also belongs to EGDMA and MAA. The peak at 1454.03 cm−1shows the stretching band of 4-VP functional groups and the band at 1161 cm−1 shows the vibration band of C=C which present in EGDMA and MAA. The last peak at 827.55 cm−1 which is not present in NIP, it may belong to Ni2+ ions which are successfully imprinted in the polymer matrix. 3.2. Effect of preparation conditions of imprinted polymer 3.2.1. Effect of ligand Ligand plays a significant role in complexation with a metal ion in the polymerization process. In this study, several types of ligand i.e. 1VI, 2-VP and 4-VP were employed to attained maximum adsorption capacity and results shown in Fig. 5. The results revealed that Ni-IP prepared with 4-VP gives better adsorption capacity than others. Therefore 4-VP was used for further study.
Fig. 5. Various ligands used to obtain maximum adsorption capacity.
Fig. 4. FT-IR spectra of Ni-IP (Red) and NIP (Black). 4
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Fig. 8. Amount of initiator used to obtained maximum adsorption capacity.
Fig. 6. Various Functional monomers used to obtain maximum adsorption capacity.
3.2.4. Effect of initiator dose The initiator is used in the polymerization process to initiate the reaction and form the radicals. In this study, different amount of 2,2azobisisobutyronitrile (AIBN) is used as an initiator to achieve the maximum adsorption capacity and results are shown in Fig. 8. The result shows that polymerization reaction initiated at low quantity by using 25 mg of the initiator. The polymerization reaction was initiated when the amount of initiator was up to 50 mg but its adsorption capacity is still low. The maximum adsorption capacity was obtained at 100 mg, therefore, the 100 mg dose of initiator was used for the preparation of Ni-IP.
3.2.2. Effect of functional monomer The functional monomer is an important key factor in an ion imprinted polymer polymerization process. In this study, various types of functional monomers i.e. 2-(Hydroxyethyl)methacrylate (2-HEMA), Acrylamide (AM) and methacrylic acid (MAA) were used to obtained maximum adsorption capacity and results are shown in Fig. 6. It can be observed that Ni-IP synthesized with methacrylic acid as functional monomer gives maximum adsorption capacity than others due to carboxylic functionalities present in MAA which can form complex with Ni2+ metal ion and prepared a stable polymer. Hence this functional monomer is used for the preparation of Ni-IP.
3.3. Adsorption conditions In order to obtain maximum adsorption efficiency, several parameters i.e. pH, time, adsorbent dose and shaking speed were optimized.
3.2.3. Effect of cross-linker Cross-linker is an important constituent for the formation of the imprinted polymer as it fixes the functionality and gives a stable structure to the polymer matrix. In this study, various cross-linkers i.e. Divinylbenzene (DVB), Pentaerythrityl triacrylate (PETA), Trimethylolpropane triacrylate (TMPTA) and Ethylene glycol dimethyacrylate (EGDMA) were employed to obtain maximum adsorption capacity and results are shown in Fig. 7. The result shows that EGDMA gives maximum adsorption capacity as compared to others because EGDMA forms stable imprinting cavity. The functional monomer (MAA) and cross-linker (EGDMA) mainly consist of C,H,O and they also contain C=C double bond which gives similar chemical properties. The C=C double play an important role in the crosslinking during the polymerization process. Therefore EGDMA is selected for the preparation of Ni-IP.
3.3.1. pH optimization pH is very important in adsorption method to attain the maximum adsorption and it affects the concentration of metal ions in aqueous solution. The pH effect for the extraction of Ni2+ ions on the polymer surface was investigated ranging from 2 to 8. It was found that adsorption of Ni2+ ions was increased from 2 to 6 on polymer surface and then slightly decreased the Ni2+ adsorption above the pH 6.0 as shown in Fig. 9. At acidic pH, the adsorption efficiency is low due to excess number ofH+ions which interfering the coordination of Ni2+ ions with a functional monomer. At higher pH the adsorption efficiency was decreased due to the precipitation of Ni2+ ions with OH−ions Therefore pH 6.0 was optimized for further studies. 3.3.2. Agitation time optimization The effect of agitation time was checked ranging from 2 to 12 min to find the maximum adsorption efficiency and keeping all other parameters constant. It was found that the maximum adsorption was at 6minof contact time as shown in Fig. 10. After 6min no significant change was observed in adsorption uptake, therefore 6min time was optimized for further studies. 3.3.3. Adsorbent dose optimization The effect of dose was also checked to obtain maximum efficiency of adsorbent. The amount of adsorbent dose in adsorption studies was varying in the range of 30–140 mg at the same concentration of solution shown in Fig. 11. The result shows that the amount of adsorbent increases the percent sorption also increases and obtained maximum adsorption at 100 mg. After 100 mg there was no change in the adsorption process, therefore, the 100 mg dose was optimized for further studies.
Fig. 7. Various Cross-linkers used to obtain maximum adsorption capacity. 5
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Fig. 9. pH effect on the adsorption capacity of Ni2+ions.
3.3.4. Shaking speed optimization The effect of shaking speed was also checked in the range of 50–200 rpm. The batch adsorption process was used to carry out and keep all other parameters constant as shown in Fig. 12. The result shows that adsorption efficiency is increased by increasing the shaking speed and obtained maximum adsorption efficiency was found at 150 rpm after that the adsorption was decreased. The decrease in adsorption is due to high kinetic energy which decreases the interaction of adsorbent and the initial concentration of the ion. Therefore 150 rpm was optimized for further studies.
3.4. Adsorption isotherm To check the adsorption performance of Ni2+ ion on the surface of Ni-IP, several adsorption isotherm models are reported. But Langmuir and Freundlich isotherm models are mostly used to check the binding properties. The adsorption mechanism was investigated by using 100 mg of polymer in 10 mL of Ni2+ solution of varying concentration from 5 mg/L to 25 mg/L at 50 °C temperature. The Langmuir equation can be expressed as
Fig. 10. Agitation time effect on the adsorption capacity of Ni2+ions. 6
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Fig. 11. Adsorbent dose effect on the adsorption capacity of Ni2+ions.
adsorbate interaction on the surface of the polymer. The Freundlich equation can be expressed as
Ce 1 Ce = + qe qmax b qmax where Ceis the equilibrium concentration of a metal ion in the equilibrium phase, mg/L; qeis the adsorption capacity of ions on the polymer in mg/g; qmaxshows the maximum adsorption capacity and b shows the Langmuir constant. Apart from Langmuir isotherm which only gives the information about the monolayer adsorption on the surface of the polymer, Freundlich isotherm model also has been applied to investigate the
log qe = logk f +
1 log Ce n
where n and Kfare the Freundlich constants. The adsorption method will be favorable if the values of 1/n will be in the range of 0–1. It is concluded that our experimental data is well fitted towards the Langmuirmodel as compared to the Freundlich model on the basis of
Fig. 12. Shaking speed effect on the adsorption capacity of Ni2+ions. 7
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Fig. 13. Langmuir isotherm (A) and Freundlich isotherm (B) for the sorption of Ni2+ions. Table 1 Langmuir and Freundlich isotherm constants for the sorption of Ni2+ions. Langmuir q0 (mgg−1) b(Lmg−1) RL R2 125 0.8 0.037–0.147 0.997
Table 2 Kinetic adsorption model for the sorption of Ni2+ ions.
Freundlich n 1/n KfR2 2.017 0.4957 2.4880.9867
Pseudo first order K1(min−1) qe(mg/g) R2 0.1303 1.3648 0.9777
the correlation coefficient and adsorption capacity of Ni-IP. The correlation coefficient value of Langmuir isotherm is 0.994 whereas the correlation coefficient value of Freundlich isotherm is 0.9867 shown in Fig. 13 and isotherm constants value are shown in Table 1. The Langmuir adsorption capacity of Ni-IP was 125 mg/g whereas the Freundlich adsorption capacity was 2.488 mg/g respectively.
log(qe
qt ) = logqe
Pseudo-second order K2 (mg−1 min−1) qe(mg/g) R2 5.160 2.6570.9996
k1
t 2.303
where qeand qtshow the adsorption capacities at the time (t) and at equilibrium time and k1 denotes the adsorption rate constant. The equation of pseudo-second order is
t 1 t = + qt qe k2 qe2
3.5. Adsorption kinetics
where qt is the adsorption capacity at the time (t), mg/g and k2represent the rate constant. It is concluded that our experimental data is followed by the pseudosecond-order kinetic model on the basis of the regression coefficient. The pseudo-second-order Fig. 14 shows the regression coefficient value with R2 = 0.9996 whereas the regression coefficient value of pseudofirst order is R2 = 0.9777 and kinetic constants are represents in Table 2.
The time reaches the equilibrium is important for evaluating the maximum adsorption the efficiency of adsorbent. To check the equilibrium time of adsorption method, pseudo first and pseudo-second-order kinetic models were used in obtained experimental data. The equation of the pseudo-first-order kinetic model is
Fig. 14. Pseudo First order (A) and Pseudo Second order (B) for the sorption of Ni2+ions. 8
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Fig. 15. Van't Hoff plot, log kc versus 1/T (A), Effect of temperature on ΔG0(B).
S° =
Table 3 Thermodynamic Parameters of Ni2+adsorption onto the Ni-IP at different Temperatures T(K)
ΔG0 (kJ/mol)
ΔH0 (kJ/mol)
ΔS0 (kJ/mol k)
R2
303 308 313 318 323
−4.028 −4.561 −5.508 −7.034 −7.117
49.43
0.173
0.936
where ΔG is the Gibbs free energy (KJ/mol), T is the temperature (K), R is the universal gas constant (0.008314 kJ/mol/K), Kc is the equilibrium constant, ΔH0 is the enthalpy (kJ/mol) and ΔS0 is the entropy of system (J/K). The positive value of ΔH suggests that reaction is endothermic in nature. The positive value of ΔS0 shows that randomness is increased during the adsorption process and the negative value of ΔG0 indicates that reaction is spontaneous in nature (Table 3). 3.7. Selectivity studies
Binary Mixtures
Adsorbent
Kd Ni2+(L/g)
Kd(M)
(K)
(K′)
Ni2+/Zn2+
Ni-IP NIP Ni-IP NIP Ni-IP NIP Ni-IP NIP Ni-IP NIP Ni-IP NIP Ni-IP NIP Ni-IP NIP Ni-IP NIP
1.582 0.436 2.996 0.126 0.384 0.037 1.264 0.341 0.635 0.194 1.851 0.167 0.477 0.182 0.372 0.158 0.591 0.274
0.259 0.184 0.472 0.098 0.091 0.029 0.287 0.398 0.279 0.155 0.749 0.139 0.261 0.114 0.195 0.127 0.385 0.199
6.108 2.369 6.347 1.28 4.21 1.275 4.407 0.856 2.275 1.251 2.471 1.201 1.827 1.596 1.907 1.244 1.535 1.376
2.578
Ni
/Pb
2+
Ni2+/Co2+ Ni2+/Cu2+ 2+
Ni
3+
/Cr
Ni2+/Cl− Ni2+/NO3− 2+
/SO42-
2+
/CO32-
Ni Ni
To investigate the adsorption selectivity of Ni-IP and NIP, the selectivity experiments were conducted by selecting Zn2+, Pb2+,Co2+, Cu2+, and Cr3+ as competitive ions due to similar charge and size. The data of Kd, K and K′ of Ni2+ ions as compared to other ions are listed in Table 4. Kdvalues of Ni-IP for Ni2+ ions was greater than NIP, while the Ni-IP possess much greater K value than other competitive ions. The K’ values of Ni2+/Zn2+, Ni2+/Pb2+, Ni2+/Co2+, Ni2+/Cu2+ and Ni2+/ Cr3+ were 2.578, 4.958, 3.301, 5.148 and 1.583 which are greater than 1. The results show that Ni-IP had high selectivity for Ni2+ ions in binary mixtures. Ni-IP has strong selectivity towards Ni2+ ions as compared to other interfering ions. There are maybe two reasons. First, it may be the selectivity of cavity size. The ionic radii of Ni2+, Zn2+,Pb2+, Co2+, Cu2+ and Cr3+ is 0.069, 0.074,0.119, 0.065, 0.073 and 0.062 A. Zn2+, Cu2+, and Pb2+ have a greater size than Ni2+, therefore they cannot enter in the imprinting cavity of Ni-IP while Co2+and Cr3+have a smaller size than Ni2+ which does not match the size of the imprinting cavity. Second, it may be due to the coordination geometry of Ni-IP. It may provide certain functional groups which can only bind with Ni2+ ions in a specific structure. The results revealed that these two possibilities are responsible for the selective adsorption of Ni2+ ions.
4.958 3.301 5.148 1.818 2.057 1.144 1.532 1.115
3.6. Adsorption thermodynamics Thermodynamics study was carried out to examine the temperature effect on the adsorption of Ni2+ ion. In the thermodynamic study, temperature varies from the range of 308 K-323 K (Fig. 15). It was noticed that when the temperature increases the adsorption is also increases and obtained maximum adsorption at 50 °C. After 50 °C there was no significant change. The thermodynamic parameters i.e. Gibbs free energy (ΔG0), Enthalpy (ΔH0), and Entropy (ΔS0) were also investigated by using the equation.
G° =
ln k =
G° / T .
0
Table 4 The selectivity studies of the Ni2+ion.
2+
H°
3.8. Reusability studies Reusability is one of an important parameter to evaluating the adsorbent for practical usage. Ni2+ removal by using Ni-IP in water samples mostly depends on the times of reuse. Later desorption experiments of Ni2+ contained Ni-IP were conducted by using the batch method. The adsorbed Ni2+ ions on Ni-IP leached by using 20 mL of 1.0 M HCl. HCl was an effective solvent to leach the Ni2+ ions from polymer and the recovery was obtained up to 99.5%. The adsorptiondesorption cycle of Ni2+ ion on Ni-IP was repeated upto ten times and
RT ln k
H °/ RT + S°/ R 9
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Fig. 16. Reusability studies of Ni-IP
make results more accurate and reliable results, the Ni-IP was applied for the trace removal of Ni2+ from water samples to evaluate practically of synthesized adsorbent. For this purpose, various recovery experimentations were performed using a standard addition method by spiking 15 μgL−1ofNi2+solutions in real water samples and results are listed in Table 5b.
Table 5a Analytical performance of the prepared pre-concentration method. Range
10-100 (μg/L)
2
R %RSD (n = 10) LOD LOQ
0.9981 4.2 0.85 2.83
3.10. Comparison study To check the advantages of Ni-IP which synthesized in this work, it was compared with other adsorbents for the adsorption of Ni2+ ions in the terms of adsorption capacity, pH and equilibrium time shown in Table 6. The maximum adsorption capacity was obtained at a concentration of 5 mg/L. It can be seen that Ni-IP possesses good adsorption capacity and higher selectivity for the extraction of Ni2+ions in a shorter time 6min. Therefore results show that synthesized novel Ni-IP is a selective adsorbent for Ni2+ions adsorption from the aqueous environment.
shown in Fig. 16. It was observed that the recovery of the polymer was decreased up to 1.87% after tenth adsorption-desorption cycles. Thus the results revealed the excellent reusable capability of synthesized imprinted polymer for the elimination of Ni2+ ions from aqueous system. 3.9. Real sample analysis The analytical features and applicability of Ni-IIP as an adsorbent for the remediation of Ni2+from aqueous medium were investigated under optimized conditions. The excellent linearity was found in the range of 10–100 μg/L concentration. The limit of detection (LOD) and limit of quantification (LOQ) were achieved as 3 and 10 times respectively, the ratio between the standard deviation of 10 readings of blank and slope of the calibration graphs. The LOD and LOQ were found 0.85 and 2.83 μg/L. The LOD was acceptable for the detection of Ni2+ from the aqueous environment. The linear concentration ranges, LOD, LOQ, %RSD and R2 values of the methods are given in Table 5a. In order to
4. Conclusion The Ni-IP was synthesized by precipitation polymerization. 4-VP, MAA, EGDMA, and AIBN were selected as ligand, functional monomer, cross-linker and initiator in the solvent of methanol. The irregular shape and porosity of Ni-IP were checked by using SEM. The FT-IR and EDX results show the completely imprinting of Ni2+ in a polymer matrix. Effects of various parameters i.e. pH, agitation time, adsorbent dose and shaking speed were optimized to obtained maximum adsorption
Table 5b Spiked recovery test of Ni2+in a real water sample. % Recovery =
found
found without addition *100 added
Sample
S1
S2
S3
S4
S5
Without addition Ni2+ added Ni2+ found % Recovery
13.3 ± 0.6 15 27.6 ± 0.73 98.2
14.1 ± 0.35 15 28.3 ± 0.42 98.5
11.28 ± 0.57 15 25.8 ± 0.68 97.6
12.9 ± 0.42 15 26.7 ± 0.57 97.9
11.72 ± 0.65 15 25.2 ± 0.71 97.1
10
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Table 6 AComparison study of different Ni2+adsorbents. Adsorbents
Adsorption Capacity (mg/g)
Equilibrium Time
pH
Ref
Ion-imprinted Alginate Based Beads poly(methacrylic acid) material (IIP/CTAB) Ni (II) ion-imprinted polymers (Ni-IIP) nanoparticles Ni-ion-imprinted polymer (IIP) Magnetic ion-imprinted polymer Macro porous Ni2+-imprinted chitosan foam adsorbents (F-IIP) Ni(II) ion-imprinted polymer Ni(II) ion-imprinted silica gel polymer Ni(II)-imprinted cryogels Ni(II) ion-imprinted polymer (IIP) Ni(II) ion-imprinted polymer (Ni-IP)
10.81 33.31 40.25 3.26 87 69.93 20.30 66.22 5.54 86.3 125
24 h 20min 15min 5min 5min 120min 12min 10min 60min 30min 6min
7.0 7.25 9.0 6.0 7.0 6.0 7.0 7.0 6.5 7.0 6.0
[22] [1] [5] [23] [24] [25] [26] [27] [28] [29] This study
efficiency. The adsorption of IIPs was fast and followed the Langmuir isotherm and pseudo-second-order kinetic model. The maximum adsorption capacity was obtained 125 mg/g at pH 6.0 and the initial concentration was 5 mg/L. The relative selective coefficient of all Ni2+/ competitive ions was greater than one due to the presence of specific binding sites of Ni2+ in a polymer. Therefore, the novel synthetic imprinted polymer is a selective polymer for the selective adsorption of Ni2+ ions from the aqueousenvironment at trace level.
[11] S. Sabir, M.K. Samoon, Arsenic remediation by synthetic and natural adsorbents, Pak. J. Anal. Environ. Chem. 18 (2017) 18–36. [12] V. Khandegar, A.K. Saroha, Electrocoagulation for the treatment of textile industry effluent–a review, J. Environ. Manag. 128 (2013) 949–963. [13] T.A. Kurniawan, G.Y. Chan, W.h. Lo, S. Babel, Comparisons of low-cost adsorbents for treating wastewaters laden with heavy metals, Sci. Total Environ. 366 (2006) 409–426. [14] A. Azimi, A. Azari, M. Rezakazemi, M. Ansarpour, Removal of heavy metals from industrial wastewaters: a review, Chem. Bioeng. Rev 4 (2017) 37–59. [15] F. Fu, Q. Wang, Removal of heavy metal ions from wastewaters: a review, J. Environ. Manag. 92 (2011) 407–418. [16] C. Branger, W. Meouche, A. Margaillan, Recent advances on ion-imprinted polymers, React. Funct. Polym. 73 (2013) 859–875. [17] A. Kumar, A. Balouch, A.A. Pathan, Abdullah, M.S. Jagirani, A.M. Mahar, M.-U.H. Rajput, Novel chromium imprinted polymer: synthesis, characterization and analytical applicability for the selective remediation of Cr (VI) from an aqueous system, Int. J. Environ. Anal. Chem. (2019) 1–20. [18] Ç. Arpa, N. Bereli, E. Özdil, S. Bektaş, A. Denizli, Reactive green HE-4BD functionalized supermacroporous poly (hydroxyethyl methacrylate) cryogel for heavy metal removal, J. Appl. Polym. Sci. 118 (2010) 2208–2215. [19] M. Andaç, A. Denizli, Affinity-recognition-based polymeric cryogels for protein depletion studies, RSC Adv. 4 (2014) 31130–31141. [20] A. Balouch, F.N. Talpur, A. Kumar, M.T. Shah, A.M. Mahar, Synthesis of ultrasonicassisted lead ion imprinted polymer as a selective sorbent for the removal of Pb2+ in a real water sample, Microchem. J. 146 (2019) 1160–1168. [21] F.A. Mustafai, A. Balouch, N. Jalbani, M.I. Bhanger, M.S. Jagirani, A. Kumar, A. Tunio, Microwave-assisted synthesis of imprinted polymer for selective removal of arsenic from drinking water by applying Taguchi statistical method, Eur. Polym. J. 109 (2018) 133–142. [22] B. Özkahraman, Z. Özbaş, A.B. Öztürk, Synthesis of ion-imprinted alginate based beads: selective adsorption behavior of nickel (II) ions, J. Polym. Environ. 26 (2018) 4303–4310. [23] N. Ashouri, A. Mohammadi, M. Shekarchi, R. Hajiaghaee, H. Rastegar, Synthesis of a new ion-imprinted polymer and its characterization for the selective extraction and determination of nickel ions in aqueous solutions, Desalination Water Treat. 56 (2015) 2135–2144. [24] H. Faghihian, Z. Adibmehr, Comparative performance of novel magnetic ion-imprinted adsorbents employed for Cd 2+, Cu 2+ and Ni 2+ removal from aqueous solutions, Environ. Sci. Pollut. Control Ser. 25 (2018) 15068–15079. [25] N. Guo, S.-j. Su, B. Liao, S.-l. Ding, W.-y. Sun, Preparation and properties of a novel macro porous Ni2+-imprinted chitosan foam adsorbents for adsorption of nickel ions from aqueous solution, Carbohydr. Polym. 165 (2017) 376–383. [26] H. He, Q. Gan, C. Feng, Preparation and application of Ni (II) ion-imprinted silica gel polymer for selective separation of Ni (II) from aqueous solution, RSC Adv. 7 (2017) 15102–15111. [27] H.-X. He, Q. Gan, C.-G. Feng, An ion-imprinted silica gel polymer prepared by surface imprinting technique combined with aqueous solution polymerization for selective adsorption of Ni (II) from aqueous solution, Chin. J. Polym. Sci. 36 (2018) 462–471. [28] E. Tamahkar, M. Bakhshpour, M. Andaç, A. Denizli, Ion imprinted cryogels for selective removal of Ni (II) ions from aqueous solutions, Separ. Purif. Technol. 179 (2017) 36–44. [29] Z. Zhou, D. Kong, H. Zhu, N. Wang, Z. Wang, Q. Wang, W. Liu, Q. Li, W. Zhang, Z. Ren, Preparation and adsorption characteristics of an ion-imprinted polymer for fast removal of Ni (II) ions from aqueous solution, J. Hazard Mater. 341 (2018) 355–364.
Acknowledgment This work was supported and funded by Pakistan Science Foundation, Pakistan under research grant number PSF/Res/S-SU/ Chem (465). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.polymertesting.2019.04.018. References [1] F.M. de Oliveira, B.F. Somera, E.S. Ribeiro, M.G. Segatelli, M.J. Santos Yabe, E. Galunin, C.s.R.T. Tarley, Kinetic and isotherm studies of Ni2+ adsorption on poly (methacrylic acid) synthesized through a hierarchical double-imprinting method using a Ni2+ ion and cationic surfactant as templates, Ind. Eng. Chem. Res. 52 (2013) 8550–8557. [2] M. Yebra, S. Cancela, R. Cespón, Automatic determination of nickel in foods by flame atomic absorption spectrometry, Food Chem. 108 (2008) 774–778. [3] O. Sadeghi, N. Tavassoli, M. Amini, H. Ebrahimzadeh, N. Daei, Pyridine-functionalized mesoporous silica as an adsorbent material for the determination of nickel and lead in vegetables grown in close proximity by electrothermal atomic adsorption spectroscopy, Food Chem. 127 (2011) 364–368. [4] L.N. Li, N.B. Li, H.Q. Luo, A new chemiluminescence method for the determination of nickel ion, Spectrochim. Acta Mol. Biomol. Spectrosc. 64 (2006) 391–396. [5] S. Abbasi, M. Roushani, H. Khani, R. Sahraei, G. Mansouri, Synthesis and application of ion-imprinted polymer nanoparticles for the determination of nickel ions, Spectrochim. Acta Mol. Biomol. Spectrosc. 140 (2015) 534–543. [6] A. Özverdi, M. Erdem, Cu2+, Cd2+ and Pb2+ adsorption from aqueous solutions by pyrite and synthetic iron sulphide, J. Hazard Mater. 137 (2006) 626–632. [7] R.-S. Juang, R.-C. Shiau, Metal removal from aqueous solutions using chitosanenhanced membrane filtration, J. Membr. Sci. 165 (2000) 159–167. [8] J.-J. Qin, M.-N. Wai, M.-H. Oo, F.-S. Wong, A feasibility study on the treatment and recycling of a wastewater from metal plating, J. Membr. Sci. 208 (2002) 213–221. [9] M. Owlad, M.K. Aroua, W.A.W. Daud, S. Baroutian, Removal of hexavalent chromium-contaminated water and wastewater: a review, Water Air Soil Pollut. 200 (2009) 59–77. [10] R. Dave, G. Dave, V. Mishra, Removal of nickel from eletroplating wastewater by weakly basic chelating anion exchange resins: Dowex 50x4, Dowex 50x2 and Dowex M-4195, J. Appl. Sci. Environ. Sanit. 6 (2011).
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Synthesis, adsorption and analytical applicability of Ni-imprinted polymer for selective adsorption of Ni2+ ions from the aqueous environment
T
Ameet Kumara, Aamna Baloucha,*, Abdullaha, Ashfaque Ahmed Pathanb a b
National Centre of Excellence in Analytical Chemistry, University of Sindh, Jamshoro, Pakistan Department of Civil Engineering, Mehran University of Engineering and Technology, Jamshoro, Pakistan
ARTICLE INFO
ABSTRACT
Keywords: Precipitation polymerization Nickel imprinted polymer Selectivity Water treatment
A novel nickel imprinted polymer (Ni-IP) was synthesized by using precipitation polymerization method with 4vinyl pyridine as ligand and methacrylic acid as functional monomer and it was employed to eliminate the nickel (II) ions from the aqueous media. In this work, various effects of preparation conditions on adsorption performance were also investigated. Several ligands, functional monomers, and cross-linkers were employed in the polymerization process to obtained maximum adsorption capacity. The synthesized Ni-IP was characterized by scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX) and Fourier transform infrared spectroscopy (FT-IR). Several parameters i.e. pH, adsorbent dose, agitation time and shaking speed were optimized to obtained maximum adsorption capacity. The maximum adsorption capacity of Ni-IP was 125 mg/g and obtained in 6min of agitation time with the concentration of 5 mg/L at pH 6.0. The kinetic data were well described by pseudo-second-order while Langmuir model is best fitted to isotherm data. The relative standard deviation, the limit of detection and quantification of this method was found to be 4.2%, 0.85μg/L and 2.83μg/L. The relative selective coefficient of Ni2+ ions and selective ions are greater than one due to the imprinting effect. This Ni-IP polymer also possesses good reproducibility and stability.
1. Introduction Nickel is one of the toxic and carcinogenic elements found in water, air, and soil. The major sources of its discharges into air i.e. waste incineration, volcanic eruptions, fossil fuel combustion, and rock weathering while metal alloys, kitchen utensils, agricultural and industrial activities are the main sources of water and soil contamination [1–3]. The metal exposure to humans occurs due to the inhalation of gases and ingestion of food and drinks. Although nickel (II) is an essential and crucial element in the organism and strengthens the secretion of insulin, endorse the regeneration of red corpuscles and treat against anemia and cirrhosis but nickel at higher concentration causes the skin disorder i.e. dermatitis, damage kidneys, nickel eczema mostly in women and allergic reactions. The chronic exposure of nickel may cause cardiovascular disease, pulmonary fibrosis and carcinogenic activity [4,5]. In an aqueous environment, nickel is present in oxides, hydroxide and in sulfide compounds, therefore its availability depends on pH, concentration, and salinity. Due to these reasons nickel is known as toxic and carcinogenic to environment and it is necessary to remove the nickel from industrial wastewater by using common treatment *
techniques i.e. chemical precipitation [6], membrane filtration [7,8], ion exchange [9–11], electrochemical treatment [12] and adsorption [13] to save our environment. Adsorption is one of the simplest, inexpensive and most effective method for the remediation of heavymetal ions from the aqueous system [14]. Wide ranges of adsorbents are used for the remediation of heavy metal ions i.e. activated carbon, bio-sorbents, agricultural waste sorbents, zeolites and polymers [15]. Today ion-imprinted polymers have received great attention for the selective remediation of toxic metals from industrial wastewater. These polymers are prepared by using metal ion as a template and form complexation by using functional monomer. Then cross-linking agents and initiators are added in the polymerization reaction. After the formation of the polymer, the metal ion is leached out from the polymer by using strong acids and form selective recognition cavities [16,17]. Ion-imprinted polymers have many advantages ascompared to other adsorbents. They are stored for a long period without loss of any affinity, cost-effective, easily prepared and they are highly reusable for many time [18–21]. In this work, we have prepared novel Ni-imprinted polymer (Ni-IP) by using 4-vinyl pyridine (4-VP) and methacrylic acid (MAA) as ligand and a functional monomer for the faster removal of Ni2+ ions from
Corresponding author. E-mail address: [email protected] (A. Balouch).
https://doi.org/10.1016/j.polymertesting.2019.04.018 Received 14 March 2019; Received in revised form 17 April 2019; Accepted 18 April 2019 Available online 24 April 2019 0142-9418/ © 2019 Elsevier Ltd. All rights reserved.
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aqueous solution. The prepared polymer was characterized by using scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX) and Fourier transform infrared spectroscopy (FT-IR). The effect of various parameters has also been studied i.e. adsorption time, pH, temperature, adsorbent dose and initial concentration of Ni2+ ion. The selectivity study of Ni-IP was also investigated by choosing Zn2+, Pb2+, Cu2+, Co2+, Cr3+as interfering ions and applied on real water samples.
2.4. Instrumentation The characterization of Ni-IP and NIP were carried out by using SEM, EDX, and FT-IR. SEM was carried out to check the surface morphology of non-imprinted (NIP), Ni-imprinted (Ni-IP), and leached imprinted polymer (L-IP) by using (JSM 6380 of Joel, Japan) operating at 20 kV. The elemental composition of Ni-imprinted, non-imprinted and leached imprinted polymers were analyzed by Energy-dispersive Xray (EDX) analyzer which was equipped Scanning electron microscope. Fourier transform infrared microscopy of IIP and NIP were carried out on the Thermo Nicolet 5700 in transmission mode wave number range (4000-500 cm−1) with deuterated triglycine sulfate detector was employed.
2. Material and methods 2.1. Reagents Nickel chloride hexahydrate (NiCl2·6H2O),2,2-azobisisobutyronitrile (AIBN) and Hydrochloric acid were obtained from Daejung chemicals (South Korea), dimethylglyoxime (DMG) was obtained from Ever chemicals (China), 2-(Hydroxyethyl)methacrylate (2-HEMA), Methacrylic acid (MAA), Acrylamide (AM), Ethylene glycol dimethacrylate (EGDMA), Pentaerythrityl triacrylate (PETA), Trimethylolpropane triacrylate (TMPTA) were obtained from Alfa Aesar (England), lead nitrate (Pb(NO3)2) and Copper sulfate pentahydrate (CuSO4·5H2O) Merck AG (Darmstadt, Germany), 2-vinyl pyridine (2-VP),4-Vinylpyridine (4-VP), 1-vinyl imidazole (1-VI), Divinylbenzene (DVB) and methanol were purchased from Sigma Aldrich (USA),CobaltChloride hexahydrate (CoCl2·6H2O), Zinc Chloride tetrahydrate (ZnCl2·4H2O), Chromium Chloride hexahydrate (CrCl3·6H2O) (BDH AnalaR) (England) and ELGA ultrapure Milli-Q water (USA) used throughout the experiment. All the above chemicals belong to analytical grade and used without any purification.
2.5. Adsorption procedure Batch experiments were used to check the adsorption performance of Ni2+ ion from the aqueous system. The Ni-IP was added into 10 mL of an aqueous solution which contains the concentration of Ni2+ ion ranging from 5 to 50 mg/L. A 0.1 M solution of HCl and NaOH were prepared for adjusting the pH from 2.0 to 8.0. Adsorption method can take place by shaking the solution for 15 min at 50 °C. After shaking the solution was analyzed on UV–visible spectrophotometer. The adsorption capacity of the Ni-IP was calculated by the following formula. Adsorption capacity:
Q=
Ci
Cf m
*V
where Q represents the adsorption capacity (mg/g) of Ni2+ ion on imprinted polymer surface, Ci and Cf are the initial and final concentration (mg/L) of Ni2+ metal ion in aqueous solution, V is the volume (L) of aqueous solution and m is the mass (g) of imprinted polymer added into the aqueous solution. The selectivity studies of Ni-IP were also investigated by using batch adsorption method. 20 mL of the aqueous solution was prepared which contains 10 mg/L of Ni2+ ion and another competitive ion. The competitive ions were done on the basis of atomic radii. The 20 mg of Ni-IP was added into the aqueous solution and shacked for 15 min at pH 6.0 and temperature 50 °C. The distribution ratio (Kd), selectivity coefficient (K) and relative selective coefficient (K’) values were calculated by using these formulas
2.2. Real water sample To conduct the applicability of this imprinted polymer, real water samples were collected from Channel canal near Hyderabad, Sindh, Pakistan. Samples were collected in cleaned plastic containers which washed by detergent then rinsed with distilled water and soaked in 8% HNO3 for 15 h finally rinsed D/D water prior to usage. The water samples were filtered by 0.45 μm pore size membrane to eliminate the suspended particles. Then all samples were transferred into plastic bottles, labeled and stored in the refrigerator at about 4 °C prior to analysis.
Kd = 2.3. Preparation of Ni-IP
K=
In order to synthesize the Ni-IP, a precipitation polymerization technique was carried out as shown in Fig. 1. In the first step, 1-mmol of Ni2+ ion as template dissolved in 50 mL of methanol as a solvent for 30min. In the second step, 2-mmol of 4-VP and 4-mmol of MAA was added as ligand and functional monomer to form complexation with the template. In the third step, 20-mmol of EGDMA was added in the solution as cross-linker and 100 mg of AIBN was added as initiator and continuously stirring for 10 min at room temperature. Finally, the solution was purged with nitrogen gas for 20min to eliminate dissolved oxygen from the reaction mixture. The molar ratio of Ni2+, 4-VP, MAA and EGDMA were 1:2:4:20. The prepared solution was sealed and starts the polymerization reaction for 12 h at 60 °C. After polymerization, the polymer was washed with 1:4 v/v of water and methanol to remove the unreacted material and in last the imprinted polymer was washed with 0.1 M hydrochloric acid to leach the Ni2+ ion from the polymer matrix and form the selective recognition cavities and then rinsed the polymer with Milli-Q water up to the neutral pH. The resulting polymer was dried at 55 °C and used for the adsorption studies. The same procedure used for the synthesis of non-imprinted polymers (NIPs) without the addition of Ni2+ ion as a template.
K =
Ci
Cf Ci
*
V m
K d (targeted ion) K d (Mint.)
Kimprinted K (non
.
imprinted )
where Kd, K and K'represents the distribution ratio, selectivity coefficient, and relative selectivity coefficient and Mint. Shows the other selected competitive metal ion respectively. 2.6. Sample analysis A digital pH meter (Metrohm 781) was employed to measure the pH of the solution. The concentration of Ni2+ ion was measured by UV–Visible spectrophotometer (Biochrom Libra S22) with dimethylglyoxime (DMG) as a complexing agent at 465 nm. The concentration of other competitive metal ions was determined by using atomic absorption spectrophotometer (AAS) was used for metals quantification (Perkin Elmer Model Analyst 700, Norwalk, CT) and Ion Chromatography (IC) was used for anions quantifications (Metrohm, 861 Advanced Compact IC). 2
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Fig. 1. Graphical presentation for the preparation of Ni-IP.
3. Results and discussions
3.1.2. EDX EDX analysis was used to determine the composition of NIP, Ni-IP, and L-IP. EDX spectrum of NIP Fig. 3a shows two major peaks of carbon and oxygen. It shows that the polymer has no impurity. However, Fig. 3b shows the peak of Ni2+ ions in the spectrum which confirms that nickel is successfully imprinted into the polymer matrix. However, the nickel ion was leached from the polymer to obtain the selective cavities into the polymer matrix of similar size and shape. In Fig. 3c, the absence of Ni2+ peak confirmed that Ni2+is completely removed from the polymer matrix.
3.1. Characterization 3.1.1. SEM The morphological analysis of non-imprinted polymer (NIP), Niimprinted polymer(Ni-IP) and leached imprinted polymer (L-IP) was done by using SEM as shown in Fig. 2. Fig. 2a and b displayed some noticeable changes in the surface of Ni-IP and NIP. In Fig. 2a, particles are closely packed to each other and show less porous surface while the SEM image of Ni-IPpossesses a fractured, irregular and rough surface due to the imprinting of Ni2+ ions in the polymer matrix as shown in Fig. 2b. It could be seen in Fig. 2c that after the extraction of Ni2+ ions from polymer matrix the roughness is increased. The increase in roughness is may be due to the elimination of Ni2+ ions which generates selective recognition sites in the polymer matrix.
3.1.3. FT-IR FT-IR analysis was carried out to determine the interaction between the ligand and Ni2+ ion. The FT-IR spectra of Ni-imprinted polymer and non-imprinted polymer are shown in (Fig. 4). Both spectra are almost the same which shows they have a similar backbone. The peak at 3452.73 cm−1 shows the stretching vibration of the O-H group present
Fig. 2. SEM images of Non-Imprinted Polymer (NIP) (a), Nickel imprinted polymer Ni-IP (b) and leached imprinted polymer (c). 3
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Fig. 3. EDX images of Non-Imprinted Polymer (NIP) (a), Nickel imprinted polymer Ni-IP (b) and leached imprinted polymer (c).
in EGDMA. The peak at 2973.82 cm−1 and 1728.27 cm−1show the stretching vibration of C-H and C=O which also belongs to EGDMA and MAA. The peak at 1454.03 cm−1shows the stretching band of 4-VP functional groups and the band at 1161 cm−1 shows the vibration band of C=C which present in EGDMA and MAA. The last peak at 827.55 cm−1 which is not present in NIP, it may belong to Ni2+ ions which are successfully imprinted in the polymer matrix. 3.2. Effect of preparation conditions of imprinted polymer 3.2.1. Effect of ligand Ligand plays a significant role in complexation with a metal ion in the polymerization process. In this study, several types of ligand i.e. 1VI, 2-VP and 4-VP were employed to attained maximum adsorption capacity and results shown in Fig. 5. The results revealed that Ni-IP prepared with 4-VP gives better adsorption capacity than others. Therefore 4-VP was used for further study.
Fig. 5. Various ligands used to obtain maximum adsorption capacity.
Fig. 4. FT-IR spectra of Ni-IP (Red) and NIP (Black). 4
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Fig. 8. Amount of initiator used to obtained maximum adsorption capacity.
Fig. 6. Various Functional monomers used to obtain maximum adsorption capacity.
3.2.4. Effect of initiator dose The initiator is used in the polymerization process to initiate the reaction and form the radicals. In this study, different amount of 2,2azobisisobutyronitrile (AIBN) is used as an initiator to achieve the maximum adsorption capacity and results are shown in Fig. 8. The result shows that polymerization reaction initiated at low quantity by using 25 mg of the initiator. The polymerization reaction was initiated when the amount of initiator was up to 50 mg but its adsorption capacity is still low. The maximum adsorption capacity was obtained at 100 mg, therefore, the 100 mg dose of initiator was used for the preparation of Ni-IP.
3.2.2. Effect of functional monomer The functional monomer is an important key factor in an ion imprinted polymer polymerization process. In this study, various types of functional monomers i.e. 2-(Hydroxyethyl)methacrylate (2-HEMA), Acrylamide (AM) and methacrylic acid (MAA) were used to obtained maximum adsorption capacity and results are shown in Fig. 6. It can be observed that Ni-IP synthesized with methacrylic acid as functional monomer gives maximum adsorption capacity than others due to carboxylic functionalities present in MAA which can form complex with Ni2+ metal ion and prepared a stable polymer. Hence this functional monomer is used for the preparation of Ni-IP.
3.3. Adsorption conditions In order to obtain maximum adsorption efficiency, several parameters i.e. pH, time, adsorbent dose and shaking speed were optimized.
3.2.3. Effect of cross-linker Cross-linker is an important constituent for the formation of the imprinted polymer as it fixes the functionality and gives a stable structure to the polymer matrix. In this study, various cross-linkers i.e. Divinylbenzene (DVB), Pentaerythrityl triacrylate (PETA), Trimethylolpropane triacrylate (TMPTA) and Ethylene glycol dimethyacrylate (EGDMA) were employed to obtain maximum adsorption capacity and results are shown in Fig. 7. The result shows that EGDMA gives maximum adsorption capacity as compared to others because EGDMA forms stable imprinting cavity. The functional monomer (MAA) and cross-linker (EGDMA) mainly consist of C,H,O and they also contain C=C double bond which gives similar chemical properties. The C=C double play an important role in the crosslinking during the polymerization process. Therefore EGDMA is selected for the preparation of Ni-IP.
3.3.1. pH optimization pH is very important in adsorption method to attain the maximum adsorption and it affects the concentration of metal ions in aqueous solution. The pH effect for the extraction of Ni2+ ions on the polymer surface was investigated ranging from 2 to 8. It was found that adsorption of Ni2+ ions was increased from 2 to 6 on polymer surface and then slightly decreased the Ni2+ adsorption above the pH 6.0 as shown in Fig. 9. At acidic pH, the adsorption efficiency is low due to excess number ofH+ions which interfering the coordination of Ni2+ ions with a functional monomer. At higher pH the adsorption efficiency was decreased due to the precipitation of Ni2+ ions with OH−ions Therefore pH 6.0 was optimized for further studies. 3.3.2. Agitation time optimization The effect of agitation time was checked ranging from 2 to 12 min to find the maximum adsorption efficiency and keeping all other parameters constant. It was found that the maximum adsorption was at 6minof contact time as shown in Fig. 10. After 6min no significant change was observed in adsorption uptake, therefore 6min time was optimized for further studies. 3.3.3. Adsorbent dose optimization The effect of dose was also checked to obtain maximum efficiency of adsorbent. The amount of adsorbent dose in adsorption studies was varying in the range of 30–140 mg at the same concentration of solution shown in Fig. 11. The result shows that the amount of adsorbent increases the percent sorption also increases and obtained maximum adsorption at 100 mg. After 100 mg there was no change in the adsorption process, therefore, the 100 mg dose was optimized for further studies.
Fig. 7. Various Cross-linkers used to obtain maximum adsorption capacity. 5
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Fig. 9. pH effect on the adsorption capacity of Ni2+ions.
3.3.4. Shaking speed optimization The effect of shaking speed was also checked in the range of 50–200 rpm. The batch adsorption process was used to carry out and keep all other parameters constant as shown in Fig. 12. The result shows that adsorption efficiency is increased by increasing the shaking speed and obtained maximum adsorption efficiency was found at 150 rpm after that the adsorption was decreased. The decrease in adsorption is due to high kinetic energy which decreases the interaction of adsorbent and the initial concentration of the ion. Therefore 150 rpm was optimized for further studies.
3.4. Adsorption isotherm To check the adsorption performance of Ni2+ ion on the surface of Ni-IP, several adsorption isotherm models are reported. But Langmuir and Freundlich isotherm models are mostly used to check the binding properties. The adsorption mechanism was investigated by using 100 mg of polymer in 10 mL of Ni2+ solution of varying concentration from 5 mg/L to 25 mg/L at 50 °C temperature. The Langmuir equation can be expressed as
Fig. 10. Agitation time effect on the adsorption capacity of Ni2+ions. 6
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Fig. 11. Adsorbent dose effect on the adsorption capacity of Ni2+ions.
adsorbate interaction on the surface of the polymer. The Freundlich equation can be expressed as
Ce 1 Ce = + qe qmax b qmax where Ceis the equilibrium concentration of a metal ion in the equilibrium phase, mg/L; qeis the adsorption capacity of ions on the polymer in mg/g; qmaxshows the maximum adsorption capacity and b shows the Langmuir constant. Apart from Langmuir isotherm which only gives the information about the monolayer adsorption on the surface of the polymer, Freundlich isotherm model also has been applied to investigate the
log qe = logk f +
1 log Ce n
where n and Kfare the Freundlich constants. The adsorption method will be favorable if the values of 1/n will be in the range of 0–1. It is concluded that our experimental data is well fitted towards the Langmuirmodel as compared to the Freundlich model on the basis of
Fig. 12. Shaking speed effect on the adsorption capacity of Ni2+ions. 7
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Fig. 13. Langmuir isotherm (A) and Freundlich isotherm (B) for the sorption of Ni2+ions. Table 1 Langmuir and Freundlich isotherm constants for the sorption of Ni2+ions. Langmuir q0 (mgg−1) b(Lmg−1) RL R2 125 0.8 0.037–0.147 0.997
Table 2 Kinetic adsorption model for the sorption of Ni2+ ions.
Freundlich n 1/n KfR2 2.017 0.4957 2.4880.9867
Pseudo first order K1(min−1) qe(mg/g) R2 0.1303 1.3648 0.9777
the correlation coefficient and adsorption capacity of Ni-IP. The correlation coefficient value of Langmuir isotherm is 0.994 whereas the correlation coefficient value of Freundlich isotherm is 0.9867 shown in Fig. 13 and isotherm constants value are shown in Table 1. The Langmuir adsorption capacity of Ni-IP was 125 mg/g whereas the Freundlich adsorption capacity was 2.488 mg/g respectively.
log(qe
qt ) = logqe
Pseudo-second order K2 (mg−1 min−1) qe(mg/g) R2 5.160 2.6570.9996
k1
t 2.303
where qeand qtshow the adsorption capacities at the time (t) and at equilibrium time and k1 denotes the adsorption rate constant. The equation of pseudo-second order is
t 1 t = + qt qe k2 qe2
3.5. Adsorption kinetics
where qt is the adsorption capacity at the time (t), mg/g and k2represent the rate constant. It is concluded that our experimental data is followed by the pseudosecond-order kinetic model on the basis of the regression coefficient. The pseudo-second-order Fig. 14 shows the regression coefficient value with R2 = 0.9996 whereas the regression coefficient value of pseudofirst order is R2 = 0.9777 and kinetic constants are represents in Table 2.
The time reaches the equilibrium is important for evaluating the maximum adsorption the efficiency of adsorbent. To check the equilibrium time of adsorption method, pseudo first and pseudo-second-order kinetic models were used in obtained experimental data. The equation of the pseudo-first-order kinetic model is
Fig. 14. Pseudo First order (A) and Pseudo Second order (B) for the sorption of Ni2+ions. 8
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Fig. 15. Van't Hoff plot, log kc versus 1/T (A), Effect of temperature on ΔG0(B).
S° =
Table 3 Thermodynamic Parameters of Ni2+adsorption onto the Ni-IP at different Temperatures T(K)
ΔG0 (kJ/mol)
ΔH0 (kJ/mol)
ΔS0 (kJ/mol k)
R2
303 308 313 318 323
−4.028 −4.561 −5.508 −7.034 −7.117
49.43
0.173
0.936
where ΔG is the Gibbs free energy (KJ/mol), T is the temperature (K), R is the universal gas constant (0.008314 kJ/mol/K), Kc is the equilibrium constant, ΔH0 is the enthalpy (kJ/mol) and ΔS0 is the entropy of system (J/K). The positive value of ΔH suggests that reaction is endothermic in nature. The positive value of ΔS0 shows that randomness is increased during the adsorption process and the negative value of ΔG0 indicates that reaction is spontaneous in nature (Table 3). 3.7. Selectivity studies
Binary Mixtures
Adsorbent
Kd Ni2+(L/g)
Kd(M)
(K)
(K′)
Ni2+/Zn2+
Ni-IP NIP Ni-IP NIP Ni-IP NIP Ni-IP NIP Ni-IP NIP Ni-IP NIP Ni-IP NIP Ni-IP NIP Ni-IP NIP
1.582 0.436 2.996 0.126 0.384 0.037 1.264 0.341 0.635 0.194 1.851 0.167 0.477 0.182 0.372 0.158 0.591 0.274
0.259 0.184 0.472 0.098 0.091 0.029 0.287 0.398 0.279 0.155 0.749 0.139 0.261 0.114 0.195 0.127 0.385 0.199
6.108 2.369 6.347 1.28 4.21 1.275 4.407 0.856 2.275 1.251 2.471 1.201 1.827 1.596 1.907 1.244 1.535 1.376
2.578
Ni
/Pb
2+
Ni2+/Co2+ Ni2+/Cu2+ 2+
Ni
3+
/Cr
Ni2+/Cl− Ni2+/NO3− 2+
/SO42-
2+
/CO32-
Ni Ni
To investigate the adsorption selectivity of Ni-IP and NIP, the selectivity experiments were conducted by selecting Zn2+, Pb2+,Co2+, Cu2+, and Cr3+ as competitive ions due to similar charge and size. The data of Kd, K and K′ of Ni2+ ions as compared to other ions are listed in Table 4. Kdvalues of Ni-IP for Ni2+ ions was greater than NIP, while the Ni-IP possess much greater K value than other competitive ions. The K’ values of Ni2+/Zn2+, Ni2+/Pb2+, Ni2+/Co2+, Ni2+/Cu2+ and Ni2+/ Cr3+ were 2.578, 4.958, 3.301, 5.148 and 1.583 which are greater than 1. The results show that Ni-IP had high selectivity for Ni2+ ions in binary mixtures. Ni-IP has strong selectivity towards Ni2+ ions as compared to other interfering ions. There are maybe two reasons. First, it may be the selectivity of cavity size. The ionic radii of Ni2+, Zn2+,Pb2+, Co2+, Cu2+ and Cr3+ is 0.069, 0.074,0.119, 0.065, 0.073 and 0.062 A. Zn2+, Cu2+, and Pb2+ have a greater size than Ni2+, therefore they cannot enter in the imprinting cavity of Ni-IP while Co2+and Cr3+have a smaller size than Ni2+ which does not match the size of the imprinting cavity. Second, it may be due to the coordination geometry of Ni-IP. It may provide certain functional groups which can only bind with Ni2+ ions in a specific structure. The results revealed that these two possibilities are responsible for the selective adsorption of Ni2+ ions.
4.958 3.301 5.148 1.818 2.057 1.144 1.532 1.115
3.6. Adsorption thermodynamics Thermodynamics study was carried out to examine the temperature effect on the adsorption of Ni2+ ion. In the thermodynamic study, temperature varies from the range of 308 K-323 K (Fig. 15). It was noticed that when the temperature increases the adsorption is also increases and obtained maximum adsorption at 50 °C. After 50 °C there was no significant change. The thermodynamic parameters i.e. Gibbs free energy (ΔG0), Enthalpy (ΔH0), and Entropy (ΔS0) were also investigated by using the equation.
G° =
ln k =
G° / T .
0
Table 4 The selectivity studies of the Ni2+ion.
2+
H°
3.8. Reusability studies Reusability is one of an important parameter to evaluating the adsorbent for practical usage. Ni2+ removal by using Ni-IP in water samples mostly depends on the times of reuse. Later desorption experiments of Ni2+ contained Ni-IP were conducted by using the batch method. The adsorbed Ni2+ ions on Ni-IP leached by using 20 mL of 1.0 M HCl. HCl was an effective solvent to leach the Ni2+ ions from polymer and the recovery was obtained up to 99.5%. The adsorptiondesorption cycle of Ni2+ ion on Ni-IP was repeated upto ten times and
RT ln k
H °/ RT + S°/ R 9
Polymer Testing 77 (2019) 105871
A. Kumar, et al.
Fig. 16. Reusability studies of Ni-IP
make results more accurate and reliable results, the Ni-IP was applied for the trace removal of Ni2+ from water samples to evaluate practically of synthesized adsorbent. For this purpose, various recovery experimentations were performed using a standard addition method by spiking 15 μgL−1ofNi2+solutions in real water samples and results are listed in Table 5b.
Table 5a Analytical performance of the prepared pre-concentration method. Range
10-100 (μg/L)
2
R %RSD (n = 10) LOD LOQ
0.9981 4.2 0.85 2.83
3.10. Comparison study To check the advantages of Ni-IP which synthesized in this work, it was compared with other adsorbents for the adsorption of Ni2+ ions in the terms of adsorption capacity, pH and equilibrium time shown in Table 6. The maximum adsorption capacity was obtained at a concentration of 5 mg/L. It can be seen that Ni-IP possesses good adsorption capacity and higher selectivity for the extraction of Ni2+ions in a shorter time 6min. Therefore results show that synthesized novel Ni-IP is a selective adsorbent for Ni2+ions adsorption from the aqueous environment.
shown in Fig. 16. It was observed that the recovery of the polymer was decreased up to 1.87% after tenth adsorption-desorption cycles. Thus the results revealed the excellent reusable capability of synthesized imprinted polymer for the elimination of Ni2+ ions from aqueous system. 3.9. Real sample analysis The analytical features and applicability of Ni-IIP as an adsorbent for the remediation of Ni2+from aqueous medium were investigated under optimized conditions. The excellent linearity was found in the range of 10–100 μg/L concentration. The limit of detection (LOD) and limit of quantification (LOQ) were achieved as 3 and 10 times respectively, the ratio between the standard deviation of 10 readings of blank and slope of the calibration graphs. The LOD and LOQ were found 0.85 and 2.83 μg/L. The LOD was acceptable for the detection of Ni2+ from the aqueous environment. The linear concentration ranges, LOD, LOQ, %RSD and R2 values of the methods are given in Table 5a. In order to
4. Conclusion The Ni-IP was synthesized by precipitation polymerization. 4-VP, MAA, EGDMA, and AIBN were selected as ligand, functional monomer, cross-linker and initiator in the solvent of methanol. The irregular shape and porosity of Ni-IP were checked by using SEM. The FT-IR and EDX results show the completely imprinting of Ni2+ in a polymer matrix. Effects of various parameters i.e. pH, agitation time, adsorbent dose and shaking speed were optimized to obtained maximum adsorption
Table 5b Spiked recovery test of Ni2+in a real water sample. % Recovery =
found
found without addition *100 added
Sample
S1
S2
S3
S4
S5
Without addition Ni2+ added Ni2+ found % Recovery
13.3 ± 0.6 15 27.6 ± 0.73 98.2
14.1 ± 0.35 15 28.3 ± 0.42 98.5
11.28 ± 0.57 15 25.8 ± 0.68 97.6
12.9 ± 0.42 15 26.7 ± 0.57 97.9
11.72 ± 0.65 15 25.2 ± 0.71 97.1
10
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Table 6 AComparison study of different Ni2+adsorbents. Adsorbents
Adsorption Capacity (mg/g)
Equilibrium Time
pH
Ref
Ion-imprinted Alginate Based Beads poly(methacrylic acid) material (IIP/CTAB) Ni (II) ion-imprinted polymers (Ni-IIP) nanoparticles Ni-ion-imprinted polymer (IIP) Magnetic ion-imprinted polymer Macro porous Ni2+-imprinted chitosan foam adsorbents (F-IIP) Ni(II) ion-imprinted polymer Ni(II) ion-imprinted silica gel polymer Ni(II)-imprinted cryogels Ni(II) ion-imprinted polymer (IIP) Ni(II) ion-imprinted polymer (Ni-IP)
10.81 33.31 40.25 3.26 87 69.93 20.30 66.22 5.54 86.3 125
24 h 20min 15min 5min 5min 120min 12min 10min 60min 30min 6min
7.0 7.25 9.0 6.0 7.0 6.0 7.0 7.0 6.5 7.0 6.0
[22] [1] [5] [23] [24] [25] [26] [27] [28] [29] This study
efficiency. The adsorption of IIPs was fast and followed the Langmuir isotherm and pseudo-second-order kinetic model. The maximum adsorption capacity was obtained 125 mg/g at pH 6.0 and the initial concentration was 5 mg/L. The relative selective coefficient of all Ni2+/ competitive ions was greater than one due to the presence of specific binding sites of Ni2+ in a polymer. Therefore, the novel synthetic imprinted polymer is a selective polymer for the selective adsorption of Ni2+ ions from the aqueousenvironment at trace level.
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