SEARCH

SEARCH BY CITATION

Keywords:

  • N-vinyl-2-pyrrolidone;
  • Itaconic acid;
  • heavy metal ion;
  • water absorption;
  • adsorption;
  • kinetic study

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. References

In this study, a series of crosslinked N-vinyl-2-pyrrolidone–itaconic acid (NVP-IA) hydrogels was prepared by free radical polymerisation in aqueous solution. Swelling, mechanical and adsorption properties for the removal of Cu2+ and Pb2+ ions from aqueous solutions of these hydrogels were investigated. Metal ion adsorption capacities of hydrogels for Cu2+ and Pb2+ ions were determined as 2.1 and 0.6 mmol g −1, respectively. Adsorption processes of metal ions onto the NVP-IA hydrogels follow pseudo-second-order type adsorption kinetics. The equilibrium adsorption data have been evaluated using Freundlich and Langmuir isotherm models. The results illustrated that the adsorption follows Freundlich isotherm.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. References

Agricultural and industrial activities of humans have caused pollution of water resources. As a result of these activities, different types of toxic pollutants such as heavy metals, cationic and anionic dyes occur in waste water.([1]) These toxic pollutants cause important environmental problems. Waste waters containing different heavy metals are often encountered in mining operations, electronic device manufactures, metal plating facilities, battery manufactures and alloy industries.([2, 3]) Heavy metals are highly toxic and non-degradable pollutants.([2]) The removal of heavy metal content from water is extremely important because of its toxic effects.([4]) Various methods to remove heavy metal ions from contaminated water have been developed such as flocculation, ion-exchange, evaporation, membrane filtration, electrodialysis, chemical precipitation, adsorption and reverse osmosis techniques.([4-7]) Adsorption is the most widely used method among the methods developed to remove the heavy metals from waters because it is simple, nontoxic and inexpensive. In addition adsorbents are also easily separated from the treated water.([8-10])

A wide range of materials including activated carbon,([11]) carbon nanotubes,([12]) natural clays,([13, 14]) modified clays,([15, 16]) some agricultural residues,([17]) polyaniline, ([18]) natural polymers and its derivatives, such as chitosan,([2, 19]) starch([4, 8, 20, 21]) and cellulose([22]) were used as adsorbents. Synthetic polymer-based hydrogels were widely used as adsorbents in removal of metal ions from aqueous solutions. Different synthetic-based copolymeric hydrogels such as poly(2-acrylamido-2-methyl-1-propane sulphonic acid-co-itaconic acid),([23]) poly(acrylic acid-co-metacrylamide),([24]) acrylamide/maleic acid([25]) homopolymeric hydrogels such as poly(N-hydroxymethylacrylamide),([3]) poly(N-vinyl imidazole)([26]) and nanocomposite hydrogels such as Poly(acrylamide)/attapulgite([27]) were used in heavy metal ion removal studies.

In recent years, many researchers studied synthesis and characterisation of hydrogels based N-vinyl-2-pyrrolidone copolymer and terpolymer. N,N-dimethylaminoethyl metacrylate,([28]) methacrylic acid,([29]) acrylic acid,([30]) neutralised acrylic acid,([31]) citric acid([32]) and methacrylamide–itaconic acid([33]) were used as comonomers. There have been limited studies on the use of these hydrogels as adsorbents. Poly(N-vinyl-2-pyrrolidone/itaconic acid) hydrogels([34]) and poly(N-vinyl-2-pyrrolidone/methacrylic acid) hydrogels([35]) were used for the adsorption of α-amylase and methyl violet, respectively. A literature survey has not yielded any research on the removal of heavy metal ions from aqueous solutions by poly(N-vinyl-2-pyrrolidone/itaconic acid) hydrogels. The present work describes the application of poly(N-vinyl-2-pyrrolidone/itaconic acid) hydrogels for the removal of heavy metal ions.

In this study, copolymeric hydrogels were synthesised by using the equal mole amounts of N-vinyl-2-pyrrolidone and itaconic acid monomers, according to the free radical addition polymerisation mechanism in the aqueous media. Different amounts (0.5%, 1%, 2%, 3% and 4% of total monomer mole) of N,N-methylenebisacrylamide (NMBA) was used as a crosslinking agent. Purified hydrogels were characterised by using Fourier transform infrared spectroscopy (FTIR). The effects of crosslinker content on swelling behaviours, mechanical properties and adsorption properties of hydrogels was also investigated and compared in detail.

EXPERIMENTAL

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. References

Materials

The monomers, itaconic acid (IA) and N-vinyl-2-pyrrolidone (NVP) were obtained from Fluka (Saint-Louis, MO). The crosslinking agent, NMBA, and the water soluble initiator, ammonium persulphate (APS), were purchased from Merck (Darmstadt, Germany); the accelerator, N,N,N,N-tetrametiletilendiamin (TEMED), was purchased from ABCR (Karlsruhe, Germany), and chitosan (Ch) was purchased from Aldrich (Saint-Louis, MO). Other reagents were chemically pure grade, and all solutions and standards were prepared with distilled water.

Preparation of Hydrogels

Hydrogels were synthesised by using the equal mole amounts of IA (0.005 mol) and NVP (0.005 mol), along with APS as an initiator, TEMED as an accelerator and NMBA as a crosslinking agent in aqueous solution. Total amount of water is 3.5 mL. The feed compositions of hydrogels were listed in Table 1.

Table 1. Symbols and feed compositions of hydrogels
Symbols ofNVP/IAAPSTEMEDNMBA
hydrogels(mol)(% mol)(% mol)(% mol)
H-051120.5
H-11121
H-21122
H-31123
H-41124

The monomers (IA and NVP) and the crosslinking agent (NMBA) were taken in 12-cm long glass tubes of 2 cm diameter. APS solution and TEMED were added to the reaction mixture in the tubes. After sealing the tubes with rubber caps, oxygen-free nitrogen gas was purged in the solution for 20 min. Then, the sealed tubes were placed in a water bath at 20 ± 1∘C. At the end of the polymerisation, the glass tubes were carefully broken, then hydrogels were taken and cut into discs 5-mm in length and put in distilled water. The discs were held in water at room temperature for 5 days, and in order to remove unreacted monomers, the water was replaced with distilled water twice in a day. After the treatment process, some of the swollen disc samples were used in determination of mechanical properties, and the rest of them were dried under vacuum at 40C. Dried hydrogels were used for the water absorption and heavy metal adsorption experiments.

FT-IR Analysis

The infrared spectra of hydrogels were taken with Digilab Excalibur-FTS 3000MX model FT-IR spectrophotometer (Randolph, MA) using KBr pellets.

Determination of Mechanical Properties

Mechanical properties of hydrogels were determined of disc samples using a Zwick/Roell Z0.5 Analyser (Germany) with a 500 N load cell. Mechanical properties of the samples were determined using a software-controlled materials testing machine. Test conditions were speed, 10 mm/min and probe diameter, 12 mm. Data collection and calculation were performed using the testXpert II V3.2 software package of the instrument. Deformation (dL; %), the compressive modulus (Emod; MPa) and maximum strength (Fmax; N) values were determined. Three specimens were tested for each sample.

Determination of Water Absorption Capacities

The swelling properties of the products were investigated in distilled water. Water absorption capacities and swelling rate were determined by tea-bag method.([36-38]) The equilibrium water absorption capacities (Qe; g of water/g of sample) of hydrogels were calculated to the following equation:

  • display math(1)

where Wwet and Wdry are the weight of the swollen and dried hydrogel samples, respectively.

Swelling rate of the products was measured. The measurement condition is the same as that for equilibrium water absorbency. At certain time intervals, the water absorbency of the sample Qt was measured and calculated using the following equation:

  • display math(2)

where Qt is the water absorbency at certain time t, Wtwet and Wdry are the weights of the swollen sample at certain time t and the dry sample as g, respectively. Each experiment was carried out in triplicates, and the averages of the results were reported.

image

Figure 1. FTIR spectra of monomers and hydrogel.

Download figure to PowerPoint

Table 2. Mechanical properties of hydrogels
HydrogelsElastic modül (kPa)Fmax (N)Max. def. (%)
H-05
H-11712.8411
H-29154.36.13
H-315204.975.58
H-418305.645.47

Metal Ion Adsorption Studies

Stock solution containing Cu2+ and Pb2+ or both of these ions was prepared by dissolving metal acetate salts in distilled water. Hydrogel samples (0.2 g) were added in 50 mL of stock solution (4 mmol metal ion/L), and the mixture was stirred with a magnetic stirrer. The amount of residual metal ions in the aliquots of withdrawn solution was followed by an atomic absorption spectrometer (AAS) (Varian SpectrAA FS-220) up to 48 h. Metal ion removal capacities of the copolymers were calculated as follows:

  • display math(3)

Ci, initial concentration of metal ions in the solution (mmol/L); Ct, concentrations of metal ions in the solution after metal ion removal (mmol/L); V, volume of the solution (L); Mcopolymer, weight of hydrogel sample (g).

image

Figure 2. The effect of NMBA on the water absorbency of hydrogels.

Download figure to PowerPoint

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. References

Synthesis and Characterisation of Hydrogels

N-vinyl-2-pyrrolidone–itaconic acid (NVP-IA) hydrogels were synthesised by free radical polymerisation technique using different amounts of NMBA as a crosslinking agent.

Hydrogels were characterised by using the FTIR technique. FTIR spectra of monomers and hydrogel are shown in Figure 1. The band corresponding to the C-C bond stretching vibrations of vinyl groups at 1630 cm −1 was observed in the spectra of NVP and IA monomers. This band disappeared in the spectrum of hydrogels after copolymerisation and further procedure to remove unreacted monomers. In the spectrum of hydrogel, absorption band observed at 1720 cm −1 according to the carbonyl groups in hydrogel structure.

Table 3. Diffusion parameters and the correlation coefficients for the hydrogels
Hydrogelsnk × 10 −2 (min −1)R2
H-050.387.820.9983
H-10.2212.640.9928
H-20.1121.040.9927
Table 4. Kinetic parameters for the adsorption of metal ions onto hydrogels
   Pseudo-first-orderPseudo-second-order
HydrogelIonqe (exp)R2qe (theorical)k1R2qe (theorical)k2
H-05Cu2+1.8600.80570.3746.909 × 10 −40.99961.85811.70 × 10 −3
H-2Cu2+2.1000.76230.8806.909 × 10 −40.99952.1343.397 × 10 −3
H-4Cu2+2.1100.81961.1654.79 × 10 −20.99792.2061.683 × 10 −3
H-05Pb2+0.5570.83430.1749.212 × 10 −40.99990.5622.576 × 10 −2
H-2Pb2+0.5880.79740.2009.212 × 10 −40.99980.5961.874 × 10 −2
H-4Pb2+0.5880.79520.2116.909 × 10 −40.99970.5901.910 × 10 −2
image

Figure 3. Swelling kinetics of hydrogels. (a) H-05; (b) H-1 and (c) H-2.

Download figure to PowerPoint

Mechanical Properties of Hydrogels

Superabsorbent hydrogels are widely used in many fields. However, the applications of the hydrogels are frequently restricted in some fields mainly due to their poor mechanical properties. Mechanical properties of the hydrogels used as adsorbent are also very important. Our aim is synthesis of the hydrogels have adequate mechanical properties for use as an adsorbent in this study. For this reason, hydrogels synthesised in different crosslinking agent rate and mechanical properties of synthesised hydrogels were determined. To evaluate the effects of the crosslinking agent amount on the mechanical properties of the hydrogels, maximum deformation %, the compressive modulus (Ea) and maximum strength (Fmax) measurements were carried out as described in this study. Data obtained from these test are presented in Table 2. Three specimens were tested for each sample. The average values of the three mentioned measurements are given in Table 2. Since the results obtained from mechanical tests are very close to each other, the standard deviations of these samples are not shown in the table. The mechanical properties of the H-05 sample are not determined, since it cannot be obtained as disc-shaped. The mechanical properties of this hydrogel are very poor, and these properties are not in a measurable range for the H-05 hydrogel. Ea and Fmax values considerably increase (from 171 to 1830 kPa and 2.84 to 5.84 N, respectively) with an increase in the NMBA content of the hydrogels in the range of 1–4 wt.% (Table 2). As expected, maximum deformation values of the hydrogels decreased with increasing crosslink density. This situation can be explained with highly reduced elasticity of the hydrogel structure. Swelling behaviours of hydrogels have also confirmed this matter.

Swelling Properties of Hydrogels

The effect of the NMBA on the swelling behaviour of the hydrogels was shown in Figure 2. As seen when the NMBA content increased, equilibrium swelling capacity of the hydrogels decreased because of increasing the crosslinking points of the copolymer chains. At the same time, Figure 2 shows swelling kinetics of the hydrogels. All hydrogels have similar swelling behaviour. In the first 4 h, water absorption rate of hydrogels is significantly higher, then water absorption rate is reduced and hydrogels reached the equilibrium swelling value.

Fickian diffusion models were applied to determine the diffusion mechanism of water into hydrogels, and the following equation was used:

  • display math(4)

In this equation, F denotes fractional uptake at time t, k is a constant related to the structure of the network, and n is the diffusional exponent, which is indicative of the transport mechanism.([39])

The five basic categories of the absorption for cylindrical shaped hydrogels according to ‘n’ value may be described as follows([37]):

  1. n  = 0.5 classical Fickian or Case I transport (the system is controlled by diffusion).
  2. n  = l Case II transport (the system is controlled by relaxation).
  3. 0.5 <  n <  1: diffusion behaviour, which is an intermediate between that of Case I and Case II and is regarded as non-Fickian behaviour.
  4. n >  l Supercase II.
  5. When n <  0.5, it is called as pseudo-Fickian.
image

Figure 4. The effect of adsorption time on the metal ion adsorption efficiency of hydrogels. (a) Cu2+ and (b) Pb2+.

Download figure to PowerPoint

Fick's laws were applied to swelling kinetic data of H-05, H-1 and H-2 hydrogels. The plots of ln F versus ln t are represented in Figure 3. The exponent n and k were calculated from the slopes and intercepts of the lines, respectively, and are listed in Table 3. As seen in the table, the values of the diffusion exponent range between 0.11 and 0.38 and are found to be less than 0.5. Hence, the diffusion of water into these gel systems is assumed to be a pseudo-Fickian behaviour.

image

Figure 5. Pseudo-second-order kinetic model for Cu2+ adsorption. (a) H-05; (b) H-2 and (c) H-4.

Download figure to PowerPoint

image

Figure 6. Pseudo-second order kinetic model for Pb2+ adsorption. (a) H-05; (b) H-2 and (c) H-4.

Download figure to PowerPoint

Metal Ion Adsorption Kinetics of Hydrogels

Contact time is an important parameter because this factor can reflect the adsorption kinetics of an adsorbent for a given initial concentration of the adsorbate.([37]) Figure 4 illustrates the effect of contact time on the metal ion adsorption capacities of selected hydrogels. All hydrogels (H-05, H-2 and H-4) show similar adsorption behaviour. The adsorption capacities of hydrogels increase with the increase of the adsorption time until it reaches the equilibrium value. As shown in Figure 4, Cu2+ removal capacity of H-05 hydrogel is only 5% lower than Cu2+ removal capacities of other hydrogels. There is no significant difference between the adsorption capacities of hydrogels. The main adsorption mechanism of Cu2+ and Pb2+ ions occurred by the interactions of negative charges caused by ionisation of the carboxyl group on the hydrogels and the positive charge of the metal ions. We explained the adsorption mechanism of the hydrogel just as we did in previous works. This mechanism is shown below([4, 21, 22]):

  • display math

To investigate the controlling mechanism of adsorption process, the pseudo-first-order and pseudo-second-order equations were used to test the experimental data. The pseudo-first-order kinetic model was suggested by Lagergren for the adsorption of solid/liquid systems.([37]) The first-order rate expression of Lagergren([27]) is given as follows:

  • display math(5)
image

Figure 7. Freundlich izotherm model for H-2. (a) Cu2+ and (b) Pb2+.

Download figure to PowerPoint

qe, the amounts of metal ion adsorbed on NVP-IA hydrogels at equilibrium (mmol g −1); qt, the amounts of metal ion adsorbed on NVP-IA hydrogels at time t (mmol g −1); k1, the rate constant of first-order adsorption (min −1).

A straight line of log(qe - qt) versus t suggests the applicability of this kinetic model to fit the experimental data. First-order rate constant k1 and calculated qe value were determined from slopes and intercepts of the linear plot of log(qe - qt) versus t, respectively. Correlation coefficient (R2) was calculated using this plot. The results are given in Table 4.

The pseudo-second-order kinetic model was suggested by Ho and McKay, and this kinetic model can be expressed as([40, 42]):

  • display math(6)

k2, the rate constant of second-order adsorption (g mmol −1 min −1); qe, the amounts of metal ion adsorbed on NVP-IA hydrogels at equilibrium (mmol g −1).

This kinetic model is more likely to predict the behaviour over the whole range of adsorption and is in agreement with chemical sorption being the rate-controlling step.([4, 40, 41]) If second-order kinetic is applicable, the plot of t/qt versus t should show a linear relationship. The slopes and intercepts of plots of t/qt versus t (Figures 5 and 6) were used to determine the second-order rate constant k2 and equilibrium adsorption qe. The parameters obtained for the two models are presented in Table 4. The results show that the pseudo-second-order kinetic model fit better than the data obtained from pseudo-first-order model. High correlation coefficients (0.9979–0.9999) were obtained when the second-order kinetic model was used. The calculated qe values almost agree with the experimental data for the pseudo-second-order kinetic model. These results suggest that the second-order kinetic model was suitable for the adsorption of Cu2+ and Pb2+ ions onto the NVP-IA hydrogels.

image

Figure 8. Competitive Removal of Cu2+ and Pb2+ from aqueous solutions.

Download figure to PowerPoint

Table 5. Freundlich isotherm coefficients for H-2
IonR2Kn
Cu2+0.99213.1371.1
Pb2+0.99291.4371.1

Adsorption Isotherms

The equilibrium adsorption isotherms are very important data to understand the mechanism of the adsorption process.([27]) Adsorption isotherms are important to describe how adsorbates interact with adsorbents.([37]) Several models have been published in the literature to describe adsorption isotherms. The Freundlich and Langmuir isotherms are commonly used to describe the adsorption characteristics of adsorbents used in aqueous solutions.([42-44])

image

Figure 9. Cu2+ adsorption capacities of regenerated hydrogel samples (a) H-2 and (b) H-4.

Download figure to PowerPoint

image

Figure 10. Pb2+ adsorption capacities of regenerated hydrogel samples (a) H-2 and (b) H-4.

Download figure to PowerPoint

The Freundlich equation is basically an empirical equation, and this model is suitable for use with heterogeneous surfaces.([37, 45]) This isotherm model is the earliest known relationship describing the adsorption process.([46]) The adsorption isotherm data were correlated with the Freundlich equations, and the Freundlich constants Kf (mg/g) and n (intensity of adsorption) were calculated from the following equations([47-49]):

  • display math(7)
  • display math(8)

qe, the amount of Cu2+ and Pb2+ adsorbed onto NVP-IA hydrogels (mg g −1); Ce, adsorbate equilibrium concentration (mg L −1); ‘n’ values indicate the type of isotherm to be favourable (1 <  n < 10;([49, 50])). Kf and n can be determined from the linear plot of log qe and log Ce (Figure 7). The Freundlich isotherm parameters, Kf, n and R2 are given in Table 5. As seen from the table and figure, the Freundlich isotherm fits well with the experimental data (R2 = 0.9921–0.9929), and n values are higher than 1, which demonstrates beneficial adsorption.

The expression for the Langmuir model is given as follows:

  • display math(9)

The Langmuir constants q0 (mg g −1) and b (L mg −1) are related to the adsorption capacity and the energy of adsorption, respectively. Regression coefficients R2 were determined as 0.3086 and 0.3432 from the linear plot of Ce /qe versus Ce. The results showed that the Freundlich model exhibited a better fit to the equilibrium adsorption data of Cu2+ and Pb2+ onto the NVP-IA hydrogels than the Langmuir model.

Competitive Adsorption of Cu2+ and Pb2+

In this section, the competitive removal of Cu2+ and Pb2+ ions from aqueous solutions was described. In competitive ion removal studies, the concentration of each ion in aqueous solution was fixed to 4 mmol/L. The results for competitive removal of Cu2+, Pb2+ ions are shown in Figure 8. In the case of competitive adsorption, the adsorption capacity of the hydrogels is found to be Cu2+ >  Pb2+. Since Pb2+ ion has a bigger ion radius than Cu2+, diffusion of the Pb2+ ion into the hydrogel network is more difficult. As a result, copper ions are more easily adsorbed onto the crosslinked NVP-IA hydrogels.

Regeneration of Hydrogels

Regeneration of the selected hydrogels, which were used in competitive adsorption experiments, was achieved using 1 M HNO3 as regeneration agent. Regeneration of hydrogels is an important property because it determines the eligibility of the hydrogel for being able to be used again in adsorption experiments. Since NVP-IA hydrogels (H-2 and H-4) have high crosslink density, regeneration of the samples was relatively difficult. A diminution in the adsorption capacities of these hydrogels after regeneration period was observed in the range of 25–30% (Figures 9 and 10).

CONCLUSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. References

In this study, a series of new polymeric adsorbents was prepared using IA and NVP as monomers by free radical polymerisation in aqueous solution. Swelling properties and mechanical properties of these hydrogels were investigated. To evaluate the mechanical properties of the hydrogels, maximum deformation %, Ea and Fmax values were determined. Ea and Fmax values considerably increase with increasing the NMBA content of the hydrogels in the range of 1–4 wt.%. When the NMBA content increased, equilibrium swelling capacity of the hydrogels decreased because of increasing the crosslinking points of the copolymer chains.

In addition, we describe the removal of Cu2+ and Pb2+ ions from aqueous solutions using NVP-IA hydrogels for the first time. Formation of NVP-IA structure was clearly confirmed using data in the literature about FTIR analysis.([51]) Kinetic studies were made for the adsorption of metal ions, and metal ion adsorption capacities of hydrogels for Cu2+ and Pb2+ ions were determined as 2.1 and 0.6 mmol g −1, respectively. The adsorption capacities of hydrogels for Cu2+ are approximately 3.5 times higher than that of hydrogels for Pb2+. This may be attributed to bigger ion radius of Pb2+ ion compared with Cu2+. Adsorption processes of metal ions onto the NVP-IA hydrogels follow pseudo-second-order type adsorption kinetic. The straight lines in plots of t/qt versus t indicated good agreement of experimental data with the second-order kinetic model. High correlation coefficients (R2 = 0.9979–0.9999) were obtained when the second-order kinetic model was used. The equilibrium adsorption data have been evaluated using Freundlich and Langmuir Isotherm models. Linear plots of log qe versus log Ce for the different initial metal ion concentrations illustrated that the adsorption follows the Freundlich isotherm (R2 = 0.9921–0.9929).

References

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. References