Synthesis of Poly(N-methylol Methacrylamide/Vinyl Sulfonic Acid) Hydrogels for Heavy Metal Ion Removal

Abstract

In this study, poly(N-methylol methacrylamide) (NMMAAm) and poly(N-methylol methacrylamide/vinyl sulphonic acid) (NMMAAm-VSA) hydrogels were synthesized by $^{60}Co-{\gamma}$ ray irradiation at an ambient temperature. The graphs belonging to the gelation percent- percent-dose and swelling curves were drawn by using data which were obtained from water and different pH solutions. Characterization of hydrogels was performed by FTIR and DSC-TGA analysis. Heavy metal ion ($Ni^{2+}$, $Co^{2+}$) removal capacities of hydrogels were investigated in aqueous solutions, which had different concentrations (100-1500 mg/L). In metal ion removal studies, pH value of aqueous medium was kept constant at 5.0. Maximum metal ion removal values were obtained for NMMAAm-VSA (1:3 mole ratio) hydrogels. Metal ion removal capacities of NMMAAm-VSA (1:3 mole ratio) hydrogels were found as 82 mg/g and 98 mg/g for $Ni^{2+}$ and $Co^{2+}$ ions, respectively.

Introduction

Heavy metals pose a threat to the environment and living things. Some heavy metals are essential for the fulfilment of vital functions, but while the amount is exceeded, they display toxic effects. Accumulation of heavy metals in water and soil is come out from the industrial activities such as electronics, automotive, mining and metallurgical processes, etc. Nickel and cobalt ions placed in 4th period of periodic table have toxic effects on human beings. Cobalt is essential to all animals, including humans but repeated or long term contact to cobalt compounds may cause skin sensitization and exposure to repeated or long term inhalation may cause asthma. Some cobalt compounds may have effects on the heart, thyroid and bone marrow. Cobalt is used in preparation of magnetic materials, petrochemical and plastic industry, ceramic industry, detergent production industry etc..1 Nickel also has important roles in the biology of microorganisms and plants. The main ways of nickel intake for mammalian are ingestion, inhalation and absorption through the skin. Intake of overdose nickel may cause allergic reactions and kidney damage. Nickel particles as dust or fumes can be a carcinogen. Nickel is used in electroplating, transportation, chemical industry, electrical equipments etc..2 For these reasons, removal of heavy metals from water and soil is an important issue. Physical, chemical and biological processes are implemented to remove heavy metal ions.3,4 The removal of heavy metal ions from aqueous solutions by using polymers especially hydrogels has become a popular and widely studied research subject.5-9 Hydrogels are hydrophilic polymer networks including several cross-links, crystalline regions or entanglements, so hydrogels are insoluble materials. They swell in water more than 10-100 times of their own masses due to the presence of hydrophilic functional groups such as -OH, -COOH, -NH2, -CONH2, etc. along the polymer chains. Therefore, hydrogels are used as adsorbent materials to remove heavy metal ions from aqueous systems, as biomedical devices for controlled drug delivery, as artificial materials for treatment of some damaged organs, etc..3-10 Compared to conventional methods (solution polymerization, suspension polymerization, etc.) for preparation of hydrogels, the usage of ionizing radiation in hydrogel preparation has some advantages and these advantages are as follows; it provides easy process control, enables to combine hydrogel formation and sterilization in one technological stage, there is no need to add any initiators, cross-linkers, etc., which may be harmful and difficult to remove, there is no waste, and it provides relatively low running costs. All these advantages make irradiation the method of choice in the synthesis of hydrogels, especially for biomedical applications. 11

In this study, polymers having –SO3−, –NH and –OH groups functional groups playing a significant role in the metal adsorption process were preferred. Hydrophilic copolymers were synthesized by irradiation of binary mixtures of Nmethylol methacrylamide and vinyl sulphonic acid. Ni2+ and Co2+ removal capacities of copolymeric hydrogels from aqueous solutions were investigated by using a batch adsorption experiments.

 

Experimental

Chemicals. N-Methylol methacrylamide (NMMAAm) (60 wt % in H2O, 1.1 g/mL at 25 °C) was provided from Meryer Chemicals (China) and vinyl sulphonic acid (VSA) sodium salt (25 wt % in H2O, 1.176 g/mL at 25 °C) was provided from Aldrich (Milwaukee, WI), both reagents were used as they were purchased.

Synthesis of Hydrogels. 1:1, 1:2, and 1:3 NMMAAm: VSA mole ratios were used in the present study. NMMAAm was mixed with VSA at the ratios given above; placed into PVC straws; then exposed to irradiation at different doses by means of 60Co-γ source having a dose rate of 64.6 Gy/h (Gammacell 220) in air at room temperature. The reaction between the monomers was given below.

Determination of Gelation (%). After irradiation, hydrogels obtained in long cylindrical shapes were cut into pieces of 1–2 mm length. After dried in vacuum oven at 40 °C, irradiated mixtures were immersed into water to remove the un-reacted monomers and uncross-linked soluble fractions. Then hydrogels were dried in vacuum oven again. Percentage of their gelations to insoluble network was gravimetrically determined by using Eq. (1).11 The amount of sulfonic acid in monomer, polymer and/or copolymer form was determined by the titration of the solution containing residues which are unreacted materials against NaOH (0.05 M) to phenolphthalein end point. The percentage of gelation was calculated by using the following equation:

where, wo is the initial weight of dry gel and w is the weight of dry gel after standing in water.

Swelling Studies. Dried hydrogels following water extraction were accurately weighed and transferred into water and phosphate buffer solutions at different pHs (2-13).11 During all swelling experiments, ionic strength of buffer media was kept constant at I = 0.02 M as obtained from preliminary experiences. The higher the ionic strength becomes, the more negatively charged sulphonyl groups can be masked and as a result of that the swelling reduces. Swollen gels were periodically removed from the swelling medium, lightly blotted to remove excess water from their surface, and then reweighed at room temperature. The following equation12was used to calculate the percentage of swelling:

where wo is the weight of the dry gel before swelling and wt is the weight of the dry gel after swelling at time t.

Characterization of Hydrogels. In order to investigate structural and thermal characteristics, hydrogels were ground into small particles and then were pressed into pellets with KBr. The infrared spectra of samples were recorded in a Spectrum One FT-IR spectrometer, which is one of the Perkin Elmer Instruments. The thermal characterization was performed by using Shimadzu DTG-60H model thermal analyzer, in dynamic N2 atmosphere (100 mL/min) at a heating rate of 10 °C/min.

Removal of Metal Ions. The adsorption trials for Ni2+ and Co2+ (Me2+) were carried out with pre-determined pH (5.0, adjusted with phosphate buffer) and different concentrations of solutions of NiCl2 and CoCl2 (obtained from BDH-Poole, UK) to display the metal ion uptake behaviour of hydrogels. Plain NMMAAm and NMMAAm-VSA hydrogels with a weight of 0.01 g were added into 10 mL of Me2+ solutions, and then they were stirred at 25 ± 1 °C for a pre-determined period (2 days). Adsorption experiments were conducted for 2 days to ensure access to the functional groups that were placed in the inner part of hydrogels. Hydrogel particles were decanted by using centrifugation; clear solutions were analyzed for remaining metal ions by means of UV-Vis Spectrophotometer (Shimadzu 1601 model). The amount of metal ions adsorbed on hydrogels (mg metal ions/g dry hydrogel) was calculated by using Eq. (3).13

where qe is the amount of adsorbed metal ions per unit weight of hydrogel (mg/g); Co and Ce are initial and equilibrium concentration of the metal ions in the aqueous solution (mg/L), respectively; V is the volume of the aqueous solution (L) and m is the weight of the dry hydrogel (g).

 

Results and Discussion

Gelation (%). Gelation (%) values were calculated by using Eq. (1). Figure 1 has shown the gelation % curves of NMMAAM, NMMAAm-VSA (1:1 mole ratio), NMMAAm-VSA (1:2 mole ratio) and NMMAAm-VSA (1:3 mole ratio) gel systems. Maximum gelation (about 80%) reached approximately at 2 kGy dose. 2 kGy dose-irradiated samples were used for further experiments.

Figure 1.Gelation % - dose curves for NMMAAm and NMMAAm-VSA hydrogels.

As shown in Figure 1, the extent of gelation decreased with the increase of VSA content in the feed composition. This was probably caused by VSA, which acted as an effective chain transfer agent in the copolymerization of NMMAAm. As it is well-known,14 chain transfer occurs when a radical species reacts with a nonradical species called as chain transfer agents. After that, the current chain finishes. A new chain may start or not, which depends on new radical reactivity. In this study, the increase in the amount of VSA led copolymerization to finish in the beginning of copolymer composition, which was followed by the formation of chain transfer reactions.

Swelling (%) Properties of Hydrogels. Figure 2 has shown the dependence of swelling on the composition of the hydrogel systems. The highest swelling degree was obtained for NMMAAm-VSA hydrogels of 1:3 mole ratio. There was a substantial increase in the swelling degree of NMMAAm hydrogel by the incorporation of VSA resulting from the hydrophilic –SO3− groups. Increase of the amount of VSA decreased the conversion into polymer but increased the average molecular weight between the crosslinks and decreased the crosslink density, which resulted in higher swelling ratios.12 Swelling behaviour of hydrogels was investigated in solutions of different pH ranging from 2 to 13. The effect of pH on the degree of swelling of hydrogels was given in Figure 3. As it can be monitored from the figure, NMMAAm is a slightly ionic polymer which showed no considerable amount of swelling in the whole range of the investigated pH. However, VSA has a strong acidic character due to the presence of ionizable –SO3H groups in its structure, which caused higher swelling values. Two maxima have been observed in all swelling-pH curves obtained for NMMAAm-VSA hydrogel systems. Reasonable assumptions can be made to explain these two maxima. The pKa value of sulfonic acid is known from literature as −2 (showing very strong acidity).15 At pH 4, all –SO3Na groups are in form of –SO3−. In the first maximum observed around pH 4.5, when the all –SO3Na groups of VSA have released Na+ into the solution, the strong repulsion between –SO3− groups has led to increase in swelling. It is known that a high concentration of charged ionic groups in the hydrogel increases the swelling due to osmosis and charge repulsion.16 In the region of pH 5–8, –SO3− groups have formed potential Hbonded bridge complexes with –NH and −OH groups of NMMAAm through the presence of water molecules, which caused decrease in swelling. In the second maximum observed around pH 10, there were a great number of OH− groups in the solution. These OH− groups blocked the interactions between –SO3− and –NH and –OH groups of NMMAAm, then large amounts of disentangling –SO3− groups repelled each other through strong anionic interactions, so the swelling reached to higher values. When the pH value exceeds 10, the swelling ratio decreases due to the shielding effect of other ions, which prevents efficient anion–anion repulsion. Similarly at a high value of pH, the Na+ belonging to –SO3 groups and other cations have come out from phosphate buffer chemicals; therefore, this interaction has caused hydrogels to be swollen in less proportion.17,18

Figure 2.Swelling curves for NMMAAm and NMMAAm-VSA hydrogels in deionized water.

Figure 3.Effect of pH on the degree of swelling of hydrogels.

Characterization of Hydrogels. Fourier Transform Infrared Spectroscopy was used to confirm the structure of both NMMAAm and NMMAAm-VSA hydrogels. Results have been presented in Figure 4. In order to observe the changes in detail, spectra were given in the range of 2000-400 cm−1 and full range spectra were inserted into it. At a first glance, there are two important points to be remarked about these spectra: Firstly, there are no peaks appeared at 900-1000 cm−1 of the evidence of monomeric double bonds, so both components were considered to be successfully polymerized. Secondly, single broad band at 3330 cm−1 which was free –NH and –OH groups of NMMAAm shifted to 3430 cm−1 due to the formation of H-bonds between OH and NH groups by the contribution of –SO3− groups of VSA. A very broad band was centred at 3430 cm−1, which represents the distribution of the hydrogen bonded OH groups. The wide signal for the hydrogen-bonded OH groups has shifted to higher frequency− to 3423, 3459, 3466 and 3470 cm−1. These changes have derived from transformations of strong intra-molecular OH···OH, and SO3···OH into weak intermolecular OH···C=O bonds.19,20 The width of this band has increased with the increase of VSA content.18,19 The band at 2900 cm−1 has been responsible for main chain asymmetric – CH2 stretching vibrations. The C=O stretching vibration of NMMAAm was located at 1650 cm−1. From the copolymer spectra, the bands, which were located at 1170 cm−1, 1035 cm−1, 750 cm−1, 713 cm−1, and minor shoulder at 1270 cm−1 were attributed to symmetrical stretching mode of SO2, stretching mode of S=O, antisymmetrical and symmetrical stretching modes of C-S and antisymmetrical stretching mode of SO2, respectively.21 Based on this information, it is possible to say that copolymers were formed so as to include both NMMAAm and VSA components.

Figure 4.FT-IR spectra of homo- and co-polymeric hydrogels.

Thermal stability studies provide useful information on the selection of materials with the best properties for specific applications.22 In order to observe the thermal behaviour of the NMMAAm and NMMAAm-VSA hydrogels, they were ground into small pieces, and thermograms were taken in an N2 atmosphere. The thermal degradation behaviours of plain NMMAAm hydrogel and its copolymers with VSA were studied in the range of 50-600 °C. Figure 5(a) showed the two main stages of NMMAAm degradation with maximal decomposition rates at 319 °C and 442 °C. The first stage of decomposition occured mainly due to the rupture of the side chain. The major decomposition came out because of the main chain decomposition. TGA data on NMMAAm-VSA copolymers (with a mole ratio of 1:3) displayed again a twostage decomposition (Figure 5(b)). The first decomposition at maximum 310 °C may also be attributed to rupture of the side chain as NMMAAm. The second decomposition stage at maximum 399 °C occured due to the decomposition of copolymers NMMAAm-VSA. The prominent matter of this figure is that the maximum decomposition temperatures shifted to lower temperatures depend on incorporation of VSA, which has led to a more open pore structure of the copolymer. In addition, you could pay attention to a point where polymer stability increases further with the increase of VSA. This is because of the increase of hydrogen bond formation between the two components. The other important points about Figure 5(a) and (b) are the amounts of residues. After heating up to 600 °C, maximum residual weights were observed on polymers containing VSA. At 600 °C, NMMAAm was completely carbonized. On the other hand, VSA containing polymers have been more stable than NMMAAm itself due to the presence of the –SO3− groups which made more stable structures via H-bond formation. Moreover, the heating process up to 600 °C has not been sufficient for the removal of S-O compounds from the structures completely. All these statements have been acceptable for other two copolymers (with mole ratios of 1:1 and 1:2) and results were tabulated in Table 1.

Figure 5.TG-DTG curves for NMMAAm hydrogel (a) NMMAAm-VSA (1/3 mole ratio) hydrogels (b).

Table 1.Weight loss and corresponding decomposition temperatures for NMMAAm and different mole ratio of NMMAAm-VSA hydrogel systems

In order to improve interpretations made about thermal behaviour of both hydrogel systems, DSC thermograms were taken. As shown in the Figure 6, Tg values, at which the polymer starts to display a rubbery appearance, an increase came out with the increase of VSA content of co-polymeric hydrogel systems. As mentioned previously, this has been come out from H-bond formation between amide and sulphonyl groups of hydrogel. The Tg value of NMMAAm hydrogel was found at 125 °C. Incorporation of VSA to NMMAAm chains increased Tg into higher values. This could probably be caused by higher extent of H-bonding which depended on increasing amount of VSA in copolymeric hydrogel.

Figure 6.DSC thermograms of homo- and co-polymeric hydrogels.

Co2+ and Ni2+ Adsorption Behavior of Hydrogels. The adsorption of NMMAAm-VSA hydrogels was tested in the aqueous solutions containing cobalt and nickel ions. The heavy metal ions removal studies were carried out with predetermined constant pH of 5.0. As it is known, the metal salt solutions are acidic. Due to the increase of the basicity of the environment, metal ions are precipitated in the form of hydroxides. Conversely, in acidic pH region, nitrogen group of amide in copolymers are slightly protonated, which causes decrease in metal ion uptake. Also, pKa value of sulphonic acid is around −2.15 Therefore, at pH 5.0 value, all sulphonyl groups of copolymers are ionized forms so as to capture the metal ions easily. Another reason for choosing the pH value of 5 is that copolymers reach the highest swelling value at this pH. In order to evaluate the effect of initial metal ion concentration on adsorption characteristics of hydrogels, a series of experiments were conducted. Figure 7 and 8 show the adsorption behaviours of hydrogels in different initial concentrations of Co2+ and Ni2+. It can be seen that the amount of adsorbed Co2+ ions on NMMAAm gel increased with a slight increase of initial Co2+ concentration. The amount of adsorbed Co2+ ions reached 19 mg/g, which was a maximum value for NMMAAm hydrogel. As shown in the figure, the amount of Co2+ removed from aqueous solution increased with an increasing amount of VSA content in the hydrogel. The maximum amount of adsorbed metal ion on NMMAAm-VSA hydrogel (1:3 mole ratio) reached to a value of 98 mg Co2+/g gel in 1000 mg/L initial metal concentration. The amount of adsorbed Ni2+ on NMMAAm gel increased with the increase of initial Ni2+ concentration and this value reached to a maximum value around 64 mg/g. Like Co2+ ion, the amount of adsorbed Ni2+ ion increased with an increasing amount of VSA content in the hydrogel. NMMAAm-VSA hydrogel (1:3 mole ratio) has had maximum adsorption capacity, which reached to 82 mg/g in 1000 mg/L initial metal concentration. A remarkable thing at this point is that the amount of adsorbed Ni2+ ions on the NMMAAm hydrogel is not too different from the values of copolymers. Maximum adsorption capacity is nearly equal to other three type hydrogels that have a composition ratio of 1:1, 1:2, 1:3. The results of batch experiments showed that NMMAAm-VSA copolymers were more effective than NMMAAm hydrogels to remove Co2+ and Ni2+ ions from aqueous media. The reason why there is a differentiation between them is that homopolymer contained –OH and –NH groups, copolymers also contained –SO3− groups in addition to these two groups. Thus, the interactions between metal ions and –SO3− groups increased and more metal ions were removed by copolymer from aqueous solutions. Similar results were observed in the Ozay’s et al. study.23 Generally, it was observed that nearly all hydrogels were more effective in removal of Co2+ ions than removal of Ni2+ ions at the whole range of the studied concentration and certain temperature. The main reason is that this comes from the periodical properties of both Co2+ and Ni2+. It is known that the ionic radius of Ni2+ ions is larger than that of Co2+ ions.24 When the complex formation between polymeric ligands and metal ions has been discussed, it was concluded that metal ions with smaller radii interacted more easily with polymeric ligands. On the other hand, the Ni2+ ion has more electronegativity than the Co2+ ions, which tends to have less interaction with –SO3− groups. In addition to difference in their sizes, degree of hydration and the value of the binding constant with the adsorbent can also be regarded as the reason for differences of the amount of the adsorbed metal ions.25,26 It is possible to find various studies in the literature for removal of Co2+ and Ni2+ ions from aqueous systems by using different adsorbents.27-33 Adsorption results using different adsorbents for removal of Co2+ and Ni2+ have been listed in Table 3. As a result, it can be easily seen that the maximum amount of adsorbed Co2+ and Ni2+ was relatively higher than most of the study in the literature.

Figure 7.Adsorption isotherms for Co2+ (m = 0.01 g gel, V = 10 mL, pH 5.0).

Figure 8.Adsorption isotherms for Ni2+ (m = 0.01 g gel, V = 10 mL, pH 5.0).

Table 2.− R2 values are smaller than 0.96.

Table 3.ainitial ion concentration: 500 mg/L, approximately 0.2 g dry absorbents. bclay 2 g/L; initial metal ion concentrations 50 mg/L; pH: Co2+ 5.8, Ni2+ 5.7; 303 K. ctemperature 25 °C, initial solution concentration 2.5 g/L, pH 5.0. dinitial concentration of 10 mg/L were 2 g/L of dose, pH 3, 50 rpm of agitation speed and 4 h of contact time. epH 6.0; adsorption time: 48 h; amount of adsorbent: 0.02 g; initial ion concentration: 20.0 mM. f,gadsorption time: 60 min; amount of adsorbent: 0.1 g; initial ion concentration: 9.64 × 10−2 mmol/dm3. hpH 5.0, amount of adsorbent: 0.03 g, adsorption time: 8 h. *ethylenediaminetetraacetic acid. **diethylenetriaminepentaacetic acid

Another remarkable thing about Figure 7 and Figure 8 is the shape of curves. The adsorption isotherms of Co2+ and Ni2+ on hydrogels correspond to Langmuir type according to the Giles classification.34 In Langmuir type isotherm, the initial portion provides information about the availability of the active sites to the adsorbate and the surface signifies the monolayer formation. In addition, it is used to describe chemical adsorption on a set of separate localized adsorption sites. Monolayer adsorption occurs on the hydrogel’s surface during the adsorption process. This result has also been supported with evaluation of the data according to the Langmuir isotherm equation (Table 2). After R2 values in the Table 2 were investigated, it was observed that the results reached nearly 1.00 R2. Langmuir isotherm was observed in adsorption of the metal ions from solution.35 As for the mechanism of adsorption; at pH 5.0 value, sulphonyl groups get ionized, and nitrogen group of amide structure in hydrogel is not positively charged, but has un-pair electrons. Because of the reasons mentioned above, we used acidic desorption agents (HCl, HNO3, etc.) to reuse hydrogels, and desorption of the adsorbed all metal ions was assigned by using UV-Vis measurements. Results have not been included in the article. In the light of all these explanations, it is possible to say that adsorption mechanism has been formed with electrostatic interactions between metal ions and –SO3−, -NH groups of hydrogel, and the adsorption has come out physically from Langmuir plots. Finally, it was offered by this study that the main mechanism could be as follows:

 

Conclusion

In this study, NMMAAm and NMMAAm-VSA hydrogels were prepared by irradiation in air at ambient temperature in a 60Co-γ source. Gel fraction values decreased with the increase of VSA content but swelling behaviour of hydrogels, in deionized water and at different pHs, increased with increase of VSA content. A significant change in swelling has been observed for all co-polymeric hydrogels at pH range of 2.0-13.0 due to the different interactions between functional groups. Two maxima have been obtained from swelling studies, one is at pH 4 and the other is at pH 11. Both homo- and co-polymeric hydrogels were used for removal of Co2+ and Ni2+ from aqueous solutions. The results of batch experiments showed that NMMAAm-VSA copolymers have been more efficient than NMMAAm hydrogels for removal of Co2+ and Ni2+ ions from aqueous systems. As a result of this study, it can be stated that NMMAAm-VSA hydrogels can be used for removal of heavy metal ions from waste water as an adsorbent due to their high sorption capacity.