radiation induced hydrogels: acrylamide/itaconic … induced hydrogels: acrylamide/itaconic acid and...

Radiation Induced Hydrogels: Acrylamide/Itaconic Acid and Acrylamide/Maleic Acid Copolymers for Adsorption of Heavy Metal ions, Dyes and Biomolecules

Dursun Saraydın1 and Erdener Karadağ2 1Department of Chemistry, Cumhuriyet University, 58140 Sivas, Turkey 2Department of Chemistry, Adnan Menderes University, 09010 Aydın, Turkey

Acrylamide based copolymeric networks such as acrylamide/maleic acid (AAm/MA) and acrylamide/itaconic acid (AAm/IA) hydrogels prepared by irradiating with γ radiating as known clear method. Spectroscopic, thermal and mechanical characterizations of the copolymers were examined.

Adsorptions of water, some heavy metal ions such as uranyl, cupric, ferric ions, some cationic dyes such as basic red 9, basic green 4, basic blue 20 and crezyl violet, and biomolecule such as nicotine by acrylamide-itaconic acid and acrylamide– maleic acid hydrogels have been investigated.

Acrylamide hydrogel had not sorbed any molecules or ions from the solutions and acrylamide/maleic acid (AAm/MA) and acrylamide/itaconic acid (AAm/IA) hydrogels had sorbed the chemical species. It can be concluded that AAm/MA and AAm/IA hydrogels can be used as water retainer for carrying some substances in aquatic fields in biomedical, bioengineering, pharmacy, agriculture and environmental applications.

Keywords radiation; hydrogel; acrylamide; dye; ion; biomolecule; adsorption

1. Inroduction

A hydrogel is defined as cross-linked hydrophilic polymeric materials which exhibit the ability to swell in water without dissolving or losing their structural integrity. The hydrogels swell or shrink in aqueous solution due to association, dissociation and binding of various ions in polymer chains. Their ability to absorb water is because of its crosslinking network structure which is formed by polymer bearing hydrophilic groups such as –OH, –CONH2, –CONH –COOH, –SO3H, and –NH2. The networks are composed of homopolymers or copolymers [1-7]. After intimate contact is established, the rate and duration of the ion/molecule adsorption depends on the swelling behavior of the hydrogel [1-7]. Because of the presence of carboxylic acid side groups, the swelling behavior of copolymeric acrylamide hydrogels is highly dependent on the surrounding medium. Hydrogels can be prepared by simultaneous copolymerization and crosslinking of one or more monofunctional and one multifunctional monomer or by crosslinking of a homopolymer or copolymer in solution. Hydrogels are synthesized using either chemical reagents or irradiation. In recent years, considerable research has been done on the characterization and swelling behavior of hydrogels prepared by simultaneous free-radical copolymerization and crosslinking in the presence of an initiator and a crosslinking agent. Hydrogels can also be synthesized by crosslinking via γ-irradiation [1-3]. However, little work is done on the characterization of hydrogels prepared by crosslinking of a homopolymer or copolymer in solution with γ-irradiation [1-3]. It is well known that the presence of an initiator and a crosslinking agent affects the macromolecular structure and phase behavior of hydrophilic polymers in solution and contributes to inhomogenity of the network structure. It is argued that more homogeneous network structures can be synthesized, if crosslinking is accomplished with γ-irradiation in the absence of an initiator and a crosslinking agent. The structural homogenity of the network affects the swelling behavior and mechanical properties. Hydrogels have been widespread applications in the fields of bioengineering, biomedicine, pharmaceutical, veterinary, food industry, agriculture, photographic technology and others. It is used as controlled release systems of drugs, for production of contact lenses and artificial organs in biomedicine, as an adsorbent for removal of some agent in environmental applications, immobilized enzyme kinetics in bioengineering and also as a carrier of water, pesticides and fertilizer in agriculture field [1-7]. Responsive behavior of hydrogels makes them also very attractive materials for some specific applications in adsorption, enrichment and separation processes. Sites showing selectivity for proteins, enzymes, biomolecules or dyes, pigments or metal ions can be easily incorporated into network structure by radiation induced polymerization. During the last decade a number of papers published from this laboratory showed clearly the advantages of irradiating aqueous monomer solutions to synthesize copolymeric hydrogels. In this context the use of even very small quantities of diprotic acid-containing monomers proved to impart fascinating properties to the hydrogels of starting monomers homopolymers [1-7].

Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.) _______________________________________________________________________________________________

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2. Hydrogel Preparation and Characterization

2.1 Monomers

In this study, the use of even very small quantities of diprotic acid-containing monomers such as maleic acid and itaconic acid proved to impart fascinating properties to the acrylamide hydrogels of starting monomers homopolymers [8, 9]. Some properties of monomer and comonomers are shown Table 1. Table 1 Some properties of monomer and comonomers

Monomer Symbol Chemical structure Molar mass (g mol–1) Manufacturer

Acrylamide AAm

116.07 B.D.H. (Poole-UK)

Maleic acid MA 71,08 B.D.H. (Poole-UK)

Itaconic acid IA

130.10 B.D.H. (Poole-UK)

2.2 Hydrogel Preparation by Radiation Technique

For the preparation of acrylamide, acrylamide/maleic acid and acrylamide/itaconic acid hydrogels, ionizing radiation processing is used [1-3, 8, 9]. Aqueous solutions of monomers of 1 g AAm and 20, 30, 40, 50 and 60 mg MA or IA were prepared in 1mL of distilled water. Monomer solutions thus prepared were placed in the PVC straws and irradiated to 2.60, 3.73, 4.65, 5.20 and 5.71 kGy in air at ambient temperature in a γ−Cell 220-type γ−irradiator at a fixed dose rate of 0.72 kGy h−1.

Scheme 1 Possible copolymerization and crosslinking reaction mechanisms between acrylamide and itaconic acid monomers.


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Ionizing radiation such as 60Co γ−radiation is very useful in producing polymers from monomeric units and in modifying the properties of pre-existing polymers. Ionizing radiation provides a very clean method for the production and modification of polymers. No chemicals or catalysts have to be added to the reaction matrix. The polymerization is achieved by free radicals (occasionally ions) created in the material at the end of process. Therefore, no chemicals or catalysts remain in the material after radiation [1-7]. When acrylamide and itaconic acid or maleic acid are irradiated with ionizing radiation such as γ-rays in aqueous solutions, free radicals are generated. Random reactions of these radicals with the monomers lead to the formation of copolymers of acrylamide/itaconic acid or acrylamide/maleic acid. When the irradiation dose is increased beyond a certain value the polymer chains crosslink and then a gel is obtained. Possible copolymerization and crosslinking reaction mechanisms between acrylamide and itaconic acid or maleic acid comonomers are shown in Scheme 1 and 2.

Scheme 2 Possible copolymerization and crosslinking reaction mechanisms between acrylamide and maleic acid monomers.

It has been reported that complete gelation of acrylamide occurs at irradiation doses of 2.00 kGy of γ−rays at ambient temperature [8]. Hence, a γ−irradiation dose of 2.00 kGy is based for the preparation of the hydrogels. The radiation technique is used as a sterilization method in many applications; during polymerization and crosslinking reactions, all monomers are reacted at the applied γ−irradiation. This process is used for simultaneous sterilization of hydrogel systems. It can be stated that there is no monomer (such as toxic acrylamide) at the end of the copolymerization and crosslinking reaction between acrylamide and maleic acid or itaconic acid, because 2.00 kGy is a sufficient dose for gelation [9]. The dried copolymers described here are glassy and very hard, but the swollen gels are very soft. The crosslinked copolymers were obtained in the form of cylinders. Upon swelling the hydrogels retained their shapes.

2.3. Spectroscopic Analysis

For the understanding of binding and crosslinking of the samples during the polymerization, FTIR spectra of the dried crosslinked copolymers are evaluated. FTIR spectra of AAm hydrogel and the copolymers of AAm/IA and AAm/MA [8, 9] are presented in Fig 1. In the spectra, the bands at about 1700 cm–1 and 3200– 3600 cm–1 are important. The bands at about 1700 cm–1 could be attributed to the shift in the stretching vibration associated with hydrogen that is bonded directly to a carbonyl carbon atom. The peak at 1650 cm–1 indicates the carbonyl group of the amide functions. Absorption peaks in the region of 3200 and 3600 cm–1 are attributed to –OH and –NH bands. The peak at 3600 cm–1 is characteristic of a primary amine, the 1550 cm–1 signal is attributed to an –OH band related to a carbonyl group, and peaks at 1725–1700 cm–1 characterize –C=O groups in carboxylic acids. It is stated that the peak at 1200 cm–1 is related to a –C–N– band and the peak at 1400 cm–1 is related to –CH2– groups on the chains [8]. It can be seen from Fig 1 that polymerization of the hydrogels took place involving the double bonds of acrylamide and itaconic acid, because –NH– bonds are found in large amounts in the crosslinked structure. Specially, the peak at about 3200–3600 cm–1 (–NH–) is important for the examination of binding and crosslinking. Furthermore, the bands at about 1700 and 3200–3600 cm–1 increased gradually with the increase of the content of IA in the hydrogels. From


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Figure 1 it can be said that there is random copolymerization. For the investigation of this property, commercial PAAm and IA were irradiated together, but no gelation was found at the end of irradiation.

Tra

nsm

itta

nce

/ %

Tra

nsm

itta

nce

/ %

I wave number / cm–1 II wave number / cm–1

Fig. 1 FTIR spectra of AAm/IA (I) and AAm/MA (II) hydrogels. 1; 0 mg, 2; 20 mg, 3; 40 mg, 4; 60 mg of comonomer in hydrogels. Total dose given was 520 kGy. It can be seen from Fig 1 that polymerization of the hydrogels took place involving the double bonds of acrylamide and itaconic acid, because –NH– bonds are found in large amounts in the crosslinked structure. Specially, the peak at about 3200–3600 cm–1 (–NH–) is important for the examination of binding and crosslinking. Furthermore, the bands at about 1700 and 3200–3600 cm–1 increased gradually with the increase of the content of IA in the hydrogels. From Figure 1 it can be said that there is random copolymerization. For the investigation of this property, commercial PAAm and IA were irradiated together, but no gelation was found at the end of irradiation. It is said that random copolymerization of AAm occurred with IA in the presence of ionizing radiation and not any graft copolymerization of IA in the crosslinked copolymeric system was observed. The same evaluations can be made for AAm/MA hydrogels.

2.4 Thermal Analysis

For the examination of the thermal properties of the dried crosslinked copolymers of AAm/IA and AAm/MA it was investigated whether the glass transition temperature (Tg) of the copolymers is changed after crosslinking has occurred. DSC thermograms of the dried crosslinked copolymers are taken [8, 9] and Tg values of AAm/IA and AAm/MA hydrogels were given Table 2.

Table 2 The values of glass transition temperature of AAm/IA and AAm/MA hydrogels

Comonomer content mg

Tg of AAm/MA oC

Tg of AAm/IA oC

Irradiaton Dose kGy

Tg of AAm/MA oC

Tg of AAm/IA oC

0 195 195 2,00 173 164 20 172 176 4.65 178 160 40 178 159 5.71 183 171 60 179 163

The glass transition temperature of crosslinked PAAm is 195 oC (Table 2), while Tg commercial PAAm is 164 0C. [8] It is seen that the glass transition temperature of the crosslinked PAAm is higher than that of commercial PAAm. The reason of this increase is crosslinking of the monomers [8, 9]. In Table 2, it is seen that Tg of AAm/IA and AAm/MA copolymer samples irradiated at 5.71 kGy is about 171 0C and 183 oC, respectively. It is said that if the γ−ray dose is increased, crosslinking of the monomers is more than and Tg is bigger than in the previous state. From Table 2, it can be seen that Tg is very high for the crosslinked AAm, but this temperature for hydrogel samples has been decreased with the adding of diprotic acid comonomers. The reason for this decrease of the glass transition temperature may be due to the plasticizer effect of itaconic acid and maleic acid [8, 9]. Then, the glass transition temperature has increased with the comonomer content in the crosslinked copolymers. The interactions of carboxyl groups in itaconic acid have caused this increase of the glass transition temperature. If the IA or MA content in the copolymers is increased, the number-average molar mass between the crosslinks of the hydrogels has increased and the crosslinking density of the hydrogels has decreased. Translating into the elastic state, if crosslinking density has increased, the glass transition temperature is higher than in the previous state [8, 9].


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2.5 Mechanical characterization

To find the mechanical stability of fresh AAm/IA or AAm /MA hydrogels, mechanical tests were applied to 1 cm length of the hydrogels. At the end of the mechanical test, some parameters such as strain percentage at break, stress at break, Young’s modulus, energy to break and tensile energy absorption were found [8, 9] and the parameters are listed in Table 3.

Table 3 Some parameters obtained from mechanical test of the hydrogels*

Comonomer content /mg

Strain at break %

Stress at break kPa

Young’s modulus kPa

Energy to break point

J

Tensile Energy absorption / N mm-

1

IA

0 728 382 262 0.16 180 20 1276 409 289 0.31 351 40 1567 491 268 0.38 430 60 1313 388 271 0.30 338

MA

0 506 418 401 0.11 127 20 1649 486 331 0.44 495 40 2002 523 342 0.56 628 60 2392 579 304 0.67 758

*Total dose given was 3.73 kGy.

Table 3 shows that the mechanical properties of the hydrogels increase of comonomer content and irradiation doses (some deviations were in the values of mechanical properties of AAm/IA hydrogels). So, the mechanical stabilities of the AAm/IA and AAm/MA hydrogels are increased in regard to the mechanical stability of AAm hydrogel [8, 9].

2.6. Swelling Studies

A fundamental relationship exists between the swelling of a polymer in a solvent and the nature of the polymer and the solvent, respectively. The swelling is the most important parameter about swelling studies [8, 9]. The swelling, S %, is calculated following equation:

% 100

where m0 is the mass of the dry gel at time 0, mt is the mass of the swollen gel at time t. The water uptake of initially dry hydrogels was followed for a long time. Swelling curves of AAm/MA hydrogels are shown for a representative graphs in Fig. 2.

Sw

ellin

g / %

Sw

ellin

g / %

time / minute Irradiation dose / kGy

Fig. 2 Combined effect of water and maleic acid on the swelling of AAm/MA hydrogels.

Total dose given was 2.00 kGy:

−○− 0 mg MA; −■− 20 mg MA; −Δ− 30 mg MA; −●− 40 mg MA; −□− 50 mg MA; −▲− 60 mg MA.

Fig. 3 Variation of swelling in hydrogels with the irradiation dose.

−○− 0 mg MA; −■− 20 mg MA; −Δ− 30 mg MA; −●− 40 mg MA; −□− 50 mg MA; −▲− 60 mg MA.

In Fig. 2 it is shown that swelling is increased by time, but after a certain time it remains constant. This value of swelling may be named “equilibrium swelling”. The values of equilibrium swelling of the all hydrogels are used for the


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calculation of some characterization parameters. It is shown that the value of equilibrium swelling of AAm hydrogel is lower than AAm/IA and AAm/MA hydrogel in all conditions. The hydrophilic group number of copolymeric hydrogels is higher than the hydrophilic group number of the homopolymeric acrylamide hydrogels. Thus, AAm/IA and AAm/MA hydrogels have swelled more than AAm hydrogels. For a better understanding of the effect of the IA or MA content in the hydrogels and γ−irradiation doses, the equilibrium swelling of the hydrogels is plotted versus the content of IA, MA and γ−irradiation doses [8, 9]. A representative plot presented in Fig.3. It is shown in these figures that the equilibrium swelling of the superabsorbent hydrogels is decreased with the increase of irradiation dose and is increased with the increase of the IA or MA content in the copolymers. On the other hand, as mentioned before, the equilibrium swelling of AAm/IA or AAm/MA hydrogels is higher than the equilibrium swelling of acrylamide hydrogels. The reason for this is the hydrophilic groups on the itaconic acid or maleic acid which favor the swelling of the AAm/IA or the AAm/MA hydrogels. If the γ−irradiation dose is increased during ionizing radiation of acrylamide, itaconic acid (or maleic acid) and water ternary mixtures, the number of the small chains increases at unit copolymerization time, and the crosslink density of the hydrogels is higher than the lower c-irradiation doses. At the same time, the number-average molar mass of the polymer between crosslinks, is smaller than the lower γ−irradiation doses.

2.7. Diffusion

When a glassy hydrogel is brought into contact with water, water diffuses into the hydrogel and the hydrogel swells. Diffusion involves migration of water into preexisting or dynamically formed spaces between hydrogel chains. Swelling of the hydrogel involves larger scale segmental motion resulting, ultimately, in an increased distance of separation between hydrogel chains [8, 9]. Analysis of the mechanisms of water diffusion in swellable polymeric systems has received considerable attention in recent years because of important applications of swellable polymers in biomedical, pharmaceutical, environmental, and agricultural engineering. The following equation is used to determine the nature of water diffusion into hydrogels.

F = ktn

Here, F is the fractional uptake, k is a constant incorporating characteristic of the macromolecular network system and the penetrant, and n is the diffusional exponent, which is indicative of the transport mechanism. This equation is valid for the first 60% of the normalized solvent uptake. Fickian diffusion and Case II transport are defined by n equal to and n equal to 1, respectively. Anomalous transport behavior (non-Fickian diffusion) is intermediate between Fickian and Case II, this is reflected by the fact that anomalous behavior is defined by n values between 1/2 and 1[8, 9]. ln F – t graphs are plotted for all hydrogels [8, 9].. The exponents are calculated from the slope of the lines and are listed in Table 4.

Table 4 The values of n the hydrogels

Dose / kGy → 2.00 2.60 3.73 4.65 5.20 5.71 Comonomer content / mg ↓

IA MA IA MA IA MA IA MA IA MA IA MA

0 0.61 0.62 0.64 0.64 0.59 0.59 0.53 0.53 0.53 0.53 0.53 0.53 20 0.57 0.70 0.58 0.65 0.62 0.63 0.61 0.61 0.54 0.71 0.73 0.74 30 0.51 0.74 0.72 0.64 0.61 0.69 0.71 0.71 0.60 0.74 0.61 0.64 40 0.56 0.64 0.65 0.66 0.64 0.66 0.56 0.56 0.59 0.68 0.57 0.81 50 0.61 0.74 0.69 0.74 0.50 0.69 0.56 0.56 0.55 0.74 0.75 0.76 60 0.64 0.71 0.64 0.62 0.56 0.74 0.53 0.53 0.44 0.76 0.57 0.63

It can be seen that the number to determine the type of diffusion (n) is found over 0.50. Hence, the diffusion of water into the superabsorbent hydrogels has generally taken a non-Fickian character [8, 9]. When the diffusion type is of anomalous behavior, the relaxation and diffusion time are of the same order of magnitude. As the solvent diffuses into the hydrogel, rearrangement of the chains does not occur immediately.

3. Adsorption of chemical species

3.1. Adsorption of heavy metal ions

Water pollution due to the disposal of heavy metals continues to be a great concern worldwide. Consequently, the treatment of polluted industrial wastewater remains a topic of global concern since wastewater collected from municipalities, communities and industries must ultimately be returned to receiving waters or to the land. Heavy metals pollution occurs in much industrial wastewater such as that produced by metal plating facilities, mining operations, battery manufacturing processes, the production of paints and pigments, and the ceramic and glass industries. Whenever


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toxic heavy metals are exposed to the natural eco-system, accumulation of metal ions in human bodies will occur through either direct intake or food chains. Therefore, heavy metals should be prevented from reaching the natural environment [10]. In this stage, for adsorption of heavy metal ions such as UO2

2+, Cu2+, Fe3+ onto AAm/IA or AAm/MA hydrogels, the solutions of uranyl acetate (UA), uranyl nitrate (UN), copper acetate and iron (III) nitrate were used. The hydrogels were transferred into the solutions and allowed to the equilibrium for 24 hours at 25 OC. Suitable complexing agents (for uranyl ions, potassium hexacyanoferrate (II), for cubric ions, ammonium hydroxide) were added to the final solutions. Spectrophotometric measurements were carried out using a UV-VIS spectrophotometer at ambient temperature. The absorbencies of colored solutions were read at the suitable wavelengths for uranyl complex at 525 nm, copper complex at 620 nm and iron ions at 400 nm. The equilibrium concentrations of heavy metal ions solutions were determined by means of precalibrated scales. Hydrogels separated from the solutions of heavy metal ions were left for 3 days in the distilled water at 25 oC to investigate their desorptions.

Scheme 3 The possible interactions between heavy metal ions (M) and AAm/MA hydrogels. Many studies have been published on the polyelectrolyte behaviors of copolymers of diprotic acid containing comonomers in aqueous solution. Several characteristic properties has been observed: conformational transition, a two-step dissociation process of dicarboxylate groups, binding of counterions, etc. These properties of diprotic acid copolymers are attributed to two factors: I. hydrophobic interaction of nonpolar side chain, and II. short-range electrostatic interaction of a pair of carboxylate groups [11, 12].

Q

/ m

g g−

1

Fig 4. The variation of adsorption of heavy metal ions onto AAm/IA (■) and AAm/MA (□) hydrogels containing 60 mg comonomer. Total dose given 2.60 kGy.


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In order to observe the adsorption of some heavy metal ions, AAm, AAm/IA and AAm/MA hydrogels were transferred into aquatic solutions of uranyl acetate, uranyl nitrate, copper acetate, iron (III) nitrate, cobalt nitrate, nickel chloride, and chrome nitrate, and allowed to stand for two days. At the end of this time, AAm/IA and AAm/MA hydrogels in the solution of uranyl acetate, uranyl nitrate, copper acetate, iron (III) nitrate showed dark coloration of the solutions, while AAm hydrogel had not sorbed any heavy metal ions from their solutions. Since polyacrylamide is a nonionic polymer, ionizable groups on the polymer were increased by the addition of itaconic acid or maleic acid to acrylamide monomer. Therefore, these hydrogels have many carboxylic groups which can be cause an increase of interaction between heavy metal ions such as uranyl, cupric and ferric ions, and the carboxylic groups in the hydrogels [11, 12]. The possible interactions between heavy metal ions (M) are shown in Scheme 3. The mass of adsorbate per unit mass of copolymer adsorbent (Q) were calculated for all systems [11, 12] and are presented in Figure 4. Hydrogels taken out of the solutions of uranyl ions were allowed to stand for 3 days in distilled water, and the desorption of uranyl ions were shown by the light yellowish coloration of the water and return of hydrogels to their original colors. It can be said that is adsorption of uranyl ions is physical adsorption.

3.2 Adsorption of dyes

Effluent from the dyeing industry contain highly colored species; such highly colored wastes are not only aesthetically displeasing but also hinder light penetration and may in consequence disturb biological processes in water-bodies. In addition, dyes are toxic to some organisms and hence harmful to aquatic animals. Furthermore, the expanded uses of azo dyes have shown that some of them and their reaction products such as aromatic amines are highly carcinogenic. Therefore, removal of dyes before disposal of the wastewater is necessary [13]. The present mini-review reports a study of a convenient method for removing some cationic dyes from water by adsorption onto AAm/IA and AAm/MA hydrogels. Water-soluble cationic triphenyl arin dyes such as Basic Red 9 (BR 9), Basic Green 4 (BG 4), Basic Blue 20 (BB 20) and an oxazin dye such as Crezyl Violet (CV) resemble the large molecular dyes found in waste waters.


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Table 5 Some Properties of Dyes

Name Chemical Sctructure Molar mass

Color Index Number

λmax /nm

Basic Blue 20 BB20 458,48 42590 630

Basic Red 9 BB9 305.38 42 500 544

BasicGreen 4 BG4 364.91 42 000 617

Cresyl Viyolet CV 339.82 51 180 596

In this stage, the synthetic aqueous solutions of cationic dyes were prepared in the concentration ranges; 4-20 mg L−1 for BR 9, BG 4 and CV, and 10-50 mg L−1 for BB 20. 0.1 g of AAm/IA hydrogel or AAm/MA hydrogel containing 60 mg comonomer and irradiated to 5.71 kGy were transferred into 50 mL of the synthetic aqueous solutions of the dyes, and allowed the equilibrate for 24 hours at 25 oC These solutions were separated by decantation from the hydrogels. Spectrophotometric measurements were carried out using a Shimadzu 160 A model UV-VIS spectrophotometer at ambient temperature. The absorbances of these solutions were read at the wavelengths given in Table 5. Hydrogels separated from the solutions were left for 3 days in the distilled water at 25 oC to investigate their desorptions. To observe uptake of some dyes, AAm, AAm/IA and AAm/MA hydrogels were placed in aqueous solutions of cationic dyes such as BR 9, BG 4, CV, and BB 20, and the aqueous solutions of anionic dyes such as alizarin yellow R, congo red, indigo blue, eosin yellowish, bromocresol purple, naphtol green and evans blue, and allowed to equilibrate for 2 days. At the end of this time, AAm/IA and AAm/MA hydrogels in the aqueous solutions of BR 9, BG 4, CV, and BB 20 showed the dark colorations of the original solutions. AAm hydrogel had not sorbed any dyes from the solutions, and AAm/IA or AAm/MA hydrogels had not sorbed the anionic dyes. Since poly(acrylamide) is a nonionic polymer, ionizable groups on the polymer were increased by the addition of acidic comonomers to acrylamide monomer [14-19]. To observe uptake of some dyes, AAm, AAm/IA and AAm/MA hydrogels were placed in aqueous solutions of cationic dyes such as BR 9, BG 4, CV, and BB 20, and the aqueous solutions of anionic dyes such as alizarin yellow R, congo red, indigo blue, eosin yellowish, bromocresol purple, naftol green and evans blue, and allowed to equilibrate for 2 days. At the end of this time, AAm/IA and AAm/MA hydrogels in the aqueous solutions of BR 9, BG 4, CV, and BB 20 showed the dark colorations of the original solutions.


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Scheme 4 The possible interactions between dyes (D) and AAm/IA hydrogels. AAm hydrogel had not sorbed any dyes from the solutions, and AAm/IA or AAm/MA hydrogels had not sorbed the anionic dyes. Since poly(acrylamide) is a nonionic polymer, ionizable groups on the polymer were increased by the addition of acidic comonomers to acrylamide. Therefore these hydrogels have many carboxyl groups that can increase interaction between the cationic groups of cationic dyes and carboxyl groups of hydrogels. On the other hand, there will be anionic repulsion between anionic groups of anionic dyes and carboxyl group of MA or IA in the hydrogels and therefore little interaction between the anionic dyes and AAm/IA or AAm/MA hydrogels [14-19]. The other types of interaction between gel and dyes may be hydrophobic and hydrogen bond. Hydrophobic effects are specifically aqueous solutions interactions, which in the present case will involve the aromatic rings and the methyl and methine groups on the dyes molecules and the methine groups on the gel. Hydrogen bond will be expected to occur between oxygen atom on the dye molecules and the amine and carbonyl groups on the monomer unit of crosslinked copolymer. But, electrostatic interactions between dye molecules and the hydrogel is very dominant due to hydrophobic and hydrogen bonds (Scheme 4). As it is said before, AAm hydrogel had not sorbed any dye molecules while AAm/IA or AAm/MA hydrogels sorbed the dyes. These cationic dye solutions were used in the experiments of adsorption of dyes onto AAm/IA or AAm/MA hydrogels. In a batch adsorption system at equilibrium, total solute concentration (CB mol L−1) is

where, CB is the equilibrium concentration of the solute on the adsorbent in mol L−1 (bound solute concentration) and C is the equilibrium concentration of the solute in the solution in mol L−1 (free solute concentration). The value of the bound concentration may be obtained by differing the use of this equation. For a fixed free solute concentration, CB, is proportional to the polymer concentration on the binding system; the amount bound can therefore be conveniently expressed as the binding ratio, r, defined by

Thus with CB in mol L−1 and P in base mol (moles of monomer units) L−1, r then represents the average number of molecules of solute bound each monomer unit at that free solute concentration [9-14]. Plots of the binding ratio (r) against the free concentrations of the dyes in the solutions (C) are shown in Figure 5 for AAm/MA hydrogels.


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r

x 10

3

C / μmol dye

Fig. 5 Adsorption isotherms of dye-AAm/MA hydrogel systems,

−●− BR 9; −○− BG 4; −■− CV; −□− BB 20.

Adsorption isotherms show that adsorptions of the dyes within AAm/IA and AAm/MA hydrogels correspond to type L (Langmuir type) adsorption isotherms in the Giles classification system for adsorption of a solute from its solution. In this type of adsorption isotherm, the initial curvature shows that as more sites in the substrate are filled it becomes increasingly difficult for a bombarding solute molecule to find a vacant site available. This implies either that the adsorbed solute molecule is not vertically oriented or that there is no strong competition from the solvent [14-19]. The types of systems which give this curve do in fact fulfil these conditions. Thus they have one of the following characteristic: (i) the adsorbed molecules are most likely to be adsorbed flat or (ii) if adsorbed end-on, they suffer little solvent competition; examples of (ii) are (a) systems with highly polar solute and adsorbent, and a non-polar solvent, and (b) systems in which monofunctional ionic substances with very strong intermolecular attraction are adsorbed from water by ion-ion attraction. It is possible that in these cases (systems b) the adsorbed ions may have become associated into very large dusters, and just adsorption take places [14-19]. The monolayer coverages of AAm/IA and AAm /MA hydrogels for the dyes in the aqueous solutions were found by method of Point B from adsorption isotherms. The reciprocal of n is the site-size, u, which may be taken to represent either the average number of monomer units occupied by the bound solute molecule, or more generally the average spacing of solute molecules when the chain saturates [14-19]. The values of monolayer coverage and the site-size are listed Table 6. Table 6 The values of monolayer coverage of hydrogel-dye systems

Hydrogel AAm AAm/IA AAm /MA Solution nxl03 u nxl03 u nxl03 u Basic green 4 0 0 0.10 9 933 1,34 748 Basic red 9 0 0 0.22 4 553 0,30 3370 Cresyl violet 0 0 0.23 4 417 1,88 532 Basic blue 20 0 0 0.47 2 141 3,10 255

Dyes were removed from hydrogels by contact with distilled water for 3 days. We have observed that desorptions of the dyes were shown by the suitable coloration in the water and also hydrogels returned to the original colors. It can be said that is adsorption of the dyes is physical adsorption.

3.3 Adsorption of biomolecules

Despite the well-known risks of tobacco use, cigarette smoking remains a global concern: the negative effects of nicotine on public health include heart disease, cancer and respiratory disorders and even it was implied that nicotine might be genotoxic and. In addition to its addictive effects, nicotine is a highly toxic compound. It is not so well known that nicotine is one of the components of wastewaters produced during tobacco processing. Being soluble in water, a large amount of nicotine is transferred to aqueous solutions during tobacco processing and manufacturing tobacco products. One of by-products of cigarette manufacture is tobacco dust; these powdery solid wastes with high content of nicotine as the main toxic compound are accumulated and cannot be recycled; from there, nicotine can be easily extracted and transported into ground-water [20].


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In this stage of the study, nicotine, nicotinic acid, nicotinamide and nikethamide were selected as biomolecules, and the interactions between these biomolecules and the AAm, AAm/IA and AAm/MA hydrogels. Aqueous solutions of nicotine, nicotinic acid, nicotinamide, and nikethamide were prepared in the concentration of 50 mg L−1. AAm/IA and AAm/MA hydrogels containing 60 mg comonomer and irradiated to 5.71 kGy were transferred into 50 mL of aqueous solutions and allowed to equilibrate for 24 h at 25 oC. These solutions were separated by decantation from the hydrogels. Spectrophotometric measurements of these solutions were carried out using a Shimadzu A160 model UV-VIS double beam spectrophotometer. The absorbance of these solutions was recorded at a wavelength of 261 nm. To observe binding of nicotine and its pharmaceutical derivatives to the hydrogels were placed in aqueous solutions of nicotine and its pharmaceutical derivatives nicotinic acid, nicotinamide and nikethamide, and allowed to equilibrate for one day. At the end of this time, a significant difference was measured between initial and final concentration of the nicotine solution containing AAm/IA and AAm/MA hydrogel, while no significant differences were measured between initial and final concentration of the other pharmaceutical solutions containing AAm, AAm/IA and AAm/MA hydrogels. So, AAm hydrogel did not sorb nicotine or its pharmaceutical derivatives from the solutions, and AAm/IA hydrogel or AAm/MA hydrogel sorbed only nicotine. Nicotine was therefore selected for binding studies to radiation-induced AAm/IA hydrogel and AAm/MA hydrogels. AAm is a non-ionic polymer, and thus acrylamide hydrogel generally does not interact with many small molecules. By the addition of diprotic acid such as MA or IA to acrylamide, acrylamide copolymer contains ionizable groups. These groups can interact with the small molecules and these molecules bind to acrylamide the copolymers with non-covalent or covalent bonds [21-22]. In nicotine−AAm/IA and nicotine−AAm/MA hydrogel systems, nicotine has a weak basic character, and the polar pyrrolidine ring in nicotine molecule, when nicotine has a positive charge, can interact by electrostatic forces with the carboxyl groups of itaconic acid or maleic acid in the hydrogel. Amide groups of nikethamide and nicotinamide cannot interact with itaconic acid and maleic acid, because amide groups do not have ionic or polar character. On the other hand, there will be anionic repulsion between carboxyl groups of nicotinic acid and IA or MA in the hydrogel, and therefore nicotinic acid does not bind to the hydrogel. According to these results, it could be concluded that the pyrrolidine-N presumably plays more important role than the pyridine-N. The difference between the pyrrolidine-N and pyridine-N is their basicity. There- fore, the basicity of the functional group may be an important factor for binding to acrylamide hydrogel containing protic moieties [21-22]. On the other hand, for a substrate molecule to get bounded to AAm/MA hydrogel, it has to penetrate first the outside aqueous layer and then with the specifically bound water molecules for the sites. Not only that, the structure of crosslinked polymer in the three-dimensional matrix would also change with changes in the water structure. Hence, any change in the amount of water sorbed could affect the binding sites and hence alter the type of interacting forces. For the adsorption of nicotine onto the hydrogel, AAm/IA and AAm/MA AAm/IA hydrogels containing 0, 20, 30, 40, 50 and 60 mg comonomers and irradiated to 2.6 kGy. The binding ratios of nicotine-hydrogel were calculated for all hydrogels. While, AAm hydrogel had not sorbed nicotine, nicotine binding ratio of AAm/IA hydrogels and AAm/MA hydrogels were found as ~ 6.1x10−3 and ~ 3.7x10−3 mmol nicotine/mol hydrogel, respectively. The binding ratios of AAm hydrogels increased after adding IA and MA. But, the binding of nicotine is approximately constant with the increase of comonomer content in the hydrogels.

4. Conclusion

Polymeric substrates are being continuously developed and used for the purpose of complexation with metal ions either for ion-exchange or selective adsorption purposes. These polymeric ligands are tailor synthesized to remove certain metal ions or groups from aqueous media. The introduction of maleic or itaconic acid type monomeric units into acrylamide based hydrogels imparted chelating properties into these hydrogels. The complexation of these diprotic acidic groups with some heavy metal ions provided an efficient way of their removal from aqueous systems. Same hydrogels were proven to be very effective in the uptake of uranyl ions from aqueous solutions. Adsorptive and binding capacities of acrylamide/maleic acid and acrylamide/itaconic acid based hydrogels were also utilized in the removal of some dyes from aqueous solutions. While pure acrylamide hydrogels showed no adsorption towards a number of basic dyes tested, the presence of diprotic acids in these gels significantly increased their absorptive capacities. The results of adsorption studies on some basic dyes and nicotine from aqueous solutions onto acrylamide/maleic acid and acrylamide/itaconic acid hydrogels have shown the beneficial effect of diprotic acid moieties. The adsorption studies clearly showed that these superswelling hydrogels are potential sorbents to be used for the removal of basic dyes and biomolecules from wastewaters and aqueous effluents. The results achieved on bioapplications, heavy metal ion uptake and removal of dyes from aqueous systems showed that the copolymeric hydrogels prepared from hydrophilic monomers and units with doubly ionizable groups possess some unique properties. Among various methods of synthesis, use of ionizing radiations in the initiation and further cross-linking of starting monomer mixtures seems to provide the best alternative in the preparation of these new classes of materials [1]. As a result, radiation synthesized AAm/IA and Am/MA hydrogels can be used as a super water retainer for carrying of some substances in aquatic fields in pharmaceutical, agricultural, environmental and biomedical applications, or in the applications of immobilized biologically active molecules and the hydrogels can be used as an adsorbent for the


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water pollutants such as heavy metal ions, basic dyes, nicotine an important problems for the metallurgy, mine working, textile and tobacco industry.

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