- Email: [email protected]
acrylic acid binary monomers
Nuclear Instruments and Methods in Physics Research B 236 (2005) 580–586 www.elsevier.com/locate/nimb
Preparation of poly(vinyl alcohol) membranes grafted with N-vinyl imidazole/acrylic acid binary monomers Zaki Ajji a
a,*
, Ali M. Ali
b
Polymer Technology Division, Department of Radiation Technology, Atomic Energy Commission, P.O. Box 6091, Damascus, Syria b Department of Chemistry, Faculty of Science, Tishreen University, Latakia, Syria Available online 23 May 2005
Abstract Poly(vinyl alcohol) films were grafted with two monomers using gamma radiation, acrylic acid and N-vinyl imidazole. The influence of different parameters on the grafting yield was investigated as: type of solvent and solvent composition, comonomer concentration and composition, addition of mineral acids, and irradiation dose. Water uptake in respect to the grafting yield was also evaluated. The ability of the grafted films to adsorb copper ions was elaborated and discussed for different grafting yields and pH values of the solution. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Acrylic acid; N-vinyl imidazole; Poly(vinyl alcohol); Radiation grafting
1. Introduction The effective treatment of heavy metals in the environment has become one the major issues of public interest due to their toxicity. The treatment of aqueous waste, including soluble heavy metals, needs concentration of the metallic solution into small volume, followed by recovery or secure disposal. The modification of hydrophilic polymer membranes to an adsorbent has been reported to
*
Corresponding author. Fax: +963 11 611 2289. E-mail address: [email protected] (Z. Ajji).
be useful for collecting target ions and molecules [1]. Poly(vinyl alcohol) PVA is a highly hydrophilic polymer and well-known as a membrane material with good film-forming ability and has easy availability. Investigations have been done on its use in the field of separation and adsorption processes [2,3]. However, this polymer suffers from poor water resistance and low mechanical strength in aqueous solutions. Therefore, it has to be turned into insoluble stable material with good mechanical properties. A number of investigations have been reported in the literature to modify PVA by cross-linking with different ways such as heat [4], formaldehyde [5], dicaroboxylic acid [6] and
0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.04.046
Z. Ajji, A.M. Ali / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 580–586
581
radiation [7]. Graft polymerization of vinyl monomers onto polymer substrates has attracted considerable interest because it imparts some desirable properties, such as chelation, ion exchange, biocompatibility, and protein adsorption. Poly(N-vinyl imidazole) hydrogels were applied to immobilize glucose oxidase [8,9] and for ion uptake [10–12]. Also hydrogels, based on N-vinyl imidazole and acrylonitrile, were prepared regarding chelation and separation of metals [13,14]. The purpose of the present work was the preparation of PVA membranes, which are grafted with acrylic acid and N-vinyl imidazole monomers using gamma radiation. Also the factors affecting the grafting process and the possible use of the modified membranes in the field of ion adsorption and separation of heavy metals have been studied.
immersed in the monomer or binary monomer solution in glass ampoules. The glass ampoules containing all the reactants and polymer substrates were subjected to 60Co gamma rays at a dose rate of about 2.8 kGy/h. The grafted films were removed and washed thoroughly with water/methanol mixture, which is a good solvent for AAc and Zol and then immersed in water to extract the residual monomer or homopolymer may be accumulated in the grafted films. The films were then dried in vacuum oven at 50 °C. The degree of grafting was determined by the percent increase in weight as follow:
2. Experimental
2.4. Maximum swelling
2.1. Materials
The dried grafted films (after the washing procedure mentioned above) were soaked in water up to a constant weight (equilibrium swelling) and the maximum swelling (Smax %) was calculated by the following equation:
PVA (MW = 72,000, for synthesis), methanol (purity > 99.8%, Merck, Germany), acetone (purity > 99.8%), dioxane (purity > 99.5%) were supplied by Merck, Germany. N-vinyl imidazole (purity > 99%), acrylic acid (purity > 99%) were supplied by Fluka, Germany. Ethanol (purity 99.8%) was purchased by Riedel de Hae¨n, Germany. 2.2. Preparation of PVA films
Degree of grafting% ¼
WgW0 100; W0
where W0 and Wg are the weights of initial and grafted films, respectively.
S max % ¼
WSW0 100; W0
where WS is the weight of membrane at equilibrium swelling, and W0 is the weight of dried membrane. 2.5. Ion uptake
A solution of PVA was prepared by dissolving 7 g of PVA, in a conical flask with a standard joint, in 100 ml distilled water at 80 °C. The PVA films were cast from this aqueous solution on a glass plate and then subsequently dried in air at ambient temperature. The obtained film had the thickness in the range 70–90 lm.
Dry hydrogels were transferred into solutions of copper sulfate with a concentration of 2000 ppm. The concentration of the solution was measured using a double-beam UV–VIS spectrophotometer (Shimadzu, type UV-1601).
2.3. Graft copolymerization
3. Results and discussion
The direct radiation-induced grafting of acrylic acid/imidazole monomers onto PVA films was used as a preparation technique in an air atmosphere. Strips of PVA films were weighed and then
3.1. Preparation of bi-functional grafted membranes The preparation of the membranes was carried out using the technique of direct radiation graft
582
Z. Ajji, A.M. Ali / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 580–586
copolymerization of acrylic acid (AAc) and imidazole (Zol) onto PVA. The influence of some grafting parameters on the yield of grafting was investigated. 3.1.1. Effect of solvent Solvents play an important role in enhancing the grafting process of a monomer onto a trunk polymer. The solvent may influence the grafting process by diluting the monomer, thus reducing the rate of propagation and kinetic chain length. The solvent may also swell the polymer substrate to facilitate accessibility and diffusion of the monomer to the active sites and/or may modify the thermodynamic equilibrium of the copolymer in the particular monomer solvent mixture. The effect of different solvents on the graft copolymerization of AAc/Zol binary monomers onto PVA films was investigated and the results are listed in Table 1. It can be seen that a high degree of grafting is obtained when a water/solvent mixture is used, and the highest value was observed using a water/methanol mixture. The presence of water with any solvent used here resulted in enhancement of the grafting yield as compared to that obtained when the solvent was used alone. The results show that solvents influence the grafting yield of the AAc/Zol comonomer onto the PVA membrane, and the enhancement effect is more pronounced using polar solvents such as methanol/water mixture. In addition, methanol has low molecular weight, and thus diffuses easier
through polymer material carrying the monomer, and leading to a higher degree of grafting. 3.1.2. Effect of mineral acids The influence of addition of mineral acids as HCl, H2SO4 and HNO3 on the grafting process of AAc/Zol comonomer was investigated. The presence of mineral acids enhances generally the grafting yield, and the highest value was obtained by addition of H2SO4. As the concentration of H2SO4 increases, the degree of grafting increases to reach a maximum value at 2%. Thereafter, it sharply decreases as the H2SO4 concentration increases. 3.1.3. Effect of water/Me–OH composition Fig. 1 shows the grafting yield by using various water/methanol compositions. The degree of grafting increases as the water content increases, reaching a maximum value at a water/methanol composition of 40/60 for a AAc/Zol composition of 3/1 mol/mol. Thereafter, it gradually decreases as the water content increases in the solvent mixture. No grafting was found in absence of water. This behavior can be explained that in the presence of water, the packing of a PVA polymer is loosened through the swelling action of water; thus an easy penetration of vinyl monomers can occur resulting in higher grafting yield. Also, radiolytic fragments from H2O may activate PVA by abstraction of tertiary atom in the following manner [15]: H O ! R ðH þ OH Þ 2
Table 1 Effect of different solvents on the grafting process of (AAc/Zol) binary monomer system (3/1 mol/mol) onto PVA films; irradiation dose = 7 kGy; comonomer concentration = 20 wt.% Solvent
Solvent composition%
Degree of grafting (%)
Me–OH Me–OH/H2O
100 80/20
— 62
Acetone Acetone/H2O
100 80/20
— 58
Dioxane Dioxane/H2O
100 80/20
— 56
Et-OH Et-OH/H2O
100 80/20
— 38.6
R þ PVA ! RH þ PVA However, at higher water content beyond optimum, radiolysis of additional water produces a radical species, which mutually annihilate the growing grafted chains leading to decrease in percent grafting. 3.1.4. Effect of comonomer concentration The influence of comonomer concentration during radiation grafting may affect the kinetic parameters of this process. Therefore, the suitable comonomer concentration differs from system to another, depending on the diluent used, type of
Z. Ajji, A.M. Ali / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 580–586
medium. The increase in the grafting yield with comonomer concentration may be due to increases of the diffused monomer into the bulk polymer. The high concentration of free radicals at grafting sites favors propagation of growing chains and consequently, the grafting yield increases [16].
140 120
Degree of grafting [%]
583
100 80 60 40 20 0 -20 0
10
20
30
40
50
Water Methanol
100
90
80
70
60
50
Solvent composition [wt.%]
Fig. 1. Effect of solvent composition (Me–OH/water) on the grafting yield of AAc/Zol binary monomers with a composition of 3/1 mol/mol onto PVA films; comonomer concentration = 20 wt.%; irradiation dose = 7 kGy.
polymer support materials, comonomer composition, irradiation dose and dose rate, etc. The effect of dilution of AAc/Zol binary monomers mixture using a water-methanol mixture (30:70) at comonomer composition (3/1 mol/mol) on the graft copolymerization onto PVA is investigated and the results are represented in Fig. 2. It can be seen that the degree of grafting increases with increasing the comonomer concentration in the reaction
3.1.5. Effect of comonomer composition The synergistic effect of two monomers may lead to more efficient grafting processes [17]. Therefore, the influence of the comonomer composition on the grafting process has been studied. The grafting yield of AAc/Zol binary monomer systems of various relative compositions was determined at a total comonomer concentration of 20 wt.% in water-methanol mixture, and the obtained data are represented in Fig. 3. It is clear that the grafting yield increases with increasing the content of AAc in comonomer feed solution to reach a maximum value at a AAc/Zol composition of 75/ 25 mol/mol. Thereafter, at higher contents of AAc, the degree of grafting decreases to a lower value at an AAc/Zol composition of (90/10 mol/ mol). At content of AAc higher than 90%, a dense gelled homopolymer is formed. It can be suggested that vinyl imidazole is a retarding agent by dissipation of the radiation 140 120 100
Degree of Grafting (%)
Degree of grafting [%]
150
100
50
80 60 40 20 0 -20 0
20
100
80
40
60
80
azol
60
40
20
AAc 0
0 0
5
10
15
20
25
30
Comonomer concentration [wt.]
Fig. 2. Effect of comonomer concentration on the grafting yield of AAc/Zol onto PVA; comonomer concentration = 3/1 mol/ mol; Me–OH/water composition 70/30 wt.%; irradiation dose = 7 kGy.
Comonomer Composition (%)
Fig. 3. Effect of AAc/Zol comonomer composition on the degree of grafting onto PVA Films; comonomer concentration = 20 wt.%; Me–OH/water mixture composition = 70/ 30 wt.%; irradiation dose = 7 kGy.
Z. Ajji, A.M. Ali / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 580–586
energy forming a stable exciting state that prevents the free radical formation responsible for the initiation of grafting sites as by vinyl pyridine [18]. Therefore, the feed solution containing excess of vinyl imidazole hinders the grafting of the comonomer onto polymer substrate. However, the presence of vinyl imidazole with AAc retards the homopolymerization process of the latter. This resulted in obtaining higher degrees of grafting if compared with those obtained for the individual grafting of the AAc or vinyl imidazole [18,19]. In addition, these results indicated the possibility that AAC/Zol may form 1:1 charge transfer complexes, which may graft as such or participate as active intermediates in grafting [18,20–24]. 3.1.6. Effect of irradiation dose It is known that the increase in the irradiation time results in increasing the concentration of free radicals formed in the polymer substrate as well as in the monomer itself. Therefore, the influence of dose on the grafting yield of AAc/Zol in watermethanol mixture as a diluent onto PVA film is investigated, and the results are illustrated in Fig. 4. The grafting yield initially increases with increasing irradiation dose till an optimum value (14 kGy) and then tends to level off at higher doses.
500 450 400 350 300 250 200 150 100 0
50
100
150
Degree of grafting [%]
Fig. 5. Water uptake versus the grafting yield for PVA-g-AAc/ Zol membranes; comonomer composition = AAc/Zol:3/1.
3.2. Properties of grafted membranes 3.2.1. Swelling behavior Fig. 5 represents the swelling% with respect to the degree of grafting. It can be seen that the water uptake decreases as the degree of grafting increases. This behavior can be explained that the pores in gel become smaller due to an increase in the cross-linking density between the polymeric chains due to increased grafting yield. 3.2.2. Gel fraction The gelation% of the PVA-g-AAc/Zol membranes has been determined versus the grafting yield by extraction in boiled water for 4 h. Fig. 6 represents the gelation% against the degree of grafting. It can been that gel content increases with the degree of grafting to reach a maximum value at 185% grafting yield, at which the graft copolymers becomes completely insoluble in boiled water. This is due to an increase in the cross-links between the polymeric chains as the grafting yield increases.
140 120
Degree of grafting [%]
550
Water uptake [%]
584
100 80 60 40 20 0
0
5
10
15
20
25
3.3. Ion uptake
Irradiation dose [kGy]
Fig. 4. Effect of irradiation dose on the degree of grafting of AAc/Zol onto PVA Films; comonomer concentration = 20 wt.%; Me–OH/water mixture composition = 70/ 30 wt.%.
Heavy metals are toxic pollutants, which should be removed from wastewater due to their undesired effects on human physiology and ecological systems even at very low concentrations [25].
Z. Ajji, A.M. Ali / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 580–586
585
110
1000
100 90
Copper ion uptake [mg/g]
80
Gelation [%]
70 60 50 40 30 20
800
600
400
200
10
0
0 -10 0
50
100
150
200
1
Fig. 6. Gelation% versus the degree of grafting for PVA-gAAc/Zol membranes; time of extraction 4 h in hot water (98 °C).
3.3.1. Influence of grafting yield The ion uptake of PVA-g-Zol/AAc membranes versus the degree of grafting is represented in Fig. 7. As the degree of grafting increases, the ion uptake of the membrane increases. This behavior can be reasonably attributed to the increase in the number of functional groups grafted onto the polymer backbone with the increase in the grafting yield. It is also observed that, efficiency of PVA-gAAc/Zol membrane having degree of grafting
Copper ion uptake [mg/g]
1000
800
600
400
200
0 0
20
40
60
80
2
3
4
5
6
7
pH of the copper sulfate solution
Degree of grafting [%]
100
Fig. 8. Effect of pH on amount of copper ion adsorbed onto PVA-g-AAc/Zol membranes (grafting yield = 119%); initial feed concentration = 2000 ppm; time of adsorption = 8 h; ambient temperature = 22 °C.
119% is very high, and the amount of recovered copper ions is about 970 mg/g after 8 h immersion in copper solution. This result can be explained with the availability of different functional groups, which can interact with copper ions in various ways forming ionic bonds (carboxyl group) or/ and complex bonds (hydroxyl or N-imide group). 3.3.2. Effect of pH The complexation of heavy metal ions by chelating-ligand supported on polymeric network is affected by the pH of the solutions [26]. In the present study we changed the pH range between 1 and 6.2. The adsorbed copper ions are illustrated with respect to the pH of the solution in Fig. 8. The copper ions adsorption is very limited at low pH, and the adsorption capacity increases with increasing pH, reaching a plateau values at pH = 6.2. This behavior maybe explained that the carboxylate ion exists in the acidic form, and the nitrogen atom of the imidazole ring is protonated in the strong acidic medium; thus both ligands are not available to form a bond with the metal ion.
120
Degree of grafting [%]
Fig. 7. Effect of degree of grafting on the copper ion uptake for PVA-g-AAc/Zol membranes; initial feed concentration = 2000 ppm; time = 8 h; pH = 5.2; ambient temperature = 22 °C.
4. Conclusion Polymer membranes were prepared based on poly(vinyl alcohol) grafted with acrylic acid and
586
Z. Ajji, A.M. Ali / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 580–586
N-vinyl imidazole using gamma radiation. The grafting conditions were determined regarding their influence on the grafting yield as: type of solvent and solvent composition, comonomer concentration and composition, addition of mineral acids, and irradiation dose. The ability of the grafted films to adsorb copper ions was evaluated with respect to the grafting yield. It increases with increasing the grafting yield. In strong acidic medium (pH = 1) there is a very limited ion adsorption, and at pH ffi6.2 the adsorption achieved a high value of around 970 mg/g; the grafting yield of the membrane was 119%. Thus the prepared membrane may be considered for removing of copper ions from wastewater. Also the limited copper ion capacity in acidic medium may suggest the use of these membranes for separation application. Further work is in progress to elaborate this issue.
Acknowledgments The authors would like to thank Prof. I. Othman for his encouragement. They are also grateful to Mr. F. Heso and Ms. H. Dadah for their efforts during the experiments.
References [1] M. Kim, K. Saito, Radiat. Phys. Chem. 57 (2) (2000) 167. [2] K. Burczak, T. Fujisato, Y. Ikada, Proc. Jpn. Acad. Ser. (B) 67 (5) (1991) 83. [3] V. Shantora, R.Y.M. Huang, J. Appl. Polym. Sci. 26 (1981) 3223.
[4] M.G. Katz, T.J. Wydeven, J. Appl. Polym. Sci. 27 (1982) 79. [5] K. Koyama, T. Nishi, M.J. Nishimura, J. Appl. Polym. Sci. 27 (1982) 2845. [6] S. Peter, R. Stefan, in: Proceeding International Symposium, Fresh Water from the Sea, Athens, Vol. 2, 1980, p. 197. [7] M.G. Katz, T. Wydeven, J. Appl. Polym. Sci. 26 (1981) 2935. [8] N. Pekel, B. Salih, O. Guven, J. Molecular, Catal. B– Enzym. 21 (4–6) (2003) 273. [9] B. Salih, N. Pekel, O. Guven, J. Appl. Polym. Sci. 82 (2) (2001) 446. [10] N. Pekel, H. Savas, O. Guven, Colloid Polym. Sci. 280 (1) (2002) 46. [11] N. Pekel, B. Salih, O. Guven, Macromol. Symp. 169 (2001) 329. [12] N. Pekel, O. Guven, Colloid Polym. Sci. 277 (6) (1999) 570. [13] N. Pekel, N. Sahiner, O. Guven, Radiat. Phys. Chem. 59 (5–6) (2000) 485. [14] N. Pekel, O. Guven, Colloids Surf. A–Physicochem. Eng. Aspects 212 (2–3) (2003) 155. [15] B.N. Misra, Kishore, M. Kanthwal, I.K. Mehta, J. Appl. Polym. Sci. 24 (1986) 2209. [16] H.T. Lokhande, M.D. Teli, J. Appl. Polym. Sci. 29 (1984) 1843. [17] A. Hebeish, S. Shalaby, A. Bayazeid, J. Appl. Polym. Sci. 27 (1982) 197. [18] El-Sayed A. Hegazy, H.A. Abd El-Rehim, H.A. Shawky, Radiat. Phys. Chem. 57 (2000) 85. [19] A.M. Ali, M.Sc. thesis, Ain Shams University, Egypt, 1997. [20] El-Sayed A. Hegaz, H.A. Abd El-Rehim, N.A. Khalifa, A. El-Haj Ali, Radiat. Phys. Chem. 55 (1999) 219. [21] I. Kaur, R. Barsola, J. Appl. Polym. Sci. 41 (1990) 2067. [22] I. Kaur, R. Barsola, B.N. Misra, J. Appl. Polym. Sci. 51 (1994) 2067. [23] S. Dilli, J.L. Garnett, J. Polym. Sci. A-1 4 (1966) 2323. [24] J.L. Garnett, L.-T. Ng, V. Viengkhou, in: Proceedings of the RadTech. 98, North America, Chicago, 1998, p. 627. [25] C.S. Luo, S. Huang, Sep. Sci. Technol. 28 (1993) 1253. [26] B.E. Reed, M.R. Matsumoto, Sep. Sci. Technol. 28 (1993) 2179.
Preparation of poly(vinyl alcohol) membranes grafted with N-vinyl imidazole/acrylic acid binary monomers Zaki Ajji a
a,*
, Ali M. Ali
b
Polymer Technology Division, Department of Radiation Technology, Atomic Energy Commission, P.O. Box 6091, Damascus, Syria b Department of Chemistry, Faculty of Science, Tishreen University, Latakia, Syria Available online 23 May 2005
Abstract Poly(vinyl alcohol) films were grafted with two monomers using gamma radiation, acrylic acid and N-vinyl imidazole. The influence of different parameters on the grafting yield was investigated as: type of solvent and solvent composition, comonomer concentration and composition, addition of mineral acids, and irradiation dose. Water uptake in respect to the grafting yield was also evaluated. The ability of the grafted films to adsorb copper ions was elaborated and discussed for different grafting yields and pH values of the solution. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Acrylic acid; N-vinyl imidazole; Poly(vinyl alcohol); Radiation grafting
1. Introduction The effective treatment of heavy metals in the environment has become one the major issues of public interest due to their toxicity. The treatment of aqueous waste, including soluble heavy metals, needs concentration of the metallic solution into small volume, followed by recovery or secure disposal. The modification of hydrophilic polymer membranes to an adsorbent has been reported to
*
Corresponding author. Fax: +963 11 611 2289. E-mail address: [email protected] (Z. Ajji).
be useful for collecting target ions and molecules [1]. Poly(vinyl alcohol) PVA is a highly hydrophilic polymer and well-known as a membrane material with good film-forming ability and has easy availability. Investigations have been done on its use in the field of separation and adsorption processes [2,3]. However, this polymer suffers from poor water resistance and low mechanical strength in aqueous solutions. Therefore, it has to be turned into insoluble stable material with good mechanical properties. A number of investigations have been reported in the literature to modify PVA by cross-linking with different ways such as heat [4], formaldehyde [5], dicaroboxylic acid [6] and
0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.04.046
Z. Ajji, A.M. Ali / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 580–586
581
radiation [7]. Graft polymerization of vinyl monomers onto polymer substrates has attracted considerable interest because it imparts some desirable properties, such as chelation, ion exchange, biocompatibility, and protein adsorption. Poly(N-vinyl imidazole) hydrogels were applied to immobilize glucose oxidase [8,9] and for ion uptake [10–12]. Also hydrogels, based on N-vinyl imidazole and acrylonitrile, were prepared regarding chelation and separation of metals [13,14]. The purpose of the present work was the preparation of PVA membranes, which are grafted with acrylic acid and N-vinyl imidazole monomers using gamma radiation. Also the factors affecting the grafting process and the possible use of the modified membranes in the field of ion adsorption and separation of heavy metals have been studied.
immersed in the monomer or binary monomer solution in glass ampoules. The glass ampoules containing all the reactants and polymer substrates were subjected to 60Co gamma rays at a dose rate of about 2.8 kGy/h. The grafted films were removed and washed thoroughly with water/methanol mixture, which is a good solvent for AAc and Zol and then immersed in water to extract the residual monomer or homopolymer may be accumulated in the grafted films. The films were then dried in vacuum oven at 50 °C. The degree of grafting was determined by the percent increase in weight as follow:
2. Experimental
2.4. Maximum swelling
2.1. Materials
The dried grafted films (after the washing procedure mentioned above) were soaked in water up to a constant weight (equilibrium swelling) and the maximum swelling (Smax %) was calculated by the following equation:
PVA (MW = 72,000, for synthesis), methanol (purity > 99.8%, Merck, Germany), acetone (purity > 99.8%), dioxane (purity > 99.5%) were supplied by Merck, Germany. N-vinyl imidazole (purity > 99%), acrylic acid (purity > 99%) were supplied by Fluka, Germany. Ethanol (purity 99.8%) was purchased by Riedel de Hae¨n, Germany. 2.2. Preparation of PVA films
Degree of grafting% ¼
WgW0 100; W0
where W0 and Wg are the weights of initial and grafted films, respectively.
S max % ¼
WSW0 100; W0
where WS is the weight of membrane at equilibrium swelling, and W0 is the weight of dried membrane. 2.5. Ion uptake
A solution of PVA was prepared by dissolving 7 g of PVA, in a conical flask with a standard joint, in 100 ml distilled water at 80 °C. The PVA films were cast from this aqueous solution on a glass plate and then subsequently dried in air at ambient temperature. The obtained film had the thickness in the range 70–90 lm.
Dry hydrogels were transferred into solutions of copper sulfate with a concentration of 2000 ppm. The concentration of the solution was measured using a double-beam UV–VIS spectrophotometer (Shimadzu, type UV-1601).
2.3. Graft copolymerization
3. Results and discussion
The direct radiation-induced grafting of acrylic acid/imidazole monomers onto PVA films was used as a preparation technique in an air atmosphere. Strips of PVA films were weighed and then
3.1. Preparation of bi-functional grafted membranes The preparation of the membranes was carried out using the technique of direct radiation graft
582
Z. Ajji, A.M. Ali / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 580–586
copolymerization of acrylic acid (AAc) and imidazole (Zol) onto PVA. The influence of some grafting parameters on the yield of grafting was investigated. 3.1.1. Effect of solvent Solvents play an important role in enhancing the grafting process of a monomer onto a trunk polymer. The solvent may influence the grafting process by diluting the monomer, thus reducing the rate of propagation and kinetic chain length. The solvent may also swell the polymer substrate to facilitate accessibility and diffusion of the monomer to the active sites and/or may modify the thermodynamic equilibrium of the copolymer in the particular monomer solvent mixture. The effect of different solvents on the graft copolymerization of AAc/Zol binary monomers onto PVA films was investigated and the results are listed in Table 1. It can be seen that a high degree of grafting is obtained when a water/solvent mixture is used, and the highest value was observed using a water/methanol mixture. The presence of water with any solvent used here resulted in enhancement of the grafting yield as compared to that obtained when the solvent was used alone. The results show that solvents influence the grafting yield of the AAc/Zol comonomer onto the PVA membrane, and the enhancement effect is more pronounced using polar solvents such as methanol/water mixture. In addition, methanol has low molecular weight, and thus diffuses easier
through polymer material carrying the monomer, and leading to a higher degree of grafting. 3.1.2. Effect of mineral acids The influence of addition of mineral acids as HCl, H2SO4 and HNO3 on the grafting process of AAc/Zol comonomer was investigated. The presence of mineral acids enhances generally the grafting yield, and the highest value was obtained by addition of H2SO4. As the concentration of H2SO4 increases, the degree of grafting increases to reach a maximum value at 2%. Thereafter, it sharply decreases as the H2SO4 concentration increases. 3.1.3. Effect of water/Me–OH composition Fig. 1 shows the grafting yield by using various water/methanol compositions. The degree of grafting increases as the water content increases, reaching a maximum value at a water/methanol composition of 40/60 for a AAc/Zol composition of 3/1 mol/mol. Thereafter, it gradually decreases as the water content increases in the solvent mixture. No grafting was found in absence of water. This behavior can be explained that in the presence of water, the packing of a PVA polymer is loosened through the swelling action of water; thus an easy penetration of vinyl monomers can occur resulting in higher grafting yield. Also, radiolytic fragments from H2O may activate PVA by abstraction of tertiary atom in the following manner [15]: H O ! R ðH þ OH Þ 2
Table 1 Effect of different solvents on the grafting process of (AAc/Zol) binary monomer system (3/1 mol/mol) onto PVA films; irradiation dose = 7 kGy; comonomer concentration = 20 wt.% Solvent
Solvent composition%
Degree of grafting (%)
Me–OH Me–OH/H2O
100 80/20
— 62
Acetone Acetone/H2O
100 80/20
— 58
Dioxane Dioxane/H2O
100 80/20
— 56
Et-OH Et-OH/H2O
100 80/20
— 38.6
R þ PVA ! RH þ PVA However, at higher water content beyond optimum, radiolysis of additional water produces a radical species, which mutually annihilate the growing grafted chains leading to decrease in percent grafting. 3.1.4. Effect of comonomer concentration The influence of comonomer concentration during radiation grafting may affect the kinetic parameters of this process. Therefore, the suitable comonomer concentration differs from system to another, depending on the diluent used, type of
Z. Ajji, A.M. Ali / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 580–586
medium. The increase in the grafting yield with comonomer concentration may be due to increases of the diffused monomer into the bulk polymer. The high concentration of free radicals at grafting sites favors propagation of growing chains and consequently, the grafting yield increases [16].
140 120
Degree of grafting [%]
583
100 80 60 40 20 0 -20 0
10
20
30
40
50
Water Methanol
100
90
80
70
60
50
Solvent composition [wt.%]
Fig. 1. Effect of solvent composition (Me–OH/water) on the grafting yield of AAc/Zol binary monomers with a composition of 3/1 mol/mol onto PVA films; comonomer concentration = 20 wt.%; irradiation dose = 7 kGy.
polymer support materials, comonomer composition, irradiation dose and dose rate, etc. The effect of dilution of AAc/Zol binary monomers mixture using a water-methanol mixture (30:70) at comonomer composition (3/1 mol/mol) on the graft copolymerization onto PVA is investigated and the results are represented in Fig. 2. It can be seen that the degree of grafting increases with increasing the comonomer concentration in the reaction
3.1.5. Effect of comonomer composition The synergistic effect of two monomers may lead to more efficient grafting processes [17]. Therefore, the influence of the comonomer composition on the grafting process has been studied. The grafting yield of AAc/Zol binary monomer systems of various relative compositions was determined at a total comonomer concentration of 20 wt.% in water-methanol mixture, and the obtained data are represented in Fig. 3. It is clear that the grafting yield increases with increasing the content of AAc in comonomer feed solution to reach a maximum value at a AAc/Zol composition of 75/ 25 mol/mol. Thereafter, at higher contents of AAc, the degree of grafting decreases to a lower value at an AAc/Zol composition of (90/10 mol/ mol). At content of AAc higher than 90%, a dense gelled homopolymer is formed. It can be suggested that vinyl imidazole is a retarding agent by dissipation of the radiation 140 120 100
Degree of Grafting (%)
Degree of grafting [%]
150
100
50
80 60 40 20 0 -20 0
20
100
80
40
60
80
azol
60
40
20
AAc 0
0 0
5
10
15
20
25
30
Comonomer concentration [wt.]
Fig. 2. Effect of comonomer concentration on the grafting yield of AAc/Zol onto PVA; comonomer concentration = 3/1 mol/ mol; Me–OH/water composition 70/30 wt.%; irradiation dose = 7 kGy.
Comonomer Composition (%)
Fig. 3. Effect of AAc/Zol comonomer composition on the degree of grafting onto PVA Films; comonomer concentration = 20 wt.%; Me–OH/water mixture composition = 70/ 30 wt.%; irradiation dose = 7 kGy.
Z. Ajji, A.M. Ali / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 580–586
energy forming a stable exciting state that prevents the free radical formation responsible for the initiation of grafting sites as by vinyl pyridine [18]. Therefore, the feed solution containing excess of vinyl imidazole hinders the grafting of the comonomer onto polymer substrate. However, the presence of vinyl imidazole with AAc retards the homopolymerization process of the latter. This resulted in obtaining higher degrees of grafting if compared with those obtained for the individual grafting of the AAc or vinyl imidazole [18,19]. In addition, these results indicated the possibility that AAC/Zol may form 1:1 charge transfer complexes, which may graft as such or participate as active intermediates in grafting [18,20–24]. 3.1.6. Effect of irradiation dose It is known that the increase in the irradiation time results in increasing the concentration of free radicals formed in the polymer substrate as well as in the monomer itself. Therefore, the influence of dose on the grafting yield of AAc/Zol in watermethanol mixture as a diluent onto PVA film is investigated, and the results are illustrated in Fig. 4. The grafting yield initially increases with increasing irradiation dose till an optimum value (14 kGy) and then tends to level off at higher doses.
500 450 400 350 300 250 200 150 100 0
50
100
150
Degree of grafting [%]
Fig. 5. Water uptake versus the grafting yield for PVA-g-AAc/ Zol membranes; comonomer composition = AAc/Zol:3/1.
3.2. Properties of grafted membranes 3.2.1. Swelling behavior Fig. 5 represents the swelling% with respect to the degree of grafting. It can be seen that the water uptake decreases as the degree of grafting increases. This behavior can be explained that the pores in gel become smaller due to an increase in the cross-linking density between the polymeric chains due to increased grafting yield. 3.2.2. Gel fraction The gelation% of the PVA-g-AAc/Zol membranes has been determined versus the grafting yield by extraction in boiled water for 4 h. Fig. 6 represents the gelation% against the degree of grafting. It can been that gel content increases with the degree of grafting to reach a maximum value at 185% grafting yield, at which the graft copolymers becomes completely insoluble in boiled water. This is due to an increase in the cross-links between the polymeric chains as the grafting yield increases.
140 120
Degree of grafting [%]
550
Water uptake [%]
584
100 80 60 40 20 0
0
5
10
15
20
25
3.3. Ion uptake
Irradiation dose [kGy]
Fig. 4. Effect of irradiation dose on the degree of grafting of AAc/Zol onto PVA Films; comonomer concentration = 20 wt.%; Me–OH/water mixture composition = 70/ 30 wt.%.
Heavy metals are toxic pollutants, which should be removed from wastewater due to their undesired effects on human physiology and ecological systems even at very low concentrations [25].
Z. Ajji, A.M. Ali / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 580–586
585
110
1000
100 90
Copper ion uptake [mg/g]
80
Gelation [%]
70 60 50 40 30 20
800
600
400
200
10
0
0 -10 0
50
100
150
200
1
Fig. 6. Gelation% versus the degree of grafting for PVA-gAAc/Zol membranes; time of extraction 4 h in hot water (98 °C).
3.3.1. Influence of grafting yield The ion uptake of PVA-g-Zol/AAc membranes versus the degree of grafting is represented in Fig. 7. As the degree of grafting increases, the ion uptake of the membrane increases. This behavior can be reasonably attributed to the increase in the number of functional groups grafted onto the polymer backbone with the increase in the grafting yield. It is also observed that, efficiency of PVA-gAAc/Zol membrane having degree of grafting
Copper ion uptake [mg/g]
1000
800
600
400
200
0 0
20
40
60
80
2
3
4
5
6
7
pH of the copper sulfate solution
Degree of grafting [%]
100
Fig. 8. Effect of pH on amount of copper ion adsorbed onto PVA-g-AAc/Zol membranes (grafting yield = 119%); initial feed concentration = 2000 ppm; time of adsorption = 8 h; ambient temperature = 22 °C.
119% is very high, and the amount of recovered copper ions is about 970 mg/g after 8 h immersion in copper solution. This result can be explained with the availability of different functional groups, which can interact with copper ions in various ways forming ionic bonds (carboxyl group) or/ and complex bonds (hydroxyl or N-imide group). 3.3.2. Effect of pH The complexation of heavy metal ions by chelating-ligand supported on polymeric network is affected by the pH of the solutions [26]. In the present study we changed the pH range between 1 and 6.2. The adsorbed copper ions are illustrated with respect to the pH of the solution in Fig. 8. The copper ions adsorption is very limited at low pH, and the adsorption capacity increases with increasing pH, reaching a plateau values at pH = 6.2. This behavior maybe explained that the carboxylate ion exists in the acidic form, and the nitrogen atom of the imidazole ring is protonated in the strong acidic medium; thus both ligands are not available to form a bond with the metal ion.
120
Degree of grafting [%]
Fig. 7. Effect of degree of grafting on the copper ion uptake for PVA-g-AAc/Zol membranes; initial feed concentration = 2000 ppm; time = 8 h; pH = 5.2; ambient temperature = 22 °C.
4. Conclusion Polymer membranes were prepared based on poly(vinyl alcohol) grafted with acrylic acid and
586
Z. Ajji, A.M. Ali / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 580–586
N-vinyl imidazole using gamma radiation. The grafting conditions were determined regarding their influence on the grafting yield as: type of solvent and solvent composition, comonomer concentration and composition, addition of mineral acids, and irradiation dose. The ability of the grafted films to adsorb copper ions was evaluated with respect to the grafting yield. It increases with increasing the grafting yield. In strong acidic medium (pH = 1) there is a very limited ion adsorption, and at pH ffi6.2 the adsorption achieved a high value of around 970 mg/g; the grafting yield of the membrane was 119%. Thus the prepared membrane may be considered for removing of copper ions from wastewater. Also the limited copper ion capacity in acidic medium may suggest the use of these membranes for separation application. Further work is in progress to elaborate this issue.
Acknowledgments The authors would like to thank Prof. I. Othman for his encouragement. They are also grateful to Mr. F. Heso and Ms. H. Dadah for their efforts during the experiments.
References [1] M. Kim, K. Saito, Radiat. Phys. Chem. 57 (2) (2000) 167. [2] K. Burczak, T. Fujisato, Y. Ikada, Proc. Jpn. Acad. Ser. (B) 67 (5) (1991) 83. [3] V. Shantora, R.Y.M. Huang, J. Appl. Polym. Sci. 26 (1981) 3223.
[4] M.G. Katz, T.J. Wydeven, J. Appl. Polym. Sci. 27 (1982) 79. [5] K. Koyama, T. Nishi, M.J. Nishimura, J. Appl. Polym. Sci. 27 (1982) 2845. [6] S. Peter, R. Stefan, in: Proceeding International Symposium, Fresh Water from the Sea, Athens, Vol. 2, 1980, p. 197. [7] M.G. Katz, T. Wydeven, J. Appl. Polym. Sci. 26 (1981) 2935. [8] N. Pekel, B. Salih, O. Guven, J. Molecular, Catal. B– Enzym. 21 (4–6) (2003) 273. [9] B. Salih, N. Pekel, O. Guven, J. Appl. Polym. Sci. 82 (2) (2001) 446. [10] N. Pekel, H. Savas, O. Guven, Colloid Polym. Sci. 280 (1) (2002) 46. [11] N. Pekel, B. Salih, O. Guven, Macromol. Symp. 169 (2001) 329. [12] N. Pekel, O. Guven, Colloid Polym. Sci. 277 (6) (1999) 570. [13] N. Pekel, N. Sahiner, O. Guven, Radiat. Phys. Chem. 59 (5–6) (2000) 485. [14] N. Pekel, O. Guven, Colloids Surf. A–Physicochem. Eng. Aspects 212 (2–3) (2003) 155. [15] B.N. Misra, Kishore, M. Kanthwal, I.K. Mehta, J. Appl. Polym. Sci. 24 (1986) 2209. [16] H.T. Lokhande, M.D. Teli, J. Appl. Polym. Sci. 29 (1984) 1843. [17] A. Hebeish, S. Shalaby, A. Bayazeid, J. Appl. Polym. Sci. 27 (1982) 197. [18] El-Sayed A. Hegazy, H.A. Abd El-Rehim, H.A. Shawky, Radiat. Phys. Chem. 57 (2000) 85. [19] A.M. Ali, M.Sc. thesis, Ain Shams University, Egypt, 1997. [20] El-Sayed A. Hegaz, H.A. Abd El-Rehim, N.A. Khalifa, A. El-Haj Ali, Radiat. Phys. Chem. 55 (1999) 219. [21] I. Kaur, R. Barsola, J. Appl. Polym. Sci. 41 (1990) 2067. [22] I. Kaur, R. Barsola, B.N. Misra, J. Appl. Polym. Sci. 51 (1994) 2067. [23] S. Dilli, J.L. Garnett, J. Polym. Sci. A-1 4 (1966) 2323. [24] J.L. Garnett, L.-T. Ng, V. Viengkhou, in: Proceedings of the RadTech. 98, North America, Chicago, 1998, p. 627. [25] C.S. Luo, S. Huang, Sep. Sci. Technol. 28 (1993) 1253. [26] B.E. Reed, M.R. Matsumoto, Sep. Sci. Technol. 28 (1993) 2179.