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Synthesis and chelation properties of Mannich polymers derived from piperazine and some hydroxy benzaldoximes
REACTIVE & FUNCTIONAL POLYMERS
Reactive & Functional Polymers 66 (2006) 789–794
www.elsevier.com/locate/react
Synthesis and chelation properties of Mannich polymers derived from piperazine and some hydroxy benzaldoximes Nashwa N. Shafa-Amry a, Fawwaz I. Khalili a, Kais A.K. Ebraheem b, Mohammad S. Mubarak a,* a b
Department of Chemistry, Faculty of Science, University of Jordan, Amman 11942, Jordan Department of Chemistry, Faculty of Science, University of Petra, Amman 11196, Jordan
Received 7 May 2005; received in revised form 14 October 2005; accepted 22 November 2005 Available online 4 January 2006
Abstract Two new oxime containing polymers, poly(salicylaldoxime-3,5-diyl (piperazine N,N 0 -bismethylene)), polymer I, and poly(2,4-dihydroxy-benzaldoxime-3,5-diyl (piperazine N,N 0 -bismethylene)), polymer II, were prepared through Mannichtype condensation of the appropriate hydroxy benzaldoxime with formaldehyde and piperazine. The sorption properties of these polymers towards the trivalent lanthanide ions, Gd3+, Tb3+, Sm3+, Nd3+, and La3+ were studied by a batch equilibration technique as a function of contact time, pH, and counter ion. Polymer II exhibited improved chelation characteristics in comparison with polymer I and displayed faster rates of metal ion uptake and relatively higher capacities and selectivities. 2005 Elsevier B.V. All rights reserved. Keywords: Oxime-containing polymers; Mannich condensation; Trivalent lanthanide ions; Counter ion
1. Introduction Chelate-forming polymers have found widespread applications in the separation, recovery and monitoring of trace heavy metal ions from aqueous solutions [1–13]. The use of chelating polymers in radioactive nuclear waste treatment is attracting a great deal of interest. The chelation characteristics of these materials is largely dependent on the nature of the active chelating groups incorporated into the polymeric matrix and the type of intervening spacer groups connecting the active chelating ligands [14]. * Corresponding author. Tel.: +962 6 5355000x2320; fax: +962 6 5348932. E-mail address: [email protected] (M.S. Mubarak).
Oximes, C@N–OH, an important class of chelating agents [15], have found numerous applications as highly selective reagents for the separation and determination of a number of metal ions [15–17]. Hence, the incorporation of the oxime group in polymers is an attractive route to the preparation of selective chelating polymers. Several oxime-containing polymers, along with methods to incorporate the oxime groups in chelate-forming polymers, have been described in the literature [7,18–28]. Recently, [7], we have demonstrated that a phenol-formaldehyde chelating polymer derived from salicylaldoxime and formaldehyde exhibited high capacity and selectivity towards Cu2+ ions. Mannich-type condensation reaction [29] is used to introduce hydrophilic spacer groups through condensation with a bifunctional amine
1381-5148/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2005.11.009
790
N.N. Shafa-Amry et al. / Reactive & Functional Polymers 66 (2006) 789–794
OH
OH H
H
N OH
R
OH
O H
N
HN H
N
NH
(exces)
N R
n
Polymer I R = H Polymer II R = OH Fig. 1. Synthesis and structure of polymers I and II.
[14,30–34]. In this paper, the phenol, salicylaldoxime or 2,4-dihydroxy-benzaldoxime, is condensed with formaldehyde and piperazine as the bifunctional amine. Two new Mannich chelating polymers containing the oxime group are prepared; poly(salicylaldoxime-3,5-diyl (piperazine N,N 0 -bismethylene)), polymer I, and poly(2,4-dihydroxy-benzaldoxime3,5-diyl(piperazine N,N 0 -bismethylene)), polymer II. Synthesis and structures of polymers I and II are depicted in Fig. 1. The chelation properties of these polymers towards a number of trivalent lanthanide metal ions in aqueous solutions are investigated. 2. Experimental 2.1. Reagents Unless otherwise indicated, all chemicals used were of analytical grade and were used as received; salicylaldoxime, 2,4-dihydroxy-benzaldehyde and piperazine were obtained from Fluka (Switzerland); formaldehyde solution (37–41%), sodium perchlorate (98%), and standard solutions of metal ions were obtained from BDH Chemicals Ltd. (England); hydroxylamine hydrochloride, and the disodium salt of ethylene diamine tetraacetate (EDTA) were obtained from Greenland Chemical Company (UK). The following metal ion salts were also used without further purification; LaCl3 Æ 6 H2O (Aldrich), NdCl3 Æ 6 H2O (Aldrich), SmCl3 Æ 6 H2O (Aldrich), GdCl3 Æ 6 H2O (Aldrich), and TbCl3 Æ 6 H2O (K & K Laboratories Inc.). 2.2. Instrumentation Infrared spectra of the monomers, polymers and their metal chelates were recorded as KBr discs using a Nicolet Impact 400 FTIR-Spectrophotometer from 4000 to 400 cm1. Elemental analyses (C,H,N) were
carried out at Tuebingen University (Germany). Complexometric titrations were performed with a Metrohm 655 Dosimat Titrator (Switzerland). Polymer-metal ion batch equilibration samples were shaken using a GFL-1083 shaker thermostated water bath maintained at 25 C. 2.3. Polymer preparation Polymer I was prepared by dissolving 13.7 g (0.1 mol) of salicylaldoxime and 19.4 g (0.1 mol) of piperazine hexahydrate in 150 mL of 80% aqueous ethanol as the reaction medium. To this solution, 20 mL (0.26 mol) of 37% aqueous formaldehyde was slowly added with stirring, using a magnetic stirrer at room temperature. After 2 h, the mixture was heated, gradually in an oil bath, to 70 C and heating was then continued for 16 more hours. The reflux condenser was converted to distillation and the ethanol was slowly distilled, solution depth was kept constant by the slow addition of water and occasional addition of small portions (5 · 5 mL) of 37% aqueous formaldehyde solution. After most of ethanol had been removed, the reaction mixture was cooled, and the yellow-orange resin lumps were filtered-off and washed well with ethanol and water. Further purification of the resin was achieved by soxhlet extraction with ethanol for 24 h. The resin was then dried in a vacuum oven at room temperature and sieved through mesh size 35–60 (0.25– 0.50 mm). Polymer I decomposes at 150 C. Satisfactory elemental analyses were obtained. Found: %C 61.62, %H 6.96, %N 15.76. Calculated for [C13H17N3O2]n: %C 63.14, %H 6.93, %N 16.99. Polymer II was prepared using the same aforementioned procedure by condensing 2,4-dihydroxy-benzaldoxime, prepared from 2,4-dihydroxy-benzaldehyde and hydroxylamine as described elsewhere [30], with formaldehyde and piperazine in 70% aqueous etha-
N.N. Shafa-Amry et al. / Reactive & Functional Polymers 66 (2006) 789–794
2.4. Sorption of metal ions on the polymer The metal chelation characteristics of the polymer for each metal ion were studied by the batch equilibrium technique; duplicate experiments involving dry 0.100 g resin samples were suspended in 15 mL of sodium acetate–acetic acid buffer solutions adjusted to the desired pH and total ionic strength of 0.10 M using sodium perchlorate solution and were left to equilibrate for 4 h. Then, 10.0 mL of metal ion solution containing a total of 20.0 mg of lanthanide metal ion was added. After being shaken for a definite period of time (1–24 h) at 25 C, the mixture was filtered and the amount of metal ions remaining in the filtrate was determined by compleximetric titration with standard 0.01 M EDTA solution using xylenol orange as the indicator. Experiments on the rate of metal-ion uptake were conducted at a fixed pH of 7, whereas, experiments on the effect of pH were carried out at different pH values ranging from 4.0 to 7.5 under a fixed contact time of 6 h. The effect of counter ions on the metal-ion uptake was also investigated using the same general procedure, where the perchlorate ðClO 4 Þ is replaced by chloride (Cl ), nitrate 2 ðNO3 Þ or sulfate ðSO4 Þ ions at a fixed pH of 7 and for a fixed contact time of 6 h. 3. Results and discussion 3.1. Characterization of the polymer Polymers I and II are insoluble in all common organic solvents. They are also insoluble in strong acid and strong base solutions. The hydrophilic characters of the polymers, as reflected by water regain are 2.996 g water/g resin after 2 h and 3.100 g water/g resin after 24 h for polymer I and 3.228 g water/g resin after 2 h and 3.608 g water/ g resin for polymer II; these values are relatively higher than that of poly(salicylaldoxime-3,5-diylmethylene) [7]. Clearly, the introduction of piperazine spacer group enhances the hydrophilic character. In addition, the presence of an extra hydroxyl group on
the monomer (2,4-dihydroxy-benzaldoxime) in polymer II makes it more hydrophilic than polymer I. These findings agree well with our recent work on related Mannich polymer derived from N,N 0 dimethylethylenediamine and salicylaldoxime [31]. IR spectra of the polymers are consistent with the structures (Fig. 1) assigned to the polymers. The broad bands in the 3000–3550 cm1 region are assigned to the intramolecularly hydrogen bonded O–H stretching vibration [35–37]. These bands are also observed in the spectra of the metal chelates, since only a fraction of the ligands moieties on the polymers are involved in chelate formation. The absorption bands observed at about 2950 and 2860 cm1 have been attributed to the C–H stretching of the methylene groups connecting the aromatic rings with the amine. The C@N stretching vibrations of the oxime groups were observed at 1612 and 1618 cm1 for polymers I and II, respectively. 3.2. Sorption of metal ions on polymers The sorption of various trivalent lanthanide ions (Gd3+, Tb3+, Sm3+, Nd3+, and La3+) on polymers I and II was investigated by a batch equilibrium tech150
140
Metal-ion uptake (mg/g resin)
nol as the reaction medium. Polymer II was obtained as a fine yellow powder which decomposes at 150 C. Satisfactory elemental analyses were obtained. Found: %C 57.26, %H 6.53, %N 15.57. Calculated for [C13H17N3O3]n: %C 59.30, %H 6.51, %N 15.96. Water regain of polymers I and II was determined using the procedure of Sugii et al. [18].
791
130
Tb(III)
120
Gd(III) Sm(III) Nd(III) La(III)
110
100 0
3
6
9 12 15 18 Contact Time (Hours)
21
24
Fig. 2. Metal ion uptake by polymer I as a function of contact time.
792
N.N. Shafa-Amry et al. / Reactive & Functional Polymers 66 (2006) 789–794
nique as a function of contact time at fixed pH of 7.0. The results for the dependence of the metal ion uptake on contact time for polymers I and II are presented in Figs. 1 and 2, respectively. These results indicate fast rates of equilibration; the rates of metal ion uptake increases in the first 1–3 h and reaches a steady state after 6–9 h. It is interesting to note that, during the first 30 min, over 60–70% of the metal ions are taken by the polymers. This constitutes a considerable improvement over the previously published rates of metal ion uptake by various chelating polymers derived from the same active chelating groups [7,30,31]. The chelation characteristics of polymers are more conveniently compared in terms of the distribution coefficients, Kd, [7] defined as Kd ¼
½Mresin ½Msolution
where [M]resin is the amount (in mg) of metal ions taken up by 1 g of resin and [M]solution is the concentration (in mg/mL) of metal ions remaining in solution. The calculated values of Kd at a contact time of 24 h are used to compare the relative sorption capacity of chelate-forming polymers towards vari-
ous metal ions. The following trends are found based on Kd values; enclosed between brackets. Polymer I: Tb3þ ð701:7Þ > Gd3þ ð687:4Þ > La3þ ð478:8Þ > Sm3þ ð449:9Þ > Nd3þ ð439:2Þ Polymer II: Tb3þ ð900:0Þ > Gd3þ ð843:1Þ > La3þ ð515:9Þ > Sm3þ ð484:2Þ > Nd3þ ð452:8Þ This pattern is partially consistent with the reported chelation behavior of the Mannich polymer, poly(8-hydroxyquinoline-5,7-(piperazine-N,N 0 -diylmethylene)) [32] with the trend: Gd3+ > Sm3+ > Nd3+ > La3+. This order follows closely the increase in the hydrated ionic radius in going from Gd3+ to La3+. It is noteworthy that Tb3+ and Gd3+ ions have almost the same ionic radius of 97 pm. The results presented in Figs. 2 and 3 clearly show that polymer II is a more efficient chelating agent than polymer I and shows enhanced selectivity for Tb3+ and Gd3+ ions. The pH dependence of metal ion uptake by polymers I and II was studied in the pH range 4–7.5 for 150
160
140 Metal-on uptake (mg/g resin)
Metal-ion uptake (mg/g resin)
150
140
130
Tb(III)
120
Gd(III)
130
120 Tb(III) Gd(III)
Sm(III)
Sm(III)
Nd(III)
110
110
Nd(III)
La(III)
La(III)
100
100 0
3
6 9 12 15 Contact Time (Hours)
18
21
24
Fig. 3. Metal ion uptake by polymer II as a function of contact time.
4
5
6 pH
7
8
Fig. 4. pH-binding capacity profiles of metal ion uptake by polymer I.
N.N. Shafa-Amry et al. / Reactive & Functional Polymers 66 (2006) 789–794
SO2 4
<
NO 3
< Cl <
ClO 4
whereas, for polymer II, the following trend is obtained
Metal-ion uptake (mg/g resin)
160 Perchlorate
Chloride
Nitrate
Sulfate
150 140 130 120 110 100 Tb(III)
Gd(III)
Sm(III)
Nd(III)
La(III)
Fig. 6. Effects of counter ions on the metal ion uptake by polymer I.
160
Metal-ion uptake (mg/g resin)
a fixed contact time of 6 h. At higher pH values, hydrolysis of the metal ions investigated becomes significant and may compete with polymer chelate formation. The pH-uptake profiles are displayed in Figs. 4 and 5. In general, metal ion uptake starts to increase at pH values higher than 5.5 with polymer II exhibiting higher sensitivity towards changes in pH than polymer I. These findings are in agreement with the pH-profiles of most chelating polymers with N,O binding sites [5–8,35]. The effect of counter ion on the chelation process was investigated by determining the metal-ion uptake as a function of the counter ion at 25 C and in an acetate/acetic acid buffer of pH 7.0. Ionic strength was maintained at 0.100 M using different types of salts such as NaNO3, NaClO4, NaCl, and Na2SO4. The results are displayed in Figs. 6 and 7 for polymers I and II, respectively. In polymer I, the metal ion uptake for all lanthanides increases in the order:
793
Perchlorate
Chloride
Nitrate
Sulfate
150 140 130 120 110 100 Tb(III)
Gd(III)
Sm(III)
Nd(III)
La(III)
150
Fig. 7. Effects of counter ions on the metal ion uptake by polymer II.
Metal-ion uptake (mg/g resin)
140
SO2 4 < Cl < NO3 < ClO4
The use of perchlorate as the counter ion enhances the metal ion uptake by both polymers for all lanthanide ions investigated in agreement with our previous findings for other Mannich polymers derived from 8-hydroxyquinoline [32]. However, the relative influence of the other anions differs. This suggests a rather complicated combination of factors including the relative free energy of hydration of anions [32], the relative stability of lanthanide–anion complexes, and the relative stabilizing effect of anions on polymer–lanthanide chelates.
130
120 Tb(III) Gd(III) Sm(III)
110
Nd(III) La(III)
4. Conclusions 100 4
5
6 pH
7
8
Fig. 5. pH-binding capacity profiles of metal ion uptake by polymer II.
The chelation properties of two chelate-forming polymers derived from piperazine and each of benzaldoxime and 2,4-dihydroxy-benzaldoxime towards some trivalent lanthanide metal ions in aqueous
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N.N. Shafa-Amry et al. / Reactive & Functional Polymers 66 (2006) 789–794
solutions were investigated using a batch equilibration method. The investigation has revealed polymer II has higher metal-ion uptake capacity than polymer I and the extent of metal-ion uptake followed the order Tb3+ > Gd3+ > La3+ > Sm3+ > Nd3+. The effect of exposure time on the metal-ion uptake was also investigated; it was found that the rates of metal ion uptake increases in the first 1– 3 h and reaches a steady-state after 6–9 h and that, during the first 30 min, over 60–70% of the metal ions are taken by the polymers. The pH-binding capacity profiles showed that the metal-ion uptake of the resins increased with increasing pH and reached a maximum at pH 7.0. The effect of counter ions on the sorption properties of the resins was also studied; since perchlorate anions are considered non-complexing, it was not surprising that the metal-ion uptake capacity of the resins was highest when perchlorate was employed as a counter-ion. References [1] C. Kantipuly, S. Katragadda, A. Chow, H.D. Gesser, Talanta 37 (1990) 491. [2] A. Warshawsky, Chelating ion exchangers, in: Critical Reports on Applied Chemistry, Blackwell Scientific Publications, London, 1987, pp. 166–225. [3] J.H. Hogkin, in: H.F. Mark, N.M. Bikales, C.G. Overberger, G. Menges (Eds.), Encyclopedia of Polymer Science and Engineering, vol. 3, John Wiley & Sons, New York, 1985, pp. 363–381. [4] N. Kabay, H. Egawa, Separ. Sci. Technol. 29 (1994) 135. [5] K.A.K. Ebraheem, M.S. Mubarak, Z.J. Yassien, F. Khalili, Solvent Extr. Ion Exc. 16 (1998) 637. [6] K.A.K. Ebraheem, M.S. Mubarak, Z.J. Yassien, F. Khalili, Separ. Sci. Technol. 35 (2000) 2115. [7] K.A.K. Ebraheem, S.T. Hamdi, React. Funct. Polym. 34 (1997) 5. [8] K.A.K. Ebraheem, S.T. Hamdi, J.A. Al-Duhan, J. Macromol. Sci.-Pure Appl. Chem. A34 (1997) 1691. [9] H. Matsuyama, Y. Miyamoto, M. Teramoto, M. Goto, F. Nakashio, Separ. Sci. Technol. 31 (1996) 687; H. Matsuyama, Y. Miyamoto, M. Teramoto, M. Goto, F. Nakashio, Separ. Sci. Technol. 31 (1996) 799.
[10] A. Lezzi, S. Cobianco, A. Roggero, J. Polym. Sci., Polym. Chem. Ed. 32 (1994) 1817. [11] D. Zhao, A.K. Sengupta, Y. Zhu, Ind. Eng. Chem. Res. 34 (1995) 2676. [12] A. Ramanta, A.K. Sengupta, J. Environ. Eng. 118 (1992) 755. [13] M.B. Colella, S. Siggia, R.M. Barnes, Anal. Chem. 52 (1980) 2347. [14] N.M. Salem, K.A.K. Ebraheem, M.S. Mubarak, React. Funct. Polym. 59 (2004) 63. [15] R.B. Singh, B.S. Garg, R.P. Singh, Talanta 26 (1979) 425. [16] A.W. Ashbrook, J. Chromatogr. 105 (1975) 141. [17] M. Hemmes, J.R. Parrish, Anal. Chim. Acta 94 (1977) 307. [18] A. Sugii, N. Ogawa, H. Hashizume, Talanta 26 (1979) 189. [19] F. Vernon, T. Shah, React. Polym. 1 (1983) 301. [20] D.J. Walsh, P. Crosby, R.F. Dalton, Polymer 1 (1983) 423. [21] A. Warshawsky, Inst. Min. Metall., Trans., Sec. C 83 (1974) 101. [22] A. Patchornik, R. Klair, M. Fridkin, A. Warshawsky, Ger. Offen. 1975, 2,504,397 (Chemical Abstracts, 83: 206988a). [23] A. Warshawsky, R. Klair, H. Bercovitz, Extended Abstract. Proceedings of the International Symposium on Analytical Chemistry Explor. Min. Process. Mater., 1977, p. 139 (Chemical Abstracts, 90: 80261g). [24] A. Warshawsky, H. Bercovitz, Inst. Min. Metall., Trans., Sec. C 88 (1979) 36. [25] L.D. Prabhakar, C. Umarani, J. Macromol. Sci. -Pure Appl. Chem., Sci. 100 (1995) 129. [26] J.S. Parmer, M.R. Patel, M.M. Patel, J. Macromol. Sci., Chem. A 20 (1983) 79, and references cited therein. [27] B.K. Patel, M.M. Patel, Indian Acad. Sci., Chem. Sci. 100 (1988) 405. [28] B.K. Patel, M.M. Patel, Indian J. Chem. 29 A (1990) 90. [29] F.F. Blicke, The mannich reacionOrganic Reactions, vol. 1, John Wiley & Sons, Chichester, 1947, p. 303. [30] S.I. Al-Gharabli, K.A.K. Ebraheem, M.S. Mubarak, J. Saudi Chem. Soc. 5 (2001) 399. [31] K.A.K. Ebraheem, M.S. Mubarak, S.I. Al-Gharabli, J. Macromol. Sci.-Pure Appl. Chem. A39 (2002) 217. [32] A.I. Ismail, K.A.K. Ebraheem, M.S. Mubarak, F.I. Khalili, Solvent Extr. Ion Exc. 21 (2003) 125. [33] J.H. Hogkin, J. Polym. Sci., Part A – Polym. Chem. Ed. 24 (1986) 3117. [34] J.H. Hogkin, R. Eibl, React. Polym. 3 (1985) 83; J.H. Hogkin, R. Eibl, React. Polym. 4 (1986) 285. [35] K.A.K. Ebraheem, J.A. Al-Duhan, S.T. Hamdi, Eur. Polym. J. 21 (1985) 97. [36] L. Bellamy, third ed.Infrared Spectra of Complex Molecules, vol. 1, Chapman and Hall, London, 1975. [37] A. Bigotto, J. Mol. Struct. 175 (1985) 377.
Reactive & Functional Polymers 66 (2006) 789–794
www.elsevier.com/locate/react
Synthesis and chelation properties of Mannich polymers derived from piperazine and some hydroxy benzaldoximes Nashwa N. Shafa-Amry a, Fawwaz I. Khalili a, Kais A.K. Ebraheem b, Mohammad S. Mubarak a,* a b
Department of Chemistry, Faculty of Science, University of Jordan, Amman 11942, Jordan Department of Chemistry, Faculty of Science, University of Petra, Amman 11196, Jordan
Received 7 May 2005; received in revised form 14 October 2005; accepted 22 November 2005 Available online 4 January 2006
Abstract Two new oxime containing polymers, poly(salicylaldoxime-3,5-diyl (piperazine N,N 0 -bismethylene)), polymer I, and poly(2,4-dihydroxy-benzaldoxime-3,5-diyl (piperazine N,N 0 -bismethylene)), polymer II, were prepared through Mannichtype condensation of the appropriate hydroxy benzaldoxime with formaldehyde and piperazine. The sorption properties of these polymers towards the trivalent lanthanide ions, Gd3+, Tb3+, Sm3+, Nd3+, and La3+ were studied by a batch equilibration technique as a function of contact time, pH, and counter ion. Polymer II exhibited improved chelation characteristics in comparison with polymer I and displayed faster rates of metal ion uptake and relatively higher capacities and selectivities. 2005 Elsevier B.V. All rights reserved. Keywords: Oxime-containing polymers; Mannich condensation; Trivalent lanthanide ions; Counter ion
1. Introduction Chelate-forming polymers have found widespread applications in the separation, recovery and monitoring of trace heavy metal ions from aqueous solutions [1–13]. The use of chelating polymers in radioactive nuclear waste treatment is attracting a great deal of interest. The chelation characteristics of these materials is largely dependent on the nature of the active chelating groups incorporated into the polymeric matrix and the type of intervening spacer groups connecting the active chelating ligands [14]. * Corresponding author. Tel.: +962 6 5355000x2320; fax: +962 6 5348932. E-mail address: [email protected] (M.S. Mubarak).
Oximes, C@N–OH, an important class of chelating agents [15], have found numerous applications as highly selective reagents for the separation and determination of a number of metal ions [15–17]. Hence, the incorporation of the oxime group in polymers is an attractive route to the preparation of selective chelating polymers. Several oxime-containing polymers, along with methods to incorporate the oxime groups in chelate-forming polymers, have been described in the literature [7,18–28]. Recently, [7], we have demonstrated that a phenol-formaldehyde chelating polymer derived from salicylaldoxime and formaldehyde exhibited high capacity and selectivity towards Cu2+ ions. Mannich-type condensation reaction [29] is used to introduce hydrophilic spacer groups through condensation with a bifunctional amine
1381-5148/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2005.11.009
790
N.N. Shafa-Amry et al. / Reactive & Functional Polymers 66 (2006) 789–794
OH
OH H
H
N OH
R
OH
O H
N
HN H
N
NH
(exces)
N R
n
Polymer I R = H Polymer II R = OH Fig. 1. Synthesis and structure of polymers I and II.
[14,30–34]. In this paper, the phenol, salicylaldoxime or 2,4-dihydroxy-benzaldoxime, is condensed with formaldehyde and piperazine as the bifunctional amine. Two new Mannich chelating polymers containing the oxime group are prepared; poly(salicylaldoxime-3,5-diyl (piperazine N,N 0 -bismethylene)), polymer I, and poly(2,4-dihydroxy-benzaldoxime3,5-diyl(piperazine N,N 0 -bismethylene)), polymer II. Synthesis and structures of polymers I and II are depicted in Fig. 1. The chelation properties of these polymers towards a number of trivalent lanthanide metal ions in aqueous solutions are investigated. 2. Experimental 2.1. Reagents Unless otherwise indicated, all chemicals used were of analytical grade and were used as received; salicylaldoxime, 2,4-dihydroxy-benzaldehyde and piperazine were obtained from Fluka (Switzerland); formaldehyde solution (37–41%), sodium perchlorate (98%), and standard solutions of metal ions were obtained from BDH Chemicals Ltd. (England); hydroxylamine hydrochloride, and the disodium salt of ethylene diamine tetraacetate (EDTA) were obtained from Greenland Chemical Company (UK). The following metal ion salts were also used without further purification; LaCl3 Æ 6 H2O (Aldrich), NdCl3 Æ 6 H2O (Aldrich), SmCl3 Æ 6 H2O (Aldrich), GdCl3 Æ 6 H2O (Aldrich), and TbCl3 Æ 6 H2O (K & K Laboratories Inc.). 2.2. Instrumentation Infrared spectra of the monomers, polymers and their metal chelates were recorded as KBr discs using a Nicolet Impact 400 FTIR-Spectrophotometer from 4000 to 400 cm1. Elemental analyses (C,H,N) were
carried out at Tuebingen University (Germany). Complexometric titrations were performed with a Metrohm 655 Dosimat Titrator (Switzerland). Polymer-metal ion batch equilibration samples were shaken using a GFL-1083 shaker thermostated water bath maintained at 25 C. 2.3. Polymer preparation Polymer I was prepared by dissolving 13.7 g (0.1 mol) of salicylaldoxime and 19.4 g (0.1 mol) of piperazine hexahydrate in 150 mL of 80% aqueous ethanol as the reaction medium. To this solution, 20 mL (0.26 mol) of 37% aqueous formaldehyde was slowly added with stirring, using a magnetic stirrer at room temperature. After 2 h, the mixture was heated, gradually in an oil bath, to 70 C and heating was then continued for 16 more hours. The reflux condenser was converted to distillation and the ethanol was slowly distilled, solution depth was kept constant by the slow addition of water and occasional addition of small portions (5 · 5 mL) of 37% aqueous formaldehyde solution. After most of ethanol had been removed, the reaction mixture was cooled, and the yellow-orange resin lumps were filtered-off and washed well with ethanol and water. Further purification of the resin was achieved by soxhlet extraction with ethanol for 24 h. The resin was then dried in a vacuum oven at room temperature and sieved through mesh size 35–60 (0.25– 0.50 mm). Polymer I decomposes at 150 C. Satisfactory elemental analyses were obtained. Found: %C 61.62, %H 6.96, %N 15.76. Calculated for [C13H17N3O2]n: %C 63.14, %H 6.93, %N 16.99. Polymer II was prepared using the same aforementioned procedure by condensing 2,4-dihydroxy-benzaldoxime, prepared from 2,4-dihydroxy-benzaldehyde and hydroxylamine as described elsewhere [30], with formaldehyde and piperazine in 70% aqueous etha-
N.N. Shafa-Amry et al. / Reactive & Functional Polymers 66 (2006) 789–794
2.4. Sorption of metal ions on the polymer The metal chelation characteristics of the polymer for each metal ion were studied by the batch equilibrium technique; duplicate experiments involving dry 0.100 g resin samples were suspended in 15 mL of sodium acetate–acetic acid buffer solutions adjusted to the desired pH and total ionic strength of 0.10 M using sodium perchlorate solution and were left to equilibrate for 4 h. Then, 10.0 mL of metal ion solution containing a total of 20.0 mg of lanthanide metal ion was added. After being shaken for a definite period of time (1–24 h) at 25 C, the mixture was filtered and the amount of metal ions remaining in the filtrate was determined by compleximetric titration with standard 0.01 M EDTA solution using xylenol orange as the indicator. Experiments on the rate of metal-ion uptake were conducted at a fixed pH of 7, whereas, experiments on the effect of pH were carried out at different pH values ranging from 4.0 to 7.5 under a fixed contact time of 6 h. The effect of counter ions on the metal-ion uptake was also investigated using the same general procedure, where the perchlorate ðClO 4 Þ is replaced by chloride (Cl ), nitrate 2 ðNO3 Þ or sulfate ðSO4 Þ ions at a fixed pH of 7 and for a fixed contact time of 6 h. 3. Results and discussion 3.1. Characterization of the polymer Polymers I and II are insoluble in all common organic solvents. They are also insoluble in strong acid and strong base solutions. The hydrophilic characters of the polymers, as reflected by water regain are 2.996 g water/g resin after 2 h and 3.100 g water/g resin after 24 h for polymer I and 3.228 g water/g resin after 2 h and 3.608 g water/ g resin for polymer II; these values are relatively higher than that of poly(salicylaldoxime-3,5-diylmethylene) [7]. Clearly, the introduction of piperazine spacer group enhances the hydrophilic character. In addition, the presence of an extra hydroxyl group on
the monomer (2,4-dihydroxy-benzaldoxime) in polymer II makes it more hydrophilic than polymer I. These findings agree well with our recent work on related Mannich polymer derived from N,N 0 dimethylethylenediamine and salicylaldoxime [31]. IR spectra of the polymers are consistent with the structures (Fig. 1) assigned to the polymers. The broad bands in the 3000–3550 cm1 region are assigned to the intramolecularly hydrogen bonded O–H stretching vibration [35–37]. These bands are also observed in the spectra of the metal chelates, since only a fraction of the ligands moieties on the polymers are involved in chelate formation. The absorption bands observed at about 2950 and 2860 cm1 have been attributed to the C–H stretching of the methylene groups connecting the aromatic rings with the amine. The C@N stretching vibrations of the oxime groups were observed at 1612 and 1618 cm1 for polymers I and II, respectively. 3.2. Sorption of metal ions on polymers The sorption of various trivalent lanthanide ions (Gd3+, Tb3+, Sm3+, Nd3+, and La3+) on polymers I and II was investigated by a batch equilibrium tech150
140
Metal-ion uptake (mg/g resin)
nol as the reaction medium. Polymer II was obtained as a fine yellow powder which decomposes at 150 C. Satisfactory elemental analyses were obtained. Found: %C 57.26, %H 6.53, %N 15.57. Calculated for [C13H17N3O3]n: %C 59.30, %H 6.51, %N 15.96. Water regain of polymers I and II was determined using the procedure of Sugii et al. [18].
791
130
Tb(III)
120
Gd(III) Sm(III) Nd(III) La(III)
110
100 0
3
6
9 12 15 18 Contact Time (Hours)
21
24
Fig. 2. Metal ion uptake by polymer I as a function of contact time.
792
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nique as a function of contact time at fixed pH of 7.0. The results for the dependence of the metal ion uptake on contact time for polymers I and II are presented in Figs. 1 and 2, respectively. These results indicate fast rates of equilibration; the rates of metal ion uptake increases in the first 1–3 h and reaches a steady state after 6–9 h. It is interesting to note that, during the first 30 min, over 60–70% of the metal ions are taken by the polymers. This constitutes a considerable improvement over the previously published rates of metal ion uptake by various chelating polymers derived from the same active chelating groups [7,30,31]. The chelation characteristics of polymers are more conveniently compared in terms of the distribution coefficients, Kd, [7] defined as Kd ¼
½Mresin ½Msolution
where [M]resin is the amount (in mg) of metal ions taken up by 1 g of resin and [M]solution is the concentration (in mg/mL) of metal ions remaining in solution. The calculated values of Kd at a contact time of 24 h are used to compare the relative sorption capacity of chelate-forming polymers towards vari-
ous metal ions. The following trends are found based on Kd values; enclosed between brackets. Polymer I: Tb3þ ð701:7Þ > Gd3þ ð687:4Þ > La3þ ð478:8Þ > Sm3þ ð449:9Þ > Nd3þ ð439:2Þ Polymer II: Tb3þ ð900:0Þ > Gd3þ ð843:1Þ > La3þ ð515:9Þ > Sm3þ ð484:2Þ > Nd3þ ð452:8Þ This pattern is partially consistent with the reported chelation behavior of the Mannich polymer, poly(8-hydroxyquinoline-5,7-(piperazine-N,N 0 -diylmethylene)) [32] with the trend: Gd3+ > Sm3+ > Nd3+ > La3+. This order follows closely the increase in the hydrated ionic radius in going from Gd3+ to La3+. It is noteworthy that Tb3+ and Gd3+ ions have almost the same ionic radius of 97 pm. The results presented in Figs. 2 and 3 clearly show that polymer II is a more efficient chelating agent than polymer I and shows enhanced selectivity for Tb3+ and Gd3+ ions. The pH dependence of metal ion uptake by polymers I and II was studied in the pH range 4–7.5 for 150
160
140 Metal-on uptake (mg/g resin)
Metal-ion uptake (mg/g resin)
150
140
130
Tb(III)
120
Gd(III)
130
120 Tb(III) Gd(III)
Sm(III)
Sm(III)
Nd(III)
110
110
Nd(III)
La(III)
La(III)
100
100 0
3
6 9 12 15 Contact Time (Hours)
18
21
24
Fig. 3. Metal ion uptake by polymer II as a function of contact time.
4
5
6 pH
7
8
Fig. 4. pH-binding capacity profiles of metal ion uptake by polymer I.
N.N. Shafa-Amry et al. / Reactive & Functional Polymers 66 (2006) 789–794
SO2 4
<
NO 3
< Cl <
ClO 4
whereas, for polymer II, the following trend is obtained
Metal-ion uptake (mg/g resin)
160 Perchlorate
Chloride
Nitrate
Sulfate
150 140 130 120 110 100 Tb(III)
Gd(III)
Sm(III)
Nd(III)
La(III)
Fig. 6. Effects of counter ions on the metal ion uptake by polymer I.
160
Metal-ion uptake (mg/g resin)
a fixed contact time of 6 h. At higher pH values, hydrolysis of the metal ions investigated becomes significant and may compete with polymer chelate formation. The pH-uptake profiles are displayed in Figs. 4 and 5. In general, metal ion uptake starts to increase at pH values higher than 5.5 with polymer II exhibiting higher sensitivity towards changes in pH than polymer I. These findings are in agreement with the pH-profiles of most chelating polymers with N,O binding sites [5–8,35]. The effect of counter ion on the chelation process was investigated by determining the metal-ion uptake as a function of the counter ion at 25 C and in an acetate/acetic acid buffer of pH 7.0. Ionic strength was maintained at 0.100 M using different types of salts such as NaNO3, NaClO4, NaCl, and Na2SO4. The results are displayed in Figs. 6 and 7 for polymers I and II, respectively. In polymer I, the metal ion uptake for all lanthanides increases in the order:
793
Perchlorate
Chloride
Nitrate
Sulfate
150 140 130 120 110 100 Tb(III)
Gd(III)
Sm(III)
Nd(III)
La(III)
150
Fig. 7. Effects of counter ions on the metal ion uptake by polymer II.
Metal-ion uptake (mg/g resin)
140
SO2 4 < Cl < NO3 < ClO4
The use of perchlorate as the counter ion enhances the metal ion uptake by both polymers for all lanthanide ions investigated in agreement with our previous findings for other Mannich polymers derived from 8-hydroxyquinoline [32]. However, the relative influence of the other anions differs. This suggests a rather complicated combination of factors including the relative free energy of hydration of anions [32], the relative stability of lanthanide–anion complexes, and the relative stabilizing effect of anions on polymer–lanthanide chelates.
130
120 Tb(III) Gd(III) Sm(III)
110
Nd(III) La(III)
4. Conclusions 100 4
5
6 pH
7
8
Fig. 5. pH-binding capacity profiles of metal ion uptake by polymer II.
The chelation properties of two chelate-forming polymers derived from piperazine and each of benzaldoxime and 2,4-dihydroxy-benzaldoxime towards some trivalent lanthanide metal ions in aqueous
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solutions were investigated using a batch equilibration method. The investigation has revealed polymer II has higher metal-ion uptake capacity than polymer I and the extent of metal-ion uptake followed the order Tb3+ > Gd3+ > La3+ > Sm3+ > Nd3+. The effect of exposure time on the metal-ion uptake was also investigated; it was found that the rates of metal ion uptake increases in the first 1– 3 h and reaches a steady-state after 6–9 h and that, during the first 30 min, over 60–70% of the metal ions are taken by the polymers. The pH-binding capacity profiles showed that the metal-ion uptake of the resins increased with increasing pH and reached a maximum at pH 7.0. The effect of counter ions on the sorption properties of the resins was also studied; since perchlorate anions are considered non-complexing, it was not surprising that the metal-ion uptake capacity of the resins was highest when perchlorate was employed as a counter-ion. References [1] C. Kantipuly, S. Katragadda, A. Chow, H.D. Gesser, Talanta 37 (1990) 491. [2] A. Warshawsky, Chelating ion exchangers, in: Critical Reports on Applied Chemistry, Blackwell Scientific Publications, London, 1987, pp. 166–225. [3] J.H. Hogkin, in: H.F. Mark, N.M. Bikales, C.G. Overberger, G. Menges (Eds.), Encyclopedia of Polymer Science and Engineering, vol. 3, John Wiley & Sons, New York, 1985, pp. 363–381. [4] N. Kabay, H. Egawa, Separ. Sci. Technol. 29 (1994) 135. [5] K.A.K. Ebraheem, M.S. Mubarak, Z.J. Yassien, F. Khalili, Solvent Extr. Ion Exc. 16 (1998) 637. [6] K.A.K. Ebraheem, M.S. Mubarak, Z.J. Yassien, F. Khalili, Separ. Sci. Technol. 35 (2000) 2115. [7] K.A.K. Ebraheem, S.T. Hamdi, React. Funct. Polym. 34 (1997) 5. [8] K.A.K. Ebraheem, S.T. Hamdi, J.A. Al-Duhan, J. Macromol. Sci.-Pure Appl. Chem. A34 (1997) 1691. [9] H. Matsuyama, Y. Miyamoto, M. Teramoto, M. Goto, F. Nakashio, Separ. Sci. Technol. 31 (1996) 687; H. Matsuyama, Y. Miyamoto, M. Teramoto, M. Goto, F. Nakashio, Separ. Sci. Technol. 31 (1996) 799.
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