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Chelating resins with N-substituted diamides of malonic acid as ligands
PII:
Eur. Polym. J. Vol. 34, No. 11, pp. 1657±1662, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0014-3057/99 $ - see front matter S0014-3057(98)00018-4
CHELATING RESINS WITH N-SUBSTITUTED DIAMIDES OF MALONIC ACID AS LIGANDS ANDRZEJ W. TROCHIMCZUK Institute of Organic and Polymer Technology, Technical University of Wroclaw, Wybrzeze St.Wyspianskiego 27, 50-370 Wroclaw, Poland (Received 14 April 1997; accepted in ®nal form 18 September 1997) AbstractÐNew chelating resins with improved uptake of metal ions have been synthesized by functionalizing a vinylbenzyl chloride±divinylbenzene copolymer with a sodium salt of diethyl malonate followed by reacting modi®ed beads with diamines (ethylenediamine, diethylenetriamine, 2-aminomethyl pyridine, 3-aminopropyl imidazol). Their chelating properties towards Cu(II), Cd(II), Ni(II) and Zn(II) from an acetate buer of pH 3.7 and 5.6 have been studied. It was found that the sorption of the above cations on studied resins was higher than the sorption on reference ones containing respective Nsubstituted amides of monocarboxylic acid. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
Complexing agents immobilized on insoluble polymers are well known materials for the removal of metal cations from aqueous solutions. Main advantages of such materials are easy loading and in most cases stripping of cations with simple chemicals, reusability and the possibility of semi-continuous operation. A vast number of commonly used low-molecular complexants was immobilized on polymer matrices; among them those containing nitrogen received a lot of attention due to their strong anity for divalent transition metal cations. Examples of such materials include oligoethyleneimines [1±3], hydrazine, ethylenediamine, diethylenetriamine and triethylenetetraamine, bound to acrylonitrile±divinylbenzene copolymers [4]. However, in the case of work [1], the resin contained also mercapto groups. Some resins were obtained by polymerization of suitable monomers like for example one obtained from vinylamine precursor [5]. In the above materials amino groups capable of complexing metal cations are deposited randomly and for an ecient metal uptake they rely on ¯exibility of the ligand or in the case of only slightly crosslinked systems also on chain movements. As a result only a fraction of ligands is used. In some papers immobilization of more rigid compounds with multiple nitrogens atoms in one ligand was reported Ð for example 3-amino-1,2,4 triazole was immobilized on poly(vinyl phenol) and showed high anity for divalent metal cations at pH 5.3 [6]. In Ref. [7] di- and tridentate pyrazole containing ligands were immobilized on a glycidyl methacrylate resin. Also low molecular complexing agents such as bipyridyl having ®xed distances between nitrogens have been shown to retain complexing properties after immobilization [8]. Similar compound Ð dipyridylamine has been immobilized on chloromethylstyrene polymer [9]. Recently, bis(imidazo-2-yl)-methylaminomethane has been im-
mobilized on hydrophilic resins, crosslinked with trimethylolpropane trimethacrylate and proved to be Cu(II) selective [10]. Low molecular amides and diamides of malonic acid have been receiving great attention since they are known to be good complexing agents. For example in Ref. [11] synthesis of N-substituted diamides of malonic acid containing lipophilic chain was presented recently. In [12] N-substituted diamides of malonic acid, containing terminal amino groups, were investigated to show the eect of ring size on the stability of Cu(II) complexes. Another group presented the complexing ability of tetraethylmalonodiamide towards lanthanides and the structure of this compound Ð La(III) complex [13]. In [14] diamides of malonic and glutaric acids were synthesized and their complexing ability towards Cu(II) was used in the transport of cations through liquid membrane. The aim of the present work is to synthesize series of complexing resins using reactions of various amines with polymer immobilized diethyl malonate and to compare their properties with reference material containing respective N-substituted amides of monocarboxylic acid. EXPERIMENTAL
Materials Copolymers of vinylbenzyl chloride (VBC) and technical divinylbenzene (DVB), nominal crosslinking degree 2 wt%, is prepared by suspension polymerization of monomers with 0.5 wt% of benzoyl peroxide as initiator. The polymerization is carried out in the presence of 50 wt% of toluene. The organic phase is suspended in a continuous aqueous phase Ð 2 wt% sodium chloride solution containing 1 wt% of poly(vinyl alcohol) as stabilizer and heated at 608C for 1 h, at 708C for 1 h, at 858C for 2 h and at 958 for 5 h. After polymerization, the beads are washed with water, hot water and methanol, dried and extracted with toluene.
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Such polymers react with a sodium derivative of diethyl malonate. Thus, 0.375 mol (60.0 g) of diethyl malonate is dissolved in 400 ml of dry dioxane and subsequently 0.375 mol of NaH (60% suspension in mineral oil) is added in portions. After this reaction is ®nished 20 g of VBC/DVB polymer is added and the entire mixture is re¯uxed for 40 h. Next, the beads are washed with dioxane, acetone, water and acetone. A small sample of the modi®ed polymer is hydrolysed using 3 M NaOH. Thus, 2 g of modi®ed beads is re¯uxed for 8 h with 50 ml of 3 M NaOH followed by treating with water, 1 M HCl, water. This conditioning procedure is repeated three times. A polymer having diethyl malonate ligands is modi®ed using the following amines: ethylenediamine, diethylenetriamine, 2-aminomethylpyridine and 3-aminopropylimidazole (Resins 1±4). The reaction is carried out in each case by heating 5 g of polymer with 50 ml of amine at 1258C for 24 h. After the reaction, polymers are washed with diluted hydrochloric acid, water, 1 M HCl, water, 1 M NaOH, water, 1 M HCl and water. The conditioning procedure is repeated three times. Reference polymers are prepared by reaction of VBC/ DVB polymer with KCN in dimethyl formamide. Thus, 15 g of polymer is reacted with 18 g of potassium cyanide in 215 g of DMF at 80±908C for 8 h. The resulting polymer is reacted with the same four amines (Resins 5±8). Reaction conditions are the same as the synthesis of Resins 1±4.
RESULTS AND DISCUSSION
Synthesis of resins The starting polymer containing 6.25 mmol of Cl/ g was reacted with the sodium derivative of diethyl malonate under conditions described in Section 2. The sample of the resulting polymer was subjected to hydrolysis with 3 M NaOH and an acid capacity 6.20 mmol/g was found. Since during hydrolysis a net weight loss was observed it could be calculated that the amount of ester groups (i.e. before hydrolysis) was 5.28 mmol/g. This value corresponds to a 60% yield of VBC/DVB polymer modi®cation with diethyl malonate. However, chlorine analysis showed only a very small amount of that element Ð 0.12 mmol/g. The possible explanation is that sodium from diethyl malonate is transferred to an already modi®ed site and in a subsequent step it reacts with the neighboring chloromethyl group in a reaction leading to additional chemical crosslinks and thus lowering the Cl concentration. The same explanation was given by Finnish authors in the case of chemical modi®cation of chloromethyl groups with a sodium salt of methylenediphosphonate [15]. In the presented case, a negligible concentration of chloromethyl groups is essential since it allows for the next step of planned modi®cation Ð aminolysis of the ester groups without introducing unwanted ligands into the polymer structure. As can be seen in Fig. 1, the next step is leading to
Methods The water regain is measured using a centrifugation method. Thus, approx. 1 g of resin swollen with water is placed in a fritted column and centrifuged at 3000 rpm for 5 min, weighted and dried for 17 h at 808C. After cooling in a desiccator, the column is weighted with, and without polymer. The water regain is calculated as: (mwÿmd)/md where: mw Ð wet weight, md Ð dry weight. The amount of carboxylic groups is measured by converting the sample into the acid form and bringing a known amount in contact with 100 ml of a 0.1 M NaOH solution for 24 h. After this time 50 ml aliquot is taken and titrated with 0.1 M HCl. The nitrogen content is determined using the Kiejdahl method. The chlorine content is measured by burning approx. 25 mg sample of polymer in an oxygen ®lled ¯ask containing 15 ml of 3% hydrogen peroxide solution. After 30 min, walls are washed with water and the Volhardt method is used for Cl determination. In kinetic experiments, the amount of swollen resins equivalent to 0.5 mmol of ligand is contacted with 100 ml of Cu(II) solution at pH 3.7 and small samples are collected at appropriate intervals for AA analysis. In order to determine the sorption capacity of resins towards Cu(II), Cd(II), Ni(II) and Zn(II), they are contacted with 1 10ÿ4 N Me(II) solutions in 0.2 M acetate buer pH 3.7 and 5.6. Thus, resin equivalent to 0.05 mmol of ligand (0.1 mmol of N for Resins 1, 2, 5 and 6 and 0.15 mmol of N for Resins 3, 4, 7 and 8) is shaken with 10 ml of the given metal solution for 48 h. The solution is then ®ltered and the metal ion concentration is measured using atomic absorption spectroscopy on a Varian 250+ spectrophotometer with wavelength set at 324.8, 228.8, 232.0 and 213.9 nm for Cu(II), Cd(II), Ni(II) and Zn(II), respectively.
Fig. 1. Scheme of investigated resins synthesis.
Chelating resins with N-substituted diamides Table 1. Characteristics of amino resins Resin number 1 2 3 4 5 6 7 8
N content (mmol/g) 6.45 7.02 4.29 7.81 5.92 9.36 4.00 6.98
Ligand* content Water regain (g/ (mmol/g) g) 3.23 2.34 2.15 2.60 2.96 3.12 2.00 2.33
0.97 1.06 0.27 0.80 0.77 1.10 0.33 0.36
*Ligand de®ned as -C(O)NHR.
amides of malonic acid. In order to assure the conversion of both ester groups of malonate ligand, an 20±30 fold excess of amine is used in all cases. Resin 1 is obtained using ethylenediamine and displays a nitrogen capacity of 6.45 mmol/g (Table 1). This gives an 80% yield of modi®cation. Repeated aminolysis did not result in an increase of nitrogen capacity. The Hecker method [16] was applied to determine the content of amino groups. The obtained value has been 3.08 mmol/g and indicates that despite a huge excess of amine, some secondary reactions are possible. These could be reactions of the terminal amino group in already formed amide with either ester originating from the same malo-
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nate ligand, thus forming a seven member ring, or ester from another ligand with formation of chemical crosslink. Similar modi®cations but with diethylenetriamine (Resin 2) resulted in polymer with a nitrogen capacity of 7.02 mmol/g, which corresponded to a 75% yield. Better results were obtained in the case of modi®cation with 2-aminomethyl pyridine and 3aminopropyl imidazol. The obtained yield for Resin 3 and Resin 4 was 84 and 89%, respectively. In both of the above reactions there was no possibility of secondary crosslinks as each of these amines had only one primary amino group. Next, a set of reference resins was prepared (Resins 5±8). In order to make amides of monocarboxylic acid a synthesis route depicted in Fig. 2 was chosen. It is well known [17] that modi®cation of VBC/DVB copolymer with KCN in dimethylformamide proceeds quantitatively. Chloromethyl groups were not detected after this reaction and in a subsequent step, polymer with 5.53 mmol of nitrile groups per gram was functionalized with ethylenediamine (Resin 5), diethylenetriamine (Resin 6), 2aminomethyl pyridine (Resin 7) and 3-aminopropyl imidazol (Resin 8). In the case of Resin 5, the nitrogen capacity was found to be 5.92 mmol/g, giving an 82% yield. Again, checking the concentration of
Fig. 2. Scheme of reference resins synthesis.
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Table 2. Logarithm of Cu(II) distribution coecient as a function of contact time Time (h) Resin
2
4
8
24
48
1 2 3 4 5 6 7 8
1.18 2.62 1.51 3.87 0.98 2.11 1.42 1.60
1.57 3.00 1.84 4.51 1.20 2.53 1.76 1.83
1.92 3.49 2.03 4.73 1.71 3.02 2.11 2.31
2.00 3.51 2.36 5.18 1.82 3.07 2.30 2.63
2.00 3.51 2.40 5.18 1.82 3.07 2.30 2.63
amino groups using the Hecker method gave 2.74 mmol/g, which was less than could be expected from nitrogen analysis (5.92:2 = 2.96 mmol/g) and indicated a slight crosslinking. Similarly, in the case of Resin 6 N = 9.36 mmol/g corresponded to an 89% yield. In both cases, IR spectra showed an absence of peaks at 2240 cmÿ1 characteristic for the stretching of the carbon±nitrogen bond in nitriles. This combined with lower than expected nitrogen content proves the reaction of terminal amino groups from N-substituted amide with nitrile groups. Resins 7 and 8 display nitrogen capacity 5.20 and 6.98 mmol/g, respectively. This gives a 75 and 88% yield. As can be seen in Table 1, resins obtained with linear amines posses moderate hydrophilicity, whereas those synthesized with amines containing pyridyl or imidazole ring are less hydrophilic as manifested by their lower water uptake. It should be also noted that respective pairs of resins, which complexing properties are to be compared, display quite similar ligand capacity and water uptake.
Batch experiments Before batch experiments started, the kinetics of copper (II) uptake by the resins was investigated. Resins were shaken with a solution of Cu(NO3)3 at pH 3.7 (for details see Section 2) and the results obtained are presented in Table 2. It could be concluded that for obtaining an equilibrium distribution coecient it was necessary to measure the metal concentration after a 48 h contact time. It can be seen that the kinetics of copper ion complexation are better for resins with higher water uptake (Resins 2, 4 and 6). In every case, the kinetics of metal uptake is rather slow comparing to the behavior of more hydrophilic chelating ionexchangers such as those presented in Ref. [18]. Complexation of divalent metal cations was studied at pH 3.7 and 5.6. At lower pH, distribution coecients are smaller than at pH 5.6 for all metals. This eect is ascribed to partial protonation of nitrogen atoms in ligands. A similar behavior of resins with ligands with amino groups was observed earlier [18]. As can be seen in Fig. 3 for Resins 5±8 with amides of monocarboxylic acid, the highest sorption is observed for Cu(II), then for linear amides, followed by Ni(II) and Cd(II), whereas for amides substituted with pyridine and imidazol ring, Cd(II) is followed by Ni(II). In all cases Zn(II) has the lowest anity for reference resins, and for Resin 5, containing 2-aminoethyl amide, no sorption of this metal can be observed. When the above results are compared with log D for Resins 1±4, containing amides of dicarboxylic acid, it can be seen that for all of them, sorption of copper (II) is improved. The most signi®cant increase is obtained for Resin 4 for which log D is 5.18. Its counterpart, Resin 8, displays log D 2.58. In terms of sorption it
Fig. 3. Metal ions uptake at pH 3.7.
Chelating resins with N-substituted diamides
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Fig. 4. Metal ions uptake at pH 5.6.
corresponds to 1.69 and 0.56 mg Cu/g, respectively. The uptake of other metal cations by Resin 4 is also improved. Cd(II) log D for it is 1.97 vs 1.40 for Resin 8 (sorption 0.45 vs 0.13 mg Cd/g), for Ni(II) respective log D is 2.53 vs 1.18, giving sorption values 0.60 and 0.04 mg Ni(II)/g and in the case of Zn(II), log D is 2.58 vs 0.98 giving sorption 0.72 vs 0.03 mg Zn/g of resin. As can be seen in Fig. 3, an increase of log D for all metals is signi®cant also in the case of Resin 3 (comparing to Resin 7) but for Resin 1 and 2, with linear substituted amides, only a small improvement is observed for Cu(II), whereas Ni(II) and Cd(II) sorption decreases. The nature of this phenomenon is not clear. It may be that the possibility of secondary reactions (reaction of terminal amino groups from already formed amide with another malonate) is resulting in a decrease of the number of available amino groups, thus lowering sorption. Such a reaction would also make polymers more crosslinked and rigid and thus some groups could be less accessible. Figure 4 presents log D obtained at pH 5.6. In general they are higher as the result of deprotonation of part of amino groups and follow the same order as at pH 3.7. Again, introducing ligands next to each other (Resins 1±4) results in an improvement of the Cu(II) uptake, whereas other metal cations display only a moderate increase of anity towards resins. It should be noted that at both pHs, Resin 5 is not able to remove any Zn(II) from the acetate solution, whereas its counterpart Resin 1 posseses this ability but to a very small extend only. Improvement of the metal ion uptake, by using amides of dicarboxylic acid, was observed in the case of low molecular extractants. In [19±23] dicarboxylic acid diamides were found to be more
powerful in metal ion removal than monocarboxylic ones. Large dierences in log D for Cu(II) and other metal cations prompted experiments with metal removal under competitive conditions. Resin 2 contacted with a solution of Cu(II)/Cd(II) at pH 3.7 gives log D 3.44 and 0.66, respectively. It is a little less than in the case of non-competitive conditions but gives a separation factor of 606 (de®ned as the ratio of Ds for Cu(II) and Cd(II)). The same resin contacted with a Cu(II)/Zn(II) solution at pH 3.7 gives log D = 3.40 for Cu(II) and does not recognize Zn(II) at all. Resin 4 at pH 3.7 contacted with a Cu(II)/Cd(II) solution displays log D 5.18 and 1.91, respectively. This corresponds to a separation factor of 1845. The same resin in the Cu(II)/Zn(II) system gives log D 5.17 and 2.33, resulting in a separation factor of 695. Conclusion This investigation has shown that it is possible to signi®cantly increase the uptake of metal cations by chelating resins by placing ligands next to each other and thereby allow them to interact to a higher extend. This eect can be observed since other parameters which could have in¯uenced the metal uptake are similar for both investigated and reference resins (water uptake, total ligand concentration, type of polymer matrix). Obtained resins display high anity towards transition metal cations. Cu(II) is the most preferred one and can be removed with a high separation factor from acetate solutions containing Cd(II), Ni(II) and Zn(II). AcknowledgementÐThe author would like to gratefully acknowledge ®nancial support from Scienti®c Research Council through grant 3T 09B 09311.
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1. Egawa, H. and Nonaka, T., J. Appl. Polym. Sci., 1986, 31, 1677. 2. Svec, F., Acta Polym., 1980, 31, 68. 3. Warshawsky, A., Deshe, A., Rossey, G. and Patchornik, A., React. Polym., 1984, 2, 301. 4. Trochimczuk, A., Kolarz, B. N. and Wojaczynska, M., React. Polym., 1988, 7, 197. 5. Janus, L., Morcellet, J., Delporte, M. and Morcellet, M., Eur. Polym. J., 1992, 28, 1185. 6. Onari, Y., J. Appl. Polym. Sci., 1986, 31, 1663. 7. van Berkel, P. M., Verweij, P. D., Driessen, W. L., Reedijk, J. and Sherrington, D. C., Eur. Polym. J., 1992, 28, 747. 8. Kaneko, M., Ochiai, M., Kunosita, K. and Yamada, A., J. Polym. Sci. Polym. Chem. Edn, 1982, 20, 1011. 9. Kratz, M. R. and Hendricker, D. J., Polymer, 1986, 27, 1641. 10. van Berkel, P. M., Driessen, W. L., Kodhaas, G. J. A. A., Reedijk, J., Sherrington, D. C., J. Chem. Soc. Chem. Commun., 1995, 147. 11. Gupta, A., Skarzewski, J., J. Chem. Res.(S), 1994, 206.
12. Jackson, G. A., Linder, P. W. and Voye, A., Polyhedron, 1991, 10, 883. 13. Byers, P., Drew, M. G. B., Hudson, M. J., Isaacs, N. S. and Madic, C., Polyhedron, 1994, 13, 349. 14. Hiratani, K., Hirose, T., Fujiwara, K., Saito, K., Chem. Lett., 1990, 1921. 15. Sundell, M. J., Eckman, K. B., Svarfar, B. L. and Nasman, J. H., React. Polym., 1995, 25, 1. 16. Hecker, H., J. Chromatogr., 1974, 102, 135. 17. Frechet, J. M. J., de Smet, M. D. and Farrall, M. J., J. Org. Chem., 1979, 44, 1174. 18. Verweij, P. D., Sital, S., Haanepen, M. J., Driessen, W. L. and Reedijk, J., Eur. Polym. J., 1993, 29, 1603. 19. Nigond, L., Condamines, N., Cordier, P. Y., Livet, J., Madic, C., Cuillerdier, C. and Musikas, C., Sep. Sci. Technol., 1995, 30, 2075. 20. Condamines, N. and Musikas, C., Solvent Extr. Ion Exch., 1988, 6, 1007. 21. Condamines, N. and Musikas, C., Solvent Extr. Ion Exch., 1992, 10, 69. 22. Cuillerdier, C., Musikas, C., Hoel, P., Nigond, C. and Vitart, X., Sep. Sci. Technol., 1991, 26, 1229. 23. Cuillerdier, C., Musikas, C. and Nigond, L., Sep. Sci. Technol., 1993, 28, 155.
Eur. Polym. J. Vol. 34, No. 11, pp. 1657±1662, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0014-3057/99 $ - see front matter S0014-3057(98)00018-4
CHELATING RESINS WITH N-SUBSTITUTED DIAMIDES OF MALONIC ACID AS LIGANDS ANDRZEJ W. TROCHIMCZUK Institute of Organic and Polymer Technology, Technical University of Wroclaw, Wybrzeze St.Wyspianskiego 27, 50-370 Wroclaw, Poland (Received 14 April 1997; accepted in ®nal form 18 September 1997) AbstractÐNew chelating resins with improved uptake of metal ions have been synthesized by functionalizing a vinylbenzyl chloride±divinylbenzene copolymer with a sodium salt of diethyl malonate followed by reacting modi®ed beads with diamines (ethylenediamine, diethylenetriamine, 2-aminomethyl pyridine, 3-aminopropyl imidazol). Their chelating properties towards Cu(II), Cd(II), Ni(II) and Zn(II) from an acetate buer of pH 3.7 and 5.6 have been studied. It was found that the sorption of the above cations on studied resins was higher than the sorption on reference ones containing respective Nsubstituted amides of monocarboxylic acid. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
Complexing agents immobilized on insoluble polymers are well known materials for the removal of metal cations from aqueous solutions. Main advantages of such materials are easy loading and in most cases stripping of cations with simple chemicals, reusability and the possibility of semi-continuous operation. A vast number of commonly used low-molecular complexants was immobilized on polymer matrices; among them those containing nitrogen received a lot of attention due to their strong anity for divalent transition metal cations. Examples of such materials include oligoethyleneimines [1±3], hydrazine, ethylenediamine, diethylenetriamine and triethylenetetraamine, bound to acrylonitrile±divinylbenzene copolymers [4]. However, in the case of work [1], the resin contained also mercapto groups. Some resins were obtained by polymerization of suitable monomers like for example one obtained from vinylamine precursor [5]. In the above materials amino groups capable of complexing metal cations are deposited randomly and for an ecient metal uptake they rely on ¯exibility of the ligand or in the case of only slightly crosslinked systems also on chain movements. As a result only a fraction of ligands is used. In some papers immobilization of more rigid compounds with multiple nitrogens atoms in one ligand was reported Ð for example 3-amino-1,2,4 triazole was immobilized on poly(vinyl phenol) and showed high anity for divalent metal cations at pH 5.3 [6]. In Ref. [7] di- and tridentate pyrazole containing ligands were immobilized on a glycidyl methacrylate resin. Also low molecular complexing agents such as bipyridyl having ®xed distances between nitrogens have been shown to retain complexing properties after immobilization [8]. Similar compound Ð dipyridylamine has been immobilized on chloromethylstyrene polymer [9]. Recently, bis(imidazo-2-yl)-methylaminomethane has been im-
mobilized on hydrophilic resins, crosslinked with trimethylolpropane trimethacrylate and proved to be Cu(II) selective [10]. Low molecular amides and diamides of malonic acid have been receiving great attention since they are known to be good complexing agents. For example in Ref. [11] synthesis of N-substituted diamides of malonic acid containing lipophilic chain was presented recently. In [12] N-substituted diamides of malonic acid, containing terminal amino groups, were investigated to show the eect of ring size on the stability of Cu(II) complexes. Another group presented the complexing ability of tetraethylmalonodiamide towards lanthanides and the structure of this compound Ð La(III) complex [13]. In [14] diamides of malonic and glutaric acids were synthesized and their complexing ability towards Cu(II) was used in the transport of cations through liquid membrane. The aim of the present work is to synthesize series of complexing resins using reactions of various amines with polymer immobilized diethyl malonate and to compare their properties with reference material containing respective N-substituted amides of monocarboxylic acid. EXPERIMENTAL
Materials Copolymers of vinylbenzyl chloride (VBC) and technical divinylbenzene (DVB), nominal crosslinking degree 2 wt%, is prepared by suspension polymerization of monomers with 0.5 wt% of benzoyl peroxide as initiator. The polymerization is carried out in the presence of 50 wt% of toluene. The organic phase is suspended in a continuous aqueous phase Ð 2 wt% sodium chloride solution containing 1 wt% of poly(vinyl alcohol) as stabilizer and heated at 608C for 1 h, at 708C for 1 h, at 858C for 2 h and at 958 for 5 h. After polymerization, the beads are washed with water, hot water and methanol, dried and extracted with toluene.
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Such polymers react with a sodium derivative of diethyl malonate. Thus, 0.375 mol (60.0 g) of diethyl malonate is dissolved in 400 ml of dry dioxane and subsequently 0.375 mol of NaH (60% suspension in mineral oil) is added in portions. After this reaction is ®nished 20 g of VBC/DVB polymer is added and the entire mixture is re¯uxed for 40 h. Next, the beads are washed with dioxane, acetone, water and acetone. A small sample of the modi®ed polymer is hydrolysed using 3 M NaOH. Thus, 2 g of modi®ed beads is re¯uxed for 8 h with 50 ml of 3 M NaOH followed by treating with water, 1 M HCl, water. This conditioning procedure is repeated three times. A polymer having diethyl malonate ligands is modi®ed using the following amines: ethylenediamine, diethylenetriamine, 2-aminomethylpyridine and 3-aminopropylimidazole (Resins 1±4). The reaction is carried out in each case by heating 5 g of polymer with 50 ml of amine at 1258C for 24 h. After the reaction, polymers are washed with diluted hydrochloric acid, water, 1 M HCl, water, 1 M NaOH, water, 1 M HCl and water. The conditioning procedure is repeated three times. Reference polymers are prepared by reaction of VBC/ DVB polymer with KCN in dimethyl formamide. Thus, 15 g of polymer is reacted with 18 g of potassium cyanide in 215 g of DMF at 80±908C for 8 h. The resulting polymer is reacted with the same four amines (Resins 5±8). Reaction conditions are the same as the synthesis of Resins 1±4.
RESULTS AND DISCUSSION
Synthesis of resins The starting polymer containing 6.25 mmol of Cl/ g was reacted with the sodium derivative of diethyl malonate under conditions described in Section 2. The sample of the resulting polymer was subjected to hydrolysis with 3 M NaOH and an acid capacity 6.20 mmol/g was found. Since during hydrolysis a net weight loss was observed it could be calculated that the amount of ester groups (i.e. before hydrolysis) was 5.28 mmol/g. This value corresponds to a 60% yield of VBC/DVB polymer modi®cation with diethyl malonate. However, chlorine analysis showed only a very small amount of that element Ð 0.12 mmol/g. The possible explanation is that sodium from diethyl malonate is transferred to an already modi®ed site and in a subsequent step it reacts with the neighboring chloromethyl group in a reaction leading to additional chemical crosslinks and thus lowering the Cl concentration. The same explanation was given by Finnish authors in the case of chemical modi®cation of chloromethyl groups with a sodium salt of methylenediphosphonate [15]. In the presented case, a negligible concentration of chloromethyl groups is essential since it allows for the next step of planned modi®cation Ð aminolysis of the ester groups without introducing unwanted ligands into the polymer structure. As can be seen in Fig. 1, the next step is leading to
Methods The water regain is measured using a centrifugation method. Thus, approx. 1 g of resin swollen with water is placed in a fritted column and centrifuged at 3000 rpm for 5 min, weighted and dried for 17 h at 808C. After cooling in a desiccator, the column is weighted with, and without polymer. The water regain is calculated as: (mwÿmd)/md where: mw Ð wet weight, md Ð dry weight. The amount of carboxylic groups is measured by converting the sample into the acid form and bringing a known amount in contact with 100 ml of a 0.1 M NaOH solution for 24 h. After this time 50 ml aliquot is taken and titrated with 0.1 M HCl. The nitrogen content is determined using the Kiejdahl method. The chlorine content is measured by burning approx. 25 mg sample of polymer in an oxygen ®lled ¯ask containing 15 ml of 3% hydrogen peroxide solution. After 30 min, walls are washed with water and the Volhardt method is used for Cl determination. In kinetic experiments, the amount of swollen resins equivalent to 0.5 mmol of ligand is contacted with 100 ml of Cu(II) solution at pH 3.7 and small samples are collected at appropriate intervals for AA analysis. In order to determine the sorption capacity of resins towards Cu(II), Cd(II), Ni(II) and Zn(II), they are contacted with 1 10ÿ4 N Me(II) solutions in 0.2 M acetate buer pH 3.7 and 5.6. Thus, resin equivalent to 0.05 mmol of ligand (0.1 mmol of N for Resins 1, 2, 5 and 6 and 0.15 mmol of N for Resins 3, 4, 7 and 8) is shaken with 10 ml of the given metal solution for 48 h. The solution is then ®ltered and the metal ion concentration is measured using atomic absorption spectroscopy on a Varian 250+ spectrophotometer with wavelength set at 324.8, 228.8, 232.0 and 213.9 nm for Cu(II), Cd(II), Ni(II) and Zn(II), respectively.
Fig. 1. Scheme of investigated resins synthesis.
Chelating resins with N-substituted diamides Table 1. Characteristics of amino resins Resin number 1 2 3 4 5 6 7 8
N content (mmol/g) 6.45 7.02 4.29 7.81 5.92 9.36 4.00 6.98
Ligand* content Water regain (g/ (mmol/g) g) 3.23 2.34 2.15 2.60 2.96 3.12 2.00 2.33
0.97 1.06 0.27 0.80 0.77 1.10 0.33 0.36
*Ligand de®ned as -C(O)NHR.
amides of malonic acid. In order to assure the conversion of both ester groups of malonate ligand, an 20±30 fold excess of amine is used in all cases. Resin 1 is obtained using ethylenediamine and displays a nitrogen capacity of 6.45 mmol/g (Table 1). This gives an 80% yield of modi®cation. Repeated aminolysis did not result in an increase of nitrogen capacity. The Hecker method [16] was applied to determine the content of amino groups. The obtained value has been 3.08 mmol/g and indicates that despite a huge excess of amine, some secondary reactions are possible. These could be reactions of the terminal amino group in already formed amide with either ester originating from the same malo-
1659
nate ligand, thus forming a seven member ring, or ester from another ligand with formation of chemical crosslink. Similar modi®cations but with diethylenetriamine (Resin 2) resulted in polymer with a nitrogen capacity of 7.02 mmol/g, which corresponded to a 75% yield. Better results were obtained in the case of modi®cation with 2-aminomethyl pyridine and 3aminopropyl imidazol. The obtained yield for Resin 3 and Resin 4 was 84 and 89%, respectively. In both of the above reactions there was no possibility of secondary crosslinks as each of these amines had only one primary amino group. Next, a set of reference resins was prepared (Resins 5±8). In order to make amides of monocarboxylic acid a synthesis route depicted in Fig. 2 was chosen. It is well known [17] that modi®cation of VBC/DVB copolymer with KCN in dimethylformamide proceeds quantitatively. Chloromethyl groups were not detected after this reaction and in a subsequent step, polymer with 5.53 mmol of nitrile groups per gram was functionalized with ethylenediamine (Resin 5), diethylenetriamine (Resin 6), 2aminomethyl pyridine (Resin 7) and 3-aminopropyl imidazol (Resin 8). In the case of Resin 5, the nitrogen capacity was found to be 5.92 mmol/g, giving an 82% yield. Again, checking the concentration of
Fig. 2. Scheme of reference resins synthesis.
1660
A. W. Trochimczuk
Table 2. Logarithm of Cu(II) distribution coecient as a function of contact time Time (h) Resin
2
4
8
24
48
1 2 3 4 5 6 7 8
1.18 2.62 1.51 3.87 0.98 2.11 1.42 1.60
1.57 3.00 1.84 4.51 1.20 2.53 1.76 1.83
1.92 3.49 2.03 4.73 1.71 3.02 2.11 2.31
2.00 3.51 2.36 5.18 1.82 3.07 2.30 2.63
2.00 3.51 2.40 5.18 1.82 3.07 2.30 2.63
amino groups using the Hecker method gave 2.74 mmol/g, which was less than could be expected from nitrogen analysis (5.92:2 = 2.96 mmol/g) and indicated a slight crosslinking. Similarly, in the case of Resin 6 N = 9.36 mmol/g corresponded to an 89% yield. In both cases, IR spectra showed an absence of peaks at 2240 cmÿ1 characteristic for the stretching of the carbon±nitrogen bond in nitriles. This combined with lower than expected nitrogen content proves the reaction of terminal amino groups from N-substituted amide with nitrile groups. Resins 7 and 8 display nitrogen capacity 5.20 and 6.98 mmol/g, respectively. This gives a 75 and 88% yield. As can be seen in Table 1, resins obtained with linear amines posses moderate hydrophilicity, whereas those synthesized with amines containing pyridyl or imidazole ring are less hydrophilic as manifested by their lower water uptake. It should be also noted that respective pairs of resins, which complexing properties are to be compared, display quite similar ligand capacity and water uptake.
Batch experiments Before batch experiments started, the kinetics of copper (II) uptake by the resins was investigated. Resins were shaken with a solution of Cu(NO3)3 at pH 3.7 (for details see Section 2) and the results obtained are presented in Table 2. It could be concluded that for obtaining an equilibrium distribution coecient it was necessary to measure the metal concentration after a 48 h contact time. It can be seen that the kinetics of copper ion complexation are better for resins with higher water uptake (Resins 2, 4 and 6). In every case, the kinetics of metal uptake is rather slow comparing to the behavior of more hydrophilic chelating ionexchangers such as those presented in Ref. [18]. Complexation of divalent metal cations was studied at pH 3.7 and 5.6. At lower pH, distribution coecients are smaller than at pH 5.6 for all metals. This eect is ascribed to partial protonation of nitrogen atoms in ligands. A similar behavior of resins with ligands with amino groups was observed earlier [18]. As can be seen in Fig. 3 for Resins 5±8 with amides of monocarboxylic acid, the highest sorption is observed for Cu(II), then for linear amides, followed by Ni(II) and Cd(II), whereas for amides substituted with pyridine and imidazol ring, Cd(II) is followed by Ni(II). In all cases Zn(II) has the lowest anity for reference resins, and for Resin 5, containing 2-aminoethyl amide, no sorption of this metal can be observed. When the above results are compared with log D for Resins 1±4, containing amides of dicarboxylic acid, it can be seen that for all of them, sorption of copper (II) is improved. The most signi®cant increase is obtained for Resin 4 for which log D is 5.18. Its counterpart, Resin 8, displays log D 2.58. In terms of sorption it
Fig. 3. Metal ions uptake at pH 3.7.
Chelating resins with N-substituted diamides
1661
Fig. 4. Metal ions uptake at pH 5.6.
corresponds to 1.69 and 0.56 mg Cu/g, respectively. The uptake of other metal cations by Resin 4 is also improved. Cd(II) log D for it is 1.97 vs 1.40 for Resin 8 (sorption 0.45 vs 0.13 mg Cd/g), for Ni(II) respective log D is 2.53 vs 1.18, giving sorption values 0.60 and 0.04 mg Ni(II)/g and in the case of Zn(II), log D is 2.58 vs 0.98 giving sorption 0.72 vs 0.03 mg Zn/g of resin. As can be seen in Fig. 3, an increase of log D for all metals is signi®cant also in the case of Resin 3 (comparing to Resin 7) but for Resin 1 and 2, with linear substituted amides, only a small improvement is observed for Cu(II), whereas Ni(II) and Cd(II) sorption decreases. The nature of this phenomenon is not clear. It may be that the possibility of secondary reactions (reaction of terminal amino groups from already formed amide with another malonate) is resulting in a decrease of the number of available amino groups, thus lowering sorption. Such a reaction would also make polymers more crosslinked and rigid and thus some groups could be less accessible. Figure 4 presents log D obtained at pH 5.6. In general they are higher as the result of deprotonation of part of amino groups and follow the same order as at pH 3.7. Again, introducing ligands next to each other (Resins 1±4) results in an improvement of the Cu(II) uptake, whereas other metal cations display only a moderate increase of anity towards resins. It should be noted that at both pHs, Resin 5 is not able to remove any Zn(II) from the acetate solution, whereas its counterpart Resin 1 posseses this ability but to a very small extend only. Improvement of the metal ion uptake, by using amides of dicarboxylic acid, was observed in the case of low molecular extractants. In [19±23] dicarboxylic acid diamides were found to be more
powerful in metal ion removal than monocarboxylic ones. Large dierences in log D for Cu(II) and other metal cations prompted experiments with metal removal under competitive conditions. Resin 2 contacted with a solution of Cu(II)/Cd(II) at pH 3.7 gives log D 3.44 and 0.66, respectively. It is a little less than in the case of non-competitive conditions but gives a separation factor of 606 (de®ned as the ratio of Ds for Cu(II) and Cd(II)). The same resin contacted with a Cu(II)/Zn(II) solution at pH 3.7 gives log D = 3.40 for Cu(II) and does not recognize Zn(II) at all. Resin 4 at pH 3.7 contacted with a Cu(II)/Cd(II) solution displays log D 5.18 and 1.91, respectively. This corresponds to a separation factor of 1845. The same resin in the Cu(II)/Zn(II) system gives log D 5.17 and 2.33, resulting in a separation factor of 695. Conclusion This investigation has shown that it is possible to signi®cantly increase the uptake of metal cations by chelating resins by placing ligands next to each other and thereby allow them to interact to a higher extend. This eect can be observed since other parameters which could have in¯uenced the metal uptake are similar for both investigated and reference resins (water uptake, total ligand concentration, type of polymer matrix). Obtained resins display high anity towards transition metal cations. Cu(II) is the most preferred one and can be removed with a high separation factor from acetate solutions containing Cd(II), Ni(II) and Zn(II). AcknowledgementÐThe author would like to gratefully acknowledge ®nancial support from Scienti®c Research Council through grant 3T 09B 09311.
1662
A. W. Trochimczuk REFERENCES
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