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Ion-selective polymer-supported reagents: the principle of bifunctionality
PERGAMON
European Polymer Journal 35 (1999) 431±436
Ion-selective polymer-supported reagents: the principle of bifunctionality S.D. Alexandratos *, Subramanian Natesan University of Tennessee, Department of Chemistry, Knoxville, Tennessee 37996, USA Received 10 February 1998; accepted 31 March 1998
Abstract Phosphonic acid ion exchange resins have been synthesized from polystyrene beads and sorption studies with Eu(III) and Fe(III) from acid solutions are reported. The eect of increasing matrix rigidity by crosslinking with 5, 12, and 20% divinylbenzene (DVB) on the extent of complexation is quanti®ed. In a representative example, complexation decreases from 97.7% to 3.60% Eu(III) at a 0.5 h contact time from 10 ÿ 4 N Eu(NO3)3/0.10 N HNO3 solution as the crosslink level in microporous (i.e., gel) resins changes from 5 to 20% DVB. Complexation at the two crosslink levels decreases to 31.7% and 0.61%, respectively, from 1.00 N HNO3 solution. Macroporosity can increase the level of complexation, but the eect under the present conditions is minimal for 5 and 20% DVB macroporous resins (98.3% and 31.2% Eu(III) from 0.10 N HNO3; 28.8% and 1.26% Eu(III) from 1.00 N HNO3). It is now reported that immobilizing sulfonic acid ligands on to the aromatic rings bearing the phosphonic groups allows the latter to rapidly complex metal ions to levels exceeding 90% from highly acidic solutions. Under the conditions noted above, bifunctional 5 and 20% DVB gel resins complex >99% Eu(III) from 0.10 N HNO3 and 94% Eu(III) from 1.00 N HNO3 solutions. The principle of bifunctionality is thus proposed as an alternative to macroporosity: polymer-supported reagents with enhanced metal ion complexation kinetics require the presence of an access ligand along with a recognition ligand for the targeted selectivity. # 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction The design and development of ion-selective polymer-supported reagents is important to applications in environmental remediation, the removal of toxic metals from industrial process streams, and the recovery of precious metals from low grade ores. The advantages of polymeric reagents include their ease of use, especially under continuous conditions, and their compatibility with the environment [1]. The commercially available sulfonic acid ion exchange resin is insuciently selective to permit the complexation of a targeted ion present in trace quantities in a solution with ions such as sodium, magnesium and calcium present in much higher concentrations [2]. Ion exchange resins
* Corresponding author.
with carboxylic acid ligands can be more selective but are limited by a relatively high pKa value. Chelating resins have been prepared with targeted selectivities: dioxocyclam has been immobilized on crosslinked methacrylates and used to complex Cu(II) [3]; immobilized pyridinone is selective for Fe(III) [4]; and polymeric dioximes have a high anity for Ni(II) [5]. Their application can be problematic, however, because high levels of complexation can be dependent on long contact times [6], use of a very small particle size [7], or the presence of non-aqueous solvents [8]. The ideal polymer-supported reagent would have immobilized ligands that selectively and rapidly complex a targeted metal ion from aqueous solutions. The present report describes the manner in which this can be accomplished through the synthesis of bifunctional polymers. Research from this laboratory has described a category of ion-selective resins termed dual mechanism
0014-3057/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 8 ) 0 0 1 4 2 - 6
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S.D. Alexandratos, S. Natesan / European Polymer Journal 35 (1999) 431±436
bifunctional polymers [9]. These polymers combine a ligand responsible for a recognition mechanism (i.e., a selective reaction with a targeted metal ion) with a ligand responsible for an access mechanism (i.e., for allowing the ions into the matrix rapidly). A recent example of this category of polymers is the Diphonix1 ion exchange resin: the combination of gem-diphosphonate with sulfonate ligands leads to the selective complexation of certain ions (e.g., uranium from highly acidic solutions) at a rapid rate [10]. The sulfonic acid ligand is responsible for the observed kinetics, probably through its hydrophilicity (and thus compatibility with the hydrated metal ions) and its inherently rapid, though non-selective, exchange kinetics; once certain ions enter the matrix, they are then complexed by the far more selective diphosphonic acid ligand. We now ®nd that combining the sulfonic acid ligand with the monophosphonic acid ligand allows the latter to complex ions rapidly from highly acidic solutions. Additionally, this is found to be a more powerful technique for enhancing exchange kinetics than the conventional procedure of utilizing macroporous supports.
2. Experimental The copolymer support consisted of beads prepared from vinylbenzyl chloride (Aldrich Chemical Co.) and divinylbenzene (Aldrich; technical grade) via suspension polymerization [11]. The eect of matrix rigidity was studied at three dierent crosslink levels: 5, 12, and 20 wt% divinylbenzene (DVB). Both microporous (gel) and macroporous (MR) beads were prepared, the latter with 50 wt% 4-methyl-2-pentanol in the monomer solution [12]. The functionalization reactions were carried out on beads sieved to 0.42±0.60 mm because rate studies at this particle size are sensitive to dierences in polymer architecture and because it is a size that is useful in process applications.
The monofunctional phosphonic acid resins were synthesized by reaction of the copolymer beads with triisopropyl phosphite followed by hydrolysis with concentrated HCl; the bifunctional phosphonic/sulfonic acid resins were prepared by reacting the phosphorylated precursors with chlorosulfonic acid [13]. All resins were conditioned with 1 L elutions of H2O, 1 N NaOH, H2O, 1 N HCl, and H2O, each with a 1 h elution time. The resins were characterized by the percent solids ( g dry/g wet 100), total acid capacity, and phosphorus elemental analysis. Metal ion studies were performed by contacting enough of the 5 and 12% DVB resins to give 1 mequiv of phosphorus ligands (based on the phosphorus analysis) with 10 mL of 10 ÿ 4 N Eu(NO3)3 and Fe(NO3)3 solutions in 0.10 N and 1.00 N HNO3 at 0.5 and 24 h contact times. The 20% DVB resins were treated in the same manner except that 0.5 g (wet weight) was used in the contact due to their low phosphorus capacities. Each resin was pre-equilibrated with either 0.10 or 1.00 N HNO3 prior to contact with metal-containing solution. The amount of metal complexed was determined with a Perkin±Elmer model 3100 atomic absorption spectrometer. Europium was evaluated in the emission mode at 459.4 nm and iron in the absorption mode at 248.3 nm. Results are reported as percent Mn+ complexed and as the distribution coecient, D (mequiv Mn+ on the resin per g dry resin/mequiv Mn+ in solution per mL solution).
3. Results and discussion The resins were synthesized via an Arbusov reaction on the copolymer beads with triisopropyl phosphite. Hydrolysis gave phosphonic acid ligands and sulfonation gave both phosphonic and sulfonic acid ligands on the phenyl rings (Scheme 1) The resins that were synthesized fall into three categories: monofunctional
Scheme 1.
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Table 1. Characterization of monofunctional phosphonic and bifunctional phosphonic/sulfonic acid resins % DVB
Category
Percent solids
P capacity*
Acid capacity*
5 12 20 5 12 20 5 12 20
P$/gel P/gel P/gel P/MR P/MR P/MR P±S%/gel P±S/gel P±S/gel
63.9 72.8 87.5 33.6 29.5 34.7 42.0 60.0 75.2
4.33 3.68 0.91 4.08 3.37 2.45 3.30 2.80 0.85
8.68 7.25 1.70 8.57 6.33 5.00 9.70 8.47 2.13
*mmol/g.$Monofunctional phosphonic acid resin.%Bifunctional phosphonic/sulfonic acid resin
gel, monofunctional MR, and bifunctional gel. Each was prepared at crosslink levels of 5, 12, and 20% DVB. Gel resins have a high volume capacity, which is advantageous in large scale applications, but high crosslink levels may be needed to increase attrition resistance and decrease swelling (in order to minimize the pressure drop in column applications). Higher crosslink levels can limit substrate accessibility into the polymer matrix and, to counter this, macroporous supports are commonly prepared. Macroporous resins, however, can be less resistant to attrition and often require higher levels of crosslinking; they also have lower volume capacities. The objective of this research is to determine how the complexation kinetics of highly crosslinked gel resins can be maximized. The method chosen is to introduce a second type of ligand into the matrix that is responsible for the rapid access of ions into the matrix and works in tandem with a ligand responsible for the observed selectivity. The properties of the resins are reported in Table 1. The phosphonic acid gel resins show an increase in percent solids with increasing crosslink level. The 5 and 12% DVB resins are almost completely functionalized, as determined by comparing their actual phosphorus capacities (4.33 and 3.68 mmol/g, respectively) with the values calculated assuming complete phosphorylation of the copolymer-CH2Cl groups (4.65 and 4.12 mmol P/g, respectively). The 20% DVB resin is functionalized to only 26% of its maximum value (ex-
perimental and theoretical phosphorus capacities of 0.91 and 3.47 mmol/g, respectively): it is more dicult to functionalize due to limited reagent accessibility into the highly crosslinked matrix. The ligands are, however, accessible to ions in solution, as indicated by the fact that its acid capacity is almost twice its phosphorus capacity. The solids level drops signi®cantly with the macroporous resins, regardless of the crosslink level. The 20% DVB resin is now 71% functionalized, indicating a greater substrate accessibility due to the macroporosity. In each case, the acid capacity is, within experimental error, twice the phosphorus capacity. The bifunctional phosphonic/sulfonic acid gel resins show an eect of crosslink level on percent solids though each has lower solids than the comparably crosslinked monofunctional gel resin. The 5 and 12% DVB resins now have acid capacities that are three times their phosphorus capacities and the 20% DVB resin has an acid capacity 2.6 times its phosphorus capacity due to the presence of the sulfonic acid ligands. The ability of the monofunctional gel resins to complex Eu(III) from 0.10 N and 1.00 N HNO3 is summarized in Table 2. At 0.5 and 24 h contact times, the 5 and 12% DVB resins complex >94% Eu(III) from 0.10 N HNO3. Complexation by the 20% DVB resin is at a much lower level due to limited ionic accessibility into the highly crosslinked resin, even from a dilute acid solution. Complexation from 1.00 N HNO3 is
Table 2. Complexation of Eu(III) from 10 ÿ 4 N Eu(NO3)3 in 0.10 N and 1.00 N HNO3 at 0.5 h and 24 h contact times by monofunctional gel resins 0.10 N HNO3
5% DVB 12% DVB 20% DVB
1.00 N HNO3
0.5 h
24 h
0.5 h
24 h
97.7%* (1774)$ 94.4% (705) 3.60% (0.95)
99.4% (6877) 99.0% (4024) 25.5% (16.1)
31.7% (21.3) 3.20% (1.24) 0.61% (0.15)
35.0% (24.7) 8.34% (3.41) 4.37% (1.11)
*Percent complexed.$Distribution coecient
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Table 3. Complexation of Fe(III) from 10 ÿ 4 N Fe(NO3)3 in 0.10 N and 1.00 N HNO3 at 0.5 h and 24 h contact times by monofunctional gel resins 0.10 N HNO3
5% DVB 12% DVB 20% DVB
1.00 N HNO3
0.5 h
24 h
0.5 h
24 h
98.0%* (2466)$ 43.9% (28.6) 0% (0)
100% (1) 98.0% (1817) 3.26% (0.34)
74.3% (131) 13.9% (5.98) 3.95% (1.04)
96.7% (1335) 74.3% (129) 3.95% (1.05)
*Percent complexed.$Distribution coecient Table 4. Complexation of Eu(III) from 10 ÿ 4 N Eu(NO3)3 in 0.10 N and 1.00 N HNO3 at 0.5 h and 24 h contact times by monofunctional MR resins 0.10 N HNO3
5% DVB 12% DVB 20% DVB
1.00 N HNO3
0.5 h
24 h
0.5 h
24 h
98.3%* (2386)$ 99.1% (3506) 31.2% (26.1)
99.5% (7749) 100% (1) 90.4% (553)
28.8% (16.8) 22.2% (8.94) 1.26% (0.74)
30.8% (18.4) 25.9% (10.9) 8.32% (5.23)
*Percent complexed.$Distribution coecient
very low for all three resins, though the eect of crosslinking on the amount of Eu(III) complexed continues to be 5%>12%>20% DVB. Results with Fe(III) under identical conditions are given in Table 3 with the same general trends being observed. The low level of Eu(III) or Fe(III) complexation from 1.00 N HNO3 with the monofunctional gel resins can be due to thermodynamics (i.e., eective competition for the ligands by the high H + concentration) or kinetics (i.e., accessibility of the metal ions into the polymer matrix). Results with the monofunctional MR resins show that kinetics is an important variable that must be evaluated before conclusions regarding ligandion anities can be made. Eu(III) complexation from 0.10 N HNO3 (Table 4) is almost quantitative for both the 5 and 12% DVB resins at both contact times. The 20% DVB resin, which complexed very low levels as a gel, complexes >90% as an MR at 24 h but approximately 30% at 0.5 h, which indicates that equilibrium
is reached slowly. Eu(III) complexation is very low from 1.00 N HNO3. Results with Fe(III) are similar to those for Eu(III) from 0.10 N HNO3 (Table 5); complexation remains high from 1.00 N HNO3 for both the 5 and 12% DVB resins while the 20% DVB resin complexes 90.6% Fe(III) at 24 h but only 13.7% at 0.5 h. A comparison of the results in Tables 2±5 leads to the conclusion that macroporosity enhances the rate of complexation, at least when the degree of crosslinking does not exceed 12% DVB. Additionally, it would seem that this is possible only if there is an inherently high anity for the metal ion and that the phosphonic acid ligand has a higher anity for Fe(III) than for Eu(III) from highly acidic solutions. Results with the bifunctional resins, however, emphasize the importance of ensuring that the ligand±ion pair are at equilibrium before conclusions about inherent anities are made (Tables 6 and 7): introducing an access ligand into the
Table 5. Complexation of Fe(III) from 10 ÿ 4 N Fe(NO3)3 in 0.10 N and 1.00 N HNO3 at 0.5 h and 24 h contact times by monofunctional MR resins 0.10 N HNO3
5% DVB 12% DVB 20% DVB
1.00 N HNO3
0.5 h
24 h
0.5 h
24 h
100%* (1)$ 98.2% (1673) 32.3% (27.3)
100% (1) 98.2% (1670) 100% (1)
94.5% (729) 99.5% (6228) 13.7% (9.09)
96.7% (1247) 100% (1) 90.6% (551)
*Percent complexed.$Distribution coecient
S.D. Alexandratos, S. Natesan / European Polymer Journal 35 (1999) 431±436
435
Table 6. Complexation of Eu(III) from 10 ÿ 4 N Eu(NO3)3 in 0.10 N and 1.00 N HNO3 at 0.5 h and 24 h contact times by bifunctional gel resins 0.10 N HNO3
5% DVB 12% DVB 20% DVB
1.00 N HNO3
0.5 h
24 h
0.5 h
24 h
99.6%* (8646)$ 100% (1) 100% (1)
100% (1) 100% (1) 100% (1)
94.2% (6019) 95.5% (624) 93.8% (395)
100% (1) 100% (1) 97.5% (1038)
*Percent complexed.$Distribution coecient
Table 7. Complexation of Fe(III) from 10 ÿ 4 N Fe(NO3)3 in 0.10 N and 1.00 N HNO3 at 0.5 h and 24 h contact times by bifunctional gel resins 0.10 N HNO3
5% DVB 12% DVB 20% DVB
1.00 N HNO3
0.5 h
24 h
0.5 h
24 h
100%* (1)$ 100% (1) 100% (1)
100% (1) 100% (1) 100% (1)
100% (1) 100% (1) 97.3% (939)
100% (1) 100% (1) 100% (1)
*Percent complexed.$Distribution coecient
polymer matrix leads to almost quantitative recovery of both Eu(III) and Fe(III) regardless of the degree of crosslinking from both low and high acid solutions at even a 0.5 h contact time. Experiments have shown that the sulfonic acid ligand is not responsible for a signi®cant amount of complexation from high acidic solutions [14]. The low levels of complexation for the 20% DVB resin noted in Tables 2±5 are thus found to be entirely a function of ligand accessibility. Bifunctionality allows for highly crosslinked resins to rapidly complex metal ions far more eciently than making such a resin macroporous.
resins have signi®cantly lower percent solids than the bifunctional gel resins, yet they are outperformed by the latter. This is because the MR resins have a low percent solids due to the water in their macropores and metal ion complexation is dependent on water in the microporesÐa condition best achieved through bifunctionality. This research thus establishes the principle of bifunctionality: polymer-supported reagents with enhanced metal ion complexation kinetics require the presence of an access ligand along with a recognition ligand for the targeted selectivity. Bifunctionality is found to be an important alternative to macroporosity for rapid metal ion separations.
4. Conclusions A low level of complexation by an inherently selective resin in high acid solutions is most often attributed to competition by H + for the binding sites. At least for phosphorus acid ligands, this is seen not to be the case. Results from this research suggest that collapse of the microporous structure in high ionic strength solutions is an alternate cause for low levels of metal ion complexation. This collapse may be brought about by insucient hydration of the ligands when the polymer is in such solutions. Introducing the polar sulfonic acid ligand into the polymer matrix increases the resin's hydrophilicity and this leads to an increase in ionic accessibility. The percent solids is not a sucient measure of matrix hydrophilicity. The macroporous
Acknowledgements We gratefully acknowledge support from the U.S. Department of Energy, Oce of Basic Energy Sciences, Division of Chemical Sciences, through grant DE-FG05-86ER13591.
References [1] Sherrington DC, Hodge P, editors. Syntheses and separations using functional polymers. New York: Wiley, 1988.
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[2] Boyd GE, Vaslow F, Lindenbaum S. J Phys Chem 1967;71:2214. [3] Jyo A, Sassa M, Egawa H. J Appl Poly Sci 1996;59:1049. [4] Feng M, van der Does L, Bantjes A. J Appl Poly Sci 1994;52:21. [5] Bicak N, Atay T, Koza G. Angew Makromol Chem 1992;197:83. [6] Lehto J, Vaaramaa K, Leinonen H. React Funct Polym 1997;33:13. [7] Gauthier M, Frank PC. React Funct Polym 1996;31:67.
[8] Yatsimirskii KB, Zicmanis A, Strizhak PE, Pavlishchuk VV. Dokl Akad Nauk SSR 1989;307:888. [9] Alexandratos SD. Sep Purif Methods 1992;21:1. [10] Chiarizia R, Horwitz EP, Alexandratos SD. Solv Extr Ion Exch 1994;12:211. [11] Tomoi M, Ford WT. J Am Chem Soc 1981;103:3821. [12] Kun KA, Kunin R. J Poly Sci, Part A-1 1968;6:2689. [13] Trochimczuk AW, Alexandratos SD. J Appl Poly Sci 1994;52:1273. [14] Horwitz EP, Chiarizia R, Diamond H, Gatrone RC, Alexandratos SD, Trochimczuk AW, Crick DW. Solv Extr Ion Exch 1993;11:943.
European Polymer Journal 35 (1999) 431±436
Ion-selective polymer-supported reagents: the principle of bifunctionality S.D. Alexandratos *, Subramanian Natesan University of Tennessee, Department of Chemistry, Knoxville, Tennessee 37996, USA Received 10 February 1998; accepted 31 March 1998
Abstract Phosphonic acid ion exchange resins have been synthesized from polystyrene beads and sorption studies with Eu(III) and Fe(III) from acid solutions are reported. The eect of increasing matrix rigidity by crosslinking with 5, 12, and 20% divinylbenzene (DVB) on the extent of complexation is quanti®ed. In a representative example, complexation decreases from 97.7% to 3.60% Eu(III) at a 0.5 h contact time from 10 ÿ 4 N Eu(NO3)3/0.10 N HNO3 solution as the crosslink level in microporous (i.e., gel) resins changes from 5 to 20% DVB. Complexation at the two crosslink levels decreases to 31.7% and 0.61%, respectively, from 1.00 N HNO3 solution. Macroporosity can increase the level of complexation, but the eect under the present conditions is minimal for 5 and 20% DVB macroporous resins (98.3% and 31.2% Eu(III) from 0.10 N HNO3; 28.8% and 1.26% Eu(III) from 1.00 N HNO3). It is now reported that immobilizing sulfonic acid ligands on to the aromatic rings bearing the phosphonic groups allows the latter to rapidly complex metal ions to levels exceeding 90% from highly acidic solutions. Under the conditions noted above, bifunctional 5 and 20% DVB gel resins complex >99% Eu(III) from 0.10 N HNO3 and 94% Eu(III) from 1.00 N HNO3 solutions. The principle of bifunctionality is thus proposed as an alternative to macroporosity: polymer-supported reagents with enhanced metal ion complexation kinetics require the presence of an access ligand along with a recognition ligand for the targeted selectivity. # 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction The design and development of ion-selective polymer-supported reagents is important to applications in environmental remediation, the removal of toxic metals from industrial process streams, and the recovery of precious metals from low grade ores. The advantages of polymeric reagents include their ease of use, especially under continuous conditions, and their compatibility with the environment [1]. The commercially available sulfonic acid ion exchange resin is insuciently selective to permit the complexation of a targeted ion present in trace quantities in a solution with ions such as sodium, magnesium and calcium present in much higher concentrations [2]. Ion exchange resins
* Corresponding author.
with carboxylic acid ligands can be more selective but are limited by a relatively high pKa value. Chelating resins have been prepared with targeted selectivities: dioxocyclam has been immobilized on crosslinked methacrylates and used to complex Cu(II) [3]; immobilized pyridinone is selective for Fe(III) [4]; and polymeric dioximes have a high anity for Ni(II) [5]. Their application can be problematic, however, because high levels of complexation can be dependent on long contact times [6], use of a very small particle size [7], or the presence of non-aqueous solvents [8]. The ideal polymer-supported reagent would have immobilized ligands that selectively and rapidly complex a targeted metal ion from aqueous solutions. The present report describes the manner in which this can be accomplished through the synthesis of bifunctional polymers. Research from this laboratory has described a category of ion-selective resins termed dual mechanism
0014-3057/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 8 ) 0 0 1 4 2 - 6
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bifunctional polymers [9]. These polymers combine a ligand responsible for a recognition mechanism (i.e., a selective reaction with a targeted metal ion) with a ligand responsible for an access mechanism (i.e., for allowing the ions into the matrix rapidly). A recent example of this category of polymers is the Diphonix1 ion exchange resin: the combination of gem-diphosphonate with sulfonate ligands leads to the selective complexation of certain ions (e.g., uranium from highly acidic solutions) at a rapid rate [10]. The sulfonic acid ligand is responsible for the observed kinetics, probably through its hydrophilicity (and thus compatibility with the hydrated metal ions) and its inherently rapid, though non-selective, exchange kinetics; once certain ions enter the matrix, they are then complexed by the far more selective diphosphonic acid ligand. We now ®nd that combining the sulfonic acid ligand with the monophosphonic acid ligand allows the latter to complex ions rapidly from highly acidic solutions. Additionally, this is found to be a more powerful technique for enhancing exchange kinetics than the conventional procedure of utilizing macroporous supports.
2. Experimental The copolymer support consisted of beads prepared from vinylbenzyl chloride (Aldrich Chemical Co.) and divinylbenzene (Aldrich; technical grade) via suspension polymerization [11]. The eect of matrix rigidity was studied at three dierent crosslink levels: 5, 12, and 20 wt% divinylbenzene (DVB). Both microporous (gel) and macroporous (MR) beads were prepared, the latter with 50 wt% 4-methyl-2-pentanol in the monomer solution [12]. The functionalization reactions were carried out on beads sieved to 0.42±0.60 mm because rate studies at this particle size are sensitive to dierences in polymer architecture and because it is a size that is useful in process applications.
The monofunctional phosphonic acid resins were synthesized by reaction of the copolymer beads with triisopropyl phosphite followed by hydrolysis with concentrated HCl; the bifunctional phosphonic/sulfonic acid resins were prepared by reacting the phosphorylated precursors with chlorosulfonic acid [13]. All resins were conditioned with 1 L elutions of H2O, 1 N NaOH, H2O, 1 N HCl, and H2O, each with a 1 h elution time. The resins were characterized by the percent solids ( g dry/g wet 100), total acid capacity, and phosphorus elemental analysis. Metal ion studies were performed by contacting enough of the 5 and 12% DVB resins to give 1 mequiv of phosphorus ligands (based on the phosphorus analysis) with 10 mL of 10 ÿ 4 N Eu(NO3)3 and Fe(NO3)3 solutions in 0.10 N and 1.00 N HNO3 at 0.5 and 24 h contact times. The 20% DVB resins were treated in the same manner except that 0.5 g (wet weight) was used in the contact due to their low phosphorus capacities. Each resin was pre-equilibrated with either 0.10 or 1.00 N HNO3 prior to contact with metal-containing solution. The amount of metal complexed was determined with a Perkin±Elmer model 3100 atomic absorption spectrometer. Europium was evaluated in the emission mode at 459.4 nm and iron in the absorption mode at 248.3 nm. Results are reported as percent Mn+ complexed and as the distribution coecient, D (mequiv Mn+ on the resin per g dry resin/mequiv Mn+ in solution per mL solution).
3. Results and discussion The resins were synthesized via an Arbusov reaction on the copolymer beads with triisopropyl phosphite. Hydrolysis gave phosphonic acid ligands and sulfonation gave both phosphonic and sulfonic acid ligands on the phenyl rings (Scheme 1) The resins that were synthesized fall into three categories: monofunctional
Scheme 1.
S.D. Alexandratos, S. Natesan / European Polymer Journal 35 (1999) 431±436
433
Table 1. Characterization of monofunctional phosphonic and bifunctional phosphonic/sulfonic acid resins % DVB
Category
Percent solids
P capacity*
Acid capacity*
5 12 20 5 12 20 5 12 20
P$/gel P/gel P/gel P/MR P/MR P/MR P±S%/gel P±S/gel P±S/gel
63.9 72.8 87.5 33.6 29.5 34.7 42.0 60.0 75.2
4.33 3.68 0.91 4.08 3.37 2.45 3.30 2.80 0.85
8.68 7.25 1.70 8.57 6.33 5.00 9.70 8.47 2.13
*mmol/g.$Monofunctional phosphonic acid resin.%Bifunctional phosphonic/sulfonic acid resin
gel, monofunctional MR, and bifunctional gel. Each was prepared at crosslink levels of 5, 12, and 20% DVB. Gel resins have a high volume capacity, which is advantageous in large scale applications, but high crosslink levels may be needed to increase attrition resistance and decrease swelling (in order to minimize the pressure drop in column applications). Higher crosslink levels can limit substrate accessibility into the polymer matrix and, to counter this, macroporous supports are commonly prepared. Macroporous resins, however, can be less resistant to attrition and often require higher levels of crosslinking; they also have lower volume capacities. The objective of this research is to determine how the complexation kinetics of highly crosslinked gel resins can be maximized. The method chosen is to introduce a second type of ligand into the matrix that is responsible for the rapid access of ions into the matrix and works in tandem with a ligand responsible for the observed selectivity. The properties of the resins are reported in Table 1. The phosphonic acid gel resins show an increase in percent solids with increasing crosslink level. The 5 and 12% DVB resins are almost completely functionalized, as determined by comparing their actual phosphorus capacities (4.33 and 3.68 mmol/g, respectively) with the values calculated assuming complete phosphorylation of the copolymer-CH2Cl groups (4.65 and 4.12 mmol P/g, respectively). The 20% DVB resin is functionalized to only 26% of its maximum value (ex-
perimental and theoretical phosphorus capacities of 0.91 and 3.47 mmol/g, respectively): it is more dicult to functionalize due to limited reagent accessibility into the highly crosslinked matrix. The ligands are, however, accessible to ions in solution, as indicated by the fact that its acid capacity is almost twice its phosphorus capacity. The solids level drops signi®cantly with the macroporous resins, regardless of the crosslink level. The 20% DVB resin is now 71% functionalized, indicating a greater substrate accessibility due to the macroporosity. In each case, the acid capacity is, within experimental error, twice the phosphorus capacity. The bifunctional phosphonic/sulfonic acid gel resins show an eect of crosslink level on percent solids though each has lower solids than the comparably crosslinked monofunctional gel resin. The 5 and 12% DVB resins now have acid capacities that are three times their phosphorus capacities and the 20% DVB resin has an acid capacity 2.6 times its phosphorus capacity due to the presence of the sulfonic acid ligands. The ability of the monofunctional gel resins to complex Eu(III) from 0.10 N and 1.00 N HNO3 is summarized in Table 2. At 0.5 and 24 h contact times, the 5 and 12% DVB resins complex >94% Eu(III) from 0.10 N HNO3. Complexation by the 20% DVB resin is at a much lower level due to limited ionic accessibility into the highly crosslinked resin, even from a dilute acid solution. Complexation from 1.00 N HNO3 is
Table 2. Complexation of Eu(III) from 10 ÿ 4 N Eu(NO3)3 in 0.10 N and 1.00 N HNO3 at 0.5 h and 24 h contact times by monofunctional gel resins 0.10 N HNO3
5% DVB 12% DVB 20% DVB
1.00 N HNO3
0.5 h
24 h
0.5 h
24 h
97.7%* (1774)$ 94.4% (705) 3.60% (0.95)
99.4% (6877) 99.0% (4024) 25.5% (16.1)
31.7% (21.3) 3.20% (1.24) 0.61% (0.15)
35.0% (24.7) 8.34% (3.41) 4.37% (1.11)
*Percent complexed.$Distribution coecient
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Table 3. Complexation of Fe(III) from 10 ÿ 4 N Fe(NO3)3 in 0.10 N and 1.00 N HNO3 at 0.5 h and 24 h contact times by monofunctional gel resins 0.10 N HNO3
5% DVB 12% DVB 20% DVB
1.00 N HNO3
0.5 h
24 h
0.5 h
24 h
98.0%* (2466)$ 43.9% (28.6) 0% (0)
100% (1) 98.0% (1817) 3.26% (0.34)
74.3% (131) 13.9% (5.98) 3.95% (1.04)
96.7% (1335) 74.3% (129) 3.95% (1.05)
*Percent complexed.$Distribution coecient Table 4. Complexation of Eu(III) from 10 ÿ 4 N Eu(NO3)3 in 0.10 N and 1.00 N HNO3 at 0.5 h and 24 h contact times by monofunctional MR resins 0.10 N HNO3
5% DVB 12% DVB 20% DVB
1.00 N HNO3
0.5 h
24 h
0.5 h
24 h
98.3%* (2386)$ 99.1% (3506) 31.2% (26.1)
99.5% (7749) 100% (1) 90.4% (553)
28.8% (16.8) 22.2% (8.94) 1.26% (0.74)
30.8% (18.4) 25.9% (10.9) 8.32% (5.23)
*Percent complexed.$Distribution coecient
very low for all three resins, though the eect of crosslinking on the amount of Eu(III) complexed continues to be 5%>12%>20% DVB. Results with Fe(III) under identical conditions are given in Table 3 with the same general trends being observed. The low level of Eu(III) or Fe(III) complexation from 1.00 N HNO3 with the monofunctional gel resins can be due to thermodynamics (i.e., eective competition for the ligands by the high H + concentration) or kinetics (i.e., accessibility of the metal ions into the polymer matrix). Results with the monofunctional MR resins show that kinetics is an important variable that must be evaluated before conclusions regarding ligandion anities can be made. Eu(III) complexation from 0.10 N HNO3 (Table 4) is almost quantitative for both the 5 and 12% DVB resins at both contact times. The 20% DVB resin, which complexed very low levels as a gel, complexes >90% as an MR at 24 h but approximately 30% at 0.5 h, which indicates that equilibrium
is reached slowly. Eu(III) complexation is very low from 1.00 N HNO3. Results with Fe(III) are similar to those for Eu(III) from 0.10 N HNO3 (Table 5); complexation remains high from 1.00 N HNO3 for both the 5 and 12% DVB resins while the 20% DVB resin complexes 90.6% Fe(III) at 24 h but only 13.7% at 0.5 h. A comparison of the results in Tables 2±5 leads to the conclusion that macroporosity enhances the rate of complexation, at least when the degree of crosslinking does not exceed 12% DVB. Additionally, it would seem that this is possible only if there is an inherently high anity for the metal ion and that the phosphonic acid ligand has a higher anity for Fe(III) than for Eu(III) from highly acidic solutions. Results with the bifunctional resins, however, emphasize the importance of ensuring that the ligand±ion pair are at equilibrium before conclusions about inherent anities are made (Tables 6 and 7): introducing an access ligand into the
Table 5. Complexation of Fe(III) from 10 ÿ 4 N Fe(NO3)3 in 0.10 N and 1.00 N HNO3 at 0.5 h and 24 h contact times by monofunctional MR resins 0.10 N HNO3
5% DVB 12% DVB 20% DVB
1.00 N HNO3
0.5 h
24 h
0.5 h
24 h
100%* (1)$ 98.2% (1673) 32.3% (27.3)
100% (1) 98.2% (1670) 100% (1)
94.5% (729) 99.5% (6228) 13.7% (9.09)
96.7% (1247) 100% (1) 90.6% (551)
*Percent complexed.$Distribution coecient
S.D. Alexandratos, S. Natesan / European Polymer Journal 35 (1999) 431±436
435
Table 6. Complexation of Eu(III) from 10 ÿ 4 N Eu(NO3)3 in 0.10 N and 1.00 N HNO3 at 0.5 h and 24 h contact times by bifunctional gel resins 0.10 N HNO3
5% DVB 12% DVB 20% DVB
1.00 N HNO3
0.5 h
24 h
0.5 h
24 h
99.6%* (8646)$ 100% (1) 100% (1)
100% (1) 100% (1) 100% (1)
94.2% (6019) 95.5% (624) 93.8% (395)
100% (1) 100% (1) 97.5% (1038)
*Percent complexed.$Distribution coecient
Table 7. Complexation of Fe(III) from 10 ÿ 4 N Fe(NO3)3 in 0.10 N and 1.00 N HNO3 at 0.5 h and 24 h contact times by bifunctional gel resins 0.10 N HNO3
5% DVB 12% DVB 20% DVB
1.00 N HNO3
0.5 h
24 h
0.5 h
24 h
100%* (1)$ 100% (1) 100% (1)
100% (1) 100% (1) 100% (1)
100% (1) 100% (1) 97.3% (939)
100% (1) 100% (1) 100% (1)
*Percent complexed.$Distribution coecient
polymer matrix leads to almost quantitative recovery of both Eu(III) and Fe(III) regardless of the degree of crosslinking from both low and high acid solutions at even a 0.5 h contact time. Experiments have shown that the sulfonic acid ligand is not responsible for a signi®cant amount of complexation from high acidic solutions [14]. The low levels of complexation for the 20% DVB resin noted in Tables 2±5 are thus found to be entirely a function of ligand accessibility. Bifunctionality allows for highly crosslinked resins to rapidly complex metal ions far more eciently than making such a resin macroporous.
resins have signi®cantly lower percent solids than the bifunctional gel resins, yet they are outperformed by the latter. This is because the MR resins have a low percent solids due to the water in their macropores and metal ion complexation is dependent on water in the microporesÐa condition best achieved through bifunctionality. This research thus establishes the principle of bifunctionality: polymer-supported reagents with enhanced metal ion complexation kinetics require the presence of an access ligand along with a recognition ligand for the targeted selectivity. Bifunctionality is found to be an important alternative to macroporosity for rapid metal ion separations.
4. Conclusions A low level of complexation by an inherently selective resin in high acid solutions is most often attributed to competition by H + for the binding sites. At least for phosphorus acid ligands, this is seen not to be the case. Results from this research suggest that collapse of the microporous structure in high ionic strength solutions is an alternate cause for low levels of metal ion complexation. This collapse may be brought about by insucient hydration of the ligands when the polymer is in such solutions. Introducing the polar sulfonic acid ligand into the polymer matrix increases the resin's hydrophilicity and this leads to an increase in ionic accessibility. The percent solids is not a sucient measure of matrix hydrophilicity. The macroporous
Acknowledgements We gratefully acknowledge support from the U.S. Department of Energy, Oce of Basic Energy Sciences, Division of Chemical Sciences, through grant DE-FG05-86ER13591.
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