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Synthesis of hydrogel beads having phosphinic acid groups and its adsorption ability for lanthanide ions
REACTIVE & FUNCTIONAL POLYMERS
Reactive & Functional Polymers 66 (2006) 625–633
www.elsevier.com/locate/react
Synthesis of hydrogel beads having phosphinic acid groups and its adsorption ability for lanthanide ions Tomonari Ogata *, Kana Nagayoshi, Tadashi Nagasako, Seiji Kurihara, Takamasa Nonaka Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan Received 17 April 2005; received in revised form 8 October 2005; accepted 19 October 2005 Available online 15 December 2005
Abstract Hydrophilic copolymer beads, APPO–DMAAm–4G having phosphinic acid groups were prepared by suspension copolymerization of acryloyloxypropyl n-octylphosphinic acid (APPO), N, N-dimethylacrylamide (DMAAm), and tetraethyleneglycol dimethacrylate (4G) dissolved in chloroform in a saturated Na2SO4 aqueous solution in the presence of surfactant and MgCO3. The adsorption abilities for metal ions of the copolymer beads were studied. The APPO– DMAAm–4G copolymer beads had higher adsorption ability for lanthanide metal ions (Eu3+, Sm3+, Nd3+ or La3+). Furthermore, it was also found that the APPO–DMAAm–4G copolymer beads had selective adsorption ability for lanthanide metal ions in the following order: Eu3+ > Sm3+ > Nd3+ >La3+. 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogel beads; Suspension polymerization; Adsorption of lanthanide ion; Phosphinic acid group
1. Introduction Many types of ion-exchange and chelating polymer resins are used for removing or recovering metal ions from industrial waste water, and for chemical analysis such as chromatography. In particular, the chelating polymers having various types of ligands, for example, iminodiacetic acid group [1,2], amino group [3], amidoxime group [4,5], phosphinic acid group [6–9], etc. are suitable when selec-
* Corresponding author. Tel.: +81 96 342 3677; fax: +81 96 342 3679. E-mail address: [email protected] (T. Ogata).
tivity is required. In many ligands, phosphoric acid derivatives are much employed due to its high adsorption ability for rare metal ions [10–16]. Usually, these chelating polymers consist of hydrophobic polymer and have rigid structures to maintain durability. On the other hand, hydrogels are hydrophilic materials which have high water content and softness. These hydrogels acquire much attention in many fields such as water absorbent material, smart materials and biomaterials. Recently, hydrogels having metal ion adsorption ability have been reported [17–25]. Due to the high hydrophilicity, hydrogels bear much free water and have been expected to exhibit high and rapid adsorption ability for metal ions in water.
1381-5148/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2005.10.015
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We previously reported that water-soluble thermosensitive copolymers consisting of acryloyloxypropyl phosphinic acid (APPA) and N-isopropylacrylamide (NIPAAm) had adsorption ability for lanthanide metal ions or main transition metal ions, in particular, they had higher adsorption abilities for lanthanide metal ions than that for main transition metal ions [26]. The APPA–NIPAAm copolymer–metal complexes exhibited different thermosensitive behavior depending on the type of metal ions adsorbed [26]. Furthermore, we also found that selective adsorption of lanthanide metal ions from their mixed solutions containing main transition metal ions could be performed by changing the temperature of the solution, in which insoluble copolymer–metal complexes were formed. In our previous report [27], we described about the preparation of cross-linked thermosensitive copolymer beads having phosphinic acid groups (APPO– NIPAAm–4G) by the suspension copolymerization of acryloyloxypropyl n-octylphosphinic acid (APPO), N-isopropylacrylamide (NIPAAm), and tetraethyleneglycol dimethacrylate (4G). Although the swelling of APPO–NIPAAm–4G copolymer in water was greatly affected by temperature, the metal adsorption ability of the copolymer was almost constant at various temperatures. On the other hand, the selective adsorption ability of the APPO–NIPAAm–4G copolymer beads for lanthanide metal ions was significantly higher than that for typical elements. In this work, we synthesized hydrophilic copolymer beads having phosphinic group by suspension copolymerization of acryloyloxypropyl n-octylphosphinic acid (APPO), N,N-dimethylacrylamide (DMAAm), and tetraethyleneglycol dimethacrylate (4G) and the adsorption abilities of the copolymer beads obtained for lanthanide metal ions were measured. 2. Experimental 2.1. Materials APPO and DMAAm were kindly supplied by Nippon Kagaku Kogyo Co. (Tokyo, Japan) and Kohjin Co. (Tokyo, Japan), respectively, and used without further purification. Tetraethyleneglycol dimethacrylate (4G) as a cross-linking agent was purchased from Sigma-Aldrich Japan Co. (Tokyo, Japan) and used without further purification. a,a 0 Azobisisobutyronitrile (AIBN) (Nacalai tesque
Co., Tokyo, Japan) was purified by recrystallization from ethanol (Wako Pure Chemical Industries, Ltd., Osaka, Japan). All of lanthanide salts, lanthanum chloride (Wako Pure Chemical Industries, Ltd., Osaka, Japan), neodymium chloride (Junsei Chemical, Ltd., Tokyo, Japan), samarium chloride (Wako Pure Chemical Industries, Ltd., Osaka, Japan), and europium chloride (Kanto Chemical Co., Tokyo, Japan) were used as received. Other chemical compounds used were of reagent grade. 2.2. Synthesis of Cross-linked APPO–DMAAm–4G copolymer beads The APPO–DMAAm–4G copolymer beads were produced in aqueous dispersant by suspension polymerization as follows [27]. Monomers, 0.99 g (3.41 mmol) of APPO, 2.79 g (28.1 mmol) of DMAAm, 0.22 g (0.666 mmol) of 4 G were mixed, and 4, 2 cm3 or none of chloroform was added to make a solution of 50, 75, and 100 wt./vol.% of concentration,respectively. The monomer solution was bubbled with nitrogen for 1 h in ice bath and AIBN was added to the solution as an initiator. Four hundred cubic centimeter of dispersant media were prepared by dispersing 4 g of magnesium carbonate in saturated sodium sulfate aqueous solution containing 0.1 g of sodium sulfodiisooctyl succinate (AOT) as a surfactant. The dispersant medium was treated with ultrasonic to make the solution homogenous and placed in three-neck flask equipped with a condenser, a nitrogen inlet and a PTFE stirrer stick. The dispersant medium was bubbled with nitrogen for 1 h and the monomer solution was added under stirring. Polymerization was carried out with stirring at 300, 400 or 500 rpm at 50 C for 3 h and further at 60 C for 1 h under a nitrogen atmosphere. After polymerization, the product was filtered off and washed with 0.5 N HCl to remove magnesium carbonate. After that, the copolymer beads were washed with acetone using soxhlet extractor for over 24 h and they were dried in the air. Additionally, the copolymer beads were alternately soaked in water of 10 and 60 C to eliminate exudation in the case of aqueous experiments. After drying, the copolymer beads were used for experiments. The content of phosphinic acid groups in the copolymer beads was calculated from the phosphorus content of the dried copolymer beads. The phosphorus content was determined by phosphovanadomolybdate method after combustion. The nitrogen content,
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which corresponds to the DMAAm content in the copolymer beads, was determined by elemental analysis (Yanaco CHN Corder MT-6). 2.3. Measurement of the swelling volume of the APPO–DMAAm–4G copolymer beads To evaluate the swelling behavior of copolymer beads, swelling volumes were measured as follows: 0.2 g of dried copolymer bead was soaked for 24 h at room temperature (24–25 C) in deionized water in 5 cm3 measuring cylinder, and then the temperature was varied and the copolymer beads were allowed to stand at each temperature for 1 h. The swelling volume of the copolymer beads at each temperature was calculated using Eq. (1). Swelling volumeðcm3 g1 Þ ¼ Apparent volumeðcm3 Þ =Weight of copolymer beadsðgÞ
ð1Þ
2.4. Measurement of the adsorption capacity of the copolymer beads for metal ions The quantity of adsorbed metal ions to the copolymer beads were measured as follows: 0.1 g of dried copolymer beads was placed into 25 cm3 of buffer solution (pH = 3.5, 5.0, 6.5; CH3COOH– CH3COONa) in a 50 cm3 Erlenmeyer flask and the flask was shaken at 20 C for 24 h to complete the swelling of the copolymer beads. And then 25 cm3 of metal ion solution containing 200 mol% of metal ion of a phosphinic group in copolymer were poured into the flask and shaken for further 24 h at 20 C. After that, the copolymer beads were filtered. The adsorption capacity was calculated by determining the concentration of residual metal ions in the supernatant with inductively coupled argon plasma atomic emission spectrophotometer (Shimadzu ICPS-5000). 3. Results and discussions 3.1. Synthesis of the APPO–DMAAm-4G copolymer beads The monomers used in this study have different solubility in water, DMAAm is an amphiphilic monomer and APPO and 4G show poor solubility in water. When the aqueous media were used for suspension polymerization, the water soluble monomer will be extracted to aqueous phase
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resulting in reduction of the monomer component in copolymer. Therefore, suspension polymerization of water-soluble monomer in aqueous media is difficult [28,29]. In our previous report, the conditions for suspension polymerization of amphiphilic monomer such as NIPAAm in aqueous solution have been studied [27]. In this study, we obtained spherical APPO–DMAAm–4G copolymer beads by suspension polymerization as described in the previous report as follows: A saturated sodium sulfate aqueous solution was used as a dispersing medium in the presence of suspended magnesium carbonate powder. The monomer solution contained AOT was poured into the dispersant medium placed in a reaction flask and the mixture solutions were stirred at 300, 400 or 500 rpm to make droplets containing the monomers. In the polymerization reaction, saturated sodium sulfate solution played an important role to hold DMAAm monomer in chloroform, AOT and magnesium carbonate also acted as auxiliary dispersing agents. In the preparation of copolymer beads, chloroform was used for solvent of monomers. However, chloroform is not suitable to polymerization solvent due to its high chain transfer constant, there was no reasonable solvent in this study except chloroform. The polymerization reactions were carried out at 50 C for 3 h and further at 60 C for 1 h (Fig. 1). After the reaction, well dispersed copolymer beads were obtained and they were washed with hydrochloric acid and purified using acetone and water. Table 1 shows the results of the APPO–DMAAm– 4G copolymer beads obtained at various stirring speeds and monomer concentrations. The yields of copolymers were changed by the conditions. Higher stirring speed and higher monomer concentration increased the yield of copolymer. The yield decreased at higher concentration of chloroform which acted as a chain transfer agent as described above. The contents of APPO and DMAAm in the copolymer were estimated from the phosphorous and nitrogen contents, respectively. It was found that the composition of copolymer was affected by conditions of copolymerization. In all conditions, the contents of APPO were decreased from feed composition. On the contrary, content of 4G increased from feed composition and it would make the copolymer gel to lower swelling. In our previous report, we have investigated about the influences of crosslinking degree on the swelling and the adsorption ability of metal ion. It was found
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Fig. 1. Synthesis of APPO–DMAAm–4G copolymer beads.
Table 1 Results of APPO–DMAAm–4G copolymer beads obtained Sample
1 2 3 4 5 a b
Composition in copolymera
Stirring speed (rpm)
Monomer conc. (wt./vol.%)
Elementary assay N (mmol g )
P (mmol g )
DMAAm mol%
APPO mol%
Average diameterb (lm)
300 400 500 400 400
50 50 50 75 100
7.1 6.7 7.1 6.3 6.2
0.54 0.61 0.65 0.77 0.87
88 86 88 84 83
6.7 7.8 8.0 10.3 11.7
27 22 21 33 –
1
1
Calculated from results of elementary assay. Estimated from SEM images.
that the adsorption of metal ion was suppressed only at lower swelling volume [27]. In this study, since all of APPO–DMAAm–4G copolymers exhibited higher swelling volume, the influence of the swelling ratio on the metal adsorption was low. Fig. 2 shows the scanning electron micrograph of the APPO–DMAAm–4G copolymer beads obtained under various conditions. The average diameters of copolymer beads were estimated from these pictures. With decreasing of monomer concentration in feed, the size of beads became larger, and the irregular shaped beads were increased, and the surface of bead became rough. The best appearances of beads were obtained by using 50 vol% monomer solution. Although the size of beads tended to be smaller with increasing of stirring speed, they maintained spherical shape and smooth surface. These copolymer beads obtained
were used for experiments as having different APPO content. 3.2. Swelling behavior of the APPO–DMAAm–4G copolymer beads In our previous report [27], NIPAAm was copolymerized with APPO and 4G instead of DMAAm. The APPO–NIPAAm–4G copolymer exhibited a typical thermosensitivity which shows reversible swelling transition around 35 C. It was found that although the swelling of NIPAAm–4GAPPO was significantly suppressed above 35 C by the thermosensitivity, the adsorption ability was not affected. It is a significant difference between APPO–NIPAAm–4G and APPO–DMAAm–4G that thermosensitive moiety is contained or not contained.
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Fig. 2. SEM images of the APPO–DMAAm–4G copolymer beads obtained by suspension polymerization.
In Fig. 3, the swelling volume of the copolymer beads having different APPO content in deionized water at various temperatures was measured. The decrease in the swelling volume of the copolymer beads with increasing temperature indicates that the copolymer beads become hydrophobic at higher temperatures.
The swelling volume of the APPO–DMAAm–4G copolymer beads increased with increasing APPO content in the copolymer beads. This indicates that the hydrophilicity of the copolymer beads became higher as the quantity of phosphinic acid groups in the copolymer beads increased. Consequently, the adsorption experiments were carried out at 30 C, since the magnitude of swelling at this temperature was appropriate for handling and was a suitable temperature to maintain constantly during experiments. 3.3. pH dependence of the swelling volume of the APPO–DMAAm–4G copolymer beads
Fig. 3. Temperature dependence of the swelling of APPO– DMAAm–4G copolymer beads in deionized water.
The swelling volume (cm3 g1) of the copolymer beads was measured in buffer solutions having different pHs at various temperatures (Fig. 4). Although the shape of curve is almost the same as the result in the deionized water (Fig. 3), position of the curve was shifted to a value of lower swelling volume by lowering pH of the buffer solution. In the higher pH region (pH = 5.0, 6.0, 7.0), the swelling volumes were almost the same. This result was readily explained in the pKa of phosphinic group in APPO monomer. Ionizing ratio was calculated from pKa of APPO monomer (= 4.3) [30] in each buffer solutions and the results of ionizing ratio of phos-
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Fig. 4. Temperature dependences of the swelling of APPO– DMAAm–4G copolymer beads at various pHs. P content in copolymer = 0.61 mmol g1.
phinic group are 50%, 80%, and 98% at pH = 3.5, 4.5, 5.0, respectively. When the pH is higher than 5.0, almost phosphinic group will be ionized and consequently, the swelling ratio of copolymer showed the same curve. The swelling volume of APPO–DMAAm–4G copolymer beads was affected by pH and remarkable change was found in the range of pH 3.5–5.0 according to the pKa of APPO. 3.4. Adsorption rate of metal ions with the APPO– DMAAm–4G copolymer beads The phosphinic acid groups in APPO are used to coordinate and/or ion exchange with metal ions, in particular, with lanthanide ions [11,12,31]. In this study, the adsorption ability of APPO–DMAAm– 4G copolymer beads for lanthanide ions (La3+, Nd3+, Sm3+, Eu3+) was measured under various conditions. To investigate the adsorption speed, the time course of adsorption of Sm3+ by APPO–DMAAm– 4G copolymer bead was measured by a batch method at 30 C in buffer solution (pH = 5.0) for various adsorption periods (Fig. 5). The quantities of adsorption increased significantly in first 4 h and reached equilibrium at 12 h. From this result, the adsorption time is evaluated to 24 h enough to reach the adsorption equilibrium completely. The maximum quantity of adsorbed metal ions was about 1/4–1/5 in mol ratio to APPO content in the copolymer.
Fig. 5. Time course of the adsorption of Sm3+ with APPO– DMAAm–4G copolymer beads at pH 5.0. Weight of gel: 0.10 g; Metal ion solution: 3.10 · 103 mol dm3, 50 cm3; Buffer solution (pH 5.0): CH3COOH–CH3COONa.
3.5. Effect of pH on the adsorption of metal ions with the APPO–DMAAm–4G copolymer beads from its single metal ion solution The adsorption ability of the APPO–DMAAm– 4G copolymer beads for lanthanide metal ions (La3+, Nd3+, Sm3+, Eu3+) from each single metal ion solution was measured at different pHs (3.5, 5.0, 6.5) at 20 C. Fig. 6 shows the pH dependence of the adsorption of each metal ion with the APPO– DMAAm–4G copolymer beads. The adsorption abilities for each metal ion increased in higher pH solution. The adsorption behaviors of the metal ions were almost the same except La3+ that exhibited less adsorption compared to other metal ions. The degree of ionization of phosphinic acid increased with increasing pH value and almost all of phosphinic acid groups are ionized pH = 5.0 as described above. The increase of the metal ion adsorption supports the estimation which the metal ions were adsorbed via ion-exchange adsorption with ionized phosphinic acid groups in the copolymer beads. 3.6. Adsorption of metal ions with the APPO– DMAAm–4G copolymer beads from mixed metal ion solutions In the previous section, the adsorption ability for lanthanide metal ions of APPO–DMAAm–4G copolymer beads was found. Fig. 7 shows the selec-
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Selectivity coefficientðKÞ 3þ ¼ ðM 3þ 1 =M 2 molar ratio adsorbed with the copolymer beadsÞ 3þ =ðM 3þ 1 =M 2 molar ratio in solutionÞ
ð2Þ
The APPO–DMAAm–4G copolymer beads had higher adsorption ability for Eu3+ than Sm3+, Sm3+ than Nd3+, and Nd3+ than La3+ from each binary solutions, although a significant difference in the metal ion adsorption was not observed from its single metal ion solution. From this result, the order of adsorption ability with the APPO–DMAAm–4G copolymer beads for lanthanide metal ions was estimated as follows: Eu3þ > Sm3þ > Nd3þ > La3þ .
Fig. 6. Effect of pH on the adsorption of metal ions with APPO– DMAAm–4G copolymer beads. Weight of gel: 0.10 g (P content = 0.68 mmol g1; Metal ion solution: 2.40 · 103 mol dm3, 50 cm3; Shaking at 30 C for 24 h.
tive adsorption between two kinds of lanthanide ion from their binary mixture solutions (1/1 in molar ratio) at pH 3.5, 5.0, and 6.5. The adsorption behavior was different from the result obtained for single metal ion solution. That is, there is no change in quantity of adsorption at different pHs, while the adsorption from single metal ion solution increased with an increase in the value of pH. In addition, the quantity of adsorption was significantly different between the two metal ions. The selectivity coefficient (K) was calculated by using Eq. (2) and the results are shown in Table 2.
This order is in good agreement with the order of an atomic number of these metal ions [11]. It is known as lanthanide contraction that lanthanide metal ion with larger atomic number has lower ionic radius than that with smaller atomic number, because, in lanthanide metal series, inner shell is filled faster with electron than the outer shell [32]. Therefore, lanthanide metal ion with larger atomic number has higher charge density on its surface than that with smaller atomic number. As a consequence, lanthanide metal ion with lower atomic number exhibited higher adsorption abilities. The quantity of adsorption in binary mixture solution of two metal ions decreased from the case of adsorption from single metal ion solution. The reduction is due to lower concentration in each metal ion compared to the case in single metal ion adsorption. Although the decreased in adsorption
Fig. 7. Adsorption of metal ions with APPO–DMAAm–4G (15:97:3) copolymer beads from mixed metal ion solution in the pH range from 3.5 to 6.5. Weight of gel: 0.10 g (P content = 0.68 mmol g1); Metal ion solution: 2.72 · 103 mol dm3, 50 cm3; Shaking at 30 C for 24 h.
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Table 2 Selectivity coefficient of adsorption (K) from binary mixed metal ions solution
Table 3 Selectivity coefficient of adsorption (K) from Eu3+ and La3+ mixed metal ions solution with various concentration ratio
Metal ion solution
Concentration ratio (Eu3+/La3+)
Selectivity coefficient (K)
4/1 2/1 1/1 1/2 1/4
2.6 2.9 4.7 5.4 4.2
Eu3+/Sm3+ Sm3+/Nd3+ Nd3+/La3+
Selectivity coefficient (K) pH = 3.5
pH = 5.0
pH = 6.5
5.1 1.7 3.3
5.5 2.2 2.1
2.7 6.3 2.7
Selectivity coefficient (K); K = a/b. a: ratio of adsorption of metal ions. b: ratio of concentration of metal ions in solution.
of each metal ion is agreeable, decreasing in total of the quantity of adsorption of two kinds of metal ions compared to the quantity of adsorption from single metal ion is unexplainable phenomena at this moment. To evaluate selective adsorption between Eu3+ and La3+, the adsorption was measured from binary mixed solutions with different compositions (Eu3+/La3+ = 8/2 2/8 molar ratio; total concentration = 2.72 · 103 mmol dm3) at 20 C and at pH of 6.5 (buffer solution). As shown in Fig. 8, adsorption of Eu3+ was higher than La3+, even from mixed solutions with smaller content of Eu3+ than La3+. The selective coefficient of adsorption for Eu3+/La3+ (Table 3) exhibited highest value in adsorption from Eu3+/La3+ = 1/2 (molar ratio) mixture solution. As observed above, it was found that the APPO–DMAAm–4G copolymer beads had high selective adsorption ability for Eu3+ than La3+.
Selectivity coefficient (K); K = a/b. a: ratio of adsorption of metal ions. b: ratio of concentration of metal ions in solution.
4. Conclusion APPO–DMAAm–4G copolymer beads were prepared by suspension copolymerization in aqueous media. The composition of copolymer gel was affected by copolymerization conditions such as stirring speed and monomer concentration. The swelling behavior and metal ion adsorption capacity of copolymer beads were influenced by pH. The copolymer beads exhibited higher adsorption capacity for lanthanide metal ions. The selectivity of APPO–DMAAm–4G copolymer beads for lanthanide metal ions is in the following order: Eu3+ > Sm3+ > Nd3+ > La3+, same as the order of atomic number. In selective adsorption of Eu3+ and La3+, selectivity for Eu3+ was higher even from mixed solutions with smaller content of Eu3+ than La3+. Thus, APPO–DMAAm–4G copolymer beads is a hydrogel having high hydrophilicity and selective adsorption ability for lanthanide metal ions. References
Fig. 8. Adsorption of metal ions (Eu3+, La3+) with APPO– DMAAm–4G copolymer beads from mixed metal ion solutions with various concentration ratios of pH = 6.5 at 20 C. Weight of gel: 0.10 g (P content = 0.68 mmol g1); Metal ion solution: 2.72 · 103 mol dm3, 50 cm3; Shaking at 20 C for 24 h.
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Reactive & Functional Polymers 66 (2006) 625–633
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Synthesis of hydrogel beads having phosphinic acid groups and its adsorption ability for lanthanide ions Tomonari Ogata *, Kana Nagayoshi, Tadashi Nagasako, Seiji Kurihara, Takamasa Nonaka Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan Received 17 April 2005; received in revised form 8 October 2005; accepted 19 October 2005 Available online 15 December 2005
Abstract Hydrophilic copolymer beads, APPO–DMAAm–4G having phosphinic acid groups were prepared by suspension copolymerization of acryloyloxypropyl n-octylphosphinic acid (APPO), N, N-dimethylacrylamide (DMAAm), and tetraethyleneglycol dimethacrylate (4G) dissolved in chloroform in a saturated Na2SO4 aqueous solution in the presence of surfactant and MgCO3. The adsorption abilities for metal ions of the copolymer beads were studied. The APPO– DMAAm–4G copolymer beads had higher adsorption ability for lanthanide metal ions (Eu3+, Sm3+, Nd3+ or La3+). Furthermore, it was also found that the APPO–DMAAm–4G copolymer beads had selective adsorption ability for lanthanide metal ions in the following order: Eu3+ > Sm3+ > Nd3+ >La3+. 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogel beads; Suspension polymerization; Adsorption of lanthanide ion; Phosphinic acid group
1. Introduction Many types of ion-exchange and chelating polymer resins are used for removing or recovering metal ions from industrial waste water, and for chemical analysis such as chromatography. In particular, the chelating polymers having various types of ligands, for example, iminodiacetic acid group [1,2], amino group [3], amidoxime group [4,5], phosphinic acid group [6–9], etc. are suitable when selec-
* Corresponding author. Tel.: +81 96 342 3677; fax: +81 96 342 3679. E-mail address: [email protected] (T. Ogata).
tivity is required. In many ligands, phosphoric acid derivatives are much employed due to its high adsorption ability for rare metal ions [10–16]. Usually, these chelating polymers consist of hydrophobic polymer and have rigid structures to maintain durability. On the other hand, hydrogels are hydrophilic materials which have high water content and softness. These hydrogels acquire much attention in many fields such as water absorbent material, smart materials and biomaterials. Recently, hydrogels having metal ion adsorption ability have been reported [17–25]. Due to the high hydrophilicity, hydrogels bear much free water and have been expected to exhibit high and rapid adsorption ability for metal ions in water.
1381-5148/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2005.10.015
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We previously reported that water-soluble thermosensitive copolymers consisting of acryloyloxypropyl phosphinic acid (APPA) and N-isopropylacrylamide (NIPAAm) had adsorption ability for lanthanide metal ions or main transition metal ions, in particular, they had higher adsorption abilities for lanthanide metal ions than that for main transition metal ions [26]. The APPA–NIPAAm copolymer–metal complexes exhibited different thermosensitive behavior depending on the type of metal ions adsorbed [26]. Furthermore, we also found that selective adsorption of lanthanide metal ions from their mixed solutions containing main transition metal ions could be performed by changing the temperature of the solution, in which insoluble copolymer–metal complexes were formed. In our previous report [27], we described about the preparation of cross-linked thermosensitive copolymer beads having phosphinic acid groups (APPO– NIPAAm–4G) by the suspension copolymerization of acryloyloxypropyl n-octylphosphinic acid (APPO), N-isopropylacrylamide (NIPAAm), and tetraethyleneglycol dimethacrylate (4G). Although the swelling of APPO–NIPAAm–4G copolymer in water was greatly affected by temperature, the metal adsorption ability of the copolymer was almost constant at various temperatures. On the other hand, the selective adsorption ability of the APPO–NIPAAm–4G copolymer beads for lanthanide metal ions was significantly higher than that for typical elements. In this work, we synthesized hydrophilic copolymer beads having phosphinic group by suspension copolymerization of acryloyloxypropyl n-octylphosphinic acid (APPO), N,N-dimethylacrylamide (DMAAm), and tetraethyleneglycol dimethacrylate (4G) and the adsorption abilities of the copolymer beads obtained for lanthanide metal ions were measured. 2. Experimental 2.1. Materials APPO and DMAAm were kindly supplied by Nippon Kagaku Kogyo Co. (Tokyo, Japan) and Kohjin Co. (Tokyo, Japan), respectively, and used without further purification. Tetraethyleneglycol dimethacrylate (4G) as a cross-linking agent was purchased from Sigma-Aldrich Japan Co. (Tokyo, Japan) and used without further purification. a,a 0 Azobisisobutyronitrile (AIBN) (Nacalai tesque
Co., Tokyo, Japan) was purified by recrystallization from ethanol (Wako Pure Chemical Industries, Ltd., Osaka, Japan). All of lanthanide salts, lanthanum chloride (Wako Pure Chemical Industries, Ltd., Osaka, Japan), neodymium chloride (Junsei Chemical, Ltd., Tokyo, Japan), samarium chloride (Wako Pure Chemical Industries, Ltd., Osaka, Japan), and europium chloride (Kanto Chemical Co., Tokyo, Japan) were used as received. Other chemical compounds used were of reagent grade. 2.2. Synthesis of Cross-linked APPO–DMAAm–4G copolymer beads The APPO–DMAAm–4G copolymer beads were produced in aqueous dispersant by suspension polymerization as follows [27]. Monomers, 0.99 g (3.41 mmol) of APPO, 2.79 g (28.1 mmol) of DMAAm, 0.22 g (0.666 mmol) of 4 G were mixed, and 4, 2 cm3 or none of chloroform was added to make a solution of 50, 75, and 100 wt./vol.% of concentration,respectively. The monomer solution was bubbled with nitrogen for 1 h in ice bath and AIBN was added to the solution as an initiator. Four hundred cubic centimeter of dispersant media were prepared by dispersing 4 g of magnesium carbonate in saturated sodium sulfate aqueous solution containing 0.1 g of sodium sulfodiisooctyl succinate (AOT) as a surfactant. The dispersant medium was treated with ultrasonic to make the solution homogenous and placed in three-neck flask equipped with a condenser, a nitrogen inlet and a PTFE stirrer stick. The dispersant medium was bubbled with nitrogen for 1 h and the monomer solution was added under stirring. Polymerization was carried out with stirring at 300, 400 or 500 rpm at 50 C for 3 h and further at 60 C for 1 h under a nitrogen atmosphere. After polymerization, the product was filtered off and washed with 0.5 N HCl to remove magnesium carbonate. After that, the copolymer beads were washed with acetone using soxhlet extractor for over 24 h and they were dried in the air. Additionally, the copolymer beads were alternately soaked in water of 10 and 60 C to eliminate exudation in the case of aqueous experiments. After drying, the copolymer beads were used for experiments. The content of phosphinic acid groups in the copolymer beads was calculated from the phosphorus content of the dried copolymer beads. The phosphorus content was determined by phosphovanadomolybdate method after combustion. The nitrogen content,
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which corresponds to the DMAAm content in the copolymer beads, was determined by elemental analysis (Yanaco CHN Corder MT-6). 2.3. Measurement of the swelling volume of the APPO–DMAAm–4G copolymer beads To evaluate the swelling behavior of copolymer beads, swelling volumes were measured as follows: 0.2 g of dried copolymer bead was soaked for 24 h at room temperature (24–25 C) in deionized water in 5 cm3 measuring cylinder, and then the temperature was varied and the copolymer beads were allowed to stand at each temperature for 1 h. The swelling volume of the copolymer beads at each temperature was calculated using Eq. (1). Swelling volumeðcm3 g1 Þ ¼ Apparent volumeðcm3 Þ =Weight of copolymer beadsðgÞ
ð1Þ
2.4. Measurement of the adsorption capacity of the copolymer beads for metal ions The quantity of adsorbed metal ions to the copolymer beads were measured as follows: 0.1 g of dried copolymer beads was placed into 25 cm3 of buffer solution (pH = 3.5, 5.0, 6.5; CH3COOH– CH3COONa) in a 50 cm3 Erlenmeyer flask and the flask was shaken at 20 C for 24 h to complete the swelling of the copolymer beads. And then 25 cm3 of metal ion solution containing 200 mol% of metal ion of a phosphinic group in copolymer were poured into the flask and shaken for further 24 h at 20 C. After that, the copolymer beads were filtered. The adsorption capacity was calculated by determining the concentration of residual metal ions in the supernatant with inductively coupled argon plasma atomic emission spectrophotometer (Shimadzu ICPS-5000). 3. Results and discussions 3.1. Synthesis of the APPO–DMAAm-4G copolymer beads The monomers used in this study have different solubility in water, DMAAm is an amphiphilic monomer and APPO and 4G show poor solubility in water. When the aqueous media were used for suspension polymerization, the water soluble monomer will be extracted to aqueous phase
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resulting in reduction of the monomer component in copolymer. Therefore, suspension polymerization of water-soluble monomer in aqueous media is difficult [28,29]. In our previous report, the conditions for suspension polymerization of amphiphilic monomer such as NIPAAm in aqueous solution have been studied [27]. In this study, we obtained spherical APPO–DMAAm–4G copolymer beads by suspension polymerization as described in the previous report as follows: A saturated sodium sulfate aqueous solution was used as a dispersing medium in the presence of suspended magnesium carbonate powder. The monomer solution contained AOT was poured into the dispersant medium placed in a reaction flask and the mixture solutions were stirred at 300, 400 or 500 rpm to make droplets containing the monomers. In the polymerization reaction, saturated sodium sulfate solution played an important role to hold DMAAm monomer in chloroform, AOT and magnesium carbonate also acted as auxiliary dispersing agents. In the preparation of copolymer beads, chloroform was used for solvent of monomers. However, chloroform is not suitable to polymerization solvent due to its high chain transfer constant, there was no reasonable solvent in this study except chloroform. The polymerization reactions were carried out at 50 C for 3 h and further at 60 C for 1 h (Fig. 1). After the reaction, well dispersed copolymer beads were obtained and they were washed with hydrochloric acid and purified using acetone and water. Table 1 shows the results of the APPO–DMAAm– 4G copolymer beads obtained at various stirring speeds and monomer concentrations. The yields of copolymers were changed by the conditions. Higher stirring speed and higher monomer concentration increased the yield of copolymer. The yield decreased at higher concentration of chloroform which acted as a chain transfer agent as described above. The contents of APPO and DMAAm in the copolymer were estimated from the phosphorous and nitrogen contents, respectively. It was found that the composition of copolymer was affected by conditions of copolymerization. In all conditions, the contents of APPO were decreased from feed composition. On the contrary, content of 4G increased from feed composition and it would make the copolymer gel to lower swelling. In our previous report, we have investigated about the influences of crosslinking degree on the swelling and the adsorption ability of metal ion. It was found
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Fig. 1. Synthesis of APPO–DMAAm–4G copolymer beads.
Table 1 Results of APPO–DMAAm–4G copolymer beads obtained Sample
1 2 3 4 5 a b
Composition in copolymera
Stirring speed (rpm)
Monomer conc. (wt./vol.%)
Elementary assay N (mmol g )
P (mmol g )
DMAAm mol%
APPO mol%
Average diameterb (lm)
300 400 500 400 400
50 50 50 75 100
7.1 6.7 7.1 6.3 6.2
0.54 0.61 0.65 0.77 0.87
88 86 88 84 83
6.7 7.8 8.0 10.3 11.7
27 22 21 33 –
1
1
Calculated from results of elementary assay. Estimated from SEM images.
that the adsorption of metal ion was suppressed only at lower swelling volume [27]. In this study, since all of APPO–DMAAm–4G copolymers exhibited higher swelling volume, the influence of the swelling ratio on the metal adsorption was low. Fig. 2 shows the scanning electron micrograph of the APPO–DMAAm–4G copolymer beads obtained under various conditions. The average diameters of copolymer beads were estimated from these pictures. With decreasing of monomer concentration in feed, the size of beads became larger, and the irregular shaped beads were increased, and the surface of bead became rough. The best appearances of beads were obtained by using 50 vol% monomer solution. Although the size of beads tended to be smaller with increasing of stirring speed, they maintained spherical shape and smooth surface. These copolymer beads obtained
were used for experiments as having different APPO content. 3.2. Swelling behavior of the APPO–DMAAm–4G copolymer beads In our previous report [27], NIPAAm was copolymerized with APPO and 4G instead of DMAAm. The APPO–NIPAAm–4G copolymer exhibited a typical thermosensitivity which shows reversible swelling transition around 35 C. It was found that although the swelling of NIPAAm–4GAPPO was significantly suppressed above 35 C by the thermosensitivity, the adsorption ability was not affected. It is a significant difference between APPO–NIPAAm–4G and APPO–DMAAm–4G that thermosensitive moiety is contained or not contained.
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Fig. 2. SEM images of the APPO–DMAAm–4G copolymer beads obtained by suspension polymerization.
In Fig. 3, the swelling volume of the copolymer beads having different APPO content in deionized water at various temperatures was measured. The decrease in the swelling volume of the copolymer beads with increasing temperature indicates that the copolymer beads become hydrophobic at higher temperatures.
The swelling volume of the APPO–DMAAm–4G copolymer beads increased with increasing APPO content in the copolymer beads. This indicates that the hydrophilicity of the copolymer beads became higher as the quantity of phosphinic acid groups in the copolymer beads increased. Consequently, the adsorption experiments were carried out at 30 C, since the magnitude of swelling at this temperature was appropriate for handling and was a suitable temperature to maintain constantly during experiments. 3.3. pH dependence of the swelling volume of the APPO–DMAAm–4G copolymer beads
Fig. 3. Temperature dependence of the swelling of APPO– DMAAm–4G copolymer beads in deionized water.
The swelling volume (cm3 g1) of the copolymer beads was measured in buffer solutions having different pHs at various temperatures (Fig. 4). Although the shape of curve is almost the same as the result in the deionized water (Fig. 3), position of the curve was shifted to a value of lower swelling volume by lowering pH of the buffer solution. In the higher pH region (pH = 5.0, 6.0, 7.0), the swelling volumes were almost the same. This result was readily explained in the pKa of phosphinic group in APPO monomer. Ionizing ratio was calculated from pKa of APPO monomer (= 4.3) [30] in each buffer solutions and the results of ionizing ratio of phos-
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Fig. 4. Temperature dependences of the swelling of APPO– DMAAm–4G copolymer beads at various pHs. P content in copolymer = 0.61 mmol g1.
phinic group are 50%, 80%, and 98% at pH = 3.5, 4.5, 5.0, respectively. When the pH is higher than 5.0, almost phosphinic group will be ionized and consequently, the swelling ratio of copolymer showed the same curve. The swelling volume of APPO–DMAAm–4G copolymer beads was affected by pH and remarkable change was found in the range of pH 3.5–5.0 according to the pKa of APPO. 3.4. Adsorption rate of metal ions with the APPO– DMAAm–4G copolymer beads The phosphinic acid groups in APPO are used to coordinate and/or ion exchange with metal ions, in particular, with lanthanide ions [11,12,31]. In this study, the adsorption ability of APPO–DMAAm– 4G copolymer beads for lanthanide ions (La3+, Nd3+, Sm3+, Eu3+) was measured under various conditions. To investigate the adsorption speed, the time course of adsorption of Sm3+ by APPO–DMAAm– 4G copolymer bead was measured by a batch method at 30 C in buffer solution (pH = 5.0) for various adsorption periods (Fig. 5). The quantities of adsorption increased significantly in first 4 h and reached equilibrium at 12 h. From this result, the adsorption time is evaluated to 24 h enough to reach the adsorption equilibrium completely. The maximum quantity of adsorbed metal ions was about 1/4–1/5 in mol ratio to APPO content in the copolymer.
Fig. 5. Time course of the adsorption of Sm3+ with APPO– DMAAm–4G copolymer beads at pH 5.0. Weight of gel: 0.10 g; Metal ion solution: 3.10 · 103 mol dm3, 50 cm3; Buffer solution (pH 5.0): CH3COOH–CH3COONa.
3.5. Effect of pH on the adsorption of metal ions with the APPO–DMAAm–4G copolymer beads from its single metal ion solution The adsorption ability of the APPO–DMAAm– 4G copolymer beads for lanthanide metal ions (La3+, Nd3+, Sm3+, Eu3+) from each single metal ion solution was measured at different pHs (3.5, 5.0, 6.5) at 20 C. Fig. 6 shows the pH dependence of the adsorption of each metal ion with the APPO– DMAAm–4G copolymer beads. The adsorption abilities for each metal ion increased in higher pH solution. The adsorption behaviors of the metal ions were almost the same except La3+ that exhibited less adsorption compared to other metal ions. The degree of ionization of phosphinic acid increased with increasing pH value and almost all of phosphinic acid groups are ionized pH = 5.0 as described above. The increase of the metal ion adsorption supports the estimation which the metal ions were adsorbed via ion-exchange adsorption with ionized phosphinic acid groups in the copolymer beads. 3.6. Adsorption of metal ions with the APPO– DMAAm–4G copolymer beads from mixed metal ion solutions In the previous section, the adsorption ability for lanthanide metal ions of APPO–DMAAm–4G copolymer beads was found. Fig. 7 shows the selec-
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Selectivity coefficientðKÞ 3þ ¼ ðM 3þ 1 =M 2 molar ratio adsorbed with the copolymer beadsÞ 3þ =ðM 3þ 1 =M 2 molar ratio in solutionÞ
ð2Þ
The APPO–DMAAm–4G copolymer beads had higher adsorption ability for Eu3+ than Sm3+, Sm3+ than Nd3+, and Nd3+ than La3+ from each binary solutions, although a significant difference in the metal ion adsorption was not observed from its single metal ion solution. From this result, the order of adsorption ability with the APPO–DMAAm–4G copolymer beads for lanthanide metal ions was estimated as follows: Eu3þ > Sm3þ > Nd3þ > La3þ .
Fig. 6. Effect of pH on the adsorption of metal ions with APPO– DMAAm–4G copolymer beads. Weight of gel: 0.10 g (P content = 0.68 mmol g1; Metal ion solution: 2.40 · 103 mol dm3, 50 cm3; Shaking at 30 C for 24 h.
tive adsorption between two kinds of lanthanide ion from their binary mixture solutions (1/1 in molar ratio) at pH 3.5, 5.0, and 6.5. The adsorption behavior was different from the result obtained for single metal ion solution. That is, there is no change in quantity of adsorption at different pHs, while the adsorption from single metal ion solution increased with an increase in the value of pH. In addition, the quantity of adsorption was significantly different between the two metal ions. The selectivity coefficient (K) was calculated by using Eq. (2) and the results are shown in Table 2.
This order is in good agreement with the order of an atomic number of these metal ions [11]. It is known as lanthanide contraction that lanthanide metal ion with larger atomic number has lower ionic radius than that with smaller atomic number, because, in lanthanide metal series, inner shell is filled faster with electron than the outer shell [32]. Therefore, lanthanide metal ion with larger atomic number has higher charge density on its surface than that with smaller atomic number. As a consequence, lanthanide metal ion with lower atomic number exhibited higher adsorption abilities. The quantity of adsorption in binary mixture solution of two metal ions decreased from the case of adsorption from single metal ion solution. The reduction is due to lower concentration in each metal ion compared to the case in single metal ion adsorption. Although the decreased in adsorption
Fig. 7. Adsorption of metal ions with APPO–DMAAm–4G (15:97:3) copolymer beads from mixed metal ion solution in the pH range from 3.5 to 6.5. Weight of gel: 0.10 g (P content = 0.68 mmol g1); Metal ion solution: 2.72 · 103 mol dm3, 50 cm3; Shaking at 30 C for 24 h.
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Table 2 Selectivity coefficient of adsorption (K) from binary mixed metal ions solution
Table 3 Selectivity coefficient of adsorption (K) from Eu3+ and La3+ mixed metal ions solution with various concentration ratio
Metal ion solution
Concentration ratio (Eu3+/La3+)
Selectivity coefficient (K)
4/1 2/1 1/1 1/2 1/4
2.6 2.9 4.7 5.4 4.2
Eu3+/Sm3+ Sm3+/Nd3+ Nd3+/La3+
Selectivity coefficient (K) pH = 3.5
pH = 5.0
pH = 6.5
5.1 1.7 3.3
5.5 2.2 2.1
2.7 6.3 2.7
Selectivity coefficient (K); K = a/b. a: ratio of adsorption of metal ions. b: ratio of concentration of metal ions in solution.
of each metal ion is agreeable, decreasing in total of the quantity of adsorption of two kinds of metal ions compared to the quantity of adsorption from single metal ion is unexplainable phenomena at this moment. To evaluate selective adsorption between Eu3+ and La3+, the adsorption was measured from binary mixed solutions with different compositions (Eu3+/La3+ = 8/2 2/8 molar ratio; total concentration = 2.72 · 103 mmol dm3) at 20 C and at pH of 6.5 (buffer solution). As shown in Fig. 8, adsorption of Eu3+ was higher than La3+, even from mixed solutions with smaller content of Eu3+ than La3+. The selective coefficient of adsorption for Eu3+/La3+ (Table 3) exhibited highest value in adsorption from Eu3+/La3+ = 1/2 (molar ratio) mixture solution. As observed above, it was found that the APPO–DMAAm–4G copolymer beads had high selective adsorption ability for Eu3+ than La3+.
Selectivity coefficient (K); K = a/b. a: ratio of adsorption of metal ions. b: ratio of concentration of metal ions in solution.
4. Conclusion APPO–DMAAm–4G copolymer beads were prepared by suspension copolymerization in aqueous media. The composition of copolymer gel was affected by copolymerization conditions such as stirring speed and monomer concentration. The swelling behavior and metal ion adsorption capacity of copolymer beads were influenced by pH. The copolymer beads exhibited higher adsorption capacity for lanthanide metal ions. The selectivity of APPO–DMAAm–4G copolymer beads for lanthanide metal ions is in the following order: Eu3+ > Sm3+ > Nd3+ > La3+, same as the order of atomic number. In selective adsorption of Eu3+ and La3+, selectivity for Eu3+ was higher even from mixed solutions with smaller content of Eu3+ than La3+. Thus, APPO–DMAAm–4G copolymer beads is a hydrogel having high hydrophilicity and selective adsorption ability for lanthanide metal ions. References
Fig. 8. Adsorption of metal ions (Eu3+, La3+) with APPO– DMAAm–4G copolymer beads from mixed metal ion solutions with various concentration ratios of pH = 6.5 at 20 C. Weight of gel: 0.10 g (P content = 0.68 mmol g1); Metal ion solution: 2.72 · 103 mol dm3, 50 cm3; Shaking at 20 C for 24 h.
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