Fibrous Iminodiacetic Acid Chelating Cation Exchangers with a Rapid

Cross-linked polystyrene-based iminodiacetic acid resins are the most representative chelating resins used widely from laboratory to industrial scales...
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Ind. Eng. Chem. Res. 2004, 43, 1599-1607

1599

Fibrous Iminodiacetic Acid Chelating Cation Exchangers with a Rapid Adsorption Rate Akinori Jyo,* Jameson Kugara, Haris Trobradovic, and Kazunori Yamabe Department of Applied Chemistry and Biochemistry, Kumamoto University, Kumamoto 860-8555, Japan

Takanobu Sugo, Masao Tamada, and Tamio Kume Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute, Takasaki 370-1292, Gunma, Japan

Fibrous chelating cation exchangers were derived from chloromethylstyrene-grafted polyethylenecoated polypropylene filamentary fiber and its nonwoven cloth. Ligand contents and acid capacities of the resulting chelating filamentary fiber (FIDA-f) and cloth (FIDA-c) were ca. 2 mmol/g and ca. 4 mequiv/g, respectively. A distribution study by using FIDA-c clarified that the selectivity sequence of divalent metal ions is Mg(II) ∼ Ca(II) < Co(II) ∼ Zn(II) < Cd(II) ∼ Ni(II) < Pb(II) < Cu(II). Capacities of FIDA-c in mmol/g at around pH 5 were as follows: for divalent ions, Ca(II) ) 0.91, Mg(II) ) 0.98, Cd(II) ) 1.5, Ni(II) ) 1.5, Pb(II) ) 1.6, and Cu(II) ) 1.8; for trivalent ions, La(III) ) 0.75, Gd(III) ) 0.92, and Lu(III) ) 1.0. A column-mode study using a FIDA-f packed column revealed that breakthrough capacities for Cu(II) (ca. 1 mmol/g) were not dependent on flow rates up to 200-300 h-1 in space velocity but slightly decreased with a further increase in the flow rate. Conditioning-adsorption-elution operations of the FIDA-f column were repeated many times (n ) 21) under various conditions; averages of the column capacity for Cu(II) and the amount of eluted Cu(II) were 1.9 ( 0.1 and 1.9 ( 0.2 mmol/g, respectively. 1. Introduction Cross-linked polystyrene-based iminodiacetic acid resins are the most representative chelating resins used widely from laboratory to industrial scales. However, one of their serious disadvantages is slow kinetics in the adsorption of metal ions.1 Three-dimensionally cross-linked matrixes of these resins are not suitable for rapid diffusion of metal ions in resin particles as well as for a fast chelating reaction of the polymer bound functional groups with metal ions. To resolve these kinetic problems, fibrous adsorbents with iminodiacetic acid groups have been studied from the end of the 1960s.1-3 Although a few iminodiacetic acid fibers are now commercially available,1,3 there is still room for improvement of capacities and adsorption rates. For instance, the capacity of a commercially available iminodiacetic acid fiber named Ionex IDA-Na (Toray Industries, Inc., Tokyo) was reported to be 0.9-1.1 mmol/g for Cu(II).3 In addition, its breakthrough capacity in column-mode removal of Mn(II) at pH 5.4 decreased by ca. 20% when the flow rate of feeding solutions was increased from 5 to 80 h-1 in space velocity.1,3 From the beginning of the 1990s, Sugo’s group4-8 and Choi and Nho9 have been studying the preparation, properties, and application of nonporous and porous polymeric membranes having iminodiacetic acid groups. These membranes, including hollow fibers, were prepared by two steps: the first step is radiation-induced graft polymerization of chloromethylstyrene (CMS) or glycidyl methacrylate onto trunk polymeric membranes, * To whom correspondence should be addressed. Tel., Fax: +81-96-342-3871. E-mail: [email protected].

and the second step is the introduction of iminodiacetic acid groups onto the grafted poly(CMS) or poly(glycidyl methacrylate) chains. These membranes exhibited rapid adsorption rates of metal ions, but they are difficult to use in conventional batchwise and columnar adsorption modes.4-9 Then, the present work was planned to prepare iminodiacetic acid fibers for conventional batchwise and columnar uses. Because we have proved that polyethylene-coated polypropylene fiber (PPPE) was suitable for the preparation of chemically and physically stable phosphoric and phosphonic acid cation-exchange fibers with rapid cation-exchange rates,10,11 PPPE was adopted as trunk fibers in this work. According to the synthetic route shown in Scheme 1, graft polymerization of CMS onto PPPE was conducted by an electron preirradiation-induced liquid-phase graft polymerization method, and then CMS-grafted PPPE was functionalized by the reaction with diethyl iminodiacetate and subsequent alkali hydrolysis of the introduced diethyl iminodiacetate. Metal ion adsorption abilities of the resulting iminodiacetic acid fibers were studied by batchwise and columnar methods. In this work, two types of fibrous chelating exchangers were prepared from PPPE trunk fibers: one was from nonwoven cloth of PPPE for use in batchwise studies and the other from filamentary PPPE for column-mode use. 2. Experimental Section 2.1. Materials. CMS, named CMS14, was provided by Seimi Chemical Co., Chigasaki, Japan, and its purity was 95% as a para isomer. Diethyl iminodiacetate was purchased from Tokyo Kasei Kogyo Co. Ltd., and its purity was 97%. Porous-type iminodiacetic acid resin Diaion CR10 (Mitsubishi Chemical Co., Tokyo, Japan)

10.1021/ie030146h CCC: $27.50 © 2004 American Chemical Society Published on Web 03/06/2004

1600 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 Scheme 1. Preparation Scheme of Iminodiacetic Acid Fibers from PPPE

was used to compare breakthrough behavior in columnmode uptake of Cu(II) with that of the fiber. This is selected because it is one of the iminodiacetic acid resins used in wide fields of industries. First the provided Diaion CR10 was dried in a vacuum and then meshed to collect particles from 32 to 60 mesh. The selected particles were conditioned by treatment with 1 M NaOH and then with 1 M HCl in a column and were finally equilibrated with a dilute hydrochloric acid of pH 2. After air-drying, it was dried in a vacuum. Thus, conditioned Diaion CR10 in hydrogen ion form was used in the column-mode experiments, and its nitrogen content and acid capacity in the hydrogen ion form were 3.24 mmol/g and 6.7 mequiv/g, respectively. Other chemicals were of reagent grade unless otherwise specified. 2.2. Electron Irradiation to Trunk Fibers. Polyethylene-coated polypropylene filamentary fiber (PPPEf; 0.9 denier, length ca. 3.8 cm) and nonwoven cloth of polyethylene-coated polypropylene fiber (PPPE-c; 1.5 denier) were used as trunk polymers. They were provided by Kurashiki Textile Mfg. Co., Osaka, Japan. In the case of PPPE-c, it was cut into pieces of 7.5 cm × 10 cm. After PPPE-f or PPPE-c was dried in a vacuum oven at 40 °C for a day, it was packed into a polyethylene bag, and air in the bag was displaced by nitrogen. Then, the bag containing PPPE-f or PPPE-c was irradiated with the electron beam (2 MeV, 1 mA, 10 kGy/min) for 20 min (a total of 200 kGy), and the electronirradiated PPPE-f and PPPE-c were stored in an electric refrigerator at -60 °C. The electron accelerator used was a cascade-type Dynamitron No. 2 at Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute. 2.3. Graft Polymerization of CMS. After the removal of the polymerization inhibitor with an alumina column from CMS14, dimethyl sulfoxide solutions of CMS14 (50 and 80 wt % for PPPE-c and PPPE-f, respectively) were prepared, and nitrogen was bubbled into the resulting CMS14 solutions for 30 min to expel dissolved oxygen. The electron-irradiated PPPE-f or PPPE-c (200 kGy) was taken into glass ampules, and

the air in the ampules was eliminated by vacuum suction. Then, the dimethyl sulfoxide solutions of CMS14 (150 mL) were sucked into the ampule, and the resulting mixture was maintained at 40 °C for 6.5 and 6 h for PPPE-f and PPPE-c, respectively. The CMS14-grafted PPPE-f and PPPE-c were washed with N,N-dimethylformamide and then with methanol. They were air-dried and then dried in a vacuum. From an increase in weight (Win) by graft polymerization of CMS14, the degree of grafting (dg) was calculated. The dg is designated by 100(Win/Wo). Here, Wo is the weight of PPPE-c or PPPE-f before grafting. Under conditions described above, values of dg became ca. 100%; CMS14-grafted PPPE-c and PPPE-f used in this work had dg values of 105108%, because peeling and exfoliation products during functionalization processes were minor as long as the dg of the grafted precursory fibers was less than ca. 150%.10,11 2.4. Functionalization of CMS14-Grafted Fibers. Here, the only optimized functionalization method of CMS14-grafted PPPE-c is described as an example. First, CMS14-grafted PPPE-c was chopped into small pieces (ca. 1 cm × ca. 2 cm). These pieces (0.67 g) were heated with a mixture of diethyl iminodiacetate (9 mL) and ethanol (6 mL) at 120 °C for 3 h in a stainless steel autoclave, and then hydrolysis of diethyl iminodiacetate introduced onto the fiber was carried out by heating in 1 M NaOH (150 mL) under reflux for 9 h. After hydrolysis, the functionalized PPPE-c was treated with 1 M hydrochloric acid and then equilibrated with dilute hydrochloric acid of pH 2. The functionalized PPPE-c (hereafter named FIDA-c) in the hydrogen ion form was air-dried and then dried in a vacuum at 40 °C for 24 h. The CMS14-grafted PPPE-f was also functionalized by almost the same procedures as those mentioned previously. The functionalized PPPE-f was named FIDA-f hereafter. Nitrogen contents of both FIDA-c and FIDA-f before the hydrolysis were determined by CHN analysis, and acid capacities of FIDA-c and FIDA-f after the hydrolysis were determined by measuring the amounts of sodium hydroxide consumed by both fibers in the

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1601

hydrogen ion form after their contact with 0.1 M NaOH for 24 h at 30 °C. 2.5. Batchwise Study. Batchwise study was conducted by using FIDA-c derived from nonwoven cloth of PPPE, which is much more convenient for use in batch-mode experiments than FIDA-f.10,11 2.5.1. Distribution Ratios of Metal Ions. Pieces (ca. 1 cm × ca. 2 cm) of FIDA-c (0.040 g) and a metal ion solution (10-4 M, 25 mL) were placed into 50-mL Erlenmeyer flasks. FIDA-c and the solution were equilibrated by shaking the flasks with a mechanical shaker at 30 °C for 24 h. Metal ion concentrations in the aqueous phases were determined by means of inductively coupled plasma atomic emission spectrometry (ICP-AES). From the following equations, extraction percentages (E %) and distribution ratios (D) were calculated.

E % ) 100(Co - Ce)/Co D ) (V/m)E %/(100 - E %) where Co and Ce are the initial and equilibrium concentrations of the target metal ion, respectively, and where V and m represent the volume of a contacting solution and the weight of dried FIDA-c in the hydrogen form, respectively. In the calculation of D, E % values between 2 and 98% were adopted because errors in log D values markedly increase with an approach of E % to 0 or 100%. In the adjustment of the pH of aqueous phases, only nitric acid was used. 2.5.2. Capacity for Uptake of Metal Ions. Capacities for the uptake of a metal ion were measured by contacting pieces of FIDA-c (0.050 g) with 25 mL of the metal ion solution for 24 h at 30 °C. By measurement of the metal ion concentration in the supernatant by means of ICP-AES, metal ion uptake (MU) in mmol/g was calculated from the following equation:

MU ) (Co - Ce)V/m The pH of solutions below 3 was adjusted with nitric acid and that above 3 with acetic acid/sodium acetate buffers. Three kinds of buffers of pH 3.2, 4.1, and 5.0 were prepared by mixing 0.1 M sodium acetate with 0.1 M acetic acid in ratios of 1:32, 1:4, and 2:1, respectively. In these buffer solutions was dissolved nitrate salt of a target metal ion, and an exact concentration of each metal ion was determined by ICP-AES. 2.6. Column-Mode Study. We have already reported that filamentary type fibers are much more suitable for packing into a column than nonwoven cloth type ones, and the filamentary type fiber packed columns give smooth breakthrough profiles comparable to those observed in spherical resin-packed columns.10-13 Then, filamentary type fiber FIDA-f was used in the columnmode study. Dried FIDA-f (0.4 g) in the hydrogen ion form was swollen with water and then packed into glass columns (i.d. 0.60 cm), and then the top of the fiber bed was pressed with a glass rod with flat ends (diameter ca. 0.4 cm) until the fiber bed height became constant. Then, the FIDA-f-packed column was equilibrated with dilute hydrochloric acid of pH 2.0. The bed volume of the fiber equilibrated with dilute hydrochloric acid of pH 2.0 was 1.66 mL (bed height 5.9 cm). This fiber bed volume was used as the reference volume to convert flow rates (in mL/h) into space velocities (in h-1) throughout.

Table 1. Results of the Condensation Reaction of CMS14-Grafted PPPE-c with Diethyl Iminodiacetatea entry no.

temp (°C)

reaction time (h)

nitrogen contentb (mmol/g)

1 2 3 4 5 6 7 8

100

1 2 3 4 1 2 3 4

1.84 2.00 1.99 2.04 1.96 1.99 1.96 2.01

120

a CMS14-grafted PPPE-c, 0.20 g; diethyl iminodiacetate, 6.0 mL; ethanol, 2.7 mL. The reaction was conducted by using a stainless steel autoclave. b Nitrogen contents are values before hydrolysis.

Table 2. Dependence of Acid Capacities on the Time of Hydrolysis Reaction entry hydrolysis acid capacity entry hydrolysis acid capacity no.a timeb (h) (mequiv/g) no.a timeb (h) (mequiv/g) 1 2 3 4

3 6 9 12

3.6 4.5 5.1 5.2

5 6 7 8

3 6 9 12

1.9 5.0 5.4 5.4

a Entry number is the same as that in Table 1. b Hydrolysis conditions: immersed in 50 mL of 1 M NaOH under reflux.

A feed containing a target metal ion was downflowed through the column. Flow rates of the feed were adjusted with a peristaltic pump. After the column was washed with water, metal ions adsorbed on the column were eluted with 1 M nitric acid at a space velocity of 3 h-1. All column effluents including washings were collected on a fraction collector. Volumes of fractions were 4 and 2 mL for adsorption and elution operations, respectively. Metal ions in each fraction were determined by means of ICP-AES after dilution when necessary. The breakthrough point of the metal ion is designated as the feed volume where the value of C/Co corresponds to 0.1. After elution of the adsorbed metal ion, the column was equilibrated again with a dilute hydrochloric acid of pH 2 for the next adsorption experiment. Although the height of the fiber bed was slightly shortened after elution operation with 1 M nitric acid, it swelled back to its initial height under the equilibrated condition with dilute hydrochloric acid of pH 2. In the case of the Diaion CR10 column, the resin in the hydrogen ion form (dry weight 0.80 g; wet volume 1.69 mL) was packed into a glass column (i.d. 0.60 cm). Detailed experimental conditions for both columns will be given in later sections with the results. 3. Results and Discussion 3.1. Functionalization. Optimization of the condensation reaction of CMS14-grafted PPPE-c with diethyl iminodiacetate was conducted at 100 and 120 °C by changing the reaction time. The results are shown in Table 1. Although heating at 100 °C for 1 h gave a somewhat lower nitrogen content of 1.84 mmol/g, nearly constant nitrogen contents of 1.99 ( 0.03 mmol/g were observed under other conditions. From the results given in Table 1, heating at 120 °C for 3 h was selected in the further experiment. Table 2 gives the results of the optimization of the reaction time in the alkali hydrolysis process of introduced diethyl iminodiacetate; the reaction time for 9 h is required to complete hydrolysis. Then, this reaction time for hydrolysis was adopted in

1602 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 Table 3. Properties of FIDA-c and FIDA-f for Use in the Investigation of the Uptake of Metal Ions entry no.

chelating fiber

9 10 11 12 13 14

FIDA-ca FIDA-fb

nitrogen content before hydrolysis (mmol/g)

acid capacity after hydrolysisc (mequiv/g)

2.04 2.05 2.06 2.12 2.11 2.14

4.2 3.9 4.1 4.3 4.3 4.3

a CMS14-grafted PPPE-c, 0.67 g; diethyl iminodiacetate, 9.0 mL; ethanol, 4.0 mL. b CMS14-grafted PPPE-c, 0.50 g; diethyl iminodiacetate, 9.0 mL; ethanol, 4.0 mL. c Hydrolysis conditions: immersed in 150 mL of 1.0 M NaOH under reflux for 9.0 h.

the further study. Because the nitrogen content values are values before hydrolysis, they are not directly correlated to acid capacities. During alkali hydrolysis and subsequent acidification processes, two ethyl groups were replaced by two hydrogen ions, resulting in a decrease in the weight of the fiber. Then, acid capacities became greater than twice the nitrogen contents before hydrolysis. Next, FIDA-c and FIDA-f for use in the evaluation of MU were prepared under the optimized conditions for both condensation and hydrolysis processes. Each of FIDA-c and FIDA-f was separately prepared three times. The results are shown in Table 3, which shows high reproducibilities of the functionalization processes; the averages of nitrogen contents before hydrolysis and acid capacities of three separately prepared FIDA-c were 2.05 mmol/g and 4.1 mequiv/g, respectively, and those of FIDA-f were 2.12 mmol/g and 4.3 mequiv/g. FIDA-c and FIDA-f given in Table 3 were used in the further study. 3.2. Distribution Ratios of Metal Ions. To evaluate the metal ion selectivity of FIDA-c, distribution ratios (D) of metal ions between FIDA-c in the hydrogen ion form and a nitric acid solution were measured. Because iminodiacetic acid is diprotic, FIDA-c in the hydrogen ion form is represented by a symbol FH2. It was reported that divalent metal ions were adsorbed by two pathways as follows:14

M2+ + 2FH2 S M2+(FH-)2 + 2H+ M2+ + FH2 S M2+F2- + 2H+

K1 (3.1) K2

(3.2)

The metal ion binds to both nitrogen and oxygen donor atoms in eq 3.2, whereas the metal ion binds to oxygen donors only in eq 3.1. From eqs 3.1 and 3.2, the following relation can be obtained:

D ) ([M2+(FH-)2] + [M2+F2-])/[M2+] ) (K1[FH2]2 + K2[FH2])/[H+]2 (3.3) Under the conditions that the quantity of a metal ion (AM) is much less than that of the functional group (AF), [FH2] can be regarded as constant; here, the molar ratio of AM/AF is 0.032 in this work. Then, [FH2] can be approximated to be constant irrespective of E % values. However, [FH2] cannot be regarded as constant when the pH of aqueous phases was adjusted with buffers. For instance, if an acetic acid/sodium acetate buffer is used, the formation of species such as FH-Na+ will become marked in the exchanger phase. In the distribu-

Figure 1. log D vs pH plots for divalent metal ions: (O) Cu; (b) Pb; (0) Ni; (9) Cd; (4) Zn; (2) Co; (1) Ca; (3) Mg. Adsorbent: FIDAc, 0.040 g. Metal ion solution: 0.0001 M solution of each metal ion, 25 mL. The pH was adjusted with nitric acid. Table 4. Slopes and Intercepts of log D vs pH Plots, Calculated pH1/2 Values, and Stability Constants for 1:1 Complexes metal

slope

intercept

pH1/2

log β1

Cu Pb Ni Cd Zn Co Ca Mg Al La

2.38 2.00 2.03 1.96 2.08 2.09 2.21 1.89 2.42 2.69

-0.83 -1.31 -2.41 -2.40 -3.12 -3.29 -4.68 -3.80 -2.61 -2.59

1.52 2.05 2.56 2.65 2.84 2.92 3.38 3.49 2.23 2.00

10.63a 7.45a 8.19a 5.73a 7.27a 6.97a 2.59a 2.94a 8.10b 5.70a

a Reference 16 at 20 °C in 0.1 M KNO . b Reference 15. The 3 concentration of each metal ion was 0.0001 M. For more detailed experimental conditions, refer to the captions of Figures 1 and 2.

tion study, then the pH of aqueous phases was adjusted with nitric acid only in order to avoid participation of the third cation, such as sodium ion, in distribution equilibria of the target metal ions. As long as the two requirements mentioned above are satisfied, eq 3.3 can be simplified to eq 3.4.

log D ) constant + 2pH

(3.4)

Figure 1 shows log D vs pH plots for divalent metal ions, and Table 4 summarizes the slopes and intercepts of log D vs pH plots. As judged from data given in Figure 1 and Table 4, eq 3.4 approximately holds for all tested divalent metal ions. The half extraction pH (pH1/2) is thought to be one of the semiquantitative indices of the metal ion selectivity; a given exchanger prefers a metal ion with the smaller value of pH1/2. Table 4 lists values of pH1/2 calculated from the least-squares linear equation for each metal ion and stability constants for 1:1 complexes of iminodiacetate with the metal ions as well.15,16 From values of pH1/2, increasing order of the affinity of the divalent metal ions to FIDA-c is Mg(II) ∼ Ca(II) < Co(II) ∼ Zn(II) < Cd(II) ∼ Ni(II) < Pb(II) < Cu(II). This order does not exactly coincide with the increasing order of the stability constants: Ca(II) < Mg(II) < Cd(II) < Co(II) < Zn(II) < Pb(II) < Ni(II) < Cu(II). As reported by Pesavento et al.,14 eq 3.1 corresponds to the adsorption by carboxylic acid groups only and eq 3.2 to the adsorption by a chelating reaction. If the contribution of eq 3.1 is higher than that of eq 3.2, values of D can no longer be simply correlated to stability constants of 1:1 metal-ligand complexes. Indeed, it was pointed out

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1603

Figure 2. log D vs pH plots for trivalent metal ions: (O) La; (0) Al. Adsorbent: FIDA-c, 0.040 g. Metal ion solution: 0.0001 M solution of each metal, 25 mL. The pH was adjusted with nitric acid.

that Ca(II) is extracted at a much lower pH than the anticipated pH from the stability constant of the 1:1 complex (ML).14 Because logarithmic values of side reaction coefficients of hydrogen ion for iminodiacetate are 6.7 and 8.3 at pH 3 and 2,17 respectively, the formation of the complex (ML) is difficult in the pH region less than 3 except for Cu(II). Then, the present results suggest that the pathway of eq 3.1 also plays an important role in the adsorption of most divalent metal ions by FIDA-c except for Cu(II). For the adsorption of trivalent metal ions by iminodiacetic acid chelating resins (LH2), Yuchi et al.15 have proposed the following two pathways:

M3+ + 3LH2 S M3+(HL-)3 + 3H+ M3+ + 2LH2 S (HL-)M3+L2- + 3H+

K3

(3.5)

K4 (3.6)

In the case of FIDA-c (FH2), D can be expressed from eqs 3.5 and 3.6 as follows:

D ) ([M3+(FH-)3] + [(FH-)M3+F2-])/[M3+] ) (K3[FH2]3 + K4[FH2]2)/[H+]3 (3.7) Under foregoing assumptions, eq 3.7 can be reduced to eq 3.8.

log D ) constant + 3pH

(3.8)

Figure 2 shows log D vs pH plots of Al(III) and La(III), and Table 4 summarizes their distribution data. Although eq 3.8 predicts slopes of log D vs pH plots to be 3, Al(III) gives the smaller slope of 2.42. Because the slope of log D vs pH plots gives the number of hydrogen ions given to solution as a result of metal ion adsorption, lower slopes probably come from the contribution of the hydrolyzed species such as [Al(OH)]2+ with an increase in the pH as reported.15,18 As opposed to the prediction from the stability constants of 1:1 complexes of iminodiacetate with Al(III) and La(III), La(III) was more highly distributed into FIDA-c. This observation is also in good agreement with the one reported by Yuchi et al. in the adsorption of trivalent metal ions by iminodiacetic acid resins below pH 2.8.15 In conclusion, the present study clarified that the metal ion selectivities

Figure 3. Dependence of divalent MU on pH: (b) Cu; (O) Pb; (4) Ni; (9) Cd; (0) Ca; (2) Mg. Adsorbent: FIDA-c, 0.050 g. Metal ion solution: 0.006 M of each metal ion, 25 mL. The pH was adjusted with nitric acid (pH 1 and 2) and with sodium acetate/ acetic acid buffers (pH 3-5). Final concentrations of sodium acetate in test solutions were 3.0, 20, and 67 mM for pH 3, 4, and 5, respectively, and those of acetic acid were 97, 80, and 33 mM for pH 3, 4, and 5, respectively.

of iminodiacetic acid groups bound to non-cross-linked matrixes are also different from those of complexation reactions of iminodiacetate in homogeneous aqueous solutions, as observed in the metal ion adsorption by iminodiacetic acid resins based on cross-linked polystyrene matrixes.14,15 3.3. Capacities of FIDA-c for the Uptake of Metal Ions. As opposed to the case of the distribution study, the molar ratio of the metal ion to the functional groups (AM/AF) was set at 1.5 in the capacity measurement; namely, the quantity of a target metal ion is greater than that of the functional groups. The uptake of metal ions above pH 5 was not tested because there is a fear that some heavy-metal ions precipitate as hydroxides. In the case of Al(III), the highest pH tested was 3.7 to avoid its precipitation as hydroxide.17 Figure 3 shows capacities for the uptake of divalent metal ions as a function of pH. The increasing order of capacities at ca. pH 5 is Ca(II), Mg(II) < Cd(II), Ni(II), Pb(II) < Cu(II). It seems that this order roughly coincides with the increasing order of stability constants of 1:1 complexes of these metal ions with iminodiacetate.16 Because the side reaction coefficient of iminodiacetate for hydrogen ion at pH 5 is 4.5,17 conditional stability constants of 1:1 complexes at pH 5 are Ca(II) ) 10-1.9, Mg(II) ) 10-1.6, Cd(II) ) 101.2, Pb(II) ) 103.0, Ni(II) ) 103.7, and Cu(II) ) 106.1. Then, iminodiacetate is not able to form 1:1 complexes with Ca(II) and Mg(II) at pH 5, whereas it forms the 1:1 complexes with other tested divalent metal ions at pH 5. As described in the preceding section, polymer-bound iminodiacetates take up metal ions by both cation exchange (eq 3.1) and chelating as well (eq 3.2). The latter becomes predominant with an increase of pH. Therefore, there is a possibility that sodium ion interferes with the adsorption of the target metal ion through a cationexchange mechanism and that acetate ion also interferes indirectly with the adsorption of the metal ion through the formation of acetato complexes in the aqueous phases. Therefore, a detailed explanation on pH profiles of capacities for the uptake of each metal mentioned above is difficult to achieve at the present because sodium and acetate ions from the buffers simultaneously influence the equilibrium of the cationexchange reaction. However, we would like to point out

1604 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004

Figure 4. Dependence of trivalent MU on pH: (b) Lu; (4) Gd; (O) La; (0) Al. Adsorbent: FIDA-c, 0.050 g. Metal ion solution: 0.006 M of each metal ion, 25 mL. The pH was adjusted with nitric acid (pH 1 and 2) and with sodium acetate/acetic acid buffers (pH 3-5). Concentrations of sodium acetate were 3.0, 20, and 67 mM for pH 3, 4, and 5, respectively, and those of acetic acid were 97, 80, and 33 mM for pH 3, 4, and 5, respectively.

that the capacity of FIDA-c for Cu(II) at ca. pH 5 (1.71.8 mmol/g) is comparable to that (1.6 mmol/g) of an iminodiacetic acid resin Chelex 10014 and is higher than that of a commercialized iminodiacetic acid fiber named Ionex (0.9-1.1 mmol/g).1,3 Figure 4 shows pH profiles of capacities for uptake of four trivalent metal ions: Lu(III), Gd(III), La(III), and Al(III). Uptake of three lanthanide ions at ca. pH 5 increases in the order of La(III) < Gd(III) < Lu(III). This is exactly the increasing order of the stability constants of 1:1 and 1:2 complexes of these ions with iminodiacetate.16 At pH 5, conditional overall stability constants for 1:1 and 1:2 complexes (β1′ and β2′, respectively) are as follows: for La(III), β1′ ) 101.4, β2′ ) 101.0; for Gd(III), β1′ ) 102.2, β2′ ) 103.1; and for Lu(III), β1′ ) 103.1, β2′ ) 104.7.16,17 Thus, capacities for the three tested lanthanides ions at ca. pH 5 can be qualitatively correlated to complexing equilibria. At pH 2, on the other hand, conditional overall stability constants of these complexes become essentially equal to 0, and then FIDA-c takes up lanthanide ions through the ion-exchange mechanism. Indeed, the increasing order of their uptake below pH 3 is not correlated to the complexing equilibria of iminodiacetate. 3.4. Column-Mode Study. Because the pKa1 of polymer-bound iminodiacetic acid is reported to be 2.73-3.25,14 the majority of polymer-bound iminodiacetic groups exists as electrically neutral twice-protonated species (FH2 or in hydrogen ion form) at pH 2. Weakly acidic cation exchangers in the hydrogen ion form do not highly swell, and low swelling of cross-linked polymers reduces not only diffusion coefficients of ionic species in the exchanger phases but also reaction rates of complexation of the fixed ligands with metal ions, resulting in the slow adsorption rate of metal ions. In FIDA-f, on the other hand, iminodiacetic acid groups are bound to grafted linear polymer chains without cross-linked network structures. Therefore, of interest is the difference in the adsorption rates of metal ions between FIDA-f and granular iminodiacetic acid resins in the hydrogen ion form. In this work, then, the rate of the metal ion adsorption by FIDA-f was indirectly evaluated from the dependence of breakthrough profiles of Cu(II) on the flow rate of the feed in the column-mode adsorption. As a target metal ion, Cu(II) was selected

Figure 5. Dependence of breakthrough profiles of Cu(II) on the flow rate of the feed in the column-mode adsorption of Cu(II) by FIDA-f and Diaion CR10 conditioned at pH 2. Columns: FIDA-f, 0.40 g; Diaion CR10, 0.80 g. Feed: aqueous solution of cupric nitrate (ca. 10 mM). Breakthrough profiles of Cu(II) for the FIDA-f column: 50 h-1 (O, 1); 100 h-1 (9, 2); 200 h-1 (b, 3); 400 h-1 (2, 6); 600 h-1 (0, 7). Breakthrough profiles of Cu(II) for the Diaion CR10 column: 50 h-1 (×, 12); 100 h-1 (+, 14). The number after the symbol in each set of parentheses shows the entry number in Table 5. Refer to these entry numbers in Table 5 for detailed experimental conditions.

because it forms the most stable complex with iminodiacetate among the tested metal ions as shown in Table 4. If the adsorption rate of Cu(II) by FIDA-f in the hydrogen ion form is extremely fast, the breakthrough capacity for Cu(II) will be independent of the flow rates of the feed. On the contrary, if the adsorption rate is slow, the breakthrough capacity will markedly decrease with an increase in the feed flow rates. Figure 5 shows breakthrough profiles of Cu(II) in the column-mode uptake by FIDA-f as well as Diaion CR10 in both hydrogen ion forms, and Table 5 summarizes the detailed experimental conditions and performances of both exchangers. In the case of Diaion CR10, Cu(II) leaked out through the column before 10 mL of the feed, and the C/Co value rapidly increased up to 0.5-0.6, but the further increase in C/Co is gentle, indicating a slow rate in the adsorption of Cu(II) by Diaion CR10 in hydrogen ion form. Breakthrough capacities of Diaion CR10 are much less than those of FIDA-f even at lower feed flow rates of 50 and 100 h-1. In the case of FIDA-f, on the other hand, breakthrough profiles of Cu(II) were not markedly dependent on the flow rate of the feed, and C/Co attained unity at ca. 180 mL of the feed, whereas breakthrough capacities are independent of the flow rate up to 400 h-1, suggesting that the adsorption rate of Cu(II) by FIDA-f in the hydrogen ion form is much faster than that by Diaion CR10. At pH 4.6, polymer-bound iminodiacetic acid groups exist as monovalent anions (FH-), and negative charges are generated on grafted polymer chains in FIDA-f, increasing the swelling of FIDA-f. Then, it is expected that the adsorption rate of Cu(II) by FIDA-f conditioned at pH 4.6 will be faster than the case conditioned at pH 2.0. When the FIDA-f column was conditioned with the buffer of pH 4.6, it became difficult to supply the feed to the FIDA-f column at flow rates higher than 300 h-1 because the increased swelling tightened the packing

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1605 Table 5. Summary of the Column-Mode Adsorption of Cu(II) by FIDA-f and Diaion CR10 conditions for adsorption operation entry no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 a

column FIDA-f (0.40 g)

column conditioning pH

flow rate of the feed (h-1)

feed volume (mL)

Coa of Cu(II) in the feed (mM)

2.0

50 100 200 200 400 400 600 100 100 300 300 50 100 100

184 196 189 195 197 202 197 215 204 200 206 197 201 201

8.32 8.59 9.13 9.40 9.57 9.11 9.04 10.4 11.2 9.16 8.72 9.70 9.40 9.28

4.6

Diaion CR10 (0.80 g)

2.0

breakthrough total uptake eluted amount capacity of Cu(II) of Cu(II) recovery (mmol/g) (mmol/g) (mmol/g) (%) 1.1 1.0 1.1 1.1 0.95 0.75 0.70 1.2 1.3 1.2 1.2 0.15 0.11 0.13

1.6 1.6 1.7 1.9 1.9 1.9 1.7 2.0 1.9 1.9 1.9 0.96 0.69 0.80

1.9 2.1 1.7 1.6 2.1 2.1 2.0 1.7 1.7 1.7 1.7 0.80 0.59 0.66

119 131 100 84 110 110 118 85 89 89 89 83 86 82

Co is the concentration of Cu(II) in feeds.

Figure 6. Breakthrough profiles of Cu(II) in the adsorption by the FIDA-f column conditioned at pH 4.6. Adsorbent: FIDA-f, 0.40 g. Feed: aqueous solution of cupric nitrate (ca. 10 mM). Flow rate of the feed: 100 h-1 (O, 8); 100 h-1 (b, 9); 300 h-1 (0, 10); 300 h-1 (9, 11). The number after the symbol in each set of parentheses shows the entry number in Table 5. Refer to these entry numbers in Table 5 for detailed experimental conditions.

state of FIDA-f. Figure 6 shows breakthrough profiles of Cu(II) in the adsorption by the FIDA-f column conditioned at pH 4.6, and numerical data are also summarized in Table 5. Breakthrough profiles of Cu(II) are essentially independent of the flow rate of the feeds. Although breakthrough capacities of FIDA-f conditioned at pH 4.6 slightly increased compared with the case conditioned at pH 2.0, marked improvement of breakthrough performances was not observed as judged from Figures 5 and 6 and Table 5. This means that FIDA-f takes up very rapidly the metal ions even in the hydrogen ion form. Ca(II) and Mg(II) are the main cationic species in water on the Earth’s surface. Then, their effect on uptake of Cu(II) was tested by using the FIDA-f column conditioned at pH 2. Figure 7 shows breakthrough curves of Cu(II) and Ca(II) in the adsorption by FIDA-f from solutions containing both Cu(II) and Ca(II). Both breakthrough profiles of Cu(II) and Ca(II) are independent of the flow rate of the feed. In the case of a Cu(II)Mg(II) binary system, similar results were observed. Table 6 summarizes numerical data for Cu(II)-Ca(II)

Figure 7. Breakthrough profiles of Cu(II) and Ca(II) in the adsorption from solutions containing both Cu(II) and Ca(II). Flow rate: 50 h-1 (O, b, 15); 100 h-1 (0, 9, 17); 200 h-1 (4, 2, 18); 400 h-1 (tilted 3, tilted 1, 20). Open and filled symbols stand for Cu(II) and Ca(II), respectively. The number after the symbols in each set of parentheses shows the entry number in Table 6. Refer to these entry numbers in Table 6 for detailed experimental conditions.

and Cu(II)-Mg(II) binary systems. Breakthrough capacities for Cu(II) are somewhat lowered with an increase in the flow rate, in particular, at a flow rate of 400 h-1. Because of the high selectivity of FIDA-f to Cu(II) over Mg(II) and Ca(II), Ca(II) and Mg(II) were not detected in eluted solutions. Even if small amounts of Ca(II) and Mg(II) were adsorbed by the FIDA-f column at the beginning stage of the adsorption operation, they were easily replaced by the much more preferred Cu(II). Thus, Ca(II) and Mg(II) do not interfere with the adsorption of Cu(II) at all. Because C/Co values of Cu(II) attained to 1 at 200 mL of the feed in the case of a FIDA-f-packed column, the total uptake of Cu(II) can be regarded as the equilibrium capacity for Cu(II). Averages of the total uptake and eluted amount of Cu(II) on the FIDA-f column are 1.9 ( 0.1 and 1.9 ( 0.2 mmol/g (total of 21 runs; refer to Tables 5 and 6), respectively. Rather large standard deviations were ascribable to the fact that column effluents in adsorption operations were divided into many fractions to observe breakthrough points as well

1606 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 Table 6. Effect of Ca(II) and Mg(II) on the Column-Mode Adsorption of Cu(II) by FIDA-f in Hydrogen Ion Form conditions for adsorption operation Coa in the feed (mM)

entry no.

flow rate of the feed (h-1)

feed volume (mL)

15 16 17 18 19 20

50 50 100 200 200 400

210 205 196 203 204 203

10.4 10.5 10.2 10.4 10.3 9.87

21 22 23 24

100 100 400 400

217 210 196 202

10.5 10.7 10.4 10.3

11.6 10.3 11.8 10.2

a

Cu(II)

total uptake of Cu(II) (mmol/g)

amount of eluted Cu(II) (mmol/g)

recovery of Cu(II) (%)

Cu-Ca Binary System 9.88 1.1 9.80 1.1 10.1 1.2 11.7 0.93 11.9 0.90 11.8 0.85

1.9 2.0 1.9 2.0 1.9 2.2

2.0 2.3 1.6 1.9 1.9 1.8

105 115 84 95 100 81

Cu-Mg Binary System 1.0 1.1 0.70 0.67

1.9 1.9 2.0 2.1

1.9 1.9 1.8 1.9

100 100 95 90

foreign cation

breakthrough capacity of Cu(II) (mmol/g)

Co is the concentation of each metal ion in feeds.

Figure 8. Typical elution curves of Cu(II) adsorbed on the FIDA-f column with 1 M nitric acid. For detailed numerical conditions and results, refer to entry number 4 in Table 5.

as smooth breakthrough curves; measurements of metal ions in many fractions and dilution led to expanded errors. However, the average of the total uptake of Cu(II) is in good agreement with that of the eluted amount of Cu(II), indicating that Cu(II) adsorbed on the FIDA-f column is quantitatively recovered by elution with 1 M nitric acid. As illustratively shown in Figure 8, Cu(II) adsorbed on the FIDA-f column was completely eluted with 10 mL of 1 M nitric acid. The conditioning, adsorption, and elution operations on the FIDA-f column were repeated more than 21 times without changes of the packed FIDA-f, and no deterioration of the packed FIDA-f was observed, indicating its high chemical and physical stabilities. Last, we discuss briefly the difference in the adsorption rates of Cu(II) between FIDA-f and Diaion CR10 based on plots of MU values against time. Figure 9 shows the results; here, data shown in Figure 5 and Table 6 were used. Within the breakthrough point, MU values at a given time t (mt) is proportional to t under a constant flow rate of the feed. Therefore, plots of log mt against log t before the breakthrough point must be a straight line with a slope of 1. After the breakthrough point, on the other hand, the slope becomes smaller than 1 and finally attains to 0 after the establishment of equilibrium between the packed exchanger and feed. In

Figure 9. Plots of MU values against time for column-mode adsorption of Cu(II) by FIDA-f and Diaion CR10 in hydrogen ion form. Data given in Table 5 were used. Column: FIDA-f, 0.40 g; Diaion CR10, 0.80 g. Feed: aqueous solution of cupric nitrate (ca. 10 mM). Flow rate of the feed for the FIDA-f column: 50 h-1 (O, 1); 100 h-1 (], 2); 200 h-1 ([, 3); 400 h-1 (4, 6); 600 h-1 (2, 7). Flow rate of the feed for the Diaion CR10 column: 50 h-1 (×, 12); 100 h-1 (0, 13); 100 h-1 (9, 14). The number after the symbol in each set of parentheses shows the entry number in Table 5. Refer to these entry numbers in Table 5 for more detailed experimental conditions.

the case of Diaion CR10, the linear relationship with a slope of 1 holds in the region that mt < ca. 0.1 mmol/g and the slope becomes less than 1 in the region that mt > 0.1 mmol/g even at the slowest feed flow rate of 50 h-1. In the case of FIDA-f, on the contrary, the linear relationship with the slope of 1 expanded up to a mt value of ca. 1 mmol/g in the flow rate range of 50-400 h-1, whereas the linear region with a slope of 1 becomes slightly narrower at the highest flow rate of 600 h-1. These results mean that functional groups in FIDA-f can very effectively work even at high flow rate. Although Diaion CR10 has a higher ligand content of 3.24 mmol/g, its breakthrough capacity for Cu(II) is only ca. 0.1 mmol/g at feed flow rates of 50-100 h-1. This means that the breakthrough capacity corresponds to only 3% of ligand contents. On the other hand, the breakthrough capacity of FIDA-f corresponds to ca. 50% of its ligand content even in the wide feed flow rate range from 50 to 600 h-1. Thus, iminodiacetate groups

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1607

in FIDA-f can much more effectively take up Cu(II) even at high feed flow rates than those in Diaion CR10 do. The rapid and effective metal uptake of Cu(II) by FIDA-f may be probably ascribable to the small diameter of the fiber and non-cross-linked structures of grafted polymer chains bearing functional groups. A decrease in the diameters of spherical and cylindrical exchangers markedly enhances the adsorption rates.19-21 In addition, functional groups fixed onto flexible grafted polymer chains are more suitable for rapid complexation with the target species than those fixed on cross-linked polymer networks, as was already shown in the study of the improvement of the adsorption rates of borate adsorbents.22 4. Conclusions In this work, novel iminodiacetic acid fibers (filamentary FIDA-f and cloth FIDA-c) were developed from CMS-grafted PPPEs and its cloth. Although the metal ion selectivity of the present fiber was close to that of iminodiacetic acid resins, the metal adsorption rate of FIDA-f is much higher than that of commercially available granular and fibrous iminodiacetic acid exchangers having cross-linked polystyrene matrixes. In the column-mode adsorption of Cu(II), breakthrough capacities of Cu(II) were independent of the flow rates of feeds up to 200-300 h-1 in space velocity but somewhat decreased with a further increase in the flow rate. The main reasons for the extremely fast adsorption rate of FIDA-f can be ascribed to the diameter of the fiber being much less than those of the resins and the functional groups in FIDA-f being introduced onto noncross-linked grafted polymer chains. In addition, the chemical and physical stabilities of FIDA-f are comparable to those of commercially available iminodiacetic acid resins. Therefore, FIDA-f is promising in the significant reduction of column operation time for the removal of heavy-metal ions from water. Acknowledgment The authors are grateful to Kurashiki Textile Mfg. Co. and Seimi Chemical Co. for the provision of trunk fibers and CMS, respectively. This work was partly supported by Ministry of Education and Science of Japan. Literature Cited (1) Yoshioka, T.; Hirata, N.; Yamada, S. Kireto Seni (Chelating Fibers). In Kyucyaku Gijutsu Handobukku (in Japanese, Handbook of Adsorption Technology); Simizu, H., Ed.; NTS Pub. Co.: Tokyo, 1993; pp 395-400. (2) D’Alelio, G. F.; Monacelli, W. J. Grafted Chelating Polymers Containing Iminodiacetic Acid Groups. U.S. Patent 3,395,197, 1967. (3) Yoshioka, T. Studies of Polystyrene-Based Ion-Exchange Fiber. III. A Novel Fiber-Form Chelating Exchanger and Its Adsorption Properties for Heavy-Metal Ions. Bull. Chem. Soc. Jpn. 1985, 58, 2618.

(4) Tsuneda, S.; Saito, K.; Furusaki, S.; Sugo, T.; Okamoto, J. Metal Collection Using Hollow Fiber Membranes. J. Membr. Sci. 1991, 58, 221. (5) Yamagishi, H.; Saito, K.; Furusaki, S.; Sugo, T.; Ishigaki, I. Introduction of a High-Density Chelating Group into a Porous Membrane without Lowering Flux. Ind. Eng. Chem. Res. 1991, 30, 2234. (6) Konishi, S.; Saito, K.; Furusaki, S.; Sugo, T. Sorption Kinetics of Cobalt in Chelating Porous Membrane. Ind. Eng. Chem. Res. 1992, 31, 2722. (7) Li, G.; Konishi, S.; Saito, K.; Sugo, T. High Collection Rate of Pd in Hydrochloric Acid Medium Using Chelating Microporous Membrane. J. Membr. Sci. 1994, 95, 63. (8) Lee, W.; Oshikiri, T.; Saito, K.; Sugita, K.; Sugo, T. Comparison of Formation Site of Graft Chain between Nonporous and Porous Films Prepared by RIGP. Chem. Mater. 1996, 8, 2618. (9) Choi, S.; Nho, Y. C. Adsorption of Co2+ and Cs+ by Polyethylene Membrane with Iminodiacetic Acid and Sulfonic Acid Modified by Radiation-Induced Graft Polymerization. J. Appl. Polym. Sci. 1999, 71, 999. (10) Jyo, A.; Aoki, S.; Kishita, T.; Yamabe, K.; Tamada, T.; Sugo, T. Phosphonic Acid Fiber for Selective and Extremely Rapid Elimination of Lead(II). Anal. Sci. 2001, 17, i201. (11) Aoki, S.; Saito, K.; Jyo, A.; Katakai, A.; Sugo, T. Phosphoric acid Fiber for Extremely Rapid Elimination of Heavy Metal Ions from Water. Anal. Sci. 2001, 17, i205. (12) Jyo, A.; Okada, K.; Nakao, M.; Sugo, T.; Tamada, M.; Katakai, A. Bifunctional Phosphonate Fiber Derived from Vinylbiphenyl-grafted Polyethylene-coated Polypropylene Fiber for Extremely Rapid Removal of Iron(III). J. Ion Exchnage 2003, 14 (Supplement), 69. (13) Kugara, J.; Trobradovic, H.; Jyo, A.; Sugo, T.; Tamada, M.; Katakai, A. Behavior of Iminodiacetate Fiber in Column-mode Adsorption of Lead(II). J. Ion Exchange 2003, 14 (Supplement), 77. (14) Pesavento, M.; Biesuz, R.; Gallorini, M.; Profumo, A. Sorption Mechanism of Trace Amounts of Divalent Metal Ions on a Chelating Resin Containing Iminodiacetate Groups. Anal. Chem. 1993, 65, 2522. (15) Yuchi, A.; Sato, T.; Morimoto, Y.; Mizuno, H.; Wada, H. Adsorption Mechanism of Trivalent Metal Ions on Chelating Resins Containing Iminodiacetic Acid Groups with Reference to Selectivity. Anal. Chem. 1997, 69, 2941. (16) Sillen, L. G., Martell, A. E., Compilers. Stability Constants of Metal-Ion Complexes; Special Publication No. 25; The Chemical Society, Burlington House: London, 1971. (17) Ringbom, A. Complexation in Analytical Chemistry; Interscience Pub.: New York/London, 1963. (18) Pesavento, M.; Biesuz, R.; Palet, C. Study of Aluminium Speciation in Freshwater by Sorption on Chelating Resin. Analyst 1998, 123, 1295. (19) Helfferich, F. Ion Exchange; Dover Pub. Inc.: New York, 1995; Chapter 6. (20) Lin, W.; Hsieh, Y. Kinetic of Metal Ion Adsorption on IonExchange and Chelating Fibers. Ind. Eng. Chem. Res. 1996, 35, 3817. (21) Chen, L.; Yang, G.; Zhang, J. A Study on the Exchange Kinetic of Ion-Exchange Fiber. React. Funct. Polym. 1996, 29, 139. (22) Jyo, A.; Aoki, S.; Uchimura, M.; Yamabe, K.; Sugo, T. Behavior of Chelating Fibers Having Polyol Groups in Column Mode Adsorption of Boric Acid. Anal. Sci. 2001, 17, i1211.

Received for review February 18, 2003 Revised manuscript received October 27, 2003 Accepted November 6, 2003 IE030146H