- Email: [email protected]
A study on the exchange kinetics of ion-exchange fiber
Reactive & Functional Polymers 29 ( 1996) 139- 144
REACTIVE 84 FUNCTIONAL POLYMERS
A study on the exchange kinetics of ion-exchange fiber Liquan
Chen a,*, Gengliang
Yang b, Jin Zhang a
” Department of Modem Physics, Lanzhou University Lanzhou. 730001, Gansu, PR China ‘Department of Chemistq: Hebei University, Baoding 071002, Hebei Province, PR China
Received 19 September 1994; revised version accepted I1 June 1995
Abstract
In this paper, the ion-exchange kinetics of ion-exchange fiber has been studied. Under Finite Solution Volume (FSV), not only the effect of temperature, speed of stir, concentration and Ph value of the solution on the heterophase isotope exchange has been investigated, but the analytical solution of the exchange equation have also been obtained. By use of these solutions, the intraparticle diffusion coefficients D and the film transmitting coefficients D/S under different conditions were calculated. Meanwhile, the reason why the ion-exchange rate of fiber is much faster than that of conventional spherical resins was also given with a proposed mechanism. Keywords:
Ion-exchange fiber; Ion-exchange kinetics; Radioactive trace element; Gadolinium
1. Introduction Ion-exchange fiber was developed to meet the specific need of high performance in ionexchange chromatography [1,2]. Because of its high separation efficiency, fast ion-exchange rate and good electrical conductivity, it has been widely used in the separation of rare earth elements [3], enrichment of uranium from sea [4], purification of gas and solution and many other industrial applications [5--l 11. Although a lot of papers on the kinetic study of ion exchange have been published [ 12-161, the rate law and its moment expressions for ion-exchange fiber in a finite volume solution under the conditions that the ion-exchange rate is controlled by both particle diffusion and film diffusion have not been reported. In this paper, the heterophase isotope ionexchange kinetics of strong acidic ion-exchange * Corresponding author.
fiber has been studied by using radio-active trace method. In addition, the theoretical rate law and its moment expression ion-exchange fiber in a finite volume solution have been derived to predict the exchange fraction by taking into account both the diffusion in the outer shell and that in the adherent film. The diffusion coefficients are also calculated from the formula obtained for isotope exchange in strong acidic cation-exchange fiber. 2. Experimental
2.1. Apparatus and materials The experiments were carried out in finite volume solution and the installation is similar to that of the literature [4]. The experiment data were obtained by using the following instruments and materials: Atomatic Scaler N530 (England EKCO); Scintillation counter N664A; PHS-2pH meter;
1381-5148/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved. SSDI 1381-5148(95)00070-4
140
L. Chen et al. /Reactive
& Functional Polymers 29 (1996) 139-144
deionized water (its conductivity is less than 1.0 x lop6 s/m); Gd20s 99.9%; radioactive isotopes Gadolinium- 153,159 (Atomic Energy Institute of Beijing, P.R. China); VS-1 cation ionexchange fiber (Liaoyuan Science and Technology Institute, Jilin Province); all other reagents are A.R. or G.R. 2.2. Preparation of the ion-exchange fiber The fibers were cut into 5 cm lengths and put into deionized water to swell. Then they were eluted repeatedly by H20, 1 mol/dm3 NaOH, Hz0 1 mol/dm3 HCl for three times in column and conversed into H-form at pH = 6.0. At last, they were dried with saturated NaCl solution to constant weight in a desiccator. 2.3. The appearance
and diameter of thejber
The VS-1 cation ion-exchange fiber was prepared by carbonization and sulphonation of polyvinyl alcohol fiber at high temperature. The final product is an ion-exchange fiber with -SOsH as the functional group. The fiber was observed with a SEM and the morphological pictures were shown in Figs. 1 and 2. From the figures, it can be clearly seen that the surface of the fiber is composed of etched strips and spots. The average diameter is 0.036 & 0.002 mm, and the thickness of the shell of the ion-exchange fiber is 4.6 f 0.3 pm. In order to measure the
Fig. 2. The diameter of ion exchange fiber.
thickness of the ion-exchange shell of the fiber, the H-form fiber was eluted by Eu3+ solution to saturation of adsorption at first, then eluted by water until no Eu3+ ions could be found. Lastly the fibers were wrapped by paraffin was, sliced and investigated by using fluori-microscope photograph techniques. 2.4. Ion-exchange
capacity
The capacity of H-form with constant weight is 2.25 mmol/g determined by dynamic method and 3.33 mmol/g by static method. 3. Procedures 3. I. Preparation of solutions
Fig. 1. Surface of fiber (1 x 3200).
At first, a known amount of Gd203 was dissolved in concentrated hydrochloric acid. By evaporating over-amount HCl and adjusting pH value, one obtained GdCls solutions with different concentrations. Then, take a certain amount of solution to be marked by high ratio Gadolinium-153,159 isotopes (the added amount doesn’t affect the concentration) for use. The fiber was marked in the following way: 1.00 g of the swollen fiber was packed in a column equipped with a water pipe thermostat. At fixed temperature, the ion-exchange fibers were eluted by the marked GdCls solution until equilibrium was reached.
L. Chen et al. /Reactive
3.2. Measurement
& Functional Polymers 29 (19961 139-144
of exchange factor F
150 ml GdCls solution was put into a flask whose temperature was kept constant by water bath. At preset stirring speed, the marked fiber was transferred into the flask quickly. Then at certain intervals, 1.00 ml of solution was taken out and the radio-activity intensity was measured. Because the ion-exchange rate of the fiber is very fast, it is much more difficult to take sample solution with fiber than with the conventional spherical resin and the time intervals used in the first three sample solutions should be less than 3 seconds. The radio-activity intensity used in the calculation was the average of three parallel samples. As the total volume of the solution decreases with the take-out of the sample solution, the intensity was corrected by the following formula l,(l50-?Z)+kZj j=l
z,; =
150
where ZL is the corrected intensity of the nth sample and Z,,the intensity of the original sample. The exchange factor was calculated by: F, =
141
Fig. 3. Model of the ion exchange
fiber.
can not penetrate, and a shell of ion-exchange material that covers the core. Let 6 represent the thickness of the Nernst film that adheres to the outer surface of the fiber, R the outer radius of the fiber, L the thickness of the shell and ra the radius of the hard core. Because 6/R << 1, one can treat the diffusion process approximately in one dimensional space. From Fick’s law, the concentration of the ith component satisfies the following equation [5 J:
ac -D- a2c
at=
ar2 The initial and boundary conditions
Z,: - Zo z’x - 10
(1) of Eq. 1
are
where Ia is the intensity of the background. From the experiment data, the diffusion coefficient D and the film transmitting coefficient D/6 can be obtained from the derived equations.
ac dr
=o
(2)
r=rc,
a-,t>It=o= c;
(3)
-
D
4. Results and discussions J[C(t)
4.1. Mathematical
-
K-‘C(t)]
(4)
model
The mathematical treatment of ion-exchange processes in the fiber shell is similar to that in the conventional spherical resin. In this paper, the mathematical model is based on the following assumptions: (1) the shell of the ion-exchange fiber is uniform both in size and density: and (2) the diffusion is the rate-determining process and is caused by concentration gradients only. Fig. 3 is a model of the ion-exchange fiber. The fiber is composed of an inert hard core, in which ions
r
=R
dt = -VC(t>
0
where S is the effective outer area of the fiber surface; N the number of fibers; V the solution volume; and K the partition coefficient. By solving Eqs. l-5 with Laplace transformation, one gets the analytical solution of ion-exchange factor F: 1+cX F(t) = 1 - 2-
ff
W A, c-grfr c IT=1
(6)
L. Chen et al. /Reactive
142
& Functional Polymers 29 (1996) 139-144
where CX’ is the ratio of the solution volume to the total volume; g, is the nth nonzero root of equation gil
tang,Z = A,,
g,‘e-a
328+OsK * 3 18i0.5K
??
g, cos g,l - cg,’sin g,l
=
??308iO.5K
g,2Og,llcosgnl + [a - 3g,2.!j- g$l sing,1
F’(g,) = vg, +
+ (a -
0.0
K(l -a’> CY
-
one obtains the first and second kinetic moment for the ion-exchange fiber +
P2 = (1 +a)2
I
50
!
Tim”:(s)
150
I 200
’
D and D/6 derived by fitting Eq. 6 with experiment data [ 141. From the definition of the nth kinetic moment co O”dF [I + F(t)]t” dt = o dtfm dt (8) pm = s s0
1
t 0
Fig. 4. The F-time curve at different temperatures. Stirring speed = 500 + 50 r/min CG~ = 1.2 x lo-’ mol dm’; pH = 2.00 + 0.02.
and a=
‘298+0.51(
(7)
F’(g,)
KS(R;-
r”)
1
(g)
4 (Ro - ro)4 15 B2 + [ 4 (R. - ro)* KG(Ro - ro) $3
D
D
+
KG(Ro - ro) 2
+2 (
D
>I
(10)
D and D/k6 can also be obtained by solving the algebra in Eqs. 9 and 10. 4.2. The effect of temperature on the ion-exchange rate At fixed stirring speed, solution concentration and pH, the experiment results about the relations of F vs. t at different temperatures are shown in Fig. 4. The diffusion coefficients of D and D/6 calculated from Eqs. 9 and 10 are listed in Table 1; it can be seen that the ion-exchange rate increases with the increase of temperature and so do D and D/k6.
Table 1 The values of D and D/ k6 at different temperatures
298 & 0.5 308 + 0.5 318i0.5 328 f 0.5
10s x D (cm’ls)
lo4 x D/(kS) (cm/s)
1.2 1.3 1.7 2.9
0.9 1.5 1.5 2.6
4.3. The effect of stirring speed At fixed temperature, pH and solution concentration, the experiment results at different stirring speeds are shown in Fig. 5 and D and D/6 calculated from Eqs. 8 and 9 are given in Table 2. The results show that the exchange rate increases with the increase of stirring speed. This can be explained as follows: When the stirring speed increases, the thickness of the adherent film will decrease, and this in turn lead to the increase of the exchange rate. In our experiment, when the stirring speed change from 370-900 r/min, D/K6 increases about lOO%, while D increases 70%.
Table 2 The values of D and D/k6 at different stirring speeds Stirring speed (rlmin)
lOa x D (cm2/s)
IO4 x Dl(kQ (cm/s)
370 * 50 6OOk50 900 f 50
1.1 1.3 1.8
0.9 1.0 1.9
L. Chen et al. /Reactive & Functional Polymers 29 (1996) 139-144
ion pass through pores inside the fiber more easily.
1.0
0.8
4.5. E#ect of concentration of solution
by 0.6
0.4
0.0 ;
I
1
50
100
??900+50
r.min“
A600+50
r.min“
?? 370+50
r.min-'
I
200
150
Time(s)
Fig. 5. The F-time curve at different stirring speeds. T = 298 f 0.5 K; Cod = 1.2x lo-* mol dm3; pH = 2.00 f 0.02.
4.4. Effect of pH values Fig. 6 shows the experiment results at different pH values and Table 3 lists the calculated D and D/kA. From Table 3, it is shown that although D/k8 decreases with the increase of pH, the effect of pH on D is much greater than that on D/k8. The reason is that when the pH value decreases the polarized water molecules surrounding the metal ion becomes fewer. This results in the decreases of the effective volume of the hydrate metal ion, and makes the metal
1.1 , 1
143
The effect of concentration of solution onexchange rate is shown in Fig. 7 and Table 4. From Fig. 7 and Table 4, it can be clearly seen that as the concentration of the solution increases, the change of D/ k6 becomes less and less. When the concentration is over 1.6 x lo-* mol/dm3, D/ k6 is almost a constant. That means that when the concentration of solution exceeds a certain value, it is the diffusion of the metal ion inside the particles that is affected by the concentration. 5. Conclusion According to the above results, one can see that the diffusion coefficient D is of the order lo-’ cm’/s, which is of the same order as that of the conventional spherical resin. But, for conventional spherical resin, the time needed for the equilibrium to achieve is more than 1000 s [17], while for the ion-exchange fiber, the equilibrium
I
.o
0.9 0.8
0.7 0.5 0.4 0.3 0.2 t 0
, 50
I
Tigg(s)
150
Fig. 6. The F-time curve at different pH. T = 298 k 0.05 K; Cod = 1.2 x lo-’ mol dm3; stirring speed = 800 & 50 r/min.
Table 3 The values of D and D/k8 at different pH PH
lo* x D (cm*/s)
lo4 x D/(kS) (cm/s)
1.oo f 0.02 2.00 & 0.02 3.00 f 0.02
2.9 1.9 1.4
1.7 1.6 1.5
0.0
1 200
’
0
50
150
Tig$s)
2
‘0
Fig. 7. The F-time curve at different concentrations. T = 298 f 0.5 K; stirring speed = 800 f 50 rlmin; pH = 2.00 III 0.02.
Table 4 The values of D and D/k8 at different concentrations of solution (mol/dm3)
10’ x D (cm*/s)
lo4 x D/(K6) (cm/s)
0.8 1.2 1.6 2.4
2.6 1.9 1.2 0.89
2.3 1.6
10*&d
1.o 1.0
144
L. Chen et al. /Reactive
& Functional Polymers 29 (1996) 139-144
time is only about 50 s under any conditions. So it can be concluded that the thickness of the fiber shell being much thinner than the radius of the spherical resin is the main reason. For the ionexchange fiber, the average thickness of the shell is 4.6 pm, and the ion-exchange process occurs only in this thin shell - while for the conventional spherical resin, its radius ranges from 0.1 to 0.5 mm [18]. The ion will travel a much longer distance in the conventional resin than that in the fiber shell. So it is understandable that the overall exchange rate of the exchange fiber is much faster than that of the conventional resin.
References [I] S.J. Zhou, Ion Exch. Absorpt. (Chinese), 5 (1987) 51. [2] M. Tagakawa, Industr. Material (Japan), 31(5) (1983) 102. [3] T. Asami, T. Suchiro, H. Ichijo, A. Yamanchi, S. Ogawa and M. Suzuki, US Patent 4514367, 1985 (Chem. Abstr., 102 (1985) 222870).
[4] M.A. Tyuganova and M.Yu. Mazov, Zh. Vses. Khim. Obshchest (Russia), 17(6) (1972) 659. [5] L.Q. Chen and Z. Zhong, Chin. J. Chromatogr., 1 l(5) (1993) 265. [6] D.S. Darry, Anal. Chem., 52 (1980) 1874. [7] T.S. Stevens, G.H. Jewett and R.A. Bredeweg, Anal. Chem., 54 (1982) 1206. [8] P. Dasgupta, Anal. Chem.. 56 (1984) 103. [9] T.S. Stevens and J.C. Davis, Anal. Chem., 53 (1981) 1488. [IO] W. Lin, Y. Lu and H. Zeng, Chin. J. React. Polym., l(2) (1992) 133. [ll] X. Zhang and X. Jiang, Chin. J. React. Polym., 12 (1994) 92. [ 121 E. Kucera, J. Chromatogr., 9 (1965) 237. [13] M. Suzuki and J.M. Smith, Chem. Eng. Sci., 26 (1971) 221. [14] G.L. Yang and Z.Y. Tao, Chin. J. React. Polym., I (1992) 109. [I51 G.L. Yang and Z.Y. Tao, Ion Exch. Adsorpt.. 8 (1992) 70. [16] V.S. Soldutor, OP. Popova and A.A. Shunkeveg. J. Phys. Chem. (Russia), 68 (1994) 763. [17] X.Q. Chen, Z.Y. Tao and Z. Xin, J. Nucl. Radio-Chem. (Chinese), 5(4) (1985) 238. [I81 Z.Y. Tao, Z. Xing and D. Liu. J. Nucl. Radio-Chem. (Chinese), 3(3) (1981) 134.
REACTIVE 84 FUNCTIONAL POLYMERS
A study on the exchange kinetics of ion-exchange fiber Liquan
Chen a,*, Gengliang
Yang b, Jin Zhang a
” Department of Modem Physics, Lanzhou University Lanzhou. 730001, Gansu, PR China ‘Department of Chemistq: Hebei University, Baoding 071002, Hebei Province, PR China
Received 19 September 1994; revised version accepted I1 June 1995
Abstract
In this paper, the ion-exchange kinetics of ion-exchange fiber has been studied. Under Finite Solution Volume (FSV), not only the effect of temperature, speed of stir, concentration and Ph value of the solution on the heterophase isotope exchange has been investigated, but the analytical solution of the exchange equation have also been obtained. By use of these solutions, the intraparticle diffusion coefficients D and the film transmitting coefficients D/S under different conditions were calculated. Meanwhile, the reason why the ion-exchange rate of fiber is much faster than that of conventional spherical resins was also given with a proposed mechanism. Keywords:
Ion-exchange fiber; Ion-exchange kinetics; Radioactive trace element; Gadolinium
1. Introduction Ion-exchange fiber was developed to meet the specific need of high performance in ionexchange chromatography [1,2]. Because of its high separation efficiency, fast ion-exchange rate and good electrical conductivity, it has been widely used in the separation of rare earth elements [3], enrichment of uranium from sea [4], purification of gas and solution and many other industrial applications [5--l 11. Although a lot of papers on the kinetic study of ion exchange have been published [ 12-161, the rate law and its moment expressions for ion-exchange fiber in a finite volume solution under the conditions that the ion-exchange rate is controlled by both particle diffusion and film diffusion have not been reported. In this paper, the heterophase isotope ionexchange kinetics of strong acidic ion-exchange * Corresponding author.
fiber has been studied by using radio-active trace method. In addition, the theoretical rate law and its moment expression ion-exchange fiber in a finite volume solution have been derived to predict the exchange fraction by taking into account both the diffusion in the outer shell and that in the adherent film. The diffusion coefficients are also calculated from the formula obtained for isotope exchange in strong acidic cation-exchange fiber. 2. Experimental
2.1. Apparatus and materials The experiments were carried out in finite volume solution and the installation is similar to that of the literature [4]. The experiment data were obtained by using the following instruments and materials: Atomatic Scaler N530 (England EKCO); Scintillation counter N664A; PHS-2pH meter;
1381-5148/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved. SSDI 1381-5148(95)00070-4
140
L. Chen et al. /Reactive
& Functional Polymers 29 (1996) 139-144
deionized water (its conductivity is less than 1.0 x lop6 s/m); Gd20s 99.9%; radioactive isotopes Gadolinium- 153,159 (Atomic Energy Institute of Beijing, P.R. China); VS-1 cation ionexchange fiber (Liaoyuan Science and Technology Institute, Jilin Province); all other reagents are A.R. or G.R. 2.2. Preparation of the ion-exchange fiber The fibers were cut into 5 cm lengths and put into deionized water to swell. Then they were eluted repeatedly by H20, 1 mol/dm3 NaOH, Hz0 1 mol/dm3 HCl for three times in column and conversed into H-form at pH = 6.0. At last, they were dried with saturated NaCl solution to constant weight in a desiccator. 2.3. The appearance
and diameter of thejber
The VS-1 cation ion-exchange fiber was prepared by carbonization and sulphonation of polyvinyl alcohol fiber at high temperature. The final product is an ion-exchange fiber with -SOsH as the functional group. The fiber was observed with a SEM and the morphological pictures were shown in Figs. 1 and 2. From the figures, it can be clearly seen that the surface of the fiber is composed of etched strips and spots. The average diameter is 0.036 & 0.002 mm, and the thickness of the shell of the ion-exchange fiber is 4.6 f 0.3 pm. In order to measure the
Fig. 2. The diameter of ion exchange fiber.
thickness of the ion-exchange shell of the fiber, the H-form fiber was eluted by Eu3+ solution to saturation of adsorption at first, then eluted by water until no Eu3+ ions could be found. Lastly the fibers were wrapped by paraffin was, sliced and investigated by using fluori-microscope photograph techniques. 2.4. Ion-exchange
capacity
The capacity of H-form with constant weight is 2.25 mmol/g determined by dynamic method and 3.33 mmol/g by static method. 3. Procedures 3. I. Preparation of solutions
Fig. 1. Surface of fiber (1 x 3200).
At first, a known amount of Gd203 was dissolved in concentrated hydrochloric acid. By evaporating over-amount HCl and adjusting pH value, one obtained GdCls solutions with different concentrations. Then, take a certain amount of solution to be marked by high ratio Gadolinium-153,159 isotopes (the added amount doesn’t affect the concentration) for use. The fiber was marked in the following way: 1.00 g of the swollen fiber was packed in a column equipped with a water pipe thermostat. At fixed temperature, the ion-exchange fibers were eluted by the marked GdCls solution until equilibrium was reached.
L. Chen et al. /Reactive
3.2. Measurement
& Functional Polymers 29 (19961 139-144
of exchange factor F
150 ml GdCls solution was put into a flask whose temperature was kept constant by water bath. At preset stirring speed, the marked fiber was transferred into the flask quickly. Then at certain intervals, 1.00 ml of solution was taken out and the radio-activity intensity was measured. Because the ion-exchange rate of the fiber is very fast, it is much more difficult to take sample solution with fiber than with the conventional spherical resin and the time intervals used in the first three sample solutions should be less than 3 seconds. The radio-activity intensity used in the calculation was the average of three parallel samples. As the total volume of the solution decreases with the take-out of the sample solution, the intensity was corrected by the following formula l,(l50-?Z)+kZj j=l
z,; =
150
where ZL is the corrected intensity of the nth sample and Z,,the intensity of the original sample. The exchange factor was calculated by: F, =
141
Fig. 3. Model of the ion exchange
fiber.
can not penetrate, and a shell of ion-exchange material that covers the core. Let 6 represent the thickness of the Nernst film that adheres to the outer surface of the fiber, R the outer radius of the fiber, L the thickness of the shell and ra the radius of the hard core. Because 6/R << 1, one can treat the diffusion process approximately in one dimensional space. From Fick’s law, the concentration of the ith component satisfies the following equation [5 J:
ac -D- a2c
at=
ar2 The initial and boundary conditions
Z,: - Zo z’x - 10
(1) of Eq. 1
are
where Ia is the intensity of the background. From the experiment data, the diffusion coefficient D and the film transmitting coefficient D/6 can be obtained from the derived equations.
ac dr
=o
(2)
r=rc,
a-,t>It=o= c;
(3)
-
D
4. Results and discussions J[C(t)
4.1. Mathematical
-
K-‘C(t)]
(4)
model
The mathematical treatment of ion-exchange processes in the fiber shell is similar to that in the conventional spherical resin. In this paper, the mathematical model is based on the following assumptions: (1) the shell of the ion-exchange fiber is uniform both in size and density: and (2) the diffusion is the rate-determining process and is caused by concentration gradients only. Fig. 3 is a model of the ion-exchange fiber. The fiber is composed of an inert hard core, in which ions
r
=R
dt = -VC(t>
0
where S is the effective outer area of the fiber surface; N the number of fibers; V the solution volume; and K the partition coefficient. By solving Eqs. l-5 with Laplace transformation, one gets the analytical solution of ion-exchange factor F: 1+cX F(t) = 1 - 2-
ff
W A, c-grfr c IT=1
(6)
L. Chen et al. /Reactive
142
& Functional Polymers 29 (1996) 139-144
where CX’ is the ratio of the solution volume to the total volume; g, is the nth nonzero root of equation gil
tang,Z = A,,
g,‘e-a
328+OsK * 3 18i0.5K
??
g, cos g,l - cg,’sin g,l
=
??308iO.5K
g,2Og,llcosgnl + [a - 3g,2.!j- g$l sing,1
F’(g,) = vg, +
+ (a -
0.0
K(l -a’> CY
-
one obtains the first and second kinetic moment for the ion-exchange fiber +
P2 = (1 +a)2
I
50
!
Tim”:(s)
150
I 200
’
D and D/6 derived by fitting Eq. 6 with experiment data [ 141. From the definition of the nth kinetic moment co O”dF [I + F(t)]t” dt = o dtfm dt (8) pm = s s0
1
t 0
Fig. 4. The F-time curve at different temperatures. Stirring speed = 500 + 50 r/min CG~ = 1.2 x lo-’ mol dm’; pH = 2.00 + 0.02.
and a=
‘298+0.51(
(7)
F’(g,)
KS(R;-
r”)
1
(g)
4 (Ro - ro)4 15 B2 + [ 4 (R. - ro)* KG(Ro - ro) $3
D
D
+
KG(Ro - ro) 2
+2 (
D
>I
(10)
D and D/k6 can also be obtained by solving the algebra in Eqs. 9 and 10. 4.2. The effect of temperature on the ion-exchange rate At fixed stirring speed, solution concentration and pH, the experiment results about the relations of F vs. t at different temperatures are shown in Fig. 4. The diffusion coefficients of D and D/6 calculated from Eqs. 9 and 10 are listed in Table 1; it can be seen that the ion-exchange rate increases with the increase of temperature and so do D and D/k6.
Table 1 The values of D and D/ k6 at different temperatures
298 & 0.5 308 + 0.5 318i0.5 328 f 0.5
10s x D (cm’ls)
lo4 x D/(kS) (cm/s)
1.2 1.3 1.7 2.9
0.9 1.5 1.5 2.6
4.3. The effect of stirring speed At fixed temperature, pH and solution concentration, the experiment results at different stirring speeds are shown in Fig. 5 and D and D/6 calculated from Eqs. 8 and 9 are given in Table 2. The results show that the exchange rate increases with the increase of stirring speed. This can be explained as follows: When the stirring speed increases, the thickness of the adherent film will decrease, and this in turn lead to the increase of the exchange rate. In our experiment, when the stirring speed change from 370-900 r/min, D/K6 increases about lOO%, while D increases 70%.
Table 2 The values of D and D/k6 at different stirring speeds Stirring speed (rlmin)
lOa x D (cm2/s)
IO4 x Dl(kQ (cm/s)
370 * 50 6OOk50 900 f 50
1.1 1.3 1.8
0.9 1.0 1.9
L. Chen et al. /Reactive & Functional Polymers 29 (1996) 139-144
ion pass through pores inside the fiber more easily.
1.0
0.8
4.5. E#ect of concentration of solution
by 0.6
0.4
0.0 ;
I
1
50
100
??900+50
r.min“
A600+50
r.min“
?? 370+50
r.min-'
I
200
150
Time(s)
Fig. 5. The F-time curve at different stirring speeds. T = 298 f 0.5 K; Cod = 1.2x lo-* mol dm3; pH = 2.00 f 0.02.
4.4. Effect of pH values Fig. 6 shows the experiment results at different pH values and Table 3 lists the calculated D and D/kA. From Table 3, it is shown that although D/k8 decreases with the increase of pH, the effect of pH on D is much greater than that on D/k8. The reason is that when the pH value decreases the polarized water molecules surrounding the metal ion becomes fewer. This results in the decreases of the effective volume of the hydrate metal ion, and makes the metal
1.1 , 1
143
The effect of concentration of solution onexchange rate is shown in Fig. 7 and Table 4. From Fig. 7 and Table 4, it can be clearly seen that as the concentration of the solution increases, the change of D/ k6 becomes less and less. When the concentration is over 1.6 x lo-* mol/dm3, D/ k6 is almost a constant. That means that when the concentration of solution exceeds a certain value, it is the diffusion of the metal ion inside the particles that is affected by the concentration. 5. Conclusion According to the above results, one can see that the diffusion coefficient D is of the order lo-’ cm’/s, which is of the same order as that of the conventional spherical resin. But, for conventional spherical resin, the time needed for the equilibrium to achieve is more than 1000 s [17], while for the ion-exchange fiber, the equilibrium
I
.o
0.9 0.8
0.7 0.5 0.4 0.3 0.2 t 0
, 50
I
Tigg(s)
150
Fig. 6. The F-time curve at different pH. T = 298 k 0.05 K; Cod = 1.2 x lo-’ mol dm3; stirring speed = 800 & 50 r/min.
Table 3 The values of D and D/k8 at different pH PH
lo* x D (cm*/s)
lo4 x D/(kS) (cm/s)
1.oo f 0.02 2.00 & 0.02 3.00 f 0.02
2.9 1.9 1.4
1.7 1.6 1.5
0.0
1 200
’
0
50
150
Tig$s)
2
‘0
Fig. 7. The F-time curve at different concentrations. T = 298 f 0.5 K; stirring speed = 800 f 50 rlmin; pH = 2.00 III 0.02.
Table 4 The values of D and D/k8 at different concentrations of solution (mol/dm3)
10’ x D (cm*/s)
lo4 x D/(K6) (cm/s)
0.8 1.2 1.6 2.4
2.6 1.9 1.2 0.89
2.3 1.6
10*&d
1.o 1.0
144
L. Chen et al. /Reactive
& Functional Polymers 29 (1996) 139-144
time is only about 50 s under any conditions. So it can be concluded that the thickness of the fiber shell being much thinner than the radius of the spherical resin is the main reason. For the ionexchange fiber, the average thickness of the shell is 4.6 pm, and the ion-exchange process occurs only in this thin shell - while for the conventional spherical resin, its radius ranges from 0.1 to 0.5 mm [18]. The ion will travel a much longer distance in the conventional resin than that in the fiber shell. So it is understandable that the overall exchange rate of the exchange fiber is much faster than that of the conventional resin.
References [I] S.J. Zhou, Ion Exch. Absorpt. (Chinese), 5 (1987) 51. [2] M. Tagakawa, Industr. Material (Japan), 31(5) (1983) 102. [3] T. Asami, T. Suchiro, H. Ichijo, A. Yamanchi, S. Ogawa and M. Suzuki, US Patent 4514367, 1985 (Chem. Abstr., 102 (1985) 222870).
[4] M.A. Tyuganova and M.Yu. Mazov, Zh. Vses. Khim. Obshchest (Russia), 17(6) (1972) 659. [5] L.Q. Chen and Z. Zhong, Chin. J. Chromatogr., 1 l(5) (1993) 265. [6] D.S. Darry, Anal. Chem., 52 (1980) 1874. [7] T.S. Stevens, G.H. Jewett and R.A. Bredeweg, Anal. Chem., 54 (1982) 1206. [8] P. Dasgupta, Anal. Chem.. 56 (1984) 103. [9] T.S. Stevens and J.C. Davis, Anal. Chem., 53 (1981) 1488. [IO] W. Lin, Y. Lu and H. Zeng, Chin. J. React. Polym., l(2) (1992) 133. [ll] X. Zhang and X. Jiang, Chin. J. React. Polym., 12 (1994) 92. [ 121 E. Kucera, J. Chromatogr., 9 (1965) 237. [13] M. Suzuki and J.M. Smith, Chem. Eng. Sci., 26 (1971) 221. [14] G.L. Yang and Z.Y. Tao, Chin. J. React. Polym., I (1992) 109. [I51 G.L. Yang and Z.Y. Tao, Ion Exch. Adsorpt.. 8 (1992) 70. [16] V.S. Soldutor, OP. Popova and A.A. Shunkeveg. J. Phys. Chem. (Russia), 68 (1994) 763. [17] X.Q. Chen, Z.Y. Tao and Z. Xin, J. Nucl. Radio-Chem. (Chinese), 5(4) (1985) 238. [I81 Z.Y. Tao, Z. Xing and D. Liu. J. Nucl. Radio-Chem. (Chinese), 3(3) (1981) 134.