균질 및 불균질 양이온 교환막의 특성 비교

김성혜, 최재환, 문승현

광주과학기술원 환경공학과

Comparison of Characteristics

between Homogeneous and Heterogeneous Cation-exchange Membranes

Sung Hye Kim, Jae Hwan Choi, Seung Hyeon Moon

Dept. of Environmental Science and Engineering, K-JIST, Kwangju, Korea

Introduction

The general application of ion-exchange membranes is in desalting ionic species from aqueous solutions. There is an increasing number of industrial applications where ions have to be removed from a process stream such as demineralization of whey, deacidification of fruit juices, production of boiler feed water, removal of organic acids from a fermentation broth, and electrokinetic soil processing[1,2,3].

There are two different types of ion-exchange membranes, namely, "homogeneous" and "heterogeneous" membranes. In homogeneous membranes, ion-exchange groups are attached to the basic polymer structure directly so that the ionic charges are distributed over the membrane. Thus they have good electrochemical properties in terms of low resistance and high selectivity. On the other hand, heterogeneous membranes are produced by melting and pressing dry ion-exchange resins with granulated polymers[4]. The ion-exchange resins are embedded in the matrix irregularly so that the electrochemical properties of those are not as good as those of the homogeneous membranes. But their mechanical strengths are superior to homogeneous membranes and preparation is very easy compared to homogeneous membranes.

In this study, the electrochemical properties of homogeneous and heterogeneous cation-exchange membranes such as water contents(W), ion-exchange capacity(AR), transport number of counter-ion(TN), specific resistance of membrane(□) and fraction of ion permeable were compared.

Material and Methods

In this study, all experiments were performed with CMX(Neocepta, Tokuyama. Corp., Japan) as a homogeneous cation-exchange membrane and Het-C(obtained from an anonymous company, and code-named in this study) as a heterogeneous membrane. For each membrane, electrochemical properties were measured as following.

Water content(W)	A Na-type cation-exchange membrane(Sw) was dried at 65 ℃ in a vacuum oven(Sd). Water content was obtained from W(g H2O/g dry membrane)=(Sw-Sd)/Sw [6].

Ion-exchange capacity(AR)	An Na-type cation-exchange membrane was exchanged with H+ ions in 1 M HCl, immersed in 200 ml of 0.1 N NaOH solution, and OH- ions remained after reacted with H+ ions isolated from the membrane were titrated with a 0.1 N HCl solution. AR(meq/g-dry) was obtained from the analyzed values divided by the weight of the membrane dried[5,6].

Transport number of counter-ions(TN)      A Na-type cation-exchange membrane was placed between the two compartments in a cell and the potential due to the concentration difference between the two compartments(0.1 M and 0.5 M NaCl solution) was measured by Ag/AgCl electrode. The TN was calculated by the following equation[5].

[eqn: rm E_m~=~{RT}over{F}(2{bart}_+ -1) ln{(a_± )_1 }over{(a_± )_2 }]	(1)

where, Em is cell potential.;[eqn: rm {bar t}_+] is transport number of cation in the membrane phase.;[eqn: rm (a_± )_1] and[eqn: rm (a_± )_2] are the activity of the cation in each cell.; R is gas constant.; F is faraday constant.; T is temperature.

Specific Resistance(□)	Cation-exchange membranes were immersed in 0.1 N of KCl, NaCl, LiCl, CaCl2, MgCl2, AlCl3 solutions, respectively and the resistances(R) were measured by a clip cell using alternating current at 10 kHz. The specific resistances of membranes(□) were calculated from the following equation.

[eqn: rm R~=~□·{□}over{A}]	(2)

where, A and □are surface area of membrane and the thickness, respectively.

Chronopotentiogram       Chronopotentiograms were obtained in a galvanostatic mode(i.e. a constant current density is applied) using a two compartment cell, in which the voltage drop between the electrode and Ag/AgCl electrode was measured as a function of time with a sampling rate of 300 times per minute. The membrane area was 0.785 cm2. All measurements were performed with 0.025 M NaCl solution on either side of the membranes.

Result and Discussion

The water content(W), ion-exchange capacity(AR), transport number of counter ions(TN) of each membranes were shown in Table 1.

In the preparation of heterogeneous membrane, cavity was formed between polymer matrix and ion exchange resin by swelling of the resin during the water conditioning. And the ions can be exchanged with fixed charge moving through this cavity[7]. On the other hand, for the homogeneous membrane, the ions are exchanged with regular fixed charge on the pore surface. Thus heterogeneous membrane has relatively large pore and can hold more water than homogeneous one. Moreover, in the heterogeneous membrane, it is possible that the hydrated counter-ions accompanied by co-ions transport through the cavity. So TN of Het-C is lower than that of CMX due to the relatively lower Donnan exclusion[5]. From Table 1, it is observed that the ion-exchange capacity of Het-C is about half of CMX. In the heterogeneous membrane, the ion-exchange particles are embedded in the polymer matrix irregularly so that the ion exchange functional group works less effectively compared to the  homogeneous membrane.

The specific resistances(□) of the membranes with the radius of hydrated ions are shown in Fig. 1. For small molecular size ions, □ value of Het-C is higher than that of CMX because AR of the former is about double as high as that of the latter. However, as the radius of hydrated ions increases, the effect of AR on the resistance decreases. Instead, the effect of the pore size of the membrane increases. Because

Table 1. Electrochemical properties of cation-exchange membranes

Membrane	W (%)	AR (meq/g)	TN
Het-C	51	1.8	0.93
CMX	25	3.4	0.97

Figure 1. The inclination of specific resistance

according to the change of Stokes' radii of

ions for CMX and Het-C membranes.

Figure 2. Potential vs time curves of CMX

and Het-C membrane at 2.7 mA/cm2.

Figure 3. Plots of i□1/2 vs i as a function

of current density.

heterogeneous membrane has relatively large pore compared to homogeneous one due to the cavity, the specific resistance to the transport of large ions in the former is lower than that in the latter even with the low AR[7].

Perfluorinated ion exchange membranes are currently regarded as heterogeneous in terms of polymeric and ion transport regions. Rubinstein et al. experimentally determined limiting current densities which were up to four times smaller than the theoretical prediction suggesting a reduced permeable membrane area, i.e. the presence of heterogeneities even in the homogeneous membranes[9].

A technique which has successfully been applied to determine a reduced permeable area of ion exchange membrane is chronopotentiometry. Originally, this technique has frequently been used to investigate kinetic effects such as adsorption and transport phenomena near electrode surface but in a limited amount it has also been applied for studying the transport processes near and through ion exchange membranes[10].

In a chronopotentiometric method, the transition time(□) at which the concentration of electrolyte at the membrane surface reaches zero, is given by eqn.(3)[11]. Considering reduced membrane area resulted from the non-conducting region of ion exchange membrane, the eqn.(3) can be rewritten eqn.(4)

[eqn: rm i □^1/2 ~=~{C_0 z_i F}over{{bar t}_i -t_i }{(pi D)^1/2}over{2}]	(3)

[eqn: rm i □^1/2 ~=~{C_0 z_i F epsilon}over{{bar t}_i -t_i }{(pi D)^1/2}over{2}]	(4)

where i is current density, □is a fraction of the ion transport region.

Chronopotentiograms of CMX and Het-C at current density of 2.7 mA/cm2 are presented in Fig. 2. It is observed that these membranes exhibited two potential zones which were nearly horizontal. Also, Het-C shows more diffusive in nature in the inflection region than the CMX and the transition time for CMX and Het-C were 27.4, 16.8 seconds, respectively. The value of i□1/2 was plotted against i in Fig. 3. The results elucidated that the I□1/2 was fairly constant and independent of i at a given concentration of NaCl justifying the validity of eqn.(4). The i□1/2 values obtained form Fig. 3 for CMX and Het-C were 14.09 and 10.40, respectively. With these i□1/2 values and TN measured by electrochemical cell, we can find the fraction of ion transport region (□) from eqn. (4). For CMX and Het-C, □ values were 0.99 and 0.68, respectively. It could be explained that Het-C has more non -conducting region than the CMX . These result are consistent with the results of AR for both membranes(See Table 1). A membrane with a lower AR  can be regarded as being more heterogeneous than a membrane with higher AR. Base on these results, it was concluded that a reduction in transition time of Het-C compared to the CMX is due to a reduced permeable membrane area.

Acknowledgement

This research was supported by the Korea Research Foundation (Project No. 1998-001-E00576).

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