Selective permeation of Cu2+ and UO22+ through a Nafion ionomer membrane

journal of MEMBRANE SCIENCE ELSEVIER

Journal of Membrane Science 116 (1996) 31-37

Selective permeation of C u 2 + and UO 2+ through a Nation ionomer membrane Jayshree Ramkumar, K.S. Shrimal, B. Maiti, T.S. Krishnamoorthy * Analytical Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India Received 26 September 1995; accepted 29 November 1995

Abstract The selective permeation of Cu 2+ and UO 2+ in the presence of common cations through a Nation 117 ionomer membrane have been studied. EDTA served as a receiving agent for an effective permeation of Cu 2+, whereas the same has been used as masking agent for Fe 3+, Cu 2+, Ni 2+ and Zn 2+ during the selective permeation of UO~ + using N a z C O 3 or Tiron as a receiving solution. Selective permeation of Cu 2+ from a mixture of Fe 3+ and Cu 2+ after masking Fe 3+ with F , SCN and PO43- was studied in detail. A lowering of permselectivity and the permeation of anions has been attributed to the simultaneous permeation of Fe 3+ through metal speciation. Keywords: Nation membrane; Permeation of uranium and copper; Anion permeation; Metal speciation

1. I n t r o d u c t i o n Ion exchange membranes find many industrial applications primarily due to their unique property of permselectivity - a term that denotes the difference in permeability between the ions of opposite charge, The ion exchange between two solutions separated by a permselective cation exchange membrane [ 1] or anion exchange [2] membrane have been studied in considerable detail and a few possible analytical applications have also been pointed out. There are several reports available on the permeation of gases [3], neutral substances [4], heterocyclic bases [5] and cations [6,7] but no study on the selective permeation of cation from a mixture of cations has been re-

* Corresponding author,

ported. In the present paper, we have made use of the permselectivity of a perfluorinated ionomer membrane Nation 117 to bring about selective permeation of Cu 2+ and UO22+. Nation 117 is a cation exchange membrane of relatively recent origin. It is homogeneous, chemically inert and highly permselective. It has good mechanical properties and negligible water leakage. Experiments have been carried out in order to demonstrate the applicability of permselectivity to bring about separation of Cu 2+ and UO 2+ from a mixture of cations. In the first instance Cu 2+ was made to permeate selectively from a mixture of Fe 3+ and Cu 2+ by masking the Fe 3+ with a suitable masking agent like F - , S C N and PO 3 - . E D T A was used as a receiving solution for a quicker and more, effective permeation. In the second instance UO 2+ ion selectively, permeated from a mixture of cations like Cu 2+, N i 2+ , Fe 3+ and

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J. Ramkumar et al. / Journal of Membrane Science 116 (1996) 31-37

Zn 2+ , through the Nation membrane into a solution of Na2CO 3 or Tiron; the other cations in the mixture were quantitatively held up as their EDTA complexes.

A

I I I

B

I

2. Experimental The Nation 117 perfluorinated sulphonated membrane I -CF 2 -CF 2 - C F -

" ~1~

, o, - c % -~Foc% CV2 S%H . . . . .

'

I

cFa

I- J,.... J,--I Fig. 1. Permeation measurement cell.

with an equivalent weight of 1100 g of polymer/mol of -SO3H and a thickness of 0.178 mm was obtained from DuPont, USA. Circular pieces of membrane (35 mm diameter approximately) were converted into the acid form by refluxing with 1:1 HNO 3. It was washed and dried as described earlier [3]. EDTA solution: 18.6 g of disodium salt of ethylene diamine tetra acetic acid (BDH AnalaR) was dissolved in 500 ml of deionized water to give a 0.1 M stock solution. A suitable volume of this solution was added to the receiving solution in order to obtain the required concentration. Sodium acetate (E. Merck, GR): 2.72 g of CH3COONa. 3H20 was dissolved in 200 ml deionized water to give a 0.1 M solution. The acetate

solution/dilute acetic acid was used for adjustment of pH. The solutions of transition metal ions were obtained by dissolving the corresponding metal salts in dilute acid. They were standardized by titrating with standard EDTA solution using a suitable metallochromic indicator. All other reagents used were of high analytical purity. The permeation studies were carried out in a U type cell specially designed for this purpose (Fig. 1). Two flat ground ends of standard flanged joints FG 15 (Quick Fit, UK) were each connected to a glass tube bent at an angle of 90 °. A circular piece of clean swollen membrane (M), fully neutralized with the

Table 1 Experimental details of permeation studies Compartment A Permeating ion Cu 2+ Fe 3+ Cu 2+ UO 2+ UO 2+

Compartment B Interfering ion(s)

Fe 3 + Fe 3+, Cu 2+, Ni 2+ Zn 2+ Fe 3+, Cu 2+, Ni 2+ Zn 2 +

Masking agents

Receiving solution

Wavelength of measurement (nm)

F-/SCN-/PO~EDTA

EDTA (0.1 M) EDTA (0.1 M) EDTA (0.05 M) NaOH, Na2CO 3 H202 Tiron (0.1 M)

730 365 730 410 [14]

EDTA

420

J. Ramkumar et al. / Journal of Membrane Science 116 (1996) 31-37

cations of interest for permeation studies was held tightly in between the joints, which were clamped mechanically so that the joint was leak proof, A solution containing the ion or mixture of ions for the different permeation studies was placed in one of the compartments A and the other compartment B contained a definite volume (usually 20 ml) of receiving solution of a complexing agent adjusted to appropriate pH for accelerating the permeation, The solution in both the compartments were continuously stirred using magnetic stirrers and the concentration of a given ion in the compartment B due to permeation, at a given time, was monitored by measuring the absorbance of the solution in compartment B. The absorbance measurements were carried out with Hitachi 330 U V / V I S recording spectrophotometer using a 1 cm quartz cell. The pH measurements were made with ElL 7030 pH meter (India) equipped with a combination electrode. Fluoride ion concentrations were measured with a Radiometer model Ion 85 analyzer (Copenhagen) equipped with a fluoride ion sensitive electrode. The pH of the solution in compartment B was maintained at 5-5.5 using acetate buffer. The details of the experiments carried out for permeation studies are given in Table 1.

3. Results and discussion The pendant - S O 3 H groups in the perfluorinated ionomer (I) are known to form clusters of 40 A diameter separated by a distance of 50 A and interconnected through channels of 10 A when swollen in water [8,9]. The high acidity of - S O 3 H group is responsible for a quantitative neutralization of the membrane as shown by the quantitative liberation of

33

H ÷ by Na ÷. The complexometric titrations of the unreacted metal ions after equilibration for a sufficiently long time (2 h) also suggest a near quantitatire neutralization of the membrane by Cu 2÷ and Fe 3÷. When there are two solutions of a given electrolyte of different concentration separated by a cation exchange membrane, the cation from high concentration side will permeate to the low concentration side. If one of the solutions contains a mixture of cations, all the cations will tend to permeate to the other side. But if a suitable masking agent is used to mask the interfering cations by forming an anionic species only the uncomplexed cations will permeate. The C u - E D T A 2- formed in the B arm being negatively charged would be pushed away from the membrane. Negligible quantities of Cu 2÷ leached out from the membrane saturated with Cu 2+ when equilibrated with deionized water for 3 h or more, and about 10 -4 M / 1 of Cu 2+ was leached out during the same time when 5 × 10 -2 M EDTA was used as leaching solution (Table 2). The results of mass balance experiments carried out due to permeation of Cu 2+ from the feed solutions at two different concentrations are also shown in Table 2. This shows that the loss of Cu 2+ in compartment A is balanced by the gain of C u 2 ÷ in compartment B. Any leaching from membrane is automatically compensated from the feed solution. These experiments establish that the self leaching of Cu 2+ is not of any significance. Further experiments show that the number of moles of Cu 2÷ permeated in a given time depends on the initial concentration of Cu R÷ in arm A (Table 3). The decrease in the permeation could be due to the deviation of the system from the Donan distribution law at high ionic concentration. Moreover, the high cation concentration is likely to block the diffusion

Table 2 Concentration profiles in permeation cell compartments (after 3 h) Compartment A

Compartment B

Initial Cu2+ concentration (mol 1 1)

Decrease in Cu2+ concentration (tool 1-1 )

EDTA concentration (mol 1- 1)

Increase in Cu2+ concentration (moles 1- i)

NIL NIL

NIL NIL

NIL 5.0 × 10-2

< 10-5 1.0 x 10 4

3.0>( 10 3 1.0×10 2

2 . 1 X 10 - 3 4.5X10 3

5 . 0 X 10 - 2 5.0X10 2

2 . 0 X 10 - 3 4.6X10-3

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J. Ramkumar et al. / Journal of Membrane Science 116 (1996) 31-37

Table 3 Concentrationdependenceof permeation C Cu2+ (M) Permeation in 5 h (%) 2.5 x 10-3 5.0 x 10 - 3 7.5 × 10 - 3 1.0xlo -2

membrane with another cation (Cu 2+ ) in the aqueous phase is given by, 2Na+ + ~--~2+ ~ 2N--]+ + Cu 2+

75 68 56 51

where the bar denotes the membrane phase. Addition of EDTA to compartment B where a small quantity of free Cu 2+ is present in aqueous solution will result in considerable lowering of the concentration of Cu 2+ ion in that arm due to the formation of C u - E D T A 2- complex. This will continuously shift the equilibrium to the right and the deficiency in Cu 2+ concentration in the membrane phase would be compensated by taking up Cu 2÷ from compartment A. Thus free Cu 2+ from compartment A will continuously be permeated to compartment B till overall equilibrium is reached. This is shown in Fig. 2 where the permeation of Cu 2+ has been plotted as a function of time for different Cu 2÷ concentrations. The equilibrium is attained in 4 - 5 h. If it is assumed that the cations are transported

pathway and lower the permeation rate. The permeation process was influenced by other factors like pH, reagent and buffer concentration. The permeation was found to be independent of pH between 3 and 5.5 but increased marginally with increase in the EDTA concentration in the narrow range of 0.025 to 0.05 M. Since the high EDTA concentration causes a simultaneous increase in the Na÷ concentration, the permeation decreases. In compartment B, the heterogeneous equilibrium between a cation (Na ÷) in the

o

1

2 O

m. Q

3

cO

o

o

c5

o 0

0

I

I

I

I

I

I

1

2

3

4

5

6

7

Time / hr.

Fig. 2. Permeationplots for: (1) 1 X 10 2 M; (2) 7.5 X l 0 - 3 M; (3) 5 X 10-3 M Cu2+ solution through a Nation ionomer membrane. EDTA concentrationin compartmentB was kept at 5 X 10-2 M.

J. Ramkumar et a l . / Journal of Membrane Science 116 (1996) 3 1 - 3 7

across the membrane only due to ion exchange, the permeation of Cu 2+ would involve simultaneous transport of an equivalent amount of other cation(s) (e.g. Na +) from the receiving compartment to the feed compartment. The excess of Na + available from the buffer solution and also the Na + released due to the complexation of permeated Cu 2+ ion with Na 2 EDTA present in the receiving solution would maintain the equilibrium and the electroneutrality of the solutions. The experiments were carried out at pH about 5.5 where EDTA would exist in solution in its dinegative form and would readily form a stable complex with Cu 2+ permeated into the receiving solution, Besides the transport of counter ions to maintain electroneutrality, other mechanisms if any could possibly also play a role. Hence no attempt has been made to quantify the equilibrium process in this studies, The two instances of selective permeation described below are simple and make use of the permselectivity of the membrane, 0.6

35

3.1. Case L Permeation o f Cu 2 + ion f r o m a mixture o f Cu 2 + a n d Fe 3 +

It can be seen from Table 1 that the permeation of Fe 3+ from Cu 2+ and Fe 3+ mixture was prevented by complexing agents like F-, SCN- or PO43 . All the three ligands form strong anionic complexes which cannot permeate through the membrane whereas Cu 2+ could progressively permeate to the other side and be removed from the vicinity of the membrane as negatively charged EDTA complex. The concentration of the masking agent was kept at about ten times higher than that of Fe 3+ in order to keep the lower complex formation at a minimum. In spite of this, considerable amount of Fe 3+ was detected in the other compartment. The concentration of Cu 2+ and Fe 3+ was measured at different time intervals by measuring the absorbance of their EDTA complexes at 730 and 365 nm, respectively. Table 4 shows the ratio of CuZ+/Fe 3+ using different masking agents. Though the stability constant for the highest complex for F with Fe 3+ is higher than

I

I

1

I

0.5

--o

2

04

0.3

,n <

0.2

0.1

/~

0.0

0

I

I

I

100

200

300

TIME

400

/ MIN

Fig. 3. Permeation of UO 2+ (8.4 × 10 -4 M) from a synthetic mixture through Nation using: (1) Na2CO 3 (10 M) and (2) Tiron (10 -2 M) solution in compartment B.

36

J. Ramkumar et al./ Journal of Membrane Science 116 (1996) 31-37

Table 4 Separation factor for Cu using different masking agents, after 4 h of permeation Masking

agent

[Cue+ ]/[Fe 3+ ]

Separation factor

Log K for Fe3+ [ 15]

SCNF

9.70 18.67 23.00

4.3 16.1 9.75

PO 3 -

SCN- and PO 3-, the concentration ratio of [cue+]/[Fe 3+ ] is lower than that obtained using PO 3- as masking agent. This suggests that considerable amount of Fe 3+ has permeated as its cationic lower complexes. This was confirmed from the measurement of F - concentration in the B compartment, It was observed that the F - concentration was about 15 /xg/ml after an equilibration period of 6 h. This is remarkably higher than the fluoride ion permeated (1 /zg/ml) from a millimolar solution of NaF in 12 h. This suggests that the presence of Fe 3+ has apparently reduced the permselectivity and enhances the permeation of anion through the cation exchange membrane as a cationic complex. The mechanism of anion permeation may be attributed to the lower complex formation and metal specialisation [10]. Formation of cationic lower complexes is ruled out in case of PO43- though HPO~- and HzPO 3 at lower pH may lead to the formation of lower cornplexes. However, at pH 5.5 the high CuZ+/Fe 3+ is due to the low Fe 3+ concentration in the B compartment. 3.2. Case II. Separation o f UO 2 + f r o m a mixture o f cations

Fig. 3 shows the permeation of UO22+ (8.4 × 10 - 4 M) from a mixture of cations containing Cu 2+, Ni 2+, Fe 3+ and Zn 2+, in millimolar concentration. The cations are effectively masked with EDTA at pH 4.0 and a 10 - 2 M solution of Na2CO 3, NaOH and H 2 0 2 mixture or Tiron in compartment B served as a receiving solution. UO~-2 does not form a strong complex [11] with EDTA (log K = 7.40) whereas the overall stability constant for CO 2- is very high (log k = 53.7) [12]. Tiron also forms a comparably strong complex (log K = 15.9) [13]. The higher stability of the complex formed by the receiving reagent

results in a very effective permeation of uranyl ion. It was observed that 77% of UO~ + could selectively permeate with CO 2- in ann B and the value was 63% when Tiron was used in place of CO 2-. The carbonate complex of uranium being stable and specific for uranyl ion a higher percentage of permeation was obtained in shorter time. EDTA was found most suitable as a masking agent for most of the interfering cations. Unlike in case of the separation of Fe3+-Cu e+ where permeation of Cu 2+ was accompanied by that of considerable amount of Fe 3+, it was interesting to note that the permeation of other ions compared to that of UO 2+ was negligibly small in this case. The permeation of the transition metal ions from feed solution was very low and was difficult to measure without pre concentration. No attempt was therefore, made to calculate the separation factor, i.e. [UO22+]/[M n+ ] where M n+ represents transition metal ions. The high stability of the negatively charged EDTA complexes of these transition metal ions coupled with their relatively larger size make them totally unfavourable for permeation through the membrane. In spite of that, if traces of these ions permeated across the membrane they (except Zn 2+) would not remain in solution at a higher pH, particularly in the highly alkaline Na2CO 3 NaOH medium and no such precipitate or turbidity was observed in practice.

4. Conclusion Nafion 117 ionomer membrane could be successfully used for selective permeation of a given cation by masking the interfering cations as their anionic complexes. Permselectively of the membrane for F was found to be remarkably reduced during the selective permeation of Cu 2+ from a mixture of Cu 2+ and Fe 3+ where F - was used as a masking agent for Fe 3+. The lowering of permselectivity is possibly due to metal speciation. The permeation of UO~ + could also be made selective by masking the common interfering cations like Cu 2+, Fe 3+, Ni 2+ and Zn 2+ as their EDTA complex. The success of this experiment suggests possible application of the membrane for recovery and preconcentration of UO22+ through selective permeation.

J. Ramkumar et a l . / Journal of Membrane Science 116 (1996) 31-37

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[7] A. Chapotot, G. Pourcelly and C. Gavach, Transport competition between monovalent and divalent cations through cation exchange membranes. Exchange isotherms and kinetic concepts, J. Membrane Sci., 96 (1994) 167. [8] A. Eisenberg and H.L. Yeager, Perfluorinated ionomer membranes, ACS Symp. Ser., 180 (1982). [9] A. Eisenberg and M. King, Ion Containing Polymers, Academic Press, New York, 1977. [10] J.A. Cox, K. Stonawska and D.K. Gatchell, Metal speciation by Donnan dialysis, Anal. Chem., 56 (1984) 650. [11] A.E. Martel and R.M. Smith, Critical Stability Constants, Vol. l, Amino Acids, Plenum Press, New York, 1974. [12] A.E. Martel and R.M. Smith, Critical Stability Constants, Vol. 6, Second Supplement, Plenum Press, New York, 1989. [13] A.E. Martel and R.M. Smith, Critical Stability Constants, Vol. 3, Other Organic ligands, Plenum Press, New York, 1977. [14] C.J. Rodden, Analytical Chemistry of the Manhattan, Project, McGraw Hill, New York, 1950, p. 83. [15] J. Luries, Handbook of Analytical Chemistry, MIR, Moscow, 1975.