The Mechanism of the Sorption of Monovalent Cations on Cobalt(II

The Mechanism of the Sorption of Monovalent Cations on Cobalt(II)-hexacyanoferrate(II) T. Ćeranić, D. Trifunović, and R. Adamović Institute of Physical Chemistry, Faculty of Sciences, Beograd, Yugoslavia Z. Naturforsch. 33 b, 1099-1101 (1978); received July 3, 1978 Monovalent Cations, Cobalt(II)-hexacyanoferrate(II) Cobalt(II)-M-hexacyanoferrate(II), M = K, NH4, Cs, as synthetic sorbents have ion exchange properties selective for monovalent cations. They show a higher selectivity for the ions with larger crystal radius if the sorption takes place in dilute solutions, whereas in concentrated sulutions the selectivity sequence is reversed. The phenomenon is attributed to the differences in physico-chemical properties of the given pair of competing ions in the solution phase and to steric limitations in the phase of ion exchanger.

1. Introduction

Equilibrium

Potassium(I)-cobalt(II)-hexacyanoferrate(II) has been used as an inorganic ion exchanger for decontamination of radioactive waste solutions from 137Cs [1] as well as for its separation from urine, milk and see water [2], The 137Cs obtained in this way is of a high radiochemical purity (99%) [1]. The mere selectivity for Cs+ ions, when in trace concentrations, does not provide sufficient data for a more detailed explanation of its sorption mechanism. In the present work the results of the investigation are presented for the ion exchange reactions: K - > N H

4

,

CS;

N H

4

- > K ,

CS

and

C I - > K ,

N H

4

[K, NH 4 , CS being the ions incorporated in the structure of cobalt(II)-hexacyanoferrate(II)], in dilute and concentrated aqueous solutions. The objective of these investigations was to determine the reaction mechanism and also the sequence of selectivity which would lead to a more complete characterization of this inorganic ion exchanger.

experiments.

The 10 mg aliquot of

the ion exchanger (grain size 0.71 to 0.84 mm) was equilibrated with 10 cm 3 of aqueous solution of the chlorides of monovalent ions at the temperature of 298 K for 3 hours (it had been proven experimentally that the equilibrium was achieved after one hour). The degree of exchange was determined by analysing the solution after the equilibrium had been attained. The K+ concentration was determined by flame photometry, Cs+ labelled with 137Cs by radiometry with GM counter, and NH 4 + from the concentration difference of the competing ion before and after the equilibrating procedure. The distribution coefficient were determined as D3

mmol M + per g of sorbent mmol M+ per cm 3 of solution

at equilibrium (1)

Composition of the solutions. 0.005 M aqueous solutions of chlorides of monovalent cations (microion) were used for the study of the sorption mecha-

2. Experimental Synthesis and analyses.

The synthesis of M+-

cobalt(II)-hexacyanoferrate(II) was performed by precipitation with hexacyanoferric(II) acid of cobalt in the presence of K + , NH4+ and Cs+. The details have been given in a previous publication [3]. The isolated crystallites, air-dried at the ambient temperature, have the following stoichiometric compositions [3]: K 2 Co[Fe(CN) 6 ], 2.5 H 2 0 , (CoFC)K (NH4)2Co[Fe(CN)6], 2.39 H 2 0, (CoFC)NH4 Cs2Co[Fe(CN)6], 4.46 H 2 0, (CoFC)Cs Requests for reprints should be sent to Dr. T. Ceranic, Institute of Physical Chemistry, Faculty of Sciences, 11000 Beograd, Studenski trg 12, Yugoslavia.

-3

-2-1

0 1

Fig. 1 . log D M 2 a s a function of log C M I C I for reactions: 1) K NH4+ and 2) NH 4 K+.

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T. Ceranic ei al. • Mechanism of the Sorption on Cobalt(II)-hexacyanoferrate(II)

IIOO

nism. The concentration of monovalent cations the ion exchanger was compared with varied between 0.005 and 1 mol dm - 3 (macro-ion), log D of microions as a function of the concentration of macro-ions are presented in Figs. 1-3.

(0.005 mol dm - 3 ) and concentrated (1 mol dm~3) solutions of their chlorides. The ions incorporated in the ion exchanger were not present in the solutions used in these experiments. The results are presented in Table II. 3. Results and Discussion

The sorption mechanism. If the investigated sorption processes (CoFC)K ->NH 4 +, Cs+; (CoFC)NH4 -> K+, Cs+ and (CoFC)Cs->K+ NH4+ are of the ion exchange type, the law of mass action can be applied, to the reaction (CoFC)Mi+M 2 +,aq ^ (CoFC)M2 + Mi+, aq (2) where from the condition for the exchange of monovalent cations [4] follows g log Dm,

6 log 'oqCmiCI F i g ^ . log D M s as a function of log 1) K -> Cs+ and 2) Cs K+.

CMXCI

for reactions:

CMJCI

for reactions:

20 z Q Gl O 1.0

Fig. 3. log D M 2 as a function of log 1) NH 4 Cs+ and 2) Cs -> NH 4 +.

The sequence of selectivity for the investigated cations was determined from the distribution coefficients of counterions from the diluted (CoFC)K

NH 4 ,Cs*

CKCI

M2

(COFC)NH4

M2

mol dm - 3 NH

4 4

Cs+

0.5 0.5

K+

Cs+

->

K,Cs

(CoFC)Cs

Cnh4C1 mol d m - 3

M2

0.01 0.5

K+ NH4^

UMI,

aq

Eq. (3) was satisfied for all the above reactions which points out the ion exchange character of the sorption of monovalent cations on the synthetized crystallites. For the same pair of competing ions in the inverse procedure, however, the concentration of macroions at which the desorption begins was not the same (Figs. 1-3, Table I). This hysteresis is due to the difference in the behaviour of the competing ions in the solution and in the ion exchanger. If the crystal radius of the macro-ion is smaller than that of the micro-ions, the desorption of the former begins at a higher concentration and vice versa (Table I). An exception is the Cs+ macro-ion which desorbs NH4+ ions more easily than K+ ions. It is probably due to the free energy change of dehydration of competing ions in the liquid phase which for the desorption of NH4+ by Cs+ amounts to—66kJ/g-ion; for K+ by Cs+ its value is —63 kJ/g-ion [5]. The phenomenon of the Cs+ micro-ions desorption at the same and rather high concentrations (0.5 mol d m - 3 ) of K+ and NH 4 + macro-ions is due to the above-mentioned hydration energy which changes its sign in this case; it is also due to the K , N H

4

Table I. The concentration of macroion at which the desorption of microions begins (see eq. 3).

CCSCI

mol dm - 3 0.1 0.05


* The bar above the sign denotes the solid phase.

T. Ceranic et al. • Mechanism of the Sorption on Cobalt(II)-hexacyanoferrate(II)

large crystal radius of Cs+ (169 pm) which makes its diffusion through the pores of the crystallite more difficult. At the macro-ion concentrations higher than 0.5 mol dm - 3 the presence of Cl - ions can not be neglected; it enables the ion pair formation and the disturbance of the hydration shell of K+ and NH4+ ions. The ion exchanger phase becomes therefore a more suitable medium for such ions which then initiate the Cs+ desorption. If the counter ions in the solution are present in micro concentration, the synthetized ion exchangers are more selective for the ions with the larger crystal radius, whereas for the macro concentration of the counter ions, the ions with smaller radius are favorized (Table II). It has also been observed that if an ion with smaller crystal radius was exchanged by an ion with larger radius, the latter being in macro concentration (e.g. the exchange of K and NH4 by Cs+), the crystallite collapse takes place. By comparing the crystal radii of ions incorporated by the synthesis and the effective radii of the windows, rw, in the cages [3] it seen that the dehydrated K+ ion (r c =133pin) can easily leave the (CoFC)K ion exchanger's window (rw = 147 pm) which to a considerable extent holds also for NH 4 + ions (r c =148pin) and the (CoFC)NH4 cages (rw = 149 pm). The Cs+ ions (rc = 169 pm), however, are "trapped" in the (CoFC)Cs cages the rw of which amounts to 160 pm.

[1] W . E. Prout, E. R. Russell, and H. J. Groth, J. Inorg. Nucl. Chem. 27, 473 (1965). [2] A. L. Boni, Anal. Chem. 38, 89 (1966). [3] T. Ceranic, Z. Naturforsch. 33b, (1978).

1101

Table II. The values of the distribution coefficient of the M2 ion from diluted and concentrated solutions. D>i2) cm3 g - 1

Mi -> M 2

CM2CI

CM2CI

K -> NH 4

96.15 163.37

3.46

K -> Cs Selectivity N H 4 -> K NH 4 Cs

0.005 mol dm- 3

NH 4 < Cs 82.04

Selectivity

109.51 K < Cs

Ci -> K Cs NH 4

48.25 43.62

Selectivity

K > NH 4

1 mol d m - 3

2.30 NH 4 > Cs 2.72 1.20 K > Cs 1.29 1.26 K ~

NH 4

The non-equivalence of the ion exchange reaction in both directions Mi ^M 2 (hysteresis, Figs. 1-3, Table I), the obtained selectivity sequence (Table II) and the crystallite collapse in the case when M2 = Cs+, are to be attributed not only to the different properties of these ions in the solution but also to the steric limitations in the ion exchanger phase which originated from the structural parameters of the crystallites [3]. With respect to this the synthetized ion exchangers have similar properties with the zeolites in reactions with monovalent cations.

[4] C. B. Amphlett, "Inorganic Ion Exchanger", Elsevier Publ. Co., London, 1966. [5] Y . Marcus and A. S. Kertes, "Ion Exchanger and Solvent Extraction of Metal Complexes", Inter. Publ. Co., London, p. 13.
