Ion-exchange properties of hydrous ceric oxide—I

Z inorg, nucL Chem., 1976, Vol. 38, pp. 1211-1213. Pergamon Press. Printed in Great Britain

ION-EXCHANGE PROPERTIES OF HYDROUS CERIC OXIDE--I SORPTION O F SOME M E T A L A M M I N E S K. R. KAR, K, B. PANDEYA and A. K. BHADURI Departmentof Chemistry,Universityof Delhi,Delhi-110007,India

(Firstreceived 12July 1975;in revisedform 8 October 1975) Abstract--The sorption of ammines of Co(IIl), Ni(II), Cu(II), Zn(II) and Cd(II) on hydrated ceric oxide has been studied at different pH values and ionic concentrationsof the external solution. In each case, qa and Kd values increase with an increase in pH, attain a maximum and then show a fall. With an increase in the exchangingion concentration, qA first increases, attains a maximum and then decreases. The selectivity quotients (Ka~) of the exchanger materialtowards competingpairs -Cu2+-Ni2÷,CC+-Niz+, Zn2+-Ni2÷and Cd2÷-Ni2+of the complexions have been evaluated at pH = 10.5, where most of the exchangingions show the maximum sorption. The sorption capacity and selectivity (at oH = 10.5) follow the order, CC+ > Cu2+> Zn2+> Ni~+> Cdz+. INTRODUCTION THE ion-exchange properties of hydrated ceric oxide have not been previously reported[l]. Apart from the immense practical utility of selectivity studies for the purpose of chemical separation, an understanding of the factors contributing to the selectivity pattern is of considerable theoretical interest. In continuation of our work on the ion-exchange properties of hydrated oxides of Si(IV) and Zr(IV)[2,3], we are reporting here similar studies on hydrous CeO2. The sorption behaviour of ammine complexes of Co(Ill), Ni(II), Cu(II), Zn(II) and Cd(II) on hydrated CeO2 have been studied under static conditions. The study includes evaluation of (a) qA and K~ values at different pH values and at varying exchanging ion concentrations, in the presence/absence of buffer and (b) the selectivity quotient (KAB). MATERIALS AND METHODS All the reagents used were of A.R. quality, unless otherwise stated. Doubly distilled water was used for the experimental work. Hydrated CeO2 was prepared by adding aqueous ammonia solution to eerie ammonium sulphate solution [4], filtering washing free of sulphate ion and drying at 100°C. Ammine complexes of Co(III), Cu(II) and Ni(II) were prepared by the usual method[5] and those of Zn(II) and Cd(II), by adding an excess of aqueous ammonia to the saturated metal ion solution, followed by the addition of distilled ethanol in excess for the precipitation of the complexes. The metal salts used in these preparations were CoC12.6H20, CuSO4"5H20, NiSO4.6H20, ZnSO4.7H20 and 3CdSO4.8H20 respectively.

Determination of qA and Ko values The exchanger material (100 rag) in a stoppered conical flask was equilibrated by mechanical shaking with successive samples (25 ml) of the metal complex ions at different pH values or concentrations and in the presence or absence of buffer [NH4CI/(NH4)2SO4] salt. In each case, whenever buffer was added, the molarity of the buffer salt was 10-times the molarity of the complexed metal ion. When equilibrium was reached, the clear centrifuged supernatant solutions (5 ml) were analysed complexometricaUy[8] in the case of Zn(II), Cd(II) and

Ni(II); and iodometrically[5, 9] in the case of Co(Ill) and Cu(II). Knowing the initial and equilibrium concentrations of the metal ion in the external solution, the amount of the metal ion sorbed per 100 mg of the exchanger material (qa) were calculated. Kd values were calculated using the relationship [6]

ga=

100-X V X m

where K~ is the distribution coefficient, X, the metal ion concentration in solution at equilibrium (as a percentage of the original concentration); V, the volume of the solution in ml and m, the quantity of the exchanger in grams. Determination of the selectivity quotient (KAB). The selectivity quotient, KAB was determined using the relationship[7] Kfl

K~ =ra

where K a and Kd~ are the Kd values of A and B in the mixture. Experimental methods are described below:

Selectivity studies A 100 mg sample of the exchanger was equilibrated by mechanical shaking with a 20 ml sample of the competing complex ion mixture at a specified concentration, in the presence of a 10-fold molar excess of the buffer ion in each case, at pH = 10.5. For the Co3+-Ni2+ pair, the amounts of unsorbed nickel and cobalt ammines were determined in an aliquot of the solution by precipitating Ni(II) as nickei(II) dimethyl glyoximate and that of Co(Ill), iodometrically[5]. In the case of the Cu2÷-Ni2+ pair, the total unsorbed metal ammine content in an aliquot was determined complexometrically[8]; unsorbed Cu(II) was estimated iodometrically[9] and the unsorbed Ni(II) obtained by difference. Further, for the competing pairs Zn~+-Ni2+ and Cd2+-Ni2+, a radiometric method was followed, using the radioisotope Zn65 in the case of the Zn2+-Ni2+pair and Cd "s in the case of the Cde+-Ni2÷pair. From the initial and equilibrium counts, the amount of zinc and cadmium ion that had sorbed were calculated and nickel was determined as Ni(II) dimethyl glyoximate.

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K. R. KxR et al.

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RESULTS AND DISCUSSION The results are presented in Table 1-7. A perusal of the data reveals the following points: (i) The value of the sorption capacity, qA, initially increases with an increase of pH of the external solution, attains a maximum and then decreases; in most cases addition of buffer ion causes a fall in qA particularly at higher pH (Table 2). (ii) The qA value first increases with an increase of the concentration of the exchanging ions upto a certain limit and then suddenly decreases. (iii) The Co(III)-ammine complex has the greatest sorption capacity (Tables 1-3), the order being Co(III)-ammine > Cu(II) ammine > Zn(II)-ammine > Ni(II)-ammine > Cd(II)ammine at pH = 10.5. (iv) The selectivity quotients, KAB, which were measured under identical experimental conditions at pH = 10.5, parallel the order of the sorption capacity (Tables 4-7). It is well known[l] that the sorption of simple metal ions on hydrated oxides takes place by the following exchange mechanism basic medium

M-OH.

and three hydrogen ions. Keeping the above mechanism in view, the dependence of sorption capacity on various experimental factors may be examined. Effect o[ addition of ammonia and buffer. Successive addition of ammonia influences the overall exchange process in three different ways. (i) The increased OHfacilitates the release of hydrogen ions by exchange. (ii) The gradual addition of ammonia leads to the gradual completion of metal-ammine formation. (iii) Beyond a certain limit, the helpful role of ammonia is reversed by the formation of ammonium ions which starts competing with the metal ions for the exchange sites (Table 1). The fall in qa values at higher pH, caused by the addition of ammonium buffer, is considered to be due to an increased competition of the added ammonium ions for the exchange sites (Table 2). Effect of exchanging ion concentration. The decrease in the sorption capacity at higher concentration of the exchanging ion is presumably due to the possible lowering of ionic activity at higher concentration. A diminution of exchange potential at higher concentration is well known [10]. Effect of nature of exchanging ion. The observed differences in qA values for the different exchanging ionic species seem to be intimately connected with the charge and size of the ions [10]. Experimental observations reveal that, other factors remaining the same, sorption of an ion

X +

' + M-O- + H + .

• M-OX + H +

where M stands for the exchanger metal ion and X ÷ the exchanging metal ion. It is presumed that the sorption of the complex metal ions takes place at the expense of two

Table 1. Effect of pH on the extent of exchange of different metal ammines. Amount of CeO2= 100mg, [buffer ion] = 1 M, total volume= 25 ml

Ka

Amount sorbed (qA),m-equiv. Metal ion concentration

pH

Co3+

0.1 M

9.0 9.5 I0.0 10.5 11.0

0.78 0.87 0.98 0.90 0"80

Cu2+

Zn2+

Ni2÷

Cd~+

Co3+

Cu~+

Zn2+

Ni2+

Cd2~

0.43

0.05 0.16 0.30 0.22 0.20

0.01 0.02 0.10 0'13 0'10

0.03 0.05 0.08 0'10 0'05

46-2 52.6 55.3 54"0 34'9

23.5 26.5 39.8 36.4 27.7

2.5 8.2 18.4 13'3 10"4

0.5 2.0 5.1 9"0 8'2

1.6 2.5 4.0 5.1 2'5

0.48 0.68 0.56 0'50

Table 2. Effect of pH and buffer ion addition on the extent of exchange of different metal ammines.Amount of CeO2= 100 nag,metal ion conc.= 0.01 M, Total volume = 25 ml Amount sorbed (qa), m-equiv.

Amount sorbed (qA), m-equiv.

Buffer concentration

pH

Co3+

Cu~+

Zn2+

Ni2+

Cd2+

Nil

9.0 9'5 10.0 10.5 11.0

0.10 0.32 0.35 0.34 0.26

0.10 0.17 0.21 0.20 0.19

0.07 0.12 0.18 0.17 0.13

0.02 0.04 0.14 0.13 0.11

0.04 0.05 0.08 0.11 0.10

Buffer concentration

Co3+

Cui+

Zn2+

Ni2+

Cd2÷

0.1M

0.21 0.22 0.24 0.25 0.22

0.19 0.20 0.19 0.18 0.17

0.10 0-12 0.15 0.16 0.12

0.01 0.03 0.10 0.12 0.10

0.03 0.04 0.06 0.10 0.08

Table 3. Effect of concentration of the extent of exchange of different metal ammines. Amount of CeO2= 100 rag, [Buffer ion]/[metal ion]= 10, Total volume = 25 ml Amount sorbed (qA) m-equiv pH

Metal ion concentration

Co3+

Cu2+

Zn2+

Ni2+

Cd2+

10'5

0-01 M 0"03M 0'05 M 0.08 M O.IOM

0.25 0.72 0'98 0'95 0.90

0.18 0.52 0.68 0'65 0!56

0.16 0.20 0"25 0.24 0"22

0.12 0.14 0"20 O'18 0"13

0.10 0"13 O'16 O"12 0.10

Ion-exchange properties of hydrous ceric oxide--I Table 4. Selectivity dependence on the concentration of competing pair Co3+-Ni2+. Amount of Ce02 = 100 mg; pH = 10.5, [metal ion]/[buffer ion] = 0.10, total volume = 20 ml Concentrations of the competing ions

Amount sorbed (qA) in m-equiv.

Co3+

Ni2+

Co3+

Ni2+

0.01 M 0.03 M 0-05 M 0.08 M 0' 10 M

0-01 M 0.03 M 0-05 M 0.08 M 0.10 M

0,298 0.842 0.900 0.880 0.800

0.020 0.060 0.100 0.090 0.080

KC~++

55.51 44.71 15.54 13.10 12.25

Table 5. Selectivity dependence on the concentration of the competing pair Cu2+-Niz+. Amount of CeO2= 100 nag; pH = 10.5; [metal ionl/[buffer] = 0.10, total volume = 20 ml Concentrations of the competing ions

Amount sorbed (qa) m-equiv.

Cu2+

Ni2+

CC +

Ni2+

0.01 M 0.03 M 0.05 M 0-08 M 0.10M

0.01 M 0.03 M 0-05 M 0-08 M 0.10M

0.252 0.260 0-280 0.240 0-180

0.020 0.040 0.080 0.120 0.100

KC~+*

32.30 8.01 3.90 2.81 1.71

Table 6. Selectivity dependence on the concentration of the competing pair Zn2+-Ni2+. Amount of CeO2= 100 rag; pH = 10.5; [metalion]/[butter ion] = 0.10, total volume = 20 ml Concentrations of the competing i o n s

Amount sorbed (qA) m-equiv.

Zn2+

Ni2+

ZC +

Ni2.

0.01 M 0,03 M 0.05 M 0.08M 0.10M

0.01 M 0.03 M 0.05 M 0.08M 0.10M

0.162 0.182 0.200 0.198 0.150

0.01 0-03 0.06 0.10 0.09

KZ~;

25.86 6.97 3.59 2.04 1.69

Table 7. Selectivity dependence on the concentration of the competing pair, Cd2+-Ni2+. Amount of CeO~= 100 nag;pH = 10.5; [metal ion]/[bufferion] = 0.10, total volume = 20 ml Concentration of the competing ions

Amount sorbed (qA) m-equiv.

Cd~+

Ni2÷

Cd2+

Ni2+

0.01 M 0.03 M 0.05 M 0.08 M 0.10 M

0"01M 0.03 M 0.05 M 0.08 M 0' 10 M

0.143 0.215 0.170 0.270 0.020

0'147 0.245 0.290 0.470 0'260

KC~2+ +

0'98 0.93 0.55 0'53 0"07

is dependent on the ionic potential [ 11], and the higher the ionic potential, the greater is the sorption. Among the metal ions under study, Co 3÷ is the smallest in size, bears the highest ionic charge, and consequently

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has the highest ionic potential. The corresponding ammine shows the maximum sorption as expected. Other metal ions are all bivalent, their ionic radii following the order Cu ~+, 0.72 A < Zn z+, 0.74/~ < Ni 2+ 0.78 A < Cd 2+, 0.92 A. But the size of the corresponding metal ammines may have to be viewed in the light of their stereochemistries[12]. Copper(II)-ammine is squareplanar, nickel(II)-amine is octahedral and zinc(II)- and Cd(II)-amine are tetrahedral so that their sizes may follow the order Cu(II)-ammine < Zn(II)-ammine < Cd(II)-ammine < Ni(II) ammine and their ionic potentials the reverse one. For pH < 10 the order of the sorption capacity of the metal ammines is Cu(II)>Zn(II)> Cd(II) > Ni(II), in full agreement with that expected on the basic of the ionic potential values. At very high pH, the increased possibility of hydrolysis makes the situation complicated. The fact that the selectivity quotient values follow the order of qa values, particularly at lower concentrations, indicates that the ionic potential is the significant factor which decides the selectivity of the exchanger material. The decrease in the selectivity values at higher external concentration (Tables 4-7) shows that the activity of the competing ions inside the solid phase assumes a more dominant role in deciding this issue. This is in agreement with the findings of Samuelson[13] and other earlier workers[14-17] while investigating selectivity effects in ion exchange resins. Acknowledgements--Our grateful thanks are due to Council of Scientific and Industrial Research, New Delhi, for providing financial assistance. REFERENCES I. V. Vesely and V. Pekarek, Talanta, Vol. 19, pp. 219-262. Pergamon Press, Oxford (1972). 2. K. R. Kar and R. K. Srivastava, J. Rad. Anal. Chem. 20, 575 (1974). 3. K. R. Kar and Sukhbir Singh, Microchim. Acta 616 (1970). 4. J. W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. 5, p. 632. Longmans (1%0). 5. G. Marr and B. W. Rockett, Practical Inorganic Chemistry, pp. 266, 267, 270, 277. Reinhold, New York. 6. V. M. Klechkovsky, L. N. Sokolova and G. N. Tselishcheva, Proc. 2nd lnt. Con[. Peace[ul Uses of Atomic Energy, Geneva Vol. 18, p. 487 (1958). 7. C. B. Amphlett, Inorganic Ion Exchangers, p. 23. Elsevier, Amsterdam (1%4). 8. H. A. Flaschka, EDTA Titrations. Pergamon Press, Oxford (1%4). 9. O. Glethorpe and Smith, Analyst 68, 325 (1943). 10. R. Kunin, Elements o[Ion-Exchange, Vol. 12, p. 13. Reinhold, New York (1960). 11. G. H. Cartledge, J. Am. Chem. Soc. 50, 2855 (1928). 12. C. K. Jorgenson, Inorganic Complexes, p. 54. Academic Press, London (1966). 13. O. Samuelson, Studier rorande Joubytande [asta ammen, Stockholm (1944). 14. W. C. Bauman, 23rd National Colloid Symposium, Minneapolis, (June 1949). 15. G. E. Boyd, B. A. Soldano and O. D. Bonner, J. Phys. Chem. Ithaca, Vol. 58, p. 456 (1954). 16. H. P. Gregor, J. Am. Chem. Soc. 73, 642 (1951). 17. D. A. Robinson and G. F. Mills, Ind. Engng Chem. 41, 2221 (1949).