Thermodynamics of adsorption of cerium on lead dioxide

J.RADIOANAL.NUCL.CHEM.,LETTERS 186 (I) 63-74 (1994)

THERMODYNAMICS OF ADSORPTION OF CERIUM ON LEAD DIOXIDE

M. Afzal*, H. Ahmad, M. Saleem, S.M. Hasany**

Department of Chemistry, Quaid-I-Azam University, Islamabad, Pakistan

**Nuclear Chemistry Division, PINSTECH, Nilore, Islamabad, Pakistan

Received 17 May 1993 Accepted 31 May 1993

Adsorption of cerium on lead dioxide from aqueous solutions has been studied as a function of shaking time, amount of adsorb- ent, pH, concentration of the adsorSate and temperature. The adsorption process is endothermic and the distribution coefficient (K D) increases with increasing temperature. The data fit very well the Lang- muir, Freundlich, and Dubinin-Raduskevich Isotherms and their corresponding constants were Calculated. AH ~ and AS ~ were calculated from the slope and intercept of plots of ink D vs. I/T. AG ~ values decreases with increasing temperature, showing that the adsorption of Ce(III) is more favorable at high temperature. The endothermacity of the adsorption process is discussed.

INTRODUCTION

Due to the uncontrolled release of heavy metals and

radionuclides into surface and sea water, the monitor-

ing of different pollutants in water systems has in-

*Author for correspondence.

63 Elsevier Sequoia S. A., Lausanne

Akaddmial Kiadd, Budapest

AFZAL et al.: ADSORPTION OF CERIUM ON LEAD DIOXIDE

creased in recent years. As these pollutants are within

the concentration range of microamounts or traces, a

preconcentration step is usually needed before their

instrumental determination. Cerium is one of the impor-

tant fission products, so its separation/removal is es-

sential. Preconcentration/separation procedures based

on adsorption phenomena are important in analytical and

radiochemistry of trace elements, because of their sim-

plicity, efficiency and selectivity. The adsorption of

cerium on metal oxides has been reviewed in our earlier

investigations of its adsorption on lead I and manganese

dioxides 2. This study was undertaken in order to evalu-

ate the importance of lead dioxide for scavenging in

natural water or in waste water treatments. Here we re-

port the results of our investigations on the adsorp-

tion of cerium on lead dioxide from aqueous solutions

at various temperatures.

EXPERIMENTAL

The surface area, pore parameters and preparation of

buffer solutions of different pHs were reported earlier I.

All the solutions were made from doubly distilled de-

ionized water and analytical reagents. The measurement

of K D was carried out at (273-303) • 0.3 K and the error

in most cases was around •

To check the dissolution of lead dioxide, 50 mg was

shaken with 100 cm 3 buffer solutions of different pHs

for 30 min separately. The solution was then filtered

through a 0.2 ~m pore size sinter (ACE Bectan Dickenson

& Co. U.S.A.). The amount of lead due to dissolution

was then measured colorimetrically 3-5. The solubility

of the oxide was (0.93, 0.81, 0.64, 0.008, 0.005,

64


0.002, 0.001, 0.01, 0.05, 0.2) mg/100 cm 3 for pHI to

10, respectively. The high solubility observed at pH

I-3 may be due to the fact that these solutions were

made by mixing 0.1 mol dm -3 HCl and KCI solutions and

lead dioxide is sparingly soluble in dil. HCI 6. The

low solubility at pH 4 to 7 may be due to the fact that

the oxide is not dissolved in and does not react with

any component of these buffer solutions. The increase

in solubility at pH 8 to 10 may be due to the reaction

of lead dioxide with ammonium chloride present in these

buffers 7 .

The preparation of cerium tracer, the experimental

techniques used and the procedure for adsorption meas-

urement, computation of distribution coefficient and

percent adsorption are given in detail elsewhere I-3'8

RESULTS AND DISCUSSION

Experiments were conducted to determined the optimum

amount of oxide and shaking time for the adsorption of

cerium on lead dioxide and were found to be 50 mg and

less than 10 min, respectively, as reported earlier I'3

Adsorption dynamics were determined using various

rate-controlling steps. The study of adsorption dynamics

is quite significant in waste water treatment, as it

describes the solute uptake rate, which in turn controls

the residence time of the adsorbate uptake at the solid/

solution interface. The adsorption of cerium from solu-

tion to solid phase can be considered as a reversible

reaction with an equilibrium between two phases 9.

S + Ce ' S-Ce ( 1 )

65


The overall rate constant k', and rate constants for

adsorption (kl) and desorption (k 2) of this reaction

were calculated using the equations of Ref. (10) and

were found to be 0.49, 0.48799 and 2.01x10 -3 min -I , re-

spectively. The rate constant of intraparticle trans-

port (k~) was calculated using the equation of Weber --I and Moris 11 and was found to be 3.92x10 -5 mmol g

-i/2 min

The adsorption of cerium on lead dioxide was first

measured as a function of pHI . No appreciable adsorp-

tion has been observed up to pH 2. The distribution

coefficient increases with increasing pH up to 6, where

maximal adsorption is achieved 12. Thus a buffer of pH

6 was selected for all further studies I-3. Similar ob-

servation was also reported by Shiao 13, for the adsorp-

tion of cobalt on AI203. Based on hydrolysis constants 14

of cerium, up to pH 5, no hydrolysis take places and

all the cerium would be present as Ce 3+ ions. As the pH

increases above 5, hydrolysis of cerium becomes pre-

dominant, resulting in a variety of hydrolyzed species

and the adsorption of cerium starts decreasing again.

Cerium may be present as hydrolyzed ions of varying com-

position under different pH conditions in solution. The

percentages of these species, namely Ce 3+ Ce(OH) 2+ I l

Ce(OH)+ 2' Ce(OH)3 and Ce(OH)4 at different pHs have been I

estimated using the values of K I, K 2, K 3, and K 4 as

10 - 8 " 1 , 10 - 8 . 2 , 10 - 9 . 7 a n d 1 0 - 1 2 ~ r e s p e c t i v e l y , a n d a r e

given in Table I.

The adsorption isotherms of cerium at different tem-

peratures 15 were obtained by plotting the amount ad-

sorbed (mmol.g -I) vs. equilibrium concentration of cerium

in solution as shown in Fig. 1; they followed the L2 16 type of Giles classification The extent of adsorption

66


,~ 20

E_ ~6 X

o 313 o 323

do ' 100 150

C.e, pmol - dm -3

Fig. I. Adsorption isotherm of cerium on lead dioxide at different temperatures

TABLE I

Percent of hydrolyzed species of cerium formed

pH Ce 3+ Ce (OH) 2+ Ce (OH) + 2 Ce (OH) 3 Ce (OH) 4

6 99.21 00.79 - - -

7 92.81 06.76 0.43 - -

8 55.73 27.14 17.10 00.03 -

9 11.18 12.15 63.92 12.73 0.02

10 01.24 01.54 32.46 64.12 0.64

is related to Langmuir 17, Freundlich 18 and Dubinin-

Raduskevich 19 isotherms.

The data were fitted with the Langmuir adsorption

isotherm in the concentration range of 7.5 x 10 -6 to

1 x 10 -4 mol dm -3. From the slope and intercept of the

67


6

3 I o 313 o 323

I I I I 02 0.4 0.6 0,8 1.0

Ce, prno[ �9 drn "3

Fig. 2. Langmuir-type adsorption isotherm of cerium on lead dioxide at different temperatures

linear plots of Ce/X vs. C e (Fig. 2) at different tem- -I peratures the value of V (12 to 19) • 0.5 mmol g and

m constant B (1.43 to 1.42) • dm 3 ~mol -I were calcu-

lated. The V m value increases with increasing tempera-

ture, indicating the endothermicity of adsorption

process. The data follow Freundlich as well as D,R iso-

therm in the whole concentration range studied (7.5

x 10 -6 to I x 10 -3 mol dm -3) for different temperatures

as shown in Figs 3 and 4. The Freundlich constants A

(I to 28) • 0.65 mol g-1 and N (0.5 to 0.7) + 0.01,

were computed from the slope and intercept of the plots

as shown in Fig. 3. These values increase With increas-

ing temperature. N may be related to the nature and

strength of adsorption forces involved. The higher frac-

tional value of N signifies that strong adsorptive

forces are operative on the oxide surface. However, the

higher values of A signify the higher affinity of cerium

68


Fi~. .

Ig Ce - 8 - 7 - 6 - 5 - 4 - 3 _

_ _ .

�9 313 �9 323

X

- 5 9

Freundlich-type adsorption isotherm of cerium on lead dioxide at different temperatures

Fig.

s

500 1000 1500 2000 2500 " 2 t I I

Temp, K " 293 �9 303

~ " % m ~ n 313

-6 " ~ ~

-8

c -10

4. Dubinin-Radushkevich-type adsorption isotherm of cerium on lead dioxide at different temperatures

69


for lead dioxide. The D-R parameter X (79 to 278)• 0.43 m

mmol g-1 further confirmed that with increasing tempera-

ture t~e adsorption of cerium on lead dioxide is more

favorable. The sorption free energy E (11.8 to 12.1) •

0.2 kJ mo1-1 is in the energy range of ion exchange re-

actions, in good agreement with our previous observa-

tion of manganese adsorption on lead dioxide 3 and other

references therein.

The dependence of cerium adsorption on temperature

was also investigated. The concentration was varied

between 7.5 x 10 -6 and I x 10 -3 mol dm -3 and the tem-

perature was varied between 293 to 323 • 0.3 K in steps

of 10 K, while other parameters were kept constant. The

distribution coefficient increases with increasing tem-

perature, which may be due to a negative temperature

coefficient or to a steep simultaneous decrease of the

real adsorption of solvent 20-25. Belousov and Kolobov 26

have studied the adsorption of cerium on active manga-

nese dioxide, and have found that adsorption of cerium

increases with increasing temperature and with decreas-

ing cerium concentration.

The values of AH ~ and AS ~ were calculated from the

slopes and intercept of the linear variation of In K D

vs. I/T, Figs 5 and 6. The thermodynamic parameters are

given in Table 2. Both the positive values of AH ~ and

decrease in the values of AS ~ with increasing tempera-

ture show that the reaction is more favorable at high

temperature. One possible explanation of the endo-

thermacity of heats of adsorption is the well known

fact that cerium ions are well solvated in water. In

order for the ions to be adsorbed, they have to lose

part of their hydration shell. This dehydrab• process

of the ions requires energy. We have assumed that this

dehydration supersedes the exothermacity of the ion

7O


I 2 9.5~

90

Conc .:

�9 l~ox lo-_ 5 ~

�9 s ~ 16 -s ~. I o ;~x~-~ ~ I �9 1.Ox 10 -4 ~ J

8s I I J I �9 3.0 3.1 3.2 3.3 3.4 3.5

l IT , x lO "3 K -1

Fig. 5. Plot of in K D vs. I/T for cerium adsorption on lead dioxide (conc. 7.5 x 10-6 to I x 10 -4 mol dm-3)

~ 9 ~

7 i i

6

I 3 3.1 3.2 3.3 3.4 3.5

I/T ,xi0"3 K -I

Fig. 6. Plot of in K D vs. I/T for cerium adsorption on lead dioxide (conc. 2.5 x 10 -4 to I x 10 -3 tool dm -3 )

71

AF'ZAL et al.: ADSORPTION OF CERIUM ON LEAD DIOXIDE

TABLE 2

Thermodynamicparameters of cerium adsorption on lead dioxide

-AG ~ , kJ mol -I

ConcentrationA AH O AS O T, K

mol dm "3 kJ mol -I J Kmol -I 293 303 313 323

7.5 x 10 -6 7.304 102.i40 22.62 23.64 24.67 25.69

1.0 x 10 -5 7.550 105.927 23.49 24.55 25.61 26.67

2.5 x 10 -5 8.894 107.257 22.53 23.61 24;.68 223.61

5.0 x 10 -5 11.713 112.158~ 21 15 22,27 23.39 24.51

-5 7.5 x 10 14.588 120.303 20.66 21.86 23.07 24.27

1.0 x 10 -4 17.927 128.611 19.76 21.04 22'33 23.61

getting attached to the surface. The implicit assump-

tion here is that on adsorption the environment of the

metal ion is less aqueous than it is in the dissolved

state. In summary, we may say that the removal of water

from the ion is essentially an endothermic process and

it appears that endothermicity of the desolvation

process exceeds that of the heat of adsorption to a con-

siderable extent. The free energy charge is negative as

expected for a spontaneous process. The rise in -AG ~

with increasing temperature shows that the adsorption

of these metal ions on lead dioxide become favorable at

high temperature. The overall adsorption process is

endothermic because of the positive entropy change.

72


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Thermodynamics of adsorption of cerium on lead dioxide

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