Journal o f Radioanalytical and Nuclear Chemistry, Articles, Vol. 157, No. 2 (1992)321-334

SELECTIVE ADSORPTION OF THORIUM ON ACTIVATED CHARCOAL

FROM ELECTROLYTIC AQUEOUS SOLUTION

RIAZ QADEER,* JAVED HANIF,* M. SALEEM,*** M. AFZAL**

*Pakistan Institute o f Nuclear Science and Technology, P.O. Box No. 1356, Islamabad [Pakistan}

**Department o f Chemistry, Quaid4-Azam University, lslamabad [Pakistan)

(Received March 25, 1991)

The adsorption of thorium on activated charcoal has been studied as a function of shaking time, amount of adsorbent, pH, concentration of adsorbate and temperature. Adsorption of thorium obeys the Langmuir isotherm. AH ~ and AS ~ were calculated from the slope and intercept of In KD vs. I/T plots. The influence of different anions and cations on thorium adsorption has been examined. The adsorption of other metal ions on activated charcoal has been studied under specified conditions to check its selectivity. Consequently, thorium was removed from Cs, Co, Ba, Cr, Sr, Cd, Cu, Mn and Zn. More than 98% adsorbed thorium on activated charcoal can be recovered with 55 ml 3M HNO 3 solution. Wavelength dispersive X-rays fluorescence spectrometer was used for measuring thorium concentration.

Introduction

The adsorpt ion o f thor ium on various solids is impor tan t because it has numerous

applications in glass, ceramics, chemical , metallurgical and nuclear industry. Some

workers have studied the adsorpt ion o f thor ium on glass, 1 -2 thal l ium halide and

silver bromide crystals, s - 4 tungs ten ,S-7 tantalum,8 fibrin,9 hydroxide, tO rhenium,11

silica gel, 12 resins 13 - 14 and (PMBP) loaded polyure thane foam. 1 s F R E U N D L I C H

and W R E S C H N E R 16 used charcoal for separat ion o f U and Th through adsorpt ion.

The present communica t ion describes our investigations o f adsorpt ion o f thor ium on

activated charcoal and illustrates the opt imal condi t ions required for its preconcentra- t ion.

+Author for correspondence.

Elsevier Sequoia S. A., Lausanne Akaddmiai Kiad6, Budapest

RIAZ QADEER et al.: SELECTIVE ADSORPTION OF THORIUM

Experimental

Reagents and chemicals

Reagents and chemical used are: activated charcoal (BDH, item No. 33032), thorium nitrate (Fluka, item No. 89150), hydrochloric acid (RDH, item No. 30721), acetic acid (BDH, item No. 27013), sodium acetate (RDH, item No. 25023).

Instruments

Siemen's wavelength dispersive X-ray fluorescence spectrometer (WDXRFS), SRS-200 was used with the following attachments: Cr X-ray tube, Soller slit with angular divergence of 0.15 ~ LiF(100) crystal; NaI(T1) scintillation counter, linked through universal interface LC-200 to a PDP-11/04 computer for measuring thorium concentration. The pH measurements were made with a digital pH meter-605 from M/s Metrohm. The pH of the solution was adjusted with hydrochloric acid and sodium acetate solution. Edmund Buhler-SM 25 shaker was used for shaking at a constant speed of 150 revolutions per minute.

Procedure

Adsorption measurements were carried out by a batch technique at room tem- perature [(22+1) ~ except where otherwise specified. Known amount of activated charcoal in 250 ml reagent bottles containing 10 ml of thorium solution were shaken for a given time period. The solutions were then filtered and the concentration of thorium before and after shaking was measured by WDXRFS technique. The sample solutions were presented to the spectrometer in a 0.1 mm thick walled polyethylene bottle.17-18 The percentage adsorption and distribution coefficient (KD) were comp'uted in the usual way. 19

Results and discussion

The adsorption of thorium on activated charcoal was studied as a function of shaking time. 10 ml of thorium solution (2000/ag/ml) was shaken with 100 mg of solid for different intervals of time ranging from 2 to 60 minutes. Figure 1 shows the variation of percentage adsorption and distribution coefficient (K o ) with shaking time. The adsorption increases with an increase in shaking time and attains a constant value around 30 minutes when adsorption equilibrium is established. Therefore, 30 minutes shaking time was selected for all further studies.

322

RIAZ O.ADEER et al.: SELECTIVE ADSORPTION OF THORIUM

50 c" .s

.o 45

4O

35

I

I I I I 1 6

b a

80

2

70

60

3 o ,o 2o 3o 4o

Shaking time, rain

Fig. 1. Adsorption of thorium on activated charcoal as a function o f shaking time; a - percentage adsorption, b - KD

o~ 120 c" ._o K

80

60

40

20 J 0.05

400O

3000

2000

1000

1 I I 1 /0 01 015 0.20 025 030

Amount of activated charcoal, g

5000

E &

Fig. 2. Dependence of thorium adsorption on the amount of activated charcoal; a - pereentase adsorption, b - KD

The results of the dependence of thorium adsorption on the amount of activated charcoal are shown in Fig. 2. The concentration of thorium and shaking time were timed at 1000/ag/ml and 30 minutes, respectively, while the amount of activated charcoal was varied from 20 to 300 rag. The percentage adsorption and distribution

323

R I A Z Q A D E E R et al.: S E L E C T I V E A D S O R P T I O N OF T H O R I U M

450

4 0 0 -

350 -

300 -

250 -

200 -

150 -

100

50

0 0

tO0

b C

8O

20

1 2 3 4 5 i 0

pH

o

o _

ffl " u

Fig. 3. Influence of pH on thorium adsorption on activated charcoal; a - percentage adsorption, b - K D

100 I0

8O

6

4

6O

! 2

4c 500 1000 1500 2000 2500 3000

Th concentrat ion, u g / m l

Fig. 4. E f f ec t o f t h o r i u m concen t ra t ion on its adsorp t ion on act ivated charcoal ; a - percentage adsorpt ion, b - K D

coefficient (KD) increase with increasing amount of adsorbent up to 100 mg. The

percentage adsorption remains constant above 100 mg but KD decreases drastically.

Therefore, for all further studies 100 mg of activated charcoal was used.

Figure 3 shows the influence of pH on the adsorption of thorium on activated

charcoal. The percentage adsorption and distribution coefficients (KD) increase with

324

RIAZ QADEER et al.: SELECTIVE ADSORPTION OF THORIUM

A 12

E~

~O

8 E

J

0.5 ~.0 15

Fig. 5. Langmuir plot for thorium adsorption on activated charcoal

I ppm 20OO ~

A 1500 f 175o �9 2000 /

~ 2500 / 1500 �9 3 0 0 0 /

o 3 5 0 0 , / 1250

1000 ~ ~

75O

500 -

250

20o .,,.,.,....I-

lOO

5o-

o I 1 2 280 300 320 3 _ ~'u P"

Temperoture, K

Fig. 6. Effect o f temperature on K D for thorium adsorption on activated charcoal

325

RIAZ QADEER et al.: SELECTIVE ADSORPTION OF THORIUM

increasing pH up to 3.8 and then starts decreasing. Maximum adsorption occurs at pH 3.8 and hence a buffer of pH 3.8 (acetic acid and sodium acetate) was used for all further studies. The influence of pH on thorium adsorption can be explained in the following way. Up to pH 3.8, the adsorption of simple hydrolysis product species

28 A p p m - a 1500

�9 2000

�9 3000 o 3500 �9 4000

3 I I I I I ] 3.0 3.1 3.2 3.3 3.4 3.5 3.6

1 K_ 1 "T ' x10-3

Fig. 7. Plot o f l n KD vs. 1IT for thor ium adsorption on activated charcoal

takes place. As the pH goes beyond 3.8 due to coagulation, a dimer is formed which reduces the mobility of thorium ions and hence lowering the adsorption. 2~

The effect of thorium concentration on the adsorption was studied under the optimized conditions of 30 minute shaking time, 100 mg of solid and pH (3.8). The concentration of thorium was varied from 500 to 2500/~g/ml. The results in Fig. 4, show that the adsorption and distribution coefficient (KD) remains constant up to 1000/~g[ml and then decreases as the thorium concentration increases. The increase of thorium concentration contributes to the formation of larger polymer particles that lead to the lowering of adsorption. The results are then analyzed in terms of FREUNDLICH,21 LANGMUIR 22 and DUBININ-RADUSHKEVISH (D-R) 23 iso- therms. The data does not fit the FREUNDLICH and D - R equations. A straight line was obtained, Fig. 5, when Cs/X/m was plotted against Cs, the equilibrium con. centration of thorium. It follows that the adsorption of thorium on activated charcoal obeys LANGMUIR's isotherm.

Adsorption dependence of thorium on temperature was investigated. The temperature was varied from 10 to 60 ~ in steps of 10 ~ while other parameters were kept

326

RIAZ QADEER et al.: SELECTIVE ADSORPTION OF THORIUM

o

0

0

~ E ~.~

0

I I I I I 1

I I I I I I

I I I I I

I I I I

I I I I

O O O ~

327

RIAZ QADEER et al.: SELECTIVE ADSORPTION OF THORIUM

.~ 110 c" Th in presence of .2 �9 Cs 0.599"~ ~" 100 m Rb 0.6801 o%,

= K 0.752~ Z/ r 90 o No 1.031|

�9 Li 1A71 J A Pure thorium

80

70

60

50

I --1 "~~ 000 [ I I I 1500 2000 2500 3000 Th concent ro t ion, ,ug/rnl

Fig. 8. Variation of percentage adsorption of thorium in presence of monovalent cations

constant. Figure 6 shows that distribution coefficient (K D) values increase with the

rise in temperature. Similar observations have been reported earlier. ~ 4 This increase

may be due to a negative temperature coefficient or to a steep simultaneous decrease

of real adsorption of solvent. 2a The values of AH ~ and AS ~ are calculated from the

slopes and intercepts of the linear variation of In K D with reciprocal temperature,

l/T, Fig. 7, using the relation:

A S o /~ H ~ In KD = - - (1)

R RT

The values are given in Table 1. The free energy of specific adsorption A G o is

calculated using the equation

AG O = AH ~ - - TAS o (2)

328

RIAZ QADEER et al.: SELECTIVE ADSORPTION OF THORIUM

s G o values are also given in Table 1. Positive values o fAH ~ and decrease in the value of

~x G O with rise in temperature show that the adsorption is more favorable at high

temperature. The influence of monovalent cations (Li, Na, K, Rb and Cs) and cations of dif-

ferent valencies (Na, K, Co, Zn, Ce and Cr) on the adsorption of thorium on activated

10 ~ "~ Th in of presence

o Cs 0.599] ~" �9 Rb 0.680[

/

a K 0.752~ Z/r �9 Li 1.471 |

/

�9 Na 1.031 J zx Th witnout

103 cations

102 i"

~ ' ' 1500 2000 2500 3000 Th concentrotion, ug/mt

Fig. 9. Variation of KD for thorium adsorption in presence of monovalent cations

charcoal was also investigated. The concentration of each cation was fixed at 1000/~g/ml and the thorium concentration was varied from 1000 to 3000/ag/ml.

The results of these investigations are shown in Figs 8-11 . It is seen that the greater the ionic potential (Z/r) of the added cation, the smaller is the adsorption of thorium, potassium being the exception. Similar observations have been reported earlier)9

To examine the adsorption behavior of thorium in the presence of thiosulfate, iodide, chloride, bromide, nitrate and EDTA, the concentration of each anion was taken as 1000/lg/ml and the concentration of thorium was varied from 1000 to 3000 #g/ml. The results are shown in Figs 12 and 13. The thiosulfate ion enhanced

329

RIAZ QADEER et al.: SELECTIVE ADSORPTION OF THORIUM

8

1 .e. ~ g

~ ~ o o ~ ~ m ," mJ ~

o lli:, ~ . ~ ~ o ~ o ~ ~.~ CI E ~' ~ 0 :~ ~ '

z N U U t9 ,,,"a_ A ' / ' a

///l,';, - -

K �9 o a s <J ~1 o ~ ~

' ' , i I l l l l I t I t I h J l I I I I I . ~

,~/Tt ~ o ~ ~

~" ~ / / / '7/ / / ~ a

oo / ~ / ~

~,o.o..~ ///// I o ~~

~g

N ~

.~ I ~ ~ ~ i I 1 I l o ~ >~3 o ~ o o o o o o o~ o~

�9 /0 ' u o ! ~ d J o s p v u~

330

RIAZ qADEER et al.: SELECTIVE ADSORPTION OF THORIUM

8

o .=. ~ . ~ r i ~t#~ -I <~ ~ ~ I I #lll | <" "

~ 1 7 6 I Fi l l ~ 0 I " ' ~ I,- ~ O0 ~ i~ ~ '~. n ,.. L 0 Q ~ J ~ - 0

U ~

~ _~ ~ =

I I I I I I

~/jw ,0~

If; / 3 1 / 7 ' I �9 ~a ~ ql{ t~ <> ~ ~_ , , . / V / l # ~> .~ "-""~176 J M.#' "! c ~ L m : z " ' u a-

4

i . i

e 4 o o

% ' u o ! I d J o s p v

331

RIAZ QADEER et al.: SELECTIVE ADSORPTION OF THORIUM

c"

K

A

100

90

8 0 -

7 0 -

60

5 0 -

3 0 -

2 0 -

10 -

0 Th Cs Cd Co Ba Mn Zn Cr Sr Cu

Metals

Fig. 14. Percentage adsorption of Th and other metals from a mixture containing Th, Cs, Cd, Co, Ba, Mn, Zn, Cr, Sr and Cu

A : op

.O-LA o H20

30 r 1 �9 3M HNO3

2O

1 ~.

20 30 40 50 60 70

Volume of etuont, mL

Fig. 15. Elution of adsorbed thorium from activated charcoal with H~ O and 3M HNO 3 solution

3 3 2

RIAZ QADEER et al.: SELECTIVE ADSORPTION OF THORIUM

Table 2 Percentage adsorption and distribution coefficient (KD)

of other metals on activated charcoal

Metals* Adsorption, *~ % KD, ml/g

Pb 81.45 439.08 Er 74.75 296.04 U 72.20 259.71 Th 69.75 230.58 Sm 67.30 205.81 Eu 66.85 201.66 Dy 65.40 189.02 Gd 64.35 182.09 Hg 56.40 129.36 La 49.50 98.02 Zr 41.10 69.78 Y 36.50 57.48 Ce 24.40 32.28 Fe 15.05 17.72 Ga 14.00 16.28 V 12.80 14.68 Cs 9.55 10.56 Ba 9.05 9.95 Zn 8.00 8.70 Sr 5.75 6.10 Cr 5.50 5.82 Cd 4.60 4.82 Co 2.65 2.72 Cu 1.95 1.99 Mn 0.10 0.10

*Concentration of all cations fixed at 2000/~g/ml. **The order of entries is on the basis of decreasing

tion. ad so rp -

the adsorption of thorium while the other anions reduce the adsorption. This shows

that the anionic complex of thiosulfate with thorium is more strongly adsorbed on

the activated charcoal than the thorium ions themselves. The decrease in adsorption

in the presence of I-, CI-, Br-, NO~ and EDTA ions may be due to the lower affinity

o f their complexes for adsorption. The anions reduce the adsorption in the order

of EDTA > NO~ > C1- > Br- > I-. The effect of fluoride, phosphate and oxalate

ions on the adsorption of thorium could not be studied because of their insolubility.

To check the selectivity of activated charcoal for adsorption of thorium, the ad-

sorption of U, Sm, Gd, Er, Eu, Pb, Dy, Hg, Zr, La, Y, Ce, Fe, V, Ga, Cs, Co, Ba,

Cr, Sr, Cd, Cu, Mn and Zn on the adsorbent was examined under the opt imum con-

333

RIAZ QADEER et al.: SELECTIVE ADSORPTION OF THORIUM

ditions. The results are given in Table 2. It is obvious from the data that U, Sin, Gd,

Er, Dy, Eu, Pb, Hg and La have rather high value o f KD and adsorption, hence would

be coadsorbed along with thorium on activated charcoal. Zr, Y, Ce, Fe, V, Ga, Cs, Co,

Ba, Cr, Sr, Cd, Cu, Mn and Zn are poorly adsorbed; hence, separation of thorium

from these metals can be achieved. The separation factor for thorium is larger in the

presence of Cs, Co, Ba, Cr, Sr, Cd, Cu, Mn and Zn, because they have much lower

K D values. The separation of thorium in the presence of these metals is shown in

Fig. ~ 14. The feasibility o f using activated charcoal for preconcentrat ion of thorium

was further assessed by elution studies. These studies were performed with H20~'and

3M HNOa solutions. Results as summarized in Fig. 15.

The authors are grateful to Mr. Abdul MAJEED, Technical Manager, for cooperation and encouragement during this work. Services of Mr. M. KHAN are highly acknowledged. Thanks are also due to Mr. RABBANI for typing the manuscript.

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