Uptake of thorium ions from aqueous solutions by a molecular sieve (13X type) powder

Journal of Radioanalytical and Nuclear Chemistry, Articles, Vol. 190, No. 1 (1995) 103-112

UPTAKE OF THORIUM IONS FROM AQUEOUS SOLUTIONS BY A M O L E C U L A R SIEVE (13X TYPE) POWDER

IL QADEER, J. HANIF, I. HANIF

Pakistan Institute of Nuclear Science and Technology, P.O. Box 1356, Islamabad (Pakistan)

(Received November 1. 1994)

The adsorption of thorium(IV) ions on molecular sieve (13X type) powder from aqueous solutions has been studied as a function of shaking time, pH, thorium ion concentration and temperature. The conditions of maximum adsorption of thorium ions obeys Langmuir and D-R isotherms over the entire concentration range studied. Thermodynamic quantities such as AH, ziG and AS have been calculated from K D values determined at various temperatures. The results show endothermic heat of adsorption, but negative free energy value indicates that the process of thorium adsorption on molecular sieve powder is favored at high temperature. The influence of various cations and anions on thorium(IV) ion adsorption was examined. A wavelength dispersive X-ray fluorescence spectrometer was used for measuring the thorium ion concentration in solutions.

Molecular sieve ~ is applied to the natural and synthetic crystalline alumino-silicates which are also named crystalline zeolites, Zeolites are hydrated alumino-silicates com- posed of SiO 4 and A104 tetrahedra in which every oxygen is shared between two tetrahedra, so that (AI + Si) to oxygen ratio is 0.50. These alumino-silicates have rigid ionic framework containing well defined channels (pores) and cavities. These cavities are inter- connected in one, two or three dimensions..These contains mobile exchangeable cations, that can be exchanged according to valency principle by other cations without deslroying the alumino-silicate framework. The water molecules in the alumino-silicate framework acts as guests and can be outgassed continously over a wide temperature range, leaving an open empty porous structure which can be filled by other suitable molecules. 2 Molecular sieves are widely used as adsorbents or catalysts. Nevertheless their behavior depends on their specific surface area, their partial or total ax, ailability to molecules, and to the chemical state of their surface, which in turn are highly dependent upon the initial outgassing, extent of dehydration and other parameters such as pore volume, porosity and pore size and pore surface distributions. In this paper,~an attempt was made to check the efficiency of molecular sieve (13X type) powder for the removal of thorium(IV) ions from aqueous solutions and to determine the optimal conditions required for its preconcentration. The preconcentration/separatiori of thorium(IV)ions through adsorption phenomenon is important ' in nuclear/radiation chemistry, environmental and waste treatment points of view. The adsorption of thorium ions on glass , 3 tungsten, a fibrin, 5 silica gel, 6 resin, 7 tantalum, s activated charcoal 9 has been examined previously, no data were available for its adsorption on molecular sieve.

Elsevier Science S. A., Lausanne Akad~miai Kiadt, Budapest

R. QADEER, J. HANIF: UPTAKE OF THORIUM IONS

Experimental

Molecular sieve 13X type (Aldrich; item No. 20863-9) was ground to fine powder in Retsch's ball mill. The particle size of the powder was determined by sieve analysis and comes out to be 165 + 6 pro. The dehydration of the molecular sieve powder was carried out using simultaneous thermal analyzer, STA 409 (NETZSCH). A known amount of molecular sieve powder was taken in A1203 crucible and its dehydration was carried out from room temperature to 700 ~ at a heating rate of 10 ~ The experiment was performed in nitrogen atmosphere; flow rate 200 ml/min.

Surface area of the powder was determined using Quantasorb (Quantachrom Corporation, N.Y.) by continuous flow method. 1~ Nitrogen gas was adsorbed on the sample at liquid nitrogen temperature from a gas stream of nitrogen and helium. It was then desorbed and the released nitrogen was measured by a thermal conductivity detector. The BET equation was used to calculate surface area and its value comes out to be 216 m2/g.

The pore volume and porosity of molecular sieve powder were measured by Autoscan-33 mercury porosimeter (Quantachrom Corporation, N.Y.). The mercury was intruded into the pores of molecular sieve as a function of pressare. The data was corrected for compression of mercury by taking blank measurement with mercury. The determined pore volume and porosity were 0.62 cm3/g and 61%, respectively. From the mercury inlrusion data, the pore size, pore surface and pore length distributions curves were obtained.

The adsorption of thorium(IV) ions on molecular sieve from aqueous solutions was carried out via a batch technique at room temperature (25 + 0.I ~ except where otherwise specified. Accordingly a solution of 10 ml of thorium(IV) ions of known concentration were added to 250 ml glass reagent bottle and shaken with 0.5 g of dry molecular sieve powder in a thermostat shaker, for a given time period. Thorium ion stock solution was prepared from thorium nitrate: Fluka, item No. 89150. The solutions were then filtered through Whatman filter paper No. 40 (circular, diam. 14.0 cm). The first 2-3 ml portion of the filtrate was rejected because of the adsorption of thorium(IV) ions by filter paper. The concentration of thorium(IV) ions in the filtrate was measured using a wavelength dispersive X-ray fluorescence spectrometer-SRS-200 (SIEMENS, Germany). The concentration of thorium(IV) ions was corrected for the loss of thorium(IV) ions through adsorption on the glass bottle by running blank experiments (i.e., without molecular sieve powder added). The percentage adsorption and distribution coefficient (Ko) were computed using the following relations:

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Adsorption= CO -.____~C . 100% (1) Co

P.. QADEER, J. HANIF: UPTAKE OF THORIUM IONS

and Ko = C o - C V C . -~- mug (2)

where C O and C ai'e the initial and final conentration of thorium(IV) ions in the solution; V is the volume of the solution (ml) and M is the weight of the molecular sieve powder (g).

Results and discussion

TG curve of molecular sieve powder shows that the weight loss occurs in a single step starting at temperature > 50 ~ and being complete at 700 ~ The total weight loss from 60 to 700 ~ is 17.75% out of which 17.5% occurs betwen 60-350 ~ This corresponds to the loss of 132 water molecules. The pore size distribution curve shows two maxima at 5.37 cma/A and 2.52 cm3//~ which corresponds to the pore radii of 51 A and 237 A, respectively. The pore surface distribution curve supports the above observation and shows a maximum at 2.3 m2/A which corresponds to pore radii 39.5 A. The same value is obtained from the pore length distribution maxima of 0.93 �9 109 cm//~. Thus it can be concluded that the

.~80

?0

~6o 5O

- -_ 'o,

60

50

t~ o ~o ~0

I I I I I 300 5 I0 15 20 25 3030

5hQking tirne,min

Fig. 1. Adsorption of thorium ions on molecular sieve powder as a function of shaking time

molecular sieve powder of the particle size 165 + 6 Ixm having a surface area 216 m2/g with pore volume 0.62 cm3/g and 61% porosity has a large fraction of

the surface area in the mesopores with an average pore radius of 39.5 A. Preliminary investigations were conducted to ascertain the time required for an

equilibrium between thorium(IV) ions and molecular sieve powder. This was performed by shaking 10 ml thorium(IV) ion solutions of 2.0 g/l with 0.5 g of molecular sieve powder for different intervals of time ranging from 2 to 30 minutes. Figure 1 shows the

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variation of percentage adsorption (%) and distribution coefficient (KD) with shaking time. This figure indicates that initially the adsorption of thorium(IV) ions increases rapidly, but then the process slows down and subsequently attains a constant value around 15 minutes, i.e., when the adsorption equilibrium is established. Therefore 15 minutes shaking time was selected for all further studies. In the initial stages the surface coverage is too low and the adsorptive species accumulate rapidly at the adsorbent surface and occupy active sites, resulting .in the higher uptake in the early shaking time. As a consequence, some portion of the active adsorbent sites may be blocked with the passage of time, hence the rate becomes slower and reaches equilibrium when the surface becomes almost saturated. The time required to accomplish equilibrium, i.e., 15 minutes, demonstrates that adsorption of thorium(IV) ions on molecular sieve involves fast processes.

The influence of pH on the thorium(IV) ion adsorption on molecular sieve powder was studied while the thorium(IV) ion concentration, shaking time and amount of molecular sieve powder were fixed as 2.0 g/l, 15 minutes and 0.5 g, respectively. The pH of the solution was varied from 1 to 6. Figure 2 shows the variation of percentage

IO0

i ,~ 60

0

80O

2 3 4 5 pH 6 ~0

2

Fig. 2. Influence of pH on the adsorption of thorium ions on molecular sieve powder

adsorption and distribution coefficient (Ko) with pH. The adsorption increases sharply upto pH 2 and attains a maximum value of 97%. A futher increase of the pH resulted in a linear decrease of adsorption over the pH range 3 to 6. Aqueous solution of pH 2 was selected for further investigations and found to be an effective pH to remove the thorium(IV) ions by molecular sieve powder from solutions. The influence of pH on the thorium(IV) ions may be explained as follows. Up to pH 2, the adsorption of simple Th4+ ions takes place. Over the pH range 3 to 6, Th4+ ions undergo hydrolysis resulting

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in the formation of hydroxy ions. These hydroxy ions are weakly adsorbed relative to

Th 4+ ions, hence the adsorption of thorium ions on molecular sieve powder decreases. Above pH 6, the adsorption process could not be followed because of the formation of insoluble thorium complexes.

The effect of thorium(IV) ion concentration on its own adsorption on molecular sieve

powder has been studied under optimum conditions of 15 minutes shaking time, pH 2

I \ �9

0, 0 1 2 3 ~ 5 6

Concentrotion of t h r u m , g. f ~

Fig. 3. The effect of thorium concentration on its own adsorption on molecular sieve powder

and 0.5 g molecular sieve powder. The concentration of thorium(IV) ions was varied from 1.0 g/l to 7.0 g/l. The results shown in Fig. 3 show that adsorption percentage and distribution coefficient (KD) decreased as the thorium ion concentration increased, indicating that energetically less favorable sites become involved in the process with increasing concentration. The data concerning the dependence of the adsorption on the thorium ion concentration were studied by Freundlich, Langmuir and Dubinin- Radushkevich (D-R) isotherms.

The Freundlich isotherm equation was used in the form: xl

F ~ = A �9 C 1In (3)

where /'Th is the amount of thorium ions adsorbed per gram of the molecular sieve powder, C is the equilibrium concentration of thorium ions in solutions, A and n are constants that can be related to the strength of the adsorptive bond and bond distribution, respectively. The Freundlich plot of log /'Th versus log C shown in Fig. 4, curve 1 demonstrates the non-validity of the equation over the whole range of thorium ions concentration.

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log c -25 -2.0

-0.5 I

P..., 2 - -

-1~ _--2.5 ~; S 1 -

C ,g. f'

-1.5 -1.0 - 0 5 0 05 1.0 I I 1 I I 7 0

O)

60v_.. 13.

5O T ,)--

zo,3

30

20

10

0

Fig. 4. Freundlieh (curve 1), Langmuir (curve 2) and D-R isotherm (curve 3) plots for the adsorption of thorium ions on molecular sieve powder

~es' I I I I ~ |

7 2 1

6

5

4

3

21 0 3J 3.1 32 33 3.4 35 3.6

I/Tjxl0~K -I

Fig. 5. Variation of K D with temperature (curve 1) and plot of In K D vs. 1/T for thorium ions adsorption on molecular sieve powder (curve 2)

Tern~rOue ,%.

10 2o 3o 4o 5o 6~

800 2

600

4o0

200

108


The Langmuir isotherm equation was applied in the form: n

C 1 E _ _ + _ _ ( 4 ) r ~ Kadsrm~x r~a~

where/"a~ and C have already been defined. / " ~ is the measure of the monolayer capacity and K,~ is the constant related to the heat of adsorption. In general F,~ x and Ka~ are functions of pH, ionic media and ionic slrength. A straight line may be obtained by plotting C/1-'Xh versus C (Fig. 4, curve 2), indicated the conformity of the data, Values of constants Fm~ x and/(ads calculated from the slopes and intercept of the plot in Fig. 4, curve 2 were 0.063 g/g and 19.06 l/g, respectively.

The linearized D-R isotherm equation is: 12

ln F ~ = ln F r ~ x - K" e 2 (5)

where e = R T In (1 + l/C), K" is the constant related to adsorption energy (mol2/kJZ), R the gas constant and T is the absolute temperature. The quantities F~,/"m~x and C have their usual meanings. A straight line is obtained on plotting In Fa~ versus e 2 as shown in Fig. 4, curve 3, indicating that thorium ion adsorption onto molecular sieve powder also obeys the D-R isotherm equation. Values of/'max and K' calculated from the intercept and slope of the plot were 0.061 g/g and 8.568.10 -3 molZ/kJ 2, respectively. From the values of K" it is possible to calculate the adsorption energy, E~, using the following equation: 12

E~ = ( - 2K') -1/2 (6)

where a value of 7.64 kJ/mol was obtained for the adsorption of thorium ions onto molecular sieve powder.

The dependence of thorium(IV) ion on the temperature was also investigated. The temperature was varied from 10 to 50 ~ in t0 ~ steps, while the thorium concentration, shaking time, pH and amount of molecular sieve were kept constant as 3.0 g/l, 15 minutes, 2 and 0.5 g, respectively. Figure 5, curve 1 shows that K D increases with an increase in adsorption temperature, which indicates that thorium(IV) ions dehydrate considerably at higher temperature before adsorption and thus their size during adsorption is smaller yielding a larger K D values. 13 The thermodynamic quantities such as AG, A H and z~S of thorium ion adsorption on molecular sieve powder were calculated from the K D values using the following relations:

A G = - R T In K o

In K D = - A H / R T + constant

(7):

(8)

109


and A S = ( A l l - A G ) / T (9)

The variation o f In K n with reciprocal temperature, 1/T is given in Fig. 5, curve 2.

F rom the s lope o f this curve, the quanti ty AH is calculated and is given in Table 1 along

with the de te rmined values o f ACT and AS. The posi t ive value o f A H show that thorium

ion adsorption on molecu la r s ieve is an endothermic process which is quite cona-ary to

the usual observat ion o f exothermici ty . Similar observations have been reported earlier

for the adsorption o f different metal ions on solids. 14 The possible explanation o f

Table 1 Calculated values of thermodynamic parameters for thorium(IV) ion

adsorption on molecular sieve powder

Temperature, ziG, AH, AS, K kJ. mo1-1 kJ �9 mo1-1 kJ �9 deg -1 �9 tool -1

283 -8.40 64.27 0.256 293 -9.36 0.251 303 -12.81 0.254 313 -14.96 0.253 323 -18.49 0.265

Table 2 Effect of different cations on thorium ion adsoption on molecular sieve

Cation* Z/r Adsorption, Distribution coeffi- % dent (KD), ml/g

Nil - 99.0 1980.00 Cs + 0.5988 98.00 980.00 Na + 1.0309 97.05 658.00 Li + 1.4706 96.75 595.00 Sr 2+ 1.7857 94.23 326.62 Ca 2+ 2.0202 88.06 147.50 Zn 2+ 2.7027 86.19 128.82 Co 2+ 2.7778 85.73 120,15 Ce 3+ 2.9013 82.16 92.11 Sm 3+ 3.1120 80.29 81.47 Gd 3+ 3.4052 78.37 72.46 Er 3+ 3.4052 77.23 67.83 Cr 3+ 4.7619 72.44 52.57

*Nitrate salts were used.

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endothermic heat of adsorption is given in our earlier paper. 14 The values of AG are

negative as expected for a spontaneous process. The decrease in AG values with increasing temperature reveals that thorium ion adsorption on molecular sieve becomes favorable at higher temperature, because thorium ions are more readily desolvated and hence its adsorption becomes more favorable. The AS values are positive and no

appreciable change in AS values is observed with increasing temperature. That means that the magnitude of AS is not affected by the temperature. The adsorption process is

Table 3 Influence of different anions on the adsorption of thorium on

molecular sieve powder

Anion* Adsorption, Distribution coeffi- % cient (KD), ml/g

Nil 99.0 1980.0 Acetate 97.8 889.0 Thiocyanate 96.0 480.0 Oxalate 92.0 230.0 Fluoride 88.4 152.4 Iodide 87.3 137.50 Citrate 85.9 121.84 Tartrate 85.3 116.05

*Sodium salts were used.

endothermic, so under these conditions the process becomes spontaneous because of positive entropy change.

The influence of various cations on the adsorption of thorium(IV) ions on the molecular sieve powder has also been examined. The concentration of each cation and

thorium(IV) ions were fixed at 1.0 g/l, respectively. The results of these investigations are given in Table 2. It is obvious from this table that the presence of these cations in solutions has reduced the adsorption of thorium ions on molecular sieve. The adsorption of thorium is lowered because these cations are co-adsorbed along with thorium on molecular sieve powder. It is also seen in Table 2 that cation with larger Z/r (ionic potential) reduced the adsorption of thorium more than the cations with smaller Z/r values.

We have also examined the adsorption behavior of thorium ions on molecular sieve in the presence of acetate, thiocyanate, oxalate, fluoride, iodide, citrate and tartrate. The concentration of each anion and thorium ions were taken as 1.0 g/l. The results are shown in Table 3. The presence of these anions induceda negative

111


efect on the adsorption of thorium on molecular sieve powder. The reduction of thorium ions adsorption in the presence of anions was in the order of tartrate > > citrate > iodide > fluoride > oxalate > thiocyanate > acetate. The decrease in adsorption of thorium ions in the presence of above mentioned anions may be explained with the lower afinity of their complexes for adsorption.

Conclusions

From the present study it can be concluded that the thorium adsoi'ption on molecular sieve powder obeys the Langmuir and D-R isotherm equations over the entire range of study. The adsorption equilibrium is achieved within 15 minutes. Higher temperature and acidic conditions favour adsorption, whereas the presence of different cations and anions compete with thorium ion adsorption. Hence, molecular sieve (13X type) powder having surface area 216 m2/g, porosity 61%, pore volume 0.62 cma/g, average particle diameter 165 + 6 gtm can be used for the fixation of thorium ions from thorium processing sites and from radioactive waste.

References

1. O. GRUBNER, P. JIRU, M. RALEK, Molekularsiebe, Deutscher Akademie-Verlag, Berlin, 1968. 2. M. AFZAL, H. AHMAD, Sci. Intl. (LHR), 2 (1990) 289. 3. J. F. KING, A. ROMER, J. Phys. Chem., 37 (1933) 366. 4. W. H. BRATI'IN, J. h, BECKER, Phys. Rev., 43 (1933) 428. 5. W. H. SEEGERS, A. M. NIEVI', E. C. LOOMIS, Science, 101 (1945) 520. 6. H. W. KOHLSCHUETYER, W. KATZENMAYER, Z. Anorg. AUgem. Chem., 329 (1964) 163. 7. C. TH. KAWASSIADES, O. CH. PAPAVASSILIOV, Chim. Chronika, 31 (1966) 74. 8. C. J. GALLAGHER, Phys. Rev., 65 (1944) 46. 9. R. QADEER, J. HANIF, M. SALEEM, M. AFZAL, J. Radioanal. Nucl. Chem., 157 (1992) 321.

10. S. KARP, S. LOWELL, A. MUSTACC!UOLO, Anal. Chem., 44 (1972) 2395. 11. B. E. REED, M. R. MATSUMOTO, Separ. Sci. Technol., 28 (1993) 2179. 12. K. AKSOYOGLU, J. Radioanal. Nucl. Chem., 134 (1989) 393. 13. M. SALEEM, M. AFZAL, F. MAHMOOD, A. ALl, Ads. Sci. Technol., 9 (1992) 17. 14.R. QADEER, J. HANIF, M. SALEEM, M. AFZAL, Colloid. Polym. Sci., 271 (1993) 83.

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Uptake of thorium ions from aqueous solutions by a molecular sieve (13X type) powder