Thorium interaction with the silanol group, water, and nitrate in the pores of silica gel

JOURNAL OF COLLOID SCIENCE 18, 8 7 8 - 8 8 5

(1963)

THORIUM INTERACTION WITH THE SILANOL GROUP, WATER, AND NITRATE IN THE PORES OF SILICA GEL 1 Jimmy Stanton and Russell W. Maatman 2 University of Mississippi, University, Mississippi Received February 25, 1963; revised June 3, 1963 ABSTRACT Solvation of T h +4 is observed b y measuring the exclusion of Th(C104)4 from 50% of the pore volume of a high surface area silica gel u n d e r conditions of no cation exchange. C o m p e t i t i o n between T h +4 ions for water molecules increases as the conc e n t r a t i o n is increased from 0 to 0.4 M. E n o u g h water of h y d r a t i o n is removed from T h +4 h y d r a t e b y HC104 to increase t h e pore volume availability to 100% in 8 M HC104. If HNO~ replaces HC104 at 6-8 M acid, the size of the t h o r i u m species increases markedly, presumably owing to Th+4--N03 - interaction. If, however, HNO3 replaces HC104 a t 1.5-2.7 M acid, a n y Th+4--NO3 - i n t e r a c t i o n merely replaces Th+4--H~O i n t e r a c t i o n and a size change is not observed. The t h e r m o d y n a m i c values for the exchange T h +4 + 4 SiOH ~ Th(OSi)4 + 4 H + are: AF~gs = 8.3 kcal., AH ° = 11.0 kcal., and AS~gs = 9.1 e.u. INTRODUCTION

Previously two aspects of the behavior of a cation in the pores of a high surface area silica gel have been investigated (1). First, the per cent of the pore volume available to the ion has been determined and deductions concerning the size of the ion hydrate have been made. Second, the exchange of the cation with the hydrogen of the surface silanol group without foreign cations present has been studied. Procedures and techniques developed in the earlier work have been extended to a study of these two aspects for the aqueous thorium ion.

1. Pore Volume Availability When a solution and a small pore gel are mixed and equilibrium is achieved, the solution external to the gel particles is not necessarily the same concentration as it was before mixing. Reaction of solute or solvent with the surface will of course affect concentration, but even if there is 1 P r e s e n t e d a t the American Chemical Society M e e t i n g before t h e Division of Colloid a n d Surface C h e m i s t r y at the A t l a n t i c City meeting in September, 1962; presented to the g r a d u a t e faculty of The University of Mississippi by J i m m y S t a n t o n in partial fulfillment of t h e requirements for the P h . D . degree. To whom correspondence concerning this paper should be addressed at D o r d t College, Sioux Center, Iowa. 878

THORIUM INTERACTION IN THE PORES

OF S I L I C A G E L

879

no reaction there will be a concentration change if the solute and solvent particles are not the same size. The smaller particle is able to enter pores the larger particles cannot enter, and in addition the center of the large particle will not be able to approach a pore wall as closely as the center of the smaller particle. This is discussed in detail in earlier work (1, 2), where the per cent availability (the per cent of the pore volume the solute has access to), A, is shown to be

_-

÷ 11

100,

Ill

when reaction with the surface is not a complicating factor. Equation [1] holds for experiments in which V ml. of solution of concentration C~ are mixed with W g. of gel of pore volume P ml./g., and the final concentration is Cj. When silica gel is the small pore material, surface reaction--assumed to be ion exchange--has been suppressed with excess acid. Under such conditions ion solvate radii have been determined (2). In the present work the effect of large concentrations of HCI04 on Th(C10,)4 solutions is reported, and it is concluded that the large excess of hydrogen ion removes some water from the hydration sphere of T h +4. In addition, when HNO~ replaces HC104 at a high acid level, the T h +4 species becomes larger, suggesting that Th+*--NO~ pairs are forming. 2. Reaction of Th +~ with the Silanol Surface

The study of the reaction of T h +4 with the surface of silica gel is a part of a larger program to study the reactions of many cations on several different oxide surfaces. The reaction with silica gel is an ion exchange reaction, M +" -t- m SiOH ~ M(OSi): -m q- m H +

[I]

Since the silanol group is a very weak acid, Reaction (I) is easily repressed by an excess of acid, and the A values discussed above can then be determined. But this also means the acid level must be low in order that the reaction be studied. Under such conditions hydrolysis of the cation may be a complicating factor, and the choice of pH range to use must take these two conflicting factors into account. In the thorium perchlorate solutions of the exchange studies the pH was never above 1.34; Kraus and Holmberg (3) indicate that at a pH of 2, with 0.015 M T h +4, there is essentially no hydrolysis. The hydrolysis constants of Pan and Hseu (4) indicate that at no ionic strength would there be more than 1% of the thorium hydrolyzed in a solution of p H = 1.34. In addition, the adsorbed thorium species is not a hydrolyzed form of thorium, since we show for thorium at low concentration n = m = 4 in Reaction (I). The free energy, heat, and entropy of l~eaction (I) for T h +4 were de-

880

STANTON AND MAATMAN

termined by the method developed in the study of the reaction of the uranyl ion with the silica gel surface (l(b)). EXPERIMENTAL

1. Materials The silica gel is the same as was used in the uranyl ion studies (l(b)). This is Davison Code 40 Silica Gel, 6-12 mesh, washed with nitric acid, followed by a water wash, dried, and finally heated for 2 hours at 450°C. in air. The acid wash removes metal oxide and cationic impurities (5). The pore volume of the acid washed gel is 0.40 4- .01 ml./g., and the surface area is 498 m3/g. (by the B. E. T. method3). The other chemicals were of reagent grade, except for Th(Cl04)4 (from K and K Laboratories), which contained a little HCIO~.

2. Procedure Two types of experiments were carried out. In each type of experiment 25 ml. of solution was mixed with 15 g. gel and the mixture was allowed to stand, with frequent shaking, for at least two days. In the one type of experiment acid was added to the salt-water-gel system to repress exchange with the surface; here the purpose was to determine the size of the thorium hydrate under various experimental conditions and to observe ion-pairing tendencies. These experiments were carried out at room temperature. In the other type of experiment acid was not added, and the equilibrium constant for the exchange of thorium with surface silanol hydrogen was determined. These experiments were carried out at several temperatures, and the procedure is the same as was used earlier (l(b)).

3. Analytical Thorium was analyzed by EDTA titration, using alizarin sodium monosulfonate as indicator. The determination of the hydrogen ion freed from the surface, as distinguished from other hydrogen ions, was determined by the titration procedure already described (l(b)). RESULTS AND DISCUSSION

1. Availability of Thorium Species The availability of 0.05-0.4 M Th(CI04)4, in 0.5 M HCI04, is given in Fig. 1. The increase in A, or decrease in size of the thorium species, with 3 Previously the surface area of the gels used was determined by the m e t h o d referred to in Reference l(a). B. E. T. determinations indicate the previous values wrong, and for the uranyl-silica gel reaction the corrected free energy and entropy of reaction at 298 ° K. are 7.0 kcal. and - 1 2 e.u., respectively, instead of 8.2 kcal. and --16 e.u. reported in Reference l(b). The heat of reaction is not affected. The correction does not alter the conclusions which were made.

THORIUM INTERACTION

IN THE PORES OF SILICA GEL

881

increasing concentration is probably due to the greater cation competition for water at the higher concentration. In Fig. 2 A values for 0.2 M Th(ClO4)4 in perchloric acid varying from 0.5-8 M are given. The strong acid effect probably reflects competition between H + and T h +~ for water molecules of solvation. The curve is comparatively flat in the region of 0.5 M acid, indicating that the 0.5 M experiments of Fig. 1 are not complicated by H + - - T h +4 competition for water. T h + ~ - N O ~ complex ion formation is known to be significant even at low concentrations (6). Experiments were carried out varying the nitrate concentration at 0.2 M thorium concentration. The thorium was introduced as the perchlorate or the nitrate, and additional nitrate as nitric

80

0.5 M. HClO4

_ ~ •

60 A,%

40

2%

I

oi,

I

o.2

o14

M., Th (ClO4)

J~'IG. 1. Per cent availability of Th(CIO4)4 vs. equilibrium Th(CIO4)4 molarity, at 0.5 M HC104.

I00

J 80 A,% 60

O ~

40 I

I

I

2

4

6

M., HCIO4

Fro. 2. Pet' cent availability of 0.2 M Th(CI()4)~ vs. IICI04 molarity.

_L__

8

882

STANTON AND MAATMA_N

°°I 8°t

•

A,%

70t

O

• 6-8 M.•H+

i

I

i

2

4

6

M.

,

N0 3

FIG. 3. Per cent availability of 0.2 M Th(CI04)4 in HCIO4--HN03 mixtures, vs.

HNO3 molarity, at constant total acid. acid. Since there is also an acid effect (Fig. 2) the acid level was held approximately constant by adding enough perchloric acid to the nitric acid to achieve the same acid level in a given set of experiments. There were two sets of experiments, one at a total acid level of 1.5-2.7 M, in which the nitrate varied from 0-2.3 M, and another at a total acid level of 6-8 M, in which the nitrate varied from 0 to 7 M. The results are in Fig. 3. If an ion hydrates more as the charge density at its "surface" increases, then as T h +4 adds one or more nitrate ions, the greater size and decreased net charge would make the hydrate smaller and increase the availability. There may be a slight tendency in this direction in the lower curve of Fig. 3. The nitrate concentration cannot be made high enough to ascertain if there is a real trend. Of considerably more interest is the upper curve. The acid level is very high, and without nitrate there is apparently much less hydration than there is without competing hydrogen ion. In such a situation nitrate addition, with the formation of Th(NO3) +3, Th(NO3) +2, etc., actually brings about increased ion size, indicated by the striking drop in the upper curve. Possibly NOT does not replace H20 in the T h +4 co-sphere here.

2. Exchange with the Surface The value of m in Reaction (I) is shown to be four at low thorium concentrations by Fig. 4, which gives the number of hydrogen ions released per pore volume thorium ion. The ordinate value in the figure is a lower

THORIUM INTERACTION IN THE PORES OF SILICA GEL

883

4.0

5.0

H~T~ 4 2.0

l.O

0.fi

0

012

0~.1

0.5

M., Th {C104) 4 FIG. 4. N u m b e r of H + released f r o m s u r f a c e p e r T h +4 in p o r e s vs. e q u i l i b r i u m T h (C104) 4 lnolarity.

limit to the number released per reacted thorium ion. Thus for low concentrations, assuming SiOH and Th(OSi)4 constitute a surface solid solution, the equilibrium constant is

K---l-° 404

EE

o+

C~tI+ ,

CTh+"

[~]

where 0 is the fraction of sites in the hydrogen form. The log K values at 20°C. are plotted against equilibrium thorium concentration in Fig. 5. The scatter is due almost entirely to small errors in the measurement of pH: a pH error causes the error in log K to be four times as large. At finite concentrations the system is nonideal and consequently the extrapolated, infinite dilution log K is used. Therefore the standard state for SiOH is the surface fully covered with hydrogen and the Th(OSi),~ standard state is a hypothetical surface fully covered with thorium, with no interaction between sites. The temperatures and extrapolated log K values, respectively, are: 5°C., -6.68; 20°C., -6.34; 35°C., -5.91; 50°C., -5.43; 65°C., -5.19. A

884

STANTON AND MAATMA_N

-4.5

•

log 15.0 -5.5

-6.0

•

0

• / • !

!

0.1

0.2

0.3

M., Th(Cl04) 4 FIG. 5. Apparent log K for exchange reaction vs. equilibrium Th(CIO4)4 molarity. plot of log K vs. 1/T is linear, and the thermodynamic values at 298°K. are: AF °, 8.3 ± 0.2 kcal.; AH °, 11.0 ± 1.0 kcal.; AS °, 9.1 ± 3.0 e.u. Errors are estimated from plots of the type of Fig. 5, and the log K vs. 1/T plot; the error in AS ° is cumulative. In the uranyl ion study the results were consistent with a model of the surface complex in which the linear uranyl ion lay in a "trough" on the surface, parallel to and between rows of hydroxyl groups; the silica surface was assumed to have the cristobalite structure (7). The centers of four adjacent silanol groups, of 1.26 A. radius before H + removal, lie at the corners of a rectangle 5.04 A. × 2.52 A. A symmetrical placement of T h +4, of 0.95 A. crystal radius, assumed spherical, in this rectangle would mean there is a gap of 0.65 A. between the metal ion and the four silanol groups if these groups do not move or change size as they bond to thorium. Probably some surface distortion can lead to a fairly stable system. This work was supported under Atomic Energy Commission Contract No. AT-(40-1)-2759. REFERENCES

1. (a) DALTON, R. W., McCLANAHAN, J. L., AND MAATMAN, R. W., J. Colloid Sci17,207 (1962). (b) STANTON,J., AND ~/IAATMAN,R. W., ibid. 18, 132 (1963). (c) MC.

THORIUM INTERACTION

IN THE PORES OF SILICA GEL

880

CONNELL, B. L., AND MAATMAN, R. W., J. Mississippi Acad. Sei. 8, 169 (1962).

2. 3. 4. 5. 6.

7.

(d) GABRIEL, E. D., YAO, K. Y., AND MAATMAN, a. W., J. Mississippi Acad. Sci., 8, 185 (1962). (e) DANIEL, J. L., NETTERVILLE,J., AND MAATMAN, R. W., J. Mississsippi Acad. Sci. 8, 193 (1962). MAATMAN, R. W., NETTERVILLE, J., HUBBERT, H., AND IRBY, B., J. Mississippi Acad. Sei. 8, 201 (1962). KRAUS, K. A., AND HOLMBERG, R. W., J. Phys. Chem. 58, 325 (1954). PAN, :K., AN]) HSEU, T. M,, Bull. Chem. Soc. Japan 28, 162 (1955). AHRLAND, S., GRENTHE, I., AND NOREN, B., Acta Chem. Scand. 14, 1059 (1960). (a) DAY, R. A., JR., AND STOUGHTON, l:~. W., J. Am. Chem. Soc. 72, 5662 (1950). (b) ZEBROSKI, E. L., ALTER, H. W., AND HEUMANN, F. D., J. Am. Chem. Soc. 73, 5646 (1951). ILER, R. K., "The Colloid Chemistry of Silica and Silicates," p. 243. Cornell University Press, Ithaca, New York, 1955.

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