Aging of Amorphous Silica in Salt Water Solutions - ACS Symposium

10

Downloaded by KTH ROYAL INST OF TECHNOLOGY on November 22, 2016 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/bk-1982-0194.ch010

A g i n g of Amorphous Silica i n S a l t W a t e r Solutions JOAN D. WILLEY University of North Carolina at Wilmington, Department of Chemistry, Wilmington, NC 28406

Attempts to determine the solubility of amorphous silica in salt water solutions at near neutral pH and 0 to 5°C or 22 to 25°C have yielded a wide range of values, which results in part from aging of the silica surface in contact with solution. This makes determination of an initial solubility for silica difficult. Low temperature aging in salt water solutions or seawater causes a decrease in surface area, in specific pore volume, and in solubility. Solubilities determined at pressures to 1000 atmospheres (1 x 10 pascals) indicate that aging causes an increase in density of the surface silica; this data also allows calculation of the partial molal volume of dissolved silica. Identification of specific processes involved with aging of an amorphous surface are necessary for understanding silica solubility. 8

The solubility of amorphous silica in salt water solutions at 0 to 25°C has been the subject of much study in recent years, and it is interesting that determination of such an apparently simple number can yield such a wide range of results (Table I). As an illustration of the problem, Willey (1) showed a plot of the solubility of amorphous silica in seawater at 0°C and at pressures from 1 to 1000 atmospheres pressure. A later study using the same experimental apparatus (2) reproduced the same plot. However, two months later during the next experiment with the same equipment and the same silica, a solubility decrease occurred at all pressures, and the pressure dependence became slightly different than in both previous studies. This aging effect caused a solubility decrease of approximately 20%. Two other pressure studies (_3, _4), which used different experimental techniques, reported results which agreed with the latter study after aging of the solid silica (Table II and Figure 1). 0097-6156/82/0194-0149S06.00/0 © 1982 American Chemical Society Falcone; Soluble Silicates ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Falcone; Soluble Silicates ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

1410 1050 1550

39.7 29.5 43.6

25

20 3 25

Artificial Artificial

0.9% NaCl + 0.1% NaHC0 1M NaCIO, 4 Artificial N a t u r a l sw N a t u r a l sw

Silica gel S y n t h e t i c opal Vitreous

Precipitated amorphous SiÛ2 S i l i c a gel Acid-cleaned Biogenic o p a l

Siever, 1962

Stober, 1967

Jorgensen, 1968

Kato, e t

Hurd, 1972

a l . , 1968

36.3

19 19

silica

sw

3

sw sw

tris

N a t u r a l sw 2.5% NaCl +

Acid-cleaned Biogenic s i l i c a

2290 1990 1830

64.1 56.0 51.5

25 25 25

1290

1500 1700 43 48

1160

Lewin, 1961

32.7

2

N a t u r a l sw

silica

Amorphous

1750 1760 1160

49.0 49.4 32.7

25 25 0

K i t a h a r a , 1960

ym

[Si] ppm

Τ OC

Solution

A r t i f i c i a l sw N a t u r a l sw N a t u r a l sw

Phase

Silica gel Silica gel Amorphous s i l i c a

Solid

Reported s o l u b i l i t i e s f o r amorphous s i l i c a i n seawater and i n aqueous s a l t water s o l u t i o n s s i m i l a r to seawater. The s o l i d phase d e s c r i p t i o n comes from the primary r e f e r e n c e , "sw/" r e f e r s t o seawater; " t r i s " r e f e r s to t r i s b u f f e r . Temperature i s i n d i c a t e d by T. The d i s s o l v e d s i l i c a c o n c e n t r a t i o n , Q>i] , i s reported as ppm S i and ym.

Krauskopf, 1956

Reference

Table I .



continued.

Silica gel

Amorphous s i l i c a

Silica gel


Jones, et a l . , 1973

W i l l e y , 1974

M a r s h a l l , 1980

W i l l e y , 1980

*


Aged amorphous

Many other s o l u t i o n s reported a l s o .

G r i f f i n , e t a l . , 1981

Colloidal

H e r , 1973 silica

Acid-cleaned Biogenic opal

S o l i d Phase

Hurd, 1973

Reference

Table I .

+ + + +

0.7 M NaCl

3

3

tris tris tris tris

0.9% NaCl + 0.1% NaHC0 0.9% NaCl + 0.1% NaHC0

1 M NaCl*

N a t u r a l sw

A r t i f i c i a l sw

0.015 N NaCl

N a t u r a l sw 0.7 M NaCl N a t u r a l sw 0.7 M NaCl

Solution

2

2 22 2 22

25

0

2

25

23 23 3 3

T °C


24.7

31.0 46.6 26.6 36.8

54.3

30.5

26.3

45

44 44 26 25

ppm

879

1100 1660 947 1310

1930

1090

936

1600

1600 1600 930 890

ym


Baker S i l i c i c

Baker S i l i c i c

Baker S i l i c i c

Baker S i l i c i c

W i l l e y , 1975

W i l l e y , 1980

W i l l e y , 1980

G r i f f i n , et a l . , 1981

•

Baker S i l i c i c

Acid

Acid

Acid

Acid

Acid

Baker S i l i c i c A c i d

W i l l e y , 1974

W i l l e y , 1975

Matheson, Coleman and B e l l Chromat ographic Absorbent S i l i c a Gel

S o l i d Phase

Jones and Pytkowicz, 1973

Reference

none

size fractionate to remove Ρ i s pressure i n atmospheres, Δν i s the change i n volume caused by the r e a c t i o n , R i s the u n i v e r s a l gas constant, Τ i s temperature ,_jAk^ i s the change i n c o m p r e s s i b i l i t y caused by the r e a c t i o n , V i n d i c a t e s p a r t i a l molal volume, and subs c r i p t s r e f e r to pressure i n atmospheres. Determination of Δν^ would a l l o w c a l c u l a t i o n of Vgj; (QH) A(aq) » q u a n t i t y necessary f o r p r e d i c t i n g the d i r e c t i o n or tne pressure e f f e c t f o r s e v e r a l geochemical r e a c t i o n s which are d i f f i c u l t to i n v e s t i g a t e ex p e r i m e n t a l l y , f o r example, m i n e r a l formation a t sea f l o o r con d i t i o n s . However, t h i s c a l c u l a t i o n demands accuracy i n know ledge of the atmospheric pressure s o l u b i l i t y , and confidence i n the r e p r o d u c i b i l i t y of the pressure versus s o l u b i l i t y p l o t s . a


156

SOLUBLE

SILICATES

Therefore, the causes of the v a r i a t i o n i n p u b l i s h e d s o l u b i l i t i e s and p r e s s u r e dependencies o f the s o l u b i l i t y must be understood, and that i s the i n t e n t i o n o f the present study.


Experimental Methods Important experimental c o n d i t i o n s used to o b t a i n the data considered i n t h i s paper a r e given i n T a b l e s I and I I . The v a r i o u s methods used to determine s i l i c a s o l u b i l i t y are d e s c r i b e d i n d e t a i l i n the primary r e f e r e n c e s . The s u r f a c e area measure ments were made u s i n g n i t r o g e n a d s o r p t i o n ; two measurements were made f o r comparison u s i n g a Sears t i t r a t i o n w i t h 0.1 Ν NaOH. Other s u r f a c e measurements were made u s i n g n i t r o g e n a d s o r p t i o n studies. R e s u l t s and D i s c u s s i o n When s o l i d amorphous s i l i c a i s p l a c e d i n contact with s a l t water, the s o l i d s i l i c a s u r f a c e undergoes s i g n i f i c a n t changes which can a f f e c t experimental r e s u l t s . The amount of time r e q u i r e d f o r changes to be observed i s not c l e a r l y e s t a b l i s h e d , however, the necessary d u r a t i o n o f contact w i t h s o l u t i o n i s prob ably i n the range from days (8) to months (2) a t temperatures below approximately 25°C. S e v e r a l s t u d i e s of amorphous s i l i c a exposed to aqueous s o l u t i o n have r e p o r t e d a decrease i n s u r f a c e area w i t h time (8-12). W i l l e y (2) r e p o r t e d a decrease i n s u r f a c e area o f amorphous s i l i c a which o c c u r r e d along w i t h a s o l u b i l i t y decrease. A d d i t i o n a l analyses o f the s i l i c a sample used i n W i l l e y s . (2) study show t h a t the s u r f a c e area decrease and s o l u b i l i t y decrease occurred along w i t h a decrease i n s p e c i f i c pore volume (Table I I I ) . The agreement between the r e s u l t s of the BET s u r f a c e area determinations and the two Sears t i t r a t i o n s shows that the experimental s i l i c a d i d not have a s i g n i f i c a n t p r o p o r t i o n of pores s m a l l e r than the n i t r o g e n molecules. No c l e a r trend was observed f o r mean pore diameter o f the s o l i d s i l i c a , although l o g i c a l l y t h i s should i n c r e a s e w i t h decreasing s u r f a c e a r e a . These data are presented i n T a b l e I I I along w i t h i n f o r m a t i o n about b i o g e n i c s i l i c a (11, 12) f o r comparison. The s i m i l a r i t y i n trends and numbers f o r the c h e m i c a l l y pure amorphous s i l i c a com pared w i t h the b i o g e n i c s i l i c a suggests t h a t t h i s i n i t i a l aging step may be p a r t of the process t h a t occurs i n nature as b i o g e n i c s i l i c a changes w i t h time i n sediments. I n d i r e c t evidence suggests that the d e n s i t y of the s i l i c a s u r f a c e a l s o i n c r e a s e s as a r e s u l t of aging although the d e n s i t y of the b u l k s i l i c a does not change (Table I I ) . W i l l e y (2) c a l c u l a t e d Δν-L f o r aged and f r e s h s i l i c a based on p r e s s u r e s t u d i e s and found a change to a l e s s n e g a t i v e number. Since the other p a r t s of the r e a c t i o n (water and d i s s o l v e d s i l i c a ) cannot have changed i n volume, t h i s i s i n t e r p r e t e d to i n d i c a t e a change i n 1


Falcone; Soluble Silicates ACS Symposium Series; American Chemical Society: Washington, DC, 1982. J

* ** *** 1

1

250 2-20

solution 110* 64**

320 90

1

0.45 0.00

0.43 0.26

0.51 0.46

J

2

A

Specific Pore Volume cm3 g-1

Sears t i t r a t i o n 110 τατ g " Sears t i t r a t i o n 80 m g " data from Hurd and Theyer (11) and Hurd, Wenkam, Pankratz, and Fugate (12)

Biogenic s i l i c a i n seawater*** Recent Aged 40 m i l l i o n y e a r s

Dehydrated s i l i c a i n 0.9% NaCl + 0 . 1 % NaHCO Initial Aged > 60 days

Hydrated s i l i c a i n seawater Initial Aged 3 y e a r s

2

Surface Area m g"

17 17

4 20

Mean Pore Diameter nm

Table I I I . Surface c h a r a c t e r i s t i c s and s o l u b i l i t i e s f o r amorphous s i l i c a before and a f t e r a g i n g i n s a l t water, and f o r recent and 40 m i l l i o n year o l d b i o g e n i c s i l i c a .


1.0 0.1 - 0.8

1.10 0.87

1.09 0.88

Solubility mmol kg""-*0-3°C

5*

I

s

4'

w

SOLUBLE

158

SILICATES

the s i l i c a s u r f a c e to a higher d e n s i t y phase. A d d i t i o n a l e v i dence has been obtained by measuring the r a t e o f d i s s o l u t i o n o f aged v e r s u s f r e s h s i l i c a and t h i s r a t e was found t o be slower f o r the aged s i l i c a . I n an experiment s i m i l a r t o t h a t d e s c r i b e d by Stober (13) and u s i n g a s u r f a c e area to volume r a t i o o f 0.1 m cm" i n a s o l u t i o n o f 0.9% NaCl + 0.1% NaHCOo i n water, the r a t e o f d i s s o l u t i o n was found to be 2600 \xg nT day""l f o r f r e s h s i l i c a and 1400 pg m~ day~^ f o r aged s i l i c a . The f r e s h sample r a t e i s higher than t h a t r e p o r t e d by Stober (13) f o r v i t r e o u s s i l i c a (2000 yg m~ day~l), probably because Stober used a time i n t e r v a l o f one day and t h e present study used a time i n t e r v a l o f one hour. The r a t e constants obtained from the present study (2 χ 10-8 to 7 χ 1 0 ~ cm s e c " ) a r e s i m i l a r to those r e p o r t e d by Hurd (14) f o r a c i d cleaned b i o g e n i c s i l i c a , using s i m i l a r ex perimental c o n d i t i o n s . Stober (13) found that t h e d a i l y r e l e a s e of s i l i c i c a c i d from s i l i c a polymorphs decreased i n the order: v i t r e o u s s i l i c a , s t i s h o v i t e , e r y s t o b a l i t e , t r i d y m i t e , quartz, c o e s i t e . The order i s from low t o h i g h d e n s i t y w i t h the ex c e p t i o n o f s t i s h o v i t e which has a h i g h d e n s i t y but a l s o a higher s o l u b i l i t y and a d i f f e r e n t c o o r d i n a t i o n number than the other c r y s t a l l i n e polymorphs. The decrease i n the r a t e o f s i l i c a d i s s o l u t i o n i n the present study may t h e r e f o r e r e f l e c t an i n c r e a s e i n order o r d e n s i t y a t l e a s t on the s i l i c a s u r f a c e . The trends observed f o r the aging of amorphous s i l i c a i n s a l t water s o l u t i o n are summarized i n Table IV. B i o g e n i c s i l i c a e x h i b i t s s i m i l a r trends except that the r a t e o f d i s s o l u t i o n does not c l e a r l y change; i n a d d i t i o n , w i t h b i o g e n i c s i l i c a the water content decreases and c r y s t a l l i n i t y i n c r e a s e s w i t h time (11, 12). 2

3

2


2

2

8

Table IV.

x

Trends i n Aging o f Amorphous S i l i c a i n Contact w i t h S a l t Water

Decrease S p e c i f i c Surface Area S p e c i f i c Pore Volume Solubility Rate o f D i s s o l u t i o n

Increase Density

These trends i n d i c a t e t h a t the s o l i d s i l i c a s u r f a c e i n con t a c t w i t h s o l u t i o n i s changing w i t h time. Small p a r t i c l e s or areas w i t h h i g h p o s i t i v e r a d i u s o f curvature are d i s s o l v i n g , and more dense and more ordered s i l i c a i s p r e c i p i t a t i n g t o give a f l a t t e r s u r f a c e wj.th a lower s o l u b i l i t y and lower s u r f a c e area (15). T h i s process takes time. I t occurs w i t h many d i f f e r e n t kinds of s i l i c a including biogenic s i l i c a . The r e s u l t i s to r e duce d i f f e r e n c e s i n the s o l u b i l i t i e s o f v a r i o u s s i l i c a s o l i d s be cause the s u r f a c e p r e c i p i t a t e d from a given s o l u t i o n i s a t l e a s t p a r t l y a f u n c t i o n o f the s o l u t i o n and not t o t a l l y dependent on the



10.

WILLEY

Aging of Amorphous Silica

159

s o l i d amorphous s i l i c a . This interpretation i s consistent with i d e a s presented by Okkerse and de Boer (9, 10), Stober (13), Jorgensen (16), S h e i n f a i n and Neimark (8), V y s o t s k i i , e t a l . (17), and l i e r (15). These i d e a s can be used to i n t e r p r e t some o f the v a r i a t i o n i n the pressure s t u d i e s mentioned e a r l i e r . I n one o f t h e i n i t i a l s t u d i e s (Willey (1), and i n d i c a t e d by + on F i g u r e 1) the s o l u b i l i t y was determined s e v e r a l times a t a pressure, and then the pressure was i n c r e a s e d . T h i s process continued, w i t h some lower pressure determinations i n the middle o f the experiment, u n t i l the highest p r e s s u r e was a t t a i n e d , a t which p o i n t the i n t e r n a l s i l i c a columns ruptured which prevented a rechecking of the a t mospheric pressure s o l u b i l i t y . I t i s proposed that the s o l i d s i l i c a was aging during the experiment, which causes the de f l e c t i o n to s l i g h t l y lower than expected s o l u b i l i t i e s a s the experiment progressed to h i g h e r p r e s s u r e s . The s e p a r a t i o n o f p o i n t s i n t o two groups on F i g u r e 1 a l s o i s a r e s u l t o f aging w i t h the aged s i l i c a s o l u b i l i t i e s (•, 0, and Δ on F i g u r e 1) f a l l i n g i n t o the lower s o l u b i l i t y group. The magnitude o f t h i s aging process i s s i g n i f i c a n t (approximately 20%), and i t con t r i b u t e s t o the s c a t t e r i n the p u b l i s h e d s o l u b i l i t y v a l u e s ( 2 ) . Because o f t h i s aging, i t i s d i f f i c u l t to determine the s o l u b i l i t y f o r amorphous s i l i c a i n water a f t e r a s h o r t time p e r i o d . Not a l l i n v e s t i g a t o r s have observed a decrease i n s i l i c a s o l u b i l i t y w i t h time (13, 18-23). Some evidence f o r aging ( e i t h e r a decrease i n s o l u b i l i t y o r a change i n s u r f a c e charac t e r i s t i c s ) has been observed i n s e v e r a l other s t u d i e s (1, J2, 11, 16, 19, 24). S e v e r a l s t u d i e s (_3, 23, 25) obtained s o l u b i l i t y values c h a r a c t e r i s t i c o f aged s i l i c a , but no s o l u b i l i t y change was observed. One experimental parameter which i s d i f f erent i n the group which observed a g i n g and the group that d i d not was the r a t i o o f s o l i d s i l i c a s u r f a c e area to s o l u t i o n volume. Because o f i n s u f f i c i e n t i n f o r m a t i o n , t h i s could not be evaluated i n a l l s t u d i e s ; however, e s t i m a t i o n (based on product l i t e r a t u r e from s e v e r a l companies that manufacture amorphous s i l i c a ) o f a s p e c i f i c s u r f a c e area f o r s i l i c a g e l o f 300 m g*~l and o f 100 nrg f o r d i s t i l l e d water washed s i l i c a g e l allows e s t i m a t i o n of t h i s r a t i o i n most o f t h e experiments l i s t e d above. I n a l l the s t u d i e s i n which aging was i n d i c a t e d , the s u r f a c e area to s o l u t i o n volume r a t i o was g r e a t e r than 0.1 m cm" ; i n those which d i d not observe aging t h i s r a t i o was l e s s than 0.1 m cm~ or the experimental time p e r i o d was l e s s than 60 days. Apparent l y the higher s u r f a c e area to volume r a t i o enhances the s i l i c a p r e c i p i t a t i o n that occurs during aging. The s i l i c a s u r f a c e that r e s u l t s from the p r e c i p i t a t i o n has a lower s o l u b i l i t y than the o r i g i n a l s u r f a c e . I t i s i n t e r e s t i n g to note t h a t the lower s o l i d s u r f a c e area to s o l u t i o n volume r a t i o (which o f t e n i s a l s o a s m a l l e r mass o f s i l i c a i n contact w i t h a s p e c i f i c volume of s o l u t i o n ) produces a higher s o l u b i l i t y v a l u e . 2

- 1

2

3

2


3

160

SOLUBLE

SILICATES

I d e n t i f i c a t i o n of aging as a f a c t o r i n s i l i c a s o l u b i l i t y allows use of a p p r o p r i a t e data f o r c a l c u l a t i o n of AV^, using Equation 6. For t h i s c a l c u l a t i o n , o n l y data f o r aged s i l i c a ( i n c l u d i n g the atmospheric pressure s o l u b i l i t y ) was used (2, _3, 4). T h i s c a l c u l a t i o n g i v e s Δν^ = - 9.9 cm^mol "^ and hence from Equation 7, s i ( 0 H ) 4 ( a q ) " cnAnol" . The s t a t i s t i c s of t h i s c a l c u l a t i o n are discussed elsewhere (26). Several other p o s s i b l e explanations i n a d d i t i o n to short term aging were considered to e x p l a i n the v a r i a t i o n i n p u b l i s h e d data (Figure 1) f o r the s o l u b i l i t y o f amorphous s i l i c a a t elevated pressures: 1. E f f e c t o f h y d r a t i o n of the s o l i d phase. Comparison of the s o l u b i l i t y behavior of unaged dehydrated s i l i c a (2_, φ i n F i g u r e 1) with unaged hydrated s i l i c a (27, χ or • i n F i g u r e 1) shows no d i f f e r e n c e . S i m i l a r com p a r i s o n of aged dehydrated s i l i c a (2, 0 i n F i g u r e 1) with aged hydrated s i l i c a (^, Δ i n F i g u r e 1) a l s o shows no d i f f e r e n c e . There f o r e , h y d r a t i o n of the s o l i d phase does not a f f e c t the pressure dependence of s i l i c a s o l u b i l i t y i n the pressure range to 1000 atmospheres. 2. Seawater e f f e c t . The s o l u b i l i t y data p o i n t s i n F i g u r e 1 were obtained i n s e v e r a l d i f f e r e n t s o l u t i o n s , i n c l u d i n g a r t i f i c i a l seawater (3, • i n F i g u r e 1), 0.7 M NaCl (4, Δ i n F i g u r e 1 ) , and 0.9% NaCl +0.1% NaHC0 (2, 0 i n F i g u r e 1 ) . The s i m i l a r i t y among these data sets shows t h a t seawater does not have a d i f f e r e n t s o l u b i l i t y than the simpler s a l t s o l u t i o n s . 3. I n h i b i t i o n of s i l i c a d i s s o l u t i o n by s e p i o l i t e formation i n seawater. W o l l a s t , e t a l . (28) suggested that s i l i c a c o n c e n t r a t i o n s i n seawater may be l i m i t e d by the p r e c i p i t a t i o n of s e p i o l i t e (Mg Si 0g-2 H a ( s ) ) which can be w r i t t e n as follows : -


v

5 5

1

3

2

2

3

2+

2 Mg (aq)

+

3 Si0 (aq) 2

Mg Si O -2 H 0(s) 2

3

g

2

+

+

4 Η 0(£) 2

—>

+

4 H (aq)

W o l l a s t , e t a l . (28) report, that the e q u i l i b r i u m constant f o r t h i s r e a c t i o n i s i n the range of 10"^^ to 10~19 t 25°C i n seawater. Ion a c t i v i t y products c a l c u l a t e d f o r the data i n W i l l e y (1) a l l i n d i c a t e s o l u t i o n s undersaturated by s e v e r a l orders of mag nitude w i t h respect to s e p i o l i t e , and s l i g h t l y a


(8)

WILLEY

Aging

of Amorphous

Silica

under s a t u r a ted f o r data i n Jones and Pytkowicz (3) assuming a small temperature and p r e s s u r e e f f e c t on t h i s e q u i l i b r i u m constant. In order to c a l c u l a t e these i o n a c t i v i t y products, 0.22 was used as the a c t i v i t y c o e f f i c i e n t f o r M g , 1.0 was used as the a c t i v i t y c o e f f i c i e n t f o r d i s s o l v e d s i l i c a , the pH o f the s o l u t i o n s used by Jones and Pytkowicz (3) was assumed to be between 7.5 and 8.0. The data produced by W i l l e y (2) and G r i f f i n , e t a l . (4) c o u l d n o t have been a f f e c t e d by s e p i o l i t e formation b e cause no magnesium was present i n the e x p e r i mental s o l u t i o n s . The s o l u b i l i t y o f s e p i o l i t e should i n crease w i t h i n c r e a s i n g p r e s s u r e based on a simple c a l c u l a t i o n o f AV^ f o r the d i s s o l u t i o n of s e p i o l i t e . Using p a r t i a l molal volume data for M g and IT*" compiled by Berner (29), along w i t h the value f o r s i ( O H ) 4 ( a q ) c a l c u l a t e d i n t h i s study and a molal volume f o r s e p i o l i t e c a l c u l a t e d from d e n s i t y data (2.08 to 2.45 gcm""3) compiled i n Donnay and Ondik (30), AV^ should be between - 37 and - 59 cm per mole of s e p i o l i t e d i s s o l v e d . The s i g n o f t h i s num ber i n d i c a t e s t h a t the s o l u b i l i t y o f s e p i o l i t e should i n c r e a s e w i t h i n c r e a s i n g p r e s s u r e . Based on t h i s c a l c u l a t i o n , s e p i o l i t e should not l i m i t s i l i c a s o l u b i l i t y any more a t higher pressure than i t does a t lower p r e s s u r e ; t h i s cannot be s a i d w i t h c e r t a i n t y , however, u n t i l more i n f o r m a t i o n on the s o l u b i l i t y o f s e p i o l i t e as a f u n c t i o n o f p r e s s u r e i s a v a i l a b l e . A s i m i l a r c a l c u l a t i o n has been done by Sayles (31). The pH decrease (Table I I ) observed i n the e a r l y experiment when seawater came i n t o contact w i t h the amorphous s i l i c a s u r f a c e suggested p o s s i b l e s e p i o l i t e formation. How ever, i n subsequent experiments, a s i m i l a r pH change was observed f o r unwashed s i l i c a s u r faces i n contact w i t h 0.9% NaCl + 0 . 1 % NaHC03 s o l u t i o n . The amount o f base r e q u i r e d to t i t r a t e each s o l u t i o n back t o pH 8 a f t e r the s o l i d phase was removed was g r e a t e r i n the s a l t s o l u t i o n than i n seawater. T h i s e x p e r i ment shows that the pH decrease occurs i n s o l u t i o n s with no Mg2+(aq), so s e p i o l i t e formation i s n o t n e c e s s a r i l y i n v o l v e d w i t h the pH change.


2+

2 +

v

3


SOLUBLE


162

SILICATES

The c o n c e n t r a t i o n of d i s s o l v e d s i l i c a i n s a l t water s o l u t i o n s i n contact w i t h s o l i d amorphous s i l i c a may decrease f o r a time p e r i o d of s e v e r a l weeks to s e v e r a l months a f t e r the i n i t i a l d i s s o l u t i o n , and then a f t e r t h i s i n i t i a l aging process i s com p l e t e d the c o n c e n t r a t i o n remains s t a b l e f o r months or years ( 2 ) . Several s t u d i e s have r e p o r t e d t h i s e q u i l i b r i u m s o l u b i l i t y . Jorgensen (16) found that three to f i v e months were r e q u i r e d to achieve e q u i l i b r i u m i n h i s experiments, and a f t e r that time the same s o l u b i l i t y was determined from undersaturated or overs a t u r a t e d s o l u t i o n s i n contact w i t h s i l i c a f o r time p e r i o d s up to two y e a r s . Jones and Pytkowicz (3) found the same s o l u b i l i t y f o r aged s i l i c a a f t e r 66 o r 123 days of e q u i l i b r a t i o n time. W i l l e y (2) found no change i n the s o l u b i l i t y of s i l i c a aged f o r two months i n s a l t water s o l u t i o n a f t e r time p e r i o d s of up to f i v e y e a r s . G r i f f i n , e t a l . (4) determined s o l u b i l i t i e s u s i n g the crossover method of S i e v e r and Woodford (32) which does not r e q u i r e attainment o f e q u i l i b r i u m . With t h i s method (32), s o l u t i o n s which have d i f f e r e n t d i s s o l v e d s i l i c a concentrations are p l a c e d i n contact with s o l i d amorphous s i l i c a . The change i n c o n c e n t r a t i o n which r e s u l t s when e i t h e r d i s s o l u t i o n or p r e c i p i t a t i o n occurs i n the s e v e r a l s o l u t i o n s i s used to c a l c u l a t e the solubility. In the study by G r i f f i n , et a l . (4), c o n c e n t r a t i o n change measurements were made a f t e r three weeks. Kato and Kitano (20) used an e q u i l i b r a t i o n time o f 500 days i n t h e i r s o l u b i l i t y experiments. A l l of these l o n g term s t u d i e s obtained s i m i l a r values f o r the s o l u b i l i t y o f aged amorphous s i l i c a i n s a l t water s o l u t i o n s . S i e v e r (22) obtained a s l i g h t l y higher s o l u b i l i t y value a f t e r an e q u i l i b r a t i o n time o f two y e a r s . These s t u d i e s show t h a t the s o l u b i l i t y o f aged amorphous s i l i c a i n s a l t water s o l u t i o n s i s s t a b l e f o r many months or years a f t e r an i n i t i a l aging time of s e v e r a l months. The trends observed f o r the aging of b i o g e n i c s i l i c a (11, 12) and thermodynamic c a l c u l a t i o n s (_7) suggest that t h i s i s not the u l t i m a t e e q u i l i b r i u m ; e v e n t u a l l y the amorphous s i l i c a should change to quartz which has a much lower s o l u b i l i t y than amorphous s i l i c a (4, _7, 13, 15, 21). Conclusions 1. The s o l u b i l i t y of amorphous s i l i c a i n s a l t water s o l u t i o n s (at 0-3°C or 19-26°C, and over the pressure range from 1 to 1000 atmospheres) decreases by approximately 20% with time due to aging o f the s o l i d s i l i c a . 2. T h i s s o l u b i l i t y change makes amorphous s i l i c a s o l u b i l i t y d i f f i c u l t to determine, and c o n t r i b u t e s to the s c a t t e r i n pub lished s o l u b i l i t y values. 3. Other trends a s s o c i a t e d with t h i s aging of s i l i c a i n clude a decrease i n s p e c i f i c s u r f a c e area and pore volume, and an i n c r e a s e i n d e n s i t y . S i m i l a r trends have been i d e n t i f i e d f o r biogenic s i l i c a .


10. WILLEY

Aging of Amorphous Silica

163


4. The rate of silica aging depends on experimental conditions, including the ratio of solid surface area to solution volume. 5. The solubility of amorphous silica in seawater or in salt water solutions similar to seawater is not affected by the extent of hydration of the solid phase, and is not limited by sepiolite formation. Acknowledgments Discussions of importance regarding this work were held with R. Dayal and R. K. lier, and an earlier version of the manuscript was reviewed by L. M. Mayer. E. Malik typed the manuscript. All of this assistance is appreciated. Literature Cited 1. Willey, J. D. Mar. Chem. 1974, 2, 239-250. 2. Willey, J. D. Geochim. Cosmochim. Acta 1980, 44, 573-578. 3. Jones, M. M.; Pytkowicz, R. M. Bull. Soc. R. Sci. Liege 1973, 42, 118-120. 4. Griffin, J. W. ; Hurd, D. C.; Commeau, J . ; Poppe, L. Am. J. Sci. (in preparation). 5. Duedall, I. W.; Dayal, R.; Willey, J. D. Geochim. Cosmochim. Acta 1976, 40, 1185-1189. 6. Owen, B. B.; Brinkley, S. R. Chem. Rev. 1941, 29, 461-473. 7. Walther, J. V.; Helgeson, H. C. Am. J. Sci. 1977, 277, 1315-1351. 8. Sheinfain, R. Y.; Neimark, I. E. Chapter 8, in "Adsorption and Adsorbents" (ed. D. N. Strazhesko), Wiley, 1973, pp. 87-95. 9. Okkerse, C.; de Boer, J. H. Chapter 25 in "Reactivity of Solids" (ed. J. H. de Boer), Elsevier, 1961, pp. 240248. 10. Okkerse, C.; de Boer, J. H. Silic. Ind. 1962, 27, 195-202. 11. Hurd, D. C.; Theyer, F. Adv. Chem. Ser. 1975, 147, 211-230. 12. Hurd, D. C.; Wenkam, C.; Pankratz, H. S.; Fugate, J. Science 1979, 203, 1340-1343. 13. Stöber, W. Adv. Chem. Ser. 1967, 67, 161-182. 14. Hurd, D. C. Earth Planet. Sci. Lett. 1972, 15, 411-417. 15. Iler, R. K. "The Chemistry of Silica". Wiley, 1979. 16. Jorgensen, S. S. Acta Chem. Scand. 1968, 22, 335-341. 17. Vysotskii, Z. Z.; Galinskaya, V. I.; Kolychev, V. I.; Strelko, V. V.; Strazhesko, D. N. Chapter 7 in "Adsorption and Adsorbents" (ed. D. N. Strazhesko). Wiley 1973, 72-86. 18. Krauskopf, Κ. B. Geochim. Cosmochim. Acta 1956, 10, 1-26. 19. Kato, K.; Kitano, Y. J. Oceanogr. Soc. Jpn. 1968, 24, 147-152. 20. Lewin, J. C. Geochim. Cosmochim. Acta 1961, 21, 182-198.


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Siever, R. J. Geol. 1962, 70, 127-150. Iler, R. J. Colloid Interface Sci. 1973, 43, 399-408. Kitahara, S. Rev. Phys. Chem. Jpn. 1960, 30, 131-137. Marshall, W. L. Geochim. Cosmochim. Acta 1980, 44, 907-914. Hurd, D. C. Geochim. Cosmochim. Acta 1973, 37, 2257-2282. 26. Willey, J. D. Geochim. Cosmochim. Acta (in press). 27. Willey, J. D. "The Physical Chemistry of Silica in Sea Water and Marine Sediments"; Ph.D. Thesis, Dalhousie University, 1975, 195 pp. 28. Wollast, R.; MacKenzie, F. T.; Bricker, O. P. Am. Mineral.


21. 22. 23. 24. 25.

1968, 53, 1645-1662.

29. Berner, R. A. "Principles of Chemical Sedimentology"; McGraw-Hill Book Company, 1971, p. 212. 30. Donnay, J. D. H.; Ondik, H. M. "Crystal Data Determinative Tables"; Volume 12, U. S. Department of Commerce, National Bureau of Standards, and Joint Committee on Powder Diffraction, 1973, p. 0-51. 31. Sayles, F. T. Geochim. Cosmochim. Acta 1981, 45, 1061-1086. 32. Siever, R.; Woodford, N. Geochim. Cosmochim. Acta 1973, 37, 1851-1880. RECEIVED March 2, 1982.


Aging of Amorphous Silica in Salt Water Solutions - ACS Symposium

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