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4 Health, Safety, and Environmental Aspects of

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Soluble Silicates W. L. SCHLEYER and J. G. BLUMBERG The PQ Corpartion, Research and Development Center, Lafayette Hill, PA 19444

The alkalinity of soluble silicates is their primary hazard. Contact exposure effects can range from irritation to corrosion. Inhaled or ingested sodium silicates are rapidly eliminated in the urine. Trace quantities of dissolved silica are essential to nutrition, but i f normal dietary amounts are exceeded, siliceous urinary calculi may result. Dissolved silica is a minor but ubiquitous constituent of the environment. When dissolved silica becomes depleted in natural waters, diatoms are displaced by species that accelerate eutrophication. Commercial soluble silicates rapidly depolymerize upon dilution to molecular species indistinguishable from natural dissolved silica.

Soluble s i l i c a t e s have been known since ancient times, but it was not u n t i l the middle of the 19th Century that s o l u b l e s i l i c a t e s were produced on a commercial s c a l e . In 1877, a 46-page pamphlet(1) was s u f f i c i e n t to encompass most of the knowledge about s o l u b l e s i l i c a t e s then a v a i l a b l e . By 1928, over 4 0 0 pages were required f o r V a i l ' s first American Chemical S o c i e t y Monograph on the s u b j e c t ( 2 ) . This monograph contained i n f o r m a t i o n on the a m e l i o r a t i o n of the adverse environmental e f f e c t s of emissions from the now abandoned s u l f a t e process f o r s o l u b l e s i l i c a t e p r o d u c t i o n ( 3 ) , the use of s o l u b l e s i l i c a t e s i n aqueous e f f l u e n t treatment ( 4 7 , and a short chapter on the p h y s i o l o g i c a l e f f e c t s of s o l u b l e s i l i c a t e s ^ ) . In recent years, there has been an i n c r e a s i n g emphasis on b i o l o g i c a l t e s t i n g f o r the q u a n t i t a t i v e determination of environmental and h e a l t h e f f e c t s of chemical products and processes. But since b i o l o g i c a l t e s t i n g i s both time consuming and expensive, those who fund t h i s type of r e s e a r c h , government, i n d u s t r y , l a b o r or academic o r g a n i z a t i o n s , tend to give higher p r i o r i t y f o r t e s t i n g to newer chemicals about which l i t t l e i s 0097-6156/82/0194-0049$06.25/0 © 1982 A m e r i c a n C h e m i c a l Society

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

SOLUBLE


50

SILICATES

known, rather than t e s t e s t a b l i s h e d chemicals, such as s o l u b l e s i l i c a t e s , with which there has been over a century of human experience. Nevertheless, there has been a l i m i t e d amount of b i o l o g i c a l t e s t i n g conducted on s o l u b l e s i l i c a t e s by government, i n d u s t r y and academic s c i e n t i s t s . There have also been s e v e r a l c r i t i c a l e v a l u a t i o n s of the a v a i l a b l e information on s o l u b l e s i l i c a t e s by expert groups impaneled to assess to the environmental or h e a l t h r i s k s o f v a r i o u s uses o f these substances. The o b j e c t i v e o f t h i s review i s to draw together and b r i e f l y d i s c u s s the a v a i l a b l e information on the h e a l t h , s a f e t y and environmental aspects o f the s o l u b l e s i l i c a t e s . The sources o f information f o r t h i s review i n c l u d e s c i e n t i f i c p u b l i c a t i o n s , r e p o r t s of r e g u l a t o r y bodies and government agencies, and the i n c i d e n t a l records o f a c o r p o r a t i o n which has manufactured s o l u b l e s i l i c a t e s for over 120 y e a r s ( 6 ) . HEALTH AND

SAFETY ASPECTS

Ingestion O r a l L D 5 0 , the dose l e v e l where 50% of an exposed population of r a t s w i l l d i e w i t h i n a s p e c i f i e d time, i s a u s e f u l expression of the approximate magnitude of t o x i c i t y o f a substance. It a l s o provides a standard measure o f comparison among many substances. The L D ^ Q values for sodium s i l i c a t e s i n Table I and Figure 1 were compiled from the r e s u l t s o f a number of s t u d i e s . It should be noted that these s t u d i e s were conducted at d i f f e r e n t times, and vary somewhat i n t h e i r t e s t c o n d i t i o n s such as, length o f o b s e r v a t i o n p e r i o d , and s t r a i n , number and sex d i s t r i b u t i o n o f the animals. Nevertheless, we b e l i e v e the comparison i s u s e f u l f o r the purpose of i l l u s t r a t i n g , i n a general way, the i n f l u e n c e of s i l i c a t e composition on acute o r a l toxicity. Even a very c l o s e l y c o n t r o l l e d L D 5 0 study would not y i e l d data from which c o n c l u s i o n s could be drawn with g r e a t e r c e r t a i n t y unless a great number of animals were used. Thus, i n the l e t h a l range o f sodium s i l i c a t e s , l a r g e doses are r e q u i r e d , and the 95% confidence i n t e r v a l s are on the order of 0.5 g/kg. The autopsy r e s u l t s for the reported s t u d i e s , acute g a s t r o e n t e r i t i s , v a s c u l a r congestion, mottled l i v e r s ( 8 ) , were c o n s i s t e n t with n o n s p e c i f i c causes of death, e.g., changes i n pH of body f l u i d s , shock, chemical i r r i t a t i o n or c o r r o s i o n of the v i s c e r a , e t c . It appears that the Si02/Na 0 r a t i o o f sodium s i l i c a t e s has a greater i n f l u e n c e on t h e i r t o x i c i t y than t h e i r c o n c e n t r a t i o n . T h i s r e l a t i o n i s perhaps not unexpected when i t i s considered i n l i g h t o f the sodium silicate°s property o f y i e l d i n g aqueous s o l u t i o n s o f r e l a t i v e l y constant pH over a range of c o n c e n t r a t i o n s , while at constant c o n c e n t r a t i o n , pH v a r i e s i n v e r s e l y with r a t i o (see F i g u r e s 2 and 3 ). In an attempt to develop a more s p e c i f i c t e s t f o r modeling i n g e s t i o n hazard than o r a l L D ^ Q , the FDA conducted a s e r i e s of 2


4.

SCHLEYER

Table I.

Si02/Na20 wt. ratio


Environmental

AND BLUMBERG

51

Median Lethal Dose (Oral, Rat) Sodium Silicate LD-50

CONCENTRATION wt. percent

3.2 3 2 2.0 2.0 1.6 1.0 1.0 0.7 0.7 0.5

Aspects

REFERENCE

g/kg >3 3.2 1.6-8.6 1.3-2.1 1.5-2.2 1.6 2.0-2.5 0.6 0.8 1.5 1.0 0.5

36

81 81 51 99 50 61 61 90

7. 8. 9. 9. 10. 11. 10. 11. 11. 10. 11. 10.

LD-50

1.5 2.0 2.5 Si02/Na20 RATIO Figure 1.

Ratio vs. LD

50

sodium silicate.


3.5


52 s o l u b l e silicates



4.

SCHLEYER

AND

BLUMBERG

Environmental

Figure 3.

Ratio vs.

Aspects

pH.


53


54

SOLUBLE

SILICATES

t e s t s using r a b b i t s during the l a t e 1960s and e a r l y 1970s. I n i t i a l l y a d e s c r i p t i o n of the f i n d i n g s upon macroscopic examination were the only r e s u l t s reported. Two samples of a 2.0 r a t i o sodium s i l i c a t e powder ( 80% s o l i d s ) and two samples of sodium m e t a s i l i c a t e were tested at t h i s time. "No l e s i o n s ..." and "submucosal edema" were noted i n the animals exposed to 2.0 r a t i o sodium s i l i c a t e , but "severe u l c e r " and " a c t i v e hyperemia" r e s u l t e d from m e t a s i l i c a t e exposure(12). In 1973, Bierbower,(13) reported on a s e r i e s of s i m i l a r t e s t s , conducted under the a u s p i c i e s of the Consumer Product Safety Commission. Microscopic examination of the esophagus was used as the primary c r i t e r i o n for c a t e g o r i z i n g r e s u l t s as e i t h e r " c o r r o s i v e " or "negative." This data i s summarized on Table I I . The data i n d i c a t e a c o r r e l a t i o n of hazard with r a t i o only at the extremes of r a t i o . In the intermediate range, the r e s u l t s for l i q u i d s vary with concentration (independent of pH which i s v i r t u a l l y constant see Figure 2), and the r e s u l t s for powders i n t h i s range are equivocal. In man, the l e t h a l o r a l dose of sodium s i l i c a t e s has been estimated as 0.5-5 g / k g O ) . Ingestion of 200 ml of sodium s i l i c a t e egg preserving s o l u t i o n (these s o l u t i o n s t y p i c a l l y contain 5-36% of 3.2 Si0 /Na20) caused severe vomiting, d i a r r h e a and bleeding, elevated blood pressure, and r e n a l damage, but was not f a t a l ( 1 4 ) . In the past, sodium s i l i c a t e has been administered o r a l l y for m e d i c i n a l purposes i n doses of 1 to 3 g/day without reported adverse e f f e c t s ( 1 5 ) , however, i t i s not p r e s e n t l y known to be used as a drug. In an e a r l y feeding study, King et al.(16) attempted to administer soluble s i l i c a t e s to dogs as 5% s o l u t i o n s , but found they had to p r e n e u t r a l i z e the s o l u t i o n s or the dogs i n v a r i a b l y vomited them. Such s o l u b l e s i l i c a that was absorbed by the dogs from the n e u t r a l i z e d s o l u t i o n was found to be q u i c k l y eliminated i n the u r i n e . The l e v e l of s i l i c a i n the blood remained low, and i t was suggested that these animals have a low r e n a l threshold for d i s s o l v e d s i l i c a . Newberne and Wilson(17) succeeded i n feeding dogs and r a t s sodum s i l i c a t e incorporated i n t o an semisynthetic d i e t at l e v e l s equivalent to 0.8 g Si02/kg/day. The only untoward c l i n i c a l signs observed were p o l y d i p s i a , p o l y u r i a , and s o f t s t o o l s . Renal l e s i o n s were observed i n the dogs upon h i s t o p a t h o l o g i c a l examination. S i m i l a r e f f e c t s were not observed i n the r a t s . Smith(18) studied the e f f e c t s of 3.22 r a t i o sodium s i l i c a t e added to the d r i n k i n g water of r a t s at l e v e l s of 600 and 1200 mg/1. Two t r i a l s were conducted. The f i r s t , with a n u t r i t i o n a l l y adequate d i e t , l a s t e d 180 days. The second, i n which a d i e t inadequate for normal growth was provided, l a s t e d 84 days. The r a t s used i n the second t r i a l were the o f f s p r i n g of those used i n the c o n t r o l group of the f i r s t trial. Nitrogen and phosphorus r e t e n t i o n was measured by assaying the d i e t and wastes for these elements. Weight gain and reproductive a b i l i t y were recorded. Consumption of the water was 2



SCHLEYER A N DBLUMBERG

Table IL

Si02/Na20 wt. ratio 3.2 3.2 2.9 2.9 2.9 2.4 2.4 2.4 2.0 2.0 2.0 1.0 0.7

Environmental

Aspects

Esophageal Test (Oral, Rabbit) Sodium Silicate

CONCENTRATION 5% w/v 10% w/v 10% w/v 15% w/v Neat liq.(43%) 10% v/v 15% v/v Neat pwd. 5% V/V 10% v/v Neat pwd. 10% w/v 10% w/v

RESULTS + = corrosive

+ +

+

+,+ +.+,+ +



56

SOLUBLE

SILICATES

only noted by casual observation, but i t was reported to be s i m i l a r f o r a l l groups. In the t r i a l r e c e i v i n g an adequate d i e t , the male r a t s r e c e i v i n g sodium s i l i c a t e at the 600 ppm S 1 O 2 l e v e l (about 790 ppm sodium s i l i c a t e ) , experienced a 6% greater weight g a i n over c o n t r o l s r e c e i v i n g deionized H 2 O . Females from t h i s same group gained 5% l e s s weight than the c o n t r o l s . At the higher l e v e l (about 1580 ppm sodium s i l i c a t e ) weight gains by both sexes did not d i f f e r s i g n i f i c a n t l y (P = 0.05) from c o n t r o l s . In the second t r i a l , there was no s i g n i f i c a n t d i f f e r e n c e i n weight gain of the s i l i c a t e treated versus the c o n t r o l animals at the lower l e v e l , although the males at the higher l e v e l s were 6% l i g h t e r than c o n t r o l s . The g r e a t e s t v a r i a t i o n i n n i t r o g e n r e t e n t i o n was a 13% r e t e n t i o n o f u r i n a r y n i t r o g e n i n the f i r s t t r i a l group at the lower l e v e l o f s i l i c a t e consumption - the same group that gained weight. The l a r g e s t v a r i a t i o n i n phosphorus r e t e n t i o n was a 9% increase i n the second t r i a l group that consumed s i l i c a t e at the higher l e v e l , but i t was not apparent whether t h i s d i f f e r e n c e was due to the s i l i c a t e treatment or the greater body s i z e o f these animals. The r e s u l t s o f the study o f the r a t s reproductive performance are given i n Table I I I . I t appears that o f a l l the f a c t o r s observed, the number o f o f f s p r i n g to survive u n t i l weaning the i s the only one to c o n s i s t e n t l y c o r r e l a t e with increased s i l i c a t e consumption. In view o f the high m o r t a l i t y o f the c o n t r o l o f f s p r i n g (only 35% s u r v i v e d ) , any a d d i t i o n a l s t r e s s might have produced the same e f f e c t . The author concludes that " s o l u b l e s i l i c a ... exerts b i o l o g i c a l l y important e f f e c t s on growth and reproductive performance,"(19) but i t i s not c l e a r from h i s data whether there i s an e f f e c t , and i f there i s , whether i t can be a t t r i b u t e d to the d i s s o l v e d s i l i c a or the a l k a l i n i t y o f the d r i n k i n g water. Ito et a l . ( 2 0 ) , fed r a t s d r i n k i n g water c o n t a i n i n g from 200 to 1800 ppm sodium s i l i c a t e f o r 3 months. They reported an increase i n serum a l k a l i n e phosphatase a c t i v i t y at a c o n c e n t r a t i o n o f 1800 ppm i n males, and an increase o f serum glutamic-pyruvic transaminase a c t i v i t y at 200 and 600 ppm sodium s i l i c a t e i n females. A decrease i n leukocyte count occurred i n both sexes at 600 ppm. No s p e c i f i c change i n the r a t s due to the sodium s i l i c a t e was observed upon h i s t o p a t h o l o g i c a l examination. Benke and Osborne(21) studied the r a t e and extent o f u r i n a r y e x c r e t i o n o f s i l i c o n i n r a t s a f t e r o r a l a d m i n i s t r a t i o n of s i n g l e doses of s e v e r a l s i l i c a t e s , i n c l u d i n g a 2.4 r a t i o sodium s i l i c a t e , to r a t s . Two t r i a l s were conducted: i n the f i r s t t r i a l , a dose o f 40 mg/kg was administered, i n the second t r i a l the dose was 1000 mg/kg. At the 40 mg/kg l e v e l , 18.9% o f the administered s i l i c a t e was excreted i n the u r i n e , and elevated l e v e l s of S i i n the urine were observed only i n the f i r s t 24 hours a f t e r the o r a l dose. At the 1000 mg/kg l e v e l , 2.8% of the t o t a l administered s i l i c a t e was excreted i n the u r i n e , and the data i n Figure 4 were obtained f o r the r a t e o f e x c r e t i o n Benke


4.

SCHLEYER

Environmental

AND BLUMBERG

57

Aspects

Table III. Reproductive Ability of Rats Fed Sodium Silicate in Drinking Water

CONCENTRATION (as ppm Si02) 1200 600


0 Matings Litters No. Born No. Weaned % Weaned

77 54 517 182 35

Difference as % of Controls Born Weaned

77 51 346 83 24

77 49 414 44 11

67 46

80 24

Excreted Silicate as ug Si

1000 h

800 h

600 h

400 h

200 h

' / ' ' ' ' ' ' ' ' ' ' / ' ' ' / ' ' ' ' ' ' / ' / ' '

// / / / / / / / / / / // / / / / // / / / / // / / / / / /7~/ / / // //// // //// // //// // //// // //// // //// / // / / / / // / / / / // / / / / / / / / / / // / / / / // / / / / / / // / / / / // / / / / / / / / / / // / / / / / / / / / / / / // // // // //

' ' ' ' ' ' ' ' ' ' ' '

/ / / / / / / / / / / /

/ γ// / /' Δ/ / / /' / / / / /' / / / / /' / / / / /' / / / / /' / / / / /' / / / / ///// /' / / / / /* / / / / *'/'/ f

y / / λ// / / / / A/ / / / / / λ/ / / / / / As / / /////// /////// '//////

72-96

48-72 24-4β Time After Administration

0-24 Figure 4.

Urinary excretion of sodium

*/////•/ '////// /////// '////// '//////

silicate.


SOLUBLE


58

SILICATES

and Osborne c a l c u l a t e d the u r i n a r y e x c r e t i o n h a l f - l i f e for ingested sodium s i l i c a t e to be 24 hours. Sauer et a l . ( 2 2 ) , measured the t o t a l s i l i c a eliminated ( i . e . , u r i n a r y and f e c a l S 1 O 2 ) by guinea pigs a f t e r o r a l a d m i n i s t r a t i o n of 1) a s i n g l e dose of sodium m e t a s i l i c a t e pentahydrate, equivalent to 80 mg S 1 O 2 , and 2) four doses of sodium m e t a s i l i c a t e pentahydrate, equivalent to 80 mg S 1 O 2 , at 48 hr. i n t e r v a l s . Within 8 days, 60% of the s i l i c a administered as a s i n g l e dose and 96% of the s i l i c a administered as m u l t i p l e doses was excreted. Although there are no r e p o r t s i n the s c i e n t i f i c l i t e r a t u r e of chronic t e s t i n g or c a r c i n o g e n i c i t y of sodium s i l i c a t e s , a number of s t u d i e s on z e o l i t e type A, which r a p i d l y decomposes to amorphous aluminates and sodium s i l i c a t e i n the stomach and t i s s u e s , were r e c e n t l y reported(23). Among these studies was a l i f e t i m e feeding study i n r a t s which concluded that chronic feeding of high doses (.001, .01 and .1% i n d i e t ) of type A z e o l i t e d i d not produce cancer or chronic organ t o x i c i t y i n rodents(24). It i s a l s o r e l e v a n t to note that sodium s i l i c a t e s have had a long h i s t o r y of safe use i n numerous food-related applications(25). Sodium s i l i c a t e and potassium s i l i c a t e are considered GRAS (Generally Recognized as Safe) by the U.S. FDA for a d d i t i o n to canned d r i n k i n g water as a c o r r o s i o n preventative at concentrations up to 100 ppm(26). Skin Contact Tests for the e f f e c t s of s k i n contact of sodium s i l i c a t e s have been undertaken by both i n d u s t r y and governmental agencies. Since many s o l u b l e s i l i c a t e s are not s t o i c h i o m e t r i c compounds, but rather can be prepared with v a r i a b l e Si02/Na20 r a t i o s , t e s t s have been conducted at v a r i o u s points on the continuum of p o s s i b l e r a t i o s , u s u a l l y at points w i t h i n the s p e c i f i c t i o n s of commercial products. Two s i m i l a r experimental procedures have been used to q u a n t i f y the s k i n contact e f f e c t s of s o l u b l e s i l i c a t e s ; both are based on the Draize method(27). The f i r s t i s the p r o t o c o l adopted by the U.S. Food and Drug A d m i n i s t r a t i o n and Consumer Product Safety Commission for determining the contact hazard of substances under the Federal Hazardous Substances Act, and i s s p e c i f i e d i n 16 C.F.R. $ 1500.41 et seq. The second, i s the p r o t o c o l adopted by the U.S. Department of T r a n s p o r t a t i o n for determining the contact hazard of substances under the Federal Hazardous M a t e r i a l s Transporation Act, and i s s p e c i f i e d i n 49 C.F.R. 5173.240. In the FHSA t e s t , 0.5 g or 0.5 ml of the t e s t substance i s moistened with p h y s i o l o g i c a l s a l i n e and a p p l i e d to the i n t a c t and abraded skin of r a b b i t s for 24 hours. The s i t e of contact i s examined a f t e r 24 and 72 hours and the extent of i r r i t a t i o n i s ranked on a s c a l e (Primary I r r i t a t i o n Index) of i n c r e a s i n g



4.

SCHLEYER A N D BLUMBERG

Environmental

Aspects

59

s e v e r i t y o f from 1 to 4. Corrosivity, i . e . , nonreversible i n j u r y , i s also noted. Tables IV. and V. l i s t the values obtained f o r s o l u b l e s i l i c a t e s i n a number o f l a b o r a t o r i e s over the course o f about 20 years. The PII values l i s t e d are the sum of i n t a c t and abraded scores unless otherwise noted.Too few determinations have been done to compute the standard e r r o r o f the P I I , but the s u b j e c t i v e nature o f the s c a l i n g system makes the inference reasonable that i t i s l a r g e enough to account f o r the otherwise anomalous values f o r 3.2 r a t i o at 80 and 36 percent, r e s p e c t i v e l y . It appears that the breakpoint between i r r i t a n t and c o r r o s i v e s o l i d sodium s i l i c a t e s occurs between 2.0 and 2.4 r a t i o . Potassium s i l i c a t e s are e v i d e n t l y more i r r i t a t i n g than sodium s i l i c a t e s o f e q u i v a l e n t mole r a t i o . Perhaps t h i s i s the r e s u l t of the greater aqueous s o l u b i l i t y o f potassium s i l i c a t e s . Test Results The DOT t e s t d i f f e r s from the FHSA t e s t p r i n c i p a l l y i n that the exposure period i s 4 hours instead of 24 hours, and dry substances are tested dry - they are not moistened with s a l i n e solution. Consequently, t h i s t e s t i s l e s s s e n s i t i v e to small d i f f e r e n c e s i n the a c t i v i t y o f compounds, but i t provides a more r e a l i s t i c model of a c c i d e n t a l human exposure. The data i n Table VI. i n d i c a t e s that the breakpoint between i r r i t a n t and c o r r o s i v e l i q u i d sodium s i l i c a t e s occurs between 1.6 and 1.8 r a t i o , but i t i s also probably i n f l u e n c e d by the c o n c e n t r a t i o n o f the s o l u t i o n s . In i n d u s t r i e s using sodium s i l i c a t e s , d e r m a t i t i s has been a t t r i b u t e d to sodium s i l i c a t e exposure(34). Where adequate p r o t e c t i o n o f the hands i s not undertaken, p h y s i c a l i n j u r y by p r o j e c t i n g p o i n t s o f d r i e d s i l i c a t e i s f u r t h e r aggravated by a l k a l i n e i r r i t a t i o n ( 3 5 ) . Workers w i t h i n the s o l u b l e s i l i c a t e i n d u s t r y have been reported to s u s t a i n burns from hot g l a s s and d e r m a t i t i s from a l k a l i n e m a t e r i a l s . ( 3 6 ) In our experience, the most common type of a c c i d e n t s i n v o l v e s p i l l i n g or splashing s i l i c a t e s into shoes or g e t t i n g i t between the s k i n and c l o t h i n g at the c o l l a r and c u f f s where abrasion occurs.(37) Safety boots and gloves with g a u n t l e t s are recommended to avoid these types o f exposure. Eye

Contact

The e f f e c t s of eye contact with sodium s i l i c a t e s have been tested by i n d u s t r y and i n government l a b o r a t o r i e s . The standard t e s t for determining the hazard of eye contact i s the FHSA Draize method s p e c i f i e d i n 16 C.F.R. f1500.42. The data i n Table V I I . i n d i c a t e s that at the r a t i o s and concentrations t e s t e d , s o l u b l e s i l i c a t e s are i r r i t a t i n g to the eyes, and s e v e r e l y i r r i t a t i n g at high c o n c e n t r a t i o n s . A new t e s t for assessment of eye contact e f f e c t s i s c u r r e n t l y under development.


SOLUBLE

SILICATES


Table IV. FHSA Skin Contact Data Sodium Silicate

Si02/Na20 wt. ratio 3.2 3.2 3.2 2.9 2.5 2.4 2.0 2.0 2.0 1.0 1.0

CONC. wt.% 99 80 36 43 37 24 99 54 8 10 6

ΡΠ 4 0 3 3 3 4 8 4 >4* 5.6* >8*

CORROSIVITY + = corrosive

+

+ +

Ref. 28. 28. 28. 28. 28. 28. 28. 28. 29. 29. 29.

* 2x average of intact and abraded score.

Table V. FHSA Skin Contact Data Potassium Silicate

SÎ02/K20 mol. ratio

CONC. wt.%

PII

3.45

29

0

—

28.

3.33

39

2

-

28.

2.5

85

8

_

30.


Ref.


4.

Environmental

SCHLEYER AND BLUMBERG

61

Aspects


Table VI. DOT Skin Contact Data Sodium Silicate Si02/Na20 wt. ratio

CONC. wt.%

2.9 2.5 2.4 2.0 2.0 1.8 1.6 1.0 0.7 0.5

43 37 47 44 54 38 51 99 61 90

PII


3.3 0 4.2 4.2 4.7 3.2 * * • *

+

+

Ref.

31. 31. 31. 31. 32. 32. 33. 33. 33. 33.

* not reported.

Table VII. FHSA Eye Contact Data Soluble Silicates

Si02/Na20 wt. ratio 3.2 2.9 2.0 2.0 1.0 1.0 1.0 1.0 0.7 0.7 2.5*

CONC. wt.% 36 43 8 44 10 β 5 3 β 3 80

IRRITATION +(severe) + +(severe) + + + + + + +(severe)

Ref. 37. 37. 37. 37. 37. 37. 37. 37. 37. 37. 38.

• S102/K20 ratio


62

SOLUBLE

SILICATES


Inhalation Michon, et a l . ( 4 0 ) , studied the s i l i c o n metabolism of r a b b i t s a f t e r i n h a l a t i o n of a sodium s i l i c a t e a e r o s o l . They concluded that sodium s i l i c a t e d i s s o l v e s i n the lungs and i s r a p i d l y eliminated i n the u r i n e . Beeking(41) summarized two i n h a l a t i o n studies of type A sodium z e o l i t e which r a p i d l y decomposes to sodium s i l i c a t e and amorphous aluminates under p h y s i o l o g i c a l c o n d i t i o n s . In the f i r s t study, hamsters were exposed to approximately 20 mg/w? of type A z e o l i t e 3 days per week, 5 hours per day f o r 52 weeks. In the second study, Cynomolgus monkeys were exposed to 1 and 6 mg/m of type A z e o l i t e for 24 months, and 50 mg/m^ type A z e o l i t e for 12 months. No evidence of f i b r o s i s was observed i n the animals i n e i t h e r study. 2

ENVIRONMENTAL ASPECTS Occurrence Compounds of s i l i c o n and oxygen are the primary c o n s t i t u e n t s of earth*s land masses. Dissolved s i l i c a i s a minor but ubiquitous c o n s t i t u e n t of e a r t h s hydrosphere. Ground waters contain the highest concentrations o f d i s s o l v e d s i l i c a : the median value i n the U.S. i s 17 ppm(42). Of e a r t h s surface waters, streams and r i v e r s contain the most d i s s o l v e d s i l i c a . The median value for streams i n the U.S. i s 14 ppm(43). For r i v e r s , the worldwide mean concentration i s 13 ppm(44). Lakes are reported to contain about 4 ppm(45), while the mean concentration of d i s s o l v e d s i l i c a i n the oceans i s about 6 ppm(46). The median value for d i s s o l v e d s i l i c a i n the p u b l i c water s u p p l i e s of the 100 l a r g e s t U.S. c i t i e s i s 7.1 ppm(47). Earth*s biomass a l s o contains appreciable s o l u b l e s i l i c a . R e l a t i v e l y large amounts of s i l i c a are absorbed from s o l u t i o n , concentrated, and p r e c i p i t a t e d by the s i l i c e o u s sponges (Hyalospongiae) and the protozoan orders R a d i o l a r i a and H e i l o z o a , while the m a j o r i t y o f species i n the Animal Kingdom only c o n t a i n d i s s o l v e d s i l i c a i n the parts per m i l l i o n range.(48) The p r e c i s e amount of s o l u b l e s i l i c a found i n plants i s determined by both species and s o i l f a c t o r s . Lower p l a n t s , such as grasses (Gramineae) are very r i c h i n s i l i c a , wet-land v a r i e t i e s u s u a l l y c o n t a i n i n g the highest concentrâtions(49). In g e n e r a l , legumes and dicotyledonous plants contain l e s s s o l u b l e s i l i c a than monocotyledons.(50) Infrared absorption studies have shown that most biogenic s i l i c a i s present as g e l or d i s s o l v e d s i l i c a . However, emission spectroscopy studies have i n d i c a t e d that some of the s o l u b l e s i l i c a found i n animals i s bound to organic molecules, such as glycosaminoglycans, whose s t r u c t u r e has yet to be i d e n t i f i e d ( 5 1 ) . f

f


4.

SCHLEYER

Environmental

AND BLUMBERG

Aspects

Environmental Chemistry The s o l u b i l i t y o f s i l i c a can be c h a r a c t e r i z e d by the f o l l o w i n g e q u i l i b r i a at 25°C. M o n o s i l i c i c acid has been w r i t t e n H S i 0 ( 0 H ) 2 , r a t h e r than S i i O H ) ^ or H4S1O4 i n order to emphasize i t s d i a b a s i c c h a r a c t e r , and the tendency o f s i l i c o n l i k e other m e t a l l o i d s , t o coordinate with hydroxo and oxo l i g a n d Downloaded by QUEENSLAND UNIV OF TECHNOLOGY on October 31, 2014 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/bk-1982-0194.ch004

2

2

Quartz + 2H 0 2

>H Si0 (OH) 2

2

Amorphous s i l i c a + 2H 0

*H Si0 (OH)

2

H^SiO^OH)HSi0 (OH)^ 2

4H Si0 (0H)^ 2

2

2

*HSi0 (OH) 2

*Si0 (OH) 2

2 2

2

6

2

+ H + H

>Si 0 (OH)^ 4

2

2

2

+

+

+ 2H

+

+ 4H

2

log

Κ =

-3.7

log

Κ =

-2.7

log

Κ =

-9.46

log

Κ = -12.56

log

Κ = -12.57

Stumm(52) used these e q u i l i b r i a to construct the diagram i n Figure 5 which 'describes the s p e c i a t i o n o f s i l i c a i n aqueous s o l u t i o n . His data i n d i c a t e that at normal environmental pH values (pH 9) d i s s o l v e d s i l i c a e x i s t s e x c l u s i v e l y as monos i l i c i c a c i d . This c o n c l u s i o n i s supported by the f i n d i n g that s o l u b l e s i l i c a has a d i f f u s i o n c o e f f i c i e n t o f 0.53 i n d i c a t i n g a molecular s i z e about equivalent to m o n o s i l i c i c a c i d ( 5 3 ) . Below about pH = 9.4 the s o l u b i l i t y o f amorphous s i l i c a i s about 120 ppm(54). Quartz has a s o l u b i l i t y o f only about 6 ppm, but i t s r a t e o f c r y s t a l l i z a t i o n i s so slow at ordinary temperatures and pressures that the s o l u b i l i t y o f amorphous s i l i c a represents the upper l i m i t o f d i s s o l v e d s i l i c a concentration i n n a t u r a l waters. Dissolved s i l i c a i s supplied to the environment by chemical and biochemical weathering processes which involve the t r a n s f e r of energy from b i o l o g i c a l systems t o s i l i c a t e minerals as well as i o n s u b s t i t u t i o n and c h e l a t e forming r e a c t i o n s which remove mineral l a t t i c e c a t i o n s ( 5 5 ) . The concentration o f d i s s o l v e d s i l i c a i n n a t u r a l waters i s c o n t r o l l e d by a b u f f e r i n g mechanism which i s thought to i n v o l v e the s o r p t i o n and desorption o f d i s s o l v e d s i l i c a by s o i l p a r t i c l e s and sediments(56, 57). The average s i l i c a weathering rate o f watersheds i s 20 kg/ha/hr(58). The processes o f the n a t u r a l s i l i c a c y c l e are depicted i n Figure 6. Any s o l u b l e s i l i c a input to t h i s n a t u r a l c y c l e as a r e s u l t o f the production or use o f commercial s o l u b l e s i l i c a t e s would be a t r i v i a l amount i n view of the high f l u x o f the n a t u r a l s i l i c a c y c l e . Dissolved s i l i c a from commercial s o l u b l e s i l i c a t e s i s i n d i s t i n g u i s h a b l e from n a t u r a l d i s s o l v e d s i l i c a s i n c e depolymerization o f p o l y s i l i c a t e anions to monomeric d i s s o l v e d s i l i c a occurs very r a p i d l y when



64

SOLUBLE

Figure 5.

Soluble silicate speciation.

Mineral Silica

&

Silicates Mineralization

Weathering

Sedimentation \ \

Dissolution /

Dissolved

Adsorption

Silica

Biogenous Silica ^

Assimilation

DesorptionSorbed ^

Figure 6.

Silica

The natural silica cycle.


SILICATES

4.

SCHLEYER AND

BLUMBERG

Environmental

commercial s o l u b l e s i l i c a t e s o l u t i o n s are d i l u t e d water(59).


Aquatic

65

Aspects

with

Toxicity

Aquatic t o x i c i t y data i s u s u a l l y expressed i n terms o f the median t o l e r a n c e l i m i t . TLm, which i s defined as that c o n c e n t r a t i o n of a substance that i t l e t h a l to 50 percent of the t e s t population i n an a r b i t r a r y time p e r i o d . Table V I I I . l i s t s the TLm values obtained for sodium s i l i c a t e . N u t r i t i o n a l Aspects The e s s e n t i a l nature of s i l i c o n as a n u t r i e n t has long been recognized i n p r i m i t i v e plant and animal species that u t i l i z e i t i n the form of s i l i c a as a s t r u c t u r a l m a t e r i a l ( 6 0 ) . Until r e c e n t l y , i t had been thought that since the b o n e - c a r t i l a g e system had evolved i n animals and the c e l l u l o s e - l i g n i n system had evolved i n p l a n t s s i l i c a had become o b s o l e t e ; that the presence of s i l i c a i n higher species was simply a t t r i b u t a b l e to t h e i r contamination by the vast q u a n t i t i e s of s i l i c a i n the n a t u r a l environment. In the past few years, however, a number of experiments have i n d i c a t e d that s i l i c o n i s necessary, a l b e i t i n t r a c e q u a n t i t i e s , f o r the normal growth, development and f u n c t i o n i n g o f a l a r g e v a r i e t y of higher animals(61), and i t i s a n t i c i p a t e d that s i l i c o n w i l l become recognized as an e s s e n t i a l n u t r i e n t f o r most i f not a l l s p e c i e s . The problems a s s o c i a t e d with "blooms" of algae which occur i n eutrophic bodies of water have motivated much research i n t o determining the l i m i t i n g n u t r i e n t s which c o n t r o l the growth of a l g a l populations. It has been demonstrated that at concentrations o f l e s s than 0.1 ppm, s i l i c a i s a l i m i t i n g n u t r i e n t f o r diatoms(62), and a few other a l g a l s p e c i e s ( 6 3 ) . Thus, only i n bodies of water which are orders of magnitude lower i n s i l i c a c o n c e n t r a t i o n than normal environmental l e v e l s , could s i l i c a become a l i m i t i n g f a c t o r to a l g a l growth. The a d d i t i o n o f excess s o l u b l e s i l i c a over the l i m i t i n g c o n c e n t r a t i o n w i l l not stimulate the growth o f diatom p o p u l a t i o n s ; t h e i r growth r a t e i s independent of s i l i c a conc e n t r a t i o n , once the l i m i t i n g c o n c e n t r a t i o n i s exceeded(64, 65). It has been observed that when a body of water becomes eutrophic due to l a r g e inputs of phosphorus, diatom populations i n c r e a s e , and t h i s r e s u l t s i n a d e c l i n e i n the d i s s o l v e d s i l i c a content of the water, e s p e c i a l l y the surface water(66). I f t h i s process continues u n t i l the a v a i l a b l e s i l i c a becomes depleted below the l i m i t i n g c o n c e n t r a t i o n for diatoms, they are replaced by obnoxious green and blue-green a l g a l species which have much lower requirements f o r s i l i c o n ( 6 7 ) . Thus, i t i s b e n e f i c i a l to maintain an adequate supply of s o l u b l e s i l i c a i n a phosphorus-rich body o f water i n order to promote diatoms as



SOLUBLE

SILICATES

Table VIII. Aquatic Toxicity Sodium Silicate Animal

Time

Annelids Negris grubei Capitella capitata Mosquitofish Gambusia affnis It

It

ti

Dose

Ref.

28days 28days

250g-at/l. 210g-at/L

68. 68.

24hr. 48hr. 96hr.

3200ppm 2400ppm 2320ppm

69. 69. 69.

Water flea Daphnia magna Snail eggs Lymnea

96hr.

247ppm

70.

96hr.

632ppm

70.

Amphipoda

96hr.

160ppm

70.


4.

SCHLEYER AND BLUMBERG

Environmental

Aspects

67

the dominant algae. Sodium s i l i c a t e has been reported to i n h i b i t the growth o f a troublesome species o f blue-green algae(68).


LITERATURE CITED 1. Zwick, Η., Das Wasserglas, Fussli, Zurich, 1877. 2. Vail, J.G., Soluble Silicates, ACS Monograph (46), Chemical Catalog Co., NY, 1928. 3. Ibid., 95. 4. Ibid., 410-11. 5. Ibid., 413-14. 6. PQ Corporation, unpublished documents and records. 7. Joint FAO/WHO Expert Committee on Food Additives, WHO Food Additive Ser. (5), 21-30. 8. Gaskins, J.R., "Analytical Report," No. 016-2583, U.S. FDA, Div. Toxicological Evaluation, Bureau of Science, Washington, D.C., 1966, 2. 9. Calandra, J.C.: Fancher, O.E., The Soap and Detergent Association Scientific and Technical Report 1972, (5R), 24. 10. PQ Corporation, "Biological Study No. LH57085-1-4," 1961, 4. 11. Hehir, R.M., "Research Data on Silicates - Memorandum" E.W. Ligon/R.M. Hehir, U.S. FDA, Div. Toxicological Evaluation, Bureau of Science, Washington, D.C. 1967, 1. 12. Ibid, 9-10. 13. Burbower, G.W., "Experimental Data From Consumer Product Safety Commission Studies on the Provisional Rabbit Test," 1973, 5. 14. Eichhorst, Η., Schweiz Med. Wochschr., 1920, 50, 1081. 15. Scheffler, L., Comptes Rendus, 1920, 171, 416-18.16. King, E.J.; Stantial, H.; Dolan, Μ., Biochem. J., 1933, 27, (4), 1002-6. 17. Newberne, P.M.; Wilson, R.B., Proceedings National Academy of Science U.S., 1970, 65, (4), 872-75. 18. Smith, G.S.; Neumann, A. L.; Gledhill, V. Η.,; Arzola, C. Α., J. Anim. Sci., 1973, 36, (2), 271-8. 19. Ibid., 876. 20. Ita, R., Toho Igakkai Zasshi, 22, (2), 223-7. 21. Benke, G.M.; Osborne, T.W., Fd. Cosmet. Toxicol., 1979, 17, 123-127. 22. Sauer, F.; Laughland, D. H.; Davidson, W. Μ., Can. J. Biochem. & Physio., 1959, 37, 183-91. 23. Becking, G.C., Report of the Task Force on the Health Effects of Non-NTA Detergent Builders to the International Joint Commission Great Lakes Advisory Board, Windsor, Ontario, 1981, 57-69. 24. Ibid., 64. 25. Blumberg, J.G.; Schleyer, W.L., "Current Regulatory Status of Soluble Silicates," American Chemical Society Symposium on Soluble Silicates, New York, 1981.


68

SOLUBLE


26.

SILICATES

Select Committee on GRAS Substances, Evaluation of the Health Aspects of Certain Silicates as Food Ingredients, SCOGS-61, NTIS Pb 301-402/AS, 1979, 4. 27. Draize, J.H., J. Pharm. and Exp. Ther., 1944, 82, 337. 28. PQ Corporation, "Biological Study No. LH57393," 1961,2-3. 29. Hehir, R.M., Op. c i t . , 2. 30. PQ Corporation, "Biological Study No. FDRL800573," 1980, 1. 31. PQ Corporation, "Biological Study No. HL790101," 1972, 2-4. 32. PQ Corporation, "Biological Study No. HL790104," 1973, 2-3. 33. PQ Corporation, "Biological Study No. HL790106," 1973, 1. 34. Arch. Ind. Hyg. Occ. Health, 7, 1953, 411-23. 35. White, R.P., The Dermatergoses or Occupational Affections of the Skin, Lewes, London 1934. 36. Schwartz, L.T.L., Birmingham, D.J., Occupational Diseases of the Skin, 3rd ed., Lea & Febiger, Philadelphia, 1957 248. 37. PQ Corporation, unpublished records. 38. Hehir, R.M., Op. c i t . , 5. 39. PQ Corporation, "Biological Study No. FDRL800573," 1980, 1. 40. Michon, R.; Sue, P.; Merinis, J., Comtes Rendus, 1956, 243, 2194-5. 41. Becking, G.C., Op. c i t . , 64. 42. Davis, S.N., American Journal of Science, 1964, 262, 870. 43. Ibid., 870. 44. Edwards, A.M.C.; Liss, P.S., Nature 1973, 243, 341. 45. Sutherland, J.C., Envt. Sci. Tech., 4, (10), 826. 46. Kido, Κ., Marine Chemistry, 1974, 2, (4), 277-86. 47. Anon., "Public Water Supplies of the 100 Largest Cities in the United States," U.S.G.S. Paper No. 1812, 1962. 48. Levier, R.R., Bioinorganic Chemistry, 1975, 4, (2), 109-16. 49. D'Hoore, J.; Coulter, J.K., Soils of the Humid Tropics Nat. Acad. Sci., Washington, D.C. 1972, 163-73, 50. Lewin, J.; Reimann, B.E.F., Annual Review of Plant Growth, 1969, 20, 289. 51. Schwarz, Κ., Trace Elements Metab. Anim., Proc. Int. Symp. 2nd, 1974, 355. 52. Stumm, W.; Morgan, J.J., Aquatic Chemistry, New York, Wiley, 1970. 53. Iler, R., The Colloid Chemistry of Silica and Silicates, 1955, Cornell U. Press, Ithaka, NY, 12.54. Alexander, G.B.; Heston, W.M.; Iler, R.K., J. Phys. Chem., 1954, 58, 453. 55. Boyle, J.R.; Voight, G.K., "Biological Weathering of Silicate Minerals," Plant Soil, 1973, 38, (1), 191-201. 56. Edwards, A.M.C.; Liss, P.S., Nature 1973, 243, 341. 57. Oehler, J.H., Biogeochemical Cycling of Mineral Forming Elements," Trudinger, P.A.; Swaine, D.J., eds., Elsevier Publ. Co., 1979, 467-483.


4.

SCHLEYER A N DBLUMBERG

Environmental

Aspects

69

Soukup, M.A., "The Limnology of a Eutrophic Hardwater New England Lake, with Major Emphasis on the Biogeochemistry of Dissolved Silica," Xerox Univ. Microfilms, Ann Arbor, Michigan, Order No. 75-270527. 59. O'Connor, T.L., J. Phys. Chem., 1961, 65, (1), 1. 60. Carlisle, E., Trace Elem. Metab. Anim., Proc. Int. Symp. 2nd 1973(pub 1974), 407-423. 61. Schwarz, K., Op. c i t . , 357. Downloaded by QUEENSLAND UNIV OF TECHNOLOGY on October 31, 2014 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/bk-1982-0194.ch004

58.

62. Kilham, P., Limnology and Oceanography, 1971, 16, (1), 10. 63. Klaveness, D.; Guillard, R.R.L., J. Phycol., 1975, 11, 3, 349-55. 64. Jorgensen, E.G., Dansk Botanisk Arkiv, 18, (1), 1957, 5. 65. Schwartz, A.M., Interim Report for Environmental Protection Agency Contract FWQA 14-12-875, 1972. 66. Schelske, C.L.; Stoermer, E.F., Science, 1971, 173, 423. 67. Kilham, P., Op. c i t . , 12. 68. Schwartz, A.M., Op. c i t . , 79. 69. Reish, D.J., Water Research, 1970, 4, 721. 70. Wallen, I.E.: Greet, W.C.; Lasater, R., Sewage and Industrial Wastes, June 1957, 695. 71. Dowden, B.F.; Bennett, H.J., J. Water Pollution Control Fed., 1965, 37, (9), p. 1308. RECEIVED

March 2,

1982.


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