The Effect of Degree of Polymerization of Silicates on Their

9 The Effect of Degree of Polymerization of Silicates on Their Interactions with Cations in Solution

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JAMES S. FALCONE, JR. The PQ Corporation, Research and Development Center, Lafayette Hill, PA 19444 The development of a better understanding of the role soluble silcates play in detergency, mineral beneficiation, enhanced oil recovery, etc., has led us to look carefully at the interactions of soluble silicate species with metal ions in solution. The acidity of ≡Si-OH increases as the complexity of the silica species increases, e.g., pK for Si(OH)2O2= is 9.91± 0.04 whereas ≡Si-OH on silca gel has a pK = 6.8± 0.2. Viewing the interaction of metal ions in solution with a soluble silica surface as an ion exchange process, one might expect differences dependent on the degree of poly merization of the silicate anion. Our results indicate that metal ion activities in solution are sensitive to factors related to the anion structure and pH value. The results support I l e r ' s obser vation that silicate solution begins to absorb multivalent metal ions in solution at pH values roughly two units below the pH at which the metal hydroxide is precipitated a

a

The water s o l u b l e g l a s s e s i n the f a m i l y Na20:mSi02 where m ( r e f e r r e d to as the r a t i o or modulus, o f the s i l i c a t e ) v a r i e s between approximately 0.5 and 4.0 on a molar b a s i s are a s i g n i f i c a n t commodity chemical i n world commerce. These m a t e r i a l s f i n d use i n a wide v a r i e t y o f i n d u s t r i e s . They are valued as a c t i v e sources o f b u i l d i n g b l o c k S 1 O 2 f o r the manufacture o f s y n t h e t i c c l a y s , g e l s and s o l s u s e f u l as c a t a l y s t s , d e s i c c a n t s , i o n exchangers, a n t i - s l i p agents, s i l i c a wafer p o l i s h i n g chemicals and beer c l a r i f i e r s , to name a few.Q) However they are a l s o valued i n other markets f o r t h e i r a b i l i t y to provide a v a r i e t y o f s o l u t i o n and surface chemistry p r o p e r t i e s . They f i n d use i n e s s e n t i a l technologies such as detergency(2), water t r e a t m e n t ( 3 ) , mineral bénéficiâtion(4) and o i l r e c o v e r y ( 5 ) . These uses are g e n e r a l l y dependent on the 0097-6156/82/0194-0133$06.00/0 © 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.

134

SOLUBLE

SILICATES

i n t e r a c t i o n o f the anionic s i l i c a t e species i n s o l u t i o n s with metal ions leading to a r e d u c t i o n i n t h e i r a c t i v i t y and/or the i n t e r a c t i o n o f s i l i c a t e anions with surfaces ( u s u a l l y oxides) . These i n t e r a c t i o n s can i n f l u e n c e the d i s p e r s i o n o f p a r t i c l e s , the e f f e c t i v e n e s s o f s u r f a c t a n t s , the w e t t a b i l i t y o f c l a y s and the c o r r o s i o n o f metals among other e f f e c t s . In many o f these processes i t has been observed that the e f f e c t i v e n e s s o f s i l i c a t e s can vary as the r a t i o changes.


Some E a r l y Studies o f S i l i c a t e Metal

Ion I n t e r a c t i o n

In an e a r l y fundamental study o f the e f f e c t o f sodium s i l i c a t e s on i r o n oxide surfaces Hazel(6) concluded that " a t a given hydrogen ion a c t i v i t y and a given s i l i c a c o n c e n t r a t i o n , the e f f e c t i v e n e s s o f the s i l i c a i n d i s c h a r g i n g and recharging i r o n oxide surfaces increased with s i l i c a r a t i o o f the o r i g i n a l sample." He a t t r i b u t e d the d i f f e r e n c e s observed to the s i l i c a t e species v a r i a t i o n s suggested by Harman (7) i n the 1920*s. Harman showed through the a n a l y s i s o f the c o l l i g a t i v e p r o p e r t i e s o f s o l u t i o n s with v a r i o u s r a t i o s o f Si02/Na20, that the s i l i c a t e s o l u t i o n s contain c o l l o i d a l species and what he c a l l e d "crystalloidal" silica. This " c r y s t a l l o i d a l " s i l i c a was considered to be analogous to the simple species then thought to be the components o f the known c r y s t a l l i n e sodium s i l i c a t e s , charged aggregates o f these unit s t r u c t u r e s and s i l i c a ( i o n i c m i c e l l e s ) , or d e f i n i t e complex s i l i c a t e i o n s . Hazel s p e c i f i c a l l y a t t r i b u t e d the recharging behavior o f a l k a l i n e (lower) r a t i o s i l i c a t e s to hydroxide or i o n i c " c r y s t a l l o i d a l " s i l i c a t e species, whereas the behavior o f the higher r a t i o s i l i c a t e s was a t t r i b u t e d to i o n i c m i c e l l e s or aggregated " c o l l o i d a l forms". In a subsequent paper Hazel, McNabb and Machemer(jO studied the i n t e r a c t i o n o f zinc s a l t s with s i l i c a t e s and suggested that the presence o f s i l i c a t e enhanced the h y d r o l y s i s o f Z n . A mutual adsorption o f Zn(OH) and c o l l o i d a l s i l i c a was suggested as part o f the i n t e r a c t i o n . S i l i c a t e s with high proportions o f S1O2 reacted with zinc ions without the formation o f p r e c i p i t a t e s . A s i g n i f i c a n t l i t e r a t u r e has developed i n the l a s t 30 years concerning the i n t e r a c t i o n o f s i l i c a surfaces with metal i o n s . It i s summarized i n "The Chemistry o f S i l i c a " by Iler(9) who mentions that many ions are held i r r e v e r s i b l y on s i l i c a surfaces by forces s t i l l p o o r l y understood i n a d d i t i o n to i o n i c a t t r a c t i o n . Most o f the studies summarized by l i e r deal with the surface o f hydroxylated s i l i c a or s i l i c a g e l . For our purposes we are i n t e r e s t e d i n these r e s u l t s as they r e l a t e to the understanding o f the e f f e c t that " s o l u b l e " s i l i c a t e species have on metal ion a c t i v i t i e s . + +

2


9.

Polymerization

FALCONE

Polymers i n S i l i c a t e

and Interaction

with

135

Cations

Solution


1

Up u n t i l the mid-1970 s, v a r i o u s i n d i r e c t e d methods o f studying the s i l i c a t e species i n s o l u t i o n , r e c e n t l y summarized by Dent Glasser and Lachowski(10), i n d i c a t e d that these s o l u t i o n s are indeed complex mixtures o f s i l i c a t e anions, with v a r y i n g degrees o f p o l y m e r i z a t i o n , i n a dynamic e q u i l i b r i u m . The use o f FT-NMR spectroscopy i n studying complex s i l i c a t e s t r u c t u r e s i n s o l u t i o n s which i s summarized i n a preceding paper\LL' has been perfected over the l a s t decade. The r e s u l t s of NMR studies have provided a somewhat c l e a r e r p i c t u r e o f the s p e c i a t i o n i n s i l i c a t e s o l u t i o n s . Andersson, Dent Glasser and S m i t h ' i / £ t h preceeding paper i n d i c a t e that the l a r g e c o l l o i d a l species observed i n a l k a l i n e s i l i c a t e s o l u t i o n s ( i n the commercial range) might be thought o f as a separate phase. One might think o f these s o l u t i o n s as c o l l o i d a l s i z e " s i l i c a g e l " l i k e p a r t i c l e s dispersed i n a true s o l u t i o n o f i o n i c s i l i c a t e species. 2

n

The

e

Relative A c i d i t y o f "Dissolved"

Silicate

Species

The pK value o f m o n o s i l i c i c a c i d i s 9.91 ± 0.04(13) and Belyakov(14) has shown that the pK value decreases to 6.5 f o r high polymers. Schindler and Kamber(15) have reported a pK value o f 6.8 +^ 0.2 for surface s i l a n o l groups o f s i l i c a g e l . Maatman, e t . al.,(16) and S c h i n d l e r , e t . al.,(17) have shown that m u l t i v a l e n t metal ions a s s o c i a t e with a s i l i c a g e l surface i n a manner that i n d i c a t e s a l i n e a r c o r r e l a t i o n between the l i g a n d p r o p e r t i e s o f the surface s i l a n o l groups and metal ion h y d r o l y s i s . For C u , F e , C d and P b , Schindler observed that the log o f the s t a b i l i t y constant o f surface complex on s i l i c a g e l was roughly 60% o f the log o f the metal ions h y d r o l y s i s constant He a l s o observed that t h i s r e l a t i o n s h i p d i d not hold f o r Ca and M g and that the a s s o c i a t i o n was d i s t i n c t l y dependent on the surface charge and pH v a l u e . James and Healy(18) looked at the adsorption o f hydrolyzable metal ions on S 1 O 2 from the point o f view o f competition between ion-solvent and ion-surface i n t e r a c t i o n s . They suggested that metal ion adsorption i s i n i t i a t e d at a pH value corresponding to surface n u c l e a t i o n , which seems t o r e l a t e to the r e d u c t i o n o f c a t i o n - s o l v e n t i n t e r a c t i o n s , as a r e s u l t o f h y d r o l y s i s or l i g a n d complex formation, leading t o c o n d i t i o n s favorable to the adsorption o f hydrated metal ions from s o l u t i o n . T h e i r model suggests that metal ion h y d r o l y s i s enhances adsorption on S 1 O 2 , whereas Schindler proposes d i r e c t p a r t i c i p a t i o n by unhydrated i o n s . Based on the s i m i l a r i t y o f high polymer s i l i c a t e pK values to that o f s i l i c a g e l one might expect these species t o i n t e r a c t with metal ion i n s o l u t i o n i n a manner analogous t o a

a

a

+ 2

+ +

+ 3

+ 2

+ 2

++

a


136

S O L U B L E SILICATES

s i l i c a g e l . It might a l s o be expected that as the degree of p o l y m e r i z a t i o n decreases, these s i l i c a t e s species would e x h i b i t reduced i n t e r a c t i o n with the metal i o n s . In order to t e s t t h i s hypothesis and develop a b e t t e r understanding of the magnitude of the e f f e c t s , three s i l i c a t e s o l u t i o n s were chosen based on the knowledge that they span the d i s t r i b u t i o n known to be present from s t r u c t u r e studies on concentrated s o l u t i o n , that i s :


m = 0.5

predominantly monomer " i o n i c species" at any c o n c e n t r a t i o n m = 2.0 predominantly multimeric " i o n i c species" except at low c o n c e n t r a t i o n s (below Ca lOOppm) m = 3.8 a c o n c e n t r a t i o n dependent mixture of m u l t i meric and l a r g e r c o l l o i d a l s p e c i e s . The i n f l u e n c e of these s o l u t i o n s on the a c t i v i t i e s o f Ca , Mg and C u were c a r e f u l l y measured using commercial i o n s e l e c t i v e e l e c t r o d e s and compared to r e s u l t s f o r NaOH. ++

+ +

Experimental The P r e p a r a t i o n o f S o l u t i o n s The water used i n making a l l s o l u t i o n s was prepared using 10 MOhm water. Stock s o l u t i o n s of sodium s i l i c a t e s were made from c a r e f u l l y assayed commercially a v a i l a b l e s o l u t i o n s f o r r a t i o (w/w) values of 2.0 and 3.8 and from sodium m e t a s i l i c a t e and NaOH for the 0.5 r a t i o s o l u t i o n . Stock s o l u t i o n s o f M g C l , C a C l and Cu(C104>2 were prepared from reagent grade s a l t s without r e c r y s t a l l i z a t i o n . Concentrations o f M g and C a were determined using EDTA. Copper c o n c e n t r a t i o n s were determined using atomic absorption (AA). The C a and M g solut ions were used without the adjustment of t h e i r pH v a l u e s , whereas C u s o l u t i o n s were i n i t i a l l y adjusted to a pH value of 4.0 using HCl. 2

++

+ +

2

+ +

++

+ +

The Ion S e l e c t i v e E l e c t r o d e s The e l e c t r o d e s used were purchased from ORION and they were as shown i n TABLE I . TABLE I Equipment S p e c i f i c a t i o n s f o r E l e c t r o d e s

Specifications E l e c t r o d e Model # Reference E l e c t r o d e # pH Range Concentration range, ppm

Calcium 93-20 90-01 5.5-11.0 8-2000

Metal Ions Magnesium 93-32 90-01 5.5-10.0 5-2000

Copper 94-29 90-01 6 5-1000


9.

FALCONE

Polymerization

and

Interaction

with

Cations

137

The ORION 7 0 1 / D i g i t a l pH meter was used f o r a l l i o n s e l e c t i v e e l e c t r o d e measurements i n order to employ an expanded s c a l e , whereas pH measurements were recorded on an ORION 6 0 1 / d i g i t a l i o n a l y z e r . The Model 476050 Corning s i n g l e pH r e f e r e n c e combination g l a s s e l e c t r o d e was used f o r the measurement o f the hydrogen i o n a c t i v i t y .


The Experimental

Procedure

In order to minimize u n c e r t a i n t i e s that could a r i s e from v a r i a t i o n s i n p o l y m e r i z a t i o n e q u i l i b r i a due to d i l u t i o n and other sources o f e r r o r due to unsystematic sampling procedures, we adopted, a f t e r some t r i a l and e r r o r , the f o l l o w i n g procedure: -

Prepare and analyze a 2000 ppm c o n c e n t r a t i o n metal ion stock s o l u t i o n . - Prepare s o l u t i o n s from the above i n the working range of 5 to 2000 ppm and check by EDTA or AA. - Measure 100 cm3 of a given c o n c e n t r a t i o n metal i o n s o l u t i o n i n t o a sample b o t t l e measuring c e l l using a 50 cm3 s y r i n g e . - Record the value o f the emf and pH a s s o c i a t e d with the above s o l u t i o n . - I n j e c t i n t o the metal i o n s o l u t i o n a 1 cur* volume of s i l i c a t e or hydroxide s o l u t i o n ( y i e l d i n g a predetermined S i 0 content from 25 to 500ppm or pH value.) - A f t e r one minute record the new emf and pH value o f the combined s o l u t i o n s . - Store the r e s u l t a n t s o l u t i o n i n the a i r t i g h t p l a s t i c sample b o t t l e . - Repeat t h i s measurement procedure with a new S i 0 and/or metal i o n c o n c e n t r a t i o n . - P e r i o d i c a l l y measure the emf and pH value o f a f r e s h standard metal ion s o l u t i o n i n order to assure i n t e r n a l c o n s i s t e n c y i n the r e s u l t s . A l l measurements i n t h i s study were c a r r i e d out at room temperature which was maintained at 21.0 +^ 0.5°C. 2

2

RESULTS The E l e c t r o d e Response Data The emf d a t a c o l l e c t e d f o r each c o n c e n t r a t i o n o f a metal i o n s o l u t i o n (approximately 13 r e p l i c a t e s ) before the a d d i t i o n of hydroxide or s i l i c a t e s o l u t i o n were averaged and the mean value used to c a l c u l a t e a Nernst response curve. Metal c o n c e n t r a t i o n s i n ppm were converted to mois /dm and the Davies equation (19) was used to estimate the values o f the mean molar a c t i v i t y c o e f f i c i e n t s o f the metal ions i n 3


SOLUBLE

138

SILICATES

s o l u t i o n . Least squares r e g r e s s i o n o f the mean emf value vs molar ion a c t i v i t y y i e l d e d the f o l l o w i n g Nernst response curves: + +

E ++ = 75.8(14)* + 26.9(5) l o g [ C a ] Ca

++

E ++ = 70.9(6) + 27.9(3) l o g [ M g ] Mg


Ecu

+ +

=

+ +

1^0-1(38) + 32.6(14) l o g [ C u ]

*Values i n parenthesis are the 95% confidence l i m i t s o f the last d i g i t s . These equations were used i n a l l subsequent c a l c u l a t i o n s o f the values o f metal ion a c t i v i t i e s . The Calcium

Results

Figure 1 represents a summary o f the r e s u l t s observed where sodium hydroxide and the three s i l i c a t e solutons are added to the v a r i o u s CaCl2 s o l u t i o n s . The values f o r 3.8 and 2.0 r a t i o s i l i c a t e s are p l o t t e d , the i n d i v i d u a l 2.0 r a t i o s i l i c a t e s p o i n t s are not shown, o n l y the l e a s t square f i t data and the 95% confidence p r e d i c t i o n l i m i t s o f the r e g r e s s i o n . These l i m i t s give an estimate o f the p r e c i s i o n o f the metal ion a c t i v i t y r e s u l t s p r e d i c t e d from the experimental data which i s u s e f u l when one wishes to compare e n t i r e r e g r e s s i o n l i n e s . The highest values o f pH obtained upon a d d i t i o n o f 0.5 r a t i o s i l i c a t e are also shown. No s i g n i f i c a n t drop i n pCa was shown for OH" or o r t h o s i l i c a t e i n the pH range s t u d i e d . I t was not p o s s i b l e t o study the e f f e c t o f OH" (that i s , reproduce the l i t e r a t u r e value shown i n F i g . 1) or o r t h o s i l i c a t e r a t i o at higher pH v a l u e s due to the l o s s o f accurate response o f the e l e c t r o d e . The r e s u l t s suggest that the a c t i v i t y o f C a i n s o l u t i o n s at constant pH value i s i n f l u e n c e d by the nature o f the s i l i c a t e s o l u t i o n s used to a t t a i n said pH v a l u e . The 3.8 r a t i o s i l i c a t e y i e l d e d a greater a c t i v i t y r e d u c t i o n at an equivalent pH value than d i d the 2.0 r a t i o s i l i c a t e . However, due to i t s lower base strength the 3.8 r a t i o system contains more S 1 O 2 when the s o l u t i o n pH v a l u e s are e q u i v a l e n t . The s o l u t i o n s when aged f o r several weeks and remeasured show an even greater r e d u c t i o n i n pCa and l e s s d i s t i n c t i o n between the 3.8 and 2.0 r a t i o s i l i c a t e . + +

The Magnesium Results The r e s u l t s for magnesium i o n are presented g r a p h i c a l l y i n Figure 2. In t h i s case a l l s o l u t i o n s could be studied completely and intercompared as well as compared to the phase boundary f o r the a c t i v e and b r u c i t e form o f Mg(0H)2- Only the l e a s t squares r e p r e s e n t a t i o n s are shown with the r e g r e s s i o n l i m i t s for the 3.8 r a t i o and the NaOH experimental l i n e s . The l i m i t s f o r the r e p r e s e n t a t i o n s for the 2.0 r a t i o and 0.5 r a t i o are s i m i l a r t o



FALCONE

Polymerization

and Interaction

with

Cations

pH Value Figure 1. Calcium ion activity in solution vs. pH value for freshly prepared and aged solutions of CaCl . The pH value has been adjusted by a sodium silicate solu tion with the indicated ratio of Si0 :Na 0. Key: +, 0.5; Δ , 2.0; O , 3.8 ratios. 2

2

2


139

S O L U B L E SILICATES


140

pH Value Figure 2. Magnesium ion activity in solution vs. pH value for freshly prepared solutions of MgCl . The pH values were adjusted by NaOH and sodium silicate solutions with the indicated ratios of Si0 :Na 0, Key: •, NaOH; +, 0.5; Δ , 2.0; O, 3.8 ratios. 2

2

2


9.

FALCONE

Polymerization

and

Interaction

with

141

Cations


those shown f o r the adjacent l i n e s . I t i s apparent that the r e s u l t s f o r sodium hydroxide and the 0.5 r a t i o s i l i c a are i n d i s t i n g u i s h a b l e and s i g n i f i c a n t l y d i f f e r e n t from those observed for the higher r a t i o s i l i c a t e s . Comparison o f the higher r a t i o r e s u l t s suggests a g r e a t e r r e d u c t i o n as the r a t i o i s i n c r e a s e d , but t h i s d i f f e r e n c e i s not s t a t i s t i c a l l y s i g n i f i c a n t at the 95% confidence l e v e l . It i s a l s o i n t e r e s t i n g to note that the r e s u l t s f o r the h i g h r a t i o s o l u t i o n s are s i m i l a r to the r e s u l t s expected i f the more s t a b l e b r u c i t e form of Mg(0H>2 were being formed. These r e s u l t s suggests that higher r a t i o s o l u t i o n s nucleate the c r y s t a l l i z a t i o n of t h i s form of Mg(0H)2« The Copper Results + +

The r e s u l t s f o r C u are represented g r a p h i c a l l y i n F i g u r e 3 i n the same manner as f o r Ca and M g and we see again a s t a t i s t i c a l l y d i f f e r e n t i n t e r a c t i o n of C u with the species present i n higher r a t i o s i l i c a t e s o l u t i o n when compared to the lower r a t i o s o l u t i o n and hydroxide. A l s o , o f i n t e r e s t , i s the f a c t that the values o f C u a c t i v i t y are somewhat higher i n a l l cases than the v a l u e s one would p r e d i c t from the e q u i l i b r i u m : + +

+ +

+ +

C u 0 ( ) + 2H

+

s

*Ξ=£ C u

+ +

+

H 0, 2

but lower than those expected Cu (0H)£ 2

+

^

2Cu

+ +

+

from:

20H-

There was a l s o a g r e a t e r d r i f t i n the emf data (towards reduced a c t i v i t y ) immediately a f t e r a d d i t i o n o f t i t r a n t making the measurement o f a meaningful " a c t i v e " e q u i l i b r i u m v e r y difficult. A much g r e a t e r s t a b i l i t y was seen i n the C a and Mg d a t a . Since the l i t e r a t u r e values i n d i c a t e that i n time the copper a c t i v i t y should decrease we measured the v a l u e s of pH and pCu f o r the c o p p e r - s i l i c a t e s o l u t i o n s at a l a t e r date and observed the r e s u l t s presented i n F i g u r e 4. In t h i s F i g u r e we see that a l l of the a c t i v i t i e s are reduced as expected based on the l i t e r a t u r e i n f o r m a t i o n , however, what i s t r u l y i n t e r e s t i n g i s the nature of the p r e c i p i t a t e s formed with time i n the v a r i o u s solutions. In the C a and M g ion studies p r e c i p i t a t e s , when observed, were seen immediately. For the copper s o l u t i o n s no p r e c i p i t a t i o n was noted immediately a f t e r a d d i t i o n o f complexing s p e c i e s ; however, the aged s o l u t i o n s do show the presence of p r e c i p i t a t e s which d i f f e r e d i n c o l o r as one v a r i e d the r a t i o o f the s i l i c a t e . The q u a l i t a t i v e observations o f the c o l o r s o f the p r e c i p i t a t e s are: + +

+ +

+ +

++



142

SOLUBLE

SILICATES

Ο

5.75

6.0

6.25

6.5

6.75

7.0

7.25

pH VALUE Figure 3. Copper ion activity in solution vs. pH value for freshly prepared solu tions of Cu(ClO ) adjusted to initial pH 4.0. The pH values were further adjusted by NaOH and sodium silicate solution with the indicated ratios of Si0 :Na 0. Key: •, NaOH; +, 0.5; and Δ , 2.0 and 3.8 ratios. h 2

2

2


FALCONE

Polymerization

and

Interaction

with

Cations


9.

Figure 4. Copper ion activities vs. pH value for solutions shown in Fig. 3 after aging approximately 1 mo compared to the initial values shown as marked solid lines. Key: +, 0.5; Δ , 2.0; and 0 , 3 . 8 ratios of aged solutions.


143

144

SOLUBLE

Material NaOH 0.5 r a t i o 2.0 r a t i o 3.8 r a t i o

SILICATES

Color black-brown b r i l l i a n t blue pale blue white

These r e s u l t s suggest s t r o n g l y that d i f f e r e n t s t r u c t u r e s e x i s t i n the s o l i d phases for d i f f e r e n t r a t i o s .


Discussion The use o f sodium s i l i c a t e as a m o d i f i e r o f d i v a l e n t ion s o l u t i o n a c t i v i t y could be studied using ion s e l e c t i v e e l e c t r o d e s . It appears based on the r e s u l t s for M g and Cu that the r e d u c t i o n i n metal ion a c t i v i t i e s observed f o r the s o l u t i o n s of sodium o r t h o s i l i c a t e are s t a t i s t i c a l l y i n d i s t i n g u i s h a b l e from those observed when NaOH was used; however, f o r C u d i f f e r e n t c o l o r p r e c i p i t a t e s are observed. No a c t i v i t y r e d u c t i o n as a r e s u l t o f base a d d i t i o n could be measured i n the CaCl2 s o l u t i o n s with 0.5 r a t i o s i l i c a t e or NaOH; however, a c t i v i t y reductions were c l e a r l y observed with s i l i c a t e s o l u t i o n s at m values of 2.0 and 3.8. This i n d i c a t e s some enhanced i n t e r a c t i o n of s i l a n o l with C a i o n s . The most meaningful system was M g C l i n that the e l e c t r o d e was s t a b l e i n a l l pMg-pH regions studied and the emf values were more s t a b l e than those observed f o r C u . For t h i s system a c l e a r d i s t i n c t i o n could be made between the r e s u l t s seen for the higher r a t i o s i l i c a t e s and the more a l k a l i n e s o l u t i o n s . This e f f e c t i s a l s o seen for C u , but the u n c e r t a i n t y i n the emf data cloud the r e s u l t s . ++

+ +

+ +

+ +

2

+ +

+ +

Looking at a l l of the r e s u l t s together i t appears that there i s a s i g n i f i c a n t d i f f e r e n c e between higher and lower r a t i o s i l i c a t e s i n t h e i r i n t e r a c t i o n with the metal ions chosen, l i e r has observed t h a t : " S i l i c a suspended i n a s o l u t i o n o f most p o l y v a l e n t metal s a l t s begins to adsorb metal ions when the pH i s r a i s e d to w i t h i n 1-2 pH u n i t s below the pH at which the p o l y v a l e n t metal hydroxide i s p r e c i p i t a t e d . " This observation from r e s u l t s using s i l i c a s o l s i s c o n s i s t e n t with the experimental r e s u l t s observed here for the higher r a t i o s o l u t i o n s . Thus, i t i s most probable that the same mechanisms a s s o c i a t e d with surface complex formation with deprotonated s i l i c a , are operating here. The r e s u l t s for M g and C a suggest that Mg(0H) and Ca(0H) species are being formed s i n c e the pH-pMe l i n e s a l l have slopes o f -2.0 w i t h i n the e r r o r (see Table II) d i s r e g a r d i n g the e r r a t i c r e s u l t s for C a at m • 3.8. + +

+ +

2

2

+ +


9.

FALCONE

Polymerization

and Interaction

with

Cations

145

The C u r e s u l t s suggest a mixture o f Cu(0H>2 and C u ( O H ) 2 . However, since no analyses were c a r r i e d out o f the s o l i d phases formed i n t h i s study, we can not r u l e out the p o s s i b l i t y o f the formation o f precursors t o (or a c t u a l ) mineral s i l i c a t e s and other b a s i c p r e c i p i t a t e s due to the presence o f C I O 4 and Cl~*. Studies are c u r r e n t l y under way i n our l a b o r a t o r y to look at the s o l i d phases formed i n d i l u t e s i l i c a t e and metal ion s o l u t i o n s . + +

+

2


TABLE I I Slopes o f pH-pMe l i n e s pH-pMe slopes Ca Silicate ratio O.O(NaOH) 0.5 2.0 3.8

+ +

-1.97(39) -3.47(172)

+ +

Mg"

Cu

-2.02(10)* -1.82(30) -2.00(26) -2.10(21)

-1.76(34) -1.68(20) -1.43(14) -1.43(16)

*Values i n parentheses are 95% CL values o f l a s t

digits.

So f a r i n our d i s c u s s i o n we have j u s t looked at the a c t i v i t i e s o f the ions versus hydrogen i o n a c t i v i t y . In Figure 5 the r e s u l t s f o r magnesium s o l u t i o n s with i n i t i a l pMg values of 3.47 and 2.49 are shown versus v a r y i n g S 1 O 2 c o n c e n t r a t i o n l e v e l . In t h i s F i g u r e we see that the 0.5 r a t i o s o l u t i o n s cause l i t t l e pMg r e d u c t i o n compared to that seen f o r the 2;0 r a t i o s i l i c a t e at constant values o f S 1 O 2 content i n s o l u t i o n ; however, we a l s o observe that the 3.8 r a t i o s o l u t i o n s are not as e f f e c t i v e as the 2.0 s i l i c a t e r a t i o when compared on t h i s b a s i s . These r e s u l t s suggest that at l e a s t for M g significant a c t i v i t y r e d u c t i o n i n the presence o f c o l l o i d a l s i l i c a species i s dependent on some other f a c t o r s , e.g. c r i t i c a l pH v a l u e , the presence o f small i o n i c s i l i c a t e s p e c i e s , a change i n the s o l i d species present, or some degree o f s u r f a c e s S i - 0 " s i t e s , as w e l l as having a polymerized form o f S 1 O 2 a v a i l a b l e i n s o l u t i o n . There was i n s u f f i c i e n t data to look at t h i s e f f e c t for C a and C u . It would appear that the most probable explanation o f the r e s u l t s i s that i n higher r a t i o s o l u t i o n s the presence o f c o l l o i d a l s i z e s i l i c a t e anions with more a c i d i c surface s i l a n o l groups r e a d i l y adsorb n u c l e a t i n g metal ion hydroxides thus enhancing the r e d u c t i o n o f pMe. This adsorption might be enhanced by the presence o f OH", or i n c r e a s e d f S i - 0 ~ s i t e s on the c o l l o i d a l s i z e d s i l i c a surfaces at higher pH v a l u e s . There i s a l s o the p o s s i b i l i t y that intermediate s i z e d s i l i c a t e species are capable o f forming l i g a n d complexes with metal ions due to t h e i r increased S i l a n o l a c i d i t y and s t r u c t u r e . The + +

+ +

+ +



146

SOLUBLE

>

pMg°=

2.49

^

pMg°=

(10.40)

(10.31)

(9.33) w

ο

Δ (9.61)^Δ^^ (9.59)""

Ε

SILICATES

•

"

(9.29)

+ ο (9.29)

• (9.66) (10.54)

3.47

°- - - - —

-

_

(10.93)

Ο- „ _

(9.67)

~~ "~ ~ ^ _ _

_

\

* *Ο - - ^

Δ^ (10.12)

(9.80)

Ν Ν

Δ

(10.43)

Ν

— Δ

5.0 0

^(10.63)

I

I

I

50

100

150

Sî0

2

I

I 200

250

CONCENTRATION, ppm

Figure 5. Reduction in magnesium ion activity vs. silica concentration for sodium silicate solutions with SiO :Na 0 ratio values of 0.5 (+), 2.0 (A), and 3.8 (O). The pH values are shown in parentheses. g

B


9.

FALCONE

Polymerization

and Interaction

with

Cations

147


behavior o f these s o l u t i o n s might be predicted using a model employing an analogy to s i l i c a g e l surface-metal i o n interactions. The species present i n the very a l k a l i n e lower r a t i o s i l i c a t e s o l u t i o n s do not possess the c a p a b i l i t y o f competing with OH" as a l i g a n d or o f adsorbing these n u c l e a t i n g hydroxide species thus they do not enhance n u c l e a t i o n and t h e i r behavior i s e s s e n t i a l l y p r e d i c t a b l e from metal i o n h y d r o l y s i s constants. Further experiments are planned to look at the above p o s s i b i l i t i e s more c a r e f u l l y .

Literature Cited 1. Falcone, J.S., Jr. Silicon Compounds, Synthetic Inorganic Silicates, ECT, 20, 3rd ed. to be issued Dec. 1982. 2. Campbell, T.C.; Falcone, J.S., Jr.; Schweiker, G.C. HAPPI 1978, 15, 33. 3. Stumm, W.; Huper, H.; Champlin, R.L. Envi. Sci. Tech. 1967, 1, 221. 4. Falcone, J.S., Jr. "Recent Advances in the Chemistry of Sodium Silicates: Implications for Ore Beneficiation" presented at Fall Meeting SME-AIME, Denver, CO, Nov. 19, 1981. 5. Krumrine, P.H.; Ailin-Pyzik, I.B.; Falcone, J.S., Jr.; Campbell, T.C. "Surfactant Flooding III: The Effect of Alkaline Chemicals on the Adsorption of Anionic Surfactants by Clays", presented at the "ACS Symposium on the Chemistry of Enhanced Oil Recovery", Atlanta, GA, April 2, 1981. 6. Hazel, J.F. J. Phys. Chem. 1945, 49, 520. 7. Harman, R.W. J. Phys. Chem. 1928, 32, 44. 8. Hazel, J.F.; McNabb, W.M.; Machemer, P.E. J. Electrochem Soc. 1952, 99, 301. 9. Iler, R.K. "The Chemistry of Silica"; John Wiley & Sons: NY 1979, Chapter 6. 10. Dent Glasser, L.S.; Lachowski, E.E. JCS Dalton 1980, 393. 11. Harris, R.K.; Knight, C.T.G.; Hull, W.E. "NMR Studies of the Chemical Structure of Silicates in Solution", in this volume. 12. Anderson, K.R.; Dent Glasser, L.S.; Smith, D.N. "Polymerization and Colloid Formation in Silicate Solutions", the preceeding paper. 13. Schwartz, R.; Müller, W.D. Z. Anorg. Allg. Chem. 1958, 296, 273. 14. Belyakov, V.N.; Soltiuskii, N.M.; Strazhesko, D.N.; Strelko, V.V. Ukr. Khim. Zh. (Russ. ed) 1974, 40, 236. 15. Schindler, P.W.; Kamber, H.R. Helv. Chim. 1968, 51, 1781. 16. Maatman, R.W.; Dugger, D.L.; Stanton, J.H.; Irby, B.N.; McConnell, B.L.; Cummings, W.W. J. Phys. Chem 1964, 68, 757.


SOLUBLE

148

SILICATES

17. Schindler, P.W.; Furst, B.; Dick, R.; Wolf, P.U. J. Colloid and Interface Sci. 1976, 55, 469. 18. James. R.O.; Healey, T.W. J. Colloid and Interface Sci. 1972, 40, 65. 19. Robinson, R.A.; Stokes, R.H. "Electrolyte Solutions"; 2nd Ed., Butterworths: London, 1959, p 232. 2, 1982.


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The Effect of Degree of Polymerization of Silicates on Their

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