Adsorption From Aqueous Solution

6 The Adsorption of Aqueous Co(II) at the

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Silica-Water Interface T. W .

HEALY

R. O. J A M E S , and R. C O O P E R

Department of Physical Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia

The

adsorption

been studied Co(II)

of Co(II)

concentration.

electrophoretic

The

mobility

the free aquo Co(II) participation

of surface

at the quartz

evidence

of mutual

tated cobalt

silica-water

interface

of pH, ionic strength, adsorption

data,

and coagulation

ion is not specifically

Co(OH)

2

at the

as a function

hydroxyls. surface

coagulation

has

and

total

together

with

data suggest adsorbed

Evidence

for

is presented

polymeric

together

of the quartz

that

without

and

with precipi-

hydroxide.

' T ' h e a d s o r p t i o n of m e t a l ions f r o m aqueous solutions is a p h e n o m e n o n of i m m e d i a t e interest to w o r k e r s i n m a n y diverse d i s c i p l i n e s .

The

i n c o r p o r a t i o n of metals i n t o g e o l o g i c a l sediments, r e m o v a l of m e t a l ions f r o m i n d u s t r i a l a n d c i v i c effluent, interference of trace m e t a l ions i n a n a l y t i c a l a n d e l e c t r o a n a l y t i c a l c h e m i s t r y , ore

flotation,

metallurgical

l e a c h i n g processes, a n d the s t a b i l i t y of c e r a m i c slips are a l l processes w h i c h are c o n t r o l l e d to a large extent b y i n t e r a c t i o n of m e t a l ions w i t h s o l i d - l i q u i d interfaces. R e c e n t studies i n d i c a t e that the a d s o r p t i o n of m e t a l ions is c o n t r o l l e d o n l y i n p a r t b y the c o n c e n t r a t i o n of t h e free ( a q u o ) m e t a l i o n ; of c o n s i d e r a b l e i m p o r t a n c e is the a b i l i t y of h y d r o x o a n d other c o m p l e x ions a n d m o l e c u l e s to adsorb. T h e r e h a v e b e e n t w o a p p a r e n t l y d i v e r g e n t approaches to d e s c r i b e the r o l e p l a y e d b y h y d r o x o m e t a l complexes i n a d s o r p t i o n at solid-aqueous electrolyte interfaces.

M a t i j e v i c et al.

h a v e p r o p o s e d t h a t specific h y d r o l y s i s products—e.g., A l ( O H ) o 8

2

4 +

(9)

i n the

A 1 ( I I I ) - H 0 system, are r e s p o n s i b l e for extensive c o a g u l a t i o n a n d charge 2

reversal of h y d r o p h o b i c c o l l o i d s . M a t i j e v i c that the free (aquo)

It has also b e e n

demonstrated

by

species of t r a n s i t i o n a n d other m e t a l ions 62

In Adsorption From Aqueous Solution; Weber, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

6.

H E A L Y

E T

A L .

Adsorption

of Aqueous

Co(II)

63

is f r e q u e n t l y u n a b l e to reverse the c h a r g e of a sol whereas the h y d r o l y s i s p r o d u c t s , often of l o w e r charge p e r i o n , c a n reverse the m o b i l i t y of A g l sols ( i n statu nascendi)

electrophoretic

a n d r u b b e r latex sols ( J O ) .

A l t e r n a t i v e l y , several w o r k e r s h a v e s h o w n that not o n l y is t h e soluble, z e r o - c h a r g e d h y d r o l y s i s p r o d u c t c o n s i d e r a b l y m o r e surface active t h a n the free ( a q u o ) i o n b u t also a p o l y m e r i c c h a r g e d or u n c h a r g e d h y d r o l y sis p r o d u c t m a y b e f o r m e d at the s o l i d - l i q u i d interface at c o n d i t i o n s w e l l Downloaded by UNIV OF MASSACHUSETTS AMHERST on October 13, 2015 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch006

b e l o w s a t u r a t i o n or p r e c i p i t a t i o n i n s o l u t i o n . H a l l ( 5 )

has

considered

the c o a g u l a t i o n of k a o l i n i t e b y a l u m i n u m ( I I I ) a n d c o n c l u d e d that sur face p r e c i p i t a t e s r e l a t e d to h y d r a t e d a l u m i n u m h y d r o x i d e c o n t r o l the adsorption-coagulation behavior.

S i m i l a r l y H e a l y a n d Jellett ( 6 )

p o s t u l a t e d that the p o l y m e r i c , soluble, u n c h a r g e d Z n ( O H ) be n u c l e a t e d c a t a l y t i c a l l y at Z n O - H 0 interfaces a n d w i l l 2

2

have

polymer can flocculate

the

c o l l o i d a l Z n O via a b r i d g i n g m e c h a n i s m . These

two

mechanisms,

t h e one

e m p h a s i z i n g the

adsorption

of

specific often p o l y n u c l e a r h y d r o l y s i s p r o d u c t s , the other e m p h a s i z i n g the role of p o l y m e r i c species, are c l e a r l y not m u t u a l l y e x c l u s i v e ; a n earlier s t u d y o n t h o r i u m ( I V ) a d s o r p t i o n suggested a c o m b i n e d m e c h a n i s m T h e present s t u d y is o n a system C o ( I I ) - H 0 - S i 0 2

2

(I).

for w h i c h i t w a s

e x p e c t e d that there w o u l d be m i n i m a l a d s o r p t i o n of p o l y n u c l e a r species of the m e t a l i o n b u t the p o s s i b i l i t y of surface catalysis to y i e l d surface p o l y m e r s of the h y d r o x i d e . T h e oxide, a - q u a r t z , w a s selected as the substrate for the present a n d c o n t i n u i n g studies of m e t a l i o n a d s o r p t i o n .

It is of c o n s i d e r a b l e i m p o r

tance i n several p r a c t i c a l situations—e.g.,

water purification and

flotation—and

ore

has the i m p o r t a n t p r o p e r t y that it is n e g a t i v e l y c h a r g e d

over a w i d e p H range since its zero-point-of-charge

(z.p.c.) is circa p H 2.

Experimental T h e quartz dispersion was prepared by milling pure, acid leached, m i l k y «-quartz specimens f r o m W a t t l e G u l l y , V i c t o r i a , A u s t r a l i a , i n a s y n t h e t i c p o r c e l a i n m i l l . T h e b a l l m i l l p r o d u c t was f u r t h e r c l e a n e d b y repeated washing-centrifugation w i t h conductivity water. E.S.R. spectra of the as d r i e d o x i d e p o w d e r itself a n d the d r i e d p o w d e r w h e n y - i r r a d i a t e d i n a C o source, s h o w e d there w e r e n o p a r a m a g n e t i c i m p u r i t i e s . T h i s t e c h n i q u e of analysis i n w h i c h p a r a m a g n e t i c centers are generated around any multi-electron contaminant atom w i l l be reported i n detail shortly; i t has p r o v e d u s e f u l i n the d e t e c t i o n of t r a n s i t i o n m e t a l i o n c o n taminants at the sub p . p . m . l e v e l o n oxide surfaces. T h e q u a r t z p o w d e r w a s f o u n d to have a B . E . T . surface area, b a s e d o n k r y p t o n a d s o r p t i o n , of 5.4 m e t e r / g r a m . 6 0

2

T h e m e t a l ions w e r e generated f r o m t h e i r p e r c h l o r a t e salts w h i c h w e r e either p r e p a r e d f r o m A . R . starting materials or p u r c h a s e d as A . R . chemicals. Solutions w e r e freshly p r e p a r e d for each experiment. A c i d -


64

ADSORPTION F R O M

AQUEOUS SOLUTION

base p H adjustment w a s m a d e w i t h A . R . p e r c h l o r i c a c i d a n d A . R . potas s i u m h y d r o x i d e , respectively. C o n d u c t i v i t y w a t e r w a s p r e p a r e d b y d o u b l e d i s t i l l a t i o n a n d d u r i n g t h e course of t h e i n v e s t i g a t i o n h a d a c o n d u c t a n c e of 0.9 ± 0.1 m i c r o m h o s c m . " . W a t e r outside this u p p e r l i m i t w a s r e jected. C o b a l t ( I I ) h y d r o x i d e , p r e c i p i t a t e d f r o m p e r c h l o r a t e s o l u t i o n w i t h K O H , was washed repeatedly w i t h conductivity water. W h i l e m e t a l ions m a y b e s a i d to adsorb s t r o n g l y at the o x i d e - w a t e r interface, a d s o r p t i o n studies are h a m p e r e d b y t h e n e e d to d e t e r m i n e m e t a l ions at t h e s u b m i c r o m o l a r l e v e l . A g a i n the s i m i l a r i t y b e t w e e n s i l i c a w a t e r a n d glass-water interfaces means that t h e m e t a l i o n species w i l l a d s o r b o n a l l glass surfaces. F o r d e t e r m i n a t i o n of t h e a d s o r p t i o n isotherms of C o ( I I ) , r a d i o a c t i v e tracer techniques w e r e e m p l o y e d u s i n g t h e C o isotope. F o r this y - e m i t t i n g isotope l i q u i d G e i g e r - M u l l e r tubes w e r e f o u n d to b e satisfactory. A t a l l times " b l a n k " runs w e r e c o n d u c t e d to correct f o r a d s o r p t i o n o n t h e glass vessels. E q u i l i b r a t i o n of t h e s i l i c a - w a t e r suspension w i t h the m e t a l i o n w a s c o n d u c t e d i n a t h e r m o s t a t t e d vessel s i m i l a r to that d e s c r i b e d p r e v i o u s l y ( 1 5 ) . H o l e s i n t h e loose fitting l i d w e r e p r o v i d e d f o r p H electrodes,


2

6 0

I

2

3

u

pH

4

5

6

Figure 1. The coagulation and electrophoretic mobility (microns sec.' /volt cm.' ) behavior of quartz in electrolyte solutions as a function of pH 1

1


6.

HEALY

Adsorption

ET AL.

of Aqueous

65

Co(II)


n i t r o g e n gas, a n d for c a p i l l a r y p l a s t i c t u b i n g for s a m p l e r e m o v a l a n d a c i d / b a s e a d d i t i o n f r o m a n a u t o m a t i c p H - s t a t f a c i l i t y . It w a s f o u n d necessary to a l l o w f r o m 3 to 12 hours for e q u i l i b r i u m to be established. T h i s k i n e t i c effect is c u r r e n t l y b e i n g i n v e s t i g a t e d i n m o r e d e t a i l .

i 0

i 2

i

i

i

4

i 6

i

i 8

i

i

i

10

i 12

PH

Figure 2.

Percent adsorption on quartz of cobalt (II) as a function of pH for 1.3 X 10~*M total Co(ClO ) Jf 2

E l e c t r o p h o r e t i c m o b i l i t i e s of the q u a r t z particles i n c o b a l t ( I I ) p e r chlorate solutions w e r e d e t e r m i n e d w i t h a c a l i b r a t e d Z e t a - M e t e r a p p a ratus. C o a g u l a t i o n s e d i m e n t a t i o n b e h a v i o r w a s f o l l o w e d u s i n g a stopflow t y p e apparatus. T h e d i s p e r s i o n is p u m p e d i n a closed l o o p f r o m a n e q u i l i b r a t i o n vessel t h r o u g h a n o p t i c a l c e l l l o c a t e d i n the sample c o m p a r t m e n t of a r e c o r d i n g spectrophotometer. F r o m the o p t i c a l d e n s i t y t i m e c u r v e o b t a i n e d f r o m the t i m e the p u m p is s w i t c h e d off, the t u r b i d i t y i n d e x ( i n a r b i t r a r y u n i t s ) is o b t a i n e d as the slope of the c u r v e at z e r o time.


66

ADSORPTION F R O M

AQUEOUS SOLUTION

Results T h e g e n e r a l surface properties of q u a r t z are s h o w n i n F i g u r e 1 i n w h i c h the electrophoretic

m o b i l i t y a n d t u r b i d i t y of q u a r t z i n s i m p l e

electrolytes is p l o t t e d as a f u n c t i o n of p H . T h e d o t t e d lines s h o w n i n F i g u r e 1 represent a r e g i o n of i o n i c strength for w h i c h inaccuracies i n m o b i l i t y d e t e r m i n a t i o n p r e v e n t the a c c u m u l a t i o n of m e a n i n g f u l

data.


A l t h o u g h the a p p r o a c h to the z.p.c. of q u a r t z is o b s c u r e d b y this effect, the t u r b i d i t y d a t a c o n f i r m that the z.p.c. is at p H 1.8-2.0 i n agreement w i t h other w o r k e r s

(13).

T h e a d s o r p t i o n of c o b a l t ( I I )

at 1.3 X

10" M C o ( C 1 0 ) 4

4

2

is s h o w n

i n F i g u r e 2. i n the p H range f r o m 1.7 to 12.0. T h i s f o r m of p l o t , percent a d s o r p t i o n vs. p H or c o n c e n t r a t i o n w h i l e u s e f u l for d e m o n s t r a t i n g the d r a m a t i c increase i n a d s o r p t i o n over a n a r r o w p H o r c o n c e n t r a t i o n range, is h o w e v e r of l i m i t e d t h e o r e t i c a l v a l u e . I n F i g u r e 3 the c o b a l t ( I I )

ad

s o r p t i o n d a t a are therefore r e d r a w n as l o g ( a d s o r p t i o n d e n s i t y ) vs. p H . T h e v e r t i c a l d a s h e d lines i n F i g u r e s 2 a n d 3 represent the m i n i m u m p H for p r e c i p i t a t i o n of 1.3 X

10" M C o (II) 4

i n the absence of

adsorption.

T h e p l a t e a u of F i g u r e 3 therefore represents a d s o r b e d a n d p r e c i p i t a t e d cobalt. I

Figure 3.

1

1

1

1

1

1

Adsorption isotherm of cobalt (II) on quartz as a of pH and at 1.3 X i O ' M total Co(ClO )

function

lt 2



6.

H E A L Y

I0"

E T A L .

Adsorption

10"*

7

EQUILIBRIUM

Figure

4.

Complete

of Aqueous

io'

I0"

8

CONCENTRATION

67

Co(II)

OF

I0~

4

8

Co(H), M / L I T E R

adsorption isotherm of Co(II) on quartz at pH 6.0 and 25°C.

T h e adsorption of C o ( I I )

at p H 6.0 is s h o w n i n F i g u r e 4, a g a i n

p l o t t e d o n a log-log scale. S i n c e c o b a l t ( I I ) h y d r o x i d e does not p r e c i p i tate at p H 6 at less t h a n 1 M t o t a l cobalt, the p l a t e a u a t t a i n e d represents saturation a d s o r p t i o n w i t h o u t interference b y p r e c i p i t a t i o n . T h e v a r i a t i o n w i t h p H o f t h e e l e c t r o p h o r e t i c m o b i l i t y of q u a r t z i n 1 0 " M c o b a l t ( I I ) p e r c h l o r a t e a n d a c o m p a r i s o n o f the m o b i l i t y o f q u a r t z 4

i n 1 0 " M K C 1 is s h o w n i n F i g u r e 5. I n c l u d e d i n F i g u r e 5 is the v a r i a t i o n 4

w i t h p H of electrophoretic m o b i l i t y o f p r e c i p i t a t e d c o b a l t ( I I ) h y d r o x i d e . It c a n b e seen that t h e s i l i c a surface w i t h a d s o r b e d

C o (II)

acts as

c o b a l t ( I I ) h y d r o x i d e f o r p H values a b o v e 8.0. T h e t u r b i d i t y vs. p H b e h a v i o r at 1 0 " M C o ( 0 0 4 ) 2 4

is s h o w n i n F i g u r e 6. T h e t w o curves

represent t h e b e h a v i o r f o r i n c r e a s i n g a n d decreasing p H a n d w i t h i n e x p e r i m e n t a l error the curves superimpose. Discussion T h e most g e n e r a l feature o f the a d s o r p t i o n b e h a v i o r o f m e t a l ions at solid-aqueous s o l u t i o n interfaces is the a b r u p t rise i n a d s o r p t i o n over a n a r r o w p H range. T h i s has b e e n i l l u s t r a t e d , for e x a m p l e , for manganese a d s o r p t i o n o n glass (2), c o b a l t o n h y d r o u s f e r r i c oxide ( 8 ) , manganese o n h y d r o u s manganese

oxide

(12), p r o t a c t i n i u m o n glass

(14), a n d


68

ADSORPTION F R O M

AQUEOUS SOLUTION


CO o o h1x1 o X CL

o

h

Figure 5. Electrophoretic mobility (microns sec.' / volt cm.' ) of Si0 in different electrolyte solutions together with the electrophoretic mobility of cobalt (II) hydroxide as a function of pH 1

1

2

— A — SiO 10-*M Co(II) Co(OH),, 10-''M KCl ti

t h o r i u m o n silver i o d i d e (11).

If w e c o m p a r e the p H range at w h i c h

i n c r e a s e d a d s o r p t i o n occurs w i t h the properties of the s o l u t i o n i n the same p H range there is often a s t r i k i n g p a r a l l e l o b s e r v e d . M a t i j e v i c et al.

F o r example,

h a v e s h o w n that t h e increase i n a d s o r p t i o n of

(11)

t h o r i u m occurs over the same p H r a n g e w h e r e the r a t i o [ T h ( O H )

3 +

]/

[ T h ( N O ) ] also increases a b r u p t l y . H

4

W e c a n g e n e r a l i z e b y n o t i n g that there w i l l be i n c r e a s e d a d s o r p t i o n at the p * K i of the a q u o complex—i.e., * K M(H 0) 2

6

n +

X

for

4- H 0 ?± M ( H 0 ) ( O H ) " " 2

2

5

(

1 ) +

+ H.O

(1)

+

T h u s , for t h o r i u m ( I V ) t h e a d s o r p t i o n increase occurs at p H 3.82

(13).

H o w e v e r , for C o ( I I ) w i t h a p * K i v a l u e of 9.8, the a d s o r p t i o n increase does not c o r r e s p o n d to p * K i b u t is d i s p l a c e d to a l o w e r p H v a l u e . C o m p a r i s o n of F i g u r e s 2 a n d 5 shows that at the p H range 6.5 to 7.5 w h e r e i n c r e a s e d a d s o r p t i o n occurs, simultaneous r e v e r s a l of c h a r g e a n d e q u i v a l e n t c o a g u l a t i o n are b o t h observed.

T h i s suggests

that strong

a d s o r p t i o n of a c a t i o n i c c o b a l t ( I I ) species is o c c u r r i n g . T h e p r i n c i p a l


6.

H E A L Y

E T

Adsorption

A L .

of Aqueous

69

Co(II)

c o b a l t ( I I ) s o l u t i o n species r e p o r t e d i n the l i t e r a t u r e ( 3 ) are represented i n F i g u r e 7 for the f o l l o w i n g self-consistent set of s t a b i l i t y constants Co(OH) ^Co 2

2 +

+ 20H-

log

Co

2 +

+ OH"^ CoOH

Co

2 +

+ H 0 5± C o O H + H

log K

+

+

2


Co(OH) Here, C o ( O H )

2

2

1

=

-14.8

(2)

= 4.2

(3)

-9.8

(4)

> log K =

-6.40

(5)

=

-5.10

(6)

X

2

+ OH"^

0

log *K =

+

Co(OH) ^±Co(OH) aq. 2

K =

S 2

C o ( O H ) - log K 3

B 8

represents the s o l i d h y d r o x i d e . T h e s o l u t i o n d a t a s h o w

that at p H values of 7.5 a n d 6.5 the d o m i n a n t c o b a l t ( I I ) species is t h e free ( a q u o ) i o n b y factors of 100 a n d 1000 respectively.

It is therefore

h i g h l y u n l i k e l y that the c o a g u l a t i o n at p H 6.5-7.5 a n d 1 0 " M C o ( I I ) 4

the r e v e r s a l of charge c a n b e c a u s e d b y the free C o O H

+

species.

and

If it is

caused b y p o l y n u c l e a r c h a r g e d species t h e n the l o g - l i n e a r r e l a t i o n s h i p ( 9 ) b e t w e e n the c r i t i c a l c o a g u l a t i o n c o n c e n t r a t i o n a n d the valence of the c o a g u l a t i n g i o n w o u l d r e q u i r e a p o l y n u c l e a r species to h a v e a charge of + 5 or + 6 .

S u c h a species has not b e e n i d e n t i f i e d . ( I t is of interest to

note t h a t i f this species d i d exist i t w o u l d have to b e a c o m p a c t

5

6

7

8 pH

9

10

Figure 6. Coagulation of silica in aqueous 10~*M Co(II) solution as a function of pH. Increase in the turbidity index indicates an increase in dispersion


ion,


70

ADSORPTION F R O M

O

2.

4

C P

H

8

10

AQUEOUS SOLUTION

12.

Adsorption From Aqueous Solution

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