Adsorption From Aqueous Solution

8 Adsorption of Selenite by Goethite F.J.

HINGTON

Division of Soils, C.S.I.R.O., W . Α., Laboratories, Wembley, Western Australia, 6014

Downloaded by UNIV OF BATH on October 2, 2014 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch008

A . M. P O S N E R and J . P.

QUIRK

Department of Soil Science and Plant Nutrition, University of Western Australia, Nedlands, Western Australia, 6009

Specific

adsorption

of the

of selenite

suspension

surface.

and

Isotherms Γ=

where Γ = tion, selenite,

the

at

constant

selenite

adsorbed,

(Se)

equilibrium

=

Γo(pH)

and KL is a constant. of K

L

KL = K

D

=

pH

are

Γo(pH)

+

+

K'2/Kw,

+

The

entropy

=

maximum

adsorp

concentration

of

and KL vary with

pH.

by,

KD), for

selenious

where K'2 is an exchange

constant

constant

gain of the reaction

release of a water molecule ion is

by,

L

for OH- and SeO32- and Kw is the dissociation water.

pH oxide

K (Se)],

is represented

K2KDH /(H

the

on the

represented

solution

the second dissociation

acid and K2 =

increases

charge

• KL(Se)/[1 +

Γo(pH)

The pH dependence

where

on goethite negative

constant

is consistent

for with

from the surface when a selenite

adsorbed.

A d s o r p t i o n of anions at m i n e r a l surfaces is i m p o r t a n t i n soils because of the l i m i t this process imposes o n the a v a i l a b i l i t y of p l a n t n u t r i e n t s s u c h as P , S, a n d M o w h i c h o c c u r n a t u r a l l y as anions a n d are a d d e d i n a n i o n i c f o r m i n fertilizers.

A n i o n a d s o r p t i o n is also relevant i n geo

c h e m i s t r y , ore processing, a n d other fields w h e r e m i n e r a l s w i t h

high

surface areas are b r o u g h t i n t o contact w i t h aqueous solutions of anions. Selenite a n d goethite w e r e chosen for this s t u d y because

in Western

A u s t r a l i a a s e l e n i u m deficiency i n pastures has b e e n s h o w n to b e r e l a t e d to the i n c i d e n c e of w h i t e m u s c l e disease i n sheep ( 3 ) , a n d a c c o r d i n g to w o r k e r s q u o t e d b y R o s e n f e l d a n d B e a t h ( 9 ) s e l e n i u m i n soils of h i g h e r 82

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

8.

HINGSTON E T A L .

Adsorption

of

83

Selenite

r a i n f a l l areas is p r o b a b l y present m a i n l y as f e r r i c selenites.

Studies of

selenite w i l l s u p p l e m e n t those of other anions s u c h as p h o s p h a t e

and

sulfate a n d s h o u l d h e l p to m a k e it possible to f o r m u l a t e a g e n e r a l m e c h a n i s m for a d s o r p t i o n of anions b y s o i l colloids. Methods A m i c r o - c r y s t a l l i n e f o r m of synthetic goethite consisting of aggregates of n e e d l e - l i k e crystals w i t h a B . E . T . surface area of 32 m e t e r / g r a m w a s u s e d as a n adsorbent. T h e excess surface c h a r g e o n this goethite w a s m e a s u r e d as a f u n c t i o n of p H a n d i o n i c strength b y the p o t e n t i o m e t r i c m e t h o d d e s c r i b e d b y P a r k s a n d de B r u y n ( 8 ) except that N a C l w a s u s e d as the s u p p o r t i n g electrolyte i n p l a c e of K N 0 . T h e a m o u n t of a d s o r p t i o n of selenite was m e a s u r e d r a d i o m e t r i c a l l y b y c o u n t i n g t a g g e d a n d s t a n d a r d i z e d solutions. T h e effects of v a r y i n g p H , s u p p o r t i n g electrolyte c o n c e n t r a t i o n , r e a c t i o n t i m e a n d t e m p e r a t u r e w e r e d e t e r m i n e d a n d the r e v e r s i b i l i t y of the r e a c t i o n w a s e x a m i n e d b y isotopic exchange. 2


3

Results and

Discussion

Reversibility. T h e r e v e r s i b i l i t y of the a d s o r p t i o n r e a c t i o n was tested b y a d d i n g r a d i o a c t i v e selenite to a suspension after r e a c t i o n of the s o l i d a n d i n a c t i v e selenite. A d s o r p t i o n w a s c o m p l e t e i n one d a y , exchange

of

r a d i o a c t i v e selenite w i t h a d s o r b e d selenite took seven days to c o m e to equilibrium.

A l t h o u g h exchange w a s slower t h a n a d s o r p t i o n the

same

e q u i l i b r i u m v a l u e was r e a c h e d , therefore the r e a c t i o n is r e v e r s i b l e u n d e r these c o n d i t i o n s . Adsorption Isotherms. T h e d e p e n d e n c e of the a m o u n t of selenite a d s o r b e d o n p H a n d s o l u t i o n c o n c e n t r a t i o n of selenite is i l l u s t r a t e d b y the curves i n F i g u r e s 1 a n d 2. T h e s e s h o w t h a t the a m o u n t of selenite t a k e n u p b y goethite reaches a m a x i m u m v a l u e , r

0 ( P

H ) , at constant p H

w h i c h cannot be e x c e e d e d b y i n c r e a s i n g the s o l u t i o n c o n c e n t r a t i o n a n d that this m a x i m u m v a l u e varies w i t h p H . I n the p H r e g i o n s t u d i e d i o n size is u n l i k e l y to b e the o n l y factor l i m i t i n g a d s o r p t i o n b e c a u s e e v e n at l o w p H , w h e r e the m a x i m u m is greatest, the area of surface a v a i l a b l e to the i o n is a l w a y s greater t h a n the area i t w o u l d b e e x p e c t e d to o c c u p y (^20

A. /ion). 2

Isotherms c a l c u l a t e d for constant p H a m o u n t of selenite a d s o r b e d d e p e n d s

( F i g u r e 2)

show

that the

o n the e q u i l i b r i u m s o l u t i o n c o n

c e n t r a t i o n as represented b y the e q u a t i o n of the L a n g m u i r f o r m , r = r

0 ( p H )

- K ( S e ) / [ l + K (Se)] L

(1)

L

w h e r e r p ) is the m a x i m u m a d s o r p t i o n at the p a r t i c u l a r p H , ( S e ) is the c o n c e n t r a t i o n of selenite ions i n s o l u t i o n , a n d K is the L a n g m u i r constant. 0 (

H

L


84

ADSORPTION F R O M A Q U E O U S

T h e relationship between K

L

SOLUTION

a n d p H is s h o w n i n the g r a p h of l o g K

L

against p H ( F i g u r e 3 ) , w h i c h is l i n e a r w i t h a slope a p p r o a c h i n g u n i t y at h i g h p H a n d zero b e l o w p H 8—i.e., K

b e c o m e s constant w i t h decreas

L

ing p H . Excess S u r f a c e C h a r g e .

T h e r e a c t i o n at the goethite surface p r o

d u c i n g c h a r g e d sites b y a d s o r p t i o n of H

a n d O H " as p o t e n t i a l deter

+

m i n i n g ions c a n b e r e p r e s e n t e d as f o l l o w s ,


I

OH,

c

OH-M/

Fe

?=?

OH;

Fe

/ | \ OH

H

P o t e n t i o m e t r i c t i t r a t i o n of a n aqueous suspension of oxides i n the p r e s ence of v a r y i n g concentrations o f indifferent electrolyte has b e e n

used

successfully to d e t e r m i n e the zero p o i n t of charge ( z . p . c . ) a n d t h e v a r i a t i o n i n excess surface c h a r g e w i t h p H (1, surface c h a r g e

8).

T h e v a r i a t i o n i n excess

( r + - r H - ) w i t h p H a n d N a C l c o n c e n t r a t i o n is s h o w n H

0

for goethite i n F i g u r e 4. T h e excess surface c h a r g e i n the presence of s p e c i f i c a l l y

adsorbed

ions w a s f o u n d b y m e a s u r i n g the a m o u n t of selenite a d s o r b e d a n d t h e a m o u n t of h y d r o x y l d i s p l a c e d i n t o the s o l u t i o n .

T h e q u a n t i t y of O H "

d i s p l a c e d ( A ) w a s e s t i m a t e d for constant p H ( a h y p o t h e t i c a l s i t u a t i o n ) f r o m the curves for t i t r a t i o n of goethite, goethite p l u s selenite, a n d selenite alone b y the e q u a t i o n . ( G + Se) - G S e = A where G = and GSe =

t i t r a t i o n v a l u e for goethite, Se =

(2)

t i t r a t i o n v a l u e for selenite,

t i t r a t i o n v a l u e for goethite p l u s selenite, a l l expressed i n

/ x e q u i v . / g r a m of goethite. T h e excess surface c h a r g e c a n t h e n b e e s t i m a t e d f r o m , 8Se = 8 w h e r e 8Se =

(Se-)

+ A

a d s

charge i n the presence of selenite, 8 =

of selenite, a n d (Se~) ds = a

(3) charge i n t h e a b s e n c e

n e g a t i v e c h a r g e a d d e d b y selenite ions a l l o w

i n g for the p r o p o r t i o n of S e 0 " a n d H S e 0 " i n s o l u t i o n . 8Se w a s p l o t t e d 3

2

3

against p H i n F i g u r e 4 to i l l u s t r a t e the decrease i n z.p.c. a n d the v a r i a t i o n i n charge w i t h i o n i c s t r e n g t h . C o m p a r i n g 8 w i t h 8

Se

i t c a n b e seen that

a d s o r p t i o n of selenite a l w a y s results i n a decrease i n the net i.e., a n increase i n the net n e g a t i v e charge.


charge—

HINGSTON

ET

AL.

Adsorption

of

85

Selenite


8.

A M o d e l f o r the M e c h a n i s m o f A d s o r p t i o n . LOPE.

T h e curve relating

r

0 ( P

H),

(1)

A D S O R P T I O N

E N V E

the v a l u e for m a x i m u m a d s o r p t i o n at a

p a r t i c u l a r p H , a n d p H is t e r m e d the " a d s o r p t i o n e n v e l o p e / ' Studies of specific a d s o r p t i o n of a series of anions to b e discussed i n d e t a i l elsewhere ( 5 ) , h a v e s h o w n that the m a x i m u m specific a d s o r p t i o n at a n y p H ,

r

0(1)H)

,

is r e l a t e d to the p K

o n the a d s o r b i n g species.

D

of the a n i o n a c i d a n d the c h a r g e

F r o m these studies essential r e q u i r e m e n t s for

specific a d s o r p t i o n of anions, or exchange of specifically a d s o r b e d ions, seem to b e as f o l l o w s : (a)

A p r o t o n s h o u l d b e a v a i l a b l e either f r o m a net excess o n the


86

ADSORPTION F R O M

AQUEOUS SOLUTION

surface o r f r o m a p r o t o n c o n t a i n i n g species i n e q u i l i b r i u m w i t h the a n i o n i n solution. (b)

A n i o n i c species w i t h a t e n d e n c y t o a c q u i r e a p r o t o n s h o u l d b e

present ( a n i o n s o f w e a k acids h a v e a t e n d e n c y to a c q u i r e a p r o t o n a t p H values near the p K

D

of the a c i d ) .

( c ) Specific a d s o r p t i o n o f anions c a n o n l y o c c u r w i t h a n increase i n the net n e g a t i v e charge o n the surface. A t t e m p t s to desorb selenite or a n y other specifically a d s o r b e d a n i o n (6,7)

b y w a s h i n g the s o l i d w i t h N a C l solutions of the same i o n i c s t r e n g t h

a n d p H are f r e q u e n t l y not successful. I t has b e e n f o u n d that l e a c h i n g at Downloaded by UNIV OF BATH on October 2, 2014 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch008

constant p H restores the surface charge of a n oxide t o the v a l u e i t h a d at that p H before specific a d s o r p t i o n o c c u r r e d .

I n t h e case o f selenite

a d s o r b e d o n goethite, this occurs t h r o u g h d e s o r p t i o n of O H " r a t h e r t h a n selenite. T h e selenite r e m a i n i n g w h e n the charge has b e e n restored c a n only be desorbed

b y i n c r e a s i n g t h e negative charge t h r o u g h

a d s o r p t i o n of another a n i o n .

1

2

3

4

5

6

7

SOLUTION CONCENTRATION ( M S e / l Figure 2.

Langmuir

8 x10 ) 4

isotherms

Experimental values shown by symbols and full lines calculated for best fit. pH values shown on curves


specific


8.

HINGSTON

E T

A L .

Figure 3.

Adsorption

of

87

Selenite

Plot of log K against pH: O—0.1M A — 1 .OM NaCl

NaCl,

L

Curve for ideal model O.IM

NaCl

A t p H values h i g h e r t h a n the z.p.c. m a x i m u m a d s o r p t i o n is deter m i n e d b y the p r o p o r t i o n of the ions i n s o l u t i o n t h a t are a b l e to d o n a t e a n d a c c e p t protons.

T h e species S e 0 ~ c a n o n l y a c c e p t a p r o t o n t h e r e 3

2

fore i t cannot b e a d s o r b e d i n the absence of H S e 0 " . 3

F o r selenite i n

s o l u t i o n , the p r o p o r t i o n of S e 0 " species is a, w h e r e a is the degree of 3

2

d i s s o c i a t i o n of the species H S e 0 " , a n d the p r o p o r t i o n of

H S e 0 " is

3

( 1 — « ) . T h e p r o b a b i l i t y of selecting S e 0

3

2

3

' f r o m a m o n g the t o t a l sele

n i t e species is o a n d the p r o b a b i l i t y of finding H S e 0 " is ( 1 — « ) t h e r e 3

fore the p r o b a b i l i t y of finding S e 0 " a n d H S e 0 " together is a ( l — a ) . 3

2

3

S i n c e this event c a n result i n a d s o r p t i o n of b o t h ions the a m o u n t selenite a d s o r b e d s h o u l d be p r o p o r t i o n a l to V « ( l — ) « =

K /(H D

+

+

K ) , where K D

selenious a c i d , shows that r p 0 (

H )

D

is the s e c o n d d i s s o c i a t i o n constant for is p r o p o r t i o n a l to V K

D

H

T h e latter f u n c t i o n reaches a m a x i m u m v a l u e at p H = acid p H than p K

D

of

s u b s t i t u t i n g for

a

+

/ ( H

+

+

K ) . D

2

p K . A t more D

the m a x i m u m a d s o r p t i o n is n o l o n g e r d e p e n d e n t

on

the p r o p o r t i o n s

of the species because H S e 0 " c a n b o t h a c c e p t a n d

donate protons.

T h e m a x i m u m a d s o r p t i o n is t h e n o n l y l i m i t e d b y the

3


88

ADSORPTION F R O M

AQUEOUS

SOLUTION

1-0M NaCl

130'

X

NO (MMNaCl

%

9 |OqiMNaCI \ 0

70 \


50-

\

* \

K)M NaCl \

Figure 4. absent

\

\

v

Excess surface charge and pH, selenite , selenite present , NaCl concentration indicated on curves

c h a r g e i n d u c e d o n the surface b y a d s o r p t i o n . D e v i a t i o n f r o m t h i s m o d e l c o u l d b e e x p e c t e d to arise f r o m interactions b e t w e e n ions o n the surface a n d b e t w e e n ions a n d the surface. Interactions of this k i n d w o u l d p r o b a b l y b e electrostatic a n d w o u l d result i n regions of the e n v e l o p e ( F i g u r e 1) h a v i n g a T e m k i n f o r m a n d the s p r e a d of the i n i t i a l rise ( h i g h p H ) o v e r a greater range of p H t h a n p r e d i c t e d b y the s i m p l e m o d e l . T H E

E X C H A N G E

F o r a d s o r p t i o n at constant p H the a m o u n t

I S O T H E R M .

of selenite o n the surface is r e l a t e d to the s o l u t i o n c o n c e n t r a t i o n b y a n e q u a t i o n of the L a n g m u i r t y p e ( E q u a t i o n 1 ) . I f S e 0 " is the species d e t e r m i n i n g a d s o r p t i o n a n d the r e a c t i o n 3

2

m e c h a n i s m i n v o l v e s exchange w i t h h y d r o x y l ions i t c a n b e s h o w n that K where K

2

=

K' /K , 2

w

= K K H /(H

L

K'

2

2

=

D

+

+

+ K ) D

the exchange constant for selenite


(4) and

HINGSTON E T A L .

8.

hydroxyl, and K pK

D

w

=

89

Adsorption of Selenite

the dissociation constant for water. A curve using

= 9.2 and the above relationship (Equation 4) is in good agreement

with experiment (Figure 4). Although p K — 9.2 is somewhat higher than p K

for selenious acid in 0.1M NaCl—i.e., p K =

2

8.2—it could be

2

correct for the p K in the vicinity of the surface where an excess negative charge must be taken into account. Substituting into Equation 1 gives,

•^/[> 5£S>>]

+

APPROXIMATE


(3)

THERMODYNAMIC

QUANTITIES.

5

Thermodynamic

quantities, not taking into account activity coefficients, can be calculated from the experimental data at constant p H for the exchange reaction, ( O H " ) . + SeO 2- ^± ( S e 0 " ) + O H " a

3

The enthalpy of exchange AH

1/2 r

(6)

g

referred to a standard state where

1/2

the surface concentration r =

2

0 ( p H

) was found using the van't Hoff

equation on results from isotherms obtained at 5 ° and 2 0 ° C . ments on isotherms for p H 9 to p H 10.5 gave A H mole.

J / 2

Measure-

of 4.5 ± 0.5 K c a l . /

Values for the partial molar free energy of exchange,

AG*i

/ 2

(referred to standard state where r = 1/2 r p ) and a hypothetical ideal 0 (

H

molar solution of selenite and hydroxyl, less a configurational entropy term) can be calcuated from the value of K ' obtained from Equation 4. 2

Since K ' — 1.1 ± 0.5 and A G * 2

1 / 2

_ = - R T In K ' ,

AG*

2

1 / 2

— 0.0 ± 0.2 5

7

Kcal./mole. A n integral entropy ( A S i ) of 16 ± 1 e./n. was then obtained / 2

using the relationships, AS* and A S 1 / 2 =

AS*I

/

2

1 / 2

= AH

1

/

2

-

AG*

1 / 2

/T

+ 2 R In 2 (4). If the reaction involves a change

in the amount of water bound at the surface the equation giving entropy changes for the reactions is as follows, S ( S e 0 2 - ) + S(OH-) + (x - y ) S ( H 0 ) = S(Se0 ") + S ( O H ) + A S 3

s

2

B

3

2

8

where S ( i ) = entropy of species " i " in solution and S ( i ) = g

species *i on the surface. Substituting values, S ( O H " ) = S(Se0 ") = 3

2

1 / 2

entropy of

—10 e./x. and

—8 e.u. given by Cobble (2) and the experimental value

of A S 2 — 16 ± 1 e.u. it follows that, V

S ( S e O " ) . - S ( O H - ) . + (x - y ) S ( H 0 ) = 18 ± 1 e.u. s

2

2

Further, if the difference in entropy between selenite and hydroxyl is the same on the surface as it is in solution, the value for the entropy of water (16.7 e.u.) indicates that (x — y ) =

1—i.e., a molecule of water

is displaced from the surface during exchange of S e 0 " for O H " . T h e 3

2


90

ADSORPTION

v a l u e for K'

2

(1.1

±

0.5)

F R O M

AQUEOUS

SOLUTION

suggests that there is l i t t l e difference i n

s e l e c t i v i t y coefficients for selenite a n d h y d r o x y l o n the goethite surface. Literature (1) (2) (3) (4) (5)


(6) (7) (8) (9)

Cited

Atkinson, R. J., Posner, A. M., Quirk, J . P . , J. Phys. Chem. 71, 550 (1967). Cobble, J . W., J. Chem. Phys. 2 1 , 1443 (1953). Gardiner, M. R., J. Dept. Agr. Western Australia, 4th series, 4, 632 (1963). Heath, N. S., Culver, R. V., Trans. Faraday Soc. 51, 1575 (1955). Hingston, F. J., Atkinson, R. J., Posner, A . M., Quirk, J . P . , Nature 215, 1459 (1967). Kafkafi, U., Posner, A . M., Quirk, J. P., Soil Sci. Soc. Amer. Proc. 3 1 , 348 (1967). M u l j a d i , D . , Posner, A. M., Quirk, J . P . , J. Soil Sci. 17, 212 (1966). Parks, C . A., de B r u y n , P. L., J. Phys. Chem. 66, 967 (1962). Rosenfeld, I., Beath, O. A . , "Selenium," Academic Press, N e w York, 1964.

RECEIVED

October 26, 1967.


Adsorption From Aqueous Solution

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