In Situ Mössbauer Studies of Metal OxideAqueous Solution Interfaces

Solution Interfaces with Adsorbed Cobalt-57 and. Antimony-119 Ions .... 0, ±1 are allowed and a symmetric sextet is observed in the absence of e l e ...
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19 In Situ Mössbauer Studies of Metal Oxide-Aqueous Solution Interfaces with Adsorbed Cobalt-57 and Antimony-119 Ions F. Ambe, S. Ambe, T. Okada, and H. Sekizawa Institute of Physical and Chemical Research (Riken), Wako-shi, Saitama 351-01, Japan In situ emission Mossbauer spectroscopy provides valuable information on the chemical structure of dilute metal ions at the metal oxide/aqueous solution interface. The principles of the method are described with some experimental results on divalent Co-57 and pentavalent Sb-119 adsorbed on hematite. The chemical structure of the adsorbed ions was found to be dependent on pH of the aqueous phase. Most of the divalent Co-57 and pentavalent Sb-119 ions form strongly bonded surface complexes under alkaline and acidic conditions, respectively. In s p i t e o f the development o f p h y s i c o c h e m i c a l t e c h n i q u e s f o r s u r f a c e a n a l y s i s , s p e c t r o s c o p i c methods a p p l i c a b l e t o t h e s t u d y o f bonding between adsorbed m e t a l i o n s p e c i e s and s u b s t r a t e a r e l i m i t e d , e s p e c i a l l y t h o s e a p p l i c a b l e t o i n s i t u measurement a t i n t e r f a c e s between s o l i d and aqueous phases ( 1 , 2 ) . In p r e v i o u s p a p e r s , we showed t h a t e m i s s i o n Mossbauer measurement i s u s e f u l i n c l a r i f y i n g the c h e m i c a l b o n d i n g environment o f d i l u t e m e t a l i o n s adsorbed on magnetic m e t a l o x i d e s u r f a c e s (3>^)· We now extend t h e work t o i n s i t u measurements on m e t a l i o n s adsorbed a t t h e m e t a l oxide/aqueous s o l u t i o n i n t e r f a c e . In t h i s r e p o r t , o u r p r e v i o u s r e s u l t s a r e combined w i t h new measurements t o y i e l d s p e c i f i c i n f o r m a t i o n on t h e c h e m i c a l s t r u c t u r e o f adsorbed s p e c i e s a t the s o l i d / a q u e o u s s o l u t i o n i n t e r f a c e . H e r e , we d e s c r i b e the p r i n c i p l e s o f e m i s s i o n Mossbauer s p e c t r o s c o p y , e x p e r i m e n t a l t e c h n i q u e s , and some r e s u l t s on d i v a l e n t Co-57 and p e n t a v a l e n t Sb-119 i o n s adsorbed a t t h e i n t e r f a c e between h e m a t i t e (a-Fe2C>3) and aqueous s o l u t i o n s . Principles A l t h o u g h r a d i o a c t i v e i s o t o p e s have been w i d e l y u t i l i z e d a s t r a c e r s i n the s t u d y o f a d s o r p t i o n e q u i l i b r i u m and k i n e t i c s , i n t h e s e t y p e s o f s t u d i e s they p r o v i d e no d i r e c t i n f o r m a t i o n on c h e m i c a l s t r u c t u r e 0097-6156/ 86/ 0323-0403S06.50/ 0 © 1986 American Chemical Society

404

GEOCHEMICAL PROCESSES AT MINERAL SURFACES

o f the adsorbed s p e c i e s . However, when a Mossbauer s o u r c e n u c l i d e i s adsorbed on a magnetic o x i d e , s t r u c t u r a l i n f o r m a t i o n about adsorbed i o n s can be o b t a i n e d from t h e i r e m i s s i o n Mossbauer spectra. The p r i n c i p l e s o f t h e method a r e d e s c r i b e d below w i t h t h e examples Co-57 and Sb-119 ( F i g u r e 1 ) , which a r e the s o u r c e s o f the most p o p u l a r Mossbauer n u c l i d e s , F e - 5 7 and S n - 1 1 9 . The two s o u r c e n u c l i d e s may be r e g a r d e d as r e p r e s e n t a t i v e s o f a t r a n s i t i o n and n o n - t r a n s i t i o n element, r e s p e c t i v e l y . The r a d i o a c t i v e n u c l i d e Co-57 decays t h r o u g h the 136 keV second n u c l e a r e x c i t e d l e v e l t o the f i r s t e x c i t e d l e v e l o f F e - 5 7 , which then e m i t s t h e 14.4 keV Mossbauer gamma-ray w i t h a h a l f - l i f e o f 98 ns. I f Co-57 i o n s a r e adsorbed on a c e r t a i n s u r f a c e f i r m l y enough t o p r o v i d e an a p p r e c i a b l e r e c o i l l e s s f r a c t i o n on the e m i s s i o n o f the gamma-rays, the c h e m i c a l environment o f the F e - 5 7 i n the f i r s t e x c i t e d l e v e l can be d e t e r m i n e d by a n a l y z i n g the r e s o n a n t gamma-rays w i t h a s t a n d a r d a b s o r b e r ( e m i s s i o n Mossbauer s p e c t r o s c o p y ) . Simil a r l y , the c h e m i c a l s t r u c t u r e o f Sn-119 a r i s i n g from adsorbed p e n t a v a l e n t Sb-119 can be d e t e r m i n e d t h r o u g h e m i s s i o n Mossbauer spectra. S i n c e the o b s e r v a t i o n o f Mossbauer s p e c t r a on F e - 5 7 and Sn-119 i s made i m m e d i a t e l y a f t e r the EC ( e l e c t r o n c a p t u r e ) decays o f Co-57 and Sb-119 ( a f t e r 141 ns and 2 5 . 7 ns on a v e r a g e ) , the c h e m i c a l s t r u c t u r e o f Co-57 and Sb-119 can be r e g a r d e d as e s s e n t i a l l y t h a t r e s u l t i n g from F e - 5 7 and Sn-119 e m i s s i o n Mossbauer s p e c t r a . The Auger cascade f o l l o w i n g the EC decay r e s u l t s i n m u l t i p l e i o n i z a t i o n o f the d e c a y i n g atom. In the case o f C o - 5 7 , charge s t a t e s up t o 7+ a r e t h e o r e t i c a l l y p r e d i c t e d f o r the daughter n u c l i d e F e - 5 7 ( 5 ) . Such h i g h l y i o n i z e d s p e c i e s have been d e t e c t e d f o r C l - 3 7 produced by the EC decay o f A r - 3 7 i n gaseous phase ( 6 ) . In s o l i d s , however, such anomalous s t a t e s a r e n o t r e a l i z e d o r t h e i r l i f e time i s much s h o r t e r than the h a l f - l i f e o f the Mossbauer l e v e l ( F e - 5 7 : 98 ns and S n - 1 1 9 : 17.8 n s ) because o f f a s t e l e c t r o n t r a n s f e r , and u s u a l l y s p e c i e s i n o r d i n a r y v a l e n c e s t a t e s (2+, 3+ f o r F e - 5 7 and 2+, 4+ f o r Sn-119) are o b s e r v e d i n e m i s s i o n Mossbauer s p e c t r a ( 7 , 8 ) . The d i s t r i b u t i o n o f F e - 5 7 and Sn-119 between the two v a l e n c e s t a t e s depends on the p h y s i c a l and c h e m i c a l e n v i r o n m e n t s o f the d e c a y i n g atom i n a v e r y c o m p l i c a t e d way, and d e t e c t i o n o f the c o u n t e r p a r t s o f the redox r e a c t i o n i s g e n e r a l l y v e r y d i f f i c u l t . The r e c o i l energy a s s o c i a t e d w i t h the EC decays o f C o - 5 7 and Sb-119 i s e s t i m a t e d t o be i n s u f f i c i e n t to i n d u c e d i s p l a c e m e n t o f the atom i n s o l i d s . In a b s o r p t i o n Mossbauer s p e c t r o s c o p y , a s o u r c e n u c l i d e i n a s t a n d a r d form ( u s u a l l y i n a m e t a l l i c m a t r i x ) i s c o u p l e d w i t h a sample to be i n v e s t i g a t e d . T h i s method r e q u i r e s a t l e a s t 100 pg o f Fe or Sn i n the u s u a l e x p e r i m e n t a l s e t u p even i f a Mossbauer s e n s i t i v e e n r i c h e d s t a b l e i s o t o p e F e - 5 7 o r Sn-119 i s employed. In e m i s s i o n Mossbauer s p e c t r o s c o p y , however, 1 mCi o f Co-57 o r S b - 1 1 9 , which c o r r e s p o n d s n o m i n a l l y t o 120 ng o f Co-57 o r 1.4 ng o f S b - 1 1 9 , i s s u f f i c i e n t t o p e r m i t measurement. This technique enables study o f v e r y d i l u t e s y s t e m s , e s p e c i a l l y t h o s e w i t h i o n s d i r e c t l y bound t o the s u b s t r a t e . Mossbauer isomer s h i f t and q u a d r u p o l e s p l i t t i n g a r e commonly used to o b t a i n i n f o r m a t i o n about the bonding environment around source n u c l i d e s . The isomer s h i f t a r i s e s from the e l e c t r i c monopole i n t e r a c t i o n o f the n u c l e u s w i t h the e l e c t r o n s and depends on the

19.

Metal Oxide-Aqueous

A M B E ET AL.

s-electron the of

line Fe-57

excited ±3/2)

density

or

Sn-119 nucleus

a

Mossbauer and they

(I

This line

are

not an

and

parameter, structure

the

and

3/2)

and

m = -3/2, are

absence field

of

becomes the

-1/2,

When

is

not

Fe-57

oxide,

the

induced

Fe-57

by

bonded

the to

the

in

the

(for

oxide the

or

can

Sb-119

be

present

Experimental ferric

oxide,

in

air

structure

checked

by

measurements. sextet

to

a

distri-

the

broadening

are

magnetic

magnetic ions as

more

or

intoxide

those ions

ions

in

surfaces),

chemical the

the

broadening

through For

of

fields

of

magnetic

oxide

the

is

but

structure magnetic

splitting

of

applicable

also

to

not

only

to

ferromagnetic

antiferromagnetic

oxides,

and

which

are

magnets.

with

run. to

see of

used

present

w o r k was

of

Some m e a s u r e m e n t s

were

made

the

no

the

area

Mossbauer with

in

BET s u r f a c e

the

a

effects adsorbed

of

sintering

metal

absorption

The M o s s b a u e r

and

absorption

on

high 30 mg

the

surface

The

hematite

powder

X-ray

on

the

samples

diffraction

consisted

component

was

hematite

ions. spectra

superparamagnetic

a

27 m 2 / g ;

due

of

to

a

fine

(9).

The M o s s b a u e r was

hydrogen-bonded

or

analyzing

hematite,

chemical

Sb-119

two

surfaces

reagent

calcined

particles

Am

values,

Sb-119

substrate.

the

not

each

magnetic

a

low

of

bonded

on

by

has

observed

are

through

with

the

quadrupole

metal

ions

method

in

were

the

=

1/2

Section

powder

employed

or

ions

Substantial that

in

of

hyperfine are

(I

spectrum

field

magnetic

fields

nucleus

levels

observed.

metal

the

chemical

m = -1/2,

region

Therefore,

oxides

macroscopically

The

the

metal

lines.

ferrimagnetic

purity

feel

ordered

adsorbed

not

the

electric

Co-57

of

oxygen However,

Sn-119

Mossbauer

from

ions

species

an

magnetic in

surface

ions

field.

transitions

much w e a k e r .

estimated

emission The

is

If

or

observed

arising

ions

the

Mossbauer

with

The is

the

spectra.

magnetic

example,

sextet

the

magnetic

first

the

by or

excited

is

on a d s o r b e d

surface

first

band

the

magnetic

Fe-57

broad

the

Mossbauer

ordered

the

When a

metal

investigate

sublevels

field,

Sn-119 n u c l e i

only

interaction Co-57

to

hyperfine

occurs

sequence of

The

splitting to

or

and

fraction

ions

ions

to When

field.

and

interaction with

substrate. eraction

large

magnetic of

another

respectively.

sextet.

Sn-119

oxide

of

of

and

s p l i t t i n g of

respectively.

study

used

symmetric

its

±1/2

h y d r o l y t i c a l l y adsorbed

this

four

magnetic

=

d e t e r m i n i n g whether

ground

3/2, a

(m

shift

environment

interaction with

absence

ions.

and

the

symmetric,

quadrupole

the

was

metal

resolved

or

In

quadrupole

large

relatively

through

ions

1/2,

in

between

two

and

an a s y m m e t r i c

sextet

enough

their

into

quadrupole the

as

electronic

sublevels

environment,

splitting,

electric

with

oxygen

field,

observed the

c o o r d i n a t i o n number

substrate.

allowed

bonded

or

of

in

is If

two

the

405

Interfaces

spherically

into

as

doublet with

adsorbed

a

not

electric

formed

split

overlaps

bution

a

magnetic of

a magnetic

±1

the

vary

are

is

observed

into

oxide

1/2 0,

of is

It

spectrum.

splits

informative

ions

in

3/2)

distortion bonds

=

-

result

chemical

is

Mossbauer

parameters

with

nucleus.

the

electrons. These

the

in

level

as

at

Solution

produced

source by

the

nuclide

C o - 5 7 was

obtained

160 cm R I K E N c y c l o t r o n a t

commercially; our

institute.

406

GEOCHEMICAL PROCESSES AT MINERAL SURFACES

The

procedure

target and

for separating

has been

antimony

about

described

coexisting

with

smaller

than

that

the

hematite

1 -

2 mCi o f d i v a l e n t

required

sample.

with

Fe-57-enriched

detected

counter

X-rays.

standard

manner

with

o f aqueous

spectra

obtained

appreciable Since

The

change

Co-57

Situ

essentially

occurred

at

24±1°C

or

barium

700-series TN-7200

o f Co-57 and

carbon

dioxide)scintilla-

absorber

f o r Sn Κ

velocity

were

calibration

estimated

measurements

measurements, suspensions were

5 - 1 2

performed of

t h e same

i n d i c a t i n g that no

of dispersion

contains

inevitable

Fe-57,

i n the case

of the absorption

were

similar

to check the

The a b s o r p t i o n

consisted

powder,

Mossbauer in a

and p a r t i c l e

pH r a n g e .

hematite

is

in situ treated

certain

i s considered

o f the spectra

was k e p t

constant

uniformly analyzed

a

Moss­

o f measurements is

and the

on

t o be n o t concerned, adsorbed

in i t .

with

a FACOM M 3 8 0

computer.

Results Measurement

behavior

as that

from

particles

adsorption

show

a marked

sion

lines

begins

complete.

a n d t h e 0.1

ascribable

reported

a t about Emission

mol/dm3

ions NaCl

i n Figure iron

The was

and Healy

pH 4 f o l l o w e d Beyond

Mossbauer

solutions 3·

of

spectra

species

of

between

different

The e m i s s i o n phase.

by a n

pH 9,

at the interface

o n t h e pH o f t h e aqueous

to paramagnetic

Co-57. surfaces

by James

p H 6 a n d 8.

Co-57

a r e shown

dependence

on hematite

between

the divalent

temperature

ions

on s i l i c a

i n adsorption

is practically

arising

on Hematite/Divalent

o f cobaltous

t h e same

increase

pH a t r o o m

30

with the

2 mm-thick Nal(Tl)

i n the state

was d i s p e r s e d

Appreciable

hematite

Ranger

gamma-rays

Pd c r i t i c a l

as dry hematite

obtained

(22).

Fe-57

by a

a Kr(+3Î

a

measurements

o f hematite

abrupt

adsorption

for

The a b s o r b e r ,

2.

i n the r e l a t i v e

i n the studied

Mossbauer

adsorption

0.5

a t the bottom

Tracor-Northern

a s f a r a s t h e pH d e p e n d e n c e

data

was s h a k e n

measurement

pH o n t h e s u b s t r a t e .

the substrate

Experimental In

phase

sextet

t h e amount

divalent

a

hematite

( 0 . 5 mg F e - 5 7 / c m 2 )

with

and with

on hematite

But, the effect

important since

i n Figure

the emission

self-absorption

Co-57.

3 0 mg o f

or Sb-119 along

Mossbauer

to a

i n t h e pH r e g i o n

o f hematite

bauer

with

o f t h e powder

0 . 0 5 mm/s b y r e p e a t e d

as i n the emission

well-defined

vessel

and about

The Mosssbauer

measurements

effects

size

connected

errors

of

Teflon

absorbers.

parallel

absorption

S b - 1 1 9 was

in a

was d r i v e n

65 y m - t h i c k

The i n t e g r a l

In

shown

counter

through

be o f t h e o r d e r

using

containing

Co-57

respectively

proportional

3 0 mg o f

1 mCi o f pentavalent

ferrocyanide

analyzer.

of

been

been

remeasured.

setup

spectrometer

S b - 1 1 9 were

coverage

The s u s p e n s i o n

to emission

potassium

cobalt

t o have

solution

settling

adsorbed

2

Mossbauer

to

After

( 0 . 9 mg S n - 1 1 9 / c m ) ,

multi-channel

tion

with

the experimental

filled

pH v a l u e

t h e pH was

was s u b j e c t e d

stannate

-

to the s o l u t i o n .

hematite

solution

0.1

t i n

of

i . e . , t o have

f o r monolayer

window a t t h e b o t t o m ,

a t room t e m p e r a t u r e . The

respectively,

or

Teflon

the vessel,

are estimated

Co-57

mm-thick

was a d d e d

The amounts

10 cm3 o f a n a q u e o u s

to an appropriate

of

(10,11).

About

adjusted powder

from an a l p h a - i r r a d i a t e d

the nuclides

4 0 0 n g / m C i a n d 300 n g / m C i ,

much

min

Sb-119

elsewhere

spectra

No e m i s ­

are recognized

in

AMBE ETAL.

Metal

Oxicfe-Aqueous

Solution

Interfaces

Co 271 d

57

1 1 9

EC

8.6 n s

136 keV fr*

98 ns

Sb

38.0 h

1

7

? "

S

' 23.9 keV

- 14.4 • 0

'Fe Figure

"Sn

Simplified

1.

decay

schemes

of

the Mossbauer

source

nuclides.

Jl Aq.

TL

solution 5

7

Co

2

+

η

c α

-

^

"

2

3"

57

r

2+

T e f l o n Vessel Absorber: K C Fe(CN) ] 5 7

4

6

•3H 0 2

γ-Rays S h i e l d : Pb

Figure

2.

Experimental setup

measurement.

Setup

for

Co-57

for is

i n s i t u emission shown.

Mossbauer

GEOCHEMICAL PROCESSES AT MINERAL SURFACES

408

lOOh * 99h

CF 3

I • -15

Figure from



In

interface 9.6,

(D)

pH

11.0,

readjustment relative

in

is

defined

composed Figure

10.

the

Mossbauer

pH v a l u e s

to as

from

(E)

of

pH

of

in the

12.7,

pH f r o m

metallic

spectra

hematite/0.1

room t e m p e r a t u r e ) :

given city

emission

various at

after

are

-5 0 5 10 RELATIVE VELOCITY CMM/S VS M E T A L L I C I R O N ]

Co-57 a t

for

(measurement pH

situ

divalent

ι—I 15

-10

ordinary results

and

Fe-57 NaCl

arising solution

aqueous

phase

(A)

pH 5 . 7 ,

(B)

pH 7.4,

(F)

was

to

pH 3.0. 3-0.

the

The

sign

absorption of

of

dm~3

the (F)

12.7

iron

mol

of

isomer

shift

relative

spectra.

Hesse-Rubartsch

The

(C)

measured is

velo­ curves

analysis

given

19.

the

spectra

indicates face it

Metal Oxide-Aqueous

A M B E ET AL.

within

that

the experimental

Fe-57

ions

are predominantly

i s extremely

ions

detected species

with

field

3(A)).

o f the sextet

line

diminishes When

obtained pH o f a

that

alkaline

Adsorption

is

ions

lowered

form

pentavalent

Sb Ions

Sb i o n s

Carrier-free

LiCl

in

run i s estimated

solutions

at

at

rate

on H e m a t i t e . oxides.

900°C

24±1°C.

Sb-119

ions

50 n g .

diminishes

adsorbed

on h e m a t i t e

abruptly from

subsequent

adjustment

of

pre-adsorbed

studied

the

fore, on

In

they

the observed

Situ

shaking

are charged pH d e p e n d e n c e

i s apparently and r e p u l s i o n

the p o s i t i v e l y Mossbauer

Mossbauer

charged

between

Measurement

measurement

the suspension

22 of

the percent

Results

i n Figure

ions

The z e r o

over

point

(J_5).

negatively

above

[Sb(0H)o]~

o f charge

surface

i n adsorption

(ZPC) o f

of

hematite

protons

the ZPC.

of

the negatively

below

There-

o f pentavalent

i n terms

charged

the

t h e pH r a n g e

The s u r f a c e

due t o e x c e s s

by

on desorption

4.

Sb i o n s ,

Sb-119

electrostatic

charged

Sb

complex

surface.

on Hematite/Pentavalent

on Sb-119 for

after

adsorption

o f t h e Sb

conditions.

interpreted

or negatively

with

i n the solution

obtained

to predominate

(J_4).

proceeds after

o f pH 2 - 5 a r e n o t d e s o r b e d

t o be pH 6 . 5 - 8 . 6

are positively

hematite

attraction and

work

order

Sb i o n s

Most

o n 30

employed

equilibrium

pH 7 , w h i l e

that.

adsorbed

10 cm3 o f 0 . 2 5

o f antimony

4, strong

a t pH 4 a r e shown

i s reported

i s reported

ZPC, while

above

solutions

to alkaline

i n the present

particles

below

t h e pH

The a d s o r p t i o n

4 are those

i n Figure

were

i s second

concentration o f pentavalent

o f the complex

hematite

i n Figure

As s e e n

know,

o n h e m a t i t e was

from

an apparent

The r e a c t i o n

i s observed

adsorbed

form

lowered

equilibrium of

Sb i o n s

The amount

and a t t a i n s

given

Sb i o n s

dilute

from an is

S o f a r a s we

for 2 hours)

t o be a b o u t

hours.

equilibration.

In

spectrum

observations

Therefore,

o f pentavalent

to the concentration o f pentavalent

Sb i o n s

These

when t h e slow

Co-57 adsorbed

for several

pentavalent

to 3.0,

and the Mossbauer

3(F)).

shaking

The v a l u e s

a s t h e one

However,

1 2 . 7 down

respect (Jj3).

each

the s p l i t t i n g

t h e same

on the a d s o r p t i o n

pentavalent

(prefired

mol/dm3

hours

of

t h e pH o f t h e s o l u t i o n

on metal

measured.

a measurable

the width

value,

of divalent when

data

o f the adsorption

with

the s p l i t -

pH v a l u e .

dependence

each

from

(Figure

i s retained

o f Pentavalent

mg o f h e m a t i t e

phase

pH i s o b s e r v e d .

was o b s e r v e d ,

a r e no e x p e r i m e n t a l

dilute

field

and simultaneously

unchanged

the chemical

to an a c i d i c

there

magnetic

o f magnetic

3(B)-(E)).

solution

solution

are

the

a hyperfine

i n pH o f t h e a q u e o u s

at the higher

o f Co-57

while

Co-57

ions

t o low v a l u e s

is essentially

remained

Fe-57

suggesting

to a higher

desorption

above,

of divalent

gamma-rays,

t h e pH i s r e a d j u s t e d originally

virtually

down

increases

(Figure

shift

A t pH 5 . 7 , t h e s p e c t r u m

sextet

increase

As d e s c r i b e d

the trivalent

by t h e i r

down

The isomer

Co-57 a t the i n t e r -

state.

and a spectrum which

sample

suggest

extending

With

ting

increases

resolved

409

the oxidants

are not radioactive.

of a partly

divalent

i s because

sensitivity

distribution

(Figure

to identify It

Interfaces

uncertainties.

from

i n the trivalent

Fe-57.

high

reduced

consists

arising

difficult

to t r i v a l e n t

Solution

was c o n t i n u e d

30 m i n a n d w a i t i n g

for

1 -

Sb-119. 3 days

for settling

The after

o f the

GEOCHEMICAL PROCESSES AT MINERAL SURFACES

410

Figure

4.

pH d e p e n d e n c e

carrier-free (30 LiCl

mg o f

pentavalent

hematite

solutions).

prefired

at

tion

5 days

was

pH 4 . for

the

adsorption

Sb-119

Desorption

pre-adsorbed and

of

at

and

hematite

900°C

was

Shaking the

on

in

10 cm3

measured

t i m e was

desorption.

at

on

desorption room of

0.25

mol/dm3

pentavalent

22 h o u r s

for

of

temperature

the

Sb-119 adsorp-

19.

Metal Oxide-Aqueous

A M B E ET AL.

hematite ties,

powder

The

effect only

essentially

symmetric

the interface

expected The

width

chemical

shows

structure

dependent

is

width

at

the h a l f

value

i s about

single the

certain

temperature work

magnetic

full

width

at the h a l f

2.5,

which

suggests

ferric

ions

In the

of

sely,

(Figure

to remain

In

Thus,

the s o l u t i o n

Figure

different

heating formed with pH

at

region

heating sion the

98°C

for

line

show metal

no f u r t h e r In

order

than

pH 6 . 6 .

suggesting ion sites.

appreciable

to study

ions

on a d s o r p t i o n

made

on S b - 1 1 9

state,

adsorbed

change

in situ

of

above,

Conver-

t o a pH o f raised

2.5

to 8.6

spectra

solution

Sb-119 when

900°C,

and

which

sample

was

per-

gives over

immature

spectra

t h e whole

the

after emis-

s p l i t t i n g to a

o f more

heating

i n the

studied,

Sb-119

up t o

of

after

i n pH was o b s e r v e d

pH v a l u e

sextet

ions

in

100 m i n b r o u g h t

spectra.

t h e amount with

o f the

systems

The e x p e r i m e n t at

of

pentavalent

e m i s s i o n Mossbauer

on hematite

with

described o f pH.

((A1)-(C1))

incorporation Further

Sn-119

pentavalent

Mossbauer

change

o r even

the effect

of

and the

7 mm/s a t p H

7.

heating

At each

aqueous

t o be r e t a i n e d

mol dm-3 L i C l

A considerable

the

region.

adjusted of

o f the

remarkably

ions

considered

prefired

a

employed

ions

o f about

the non-pretreated

broadening

the heating,

surface

sample

width

studied.

before

as

i n pH o f

acidic

structure

give

a

Co-57

30 m i n ( ( A 2 ) - ( C 2 ) ) .

the system o f

spectra

about

pH v a l u e s

magnetic

1 . 3 mm/s a b o v e

ferric

the lowering

above

from

o f the

t h e pH h a d been

is

This

have

interactions

the emission

Sb-119/0.25

on t h e h e m a t i t e

larger

after

i s raised

6 a r e shown

hematite/pentavalent three

solution

full

5(A)).

particles

increases

previously after

the chemical

from an a c i d i c

pH o f

sample

quality the

most

a value

against

the

that

the decrease

of divalent

broadened

a poor Sb-119>

absorber

the ordered width

region.

that

considerably

maximum o f

i n the weakly

t o the case

due t o

field

without

we c o n c l u d e

magnetic

show n o h y s t e r e s i s

5(F)).

adsorbed the

stronger

of

Sn-119

The

on c h r o m i c o x i d e

t h e same

maximum a t t a i n s

the spectrum o f a

found

With

ions

on hematite

the line

the substrate

contrast

spectra

was

ions

interaction with

5(B)-(E)),

is

surface

the h a l f

Therefore,

a t t h e pH v a l u e .

(Figure

at

(303 K) a g a i n s t Sb-119

band

suggesting

Sb-119

velocity

state.

broad

Sb-119 are

that

f o r Sn-119 a r i s i n g

Sb-119

spectra

relative

t o 2 . 5 mm/s ( F i g u r e oxide

room

spectra

i n low magnetic

that

at

o f the

the

demonstrates

single

from

parti-

pentavalent

A t pH 8 . 5 , where

as

width

(3))·

pentavalent

substrate phase

full

the zero

low a d s o r p t i o n

on corundum-type

a

pH v a l u e s

the emission

pentavalent

of

large

hematite

the absence

from

pH d e p e n d e n c e ,

(The p e n t a v a l e n t

with

as a

maximum a m o u n t s as

of

This

distributed

strong

Sn-119 a r i s i n g

i n the tetravalent

appears

because

twice

the present

adsorbed

at

the s o l u t i o n .

Sb-119

line

Neel

a

obtained

interactions.

centered

uncertain-

each r u n .

i n the solution,

absorber.

o f adsorbed

on pH o f

spectrum

pentavalent

Because

5,

exclusively

sextet

of

of different

5.

i n Figure

lines

o f components

during

on Sn-119 a r i s i n g

stannate

is

magnetic

line

the experimental

spectra

solutions

i n Figure

As s e e n

411

on n o n - p r e t r e a t e d

for chemical species

the barium

overlapping

Mossbauer

information

the i n t e r f a c e .

against

Within

adsorbed

LiCl

a r e shown

represent

in

ions

i n 0 . 2 5 mol/dm3

Mossbauer

at

emission

Sb-119

temperature

at

the bottom.

Interfaces

i n t h e s p e c t r u m was o b s e r v e d

in situ

pentavalent cles

at

no change

Solution

Sb

measurement

non-radioactive

was

pentavalent

412

GEOCHEMICAL PROCESSES AT MINERAL SURFACES

Figure from

5.

In

solution 4.6,

(D)

at

pH 3 . 4 ,

readjustment

given

relative

curves given

is

are in

for

to

Mossbauer the

(E) of

pH v a l u e s

pH 2 . 5 ,

pH f r o m

defined 11.

as

in

from

(A)

(F)

2.5

stannate ordinary the

spectra

hematite/0.25

various

barium

composed

Figure

at

room t e m p e r a t u r e ) :

after

velocity

emission

Sb-119

interface

(measurement pH

situ

pentavalent

to

the

8.6.

8.6.

The

the

sign

of

(B)

(F)

absorption

results

Sn-119 dm-3

phase

pH 6 . 6 ,

was

(C)

measured

isomer of

arising

LiCl

aqueous

pH 8 . 5 ,

pH

and

of

of mol

shift

is

relative

spectra.

Hesse-Rubartsch

The analysis

AMBE E T A L .

Metal Oxide-Aqueous

L-i -15

, -10

Solution

Interfaces

• , ι -5 0 5 10 RELATIVE VELOCITY CflM/S VS B f l S N 0 ]

413

U 15

3

Figure

6.

for

min on

30

arising LiCl

Effects

from

solution

Before

the

of in

preheating situ

pentavalent interface

heating,

pH

6.6

of

emission Sb-119

at

sample the

(measurement and

(A2)

suspensions

Mossbauer

spectra

hematite/0.25

at

at

of

98°C

Sn-119

mol

dm-3

room t e m p e r a t u r e ) :

after

heating,

pH 7 . 9 ;

(B1)

Before

heating,

pH 4 . 4

and

(B2)

after

heating,

pH 4 . 3 ;

(C1)

Before

heating,

pH 2 . 6

and

(C2)

after

heating,

pH 2 . 6 .

The

curves

are

given

in

composed

Figure

12.

from

the

results

of

Hesse-Rubartsch

(A1)

analysis

414

GEOCHEMICAL PROCESSES AT MINERAL SURFACES

Sb c a r r i e r i o n s . The Sb-119 i o n s were adsorbed on 30 mg o f h e m a t i t e from 10 cm3 o f a 0 . 2 5 mol/dm3 KC1 s o l u t i o n c o n t a i n i n g about 1 mg o f p e n t a v a l e n t Sb i o n s . About 0 . 3 mg o f Sb was adsorbed a t pH 2 . 5 and 4.0. The amounts o f Sb a d s o r b e d a r e l e s s than t h a t r e q u i r e d t o c o v e r a l l the h e m a t i t e s u r f a c e s a s a m o n o l a y e r . The e m i s s i o n Mossbauer s p e c t r a o b t a i n e d a r e shown i n F i g u r e 7. I t i s seen from F i g u r e 7 t h a t the w i d t h o f t h e e m i s s i o n Mossbauer spectrum a t pH 2 . 5 i s much s m a l l e r than t h a t o f t h e c a r r i e r - f r e e o n e , w h i l e e s s e n t i a l l y no e f f e c t o f c a r r i e r Sb i o n s i s o b s e r v e d a t pH 4 . 0 . E f f e c t s o f P e n t a v a l e n t Sb Ions on the A d s o r p t i o n o f D i v a l e n t C o - 5 7 on H e m a t i t e . Benjamin and Bloom r e p o r t e d t h a t a r s e n a t e i o n s enhance the a d s o r p t i o n o f c o b a l t o u s i o n s on amorphous i r o n o x y h y d r o x i d e (j_6). S i m i l a r l y , when d i v a l e n t Co-57 i o n s were adsorbed on h e m a t i t e t o g e t h e r w i t h p e n t a v a l e n t Sb i o n s , an i n c r e a s e o f a d s o r p t i o n i n t h e weakly a c i d i c r e g i o n was o b s e r v e d . F o r example, when 30 mg o f h e m a t i t e was shaken w i t h 10 cm3 o f 0.1 mol/dm3 KC1 s o l u t i o n a t pH 5.5 c o n t a i n i n g c a r r i e r - f r e e C o - 5 7 and about 1 mg o f p e n t a v a l e n t Sb i o n s , 95 % o f Co-57 and about 30 % o f Sb i o n s were a d s o r b e d . The e m i s s i o n s p e c t r a o f the d i v a l e n t Co-57. i o n s adsorbed under t h e s e c o n d i t i o n s a r e shown i n F i g u r e 8 t o g e t h e r w i t h t h e r e s u l t s o b t a i n e d under d i f f e r e n t c o n d i t i o n s . As seen i n F i g u r e 8, t h e s p e c t r a o f d i v a l e n t Co-57 c o - a d s o r b e d w i t h p e n t a v a l e n t Sb i o n s a r e much d i f f e ­ r e n t from t h o s e o f Co-57 adsorbed a l o n e ( F i g u r e 3 ) . These o b s e r v a ­ t i o n s show a marked e f f e c t o f t h e . c o - a d s o r b e d p e n t a v a l e n t Sb i o n s on the c h e m i c a l s t r u c t u r e o f a d s o r b e d C o - 5 7 . A n a l y s i s o f the Mossbauer

Data

M a g n e t i c I n t e r a c t i o n s on Hematite S u r f a c e s . In magnetic m e t a l o x i d e s , t h e l o c a l i z e d s p i n d e n s i t i e s on t h e m e t a l i o n s i n t e r a c t w i t h each o t h e r t h r o u g h t h e superexchange i n t e r a c t i o n ( 1 7 - 1 9 ) . The main component o f the h y p e r f i n e magnetic f i e l d on t r i v a l e n t F e - 5 7 a r i s i n g from adsorbed Co-57 i o n s o r i g i n a t e s i n t h e i r own d - e l e c t r o n s o r d e r e d by the superexchange i n t e r a c t i o n w i t h t h e n e i g h b o r i n g f e r r i c i o n s . In the s i m p l e s t c a s e i n which t h e t r i v a l e n t F e - 5 7 i o n s a r e com­ p l e t e l y i n c o r p o r a t e d i n t o t h e c o o p e r a t i v e a n t i f e r r o m a g n e t i c system o f the b u l k s u b s t r a t e , t h e F e - 5 7 i o n s a r e e x p e c t e d t o a l i g n p a r a l l e l o r a n t i p a r a l l e l t o t h e magnetic i o n s o f t h e s u b s t r a t e i n a s i m i l a r manner as t h e f e r r i c i o n s o f t h e s u b s t r a t e . When t h e t r i v a l e n t F e - 5 7 i o n s a r e on t h e s u r f a c e , however, t h e i r m a g n e t i z a t i o n i s c o n s i d e r e d t o be r e d u c e d t o some e x t e n t due t o r e d u c t i o n i n t h e number o f n e i g h b o r i n g magnetic m e t a l i o n s i n t e r a c t i n g w i t h them. In the f o l l o w i n g d i s c u s s i o n , we t r e a t t h e s u r f a c e e f f e c t on t h e b a s i s o f the Weiss f i e l d ( m o l e c u l a r f i e l d ) a p p r o x i m a t i o n (17-19)> assuming no r e l a x a t i o n ( f l u c t u a t i o n o f t h e e l e c t r o n s p i n s ) . In t h e treatment, the reduced magnetization m (magnetization a t a c e r t a i n t e m p e r a t u r e d i v i d e d by t h a t a t O K ) o f the s u r f a c e f e r r i c i o n s a t t e m p e r a t u r e Τ Κ i s d e s c r i b e d by m(surface

Fe

3 +

)

= B (g3Sf H /kT) s

r

w

H e r e , Bs i s the B r i l l o u i n f u n c t i o n f o r a p a r a m a g n e t i c i o n w i t h s p i n quantum number S (= 5/2 f o r f e r r i c i o n ) , w h i l e g , 3, and k a r e t h e

AMBE E T A L .

Metal Oxide-Aqueous

Figure

7.

Sb-119

adsorbed

KC1

In

situ

solution:

on

(A)

Solution

emission

Mossbauer

hematite

with

pH 4 . 0

and

Sb

(B)

Interfaces

spectra

carrier

of

from

pentavalent 0.25

mol/dm3

2.5.

100 CAD

μ— giooC Β ]

LU 9 9

v>y

> I—

5 loo Lu or C

C

)

\ Λ

ν

y

99 -

-10

Figure

8.

adsorbed KC1

In on

situ (A)

v

Λ

/

ν

-8 0 8 RELATIVE VELOCITY C MM/S VS M E T A L L I C IROND emission

hematite

solution:

Λ

with

pH 5 . 5 ,

Mossbauer

pentavalent (B)

pH 9 . 2 .

Ιό

spectra

of

Sb

from

ions

(C)

divalent

From 0 . 3

0.1

Co-57

mol/dm3

M KOH.

416

GEOCHEMICAL PROCESSES AT MINERAL SURFACES

g-factor(~ constant, Hw d u e

2 for ferrie

ion),

respectively.

to the surface

effect

f

/

=

r

Γ

n.G. ι ι

L %

i o nacting

number

o f ions

The Sn-119 own

ions

chemical

(STHF)

cations

metal

proportional ions H =

spins,

that

ions.

1

1

9

4

Magnetic

corundum-type

In the corundum-type

structure

model

down

sites

i n the zeroth

ionsites

from

results

f o r room

andpentavalent

(bulk

1

1

9

temperature

ions

sites

sites

we c a n e s t i m a t e

with

hyperfine

ions

i s

mio f the

Sn

4

)

+

hematite

pairs the

i n each

pair

and

U

A

D

are shifted We d e n o t e

) ,

ions

these

respectively.

(111) surfaces that

i o nlayers metal

with

below 963

of ferric

along

zation

o f surface

on t e t r a v a l e n t

calculation

ferric

i n Figure

can

s i x ligand

metal

fields

o f hematite,

i s , up a n d

( A Q , A ^ , AQ a n d

i o nlayer

oxide

pentavalent STHF

i s not

ions

Sn-119

(3x0.76/9)H

andbulk ions

at the

Since

both

or distorted hematite

i s

i n themetal i o n

on t e t r a v a l e n t Using

temperature,

a t the surface

(bulk)

10.

o f hematite.

a t room ions

h f

ions Sb-119

ions

6 o f r e f 4, and t h e

octahedral

interactions

i o nsites

on f e r r i c

i n Figure

occupy

t o be =

( A

metal

a r e shown

Sb i o n s

t o accommodate

at the surface

The s u b s t r a t e

t h e second

field

considered

( A Q )

h f

are distinguished,

a r e given

octahedral

F

. H

sites

sites

and f i r s t

o f Weiss

i o nsites

ferric

H

t o the ordered

the bulk.

values

H

)]

i n t h e d i r e c t i o n .

In the model,

9.

distinguishable

lated

3 +

f o r t h e dominant

o f metal

(3),

ions Sn-119

o f hematite,

vacant

a s u p a n d down

kinds

ions

through t o such

i s an antiferromagnet

of ferric

o r downward

our simplified

metal

their

mag­

Namely,

Surfaces.

by s i n g l e

four

The

on t h e surface

structure

The p o s i t i o n s

upward

positions

surface

oxide

ions only

thesupertransferred

n.m.(Fe

Oxide

crystal

i n a row spaced

i n Figure

occurs

through

have

the ordered

t o t h e Sn-119

that

Although

don't

bulk

K.

A^

Fe-57.

they

from

transfer

them.

£

3

are

In

η being the

)

+

n.m.tFe *)/

direction.

ordered

on tetravalent

sum o f t h e m a g n e t i z a t i o n

with

Sn

i.e.,

densities

spin

Hhf a c t i n g

interacting

Corundum-type

lattice

spin

to thealgebraic

[ Σ

slightly

interaction

on paramagnetic

a r e bound

surface

the

that

We a s s u m e

field

(surface

h f

f o r each

i o ni n question,

are diamagnetic,

Substantial

magnetic

ferric

constant

ferric

c a nbe " s u p e r t r a n s f e r r e d "

diamagnetic magnetic

field

hyperfine

from

ions

electron

bonds.

field

t h e same G .

o f magnetic

Sn-119

unpaired

netic

Weiss

i s different

tetravalent

andt h e Boltzmann

f r f o r t h eWeiss

i s given by

on the surface

with

nature

ions

factor

bulk

G i i s a microscopic

ferric

magneton,

V n.O. j j

L

surface

where

t h e Bohr

Ther e d u c t i o n

t h e STHF

sites

Sn-119

the magneti­ magnetic

are calcu­

AMBE ET AL.

Metal

Oxide-Aqueous

Solution

Interfaces

417

(At) -

©

i i

a

b) Oth Metal layer

1st 0 " layer

®

1st

ÎS>

2

©- 2 n d

©

»

2nd

ο ο

οΟ

(d)

protons 3 Ο " 2

®

30 -

W

2

protons

O ~ layer z

© ©

© © 0

0

O

Bulk

Figure

9.

Simplified

corundum-type direction zeroth, the

slightly

first,

surface

protons Sb-119 complex to

the

are ion of

second the

oxide not

on

the

surface

Co-57 ion

surface

of

are

the

are

A divalent ion

Only in

shown

layers

of

metal (b)

part

Co-57

layer,

ions

or

(d)

a of

the of

Hexagonally

lines.

Sb-119

hematite.

the from

A section

a.

with

pentavalent

of

surface

shown,

depicted

metal or

(111)

.

layers

(c)

zeroth

oxide

from

layers

shown,

the

A view

arrows

ion

divalent

of

(a)

shifted

and

along

close-packed

model

structure,

Surface

pentavalent Aquo

or

hydroxyl

hydrogen-bonded

GEOCHEMICAL PROCESSES AT MINERAL SURFACES

418

CA)

0

Figure

10.

trivalent spectra

200 400 600 H Y P E R F I N E M A G N E T I C F I E L D CKOED

Distribution Fe-57

shown

obtained

i n Figure

of

the hyperfine

magnetic

by t h e H e s s e - R u b a r t s c h 3.

fields

analysis

on of

the

19.

AMBE ET AL.

Metal Oxide-Aqueous

H^CAQ)

=

((3x0.95+3x0.76)/9)H

\

)

=

(3/9)H

h f

(bulk)

\ (Αι)

=

(6/9)H

h f

(bulk)

f

( A

U

Γ

A bulk itation

of

hematite/pentavalent ferric

STHF

and

a

room

temperature.

Using

STHF

fields

on

surface

11.

In

earlier

A 's, of

our (3),

A 's

u

field

d

of

the

we

did

not

the

bulk

the

Regions

I,

arising

from

smaller

than

magnetic

the

field

interactions

like

sharp

Because limit

of

analysis tal

of

is

error,

ref

3,

species

values

field

and

a

estimated

give

rise

to

a

for limits

tetravalent

surfaces be

by

much

near

ions

to

Figure

fields

For

Sn-119

of

upper

distribution

The

at

the

pentavalent the

estimated

broad

appear.

the

having

zero

no

delta-function­

field.

simplifications

the

is

Sb-119

obtain

the

the

coprecip­

bottom

of

to

on

from

the

But,

by

calcination

we

respectively.

substrate

the

at

roughly

adsorbed

with zero

shown

treatment.

to

the

namely

of

by

arising

measurements

expected

at

fitting,

ions

this

and

Hhf(bulk),

correspond

hyperfine

above

STHF

peak

III

Sb-119

the

is

(B)

prepared

ions

ions

for

situ

419

(bulk)

sample

Sn-119

value

ex

Interfaces

Sb-119

Sn-119

on

apply

II,

bonding,

on

this

work

and

Sn-119

hydrogen

122 k O e

h f

Sb-119

pentavalent

gave

Sb-119

Solution

described

associated

to

by

one

order

larger

15 k O e

for

Fe-57

and

about

be

with

above

error

the

as

well

as

of

the

results than

3 kOe

the

for

the

experimen­

Sn-119.

Discussion Divalent of

from of

Co-57

hyperfine the

emission

Hesse

from

Ions

on

magnetic

and

Hematite. fields

Mossbauer

Rubartsch

Figure

acting

on

spectra

(20)

10 s h o w s

the

given

assuming

no

10 t h a t

the

adsorbed

divalent

two

chemical

forms:

one

giving

peak

the

calculated

another

giving

former,

whose

to ble

the

values the

for

broad

fraction

divalent

forms

Co-57

the

n

in

Figure

Fe(substrate)

bonds the

half

of

states.

With

surface

sites

alkaline after

region.

adsorption

of

divalent

of

the

based, due

to

A's

Co-57

aqueous the

fore,

that

sites

A's

pH

are

pH,

the

the

ions was

lowered

desorption occurs

in

from

At are

pH o f

the

observed the 12.7

the

by

to

3.0.

bound

of

slow on

surface

in

corresponding

to

9)

and

fields.

The

pH,

is

attributed

One

of

the

one

or

proba­

two

weakly

Co-57 in

10(A)), in

reajusted

but

steady

10(F) from

below

5

desorption

radioactivity

Figure

Mossbauer

the

strongly

was

the

as

Co-0-

bound

ions the

form

surface

(Figure

ions

seen

least

5.7

Figure

Co-57

method be

at

these

which

the

are

hematite

measuring

spectrum

divalent

strongly

pH in

the

hydrolyzed

with

solution

continuing during from

some

dominant

region,

For

in

the

fraction

was

of

to

becomes

alkaline

lower

calculated can

(Figure

A's.

or

forms

ions

the

and

the

2 +

It

ions

A's

sites

possible.

Co-57

increases When

surface

distribution

3 by

region

increase

chemical

also

the

in

with

Figure

Co-57

sites

[Co(H20)6]

Other

phase.

desorption

in

is

adsorbed

in

surface

hydrogen-bonded

+

increase

in

distribution

ions

9(d).

about

the

increases

latter

[Co(H20)n(0H)6-n]^ "^^ shown

for

in

relaxation.

Figure

a

the

Fe-57 n u c l e i

10(F)

is

measurement shows, the

complexes.

there­

surface

420

GEOCHEMICAL PROCESSES AT MINERAL SURFACES Our

namely

d i s c u s s i o n has so f a r ignored

spin

precession

of electrons

period

component However,

flip

(2J_).

the decrease

reflect

the decrease

adsorbed

The a p p a r e n t

i n the analysis

metal

described

Pentavalent

Sb-119 I o n s

spectra

given

i n Figure

acting

Sb-119 i o n s

adsorbed

the tetravalent

pH o f t h e a q u e o u s the

STHF

fields

magnetic

field.

Sn-119 s p e c i e s STHF

interface.

phase.

bonding

excess

favorable

probable

in this

region,

protons

for the adsorption

acidic

pH v a l u e s ,

chemical

a

certain

a t the

form

o f the

p H 4 ) , we p r o p o s e

surfaces

by

9(d).

the

hydrogen

In the n e u t r a l and

situation

o f the hydroxyl

zero

tetravalent

are not negatively

This

substrate

decreasing

but having

(above

i n Figure

on them.

trivalent

the

with

that

sites

dominant

the surfaces

exist

to

d i s t r i b u t i o n near

to the oxide shown

gradually

are overwhelming

pH r e g i o n

attached

magnetic

pentavalent

In c o n t r a s t

and s l i g h t l y

metal

the substrate

the structure

acidic

Mossbauer

from

demonstrates

not i n the surface

As t h e most

like

11 a r e s h o w n t h e

i n t e r a c t i o n between

increases

The o b s e r v a t i o n

Sb-119 i o n s

slightly

In Figure

At n e u t r a l

to

between t h e

the conclusion

o f the emission

show d o m i n a n t l y a b r o a d

[Sb(0H)6]~ complex

few

Sn-119 i o n s

interaction with

adsorbed

Therefore,

surface.

the magnetic

relaxation.

considered

5 o n t h e d i s t r i b u t i o n o f STHF

on h e m a t i t e

above

bonding)

of

valid.

o n t h e n u c l e i o f Sn-119 a r i s i n g

Fe-57 d e s c r i b e d and

i s also

to remain

analysis

(weak

o f c h e m i c a l bonds

on H e m a t i t e .

o f Hesse-Rubartsch

fields

time

and the substrate.

results

relaxation,

t o t h e Larmor

be t h e r e s u l t

i n the strength

i s considered

of

comparable

low f i e l d

above might

in relaxation

species

above

the effect

i n a time

is

charged and

considered

complex

by

hydrogen-

bonding. With field

the decrease

(Figure

This

11).

bonds

occurs

large

fraction

for

surface

Sb-119 i o n s ion

layer

aqueous

is

the

bulk

(the second

or deeper

Sb-119 i o n s Moreover, heating

the fact

that

into

tite

particles

a r e exposed ferric

ions

reported

9), based

previously,

into

o f the f i e l d o f hematite

the second change

t o aqueous

or deeper

i s observed occurs

solutions from

at

apprecia-

or

deeper

hours distribu-

with

adsorbed

o f simple

A plausible

o f the surface

are released

no

C for 2

t o be t h e r e s u l t

mechanism.

a t pH

are i n

on t h e

the second

a t 200

the change

no f u r t h e r

a different

a chemical rearrangement some

metal

when t h e

In the d r i e d

12.

the suspension

Sb-119 i o n s

suggests

9),

o f Sb-119 i o n s

i n Figure

t h e sample

Therefore,

heating

that

example,

layers

98°C i s n o t l i k e l y

o f surface

fields

pentavalent

Sb-119 s u s p e n s i o n

amount

Sb-119 i o n s

on h e a t i n g

after

at

Sb-O-Fe

or i n the f i r s t

i n Figure

i n Figure

Sb-119 s a m p l e

3 o f r e f 3).

observed

shown

o f the surface

was o b s e r v e d

(Figure

(A's

of

relatively

to the magnetic

t o be i n t h e z e r o t h surface

magnetic

fields

formation

o f the adsorbed

the hematite/pentavalent

hematite/pentavalent layers

most

(C2), a c o n s i d e r a b l e

of calculation

diffusion

the zero

to higher

acidic.

heating

(B2) a n d 2.6

sion

near

t o show t h a t

corresponding

Therefore,

are considered

4.3

tion

peak

The d i s t r i b u t i o n h a s a

i n the region

sites.

phase

results

is interpreted

on t h e s u r f a c e .

o f the hematite

After

ble

i n pH, the broad

diminishes and the d i s t r i b u t i o n extends

diffu-

layers. after

30

min's

explanation when

98 C .

the surfaces

is

the hemaFor of

hematite

AMBE ETAL.

Metal

I 0

Figure lent the

11.

I

shown

Solution

Interfaces

I

50 100 H Y P E R F I NE M A G N E T I C F I E L D

Distribution of

Sn-119 n u c l e i spectra

Oxide-Aqueous

obtained i n Figure



150 C Κ 0E 3

t h e STHF m a g n e t i c

fields

by t h e H e s s e - R u b a r t s c h 5.

on tetrava>

analysis

of

422

GEOCHEMICAL PROCESSES AT MINERAL SURFACES

cm ]

C A2 3

CB1 3

CCI 3

CC23

1

1

1

A^A? 0

Figure lent the

12.

particles of

second a

or deeper

layers.

few l a y e r s

of

ions

t o them so a s

into

Since

concluded

fields

on

tetrava­

analysis

of

6.

attached

Sb-119

150 C Κ0E 3

by t h e H e s s e - R u b a r t s c h

i n Figure

is

1

o f t h e STHF m a g n e t i c

obtained

and a r e a g a i n

30 m i n , i t

among

the metal

the change

that

the surfaces

to incorporate ion sites

does

not proceed

the rearrangement

and the Sb-119

ions

a

part

o f the

is

further

limited

are

only

distributed

them. The

observation

pentavalent smaller energy Sb,

shown

the pentavalent

after to

nuclei

spectra

1

Β

50 100 HYPERFINE MAGNETIC F I E L D

Distribution

Sn-119

1

AgA?

Sb-119

than

the sites

tially

that

bonding

at

the l i n e

adsorbed

lower

coverages,

stronger

These

width

with

of carrier-free

forming

by S b - 1 1 9 .

that

ions

of

the emission

Sb c a r r i e r

Sb-119

is

indicative

i . e . , i n the absence

chemical

interactions

of

bonds

spectrum

a t pH 2 . 5 i s of

higher

of

are occupied

Sb-119

with

of

much

strong

carrier preferen­ bonding

19.

A M B E ET AL.

Metal

Oxide-Aqueous

Solution

423

Interfaces

s i t e a r e d i l u t e d o u t i n the p r e s e n c e o f c a r r i e r s m a l l e r l i n e w i d t h , weaker a v e r a g e b o n d i n g .

Sb l e a d i n g

to

the

E f f e c t s o f P e n t a v a l e n t Sb on t h e A d s o r p t i o n o f D i v a l e n t C o - 5 7 . The e m i s s i o n Mossbauer s p e c t r a o f d i v a l e n t C o - 5 7 a d s o r b e d on h e m a t i t e w i t h p e n t a v a l e n t S b i o n s ( F i g u r e 8) a r e complex and we have n o t y e t succeeded i n t h e i r a n a l y s i s . I t i s c e r t a i n , however, from the s p e c t r a t h a t t r i v a l e n t F e - 5 7 i o n s produced by the EC decay o f C o - 5 7 a r e i n t e r a c t i n g m a g n e t i c a l l y w i t h the f e r r i c i o n s o f the s u b s t r a t e . T h i s means t h a t the d i v a l e n t C o - 5 7 a r e n o t a d s o r b e d on the p e n t a v a l e n t Sb i o n s , but on h e m a t i t e d i r e c t l y . The [ S b ( 0 H ) 6 ] ~ a n i o n s a r e c o n s i d e r e d t o f a c i l i t a t e d i r e c t a d s o r p t i o n o f d i v a l e n t C o - 5 7 i o n s on the p o s i t i v e l y charged s u r f a c e s o f h e m a t i t e i n the a c i d i c r e g i o n . Conclusion In s i t u e m i s s i o n Mossbauer s p e c t r o s c o p i c measurement o f the h y p e r f i n e magnetic f i e l d s on t r i v a l e n t F e - 5 7 and t e t r a v a l e n t S n - 1 1 9 a r i s i n g from d i v a l e n t C o - 5 7 and p e n t a v a l e n t S b - 1 1 9 , r e s p e c t i v e l y , y i e l d s v a l u a b l e i n f o r m a t i o n on the c h e m i c a l s t r u c t u r e o f a d s o r b e d m e t a l i o n s a t the i n t e r f a c e between h e m a t i t e and an aqueous solution. In the s l i g h t l y a c i d i c r e g i o n , a d s o r b e d C o - 5 7 i o n s a r e d i s t r i buted between a t l e a s t two c h e m i c a l f o r m s : one a t t r i b u t a b l e t o t h e C o - 5 7 i o n s c o o r d i n a t i v e l y bound t o s u r f a c e s i t e s and the o t h e r t o C o - 5 7 weakly bound t o t h e h e m a t i t e s u r f a c e s . In the a l k a l i n e r e g i o n , most o f the a d s o r b e d C o - 5 7 i o n s a r e i n the z e r o t h o r f i r s t m e t a l - i o n l a y e r s o f the s u b s t r a t e f o r m i n g C o - O - F e bonds. Desorption o f d i v a l e n t C o - 5 7 from s t r o n g l y c o o r d i n a t e d s u r f a c e complexes o c c u r s when the pH i s lowered from a l k a l i n e t o a c i d i c v a l u e s . In c o n t r a s t , the p e n t a v a l e n t S b - 1 1 9 i o n s a t t h e i n t e r f a c e s a r e weakly bonded t o the o x i d e i o n l a y e r o f the h e m a t i t e s u r f a c e s i n n e u t r a l and s l i g h t l y a c i d i c r e g i o n , w h i l e i n the a c i d i c r e g i o n most o f the adsorbed S b - 1 1 9 i o n s a r e i n t h e z e r o t h o r f i r s t m e t a l i o n l a y e r s o f the s u b s t r a t e f o r m i n g S b - O - F e bonds. The p e n t a v a l e n t S b - 1 1 9 i o n s h a v i n g once been i n c o r p o r a t e d i n t o the s u r f a c e m e t a l i o n s i t e s r e t a i n t h e i r c h e m i c a l f o r m , even when the pH o f the aqueous phase i s r a i s e d above 7. H e a t i n g o f s u s p e n s i o n s a t 98 C r e s u l t s i n c h e m i c a l rearrangement o f the h e m a t i t e s u r f a c e s t o y i e l d p e n t a v a l e n t Sb-119 i o n s i n the second o r deeper m e t a l i o n l a y e r s . Future

Prospect

The p r e s e n t method i s s t i l l i n i t s e a r l y s t a g e o f a p p l i c a t i o n . Both ex s i t u and i n s i t u t y p e measurements a r e a p p l i c a b l e t o a v a r i e t y o f mineral/aqueous s o l u t i o n i n t e r f a c e s . F o r example, t h e mechanism o f s e l e c t i v e a d s o r p t i o n o f c o b a l t o u s i o n s on manganese m i n e r a l s can be s t u d i e d by t h i s method. In a d d i t i o n t o t h e two Mossbauer s o u r c e n u c l i d e s d e s c r i b e d i n t h e p r e s e n t a r t i c l e , t h e r e a r e a number o f o t h e r n u c l i d e s which can be s t u d i e d . We have r e c e n t l y s t a r t e d a s e r i e s o f e x p e r i m e n t s u s i n g Gd-151 which i s a s o u r c e n u c l i d e o f Eu-151 Mossbauer s p e c t r o s c o p y . Development o f t h e o r y on s u r f a c e magnetism, e s p e c i a l l y one i n c l u d i n g r e l a x a t i o n i s d e s i r a b l e . Such a t h e o r y would f a c i l i t a t e t h e i n t e r p r e t a t i o n o f the e x p e r i m e n t a l results.

424

G E O C H E M I C A L

PROCESSES A T M I N E R A L

S U R F A C E S

Acknowledgments C o o p e r a t i o n o f t h e s t a f f o f RIKEN c y c l o t r o n i n many i r r a d i a t i o n f o r the p r o d u c t i o n o f Sb-119 i s g r a t e f u l l y acknowledged.

runs

Literature Cited 1. Jenne, E. A. "Chemical Modeling in Aqueous System"; ACS SYMPO­ SIUM SERIES No. 93, American Chemical Society: Washington, D.C., 1979. 2. Tewari, P. H. "Adsorption from Aqueous Solutions"; Plenum Press: New York, 1981. 3. Okada, T.; Ambe, S.; Ambe, F.; Sekizawa, H. J. Phys. Chem. 1982, 86, 4726. 4. Ambe, F.; Okada, T.; Ambe, S.; Sekizawa, H. J. Phys. Chem. 1984, 88, 3015. 5. Pollak, H. Phys. Status Solids 1962, 2, 720. 6. Snell, A. H.; Pleasonton, F. Phys. Rev. 1955, 100, 1396. 7. Wertheim, G. K. In "The Electronic Structure of Point Defects"; Amelinckx, S.; Gevers, R.; Nihoul, J . , Eds.; North-Holland: Amsterdam, 1971; Part 1. 8. Friedt, J. M.; Danon, J. Radiochim. Acta 1972, 17, 173. 9. Kundig, W.; Bommel, H.; Constabaris, G.; Lindquist, R. H. Phys. Rev. 1966, 142, 327. 10. Ambe, F.; Ambe, S.; Shoji, H.; Saito, N. J. Chem. Phys. 1974, 60, 3773. 11. Ambe, S. J. Radioanal. Nucl. Chem., Articles 1984, 81, 77. 12. James, R. O.; Healy, T. W. J. Colloid Interface Sci. 1972, 40, 42. 13· Ambe, S., to be published. 14. Jander, G.; Ostmann, H. J. Z. Anorg. Allg. Chem. 1962, 315, 241. 15. Parks, G. Α.; de Bruyn, P. L. J. Phys. Chem. 1962, 66, 967. 16. Benjamin, M. M.; Bloom, N. S. In "Adsorption from Aqueous Solutions"; Tewari, P. H. Ed.; Plenum Press: New York, 1981; pp. 41-60. 17. Goodenough, J. B. "Magnetism and the Chemical Bond"; Interscience-Wiley: New York, 1963. 18. Morrish, A. H. "The Physical Principles of Magnetism"; Wiley: New York, 1965. 19. Kittel, C. "Introduction to Solid State Physics", 5th ed.; Wiley: New York, 1976. 20. Hesse, J . ; Rubartsch, A. J. Phys. Ε 1974, 7, 526. 21. Hoy, G. R. In "Mossbauer Spectroscopy Applied to Inorganic Chemistry"; Long, G. J . , Ed; Plenum Press: New York, 1984; Vol. I, p. 195. RECEIVED

June 18, 1986