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
3·
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
