21 Adsorption of Organic Reductants and Subsequent Electron Transfer on Metal Oxide Surfaces Alan T. Stone Department of Geography and Environmental Engineering, Johns Hopkins University, Baltimore, MD 21218 Rates of reductive dissolution of transition metal oxide/hydroxide minerals are controlled by rates of surface chemical reactions under most conditions of environmental and geochemical interest. This paper examines the mechanisms of reductive dissolution through a discussion of relevant elementary reaction processes. Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is an important determinant of rates of individual surface chemical reactions and overall rates of reductive dissolution. Oxide/hydroxide
minerals
are
thermodynamically
but
a r e reduced
the
presence
tically for
hydroxide
sition metal
metal
ions
of aquatic
and therefore
have
geochemistry.
Catechol,
by o r g a n i c
one-equivalent
with
orders
respect
i n higher
Fe(II),
oxide/
concentrations and than
significant
compounds
to
to
in
drama-
of magnitude (1).
environments
t h e most
state
inorganic
impact
solution,
on t r a n -
reduction of
is relatively
well
under-
F e ( H 2 0 ) j H i n 0.1M HCIO4 i n
reduces
steps (2):
Fe(H 0)^
+
Fe(H 0)^
+
2
2
conditions
i n oxidation
I n homogeneous
f o r example,
anoxic
and Pb(IV)
at neutral pH,
Reduction of Fe(III)
solubility
are present
variety
Co(III),
solutions
under
Changes
b y a s much a s e i g h t
i o n complexes
stood.
metal
agents.
iron
Fe(III),
i n oxygenated
solubility.
reductants
i n a wider
reductants,
two,
their
increases
phases
Organic found
to divalent
of reducing
alter
example,
of Mn(III,IV),
stable
+ QH
— ± Fe(H 0)^
+
2
+ QH- + H
+ QH-
Fe(H 0)^
+
+ Q + H
2
2
0097-6156/ 86/ 0323-0446$06.00/ 0 © 1986 American Chemical Society
+
+
(1)
(2)
21.
STONE
Within tion
Adsorption
t h e pH r a n g e
metals
organic
are
>Fe
>Fe
i i
t h e symbol
oxide
lattice. with
dissolution, Gorichev
surface
tive tron neous tion ties
dissolution
steps and
(i)
product
arising tive
chemical from
and
interfacial
are
diffusion
species
from
focus
of
this
(for
Overall of
reduc-
surface
substitution
and
in
elec-
homoge-
i n homogeneous
arising of
from
electrical surface
results
proper-
chemical
from
and i n p r e d i c t i n g
solu-
surface
actual
the k i n e t i c
Reaction
of
occurring reductant
reaction,
molecules
and
(iii)
surface.
are influenced
In
of
by
gradient
by o r g a n i c
to
diffusion of
and e l e c t r i c a l surface
environmental
surface
of
situations fluxes
reduc-
sequence
the oxide
t h e combined
on t h e o x i d e of
oxides
i n the following
are rate-controlling,
reactions
Scheme
reaction
where
transport
charged effects
reactant of
potential (8).
Many
and g e o c h e m i c a l
chemical
reactions,
of
surface,
which
the
gradient reducinterest are the
discussion.
organic
[H+]=[Me
several
and upon
discusses
studies
Much
transfer
pertinent
and i n t e r f a c i a l
t r a n s i t i o n metal
as
by r a t e s
Transition oxidize
of
and
speciation
Ç5-7),
paper
reactions
surfaces
how b u l k
surface
rates
kinetic
in interpreting
the net charge
controlled
This
and
( 3 ) and
dissolution.
and e l e c t r o n
ligand
from
transfer,
by D i g g l e
terms
of
i n the
steps
reactions.
concentration
dissolution
of
center
reaction
electron
the impact
(5).
(4)
oxide
descriptions
experiments
the oxide
(iii)
metal
(3)
+
reviews
i n general of
between
useful
c a n be d e s c r i b e d
away
theories developed
Molecular
Reductive
formation,
substitution
speciation,
Dissolution:
iron
+
metal
surface
on d i s s o l u t i o n
oxide/hydroxide
prove
The of
(5)
the surface
t h e mechanisms
Differences
unexplored
surface
upon
solution
using
Reductive
(ii)
to
rates
ligand
reactions
dissolution
reaction.
+
2
Comprehensive
reactions
will
products
bonds
by examining
chemical
(i)
+ Q + H
(4·) o u t l i n e
with
reductive
steps:
0H~
Fe(H 0)^
has focussed
reactions
tants
I 1 [
>
influence
are discussed.
of
>Fe
complex
solution.
behavior
+ QH- + H
precursor
and on m e t a l
structure,
0H~
3 - 5 represent
reactions
by
the reduction
i : E
_
i n homogeneous
transfer
transi-
Reduction
chemical for
dissolution
> denotes
dissolution
chemical
surface
Equations
chemical
minerals.
>Fe
OH"
analogies
reactions
2
+ QH- ^
and K i p r i y a n o v
possible
447
catechol:
respectively.
interest
a
Transfer
t e r - and t e t r a - v a l e n t
oxide
c a n be p o s t u l a t e d
+ QH
OH
waters,
into
therefore,
by
O H
properties
recent and
m
I i : i
where
associated
natural
steps
sites
> F e
surface
is,
reaction
surface
of
incorporated
compounds
following oxide
of Organic Reductants and Electron
2 +
metal
oxide/hydroxides
compounds.
]=1.0M)
first-row
Table
and E
transition
f
(for
I
differ
lists
[H ]=1CT M +
in their
reduction 7
and
metals.
American Chemical Society Library 1155 16th St., N.W. Washington, D.C. 20036
ability
to
potentials
E°
[Me +]=10-6M) 2
for
448
G E O C H E M I C A L PROCESSES AT M I N E R A L S U R F A C E S
Table
I.
Reduction Potentials
of
Selected
Transition Metal
Oxide/
Hydroxides Half-Reaction h
6-Mn0 (s)
+
2
Y-MnOOH(s)
+
2H 3H
a-FeOOH(s)
+
CoOOH(s)
3H
+
^Ni 04(s)
Based
upon
decreases of
3H
+
3
the
= C 0
e " == -
N i
is
in
Littler
(14).
-0.22
(I)
+1.48
+0.59
(10)
+1.98
+0.85
(11)
and
+
+
metal
the
transfer
tion
sphere
center
rate
ligand
by
to the
transfer
leaves is
the
as
shown
fer,
and
guished.
in
the
The
the
complex,
of
and
nickel.
A*
mechanism
an
inner-sphere reductive
via
are
inner
of
to
the
complex
trans of
mechanism,
in
electron
complex
overall
reaction bound
mechanisms
is
can
oxide
an
ion
can be
pos
surface
sites,
electron
trans
s t i l l
unique,
within
(14).
reaction
mechanism.
formation,
complex
metal
rate
precursor
metal
In
coordina
the
intact; the
with
electron
l i m i t e d by
outer-sphere
successor
chemical
reactions
directly
with
of
one-equiva
reactions.
The h i g h e s t
sphere
metal
Both
complex,
by
the
outer-sphere
by
breakdown
further
outer-sphere
dissolution
centers
tervalent
produced
upon
mechanism
S u b s t i t u t i o n - i n e r t metal
an
and
a
metal
reviewed
precursor
bonds
parallel,
Precursor the
an
are
which
based
solution.
enters
is
a solu-
between
coupling
outer-sphere in
pathway.
react
and
by
radical
(14) .
through reactions
subsequent
consumed
coordination
by
fastest
surface
cobalt
developed,
of
a
and
radical
transfer
can operate
metal
comparable
inner-sphere
reduction
Reaction via
2.
participating
both
formation
quickly
inner
of
dissolved
No
transfer
i n homogeneous
free
inner-sphere
Figure
to
and
i n homogeneous
solution,
s u b s t i t u t i o n and
electron
breakdown
and
13).
occurs
been
reductant
necessarily
for
have
the
by
Rates
appear
increased
transfer
electron
mechanism,
complexes
Similarly,
surfaces
by
dominated
tulated
is
facilitated
mechanisms
is
dissolved
and
the
FeOOH.
reduced
reactions
the
this The
ligand
strength
CoOOH > sediments
ÇL2,
complexes
substitution.
contrast, These
via
prior
oxidant
flux
are
electron
for
(HA)
common t h e within
oxidant
inner-sphere
fer
phenol
electron ion
oxide
1 illustrates
complex.
lent
of
compounds
by
metal
oxides
i n homogeneous
successor
I,
and
reductant
analogous
mechanism for in
waters
release
(Με(Η2θ)β3 ) transfer
Table
> MnOOH >
iron oxides
outer-sphere +
in
> MnÛ2 the
ion
the
2
natural
mechanisms
Figure
have
2H 0
given
and
electron
2
Mechanism
organic
reactions
mechanisms
o
2
on
respects
general
of
+0.67
2
s u b s t i t u t i o n and
i n many
studies
2H 0
than
available
ligand
Two
+
H 0
manganese
quickly
d i s s o l u t i o n of
complexes
(9)
When
series
ion
(9)
+0.61
data
trend.
Reductive resemble
+0.64
+1.50
N13O4
Chemical Reaction
of
+1.29v
2H 0
+
2H 0
2
Ref.
f
+
+
+
dissolution
information
tion.
2
order:
more
+
Fe
turn anoxic,
and
Surface
+
2
2 +
2+
e~ =
e"~
similar
conditions earlier
+
+
Mn
thermodynamic in
a
e~ =
+
+
+
e"" = ^ M n
+
+
+
+
4H
reductive
follow
E
E°
be
distin
however,
in
oxide/hydroxide
that
U
C
"
cessor Complex
S
Breakdown
(C)
f
Electron Transfer
(B)
o
Precursor Complex Formation
(A)
e
W
H
solution.
1.
2
0 )
n N
+
* ΤΓ^
k
2
Μ
β
Reduction
W
of
k^
0 )
2
2
+
3+ Me(H 0)^
η
H
Μβ (Η 0)*
A
Λ Ν
II, „ 2+ Me
iA
'Α
A "
3
0
k ~
k
k
2
0 Η + HA
Ii:[
Χ 1 1
Me
me 1 1
1 T
T
T
9
(H 0)^ 2
Ά "
LLL
>Ke k
+ A
2
+ H 0
2. Reduction of tervalent (HA).
>Me ' ^
>Μθ
Figure phenol
Electron Transfer
(B)
Successor Complex
Precursor Complex Formation
(A)
I n n e r Sphere
metal
I
I
n i
i l J
T
O H + HA
γ
oxide
>Me
τ
,
k
Q
k,
"-2
2
γ
I X
by
Μβ
γ
>Me
H I ^ >Me k_^
sites
O H , A*
surface
±i
> M e O H , HA
>Me
O u t e r Sphere
Α Α
2
6
+ (Η 0)
2
+
+ A'
OH", A- + H '
OH, HA
r
m > r co c ?o
5
^
>
m
g ^
-σ
Ο
8 ac
m
Ο
21.
STONE
phase;
Adsorption
this
nuclear
coordinative
aquated
i n Figure
Action
i n several
behavior
complex
and Electron
important
of the reductive
2 c a nbe found
to the elementary
precursor
Reductants
reaction
formation
steps.
from
that
o f mono
respects.
dissolution
by applying
451
Transfer
e n v i r o n m e n t may d i f f e r
species
The k i n e t i c given
of Organic
mechanisms
the Principle of Therate
v i a an inner-sphere
Mass
expression f o r
mechanism i s g i v e n
by:
d [ >
ff
I I l A l
=
l [>Me
k
l : E I
OH][HA]
-
( k ^+ k
) [>Me
I I i :
A] + k_
[>Me
2
I 3 :
-A] (6)
For
the special
(and
assuming
Me +(aq.)
formation
2
d
A similar anism. are
[
f
rate
Based
favored
ki),
case
of steady-state
negligible
2
+
"
1
(k ^
expression upon
rates
(high k 3 ) .
high
(large k
Thenext
characteristics influence
transfer
spectroscopic
measurements
have many
oxide
rate
of ligand
Figure
ions
donor native
sites
reaction
i n homogeneous
i n part
c a nbe used often
similarities
Unfavorable
restrict
andmetal but differ
compli direct
Available oxide
surface
i n several
and differences a molecular
or
fewer
interfaces.
i o n complexes
to construct
because to monitor
as a result,
at oxide/water
Thecoordinative
t h e thermodynamic
s u b s t i t u t i o n and e l e c t r o n (a-Fe 03)
a r e used i n
description of
the inner
o f each m e t a l
are introduced
into
transfer Within array
transi
a r e shown
the lattice of
of 0
however,
center
a r e vacant solution,
of
and reaction
reactions.
structures
surface, aqueous
force
coordination spheres
occupied by a regular
At themineral
environment
driving
surface
2
of comparison.
minerals,
positions
Level
concentrations.
interfaces
molecules steps.
reactions.
3 f o r t h e sake
groups.
i o n complexes
techniques
release
how c h e m i c a l
and reductant
detail,
(large
electron
of product
contributing
mech
dissolution
formation
examines
characteristics,
and hematite
are fully
surfaces
complex
rates
techniques;
metal
These
affects
oxide/hydroxide centers
that
Environment.
metal
(aq)
made
discussions
tion
2 +
of reductive
paper
of metal
(7)
for the outer-sphere
k_i)> ( i i i ) high
surface
of these
and product
chemical
dissolution
Coordinative
Fem()H
(small
and other
been
respects.
following
concentration
° H ] EHA] ( l - e - ^ t )
rates
i n this
of oxide/water
indicates
share
reductive
I
i n considerable
intermediate,
important
I
of precursor
t h e a p p l i c a t i o n of these
sites
I
) » and (iv) high
reactions
methods
characteristics
evidence
e
of Reductive D i s s o l u t i o n on a Molecular
are understood
in
2
) [>"
7, high
rates
o f each
solution
cate
complex
f o r Β a n d C) t h e r a t e o f
c a nbe w r i t t e n
section
Electron
reactant,
2
rates
of metal
the rate
Description
k
2
Equation
by (i)
precursor
reactions
i s given by:
( i i ) low desorption
transfer
the
back
2
of metal
~ and/or
0H~
o n e o r more (15).
H2O
coordi
When
and OH"
oxide
molecules
G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S
452
0H
H 0
2
2
/I
HEMATITE
Figure and
3.
on a
Fe(III) hematite
coordinative ( a - F e ^ )
environment
surface
(adapted
in
Fe
from
Χ
0Η
48).
(aq.)
STONE
21.
bind to a
to
Adsorption
vacant
oxygen layer
for
solute
tive
for
positions
to
form of
oxide
ion
oxide
negative,
surface
>MeOH + >MeOH An e x c e s s
of
surface.
This
in
the
of
charged
charged
near
surface,
than bulk
substantially
are
Electrostatics,
therefore,
metal
oxide
Precursor tion
Complex
reactions, surface ties
sites
towards
ligand ty,
bond
which
(6.3xl0~3
change
To
different
of
are
rates few
at
a
(8) + H
2
turn,
a
(9)
+
analogous
protona-
positive,
near
charge
Near are
and
the a
charged
the
gradient
distribution
negatively
substantially
anion
constant
substantial
to
potential
disturbs
(17).
concentrations
and
other
medium
interfaces
impact
(18).
on r e a c t i o n s
limited
Water
(20);,
the
rates
possible
(2.0xl0
of
for
have
hexaquo
measure
of
(19))
and
differ
by
ligand
exchange
the
onto
number
oxides
opportunities
for
to
occur
sites.
has,
reliable
surfaces direct
eight
more
is
in
fact,
at
metallabili for
Fe(H 0) 2
orders at
^+ 6
of
Cr(III) than
ex
slower
speci
been
observed
(22-25)
comparison.
in
times
measurements
small
oxide
labili
complexes
slowly
Substantially
α-Ο^Οβ of
and
relative
example,
expected
ligand
different
approximation, surface
or
differences of
forma
substitution
complexes
Characteristic
seconds
6
complex
ligand
lability,
reflecting
exchange
one
4 -
phosphate metal
are
Dissolved
variations.
c a n be
oxide
by
c e n t e r (6).
substitution,
first
precursor
complex
t r a n s i t i o n metals
CriHoO)^
sites
of
+
a net
creates
Inner-sphere
therefore
Unfortunately,
exchange there
a
Fe(III)
adsorption
(21).
at
in
successor
solution are
seconds
surface at
the
strengths.
exchange
solutions
(11)
dielectric
has
the metal
ligand
oxide.
+
perturbed
show s u b s t a n t i a l
magnitude. oxide
of
mole
coordina
sites.
of
are of
homogeneous
water
fic
and
rate,
more
the underlying
undergo
concentrations
Formation.
and breakdown
exchange
in
surface
solution
solute
d i s t r i b u t i o n of
concentrations,
characteristics
All
(10)
interface
The
also
a
>MeO"" i m p a r t s
cation
lower.
or
by
order
2
charge
which,
solution
and
binds
(16):
>MeOH
the
4
(>MeOH)
sites
or
2
region
species
oxide
>MeOH
one
+ H
2 +
2
creating
electrical
interfacial
higher are
net
solvent
Me(H 0) (OH)
>MeO" + H
either
in
sites.
i n homogeneous
5
sites
^ ±
+
H+
covered
phenomenon:
surface H
and is
i n homogeneous
i n t e r a c t i o n by
—
2 +
453
displaced
surface
however,
complexes
reactions,
and n e u t r a l
with
be
incoming
2
5
tion/deprotonation
centers, surface
Me(H 0) (OH)
+
Me(H 0) (OH) 2
on m e t a l
complexes
sites,
Transfer
hydrated
w h i c h must
ion
from
important
2
and Electron
complexes
surface
metal
an
Me(H 0)j?
Hydrated metal
fully
metal
shielded
of
pH i s
a
interactions with
are
Hydrolysis increasing
positions
Thus,
positions
At metal
Reductants
OH" m o l e c u l e s ,
molecules
available
cules.
atoms. and
H2O
coordinative are
coordinative
donor
of
of Organic
of and
ligand so
454
G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S
If
the
electron
reductant
is
a metal
>Me
I I : E
OH +
Me (H 0) f
2
o
—
+
^ k
In
this
of
the
site, not
case,
since
Rates
the
of
of
is
2 +
may
also
precede
OMe
formation
rather
exchange the
than
l
(Η 0)ΐ
+
ο
depends
that
of
ligands
of
hydroxycarboxylic for
a
H 0
(12)
+
o
upon
the the
the
oxide
lability surface
surface
orders
of
Fe(H20)5(OH) phenols
(29)
site
rather has
2 +
with
on
higher
than
also
quickly
an
been
catechols than
phenolate
the
exchange
magnitude
(27),
much more
formation
strongly
The w a t e r
dissociative,
with
acids
complex
quite
center.
three
(20).
complexes
depend
metal
follows
mechanism
inner-sphere mechanism
I I I
- i
coordination
almost
and
+
exchange
>Me
complex
ion,
inner
ligand
Fe(H20)^ ,
below
adsorption
are
(26).
environment
Fe(H20)5(OH) of
precursor
incoming metal
exchanged
tive
ion,
transfer:
coordina rate
of
than
that
associative
shown
(28),
to
and
form
a-
Fe(H2Û)^ .
The
+
anion
(A~)
is
shown
(27): Fe(H 0)^
+
Fe(H 0)^
+
2
Fe(H 0) OH 2
5
+
2 +
H
K
+
Q
=
H
2.75xl0~ M
(13)
3
k 2
+
A"
~
^
Fe(H 0) A 2
5
+
2 +
H 0
(14)
2
k -a Fe(H 0) OH 2
5
+
2 +
A"
^ k
Fe(H 0) (OH)A 2
The
4
c o n t r i b u t i o n of
ficant.
Rate
quite
similar,
anism
is
is
rather
rounding
and
to
as
the
act
>Me0~ s i t e s
decrease
rates
of
ligand but
2
5
relative
variety a
+
+
H 0
(15)
2
to
of
(16)
2 +
reaction
15
substituted
dissociative
is
insigni-
phenols
ligand
are
exchange
of
depend and
exhibit
sites,
ligands
of
the
mech-
available by
sur-
>Me0H , 2
OH",
H2O, to
H2O,
ligand
are
at
situa-
that
shielded
lengths
ability
2
and
decrease
act
as
a
_
0 in
leaving
trend.
structure and
influences
should
Hematite example,
different
15
surface
are
bond
the
same
lattice
for
the
ability
Metal-ligand
surface
In
substitution
the
therefore
(γ-FeOOH),
may
upon
the
reaction
ligands
ligand
sites,
in
coordinated
coordinated
rates
surface
Note
ligand.
outfacing
following
of
oxide
complex .
substitution.
lepidocrocite
hydroxides,
Fe(H 0) A
14
that
c o m p o s i t i o n and
environment
$
S
a
ligands. 2
Oxide
a
leaving
other
Thus,
ative and
only
H20>OH~>0 "*,
may
for
more the
since
leaving
order
group
is
oxide.
>Me0H,
^
+
involving metal
reaction,
exchange,
4
(27).
reactions
exchange
2
-b
indicating
OH"",
Fe(H 0) (0H)A
reaction
considerably than
H
constants
operative
For tion
for
+
+
»
b
have
(Fe2Û3), are
rates
of
a l l
an
the
goethite Fe(III)
surface
coordin-
impact
on
(α-FeOOH), oxide/
chemical
21.
STONE
Adsorption
reactions
because
of Organic Reductants
of
variations
and Electron
i n surface
455
Transfer
site
bond
lengths
and
geometry. An
additional
complicating
face
sites,
discussed
tice
imperfections,
effects
broaden
surface
sites
for
others.
influenced tion faces gies
higher
these
the nonuniformity and L e c k i e
inhomogeneities,
rates
of
differences
and n e i g h b o r i n g
group
such
for
adsorbate
molecules
than
formation
are
surface
in site of
complex
energies.
rates
It
i s based
on the assumption
increase
sur Lat
energies,
to model
adsorption
of
(30).
site
affinities
likely,
is
by Benjamin
used
(31,32). for
surface
have
by
factor
length
the d i s t r i b u t i o n of
Most
has been
at
adsorption
linearly
with
that
some
The E l o v i c h
Equa
on n o n - u n i f o r m
that
activation
increasing
sur ener
surface
cover
age. Surface on
rates
complex tion, vary
of
speciation precursor
formation,
since
may d e p e n d
ligand
exchange
expected
to have
formation.
R^,
upon
rates
the extent of
>MeOH2»
a
tremendous
the rate of
of
surface
>MeOH,
impact
precursor protona
a n d >MeO" may
substantially: = k - j H A ] [>Me0H*]
R
±
Additional such
as
surface
>MeP04H", can
gradually
be
[HA] [>MeOH]
form
Experiments i n fact,
phosphate
when
2
examining
while
shown
in proportion
is
Coverage
and R e a c t i o n R a t e .
is
fast
relative
to electron
be
treated
a quasi-equilibrium
«
0H
/k .
k
S
Production [>Me rates
I I I
A].
of
surface
of
Relatively
bidentate
is
organic
[
^ '
[>Me
surface
of
phosphate
hydroquinone
(33) .
As the
Mn^+ r e l e a s e
total
dimi
coverage.
precursor
and p r o d u c t
>Me
>
by
complex
formation
release,
i t can
l i J
sites
I
l
A
I I : E
A + H 0
(18)
2
(19)
]
-0H][HA] is
ion release
directly reduced both
proportional
surface
increase
sites
as
to and
reductant
increased.
little It
^ ±
-
L
metal
and o r g a n i c
oxides.
~
sites
step:
the d i s t r i b u t i o n of
subsequent coverage
compounds, metal
1
reduced
Thus,
+ HA
groups
molecules.
ions
surface If
surface
c a l c i u m and
the rate
transfer
ions
slowly-exchanging
oxides
by a d s o r b e d
increased,
to the phosphate
i : E I
of
manganese
Surface
>Me
Non-labile
other,
the influence
inhibition
(17)
>MeP04H2»
+
incoming reductant
dissolution of
Κ
>MeOCa ,
2
by
[HA] [>MeO""]
specifically-adsorbing
are added:
reaction,
in solution
as
+ k^
>Me P04H, a n d >Me P0£. for
replaced
the reductive
nished
species
>MeP0?~,
not available
have,
+ k^
c a l c i u m and phosphate
are
on
c a n be
complex
is
known
about
reductants
the speciation
in particular,
i s known
that
surface
ligands,
such
as
coverage
catechol
when
of
organic
adsorbed
i s higher
and s a l i c y l a t e ,
to
for than
456
G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S
for
monodentate
has
been
is
shown
lowered,
analogous with
one
and
ion
another
in
a
example, enhancing
overall
surface
apart
in
may h a v e
a
oxidation a metal form
a
surface one
dramatic
which
two
or
several
acetate fides: The
metal
ASSB,
Rates
pH and
coordinate
reactive
step,
surface
in
space
for
groups, between
molecules
organic
One-equivalent couple
species
on
are
p a t t e r n of s u r f a c e
oxidation
the
of
various of
with
possible
mercaptans
oxidation
gener
coverage
forms
neighboring
organic
(RSH)
by
mercapto
radicals
to
(36).
1
1
·* R S -
1
+
H
+
+
Me
(20)
1 1
«SR -> R S S R
(21)
are
products
simultaneously
c a n be (BSH)
formed. yields
oxidized, Oxidation
three
a
mixture
of
mercapto-
possible
disul
BSSB. Step.
Inner-sphere
dissolution are,
of
the
yields
and
of
occupy
interactions,
adsorb
may
the
and
substitution at
sites,
which
inner-sphere
outer-sphere
in practice,
ligand
c o u l d be
tervalent used
reaction,
are
mechan
difficult to
not
to
and
dis
tetravalent
estimate
upward
known
any
to
level
certainty. The most
direct
tion
prior
electron
tive
dissolution
adsorbs
to
reduces tion. then
to
of
surface
sites
citrate
Results
related
to
the
calcium
and
tion
manganese
explanation inner-sphere
at
show
that
amount
of
complex
by
been
found
to
calcium or
formation
rate
complex of
citrate
(37).
Citrate
conditions,
only
under
but
illumina
in
the
d i s s o l u t i o n under citrate inhibit (33).
rates
between metal
are
(37).
directly
Adsorption
The most
of
dissolu
likely
molecules
oxide
dark,
illumi
reductive
phosphate
forma
photoreduc-
measured
dissolution
bound
hydroquinone
adsorbed
study
dark
c a n be
reductive
initial
a
by
under
coverage
of
precursor
from
particles sites
surface
has
oxide
that
surface
comes
an a p p r e c i a b l e
rates
phosphate is
oxide
surface surface
with
for
transfer
iron
oxide
correlated
of
evidence
iron
Thus,
nation.
and
It
the
Electrostatic repulsion
the
mercaptan
surface
rates
35). as
complexes
solution
to
on
Transfer
reductive
on
surface
necessarily
contrast,
other
mercaptans
ASSA,
oxide
limits of
more
η-butyl
tinguish.
(34,
increases
hydrophobic
Consider
Me
+
and
of
or
quickly
disulfide
Electron
isms
for
compounds
in
transfer
product
RS'
of
phenol
another.
surface.
disulfide
do n o t
coverage.
impact
products.
RSH +
When
constants
organic
radicals
(RS*)
and
generally
Favorable
molecules,
electron
oxide
radicals
some
from
free the
benzoate
i n homogeneous
compounds
fashion.
cause
adsorbate
When
stability complexes
organic
charged
ated
as
coverage
(35).
random
may
such
surface
that
metal
Adsorbed sites
ligands, that
surface
block sites
hydroquinone. The
surface
stoichiometry coverage
is
of
surface
important
in
complexes determining
as
well
as
electron
the
total
transfer
rate.
21.
STONE
Adsorption
Consider,
for
of Organic Reductants and Electron
example,
an
(>MeAH),
neutral
(>MeA)
plexes.
R2,
rate
quickly R
electron
+
pH d e p e n d e n c e
of
Few
studies
teristics lution. and
of
(38). oxide are
the
oxidized
more
and
tertiary
analogs,
inhibit oxide
the
Fe;
the
the
same
by
and
27
and
resorcinols,
overall
observed,
supports
rates
for
to
amines
(ii)
primary
secondary and
the
and
branching metal
Rates order
also
al.
the
com
(i)
transition
reaction
cobalt,
surface
well.
in
disso
adsorb
that
than
as
charac
iron,
alcohols,
decreased
force
by
the
length
behavior
of
amine
N i > Co
>
decreases
in
process
is
similar changes
tions
been
believed
cleavage
of
explain (n)
detachment
could
the
reaction
rate
groups
increase
the
reductive
are
Surface
with
for oxides
formation
substituents manganese rate
by
methoxyphenols, and
distinguished,
d i s s o l u t i o n by
i n many in
into
respects
oxidation
since It
on
oxides,
(39).
was
aroma while
Recent,
substituted
extensively
studied
the
Protonation detachment observed
this
has
surface
phenols
reactions
latter
class
7).
The
with
bonds
step. been sites
weakens
(7).
has
been
sites
with
be
by
Me-0
reac
metal
via
ion
molecules,
progressive Valverde
and
lattice
bonds,
respect
to
dissolution
postulated must
of
not
of
solvent
Release
Fractional orders it
rate to
postulated
detachment-controlled surface
transfer, This
This
in
observation,
electron
solution.
dissolution
(_7) .
neighboring occur
bonds
bonds of
to (5,
rate-limiting
be
Following overlying
state.
to
can
be
reaction
Sites.
released
metal-lattice
of
not
electron-withdrawing
on
except
manganese
complex
sus
aldehydes,
d i s s o l u t i o n were measured.
lattice
commonly
dissolved
oxide
examined
alcohols,
reactivity,
precursor
replacing
(5).
enhancing
of
was
hydroquinones,
ascorbate,
reductive
Reduced ions
involving
detachment,
showed no
transfer
that
saturated
Catechols, as
Mn(III,IV)
compounds
findings.
of
metal
d i s s o l u t i o n of
nonaromatic
Rates
of
research
reduced
has
well
lower
these
Dissolution
To
the
Fleischman et
found
quickly
three
by
within
also
different
acids
electron
substrates
number
by
reductive
molecules
The
pH 7 . 2 ,
acids.
as
however,
unpublished
At
rates.
electron-donating
are
(22)
chemical
of
examined
was
more
different
and
carboxylic
appreciable
Wagner
how
complex.
part,
amines
chain
reductive
(39).
oxalic
intramolecular
is
and
increased
driving
in
how
rates
corresponding
considered
aromatic
Morgan and
pyruvic
tic
(38) .
oxides
recently, by
ketones,
only
(iii)
exhibited
the
than
was
It
oxidized
of
order.
More
at
and
thermodynamic
pensions Stone
are
examined
transfer
step.
quickly
reaction
determined,
reductant
electron
type
com
by
2
alcohols
that
determined
2
electrodes
that
each
influence
aliphatic
revealed
amines
surfaces
oxidation
and
of
is
protonated
surface
k .
systematically reductants
rate-limiting
alcohols
is
and
forms HA)
" TTT k [>Me ΟΗ,ΗΑ]
+
L L L
k2»
2
have
and
within
reaction
k ,
oxide-coated
surface, is
the
organic
Experiments
plex
occurs
that
(>MeOH,
transfer,
1
of
Oxidation
nickel
electron
2
magnitude
reductant
outer-sphere
TTT + k [>Me A]
LLL
2
relative
of
transfer
TTT = k [>Me AU]
2
The
the
organic
and
457
Transfer
that
a
protonated
[H ] +
reactions. required before
458
G E O C H E M I C A L PROCESSES AT M I N E R A L S U R F A C E S
Rj. = k {>MeOH*} dissolution H 2 n
Specific rates
adsorption
by a l t e r i n g
Salicylate, (40).
u
of
ligands
c a n enhance
the strength
oxalate,
of
or
and l a b i l i t y
and c i t r a t e
In the presence
(23)
n
ligand
promote (L)
inhibit
dissolution
of Me-0 l a t t i c e
bonds.
the d i s s o l u t i o n of
the d i s s o l u t i o n
rate
alumina
becomes
(7): ι *.· dissolution Organic
ligands
surface
sites
and
without
have
non-reductive Reduction
ions, also
oxidation
quite
Table
II.
reactivity
that
to enhance
rates
rates
states
soluble
reactions
of
Product
+
2H
+
= Mn
Fe(0H) (s)
+
2H
+
= F e
2
+
Co(0H) (s)
+
2H
+
= Co
2
+
Ni(0H) (s)
+
2H
+
= N i
2
2
2
The h i g h
solubility
make m e t a l
Constants
Effect
Oxide
surface
state
surfaces
several
example, different found
of
cobalt,
and n i c k e l
are
for Divalent
Metal
Ion
[Me
2 +
]
a t pH 7
sat.
r
12.9
7.9xl0"
2
+
2H 0
12.3
2.0χ1θ"
2
+
2H 0
10.8
6.3xl0~
4
2
2
2
of
15.9 M
reduced
relative
metal
centers
to preceding
on Reductive
steps
throughout
should
i n many
during is
surface
quite
the course
redox
(a-Fe20^)
from
rates
reduc
oxidation may e x h i b i t
that
of
reductive
and maghemite
environments.
octahedral
metal a
observed
minerals.
stoichiometry but contain
octahedral
of of
and mixed reactions
different
and hydroxide
Oxide/hydrox
environment
Nonstoichiometric
may i n f l u e n c e
coordinative
Dissolution.
and t h e c o o r d i n a t i v e
Hematite
t h e same
i n both
d i
2H 0
that
mineralogy
have
metal
Reduced,
reactions.
produced
ways.
transition
+
reaction.
trigonally-distorted is
ratio
15.2
substantially
behavior
oxide
(41)
2
fast
uniform oxide
Oxide in
+
metal
reductive
2H 0
Mineralogy
dissolution
more
2
structures
may c h a n g e
dissolution with
both
+
and l a b i l i t y
dissolution
of
2 +
ion release
reductive
tive
of
logK
2
centers
coordinate
substitution. iron,
Reaction Mn(0H) (s)
(24)
II).
Hydroxides
ide
{>MeL}
(7).
to radius ligand
o f manganese,
(Table
Solubility
L
found
the charge
higher
+ k
redox
dissolution
lowers
promoting
valent
been
= k__{>MeOH^} H 2
sites,
Fe(III) while
and t e t r a h e d r a l
dissolution
(y-Fe203),
Fe(III)
in
i n hematite Fe(III)
sites
in
(42).
for
quite
occupies
maghemite Differences
21.
STONE
Adsorption
in
coordinative
of
(i)
and
Few lution of
of
mineral
seven
(NiFe2Û4)
Fe(III)
tion.
redox
present
(43).
Semiconducting potentially tion
of
energy from
of
between
molecules eV a t
conduction ture.
is
(M.
band
especially reductant
transfer
i n several reactive
state, metal
release
centers
delay
sites
originally
occurs
(49).
gap
transfer
energies
and between
electrons extent
into
at
are
the
room
In poorly
of
oxides
whether
temperaor not
ordered
oxides,
and i m p e r f e c t i o n s
between
adjacent
between metal
centers
can a l t e r
1.1
energy
has a value
o f manganese
by gaps
gap
vibrational
to k T , which
also
Adsorp-
electron
oxides,
of
are
t h e band
Band
The average
excitation
oxides
overlap
ways
(46).
metal
that
centers
on the oxide at
consumed
further
of
to metal
metal cause
centers site
reaction with
may t h e r e f o r e
the onset
on surface
the surface
reduced
surface,
the surface
deposited
at
causing
sites
within
by
the bulk oxidation
molecules
i n bulk
rate.
organic
to
Electron transfer
changes
course
a higher
to i t s o r i g i n a l
reductant
ions.
the
e x c i t a t i o n may c r e a t e
to be
returns
of
Thermal
electron holes
transferred
This
allowing
without
reac-
to Fe(II) sites
facile
are significant.
electrons
may b e
(47).
exceeding
to a s i g n i f i c a n t
orbital
since
to
reactions.
c r y s t a l l i n i t y may d e t e r m i n e
of
was
commun.).
molecules
reductants
energy
conductive;
may b e r e s t r i c t e d
importantly,
oxide
and
of
case,
prior
Fe(II)
dissolution
gap e n e r g i e s
thermal
order
reductants
reduced
than
disso-
the
(Fe3U4)>nickel
t r a n s i t i o n metal
equal
Band
may o c c u r
t h e amount
reaction
of
(44).
properties
Fox, pers.
More
(45).
transfer
Electron of
oxides
study,
(6).
to another
low that
semiconducting reduce
quickly
oxides
approximately
The degree
electron
sites
0 . 2 6 and 0 . 7 eV f o r manganese
25°C
sufficiently
more
lattice.
on t h e r e d u c t i v e
i n the l a t t i c e
Fe(III)
or photochemical
2 . 3 eV f o r i r o n
0.026
that
rates
transfer,
the oxide
i s an i n t e r e s t i n g
i n reductive
center
from
(a-Fe2Û3)>magnetite
lattice
crystalline
one metal
range and
thermal makes
electron
and t r a n s i t i o n metal
properties
important
may i n f l u e n c e
surface
made
459
Transfer
In one such
Magnetite
dissolve
i n the mineral
been
are present
indicates
reaction
have
phases.
organic
lengths
(ii)
ion release
hematite
and Fe(II)
Evidence
metal
studies
different
t o be t h e same:
ferrite
formation,
reduced
comparative
reaction with
both
and Me-0 bond
complex
rates
of
found
by
geometry
precursor
(iii)
of Organic Reductants and Electron
oxide
occur
between
composition
dissolution.
Conclusions Reductive
dissolution
formation
between
electron of
transfer
the successor
face
speciation
contributing and
reactions Rate
within complex
is
steps.
mechanism o f
occurs
reductant
surface
complex, of
available
s o l u t i o n both
f o r homogeneous
metal of
ions.
concerning this
(ii)
breakdown
each
and analogy
support
reactions
sites,
and ( i i i )
rates
evidence
reactions
complex
surface
dissolved
i n determining
chemical
precursor
and o x i d e
surface
and r e l e a s e
Limited
i n homogeneous
expressions
this
important
surface
v i a (i) molecules
to
of
Surthese
rates similar
conclusion.
are written
as
functions
460 of
G E O C H E M I C A L PROCESSES AT MINERAL SURFACES
species
tive
concentrations,
methods.
reactions
is
considerably
sions
are often
total
surface
position. tion
written
coverage)
requires
and i t s impact
that on
of
more
as
by s p e c t r o s c o p i c
surface
speciation
difficult,
functions
o r as
Comprehensive
reactions
determined
Determination
of
functions
understanding much more
of
terms
overlying reductive
be l e a r n e d
or
alterna
heterogeneous
and t h e r e f o r e
composite of
in
rate
expres
(such
solution
as com
dissolution
about
surface
specia
by N a t i o n a l
Science
Founda
rate.
Acknowledgments Support tion
for
Grant
Marye
A.
this
research
CEE-04076.
Fox i s
was p r o v i d e d
The u s e f u l
input
b y James
J . Morgan and
acknowledged.
Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
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J u n e 18, 1986
