8
Characterization o f A n i o n B i n d i n g o n Goethite U s i n g Titration Calorimetry a n d C y l i n d r i c a l Internal R e f l e c t i o n - F o u r i e r Transform Infrared Spectroscopy W. A. Zeltner, E. C. Yost, M. L. Machesky, M. I. Tejedor-Tejedor, and M. A. Anderson Water Chemistry Program, University of Wisconsin, Madison, WI 53706 Titration calorimetry and cylindrical internal reflection-Fourier transform infrared (CIR-FTIR) spectroscopy are two techniques which have seldom been applied to study reactions at the solid-liquid interface. In this paper, we describe these two techniques and their application to the investigation of salicylate ion adsorption in aqueous goethite (α-FeOOH) suspensions from pH 4 to 7. Evidence suggests that salicylate adsorbs on goethite by forming a chelate structure in which each salicylate ion replaces two hydroxyls attached to a single iron atom at the surface. Lucio Forni ( 1 ) , while discussing catalysis research, commented that: "Each technique, in fact, taken by itself, allows some useful information to be collected, but can give rise to criticisms. The combination of information obtainable by two or more of them can often be the only way to give a complete picture of the surface acid properties of the solid." To a certain extent a similar statement could be made about research on the chemistry of mineral-water interfaces. Some theoretical models (2»3) developed to date have focused primarily on their ability to fit data collected from one experimental technique, namely potentiometric titration. While these models have done much to improve our understanding of the oxide-water interface, we do not have a complete picture of the interfacial region at present. Although potentiometric titrations can s t i l l provide new insights, failure to utilize other techniques may result in the problem mentioned in Forni s statement above. Several alternative methods for examining the chemistry of interfacial reactions are currently being developed, as evidenced by the many fine chapters in this volume. While it is certainly not necessary to utilize all techniques described, many can be employed in a given system to better understand the reaction chemistry involved in these complicated interfacial processes. 1
0097-6156/ 86/ 0323-0142506.00/ 0 © 1986 American Chemical Society
8.
ZELTNER ET AL.
To
put
analytical
Anion
Binding
things
into
perspective,
into
bulk,
This
illustrating
tw o
the
(α-FeOOH)-water
goethite
cylindrical
powerful,
our
of
in
143
we
can
surface, on
techniques
the
interface:
in
to
transform
it
allows
direct
at
mineral-water
have
anion binding
titration calorimetry
CIR-FTIR
the
these
interfacial
category,
fact,
techniques
classify
situ
study
In
adsorbed
situ
broadly and
last
used
reflection-Fourier since
ions
these
dry
focuses
situ
spectroscopy.
observation of
chapter in
internal
(CIR-FTIR)
Both
Goethite
methods
techniques.
extremely
on
at
and
infrared
could
prove
to
be
spectroscopic
proven
interface.
very
beneficial
in
research.
Background Several
general
discussing in
this
chapter
polyethylene OH/Fe
experimental
these
ratio
by
of
reported
averaged
50 gas
are
yields as
the
with anion
least
1.5
pH u n i t s was
of
are
of
(14).
to
below
anion ion is
are
Titration
calorimetry
adding
determining and
less
(4y , w i t h the
a
was
primary
the
predominant
nm i n w i d t h , a
a
16_ f o r
often
to
suspensions.
of
et
surface
and area
g/L,
while
other
iron
anions as
adsorption
involves This
the
to
general
reviews
measure
adsorption adsorption
at
anion
calorimetry
for
salicylic
aqueous is
(13)
solubility 975
and
g/L.
SAL
aluminum
studied,
illuminate
measurement
the
of
salicylate
NaSAL
also
ZPC
surfaces.
constants of
a
pH v a l u e s
initial
the
is
method)
enthalpies studies
of
well
i n homogeneous
of
at
other
and
results
certain
features
process.
technique
enthalpies
found
was
oxides
were
needed
excellent
in
titration
that
(ZPC).
sodium
than
that
the
(5-7_) ,
charge
using
the
and
in
Surface
but
made
charged
here
of
net
higher
who
were
Acidity
on b o t h
given
Instead,
(11),
ensure
(12) ,
is
the
titration
0.1
(8-K0
in both
13
±
by
"charge"
surfaces.
point
This
al.
to
reported =
surface
pH 8 . 1
7.5-7.6
(SAL).
2.4
titrant.
reaction
ZPC
ZPC
affected
oxide
zero
on p o s i t i v e l y
salicylate-goethite Calorimetry
15
20
potentiometric
measurements
the
pK
adsorb
Titration
while
and
as
metal the
of
studied
and
Several
anions
a
to
for
Kavanagh
occurring
acid
shown
these the
as
indicated
directly
electrolyte.
adsorption
=3.0
salicylic been
gave
experiments
pK
are
by
pH v a l u e
inert
(2-hydroxybenzoate)
for
al.
that
plane
length
described
in
morphology
showed
100
referred
determined
that
primary
CIR-FTIR
oxides
Sample
adsorption
hydrated
ZPC v a l u e s
All
The
of
titrations
goethite
adsorption
(often
the
8.2.
has
prepared
A t k i n s o n et
the
nm i n
densities
models)
agreement
of
of
with
nitrogen
balance
balances also
perchlorate.
acid
shaped,
adsorption
proton
goethite
and
method
before
experiments
goethite
which
theoretical
Our
using
presented
/g.
surface proton
the
50 h o u r
needle
needles
Anion
which
by
be A l l
electron microscopy,
were
analysis
the
performed
will
detail.
time.
The m
in
aging
face. 81
were 2 and
B.E.T. of
points
methods
vessels of
determined particles
two
have
evolved
solution
but in
heat
established has
(see
been
for refs.
used
far
heterogeneous relied
mainly
on
the
144
G E O C H E M I C A L PROCESSES AT M I N E R A L SURFACES
determination (usually
and
several entropie
process
site
has
reactions, Most
as
previous
investigated (17-21), Proton
entropies
endothermic
studies phenomena
after
the
of
negative
the
when
energies.
on
resulting part
gations
or of
cation
relatively explained
of
as
more
that
starch
A
increased
molecules
Free estimate
energy
In
study
one
such
calcite
increased were
markedly
mineral
to
and
the
association
with
with
in
the
adsorption
see
nature metal
a
may
regions
of
were
the this
volume).
suspensions
on h e m a t i t e to
as
investi-
oxide
exothermic
attributed
which
which
of
Goulding,
adsorption less
as
revealed
surface
rearrangement
of
configurations.
temperature
However, octyl it
few
can
observed
which
resulting
water
solvent
surrounding
process,
as
that
structure,
well
as
the
published. on
adsorption the
enthalpies
entropies both
octyl
to
to
adsorption
positive
ordered
used
been
suggested
large
be
devoted
have
hydroxamate was
also
studies
phenomena
temperature, of
During
hydrous
adsorption
The
loosening
surface
during
of
starch
potassium
(26).
cation,
observed
and b a s t n a e s i t e ,
endothermic
attributed ions
of
metal
intact.
heterogeneous
well
free
minerals,
favorable
of
water
net
clay
enthalpies.
dependence
and
to
the
This
as
generally
were
became
variations
reaction
fraction.
exchanger
enthalpies the
(also
phosphates
progressively
on
T h i s was
less
temperature barite,
of
(23,24).
enthalpies
the
(25).
to
of
phenomena
process
the
have
phosphates
zirconium
exchange
sheath
studied
study
adsorption
coverage
exchange
investigations
scarce.
the
to
surfaces
Calorimetric are
due
on
E n t r o p i e s were
attributed
exchange
being
aluminosilicate
or
calorimetry
becomes
critical
hydration
exchange
discrete
information
ordering
minerals
subsequently
from measured
from
its
clay
cations
cation.
largely
structuring
and
been
adsorption
on z i r c o n i u m
c r y s t a l l i n i t y of
calculated
T h i s was
of
system
employing
and
some
the
alkali
keep
all
but
reaching
depends
nature
(22)
a l k a l i metal
exothermic,
rarely
important
increased
of
overall
enthalpic
have
dependence
exchange
with
transition
provide
one
the
occurs.
t i t a n i u m phosphate
initially
can
of
approach,
energies
adsorption
cation
exchange
this
free
reflect
energies
using
understanding
temperature
adsorption
free
experiments)
with
enthalpies and
adsorption
the
adsorption
Reaction while
While
increased
of
heterogeneity
disordering
of
isotherm
models.
components
determined.
is
adsorption
available
adsorption
on
interpretation
through
at
were
the
hydroxamate
hydrophobic
chain
effects.
The major
conclusions which
can be
drawn
from
previous
work
are: 1)
Adsorption adsorbent during
2)
A
initial
large
results
but
from
of
the
during
vary
are
stages
portion
structure 3)
enthalpies type
with
usually of
the
the
relative
s o l u t i o n phase
reactions
equipment
consists
The
Orem,
occur
over
used
Utah)
or
-
at
and least
process. change
(+
or
disrupting
of
water
-)
adsorption.
most
Inc.,
coverage
entropy
ordering
changes
(TRONAC,
adsorption
calculated
Heat
Methods.
surface
exothermic
a
period
isoperibol
of
minutes
in
contrast
(seconds). of
a
TRONAC M o d e l
450
t i t r a t i o n calorimeter
which
to
8.
ZELTNER ET AL.
Anion
Binding
on
Goethite
145
has been m o d i f i e d by a d d i n g (1) a h i g h p r e c i s i o n b u r e t d r i v e n by a s t e p p i n g motor and (2) m i c r o g l a s s and r e f e r e n c e pH e l e c t r o d e s ( M i c r o e l e c t r o d e s , I n c . , L o n d o n b e r r y , NH). T h i s system i s i n t e r f a c e d to an Apple H e computer through an ISAAC 41A (Cyborg, I n c . , Newton, MA) d a t a a c q u i s i t i o n and c o n t r o l system equipped w i t h a 2 c h a n n e l , 16 b i t A/D c o n v e r t e r t o d i g i t i z e temperature and pH d a t a , as w e l l as a b i n a r y I/O c a r d t h a t p e r m i t s automated t i t r a n t d e l i v e r y and h e a t e r c a l i b r a t i o n . O p e r a t o r c o n t r o l i s o b t a i n e d by s e t t i n g r e q u i r e d input parameters b e f o r e s t a r t i n g data a c q u i s i t i o n . These p a r a m e t e r s i n c l u d e : (1) a t h r e s h o l d d i g i t a l v a l u e t o i n i t i a t e d a t a a c q u i s i t i o n , (2) the number and l e n g t h of t i t r a n t a d d i t i o n s , (3) time between a d d i t i o n s , and (4) a " d i g i t a l window" t h a t i s used i n the pH s t a t mode t o a c t i v a t e a second b u r e t c o n t a i n i n g s t a n d a r d a c i d or b a s e . These i n p u t s a l l o w e x p e r i m e n t a l c o n d i t i o n s t o be e a s i l y a l t e r e d and o p t i m i z e d . I n o r d e r t o a d e q u a t e l y r e s o l v e the yV l e v e l t h e r m i s t o r s i g n a l s , 16 b i t A/D c o n v e r s i o n i s n e c e s s a r y , r a t h e r than the more c o n v e n t i o n a l 12 b i t . A l s o , s i g n a l t o n o i s e r a t i o i s enhanced by a v e r a g i n g 90 A/D c o n v e r s i o n s f o r each c h a n n e l e v e r y 0.5 s e c , an a v e r a g i n g time w e l l w i t h i n the t h e r m i s t o r (^3 sec) and pH e l e c t r o d e ('νΊΟ sec) r e s p o n s e t i m e s . D i g i t i z e d t h e r m i s t o r and pH d a t a ( s t o r e d on f l o p p y d i s k ) a r e c o n v e r t e d t o temperature and pH v a l u e s u s i n g a p r e v i o u s l y determined t h e r m i s t o r c o n s t a n t (°C/digital v a l u e ) and d i g i t i z e d pH buffer values. S e v e r a l c o r r e c t i o n s must be i n c l u d e d , however, b e f o r e temperature d a t a can be c o n v e r t e d c o r r e c t l y t o heat c o n t e n t v a l u e s ( i n J o u l e s ) c o r r e s p o n d i n g t o the i n t e r v a l i n c l u d i n g and f o l l o w i n g each t i t r a n t a d d i t i o n . This data reduction process must account f o r such p h y s i c a l e f f e c t s as heat produced by s t i r r i n g the s u s p e n s i o n , heat l o s t t o the s u r r o u n d i n g s , and r e s i s t a n c e h e a t i n g of the t h e r m i s t o r . The heat of water f o r m a t i o n which r e s u l t s from a c i d - b a s e n e u t r a l i z a t i o n as the system pH i s changed must a l s o be d e t e r m i n e d . T h i s i s p a r t i c u l a r l y important o u t s i d e the pH range o f 5 to 9. D e t a i l e d e x p l a n a t i o n s and e q u a t i o n s f o r t h e s e c o r r e c t i o n s a r e p u b l i s h e d elsewhere (16,27,28). The s t a n d a r d c a l o r i m e t r i c r e a c t i o n of t r i s ( h y d r o x y m e t h y l ) aminomethane (THAM) n e u t r a l i z a t i o n w i t h HC1 was used i n s e v e r a l i n i t i a l experiments t o d e t e r m i n e b o t h p r e c i s i o n and a c c u r a c y f o r the d a t a a c q u i s i t i o n and r e d u c t i o n p r o c e s s . Three t o f i v e m i n u t e s were a l l o w e d between a c i d a d d i t i o n s , s i n c e t h i s same time frame was used f o r a l l l a t e r s u s p e n s i o n t i t r a t i o n s i n o r d e r t o m i n i m i z e t h e e f f e c t s o f slow s u r f a c e r e a c t i o n s which o c c u r d u r i n g a t i t r a t i o n (9,29,30). The amount o f a c i d added i n each experiment was v a r i e d to g e n e r a t e h e a t changes of 40-400 mJ ( t y p i c a l heat changes o b s e r v e d i n our a d s o r p t i o n s t u d i e s w i t h g o e t h i t e s u s p e n s i o n s ) . P r e c i s i o n (% S t a n d a r d D e v i a t i o n (SD)) e s t i m a t e s a t 400, 160, 80 and 40 mJ a r e 3.1, 6.4, 11.1 and 19.3% r e s p e c t i v e l y . The c o r r e s p o n d i n g a c c u r a c y v a l u e s (%) a r e +0.6, +4.3, +1.5 and -13.0%. Thus, p r e c i s i o n and a c c u r a c y d e c r e a s e as heat e v o l v e d d e c r e a s e s , and the magnitude of t h i s d e c r e a s e p e r m i t s e r r o r e s t i m a t e s t o be obtained. S u s p e n s i o n s were p r e p a r e d i n 500 mL p o l y c a r b o n a t e b o t t l e s by u l t r a s o n i c a l l y dispersing freeze-dried goethite i n dilute n i t r i c
146
GEOCHEMICAL
acid
(with
λ^24
between least two
This
4.2
one
as
were
calorimetric such
at
then
s i m i l a r l y
entire
Results.
are
pH ^ 3 . 8
added
for
tbese
were
heats
on
following
filters)
capability
M NaNO^)
heats proton
the
pH
desorption,
largely
identical
of
to
well
proton
the
absolute
of were
standard
%4.
for
when
pH as
NaOH
was
Over
the
adsorption
partitioned
pH.
potentiometric-
S t a n d a r d UNO^
returned
but
starting
t i t r a t i o n s
volumes
pH M 0 .
exothermic
the
representative
A l l
equal
of
addition
measuring
Data
for i n
analyzed
only
combined
1.
pH at
pH 4
levels
desired
of
a
aged
aliquots
the
the
perform
and
u n t i l
were
when
to
Figure
at
for
at
were
kept
mL
with
suspensions.
g/L
supernate
The
to
10
Fe
Fifty
SURFACES
M solution)
of
dissolved
system
i n
0.05
Suspensions μg/L)
minutes"until
dispensed
endothermic
(
