11
Distinguishing A d s o r p t i o n f r o m Surface Precipitation Garrison Sposito
Downloaded by MONASH UNIV on November 9, 2012 | http://pubs.acs.org Publication Date: November 13, 1987 | doi: 10.1021/bk-1987-0323.ch011
Department of Soil and Environmental Sciences, University of California, Riverside, CA 92521 Measurements of the chemical composition of an aqueous solution phase are interpreted commonly to provide experimental evidence for either adsorption or surface precipitation mechanisms in sorption processes. The conceptual aspects of these measurements vis-à-vis their usefulness in distinguishing adsorption from precipitation phenomena are reviewed critically. It is concluded that the inherently macroscopic, indirect nature of the data produced by such measurements limit their applicability to determine sorption mechanisms in a fundamental way. Surface spectroscopy (optical or magnetic resonance), although not a fully developed experimental technique for aqueous colloidal systems, appears to offer the best hope for a truly molecularlevel probe of the interfacial region that can discriminate among the structures that arise there from diverse chemical conditions. The loss of a chemical species from an aqueous solution phase to a contiguous solid phase may be termed a sorption process. Among the mechanisms by which sorption processes occur, the three principal ones are: precipitation, the growth of a solid phase exhibiting a primitive molecular unit (a complex) that repeats itself in three dimensions; adsorption, an accumulation of matter at the interface between an aqueous solution phase and a solid adsorbent without the development of a three-dimensional molecular arrangement; and absorpt ion, the diffusion of an aqueous chemical species into a solid phase (1,2). A precipitation mechanism may be initiated by either homogeneous or heterogeneous nucleation, may involve the formation of a solid mixture either by inclusion or by coprecipitation, or may take place on the surface of a pre-existent solid phase (surface precipitation). Regardless of these variations, the essential characteristic of precipitation is the development of a solid phase whose molecular ordering is intrinsically three-dimensional (2). An adsorption [strictly speaking, positive adsorption (1_)] mechanism, on the other hand, involves only two-dimensional molecular arrangements 0097-6156/86/0323-0217$06.00/0 © 1986 American Chemical Society
In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
GEOCHEMICAL PROCESSES AT MINERAL SURFACES
218
on
a surface.
adsorbates eliminate sional,
solid
even
growth
for
on
layer
2:1
layers,
of
Downloaded by MONASH UNIV on November 9, 2012 | http://pubs.acs.org Publication Date: November 13, 1987 | doi: 10.1021/bk-1987-0323.ch011
form
it
i t s periphery
vein,
that
from
of
or
review,
the adsorption/surface on the c o n c e p t u a l made
distinguishing
above:
Adsorption
isotherms.
is
of
calculated
from
n^ is
with
solid,
of
solution
phase.
metric species
i
water
assigned Q ) ;
is
w
In b a t c h
hence,
to (2) be
to
-
a chemical
an
set o f is
six
in
not
a
com
approaches
The emphasis to
the
here
defining
capable
species
an aqueous
of
i
adsorbed
solution
(1):
M m. w ι
(1) species
of
water
M
i n column
w
is
i i
the
inverse
where
M
w
of
is
is
the
the
side
gravi
excess
no n e t
w on the l e f t
of
aqueous
the surface
there
suspension
mass
in the
experiments
1 represents
i n the
per unit
the adsorptive
the superscript
is
written as
result
an approach
of
interface
has read
follow
t h e mass of
region.
distinguish
popular
o f moles
Equation
t o an
likely to
depth
systems
for h i s
approach
is
penetration
precipitation?
of
experiments
whereas
content.
Corey
contacting
= n. ι
the m o l a l i t y
density,
water
M
which
should
each
the equation
number
solid,
a n d m^ i s
suspension
of
the t o t a l
kilogram
water
by
adsorbed
interfacial
which
extent
material
η
per
the
on three
surface
The q u a n t i t y
q. ι where
of
view,
only
to the
(1-3).
The d i s c u s s i o n
To what
a solid
of
adsorbate
t h e nanometer
particularly
focuses
point
previously
natural
conditions
their hydroxides
of
refer
with
in
metal
p r e c i p i t a t i o n dichotomy.
adsorption
Methods
phase
essay,
relationship
Solubility
mass
methods
does
influenced
beyond
defines
problem
instead
is
unit
this
precipitation. but
must
phase
precipitation of
to
per
solid
set out general
adsorption
c a n be
in the chemistry of
to the present
prehensive
statements
a
experimental
review
that
ordering
it
three-dimen
From t h i s succession
and not b y any o t h e r
operationally
surface
comprehensive
interlayer
to a
mixed
but
inherently
(1)].
refer
(2)],
and a r e h i n d e r e d
[e.g.,
"absorption"
into
problem
establishment
conclusions
must
forms
species
is
on surfaces reasons
not p r e c l u d e
solutions"
structure
whose m o l e c u l a r
I n t h e same
introduction
solid
aluminosilicates
on w h i c h
adsorption a
they
type
A central the
r e s t r i c t i o n does
whose
adsorption"
a chemical
from
phases
if
each
layer
layers. of
latter
stereochemical
"multilayer the
This
["two-dimensional
of
accumulation of
the
equa
tion. Adsorption the
change
phase. can
be
phenomena
frequently
in concentration of
Simple
mass-balance
rewritten
in a
form
q^ ι Δπΐ£
kilograms
= m° -
m^ a n d m °
o f water
are
species
studied i
considerations compatible
w )
η
where
a
is
with
by measuring
i n t h e aqueous (1)
show t h a t
this
solely
solution Equation
= Am.M ι Tw the m o l a l i t y
i n t h e aqueous
solution
1
methodology:
(2) of
species
phase
prior
_ i _ i n M.p to
its
In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
w
being
SPOSITO
11.
brought vides
Distinguishing
into
solution.
1 is
analysis ous
critical
the
total
is
solid of
Cd
not 2 +
variable,
Data
special
cases
instead
q^ )
of
a
of
this
of
m^,
temperature kind
the
is
(m =
l,...,n)
adsorption pression B
m
= K
0 < 3 y
m
.
(w>
=
m
or
l / ^ fit
m
are
η =
; η =
Σ m=l
adjustable (_3,j4) t h e
1), 1).
In
it
is
to
be
possible
classical
of
of
to
Langmuir
L
derive
the
basis
of
Popular
yet
Langmuir
been
more
shown
van
to
the
task
becomes
of
corre
fitted
iso
numerically
to
^
c . ι
(3) r
m
b ,
m
i
1^,
m
in B ^
special
the
larger
is
=
m
is
of
=
