13
H i g h l y Selective Ion E x c h a n g e i n C l a y M i n e r a l s a n d Zeolites A. Maes and A. Cremers K. U. Leuven, Laboratorium voor Colloïdchemie, Kard. Mercierlaan 92, B-3030 Leuven, Belgium The charge density of the clay mineral and the polarizability difference between the exchanging pair of ions are the two important factors governing the ion exchange behaviour of alkali, alkaline earth and organic ammonium cations. Highly selective ion exchange behaviour is discussed in terms of these parameters. It can essentially be ascribed to high enthalpic contributions resulting from enhanced electrostatic interactions. Complexation of transition metal ions with uncharged ligands leads to significant enhancements - up to three orders of magnitude - of ion selectivities in smectites and these effects prevail over the entire range of surface compositions. Similar enhancements in selectivity occur upon complexation in the rigid zeolite pores but are of a lesser extent. The extremely high Cs-selectivity observed in a small fraction of the charge of illites, illite-smectite interlayers and reduced charge montmorillonites, is a thermodynamically reversible ion exchange process and is discussed in terms of a multi-site Ion exchange model. In zeolites, the combination of a typical crystallographic configuration and cation properties may in certain cases (Ag, Na) lead to high selectivities in a limited number of exchange sites. The distribution of elements between the solid and the liquid phase is of primary importance for the transport processes in the environment. In addition, the uptake of elements in plants and other living organisms is determined by the speciation of the element in that phase. The distribution of the major elements (Ca, Mg, Na, K, ...) in soils is well known to be governed by ion-exchange processes (1). The behaviour of transition elements such as Co, Ni, Cd, Cu, etc. in natural systems (soils, sediments) often results from a combination of different effects such as precipitation, sorption in oxides, exchange in clay minerals and complexation with organic 0097-6156/ 86/0323-0254$ 11.50/ 0 © 1986 American Chemical Society
MAES AND CREMERS
13.
matter.
These
conditions exchange other at
which of
the
at
ruled
major
by
on
selectivity
complexes is
these
high
mostly
to
on
give
an
montmorillonites
phyllosilicate
Thermodynamic free
z
and
and
change
and
Β
for
the
cations
of
is
in
either
fraction of
surface
the
Thomas) variation appears exchanges (13)
Kg v a l u e s
t. z.B A
«-
sometimes metal
objective
which as
are
ion
of
this
involved
will
be
in
focussed
representatives
cation
z B
X
v . X +
exchange
,
A z_,A Β
-
standard
surface
viewpoint
experimental Two
( 1 )
chemical
(indicated
of
as
the
an
the
potentials
by
a bar)
theoretical
measurements
approaches
definition
approaches exchange
are ion
of
such
currently
activity
equivalent
lead
and
at
framework
equilibria
in
fraction
value
for
with that of and
identical
Such
scale
exchange
and
to
(9-10).
concentration
Although
Cd
the
species
expressed
heterovalent numerical
bonds
use.
are
They
only
the
surface,
which
(5)
or
molar
as
a
(6-8).
Both values
broken
groups.
vl>
-
thermodynamic
the
established.
differ is
a
and
well
occurs
adsorption
phase.
From (5-6)
by
exchanging
solution
The
exchanger
T
-
