Agriculture Reference
In-Depth Information
(a)
inner-sphere
complexes in which the adsorbed species is bound directly to
the surface functional group, with no intervening water molecules; and
(b)
outer-sphere
complexes in which at least one water molecule remains between
the absorbed species and the surface.
In inner-sphere complexes the bonding is covalent or ionic and the reactivity of
the surface is altered by the interaction; in outer-sphere complexes the bonding
is largely electrostatic and the reactivity of the surface is largely unaltered. Inner-
sphere complexation is usually slower than outer-sphere and is often not readily
reversible. It can occur regardless of the net surface charge and is little influenced
by the ionic strength of the external solution. If an ion is adsorbed without forming
a surface complex, neutralizing surface charge in only a delocalized way, then
it is said to be part of the
diffuse ion swarm
. Such ions are free to move about
in the soil solution near the surface. Figure 3.11 shows surface complexes on an
inorganic hydroxyl surface. It shows the distinction between inner- and outer-
sphere complexes, depending on the presence of water molecules between the
surface and complexed species. The region of the diffuse ion swarm begins at
the outer edge of the water molecules solvating ions in outer-sphere complexes.
There are a number of more loosely defined terms for different types of adsorp-
tion that are related to the form of surface complexation.
Specifically adsorbed
ions are held in inner-sphere complexes whereas
non-specifically adsorbed
ions
are in outer-sphere complexes or the diffuse-ion swarm.
Readily exchangeable
Oxygen
Metal
Inner-sphere
complexes
H
X
e.g. M
=
Zn, Pb, Cd
X
=
P, As, Si
M
H
+
X
−
Outer-sphere
complexes
H
Water
molecules
H
e.g. M
=
K, Ca, Mg, Fe
X
Cl, NO
3
, HCO
3
=
M
+
Solid
water
interface
−
Figure 3.11
Complexes formed between solutes and hydroxyl groups on oxides and
layer silicate edges (adapted from Sposito, 1984b). Reproduced by permission of Oxford
University Press
