Geoscience Reference
In-Depth Information
cationic adsorption mechanism, but this process depends on the acidity of the
medium.
Early work by Boyd et al. (
1947
), performed on zeolites, showed that the ion
exchange process is diffusion controlled and the reaction rate is limited by mass
transfer phenomena that are either film-diffusion (FD) or particle-diffusion (PD)
dependent. Under natural conditions, the charge compensation cations are held on
a representative subsurface solid phase as follows: within crystals in interlayer
positions (mica and smectites), in structural holes (feldspars), or on surfaces in
cleavages and faults of the crystals and on external surfaces of clays, clay min-
erals, and organic matter.
Cations held on external surfaces are immediately accessible to (subsurface)
water. Once removed from the solid phase, they move to a region of reduced
concentration. This movement is controlled by diffusion, and the diffusion coef-
ficient (D) can be calculated using the equation described by Nye and Tinker
(
1977
):
dC
solution
dC
ss
D ¼ D
e
hf
ð
5
:
11
Þ
where D
e
is the diffusion coefficient in water, h is the water content in the sub-
surface solid phase, f is the ''impedance'' factor related to the tortuosity, C
ss
is the
cation concentration in the subsurface (solid and water phases) expressed as mass/
volume, and C
solution
is the cation concentration in solution.
Cations held on the external surfaces of clays exhibit relatively rapid diffusion
but are subject to an additional limitation. Because the arrival rate of ingoing
cations at the exchange site is much slower than the release rate of the outgoing
cations, the rate-determining step is the influx of exchanging cations to negatively
charged sites. Sparks (
1986
) defined the following concurrent processes that take
place during Na
+
and K
+
exchange in vermiculite: (1) diffusion of Na
+
with Cl
-
through the solution film that surrounds the particle (FD), (2) diffusion of Na
+
ions
through a hydrated interlayer space and chemical reaction leading to exchange of
Na
+
in the particle (PD), (3) chemical reaction leading to exchange of Na
+
by K
+
ions on the particle surface (CR), (4) diffusion of displaced K
+
ions through the
hydrated interlayer space of the particle (PD), and (5) diffusion of displaced K
+
ions with Cl
-
through the solution film away from the particle (FD).
To enable a chemical reaction, the exchange ions must be transported to the
active sites of the particles. The film of water adhering to and surrounding the
particle, as well as the hydrated interlayer space within the particle, are zones of
low contaminant concentration that are being depleted constantly by ion adsorp-
tion to the sites. The decrease in concentration of contaminant ions in these
interfacial zones is then compensated for by ion diffusion from the bulk solution.
Ion exchange occurs when a driving force, such as a chemical potential gradient, is
maintained between solid and solution or when access to sites is kept free by the
use of a hydrated and less preferred cation for exchange.
