Geology Reference
In-Depth Information
O
O
O
Fe - O
Fe - OH
2
P
O
O
O
Fe - O
Fe - OH
2
Figure 14 Chemisorption of a phosphate anion on to an iron oxide surface by ligand
exchange
The important ions held in this way are certain anions, especially
phosphate, and some trace metals; the main surfaces involved are the
iron and aluminium oxides. Unlike ion exchange, the surface charge
plays no part in ligand exchange, which can take place onto positive,
uncharged, or negative surfaces, as these surfaces have a variable charge,
which depends on pH (Figure 7). The ion being chemisorbed replaces a
water molecule or hydroxyl ion (OH
) from the surface (Figure 14).
Phosphate, silicate, borate, arsenate, selenite, chromate, and fluoride
are anions for which ligand exchange is important. Nitrate, chloride,
bromide, and perchlorate are not held, while sulfate and selenate may be
weakly held. As a consequence, leaching of nitrate and sulfate from soil
in drainage water can be significant, but very little phosphate is lost in
solution. Of the trace metals, Co, Cu, Ni, and Pb are strongly held on
oxide surfaces by chemisorption, but the process is much less important
for Cd and Zn.
Chemisorption is often described by use of the Langmuir equation:
x/x
m
¼
Kc/(1
þ
Kc)
(5.41)
where
c
¼
concentration of chemisorbed ion in solution at equilibrium
x
¼
amount of the ion chemisorbed on to the soil
x
m
¼
maximum amount of ion chemisorbed onto the soil
K
¼
a constant related to bonding.
The equation is often used in the linearized form
c/x
¼
c/x
m
þ
1/Kx
m
(5.42)
