Environmental Engineering Reference
In-Depth Information
with stronger functional groups for M-DOM complexation. Irradiation of waters
with high contents of DOM is not able to substantially modify the degree of
M-DOM complexation, which is the most likely explanation for the cases where
the same stability constants have been observed before and after irradiation. The
effect of photoinduced degradation on M-DOM complexation will be explained
in details later.
3.6.1 Conditional Stability Constants for Ternary Complexes in Waters
Ternary complexes are operationally defined as complexes involving two newly
successive bonds between a metal ion and two different types of DOM com-
ponents or organic ligands (e.g. allochthonous fulvic acid and tryptophan). It
is assumed that fulvic acid (FA) acts as a primary ligand to form bonds with a
metal ion (M). Therefore, one initially observes binary complex formation such as
M
−
FA
(
M
+
FA
⇆
M
−
FA
)
.
Then another molecule, e.g. tryptophan (T) acts as
a secondary ligand and forms ternary complexes that can be represented as such as
T
−
M
−
FA
(
M
−
FA
+
T
⇆
T
−
M
−
FA
)
.
Formation of ternary complexes is observed in aqueous solution (Martin and
Prados
1974
; Khalil and Radalla
1998
; Khalil and Attia
1999
; Khalil
2000a
,
b
;
Khalil and Taha
2004
; Khalil and Fazary
2004
; Radalla
2010
; Rosas et al.
2010
).
For the formation of ternary complexes of the selected bivalent metal ions (M)
in the presence of resorcinol
=
R and aliphatic dicarboxylic acid
=
A, the follow-
ing equilibria may be considered (Eqs.
3.11
and
3.12
) (Radalla
2010
):
[
MA
]
[
M
][
A
]
K
MA
=
(3.11)
M
+
A
⇋
MA,
MAR
=
[
MAR
]
K
MA
MA
+
R
⇋
MAR,
(3.12)
[
M
][
A
]
In the presence of both ligands, A is presumably considered to interact first
with M forming a 1:1 MA binary complex. It follows interaction of R in a step-
wise manner. The overall stability constant
β
MAR
can be described as below
(Eq.
3.13
):
[
MAR
]
[
M
][
A
][
R
]
=
K
MA
β
MAR
=
MAR
×
K
MA
(3.13)
M
+
A
+
R
⇋
MAR,
The
β
MAR
constant expresses the stability of the mixed-ligand species and it
does not represent the binding strength between R and M
2
+
ions directly in the
presence of A. This effect is much better reflected by the equilibrium constant,
K
MA
MAR
calculated according to Eq.
3.14
:
log
10
K
MAR
⇋
log
10
β
MAR
−
log
K
MA
(3.14)
The equilibrium constant expressed in Eq.
3.14
indicates how tightly R is
bound to the simple MA binary complex in aqueous solution (Radalla
2010
).
