Environmental Engineering Reference
In-Depth Information
3 electrons in its outer d-orbitals and an empty s-orbital in its outer shell (Cr
3
+
: 1
s
2
2
s
2
2
p
6
3
d
3
3
s
0
). It can accept electrons in its two empty and three unpaired
d
-orbit-
als from functional groups in DOM to form complexes. Cu
2
+
has one unpaired
d
-orbital and one empty
s
-orbital in its outer shell (Cu
2
+
: 1
s
2
2
s
2
2
p
6
3
d
9
3
s
0
), to
which the functional groups in DOM can donate electrons when forming com-
plexes. It is assumed that donation of electrons from functional groups in DOM
to unpaired
d
-orbitals can enhance the stability of the M-DOM complexes. This
gives a strong bonding capacity of Cu
2
+
ions toward functional groups in DOM
or organic ligands. Finally, Sc
3
+
has empty
d
- and
s
-orbitals (Sc
3
+
: 1
s
2
2
s
2
2
p
6
3
s
2
3
p
6
3
d
0
4
s
0
) that can accept electrons when forming M-DOM complexes. Entirely
empty
d
-orbitals of Sc
3
+
ions gives exceptionally strong bonding properties
toward fulvic and humic acids (ca. log
10
K
=
17.57 for fulvic acid and 17.54-
20.47 for humic acid) among all the metal ions (log
10
K
=
3.26-14.58 and 0.5-
16.50, respectively) studied in aqueous solutions (Table
1
).
(iii) For alkali/alkaline earth metals and metalloids, the functional groups in
DOM can donate electrons to the outer empty
s
- and/or
p
-orbitals. For example,
H
+
has no electrons in its outer empty
s
-orbital (H
+
:1s) and can accept electrons.
Ca
2
+
has one empty
s
-and one
p
-orbital in its outer shell (Ca
2
+
: 1
s
2
2
s
2
2
p
6
3
s
0
3
p
x-
0
3
p
y
3
p
z
) that can accept electrons. Donation of electrons to
s
- and
p
-orbitals pos-
sibly explains the weak bonding properties of Ca
2
+
toward functional groups in
DOM. Al
3
+
has an empty
s
- and two
p
-orbitals in its outer shell (Al
3
+
: 1
s
2
2
s
2
2
p-
6
3
d
3
3
s
0
3
p
x
0
3
p
y
0
3
p
z
) that can accept electrons. However, the involvement of these
orbitals in complexation with functional groups in DOM would ultimately desta-
bilize the Al
3
+
-complexes. This effect can enhance the fluorescence intensity of
Al-DOM complexes in aqueous media. Sb
3
+
has empty
p
-orbitals in its outer shell
(
Sb
3
+
:
1
s
2
2
s
2
p
6
3
s
2
p
6
d
10
4
s
2
p
6
d
10
5
s
2
p
x
p
y
p
z
)
that can receive electrons from
functional groups in DOM. Therefore, alkali and metals/metalloids can form com-
plexes with DOM or organic ligands in aqueous media.
5 Factors Affecting the Metal-DOM Complexation
in Natural Waters
The formation of M-DOM complexes is greatly affected by several factors in natural
waters, which can be distinguished as (i) Quantity, nature and molecular size of DOM;
(ii) Occurrence and affinity of trace metals; (iii) Effect of pH; (iv) Effects of ions (cati-
ons and anions) and their ionic strength (
I
); (v) Effects of photoinduced processes; (vi)
Effects of microbial processes; and (vii) Effects of freshwater and sea waters.
5.1 Quantity, Nature and Molecular Size of DOM
The most common organic ligands in DOM are allochthonous fulvic acid, alloch-
thonous humic acid, tryptophan and extracellular polymeric substances (EPS).
