Environmental Engineering Reference
In-Depth Information
two functional groups of tryptophan can electronically bind metal ions, creating
strong covalent and
π
-electron bonds, respectively, with the metal-ion
d
-orbitals
(Fig.
7
). In fact, the
[−
CH
2
NH
3
CHCOO
−
]
functional group has a strong affin-
ity toward a resonance configuration, and the [C
8
H
5
(NH)-] moiety has the non-
bonding electrons of the NH- group in the aromatic ring (Fig.
7
) that can form
complexes with metal ions. The formation of the M-DOM bonding system can
greatly reduce the electron density of the functional groups, i.e. the fluorophores
in tryptophan, which results in a lower probability of electron transition of those
fluorophores and decreases as a consequence the fluorescence intensity of tryp-
tophan after complexation with the metal ions. On the other hand, an increase
in fluorescence intensity of the DOM in M-DOM complexation may arise from
enhanced probability of electron transition of the fluorophores due to M-DOM
complexation that depends on the occurrences of the trace metal ions. The fluo-
rescence peak T in the longer wavelength region is caused by the functional group
[−
CH
2
NH
3
CHCOO
−
]
and the peak T
UV
at shorter wavelengths is linked with
the group [C
8
H
5
(NH)-] (Mostofa et al.
2011
). The conditional stability constant
(log
10
K
1
) for tryptophan-like or protein-like material of biological origin is 7.82-
9.56 for peak T that has stronger bonding capacity than the humic-like component
(log
K
1
=
7.05-8.78) in lake water (Wu and Tanoue
2001b
). Moreover, the order
of complex formation of the transition metals and other metals to DOM in natural
waters depends on several associated effects (Mostofa et al.
2011
). Therefore: (i)
Fluorophores with high electron density will merely compete for the metal ions
with stabilizing effects of
d
-orbitals.
(ii) Among the transition metals, the size or atomic radius generally decreases
with increasing nuclear charge, because the electrons that experience a greater
nuclear charge are pulled more strongly towards the nucleus. However, the last
few elements (Cu, Zn, Ag, Cd, Pt, Au, Hg, etc.) in each row of the
d
-block are
slightly larger than those preceding them because in these cases the electron-
electron repulsions caused by the filling of the
d
-orbitals outweigh the increasing
nuclear charge. Therefore, the two competing effects of nuclear charge and elec-
tron-electron repulsion affect the chemical binding of the
d
-orbital metals with
the fluorophores. As one moves across a period, the increasing nuclear charge is
usually more significant than the electron-electron repulsion. These combined
effects make Cu and other metals of the same row more susceptible to complexa-
tion with DOM than the transition metals of the second and third rows. Moreover,
the ground states of Sc, Ti, Fe, Co and Ni are ferromagnetic because of the pres-
ence of one unpaired electron, while V, Cr, and Mn are antiferromagnetic (Tung
and Guo
2007
; Iota et al.
2007
). It can be proposed that the unpaired electron of
ferromagnetic transition metals would easily form a bond with an electron donated
by functional groups in DOM, which is not possible for antiferromagnetic metals.
This hypothesis can account for the rapid complexation of the ferromagnetic tran-
sition metals compared to the antiferromagnetic elements. For example, Fe
3
+
has
one electron each in its outer
d
-orbitals and no electron in its outer shell
s
-orbital
(Fe
3
+
: 1
s
2
2
s
2
2
p
6
3
d
5
3
s
0
). Therefore, it can accept electrons in its outer unpaired
d
-orbitals from functional groups in DOM to form M-DOM complexes. Cr
3
+
has
