Environmental Engineering Reference
In-Depth Information
NH 3 +
(a)
+
+
NH 3
CO 2
M n+
CH
CO 2
+
CH 2
CH
NH
NH
M n+
(b)
NH 2
NH 3 +
CH
….
CO 2
CH
CO 2 H
Fig. 7 The mechanism of the chemical bonding of two functional groups (amino-carboxylic and
amide) in tryptophan with metal ion (M n +
) ( a ) and the resonance configuration of the amino-car-
boxylic group [-CH(NH 2 )-COOH] ( b ). Data source with modifications Mostofa et al. ( 2009a ;
2011 )
from O-containing functional groups (carboxylic, amino-carboxylic, phenolic,
alcoholic and so on) of DOM or organic ligands to metal ions. Second, donation to
metal ions of non-bonding π -electrons occurs from S-, N-, and P-containing func-
tional groups in DOM or highly unsaturated π -electrons either in aromatic or ali-
phatic systems. In both cases, donation of electrons takes place from the functional
groups (F:) of DOM to the empty or partially filled d -orbitals of transition met-
als/lanthanides/actinides. Complex formation can also involve s - and p -orbitals in
alkali/alkaline earth metals (M n + ) that form a strong covalent or π -electron bond-
ing system with DOM functional groups (F:M n + ). This can simply be expressed as
follows (Eq. 4.1 ):
F :+ M N + F : M N +
(4.1)
The formation of such a bond in M-DOM complexes can greatly reduce the
electron density of the functional groups (F:) in DOM. Donation of electrons from
functional groups (F:) then causes the s -, p - or d -orbitals in metal ions (M) to be
either stabilized or destabilized. Therefore, the M-DOM complexation causes
the fluorescence properties of DOM to be either decreased or increased (Mostofa
et al. 2009a , 2011 ). A stabilizing effect from a functional group in DOM lowers
the energy of the interacting s -, p -, d -orbitals, which can considerably decrease the
electron transfer probability and decrease as a consequence the fluorescence inten-
sity of the functional groups (or fluorophores) in DOM. It is generally known that
the metal ions are excellent Lewis acids and accept electron density from many
molecules or ions that act as Lewis bases. On the other hand, M-DOM formation
may enhance the probability of an electron transfer if a destabilizing effect from
the functional groups in DOM raises the energy of the s -, p -, d -orbitals. This effect
subsequently leads to an increase in the fluorescence intensity of the functional
group (or fluorophores) in DOM.
The two types of mechanism for M-DOM complexation can be clarified by
considering tryptophan amino acid as an example, because its molecular structure
( C 8 H 5 ( NH )− CH 2
NH 3
CHCOO ) is composed of two functional groups such
CHCOO ] and [C 8 H 5 (NH)-] (Mostofa et al. 2009a , 2011 ). The
as [− CH 2
NH 3
Search WWH ::




Custom Search