810
K. M. G. Mostofa et al.
3 electrons in its outer d-orbitals and an empty s-orbital in its outer shell (Cr 3+ : 1s
2 2s 2 2p 6 3d 3 3s 0 ). It can accept electrons in its two empty and three unpaired d-orbitals 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+ : 1s 2 2s 2 2p 6 3d 9 3s 0 ), to
which the functional groups in DOM can donate electrons when forming complexes. 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+ : 1s 2 2s 2 2p 6 3s 2
3p 6 3d 0 4s 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+ : 1s 2 2s 2 2p 6 3s 0 3p x0 3p y 3p z ) that can accept electrons. Donation of electrons to s- and p-orbitals possibly 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+ : 1s 2 2s 2 2p6 3d 3 3s 0 3p x
0 3p y
0 3p z ) that can accept electrons. However, the involvement of these
orbitals in complexation with functional groups in DOM would ultimately destabilize 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+
: 1s 2 2s 2 p 6 3s 2 p 6 d 10 4s 2 p 6 d 10 5s 2 p 0
x p 0
y p 0
z ) that can receive electrons from
functional groups in DOM. Therefore, alkali and metals/metalloids can form complexes 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 (cations 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, allochthonous humic acid, tryptophan and extracellular polymeric substances (EPS).
K. M. G. Mostofa et al.
3 electrons in its outer d-orbitals and an empty s-orbital in its outer shell (Cr 3+ : 1s
2 2s 2 2p 6 3d 3 3s 0 ). It can accept electrons in its two empty and three unpaired d-orbitals 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+ : 1s 2 2s 2 2p 6 3d 9 3s 0 ), to
which the functional groups in DOM can donate electrons when forming complexes. 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+ : 1s 2 2s 2 2p 6 3s 2
3p 6 3d 0 4s 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+ : 1s 2 2s 2 2p 6 3s 0 3p x0 3p y 3p z ) that can accept electrons. Donation of electrons to s- and p-orbitals possibly 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+ : 1s 2 2s 2 2p6 3d 3 3s 0 3p x
0 3p y
0 3p z ) that can accept electrons. However, the involvement of these
orbitals in complexation with functional groups in DOM would ultimately destabilize 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+
: 1s 2 2s 2 p 6 3s 2 p 6 d 10 4s 2 p 6 d 10 5s 2 p 0
x p 0
y p 0
z ) that can receive electrons from
functional groups in DOM. Therefore, alkali and metals/metalloids can form complexes 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 (cations 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, allochthonous humic acid, tryptophan and extracellular polymeric substances (EPS).