814
K. M. G. Mostofa et al.
high variation in the conditional stability constants for alkali, alkaline, heavy
metals, transition metals, lanthanides and actinides (Table 1) (Nair and Chander
1983; Kim et al. 1990, 1991; Buckau et al. 1992; Xue et al. 1995; Takahashi
et al. 1997; Mandal et al. 1999; Sekaly et al. 2003; van Loon et al. 2004; Wu
et al. 2004b; Sonke and Salters 2006; Thakur et al. 2006; Shcherbina et al. 2007).
The trace metals have different affinities toward the functional groups and the
donor atoms in DOM. Affinity depends on the occurrence of trace metals and on
their outer-shell electronic configuration in aqueous media (Wu et al. 2004b, c;
Sonke and Salters 2006; Konstantinou et al. 2009; Fu et al. 2007; Mostofa et al.
2011; Duffus 2002; Konstantinou et al. 2007; Marang et al. 2008; Kolokassidou
and Pashalidis 2006). The complexation of various trace transitional metals
with different molecular size fractions of DOM in natural waters indicates that
the number-averaged molecular weight follows the order: Cu > Ni > (Co, Zn,
Cr) > Pb > Cd for the DOM-bound complexes. This is consistent with the IrvingWilliams series. For DOM-bound complexes with other metals, the order is followed (Fe, V, Ce) > Th > U > Mo (Wu et al. 2004b). The complexation between
fulvic acid and several metals (Cu 2+ , Ni 2+ , Co 2+ , Cd 2+ and Ca 2+ ) at pH 7 demonstrates that for the fast-reacting binding site, the rate constant and the site relative contribution shows an order of Cu 2+ > Ni 2+ > Co 2+ > Cd 2+ > Ca 2+ that
agrees with the Irving-Williams series. This indicates that the complexation kinetics is affinity-dependent (Wu et al. 2004c). The competitive binding of Cu(II),
Co(II) and Ni(II) ions by fulvic acid shows that in the absence of Cu(II) and
Co(II), the strong binding sites of the fulvic acid are occupied by Ni(II), forming
strong complexes that are inert. In contrast, in the presence of Cu(II) and Co(II),
the strong binding sites of the fulvic acid are occupied by Cu(II). Co(II), Ni(II)
and the remaining Cu(II) occupy the weak binding sites of the fulvic acid, forming weak complexes that are labile (Mandal et al. 1999). The enhanced lability
of the Ni-FA complexes in the presence of Cu(II) and Co(II) suggests that Cu(II)
and Co(II) successfully compete with Ni(II) for the strong binding sites of the
fulvic acid (Mandal et al. 1999). The order of the lability of the metal complexes,
Co(II) > Ni(II) > Cu(II) < Zn(II), follows the reverse order of the ligand field stabilization energy with the exception of Cu(II). The behavior of Cu(II) is also due
to the Jahn–Teller effect, which shortens the equatorial bonds and lengthens the
axial bonds of a tetragonally distorted Cu(II)-L6 complex (Sekaly et al. 2003).
It is known that the rate of octahedral ligand substitution reactions of 3d transition metal complexes is influenced by several factors such as effective nuclear
charge, ionic radius (Douglas et al. 1994; Margerum et al. 1978) and ligand
field stabilization energy (LFSE) (Morel and Hering 1993; Butler and Harrod
1989). The dissociation kinetics of the 3d transition metal complexes agrees
with the trend predicted by the LFSE (weak field): Co(II) d 7 > Ni(II) d 8 > Cu(II)
d 9 < Zn(II) d 10 whereas the behavior of Cu(II) is also due to the Jahn–Teller effect
cited above (Sekaly et al. 2003). A gradual increase in complexation strength is
observed with decreasing ionic radius, an expression of the lanthanide contraction
during the complexation between lanthanides and humic substances (fulvic and
humic acids) in waters (Sonke and Salters 2006).
K. M. G. Mostofa et al.
high variation in the conditional stability constants for alkali, alkaline, heavy
metals, transition metals, lanthanides and actinides (Table 1) (Nair and Chander
1983; Kim et al. 1990, 1991; Buckau et al. 1992; Xue et al. 1995; Takahashi
et al. 1997; Mandal et al. 1999; Sekaly et al. 2003; van Loon et al. 2004; Wu
et al. 2004b; Sonke and Salters 2006; Thakur et al. 2006; Shcherbina et al. 2007).
The trace metals have different affinities toward the functional groups and the
donor atoms in DOM. Affinity depends on the occurrence of trace metals and on
their outer-shell electronic configuration in aqueous media (Wu et al. 2004b, c;
Sonke and Salters 2006; Konstantinou et al. 2009; Fu et al. 2007; Mostofa et al.
2011; Duffus 2002; Konstantinou et al. 2007; Marang et al. 2008; Kolokassidou
and Pashalidis 2006). The complexation of various trace transitional metals
with different molecular size fractions of DOM in natural waters indicates that
the number-averaged molecular weight follows the order: Cu > Ni > (Co, Zn,
Cr) > Pb > Cd for the DOM-bound complexes. This is consistent with the IrvingWilliams series. For DOM-bound complexes with other metals, the order is followed (Fe, V, Ce) > Th > U > Mo (Wu et al. 2004b). The complexation between
fulvic acid and several metals (Cu 2+ , Ni 2+ , Co 2+ , Cd 2+ and Ca 2+ ) at pH 7 demonstrates that for the fast-reacting binding site, the rate constant and the site relative contribution shows an order of Cu 2+ > Ni 2+ > Co 2+ > Cd 2+ > Ca 2+ that
agrees with the Irving-Williams series. This indicates that the complexation kinetics is affinity-dependent (Wu et al. 2004c). The competitive binding of Cu(II),
Co(II) and Ni(II) ions by fulvic acid shows that in the absence of Cu(II) and
Co(II), the strong binding sites of the fulvic acid are occupied by Ni(II), forming
strong complexes that are inert. In contrast, in the presence of Cu(II) and Co(II),
the strong binding sites of the fulvic acid are occupied by Cu(II). Co(II), Ni(II)
and the remaining Cu(II) occupy the weak binding sites of the fulvic acid, forming weak complexes that are labile (Mandal et al. 1999). The enhanced lability
of the Ni-FA complexes in the presence of Cu(II) and Co(II) suggests that Cu(II)
and Co(II) successfully compete with Ni(II) for the strong binding sites of the
fulvic acid (Mandal et al. 1999). The order of the lability of the metal complexes,
Co(II) > Ni(II) > Cu(II) < Zn(II), follows the reverse order of the ligand field stabilization energy with the exception of Cu(II). The behavior of Cu(II) is also due
to the Jahn–Teller effect, which shortens the equatorial bonds and lengthens the
axial bonds of a tetragonally distorted Cu(II)-L6 complex (Sekaly et al. 2003).
It is known that the rate of octahedral ligand substitution reactions of 3d transition metal complexes is influenced by several factors such as effective nuclear
charge, ionic radius (Douglas et al. 1994; Margerum et al. 1978) and ligand
field stabilization energy (LFSE) (Morel and Hering 1993; Butler and Harrod
1989). The dissociation kinetics of the 3d transition metal complexes agrees
with the trend predicted by the LFSE (weak field): Co(II) d 7 > Ni(II) d 8 > Cu(II)
d 9 < Zn(II) d 10 whereas the behavior of Cu(II) is also due to the Jahn–Teller effect
cited above (Sekaly et al. 2003). A gradual increase in complexation strength is
observed with decreasing ionic radius, an expression of the lanthanide contraction
during the complexation between lanthanides and humic substances (fulvic and
humic acids) in waters (Sonke and Salters 2006).