781
Complexation of Dissolved Organic Matter with Trace Metal Ions in Natural Waters
From Eqs. (3.7) and (3.8), the fluorescence of the ligand can be expressed in
terms of the protonation constant and the proton concentration as follows:
Using the logarithm, Eq. (3.9) becomes
The plot of log(F H-pH –F)/(F–F L-pH ) versus pH gives the values of log K′ HL
from the intercept. For the occurrence of a further protonation at lower pH, two
protonation constants are operational that can be treated by another model (van
Den Berg 1984).
3.5 Kinetics of M-Fulvic Acid Complexation
It has been shown that allochthonous fulvic acids are the main DOM components studied in natural waters (Moran et al. 1991; Malcolm 1990; Ma et al. 2001;
Mostofa et al. 2009a). Therefore, it is vital to know how they form complexes with
metal ions. Changes in the full fluorescence spectral kinetic, i.e., in both Ex/Em
wavelengths of the fluorescence maxima occur in the EEM spectra of fulvic acid
or DOM during their complexation with trace elements (Wu et al. 2004a, c). The
complexation of a fulvic acid (extracted from Cavan Bog, Canada) with several
metals (Cu 2+ , Ni 2+ , Co 2+ , Cd 2+ and Ca 2+ ) at pH 7 shows that the fulvic acid can
react rapidly with all metals studied (Wu et al. 2004c). The result of pseudo-first
order kinetic plots demonstrates that fulvic acid has two major kinetically distinguishable binding sites, ‘fast’ and ‘slow’, having reaction half-lives of 1.3–3.9 and
34.7–69.3 s, respectively (Wu et al. 2004c). The binding of copper to fulvic acid is
found to be fairly rapid, and the reaction is virtually at equilibrium after approximately 20–30 s (Lin et al. 1995). Another study demonstrates that the three lifetimes and emission wavelength maxima for three fluorophores in fulvic acid are
as follows: ~50 ps (392 nm), ~430 ps (465 nm), and 4.2 ns (512 nm) (Cook and
Langford 1995). Kinetic changes of excitation-emission wavelengths of the fluorescence maxima also suggest the presence of two major binding sites. For the
fast-reacting binding site, the rate constant and the site relative contribution are
in the order Cu 2+ > Ni 2+ > Co 2+ > Cd 2+ > Ca 2+ , which agrees with the IrvingWilliams series, indicating affinity dependence of complexation kinetics (Wu et al.
2004c). For the fast-reacting binding site in fulvic acid, proteins and other organic
ligands, the relative contribution of rate constant for bivalent metal complexes
follows the same order (Wu et al. 2002a, 2004c; Sidenius et al. 1999; Irving and
Williams 1953; Winzerling et al. 1992). This result implies that metal ions react
(3.9)
F H−pH − F
F − F L−pH
=
[HL]
L
= K
′
HL · [H]
(3.10)
log
F H−pH − F
F − F L−pH
= log K
′
HL − pH
Complexation of Dissolved Organic Matter with Trace Metal Ions in Natural Waters
From Eqs. (3.7) and (3.8), the fluorescence of the ligand can be expressed in
terms of the protonation constant and the proton concentration as follows:
Using the logarithm, Eq. (3.9) becomes
The plot of log(F H-pH –F)/(F–F L-pH ) versus pH gives the values of log K′ HL
from the intercept. For the occurrence of a further protonation at lower pH, two
protonation constants are operational that can be treated by another model (van
Den Berg 1984).
3.5 Kinetics of M-Fulvic Acid Complexation
It has been shown that allochthonous fulvic acids are the main DOM components studied in natural waters (Moran et al. 1991; Malcolm 1990; Ma et al. 2001;
Mostofa et al. 2009a). Therefore, it is vital to know how they form complexes with
metal ions. Changes in the full fluorescence spectral kinetic, i.e., in both Ex/Em
wavelengths of the fluorescence maxima occur in the EEM spectra of fulvic acid
or DOM during their complexation with trace elements (Wu et al. 2004a, c). The
complexation of a fulvic acid (extracted from Cavan Bog, Canada) with several
metals (Cu 2+ , Ni 2+ , Co 2+ , Cd 2+ and Ca 2+ ) at pH 7 shows that the fulvic acid can
react rapidly with all metals studied (Wu et al. 2004c). The result of pseudo-first
order kinetic plots demonstrates that fulvic acid has two major kinetically distinguishable binding sites, ‘fast’ and ‘slow’, having reaction half-lives of 1.3–3.9 and
34.7–69.3 s, respectively (Wu et al. 2004c). The binding of copper to fulvic acid is
found to be fairly rapid, and the reaction is virtually at equilibrium after approximately 20–30 s (Lin et al. 1995). Another study demonstrates that the three lifetimes and emission wavelength maxima for three fluorophores in fulvic acid are
as follows: ~50 ps (392 nm), ~430 ps (465 nm), and 4.2 ns (512 nm) (Cook and
Langford 1995). Kinetic changes of excitation-emission wavelengths of the fluorescence maxima also suggest the presence of two major binding sites. For the
fast-reacting binding site, the rate constant and the site relative contribution are
in the order Cu 2+ > Ni 2+ > Co 2+ > Cd 2+ > Ca 2+ , which agrees with the IrvingWilliams series, indicating affinity dependence of complexation kinetics (Wu et al.
2004c). For the fast-reacting binding site in fulvic acid, proteins and other organic
ligands, the relative contribution of rate constant for bivalent metal complexes
follows the same order (Wu et al. 2002a, 2004c; Sidenius et al. 1999; Irving and
Williams 1953; Winzerling et al. 1992). This result implies that metal ions react
(3.9)
F H−pH − F
F − F L−pH
=
[HL]
L
= K
′
HL · [H]
(3.10)
log
F H−pH − F
F − F L−pH
= log K
′
HL − pH