782
K. M. G. Mostofa et al.
initially with the fast reacting and strong binding site on fulvic acid; after the fast
binding site is entirely occupied, the slow reacting and weak binding site starts to
bind with the metal ions (Wu et al. 2004c). In each phase, the red-shifted excitation-emission spectra suggest physical structural changes (ca. molecular conformation or rigidness) of the fulvic acid-metal complex (Wu et al. 2004c). Changes
in the molecular conformation and formation of aggregates of fulvic acid upon
complexation are also supported by other studies (Hays et al. 2004; Filella 2008;
Chakraborty et al. 2007).
Within each kinetic phase, both excitation-emission wavelengths of fluorescence maxima gradually increase with reaction time (Wu et al. 2004a, c). Such
red-shifted phenomena are also observed in humic acid extracted from sewerage
sludge (Plaza et al. 2006).The kinetics of metal-fulvic acid complexation depends
on the concentration; chemical nature and sources of DOM components; temperature; pH; molecular size; occurrence of anions and cations; salinity; and metal
affinity (da Silva et al. 1998a; Wu and Tanoue 2001a, b; Wu et al. 2004b, c; Fu
et al. 2007; Mostofa et al. 2011; Lu and Jaffe 2001; Lin et al. 1995; Plankey and
Patterson 1987). Recent studies show that all protonated and unprotonated forms
of both the ligand and the hydrated central metal ion are involved in the formation
of the precursor inner- and outer-sphere complexes, which control the kinetics of
complex formation/dissociation (van Leeuwen et al. 2007; van Leeuwen and Town
2009a, b). It has also been shown that stronger the affinity of the metal, the greater
proportion of strong and fast reacting fulvic acid binding sites are involved in the
complexation process. This finding indicates that metal affinity affects both the
thermodynamic equilibrium and the reaction kinetics (Wu et al. 2004c).
3.6 Conditional Stability Constants (log 10 K) for M-DOM
Complexation in Water
Conditional stability constants (log 10 K) for M-DOM complexation are presented for various DOM components in water (Table 1) (Zhang et al. 2010; Wu
et al. 2001, 2004a; Nair and Chander 1983; Kim et al. 1990; Sonke and Salters
2006; Shcherbina et al. 2007; Konstantinou et al. 2009; Vidali et al. 2009; Liu
and Cai 2010; Fu et al. 2007; Brooks et al. 2007; Hays et al. 2004; Cao et al.
2004; Takahashi et al. 1997; Haitzer et al. 2002; Xue and Sigg 1993; Xue et
al. 1995; Wu and Tanoue 2001b; Guibaud et al. 2004, 2006; Thakur et al. 2006;
van Loon et al. 2004; Plaza et al. 2006; Antunes et al. 2007; Midorikawa and
Tanoue 1998; Tipping 1994; Breault et al. 1996; Mcknight et al. 1983; van Den
Berg et al. 1987; Sunda and Huntsman 1991; Sunda and Hanson 1987; Sunda
and Ferguson 1983; Moffett et al. 1990; Coale and Bruland 1988, 1990; van
Den Berg 1984; Midorikawa and Tanoue 1996; Benoit et al. 2001; Buckau
et al. 1992; Comte et al. 2006; Drexel et al. 2002; Gasper et al. 2007; Hsu and
Sedlak 2003; Kim et al. 1991; Sander et al. 2005; Shank et al. 2006; Smith 1974).
Fulvic acids (FA) of different origin can exhibit two major binding sites in the
K. M. G. Mostofa et al.
initially with the fast reacting and strong binding site on fulvic acid; after the fast
binding site is entirely occupied, the slow reacting and weak binding site starts to
bind with the metal ions (Wu et al. 2004c). In each phase, the red-shifted excitation-emission spectra suggest physical structural changes (ca. molecular conformation or rigidness) of the fulvic acid-metal complex (Wu et al. 2004c). Changes
in the molecular conformation and formation of aggregates of fulvic acid upon
complexation are also supported by other studies (Hays et al. 2004; Filella 2008;
Chakraborty et al. 2007).
Within each kinetic phase, both excitation-emission wavelengths of fluorescence maxima gradually increase with reaction time (Wu et al. 2004a, c). Such
red-shifted phenomena are also observed in humic acid extracted from sewerage
sludge (Plaza et al. 2006).The kinetics of metal-fulvic acid complexation depends
on the concentration; chemical nature and sources of DOM components; temperature; pH; molecular size; occurrence of anions and cations; salinity; and metal
affinity (da Silva et al. 1998a; Wu and Tanoue 2001a, b; Wu et al. 2004b, c; Fu
et al. 2007; Mostofa et al. 2011; Lu and Jaffe 2001; Lin et al. 1995; Plankey and
Patterson 1987). Recent studies show that all protonated and unprotonated forms
of both the ligand and the hydrated central metal ion are involved in the formation
of the precursor inner- and outer-sphere complexes, which control the kinetics of
complex formation/dissociation (van Leeuwen et al. 2007; van Leeuwen and Town
2009a, b). It has also been shown that stronger the affinity of the metal, the greater
proportion of strong and fast reacting fulvic acid binding sites are involved in the
complexation process. This finding indicates that metal affinity affects both the
thermodynamic equilibrium and the reaction kinetics (Wu et al. 2004c).
3.6 Conditional Stability Constants (log 10 K) for M-DOM
Complexation in Water
Conditional stability constants (log 10 K) for M-DOM complexation are presented for various DOM components in water (Table 1) (Zhang et al. 2010; Wu
et al. 2001, 2004a; Nair and Chander 1983; Kim et al. 1990; Sonke and Salters
2006; Shcherbina et al. 2007; Konstantinou et al. 2009; Vidali et al. 2009; Liu
and Cai 2010; Fu et al. 2007; Brooks et al. 2007; Hays et al. 2004; Cao et al.
2004; Takahashi et al. 1997; Haitzer et al. 2002; Xue and Sigg 1993; Xue et
al. 1995; Wu and Tanoue 2001b; Guibaud et al. 2004, 2006; Thakur et al. 2006;
van Loon et al. 2004; Plaza et al. 2006; Antunes et al. 2007; Midorikawa and
Tanoue 1998; Tipping 1994; Breault et al. 1996; Mcknight et al. 1983; van Den
Berg et al. 1987; Sunda and Huntsman 1991; Sunda and Hanson 1987; Sunda
and Ferguson 1983; Moffett et al. 1990; Coale and Bruland 1988, 1990; van
Den Berg 1984; Midorikawa and Tanoue 1996; Benoit et al. 2001; Buckau
et al. 1992; Comte et al. 2006; Drexel et al. 2002; Gasper et al. 2007; Hsu and
Sedlak 2003; Kim et al. 1991; Sander et al. 2005; Shank et al. 2006; Smith 1974).
Fulvic acids (FA) of different origin can exhibit two major binding sites in the