818
K. M. G. Mostofa et al.
molecular dynamics (MD) study of the interactions of Na + , Mg 2+ , and Ca 2+ with
the carboxylic groups of a model DOM suggests that aggregation of the DOM
molecules occurs in the presence of Ca 2+ but not of Na + or Mg 2+ (IskrenovaTchoukova et al. 2010). These results suggest that Ca 2+ ion bridging between
NOM molecules can decrease repulsion due to the reduced net charge of the
NOM−metal complexes (Iskrenova-Tchoukova et al. 2010).
An increase of Ca 2+ can enhance the fluorescence intensity of fulvic acid
involved in Hg-DOM complexation (Fu et al. 2007). In contrast, the Mg 2+ ion has
no effect on fluorescence and, therefore, on Hg-DOM complexation under natural
conditions at pH 7.5 in urban river waters (Fu et al. 2007). On the other hand, the
Cl − ion is an inorganic ligand that can complex Hg(II) to form HgCl 2−n
n
depending on the concentration of Cl − in natural waters (Lu and Jaffe 2001). It has been
shown that the addition of Cl − to Hg–DOM complexes increases the fluorescence
emission intensity at both pH 7.50 and 9.50 due to the competition between DOM
and Cl − for Hg(II) (Fu et al. 2007). For divalent transition metal ions (Mn 2+ , Co 2+
and Zn 2+ ), carbonate complexes may be as important species as humate complexes (Takahashi et al. 1997). Hydroxide species are important for Fe 3+ , VO 2+
and Ga 3+ , and significantly affect the complexation between humic acids and trace
metal ions (Takahashi et al. 1997).
Mg 2+ generates only electrostatic interaction with humic acid during complexation with heavy metals such as Cd 2+ , Pb 2+ and Sr 2+ (Christoforidis et al.
2010). Furthermore, the two types of indigenous radicals that exist in all humic
acids are influenced by the metal cations in a similar way, because of the presence
of a unique phenolic moiety in humic acid (Christoforidis et al. 2010). Mg 2+ ions
can change the pH profile of the two radical types of humic acid, downshifting
their interconversion pK a by ca. 3 pH units (Fig. 9) (Christoforidis et al. 2010).
The competition of Al 3+ with divalent metal ions such as Pb(II) occurs primarily
at carboxylic-type groups. Aluminum can reduce by a factor of 2 to 3 the amount
of lead bound to humic acid (Pinheiro et al. 2000). In the case of Cd(II), the aluminum competition mainly affects the bound Cd, with only smaller changes in
[Cd 2+ ]. Therefore, Al 3+ competition is likely to increase Pb(II) toxicity and bioavailability and Cd(II) transport in aqueous solution (Pinheiro et al. 2000).
The conditional stability constants of the M-DOM complexation are decreased
with increasing supporting electrolyte concentration in aqueous media (Sonke and
Salters 2006; Cao et al. 2004; Benoit et al. 2001; Pinheiro et al. 2000; Stevenson et
al. 1993; Bryan et al. 2002). It has been shown that conditional stability constants
(K) values increase with decreasing ionic strength (I) (from 0.1 to 0.001 mol L −1
NaNO 3 ) for complexation of lanthanides with SRFA and extracted humic acids
(Fig. 5a) (Sonke and Salters 2006). Similarly, the conditional stability constants
of the Hg 2+ -DOM complexes were increased by 7–12 % at I = 0 compared to
those at I = 0.06 for the hydrophobic fraction of DOM extracted from the Florida
Everglades surface waters (Benoit et al. 2001). An increase of ionic strength
can lead to a decrease of Pb and Cd bound to humic acid in aqueous solution
(Pinheiro et al. 1999, 2000). With a ionic strength of 0.1 M NaCl, the EPS from P.
K. M. G. Mostofa et al.
molecular dynamics (MD) study of the interactions of Na + , Mg 2+ , and Ca 2+ with
the carboxylic groups of a model DOM suggests that aggregation of the DOM
molecules occurs in the presence of Ca 2+ but not of Na + or Mg 2+ (IskrenovaTchoukova et al. 2010). These results suggest that Ca 2+ ion bridging between
NOM molecules can decrease repulsion due to the reduced net charge of the
NOM−metal complexes (Iskrenova-Tchoukova et al. 2010).
An increase of Ca 2+ can enhance the fluorescence intensity of fulvic acid
involved in Hg-DOM complexation (Fu et al. 2007). In contrast, the Mg 2+ ion has
no effect on fluorescence and, therefore, on Hg-DOM complexation under natural
conditions at pH 7.5 in urban river waters (Fu et al. 2007). On the other hand, the
Cl − ion is an inorganic ligand that can complex Hg(II) to form HgCl 2−n
n
depending on the concentration of Cl − in natural waters (Lu and Jaffe 2001). It has been
shown that the addition of Cl − to Hg–DOM complexes increases the fluorescence
emission intensity at both pH 7.50 and 9.50 due to the competition between DOM
and Cl − for Hg(II) (Fu et al. 2007). For divalent transition metal ions (Mn 2+ , Co 2+
and Zn 2+ ), carbonate complexes may be as important species as humate complexes (Takahashi et al. 1997). Hydroxide species are important for Fe 3+ , VO 2+
and Ga 3+ , and significantly affect the complexation between humic acids and trace
metal ions (Takahashi et al. 1997).
Mg 2+ generates only electrostatic interaction with humic acid during complexation with heavy metals such as Cd 2+ , Pb 2+ and Sr 2+ (Christoforidis et al.
2010). Furthermore, the two types of indigenous radicals that exist in all humic
acids are influenced by the metal cations in a similar way, because of the presence
of a unique phenolic moiety in humic acid (Christoforidis et al. 2010). Mg 2+ ions
can change the pH profile of the two radical types of humic acid, downshifting
their interconversion pK a by ca. 3 pH units (Fig. 9) (Christoforidis et al. 2010).
The competition of Al 3+ with divalent metal ions such as Pb(II) occurs primarily
at carboxylic-type groups. Aluminum can reduce by a factor of 2 to 3 the amount
of lead bound to humic acid (Pinheiro et al. 2000). In the case of Cd(II), the aluminum competition mainly affects the bound Cd, with only smaller changes in
[Cd 2+ ]. Therefore, Al 3+ competition is likely to increase Pb(II) toxicity and bioavailability and Cd(II) transport in aqueous solution (Pinheiro et al. 2000).
The conditional stability constants of the M-DOM complexation are decreased
with increasing supporting electrolyte concentration in aqueous media (Sonke and
Salters 2006; Cao et al. 2004; Benoit et al. 2001; Pinheiro et al. 2000; Stevenson et
al. 1993; Bryan et al. 2002). It has been shown that conditional stability constants
(K) values increase with decreasing ionic strength (I) (from 0.1 to 0.001 mol L −1
NaNO 3 ) for complexation of lanthanides with SRFA and extracted humic acids
(Fig. 5a) (Sonke and Salters 2006). Similarly, the conditional stability constants
of the Hg 2+ -DOM complexes were increased by 7–12 % at I = 0 compared to
those at I = 0.06 for the hydrophobic fraction of DOM extracted from the Florida
Everglades surface waters (Benoit et al. 2001). An increase of ionic strength
can lead to a decrease of Pb and Cd bound to humic acid in aqueous solution
(Pinheiro et al. 1999, 2000). With a ionic strength of 0.1 M NaCl, the EPS from P.