and the modal distribution with the higher stability constant was labelled “L 2 ”. L 1
sites could correspond to carboxylic-type groups and L 2 sites to phenolic-type
groups, although the reality is much more complex [59].
For copper and lead complexation by DOM from receiving waters, the trace metal
complexation was modelled with a bimodal distribution, in accordance with the
literature [60]. Whereas for their complexation by EfDOM, a trimodal binding site
distribution proved to be more suitable [29, 47]. In the case of copper, there is a very
high-affinity site type (L 3 ), in addition to the typical low- (L 1 ) and high-affinity sites
(L 2 ). For lead, this is a third modal distribution corresponding to sites with an
intermediate stability constant between stability constants of low- and high-affinity
sites. This third family of binding sites could be associated with proteinic structures,
amines and amide groups present in the hydrophilic fraction of EfDOM [61].
Some of these binding sites might also be associated with amino polycarboxylates,
phosphonates and hydroxycarboxylates [62].
In contrast, for zinc, a bimodal distribution was suitable for all DOM fractions
including EfDOM, whereas for mercury only one site type was obtained for all
DOM fractions. This single site is usually associated with reduced N (amide, amine)
and especially S (thiol) groups of proteinic structures and, to a lesser extent, with
carboxylic groups.
3.3.2 Me-DOM Stability Constants and Binding Site Density
In the case of copper complexation at pH 8, for the five campaigns in receiving
waters and seven campaigns for EfDOM, the conditional stability constants
ranged from 10
7.3 M
À1 to 10
7.9 M
À1 for low-affinity sites (L 1 ) and from
10
10.2 M
À1 to 10
11.2 M
À1 for high-affinity sites (L 2 ), and no significant difference
was found across the various sampling points; moreover, no temporal variations
in stability constant were observed.
The L 1 copper-binding site density values were similar between upstream sites in
receiving waters (%1 mol of binding sites per kg of organic carbon) and higher than
for EfDOM (%0.5 mol kg
À1 ). The L 2 binding site density values were lower than for
L 1 binding sites with equivalent values for receiving waters and EfDOM, ranging
from 0.1 to 0.3 mol kg
À1 . EfDOM presented by far the lowest (L 1 /L 2 ) ratio, with an
average value, equal to 2.7, much smaller than those obtained for receiving water
DOM, whose average ratio varied between 7.3 and 7.9. In the case of lead, the L 1 /L 2
ratio value was 7.3 for receiving waters and reached only 0.7 for the EfDOM
fractions. Even more than in the case of copper, a higher presence of high-affinity
binding sites was noted in EfDOM fractions compared to receiving water DOM.
The total lead-binding site density was much higher for the HPI EfDOM fraction
compared to other EfDOM and downstream fractions.
In the case of very high-affinity (L 3 ) copper-binding sites that were highlighted
only for EfDOM, conditional stability constants ranged from 10
12.2 M
À1 to
10
13.6 M
À1 . The L 3 site density (mean, 5.7 Â 10
À2 mol kg
À1 ) was less than L 2
and L 1 densities; it amounted to between 25% and 33% of the L 2 density. The L 3
Aquatic Organic Matter in the Seine Basin: Sources, Spatio-Temporal. . .
231
sites could correspond to carboxylic-type groups and L 2 sites to phenolic-type
groups, although the reality is much more complex [59].
For copper and lead complexation by DOM from receiving waters, the trace metal
complexation was modelled with a bimodal distribution, in accordance with the
literature [60]. Whereas for their complexation by EfDOM, a trimodal binding site
distribution proved to be more suitable [29, 47]. In the case of copper, there is a very
high-affinity site type (L 3 ), in addition to the typical low- (L 1 ) and high-affinity sites
(L 2 ). For lead, this is a third modal distribution corresponding to sites with an
intermediate stability constant between stability constants of low- and high-affinity
sites. This third family of binding sites could be associated with proteinic structures,
amines and amide groups present in the hydrophilic fraction of EfDOM [61].
Some of these binding sites might also be associated with amino polycarboxylates,
phosphonates and hydroxycarboxylates [62].
In contrast, for zinc, a bimodal distribution was suitable for all DOM fractions
including EfDOM, whereas for mercury only one site type was obtained for all
DOM fractions. This single site is usually associated with reduced N (amide, amine)
and especially S (thiol) groups of proteinic structures and, to a lesser extent, with
carboxylic groups.
3.3.2 Me-DOM Stability Constants and Binding Site Density
In the case of copper complexation at pH 8, for the five campaigns in receiving
waters and seven campaigns for EfDOM, the conditional stability constants
ranged from 10
7.3 M
À1 to 10
7.9 M
À1 for low-affinity sites (L 1 ) and from
10
10.2 M
À1 to 10
11.2 M
À1 for high-affinity sites (L 2 ), and no significant difference
was found across the various sampling points; moreover, no temporal variations
in stability constant were observed.
The L 1 copper-binding site density values were similar between upstream sites in
receiving waters (%1 mol of binding sites per kg of organic carbon) and higher than
for EfDOM (%0.5 mol kg
À1 ). The L 2 binding site density values were lower than for
L 1 binding sites with equivalent values for receiving waters and EfDOM, ranging
from 0.1 to 0.3 mol kg
À1 . EfDOM presented by far the lowest (L 1 /L 2 ) ratio, with an
average value, equal to 2.7, much smaller than those obtained for receiving water
DOM, whose average ratio varied between 7.3 and 7.9. In the case of lead, the L 1 /L 2
ratio value was 7.3 for receiving waters and reached only 0.7 for the EfDOM
fractions. Even more than in the case of copper, a higher presence of high-affinity
binding sites was noted in EfDOM fractions compared to receiving water DOM.
The total lead-binding site density was much higher for the HPI EfDOM fraction
compared to other EfDOM and downstream fractions.
In the case of very high-affinity (L 3 ) copper-binding sites that were highlighted
only for EfDOM, conditional stability constants ranged from 10
12.2 M
À1 to
10
13.6 M
À1 . The L 3 site density (mean, 5.7 Â 10
À2 mol kg
À1 ) was less than L 2
and L 1 densities; it amounted to between 25% and 33% of the L 2 density. The L 3
Aquatic Organic Matter in the Seine Basin: Sources, Spatio-Temporal. . .
231