Environmental Engineering Reference
In-Depth Information
and phenolic-type groups in humic acids can bind to H
+
, whereas carboxylic-
type binding sites exhibit a smaller apparent heterogeneity than phenolic-type
sites in humic acids (Vidali et al.
2009
; Benedetti et al.
1996
; Kinniburgh et al.
1996
; Pinheiro et al.
1999
; Plaza et al.
2005
). Copper(II) complexation with DOM
suggests that sites characterized as phenolic based on alkalimetric titration, and
not carboxylic sites, account for the majority of Cu complexation under natural
water conditions. Cu-DOM complexation mainly takes place through replace-
ment of H
+
by Cu
2
+
at the phenolic binding sites (Lu and Allen
2002
). Ca/Mg-Cu
exchange experiments with DOM suggest that Ca and Mg are preferably bound by
carboxylic sites, particularly at relatively high concentrations, which results in a
weakened apparent competition effect (Lu and Allen
2002
).
Conditional distribution coefficients (
K′
DOM
) for the binding of Hg(II) to dis-
solved organic matter (extracted hydrophobic acids) shows that very strong
interactions (
K′
DOM
=
10
23.2
±
1.0
L kg
−
1
at pH
=
7.0), at Hg/DOM ratios below
approximately 1
μ
g of Hg/mg of DOM, are indicative of mercury
−
thiol
bonds. In contrast, much weaker interactions (
K′
DOM
=
10
10.7
±
1.0
L kg
−
1
at
pH
=
4.9
−
5.6), at Hg/DOM ratios above approximately 10
μ
g of Hg/mg of
DOM, are consistent with Hg binding to oxygen functional groups (Haitzer et al.
2002
). Similar results have been found in another study where conditional dis-
tribution coefficients (log
K′
DOM
) for Hg(II) binding to extracted humic acids,
fulvic acids and hydrophobic acids from diverse aquatic environments indicate
very strong interaction, suggesting the involvement of thiol groups (Haitzer et al.
2003
). It has also been shown that
K′
DOM
values decrease at low pH (4) compared
to pH 7, suggesting proton competition for the strong Hg(II) binding sites that is
consistent with bidentate binding of Hg(II) by one thiol group (p
K
a
=
10.3) and
by another group (p
K
a
=
6.3) in the DOM (Haitzer et al.
2003
). In addition, the
hydrophobic fraction of DOM is composed of thiol functional groups with high
conditional stability constant of the Hg
2
+
-DOM complexes (Benoit et al.
2001
;
Dyrssen and Wedborg
1986
,
1991
; Schuster
1991
; Xia et al.
1999
).
It has been shown that each SRFA molecule has approximately 3 carboxyl sites
available for coordination in the pH 6-9 range (Sonke and Salters
2006
). However,
coordination depends on other environmental factors such as metal concentra-
tion, ligand concentration, pH and so on (Sonke and Salters
2006
; Thomason et al.
1996
). Lanthanide ion probe spectroscopy (LIPS) suggested that an increase in the
metal/DOM ratio can result in progressively less chelated complexes, with a grad-
ual succession from 4 to 3 to 2 to 1 carboxyl groups bound to Eu
3
+
(Thomason
et al.
1996
)
.
The LIPS study indicated that Eu
3
+
complexation by Suwannee
river DOM (mixture of HA and FA) at pH 3.5 is dominated by tetra-dentate com-
plexes at low metal to ligands ratios (100 nM Eu
3
+
, 30 mg L
−
1
DOM) (Thomason
et al.
1996
). The complex formation of quinonoid-enriched humic derivatives
with actinides (ca. Np
5
+
) demonstrated that hydroquinone-enriched derivatives
have higher stability constants than the catechol ones, and that enriched humic
derivatives are more effective than the parent humic acid (Shcherbina et al.
2007
).
Moreover, interaction between HA and Np
5
+
in the neutral pH range is dominated
by carboxylate groups in aqueous media (Sachs et al.
2005
). Conditional stability
