Environmental Engineering Reference
In-Depth Information
Fig. 3
Fluorescence
quenching curves for 10 mg
L
−
1
soil fulvic acid with
Cu
2
+
in 0.1 M KNO
3
at
25 °C: (
squares
) pH 5, four
replicates; (
asterisk
) pH 6,
three replicates; (
triangles
)
pH 7, three replicates.
Data
source
Ryan and Weber
(
1982a
)
Added Cu
2+
[M × 10
4
] (C
M
)
concentration of bound metal ion with an organic ligand (ML) is the difference
between total and free metal ion concentration. A titration of naturally occurring
fulvic acid ligand with a metal ion can allow determination of the stability con-
stant (
K
) and of the complexing capacity (C
L
) of the ligand if a stoichiometry is
considered (Hart
1981
).
3.2 Theory of Multisite Stern-Volmer Equation
for Determination of M-DOM Complexation
The multisite Stern-Volmer (MSV) equation is modified, simulated, validated
for predictive capability with a suitable model compound set by Hays and his
colleagues (
2004
), and applied to fluorescence titration data in the complexa-
tion of fulvic acid with Cu
2
+
(Fig.
4
). The MSV approach assumes a simple
1:1 coordination ratio between Cu
2
+
and the fluorescent ligand components,
giving the following reversible solution equilibria: M
+
L1
⇆
ML1 and
M
+
L2
ML2 where L1 and L2 are the free ligand species (all forms of
metal-free ligand) at sites 1 and 2, respectively, M is free metal, and ML1 and
ML2 are the metal-bound species at these sites. Other reaction stoichiometries
are possible, which rely on metal loading. For example, at low metal load-
ings, chelation (2:1
=
ligand:metal) may be induced, contorting the fulvic acid
shape and affecting fluorescence. Evidences for the effect of these molecular
conformation changes on fluorescence are scarce, inconclusive, and not easily
quantified.
⇆
