Geoscience Reference
In-Depth Information
Metal-ligand complexation is sensitive to the additional variable pH (
Figure 7.3B
). Saar
and Weber (
1980
) added Cu(II) to a 5 × 10
-5
M
soil FA solution. No difference in the rel-
ative fluorescence (monitored ex = 350 nm and em = 445-450 nm) occurred until pH was
studied as another variable. pH strongly controls fluorescence emission intensity via H
+
ion quenching. Increasing the pH decreases the H
+
ion concentration, which results in a
reduced fluorescence quenching due to proton binding. Without an additional cation (i.e.,
in the Cu(II) free treatment), fluorescence generally increases to a maximum at pH ~5.0 (a
commonly observed maxima) and then slightly decreases (see
Section 7.5
on pH effects).
After metal addition, the fluorescence change is similar to the FA without the metal from
pH ~2 to 3.5, at which point the Cu(II) quenching becomes dominant. From pH 4 to 6,
the fluorescence continues to be quenched by the metal. Of six metals examined (Ni, Mn,
Co, Pb, Cu, and Cd), Cu(II) was the most effective quencher, indicating metal specificity
for DOM ligand binding sites. Further, using ion selective electrode potentiometric titra-
tions, Saar and Weber (
1980
) found that Cu(II) complexation by FA was nearly identical to
Cu(II)-DOM fluorescence quenching, indicating that indeed the formation of the metal-
ligand complex is quantitatively quenching the fluorescence. From these results, stability
and binding coefficients can then be calculated.
Ligand complexation has been assumed to display a linear relationship between the
complexing metal concentration and the change in fluorescence intensity. This assumption
has been challenged by Cabaniss and Shuman (
1986
), who used SF to study Cu
2+
binding
with fulvic acid and found nonlinear relationships. Cabaniss (1992) expanded the use of
differential synchronous fluorescence to identify metals that quench and also contribute
to fulvic acid fluorescence. Presumably, the nonlinearity is caused by differing responses
across wavelength regions and thus complexation capacity and ligand stability calculations
depended on the wavelength pairs chosen for modeling (Luster et al.,
1996
).
Al(III) appears both to decrease and increase DOM fluorescence when forming a com-
plex with fulvic acid (FA), isolated from soil (Ryan et al.,
1996
), river (Elkins and Nelson,
2002
), and marine (da Silva and Machado,
1996
) environments. The effect is presented in
Figure 7.4
(redrawn from Elkins and Nelson,
2002
), in which the change to relative fluor-
escence intensity of a river FA after addition of Al(III) is compared to additions of Cd(II)
and of Cu(II). Immediately after addition of the Al(III) metal, fluorescence (ex = 344/em =
424) increased and reached a stable intensity at 5.0 × 10
-5
M
Al(III). By contrast, additions
of up to 2 × 10
-4
M
Cd(II) did not change river FA fluorescence intensity. The addition of
Cu(II) to the river FA solution produced fluorescence quenching, as shown in
Figure 3B
.
Pullin and Cabaniss (
1997
) were able to increase synchronous fluorescence at 380 nm
excitation (SF380) by adding Al(III) to river water DOM at pH 5 but noted the effect was
smaller than that found for fulvic acid. Lakshman et al. (
1993
) used SF to investigate the
binding sites to a soil FA, separated into three size fractions by ultrafiltration, and identi-
fied changes to three SF peaks. Peaks I and II (315 and 370 nm excitation, respectively; Δ
λ
was not reported) corresponded to carboxyl or hydroxyl groups attached to simple aromatic
rings, while peak III (excitation = 470 nm) corresponded to condensed polyaromatic rings
