Geoscience Reference
In-Depth Information
2005
). Moreover, atmospheric deposition of metals in dust can greatly impact certain sur-
face water ecosystems (e.g., Psenner,
1999
). Water column effects may strongly control
metal-ligand interactions. For example, Maloney et al. (
2005
) showed that reoxygenation
of Fe-rich anoxic lake water can increase CDOM absorption within UV regions as Fe(II) is
oxidized to Fe(III). DOM ligand complexation may lead to greater metal solubility in sea-
water than expected based on ionic composition alone (Liu and Millero,
2002
). In addition,
the micronutrient metals availability (e.g. Fe) and even macronutrients (e.g., P) can be
strongly affected by metal-ligand complexes (Maranger and Pullin,
2002
), some of which
are highly photoreactive (e.g., Francko and Heath,
1982
). Similarly, photochemistry can
change metal solubility in seawater (Liu and Millero,
2002
). Thus, ligand-metal binding is
of clear biogeochemical importance.
Ligand complexation with metals will strongly affect DOM fluorescence owing to the
static quenching induced by the formation of a coordination complex (ligands, L, around
a metal, M). Collisional quenching is not as important for metals if their concentrations
are sufficiently dilute (Lakowicz,
2006
), though as seen earlier, increasing temperature
can increase the importance of collisional quenching. Fluorescence quenching titration
can be used to determine the stability of the metal-organic ligand complex. Metal salts are
added to a natural water sample containing native or extracted DOM (e.g., humic or fulvic
acids from natural waters, soils, or sediments) at a constant ionic strength, though ionic
strength may be modified to assess any secondary effects on complexation. The fluores-
cence intensity (EEM, SF, or emission spectra) is recorded during stepwise addition of the
metal salt (e.g.,
Figure 7.3B
; Saar and Weber
1980
). Any residual fluorescence must result
from fluorophores that are not quenched over the concentration range of the metal added.
Rayleigh scattering can be monitored to gauge the formation of precipitates as complexa-
tion proceeds (Ryan and Weber,
1982
).
In contrast to systematically varied pH effects on DOM fluorescence where effects are
most marked at the extremes, metal-ligand complexes that quench fluorescence intensity
do not appear to alter general spectral shape. Early studies utilized fluorescence to exam-
ine metal-ligand interactions because it is a direct measure of free ligand concentration
remaining in solution after introduction of metals (e.g., Levesque,
1972
). This methodol-
ogy was refined and compared to other quenching estimators (e.g., ion selective electrodes;
Saar and Weber,
1980
) which revealed fluorescence loss was directly related to the bound
metal and not free metal in solution. Eventually, fluorescence quenching titrations lead to
complexing capacity calculations and ligand stability constants (cf. Ryan and Weber
1982
).
Humic and fulvic acids, being the most abundant DOM in natural waters, were utilized in
these and many subsequent studies. Many metals ranging in environmental relevance have
been studied, with particular emphasis on divalent Cu, Hg, Pb, Fe, Mn, Ni, and trivalent
Al. Of these, only Al(III) can both quench and enhance fluorescence - regulated by vary-
ing solution pH. Mg(II) and Ca(II) have also been shown to enhance DOM fluorescence.
The enhancement appears to result from a newly fluorescent complex formed between the
metal and the ligand.
