Geoscience Reference
In-Depth Information
7
Physicochemical Effects on Dissolved Organic
Matter Fluorescence in Natural Waters
Christopher L. Osburn, Rossana Del Vecchio,
and Thomas J. Boyd
7.1 Introduction
Dissolved organic matter (DOM) fluorescence properties are strongly affected by physi-
cochemical properties of natural waters - increasing, decreasing, or altering DOM fluo-
rescence characteristics through a variety of mechanisms. Natural water physicochemical
properties are often interdependent at many levels, and separating their impacts on DOM
fluorescence is difficult. Natural water ionic strength varies widely due to changing con-
centration of dissolved solids from freshwater (<1000 mg L
-1
) to seawater (avg. 35,000 mg
L
-1
) to saline lakes and brines (>50,000 mg L
-1
), and much of these dissolved solids are
metal salts which contribute to solution alkalinity. In estuarine environments, freshwater
DOM in rivers mixes with seawater, dramatically changing the physicochemical environ-
ment in which freshwater DOM may be flocculated (Fox,
1983
; Sholkovitz et al.,
1976
;
Spencer et al.,
2007a
). However, such flocculation is not always observed and likely varies
with the chemical composition, perhaps DOM humic content or dissolved metals con-
centration (Mantoura and Woodward,
1983
; Spencer et al.,
2007a
) as much as the overall
chemical milieu in which these components are mixed. Layered “on top” of these inter-
active chemistries are physical effects such as mixing and sunlight. Similarly, wastewater
treatment streams, rich in DOM, can have very high ionic strengths. In all of these diverse
environments, pH and the concentration of metals can vary widely, and metal-ligand com-
plexation reactions can strongly affect DOM fluorescence. To study these effects experi-
mentally, researchers often perturb one variable (e.g., lower or raise pH) and observe the
resulting effects on DOM fluorescence.
Many reactions between fluorophores and their environment will reduce DOM fluo-
rescence and are termed
quenching reactions
. Fluorescence is related primarily to various
chemical components constituting DOM, which is reflects DOM sources and molecular
size and mass. Colloidal organic matter (COM), fractionated by a variety of ultrafiltration
and flow fractionation methods, shows distinct patterns in fluorescence, but these patterns
are not consistent among freshwater and seawater sources. In freshwater environments pro-
tein fluorescence is often found in the highest molecular size fractions, whereas in seawater
environments, fluorescence is often found in lower molecular size fractions.
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