Environmental Engineering Reference
In-Depth Information
DOM components, only a limited fraction of organic compounds show fluores-
cence properties. These compounds are termed the fluorescent DOM (FDOM).
In the pioneering works conducted by Kalle (
1949
,
1963
) and Duursma (
1974
),
the fluorescence of terrestrial humic substances served as a tracer of soil organic
matter in freshwater and seawater environments. The fluorescence of humic sub-
stances has then been used to distinguish the mixing of river water with seawater
as well as their sources (Otto
1967
; Zimmerman and Rommets
1974
; Dorsch and
Bidleman
1982
; Willey
1984
; Hayase et al.
1987
).
A number of excitation-emission (Ex/Em) maxima are detected for humic sub-
stances depending on their sources and nature (either fulvic acid or humic acid) in
soil and natural waters (Christman and Ghassemi
1966
; Ghassemi and Christman
1968
; Levesque
1972
; Almgren et al.
1975
; Brun and Milburn
1977
; Gosh and
Schnitzer
1980
; Momzikoff et al.
1992
). The effects of the molecular weight of ful-
vic and humic acids (<10,000 to >300,000 Da) on fluorescence properties have been
examined in earlier studies, particularly in lignin sulfonates (Christman and Minear
1967
), soil fulvic and humic acids (Levesque
1972
; McCreary and Snoeyink
1980
),
natural waters (Hall and Lee
1974
; Stewart and Wetzel
1980
; Visser
1984
) and sed-
iment pore waters (Hayase and Tsubota
1985
,
1983
). The results suggest that the
Ex/Em wavelength maxima for humic acid are often present at longer wavelengths
than those of fulvic acid. The position of these maxima is independent of the molec-
ular weight, whilst smaller molecules in terrestrial fulvic and humic acids typically
exhibit higher fluorescence intensity than the larger ones. It is also suggested that
the fluorophores in humic acid are aromatic compounds with higher molecular
weight compared to those in fulvic acid (Hayase and Tsubota
1985
). The fluores-
cence quantum yields of commercial, soil and aquatic humic substances excited at
350 nm have been determined by Zepp and Scholtzhauer (
1981
). Linear correlations
are observed between pH and the fluorescence intensity at the Ex/Em peaks of ful-
vic and humic acids in natural waters and in microbial cultures (Visser
1984
). The
fluorescence characteristics are different between coastal marine sedimentary humic
and fulvic acids (Hayase and Tsubota
1985
). The vertical distribution of humic-like
fluorescent substances has been examined in marine waters (Hayase et al.
1987
,
1988
; Chen and Bada
1989
,
1990
,
1992
; Hayase and Shinozuka
1995
). The fluores-
cence intensity of humic-like substances is correlated with phosphate and nitrate in
the deeper marine waters, suggesting that the production of humic-like substances
and nutrients (phosphate and nitrate) results from the decomposition of settling par-
ticles in the water column (Hayase and Shinozuka
1995
). Senesi (
1990a
) summa-
rized the fluorescence properties of fulvic acid-like components in freshwater and
seawater. All of these studies are two-dimensional and do not distinguish well the
fluorescence excitation-emission (Ex/Em) peak positions that can be used for the
characterization of the fluorophores of humic substances.
Coble et al. (
1990
) have firstly applied the three-dimensional fluorescence (exci-
tation-emission matrix) spectroscopy (EEMS) to marine FDOM to distinguish
between the humic-like and protein-like fluorescence peaks in seawater. Coble
(
1996
) summarized the various fluorescence peaks that can be distinguished in river,
lake and marine FDOM. They identified the Ex/Em wavelengths for humic-like
