Environmental Engineering Reference
In-Depth Information
Skogerboe and Wilson
1981
; Matthiesen
1994
; Struyk and Sposito
2001
). Some
studies also suggest that functional groups such as quinone or quinone-like moie-
ties in fulvic and humic acids are largely responsible for the observed reversible
redox behavior in natural waters (Scott et al.
1998
; Tratnyek and Macalady
1989
;
Schwarzenbach et al.
1990
; Nurmi and GTratnyek
2002
; Cory and McKnight
2005
; Macalady and Walton-Day
2009
). In addition, fulvic and humic acids can
donate electrons photolytically in aqueous media, which can induce the pro-
duction of oxidizing agents such as superoxide ion (O
2
•
−
) and hydrogen per-
(Mostofa and Sakugawa
2009
; Fujiwara et al.
1993
; Baxter and Carey
1983
).
The presence of diverse functional groups in the molecular structure of ful-
vic and humic acids is responsible for their redox behavior in waters. The redox
behavior of humic acids depends on the redox potential of the aqueous solu-
tions as well as on the complexation capacity with multicharged cations in water
(Österberg and Shirshova
1997
; Struyk and Sposito
2001
; Kerndorff and Schnitzer
1980
; Zauzig et al.
1993
).
4.1.2 Definition and Chemical Nature of Allochthonous Fulvic
and Humic Acids
Allochthonous DOM of vascular plant origin is primarily composed of humic
substances (fulvic and humic acids), which are also termed as hydrophobic acids.
Stream fulvic and humic acid are therefore vital to understand the nature of the
allochthonous DOM, because the chemical composition and optical properties of
these substances are greatly altered photolytically and microbially during their
transportation after leaching from soil into rivers, lakes or oceans.
Allochthonous Fulvic Acids
Allochthonous fulvic acids can be defined as molecularly heterogeneous and
supramolecular, with molecular weight ranging from less than 100 to over 300,000
Daltons and with the largest fractions ranging less than 50,000. They are opti-
cally active, typically refractory to microbial degradation, photolytically reac-
tive, biogenic, and yellow-colored. They are also soluble under all pH conditions
in water (Ma and Ali
2009
; Mostofa et al.
2005b
,
2007a
; MacFarlane
1978
; Dai
et al.
1996
; McKnight et al.
1988
,
2001
; Hayase and Tsubota
1983
; Frimmel
2004
;
Aiken et al.
1985
; Aiken and Malcolm
1987
; Aiken and Gillam
1989
; Amador
et al.
1989
; David and Vance
1991
; Allard et al.
1994
; Hummel
1997
; Fimmen
et al.
2007
). Allochthonous fulvic acids in surface waters have relatively low
contents of organic N compared to organic C, i.e. a high C:N ratio. This ratio is
in the range ~45-202, and standard SRFA (1S101F and 2S101F) have values of
73-78. Allochthonous fulvic acids also have relatively high contents of O and
organic P, low contents of S, relatively low aromaticity (17-30 % of total C) and
high aliphatic C (63 %) (Malcolm
1985
; Wetzel
1983
; McKnight et al.
2001
;