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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-
oxide (H 2 O 2 ) (see detailed description in chapter Photoinduced and Microbial
Generation of Hydrogen Peroxide and Organic Peroxides in Natural Waters )
(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 ;
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