Environmental Engineering Reference
In-Depth Information
the key factors that may significantly affect the photoinduced and microbial activities in
waters. Finally, this study summarizes the various photoproducts of DOM and depicts
their importance on biogeochemical cycles in the aquatic environments.
1.1 Biogeochemical Functions of DOM for Photoinduced
Processes in Natural Waters
Photoinduced reactions caused by solar radiation can produce various biogeo-
chemical alterations of DOM that can be listed as follows (Mostofa et al.
2011
):
(1) Photoinduced generation of free radicals, which are susceptible to induce pho-
toinduced degradation of DOM in aqueous media. The free radicals sources
include: (i) Photolysis of NO
2
−
−
and NO
3
ions inducing production of
•
hydroxyl radical (HO
) in waters (Zafiriou and True
1979a
,
b
; Takeda et al.
2004
; Vione et al.
2006
; Minero et al.
2007
) (ii) Generation of free radical
species such as superoxide ion (O
2
•
−
), hydrogen peroxide (H
2
O
2
), organic
•
peroxides (ROOH) and HO
by photolysis of CDOM or FDOM in waters
(Cooper et al.
1988
; Moore et al.
1993
; O'Sullivan et al.
2005
; Mostofa and
Sakugawa
2009
); (iii) Photoinduced production of HO
•
by the photo-Fenton
(Zepp et al.
1992
; White et al.
2003
) as well as the photo-ferrioxalate/H
2
O
2
reactions (Safarzadeh-Amiri et al.
1996
,
1997
; Southworth and Voelker
2003
).
(2) Production of new organic substances by photorespiration or assimilation
of particulate organic matter (POM: ca. algae or phytoplankton) and high
molecular weight DOM. These processes have a deep impact on the car-
bon cycling and include: (i) Photo-respiration or assimilations of algae that
can produce autochthonous fulvic acid and other organic substances as well
as nutrients in the aquatic environments (Mostofa et al.
2009b
; Thomas and
Lara
1995
; Fu et al.
2010
). (ii) Photoinduced degradation of chlorophyll
a
with production of new organic substances; this process is typically occur-
ring in the photic layer of natural lake and seawater (Rontani
2001
; Cuny
et al.
2002
). (iii) Photoinduced assimilation or degradation of algal biomass
in surface waters under natural sunlight, which may produce new DOM or
FDOM species in the aquatic environments (Rochelle-Newall and Fisher
2002
;
Thomas and Lara
1995
; Fu et al.
2010
; Henrichs and Doyle
1986
; Biddanda
and Benner
1997
; Carrillo et al.
2002
; Mostofa KMG et al. unpublished).
(iv) Microbial assimilation or degradation of algal biomass or phytoplank-
ton; in vitro experiments have shown that under dark incubation these pro-
cesses may produce new DOM or FDOM (Rochelle-Newall and Fisher
2002
;
Yamashita and Tanoue
2008
; Fu et al.
2010
; Biddanda and Benner
1997
;
Mostofa KMG et al. unpublished; Yamashita and Tanoue
2004
; Stedmon and
Markager
2005
). (v) Photoinduced transformation of high-molecular weight
DOM into low-molecular weight organic substances; in some cases the pro-
cess can lead to complete mineralization (Moran and Zepp
1997
; Dahlén et al.
1996
; Ma and Green
2004
; Vähätalo and Järvinen
2007
; Vione et al.
2009
).
