Environmental Engineering Reference
In-Depth Information
5 Photoproducts of DOM and Their Significance
on Biogeochemical Cycles in Natural Waters
Photoinduced degradation of DOM in natural waters generally occurs by the
sequential degradation of high molecular weight substances, producing low
molecular weight compounds and ending up in mineralization yielding CO 2 , CO,
DIC, COS, and so on (Table 3 ) (Reche et al. 1999 ; Ma and Green 2004 ; Graneli
et al. 1996 , 1998 ; Xie et al. 2004 ; Mopper et al. 1991 ; Miller and Zepp 1995 ;
Bertilsson and Tranvik 2000 ; Fujiwara et al. 1995 ; Bushaw et al. 1996 ; Miller
and Moran 1997 ; Stiller and Nissenbaum 1999 ; White et al. 2010 ; Valentine and
Zepp 1993 ; Mostofa K et al., unpublished data; Francko and Heath 1982 ; Fang
2004 ; Chen et al. 2001 ; Karl and Tien 1997 ; Jones 1991 ; Jones and Amador 1993 ).
Photoinduced degradation on DOM can typically lead to a variety of photo prod-
ucts, which can be distinguished in: (1) Hydrogen peroxide and organic peroxides;
(2) Low molecular weight organic substances; (3) Aromatic mono- and dibasic
acids; (4) Microbiologically labile organic photoproducts; (5) Carbon-gas end
photoproducts; (6) Nitrogenous compounds (e.g. NH 4
+
); (7) Phosphate; and (8)
Release of energy to the water ecosystem.
5.1 Photoinduced Formation of Hydrogen Peroxide and
Organic Peroxides
The formation of hydrogen peroxide (H 2 O 2 ) and organic peroxides (ROOH) is the
primary step of the photoinduced processes involving DOM in waters. The concen-
tration levels of H 2 O 2 are significantly different for a variety of waters, ranging from
4 to 3200 nM in rivers, 10-800 nM in lakes, and 0-1700 nM in seawaters as men-
tioned in chapter Photoinduced and Microbial Generation of Hydrogen Peroxide
and Organic Peroxides in Natural Waters ”. The concentration levels of ROOH com-
pounds are also highly variable in natural waters, showing low concentrations in
rivers (0-200 nM) and relatively higher levels in seawater (1-389 nM). H 2 O 2 and
ROOH may form free radicals (HO
, R = H or alkyl group), either directly
upon photolysis or indirectly by photo-Fenton reactions. The reactive radicals thus
generated contribute to the degradation of the organic substances that make up DOM.
, RO
5.2 Photoinduced Formation of Low Molecular Size Organic
Substances
Photoinduced degradation can convert the high molecular weight DOM into low
molecular size organic substances in natural waters (Table 3 ) (Moran and Zepp 1997 ;
Biddanda and Benner 1997 ; Kramer et al. 1996 ; Allard et al. 1994 ; Yoshioka et al. 2007 ;
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