Environmental Engineering Reference
In-Depth Information
peroxide radicals with organic substances in aqueous solution (Mill et al.
1980
;
Mageli and Kolczynski
1966
; Faust and Hoigne
1987
; Blough
1988
; Kieber and
Blough
1990
; Sakugawa et al.
1990
; Faust and Allen
1992
; Mostofa and Sakugawa
2009
). Furthermore, the thermal decomposition of organic peroxides can initiate
the polymerization of vinyl monomers or induce cross-linking of a polymeric sub-
strate upon formation of free radical sites on the polymer (Mageli and Kolczynski
1966
). The overall (unspeciated) photostationary-state concentration of peroxyl
radicals in sunlit cloud and fog waters is around 1-30 nM (Faust and Allen
1992
).
The superoxide radical anion (O
2
•
-
) is the one-electron reduction product
of molecular oxygen. It is an early photoinduced and short-lived intermediate
that is formed in chemical reactions occurring in natural waters, where oxy-
gen acts as the ultimate electron acceptor (Jeong and Yoon
2005
; Bielski et al.
1985
; Petasne and Zika
1987
; Zafiriou
1990
; Micinski et al.
1993
; Zafiriou et al.
1998
; Millington and Maurdev
2004
). It has been shown that the photoinduced
superoxide production rates are 0.1-6.0 nM min
-1
under full-sun irradiation in
spring, and 0.2-8.0 nM min
-1
in fall in a variety of Eastern Caribbean waters
(Micinski et al.
1993
). A key reaction of O
2
•
-
is the production of H
2
O
2
by dis-
mutation; hydrogen peroxide is then able to generate HO
•
by direct photolysis or
upon photo-Fenton type reactions in sunlit aqueous solutions (Cooper et al.
1988
;
Micinski et al.
1993
; Fischer et al.
1985
). Interestingly, the organic complexes of
Cu as well as the copper-catalyzed dismutation (involving Cu
+
and Cu
2
+
) can be
significant sinks of photoproduced O
2
•
-
in seawater (Zafiriou et al.
1998
; Voelker
et al.
2000
).
The carbon dioxide radical anion (CO
2
•
-
) is a short-lived and highly reac-
tive intermediate that is photolytically formed in the ferrioxalate reaction sys-
tem. It is a strong oxidizing agent that is able to oxidize metals and other
chemical species in aqueous solution. The CO
2
•
-
is formed photolytically (C
2
O
4
•
-
•
−
+
CO
2
;
k
=
2
×
10
6
s
-1
) upon decomposition of the oxalyl radical
anion (C
2
O
4
→
CO
2
•
-
). The latter is produced by the photoinduced decomposition of the
highly photosensitive ferrioxalate complex [Fe(C
2
O
4
)
3
]
3-
in aqueous solution
(Hislop and Bolton
1999
; Jeong and Yoon
2004
,
2005
; Mulazzani et al.
1986
). A
key role played by CO
2
•
-
is its ability to oxidize the metal ions, therefore affecting
the biogeochemical cycling of metal-containing species. These processes can have
an impact on the generation of HO
•
and of the superoxide radical anion in natural
waters (Hislop and Bolton
1999
; Jeong and Yoon
2004
,
2005
; Wang et al.
2010
).
Another potentially important process is the transformation of organic substances
induced by CO
2
•
-
, which is formed photolytically from ferrioxalate complexes in
the aqueous solution (Huston and Pignatello
1996
).
In addition, it has been reported that quinones photolytically produce species capa-
ble of hydroxylation (Alegria et al.
1997
; Pochon et al.
2002
; Gan et al.
2008
; Maurino
et al.
2008
; Maddigapu et al.
2010
; Page et al.
2011
). Some of these quinone-derived
hydroxylating species exhibit reactivity that is around one order of magnitude lower
than free HO
•
(Pochon et al.
2002
; Gan et al.
2008
). It is hypothesized that quinone-
derived hydroxylating species may contribute at least in part to the photoinduced HO
•
production by DOM (Vaughn and Blough
1998
; Page et al.
2011
).
