Environmental Engineering Reference
In-Depth Information
2.1 Sources of HO
•
in Natural Waters
•
The HO
radical is formed photolytically from various sources in natural waters. In
rivers, contributions to HO
•
photoproduction are 1-89 % from NO
2
-
, 2-70 % from
NO
3
-
, 1-50 % from H
2
O
2
, and 2-70 % from the photo-Fenton reaction and/or irra-
diated CDOM (Takeda et al.
2004
; Vione et al.
2006
; White et al.
2003
; Page et al.
2011
; Nakatani et al.
2007
; Mostofa KMG and Sakugawa H, unpublished data).
Experimental studies show that DOM isolates from rivers may contribute up to 50 %
of the hydroxylation through production of H
2
O
2
(Page et al.
2011
). The results dem-
onstrate that NO
2
-
is a key contributor (48-80 %) for HO
•
production in sewerage-
polluted river waters, but NO
3
-
can be a major contributor (16-49 %) in clean river
waters. In seawater the major sources of HO
•
are 7-75 % from NO
2
-
, 1-8 % from
NO
3
-
, 0-1 % from H
2
O
2
, and 24-93 % from unknown sources. These data were
obtained from a study carried out in Seto Inland and the Yellow Sea (Takeda et al.
2004
). The formation of HO
•
from different sources in natural waters can be distin-
guished as: (i) the photolysis of nitrite and nitrate in the aqueous solution (Mopper
and Zhou
1990
; Takeda et al.
2004
; Zepp et al.
1987
); (ii) the irradiation of CDOM
components via formation of H
2
O
2
in the aqueous solution. In this case the pro-
duction of HO
•
depends on the nature of the CDOM components (Fig.
1
) (White
et al.
2003
; Mostofa and Sakugawa
2009
; Mostofa KMG and Sakugawa H,
unpublished data), but a useful correlation has been found between the forma-
tion rate of HO
•
and the content of dissolved organic carbon in different lake
water samples (Vione et al.
2006
); (iii) the Fenton reaction (Fenton
1894
; Walling
1975
; Kang et al.
2002
), the photo-Fenton reaction (Zepp et al.
1992
; Arakaki
et al.
1998
; Southworth and Voelker
2003
) as well as the photo-ferrioxalate/H
2
O
2
system in natural waters (Southworth and Voelker
2003
; Safazadeh-Amiri et al.
1997
;
Hislop and Bolton
1999
); (iv) the direct photolysis of hydrogen peroxide, i.e. UV/
H
2
O
2
processes in aqueous solution (Draper and Crosby
1981
; Wang et al.
2001
). The
UV irradiation of natural waters can produce H
2
O
2
that further yields HO
•
(Gjessing
and Källqvist
1991
; Cooper et al.
1996
); (v) the reaction of hydroperoxide radical
(
HO
2
•
)
with
NO
(
HO
2
•
+
NO
→
HO
•
+
NO
2
)
(Sakugawa et al.
1990
); (viii) the
photolysis of dimeric
4
+
species in aqueous solution (Langford
and Carey
1975
); (vii) the photolysis of Fe
III
(OH)
2
+
in aqueous solution. The gen-
eration of HO
Fe
2
(
OH
)
2
(
H
2
O
)
8
upon photolysis of Fe
III
(OH)
2
+
is very efficient (quantum yield ~0.2),
but the Fe(III) hydroxocomplex is present in significant concentration only at strongly
acidic pH values that have little environmental significance(Jeong and Yoon
2005
;
Pozdnyakov et al.
2000
); (viii) the generation of singlet states of oxygen atoms (
1
O
1
)
by ozonolysis, followed by reaction with H
2
O to form HO
•
•
(Hoigné and Bader
1978
,
1979
; Staehelin and Hoigné
1985
; Takahashi et al.
1995
); (ix) the reaction of O
3
with
H
2
O
2
(peroxone process), which generates HO
•
•
(H
2
O
2
+
2O
3
→
2HO
+
3O
2
)
•
(Hoigné
1998
); (x) the production of HO
by auto-oxidation of cytotoxic agents
(Cohen and Heikkila
1974
); (xi) chemical effects of ultrasound, which can generate
HO
•
in aqueous solution (Makino et al.
1983
); (xii) ultrasound-induced cavitation
in aqueous solution, yielding HO
•
•
upon water splitting (H
2
O
+
ultrasound
→
HO
,
