Environmental Engineering Reference
In-Depth Information
TiO
2
, which plays a key role in the heterogeneous photocatalytic degradation
of organic contaminants (Sun and Bolton
1996
; Ullah et al.
1998
; Konstantinou
and Albanis
2004
). However, an important difference between TiO
2
photo-
catalysis and the other processes of HO
•
generation described before is that
the irradiation of TiO
2
mainly causes the production of surface-bound HO
•
•
groups, which are somewhat less reactive than homogeneous HO
(Serpone
and Pelizzetti
1989
). The hydroxyl radical has been detected in rainwater,
dew, cloud and fog (Arakaki et al.
1998
,
1999a
,
b
; Arakaki and Faust
1998
;
Nakatani et al.
2001
; Kobayashi et al.
2002
), snow (Chu and Anastasio
2005
;
Anastasio et al.
2007
; Matykiewiczová et al.
2007
), aerosols (Anastasio and
Jordan
2004
), in aqueous extracts of cigarette tar (Zang et al.
1995
), and in liv-
ing organisms (Buettner et al.
1978
; Buettner
1987
; Miller et al.
1990
; Buettner
and Jurkiewicz
1996
; Cadet et al.
1999
; Bourdat et al.
2000
; Paradies et al.
2000
; Blokhina et al.
2003
; Li et al.
2008
). The HO
•
is rapidly consumed in
natural waters by the subsequent reactions with dissolved organic compounds
(Schuchmann and von Sonntag
1979
; Neta et al.
1988
; Westerhoff et al.
1999
;
Goldstone et al.
2002
; Miller and Chin
2002
; Miller et al.
2002
; Ervens et al.
2003
) and several inorganic species (Zafiriou et al.
1984
,
1987
; Brezonik and
Fulkerson-Brekken
1998
; Neta et al.
1988
; Song et al.
1996
).
The generation of HO
•
and its interaction with the dynamics of DOM and
nutrient as well as with aquatic organisms are very important in natural waters.
There are a number of factors that can control the production and consumption of
HO
•
•
in that ecosystem. However, there is no general overview published on HO
in natural waters. A short review by von Sonntag (
2007
) covers the formation of
free radicals and their reactions in aqueous solution.
This review will provide a general overview on sources, production mecha-
nisms, steady state concentration and biogeochemical functions of HO
•
in water
environment. This paper also discusses the analytical methods that can be adopted
to measure the photoinduced generation of HO
•
, the factors controlling its produc-
tion and decay, as well as the significance and impact of HO
•
in the aquatic eco-
•
systems. It is shown how the production of HO
differs among DOM components,
as well as between freshwaters and marine environments.
2 Hydroxyl Radical (HO
•
) and Other Free Radical Species
•
The hydroxyl radical (HO
) is the most powerful oxidizing agent among the
photolytically generated ones. It is a short-lived, highly reactive and non-
selective transient, able to oxidize dissolved organic substances and other
chemical species in natural waters. The oxidation potentials for a series of com-
mon oxidants in surface waters is as follows: Fluorine (E
=
3.03 V) > HO
•
(2.80 V) > Atomic oxygen (2.42 V) > Ozone (2.07 V) > Peracetic acid (ROOH)
(1.80 V) > H
2
O
2
(1.78 V) > Perhydroxyl radical (1.70 V) > Potassium permanga-
nate (1.68 V) > Chlorine dioxide (1.57 V) > Hypochlorous acid (1.49 V) > Chlorine
