Environmental Engineering Reference
In-Depth Information
chemical species; (4) Nitrite and nitrate: Effect of wavelength spectrum, tempera-
ture, and pH; (5) H
2
O
2
: Effect of wavelength spectrum and temperature; (6) Fenton
reaction: Effect of pH, temperature and salinity; (7) Photo-Fenton reaction; and
(8) Photo-ferrioxalate/H
2
O
2
reaction: Dependence of pH and reactants. These
issues will be discussed below.
4.1 Wavelength Spectrum of Solar Radiation
•
The production of HO
significantly depends on the spectrum of the inci-
dent radiation in the ultraviolet (UVC
=
200-280 nm; UVB
=
280-320 nm;
UVA
=
320-400 nm) and visible (400-700 nm) regions. Note that UVC radiat-
ion is not present in the sunlight spectrum in the troposphere. The maximum HO
•
photo-production has been observed upon UVB irradiation of three DOM frac-
tions isolated from lake water, and it decreased at higher wavelengths (Grannas
et al.
2006
). A rough estimation shows that the HO
•
production is approximately
191-247 % and 103-178 % higher at 308 nm than at 355 and 330 nm, respec-
tively. The photoproduction of HO
•
by nitrate is only induced by UVB radia-
•
tion, while the production of HO
by nitrite and DOM can take place under both
UVB and UVA. Note that UVA radiation is able to penetrate more deeply than
UVB into the water bodies (Mopper and Zhou
1990
; Zafiriou and Bonneau
1987
;
Zellner et al.
1990
; Qian et al.
2001
). H
2
O
2
generates HO
•
(Eq.
3.2
) mostly by
non-environmental UVC with quantum yield around 0.5, but the radiation absorp-
tion by H
2
O
2
is extended to the UVB region and the photolysis quantum yield
is expected to be similar (Legrini et al.
1993
; von Sonntag et al.
1993
). During
non-ozone hole conditions, HO
•
formation from nitrate contributed ~33 % to the
•
total HO
photoinduced generation in open oceanic surface waters, while the role
of DOM plus nitrite was ~67 % (Qian et al.
2001
). During an ozone hole event
(e.g., 151 Dobson units) the corresponding results were ~40 and 60 % for nitrate
and DOM plus nitrite, respectively, because of an increase in UVB irradiance that
enhanced the photolysis of nitrate. A model estimation of HO
•
photoinduced gen-
eration in Antarctic seawater during an ozone hole (151 Dobson units) shows that
the production is enhanced by at least 20 %, mostly from nitrate photolysis and
to a minor extent from DOM photoinduced reactions (Qian et al.
2001
). Similar
results have been obtained for Arctic water (Rex et al.
1997
; Randall et al.
2005
).
4.2 Nature and Amount of DOM Components
•
The radical HO
is generated photolytically from various organic substances in
natural waters (Table
2
) (Mill et al.
1980
; Mopper and Zhou
1990
; Vaughn and
Blough
1998
; Holder-Sandvik et al.
2000
; Page et al.
2011
; Grannas et al.
2006
).
The most common dissolved organic compounds are humic substances (fulvic and
