Environmental Engineering Reference
In-Depth Information
3.7 Photoinduced Generation of HO
•
from Algae
Recent studies demonstrate that chlorophyll
a
and algae can photolytically produce
HO
•
in natural waters (Li et al.
2008
; Dykens et al.
1992
; Oda et al.
1992
). It is
hypothesized that HO
•
is produced by H
2
O
2
(Eq.
3.2
), which is initially formed
in irradiated aqueous suspensions of algae. The photo formation of H
2
O
2
from
algae is a well-known phenomenon in natural waters (Lehninger
1970
; Zepp et al.
1987
; Collen et al.
1995
), which has been explained in detail in the second chap-
ter (see chapter
“
Photoinduced and Microbial Generation of Hydrogen Peroxide
and Organic Peroxides in Natural Waters
”). As found by in vitro studies carried out
in aerated water-acetone mixtures, the generation of H
2
O
2
proceeds through the
photo-formation of superoxide ion by chlorophyll
a
(You and Fong
1986
). It has
been shown that the HO
•
photoproduction increases with increasing algal concentra-
tion and irradiation time (Li et al.
2008
). It has also been shown that for the marine
microalga
Dunaliella salina
, the HO
•
production is significantly enhanced by the
addition of Pb(II), or by Pb(II) and methylmercury. However, the HO
•
production is
decreased by addition of methylmercury only, suggesting a complex effect of metal
pollution on the HO
•
production from algae suspensions (Li et al.
2008
).
4 Factors Controlling the Production and Decay of HO
•
in Natural Waters
in irradiated natural waters are NO
2
-
, NO
3
-
, H
2
O
2
, photo-
Fenton reaction (which depends on the concentrations of Fe and H
2
O
2
), humic
substances (fulvic and humic acids), tryptophan amino acid, autochthonous and
even unknown DOM components as well as anthropogenic DOM such as com-
ponents of household detergents or fluorescent whitening agents (FWAs) such as
(DAS1) and distyryl biphenyl (DSBP) (Table
2
). The concentration values and,
in the case of DOM, also the nature of these species are very variable among
rivers, lakes and oceans, which significantly affects the HO
•
Major sources of HO
•
sources in natu-
ral waters (Takeda et al.
2004
; White et al.
2003
; Nakatani et al.
2007
; Mostofa
KMG and Sakugawa H, unpublished data; Mostofa et al.
2005
; Zika et al.
1993
).
Interestingly, the concentration values of NO
2
-
and NO
3
-
in surface seawaters are
lowered by solar irradiation that induces transformation reactions. This is a key
factor that limits the role of nitrite and nitrate as HO
•
sources in the surface sea-
water layer. Unknown or little known photoinduced HO
•
sources in natural waters
are still poorly characterized DOM components such as humic substances (fulvic
and humic acid), amino acids, proteins, components of detergents or FWAs, as
well as their daughter photoproducts, unidentified organic compounds and photo-
Fenton reactions. Therefore, the production of HO
•
in natural waters depends on
several factors, which can be listed as: (1) Wavelength spectrum of solar radiation;
(2) Nature and contents of DOM; (3) Consumption of HO
•
by DOM and other
