Environmental Engineering Reference
In-Depth Information
H
2
O
2
and CO
2
that are simultaneously produced by DOM photodegradation
can take part to photosynthesis, to form carbohydrate-type compounds (Eq.
4.8
)
(Mostofa et al.
2009a
,
b
):
x
CO
2
(
H
2
O)
+
y
H
2
O
2
(
H
2
O)
+
h
υ →
C
x
(
H
2
O
)
y
+
O
2
+
E
(±)
(4.8)
2H
2
O
2
+
photo
(
h
υ)
or biological processes
→
O
2
+
2H
2
O or unknown oxidants
(4.9)
where C
x
(H
2
O)
y
(Eq.
4.8
) represents a generic carbohydrate. In natural waters,
H
2
O
2
acts as a key component together with carbon dioxide (CO
2
) to form carbo-
hydrates and oxygen through photosynthesis (Eq.
4.8
). The formation of oxygen in
the photosynthesis process might also occur via either H
2
O
2
disproportionation or
biological processes (Eq.
4.9
) (Komissarov
2003
; Moffett and Zafiriou
1990
; Liang
et al.
2006
; Buick
2008
). Note that the contribution of H
2
O
2
decay is 65-80 % by
catalase enzyme and 20-35 % by peroxidase enzyme, respectively, as estimated
by isotopic measurements in seawater (Moffett and Zafiriou
1990
). E (
±
) is the
energy produced during photosynthesis. The new concept of photosynthesis was
firstly hypothesized in plants by Komissarov (
1994
,
2003
). He proposed that inter-
action of CO
2
in air and H
2
O
2
, instead of H
2
O, may form carbohydrate in plants. It
is interesting to note that during the diurnal cycle, H
2
O
2
production is the highest
at noon time, thereby simultaneously causing the maximum production of CO
2
or
DIC due to photodegradation by H
2
O
2
or photoinduced generation of HO
•
. The
new reaction mechanim for photosynthesis (Eq.
4.2
) will be discussed in details in
photosynthesis chapter
“
Photosynthesis in Nature: A New Look
”.
It is demonstrated that microbial consumption is the dominant sink of oce-
anic carbon monoxide (CO), and that the rate constant (
k
CO
) of microbial CO
consumption is positively correlated with chlorophyll
a
(Chl
a
). It is suggested
that Chl
a
concentration can be used as an indicator of CO-consuming bacterial
activity in natural waters (Xie et al.
2005
). Photodegradation and photosynthesis
may be important in natural waters with high contents of DOM; photodegradation
induces the production of CO
2
and peroxides, which would in turn favor photo-
sysnthesis in the aquatic environments. This would lead to the multiplication of
algae, small aquatic plants and phytoplankton. For example, high algal production
is operational in some Chinese Lakes during the summer season, which might also
be an effect of high DOM photodegradation that favor photosynthesis in lake eco-
systems (Mostofa et al.
2009b
).
4.6 Production and Decay by Photolytic and Chemical
Processes
Production of H
2
O
2
and ROOH by photolytic processes may involve their pho-
toinduced formation from DOM under natural sunlight, as explained earlier. The
decay of peroxides by photolytic processes (Moffett and Zafiriou
1990
;
1993
;
