Environmental Engineering Reference
In-Depth Information
is incorporated into epoxide products, even though oxygen exchange is observed
between the Mn
IV
catalyst species and
H
2
18
O.
Therefore, one can conclude that
O
2
transfer does not proceed by the well-known oxygen-rebound mechanism (Yin
et al.
2006
). Experiments using labeled dioxygen,
18
O
2
,
and hydrogen peroxide,
H
2
18
O
2
,
confirm that an oxygen atom is transferred directly from the
H
2
18
O
2
oxidant to the olefin substrate in the predominant pathway (Yin et al.
2006
).
Moreover, some recent experiments show that photoinduced H
2
O oxidation occurs
in the presence of inorganic catalysts (Kuznetsov et al.
2010
; Bernardini et al.
2011
). This result does not imply that H
2
O is oxidized, but rather that
O
2
•−
and
then H
2
O
2
are produced photolytically. H
2
O
2
is then photolytically decomposed
into O
2
and H
2
O.
Biological release of O
2
is observed using catalase for the decomposition of
H
2
O
2
in aqueous media, a process that can be depicted as follows (Eqs.
3.22
,
3.23
)
(Moffett and Zafiriou
1990
):
HOOH
+
Catalase
→
HOOH
∗
+
Catalase
#
(3.22)
2HOOH
∗
+
CATALASE
#
→
H
−
O
−
H
+
O
2
+
CATALASE
(3.23)
In the above reactions, catalase enzymatically activates HOOH
*
to use them
as oxidants (electron acceptors) and reductants (electron donors) (Eq.
3.22
).
Afterwards, disproportionation of activated HOOH
*
converts them into H
2
O
and O
2
(Eq.
3.23
). Therefore, H
2
O
2
can release O
2
under both photoinduced and
microbial decomposition processes. The widespread occurrence of such a process
justifies the hypothesis that the release of photosynthetic O
2
may occur from H
2
O
2
instead of H
2
O. Note that the contribution percentage decay of H
2
O
2
is 65-80 %
by catalase enzyme and 20-35 % by peroxidase enzyme, as estimated by isotopic
measurements in seawater (Moffett and Zafiriou
1990
).
Based on the current evidence, it is hypothesized that oxygenic photosynthesis
has evolved by the end of the 'Great Oxidation Event'
ca
. 2.4 Ga ago. It has per-
manently raised atmospheric oxygen above the levels produced by photolysis of
water (Buick
2008
). The latter process can produce primarily H
2
O
2
, which might
be source of oxygenic photosynthesis.
3.2 Effective Oxidation of H
2
O
2
Instead of H
2
O in Releasing
Photosynthetic O
2
The oxidation of water to molecular oxygen is described by the equation
(Rappaport and Diner
2008
): 2H
2
O
→
O
2
+
4H
+
+
4e
−
, where at pH 7.0 the
midpoint potential of the O
2
/2H
2
O couple is 810 mV. Water is a very stable mol-
ecule and its oxidation requires the successive absorption of four photons and their
photoinduced conversion into electrochemical energy. The energy of the quantum
of a visible light is relatively small, such as 1.8 eV at the maximum absorption of
chlorophyll (Komissarov
2003
).
