Environmental Engineering Reference
In-Depth Information
The reactive transient
1
O
2
is also formed from superoxide anion (O
2
−
) in the
following process (
3.28
) (Koppenol
1976
):
O
2
−
→
1
O
2
+
E
−
WHERE
E
0
=+
0. 65 V
(3.28)
In addition, any sensitizer (e.g. photoactive organic matter) can photolytically
produce
1
O
2
via the following processes (Eqs.
3.29
,
3.30
) (Braun and Oliveros
1990
):
SENS
+
H
υ →
1
SENS
∗
→
3
SENS
∗
(3.29)
3
SENS
∗
+
O
2
→
SENS
+
1
O
2
(3.30)
where Sens is the sensitizer that can absorb photons and is promoted to the singlet
excited state
1
Sens
∗
.
The latter can undergo intersystem crossing (ISC) and be
converted into the triplet state
3
Sens
∗
(Eq.
3.29
), which can react with O
2
to pro-
duce
1
O
2
(Eq.
3.30
).
On the other hand, deactivation of
1
O
2
involves two major processes such as
energy-transfer quenching and charge-ransfer quenching,through any acceptor or
sensitizer (Eqs.
3.31
,
3.32
) (Braun and Oliveros
1990
; Halliwell and Gutteridge
2007
):
1
O
2
+
Sens
→
SensO
2
(3.31)
(3.32)
SensO
2
→
O
2
+
Sens
The H
2
O
2
concentration in plant cells is approximately 0.5-1
μ
mol per milli-
gram of Chl, including Chl of photosynthetic antennae (Lyubimov and Zastrizhnaya
1992a
). Therefore, the amount of H
2
O
2
is much higher than the Chl content in the
composition of so called oxygen-evolving complexes in chloroplasts (Lobanov
et al.
2008
). Experimental studies have shown that the content of H
2
O
2
can increase
during ontogenesis of both the whole plant and populations of protoplasts of sepa-
rate leaves in the dark, and the light-dependent component of peroxide formation
increases regardless of the metabolic type of the plant antennae (Lyubimov and
Zastrizhnaya
1992b
). It is known that each molecule of the chlorophyll absorbs
light quanta ~1 time per second, even at the maximum intensity of daylight
(Komissarov
2003
). Synthetic Chl, metal complexes of porphyrins and phthalocya-
nines are photoactive and can produce H
2
O
2
under irradiation in aqueous solutions
saturated with O
2
(Lobanov et al.
2008
; Hong et al.
1987
; Bazanov et al.
1999
;
Premkumar and Ramaraj
1999
).
Lower volatility of H
2
O
2
compared to H
2
O may cause the green leaves to
be a unique concentrator of H
2
O
2
(Komissarov
2003
). It is shown that the heat
of vapour formation of pure H
2
O
2
is 12.3 kcal mole
−
1
, whilst that of water is
10.5 kcal mole
−
1
(Shamb et al.
1958
). Transpiration (evaporation of water by
plants) may evidently play the same function of H
2
O
2
concentrator in addition
to the protection of plants against overheating. For each kg of water, absorbed by
the roots from soil, only 1 g is used by the plant for the construction of tissue.
Therefore, the transpiration process may enhance the total contents of H
2
O
2
in the
