175
Photoinduced and Microbial Generation of Hydrogen Peroxide
Hong et al. 1987; Bazanov et al. 1999). The highest photoinduced activity has
been reported for porphyrin and phthalocyanine complexes with metals such as
Mg, Zn, Al, and Cd (Komissarov 2003; Vedeneeva et al. 2005), which can typically produce long-lived triplet excited states (lifetimes up to 1 ms) with a high
quantum yield (60–90 %) (Parmon 1985). Photosynthetically produced organic
matter (e.g. algae) can enhance the production of H 2 O 2 by natural sunlight in
aquatic ecosystems (Zepp et al. 1987). It can be hypothesized that the photoinduced and microbial assimilation of algae produce autochthonous fulvic acid
and other DOM components (Mostofa et al. 2009b; Fu et al. 2010; Mostofa et
al. (Manuscript In preparation), which may induce H 2 O 2 photoproduction by the
pathways already described for DOM.
In natural waters, ROOH compounds are formed upon photodegradation of
DOM (including both CDOM and FDOM) via pathways that also induce the production of H 2 O 2 (Mostofa and Sakugawa 2009; Sakugawa et al. 1990; Faust and
Hoigne 1987; Perkowski et al. 2006). A generalized chain-reaction scheme for
the formation of ROOH from DOM in natural waters can be depicted as follows
(Eqs. 3.19–3.24):
First, the photodecomposition of H 2 O 2 generates the hydroxyl radical, HO
•
(Eq. 3.19), which subsequently oxidizes DOM or DOM
•+ (the latter is formed by
3 DOM* and O 2 , see Eq. 3.20) to form the organic radical R
• (Eq. 3.20) (Mostofa
and Sakugawa 2009). Afterwards, R
• reacts with O 2 to form the organo peroxide
radical RO 2
• (Eq. 3.21). The reduction of RO 2
• , e.g. by O 2
•− , can form ROOH in
natural waters (Eq. 3.22) whereas O 2
•− is formed using (Eq. 3.14). Organic radicals (R
• and RO 2
•
) can rapidly associate with one another (Eq. 3.23), and organo
peroxide radicals can combine (Eq. 3.24) to terminate the chain reactions. The termination reactions (Eqs. 3.23, 3.24) are competitive with (3.21, 3.22), which leads
to complicated reaction kinetics (Perkowski et al. 2006).
Oxidation–reduction of transition metal ions is an important pathway for the formation of organic peroxides in natural waters. A general mechanistic scheme for these
oxidation–reduction chain reactions (Eqs. 3.25–3.27) can be expressed as follows:
First, oxidation of the metal ions (M n+ ) forms the superoxide radical anion
(O 2
•−
) (Eq. 3.25). O 2
•− then combines with H + or with an alkyl ion (R + =H + ,
positively charged alkyl group, etc.) to form an hydro-peroxide or organo-peroxide
radical (RO 2
•
, R = H or alkyl group, Eq. 3.26). RO 2
• can then associate with H +
or a metal ion (M (n+1)+ ), to form ROOH (where R = H or an alkyl group) and a
(3.19)
H 2 O 2 + hν → 2HO
•
(3.20)
DOM
•+ + HO
• → R
• + H
•
(3.21)
R
• + O 2 → RO 2
•
(3.22)
RO 2
• + O 2
•− + H
+ → ROOH + O 2
(3.23)
RO 2
• + R
• → ROOR
(3.24)
RO 2
• + RO 2
• → ROOR + O 2
Photoinduced and Microbial Generation of Hydrogen Peroxide
Hong et al. 1987; Bazanov et al. 1999). The highest photoinduced activity has
been reported for porphyrin and phthalocyanine complexes with metals such as
Mg, Zn, Al, and Cd (Komissarov 2003; Vedeneeva et al. 2005), which can typically produce long-lived triplet excited states (lifetimes up to 1 ms) with a high
quantum yield (60–90 %) (Parmon 1985). Photosynthetically produced organic
matter (e.g. algae) can enhance the production of H 2 O 2 by natural sunlight in
aquatic ecosystems (Zepp et al. 1987). It can be hypothesized that the photoinduced and microbial assimilation of algae produce autochthonous fulvic acid
and other DOM components (Mostofa et al. 2009b; Fu et al. 2010; Mostofa et
al. (Manuscript In preparation), which may induce H 2 O 2 photoproduction by the
pathways already described for DOM.
In natural waters, ROOH compounds are formed upon photodegradation of
DOM (including both CDOM and FDOM) via pathways that also induce the production of H 2 O 2 (Mostofa and Sakugawa 2009; Sakugawa et al. 1990; Faust and
Hoigne 1987; Perkowski et al. 2006). A generalized chain-reaction scheme for
the formation of ROOH from DOM in natural waters can be depicted as follows
(Eqs. 3.19–3.24):
First, the photodecomposition of H 2 O 2 generates the hydroxyl radical, HO
•
(Eq. 3.19), which subsequently oxidizes DOM or DOM
•+ (the latter is formed by
3 DOM* and O 2 , see Eq. 3.20) to form the organic radical R
• (Eq. 3.20) (Mostofa
and Sakugawa 2009). Afterwards, R
• reacts with O 2 to form the organo peroxide
radical RO 2
• (Eq. 3.21). The reduction of RO 2
• , e.g. by O 2
•− , can form ROOH in
natural waters (Eq. 3.22) whereas O 2
•− is formed using (Eq. 3.14). Organic radicals (R
• and RO 2
•
) can rapidly associate with one another (Eq. 3.23), and organo
peroxide radicals can combine (Eq. 3.24) to terminate the chain reactions. The termination reactions (Eqs. 3.23, 3.24) are competitive with (3.21, 3.22), which leads
to complicated reaction kinetics (Perkowski et al. 2006).
Oxidation–reduction of transition metal ions is an important pathway for the formation of organic peroxides in natural waters. A general mechanistic scheme for these
oxidation–reduction chain reactions (Eqs. 3.25–3.27) can be expressed as follows:
First, oxidation of the metal ions (M n+ ) forms the superoxide radical anion
(O 2
•−
) (Eq. 3.25). O 2
•− then combines with H + or with an alkyl ion (R + =H + ,
positively charged alkyl group, etc.) to form an hydro-peroxide or organo-peroxide
radical (RO 2
•
, R = H or alkyl group, Eq. 3.26). RO 2
• can then associate with H +
or a metal ion (M (n+1)+ ), to form ROOH (where R = H or an alkyl group) and a
(3.19)
H 2 O 2 + hν → 2HO
•
(3.20)
DOM
•+ + HO
• → R
• + H
•
(3.21)
R
• + O 2 → RO 2
•
(3.22)
RO 2
• + O 2
•− + H
+ → ROOH + O 2
(3.23)
RO 2
• + R
• → ROOR
(3.24)
RO 2
• + RO 2
• → ROOR + O 2