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K. M. G. Mostofa et al.
decay of H 2 O 2 was second-order overall, and first-order in both H 2 O 2 concentration and number of bacterial cells (Cooper and Lean 1992). Such a kinetic can be
expressed as follows:
where k 2 = 1.6 × 10 −9 mL cell −1 min −1 . The freshwater bacterium Enterobacter
cloaceae showed a similar rate constant, k 2 = 1.5 × 10 −9 mL cell −1 min −1 .
4.4 Production and Decay by DOM Photochemistry
Photodegradation of DOM depends on the incident light intensity, which is
directly linked to the production of H 2 O 2 and ROOH through photoinduced reactions in natural waters (Cooper and Zika 1983; Moore et al. 1993; Baxter and
Carey 1983). For example, H 2 O 2 concentration gradually increases with irradiation time in natural waters as well as in aqueous solutions of standard organic substances (Fig. 3) (Obernosterer et al. 2001; Cooper et al. 1988). Similarly, a 10–20
times lower H 2 O 2 production was observed in river waters during the cold season
compared to summer, and in the Southern Ocean in Antarctic regions (5–25 nM)
compared to other oceanic environments, respectively (Fig. 9; Table 1). The photodegradation of DOM is greatly influenced by the stratospheric ozone hole events,
particularly in Antarctic waters. The ozone hole can increase the fluxes of solar
ultraviolet radiation (UVR, 280–400 nm), which may substantially enhance the
photoinduced generation of reactive species (H 2 O 2 , ROOH, HO
•
, etc.) in natural waters (Yocis et al. 2000; Rex et al. 1997; Qian et al. 2001). For example, a
decrease in stratospheric ozone from 336 to 151 Dobson units during an ozone
hole event resulted in an increase by 19–42 % in the production of H 2 O 2 at the surface of Antarctic waters (Yocis et al. 2000). An increase in ozone hole events can
thus cause a higher degree of DOM photodegradation upon generation of highly
reactive free radicals.
4.5 Production and Decay by Photosynthesis
As a result of photodegradation of DOM, along with the production of H 2 O 2 and
ROOH compounds, several other photoproducts such as CO 2 , CO or other forms
of dissolved inorganic carbon (DIC = sum of dissolved CO 2 , H 2 CO 3 , HCO 3
− ,
and CO 3
2− ), low molecular weight (LMW) DOM, and thermal energy, E (±) are
simultaneously produced in natural waters (Mostofa et al. 2009; Wu et al. 2005;
Amador et al. 1989; Moran and Zepp 1997; Wang et al. 2009; Xie and Zafiriou
2009. A general scheme (Eq. 4.7) for the photodegradation of DOM can be
expressed as follows (Mostofa et al. 2009a, b):
(4.6)
Rate = −d [H 2 O 2 ] /dt = k 2 [H 2 O 2 ] [Number of bacterial cells]
(4.7)
DOM + hυ → H 2 O 2 + CO 2 /CO/DIC + LMW DOM + E (±)
K. M. G. Mostofa et al.
decay of H 2 O 2 was second-order overall, and first-order in both H 2 O 2 concentration and number of bacterial cells (Cooper and Lean 1992). Such a kinetic can be
expressed as follows:
where k 2 = 1.6 × 10 −9 mL cell −1 min −1 . The freshwater bacterium Enterobacter
cloaceae showed a similar rate constant, k 2 = 1.5 × 10 −9 mL cell −1 min −1 .
4.4 Production and Decay by DOM Photochemistry
Photodegradation of DOM depends on the incident light intensity, which is
directly linked to the production of H 2 O 2 and ROOH through photoinduced reactions in natural waters (Cooper and Zika 1983; Moore et al. 1993; Baxter and
Carey 1983). For example, H 2 O 2 concentration gradually increases with irradiation time in natural waters as well as in aqueous solutions of standard organic substances (Fig. 3) (Obernosterer et al. 2001; Cooper et al. 1988). Similarly, a 10–20
times lower H 2 O 2 production was observed in river waters during the cold season
compared to summer, and in the Southern Ocean in Antarctic regions (5–25 nM)
compared to other oceanic environments, respectively (Fig. 9; Table 1). The photodegradation of DOM is greatly influenced by the stratospheric ozone hole events,
particularly in Antarctic waters. The ozone hole can increase the fluxes of solar
ultraviolet radiation (UVR, 280–400 nm), which may substantially enhance the
photoinduced generation of reactive species (H 2 O 2 , ROOH, HO
•
, etc.) in natural waters (Yocis et al. 2000; Rex et al. 1997; Qian et al. 2001). For example, a
decrease in stratospheric ozone from 336 to 151 Dobson units during an ozone
hole event resulted in an increase by 19–42 % in the production of H 2 O 2 at the surface of Antarctic waters (Yocis et al. 2000). An increase in ozone hole events can
thus cause a higher degree of DOM photodegradation upon generation of highly
reactive free radicals.
4.5 Production and Decay by Photosynthesis
As a result of photodegradation of DOM, along with the production of H 2 O 2 and
ROOH compounds, several other photoproducts such as CO 2 , CO or other forms
of dissolved inorganic carbon (DIC = sum of dissolved CO 2 , H 2 CO 3 , HCO 3
− ,
and CO 3
2− ), low molecular weight (LMW) DOM, and thermal energy, E (±) are
simultaneously produced in natural waters (Mostofa et al. 2009; Wu et al. 2005;
Amador et al. 1989; Moran and Zepp 1997; Wang et al. 2009; Xie and Zafiriou
2009. A general scheme (Eq. 4.7) for the photodegradation of DOM can be
expressed as follows (Mostofa et al. 2009a, b):
(4.6)
Rate = −d [H 2 O 2 ] /dt = k 2 [H 2 O 2 ] [Number of bacterial cells]
(4.7)
DOM + hυ → H 2 O 2 + CO 2 /CO/DIC + LMW DOM + E (±)