Environmental Engineering Reference
In-Depth Information
rates (Gao and Zepp
1998
; Stumm and Lee
1961
; Miles and Brezonik
1981
).
Stumm and Lee 1961; Miles and Brezonik 1981; In iron-rich waters, the ferrous
iron is often oxidized by dissolved O
2
with production of ferric oxide floc (Stumm
and Lee
1961
). O
2
is consumed at a rate of 0.02-0.09 mg L
−
1
h
−
1
in humic
colored waters having pH 3-4 and total iron concentration of 0.1-2.0 mg L
−
1
under irradiation. The consumption rate is slightly lower (0.01-0.04 mg L
−
1
h
−
1
)
under dark conditions (Miles and Brezonik
1981
). Some standard organic com-
pounds can consume O
2
at rates of 0.01-0.83 mg L
−
1
h
−
1
under irradiation and
0.01-0.70 mg L
−
1 h
−
1
in the dark. These results have been obtained for a con-
centration of 100 mg L
−
1
of organic compounds in the presence of 6 mg L
−
1
of
Fe(III) (Miles and Brezonik
1981
). In photoexperiments conducted on Amazon
river water samples, the O
2
consumption rate was 3.68
μ
M O
2
h
−
1
under irradi-
ation and it was twelve times lower (0.30
μ
M O
2
h
−
1
) in the dark (Amon and
Benner
1996
). High rates of DOC loss and O
2
consumption are often observed
in riverine DOM, with little or no additional production of biologically labile
organic compounds. The photoinduced O
2
demand of surface water DOM in the
Atlantic Ocean varied from 0.1 to 2.8
μ
mol O
2
L
−
1
d
−
1
in 12 h irradiation periods
(Obernosterer et al.
2001
). Rivers usually exhibit a higher O
2
consumption rate
than seawaters. The O
2
consumption in waters is hypothesized to contribute to the
generation of H
2
O
2
through production of superoxide radical ion (O
2
•
−
) as inter-
mediate, upon monoelectronic reduction of O
2
by aquated electrons (e
−
) produced
by DOM (see chapter
“
Photoinduced and Microbial Generation of Hydrogen
Peroxide and Organic Peroxides in Natural Waters
”
). Photolytically produced
H
2
O
2
can participate to the production of HO
•
, by either the photo-Fenton reaction
or the direct photolysis, and such processes can contribute to the photoinduced
degradation of DOM in waters.
3.8 Depth of the Water Column
The Photoinduced degradation of DOM is significantly dependent on the depth
of the water column. Degradation is higher in the upper surface layer and gradu-
ally decreases with an increase in the water column depth (Ma and Green
2004
;
Vahatalo et al.
2000
). Solar radiation can mineralize 19 mmol C m
−
3
d
−
1
at a
depth of 1 cm, and the rate of mineralization decreases with increasing depth with
an attenuation coefficient of 23 m
−
1
(Vahatalo et al.
2000
). Most of the photo min-
eralization takes place in the top 10 cm in lakes (Vahatalo et al.
2000
). The pres-
ence of low quantity of suspended solids or particulate matter allows for a deeper
penetration of light in the water column, which can result into a greater potential
for the photoinduced degradation of deeper DOM. Both river and lake DOM have
a high potential to undergo photoinduced degradation in the surface layers (0 m),
and photoinduced degradation gradually decreases in the deeper layers (6.5 and
24 m), as has been found in an in situ experimental study (Ma and Green
2004
).
Surface waters with a high level of DOC greatly inhibit the penetration of solar
