Environmental Engineering Reference
In-Depth Information
Fig.
10
). The production rate of DIC in irradiated samples is 80-2420 nM h
−
1
in
rivers, 3180-1612800 nM h
−
1
in lakes and 40-13200 nM h
−
1
in seawaters. Under
dark incubation the production rates are 99970 nM h
−
1
in rivers, 175500 nM h
−
1
in
lakes, and 300-2300 nM h
−
1
in seawaters (Table
3
).
The production rate of CO
2
in irradiated freshwater two coastal rivers is sig-
nificantly high in air-saturation (13.2-23.0
μ
M h
−
1
) and O
2
-saturation (18.3-
26.5
μ
M h
−
1
), which are substantially decreased under air-saturation plus DFOM
(5.4-6.0
μ
M h
−
1
), O
2
-saturation plus DFOM (5.6-7.2
μ
M h
−
1
), and N
2
-saturation
(1.9
μ
M h
−
1
, measured for one river sample only) (Table
3
) (Xie et al.
2004
). Note
that DFOM is the deferoxamine mesylate that is a strong Fe(III)-complexing ligand
that forms nearly photo-inert complexes (Gao and Zepp
1998
). The CO
2
produc-
tion rate in the N
2
-saturation river water is only ca. 10 % and 20 % of those in the
O
2
-saturation and air-saturation samples, respectively (Xie et al.
2004
). This study
observes that although CO
2
production in the O
2
-saturation and DFOM samples is
consistently higher than in the air-sat and DFOM samples, the difference between
the two seldom exceeded 10 %, which indicates that in the presence of DFOM, iron
rather than O
2
is the limiting factor for CO
2
production (Table
3
) (Xie et al.
2004
).
These results suggest that although, O
2
and iron both can play very important
roles in CO
2
production, photoinduced processes without the involvement of O
2
and iron (particularly the iron independent processes) can also contribute to CO
2
production in natural waters (Xie et al.
2004
). CO
2
photoproduction rate is ~0.08-
0.63
μ
M h
−
1
in estuarine water showing high production rate (~0.63 nM h
−
1
) in
low salinity than in high salinity waters ~0.08
μ
M h
−
1
(White et al.
2010
). Studies
therefore observe that CO
2
is the largest carbon-containing product of DOM photo-
degradation in natural waters (Xie et al.
2004
; Miller and Zepp
1995
).
It is shown that the lakes produce DIC to a higher extent compared to riv-
ers and seawaters (Fig.
10
a). The DIC photoproducts in lakes are well correlated
with DOC concentration, but no correlation was observed in rivers and seawaters
(Fig.
10
a). The high production of DIC in lakes could originate from the higher
presence of low molecular weight organic substances (54-79 % in the <5 kDa
range), which are mostly originated from fulvic and humic acids (Waiser and
Robarts
2000
; Yoshioka et al.
2007
; Wu and Tanoue
2001
). The photo produc-
tion rate of DIC is higher in the upper surface layers (2.42
μ
M h
−
1
at 0 m), it
then gradually decreases (0.60
μ
M h
−
1
at 6.5 m) and then increases again
(2.73
μ
M h
−
1
at 24 m) in the deeper layers, but they are greatly lower than those
of dark incubation samples (100.0
μ
M h
−
1
) in photoexperiments conducted on
river water samples keeping in situ at different vertical depths in lake environ-
ment (Ma and Green
2004
). For lake waters the production rates of DIC are grad-
ually decreased from the surface (10.6-10.9
μ
M h
−
1
at 0-6 m) to deeper layers
(3.2
μ
M h
−
1
at 24 m). However, under dark incubation the production rate is
175.5
μ
M h
−
1
, much higher than the rate of photoproduction. An increase in the
DIC production rate of river waters kept in situ in the deeper layer of lakes (24 m
depth) is likely caused by the much lower sunlight intensity that makes this set-
up equivalent to dark incubation. Therefore, high production of DIC under dark
incubation allows the hypothesis that microbial degradation plays a significant
