Environmental Engineering Reference
In-Depth Information
(Vahatalo et al.
2000
; Wu et al.
2005
; Gennings et al.
2001
; Molot et al.
2005
).
The photo-Fenton reaction is greatly influenced by pH. A decrease in pH greatly
accelerated the photoinduced degradation of DOM in softwater stream (Molot et
al.
2005
), in Satilla river (Gao and Zepp
1998
) as well as in lake water from NW
Italy (Vione et al.
2009
). The experimental study shows that after 69 hours of arti-
ficial irradiation without addition of KI, DOC loss is decreased as pH increases
from pH 4 to 9 whilst addition of KI is significantly reduced loss of DOC at pH
4, 5 and 7 but not at pH 9 with the fraction of DOC lost by non-HO
•
mechanisms
gradually increasing from 58 % to 75 % between pH 4 and 7, and 100 % at pH 9
(Molot et al.
2005
). Photoinduced degradation rates of DOC and fluorescence are
greatly increased with a decrease in sample pH from 8 to 6 and then to 4 (Wu et
al.
2005
). Conversely, the production rates of HO
•
in the Fenton or photo-Fenton
reaction are greatly enhanced with a decrease in pH of natural waters (Zepp et al.
1992
; Vione et al.
2009
; Goldstone et al.
2002
; Moffett and Zika
1987
; Millero
and Sotolongo
1989
).
The apparent mechanism for enhanced photoinduced loss of DOC at low pH is
oxidation to dissolved inorganic carbon (DIC) by reaction with HO
•
produced via
the iron-mediated photo-Fenton pathway (Zepp et al.
1992
; Voelker et al.
1997
).
Therefore, high production rate of HO
•
at low pH can accelerate the photoinduced
degradation of DOM in waters. However, there is evidence that the production rate
of HO
•
in acidified lake water is unable to account for the rate of DOM miner-
alization, which suggests that additional mineralization processes would also be
operational (Vione et al.
2009
). The major terrestrial alkalinity-producing pro-
cesses such as ionic exchange, weathering and biological assimilation of nitrate
and other anions, mostly depend on the watershed geology, morphology, soil
characteristics, and hydrological conditions (Psenner
1988
). Watersheds of lakes
exported more SO
4
2
−
, NO
3
and H
+
than they received, and the lakes are the
dominant acidity-consuming parts of the whole ecosystem, neutralizing 50-58 %
of the H
+
input (Kopacek et al.
2003
). Terrestrial fluxes of organic acid anions
can also consume H
+
in natural lakes and are thought to be the third major inter-
nal alkalinity-producing mechanism after the biochemical reductions of NO
3
−
−
and
SO
4
2
−
(Kopacek et al.
2003
; Cook et al.
1986
; Schindler et al.
1986
). An increase
in alkalinity in waters can decrease the production of H
2
O
2
by slowing the reac-
tion of O
2
•
−
•
−
+
2H
+
→
H
2
O
2
+
O
2
). A decrease in H
2
O
2
production can reduce the photoinduced generation of HO
protonation (2O
2
•
through photo-Fenton
reaction or direct photolysis, thereby decreasing the photoinduced degradation of
DOM in natural waters.
3.7 Dissolved Oxygen
Dissolved oxygen (O
2
) can enhance the photoinduced degradation of DOM in
waters (Vahatalo et al.
2000
; Amon and Benner
1996
; Obernosterer et al.
2001
;
Laane et al.
1985
; Lindell and Rai
1994
; Reitner et al.
1997
). Addition of O
2
to
