Environmental Engineering Reference
In-Depth Information
•
H
2
O
2
. All these compounds are responsible for the production of HO
towards the
degradation of DOM or organic pollutants in aqueous solution (Zellner et al.
1990
;
Zafiriou and Bonneau
1987
; Millero and Sotolongo
1989
; Zepp et al.
1992
; Farias
et al.
2007
). It is estimated from data of Zellner et al. (Zellner et al.
1990
) that a
temperature increase from 278 to 298 K can enhance the quantum yield of HO
•
−
photoproduction (at 308 nm and at neutral pH), on average by 70 % for NO
2
−
photolysis, 129 % for NO
3
photolysis and 20 % for H
2
O
2
photolysis. The effi-
ciency of the photoinduced degradation of DOM is also significantly dependent on
the wavelength, and the quantum yields (
Φ
HO
) of HO
•
production decrease with
increasing wavelength (Zellner et al.
1990
; Zafiriou and Bonneau
1987
). At pH 8
and at 298 K the
Φ
HO
for NO
2
−
photolysis at 308 nm is on average 54 % higher
(
Φ
HO
=
0.071
±
0.009) than that at 351 nm (
Φ
HO
=
0.046
±
0.003) (Zellner et al.
1990
).
The formation of H
2
O
2
is a key step of the photoinduced processes in surface
waters (Mostofa et al.
2011
) and it is as well largely dependent on the radiation
wavelengths (Obernosterer et al.
2001
; Richard et al.
2007
). The contribution of
UV-B, UV-A and photosynthetically active radiation (PAR) to H
2
O
2
formation is
40, 33 and 27 %, respectively (Richard et al.
2007
).
An increase in the photoinduced degradation rate of DOM may extend the
water column transparency (which undergoes seasonal modifications on a variety
of time scales) and the depth of the mixed layer that influences the incident UV
radiation (Scully and Lean
1994
; Morris et al.
1995
; Morris and Hargreaves
1997
).
Diffuse light attenuation coefficients often undergo seasonal variations (39-81 %)
in surface waters, and minimum values appear during the summer season (Morris
and Hargreaves
1997
). Therefore, an extension of the summer season due to global
warming may enhance both photoinduced processes and photosynthesis, which
could for instance increase the duration of the phytoplankton or algae productiv-
ity in lake ecosystems, particularly in the Arctic and Antarctic regions (Malkin
et al.
2008
). As already indicated, an increase in UV radiation due to depletion
of the stratospheric ozone layer can accelerate the production of HO
•
, which is a
key factor for the photoinduced degradation of DOM in natural waters (Huisman
et al.
2006
; Qian et al.
2001
; Sarmiento et al.
2004
; Schmittner
2005
; Crutzen
1992
; Stolarski et al.
1992
). Therefore, global warming can enhance both the
photoinduced degradation of DOM and the release of autochthonous DOM and
nutrients from algae or phytoplankton. Such processes can be partially offset by
the fact that the production of higher amounts of H
2
O
2
, CO
2
, DIC and other low
molecular weight substances could increase photosynthesis and enhance the pri-
mary production (Mostofa et al.
2009b
).
4.3 Changes in Microbial Activity in Natural Waters
Global warming may significantly affect microbial or biological processes in
natural waters, with consequences on both autotrophs (plants, algae, bacteria) and
