Environmental Engineering Reference
In-Depth Information
3.7 Temperature
The fluorescence intensity of FDOM is inversely related to temperature because of
an increased collisional quenching of fluorescence at higher temperatures (Wehry
1973
; Vodacek and Philpot
1987
). Within the range 10-45 °C, the fluorescence
intensity can increase by approximately 1 % with a 1 °C decrease in temperature
in the case of tryptophan-like, humic-like and fulvic-like substances, depend-
ing on colloid size and fluorophore (Henderson et al.
2009
; Baker
2005
; Vodacek
and Philpot
1987
; Elliott et al.
2006
; Seredy
ska-Sobecka et al.
2007
). In contrast,
the fluorescence intensities of tryptophan standards are almost unaffected by a
temperature variation of
±
8 °C (Reynolds
2003
). The thermal quenching of fluo-
rescence can be significant because of variation in water temperature between the
summer and winter seasons as well as the high variation between boreal, tropical
and Antarctic-Arctic regions. The effect of temperature on fluorescence quenching
is linear and reversible, and it can be prevented in the laboratory by measuring the
fluorescence of samples at a constant temperature (Vodacek and Philpot
1987
). The
thermal quenching effects can also be overcome by applying simple correction fac-
tors, but such factors may be different for fluorophores of different size fractions
(Seredy
ń
ska-Sobecka et al.
2007
). The mechanism of the temperature effect on fluo-
rescence is that a rise in water temperature increases the likelihood that an excited
electron will return to its ground state by radiationless decay, leading to reduced flu-
orescence intensity (Henderson et al.
2009
). It is suggested that a variation of water
temperature across a range of 20 °C or more between summer and winter would
lead to a corresponding decrease by 20 % of the fluorescence intensity during sum-
mer (Henderson et al.
2009
). Fluorescence changes caused by temperature may have
no effect on the structure of the DOM. However, it has been shown that non-revers-
ible changes may occur, possibly as a result of the application of a light-source that
may cause photodegradation or thermal decomposition (Vodacek and Philpot
1987
).
ń
4 Kinetics of Photodegradation of the Fluorescence
Intensity of Fulvic Acid and Tryptophan
Fulvic acid and tryptophan-like fluorescence intensity (FI) decreases mono-
tonically with the number of absorbed UV photons or with integrated solar
intensity, as a result of solar effects on water (Fig.
10
) (Mostofa et al.
2007a
).
Photodegradation of fulvic acid often follows a two-step kinetics (Mostofa et al.
2007a
; Ma and Green
2004
), while tryptophan is photodegraded in a single step
(Mostofa et al.
2007a
). A decrease of FI can be best fit to a first-order kinetics as
follows (Eq.
4.1
):
(4.1)
LN
(
FI
/
FI
O
) =−
K
2
S
where
k
2
is the reaction rate constant for photodegradation of FI in waters,
FI
is
