Environmental Engineering Reference
In-Depth Information
3.5 Molecular Characteristics of the DOM
Photoinduced degradation of DOM is significantly dependent on the molecular
characteristics or the absorbing nature of the organic substances in the chromophoric
dissolved organic matter (CDOM) or fluorescent dissolved organic matter (FDOM)
in waters (Table
1
). Chromophores in CDOM and fluorophores in FDOM are con-
sidered to be equivalent components with respect to photosensitization by sunlight.
Photoinduced degradation of DOM takes place upon absorption of photons, which is
predominantly dependent on the chemical nature of the organic substances present in
DOM. The radiation absorption by organic matter increases along the spectrum from
visible toward UV regions (Amador et al.
1989
; Kieber et al.
1990
). High molecular
weight (HMW) DOM such as humic substances (fulvic and humic acids), and fluo-
rescent whitening agents (FWAs) or components of detergents (DAS1 and DSBP) can
absorb both visible and shorter wavelength regions (Kramer et al.
1996
; Kieber et al.
1990
; Sadtler
1968
; Strome and Miller
1978
). Many low molecular weight organic
acids photo-generated from large CDOM or FDOM can absorb only in UV-C range,
but they do not absorb radiation in the UV-B, UV-A or visible range (Sadtler
1968
). For
example, acetaldehyde absorbs light at 208-224 nm (Kieber et al.
1990
; Mopper et al.
1991
; Strome and Miller
1978
), acetate at 204-270 nm (Dahlén et al.
1996
; Wetzel et al.
1995
), formaldehyde at 207-250 nm (Mopper et al.
1991
; Mopper and Stahovec
1986
),
glyoxal at <240 nm (Mopper et al.
1991
; Mopper and Stahovec
1986
), malonate at 225-
240 nm (Dahlén et al.
1996
), pyruvate at 200-227 nm (Wetzel et al.
1995
; Kieber et al.
1990
; Mopper et al.
1991
), and propanal at ~230 nm (Mopper and Stahovec
1986
).
Fulvic acid absorbs radiation in both the visible and UV ranges (Fig.
1
a)
(Mostofa et al.
2005
). The DOC concentrations varied between upstream (99
μ
M
C, Kago) and downstream rivers (194
μ
M C, Yasu), but the absorption in the vis-
ible region is likely the same in both upstream (Fig.
1
a) and downstream waters
(Fig.
1
b). Such an absorption is usually caused by HMW DOM. Humic acid is
degraded more quickly than fulvic acid in water (Wu et al.
2005
), probably because
of the higher aromaticity (Malcolm
1985
; Gron et al.
1996
). The absorption of vis-
ible light by chromophores or fluorophores in HMW DOM causes decomposition,
which is usually more marked in upstream (Fig.
1
a) than in downstream waters
(Fig.
1
b). The following order for DOM photoinduced degradation can be pro-
posed: upstream DOM > downstream rivers > lake > seawaters (Table
1
; Fig.
1
).
Therefore, photoinduced degradation is greatly dependent on the molecular nature
of DOM compositions in waters. Interestingly, the residence time of water in lakes
and sea is much higher compared to the rivers. Possibly the lower photoinduced
lability of the DOM found in lake or sea water is due to the fact that labile DOM in
these environment has sufficient time to undergo photoinduced degradation.
3.6 pH and Alkalinity of Waters
Both pH and alkalinity, which can greatly vary among different waters, can
