Environmental Engineering Reference
In-Depth Information
Amador et al.
1989
; Malcolm
1990
; Mopper et al.
1991
; Bertilsson and Tranvik
2000
;
Chen et al.
1978
; Corin et al.
1996
; de Haan
1993
; Sun et al.
1993
; Hongve
1994
;
Peuravuori and Pihlaja
1997
). The major low molecular size substances examined are
polysaccharides, N-acetylamino sugars, polypeptides, lipids, proteins, n-C
16
and n-C
18
fatty acid methyl esters, etc. The conversion rate of DOM into identifiable organic pho-
toproducts is 20 % of the bleaching rate of the DOM, leaving a vast unidentified pool
of bleached organic matter in natural waters (Miller and Zepp
1995
). The unidentified
bleached DOM would account for a large proportion of the total biologically available
photoproducts (Miller and Moran
1997
).
Photoinduced degradation of DOM can produce a variety of low molecular
weight (LMW) aliphatic organic compounds which are considered to be micro-
biologically labile in the aquatic environment (Moran and Zepp
1997
; Dahlén et
al.
1996
; Wetzel et al.
1995
; Corin et al.
1996
). The most common labile LMW
organic compounds include formaldehyde, formic acid, formate, acetaldehyde,
acetate, acetic acid, hydroxyacetic acid, hydroxyacetate, acetone, propanal, oxalic
acid, oxalate, citric acid, citrate, glyoxal, methylglyoxal, glyoxylic acid, glyoxy-
late, ketomalonic acid, malonic acid, malonate, levulinic acid, levulinate, succinic
acid, succinate, pyruvic acid and pyruvate. Nine organic peroxides (ROOH) such
as methyl hydroperoxide, hydroxymethyl hydroperoxide, ethyl hydroperoxide,
1-hydroxyethyl hydroperoxide, 2-hydroxyethyl hydroperoxide, 1-hydroxypropyl
hydroperoxide, 2-hydroxypropyl hydroperoxide, 3-hydroxypropyl hydroperoxide,
and bis(hydroxymethyl)peroxide are observed in air and rainwaters (Hellpointner
and Gäb
1989
; Hewitt and Kok
1991
; Jackson and Hewitt
1996
). Precipitation is
a potential source of these peroxides into surface waters, where they are micro-
biologically labile in natural waters (Mostofa
2005
). A rapid decrease of peracetic
acid added to unfiltered river waters was detected in dark controlled samples, sug-
gesting that the organic peroxides are microbiologically labile (Mostofa
2005
).
Some long-chain aliphatic organic acids, such as 2-hydroxy propanoic acid,
3-oxobutanoic acid, 4-oxopentanoic acid, hexanoic acid, pentanedioic acid, octa-
noic acid, nonanoic acid, and decanoic acid, are produced by the Photoinduced
degradation of humic substances extracted from lakes (Corin et al.
1996
). The pro-
duction of keto acids such as 3-oxobutanoic acid and 4-oxopentanoic acid is greatly
enhanced by an increase of the UV-dose. They are mostly produced from fulvic
acid rather than humic acid (Corin et al.
1996
), probably because of the higher per-
centage of aliphatic carbon bound to fulvic acid (63 %) compared to humic acid
(47 %) (Malcolm
1985
). Carboxylic acids (oxalic, malonic, formic, acetic) are usu-
ally major products of the photoinduced degradation of DOM (25-34.4 %) (Ma and
Green
2004
; Bertilsson et al.
1999
; Bertilsson and Tranvik
2000
).
The total production rate of LMW organic substances is much higher in lakes
(2500-44200 nM h
−
1
) than in seawater (3.3-10.6 nM h
-1
) (Table
3
). Variations
of the photolytically produced LMW carboxylic acids in different lakes are linked
to the presence of fulvic and humic acids in DOM (Bertilsson and Tranvik
1998
).
The LMW organic substances undergo a rather fast disappearance in natural
waters, probably because of two major pathways. First of all, the LMW organic
compounds can be rapidly assimilated by natural microorganisms or bacterial
