Environmental Engineering Reference
In-Depth Information
and Sugiyama
2008
; Effler et al.
2010
). These studies show that UV-B penetration
depths vary from only a few centimeters in highly humic lakes to dozens of meters
in the oceans, due to variation in DOM contents. It is also observed that 99 % of
the UV-B radiation is attenuated in an approximately 0.5-m water column in the
clearest lake for DOC ranging from 408 to 725
μ
M C and for chlorophyll
a
rang-
ing from 1.6 to 16
μ
g L
−
1
(Huovinen et al.
2003
). In the UV-A region at 380 nm,
the corresponding attenuation is limited to the upper one meter.
The absorption coefficients predict that, in a small humic lake (DOC 1100-
1242
μ
M C), UV-B radiation is attenuated to 1 % of the subsurface irradi-
ance within the top 10 cm water column, whereas UV-A radiation (at 380 nm)
penetrates more than twice as deep (maximum 25 cm) (Huovinen et al.
2003
).
However, in clear lakes with low DOC concentration the contribution of phyto-
plankton to UV attenuation can be significant (Sommaruga and Psenner
1997
).
Any enhancement of photoinduced degradation of DOC by UV radiation and
acidification can substantially increase the UV transparency in lakes (Morris and
Hargreaves
1997
; Vione et al.
2009
; Schindler et al.
1996
; Yan et al.
1996
; Scully
et al.
1997
). The consequence is an enhanced penetration of UV radiation into the
water column, which can significantly damage aquatic biota. DOM is thus respon-
sible for UV attenuation in the water column and for the related protection of
aquatic organisms in natural waters.
Aggregation of DOM
Aggregation of fulvic and humic acid (humic substances) can occur at the intra-
molecular (involving a single polymer molecule) or intermolecular (involving
multiple chains) levels in aqueous solution (Wershaw
1999
; Engebretson and
von Wandruszka
1996
; Lippold et al.
2008
). The interior of the resulting aggre-
gates is relatively hydrophobic, whilst the exterior is more hydrophilic. They
can exist in a pseudomicellar form or as micelle-like aggregates in solution,
and as membrane-like aggregates on mineral surfaces (Wershaw
1999
; Sutton
and Sposito
2005
; Piccolo et al.
2001
). The results of the chemical analysis of
humic acids isolated from natural environments (water, soil, peat, sediments,
and sludge from wastewater treatment facilities) demonstrate that the per-
centage elemental composition, the contents of carboxylic groups and of aro-
matic phenolic groups is very variable. They range from 33.2 (river) to 60.7 %
(Aldrich) of C; 2.25 (river) to 5.4 % (soil) of H; 0.65 (river) to 3.7 % (peat) of
N; 34.1 (Aldrich) to 63.8 (river) of O; 0.06 (soil) to 0.10 % (sewage sludge) of
S; 1.0 (river) to 8.1 mmol g
−
1
(peat) of carboxylic groups (-COOH), and from
0.36 (bog peat) to 4.4 mmol g
−
1
of phenolic moieties (ArOH) (Klavins and
Purmalis
2010
).
Humic acids behave like surface-active substances when they are added to
solutions, which depend on their origin and molecular properties. Therefore, their
surface tension decreases as their concentration increases (Lippold et al.
2008
;
Klavins and Purmalis
2010
; Wershaw
1993
; Engebretson and von Wandruszka
1994
; Terashima et al.
2004
). Humic acids can be significantly modified in their
