Environmental Engineering Reference
In-Depth Information
Therefore, lipid photooxidation causes a 165 % increase in ketone and aldehyde
carbonyls (C-II region), which also increases the ratio of carbonyl groups to aro-
matic ones (Brooks et al.
2007
). Correspondingly, the number of metal-binding
substituents per aromatic moiety can increase, producing binding sites with
weaker conditional stability constants in the residual (or photobleached) wetland
DOM (Brooks et al.
2007
).
Photoinduced irradiation is unable to cause complete degradation in waters
having high levels of DOM, such as 2.3-32.2 mg L
−
1
in rivers, 43.3 mg L
−
1
in wetland, and 22.6-24 mg L
−
1
in estuaries. In contrast, photochemistry can
degrade most of the DOM in waters having low levels of DOM, such as
1 mg
L
−
1
in upstream rivers (Brooks et al.
2007
; Moran et al.
2000
; Mostofa et al.
2007
). Therefore, 16-23 % DOM losses can occur in waters with high levels of
DOM, whilst higher losses (32-36 %) occur in waters with low levels of DOM.
As a consequence, the effects of the photoinduced degradation on M-DOM com-
plexes are expected to be lower in high-DOM waters than in low-DOM ones.
The functional groups in protein or tryptophan are amino carboxylic acid
[-CH-(NH
2
)-COOH] and -NH group in an aromatic system [C
8
H
5
(NH)-]
(Mostofa et al.
2009a
,
2011
). Similarly, EPS is primarily composed of several ion-
isable functional groups such as carboxyl, phosphoric, amine, acidic amino acids,
hydroxyl, phenolic, sulfates and organic phosphates. These groups can form com-
plexes with trace metals depending on the environmental conditions (Beech and
Sunner
2004
; Quiroz et al.
2006
; Merroun and Selenska-Pobell
2008
; Zhang et al.
2008
; Merroun et al.
2003
; Guibaud et al.
2005
)
It has recently been shown that the functional groups of humic substances (fulvic
and humic acids) and tryptophan undergo preferential photoinduced decomposition
in natural waters (Xie et al.
2004
; Minakata et al.
2009
). Decomposition and min-
eralization of the functional groups of DOM by solar irradiation are responsible for
the disappearance of complexation between DOM and metal ions (Sachs et al.
2010
;
Kulovaara
1996
; Kulovaara et al.
1996
; Bertilsson and Tranvik
2000
). Therefore, pho-
toinduced processes can produce a marked increase of metal toxicity in natural waters.
≦
5.6 Effect of Microbial Processes
Microbial processes such as bacterial reductive precipitation, immobilization of
soluble metals and M-DOM complexation can significantly affect the mobility (or
transport) and toxicity of the trace metals and radionuclides in the aquatic environ-
ments (Bergquist and Blum
2007
; Yin et al.
2010
; Tabak et al.
2005
; Francis and
Dodge
2008
; Fletcher et al.
2010
). Key microbial processes such as biotransforma-
tion, biosorption and bioaccumulation, as well as degradation or synthesis of DOM
can alter the solubility of metals and radionuclides (Fig.
10
) (Bergquist and Blum
2007
; Yin et al.
2010
; Tabak et al.
2005
; Francis and Dodge
2008
; Fletcher et al.
2010
). Metal-reducing or sulfate-reducing microorganisms can directly or indirectly
reduce the soluble and mobile trace metals (Cr
6
+
, U
6
+
, Tc
7
+
) to metals (Cr
3
+
, U
4
+
,
