Environmental Engineering Reference
In-Depth Information
salinity or sea waters can complex with DOM (M-DOM) forming a strong
π
-elec-
tron bonding system between metal ions and the functional groups in DOM. This
π
-electron in M-DOM complex is rapidly excited photolytically, which is respon-
sible for high production of aqueous electrons (e
aq
−
) and subsequently the high
production of superoxide ion (O
2
•
−
), H
2
O
2
and HO
•
, respectively.
On the other hand, the mixing of some standard FDOM (e.g. DSBP, phenol,
and tryptophan) with seawater show that the fluorescence excitation-emission
wavelength maxima (peak C- region and peak T- region) are shifted from longer
to shorter wavelength regions compared to Milli-Q water (Nakajima
2006
). Such
changes in fluorescence excitation-emission maxima are termed as blue-shift of
the flurophores in FDOM. The blue-shift of the FDOM fluorescence peaks can
be hypothesized to occur upon loss of high molecular weight fluorescent com-
ponents by physico-chemical modifications such as flocculation, aggregation or
precipitation when the ionic strength is increased (Sholkovitz
1976
; Carlson and
Mayer
1983
; Sierra et al.
1997
; van Heemst et al.
2000
; Benner and Opsahl
2001
;
McCarthy et al.
1996
). The mechanism behind the blue-shift phenomenon in
metal-DOM complexation is presumably the fact that anions or cations can form
complexes with the fluorophores (or functional groups) of few fluorescent organic
components and may increase the excitation energy of the fluorophores associated
to the peak C or peak A-region. The result can be a change in the excitation-emis-
sion from longer to shorter wavelengths.
7 Importance of the Metal-DOM Complexation
The complexation of DOM with trace metals is connected to several major bio-
geochemical phenomena that can be distinguished as: (i) Chemical speciation
of the trace metals is of key importance for their biological effects and bio-
geochemical cycling in natural water, sediment and soil environments (Sekaly
et al.
2003
; Huber et al.
2002
; Hughes et al.
1995
; Markich
2002
). (ii) DOM
including humic substances can control the occurrence of free toxic metals
through formation of M-DOM complexes that can significantly reduce the bio-
availability and toxicity to organisms in natural waters (Shcherbina et al.
2007
;
Mostofa et al.
2011
; Filella et al.
2007
; Hörnström et al.
1984
; Markich
2002
;
Managaki and Takada
2005
; Yadav and Trivedi
2006
). The bioavailability of
toxic metals, their ability to bind to or traverse the cell surface of an organ-
ism, is generally dependent on the metal speciation or physicochemical form in
the aquatic environments. For example, U
4
+
complexes with humic substances
(fulvic and humic acids) and inorganic ligands (e.g., carbonate or phosphate)
apparently reduce the bioavailability of U by reducing the activity of UO
2
2
+
and UO
2
OH
+
, which are the major forms of U
4
+
available to organisms, rather
than U in strong complexes (e.g. uranyl fulvate) or adsorbed to colloidal and/
or particulate matter (Markich
2002
). (iii) Formation of M-DOM complexes
can influence the transport or migration of metals, the acid-base balance and
