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et al. 2005 ). Such groups can form complexes with trace metals depending on the
different environmental conditions. The complexation between EPS and metal
ions (ca. Pu 4 + ) suggests that the carboxylic groups in EPS are the primary bind-
ing sites at pH 4 and with a ionic strength of 0.1 M NaCl (Harper et al. 2008 ).
The amide functional groups of proteins in the EPS are susceptible to form com-
plexes with trace metals, which have been detected using spectroscopic analysis
(Guibaud et al. 2005a ; b ; Zhang et al. 2006 ). Polysaccharides can form complexes
with trace metal ions, and the hydroxyl groups of neutral polysaccharides and
the carboxyl groups of anionic polysaccharides are the probable binding sites
for such complexes (Guibaud et al. 2005 ; Zhang et al. 2006 ; Brown and Lester
1982 ). The organic phosphate groups of EPS of A. ferrooxidans or phosphate
groups in nucleic acids play a major role in the binding of U from aqueous solu-
tions, although this bacterium contains small amounts of phosphates (Merroun and
Selenska-Pobell 2008 ). Bacteria, algae and their exudates also consist of a mosaic
of functional groups (i.e., amino, phosphoryl, sulfhydryl and carboxylic groups)
and the net charge on the cell wall depends on the pH of the medium (Filella
2008 ). In addition, trace metals such as Th 4 + and U form complexes with organic
ligand in particulate matter that might be a nonmetal-specific chelator originating
from the cell surface of microorganisms (Hirose 2004 ).
4.1 The Mechanism for Complex Formation Between Trace
Metals and DOM in Waters
A relationship between the lability of metal DOM complexes involving 3 d transi-
tion metals in freshwater and their d -electron configuration is discussed by Sekaly
and his colleagues ( 2003 ). The order of the lability of the metal complexes, Co(II)
d 7 > Ni(II) d 8 > Cu(II) d 9 < Zn(II) d 10 , follows the reverse order of the ligand
field stabilization energy (LFSE). There is an exception for Cu(II), the behavior
of which is due to the Jahn-Teller effect that shortens the equatorial bonds and
lengthens the axial bonds of a tetragonally distorted Cu(II)-L 6 complex (Sekaly
et al. 2003 ). Another mechanism is commonly provided by several studies (Chen
et al. 1986 ; Dixon and Larive 1999 ; Sharpless and McGown 1999 ; Simpson et al.
2002 ; Wrobel et al. 2003 ), which hypothesize that macromolecules (humic sub-
stances) can form aggregations that can be induced by complexation with metal
cations. These mechanisms do not explain how the functional groups in DOM or
the organic ligands can bind to trace metal ions. Recently, Mostofa and his col-
leagues ( 2009a , 2011 ) firstly provided a comprehensive mechanism for M-DOM
complexation using tryptophan as a model compound. A strong π -electron bond
between the functional group (F:) of tryptophan and the empty d -orbitals of the
transition metal (M n + ) is formed by donation of electrons (Fig. 7 ).
Based on the available literature data that concern M-DOM complexation, it is
suggested that the functional groups in DOM (or the organic ligands) can form
complexes with trace metal ions by two ways. First, donation of electrons occurs
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