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can form complexes with DOM more rapidly than the weak metals, and that their
DOM complexes undergo a slower dissociation process (Wu et al. 2004c ; Lin et al.
1995 ). It is also shown that the distribution of transition metals in DOM is shifted
towards the larger molecular size fractions as the binding strength increases. Fe
along with V and Ce is distributed mainly in the larger size fractions, but heavy
metals such as U, Th and Mo are distributed mainly in the smaller molecular size
fractions (Wu et al. 2004b ). These studies hypothesize that the strong affinity metal
ions can form complexes with the functional groups or replace the weak affinity
metal ions in the larger molecular size fractions in the first binding site. Weaker
metals then occupy other available sites in the small molecular size fractions.
Molecular size (or mass) distribution and levels of organic ligands suggest that
new organic ligands with high molecular masses are produced during periods of
high biological productivity in natural waters (Midorikawa and Tanoue 1996 , 1998 ;
Mopper et al. 1996 ). Such new organic ligands are considered to be the autoc-
thonous fulvic acids (C-like) that have recently been found to originate under pho-
toinduced or microbial assimilation of algal or phytoplankton biomass (Fig. 1 c,
d) (Mostofa and Sakugawa 2009 ; Zhang et al. 2009 ). The fluorescence excitation-
emission maxima of autocthonous fulvic acid (C-like) are similar to those of the
allochthonous fulvic acid and humic acid (C-like), showing two fluorescence peaks
in the C- and A-regions (Fig. 1 a-d). In the molecular mass distribution of organic
ligands, the relative contribution of the fraction with <5 kDa molecular masses
is dominant (67-79 %), while 17-30 % of the total organic ligands are in the
5 kDa-0.1 μ m fraction, leaving 3-6 % in the 0.1 μ m-GF/F fraction in lake water
(Wu and Tanoue 2001a ). The contribution of organic ligands in the <1 kDa frac-
tion is 41 % of the total in estuarine water (Gordon et al. 1996 ). The contribution of
total organic ligands in DOM accounts for 10-62 % in the case of molecular masses
of >1 kDa and 50-90 % for the <1 kDa fraction in sea waters (Midorikawa and
Tanoue 1998 , 1996 ; Maurer 1976 ; Zsolnay 1979 ; Carlson et al. 1985 ; Benner et al.
1992 ; Guo et al. 1994 , 1995 , 1996 ; Buesseler et al. 1996 ; Guo and Santschi 1996 ).
Contributions of total organic ligands are 0.63-4.68 % of the bulk DOM in the water
of rivers, lakes and oceans (Wu and Tanoue 2001a , c ; Midorikawa and Tanoue 1998 ;
Wu et al. 2001 ). It has also been observed that the quantities of the weak ligands are
relatively high, approximately 0.54-1.21 % of the total DOM whilst the quantities of
strong ligands are low, approximately 0.06-0.21 % in stream waters (Wu and Tanoue
2001a ). High-affinity nitrogenous moieties account for only 2-4 % of DOM in water
(McKnight et al. 1997 ; Croue et al. 2003 ). Finally, the M-DOM complexation signifi-
cantly depends on the quantities, nature and molecular size of DOM in natural waters.
5.2 Occurrences and Affinity of Trace Metals in M-DOM
Complexation
The M-DOM complexation significantly depends on the occurrence of the trace
metal ions and on their affinity toward organic ligands. Complexation shows
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