Environmental Engineering Reference
In-Depth Information
Trace metal ions have a specific interaction affinity with organic ligands or
DOM that generally depends on the size and outer electronic configuration level
of the metal ions. Replacement of a metal ion by a higher affinity metal ion in
M-DOM complexation occurs because the number of complexing sites remains
constant for a specific amount of DOM. A decrease in fluorescence intensity is
often detected after further addition of Cu
2
+
to Hg-DOM complexes, which sug-
gests that Cu
2
+
can act as a stronger quencher for DOM than Hg(II) in aqueous
media (Fu et al.
2007
). The complexation of humic acid, olive cake and its hydro-
philic extracts with Cu
2
+
and Eu
3
+
shows that Cu
2
+
is replaced by Eu
3
+
in the
aqueous solution (Konstantinou et al.
2007
,
2009
; Konstantinou and Pashalidis
2010
). Eu
3
+
has only one electron in its outer
d
-orbital shell, which gives it strong
affinity to bind to DOM compared with Cu
2
+
. Comparison between Ca-DOM and
Cu-DOM complexation demonstrates that (i) Ca-DOM complexation increases of
much less than an order of magnitude per pH unit and decreases at higher Ca con-
centration, differently from Cu-DOM complexation; and (ii) Cu-DOM complexa-
tion is highly non-linear, in contrast to the very reduced extent of non-linearity of
Ca-DOM complexation (Lu and Allen
2002
).
The effective distribution of affinities (Conditional Affinity Spectrum, CAS) of
a metal ion binding to a humic substance under natural water conditions suggests
three groups of cations (Rey-Castro et al.
2009
): (a) Al, H, Pb, Hg, and Cr, which
are preferentially bound to the phenolic sites of the fulvic ligand; (b) Ca, Mg, Cd,
Fe(II), and Mn, which display a higher affinity for carboxylic sites, in contrast to
expectations based on the individual complexation parameters; and (c) Fe(III), Cu,
Zn, and Ni, for which phenolic and carboxylic distributions are overlapped.
The complexation of trace metals with the functional groups in EPS varies depend-
ing on the environmental conditions. The order is Pb > Cd > Zn for exopolysaccharides
of bacterial origin (Loaec et al.
1997
); Cu > Cd > Ni > Cr(III) > Cr(VI) for contami-
nated effluents (Bux et al.
1994
); Cu > Cr > Zn > Pb for activated sludge microorgan-
isms (Chua et al.
1999
); Zn > Cu > Co > Cd > Cr(III) > CrO
4
2
−
> Ni for activated
sludge (Liu et al.
2001
); Pb > Cd > Cu > Zn for polluted waters of sewage sludge and
paper mill waste (Lister and Line
2001
); Cu > Cd > Ni for pure cultures of bacteria
originated from activated sludge (Kim et al.
2002
); Pb > Cu > Zn > Ni for the first
site and Pb > Cu > Ni > Zn for the second site of an acidogenic thermophilic anaero-
bic reactor (Leighton and Forster
1997
); Cd > Pb
≈
Cu for activated sludge originated
from wastewater treatment plants (Guibaud et al.
2003
); and Cu > Ni > > Zn for acti-
vated sludge (Guibaud et al.
2003
).
Trace metals have highly variable affinity towards various functional groups
in DOM and they show strong differences in the conditional stability constants
of M-DOM complexation (Table
1
) (Sonke and Salters
2006
). The complexation
between lanthanides and humic substances (fulvic and humic acids) suggests that
a gradual increase by 2-3 orders of magnitude in the conditional stability con-
stants from La to Lu follows the decreasing ionic radius and is an expression of
the lanthanide contraction (Sonke and Salters
2006
).
Overall, the M-DOM complexation greatly depends on the outer-shell elec-
tronic configuration of the metal ions in aqueous media.
