815
Complexation of Dissolved Organic Matter with Trace Metal Ions in Natural Waters
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 suggests 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 hydrophilic 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 concentration, differently from Cu–DOM complexation; and (ii) Cu–DOM complexation 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 depending 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 contaminated effluents (Bux et al. 1994); Cu > Cr > Zn > Pb for activated sludge microorganisms (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 anaerobic 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 activated 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 constants 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 electronic configuration of the metal ions in aqueous media.
Complexation of Dissolved Organic Matter with Trace Metal Ions in Natural Waters
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 suggests 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 hydrophilic 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 concentration, differently from Cu–DOM complexation; and (ii) Cu–DOM complexation 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 depending 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 contaminated effluents (Bux et al. 1994); Cu > Cr > Zn > Pb for activated sludge microorganisms (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 anaerobic 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 activated 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 constants 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 electronic configuration of the metal ions in aqueous media.