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DOI: http://dx.doi.org/10.1065/espr2002.08.130 --- Metal complexation by natural ligands is important
for metal transport and distribution in surface and ground water.
The goal of the work was to study the ligand exchange rate
for two important metal ions in natural aquatic systems (Al, Fe)
was determined using EDTA and natural organic matter (NOM)
of humic type as ligands.
After adding EDTA to a solution containing metal-NOM complexes,
these complexes dissociated and metal-EDTA complexes
were formed. Metal-NOM complexes were separated from
metal-EDTA complexes with the help of size-exclusion chromatography
and detected by on-line inductively coupled plasmamass
spectrometry (ICP-MS). Injecting the samples into the system
over time after addition of EDTA allowed us to measure
the rate of the exchange of NOM by EDTA.
The experiments could be well described with a first-order rate
law assuming that the dissociation of the metal-NOM complexes
is the rate-determining step. The exchange rate of Fe was found
to be faster than that of Al. This corresponds well with the
exchange rate of water molecules from the coordination sphere
of the metal ions, which is also faster for Fe than for Al. Furthermore,
the UV and the fluorescence signal of the chromatograms
were measured.
The results indicate that no disaggregation of NOM molecules
took place, although about 75-85% of the aggregate-forming
metal ions exchanged NOM by EDTA in their coordination
sphere. This suggests clearly the fundamental role of NOM in
colloidal transport of metals and in their bioavailability.