Influence of Oxidation Processes on the Interaction between Natural Organic Matter and Metal Ions (NOM)
Influence of Oxidation Processes on the Interaction between Natural Organic Matter and Metal Ions
The "MetalOM" project focuses on the complexation of natural organic matter (NOM) with metal ions using steady-state and time-resolved fluorescence spectroscopy. In particular the influence of oxidation of NOM on its complexation capacity is characterised.
The intrinsic fluorescence of NOM as well as the sensitised luminescence of different lanthanides were used to study the binding of metal ions to NOM derived from different origin. Based on their interaction with NOM, the studied metal ions can be classified in two categories: a) Al(III+) and Gd(III+) increase the NOM fluorescence which is generally non-specific and b) the other metal ions induce quenching of the fluorescence signal, which is most significant for Eu(III+) and Cu(II+).
Oxidation of NOM (Ozonation, UV-irradiation, chlorination, UV/H2O2 treatment) initially causes an enhancement of the fluorescence signal. Further oxidation, however, induces mineralisation into CO2 and, consequently, the fluorescence decreases again. These changes in fluorescence intensity are not uniform but restricted to distinct spectral zones. Concerning the interaction with metal ions, on a first glance no change in the earlier mentioned induced effects is observed after oxidation. However, the complexation capacity and complexation strength are changed. The decrease of the conditional complexation constant after oxidation is connected to the decrease of the apparent molecular size of NOM. Through oxidation, degradation of NOM into smaller fractions is induced. Because the formation of chelates by these fragments is almost absent, less stable complexes are formed. The fragmentation of NOM was confirmed by size exclusion chromatography (SEC).
Future work will focus on the speciation of metal ions in the presence of NOM and the influence of NOM on the transport of metal ions in soils.