INFLUENCE OF UV-OXIDATION ON THE METAL COMPLEXING PROPERTIES OF NOM

摘要

The interaction of metals and natural organic matter (NOM) is of influence on the metal transport in aquatic systems and soil. In the presented study, NOM of a brown water lake was oxidized with UV irradiation to elucidate the influence of oxidative degradation on the metal complexing properties of NOM. UV/Vis and fluorescence spectroscopy, as well as size-exclusion chromatography (SEC) with online detection of UV absorption, fluorescence and metal concentration were used to investigate the property changes of NOM caused by oxidation. After oxidation, the fluorescence intensity of NOM increased considerably despite a decrease in the UV absorption. The SEC experiments showed a shift towards smaller molecular sizes in the oxidized NOM samples and a decrease in the stability constants of corresponding Al-NOM complexes. For Pb no such effect could be determined. The studies on the dissociation kinetics of metal-NOM complexes revealed a slower dissociation of Al and Pb complexes with original NOM compared to oxidized NOM. The determined dissociation rate constants were used to predict the migration of metal ion-NOM complexes in column experiments. The experimental data and model predictions were in good agreement for the divalent metals Pb and Zn, but differed substantially for Al. The stability of the metal complexes with original NOM decreased in the order Al > Pb > Zn. After UV-oxidation, the complexing ability of NOM towards Al- ions was decreased. In NOM equilibrated quartz columns, Pb and Zn were immobilized in the presence of all types of NOM, whereas Al was readily eluted. The original NOM was more effective for complexing Al than the oxidized form. Colloid formation however can stimulate the Al migration.