COLLOIDAL ORGANIC MATTER MEDIATED FATE OF URANIUM: SORPTION, DESORPTION AND TRANSPORT

Natural organic matter (NOM) is ubiquitous in natural environment and plays an important role in regulating the fate of radionuclides in geologic media. However, the exact interactions of NOM with radioactive contaminants are not well understood. We have made various efforts to couple the biogeochemistry of NOM and the environmental fate for important actinides, including their sorption, desorption and transport. A dialysis bag-based set-up was developed for measuring the kinetics of uranium (U) sorption onto and desorption from colloidal materials. We found that interactions between colloidal humic acids (HAs) and metal-oxide (SiO₂, ZnO and hydrous ferric oxide (HFO)) colloids can strongly influence the sorption and desorption of U. HA-ZnO interactions enhanced the desorption of U from the heterogeneous colloidal suspensions, while the association between HA and SiO₂ or HFO inhibited the U desorption. Furthermore, association with colloidal HA may largely regulate the transport of U in geological media. Maximum relative breakthrough concentrations of U(VI) in our column experiments increased from undetectable levels (less than 0.001) in the absence of HAs to 0.17 to 0.54 with HAs. The strength of the HA effect on U mobility was positively correlated with the NMR-detected content of paraffinic carbon and the hydrophobicity of HAs, which indicates the importance of hydrophobic organic matter in facilitating U transport. It is clear that colloidal/dissolved NOM strongly affects the sorption, desorption and transport of U in groundwater, and such effects are sensitive to the molecular structure of NOM.