Complexation studies of Uranium(VI) by Sulfur and Nitrogen Containing Model Ligands in Aqueous Solution

The long-term safety assessment of nuclear waste disposals requires detailed knowledge of the transport and interaction behavior of actinides within the technical, geotechnical and geological barriers. The migration and interaction behavior of actinides can be effectively influenced by humic substances, biopolymers ubiquitous in natural environments, due to their ability for complex and colloid formation and their redox properties. It is known, that especially their carboxylic and phenolic OH groups are able to complex metal ions. In addition to these oxygen containing functional groups, humic substances also offer sulfur and nitrogen containing functionalities.

The aim of this work is to determine the influence of various sulfur and nitrogen containing functional groups on the uranium(VI) complexation and to evaluate their contribution in comparison to oxygen containing functional groups. For this, simple organic model ligands that can occur as building blocks for humic substances are used in the first instance with the objective to transfer the results to humic substances. In this study, results are presented for the uranium(VI) complexation by the organic ligands benzenesulfonic acid and 4-hydrxybenzenesulfonic acid as well as anthranilic and nicotinic acid. The complex formation constants for the uranium(VI) complexation were determined applying time-resolved laser-induced fluorescence spectroscopy (TRLFS) as a function of pH.