Comparative investigation of the neptunium(V) sorption onto gibbsite by means of ATR FT-IR spectroscopy

The molecular reactions of actinides at the solid-water interface play an important role in the retardation of radionuclides in the environment. Hence, the investigation of the interactions of actinides with metal oxides such as Al(OH)3, Fe(OOH)x, TiO2, or SiO2, serving as model phases for more complex, naturally occurring minerals in aqueous solution, becomes essential for the safety assessment in the near and far field of nuclear repositories. In recent years, the sorption behavior of neptunium (Np) onto synthetic and naturally occurring minerals was insufficiently studied. The majority of these studies provide macroscopic results presenting sorption capacities of the substrates and the effect of selective parameters on the sorption behavior. However, for a better understanding of the sorption mechanisms, structural information on a molecular level of the type of surface complex is still needed.
Comprehensive studies using ATR FT-IR spectroscopy have been carried out to investigate the in situ formation of neptunyl(V) surface complexes on aluminum hydroxide, namely gibbsite. This substrate serves as a model phase for more complex mineral systems, e.g. clay minerals. The surface complexation of Np(V) on amorphous and crystalline gibbsite was studied in detail by a multiplicity of experiments in the presence and absence of atmospherically derived carbonate . In the absence of carbonate, one inner-sphere complex is formed on amorphous gibbsite, whereas no sorption occurs on the crystalline gibbsite. In the presence of carbonate and dependent on the crystal structure, different surface species (inner-, outer-sphere and ternary) were derived from the spectra