Spectroscopic identification of Np(V) sorption complexes on birnessite

Mn oxide minerals are ubiquitous in soils and sediments and participate in a variety of chemical reactions that affect groundwater and bulk soil composition. Even at tracer concentrations they play a decisive role in regulating the mobility of contaminants due to their high sorption and redox capacity and scavenging capability [1]. Due to its long half-life and its toxicity, Np-237 is considered as a major contaminant of the ecosystem in the long-term safety assessment of nuclear waste repositories. The pentavalent state is environmentally most relevant [2].
In this work, Np(V) sorption on amorphous birnessite (Bs) is investigated using a combination of ATR FT-IR and EXAFS spectroscopy on a molecular level with thermodyanamic modelling of batch sorption studies.
For the first time, in-situ Np(V) sorption is comparatively studied on the oxyhydroxides of Fe and Mn by ATR FT-IR spectroscopy under a variety of environmentally relevant sorption conditions, e.g. micromolar Np concentrations, low acidic to neutral pH, moderate ionic strength [3]. From the results, the formation of single inner-sphere complexes can be derived. In addition, time resolved spectra provide kinetic information on the surface reactions. Complementary EXAFS measurements evidence mononuclear bidentate edge-sharing Np(V) complexes on the Bs surface.