Mn and Fe oxides and hydroxides play an important role in regulating the mobility of contaminants in aquifers. Neptunium (Np) is one of the most important components of nuclear waste to consider for the long-term safety assessment of nuclear waste repositories [1]. The sorption behavior of Np(V) onto Fe oxides was studied by macroscopic experiments [2]. Mn-oxides were rarely investigated. ATR FT-IR spectroscopy is a useful tool for the in-situ identification of surface species [3]. Time-resolved measurements provide kinetic information on the surface reactions. Complementary information on molecular structure and atomic environment can be elucidated from EXAFS spectroscopy and quantum chemistry.
Np(V) sorption on amorphous birnessite (Bs) is investigated at micromolar Np concentrations by combining ATR FT-IR and EXAFS spectroscopy under a variety of environmentally relevant sorption conditions. Upon sorption of micromolar Np(V) on Bs, a band observed at 789 cm⁻¹ is assigned to the antisymmetric stretching vibrational mode (ν₃) of NpO₂. The IR spectrum obtained at equal conditions in an aqueous solution shows the absorption of ν₃(Np^VO₂) at 818 cm⁻¹ [3]. The red shift of ν₃ to 789 cm⁻¹ upon sorption can be assigned to an inner-sphere sorption complex. Kinetic experiments have shown that only one sorption complex was formed independent from Np(V) loading. No impact of ionic strength and pH on the sorbed species was found. Experiments in the presence of carbonate were performed to elucidate molecular information on ternary surface complexes. By EXAFS structural analysis of batch samples the surface complex was a binary edge-sharing Np(V) sorption species. From a spectroscopic comparison of Np(V) surface complexation on different mineral oxides of Fe, Mn, Si and Ti, similar sorption behavior was elucidated [3,4].
DFT was also applied. The 6-31G(d,p) basis set was used for H, C, O and Mn and a quasi-relativistic pseudopotential for Np with the ECP60MWB-SEG basis set. The B3LYP exchange-correlation functional was used in all cases. The derived molecular structures allows more reliable surface complexation modeling of macroscopic data.

Frank Bok

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