Redox Stability of Neptunium(V) in the Presence of Humic Substances of Varying Functionality

Naturally occurring aquatic humic substances (humic and fulvic acids) are known to effect the speciation and thus, the migration behavior of actinide contaminants in environmental systems due to their complexing and redox properties and their ability to form colloids. Therefore, to understand and predict the mobility of actinides in natural aquifer systems, amongst others, information on their redox stability in the presence of humic substances is necessary.
In the present work, the time dependence of the reduction of Np(V) to Np(IV) by humic substances of varying functionality has been studied under anaerobic conditions between pH 3.5 and pH 9. Synthetic humic acids with pronounced redox functionality (type Cat-Gly and Hyd-Glu) [1] were studied in comparison to natural humic substances (Aldrich humic acid, Kranichsee fulvic acid). For Np speciation in solution liquid-liquid extraction, laser-induced photoacoustic spectroscopy (LIPAS), NIR absorption spectroscopy, and ultrafiltration were applied. In comparison to the natural humic substances, the synthetic humic acids lead to a much stronger reduction of Np(V) to Np(IV) [2]. The Np(IV) formed in the course of the experiments is stabilized as Np(IV) humate. The tetravalent oxidation state remained constant for several months.
The redox capacities of the synthetic humic acids, which are significantly higher than those of the natural humic substances, can be attributed to their higher phenolic/acidic OH group contents compared to natural humic substances. The dominating role of phenolic/acidic OH groups for the reduction of Np(V) by humic substances could be verified applying a synthetic humic acid with blocked phenolic/acidic OH groups (type Hyd-Glu-PB). Furthermore, the influence of NO3- on the Np(V) reduction by humic substances was studied.
The results have shown that by application of the synthetic humic acids with distinct redox functionalities actinides can be stabilized in lower oxidation states, e.g. in complexation and sorption studies.

[1] Sachs, S., Schmeide, K., Brendler, V., Krepelová, A., Mibus, J., Geipel, G., Heise, K.H., Bernhard, G.: Investigation of the Complexation and the Migration of Actinides and Non-radioactive Substances with Humic Acids under Geogenic Conditions. Complexation of Humic Acids with Actinides in the Oxidation State IV Th, U, Np. FZR-399, Wissenschaftlich-Technische Berichte, Forschungszentrum Rossendorf, Dresden 2004.
[2] Schmeide, K., Geipel, G., Bernhard, G.: Study of the Neptunium(V) Reduction by Various Natural and Synthetic Humic Substances. In: FZKA 7070, Wissenschaftliche Berichte (G. Buckau, ed.). Forschungszentrum Karlsruhe, Karlsruhe 2005, 19-31.