Study of the Neptunium(V) Reduction by Various Natural and Synthetic Humic Substances

The time dependence of the reduction of Np(V) to Np(IV) by various natural and synthetic humic substances was studied under anaerobic conditions between pH 3.5 and pH 9. For Np speciation in solution liquid-liquid extraction, NIR absorption spectroscopy, laser-induced photoacoustic spectroscopy (LIPAS) and ultrafiltration were applied. In comparison to natural humic substances, synthetic humic acids lead to a stronger reduction of Np(V) to Np(IV). The Np(IV) formed in the course of the experiments is stabilized in form of Np(IV) humate complexes, whereas the remaining Np(V) occurs as NpO2+ ion or Np(V) humate depending on pH. The higher reduction potential of the synthetic humic acids can be attributed to their higher phenolic/acidic OH group contents compared to natural humic substances. The influence of phenolic/acidic OH groups on the redox behavior of humic substances was verified applying a synthetic humic acid with blocked phenolic/acidic OH groups.