Redox stability of neptunium(V) and neptunium(IV) in the presence of humic substances of varying functionality

The reducing properties of humic substances (humic acid (HA) and fulvic acid (FA)) of varying functionality towards Np(V) have been studied under anaerobic conditions between pH 3.5 and pH 9.0 in batch experiments. For Np redox speciation in solution solvent extraction, NIR absorption spectroscopy and ultrafiltration were applied. The reduction rate varied with type of humic substances, solution pH, HA to Np concentration ratio, and equilibration time. In comparison to natural humic substances, synthetic HA with designed redox properties lead to a stronger reduction of Np(V) to Np(IV). The reducing capacities of humic substances towards Np(V) could be correlated to their phenolic/acidic OH group content, which includes both hydroquinone-like moieties and non-quinonoid phenols. By applying a synthetic HA with blocked phenolic/acidic OH groups, the dominance of phenolic/acidic OH groups as the redox-active moieties of humic substances was verified.
The Np(IV) formed in the course of the experiments is predominantly humic colloid-bound. Np(IV) oxo/hydroxide colloids, that might be formed in addition, are stabilized by adsorbed humic substances. The remaining Np(V) occurs as NpO₂ ⁺ ion or Np(V) humate depending on pH. The ability of synthetic HA to effectively maintain Np in the tetravalent state during complexation experiments could be shown.