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

Abstract

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-quinoid 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 NpO 2 + ion or Np(V) humate depending on pH. The ability of synthetic HA to effectively maintain Np in the tetravalent state during humate complexation experiments could be shown.

Sorption of Np(V) and Np(IV) onto kaolinite: Effects of pH, ionic strength, carbonate and humic acid

“…The preparation of the initial Np(V) stock solution ( 237 Np, 0.03 M Np in 1 M HClO 4 ) from NpO 2 (CEA Marcoule, France) is described in Schmeide and Bernhard (2009).…”

“…This HA (elemental composition: C: 53.7 wt.%, H: 2.3 wt.%, N: 2.0 wt.%, S: 0.2 wt.%, O: 28.2 wt.%; PEC: 3.84 ± 0.27 meq/g) is an oxidation product of hydroquinone synthesized in the presence of glutamic acid. Hyd-Glu effectively reduces Np(V) to Np(IV) and stabilizes the tetravalent oxidation state during complexation experiments (Schmeide and Bernhard, 2009).…”

“…After 1 month, the Np(V) was quantitatively reduced by the HA ($99% Np(IV)) (details on the reduction process can be found in Schmeide and Bernhard, 2009). To start the sorption experiment, 1 mL of this Np(IV)/HA stock solution was added to the pre-conditioned kaolinite ([Np] 0 : 1.0 Â 10 À6 M, [HA] 0 : 50 mg/L, I: 0.01 M NaClO 4 , N 2 atmosphere).…”

“…p.a., Fluka, Taufkirchen, Germany) in xylene (puriss. p.a., Fluka) (Bertrand and Choppin, 1982;Schmeide and Bernhard, 2009) to evaluate the quality of the sorption experiments. The final HA concentration in the supernatant was determined by UV-Vis spectroscopy by recording absorbance at 320 nm from the sample supernatants and also from the supernatants of kaolinite suspensions without HA as blanks (equilibrated at pH 6.5, 8.5, 10) to take into consideration any absorbance of dissolved kaolinite.…”

“…Generally, for tetravalent radionuclides (Zr, Hf, Th, U, Np, Pu) a strong sorption onto mineral surfaces in the absence of additional complexing ligands, such as carbonate or humic material, or other groundwater colloids, has been reported (Sanchez et al, 1985;Hakanen and Lindberg, 1991;Nagasaki et al, 1999;Takahashi et al, 1999;Nakata et al, 2002;Reiller et al, 2002;Banik et al, 2007). Furthermore, it has been shown that Np(V) is reduced to Np(IV) by humic substances (e.g., Zeh et al, 1999;Shcherbina et al, 2007;Schmeide and Bernhard, 2009). The resulting formation of Np(IV) humate complexes strongly increased Np migration through sandy sediment and granitic material columns in laboratory experiments (Artinger et al, 2000;Sakamoto et al, 2000).…”