NEPTUNIUM IV AND V SORPTIN TO END-MEMBER SUBSURFACE SEDIMENTS TO THE SAVANNAH RIVER SITE

Kaplan, D.

Savannah River Site (S.C.)

关键词: Valence;    Radioactive Waste Disposal;    Sediments;    Neptunium;    Radionuclide Migration;   

DOI  :  10.2172/969288
RP-ID  :  SRNL-STI-2009-00634
RP-ID  :  DE-AC09-08SR22470
RP-ID  :  969288

美国|英语

来源: UNT Digital Library

【 摘 要 】

Migration of Np through the subsurface is expected to be primarily controlled by sorption to sediments. Therefore, understanding and quantifying Np sorption to sediments and sediments from the Savannah River Site (SRS) is vital to ensure safe disposal of Np bearing wastes. In this work, Np sorption to two sediments representing the geological extremes with respect to sorption properties expected in the SRS subsurface environment (named 'subsurface sandy sediment' and 'subsurface clayey sediment') was examined under a variety of conditions. First a series of baseline sorption tests at pH 5.5 under an oxic atmosphere was performed to understand Np sorption under typical subsurface conditions. These experiments indicated that the baseline K{sub d} values for the subsurface sandy and subsurface clayey sediments are 4.26 {+-} 0.24 L kg{sup -1} and 9.05 {+-} 0.61 L kg{sup -1}, respectively. These Np K{sub d} values of SRS sediments are the first to be reported since Sheppard et al. (1979). The previous values were 0.25 and 0.16 L kg{sup -1} for a low pH sandy sediment. To examine a possible range of K{sub d} values under various environmental scenarios, the effects of natural organic matter (NOM, also a surrogate for cellulose degradation products), the presence of various chemical reductants, and an anaerobic atmosphere on Np sorption were examined. The presence of NOM resulted in an increase in the Np K{sub d} values for both sediments. This behavior is hypothesized to be the result of formation of a ternary Np-NOM-sediment complex. Slight increases in the Np sorption (K{sub d} 13-24 L kg{sup -1}) were observed when performing experiments in the presence of chemical reductants (dithionite, ascorbic acid, zero-valent iron) or under anaerobic conditions. Presumably, the increased sorption can be attributed to a slight reduction of Np(V) to Np(IV), the stronger sorbing form of Np. The most significant result of this study is the finding that Np weakly sorbs to both end member sediments and that Np only has a slight tendency to reduce to its stronger sorbing form, even under the most strongly reducing conditions expected under natural SRS conditions. Also, it appears that pH has a profound effect on Np sorption. Based on the these new measurements and the revelations about Np redox chemistry, the following changes to 'Best K{sub d}' values, as defined in Kaplan (2006), for SRS performance assessment calculations are recommended.