The influence of citrate and oxalate on 99TcVII, Cs, NpV and UVI sorption to a Savannah River Site soil

Abstract

Batch sorption experiments were conducted with 0.5-50 ppb Tc,Cs, Np and U in the presence and absence of citrate and/or oxalate in a 25 g/L Savannah River Site (SRS) soil suspension. Citrate and oxalate were the ligands of choice due to their relevancy to plant exudates, the nuclides were selected for their wide range of biogeochemical behavior, and the soil from SRS was selected as a model Department of Energy (DOE) site soil. Batch samples were continually mixed on a rotary shaker and maintained at a pH of approximately 5. Analysis via ICP-MS indicated that sorption ofNp increased with ligand concentration compared to baseline studies, as did sorption of Tc although to a lesser extent. The increased sorption ofNp is proposed to be due to a combination of factors that are dependent on the ligand(s) present in the specific system including, ligand dissolution of the soil by citrate and formation of tertiary soil-oxalate-Np complexes. The increased Tc sorption is attributed to the dissolution of the soil by the ligands, leading to an increase in the number of available sorption sites forTc. Uranium sorption decreased and dissolution of native uranium was also observed with increasing ligand concentration, thought to be a result of the formation of strong U-ligand complexes remaining in the aqueous phase. The majority of these effects were observed at the highest ligand concentrations of 50 mg/L. No notable changes were observed for the Cs system which is ascribed to the minimal interaction of Cs with these organic ligands.

Mobility of Aqueous and Colloidal Neptunium Species in Field Lysimeter Experiments

“…The desorption coefficients are 810 and 1333 mL/g for 0.5 and 2.5 cm below source, respectively. The desorption coefficient for the sediment 15 cm below the Np­(IV) source is 10 mL/g (Figure ), which is in agreement with previous measurements of Np­(V) sorption to SRS clayey sediments , and is similar to measured K d values in the Np­(V) lysimeter, supporting the hypothesis that Np transported farther away from the Np­(IV) source is Np­(V) aqueous species. The higher K d values near the source indicate a stronger sorbing species of Np, likely Np­(IV), and these values in the Np­(IV) near-source region are similar to Pu­(IV) K d values, , suggestive of the presence of Np­(IV) just below the source.…”

“…Observed desorption coefficients from the Np­(V) source lysimeter are between 16 and 72 mL/g (Figure ), which agrees with previous measurements of Np­(V) sorption to SRS clayey sediments. , The Np­(V) lysimeter sediments show consistent behavior both above and below the source and develop a baseline of behavior for Np­(V) in the SRS sediment used in the lysimeters. Observed desorption coefficients from the Np­(IV) lysimeter do not show the same consistent behavior as a function of depth in the lysimeter.…”

“…The lysimeters were packed with a representative sandy clay loam sediment from SRS. , Selected physical and chemical properties of the soil used in these experiments are listed in SI Table S1, with full details given in Montgomery et al Notable properties include the organic matter content, which is less than 1%, and the sand/silt/clay ratio, which is 66/14/20. Of the clay fraction, greater than 95% is kaolinite.…”

Vadose-zone alteration of metaschoepite and ceramic UO2 in Savannah River Site field lysimeters

“…This sediment is a sandy clay loam representative of the vadose zone at the site. The clay fraction of this sediment (< 2 μm) contains kaolinite (> 95 %), Fe-oxyhydroxides (<2 % goethite and hematite) and quartz (< 2 %), with minor 2:1 clay phases (< 3 %, primarily as illite) (Montgomery et al, 2017). The sediment pH was 4.76.…”

“…The sediment was sieved (1 mm mesh) before packing and large pieces of organic matter were removed. The organic matter content of the sediment is 0.9 % after sieving (Montgomery et al, 2017). Duplicate sources of either UO 2 and UO 3 (see section 2.1) were placed horizontally at 25 cm and 48 cm from the top of each lysimeter and are herein termed the 'upper source' and 'lower source' for each particle type, respectively.…”