Date of Award
Master of Science (MS)
Environmental Engineering and Earth Science
Powell, Brian A
Freedman , David
Fjeld , Robert
Kaplan , Daniel
Models which can estimate environmental transport of contaminants are required to ensure minimal risk to the public. These models require accurate transport parameters in order to correctly predict how these contaminants will move in the subsurface. This work aimed to determine more accurately the distribution coefficients for neptunium, strontium, and radium for the Savannah River Site (SRS) Performance Assessment (PA). All neptunium sorption studies were performed at a pH value of 5.50 and an ionic strength of 0.01M as NaCl with neptunium concentrations ranging from 0.01 to 50 ppb. Two different sediments were examined, SRS Subsurface Clayey and SRS Subsurface Sandy. Baseline neptunium Kd values were determined to be 9.05 ± 0.61 L kg-1 and 4.26 ± 0.24 L kg-1 for the clayey and sandy sediments, respectively. Treatments were applied to the sorption experiments to examine what effects environmental perturbations would have on the Kd values. These were natural organic matter (NOM), reductants (ascorbic acid, dithionite, zero valent iron, hydrogen peroxide), and performing the experiment under an anaerobic atmosphere. The addition of NOM to the clayey sediment resulted in an increase in the Kd value most likely due to the formation of ternary soil-NOM-Np complexes. None of the reductants nor the anaerobic atmosphere resulted in large increases in Kd values for either sediment, indicating that little to no reduction of Np(V) to Np(IV) occurred. Long term equilibration experiments (71 days) indicated that even prolonged equilibration under anoxic conditions do not facilitate reduction of Np(V) to Np(IV). Desorption Kd values were calculated under the baseline and anaerobic conditions and found to approach the sorption Kd values given a long enough equilibration period which indicated fully reversible sorption. This was further confirmed with a flowcell experiment that was able to desorb >99.9% of sorbed neptunium from the clayey sediment. Sorption and desorption rate constants were calculated to be 0.0165 min-1 and 2.5x10-4 min-1, respectively. These experiments showed that neptunium sorption to sediments can be simplified to Np(V) sorbing to surficial iron. A simplified model was developed using Citrate-Dithionite-Bicarbonate (CDB) iron concentrations that can predict neptunium sorption based on three different reactions.
Radium and strontium sorption to the sediments was found to be highly dependent upon ionic strength due to competition for ion exchange sites. Radium Kd values for the clayey sediment were determined to be 185.1 ± 25.63 L kg-1 and 30.35 ± 0.66 L kg-1 for ionic strengths of 0.02M and 0.1M as NaCl which is the approximate ionic strength of groundwater. Radium Kd values for the sandy sediment were determined to be 24.95 ± 2.97 L kg-1 and 9.05 ± 0.36 L kg-1 for ionic strengths of 0.02M and 0.1M as NaCl. These values were greater than the strontium sorption Kd values which was consistent with the assumptions in the SRS PA.
Miller, Todd, "CONCEPTUAL MODEL TESTING AND DEVELOPMENT FOR NEPTUNIUM AND RADIUM SORPTION TO SRS SEDIMENTS" (2010). All Theses. 974.