(281e) Colloid-Facilitated Transport of Plutonium in Saturated Porous Med

Packed column experiments were conducted to evaluate colloid-facilitated transport of Plutonium, Pu(V), and the suitability of selected synthetic colloids as tracers to natural colloids in saturated alluvium from a well at the southwest corner of the Nevada Test Site. The well is located along a projected flow pathway from the proposed high-level nuclear waste repository at Yucca Mountain, Nevada. Natural colloids and water used in the experiments were collected directly from groundwater pumped from the same well. Pu(V), which was electrochemically prepared and verified by UV-Visible absorbance spectroscopy to be over 99% pure (in the +5 oxidation state), was adsorbed onto the natural colloids in batch sorption experiments conducted in filtered water from the same well prior to the column experiments. After adsorbing about 40% of the total Pu from solution onto natural colloids (60% in solution), the resulting tracer was injected into two separate columns having different porosities, along with tritiated water and two different sizes of fluorescent-dye-tagged carboxylate-modified-latex (CML) microspheres (500- and 190-nm in diameter). Two experiments were conducted in each column, one with a relatively low injection flow rate (~ 1.2 mL/hr, resulting in a residence time of 45-50 hrs) and the other with a higher flow rate (~ 6 mL/hr with a residence time of 8 to 9 hrs). In both columns, results showed that natural colloids traveled through the columns essentially unretarded, with almost 100% recovery at the two flow rates employed. The natural colloids arrived slightly earlier than the tritiated water in all experiments, indicating a slightly smaller effective pore volume for the colloids. No soluble Pu was detected in the column effluents, while Pu adsorbed onto natural colloids was almost completely recovered in all experiments (100% at the higher flow rate and 95% at the longer residence times), indicating that natural colloids are capable of facilitating the transport of otherwise immobile Pu in subsurface alluvium at the NTS. These results are consistent with the very slow desorption of Pu from natural colloids observed in separate batch desorption experiments, which indicated essentially no measurable desorption of Pu over the time scales of the column experiments (~ 3 to 4%). The above results combined suggest that the desorption rates in the columns, while slow, may have been enhanced by the presence of competing mineral surfaces and collisions of the Pu-bearing colloids with these surfaces. Results also showed that the 500-nm CML microspheres were significantly attenuated in the column experiments, with recoveries of about 70% at the higher flow rate and between 45 and 60% at the lower flow rate, depending on the porosity of the column. The 190-nm microspheres, however, exhibited just slightly more attenuation than the natural colloids, suggesting that these microsphers (less than 200-nm in diameter) are good representatives to the transport of natural colloids in saturated alluvium. Colloids smaller than about 200-nm in size particularly play the major role in this enhanced transport phenomenon.