THE EFFECT OF NATURAL ORGANIC MATTER ON THE FATE OF PLUTONIUM WITHIN SILICA SAND AND CONTAMINATED SOIL

Over the last several years, evidence has indicated that small amounts of Pu are mobile in saturated groundwater systems. Models of Pu transport via the ‘colloid pathway' have focused on Pu transport by inorganic colloidal species [1]. However, it is clear from our laboratory studies that natural organic matter (NOM) such as humic substances and bacterial metabolic products, including exocellular polymeric substances (EPS), are capable of complexing Pu; such complexes have the potential to affect the transport behavior of Pu. Batch studies were used to determine the effect of citric acid, fulvic acid, and Pseudomonas fluorescens EPS on the sorption of Pu to either silica sand or Pu-contaminated soil. ‘Static column' [2] studies were used to investigate the effect of fulvic acid on the transport of ²⁴¹Pu in silica sand and compared to P. fluorescens EPS data. It was found that the rate of Pu uptake was dependent on the type of NOM present. This indicates that the kinetics of NOM-Pu complexation and NOM sorption to the media must be accounted for in Pu sorption studies.

Pu contamination in soils is the weapons grade ^239,240Pu, an alpha emitter, present at low levels. Due to the low activity, tracers are often used to study the behavior of ‘aged' plutonium. Tracers eliminate the need for a labor-intensive radiochemical procedure and simulate a new deposition of Pu to the environment. Due to their ability to potentially facilitate Pu transport there is much interest in the behavior of colloids. For this study, selective sequential extraction was utilized to study the transformation of Pu colloids over time. Static column studies were used with Pu-contaminated soil from Rocky Flats, Colorado and the introduction of a ²⁴¹Pu tracer, allowed to 'age' for 1-day to 9-months. The change in Pu distribution over time may indicate a transformation of the ²⁴¹Pu colloids introduced. The indigenous microbial activity present in the soil may hasten the transformation of the ²⁴¹Pu and will be investigated by comparing abiotic and biotic columns.

[1] Honeyman, B.D. and J.F. Ranville (2002). Soil Geochemical Process of Radionuclides. Chapter 7. Soil Science Society of America Special Publication. p. 131 – 163. [2] Loveland, J.P., J.N. Ryan, G.L Amy, and R.W. Harvey (1996). Colloids and Surfaces A: Physiochemical and Engineering Aspects, 107: 205 – 221.