Plutonium partitioning to colloidal and particulate matter in an acidic, sandy sediment: Implications for remediation alternatives and plutonium migration

Plutonium partitioning within a bulk, freshwater sediment and to specific size fractions of the sediment was determined. Fission track analysis was used to observe the spatial heterogeneity of fissile isotopes of Pu and U in the sediment. For the bulk sediment, a six-step sequential extraction scheme was used to elucidate Pu partitioning. Although no direct Pu speciation is obtained from our sequential extraction scheme, this study demonstrates that the partitioning information obtain ed is more useful for evaluating ex-situ remediation treatments than information obtained from complete digestion and analysis of the bulk sediment. The majority of Pu-238 and Pu239+240 appear to be partitioned in the oxidizable fraction, suggesting that Pu is primarily associated with organic matter in the bulk sediment or may exist as a discrete, oxidizable phase. By varying filter pore size used to separate the sequential extraction leachate solution from the remaining solid phase, a fraction of (PU)-P-238 associated with colloidal material was observed, and chemical evidence suggests that this colloidal material is relatively refractory. Pu partitioning to various size fractions of the bulk sediment was also compared to the percent organic carbon present in those size fractions. Interestingly, little correlation was observed between the percentage of organic carbon and concentrations of Pu isotopes in the various size fractions, although differences were observed in the distributions of (238)pu versus Pu239+240 in the size fractions. These results suggest that other sediment phases may also be important for Pu partitioning. Our observations are described in the context of feasibility of various remediation options.