The development of nuclear weapons produced an overwhelming amount of radioactive waste that is currently being disposed of at the Waste Isolation Pilot Plant (WIPP). The WIPP is a deep geologic salt repository for long-term disposal of transuranic waste. The chemical behavior of actinide series elements (the most long-lived byproduct of the waste) and fission products is a major concern for the WIPP due to their long half-lives and unknown mobility in the halite environment. Moreover, experimental data describing the potential mobility of these contaminants under repository conditions is vital to create evidence-based risk assessment models. The objective of this research is to better understand the potential for transport of actinide elements in the +3, +4, and +6 oxidation states under high ionic strength conditions relevant to the WIPP repository including the presence of ethylenediaminetetraacetic acid (EDTA). EDTA was previously identified as the ligand that potentially poses the greatest threat due to its strong complexation of metals. Batch experiments were conducted in 0.01 to 1.0 M ionic strength brines consisting of NaCl, CaCl2, or MgCl2 with 5 mg/L of EDTA in the presence and absence of dolomite [CaMg(CO3)2] from near the WIPP repository with Nd+3, Th+4, and UO22+ up to 1000 µg/L. Although Nd is a lanthanide and not an actinide, it has been shown previously to exhibit similar chemistry to the trivalent actinide elements. Results show that the sorption affinity of EDTA-An complexes follows the trend Th &gt; Nd &gt; U with steady state reached by 30 days. Furthermore, filtration suggests that the EDTA may stabilize precipitated Th and Nd in the aqueous phase leading to potential for enhanced colloid-facilitated transport. Additional data is currently being analyzed in order to compare the effect of ionic strength and background electrolyte on the fate of these contaminants in the presence of EDTA.

Fate of actinides in the presence of EDTA and dolomite at variable ionic strength

256th National Meeting (2018)

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