Understanding the Behavior and Stability of Some Uranium Mineral Colloids

Colloids are believed to be a dominant mode of transport for radionuclides in the unsaturated zone and thus the characterization of the attributes of the colloids is important for understanding and predicting subsurface transport. As uranium-based spent nuclear fuels will be prevalent at the high-level waste repository at Yucca Mountain, we have examined the colloidal properties of a mixture of two uranium minerals. This paper presents results of analyses on colloidal suspensions of meta-schoepite, (UO ₂) ₄)(OH) ₆̇5H ₂O, and UO _2+x, in 10 mM uranyl nitrate or J-13 groundwater (from Yucca Mountain, NV). A suite of techniques was used to characterize the colloids. The colloids detected by dynamic light scattering in the 10 mM uranyl nitrate solution exhibited a mean size of ∼200 nm and a colloid concentration on the order of 10 ¹² particles/L. Likewise, large 200 nm colloids of meta-schoepite and UO _2+x were dominant in the transmission electron microscopy analyses. The colloids detected in the J-13 groundwater exhibited a bimodal distribution; large globular 100-200 nm UO _2+X colloids and needle-like colloids of meta-schoepite were observed (with extremely fine microstructure exhibited for the meta-schoepite colloids). Electrophoretic mobility measurements of the meta-schoepite and UO _2+x suspension in 10 mM uranyl nitrate indicated that the colloids are not stable at pH values less than or equal to five. While at pH values ≥ 5.5, the colloids of meta-schoepite and UO _2+x in 10 mM uranyl nitrate are stable, exhibiting zeta potentials of -30 to -60 mV. The results in this paper show that stable colloidal suspensions of uranium substrate colloids are possible under aqueous, oxidizing conditions.