ABSTRACT
Results are presented from thermodynamic and kinetic studies of dissolved rare earth element (REE) uptake by basin-fill alluvial materials from the Yucca Flat area of the Nevada Teat Site (NTS). Aqueous/particle interactions between REE and alluvial materials were investigated through stable isotope and radiotracer batda and time-series experiments in synthetic “groundwater” solutions Synthetic groundwater was prepared so that its composition duplicated the major ion composition resulting from the equilibration of distilled deionized water with the alluvial materials for a period of 120 hours. Synthetic groundwaters were prepared from analytical reagent-grade chemicals and 18 Mohm-cm distilled deionized waer. Studies were conducted as “free-drift” experiments in which the pH was adjusted to the desired value at the beginning of the experiment and subsequently recorded at the end of the experiment. Experiments were conducted with the complete suite of REE in a concentration range of 1 nanomolar to 1 micromolar and with a suspended solid load of 30-35 g/L. Separate experiments were conducted using four different alluvial material types distinguished on the basis of predominant clast mineralogy and, with in each material type, on the basis of size fraction: fines (< 53µm), said (53µm - 2mm), and gravel (>2mm). The results from stable isotope experiments using micromolar REE concentrations reveal an atomic number dependency on REE uptake from solution over a period of 48 hours. In three of the four basin-fill material types light REF (LREE) were preferentially sorbed relative to their heavy (HREE) coounterparts. The fractionation between LREE and HREE is a result of competition between REE-complexes in solution and the formation of REE-surface complexes on the sediment particles comprising the NTS alluvium. The results from nanomolar concentration stable isotope experiments showed a much lower degree of REE fractionation, owing either to less competition for available surface sites or poor analytical resolution at such low liquid-phase concentrations. The radiotracer experiments conducted at nanomolar concentration levels, indicate that there is no systematic relationship between particle size and uptake rate, and that the uniformity of uptake rate is proportional to particle size. The radiotracer experiments indicate that REE fractionation is inversely proportional to particle size. The observations presented here are generally consistent with previous experimental studies conducted using pure, synthetic mineral phases aid with solution complexation studies, indicating that the HREE have a greater tendency to form more stable solution complexes than the LREE.
