Integrating Variable Aquifer Geochemistry and Sediment Properties into Models of Uranium Retention at the Nuclear Metals Incorporated Superfund Site

The EPA-designated Superfund site Nuclear Metals, Inc. of Concord, Massachusetts, is contaminated by depleted uranium, a byproduct of defense-related uranium metals processing. Uranium retention in the solid phase, either by adsorption or precipitation, impedes its transport and dissemination from the contamination source. Geochemical parameters including pH, Ca content and alkalinity influence uranium speciation and phase partitioning and thus its transport properties. Understanding these effects is essential to the formulation of an informed remediation strategy. This research examines and attempts to model uranium adsorption in contaminated and uncontaminated sediments in a range of synthetic ground waters with compositions similar to those found at the site. Surface area and particle size were the dominant sediment characteristics that influenced the extent of uranyl adsorption. Uranyl hydroxycarbonate complexes are the dominant form of adsorbed uranium in neutral solutions; dissolved carbonate enhances uranium solubility due to the formation of these and other soluble aqueous complexes. A surface complexation model and Langmuir-type isotherm incorporating uranyl adsorption data over a range of aqueous carbonate, calcium, and pH conditions described adsorption reasonably well. Both models incorporated the formation of uranyl solution complexes, and both models were improved slightly by including the presence of ternary uranyl carbonate surface complexes. For the surface complexation model, adsorption extent was modeled most effectively at low uranium concentrations. The Langmuir isotherm based on soluble UO2^(2+)(aq) concentrations (rather than total dissolved uranyl) predicted adsorption over a wider range of uranyl compositions than could a surface complexation model.