Chemical Equilibria Model of Strontium-90 Adsorption and Transport in Soil in Response to Dynamic Alkaline Conditions

Abstract

Strontium-90 is a major hazardous contaminant of radioactive wastewater and its processing sludges at many Department of Energy (DOE) facilities. In the past, such contaminated wastewater and sludge have been disposed in soil seepage pits, lagoons, or cribs often under highly perturbed alkaline conditions (pH > 12) where 90Sr solubility is low and its adsorption to surrounding soil is high. As natural weathering returns these soils to near-neutral or slightly acidic conditions, the adsorbed and precipitated calcium and magnesium phases, in which 90Sr is carried, change significantly in both nature and amounts. No comprehensive computational method has been formulated previously to quantitatively simulate the dynamics of 90Sr in the soil-groundwater environment under such dynamic and wide-ranging conditions. A computational code, the Hydrologic Utility Model for Demonstrating Integrated Nuclear Geochemical Environmental Responses (HUMDINGER), was composed to describe the changing equilibria of 90Sr in soil based on its causative chemical reactions including soil buffering, pH-dependent cation-exchange capacity, cation selectivity, and the precipitation/dissolution of calcium carbonate, calcium hydroxide, and magnesium hydroxide in response to leaching groundwater characteristics including pH, acid-neutralizing capacity, dissolved cations, and inorganic carbonate species. The code includes a simulation of one-dimensional transport of 90Sr through a soil column as a series of soil mixing cells where the equilibrium soluble output from one cell is applied to the next cell. Unamended soil leaching and highly alkaline soil treatments, including potassium hydroxide, sodium silicate, and sodium aluminate, were simulated and compared with experimental findings using large (10 kg) soil columns that were leached with 90Sr-contaminated groundwater after treatment. HUMDINGER's simulations were in good agreement with dynamic experimental observations of soil exchange capacity, exchangeable cations, total 90Sr, and pH values of layers within the soil columns and of column effluents.

Cosorption Characteristics of SeO42– and Sr2+ Radioactive Surrogates Using 2D/2D Graphene Oxide-Layered Double Hydroxide Nanocomposites

“…Conversely, a rapid surge in the sorption capacity of Sr 2+ was observed above pH 10, which is most likely related to the formation of Sr­(OH) 2 ( K sp = 6.4 × 10 –3 ). However, the solution data indicates the decrease in the concentration of dissolved Mg 2+ at this region, which informs the possibility of the co-precipitation of Sr 2+ with Mg 2+ as Mg­{Sr}­(OH) 2 . Similar observations were demonstrated on the C-dot-decorated MgAl-LDH nanocomposites during Sr 2+ sorption .…”

“…However, the solution data indicates the decrease in the concentration of dissolved Mg 2+ at this region, which informs the possibility of the co-precipitation of Sr 2+ with Mg 2+ as Mg{Sr}(OH) 2 . 41 Similar observations were demonstrated on the C-dot-decorated MgAl-LDH nanocomposites during Sr 2+ sorption. 6 The removal capacity of SeO 4 2− was almost maintained in the broad pH range of 3−10 (Figure 8A) due to the alkalization of the solution by the dissolution of Mg 2+ to maintain the equilibrium pH (Figure 8B).…”

Carbon-Dot-Decorated Layered Double Hydroxide Nanocomposites as a Multifunctional Environmental Material for Co-immobilization of SeO42– and Sr2+ from Aqueous Solutions

“…The critical analysis of the solution data revealed that the increase in the pH of the medium as well as decrease in the dissolved Mg 2+ concentration was observed at this region. Based on the observations it can be concluded that the Sr 2+ was partially coprecipitated with Mg 2+ as Mg­{Sr}­(OH) 2 , which may be the reason for such enhanced Sr 2+ adsorption from the solution . At lower pH of the medium higher dissolution Mg 2+ occurs from solid for the alkalization of solution and it was precipitated with Sr 2+ at higher pH (Figure B).…”

Laboratory Directed Research and Development Program FY 2004 Annual Report

“…Thirdly, 85 Sr in untreated soil behaved as expected for a cation-exchangeable species in soil. Over several months, the dissolved cations in stream water replaced most, but not all, of the 85 Sr resulting in an expected degree of equilibration over time given the soil's cation exchange capacity and the concentration of dissolved cations in stream water (Spalding and Spalding 2001). Some of the spiked 85 Sr (about 8%) in the untreated soil appeared to be naturally attenuated and remained strongly adsorbed to the soil.…”