Dissolution-Desorption Dynamics of Strontium During Elution Following Evaporation: pH and Ionic Strength Effects

Abstract

Radioactive strontium-90 (90Sr2+) is a fission byproduct of uranium and plutonium production, and therefore understanding its environmental fate is of particular importance for predicting the evolution of long-term risk from historical releases. The nonradioactive strontium cation, Sr2+, is a chemical analog for 90Sr2+ that is often used in studies designed to understand the environmental behaviors of 90Sr2+. The focus of this work was on understanding the dynamics of remobilization of strontium following evaporation to dryness in porous media. Evaporation is ubiquitous in the unsaturated zone, and has the potential to significantly impact the dynamics of transport by driving adsorption or precipitation on solid surfaces. For this work, a series of transport experiments were conducted examining the behavior of strontium over a range of pH values, ionic strengths, and concentrations. Saturated transport experiments were conducted, followed by experiments designed to examine the release and transport following evaporation to dryness. Results show increasing saturated retardation with increasing pH, decreasing ionic strength, and decreasing concentration, with the concentration exhibiting the strongest effect. Breakthrough curves at low concentrations were also found to be consistent with significant rate-limited desorption. Remobilization elution curves measured following evaporation to dryness exhibited the high initial effluent concentrations, exceeding the influent strontium concentration, most likely caused by the initial dissolution and accumulation of strontium by the advancing solution. Concentrations at later times were found to be largely consistent with the dynamics of saturated transport for the systems studied.