SEQUESTERING STRONTIUM-90 BY CALCITE PRECIPITATION: PREDICTING STRONTIUM-90 UPTAKE IN THE EASTERN SNAKE RIVER PLAIN AQUIFER

Portions of the Eastern Snake River Plain Aquifer (ESRPA) and vadose zone are contaminated with ⁹⁰Sr and other divalent metals from past Department of Energy activities. We are investigating an in situ biogeochemical remediation approach that utilizes indigenous subsurface bacteria and introduced urea to sequester metals by coprecipitation into calcite. Ureolytic bacteria hydrolyze urea, raising the local pH and alkalinity. Because the ESRPA is close to saturation with respect to calcite, the increase in alkalinity expedites the precipitation of calcite and coprecipitation of metals. In situ stabilization and sequestration of this contamination is an attractive remediation strategy, especially true for ⁹⁰Sr as immobilization for ~ 290 years would reduce its radioactivity by 99.9%.

In prior free-drift batch calcite precipitation experiments pH, saturation state, and ammonium concentration all co-varied, confounding an assessment of the roles of each variable in the precipitation kinetics. This study assessed the effects of pH and ammonium concentration on calcite precipitation/strontium coprecipitation rates using a chemostat and inorganic analogs. Calcite was precipitated in a constantly-stirred, flow-through tank reactor from an artificial groundwater based on the composition of ESRPA. Ammonium carbonate (a proxy for hydrolyzed urea) was introduced while the pH was held constant. These experiments were conducted at three pH levels, two ammonium carbonate concentrations, and with or without strontium.

Experimental results were used to calculate the calcite precipitation rate constant and reaction order of an affinity based rate expression. Preliminary results were consistent with our observations from batch experiments, i.e. positive correlations between a) saturation states and precipitation rates and b) strontium incorporation and precipitation rates. However, our results supported a first order affinity based precipitation reaction rather than second order as previously assumed in the batch studies. Experimental data were used in The Geochemist's Workbench® to assess the applicability of the affinity-based rate expression R=k(&Omega-1)ⁿ for the conditions in the ESRPA and to predict strontium uptake with experimentally derived distribution coefficients.