STRONTIUM-90 REMEDIATION BY MICROBIALLY MEDIATED CALCITE PRECIPITATION: POTENTIAL APPLICATION AT HANFORD, WASHINGTON

Strontium-90 is a groundwater contaminant of concern at the US Department of Energy Hanford, Washington site. The disposal of contaminated water related to the operation of plutonium production reactors has resulted in an estimated in-ground ⁹⁰Sr inventory of 75 to 89 Ci and groundwater concentrations of up to 6,000 pCi/L at the Hanford 100-N area near the Columbia River. Although groundwater contamination is of primary concern, simple mass balance calculations suggest that greater than 99% of the ⁹⁰Sr at this site is sorbed to the aquifer medium. Because the sorbed fraction is a potential continuing source for groundwater contamination, an effective remediation strategy should treat both the dissolved and sorbed contaminant fractions.

In situ containment and stabilization of ⁹⁰Sr by its facilitated co-precipitation with calcite is a potentially cost-effective treatment strategy. The approach relies upon the microbially mediated hydrolysis of introduced urea to cause the acceleration of calcite precipitation (and trace metal co-precipitation) by increasing pH and alkalinity. In addition, the ammonium ions produced from urea hydrolysis can serve to liberate cations from the aquifer matrix by exchange reactions.. A mixed equilibrium-kinetic biogeochemical model that accounts for urea hydrolysis by ureolytic bacteria and calcite precipitation/dissolution using kinetic expressions has been developed. Cation exchange reactions and metal partitioning into the precipitated calcite are treated as equilibrium processes. Simulation of hypothetical remediation scenarios using mM levels of urea show that almost 1 mmole of calcite is precipitated per mmole of hydrolyzed urea, with most of the precipitated cations having been derived from exchange with ammonium ions on the aquifer matrix. Because of the cation exchange reactions and the near absence of ammonium ions in the groundwater, the long-term persistence of the precipitated calcite is a function of the concentration of the applied urea and the total cation exchange capacity of the aquifer matrix. Our simulation results suggest that with appropriate urea injection strategies calcite precipitation can provide for the long term in situ sequestration of ⁹⁰Sr at the Hanford site.