The nucleation and growth of calcite precipitates induced by the bacterial hydrolysis of urea (ureolysis) from a Sr-contaminant inclusive, and a Sr-free artificial groundwater (AGW) mimicking the composition of the 90Sr contaminated Snake River Plain aquifer were investigated. Sr-free experiments exhibited a gradual increase in mean calcite crystal diameter (<1000 nm) from day (D) 1 to 6, while in the Sr-inclusive experiments, daily diameters were approximately constant from D1 to D6, and crystals were smaller (mean <840 nm). These data demonstrate a steady state had been attained early in the Sr-inclusive experiments from growth inhibition by Sr. Modeling of the crystal growth mechanisms on the USGS GALOPER software suggested crystal size distributions in the Sr-inclusive and Sr-free experiments were generated in the nucleation stage by a decreasing nucleation rate with surface-controlled growth, followed by supply-controlled and random growth. This occurred despite the availability of Ca2+ and HCO3-, implying crystal growth is limited bythe rate of solute advection to the crystal surface. Calculation of the solubility constant (In KsO(A)) demonstrates smaller crystals are more soluble, reflecting a higher molar surface area. The coprecipitation of Sr therefore generates smaller and thus more soluble crystals. However, this is unlikely to dramatically reduce the long-term effectiveness of Sr immobilization because when crystal growth had ceased in the Sr-inclusive AGW, > 99% of calcite precipitated and Sr coprecipitated occurred in large crystals with a low solubility.
|Nifer y tudalennau||7|
|Cyfnodolyn||Environmental Science & Technology|
|Statws||Cyhoeddwyd - 01 Chwef 2006|