title: Effect of strontium contaminants upon the size and solubility of calcite crystals precipitated by the bacterial hydrolysis of urea.
abstract: 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.
authors.affiliation: Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario, Canada, M5S 3B1. andy@geology.utoronto.ca
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Mitchell
authors.forename: F Grant
authors.initials: FG
authors.lastname: Ferris
mesh_terms: D001419:Bacteria; D002119:Calcium Carbonate / Q000737:chemistry*; D011232:Chemical Precipitation; D003460:Crystallization; D006868:Hydrolysis; D008962:Models, Theoretical; D012990:Soil Pollutants, Radioactive; D013324:Strontium / Q000737:chemistry*; D014508:Urea / Q000378:metabolism*; D014875:Water Pollutants, Radioactive
chemical_list: D012990:Soil Pollutants, Radioactive; D014875:Water Pollutants, Radioactive; D014508:Urea; D002119:Calcium Carbonate; D013324:Strontium