HR: 0830h
AN: B41D-0925    [PDF]
TI: Partitioning of Sr$^{2+}$ into Calcite Precipitates Induced by Bacterial Ureolysis in Artificial Groundwater.
AU: * Mitchell, A C
EM: andy@geology.utoronto.ca
AF: Department of Geology, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1 Canada
AU: Ferris, F G
EM: ferris@geology.utoronto.ca
AF: Department of Geology, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1 Canada
AU: Mancini, A
AF: Department of Geology, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1 Canada
AU: Litwin, Y
AF: Department of Geology, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1 Canada
AB: A suite of experiments were performed to investigate the partitioning of Sr$^{2+}$ (to mimic the radionuclide $^{90}$Sr) between calcite and groundwater, in response to the hydrolysis of urea by {\it B. pasteurii} under simulated in-situ aquifer conditions. Three duplicate experiments were performed at $25\deg$C over 8 days in microcosms inoculated with {\it B. pasteurii} ATCC 11859 and containing an artificial groundwater (AGW). The AGW composition was based upon the aqueous chemistry of the metal and radionuclide contaminated Snake River Plain Aquifer, Idaho,USA. Microcosms also contained 25 mM urea, and 1 mM of Sr as a contaminant treatment. Control experiments were run with non urea hydrolyzing bacteria, {\it B. subtillus}. Control experiments exhibited little change in pH, and dissolved ammonium and Ca$^{2+}$ concentrations. Conversely during experiments inoculated with {\it B. pasteurii}, ammonium production increased asymptotically, peaking two days into the experiment when approximately all urea had been hydrolyzed. The production of ammonium and bicarbonate from urea hydrolysis caused an asymptotic increase in pH from 6.5 to 9.1 one day into the experiment. Dissolved Ca$^{2+}$ and Sr$^{2+}$ decreased asymptotically from the beginning of the experiment, and was accompanied by the development of solid precipitates identified as calcite by X-Ray diffraction. This caused an asymptotic decrease in the saturation state of calcite ({\it S}) after one day of the experiment. Specific rate constants were derived for calcite precipitation and critical saturation state from the time course data following a second-order chemical affinity based law. Calcite precipitation rate is fundamentally controlled by, and exhibits a positive association with {\it S}. Mass balance indicates the percentage of total Sr (dissolved and in calcite) incorporated into the calcite precipitate increases rapidly to 59 % after two days of the experiment, and increases less rapidly thereafter to a maximum of 67 %. Corresponding measured distribution coefficients (D$_{Me}$) exhibit a positive association with {\it S} and calcite precipitation rates. Therefore D$_{Me}$ is greatest after one day of the experiment (D$_{Me}$ maximum = 0.39), which corresponds to the highest {\it S} and calcite precipitation rate, and decreases thereafter (D$_{Me}$ minimum = 0.16) as {\it S} and calcite precipitation rate decreases. Therefore the extent of Sr incorporation into calcite precipitates resulting from the hydrolysis of urea by {\it B. pasteurii} appears to be a primary function of precipitation rate, which is controlled by {\it S}. The median D$_{Me}$ determined by this study (0.22) is greater than previously published coefficients for Sr in calcite by up to an order of magnitude. This demonstrates the potential of calcite precipitation by bacterial ureolysis as a remediation strategy for $^{90}$Sr in calcite saturated aquifers.
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2003 Fall Meeting