Environ. Sci. Technol. 2006, 40, 1008-1014
Effect of Strontium Contaminants upon the Size and Solubility of Calcite Crystals Precipitated by the Bacterial Hydrolysis of Urea ANDREW C. MITCHELL* AND F. GRANT FERRIS Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario, Canada, M5S 3B1
The nucleation and growth of calcite precipitates induced by the bacterial hydrolysis of urea (ureolysis) from a Srcontaminant 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 (CO3Sr+ surface groups is thought to inhibit growth at the calcite surface rather than Sr2+ binding to directly to >CO32- surface groups (17, 30). Molecular dynamics simulations of Sr incorporation at two experimentally observed steps on the (1014) calcite surface reveals that such observations are driven by changes in the energetic favorability of Sr incorporation (31). The incorporation of Sr is initially slightly endothermic (+1.8 to +35 kj mol-1 per CaCO3) in a solution with a 1:1 Ca/Sr ratio, competing effectively with Ca2+ at the calcite surface. However, over subsequent growth, Sr incorporation becomes more endothermic (+15 to +75 kj mol-1 per CaCO3) due to a mismatch between the rows of Sr-CO3 at the surface and the underlying lattice. Ultimately the presence of steps decorated with Sr-CO3 complexes inhibits further crystal growth (31), which is in agreement with experimental observations (17, 25, 28, 29). Precipitation rates measured in both the Sr-inclusive and Sr-free AGW are lower than those reported in seeded inorganic precipitation studies by 1-2 orders of magnitude at similar values of saturation, if rates are corrected for the surface area of the seed material (32). This likely reflects the inhibitory effect of other dissolved ions, such as Mg2+ and NH4+, in addition to Sr2+, and organic ligands from bacterial exudates. These are likely to screen active surfaces that would otherwise be involved in nucleation and crystal growth (21, 33) or complex with Ca2+ and CO32-, reduce reactive ion activity, and thus decrease mineral supersaturation and calcite precipitation rates (33). It is hard to assess the relative influence of inorganic and organic inhibitors from this current study. However, mature calcite crystals precipitated from both the Sr-free and Sr-inclusive AGWs exhibit somewhat roughened and poorly defined surfaces with rounded edges (Figure 1), which are similar to morphologies of mature calcite crystals precipitated from solutions of urea and CaCl2 in the presence of purified urease enzyme (3). This suggests that the distorted morphology of crystals, and the low precipitation rates measured in the Sr-inclusive and Sr-free experiments, are influenced predominately by the incorporation of NH4+ and urease derived proteins (3). Mineral distortion can allow more favorable contaminant incorporation (4, 34) which may account for the high DSr values induced by bacterial ureolysis in these experiments, even at such low rates of precipitation. Modeling Calcite Nucleation and Crystal Growth. Crystal evolution comprises a nucleation and growth stage, as can be modeled on GALOPER. Combinations of all nucleation mechanisms and all subsequent crystal growth mechanisms were modeled, as summarized in Figures S2 and S3 in the Supporting Information. The shape, variance, and mean crystal diameter of the observed log-normal CSDs from day 1 of both the Sr-free and Sr-inclusive AGW could only be generated by a decaying rate of nucleation and surfacecontrolled growth (where the growth rate is limited by the available surface area), followed by supply-controlled growth (where the growth rate is limited by the availability of reactants). Subsequent random growth (where random amounts of material are added to crystal surface) provided accurate evolution of the CSDs from days 1 to 2 in the Srinclusive AGW, after which growth ceased, and for continued growth from days 1 to 6 in the Sr-free AGW (12) (Figure 5; Supporting Information, Figure S4). Modeling suggested the larger CSD variance generated by day 1 in the Sr-free than the Sr-inclusive AGW was formed in the nucleation stage. This was generated by a lower probability of nucleation in the Sr-free AGW (0.63) than the 1012
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 3, 2006
FIGURE 5. Summary results of crystal nucleation and growth evolution modeling using USGS GALOPER software (12). (a) Initial nucleation by a decreasing rate of nucleation and surface-controlled growth. Larger CSD variance (β2) in the Sr-free AGW caused by a lower probability of nucleation in the Sr-free AGW (0.63) than the Sr-inclusive AGW (0.77). (b) Crystal growth from nucleation in the Sr-inclusive AGW to steady state by day 2, and (c) crystal growth from nucleation in the Sr-free AGW to steady state by day 4, by supply-controlled growth and random growth. Full results presented in Figure S4 of the Supporting Information. The strength of the modeled CSD fit, expressed as the residual squared of the crystal size frequency between the raw and modeled CSD, was always >0.91. Sr-inclusive AGW (0.77) after the critical saturation had been reached. This generated an initial CSD in the
