Geomicrobiology Journal, 18:93 –115, 2001 Copyright C ° 2001 Taylor & Francis 0149-0451/01 $12.00 + .00 Microbially Mediated Calcium Carbonate Precipitation: Implications for Interpreting Calcite Precipitation and for Solid-Phase Capture of Inorganic Contaminants LESLEY A. WARREN School of Geography and Geology McMaster University Hamilton, Ontario, Canada PATRICIA A. MAURICE Department of Geology Kent State University Kent, Ohio, USA NAGINA PARMAR F. GRANT FERRIS Department of Geology University of Toronto Toronto, Ontario, Canada Microbial degradation of urea was investigated as a potential geochemical catalyst for Ca carbonate precipitation and associated solid phase capture of common groundwater contaminants (Sr, UO 2 , Cu) in laboratory batch experiments. Bacterial degradation of urea increased pH and promoted Ca carbonate precipitation in both bacterial control and contaminant treatments. Associated solid phase capture of Sr was highly effective, capturing 95% of the 1 mM Sr added within 24 h. The results for Sr are consistent with solid solution formation rather than discrete Sr carbonate phase precipitation. In contrast, UO 2 capture was not as effective, reaching only 30% of the initial 1 mM UO 2 added, and also reversible, dropping to 7% by 24 h. These results likely reect differing sites of incorporation of these two elements—Ca lattice sites for Sr versus crystal defect sites for UO 2 . Cu sequestration was poor, resulting from toxicity of the metal to the bacteria, which arrested urea degradation and concomitant Ca carbonate precipitation. Scanning electron microscopy (SEM) indicated a variety of morphologies reminiscent of those observed in the marine stromatolite literature. In bacterial control treatments, X-ray diffraction (XRD) analyses indicated only calcite; while in the presence of either Sr or UO 2 , both calcite and vaterite, a metastable polymorph of Ca carbonate, were identied. Tapping mode atomic force microscopy (AFM) indicated differences in surface microtopography among abiotic, bacterial control, and bacterial contaminant systems. These results indicate that Ca carbonate precipitation induced by passive biominerali- zation processes is highly effective and may provide a useful bioremediation strategy for Ca carbonate-rich aquifers where Sr contamination issues exist. Received 6 March 2000; accepted 25 September 2000. Address correspondence to Lesley A. Warren, School of Geography and Geology, McMaster University, Hamilton, ON L8S 4K1, Canada. E-mail: warrenl@mcmaster.ca 93