Microbially mediated calcium carbonate precipitation: Implications for interpreting calcite precipitation and for solid-phase capture of inorganic contaminants

Microbial degradation of urea was investigated as a potential geochemical c atalyst for Ca carbonate precipitation and associated solid phase capture o f common groundwater contaminants (Sr, UO2, Cu) in laboratory batch experim ents. Bacterial degradation of urea increased pH and promoted Ca carbonate precipitation in both bacterial control and contaminant treatments, Associa ted 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 solut ion formation rather than discrete Sr carbonate phase precipitation. In con trast, UO2 capture was not as effective, reaching only 30% of the initial 1 mM UO2 added and also reversible, dropping to 7% by 24 h. These results li kely reflect differing sires of incorporation of these two elements-Ca latt ice sites for Sr versus crystal defect sires for UO2. Cu sequestration was poor; resulting from toxicity of the metal to the bacteria, which arrested urea degradation and concomitant Ca carbonate precipitation. Scanning elect ron microscopy (SEM) indicated a variety of morphologies reminiscent of tho se observed in the marine stromatolite literature. In bacterial control tre atments, X-ray diffraction (XRD) analyses indicated only calcite; while in the presence of either Sr or UO2, both calcite and vaterite, a metastable p olymorph of Ca carbonate, were identified. Tapping mode atomic force micros copy (AFM) indicated differences in surface microtopography among abiotic, bacterial control, and bacterial contaminant systems. These results indicat e that Ca carbonate precipitation induced by passive biomineralization proc esses is highly effective and may provide a useful bioremediation strategy for Ca carbonate-rich aquifers where Sr contamination issues exist.