Calcium carbonate precipitation by ureolytic subsurface bacteria

Coprecipitation in carbonate minerals offers a means of slowing the transpo rt of divalent radionuclides and contaminant metals (e.g., Sr-90(2+), UO22, Co2+) in the subsurface. It may be possible to accelerate this process by stimulating the native microbial community to generate chemical conditions favoring carbonate precipitation. In a preliminary evaluation of this appr oach, we investigated the ability of ureolytic subsurface bacteria to produ ce alkaline conditions conducive to calcium carbonate precipitation. Ground water samples from the Eastern Snake River Plain (ESRP) aquifer in Idaho we re screened for urea-hydrolyzing microorganisms; three isolates were select ed for further evaluation. Analysis of 16S rRNA gene sequences indicated th at two of the ESRP isolates were of the genus Pseudomonas, and the other wa s a Variovorax sp. The specific urease activities of the ESRP isolates appe ared to be similar to each other but less than that of Bacillus pasteurii, a known urease-positive organism. However, calcium carbonate was rapidly pr ecipitated in all cultures that were supplied with urea and calcium, and X- ray diffraction analyses indicated that calcite was always the predominant carbonate polymorph produced. The correspondence between measured calcium c oncentrations and equilibrium predictions suggested that the rate of calcit e precipitation was directly linked to the rate of urea hydrolysis. These r esults are promising with respect to the potential utility of this approach for in situ remediation and indicate that further evaluation of this appro ach under conditions more closely simulating environmental conditions is wa rranted.