Ureolytic CaCO₃ precipitation for immobilization of arsenic in an aquifer system

Abstract

The objective of this study was to precipitate CaCO₃ in a groundwater media to reduce dissolved arsenic concentrations. In this study a mixture of ureolytic calcite and aragonite were precipitated using groundwater as the media. Although precipitation of carbonate was successful using Ardkenneth groundwater, arsenic concentrations were not reduced. Ureolytic calcite and aragonite precipitated using broth as the media and resulted in a decrease in arsenic concentrations of up to 88% from the initial 0.7 μg Lˉ¹ concentration. Ureolytic carbonate precipitation required the inoculation of ureolytic cultures isolated from groundwater into both the groundwater and broth media. Precipitates in the inoculation experiments were identified using infrared spectroscopy techniques. The decrease in arsenic concentrations in the inoculated urea treated broth samples compared to the groundwater samples was attributed to the greater amounts of precipitate formed in the broth media. The broth had a free Ca(II) concentration of 1300 mg Lˉ¹ whereas the Ardkenneth groundwater had a free Ca(II) concentration of 36 to 42 mg Lˉ¹. The higher free Ca(II) concentrations in the broth media would account for the higher yield of carbonate precipitate, making Ca(II) concentration a limiting factor in ureolytic CaCO₃ remediation techniques. Formation of a visible precipitate required the addition of nitrate to the broth and groundwater samples. The inoculated cultures, being denitrifiers, required a nitrate source. Ca(II) ion concentrations decreased in the different media without the addition of nitrate, but no visible precipitate formed. Laboratory experiments using Ardkenneth groundwater and treatments of 0.03 M urea did not decrease the Ca(II) ion concentrations or reduce arsenic in solution. These results suggest that inoculation with selected ureolytic cultures was needed to optimize the precipitation of CaCO₃ in a natural groundwater system. The results of this study suggest that arsenic was reduced by the precipitation of ureolytic CaCO₃. Arsenic reduced by ureolytic CaCO₃ precipitation required adequate levels of Ca(II) ions, higher than those found in the Ardkenneth aquifer. Successful precipitation of CaCO₃ by ureolytic organisms also required an adequate cell density. Thus, inoculation with ureolytic cultures optimized the broth and groundwater media for CaCO₃ precipitation.