Geophysical monitoring and reactive transport modeling of ureolytically-driven calcium carbonate precipitation

Remediation of subsurface radionuclides and trace metal contaminants is one of the U.S. Department of Energy's (DOE) greatest challenges for long-term stewardship [1]. Co-precipitation in calcium carbonate is one attractive in-situ remediation strategy for divalent radionuclide and trace metal ions, such as 60Co, 137Cs, and 90Sr [2-4]. The partitioning of these trace metals, e.g. 90Sr, into the calcium carbonate lattice is partly a function of precipitation rate [5-7]. One mechanism to promote calcium carbonate precipitation is through urea hydrolysis which increases groundwater pH and alkalinity, resulting in greater mineral saturation with respect to calcium carbonate. Urea hydrolysis is catalyzed by the urease enzyme, expressed by many microorganisms in the subsurface in order to harvest nitrogen. Figure 1 illustrates the conceptual model of the proposed reaction and contaminant sequestration processes [8].The reactions involved in this process include:1. Enzymatically catalyzed urea hydrolysis produces NH4+ and HCO3- and raises pH:2. NH4+ promotes desorption of cations M2+(e.g. Ca2+ and Sr2+) from grain surfaces:3. HCO3- promotes precipitation of calcium carbonate and co-precipitation of 90Sr:The net reaction leads to the production of carbonate minerals containing Sr:Compared to direct injection of carbonate solution that could lead to clogging near the injection wells due to rapid precipitation of calcium carbonate, microbially facilitated calcium carbonate precipitation relies on the production of carbonate via urea hydrolysis, allowing urea to transport further into the subsurface before significant production of carbonate occurs. This could mitigate the risk of wellbore clogging and allow treatment of a larger area while using a single injection point. In addition, the NH4+ produced during ureolysis can exchange with cations (including radionuclides such as 90Sr2+) previously sorbed to the subsurface materials (Eq. 2), thereby promoting metal sequestration