Subsurface pH Manipulation for the Immobilization of Uranium.

Historical nuclear materials processing has created a massive contamination at the US Department of Energy managed complexes. At the Oak Ridge Y-12 S-3 site, the contaminated groundwater is characterized by a low pH (~3.5) but high contents of U (up to 60 mg/L), Al (~500 mg/L), nitrate (up to 10,000 mg/L), and other toxic metals and ions. Few treatment options currently exist to effectively remove or immobilize these contaminants in situ at the site. This study was performed to (1) evaluate the subsurface pH manipulation for immobilizing these contaminants; (2) develop a practical geochemical model to predict aqueous and solid-phase speciation during pH manipulation; and (3) evaluate potential microbial activity and community changes following the pH manipulation. Initial laboratory studies indicate that a controlled addition of strong base (KOH) provided a rapid yet effective means of sequestering U(VI), Tc(VII), and other toxic metals. Greater than 90% of soluble U(VI) and Tc(VII) could be immobilized at pH >4.5 through the co-precipitation with or adsorption onto Al-oxyhydroxides and other minerals. The wet-chemical and spectroscopic analyses (e.g., EXAFS and FTIR) provided evidence and mechanisms of the co-precipitation and adsorption processes. An equilibrium reaction model was developed and implemented in the code HydroGeoChem 5.0. Comparison of model results with experimental results indicated a close agreement. Small to intermediate scale column flow-through experiments also confirmed that more than 92% of U(VI), Tc(VII), Co(II), and Ni(II) could be retained and stabilized at a slightly acidic pH condition. No significant changes in hydraulic properties were observed, and no or little biological denitrification detected during an extended pH manipulation process. The present study provides a scientific basis and demonstrates the technical feasibility to stabilize contaminant U(VI) and Tc(VII) through the subsurface pH manipulation, which may lead to a significantly reduced U(VI)/Tc(VII) flux at this highly contaminated site.