Potential for uranium release under geologic CO2 storage conditions: The impact of Fe(III)

Abstract

Saline aquifers are considered ideal subsurface sinks for large amounts of CO2 storage. It is common to have trace levels of uranium-bearing minerals (naturally occurring radioactive material, NORM) in sandstone saline aquifers and CO2 injection may cause uranium mobilization due to the coupled chemical and physical interactions at mineral-water interfaces. In this study, we developed a TOUGHREACT model to assess the uranium mobilization potential from uraninite (UO2) dissolution induced by CO2 injection into a hypothetical CO2 storage reservoir with Fe(III)-bearing hematite in a time scale of 30 years CO2 injection and 100 years after CO2 injection. Numerical simulation results show that injection of CO2 reduced pH and induced small amounts of hematite dissolution, which released Fe3+ into aqueous phase. Fe3+ together with dissolved bicarbonate species caused oxidative dissolution of UO2 (Fe3+ serving as an oxidant), resulting in an increase of dissolved uranium concentrations in the CO2 storage reservoir. However, the released uranium was limited to a small region close to Fe3+ supply source and the maximum concentration of released uranium was only 9.00 × 10−10 mol/kg in the CO2 storage reservoir at t = 30 years. The availability of Fe3+ is the main factor that limits mineralized uranium release because the pH drop induced by CO2 injection is not low enough to cause significant Fe3+ release. For sorbed uranium, the main factor that influences sorbed uranium release is pH because uranium sorption is amphoteric. Based on our simulation results, the pH range in the CO2 storage reservoir does not cause a substantial release of sorbed uranium. Also, the potential for released uranium to migrate through a permeable zone in the caprock to the layer above the caprock and cause groundwater contamination is negligible. In summary, results from this study imply a very low risk of environmental contamination by bicarbonate and Fe(III)-induced uranium mobilization.