Multi‐Isotope Based Identification and Quantification of Oxygen Consuming Processes in Uranium Hosting Aquifers With CO2 + O2 In Situ Leaching

Abstract Although neutral in situ leaching through CO 2 + O 2 is employed to extract uranium (U) in sandstone by in situ leaching (ISL), mechanisms of U mobilization and O 2 consumption remained unclear. To address this gap, 18 groundwater samples were taken from the Qianjiadian sandstone U ore field, including seven samples from production wells in mining area M1 (mining for 5 years), six samples from production wells in mining area M2 (mining for 4 years), and five samples from monitoring wells (GC), to quantify U‐mobilizing processes in the mining aquifer by employing hydrogeochemical compositions and multi‐isotopes. The introduction of O 2 and CO 2 efficiently stimulated U mobilization in the mining aquifer. The injected CO 2 critically promoted the dissolution of carbonate minerals, which enhanced the formation of uranyl carbonate (predominantly CaUO 2 (CO 3 ) 2 2− and Ca 2 UO 2 (CO 3 ) 3 (aq)) and thus facilitated U mobility. Generally, δ 34 S SO4 and δ 18 O SO4 in M2 and M1 were significantly lower than those in GC ( p < 0.01). A Bayesian isotope mixing model of δ 34 S SO4 and δ 18 O SO4 showed that the contribution of pyrite oxidation to SO 4 2− concentration increased from 1.7% in GC to 13.6% in M2 and to 15.0% in M1. During ISL, pyrite, ammonium, and dissolved organic carbon were major compounds competing with U(IV) for introduced O 2 in the ore‐bearing aquifer. Most of the consumed O 2 was used for pyrite oxidation (56.2%) and U(IV) oxidation (39.3%), following the thermodynamic sequence of those redox reactions. The current results highlighted the significance of increasing O 2 utilization efficiency in improving the performance of ISL operations.