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

Although neutral in situ leaching through CO2 + 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 CO2 efficiently stimulated U mobilization in the mining aquifer. The injected CO2 critically promoted the dissolution of carbonate minerals, which enhanced the formation of uranyl carbonate (predominantly CaUO2(CO3)(2)(2-) and Ca2UO2(CO3)(3)(aq)) and thus facilitated U mobility. Generally, d(34)S(SO4) and d(18)O(SO4) in M2 and M1 were significantly lower than those in GC (p < 0.01). A Bayesian isotope mixing model of d(34)S(SO4) and d(18)O(SO4) showed that the contribution of pyrite oxidation to SO42- 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.

Automated summary

Although the mechanisms of uranium (U) mobilization and oxygen (O-2) consumption in the in situ leaching (ISL) of U in sandstone have remained unclear, this study found that the introduction of CO2 and O-2 efficiently stimulated U mobilization in the mining aquifer. The dissolution of carbonate minerals promoted by injected CO2 enhanced the formation of uranyl carbonate and facilitated U mobility. The results also emphasized the importance of increasing O-2 utilization efficiency to improve the performance of ISL operations.