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 CO2 + O2 is employed to extract uranium (U) in sandstone by in situ leaching (ISL), mechanisms of U mobilization and O2 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 O2 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)22− and Ca2UO2(CO3)3(aq)) and thus facilitated U mobility. Generally, δ34SSO4 and δ18OSO4 in M2 and M1 were significantly lower than those in GC (p < 0.01). A Bayesian isotope mixing model of δ34SSO4 and δ18OSO4 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 O2 in the ore‐bearing aquifer. Most of the consumed O2 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 O2 utilization efficiency in improving the performance of ISL operations.

Characteristics of Dissolved Organic Matter in Uranium Hosting Aquifers and Potential Molecular Transformation During Neutral In Situ Leaching

“…Groundwater HCO 3 − concentrations were lower in BM (average 1.71 g/L; SD 0.44 g/L) than those in M2 (average 2.42 g/L; SD 0.14 g/L) and M1 (average 2.59; SD 0.05 g/L). Our previous study showed that the injection of CO 2 facilitated the dissolution of carbonates in aquifers with contents of calcite, dolomite, and ankerite from 0.4% to 0.6%, 0.2%–0.6%, and 3.4%–6.6%, respectively (Lu et al., 2023). The neutral ISL process controlled the groundwater pH in neutral to weakly acidic conditions to prevent carbonate precipitation from clogging the ore layer, which is consistent with our observed data.…”

“…Release and oxidation processes controlled the evolutionary characteristics of DOM during ISL, and therefore quantifying their contributions was particularly important for interpreting the evolutionary characteristics of DOM. Isotope mass mixing model have been widely used in quantifying the proportion of different sources of pollutants or elements in groundwater, especially carbon isotopes (Liu et al., 2022; Lu et al., 2023). δ 13 C DOC values in the rocks, BM, M2, and M1 were −21.41‰, −13.82‰, −15.60‰, and −15.75‰, respectively, which gradually decreased from BM to M2 and to M1.…”

“…Groundwater geochemical data, optical parameters and FT-ICR MS results associated with this study are available at the Mendeley open repository via https://data.mendeley.com/datasets/vz2ky8hjyg/1 with CC BY 4.0 licence (or using the following DOI: https://doi.org/10.17632/vz2ky8hjyg.1) (Lu et al, 2024).…”

“…In situ leaching (ISL) of U is a widely used mining technique of sandstone‐hosted U (Basu et al., 2015). During the neutral ISL process, dissolved O 2 and CO 2 were injected into U‐hosted aquifers via a series of injection wells to oxidize and dissolve U minerals, and then pumped out via production wells for U recovery (Lu et al., 2023). Organic matter (OM) or reduced minerals in these environment serves as a redox buffer to scavenge molecular oxygen (Pan et al., 2018) and easily leads to anoxic groundwater (Jylhä‐Ollila et al., 2021), which may affect the efficiency of ISL by consuming dissolved oxygen (Lu et al., 2023).…”

“…The U deposit containing high OM in the southwest Songliao Basin of northeastern China has been in situ mined by injection of oxygen and CO 2 for about 5 years. The oxidative degradation of DOM consumed oxygen and decreased the efficiency of U leaching (Lu et al., 2023). However, the roles of OM in U mineralization and the relationship between the transformation characteristics of OM during ISL and U leaching are still unclear.…”