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

Dissolved organic matter (DOM) is widespread in uranium (U) hosting aquifer, a key substance affecting CO2 + O2 in situ leaching (ISL) of U by consuming injected O2. However, molecular composition and molecular transformation of DOM are poorly understood in this aquifer during the U mining. To address this issue, Fourier transform ion cyclotron resonance mass spectrometry was applied to investigate DOM molecules in groundwater samples and rock samples collected from a U mining aquifer. The high proportion of labile tyrosine‐like component and BIX value in monitoring wells (BM) indicated the biological origin of DOM. CHOS and aliphatic were the major components of water‐soluble organic matter (WSOM) in U‐bearing rocks, but CHO and highly unsaturated structures with high oxygen (HUSHO) were the main components of DOM in BM. The ISL led to the release of WSOM and the oxidation of DOM, which accounted for 24.5%–57.4% and 39.1%–65.7% of expected DOM, respectively. Potential molecular transformations (PMTs) results showed that the intragroup transformations dominated, and remarkable transformations from CHON and CHONS to CHO occurred during ISL. The positive net PMTs of condensed aromatic structure (CAS), aromatic structures (AS), and aliphatic suggested that they were transformed from HUSLO and HUSHO. A gradual decrease in (DBE−O)/Cwa and a gradual increase in NOSCwa indicated that the saturated degree and oxidation state of DOM increased during mining. This work successfully identified molecular characteristics of DOM in U hosting aquifer, and proposed a paradigm of DOM transformation pathways during ISL. Plain Language Summary: Dissolved organic matter (DOM) was a key substance widely distributed in Uranium (U) hosting aquifer, however, the characteristics and its potential molecular transformation during neutral in situ leaching (ISL) remained unclear. Here, 13 groundwater samples and 5 rock samples were collected from the Qianjiadian U deposit, a sandstone‐type U deposit by neutral ISL in China. Groundwater chemistry, 3D fluorescence spectroscopy, FT‐ICR MS, and carbon isotope approaches were employed to investigate the characteristics of DOM, identify and quantify the processes affecting the composition of DOM during ISL. Spectral and molecular characteristics of DOM revealed the biological origin of DOM and the biological activity in U‐hosting aquifers. CHOS and aliphatic were the major components of water‐soluble organic matter (WSOM) in U‐bearing rocks. The release of WSOM (24.5%–57.4%) and oxidative degradation of DOM (39.1%–65.7%) together controlled the composition of DOM during ISL. The gradual decrease of (DBE−O)/Cwa and gradual increase of NOSCwa indicated that the saturated degree and oxidation state of DOM increased as mining. This work improves our understanding of molecular characteristics of DOM in U hosting aquifer and molecular transformation pathways of DOM during ISL. [ABSTRACT FROM AUTHOR]