Deep anoxic aquifers could act as sinks for uranium through microbial-assisted mineral trapping
Ivan Pidchenko, John N. Christensen, Martin Kutzschbach, Konstantin Ignatyev, I. Puigdomènech, Eva‐Lena Tullborg, Nick M.W. Roberts, E. Troy Rasbury, Paul Northrup, Ryan Tappero, Kristina O. Kvashnina, Thorsten Schäfer, Yohey Suzuki, Henrik Drake
Abstract
Abstract Uptake of uranium (U) by secondary minerals, such as carbonates and iron (Fe)-sulfides, that occur ubiquitously on Earth, may be substantial in deep anoxic environments compared to surficial settings due to different environment-specific conditions. Yet, knowledge of U reductive removal pathways and related fractionation between 238 U and 235 U isotopes in deep anoxic groundwater systems remain elusive. Here we show bacteria-driven degradation of organic constituents that influences formation of sulfidic species facilitating reduction of geochemically mobile U(VI) with subsequent trapping of U(IV) by calcite and Fe-sulfides. The isotopic signatures recorded for U and Ca in fracture water and calcite samples provide additional insights on U(VI) reduction behaviour and calcite growth rate. The removal efficiency of U from groundwater reaching 75% in borehole sections in fractured granite, and selective U accumulation in secondary minerals in exceedingly U-deficient groundwater shows the potential of these widespread mineralogical sinks for U in deep anoxic environments.