Geomicrobiological Redox Cycling of the Transuranic Element Neptunium

摘要

Microbial processes can affect the environmental behavior of redox sensitive radnnuclides, and understanding these reactions is essential for the safe management of radioactive wastes. Neptunium, an alpha-emitting transuranic element is of particular importance because of its long half-life, high radiotoxicity, and relatively high solubility as Np(V)O_2~+ under oxic conditions. Here, we describe experiments to explore the btogeochemistry of Np where Np(V) was added to oxic sediment microcosms with indigenous microorganisms and anaerobically incubated. Enhanced Np removal to sediments occurred during microbially mediated metal reduction, and X-ray absorption spectroscopy showed this was due to reduction to poorly soluble Np(IV) on solids. In subsequent reoxidation experiments, sediment-associated Np(IV) was somewhat resistant to oxidative remobilization. These results demonstrate the influence of microbial processes on Np solubility and highlight the critical importance of radionuclide biogeochemistry in nuclear legacy management.