Fate of neptunium in an anaerobic, methanogenic microcosm

摘要

Neptunium is found predominantly as Np(IV) in reducing environments, but Np(V) in211u001eaerobic environments. However, currently it is not known how the interplay 211u001ebetween biotic and abiotic processes affects Np redox speciation in the 211u001eenvironment. In order to evaluate the effect of anaerobic microbial activity on 211u001ethe fate of Np in natural systems, Np(V) was added to a microcosminoculated with 211u001eanaerobic sediments from a metal-contaminated fresh water lake. The consortium 211u001eincluded metal-reducing, sulfate-reducing, and methanogenic microorganisms, and 211u001eacetate was supplied as the only exogenous substrate. Addition of more than 211u001e10(sup (minus)5) M Np did not inhibit methane production. Total Np volubility in 211u001ethe active microcosm, as well as in sterilized control samples, decreased by 211u001enearly two orders of magnitude. A combination of analytical techniques, including 211u001eVIS-NIR absorption spectroscopy and XANES, identified Np(IV) as the oxidation 211u001estate associated with the sediments. The similar results from the active 211u001emicrocosm and the abiotic controls suggest that microbian y produced Mn(II/HI) 211u001eand Fe(II) may serve as electron donors for Np reduction.