Biological Reduction and Removal of Np(V) by Two Microorganisms

The majority of the radionuclides generated by the nuclear fuel cycle can be removed during established remediation processes. However among the long-lived, α-emitting actinides neptunium(V) is recalcitrant to removal from solution by physicochemical or biotechnological methods. The latter include a biocrystallization process, based on the enzymatic liberation of phosphate as a precipitating ligand by a Citrobacter sp., which was previously shown to precipitate tetravalent actinides such as Th(IV) and Pu(IV) as their corresponding phosphates. Np(V) was reduced to a lower valence (probably Np(IV)) by ascorbic acid or biologically, using the reductive capability of Shewanella putrefaciens, but reduction alone did not desolubilize Np. However Np(V) was removed by the two organisms, S. putrefaciens and Citrobacter sp. in concert; bioreduction to Np(IV) by S. putrefaciens, together with phosphate liberation by the Citrobacter sp., permitted bioprecipitative removal of 237 Np as well as its daughter 233 protactinium. Tests were made possible by a novel technique permitting actinide separation by paper chromatography followed by quantification of the radioactive species using a phosphorImager. This study has implications for the development of methods to remove Np(V) from solution, by the simple combination of two biotechnological methods, which can succeed where chemical treatments are ineffective.