Biological reduction and removal of Np(V) by two microorganisms

The majority of the radionuclides generated by the nuclear fuel cycle can b e removed during established remediation processes. However among the long- lived, alpha-emitting actinides neptunium(V) is recalcitrant to removal fro m solution by physicochemical or biotechnological methods. The latter inclu de a biocrystallization process, based on the enzymatic liberation of phosp hate as a precipitating ligand by a Citrobacter sp., which was previously s hown to precipitate tetravalent actinides such as Th(IV) and Pu(IV) as thei r 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. Howe ver Np(V) was removed by the two organisms, S. putrefaciens and Citrobacter sp. in concert; bioreduction to Np(IV) by S. putrefaciens, together with p hosphate liberation by the Citrobacter sp., permitted bioprecipitative remo val of Np-237 as well as its daughter (233)protactinium. Tests were made po ssible by a novel technique permitting actinide separation by paper chromat ography followed by quantification of the radioactive species using a phosp horlmager. This study has implications for the development of methods to re move Np(V) from solution, by the simple combination of two biotechnological methods, which can succeed where chemical treatments are ineffective.