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.