Neptunium was the first synthetic transuranium actinide element created, although traces of neptunium have since been found in natural uranium ores. Neptunium is generated in nuclear reactors that produce plutonium, and the isotope ²³⁷Np has a half-life of 2.14 X 10⁶ years. Banaszak et al (1999) suggested that reduced Mn and Fe species produced by anaerobic microorganisms can subsequently reduce Np(V) to Np(IV). Neptunium can be accumulated by Citrobacter sp. in complexes with uranyl phosphate (Bonthrone et al, 1996). Factors influencing neptunium biosorption (Sasaki et al, 2001; Songkasiri, 2002) and bioaccumulation (Francis et al, 1998) by bacteria have been investigated.

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Banaszak JE, Webb SM, Rittmann BE, Gaillard J-F, Reed DT. Fate of neptunium in an anaerobic, methanogenic microcosm. Mat Res Soc Symp Proc. 1999;556:1141-1149.

Francis AJ, Fillow JB, Dodge CJ, Dunn M, Mantione K, Strietelmeier BA, Pansoy-Hjelvik ME, Papenguth HW. Role of bacteria as biocolloids in the transport of actinides from a deep underground radioactive waste repository. Radiochim Acta 1998;82:347-354.

Songkasiri W, Reed DT, Rittmann BE. Bio-sorption of neptunium(V) by Pseudomonas fluroescens. Radiochim Acta 2002;90:785-789.