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

Abstract

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 237Np as well as its daughter 233protactinium. 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.

Chemical and Biological Interactions during Nitrate and Goethite Reduction by Shewanella putrefaciens 200

“…This process has a profound effect on groundwater geochemistry and places a key control on the fate of contaminant metals (12,13,19,30,60) and organic compounds (24,42) in anoxic groundwater. As a consequence, processes that affect the rate and extent of microbial iron reduction in natural systems are of great interest.…”

Phosphate-Mediated Remediation of Metals and Radionuclides

“…Furthermore, contaminated sites that are characterized by acidic to circumneutral porewater pH represent environments that can support stable mineral formation (Figures 2(a) and 2(b)), provided that carbonates are not present in significant concentrations (i.e.,P CO 2 < 10 −3.5 atm) [176,177]. Interestingly, investigations of microbial reduction of Cr, Np, Pu, and U have been shown to support subsequent phosphate precipitation reactions via thermodynamic modeling, chromatographic separation of actinides based on valence state, and X-ray analytical methods [154,155,172,178,179]. Unlike U, that is capable of forming phosphate minerals in both hexavalent and tetravalent states [50,179], the reduction of Cr, Np, and Pu is initially required for these contaminants to participate in phosphate precipitation reactions [154,155,172,178].…”

“…Interestingly, investigations of microbial reduction of Cr, Np, Pu, and U have been shown to support subsequent phosphate precipitation reactions via thermodynamic modeling, chromatographic separation of actinides based on valence state, and X-ray analytical methods [154,155,172,178,179]. Unlike U, that is capable of forming phosphate minerals in both hexavalent and tetravalent states [50,179], the reduction of Cr, Np, and Pu is initially required for these contaminants to participate in phosphate precipitation reactions [154,155,172,178]. To date, only pure culture or coculture studies have identified such coupled microbial interactions that offer an additional approach to control contaminant toxicity and mobility that are perpetuated by valence state cycling.…”

Aerobic bioreduction of nickel(II) to elemental nickel with concomitant biomineralization

“…Microbes can also reduce a wide range of other more toxic metals such as Cr(VI) (Wang 2000), Hg(II) (Lloyd and Lovley 2001), Co(III) (Gorby et al 1998), Pd (II) (Lloyd et al 1998;Yong et al 2002), Au(III) (Kashefi et al 2001), Ag(I) (Fu et al 2000), Mo(VI) (Bautista and Alexander 1972), and V(V) (Yurkova and Lyalikova 1991). The reduction of metalloids including As(V) (Macy et al 1996), Te(IV) (Rajwade and Paknikar 2003), and Se(VI) (Klonowska et al 2005), as well as radionuclides including U(VI) , Np (V) (Lloyd et al 2000), and Tc(VII) (Lloyd et al 1997) have also been reported. However, few studies have been done on the bioreduction of Ni(II), except for biosorption (Hussein et al 2004(Hussein et al , 2005Sar et al 2001).…”