The Role of Siderophores in the Transport of Radionuclides
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THE ROLE OF SIDEROPHORES IN THE TRANSPORT OF RADIONUCLIDES *Larry E. Hersman, Life Sciences Division, ** Philip D. Palmer, and David E. Hobart, Isotope and Nuclear Chemistry Division, Los Alamos National Laboratory, Los Alamos National Laboratory, Los Alamos, NM. ABSTRACT Iron exists in aerobic soil and water environments most commonly as insoluble Fe(III). Siderophores are powerful, microbially produced chelating agents that are used to mobilize the insoluble Fe(III) cation. Over 80 siderophores have been isolated and characterized, with H me reportedly having iron-binding constants as high as 10 . Fe(III) and Pu(IV) are similar in their charge/ionic radius ratio (4.6 and 4.2, respectively); therefore, Pu(IV) may serve as analog to Fe(III). It is possible that some radioactive wastes could be chelated by naturally occurring siderophores, thereby altering the transport rates of those elements through the subsurface environment. This investigation wn 9 initiated to investigate that possibility. The binding of Pu(IV) by four chelating agents is reported in this paper: a siderophore isolated and purified from a Pseudomonas sp.; desferal, a ferrioxamine siderophore commonly used for deferration therapy; EDTA, ethylenediaminetetraacetic acid; and, citrate, trisodium salt. INTRODUCTION Yucca Mountain, Nevada, has been selected as a potential site for the high-level nuclear waste repository. A concern of the Yucca Mountain Site Characterization Project is that in the event of a break in the waste package, microorganisms present in Yucca Mountain may participate in the transport of radioactive wastes away from the repository [19]. Although the subsurface environment of the Yucca Mountain Site has not yet been sampled for microorganisms, recent studies have shown that microorganisms do exist in many other subsurface environments [6,7,8,]. It is believed that subsurface microorganisms may participate in the transport of radioactive wastes [1, 18] in one or more ways, such as, 1) changing the pH and Eh of the environment-, 2) sorbing radioactive elements, 3) interacting with the colloidal dispersion of the radioactive elements, 4) plugging the pores of the rock matrix, and 5) producing chelating agents that alter the solubility/sorption of the radionuclide elements. Siderophores are powerful, microbially produced chelating agents that are used to mobilize in the environment the insoluble Fe(III) cation. Upon uptake of the ferric siderophore, the iron is released inside the cell by the reduction to Fe(II). In recent years, interest in these compounds is increasing with respect to their ecological function in soil systems [3]. Over 80 siderophores have been isolated and characterized, with some reportedly having iron-binding constants as high as 105 2 [11]. Raymond et al. Mat. Res. Soc. Symp. Proc. Vol. 294. ©1993 Materials Research Society
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[14] have suggested that because Fe(III) and Pu(IV) are similar in their charge/ionic radius ratio (4.6 and 4.2, In respectively), Pu(IV) may serve as an analog to Fe(III). fact Bulma
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