Microbial Transformations of Plutonium

Microorganisms have been detected in transuranic- and low-level radioactive wastes, Pu-contaminated soils, and in nuclear waste-repository sites. Microbial activity could affect the chemical nature of the Pu by altering the speciation, solubility, and sorption properties and thus could affect the stability and mobility of Pu in the environment. We investigated the biotransformation of Pu(IV)-citrate by Pseudomonas fluorescens under aerobic conditions. Characterization of 242 Pu(IV)-citrate by electrospray ionization-mass spectrometry (ESI-MS) showed the presence of 1:1 mononuclear, 1:2 mononuclear biligand, and 2:2 dinuclear complex. X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) analysis of Pu-citrate confirmed Pu in the IV oxidation state and predominantly present as a 1:2 mononuclear biligand complex. Citric acid was rapidly metabolized by P. fluorescens resulting in the formation of Pu polymer. Under anaerobic conditions, reductive dissolution of Pu(IV) to Pu(III) by Clostridium sp. was observed. The presence of Pu(III) was confirmed by XANES. Addition of Pu(VI) to a mixture consisting of bacterial cells and kaolinite clay showed that the Pu was preferentially associated with bacterial cells as Pu(IV) and that Pu(VI) was reduced to Pu(V) and Pu(IV) only in the presence of bacteria. Studies with Pu contaminated soils from Nevada Test Site showed remobilization of Pu and other radionuclides due to enhanced microbial activity.