Uranyl-Pseudomonas fluorescens interactions: Implications for uranyl mobility in the subsurface

As a legacy of uranium processing and decontamination activities, there is widespread contamination of the subsurface of DOE laboratories with mixtures of radionuclides and organic acids. Organic acids such as citric acid or oxalic acid are known chelators of uranium and affect the sorption of uranyl (UO ₂2+) onto mineral surfaces. In particular, citrate strongly enhances the sorption of uranyl to goethite (α-FeOOH). Additionally, citrate is readily biodegraded by a variety of microorganisms. Therefore, in subsurface environments, where microbial activity occurs naturally, can bacteria degrade citrate that is enhancing uranyl sorption to goethite? If so, what is the fate of uranyl upon citrate degradation?; First, the interactions between uranyl, citrate and P. fluorescens were investigated. It was determined that cells dramatically shifted the uranyl speciation in a solution containing citrate and uranyl by binding >90% of the total uranyl present. The citrate initially bound in a uranyl-citrate complex was thus released into solution and was available for biodegradation. However, a uranyl concentration greater than ∼40 mg U. [g dry cells] −1 inhibited the metabolism of citrate and glucose by cells. The inhibitory effect was correlated to the association of uranyl with cell membrane(s) and was reversible.; Second, sorbed citrate was found to be biodegradable albeit at a significantly reduced rate relative to the solution citrate. Third, cells were found to exceed goethite in its affinity and capacity for uranyl sorption on a unit surface area basis.; Analysis of the full biogeochemical system revealed that after citrate biodegradation, uranyl was released from the goethite surface and rapidly adsorbed onto the cell surface. Additionally, goethite offered only limited mitigation for the inhibitory effect of uranyl.; The implications of these findings for subsurface environments are: (1) Potential biocolloidal transport of uranyl in the subsurface is of concern because of the high affinity of bacterial cells for uranyl. (2) The inhibitory effect of uranyl could hinder bioremediation efforts of areas contaminated with mixtures of uranyl and organic compounds.