Biodegradation of Uranium−Citrate Complexes:  Implications for Extraction of Uranium from Soils

Abstract

Citrate is often used as a complexing agent to mobilize sorbed and precipitated uranium in both in situ and ex situ extraction of soils and nuclear reactor components. The biodegradability of U−citrate complexes is an important control over the potential migration of residual uranium after the extraction process is complete. In solutions buffered at pH 6−7, limited biodegradation of citrate is observed within 10 days with initial U:citrate molar ratios ranging from 1:2 to 1:8; however, over 99% of the citrate is biodegraded rapidly at pH 8−9. The increase of pH may have shifted the equilibrium speciation of uranium from (UO2−citrate)2 2- to (UO2)3(OH)7 1- and, consequently, raised the bioavailability of citrate. At pH 6−7, a significant amount of uranium is also observed to associate with biomass, whereas only a negligible amount is observed at pH 8−9. Our experimental results suggest that the residual concentration of uranium−citrate complexes left in the treated soils can be reduced rapidly if the soil water pH is held between 8 and 9 after the extraction processes.

Uranium(VI) Reduction by Anaeromyxobacter dehalogenans Strain 2CP-C

“…All results were verified in at least one additional, independent experiment with duplicate cultures. Uranium precipitation, sorption, and speciation are influenced by pH (5,18,49). Therefore, we monitored pH in selected cultures over the course of the U(VI) reduction experiments.…”

Association of uranyl with the cell wall of Pseudomonas fluorescens inhibits metabolism

“…Another study by Banaszak et al (1999) suggests that the uranyl-citrate complex itself is toxic to P. fluorescens. Finally, Huang et al (1998) show that there is incomplete citrate (0.55 mmol/L) degradation by a mixed cell culture in the presence of uranyl (0.42 mmol/L) at pH 6. This study shows that uranyl inhibits cell metabolism at low, environmentally relevant concentrations and that the inhibition is associated with the binding of uranyl to the cell envelope.…”

Presence of Fe3+ and Zn2+ promoted biotransformation of Cd–citrate complex and removal of metals from solutions

“…It is due to lack of transport inside the cell and not to Cd toxicity or inhibition of cell function [7]. To maintain cell electroneutrality, proton was simultaneously transported and metabolized with the negatively charged bidentate [Fe(OH) 2 -citrate] 2− and [Zn-citrate] − [9]. Therefore, with the consumption of proton, pH of 1:1 Fe:citrate and Zn:citrate media gradually increased to the values greater than 7.9 to the end of the experiments.…”

“…Huang et al investigated the biotransformation of non-biodegradable U-citrate complex at alkaline pH, they found that over 99% of the citrate was metabolized by bacteria, and speculated that dissociation of citrate from U-citrate complex at alkaline pH raised the bioavailability of citrate [9]. Whereas, for Ni-citrate complex, limited biodegradation of citrate was observed due to the formation of tridentate Ni-citrate complex at alkaline pH [10].…”