Combined Effects of Fe(III)-Bearing Clay Minerals and Organic Ligands on U(VI) Bioreduction and U(IV) Speciation

Reduction of U­(VI) to U­(IV) drastically reduces its solubility and has been proposed as a method for remediation of uranium contamination. However, much is still unknown about the kinetics, mechanisms, and products of U­(VI) bioreduction in complex systems. In this study, U­(VI) bioreduction experiments were conducted with Shewanella putrefaciens strain CN32 in the presence of clay minerals and two organic ligands: citrate and EDTA. In reactors with U and Fe­(III)–clay minerals, the rate of U­(VI) bioreduction was enhanced due to the presence of ligands, likely because soluble Fe3+– and Fe2+–ligand complexes served as electron shuttles. In the presence of citrate, bioreduced U­(IV) formed a soluble U­(IV)–citrate complex in experiments with either Fe-rich or Fe-poor clay mineral. In the presence of EDTA, U­(IV) occurred as a soluble U­(IV)–EDTA complex in Fe-poor montmorillonite experiments. However, U­(IV) remained associated with the solid phase in Fe-rich nontronite experiments through the formation of a ternary U­(IV)–EDTA–surface complex, as suggested by the EXAFS analysis. Our study indicates that organic ligands and Fe­(III)-bearing clays can significantly affect the microbial reduction of U­(VI) and the stability of the resulting U­(IV) phase.