Systematic review of heterotrophic bacterial bioleaching and uranium sequestration

Heterotrophic bacterial leaching of uranium from rocks and ores and cellular sequestration are examined in this review. Heterotrophic microbial bioleaching solubilizes uranium by acidolysis and complexolysis and involves sequestration by organic acids, extracellular polymeric substances (EPS), lipopolysaccharides, and siderophores produced by bacteria. Citric and oxalic acids are often the dominant organic acids, but spent growth media also contain mixtures of other < C6 carboxylic acids. The leach solution (lixiviant) is based on organic acids in spent media, and the mode of leaching is proton attack on uranium ore coupled with sequestration of uranyl ions (UO22+) by organic acids, thus preventing precipitation in the lixiviant. Many heterotrophs in different bacterial genera have been tested for uranium bioleaching and sequestration from mineral resources, and some notable examples include actinomycetes, Bacillus spp., and Pseudomonas spp. Commercial applications of heterotrophic bacterial bioleaching and biomass sorption of uranium have not emerged. Uranium sequestration in biomass constituents such as EPS and siderophores can represent a significant fraction of biomass sorption and make uranyl ions biologically unavailable to the cells. Precipitation with phosphates particularly immobilizes uranyl ions and can result in crystallized mineral phases. The biological reduction of U(VI) to solid-phase U(IV) is an immobilization technique to prevent uranium migration in aquifers and reduce environmental impact, potentially as part of remediation strategies such as permeable reactive barrier construction.