Effect of Siderophore DFOB on U(VI) Adsorption to Clay Mineral and Its Subsequent Reduction by an Iron-Reducing Bacterium

Abstract

Uranium mining and nuclear fuel production have led to significant U contamination. Past studies have focused on the bioreduction of soluble U(VI) to insoluble U(IV) as a remediation method. However, U(IV) is susceptible to reoxidation and remobilization when conditions change.Here, we demonstrate that a combination of adsorption and bioreduction of U(VI) in the presence of an organic ligand (siderophore desferrioxamine B, DFOB) and the Fe-rich clay mineral nontronite partially alleviated this problem. DFOB greatly facilitated U(VI) adsorption into the interlayer of nontronite as a stable U(VI)−DFOB complex. This complex was likely reduced by bioreduction intermediates such as the Fe(II)−DFOB complex and/or through electron transfer within a ternary Fe(II)−DFOB−U(VI) complex. Bioreduction with DFOB alone resulted in a mobile aqueous U(IV)−DFOB complex, but in the presence of both DFOB and nontronite U(IV) was sequestered into a solid. These results provide novel insights into the mechanisms of U(VI) bioreduction and the stability of U and have important implications for understanding U biogeochemistry in the environment and for developing a sustainable U remediation approach.

Oxidation of Biogenic U(IV) in the Presence of Bioreduced Clay Minerals and Organic Ligands

“…In experimental treatments containing organic ligands alone, U(IV) occurred in aqueous solution (Table S3), likely because of the formation of aqueous U(IV)−ligand complexes at the experimental pH of 7 (Figure S3). 13,14 Similarly, in the NAu-2/SWy-2 + citrate and SWy-2 + EDTA treatments, biogenic U(IV) was present in aqueous phase (Table S3), likely as U(IV)-citrate/EDTA complexes. 13 However, in the NAu-2 + EDTA and NAu-2/SWy-2 + DFOB treatments (Table S3), U(IV) was associated with the solid phase, possibly due to the formation of clay-EDTA-U(IV) 13 and clay-DFOB-U(IV) 14 ternary complexes, respectively.…”

“…13,14 Similarly, in the NAu-2/SWy-2 + citrate and SWy-2 + EDTA treatments, biogenic U(IV) was present in aqueous phase (Table S3), likely as U(IV)-citrate/EDTA complexes. 13 However, in the NAu-2 + EDTA and NAu-2/SWy-2 + DFOB treatments (Table S3), U(IV) was associated with the solid phase, possibly due to the formation of clay-EDTA-U(IV) 13 and clay-DFOB-U(IV) 14 ternary complexes, respectively.…”

“…According to the XRD patterns, U(IV) was present as uraninite (Figure S2), consistent with our previous studies. 13,14 The solubility of uraninite was low (log K sp = −8.5), 54 as evidenced by the nearly identical concentrations of U(VI) and total aqueous U (Figure 1). In experimental treatments containing organic ligands alone, U(IV) occurred in aqueous solution (Table S3), likely because of the formation of aqueous U(IV)−ligand complexes at the experimental pH of 7 (Figure S3).…”

“…However, the copresence of clay minerals and organic ligands changes the direction and magnitude of U−Fe redox reactions because ligands can alter the speciation of both U(IV) and clayassociated Fe(II). 13,14,32,43 The objective of this study was therefore to explore the oxidation process of biogenic U(IV) in the presence of bioreduced clay minerals and organic ligands (e.g., EDTA, citrate, and DFOB). Batch experiments were first conducted to reduce U(VI) and structural Fe(III) in clay minerals by S. putrefaciens CN32 in the presence of organic ligands.…”

Unveiling the Pool of Metallophores in Native Environments and Correlation with Their Potential Producers

“…Metallophores are organic ligands produced by bacteria, fungi, and plants that scavenge metals from the environment (terrestrial and marine) creating a soluble complex. , They play a pivotal role in metal homeostasis, making the metal available to the cell, or contributing to the mitigation of toxic metal contamination from the environment. − The former has great repercussions, e.g., in agriculture, making the metals more available to the plant promoting its growth, whereas the latter has great impact in environmental bioremediation. , In conditions of metal scarcity, the production of metallophores by some microorganisms could participate in shaping microbial communities by promoting both cooperative and competitive interactions. , Among metallophores, the best-known and studied are those that complex iron, also known as siderophores. , Siderophores produced by bacteria and fungi that thrive at neutral pH (ideal conditions for binding) are well characterized, with hundreds of them identified, mostly using laboratory incubation cultures. , However, in the case of soils and environments rich in organic matter, iron availability is constricted, becoming growth limiting. The limited recovery of siderophores from the environment as well as the alkalinity and complexity of the matrix makes the identification of siderophores (and other metallophores by extension) in these highly challenging environments …”