Effects of aqueous complexation on reductive precipitation of uranium by Shewanella putrefaciens

Abstract

We have examined the effects of aqueous complexation on rates of dissimilatory reductive precipitation of uranium by Shewanella putrefaciens. Uranium͑VI͒ was supplied as sole terminal electron acceptor to Shewanella putrefaciens ͑strain 200R͒ in defined laboratory media under strictly anaerobic conditions. Media were amended with different multidentate organic acids, and experiments were performed at different U͑VI͒ and ligand concentrations. Organic acids used as complexing agents were oxalic, malonic, succinic, glutaric, adipic, pimelic, maleic, citric, and nitrilotriacetic acids, tiron, EDTA, and Aldrich humic acid. Reductive precipitation of U͑VI͒, resulting in removal of insoluble amorphous UO 2 from solution, was measured as a function of time by determination of total dissolved U. Reductive precipitation was measured, rather than net U͑VI͒ reduction to U͑IV͒, to assess overall U removal rates from solution, which may be used to gauge the influence of chelation on microbial U mineralization. Initial linear rates of U reductive precipitation were found to correlate with stability constants of 1:1 aqueous U͑VI͒:ligand and U͑IV͒:ligand complexes. In the presence of strongly complexing ligands ͑e.g., NTA, Tiron, EDTA͒, UO 2 precipitation did not occur. Our results are consistent with ligand-retarded precipitation of UO 2 , which is analogous to ligand-assisted solid phase dissolution but in reverse: ligand exchange with the U 4ϩ aquo cation acts as a rate-limiting reaction moderating coordination of water molecules with U 4ϩ , which is a necessary step in UO 2 precipitation. Ligand exchange kinetics governing dissociation rates of ligands from U͑VI͒-organic complexes may also influence overall UO 2 production rates, although the magnitude of this effect is unclear relative to the effects of U͑IV͒-organic complexation. Our results indicate that natural microbial-aqueous systems containing abundant organic matter can inhibit the formation of biogenic amorphous UO 2 .

The effects of uranium speciation on the rate of U(VI) reduction by Shewanella oneidensis MR-1

“…Conversely, a black U(IV)O 2 precipitate formed in the system that did not contain EDTA. These results are consistent with previous studies such as those by Haas and Northup (2004) and Suzuki et al (2010).…”

“…As pointed out by Haas and Northup (2004), the total dissolved U measurements do not distinguish between these two controls on the reduction rate as either mechanism would result in enhanced total dissolved U with increasing EDTA concentration. In the first case, the total dissolved U would be present as U(VI), and in the latter case, the aqueous U would be present as an aqueous U 4+ -EDTA complex.…”

“…The anaerobic procedures and the general composition of the experimental medium that we used for the EDTA experiments were similar to those described previously (Balch and Wolfe, 1976;Lovley et al, 1991;Haas and Northup, 2004). The experimental medium was consisted of 5 mM NH 4 Cl, 1 mM KCl, 30 mM NaHCO 3 , 40 mM Na-lactate, and vitamins solution and trace elements as per Lovley and Phillips (1988).…”

“…Haas and Northup (2004) measured the removal rate of U(VI) by S. oneidensis in the presence of a range of multi-dentate organic acids, and found that the initial rates of U removal from solution decreased with increasing stability constant values for the 1:1 aqueous U(VI):ligand and U(IV):ligand complexes. Because Haas and Northup (2004) only measured total U remaining in solution and not U(VI) specifically, it remains unclear whether the observed effects were caused by ligandretarded U(VI) reduction due to aqueous U(VI)-ligand complexation, by ligand-promoted dissolution of the solid phase UO 2 that formed on the bacteria, by ligandpromoted desorption of U 4+ from the bacterial cell wall before UO 2 could form, or by a combination of these processes. Furthermore, Suzuki et al (2010) demonstrated that strong complexing ligands like citrate, NTA and EDTA retard UO 2 precipitation in U(VI) bioreduciton experiments by forming aqueous complexes with U(IV).…”

Expanding the Crystal Chemistry of Uranyl Peroxides: Four Hybrid Uranyl-Peroxide Structures Containing EDTA

“…34 It is a flexible diaminopolycarboxylate ligand with 10 potentially coordinating sites (eight O atoms and two N atoms) that has strong chelating ability. EDTA can affect the sorption 35,36 and redox 37,38 behaviors of uranium by forming stable complexes and thus impacts the mobility of uranium in the environment. Only limited crystal structures of these complexes have been isolated, 39,40 although structures of transition metal and lanthanide based complexes containing EDTA have been widely investigated.…”