Abstract
The ubiquitous β-Proteobacterium Gallionella ferruginea is known as stalk-forming, microaerophilic iron(II) oxidizer, which rapidly produces iron oxyhydroxide precipitates. Uranium and neptunium sorption on the resulting intermixes of G. ferruginea cells, stalks, extracellular exudates, and precipitated iron oxyhydroxides (BIOS) was compared to sorption to abiotically formed iron oxides and oxyhydroxides. The results show a high sorption capacity of BIOS towards radionuclides at circumneutral pH values with an apparent bulk distribution coefficient (Kd) of 1.23 × 104 L kg−1 for uranium and 3.07 × 105 L kg−1 for neptunium. The spectroscopic approach by X-ray absorption spectroscopy (XAS) and ATR FT-IR spectroscopy, which was applied on BIOS samples, showed the formation of inner-sphere complexes. The structural data obtained at the uranium LIII-edge and the neptunium LIII-edge indicate the formation of bidentate edge-sharing surface complexes, which are known as the main sorption species on abiotic ferrihydrite. Since the rate of iron precipitation in G. ferruginea-dominated systems is 60 times faster than in abiotic systems, more ferrihydrite will be available for immobilization processes of heavy metals and radionuclides in contaminated environments and even in the far-field of high-level nuclear waste repositories.
Highlights
It is well known that in many freshwater and marine environments, where the oxygen concentrations can be very low, microaerophilic iron(II) oxidizers are common (Emerson and Moyer 1997; Edwards et al 2003)
At the end of the experiments, the uranium concentration in the bacteriogenic iron oxyhydroxides (BIOS) suspension was determined to have declined to 0.03 mM, clearly indicating that uranium had been removed from solution and immobilized on the BIOS sample
Compared with the results reported for uranium(VI), only few extended X-ray absorption fine structure (EXAFS) studies on the adsorption of neptunium(V) on iron oxides have been conducted at circumneutral pH, whereas no EXAFS studies have been conducted on the sorption of neptunium(V) on ferrihydrite in the near-neutral pH range
Summary
It is well known that in many freshwater and marine environments, where the oxygen concentrations can be very low (e.g., at anoxic-oxic transition zones), microaerophilic iron(II) oxidizers are common (Emerson and Moyer 1997; Edwards et al 2003). SE-43535 Mölnlycke, Sweden retarded at low oxygen concentrations, certain specialized species of iron oxidizers (e.g., Gallionella spp.) are able to grow (Edwards et al 2003), contributing to iron oxidation. These bacteria belong to the phylum Proteobacteria, which includes the freshwater genera Leptothrix (Fleming et al 2011), Gallionella (Kucera and Wolfe 1957), and Sideroxydans (Weiss et al 2007) as well as the marine genus Mariprofundus (Singer et al 2011; Melton et al 2014). The iron(II)-oxidizing bacterium Gallionella, for example, requires a lower pH and a lower redox potential than commonly
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