Immobilization of radionuclides and heavy metals through anaerobic bio-oxidation of Fe(II)

Abstract

Adsorption of heavy metals and radionuclides (HMR) onto iron and manganese oxides has long been recognized as an important reaction for the immobilization of these compounds. However, in environments containing elevated concentrations of these HMR the adsorptive capacity of the iron and manganese oxides may well be exceeded, and the HMR can migrate as soluble compounds in aqueous systems. Here we demonstrate the potential of a bioremediative strategy for HMR stabilization in reducing environments based on the recently described anaerobic nitrate-dependent Fe(II) oxidation by Dechlorosoma species. Bio-oxidation of 10 mM Fe(II) and precipitation of Fe(III) oxides by these organisms resulted in rapid adsorption and removal of 55 microM uranium and 81 microM cobalt from solution. The adsorptive capacity of the biogenic Fe(III) oxides was lower than that of abiotically produced Fe(III) oxides (100 microM for both metals), which may have been a result of steric hindrance by the microbial cells on the iron oxide surfaces. The binding capacity of the biogenic oxides for different heavy metals was indirectly correlated to the atomic radius of the bound element. X-ray absorption spectroscopy indicated that the uranium was bound to the biogenically produced Fe(III) oxides as U(VI) and that the U(VI) formed bidentate and tridentate inner-sphere complexes with the Fe(III) oxide surfaces. Dechlorosoma suillum oxidation was specific for Fe(II), and the organism did not enzymatically oxidize U(IV) or Co(II). Small amounts (less than 2.5 microM) of Cr(III) were reoxidized by D. suillum; however, this appeared to be inversely dependent on the initial concentration of the Cr(III). The results of this study demonstrate the potential of this novel approach for stabilization and immobilization of HMR in the environment.

FIG. 1.

Adsorption and precipitation of uranium and cobalt by Fe(III) oxides abiotically formed prior to and after addition of soluble U(VI) and Co(III).

FIG. 2.

Growth of D. suillum in the absence and in the presence of soluble uranium and cobalt. OD₆₀₀, optical density at 600 nm.

FIG. 3.

Adsorption and precipitation of soluble uranium and cobalt by Fe(III) oxides biogenically formed after addition of U(VI) and Co(III).

FIG. 4.

(A) Energy-aligned and step-height-normalized XANES data from the UO₂ [U(IV)] and UO₃ [U(VI)] standards compared with data from the uranium content of biogenically formed iron oxides. (B) Magnitude and real part of the Fourier transformed χ(k)*k best-fit model and data from biogenic solids.

FIG. 5.

Fe(II) bio-oxidation and uranium solubilization by an active culture of D. suillum with nitrate as the sole electron acceptor. conc., concentration.