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 M uranium and 81 M cobalt from solution. The adsorptive capacity of the biogenic Fe(III) oxides was lower than that of abiotically produced Fe(III) oxides (100 M 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 M) 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.

Physiological and taxonomic description of the novel autotrophic, metal oxidizing bacterium, Pseudogulbenkiania sp. strain 2002

“…As such, strain 2002 could not mobilize metals via the formation of metal-cyanide complexes, in a similar manner to C. violaceum (Brandl et al 2007;Reith et al 2007). However, the ability of strain 2002 to directly oxidize metals can directly influence metal mobility in the environment (Chaudhuri et al 2001;Cornell & Schwertmann 2003;Lack et al 2002b;Weber et al 2001;Weber et al 2006c). A recent study demonstrated the potential bioremediative applicability of this metabolism for the attenuation of soluble contaminating heavy metals and radionuclides in aquifer systems (Lack et al 2002b).…”

“…Microbial oxidation of Fe 2+ and solid-phase Fe(II)-bearing minerals can significantly affect the geochemistry of saturated soils and sediments by the formation of reactive Fe(III) minerals, including mixed Fe(II)-Fe(III) minerals, which may result in the sorption or coprecipitation of metals and/or nutrients (Chaudhuri et al 2001;Cornell & Schwertmann 2003;Lack et al 2002b;Weber et al 2001;Weber et al 2006c). In recent years, the demonstration of anaerobic microbial Fe(II) oxidation, light-dependent or nitrate-dependent, has been identified as a mechanism in which biological re-oxidation of Fe(II) in an anoxic environment can occur (Straub et al 1996;Weber et al 2006a;Widdel et al 1993).…”

“…However, similar to many other known Fe(II)-oxidizing organisms, C. violaceum did not grow by this metabolism (Weber et al 2006b). The coprecipitation of heavy metal and radionuclide contaminants with biogenic Fe(III) oxides has been proposed as a bioremediation strategy (Lack et al 2002b). Thus, implicating the role of members within the family Nesseriaceae to participate in metal biogeochemical cycling.…”

Processes of Contaminant Removal in “Fe0-H2O” Systems Revisited: The Importance of Co-Precipitation

“…The precipitation proceeds by succession and interplay of several precipitation processes, i.e., nucleation, growth, aggregation (or stabilization), and ageing [39,40]. In a contaminant-free Fe 0 -H 2 O system (e.g., early phase of barrier installation), homogeneous nucleation occurs.…”

“…Co-precipitation is a well-known unspecific specie removal mechanism from the aqueous solution [38][39][40].…”