The Effects of Gamma (γ-) Sterilization on the Redox Stability, Minerology, and Physicochemical Properties of the Synthetic Iron Oxides Ferrihydrite, Lepidocrocite, and Goethite

Abstract

Laboratory analyses were conducted on synthetic iron oxides to assess the use of gamma (γ-) irradiation as an efficient sterilization technique to remove microorganisms present in natural bacteriogenic iron oxides (BIOS) and to determine if the technique induces mineralogical changes within the Fe-rich minerals. Fe-oxides (ferrihydrite, lepidocrocite, and goethite) were synthesized with and without alginate (as a proxy for exopolysaccharides) and microbial reductions were carried out using the bacterium Shewanella putrefaciens CN32. A total of 18 Fe-oxide minerals were subjected to microbial reduction to assess redox stability, alteration due to varying levels of gamma irradiation (0, 5, and 28 kGy), and the addition of the exopolysaccharide alginate. Iron reduction rates varied for each Fe-oxide with faster Fe (III) reduction rates observed for the amorphous poorly-sorted 2-line ferrihydrite and slower Fe (III) reduction for the more crystalline Fe-oxides lepidocrocite and goethite. There was no significant impact to the Fe (III) reduction rates due to gamma irradiation (p> 0.05), which was confirmed using a t-test for statistical variance between gamma irradiated samples. However, the addition of alginate enabled lepidocrocite and goethite to achieve maximum Fe (III) reduction by an average of 7 days faster when compared to the Fe-oxides synthesized without the exopolysaccharide.