BIOAVAILABILITY OF FE(III) IN LOESS SEDIMENTS

A quantitative study was performed to understand if Fe(III) in loess soil is available for respiration by a metal reducing bacterium Shewanella putrefaciens, CN32. The loess samples were collected from three different sites: I-270, Missouri, USA; Huanxian, Shanxi Province, China; and Yanchang, Shanxi, China. The I-270 loess contains 1.69% total Fe, of which 0.48% was Fe (III); the Huanxian (Hx) loess contains 2.76% total Fe, of which 0.67% was Fe (III); the Yanchang (Ych) loess contains 3.29% total Fe, of which 1.27% was Fe (III). Mineralogy of each loess sample was characterized by X-ray diffraction (XRD) method followed by fitting the diffraction curve using Jade 7 software. The presence of illite in the sample was tested using ethylene glycol method. Bioreduction experiments were performed at loess concentration of 20mg/mL in bicarbonate buffer with lactate as the electron donor, Fe(III) in loess as sole electron acceptor in the presence and absence of AQDS, at a cell concentration of ~1.2x10⁸/ml. The unreduced and bioreduced samples were analyzed using SEM method. The time course production of biogenic Fe (II) was measured by ferrozine method. The XRD results suggest that smectite, illite, quartz, albite, and calcite were the major minerals in all loess samples, although their percentage may be different among samples. The ethylene glycol vapor treatment showed some original d-spacing (~10Å) expanded to higher values (~17Ǻ) suggesting the presence of smectite in the sample. The bioreduction experiments showed that the extent of reduction was higher in AQDS treated samples than those without AQDS. Despite the similarities among three loess samples, the extent and rate of Fe(III) reduction was very different. The effective extent of reduction was 13.4% in Hx, 16.0% in Ych, and 18.6% in I-270 in AQDS treated samples while it was 7.2% in Ych, 8.5% in Hx, and 11.9% in I-270 without AQDS. However, no appreciable change in mineralogy and morphology was detected by XRD and SEM respectively, due to bioreduction. The results of this study suggest that Fe(III) in loess can be used by bacteria for respiration.