Retention of strontium, cesium, lead and uranium by bacterial iron oxides from a subterranean environment

Bacteriogenic Fe oxides (BIOS) and groundwater samples were collected 195 m underground at the Strassa Mine in central Sweden. Ferrous iron oxidizing bacteria, including stalked Gallionella ferruginea and filamenous Leptothri x sp., were prominent in the BIOS samples. The BIOS samples were found to c ontain only poorly ordered (amorphous) hydrous ferric oxide, as determined by X-ray diffraction. Inductively coupled plasma mass spectroscopy revealed hydroxylamine-reducible Fe and Mn oxide contents that ranged from 55 to 85 % on a dry weight basis. Concentrations of Sr, Cs, Pb and U in filtered gro undwater ranged from 0.002 to 1.8 mu M. Solid phase concentrations of these heavy metals in the BIOS spanned the 0.04-2.23 mmol/kg range. Distribution coefficients (K-d values), calculated as the ratio between BIOS and dissol ved heavy metal concentrations, revealed solid phase enrichments that, depe nding on the heavy metal and Fe oxide content of the sample, extended from 10(3.0) to 10(4.7). At the same time, however, a strong inverse linear rela tionship was found between log K-d values and the corresponding mass fracti on of reducible oxide in the samples, implying that metal uptake was strong ly influenced by the relative proportion of bacterial organic matter in the composite solids. Based on the metal accumulation properties of the BIOS, an important role can be inferred for intermixed Fe oxides and bacterial or ganic matter in the transport and fate of dissolved metals in groundwater s ystems. (C) 2000 Elsevier Science Ltd. All rights reserved.