Accumulation of Metals by Bacteriogenic Iron Oxides in a Subterranean Environment

Bacteriogenic iron oxides (BIOS) and groundwater samples were collected from 66 to 432 m underground at the È Asp È o Hard Rock Laboratory near Oskarshamn, Sweden. The twisted, iron oxide± encrusted stalks of the lithoautotrophic ferrous iron± oxidizing bacterium Gallionella ferruginea were prominent in the BIOS samples. A wide variety of heterotrophic bacteria, including stalked forms resembling Caulobacter or Hyphomicrobium species, were also present. Energy dispersive x-ray spectroscopy, selected area electron diffraction, and x-ray diffraction analyses con® rmed that the BIOS samples contained only poorly ordered (amorphous) hydrous ferric oxide. Inductively coupled plasma emission spectroscopy revealed iron oxide contents that varied from 60% to 90% (dry weight basis). Metal concentrations in ® ltered groundwater ranged from » 10 mM for Na to 10¡ 4 mM or less for Co, Cu, Cr, and Zn. Intermediate concentrations were recorded for Fe and Mn ( » 10¡ 2 mM). Solid-phase metal concentrations in the BIOS spanned the 10¡ 2 to 10 ¡ 5 mmol/kg range. Metal distribution coef® cients (Kd values), calculated as the ratio between BIOS and dissolved metal concentrations, revealed solid-phase enrichments that, depending on the metal, extended from » 100 to nearly 105. At the same time, however, a distinct trend of Kd values decreasing with increasing iron oxide content was evident for each metal, implying that metal uptake was strongly