Accumulation of metals by bacteriogenic iron oxides in a subterranean environment

Bacteriogenic iron oxides (BIOS) and groundwater samples were collected fro m 66 to 432 m underground at the Aspo Hard Rock Laboratory near Oskarshamn, Sweden. The twisted, iron oxide-encrusted stalks of the lithoautotrophic f errous iron-oxidizing bacterium Gallionella ferruginea were prominent in th e 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 confirmed that the BIOS samples contained on ly poorly ordered (amorphous) hydrous ferric oxide. Inductively coupled pla sma emission spectroscopy revealed iron oxide contents that varied from 60% to 90% (dry weight basis). Metal concentrations in filtered groundwater ra nged from similar to 10 mM for Na to 10(-4) mM or less for Co, Cu, Cr; and Zn. Intermediate concentrations were recorded for Fe and Mn (similar to 10( -2) mM). Solid-phase metal concentrations in the BIOS spanned the 10(-2) to 10(-5) mmol/kg range. Metal distribution coefficients (K-d values), calcul ated as the ratio between BIOS and dissolved metal concentrations, revealed solid-phase enrichments that, depending on the metal, extended from simila r to 10(0) to nearly 10(5). At the same time, however a distinct trend of K -d values decreasing with increasing iron oxide content was evident for eac h metal, implying that metal uptake was strongly influenced by the relative proportion of bacterial organic matter in the composite solids. The metal accumulation properties of the BIOS suggest an important role for intermixe d iron oxides and bacterial organic matter in the transport and fate of dis solved metals in groundwater systems.