Accumulation of Metals by Bacteriogenic Iron Oxides in a Subterranean Environment

Abstract

Bacteriogenic iron oxides (BIOS) and groundwater samples were collected from 66 to 432 m underground at the Aspo 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 confirmed 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 filtered groundwater ranged from 10mM for Na to 10 -4 mM or less for Co, Cu, Cr, and Zn. Intermediate concentrations were recorded for Fe and Mn ( 10 -2mM). Solid-phase metal concentrations in the BIOS spanned the 10 -2 to 10 -5mmol/ kg range. Metal distribution coefficients (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 10 5. 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 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 intermixed iron oxides and bacterial organic matter in the transport and fate of dissolved metals in groundwater systems.