Application of ⁵⁷Fe-enriched synthetic ferrihydrite to speciate the product of bacterial reduction

Abstract

We have sampled a clay lens with evidence of sulfide reduction from a texturally stratified sandy aquifer at Rømø, Denmark. A minor amount of synthetic, pure ⁵⁷Fe ferrihydrite was added to this sample and allowed to react for up to three months. The initial sample, the ⁵⁷Fe ferrihydrite, and samples taken from the reaction mixture were investigated by Mössbauer spectroscopy at temperatures between 15 and 298 K as sampled and following exposure to oxygen. The initial sample only contained Fe(II) (33% of the iron) and Fe(III) in silicates. The Fe(III) in the ferrihydrite is reduced to Fe(II) as evidenced by an increase of this component by bacterial activity. The Fe(II) component remains paramagnetic at temperatures down to 15 K. Similarly to naturally reduced sediments the new-formed Fe(II) is extremely reactive towards molecular oxygen. Following oxidation the reformed Fe(III) is found as ferrihydrite. The bonding of the Fe(II) is by electrostatical bonding (adsorbed) to the layer silicates as evidenced by a temperature scanning of the sample between 80 and 270 K.