Dissimilatory bacterial reduction of Al-substituted goethite in subsurfacesediments

The microbiologic reduction of a 0.2 to 2.0 mum size fraction of an Atlanti c coastal plain sediment (Eatontown) was investigated using a dissimilatory Fe(III)-reducing bacterium (Shewanella putrefaciens, strain CN32) to evalu ate mineralogic controls on the rate and extent of Fe(III) reduction and th e resulting distribution of biogenic Fe(II). Mossbauer spectroscopy and X-r ay diffraction (XRD) were used to show that the sedimentary Fe(III) oxide w as Al-substituted goethite (13-17% Al) that existed as 1- to 5-mum aggregat es of indistinct morphology. Bioreduction experiments were performed in two buffers [HCO3-; 1,4-piperazinediethansulfonic acid (PIPES)] both without a nd with 2,6-anthraquinone disulfonate (AQDS) as an electron shuttle. The pr oduction of biogenic Fe(II) and the distribution of Al (aqueous and sorbed) were followed over time, as was the formation of Fe(II) biominerals and ph ysical/chemical changes to the goethite. The extent of reduction was comparable in both buffers. The reducibility (r ate and extent) was enhanced by AQDS; 9% of dithionite-citrate-bicarbonate (DCB) extractable Fe(III) was reduced without AQDS whereas 15% was reduced in the presence of AQDS. XRD and Mossbauer spectroscopy were used to monito r the disposition of biogenic Fe(II) and changes to the Al-goethite. Fe(II) biomineralization was not evident by XRD. Biomineralization was observed b y Mossbauer when sorbed Fe(II) concentrations exceeded a threshold value. T he biomineralization products displayed Mossbauer spectra consistent with s iderite FeCO3 (HCO3- buffer only) and green rust [(Fe(6-x)FexIII)-Fe-II(OH) (12)](x+)[(A(2-))(x/2).yH(2)O](x-). Adsorption of biogenic Fe(II) to access ory mineral phases (e.g., kaolinite) and bacterial surfaces appeared to lim it biomineralization. Al evolved during reduction was sorbed, and extractab le Al increased with reduction. XRD analysis indicated that neither crystal lite size or the Al content of the goethite was affected by bacterial reduc tion, i.e., Al release was congruent with Fe(II). Copyright (C) 2001 Elsevi er Science Ltd.