Investigating the Susceptibility of Jarosite Minerals to Reductive Dissolution by a Dissimilatory Metal Reducing Bacterium (Shewanella putrefaciens CN32)

Jarosite-group minerals (AFe3(TO4)2(OH)6) typically form under acidic (pH \u3c 3.5), oxidizing, ferric and sulfate rich conditions and commonly occur in the oxidized portion of sulfide deposits, hydrometallurgical wastes, acid sulfate soils and environments contaminated by acid rock or acid mine drainage. Jarositic wastes are typically stored in disposal ponds under circum-neutral conditions, thereby rendering the mineral phase unstable and susceptible to reductive dissolution through Fe(III) reduction by microbial populations under reducing conditions. The primary goal of this dissertation was to examine the susceptibility of a variety of jarosite-group minerals to microbial Fe(III) reduction and to examine the potential for enhanced metal release as compared to control samples during dissolution.Experiments were conducted by incubating a variety of synthetic and natural jarosites with the facultative anaerobe,Shewanella putrefaciens strain CN32 using lactate as the sole electron donor. Incubations ranged from 336 to 900 hours and solution chemistry, including elemental concentrations, Fe and As oxidation state, Eh and pH were monitored over time. Changes in solid phase, including secondary precipitation, were characterized using environmental scanning electron microscopy (ESEM) and transmission electron microscopy (TEM) coupled with energy dispersive spectroscopy (EDS). X-ray absorption spectroscopy (XAS) was also used to examine simultaneous Fe and As reduction during the reductive dissolution of a Pb-As jarosite. Results revealed that abiotic and biotic dissolution of jarosites are both incongruent reactions with the former consuming H+ and the latter producing H+. Fe(II) production was used as a proxy for Fe(III) reduction and showed that the susceptibility of jarosites to microbial Fe(III) reduction was influenced primarily by structure with susceptibility increasing in monovalent A-site substituted jarosites (K, Tl and Na). Moreover, parallel experiments with synthetic and natural jarosites demonstrated that reductive dissolution was greater in the synthetic samples. Enhanced release of structural constituents, such as Tl, Sr and As, was observed in inoculated samples as compared to control samples. Based on the instability of jarosite-group minerals under anaerobic circum-neutral conditions, however, it is expected that both abiotic and biotic processes will contribute to the release of structural constituents such as Tl and As into the environment