Microbial Dissolution of Silver Jarosite: Examining Its Trace Metal Behaviour in Reduced Environments

Abstract

Iron sulfate minerals such as jarosite-group compounds (e.g., M Fe₃(SO ₄ ) ₂ (OH) ₆ ) can be of considerable environmental importance because of their ability to scavenge trace elements and thus contribute to some degree of metal cycling. Jarosite forms in low temperature hydrothermal, acidic, sulfate-rich environments often yielding a range of elemental substituted forms: plumbojarosite (M = Pb), argentojarosite (M = Ag), jarosite (M = K), natrojarosite (M = Na), hydroniumjarosite (M = H₃O), and ammoniojarosite (M = NH₄). Anthropogenic sources of jarosite are common in mine waste environments, most often associated with the waste products resulting from base metal recovery. Few studies have investigated the effect that dissimilatory metal reducing bacteria may have in the presence of these compounds following the onset of reducing conditions. Jarosite reactivity may differ systematically as a function of its chemical properties. For example, the incorporation of Ag in the mineral lattice may have inhibitory affects on the growth of microbial strains. In this study the reductive dissolution of argentojarosite (M = Ag) in the presence of Shewanella putrefaciens CN32 (10 ⁹ cells ml ¹ , pH 7.1) was examined. Using the silver (argento)jarosite, Ag Fe ₃ (SO ₄ ) ₂ (OH) ₆ , as a sole terminal electron acceptor we observed the reduction of structural Fe(III) and Ag(I) by CN32 through the release of Fe(II) ions to solution. Aqueous silver concentrations were below instrumental detection. Environmental SEM (ESEM) and TEM micrographs of the microbial clusters revealed the progressive heterogeneous nucleation of Ag(0) nanoparticles within cellular structures and also on adjacent mineral grains. The results of this study are the first presented for the anaerobic dissolution of silver jarosite. This has implications for understanding the processes leading to the mobility or retention of silver in mine waste and industrial landfill environments. It also provides insight into the microbial mechanisms of silver resistance and nanoparticle formation with potential applications for bioleaching and/or biotechnology.

Keywords:

• Metal reduction
• silver jarosite
• nanoparticle
• electron transfer
• TEM
• Shewanella putrefaciens CN32
• silver toxicity

Acknowledgments

This research was supported by the Canadian Foundation for Innovation and the Ontario Innovation Trust (Weisener) and NSERC (Weisener). We gratefully acknowledge S. Lackie and E. Buan for ESEM analyses and sample collection and J.C. Barrette for help with the ICP-OES analyses. We would also like to thank the assistance of M. Reid at the University of McMaster for preparing the microtome samples for HRTEM analyses.