Send Orders for Reprints to reprints@benthamscience.ae 214 Current Inorganic Chemistry, 2015, 5, 214-224 Effects of Phosphate on Secondary Mineral Formation During the Bioreduction of Akaganeite (-FeOOH): Green Rust Versus Framboidal Magnetite Edward J. O'Loughlin 1,* , Christopher A. Gorski 2,3 and Michelle M. Scherer 2 1 Biosciences Division, Argonne National Laboratory, Argonne, IL 60439-4843, United States; 2 Department of Civil and Environmental Engineering, University of Iowa, Iowa City, IA 52242-1527, United States; 3 Department of Civil and Environmental Engineering, The Pennsylvania State Univer- sity, University Park, PA 16802-7304, United States Abstract: The activity of microorganisms is a key component of the biogeochemical cycle of Fe in natural systems, where green rusts are often observed as products of microbially driven redox proc- esses. To better define the factors that control green rust formation during microbial Fe(III) reduction, we examined the effects of the presence of an electron shuttle [9,10-anthraquinone-2,6-disulfonate (AQDS)] and phosphate on akaganeite (-FeOOH) bioreduction by the iron(III)-reducing bacterium (IRB) Shewanella putrefaciens CN32. Framboidal magnetite was the principal secondary mineral formed during akagane- ite bioreduction in the absence of phosphate; this is the first time framboidal magnetite has been reported as a product of microbial Fe(III) oxide reduction. Framboidal magnetite was less crystalline when formed in the presence of AQDS than without AQDS and over time was further reduced to chukanovite. Carbonate green rust was the primary secondary min- eral observed from akaganeite bioreduction in the presence of phosphate, with and without AQDS; however, siderite was also observed in the presence of AQDS. This first report of green rust as a product of akaganeite bioreduction expands the range of Fe(III) oxides that can be transformed to green rust by IRB, suggesting that the reduction of Fe(III) oxides such as ferrihydrite, lepidocrocite, and akaganeite by IRB is a key process leading to the formation of green rusts in aquatic and terrestrial environments. Keywords: AQDS, akaganeite, bioreduction, chukanovite, electron shuttle, framboidal magnetite, green rust, iron(III) oxide, iron-reducing bacteria, magnetite, phosphate, Shewanella, siderite. 1. INTRODUCTION Green rusts are a class of Fe(II)-Fe(III) layered double hydroxides with a pyroaurite-type structure. The Fe(III) con- tent of green rusts is highly variable, ranging from 0.33 in stoichiometric green rust—i.e., [Fe II 4 Fe III 2 (OH) 12 ] 2+ [(A) 2/n yH 2 O] 2- , where A is an n-valent anion (typically Cl - , SO 4 2- , or CO 3 2- ) and y denotes varying amounts of interlayer water (y = 2 to 4 for most green rusts)—to 1.0 in so called “ferric green rusts.” Green rusts are found in a wide range of natural and engineered environments with Fe(II)/Fe(III) transition zones including surface waters [1], groundwater [2-3], soils [4-8], and sediments [9-11], as well as among corrosion products in zero-valent-iron permeable reactive barriers [12- 14]. These environments typically exhibit conditions that promote the redox cycling of Fe, and green rust minerals such as fougérite, trébeurdenite, and mössbauerite [15-17] may play a central role in the biogeochemistry of Fe in these environments. Besides their importance in the biogeochemi- cal cycling of Fe, green rusts have been widely studied due *Address correspondence to this author at the Biosciences Division, Ar- gonne National Laboratory, Building 203, Room E-137, 9700 South Cass Ave., Argonne, IL, 60439-4843, USA; Tel: 630-252-9902; Fax: 630-252-9793; E-mail: oloughlin@anl.gov to their ability to reduce a range of pollutants, including chlorinated solvents, nitroaromatics, azo dyes, toxic metals, metallolids, and radionuclides [18-29]. The activity of microorganisms is a key driver of the bio- geochemical cycle of Fe in natural systems (directly by cata- lyzing Fe redox reactions and indirectly by creating redox gradients), and green rusts are frequently observed as inter- mediates or products of microbially driven oxidative and reductive processes. Green rusts have been observed as products of direct microbial or coupled biotic/abiotic oxida- tion of Fe(II) by denitrifying bacteria under anoxic condi- tions [30-32]. Conversely, iron-reducing bacteria (IRB) and archaea are diverse groups of microorganisms that can use Fe(III) oxides such as ferrihydrite, goethite (-FeOOH), aka- ganeite (-FeOOH), lepidocrocite (-FeOOH), hematite (- Fe 2 O 3 ), maghemite (-Fe 2 O 3 ), and magnetite (Fe 3 O 4 ) as ter- minal electron acceptors (TEAs) for anaerobic respiration, and in the process generate secondary minerals containing Fe(II) such as magnetite (Fe 3 O 4 ), siderite (FeCO 3 ), vivianite [Fe 3 (PO 4 ) 2 •8H 2 O], chukanovite (ferrous hydroxy carbonate), and green rusts [33-39]. The specific factors controlling the formation of green rusts during the reduction of Fe(III) ox- ides by IRB are not unambiguously defined; however, the rate and magnitude of Fe(II) production and its reaction with 1877-94 /15 $58.00+.00 © 2015 Bentham Science Publishers Bentham Science Publishers For Personal Use Only Not For Distribution