Stable Fe isotope fractionations produced by aqueous Fe(II)-hematite surface interactions Lingling Wu a,b, * , Brian L. Beard a,b , Eric E. Roden a,b , Christopher B. Kennedy c , Clark M. Johnson a,b a Department of Geoscience, University of Wisconsin–Madison, 1215 West Dayton Street, Madison, WI 53706, USA b NASA Astrobiology Institute, University of Wisconsin–Madison, Madison, WI 53706, USA c SRK Consulting, Suite 2100, 25 Adelaide St. East, Toronto, ON, M5C 3A1 Canada Received 11 December 2009; accepted in revised form 26 April 2010; available online 6 May 2010 Abstract Stable Fe isotope fractionations were investigated during exposure of hematite to aqueous Fe(II) under conditions of var- iable Fe(II)/hematite ratios, the presence/absence of dissolved Si, and neutral versus alkaline pH. When Fe(II) undergoes elec- tron transfer to hematite, Fe(II) is initially oxidized to Fe(III), and structural Fe(III) on the hematite surface is reduced to Fe(II). During this redox reaction, the newly formed reactive Fe(III) layer becomes enriched in heavy Fe isotopes and light Fe isotopes partition into aqueous and sorbed Fe(II). Our results indicate that in most cases the reactive Fe(III) that under- goes isotopic exchange accounts for less than one octahedral layer on the hematite surface. With higher Fe(II)/hematite molar ratios, and the presence of dissolved Si at alkaline pH, stable Fe isotope fractionations move away from those expected for equilibrium between aqueous Fe(II) and hematite, towards those expected for aqueous Fe(II) and goethite. These results point to formation of new phases on the hematite surface as a result of distortion of Fe–O bonds and Si polymerization at high pH. Our findings demonstrate how stable Fe isotope fractionations can be used to investigate changes in surface Fe phases during exposure of Fe(III) oxides to aqueous Fe(II) under different environmental conditions. These results confirm the coupled elec- tron and atom exchange mechanism proposed to explain Fe isotope fractionation during dissimilatory iron reduction (DIR). Although abiologic Fe(II) aq – oxide interaction will produce low d 56 Fe values for Fe(II) aq , similar to that produced by Fe(II) oxidation, only small quantities of low-d 56 Fe Fe(II) aq are formed by these processes. In contrast, DIR, which continually exposes new surface Fe(III) atoms during reduction, as well as production of Fe(II), remains the most efficient mechanism for generating large quantities of low-d 56 Fe aqueous Fe(II) in many natural systems. Ó 2010 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Iron oxides are an important component of the Fe cycle in surface environments, reflecting the end product of Fe(II) oxidation, as well as substrates for reductive processes. Re- dox transformations of iron play an important role in the fate and transport of natural and contaminant compounds in soil and groundwater environments (Lovley, 1989; Heij- man et al., 1995; Ru ¨gge et al., 1998). Under oxygen-limited conditions, dissimilatory iron-reducing bacteria produce Fe(II) through enzymatic reduction of Fe(III) oxide sur- faces (Heijman et al., 1993; Fredrickson and Gorby, 1996). Aqueous Fe(II) in the presence of mineral surfaces has been shown to reduce and attenuate contaminants in the subsurface (e.g., Buerge and Hug, 1999; Liger et al., 1999; Pecher et al., 2002; Vikesland and Valentine, 2002; Hofstetter et al., 2003; Strathmann and Stone, 2003; Elsner et al., 2004a,b). Extensive literature exists on interaction of Fe(II) with oxide/hydroxide surfaces (e.g., Liger et al., 1999; Williams 0016-7037/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2010.04.060 * Corresponding author at: Department of Geoscience, Univer- sity of Wisconsin–Madison, 1215 West Dayton Street, Madison, WI 53706, USA. Tel.: +1 608 890 0929. E-mail address: lwu@geology.wisc.edu (L. Wu). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 74 (2010) 4249–4265