Iron isotope fractionation between aqueous ferrous iron and goethite

Abstract

The equilibrium Fe isotope fractionation factor between aqueous Fe(II) and goethite has been experimentally measured to be −1.05±0.08‰ in 56Fe/54Fe (2σ) at 22°C, using the three-isotope method. Experiments were done using two sizes of goethite (81×11nm and 590×42nm), and the experimental products were subjected to serial extraction using acid partial dissolution techniques to determine if surface Fe(III) atoms have different isotopic properties than the bulk goethite. These experiments indicate that the interaction of Fe(II)aq and goethite is dynamic and results in complete or near-complete Fe isotope exchange over 30days, involving at least four components: Fe(II)aq, goethite, sorbed Fe(II), and Fe(III)surface. The equilibrium fractionation factor between Fe(II)aq and Fe(II)sorb is the same for both sizes of goethite, at Δ56FeFe(II)aq–Fe(II)sorb=−1.24±0.14‰; this fractionation factor is significantly different than the results of previous studies on Fe(II) sorption to goethite. The proportion of the Fe(III)surface component is greatest in the experiments that used the smallest goethite, and the Fe(III)surface–Fe(II)aq fractionation is estimated to be at least +2.1‰. The high Fe(III)surface–Fe(II)aq fractionation may exert a significant influence on the Fe isotope compositions of aqueous Fe(II) in natural systems that contain nanoparticulate goethite, including those involving bacterial iron reduction. These results demonstrate that the isotopic properties of nano-scale minerals may be distinct from micron-scale or larger minerals, as is the case for other thermodynamic properties of nanoparticles.