Iron isotope fractionation and atom exchange during sorption of ferrous iron to mineral surfaces

The application of stable Fe isotopes as a tracer of the biogeochemical Fe cycle necessitates a mechanistic knowledge of natural fractionation processes. We studied the equilibrium Fe isotope fractionation upon sorption of Fe(II) to aluminum oxide (gamma-Al(2)O(3)), goethite (alpha-FeOOH), quartz (alpha-SiO(2)), and goethite-loaded quartz in batch experiments, and performed continuous-flow column experiments to study the extent of equilibrium and kinetic Fe isotope fractionation during reactive transport of Fe(II) through pure and goethite-loaded quartz sand. In addition, batch and column experiments were used to quantify the coupled electron transfer-atom exchange between dissolved Fe(II) (Fe(II)(aq)) and structural Fe(III) of goethite. All experiments were conducted under strictly anoxic conditions at pH 7.2 in 20 mM MOPS (3-(N-morpholino)-propanesulfonic acid) buffer and 23 degrees C. Iron isotope ratios were measured by high-resolution MC-ICP-MS. Isotope data were analyzed with isotope fractionation models. In batch systems, we observed significant Fe isotope fractionation upon equilibrium of Fe(II) to all sorbents tested, except for aluminum oxide. The equilibrium enrichment factor epsilon(eq)(56/54), of the Fe(II)(sorb)-Fe(II)(aq) couple was 0.85 +/- 0.10 parts per thousand (+/- 2 sigma) for quartz and 0.85 +/- 0.08 parts per thousand (+/- 2 sigma) for goethite-loaded quartz. In the goethite system, the sorption-induced isotope fractionation was superimposed by atom exchange, leading to a delta(56/54) Fe shift in solution towards the isotopic composition of the goethite. Without consideration of atom exchange, the equilibrium enrichment factor was 2.01 +/- 0.08 parts per thousand (+/- 2 sigma), but decreased to 0.73 +/- 0.24 parts per thousand (+/- 2 sigma) when atom exchange was taken into account. The amount of structural Fe in goethite that equilibrated isotopically with Fe(II)(aq) via atom exchange was equivalent to one atomic Fe layer of the mineral surface (similar to 3% of goethite-Fe). Column experiments showed significant Fe isotope fractionation with delta(56/54) Fe(II)(aq) spanning a range of 1.00 parts per thousand and 1.65 parts per thousand for pure and goethite-loaded quartz, respectively. Reactive transport of Fe(II) under non-steady state conditions led to complex, non-monotonous Fe isotope trends that could be explained by a combination of kinetic and equilibrium isotope enrichment factors. Our results demonstrate that in abiotic anoxic systems with near-neutral pH, sorption of Fe(II) to mineral surfaces, even to supposedly non-reactive minerals such as quartz, induces significant Fe isotope fractionation. Therefore we expect Fe isotope signatures in natural systems with changing concentration gradients of Fe(II)(aq) to be affected by sorption. (c) 2009 Elsevier Ltd. All rights reserved.