Kinetic iron stable isotope fractionation between iron (-II) and (-III) complexes in solution

Iron stable isotope fractionation between tris (2,2 ' -bipyridine) iron-II ([Fe-II(biyp)(3)](2+)) and iron-III chloride complexes has been measured using plasma source mass spectrometry. The experimental protocol involves complexing Fell ion with 2,2 ' -bipyridine in a Fe-II/Fe-III chloride solution and then separating the Fe-II and Fe-III solution species in 6 M HCl on an anion exchange resin. Large isotopic variations of epsilon Fe-57 and epsilon Fe-56 are experimentally measured in the two separated solution fractions, with isotopic fractionations increasing from Delta (Fe-II-Fe-III) = 25 to 174 epsilon units for Fe-57/Fe-54 and 17 to 117 epsilon units for Fe-56/Fe-54. The increase in fractionations correlates with a decrease in the mole fraction of Fe-II in the solution (Fe* = (Fe-II)/[(Fe-II)+(Fe-III)]) that results from the dissociation and breakdown of [Fe-II(bipy)(3)](2+) complex in 6 M HCl solution. The data variations are mainly ascribed to a kinetic fractionation occurring during this dissociation reaction. Mass balance calculations, assuming that a Rayleigh law describes the overall isotopic trends, suggest a kinetic fractionation of ca. 1.010 (similar to 100 epsilon units). The magnitude of this fractionation is attributed to the rupturing of the strong covalent bonds between 2,2 ' -bipyridine and Fe-II ion. The experimental data confirm that the coordination chemistry of iron exhibits a profound control on its isotopic behaviour and that kinetic fractionations may play an important role in its isotope geochemistry, as was also found in the pioneering experimental studies of the sulphur isotopic system in solution. (C) 2001 Elsevier Science B.V. All rights reserved.