Sorption and desorption processes are an important part of biological and geochemical metallic isotope cycles. Here, we address the dynamic aspects of metallic isotopic fractionation in a theoretical and experimental study of Fe sorption and desorption during the transport of aqueous Fe(III) through a quartz-sand matrix. Transport equations describing the behavior of sorbing isotopic species in a water saturated homogeneous porous medium are presented; isotopic fractionation of the system (Δsorbed metal-soln) being defined in terms of two parameters: (i) an equilibrium fractionation factor, αe; and (ii) a kinetic sorption factor, α1. These equations are applied in a numerical model that simulates the sorption-desorption of Fe isotopes during injection of a Fe(III) solution pulse into a quartz matrix at pH 0-2 and explores the effects of the kinetic and equilibrium parameters on the Fe-isotope evolution of porewater. The kinetic transport theory is applied to a series of experiments in which pulses of Na and Fe(III) chloride solutions were injected into a porous sand grain column. Fractionation factors of αe = 1.0003 ± 0.0001 and α1 = 0.9997 ± 0.0004 yielded the best fit between the transport model and the Fe concentration and δ56Fe data. The equilibrium fractionation (Δ56Fesorbed Fe-soln) of 0.3‰ is comparable with values deduced for adsorption of metallic cations on iron and manganese oxide surfaces and suggests that sandstone aquifers will fractionate metallic isotopes during sorption-desorption reactions. The ability of the equilibrium fractionation factor to describe a natural system, however, depends on the proximity to equilibrium, which is determined by the relative time scales of mass transfer and chemical reaction; low fluid transport rates should produce a system that is less dependent on kinetic effects. The results of this study are applicable to Fe-isotope fractionation in clastic sediments formed in highly acidic conditions; such conditions may have existed on Mars where acidic oxidizing ground and surface waters may have been responsible for clastic sedimentation and metallic element transport.
Bibliographical noteFunding Information:
The research was supported by a Grant #2002/338 from the United States Israel Binational Science Foundation. The work of Simon Emmanuel was generously supported by a Bateman Postdoctoral Fellowship at Yale University. A.M. is Raymond F. Kravis Professor of Geology at the Hebrew University. Dr. Irena Segal of the Geological Survey of Israel is thanked for her help with the plasma mass spectrometry. Dr. Silke Severmann, two anonymous reviewers, and the Associate Editor, Dr. Mark Rehkämper, are thanked for their thoughtful and valuable critical comments.