Experimental studies of equilibrium iron isotope fractionation in ferric aquo-chloro complexes

Here we compare new experimental studies with theoretical predictions of equilibrium iron isotopic fractionation among aqueous ferric chloride complexes (Fe(H2O)(6)(3+), FeCl(H2O)(5)(2+), FeCl2(H2O)(4)(+), FeCl3 (H2O)(3), and FeCl4-), using the Fe-Cl-H2O system as a simple, easily-modeled example of the larger variety of iron-ligand compounds, such as chlorides, sulfides, simple organic acids, and siderophores. Isotopic fractionation (Fe-56/Fe-54) among naturally occuring iron-bearing species at Earth surface temperatures (up to similar to 3 parts per thousand) is usually attributed to redox effects in the environment. However, theoretical modeling of reduced isotopic partition functions among iron-bearing species in solution also predicts fractionations of similar magnitude due to non-redox changes in speciation (i.e., ligand bond strength and coordination number). In the present study, fractionations are measured in a series of low pH ([H+] = 5 M) solutions of ferric chloride (total Fe = 0.0749 mol/L) at chlorinities ranging from 0.5 to 5.0 mol/L. Advantage is taken of the unique solubility of FeCl4- in immiscible diethyl ether to create a separate spectator phase, used to monitor changing fractionation in the aqueous solution. Delta Fe-56(aq-eth) = delta Fe-56 (total Fe remaining in aqueous phase)-delta Fe-56 (FeCl4- in ether phase) is determined for each solution via MC-ICPMS analysis.