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

Abstract

Here we compare new experimental studies with theoretical predictions of equilibrium iron isotopic fractionation among aqueous ferric chloride complexes (Fe(H 2O) 6 3+, FeCl(H 2O) 5 2+, FeCl 2(H 2O) 4 +, FeCl 3 (H 2O) 3, and FeCl 4 –), using the Fe–Cl–H 2O 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 ( 56Fe/ 54Fe) among naturally occuring iron-bearing species at Earth surface temperatures (up to ∼3‰) 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 FeCl 4 – in immiscible diethyl ether to create a separate spectator phase, used to monitor changing fractionation in the aqueous solution. Δ 56Fe aq-eth = δ 56Fe (total Fe remaining in aqueous phase)−δ 56Fe (FeCl 4 – in ether phase) is determined for each solution via MC-ICPMS analysis. Both experiments and theoretical calculations of Δ 56Fe aq-eth show a downward trend with increasing chlorinity: Δ 56Fe aq-eth is greatest at low chlorinity, where FeCl 2(H 2O) 4 + is the dominant species, and smallest at high chlorinity where FeCl 3(H 2O) 3 is dominant. The experimental Δ 56Fe aq-eth ranges from 0.8‰ at [Cl –] = 0.5 M to 0.0‰ at [Cl –] = 5.0 M, a decrease in aqueous–ether fractionation of 0.8‰. This is very close to the theoretically predicted decreases in Δ 56Fe aq-eth, which range from 1.0 to 0.7‰, depending on the ab initio model. The rate of isotopic exchange and attainment of equilibrium are shown using spiked reversal experiments in conjunction with the two-phase aqueous–ether system. Equilibrium under the experimental conditions is established within 30 min. The general agreement between theoretical predictions and experimental results points to substantial equilibrium isotopic fractionation among aqueous ferric chloride complexes and a decrease in 56Fe/ 54Fe as the Cl –/Fe 3+ ion ratio increases. The effects on isotopic fractionation shown by the modeling of this simple iron–ligand system imply that ligands present in an aqueous environment are potentially important drivers of fractionation, are indicative of possible fractionation effects due to other speciation effects (such as iron–sulfide systems or iron bonding with organic ligands), and must be considered when interpreting iron isotope fractionation in the geological record.