Kinetic and equilibrium Fe isotope fractionation between aqueous Fe(II) and Fe(III)

Equilibrium and kinetic Fe isotope fractionation between aqueous ferrous and ferric species measured over a range of chloride concentrations (0, 11, 110 mM Cl super -) and at two temperatures (0 and 22 C) indicate that Fe isotope fractionation is a function of temperature, but independent of chloride contents over the range studied. Using super 57Fe-enriched tracer experiments the kinetics of isotopic exchange can be fit by a second-order rate equation, or a first- order equation with respect to both ferrous and ferric iron. The exchange is rapid at 22 C, ~60-80 percent complete within 5 seconds, whereas at 0 C, exchange rates are about an order of magnitude slower. Isotopic exchange rates vary with chloride contents, where ferrous-ferric isotope exchange rates were ~25 to 40 percent slower in the 11 mM HCl solution compared to the 0 mM Cl super - (~10 mM HNO sub 3) solutions; isotope exchange rates are comparable in the 0 and 110 mM Cl super - solutions. The average measured equilibrium isotope fractionations, Delta sub Fe(III)-Fe(II), in 0, 11, and 111 mM Cl super - solutions at 22 C are identical within experimental error at +2.76+/-0.09, +2.87+/-0.22, and +2.76+/- 0.06 ppm, respectively. This is very similar to the value measured by [Johnson et al 2002a, ] in dilute HCl solutions. At 0 C, the average measured Delta sub Fe(III)-Fe(II) fractionations are +3.25+/-0.38, +3.51+ /-0.14 and +3.56+/-0.16 ppm for 0, 11, and 111 mM Cl super - solutions. Assessment of the effects of partial re-equilibration on isotope fractionation during species separation suggests that the measured isotope fractionations are on average too low by ~0.20 ppm and ~0.13 ppm for the 22 C and 0 C experiments, respectively. Using corrected fractionation factors, we can define the temperature dependence of the isotope fractionation from 0 C to 22 C as: [equation] alt="Image" align="absbottom" border="0" height=23 width=355 > where the isotopic fractionation is independent of Cl super - contents over the range used in these experiments. These results confirm that the Fe(III)-Fe(II) fractionation is approximately half that predicted from spectroscopic data, and suggests that, at least in moderate Cl super - contents, the isotopic fractionation is relatively insensitive to Fe-Cl speciation.