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

摘要

Equilibrium and kinetic Fe isotope fractionation between aqueous ferrous and ferric species measured over a range of chloride concentrations (0, 11, 110 mM Cl-) and at two temperatures (0 and 22degreesC) indicate that Fe isotope fractionation is a function of temperature, but independent of chloride contents over the range studied. Using Fe-57-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 22degreesC, similar to60-80% complete within 5 seconds, whereas at 0degreesC, exchange rates are about an order of magnitude slower. Isotopic exchange rates vary with chloride contents, where ferrous-ferric isotope exchange rates were similar to25 to 40% slower in the 11 mM HCl solution compared to the 0 mM Cl- (similar to10 mM HNO3) solutions; isotope exchange rates are comparable in the 0 and 110 mM Cl- solutions. The average measured equilibrium isotope fractionations, Delta(Fe(III)-Fe(II)), in 0, 11, and 111 mM Cl- solutions at 22degreesC are identical within experimental error at +2.76+/-0.09, +2.87+/-0.22, and +2.76+/-0.06 parts per thousand, respectively. This is very similar to the value measured by Johnson et al. (2002a) in dilute HCl solutions. At 0degreesC, the average measured Delta(Fe(III)-Fe(II)) fractionations are +3.25+/-0.38, +3.51+/-0.14 and +3.56+/-0.16 parts per thousandfor 0, 11, and 111 mM Cl- 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 similar to0.20 parts per thousand and similar to0.13 parts per thousand for the 22degreesC and 0degreesC experiments, respectively. Using corrected fractionation factors, we can define the temperature dependence of the isotope fractionation from 0degreesC to 22degreesC as: 10(3) In alpha(Fe(III)-Fe(II)) = [0.334 +/- 0.032]*10(6)/T-2 - 0.88 +/- 0.38 where the isotopic fractionation is independent of Cl- 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- contents, the isotopic fractionation is relatively insensitive to Fe-Cl speciation. Copyright (C) 2003 Elsevier Ltd. [References: 47]