Heterogeneous electron-transfer kinetics with synchrotron 57Fe Mossbauer spectroscopy

In the first known kinetic application of the technique, synchrotron Fe-Mössbauer spectroscopy was used to follow the rate of heterogeneous electron transfer between aqueous reagents and a solid phase containing Fe. The solid, a synthetic Fe-enriched Fe(III)-bearing pyroaurite-like phase having terephthalate (TA) in the interlayer [Mg3Fe(OH)8(TA)0.5 2H2O], was reduced by Na2S2O4 and then reoxidized by K2Cr2O7 by means of a novel flow-through cell. Synchrotron Mössbauer spectra were collected in the time domain at 30-s intervals. Integration of the intensity obtained during a selected time interval in the spectra allowed sensitive determination of Fe(II) content as a function of reaction time. Analysis of reaction end member specimens by both the synchrotron technique and conventional Mössbauer spectroscopy yielded comparable values for Mössbauer parameters such as center shift and Fe(II)/Fe(III) area ratios. Slight differences in quadrupole splitting values were observed, however. A reactive diffusion model was developed that fit the experimental Fe(II) kinetic data well and allowed the extraction of second-order rate constants for each reaction. Thus, in addition to rapidly collecting high quality Mössbauer data, the synchrotron technique seems well suited for aqueous rate experiments as a result of the penetrating power of 14.4 keV X-rays and high sensitivity to Fe valence state. Copyright © 2003 Elsevier Science Ltd