posted on 2007-01-15, 00:00authored byPhilip Larese-Casanova, Michelle M. Scherer
We collected Mössbauer spectra of 57Fe(II) interacting
with 56hematite (α-Fe2O3) over a range of Fe(II) concentrations
and pH values to explore whether a sorbed Fe(II) species
would form. Several models of Fe(II) sorption (e.g.,
surface complexation models) assume that stable, sorbed Fe(II) species form on ligand binding sites of Fe(III) oxides
and other minerals. Model predictions of changes in both
speciation and concentration of sorbed Fe(II) species
are often invoked to explain Fe(II) sorption patterns, as
well as rates of contaminant reduction and microbial
respiration of Fe(III) oxides. Here we demonstrate that, at
low Fe(II) concentrations, sorbed Fe(II) species are
transient and quickly undergo interfacial electron transfer
with structural Fe(III) in hematite. At higher Fe(II)
concentrations, however, we observe the formation of a
stable, sorbed Fe(II) phase on hematite that we believe to
be the first spectroscopic confirmation for a sorbed Fe(II) phase forming on an iron oxide. Low-temperature
Mössbauer spectra suggest that the sorbed Fe(II) phase
contains varying degrees of Fe(II)−Fe(II) interaction and likely
contains a mixture of adsorbed Fe(II) species and surface
precipitated Fe(OH)2(s). The transition from Fe(II)−Fe(III) interfacial electron transfer to formation of a stable,
sorbed Fe(II) phase coincides with the macroscopically
observed change in isotherm slope, as well as the estimated
surface site saturation suggesting that the finite capacity
for interfacial electron transfer is influenced by surface
properties. The spectroscopic demonstration of two distinctly
different sorption endpoints, that is an Fe(III) coating
formed from electron transfer or a stable, sorbed Fe(II)
phase, challenges us to reconsider our traditional
interpretations and modeling of Fe(II) sorption behavior
(as well as, we would argue, of any other redox active sorbate-sorbent couple).