Effects of iron oxidation states on the surface and structural properties
of smectites
J. W. Stucki*, K. Lee, L. Zhang, and R. A. Larson
Department of Natural Resources and Environmental Sciences,
University of Illinois, W-321 Turner Hall, 1102 South Goodwin Avenue,
Urbana, IL 61801, USA
Abstract: The oxidation state of iron (Fe) in the crystal structure
of smectite clay minerals profoundly alters their physicalchemical
properties. Among the properties affected are layer charge, cation exchange
and fixation capacity, swelling in water, particle size, specific surface
area, layer stacking order, magnetic exchange interactions, octahedral
site occupancy, surface acidity, and reduction potential. Also affected
is the surface chemistry of the clay, which alters claywater and
clayorganic interaction mechanisms. Rates and extents of degradation
of pesticides are increased in the presence of reduced smectites compared
to oxidized and reducedreoxidized counterparts. A hypothesis regarding
the mechanism for Fe reduction in clay minerals was first developed
in 1963, and subsequent modifications have been proposed periodically
through the present time. Recent studies clearly reveal that the process
of Fe reduction involves more than the mere transfer of an electron
to octahedral Fe(III) in the clay crystal. Ancillary reactions occur
that produce significant structural modifications, some of which are
reversible and others of which are not. Such changes in the crystalchemical
environment of structural Fe are thought to play a dominant role in
altering the clay surface chemistry.
* Lecture presented at the 5th Conference on Solid
State Chemistry (SSC 2002), Bratislava, Slovakia, 7-12 July 2002. Other
presentations are published in this issue, pp.
2083-2168.
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