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One
of the key mineral-water reactions is ion adsorption, a process that
often controls the fate of environmental contaminants and the
composition of natural waters. Of particular concern is the
adsorption behavior of oxoanions, as many of these species, such as
arsenate, arsenite, selenate, selenite, and chromate, are
environmental toxins. Understanding their adsorption behavior is
important for predicting the environmental impacts of the natural
occurrence of oxoanion contaminants as well as the impacts of energy
production and the leakage of legacy nuclear wastes.
Work over the past two decades has sought to establish the
mechanisms through which oxoanion adsorption occurs. Spectroscopic
studies have routinely been used to identify the types of surface
complexes that form with the goal of connecting molecular-scale
mechanisms to macroscale properties. The main method by which these
disparate scales are connected is through the development of
thermodynamic models of ion adsorption, specifically surface
complexation models. While early models were based solely on
macroscopic measurements of solution concentrations and particle
charging behavior, recent models have attempted to also incorporate
molecular-level information into their developments, especially the
specific types of surface complexes present. An essential
requirement for connecting molecular-scale observations with
macroscale phenomena is that the experimental (often spectroscopic)
results capture the full complexity of the system. If multiple
types of surface complexes are present, then ideally all such
species are identified and quantified. This is not a trivial task,
and recently developed experimental methods are only now revealing
the true complexity of ion adsorption behavior.
We
have recently investigated arsenate (AsO43-)
adsorption on iron and aluminum oxide surfaces using new methods
developed by our collaborators, Paul Fenter and Changyong Park of
Argonne National Laboratory. Our results clearly demonstrate that,
under the limited conditions we have explored, arsenate forms both
inner-sphere (i.e., directly bonded to the surface) and outer-sphere
(i.e., lacking a direct bond to the surface) adsorption complexes (Figure
1). While we are confident these observations are valid,
they are troubling as they suggest that a major mechanism of
arsenate adsorption has been overlooked for the last 30 years;
inner-sphere complexation has long been accepted as the sole process
through which arsenate adsorbs to minerals. We are now actively
exploring oxoanion adsorption processes to determine the conditions
where this complex behavior exists, the major controls on this
behavior, and how this behavior manifests itself in terms of
adsorption and desorption kinetics. Ultimately, we hope to find
ways to better evaluate this complexity in environmental systems and
to explore how these new observations can be used to improve and
refine water filtration processes.
Figure 1.
Schematic model of arsenate adsorption on aluminum and iron oxide,
showing both inner- and outer-sphere species.
For more information:
Catalano J.G., Park C., Fenter P., Zhang Z. (2008) Simultaneous
inner- and outer-sphere arsenate adsorption on corundum and hematite.
Geochimica et Cosmochimica Acta,
in press. (soon to be available)
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