STRUCTURAL CHARACTERIZATION OF AMORPHOUS ALUMINUM HYDROXIDE: IMPLICATIONS FOR CONTAMINANT TRANSPORT

Amorphous or poorly crystalline metal oxyhydroxides play a central role in the transport and fate of environmental contaminants such as heavy metals and the products of radioactive fission, but their structural details are poorly defined. Understanding the structural features of these important phases is vital to a complete molecular picture of their ability of adsorb contaminants. The major impediment for determining the structural details of amorphous materials is their lack of long-range atomic ordering, rendering traditional techniques such as X-ray diffraction of limited utility. Pair distribution function analysis of high energy X-ray scattering data provides a mechanism to investigate at the short range ordering present in amorphous and poorly crystalline material. While the size of the ordered domain can easily be determined, structural interpretation of PDF analysis is difficult without good model compounds; thus, we are investigating the use of structurally characterized, synthetic nanoclusters (1-2 nm) as geochemical models for amorphous material.

The current study investigates the structural similarities of Keggin-type aluminum oxyhydroxide nanoclusters and amorphous aluminum oxyhydroxide produced from aqueous solution at pH 4.6. Keggin-type nanoclusters have been previously suggested as the building block of amorphous aluminum “floc” present in natural waters affected by acid rock drainage, but little direct evidence was initially available to confirm this observation. Several model compounds have been optimized to investigate the best geochemical model compound for the amorphous material. Insights gained from PDF anlaysis of amorphous aluminum hydroxide, as well as possible adsorption sites for transition metal contaminants will be presented.