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Micromorphologies and Stabilities of Chromium (III) Surface Precipitates Elucidated by Scanning Force Microscopy

Soil Science Division

Mickey E. Gunter

Dep. of Geology and Geological Engineering, Univ. of Idaho, Moscow ID 83844

*Corresponding author (sfendorf{at}uidaho.edu).

ABSTRACT

Ascertaining the stability of immobilized contaminants in soils is necessary to assess their potential risk in the environment. The composition, structure, and surface morphology are the principal determinants of a sorbate's reactivity. We investigated Cr(III) precipitates on goethite (-FeOOH) and silica (amorphous SiO₂) using scanning force microscopy to ascertain the morphology of the surface precipitates. Imaging was performed on both colloidal and single-particle solids. The colloidal materials were used to simulate solids found within soils, while the single particles provide in situ investigations. Scanning force micrographs reveal that Cr(III) forms a precipitate on -FeOOH that distributes over the surface, while on SiO_2(amorphous) discrete surface clusters are observed. Based on these results, one would predict that at coverages exceeding monolayer capacity, Cr(III) precipitates on silica would be more reactive and thus have a greater potential for remobilization in the environment than Cr(III) precipitates on goethite. To test this hypothesis we reacted the surface precipitates with oxalate, a common root exudate, and measured the rate and extent of Cr(III) dissolution. The expected greater reactivity of Cr(III) on silica relative to Cr(III) on goethite was supported by the more rapid and extensive oxalate-induced dissolution of Cr(III); initial dissolution rates were 0.166 mol L^-1 h^-1 for Cr(III)-silica while only 0.0420 mol L^-1 h^-1 for Cr(III)-goethite.

Received for publication May 23, 1994.