Evolution of Hematite Surface Microtopography Upon Dissolution by Simple Organic Acids

Patricia A. Maurice1, Michael F. Hochella Jr.1, George A. Parks1, Garrison Sposito2 and Udo Schwertmann3
1 Department of Geological and Environmental Sciences Stanford University, Stanford, California 94305
2 Department of Environmental Science, Policy, and Management University of California at Berkeley, Berkeley, California 94720
3 Institute of Soil Science, Technical University of Munich, Germany

Abstract: The surface microtopography of hematite over the course of dissolution in oxalic and citric acids was examined by in-situ and ex-situ atomic-force microscopy, In-situ imaging of the basal-plane surface of a centimeter-scale natural hematite sample immersed in 2 mM citric acid demonstrated that the basal-plane surface was relatively unreactive; rather, dissolution occurred along step edges and via etch-pit formation. Ex-situ imaging of synthetic hematite particles following batch dissolution in 1 mM oxalic acid showed similar dissolution features on basal-plane surfaces; in addition, etching along particle edges was apparent. The presence of etch features is consistent with a surface-controlled dissolution reaction. The results are in agreement with previous investigations suggesting that the basal-plane surface is relatively unreactive with respect to ligand exchange. Both in-situ and ex-situ imaging of particle surfaces can provide valuable information on the roles of surface structures and microtopographic features in mineral dissolution.

Key Words: Atomic force microscope • Clay mineral surfaces • Dissolution • Hematite • Organic acids

Clays and Clay Minerals; February 1995 v. 43; no. 1; p. 29-38; DOI: 10.1346/CCMN.1995.0430104
© 1995, The Clay Minerals Society
Clay Minerals Society (www.clays.org)