Influence of Organic Anions on the Crystallization of Ferrihydrite

R. M. Cornell and U. Schwertmann
Lehrstuhl für Bodenkunde der Technischen Universität München 8050 Freising-Weihenstephan, West Germany

Abstract: Hydroxy-carboxylic acids inhibit the crystallization of ferrihydrite in the pH range 9–11 in the order

citric > meso tartaric > L-tartaric ≫ lactic

and favor hematite formation relative to goethite in the order

L-tartaric > citric > meso tartaric > lactic.

The crystal shape of hematite can change from hexagonal plates to acicular in the presence of these acids. The influence of the acids on the crystallization rises with increasing concentration and with falling pH.

The effectiveness in suppressing crystallization depends on whether and how strongly the acid adsorbs on ferrihydrite and how strongly it complexes with Fe3+ in solution. Inhibition of crystallization of hematite is believed to be due to the di- and tricarboxylic acid linking ferrihydrite particles in an immobile network. Goethite formation is suppressed by the acid complexing with Fe in solution and hindering nucleation; strongly adsorbing acids also adsorb on the nuclei and hinder further growth. Certain acids can induce hematite formation because they contain a group which acts as a template for nucleation of hematite.

Key Words: Crystallization • Ferrihydrite • Goethite • Hematite • Hydroxy-carboxylic acid

Clays and Clay Minerals; December 1979 v. 27; no. 6; p. 402-410; DOI: 10.1346/CCMN.1979.0270602
© 1979, The Clay Minerals Society
Clay Minerals Society (www.clays.org)