Cation and Anion Retention by Natural and Synthetic Allophane and Imogolite

C. J. Clark1 and M. B. McBride
Department of Agronomy, Cornell University, Ithaca, New York 14853
1 Present address: Ruakura Soil and Plant Research Station, Private Bag, Hamilton, New Zealand.

Abstract: The negative surface charge of synthetic allophanes with a range of Si/Al ratios decreased and positive charge increased with increasing alumina content at a given pH. The phosphate adsorption capacity also increased with increasing Al content. That this relationship between composition and chemical reactivity was not found for the soil allophanes is attributed to the presence of specifically adsorbed organic or inorganic anions on the natural material. Both synthetic and natural imogolites had a much lower capacity to adsorb phosphate than the allophanes and adsorbed anomalously high amounts of Cl and ClO4 at high pH. It is proposed that intercalation of salt occurs in imogolite, although electron spin resonance studies using spin probes failed to reveal the trapping of small organic molecules in imogolite tubes. These spin probes in the carboxylated form did, however, suggest an electrostatic retention of carboxylate by imogolite and a more specific adsorption by allophane involving ligand exchange of surface hydroxyl. The results illustrate the inherent differences in charge and surface properties of allophane and imogolite despite the common structural unit which the two minerals incorporate.

Key Words • Adsorption • Allophane • Anion retention • Cation retention • Imogolite • SiO2/Al2O3 ratio • Surface charge

Clays and Clay Minerals; August 1984 v. 32; no. 4; p. 291-299; DOI: 10.1346/CCMN.1984.0320407
© 1984, The Clay Minerals Society
Clay Minerals Society (www.clays.org)