Reactivity of Adsorbed and Structural Iron in Hectorite as Indicated by Oxidation of Benzidine

M. B. McBride
Department of Agronomy, Cornell University, Ithaca, New York 14853

Abstract: The roles of different forms of Fe(III) impurities in a hectorite with respect to the oxidation of benzidine in aqueous suspension have been evaluated using electron spin resonance and UV-visible spectroscopy. Natural surface-adsorbed Fe(III) showed no detectable activity in the oxidation process, while very small quantities of structural octahedral Fe(III) apparently promoted a relatively rapid conversion to the radical cation. However, extremely small quantities of benzidine were oxidized in comparison to the exchange capacity of the clay. Freshly adsorbed Fe3+ cations effectively oxidized benzidine, but lost much of this ability upon aging. The Fe(III)-benzidine electron transfer could be distinguished from an O2-benzidine reaction, since the latter reaction was slow and limited by the rate of O2 diffusion into the clay-water system. The O2-benzidine reaction was also inhibited at high pH. The existence of two reaction mechanisms and the involvement of only a small fraction of the total structural iron, as shown by comparison of the hectorite and a montmorillonite, may explain the conflicting interpretations in the literature. The benzidine blue reaction not only requires an oxidizing agent to form the radical, but also a clay surface to adsorb and stabilize it against further oxidation.

Key Words: Adsorbed Iron • Benzidine • Catalysis • Electron Transfer • Hectorite • Oxidation

Clays and Clay Minerals; June 1979 v. 27; no. 3; p. 224-230; DOI: 10.1346/CCMN.1979.0270308
© 1979, The Clay Minerals Society
Clay Minerals Society (www.clays.org)