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

Abstract

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 Fe 3+ cations effectively oxidized benzidine, but lost much of this ability upon aging. The Fe(IlI)-benzidine electron transfer could be distinguished from an O2-benzidine reaction, since the latter reaction was slow and limited by the rate of O.z 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.

Structural Model for Benzidine-Vermiculite

“…Tennakoon et al (1974) presented direct Mtssbauer spectroscopic evidence for the involvement of structural iron(III) in the oxidation-reduction process leading to the formation of colored benzidine radical cations in montmorillonite. Recent work by McBride (1979) has shown that clay-edge aluminum sites are relatively unimportant in the oxidation of benzidine.…”

Transformation of 1-Aminonapththalene at the Surface of Smectite Clays

“…The formation of colored clayorganic complexes is well documented (Hauser et al, 1941;Solomon et al, 1968;Theng, 1971;Voudrias and Rienhard, 1986), and has been related to transformation reaction intermediates, and products at the clay surface Theng, 1971;Soma et al, 1984Soma et al, , 1985Soma et al, , 1986Furukawa and Brindley, 1973;Cloos et aL, 1981). Color development is often ascribed to charge transfer reactions (McBride, 1979;Soma et al, 1984Soma et al, , 1985Soma et al, , 1986Teenakoon et al, 1974), but color development and intensity are dependent on the clay type (Hauser et al, 1941;Voudrias and Rienhard, 1986;Thompson and Moll, 1973), reaction mechanism (Fenn et aL, 1973), and saturating inorganic cation (Furukawa and Brindley, 1973;Soma et al, 1983;Vansant and Yariv, 1977). The formation of a colored complex in the montmorillonite-benzidine system is the result of benzidine sorption and subsequent oxidation to a radical cation by the reduction of structural ferric iron; although 02, or freshly sorbed Fe 3 § and Cu 2 § can also act as oxidants (McBride, 1979).…”

“…Clay surfaces have the capacity to act as catalysts in chemical oxidation/reduction, acid-base neutralization, and hydrolysis reactions (McBride, 1979;Theng, 1974;Solomon, 1968). The formation of colored clayorganic complexes is well documented (Hauser et al, 1941;Solomon et al, 1968;Theng, 1971;Voudrias and Rienhard, 1986), and has been related to transformation reaction intermediates, and products at the clay surface Theng, 1971;Soma et al, 1984Soma et al, , 1985Soma et al, , 1986Furukawa and Brindley, 1973;Cloos et aL, 1981).…”

“…Color development is often ascribed to charge transfer reactions (McBride, 1979;Soma et al, 1984Soma et al, , 1985Soma et al, , 1986Teenakoon et al, 1974), but color development and intensity are dependent on the clay type (Hauser et al, 1941;Voudrias and Rienhard, 1986;Thompson and Moll, 1973), reaction mechanism (Fenn et aL, 1973), and saturating inorganic cation (Furukawa and Brindley, 1973;Soma et al, 1983;Vansant and Yariv, 1977). The formation of a colored complex in the montmorillonite-benzidine system is the result of benzidine sorption and subsequent oxidation to a radical cation by the reduction of structural ferric iron; although 02, or freshly sorbed Fe 3 § and Cu 2 § can also act as oxidants (McBride, 1979). Interestingly, with time, sorbed Fe 3 § loses its ability to oxidize benzidine, and freshly precipitated Fe(OH)3 fails to produce a reaction at all (McBride, 1979).…”

“…The formation of a colored complex in the montmorillonite-benzidine system is the result of benzidine sorption and subsequent oxidation to a radical cation by the reduction of structural ferric iron; although 02, or freshly sorbed Fe 3 § and Cu 2 § can also act as oxidants (McBride, 1979). Interestingly, with time, sorbed Fe 3 § loses its ability to oxidize benzidine, and freshly precipitated Fe(OH)3 fails to produce a reaction at all (McBride, 1979). The oxidative polymerization of aromatic compounds at the surface of transition metal exchanged clays has also been demonstrated (Soma et al, 1986;Fenn et aL, 1973).…”