Benzidine clays

Another well-studied electron transfer reaction is the oxidation of aqueous benzidine in the presence of various clays (2, 40, 43, 50, 51). An electron from the colorless benzidine molecule is abstracted by the clay with formation of a blue monovalent radical cation. Upon drying of the blue clay-benzidine system, a yellow color is produced. There is disagreement in the literature with respect to the chemical identity of the yellow product (2, 40, 52) however, in the case of hectorite, the yellow product has been suggested to be the protonated form of the radical cation (divalent radical cation) (2, 52). There is also disagreement about whether the electron-accepting sites of the clay are ferric iron at the planar surfaces, aluminum ions at the edges, or exchangeable cations <2, I). 

The role of structural Fe(III) In reactions with benzidine was also demonstrated by Mossbauer spectroscopy at very high adsorption levels (i.e., intercalation conditions) on montmorillonite (40). Upon dehydration of the clay, a yellow color and regeneration of Fe2+ to Fe3+ was observed. This was explained by the equilibria... 

Unsubstituted benzidine may be oxidized at clay surfaces when mixed with some types of elay minerals (Tennakoon et al. 1974 Theng 1971). Benzidine is oxidized to a monovalent radical cation by iron (III) in the silicate lattice and by aluminum at crystal edges. However, there is no experimental evidence that... 

Environmental Fate. It is not known if 3,3 -dichlorobenzidine, like benzidine, is oxidized by clay minerals or if cations in water ean have the same oxidizing effect. 3,3 -Dichlorobenzidine does not appear to biodegrade easily, but the few studies in this area did not state the type(s) or concentrations of mieroorganisms used in eaeh study. More systematic studies with other organisms may prove useful. A reeent study (Nyman et al. 1997) provides evidence that in the span of a year up to 80% of 3,3 -dichloro-benzidine can degrade to benzidine in anaerobic mixtures of sediment/water. Further research to identify the pathways and produets of deeomposition of 3,3 -dichlorobenzidine in various soils is needed. The toxieologieal profile for benzidine eontains information on the environmental fate of that compound (ATSDR 1995). 

The color of benzidine adsorbed on bentonite is reversibly altered by adsorbed water vapor. Vedeneeva (80) found that the intense blue color observed in the presence of excess water changed to green, yellow and finally colorless as the adsorbed water was removed. This color sequence was reversed when moisture was added. More recently, Hase-gawa (14) reexamined this so-called benzidine reaction with both benzidine and tetramethylbenzidine, using benzene as a supporting solvent for the catalyst suspension. Adsorption of benzidine on natural or acid-treated clay produced a greenish-yellow color (curves A and B, Fig. 27) with absorption maxima at 4400 A, 7800 A, and 8800 A, in close... 

Fig. 27. Absorption spectra of adsorbed benzidine curve A, on natural clay curve B, on acid-treated clay curve C, on alkali-treated clay (14).

Furukawa, T. and G. W. Brindley. 1973. Adsorption and oxidation of benzidine and aniline by montmorillonite and hectorite. Clays Clay Min. 21 279-288. 

It is well known that clays have the property to generate colored species by the adsorption of aromatic amines. The most remarkable case is the blue color generated by the adsorption of benzidine (4,4 -diaminobiphenyl) in clay [77]. Among the earliest... 

Y. Soma and M. Soma, Adsorption of benzidines and anilines on Cu- and Fe-montmorillonites studied by resonance Raman spectroscopy. Clay Miner., 23, 1-12 (1988). 

G.M. Do Nascimento, P.S.M. Barbosa, V.R.L. Constantino, and M.L.A. Temperini, Benzidine oxidation on cationic clay surfaces in aqueous suspension monitored by in situ resonance Raman spectroscopy, Coll. Surf. A Physicochem. Eng. Aspects, 289, 39-46 (2006). 

UV-v is application to study clay-organic systems Montmorillonite/benzidine H. Hasegawa (249)... 

The dependence of organic base adsorption on pH is illustrated by the case of benzidine. This base is adsorbed from aqueous solutions on Ca- and Na-montmorillonite as a mixture of monovalent and divalent cations at pH 3 and as neutral molecules when the pH is higher. Aniline is also adsorbed as a mixture of monovalent cations and neutral molecules at pH = 3.2. Amounts adsorbed are in excess of the exchange capacity of the clay, and in both cases intermolecular association by hydrogen bonds (see Fig. 1) is assumed to exist between cations and molecules in the interlayer space (23). 

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