Adsorption and Oxidation of Benzidine and Aniline by Montmorillonite and Hectorite

Abstract

Abstract--Quantitative measurements are made of the adsorption of benzidine and aniline from aqueous hydrochloride solutions by Na-, Li-, and Ca-montmorillonite and of the displaced inorganic cations. From these data, the ionic states of the adsorbed organic species are determined. Under conditions of controlled pH, the adsorption of benzidine increases as the pH increases, and involves mainly divalent species at pH < 3.2, and increasing proportions of monovalent and neutral species at pH > 3.2. With aniline, monovalent and neutral species are adsorbed, and hydrogen ions also appear to participate in the reactions.Color developments of benzidine and aniline complexes of montmorillonite and hectorite are considered qualitatively in relation to the adsorption data, to various experimental conditions including the nature of the inorganic exchangeable cations, the pH, and the presence or absence of oxygen in the system, and to relevant previous work. It is hypothesized that the blue color of the benzidine complex is due to semiquinone formation by oxidation on montmoriUonite by the clay itself, and on hectorite by dissolved oxygen or H202, and that the yellow color under acid conditions arises from reversible formation of quinoidal cations from semiquinones. The color developments of the aniline complex are due probably to oxidation of aniline by atmospheric oxygen.

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

“…Thus, the clay appeared to be necessary to stabilize the blue semiquinone product, playing a more specific role than that of a simple oxidizing agent. This has previously been indicated by the fact that H202 alone could not form benzidine-blue, but H202 plus hectorite could (Furukawa and Brindley, 1973).…”

Structural Model for Benzidine-Vermiculite

“…The surface charge densities in montmorillonite and in vermiculite can cause different packing arrangements in their benzidine intercalates. There is room for both divalent (Furukawa and Brindley, 1973) and monovalent benzidine cations to lie flat and to satisfy the exchange capacity of montmorillonite. In vermiculite, only divalent cations would have sufficient room to lie flat; monovalent species would have to stand at an angle to the surface and so give rise to spacings above 15.5/~, in agreement with our findings.…”

“…By quantitatively studying the adsorption of benzidine from aqueous hydrochloride solutions in contact with various montmorillonites, Furukawa and Brindley (1973) concluded that at pH <3.2, divalent and monovalent species were taken up by displacement of the exchangeable cations, but at higher pH, the monovalent species was preferentially adsorbed. They further considered that the total amount of benzidine adsorbed was equal to the amount of unoxidized cation taken up by exchange plus an amount of semiquinone produced by oxidation-reduction processes.…”

Chemical reactions of organic compounds on clay surfaces.

“…The structure of this yellow complex has been explained variously, such as a protonated monoradical cation, or a diimine or quinoidal cation. The following disproportionation mechanism to produce the yellow complex is supported from various experimental evidences (45)(46)(47):…”