Adsorption of Safranine by Na⁺, Ni²⁺ and Fe³⁺ Montmorillonites

Abstract

The adsorption of the cationic oxidized safranine S⁺ by a Na⁺, Ni²⁺ and Fe³⁺ montmorillonite has been studied with X-ray powder diffraction, u.v., visible and i.r. spectroscopy. In solution S⁺ may be protonated: S⁺, SH²⁺ and SH ³⁺₂ have characteristic spectra in the 500–600 nm region where the clay structure does not absorb. In the Na⁺ as well as in the Ni²⁺ and Fe³⁺ clays, the adsorption of S⁺ is a cation exchange process accompanied by the protonation of the adsorbed dye such as variable concentrations of M⁺ (Na⁺, Ni²⁺ or Fe³⁺), S⁺ and SH²⁺ are simultaneously present. Protonation activity decreases from Fe³⁺ to Ni²⁺ and Na⁺, being the protonation site the amine group as shown by i.r. In the interlamellar space it seems that a SH²⁺.. S⁺ association exists that could be described as a sandwich structure 6.5 Å thick.