Ionic migration and charge reduction in Ni²⁺-, Co²⁺- and Zn²⁺-exchanged Texas montmorillonite

Abstract

Controlled calcination of ion-exchanged Texas montmorillonite leads to layer charge reduction. Detailed chemical analyses of both exchangeable and unexchangeable metal species lead to the conclusion that, in complete contrast to the situation with Wyoming bentonite, Ni²⁺, Co²⁺ and Zn²⁺ are all capable of migrating from the interlamellar space into the octahedral region of the sheets where they are (i) isomorphously exchanged for Mg²⁺, and to a proportionately smaller extent, for Fe²⁺, (ii) trapped in the octahedral vacancies. In addition, they are converted to unexchangeable species on the interlamellar sheet surfaces by high temperature hydrolysis leading to hydroxide or oxide formation. It is suggested that protons within the octahedral region are bound as -OH⁺ ₂rather than simply physically trapped.