Effects of Wetting and Drying, and of Exchangeable Cations, on Boron Adsorption and Release by Montmorillonite

Abstract

AbstractThe effect of drying and rewetting sequences on boron adsorption and desorption by Na‐ and Ca‐montmorillonite suspensions was determined with (i) nondried suspensions, (ii) suspensions dried prior to boron addition, and (iii) suspensions dried once or twice after boron adsorption. Boron adsorption in the nondried suspensions was completely reversible for both the Na‐ and Ca‐systems. There was little effect of exchangeable cation species (Na+ vs. Ca2+) on boron adsorption in the pH values of 7.0 and 7.2. There was a significant exchangeable cation effect, however, at higher pH's, with the Ca‐system adsorbing the greatest amount of boron. Boron adsorption on clays dried prior to boron addition was essentially the same for the nondried systems, indicating that the drying and rewetting sequence does not have any effect on the adsorption sites of the clay. The amount of boron adsorbed increased with drying and wetting after boron addition for both the Na‐ and Ca‐systems, with the greatest increase occurring after the first drying‐wetting cycle for both systems. Drying and rewetting of the clay suspensions after boron addition not only increased the amount of boron adsorbed, but significantly reduced the reversibility of the adsorption process as well. The relative amount of reversibility of adsorbed boron is a complex function of the drying and rewetting cycles, pH, and type of exchangeable cations.