title: Proton Adsorption Selectivity of Zeolites in Aqueous Media: Effect of Si/Al Ratio of Zeolites.
abstract: In addition to their well-known uses as catalysts, zeolites are utilized to adsorb and remove various cations from aqueous system. The adsorption of the cations is ascribed to the negative charge of zeolites derived from isomorphous substitution of Si by Al. The amount of Na+ adsorption on 4A, X, Y, Na-P1 and mordenite type zeolites were determined in aqueous media, in a two-cation (Na+ and H+) system. Although each zeolite has a constant amount of negative charge, the amount of Na+ adsorption of each zeolite decreased drastically at low pH-pNa values, where pH-pNa is equal to log{(Na+)/(H+)}. By using the plot of the amount of Na+ adsorption versus pH-pNa, an index of the H+ selectivity, which is similar to the pKa of acids, of each zeolite was estimated, and the index tended to increase with decreasing Si/Al ratio of zeolites. These indicate that zeolites with lower Si/Al and higher negative charge density have higher H+ adsorption selectivity, and in fact, such a zeolite species (4A and X) adsorbed considerable amount of H+ even at weakly alkaline pH region. The adsorption of H+ results in the decrease of cation adsorption ability, and may lead to the dissolution of zeolites in aqueous media.
authors.affiliation: Department of Life Environmental Conservation, Faculty of Agriculture, Ehime University, 3-5-7 Tarumi, Matsuyama 790-8566, Japan. munthalimw@yahoo.co.uk.
authors.forename: Moses Wazingwa
authors.initials: MW
authors.lastname: Munthali
authors.affiliation: Department of Soil and Environment Sciences, Faculty of Agriculture, University of Khartoum, Shambat, P.O. Box 32, Khartoum North 13314, Sudan. mohmedelsheikh@gmail.com.
authors.forename: Mohammed Abdalla
authors.initials: MA
authors.lastname: Elsheikh
authors.affiliation: Department of Life Environmental Conservation, Faculty of Agriculture, Ehime University, 3-5-7 Tarumi, Matsuyama 790-8566, Japan. Johan@agr.ehime-u.ac.jp.
authors.forename: Erni
authors.initials: E
authors.lastname: Johan
authors.affiliation: Department of Life Environmental Conservation, Faculty of Agriculture, Ehime University, 3-5-7 Tarumi, Matsuyama 790-8566, Japan. matsue@agr.ehime-u.ac.jp.
authors.forename: Naoto
authors.initials: N
authors.lastname: Matsue