Adsorption of Zn(II) onto hydrous aluminosilicates

Abstract

The adsorption characteristics of Zn(II) onto various hydrous aluminosilicates were investigated. Two major types of aluminosilicates, namely, amorphous (Zeosyl 100, Zeo 49, Zeolex 23, and Blazer) and crystalline (Zeolite A and Mordenite), and two pure oxides (SiO 2 and Al 2O 3) were studied. These solids were characterized for their pH zpc and cation-exchange capacity (CEC). Isomorphous substitution and constant potential sites are the two major contributing factors to CEC. CEC increases with decreasing Si Al ratio, with Zeolite A, Mordenite, Blazer, and Zeo 23 exhibiting a CEC 2 to 10 times that of Zero 49, Zeosyl 100, SiO 2, and Al 2O 3. The CEC of pure oxide is mainly contributed by the capacity of the electrical double layer. Both constant charge sites and constant potential sites play an equally important role in Zn(II) adsorption. When adsorption takes place at constant charge sites, the reaction follows a simple ion-exchange process and is most pronounced in the low pH region. Adsorption onto constant potential sites is brought about by specific chemical interactions. Surface precipitation to some extent also contributes to Zn(II) adsorption, especially in the alkaline pH regionl.