Adsorption of Heavy Metals by Silicon and Aluminum Oxide Surfaces on Clay Minerals

Abstract The pH value of an adsorption edge (i.e., the pH value where a change occurs in the amount of aqueous ions adsorbed by a solid phase) is dependent on the surface‐metal bonding strength and, consequently, on the type of adsorption site involved. The metal‐adsorption sites of natural (untreated) kaolinite, montmorillonite, and synthetic mordenite were elucidated by comparing the pH values of their adsorption edges with those of Si and Al oxides. Cation adsorption by these clays is strongly influenced by the Si/Al surfacesite ratio, steric effects of interlayers or channels, Lewis acid strength of the metal, and pH of the medium. Using Ni, kaolinite showed additive adsorption behavior by the Si and Al oxide surfaces with well‐defined boundaries. The adsorption edge of Ni by mordenite occurred at low pH values and was attributed to the increased Ni‐binding stability inside the channels of the zeolite structure. Several heavy metals (Ni, Zn, Cd, and Pb) were used to properly identify the various adsorption edges on montmorillonite. The adsorption of Ni by montmorillonite was similar to the Si and Al oxide surfaces but with an additional adsorption edge observed at low pH values, which was attributed to the interlayer structure of the clay. The presumed pH‐independent cation‐adsorption behavior below pH 6 by 2:1 clay minerals is probably due to pH‐dependent adsorption on amorphous Si oxide sites that can induce ion‐exchange reactions at much lower pH values than previously assumed.