Mechanisms of Pb (II) sorption and desorption at some clays and goethite-water interfaces

The aim of this research is to corroborate the results obtained with Pb( II) sorption and desorption macroscopic equilibrium studies on some soil minerals ( montmorillonite, illite, kaolinite and goethite) using microscopic techniques. The sorption isotherms demonstrate that the adsorption capability of the substrates varies in the following sequence: illite > montmorillonite > kaolinite > goethite, and the desorption isotherms demonstrate the irreversibility of the bonds formed. pH adsorption edges on montmorillonite show that at a pH lower than the hydrolysis point the sorption edge is primarily due to ion exchange, while at a pH higher than the hydrolysis point, it is a combination of both ion exchange and precipitation. The EDS semi-quantitative analysis performed by SEM demonstrates that in the clays Pb replaced almost exclusively Ca ions. In the montmorillonite this replacement may also include the Ca ions in the interlayer space, and in the illite also, the replacement of protonated OH groups and the K ions situated at the edge of interlattice sites. Goethite shows an adsorption capability of the same magnitude as kaolinite.