Incidence of lead uptake on the microstructure of a (Mg, Ca)-bearing bentonite (Prrenjas, Albania)

Abstract

The article addresses Pb uptake and microstructural/morphological modifications of a (Mg, Ca)-bentonite (Prrenjas, Albania) after interaction with solutions of elevated lead content. Comparison is made with a Wyoming montmorillonite transformed to the Mg-dominated form. Characterised by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM), the Prrenjas bentonite is predominantly a smectite, containing 10% of interlayered illite, with associated kaolinite, antigorite, halloysite and minor chlorite. Significant magnesium is present in the octahedral layer in association with aluminium and iron. The CEC is rather high (above 80 meq/100 g). Magnesium and calcium are the main cations exchanged during CEC determinations and Pb uptake experiments. From TEM studies, microstructural units are composed by closely packed clay particles with 10 to 15 stacked platelets, in contrast with Wyoming montmorillonite. Exchange experiments with lead nitrate on suspended clays have been performed with concentrations ranging from 10 −5 M to 10 −2 M. A comparable capacity of lead sorption is observed by Atomic Absorption Spectrometry (AAS) for Prrenjas bentonite and Wyoming montmorillonite. At increasing lead uptake, a higher reduction of hydration in interlayer space is observed by XRD on Wyoming montmorillonite. At the highest lead concentrations, X-ray Photoelectron Spectroscopy (XPS) analyses of the Prrenjas Bentonite reveal Pb sorption in a “non interlayer” site. TEM images of the same clay show a significant size decrease of microstructural units in relation to the lead sorption. On the contrary, the Mg-exchanged Wyoming montmorillonite is not modified. Presumably, the “non interlayer lead” uptake results either from surface complexation at an edge site, or from ionic exchange with edge-exposed octahedral magnesium. Both mechanisms could initiate at edge-to-face contacts and result in a breaking process of the microstructural units.