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. Characterized 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/100g). 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 to15 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 an important size decreasing of microstructural units in relation to the lead sorption. On the contrary, the Mg-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

Structure, Energetics, and Dynamics of Smectite Clay Interlayer Hydration: Molecular Dynamics and Metadynamics Investigation of Na-Hectorite

“…80 For a given clay composition at relatively low H2O activities, hydration of the charge-balancing counter ions is the dominant interaction leading to swelling, and differences in swelling behavior are directly related to the differences in the cation hydration energy. [75][76][77]81 Previous experimental studies of hectorite swelling and its interactions with cations and water molecules have used a variety of techniques, including X-ray diffraction (XRD), 46,[82][83][84][85][86][87][88][89][90][91] thermogravimetric analysis (TGA), 40,46,91 quasi-elastic neutron scattering (QENS), 57,58,60,64,65,67 and NMR spectroscopy. 39,40,42,[44][45][46] XRD is widely used and provides information about the 6 effects of, for instance, relative humidity on the interlayer spacing.…”

“…Previous experimental studies of hectorite swelling and its interactions with cations and water molecules have used a variety of techniques, including X-ray diffraction (XRD), ,− thermogravimetric analysis (TGA), ,, quasi-elastic neutron scattering (QENS), ,,,,, and NMR spectroscopy. ,,,− XRD is widely used and provides information about the effects of relative humidity on the interlayer spacing, for example. ,,,− Due to static and dynamic disorder in the interlayer, XRD typically does not provide the specific atomic positions needed for detailed structural analyses, particularly with respect to the hydrogen positions, and provides essentially no information about dynamics. Neutron scattering also probes interlayer structure and H 2 O diffusion and has been recently used to study interlayer water diffusion in smectites. ,,,,, Diffusion of water in one- and two-layer hydrates of hectorite is typically 10 and 3–6 times slower than in bulk water, respectively. ,,,,, Unfortunately, neutron scattering techniques have difficulty distinguishing between the H atoms of interlayer H 2 O molecules and those of structural OH – groups, and structural OH – groups contribute background signal and complicate data analysis. ,, Thus, QENS has been used to investigate water diffusion in the interlayer galleries of hectorite for only fluorhectorites, in which F – substitutes fully for structural OH – , ,,,,, whereas natural hectorites often contain nearly equal mixtures of structural OH – and F – .…”

Microchemical contaminants as forming agents of anthropogenic soils

“…At high ionic strength, Pb adsorption is pHdependent, suggesting inner sphere complexation where adsorption occurs on functional groups existing on the montmorillonite edges. TEM observations showed that Pbexchanged on a natural bentonite surface displayed microaggregates or microstructure units, consisting largely of an association of small particles packed together without interparticle pore space (Jozja et al 2006).…”