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.Mécanismes d'absorption et de désorption du Pb (II) à l'interface entre solution aqueuse et certaines argiles et la goethite. L'objectif de cette recherche est d'étayer, au moyen de techniques microscopiques, les résultats obtenus par le biais d'études macroscopiques d'équilibre sur l'absorption et la désorption du Pb (II) sur certains composants minéraux du sol (montmorillonite, illite, kaolinite et goethite). Les isothermes d'absorption ont montré que la capacité d'adsorption des substrats variait de la façon suivante : illite $>$ montmorillonite $>$ kaolinite $>$ goethite, et les isothermes de désorption ont démontré l'irréversibilité des liens formés. La variation du pourcentage de Pb adsorbé par la montmorillonite en fonction du pH montrait que, lorsque l'on a une valeur de pH inférieure au point d'hydrolyse, l'absorption est principalement due à une réaction d'échange ionique alors que, lorsque l'on a une valeur de pH supérieure au point d'hydrolyse, l'absorption est due à une combinaison de réactions d'échange ionique et précipitation. Les analyses semi-quantitatives effectuées au microscope électronique à balayage (MEB) couplé à la micro-analyse aux rayons X par dispersion d'énergie (EDS) ont démontré que, dans les argiles, le Pb remplaçait exclusivement les ions Ca. Dans la montmorillonite, cette substitution pourrait intéresser aussi les ions Ca de l'espace interlamellaire et, dans l'illite, il pourrait également y avoir substitution de groupes OH protonés et d'ions K situés au bord des sites interstrates. La goethite a montré une capacité d'adsorption équivalente à celle de la kaolinite

XAS study of lead speciation in a central Italy calcareous soil

""Purpose The Pb absorption processes on a heavy textured calcareous soil, typical of central Italy, were studied using synchrotron X-ray absorption spectroscopy (XAS) in order to probe, at molecular scale, the structure and chemical nature of Pb in contaminated soils and achieve precise description of Pb ions localization into these contaminated soils.. Materials and methods In order to distinguish the role of the different components of soils in Pb retention, samples were prepared from the original soils removing the carbonate fractions, the organic matter, the metal oxides, or selecting the clay fractions. Then these samples were fortified with Pb simulating the natural interactions processes of heavy metal solutions with soils. The quantitative analysis of near edge (XANES) as well extended (EXAFS) regions of Pb LIII edge absorption spectra, in comparison with Pb XAS data of selected reference compounds, allowed the precise determination of local structure and chemical environment of Pb ions in these soil samples.. Results Four components were individuated as the major responsible of Pb retention in calcareous soils: the carbonates, the metal oxide surfaces, the organic matter, and the colloidal inorganic surfaces containing clay components. The structural analysis suggests that, within these experimental conditions, the Pb adsorbed on the soil is generally present as Pb hydroxide with poor crystallization degree. However, the presence of carbonates (CaCO3) induces the co-precipitation of PbCO3-like phases with some degree of crystallinity.. "