Study of the scavenging behavior and structural changes accompanying the interaction of aqueous Pb2+ and Sr2+ ions with calcite

Abstract

The uptake of Pb 2+ and Sr 2+ ions from aqueous solutions by calcite was studied at various initial concentrations and pH conditions using the batch method under ambient conditions. XRPD, SEM/EDS, AAS/AES, and DRIFT techniques were used in characterizing the sorption process. The results indicated that the retention mechanism of Pb 2+ and Sr 2+ ions ranged from ion incorporation to precipitate overgrowth (cerussite, hydrocerussite for Pb, strontianite for Sr) depending on the concentration and pH conditions. The calcite structure seemed to be entirely vanishing upon formation of cerussite and hydrocerussite while partial dissolution of calcite occurred upon strontianite formation. The formation of precipitates showed rapid kinetics, and equilibrium seemed to be reached within about an hour from the start of mixing. SEM analysis showed that cerussite, hydrocerussite, and strontianite had columnar prismatic-like, tabular hexagonal-like, and needlelike morphologies. Based on a nine-month observation period, dry samples of hydrocerussite seemed to show more morphological stability than cerussite. EDS analysis indicated that Pb 2+ sorption is more favored than that of Sr 2+ , particularly in the phase of precipitate overgrowth. DRIFT analysis indicated a change in the symmetry of the carbonate groups in calcite matrix upon uptake of Pb 2+ and Sr 2+ cations.Keywords: calcite, Pb 2+ , Sr 2+ , strontianite, cerussite, hydrocerussite various metals and hence affecting the biogeochemical cycles of these metals in the environment. These minerals play a major rule in the chemical regulation of aquatic environments via precipitation, dissolution, and sorption reactions which are in turn controlled by the chemical processes taking place at the interface between the mineral lattice and the bulk solution (Morse, 1986). CaCO 3 can exist in three polymorphs; calcite, aragonite, and vaterite. Among these three polymorphs, calcite is the most thermodynamically stable one at room temperature and atmospheric pressure. Calcite possesses a rhombohedral/hexagonal structure with the trigonal carbonate ions being coplanar. The planes containing carbonate ions are perpendicular to the c-axis and they are rotated by 60° from one carbonate layer to the next (Kuriyavar et al., 2000).Knowledge of the nature of interaction of the metal ions with carbonate minerals is essential in predicting the extent to which they could be fated by these ions, the thing leading to retarding their dispersion into the environment. Recently, the increased applications of various surface-sensitive spectroscopic and microscopic techniques in sorption studies is enabling an understanding of this process on molecular-level the thing providing a better understanding of the mechanisms of interactions between different ions and sorbents. A limited number of such studies were devoted to characterizing the interac- INTRODUCTIONLead is an element that is continuously discharged into the biological environment as a result of various individual and industrial activities (g...

Assessing the Lead Solubility Potential of Untreated Groundwater of the United States

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