ABSTRACT

The uptake of Pb²⁺ and Sr²⁺ 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 Pb²⁺ and Sr²⁺ 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 needle-like morphologies. Based on a nine-month observation period, dry samples of hydrocerussite seemed to show more morphological stability than cerussite. EDS analysis indicated that Pb²⁺ sorption is more favored than that of Sr²⁺, 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 Pb²⁺ and Sr²⁺ cations.