Adsorption of lead and cadmium ions on calcite in the Krka estuary

Abstract

The adsorption of Pb 2+ and of Cd 2+ ions from calcareous Krka river water of various salinities (3, 14, 20 and 38 psu) on calcite (CaCO 3, Merck p.a.) was investigated. Simultaneous adsorption of Pb 2+ and Cd 2+ ions was studied as well. The results suggest that the two ions are adsorbed at different calcite surface sites; Pb 2+ remained firmly bound to calcite at all salinities, whereas Cd 2+ was firmly bound at low salinities and released at high salinities. Dissolved natural organic ligands at the freshwater-seawater interface (FSI; at 14 psu) promoted and below the FSI (at 20 psu) decreased the adsorption of Cd 2+. The experiments were performed at metal concentrations of 8 × 10 −8 mol l −1 and at natural pH values around pH 8. Adsorbed amounts of trace metal ions were determined in filtered samples by differential pulse anodic stripping voltammetry (DPASV) with a three-electrode system, after 24 h of equilibration with calcite. Several adsorption models were tested, such as Freundlich's, Langmuir's and Schindler-Stumm's surface coordination model. Adsorption isotherms belong to S-1 class of empirical adsorption isotherms. None gave reasonable values of adsorption constants. The fractional partitioning of adsorbent to the solid phase when normalized to the quantity of adsorbent present ( K d) declined as solid concentration increased. It was found that the surface charge is not responsible for the observed effect. Instead, aggregate size increased, which effected a loss in surface area. This is a reasonable explanation for the observed S-shaped adsorption isotherms curves.