Sorption and sorption reversibility of Cd on calcite under simulated riverine, estuarine and marine conditions

Laboratory experiments using 109Cd and 45Ca tracers were carried out to study sorption and sorption reversibility of cadmium on calcite under simulated riverine, estuarine and marine conditions. Sorption of Cd on calcite increased with increasing sorption time, calcite surface area, aqueous Cd2+ activity, and decreasing aqueous Ca2+, Mg2+ and Cl− activity. Electrostatic factors which could be influenced by the ionic strength, such as surface charge or double layer thickness, had little effect on the sorption behavior of Cd. Therefore, Cd is considered to be chemisorbed and sorption data can be described with a Ca-Cd exchange model. Desorption experiments showed that Cd sorption on calcite is partly irreversible. The extent of reversibility depends on sorption time and pH. After short sorption times (<1 week) desorption and rapid isotopic exchange experiments with 109Cd showed that sorption reversibility decreases from pH 8.3 to 7.9. At lower pH, Cd may merely be sorbed at the energetically most favorable sites like kinks and steps, because of the increased competition with Ca. After long equilibration times (2–6 months) the sorption reversibility decreases as a result of recrystallization. 45Ca exchange experiments suggest that the recrystallization rate decreases from pH 7.9 to 8.2. As a result thereof, the slow removal rate of Cd decreases as well. This study indicates that although at a pH of 8.3 more Cd is rapidly adsorbed, a pH of 7.9 seems favorable for more permanent uptake of Cd by calcite. Assuming residence times of suspended calcite in rivers (lower pH) are 24 h, the release of Cd in the estuary (higher pH) is expected to be limited.