SORPTION AND SORPTION REVERSIBILITY OF CD ON CALCITE UNDER SIMULATED RIVERINE, ESTUARINE AND MARINE CONDITIONS

Laboratory experiments using Cd-109 and Ca-45 tracers were carried out to study sorption and sorption reversibility of cadmium on calcite un der simulated riverine, estuarine and marine conditions. Sorption of C d 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 ioni c strength, such as surface charge or double layer thickness, had litt le effect on the sorption behavior of Cd. Therefore, Cd is considered to be chemisorbed and sorption data can be described with a Ca-Cd exch ange model. Desorption experiments showed that Cd sorption on calcite is partly irreversible. The extent of reversibility depends on sorptio n time and pH. After short sorption times (< 1 week) desorption and ra pid isotopic exchange experiments with Cd-109 showed that sorption rev ersibility 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 equilibrati on time (2-6 months) the sorption reversibility decreases as a result of recrystallization. Ca-45 exchange experiments suggest that the recr ystallization 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 th at although at a pH of 8.3 more Cd is rapidly adsorbed, a pH of 7.9 se ems favorable for more permanent uptake of Cd by calcite. Assuming res idence 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.