Reversibility of cadmium sorption to calcite revisited

Cadmium sorption to calcite was studied in aqueous solution at calco-carbonic (CO(2)-H(2)O-CaCO(3)) equilibrium and at concentrations below the solubility product of otavite (<10(-7) mol L(-1)). Experiments were conducted in a stirred flow-through reactor, lasting for 15 and 60 h at flow rates of 1.6 and 0.5 mL min(-1), respectively. Under both flow conditions, the cadmium breakthrough curves reached steady state after a flow time of about 50 reactor volumes, but different calcite saturation levels were achieved, which implied time-dependent sorption rates. Desorption of cadmium was induced by switching from Cd-containing to Cd-free solution in the reactor inflow. Reversibility of sorption was then evaluated by comparing amounts sorbed and desorbed over identical time intervals. On average, the desorption/sorption ratio was 95±10%. Desorption rates were close to those for sorption and showed the same time dependency: Initially, rates were fast and slowed down with time. "Irreversible" surface reactions such as solid-solution formation could not be inferred. In previous research, cadmium sorption was often shown to be partly irreversible. We surmise that calcite surface ripening is conditioned by the concentration of lattice-building ions in the EDL. At calco-carbonic equilibrium, these ion concentrations at the surface are lowest. Therefore, irreversible binding of cadmium is not favoured.