Anomalous cadmium adsorption in flow interruption experiments

Adsorption of cadmium (Cd) onto fine sand was investigated using batch (static) and steady flow (dynamic) experiments at pHs of 5.5, 6.0 and 6.5. For both types of experiment, adsorption of Cd onto sand was found to increase with pH. In all experiments, greater adsorption of Cd was found in flow experiments than in batch experiments. Flow interruption was used to test whether nonequilibrium sorption occurred during transport of Cd. No concentration drop was observed immediately after the resumption of flow. Instead, for all experiments, a consistent increase in effluent concentration was observed indicating that Cd was desorbed during the interruption period. This indicates the significance of velocity on adsorption/desorption of Cd onto the sand. The amount of desorption was predicted by a transition from the dynamic to the static sorption isotherm. (C) 1998 Elsevier Science B.V. All rights reserved.