The effect of oxide coating dissolution on Cd transport through a natural quartz sand

The role of flow rate on Cd transport through a natural aquifer sand was investigated using a laboratory column set-up with particular reference to dissolution processes of external coatings. Clays and (hydr)oxides dissolution was investigated by tests performed feeding the column with Cd-free solutions at three different flow rates (0.2, 1 and 5 mL min(-1)). Flow rate significantly affected only the dissolution of Fe-containing coatings by releasing into the liquid phase different amounts of Fe. Cd transport experiments showed a strong effect of flow rate on Cd breakthrough curves and the lowest Cd adsorption was observed at the lowest flow rate. This behavior was fully explained in terms of competition between dissolved Fe and applied Cd for the same adsorption sites. Comparison of the Cd transport behavior on a pure quartz sand allowed confirmation of the strong influence of clay and (hydro) oxide coatings in determining the surface properties of the heterogeneous natural materials. An equilibrium model was proposed to describe the dynamic Cd behavior under the different experimental conditions (flow rates and Cd concentrations).