Zinc and cadmium mobility in sand: effects of pH, speciation, Cation Exchange Capacity (CEC), humic acid and metal ions

This study was undertaken to investigate the ways in which pH, speciation, cation exchange capacity (CEC), humnic acid and added metals influence Zn and Cd mobilities in porous media. The studies were conducted using columns of pure sand and sand in admixture with goethite. The mobile phases were based on aqueous NaCl solution. In all cases Zn and Cd were found to be more mobile at pH = 4.0 than pH = 6.5. Speciation calculations were used to rationalise all the observations. Introduction of a small amount of goethite in the column packing caused increased retardations. However the observed increases could not be explained simply in terms of an increased column CEC value, sorption dynamics were also involved. Increased retardations were also noted in the presence of HA. The effect reflected the amounts present in the stationary and mobile phases. Ca and Fe enhanced Zn and Cd mobilities but Al had no discernible effect.