Effect of shear and washing on metal retention in ferric flocs

Abstract

Flocculative wastewater residues present a serious threat to the environment and a costly disposal problem to industry. The composition of the sludges makes them a particularly intractable problem due to the need for either recovery or permanent isolation of the bound contaminant. The primary interest of this work was to determine the effect of conditioning processes such as shear and washing on the retention of heavy metal contaminants (nickel, cadmium, copper, zinc) within ferric flocs. Shear was found to produce an increase in the leach-out of certain heavy metals, including cadmium and nickel. Copper and nickel were unaffected by the application of shear. The extent of contaminant leaching, due to shear, increased with increasing contaminant to floc ratio. Repeated washing of the floc readily removed sodium ions, however, contaminant metal ions reformed an equilibrium with the washwater and the floc with every washing. © 2002 Society of Chemical Industry