Coagulation and Electrocoagulation of Wastes Polluted with Colloids

Abstract

The goal of this work has been to compare for both, continuous and batch processes, the efficiencies of the chemical and the electrochemical coagulation processes with hydrolyzing aluminum salts, and to determine the similarities or differences that exist between both coagulation processes. To meet the objective, experiments of both coagulation technologies have been carried out in the same operation conditions and the results have been interpreted in terms of the mechanisms previously proposed in literature for kaolin coagulation. The charge neutralization by the adsorption of monomeric hydroxocations onto the kaolin surface can be the primary coagulation‐mechanism for low concentration of aluminum and acidic pHs (below 4). In the range of pH 4–7, two primary mechanisms can explain the experimental behavior of the system: sweep flocculation for high concentration of aluminum, and a combination of precipitation‐charge‐neutralization and charge neutralization by adsorption of monomeric or polymeric aluminum, for low concentration of aluminum. In the continuously‐operated processes, the efficiency in the turbidity‐removal seems to be much related to the aluminum species present in the treated waste, and not to the way of adding aluminum to the reaction system. For the same steady‐state pH and aluminum concentration, the same turbidity removal is obtained in both, the chemical and the electrochemical coagulation processes. For high aluminum/kaolin ratios, kaolin suspensions which contain sulfate as electrolyte, achieve better removals of turbidity than those containing chloride ions. The operation mode (continuous or discontinuous) influences greatly on the efficiency of the electrocoagulation processes. Similar efficiencies are obtained for low (below 5 mg dm⁻³) and high doses of aluminum (above 20 mg dm⁻³). However, at intermediate doses a strong difference is observed, with a more marked decrease in the efficiency in the discontinuous process. This observation has been explained considering that the addition of aluminum in the continuous process is instantaneous (and not progressive as in the discontinuous one), and thus, the sweep coagulation mechanism is more favored in this operation mode.

Keywords:

• Coagulation
• electrocoagulation
• electrochemistry
• aluminum electrodes

Acknowledgement

This work was supported by the MCT (Ministerio de Ciencia y Tecnología, Spain) and by the EU (European Union) through project CTM2004‐03817/TECNO.