RETENTION MECHANISMS FOR TOXIC CATIONS USING ARTIFICIAL CONFINEMENT BARRIERS OF CLAY CEMENT MIXTURES

Artificial linings for the confinement of toxic wastes are often made of clay-cement mixtures, which sometimes contain other powdery substan ces, such as carbonates. These mixtures are characterized by: (1) high alkaline contents; and (2) strong adsorption capacities, due to the h igh specific surface of the clay and the artificial silico-aluminous c omponents. The migration of toxic cations normally occurs in acidic so lutions. When they come in contact with the clay-cement mixture, the a lkaline content of these solutions is progressively activated. The rea ctions which occur are related to the acid-base neutralization reactio n. The sorption of Pb2+ on a clay-cement mixture was studied using the batch equilibrium technique and percolation tests. The change in the retention capacity with increasing pH and the amount of Pb2+ added is similar in both cases. Pb2+ is entirely retained at pH > 7 and this ca n be attributed to precipitation as hydroxides or hydroxycarbonates. W hen the alkaline content of the mixture is neutralized, the pH of the solution lowers progressively and the clay-cement mixtures, especially the cement-rich mixture, exhibited stoichiometric uptake of Pb2+ and release of Ca2+, suggesting the formation of lead silicate.