Ground Improvement. Performance of Cement-Bentonite Slurry Wall against Heavy Metals Containment.

The containment ability of Pb++ and Cd++ by cement-bentonite slurry wall and its application to minimize the spread of groundwater contamination has been evaluated. Column specimens of cement-bentonite were permeated with 10ppm Pb(NO3)2 and 7.13ppm Cd(NO3)2 solutions, and the pH as well as heavy metal concentrations of the effluents were measured. The pH showed high alkali values at the early stages, and then gradually reached 10-10.5. Pb concentration showed values between 0.1-0.2ppm in the early stage and then decreased to 0-0.1ppm, whereas Cd concentration varies between 0-0.01ppm. The mechanism of containment of the above heavy metals in cement-bentonite has been discussed with regard to formation of complex ions, pH and hydraulic conductivity. The decrease in pH comprises the deterioration of containment ability of cement-bentonite. Since the cumulative mass of Cd leaked out from cement-bentonite is linearly related to the hydraulic conductivity, lowering the hydraulic conductivity due to larger quantity of cement additions resulted in the higher containment capacity of heavy metal. Applicable duration of cement-bentonite wall in the field (which can be parametrically calculated) showed that the thickness of cement-bentonite wall and hydraulic gradient across the wall might have a significant effect on the durability of cement-bentonite wall.