Modeling the leaching behavior of fly ash solidified/stabilized wastes

Pollution of groundwater may be caused by disposal of hazardous wastes on land. One method that has been found adequate for the disposal of wastes is the solidification/stabilization process. The solidified form is obtained by combining a pozzolanic material, such as fly ash, with the waste.; In order to establish the environmental acceptability of the solidified/stabilized waste for land disposal, the U.S. Environmental Protection Agency has developed the toxicity characteristic leaching procedure (TCLP) test. The test simulates the behavior of the waste under non-equilibrium conditions by contacting the waste with an acidic media and by measuring the concentration of material leached from the waste after 18 hours of continuous agitation.; The first purpose of the present research is to develop a mathematical model to explain the leaching behavior of heavy metals contained in fly ash solidified/stabilized wastes. The model would help in understanding the leaching behavior and preventing the leaching of heavy metals under the “worst case” leaching conditions established by the toxicity characteristic leaching procedure. In the model it is assumed that once the waste is contacted with the leaching solution, the heavy metals diffuse and then reprecipitate as hydroxide. To develop the model, a combination of Fick's diffusion theory and theoretical equations for the solubility of chromium, cadmium, and aluminum is used. The TCLP test is run in a semi-batch mode during 100 hours to simulate real environmental conditions. It was found that only cadmium behaves under the assumption given by the model, having the species Cd(OH)+1 as the limiting solubility species.; The other purpose of this research is to develop a statistical model to quantify the amount of heavy metals released from the solidified/stabilized waste to the leaching solution. The model would allow selecting the most appropriate process conditions in the formation of the solidified waste and to minimize the release of the metals. The model is based on a simplex-centroid and a 26-3 fractional factorial experimental designs. A linear polynomial equation was found to quantify the amount of chromium and cadmium release, whereas a third degree polynomial was required for the same purpose for aluminum.