Statistical analysis of process and composition factors affecting the leaching of heavy metals from fly ash solidified/stabilized wastes

The results of an extensive series of experimental Toxicity Characteristic Leaching Procedure (TCLP) tests on the leaching behavior of heavy-metal containing wastes solidified/stabilized by compacting the waste with pozzolanic fly ash are given. The test simulates the behavior of the waste by contacting it with an acidic media during 18 h with continuous agitation. The two heavy metals studied (chromium and cadmium) were selected based on the pH at which their minimum solubility occurred. Even though aluminum is not considered a heavy metal, it was also included in this study since its minimum solubility occurred at a lower pH than the other two metals. ocess factors were studied using a combined 26–3 fractional factorial design and the composition was studied using an augmented simplex-centroid design. Process factors included acid strength, acid/waste ratio, slurry pH, ash/slurry ratio, pressure, and aging time. Statistical analysis of the TCLP data obtained after 18 h of leaching showed that the factors which significantly affected the results depended on the specific response metal being studied. Only the process factor, aging time, was statistically significant for all of the responses. The composition of the waste did affect only the cadmium response. The results obtained in this study will contribute to understanding the leaching behavior of hazardous materials contained in solidified/stabilized fly ash wastes.