Investigations of ash topography/morphology and their relationship with heavy metals leachability

The leachability of heavy metals such as chromium (Cr), lead (Pb) and cadmi um (Cd) from the ash material obtained from waste combustion was studied. T he effects of ash surface topography and morphology on the leachability of these elements were examined using atomic force microscopy (AFM) and scanni ng electron microscopy (SEM). The AFM (scan size 10x10 mum) and SEM images of the simulated ash pellet obtained at various operating temperatures (100 0, 1400 and 1500 degreesC) showed significant microstructural and topograph ical changes. Ash pellets treated at 1000 degreesC contain porous and non-c ontinuous surface. On the other hand, the ash pellet obtained at higher tem perature (1500 degreesC) was found to contain a smooth, continuous and non- porous surface. The AFM height profile studies indicated that the top surfa ce Variation of the ash pellet at 1000, 1400 and 1500 degreesC were found t o be -40.0 to 25.5, -3.7 to 4.7 and -0.10 to 0.66 nm respectively. The SEM analyses also confirmed the presence of smooth, non-porous outer surface of ash formed at 1500 degreesC. In addition, it also showed the presence of c ompact and rigid interior for the same ash pellet. The leachability of the heavy metals was determined using standard toxicity characteristic leaching procedure (TCLP) test and the samples were analysed using atomic absorptio n spectroscopy. The results showed that the TCLP leaching ratios of the hea vy metals were Cr = 0.30, Pb = 0.05 and Cd = 0.09 at 1000 degreesC. However , the ash obtained at 1400 degreesC showed negligible heavy metals leaching ratio while at 1500 degreesC no leachability was detected (TCLP concentrat ion dropped to nondetectable levels). The use of high temperature treatment enabled the immobilization of heavy metals in the ash preventing their lea ching. Such ash can be considered as a non-hazardous material for reuse or safe disposal. (C) 2000 Elsevier Science Ltd. All rights reserved.