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

Abstract

The leachability of heavy metals such as chromium (Cr), lead (Pb) and cadmium (Cd) from the ash material obtained from waste combustion was studied. The effects of ash surface topography and morphology on the leachability of these elements were examined using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The AFM (scan size 10×10 μm) and SEM images of the simulated ash pellet obtained at various operating temperatures (1000, 1400 and 1500°C) showed significant microstructural and topographical changes. Ash pellets treated at 1000°C contain porous and non-continuous surface. On the other hand, the ash pellet obtained at higher temperature (1500°C) was found to contain a smooth, continuous and non-porous surface. The AFM height profile studies indicated that the top surface variation of the ash pellet at 1000, 1400 and 1500°C were found to 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°C. In addition, it also showed the presence of compact 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 absorption spectroscopy. The results showed that the TCLP leaching ratios of the heavy metals were Cr=0.30, Pb=0.05 and Cd=0.09 at 1000°C. However, the ash obtained at 1400°C showed negligible heavy metals leaching ratio while at 1500°C no leachability was detected (TCLP concentration dropped to nondetectable levels). The use of high temperature treatment enabled the immobilization of heavy metals in the ash preventing their leaching. Such ash can be considered as a non-hazardous material for reuse or safe disposal.