Leachability of metals from fly ash: Leaching tests before and after extraction with supercritical CO₂ and extractants

Residues such as sewage sludge or fly ashes are usually contaminated with toxic heavy metals. These metals leach out after contact with water, thus polluting the groundwater. If aqueous concentrations of metals are exceeded, these residues require expensive disposal. Supercritical fluid extraction (SFE) is a promising method to reduce the metal content of contaminated particles to such an extent that leachability is reduced and the demands of legislation are observed. This paper focuses on the metal leachability and the effect after extraction in a 12-L revolving vessel with supercritical CO₂ with fly ash from a municipal waste incinerator as model compound. Pre-leaching with water prior to SFE led to the removal of water-soluble compounds and structural changes on the ash surface. Standard leaching tests of fly ash after SFE with Cyanex 302, TBP, D2EHPA, or mixtures of TBP-D2EHPA focussed on metals such as Zn, Pb, Mn, Cd, Cu, V, Sb, Ni, Mo, Cr, and Co. The leaching test showed a leachability increase of Sb and the successful reduction in leachability of Zn, Pb, and Mn in particular.