Solubility of elements in waste incineration fly ash and bottom ash under various leaching conditions studied by a...

An optimized 7-step sequential extraction was applied to fly ash samples from two waste incineration plants and a bottom ash sample. The solubility of 37 elements in water under alkaline and neutral conditions, ion exchange, acid solubility as well as the influence of reducing and oxidizing agents, were investigated using reagents which are typically applied in sequential extractions. Potential error sources and the suitability and selectivity of individual steps and extracting agents were also evaluated. Additionally, the amounts of total dissolved solids were determined for each extraction step, and the results were validated by comparison with the analysis data of the elemental composition. All the investigated incineration residues exhibited a very similar solubility behavior. Only the alkali metals in the bottom ash were considerably less water-soluble than those in the fly ash. The solubility behavior among the rare earth elements was alike. The pH of the fly ash suspensions in water increased over several hours from neutral to alkaline. Concentration changes of water-soluble elements were investigated during the pH increase and to what extent precipitated elements can be re-dissolved by a subsequent neutralization. Meanwhile, it was shown that chloride ions in commonly used sequential extraction agents (e.g., MgCl₂ and NH₂OH·HCl) can influence the extractability of individual elements, which can lead to misinterpretations of the results. Using MgCl₂ to study Mg²⁺ ion exchange led to the dissolution of Ag and Cd only caused by the chloride ions. Furthermore, the order of the reducing and oxidizing step was found to be interchangeable.