Influence of test conditions on solubility controlled leaching predictions from air-pollution-control residues

Leaching of Al, Ca, Mg, Si, S, Ba, Sr, Mo, Zn, Cd, Pb, and Cu from waste incineration air-pollution-control (APC) residues was investigated. Real-life conditions, i.e. removal of readily soluble compounds and longer equilibration time, were considered. Three different pH-static leaching experiments evaluating the importance of salt level and equilibration time were performed: (i) 48-h test on untreated APC residue samples, (ii) 48-h test on washed residue samples, and finally (iii) a 172-h test on washed residue samples. Experimental data were evaluated by geochemical modelling to identify potential solubility controlling minerals. For some elements (Al, Ca, Mg, Si, S, Mo, Zn, Cd, and Cu) the same controlling minerals were suggested regardless of the equilibration period or untreated/washed character of the tested material, whereas leaching of other elements (Ba, Sr) was far better described by considering a longer equilibration time, thereby pointing out the kinetic effects. Finally, a significant fraction of total lead (57%) was found to be rather mobile in the initial stage of leaching. Both pre-washing of the residues and longer equilibration times were shown to be simple yet useful methods to identify mineral phases that could control the release of constituents after the removal of readily soluble compounds in the initial stage of leaching.