Mechanisms for the aging-induced reduction of lead solubility in scrubber residues from municipal solid waste combustion

This manuscript elucidates the mechanisms responsible for aging-induced reduction in lead leaching from scrubber residues. Leaching tests (JLT13) were conducted on 48 types of scrubber residues and lead solubility was found to be significantly reduced independent of incinerator type or type of gas treatment method. Reaction kinetics that result in lead solubility reduction were shown to be proportional to carbon dioxide partial pressure and in many cases were limited by mass transfer to the residue. With forced gas convection through the residue and a CO₂ partial pressure of 0.3%, the concentration of lead in leachate was reduced from 84 mg L^-1 to < 0.5 mg L^-1 in two days. Ettringite analogs (Ca₆Al₂(SO₄)₃ (OH)₂.26H₂O) were identified by X-Ray Diffraction in the wetted scrubber residues early in the aging process; these appear to have been converted into gypsum (CaSO₄.2H₂O) and vaterite (CaCO₃) in the aged material. All of these solid phases are believed to substitute lead into their crystal structure and evidence for this attenuation mechanism is presented.