An immobilisation mechanism for lead in fly ash subjected to mechanochemical treatment
Mechanochemical (MC) processes immobilise heavy metals in fly ash, a desirable outcome because recycled fly ash is commonly used in the synthesis of inorganic materials. We investigate the extent of immobilisation and the mechanism of lead (Pb) leaching during MC treatment of fly ash, using lead oxide (PbO) and lead dichloride (PbCl2) as model compounds. MC treatment of the fly ash inhibited Pb leaching by about 97% after a 12 h treatment. PbO was reduced to elemental Pb in a redox reaction by abraded iron (Fe) produced from the stainless steel ball mill used for MC treatment, and a phase transition of the PbO crystal structure from orthorhombic to tetragonal was confirmed. Production of Pb species with sparing solubility in water resulted in immobilisation of the Pb within the fly ash. Other insoluble intermediates detected in the milled samples, such as Pb2O3, potentially contribute to the inhibition of Pb leaching.