Mechanochemical stabilization of heavy metals in fly ash with additives

Mechanochemistry, as a non-thermal method showing remarkable degradation for persistent organic pollutants, is extended to stabilize the heavy metals in municipal solid waste incineration (MSWI) fly ash in the present study. The leaching suppression of heavy metals (i.e., Zn, Pb, Cu, Cr, Cd, and Ni) facilitated by five additives during mechanochemical (MC) treatment is systematically investigated, identifying that almost all heavy metals are effectively suppressed with the assistance of either CaO or Ca₃(PO₄)₂. The pH-dependent leaching test further reveals the superiority of Ca₃(PO₄)₂ over CaO for heavy metals stabilization. Moreover, the evolution of heavy metal speciations analysed via an optimized sequential extraction procedure shows that MC treatment with Ca₃(PO₄)₂ significantly reduces the water- and acid-soluble fraction with high mobility from 56.8, 1.39, 12.3, 8.46, 1.13, and 29.5% to 4.96, 0.17, 0.14, 7.36, 0.12, and 0.22%, respectively for Cd, Cr, Cu, Ni, Pb, and Zn. The risk assessment indicates remarkable detoxification of fly ash in terms of heavy metals after MC treatment: the Nemerow pollution index is dramatically decreased from 9.35 (far above 3.0- the threshold of seriously polluted domain) to 0.71 (slightly over 0.7- the threshold of safety domain). Finally, a hypothetical mechanism according with results in this study for MC stabilization of heavy metals in fly ash is proposed as: the conversion of heavy metal compounds from mobile to immobile form through reaction with additives after activated by mechanical energy.