Stabilization effect of chelating agents on heavy metals in two types of municipal solid waste incineration fly ash

Municipal solid waste incineration (MSWI) fly ash is complex in composition and contains large amounts of heavy metals. To investigate the effects of different passivating agents on Cd, Pb, Cr, Ni and Zn in different types of fly ash, 7 reagents were selected as passivating agents, including 3 inorganic reagents (Na2S, Na3PO4, NaH2PO4) and 4 organic reagents (SDD, DDTC, TMT, thiourea). The results showed that at low addition levels, sulfur-containing organic chemicals and phosphates were more effective in chelating these metals in both grate furnace incineration fly ash (GF FA) and circulating fluidised bed incineration fly ash (CFB FA), while Na2S was more effective in chelating heavy metals in CFB FA. In addition, agents containing -N-CS groups showed better chelation of heavy metals in GF FA, while heavy metals in CFB FA were more affinity to agents containing -SH groups. The ternary compound TMT-NaH2PO4-SDD (m1:m2:m3 = 2:3:1) was an excellent chelating agent for Pb, Cd in the GF FA, and TMT-Na2S-thiourea (m1:m2:m3 = 2:1:3) was a suitable chelating agent for these metals in CFB FA. The chelation rate of Cd and Pb after the treatments mixed ternary reagents was above 99%, and the leaching amount was much lower than the limit value of pollution control standards for domestic landfills. Moreover, under different pH leaching conditions, the leaching concentrations of Cd and Pb in the fly ashes treated with the ternary mixture were lower than the standard limits. In general, the ternary compound can effectively chelate heavy metals in fly ash, which provides a reference for selecting a suitable stabilizer combination for heavy metal in fly ash.