Early solidification/stabilization mechanism of heavy metals (Pb, Cr and Zn) in Shell coal gasification fly ash based...

Immobilizing heavy metals (HMs) from municipal solid waste incineration fly ash (MSWIFA) using shell coal gasification fly ash (SFA)-based geopolymer can solve the energy and environmental challenges simultaneously. In this study, we synthesized a geopolymer with SFA, metakaolin (MK), and steel slag (SS) to solidify and stabilize HMs (Pb, Cr, and Zn) and investigated the early immobilization mechanisms. The results show that the prepared geopolymer possessed high early-age mechanical strength and immobilization efficiency to HMs (>90%), even under the effect of excess HMs. The early immobilization mechanism of the geopolymer for the HMs could be described as follows. (1) Most of HMs were remained in the aluminosilicate. (2) The presence of amorphous zeolite precursor and clay minerals may contribute to restrain the HMs leaching; (3) Pb and Zn were trapped by the gel structure in M-O-Al and M-O-Si forms (M = Pb or Zn), whereas Cr (VI) was reduced to Cr (III). (4) Cr might involve in the geopolymerization of [SiO₄] and [AlO₄]^- units. (5) The immobilization process of Pb and Zn in the geopolymer could be described as crystal growth (NG) - phase boundary reaction (I) - NG - I - diffusion (D), whereas that of Cr is prolonged to NG-I-NG-I-NG-I-D.