The Effect of CaO in the Immobilization of Cd<sup>2+</sup> and Pb<sup>2+</sup> in Fly Ash-Based Geopolymer
Xupicheng Ren | Fan Wang | Xiang He | Xiaomin Hu
The use of geopolymers for the solidification/stabilization (S/S) of municipal solid waste incineration fly ash (MSWI FA) is promising because the Cao in MSWI FA can provide an alkaline environment to facilitate geopolymer reactions and help to form the gel phase in the solidified body. This study investigated the role of CaO in MSWI FA in immobilizing common heavy metals, especially Cd<sup>2+</sup> and Pb<sup>2+</sup>. Tests were performed to evaluate the effect of CaO on the unconfined compressive strength (UCS) of the polymer and the leaching of heavy metals. The findings revealed that as the CaO content increased, the UCS of the geopolymer samples also rose, reaching a maximum 28-day UCS of 24.8 MPa at a CaO content of 31.5%. Additionally, higher CaO levels resulted in lower leaching concentrations of heavy metals in the stabilized material. When the CaO level is 32%, the levels of heavy metals that leach out are very low, with Pb<sup>2+</sup> at 0. 02 mg/L and Cd<sup>2+</sup> at 0. 01 mg/L, achieving a stabilization rate of over 93.6% for these ions. Moreover, the geopolymer’s characteristics were analyzed by XRD, FTIR, and SEM, and the immobilization mechanisms of Cd<sup>2+</sup> and Pb<sup>2+</sup> were identified as gelation, physical encapsulation, and chemical substitution.
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