Evaluation of collaborative disposal of heavy metals in MSWI fly ash along with its environmental risk assessment

Aiming at the municipal solid waste incineration fly ash rich in Pb and Cd in a certain area of South China, cement and chelating agent are used for solidification/stabilization. The effects of cement content and curing time on the toxicity of Cu, Zn, Pb and Cd are analyzed. Based on the micro morphology, chemical components and chemical speciation of heavy metals (HMs), the mechanism of cement solidification is explored. The effects of co-disposal with cement-chelating agent and cement alone are compared, and an optimization scheme is proposed. In addition, the environmental risk of fly ash before and after disposal is evaluated by three environmental risk assessment methods. The test results show when the cement of 10% is added, the disposal efficiency of most HMs exceeds 80%. With the increase of the cement content and curing time, the leaching toxicity is gradually reduced due to the formation of hydrocalumite. The co-disposal with cement-chelating agent reduces cement consumption by 10% compared with that with cement alone, thus increasing the disposal volume of fly ash per unit volume. After the cement solidification, the environmental risk of HMs is reduced. When the cement content is 10%, 20%, 30% and 40%, the synthesis toxicity index (STI) of HMs in fly ash decreases by 10.2%, 21.4%, 41.8% and 53.2%, respectively. Based on the proposed experimental data and the literature researches, the threshold value of STI greater than 0.06 is defined as the high-risk zone, which can be widely used in the effect evaluation of disposal methods for HMs in fly ash.