Heavy metal stabilization and alkalis neutralization in municipal solid waste incineration fly ash using acid washing coupled with chelating agents

Heavy metal stabilization and alkali neutralization are crucial for the management of municipal solid waste incineration fly ash (MSWI FA). In this study, the effects of oxalic acid (OA) and citric acid (CA) washing on heavy metal migration and alkali changes were investigated. Besides, three agents were selected to stabilize acid-washed fly ash (FA), and the effects of heavy metal stabilization under different disposal environments and the changes in their form distributions were comparatively analyzed. The experimental results demonstrated that with increasing organic acid concentration, the amount of Cd and Pb extracted from MSWI FA increased, whereas the amount of alkalis tended to decrease. Moreover, the leaching concentration of heavy metals and the percentage of unstable forms increased after acid washing. The alkalis content in the products decreased to 7.91–8.75 after stabilization, which met the standards of hazardous waste landfills. Compared with other agents, sodium diethyldithiocarbamate (DDTC) exhibited excellent heavy metal stabilization performance. After the addition of 1% DDTC, the leaching concentrations of Cd and Pb decreased to values lower than the national standard limit. Additionally, the curing rates of Cd and Pb were greater than 98% under the two leaching conditions. With the addition of DDTC, the percentage of stable forms increased, and the percentages of stable Cd and Pb forms increased to 87.68% and 99.03%, respectively, after treatment with 2% DDTC. The above results corroborated that CA coupled with DDTC stabilization can significantly reduce Cd and Pb toxicity and neutralize alkalinity in FA.

Graphical abstract