Inhibiting evaporation of heavy metal by controlling its chemical speciation in MSWI fly ash

The heavy metal (HM) evaporation in the thermal treatment of fly ash (FA) from municipal solid waste incineration (MSWI) has attracted considerable attention because the inhalation of or skin exposure to heavy metal raises human health risks. In this study, the washing and chemical treatments of FA were performed to inhibit the evaporation of HM (Zn, Pb and Cd) followed by a volatilization test of the pretreated FA at a temperature of up to 1000 °C. The results indicate that the evaporation behavior of HMs in fly ash was significantly affected by chemical speciation. The evaporation ratios of HMs with different speciations decreased in the order of acid soluble (F1) > reducible (F2) > oxidizable (F3) > residual (F4). The washing pretreatment process on fly ash would largely inhibit the evaporation of HMs by removing most of the metal chlorides, which are the dominant component of the F1 fraction. Meanwhile, the fly ash after phosphate chemical pretreatment has the least evaporation via converting the F1 fraction into the chemically stable F4 fraction. Therefore, the controllable formation of chemical speciation could effectively inhibit HM evaporation in fly ash.