Speciation and mobility of volatile heavy metals (Cd, Pb, and Tl) in fly ashes

Speciation of volatile metals Cd, Pb, and Tl in fly ashes (FAs) produced from burning of hard coal in stocker-fired boilers (SFBs) was studied. Two grain fractions of fly ash collected in a multicyclone and battery cyclone of the systems of dust separation from three SFB units operating in various urban heating plants were analyzed. The characteristic feature of speciation of the three metals was a large share of labile fractions: Cd (av. 46.1 %), Pb (av. 39.8 %), and Tl (av. 21.6 %). The fraction which most clearly reflected the different chemical properties of the investigated metals was the oxidizable fraction: F(4)-Cd-0 %, F(4)-Pb-av. 10.0 %, and F(4)-Tl-av. 30.2 %. The importance of condensation of the volatile metal species on FA particles for shaping speciation of these metals was characterized using the normalized enrichment factor (NEF): Pb (2.3 ± 0.8) > Tl (1.8 ± 0.9) ≈ Cd (1.7 ± 0.6). Speciation of heavy metals may also be important economically, because the level of mobility coefficients (K _Cd = 0.46, K _Pb = 0.40, and K _Tl = 0.22) in the case of fly ashes considerably enriched with toxic metals (Cd 4.8 ± 3.4 mg/kg, Pb 293 ± 210 mg/kg, and Tl 6.3 ± 4.5 mg/kg) may limit their utilization range.