Electrodialytic removal of heavy metals from different fly ashes. Influence of heavy metal speciation in the ashes.

Electrodialytic remediation, an electrochemically assisted extraction method, has recently been suggested as a potential method for removal of heavy metals from fly ashes. In this work, electrodialytic remediation of three different fly ashes, i.e. two municipal solid waste incinerator (MSWI) fly ashes and one wood combustion fly ash was studied in lab scale, and the results were discussed in relation to the expected heavy metal speciation in the ashes. The pH-dependent desorption characteristics for Cr differed between the two MSWI ashes but were similar for Cd, Pb, Zn and Cu. Thus, it was expected that the speciation of Cd, Pb, Zn and Cu was similar in the two ashes. However, in succeeding electrodialytic remediation experiments significant differences in removal efficiencies were observed, especially for Pb and Zn. In analogous electrodialytic remediation experiments, 8% Pb and 73% Zn was removed from one of the MSWI ashes, but only 2.5% Pb and 24% Zn from the other. These differences are probably due to variations in pH and heavy metal speciation between the different ashes. Cd, the sole heavy metal of environmental concern in the wood ash, was found more tightly bonded in this ash than in the two MSWI ashes. Approximately 70% Cd was removed from both types of ashes during 3 weeks of electrodialytic remediation, although the total concentration was a factor of 10 lower in the wood ash. It was suggested that complex Cd-silicates are likely phases in the wood ash whereas more soluble, condensed phases are dominating in the MSWI ashes.