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

Electrodialytic Remediation 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.

In initial leaching experiments the pH-dependent desorption characteristics of the heavy metals Cd, Pb, Zn and Cu were analogous in the two MSWI ashes, and thus it was expected that the speciation of these metals was similar in the two ashes. On the other hand, the leaching behaviour (and concentration) of Cr was diverse.

The apparent similar speciation of Cd, Pb, Zn and Cu was only partly confirmed in the following electrodialytic remediation experiments. Significant differences in re-moval efficiencies were observed, especially for Pb and Zn. 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.

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.