Preliminary treatment of MSW fly ash as a way of improving electrodialytic remediation

Abstract

In the current work electrodialytic remediation (EDR) was applied to remove heavy metals from municipal solid waste (MSW) fly ash, a hazardous waste collected during flue gas treatment. Tests were conducted to evaluate if EDR could be improved by introducing a preliminary treatment in which very soluble salts were removed. Three different preliminary treatments were conducted with different L:S ratios and pH. Treatment in which metal release and L/S ratio were lower was selected for EDR. Electrodialytic remediation was performed at a constant current of 38 mA, for 14 days, using gluconate as a solubilisation enhancement agent. Conductivity and pH were monitored and electrolyte samples were collected every 4 days to evaluate metal release over time. It was found that the preliminary treatment reduces fouling of the ion-exchange membranes used in EDR and drastically increases the removal of metals. Remediation time was also considerably reduced. Additionally, preliminary washing reduces energy consumption during EDR, since electric current is not wasted in the transport of soluble salts. Sequential extraction was performed in the untreated and treated samples to help identify how metals are bond to the fly ash. It was seen that at the end metals are mainly found in the strongly bonded and residual phases. This indicates that the combined treatment (washing + EDR) is successful in reducing the environmental risk posed by fly ash.

Keywords:

• Electrodialytic remediation
• heavy metal
• MSW fly ash

Acknowledgments

This work was partially funded by the Portuguese Project POCTI/AGG/45073/2002, approved by FCT and POCTI with FEDER funds. The authors would like to acknowledge Ebba Cederberg Schnell for performing the sequential extraction on the treated fly ash sample.