Electrodialytic Separation of Heavy Metals from Municipal Solid Waste Incineration Fly Ash: Impact of Alternative Catholyte on Acid Dosing for System Control
Authors: Appiah Godfred , Kirkelund Gunvor M , Ottosen Lisbeth M 
Published: August 2024

Background and Purpose

The incineration of municipal solid waste (MSW) is a pivotal aspect of European waste management, particularly in Denmark, generating around 15 million tons of MSWI fly ashes annually, with nearly all of them ending up in limited landfill sites. From a resource perspective, these MSWI fly ashes are underutilised as a significant amount of critical raw materials and industrially valuable minerals, e.g., heavy metals (Cu, Ni, Cr, Co, Cd, etc.), nutrients, and rare earth elements contained in the ashes are lost during landfilling. Current landfilling practices for hazardous MSWI fly ashes are economically burdensome, ranging from 100 to 500 Euros per tonne. Future costs are anticipated to rise due to stricter EU landfill directives, increased disposal fees, waste taxes, and challenges in securing new landfill sites. Addressing these challenges requires the recovery or 'urban mining' of heavy metals from fly ash, transforming them into forms suitable for industrial use.
Over the last two decades, the electrodialytic separation (EDS) process has shown great potential in the removal of heavy metals, rare earth metals, and nutrients from particulate materials, e.g., MSWI fly ashes. This technique involves the simultaneous extraction of ions from the ash in a suspension, which is gradually acidified during the process, and the separation of the ions through anion and cation exchange membranes under the influence of applied current. These membranes serve as separators for the electromigration of ions from the suspension to concentrate compartments containing electrolytes. During the EDS process, the pH of the highly alkaline MSWI FA is reduced because of the induced water-splitting effect that occurs when the limiting current of the anion exchange membrane is exceeded (i.e., 4H2O (aq) → 4H+
(aq) + O2(g) + 4e-). This innate acidification facilitates the desorption of metals from the ash matrix into the liquid medium of the suspension so that metallic ions can be electromigrated through membranes into the electrolytes. However, an unwanted but consequent water-splitting effect can occur at the cation exchange membrane [i.e., 4H2O (aq) + 4e-→ 2H2(g) + 4OH-
(aq)], when the limiting current is exceeded here as well. To control and reduce the precipitation of the metals in the catholyte solution, there is a need to counteract OH-
(aq) formation by dosing acid into the catholyte. This acid dosage requirement is a notable challenge yet often overlooked in the literature, as it affects the cost of running the EDS system. Hence, this study investigated and compared the use of pure nitric acid (HNO3) as an alternative catholyte solution to acidified sodium nitrate in a 3-compartment EDS cell (which has been used so far).
The primary objective is to understand how the choice of catholyte solution influences the amount of acid dosage needed to control the pH of the catholyte under different currents and enhance the overall efficiency of the EDS process.

Materials and methodology

The fly ash sample was collected from the Amager Resource Centre (ARC) in Copenhagen. The fly ash was made up of ashes obtained from both the boiler and the electrostatic precipitator unit of the incineration plant. The EDS system was operated galvanostatically at 50 mA, 75 mA, 100 mA, and 125 mA for 14 days at an L/S ratio of 3.5 (distilled water). The metals under study were Cd, Co, Cu, and Zn.

Results and Conclusion

After 14 days of the EDS process, the amount of acid dosed to control the pH of the catholyte reduced considerably (4–8%) when 0.01 M HNO3 was used as compared to the reference (acidified 0.01 M NaNO3) for each current employed. Comparatively, at each current, the experiments that employed HNO3 as the catholyte recorded somewhat similar removal rates (i.e., the amount of the ions that were extracted in solution and simultaneously separated through the ion exchange membranes) of individual metals with the experimental studies that employed acidified NaNO3 as the catholyte. Overall, the removal of the metals increased considerably with currents for both catholyte solutions, with the highest recorded at 94-100% for Cd, 90-93% for Cu, 77-80% for Zn, and 69-100% for Co. The elemental distribution at the end of the EDS experimental demonstrations showed that Cu2+
, Cd2+, Zn2+, and Co2+ were mostly transported to the cathode end of the EDS system, with Cd2+ recording the highest transported metal species (90%). Based on these initial findings, pure HNO3 as a catholyte demonstrated a potential means of minimizing the amount of chemicals used to control the pH while maintaining the removal efficiency of the respective metals at each current used.