A novel carbonation washing strategy for the ultrafine fraction of incineration bottom ash under wastewater reuse conditions

Municipal solid waste incineration bottom ash (IBA) ultrafine fractions (< 0.25mm) exhibit high leaching potential, limiting their reuse despite their suitability for mineral valorization. This study evaluates a carbonation washing strategy under water reuse conditions to stabilize these fractions while enabling their use as supplementary cementitious materials (SCMs) and reducing CO2 emissions and water demand. A total of 13 laboratory-scale experiments were conducted in a 1L stirred reactor equipped with a porous stone sparger, treating 1.3kg of IBA ultrafines. Potentially toxic elements (PTEs) were monitored in both recycled process water and carbonated solids. Carbonation washing induces a strong reduction in leaching for most regulated elements after a single treatment step (up to −95% for Pb, −93% for Cu, −85% for Ni, −71% for Cd, −67% for As and −78% for Cl⁻). In contrast, increased leaching was observed for a limited number of species, notably Cr (+16%), Sb (+171%) and SO4 2⁻ (+108%). With increasing cycle number, partial re-enrichment occurs due to solute accumulation in the recirculated water; however, after 13 cycles, all metals except Sb remain below French Type 1 and Type 2 thresholds. Cr exhibits a distinct behaviour, with leaching decreasing across cycles and falling below the Type 1 limit from cycle 8 onward. The process achieves stable CO2 uptake on the order of 100 gCO2/kg IBA, unaffected by increasing salinity and metal accumulation in the recycled water. Water recovery efficiencies of approximately 88% per cycle were maintained, with dissolved species approaching asymptotic concentrations controlled by secondary phase precipitation. Overall, carbonation washing provides substantial stabilization of IBA ultrafines and constitutes a relevant process building block for their treatment, although the increased mobility of specific species, particularly Sb and SO4 2⁻, indicates that further process optimization is required.