Antimony leaching from uncarbonated and carbonated MSWI bottom ash

The recycling potential of municipal solid waste incinerator (MSWI) bottom ash may be limited by the leaching of antimony (Sb). Therefore, treatment methodologies need to be developed. The pH-dependent leaching behaviour of this oxyanion-forming element in fresh and weathered bottom ash is, however, not understood. Sb leaching was investigated in a wide range of both pH and extent of carbonation. Sb came close to equilibrium with calcium antimonate (Ca[Sb(OH)6]2) at acid and neutral pH. Therefore, adsorption experiments with synthetic calcite (CaCO3), ettringite (Ca6Al2(SO4)3(OH)12·26H2O), gypsum (CaSO4·2H2O), and portlandite (Ca(OH)2) and adsorption modelling to hydrous ferric oxides (HFO) and amorphous aluminium minerals (AAM) were conducted to investigate which minerals decrease Sb leaching below equilibrium with calcium antimonate. At pH > 12, calcium antimonate comes into solution due to portlandite formation, but the subsequent increase in Sb leaching is reduced due to strong interaction of Sb with portlandite and ettringite. Ettringite appears to be an important host mineral for Sb at the natural pH of mildly weathered bottom ash (11.8) because a minimum in leaching is observed. When pH is decreased below 10.5, ettringite dissolves and Sb comes into solution, approaching equilibrium with calcium antimonate near pH 9. Gypsum showed no affinity for Sb. The interaction of calcite with Sb was not clear. Adsorption modelling suggested that HFO, rather than AAM, control Sb leaching when pH < 9. During carbonation, Sb leaching first increased, most likely due to dissolution of ettringite. Then, Sb leaching decreased, since the pH became low enough to allow sorption by HFO.