Leaching Behavior of Trace Elements from Municipal Solid Waste Incineration Ash as Influenced by Addition of Carbonate Ions

Safe management of municipal solid waste incineration (MSWI) ash is essential to avoid detrimental effects of urbanization on the environment and human health. MSWI ash can potentially be used for carbon fixation by mineral replacement. Leaching of MSWI ash was conducted at various pH (7, 9, and 11) conditions and in the presence of additional carbonate ions (0–500 mmol/L). The concentrations of heavy metals in the resulting leachates were negligibly small (< 0.1 mmol/L), even in the presence of added carbonate ions, due to the higher stabilities of their oxide, sulfate, and carbonate species. Ca and Mg, which were significantly leached, were efficiently immobilized at higher pH or by adding carbonate ions; however, an excess of carbonate ions stimulated the release of B and F, increasing their leachate concentrations by factors of 2 to 5. X-ray diffraction and X-ray photoelectron spectroscopy analyses found that B- or F-bearing Ca/Mg minerals in the ash, such as calcium fluorite and layered double hydroxides, were converted to Ca/Mg carbonates. Thermodynamic calculations with geochemical modeling confirmed the mechanism of trace element release. The solubility of B- or F-bearing minerals was determined to be the dominant factor in preventing the leaching of these elements. The leaching potential of Ca/Mg is an essential factor in the carbon fixation capacity of MSWI ash; however, addition of excess carbonate ions induces conversion of the carbonate minerals, resulting in the release B and/or F. Highlights • Trace elements in MSW incineration ash leachate is mobilized by carbonate ions. • Carbonate ions convert Ca/Mg minerals into Ca/Mg carbonates, releasing adsorbed B and F. • Ca leaching potential is a factor for carbon fixation without releasing B and F.