The leaching of municipal solid waste incinerator (MSWI) bottom ash has been studied at different stages of natural weathering. Bottom ash samples, originating from a single incinerator, included grate siftings, unquenched, quenched, 6-week-old, 1.5-year-old and 12-year-old bottom ash. Leaching experiments were performed at various pH levels and liquid/solid ratios. The speciation code MINTEQA2 was used to evaluate whether the leachates are in equilibrium with minerals that are expected to form in MSWI bottom ash environments. Three major stages in weathering are identified, each stage having a characteristic pH that is controlled largely by Ca minerals and pCO 2 , but also by soluble Al and SO 4 : (1) unweathered bottom ash, with pH > 12 (grate siftings and unquenched samples); (2) quenched/noncarbonated bottom ash, with pH 10-10.5 (freshly quenched and 6-week-old samples); and (3) carbonated bottom ash with pH 8-8.5 (1.5-and 12-year-old samples). A comparison of the leaching mechanisms identified for these stages of weathering reveals major differences for Ca, Al, Fe, Si, and SO 4 ; for Na, K, and Cl, on the other hand, leaching mechanisms appear largely similar. The effect of weathering on the leaching of Pb, Zn, Cu, and Mo is discussed.

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In this paper, we have successfully applied surface complexation/precipitation modeling to describe the leaching of contaminants from weathered municipal solid waste incinerator (MSWI) bottom ash. A combination of 'selective' chemical extractions and leaching at pH values unfavorable for sorption has been shown to be useful for obtaining sorbent mineral and sorbate concentrations. Model calculations are based on the Diffuse Layer Model and a database of sorption constants for hydrous ferric oxide (HFO), both of which are incorporated in the computer code MINT-EQA2. This approach to sorption modeling shows that the leaching of Mo, Pb, and Cu from weathered MSWI bottom ash is well described by surface complexation, whereas the surface precipitation model is needed to describe the leaching data of Zn. The leaching of Cd cannot be explained with these models since the affinity of Cd for HFO is too low. Our results suggest that Mo and Zn are bound mainly to HFO. Additional sorbent minerals, e.g., amorphous aluminum (hydr)oxides, are required to describe the leaching of Cu and Pb.

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The complexation of Cu with dissolved organic carbon (DOC) in leachates from fresh and 1.5-year old municipal solid waste incinerator (MSWI) bottom ash was studied using a competitive ligand-exchange solvent extraction procedure. At least two different ligands appear to be involved in the complexation of copper with DOC. The dissolved Cu appears to be 95-100% organically bound in leachates from both the fresh and the weathered bottom ash, and geochemical modeling indicates that the leaching of Cu from these ashes is primarily controlled by the availability of the organic ligands in the bottom ash. The mechanism that binds Cu to the solid phase is likely to be tenorite in the fresh bottom ash, and sorption to amorphous Fe/Al-(hydr)oxides in the weathered bottom ash.